Machine Drawing 0070659923, 9780070659926

Table of contents :
Cover
About the Author
Preface
Aknowledgements
First Look
Contents
Part A: Drawing Fundamentals
Chapter 1: Introduction to Drawing
Chapter 2: Introduction to CAD
Chapter 3: Drawing Apparatus
Chapter 4: Lines and Freehand Sketching
Chapter 5: Lettering
Chapter 6: Basic Dimensioning
Part B: Methods of Projection
Chapter 7: Orthographic Projections
Chapter 8: Sectiional Views
Chapter 9: Auxiliary Views
Chapter 10: Axonometric Views and Oblique Views
Chapter 11: Perspective Views
Part C: Joints and Couplings
Chapter 12: Riveted Joints
Chapter 13: Threads
Chapter 14: Bolts and Nuts
Chapter 15: Welded Joints
Chapter 16: Shafts, Keys, Cotter and Pin Joints
Chapter 17: Couplings and Clutches
Chapter 18: Pipe Joints
Part D: Production Drawings
Chapter 19: Tolerances, Limits and Fits
Chapter 20: Geometrical Tolerances and Surface Finish
Chapter 21: Material Specifications
Chapter 22: Production Drawings
Part E: Machine Parts
Chapter 23: Springs
Chapter 24: Belts and Pulleys
Chapter 25: Bearings
Chapter 26: Gears
Part F: Machines
Chapter 27: Part and Assembly Drawings
Chapter 28: Internal Combustion Engines
Chapter 29: Steam Power Plants
Chapter 30: Machine Tools
Appendix 1: Some Useful Indian Standards
Appendix 2: Some Relevant Internet Sites
Index

Citation preview

MACHINE DRAWING Includes AutoCAD

About the Author The author graduated in Mechanical Engineering from Jodhpur University in 1963. While completing his Masters in 1970, he also served at Malviya Regional Engineering College, Jaipur. He acquired his Ph.D. degree in 1979 from Motilal Nehru Regional Engineering College, Allahabad. As one of the most distinguished faculty member of his college, he also served as Professor and Head of Mechanical Engineering Department followed by Dean Academic and Dean Research and Consultancy position. Many Ph.D. students have benefitted from his guidance including private and public companies to whom he has provided consulting services. He has many research papers to his credit in national and international journals and is also a life member of ISTE and fellow member of Institution of Engineers, India. With over 44 years of teaching experience, he is one of the most knowledgeable academicians in machine drawing and AutoCAD. With over 15 years of international teaching experience, he has served as faculty member for various universities abroad. The author’s previous book, Working with AutoCAD 2000, published by Tata McGraw-Hill, was widely accepted in India and abroad. He is pleased to introduce this new book that leverages modern day technology using AutoCAD. Use of computer in every field and drawing too has motivated him to write the book on Machine Drawing explaining the use of computer chapter by chapter.

MACHINE DRAWING Includes AutoCAD Ajeet Singh Retired Professor and Head of Mechanical Engineering Department, Motilal Nehru Institute of Technology, Allahabad

Tata McGraw-Hill Publishing Company Limited NEW DELHI McGraw-Hill Offices

New Delhi New York St Louis San Francisco Auckland Bogotá Caracas Kuala Lumpur Lisbon London Madrid Mexico City Milan Montreal San Juan Santiago Singapore Sydney Tokyo Toronto

Published by Tata McGraw-Hill Publishing Company Limited, 7 West Patel Nagar, New Delhi 110 008. Copyright © 2008, by Tata McGraw-Hill Publishing Company Limited. No part of this publication may be reproduced or distributed in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise or stored in a database or retrieval system without the prior written permission of the publishers. The program listings (if any) may be entered, stored and executed in a computer system, but they may not be reproduced for publication. This edition can be exported from India only by the publishers, Tata McGraw-Hill Publishing Company Limited. ISBN-13: ISBN-10:

978-0-07-065992-6 0-07-065992-3

Managing Director: Ajay Shukla Publishing Gen. Manager—SEM & Tech Ed: Vibha Mahajan Asst. Sponsoring Editor—SEM & Tech Ed: Shukti Mukherjee Jr. Editorial Executive: Sandhya Chandrasekhar Executive—Editorial Services: Sohini Mukherjee Senior Proof Reader: Suneeta S. Bohra Deputy General Manager—Higher Education & Sales: Michael J. Cruz Asst. Product Manager—SEM & Tech Ed: Biju Ganesan Controller—Production: Rajender P. Ghansela Asst. General Manager—Production: B.L. Dogra Information contained in this work has been obtained by Tata McGraw-Hill, from sources believed to be reliable. However, neither Tata McGraw-Hill nor its authors guarantee the accuracy or completeness of any information published herein, and neither Tata McGraw-Hill nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that Tata McGraw-Hill and its authors are supplying information but are not attempting to render engineering or other professional services. If such services are required, the assistance of an appropriate professional should be sought. Typeset at Script Makers,19, A1-B, DDA Market, Paschim Vihar, New Delhi 110 063 and Printed at Sai Printo Pack, Pvt. Ltd., A-102/4, Okhla Industrial Area, Phase-II, New Delhi-110 020. Cover Printer: Rashtriya Printer RQLYCRDXDRQCY

Dedicated to my grandchildren Gaganjit, Karanjit, Ananya, Neha and Tanvi

Preface Any machine part which is produced is first designed and then its drawing is prepared. The industry then manufactures the part according to the details given in the drawing. Thus, drawing is the only means of communication between the design office and the manufacturing shop floors. Hence Engineering Drawing is quite an important communication tool for engineers. Engineering drawings were made manually in the past but now they are made with the help of computers, and are called Computer Aided Drafting (CAD). There are many good books in the market on engineering drawing but only a few of them cover CAD. Some books give CAD as only a single small chapter in the end, which does not help a student in correlating the art of drawing with CAD. There are some good books which focus solely on CAD. They describe the tools available in the software with a few examples. This book is written as a combination of both manual and computer methods which run in parallel, chapter by chapter. The idea is that a student who makes a drawing manually, should be able to create a similar drawing with the help of a computer. It will give a better understanding of the use of the software for engineering-drawing purposes. The examples selected for AutoCAD cover a majority of the relevant commands. The software is explained by showing actual toolbars and dialog boxes which appear on the screen to enhance the understanding of students. The software adopted in this book is AutoCAD 2005. For more details on the subject of CAD, readers can consult the other book of the author, Working with AutoCAD 2000. A majority of the commands used in AutoCAD 2005 are the same as those in AutoCAD 2000, and only a few new commands have been added. This book not only gives elaborate drawings, but an effort has been made to describe all the basic knowledge required for each topic. The information given will be quite useful to answer questions asked in the viva-voce examinations. The contents of the book are selected to cover the syllabi of many good universities/institutions offering machine drawing. Some chapters like Applied Geometry, Surface Developments, Intersections, Cams, Jigs and Fixtures, etc., are not given in the book as some universities do not have these topics in their syllabi but these chapters will be put on the internet to make the book comprehensive. The chapters have been arranged in a logical order for step-by-step mastering of the subject. The problems chosen are such that a student is able to finish them in a usual class of three hours. The contents are such that at least 2 semesters are required to finish the whole book at the diploma or the degree level. The book is suitable for first year degree courses in Civil, Mechanical and Electrical engineering and the second year of Mechanical engineering courses. The production drawings section of the book on Tolerances, Limits, Fits, Geometric Tolerances and Surface Finish, etc., is very helpful

viii

Preface

for practicing engineers as well. The internet websites given in the Appendix makes the book useful for any person searching information on a topic concerning machine drawing. The conventions and terminology used in the book are mostly as per the Bureau of Indian Standards (BIS). But wherever there are different terms by BIS and AutoCAD, the terms of AutoCAD have been used so that students do not get confused while using the software. Explanations are accompanied by many pictorial drawings for better visualization of the concept. The author has had a long experience of 44 years of teaching this subject and is fully aware of the common mistakes which students commit. Therefore, these mistakes have been pointed out so that students do not repeat the same. The book has the following important features: ∑ Each chapter of the book has a summary at its beginning. This helps students revise the course quickly at the time of examination. ∑ The book has many solved examples on manual drafting as well as on CAD to give a comprehensive idea to the students. ∑ Theory questions at the end of each chapter help students know as to what type of questions can be asked on the subject. They also help the teacher to select questions for setting examination papers. ∑ Multiple choice questions are given at the end of each chapter for evaluating students quickly for short quizzes, etc. ∑ Fill up the blank questions have been so chosen that they check the key knowledge points of the chapter. ∑ A large number of unsolved problems for practice are given to create a thorough grasp on the concepts of the drawing. They also form a good question bank for the teacher to set examination papers. Solutions of the unsolved problems have not been given intentionally, otherwise students just copy from the book without understanding. The contents of the book are divided into six sections: Section A The first chapter deals with the importance of drawing and Fundamentals of Drawing while the second chapter is on the general tools available in CAD. The initial settings, and the draw and modify commands are explained with the help of solved examples. The following chapters are on the materials and tools used for drawing, standard conventions for lines, lettering, and dimensions. Section B This is about Projection Methods. Various methods of projections like Orthographic, Isometric, Oblique, Perspective, etc., are explained to fully describe a 3D object on a 2D sheet. This helps to view an object from any side and to understand the methods of visualization. Section C This is about Joints and Couplings. Permanent joints like riveted and welded joints are described. Temporary joints formed by bolts/nuts, cotter, shaft couplings and pipe joints are also explained. CAD can be of help in copying a part in rectangular or polar array form. Use of this tool for copying many parts in an array is explained. How the drawings stored in a graphic library can be used in creating drawings for the joints and couplings are demonstrated here by examples. Sections A, B and C form the first course on Engineering Drawing for a majority of the universities/colleges and deal with all the disciplines of engineering.

Preface

ix

Section D This concentrates on Production drawings to be used in the shop floor. The mating parts have to be specified by a type of standard fit required for an application. It is not possible to manufacture any part of the exact dimensions put on the drawing and hence a certain amount of tolerance is also to be put in addition to the basic dimensions. The type of machine to be used for manufacturing is decided by the type of surface finish mentioned on the drawing. Geometric tolerances like squareness, flatness, etc., are sometimes specified on production drawings. Standard methods to specify materials and all this information is described in this section. How AutoCAD helps in putting these values very easily on the drawing is explained. This section and the next few sections are mainly for mechanical engineers and is given in the second course on machine drawing. This can be useful for practicing engineers as well. Section E This deals with Machine Parts like springs, belts and pulleys, bearings, and gears to transmit power. AutoLISP is a programming language to be used in AutoCAD. Fundamentals of this language are given to create parametric drawings. Once a program is made, it can be used to create a drawing by defining the values of the variable sizes. Section F This is on the Part and Assembly Drawings. Machines are made by assembling different parts. How the parts are to be joined together are explained here. The main important parts of internal combustion engines, steam power plants are selected for these drawings. Some parts of the machine tools and hand tools have also been included. The drawings created as part drawings can be assembled very easily using AutoCAD to produce an assembly drawing. Due to development of internet facilities, useful websites for different topics are given in Appendix I. Appendix II contains Standards. The Online Learning Center of the book www.mhhe.com/singh/md provides a vast range of supplements. Instructors can take advantage of the Solution Manual and Power Point slides which can be used as effective teaching aids. For students, there are chapters on Applied Geometry, Developments, Intersections, Chains, CAMS, Rotary Machines, Jigs and Fixtures as well as Sample Question Paper with Solutions. I am sure that the contents of the book will help readers in getting sufficient proficiency and knowledge on machine drawing and CAD for mechanical drawings. No human being is perfect. Errors and omissions are always possible from any one in spite of the best efforts. I hope the readers will take them in proper spirit and inform the publisher/author for improvements and corrections in the subsequent editions. Constructive suggestions for the improvement of the book are most welcome at my email address: [email protected].

DR. AJEET SINGH 5 June 2007

Acknowledgements The manuscript of the book and drawings were finalized at Salalah College of Technology, Salalah, Sultanate of Oman where I was working during that period. I am thankful to the Ministry of Manpower and the college administration for providing excellent computing facilities in the department. I convey my sincere thanks to Dr. R.R. Gaur, Professor of Mechanical Engineering, Indian Institute of Technology, Delhi, for his valuable suggestions during the initial discussions for the book. I am thankful to my colleagues for their help and academic discussions from time to time regarding the book. I would also like to thank the editorial and production teams of Tata McGraw-Hill, who have been very cooperative and fast in communication even while they were located thousands of kilometres away. The book has undergone thorough reviews by some eminent academicians and the remarks of each have been very encouraging, mentioning the usefulness and uniqueness of the book. Their names are listed below. Name Prof. Goutam Ghosh Prof. D K Mondal Prof. Samar Chandra Mondal Prof. R S Deshmukh Prof. G N Kotwal Prof. Sundareswaran Mr. Abdul Shareiff Prof. Raman Bedi

Affiliation Department of Mechanical Engineering Future Institute of Engineering and Management, Kolkata Department of Mechanical Engineering Jadavpur University, Kolkata Department of Mechanical Engineering Jadavpur University, Kolkata Department of Mechanical Engineering Vidhya Vardhini College of Engineering and Technology, Thane Department of Mechanical Engineering Vishwakarma College of Engineering, Pune Department of Mechanical Engineering, Anna University, Chennai Department of Mechanical Engineering, Sri Siddhartha Institute of Technology, Tumkur Department of Mechanical Engineering National Institute of Technology, Jalandhar

It will not be out of place to mention, that I would have not been a professional and an author, without the encouragement of my parents right from my childhood. I thank my wife, Mrs. Kanwaljeet, for relieving me from the domestic duties to spare time for the book and bear my engagement in the book. Thanks are due to my daughter Mrs. Maneet Kaur for the proofreading of the book and my daughters Mrs. Diljeet Kaur and Mrs. Preety Singh who had inspired me to take up this work. AJEET SINGH

First Look

MEANT FOR

CONTENTS

Integrated course material for manual machine drawing and Computer Aided Drafting (CAD) ∑ First course in Engineering Drawing ∑ Detailed course on Machine Drawing ∑ Practicing Engineers Section A–Drawing Fundamentals (6 Chapters) Section B–Projection methods (5 Chapters) Section C–Joints and couplings (7 Chapters) Section D–Production drawings (4 Chapters) Section E–Machine parts (4 Chapters) Section F–Part and Assembly drawings (4 Chapters)

Arranged in a logical order QUICK STUDY

Every chapter gives a comprehensive introduction in the beginning.

AutoCAD Fundamentals of AutoCAD are described concisely in a separate chapter on AutoCAD.

RELEVANT THEORY

For viva voce examination— Key points are shown in bold.

MANUAL AND CAD HAND IN HAND

After describing manual drafting, most of the chapters have a CAD part.

STEP-BY-STEP CONSTRUCTION

Wherever necessary, construction has been demonstrated using a step-by-step method.

MANY SOLVED EXAMPLES

Every chapter has many solved examples.

PICTORIAL VIEWS

Better visualization of assembly parts are given.

SOLID MODELING

Creation of solid models using CAD is demonstrated using photographs for better visualization by viewing from any side.

ADDITONAL KNOWLEDGE

Wherever necessary, additional knowledge is given about the mechanical components, which may be useful for practicing engineers.

PRODUCTION DRAWINGS

This section is useful for engineers working in industry also. Salient features of Production drawings have been described in detail in four chapters—19 to 22.

PARAMETRIC DRAWINGS (with computer codes)

AutoLISP programming language is described in brief for parametric drawings. Computer codes are given for the solved examples

These are given to create assembly drawings with many solved examples. PART DRAWINGS

ASSEMBLY DRAWINGS

have been selected from simple problems to difficult problems to cover a variety of problems.

VARIETY OF QUESTIONS THEORY QUESTIONS

at the end of every chapter are helpful for teachers to set examination papers and also for students to understand the pattern of questions.

FILL IN THE BLANK QUESTIONS at the end of

each chapter help in setting quizzes for short duration examinations. Answers are given at the end. OBJECTIVE QUESTIONS

Multiple choice questions at the end of each chapter help in setting quizzes for short duration examinations. Answers are given at the end.

VARIETY OF ASSIGNMENT QUESTIONS

ASSIGNMENTS have been

preset for manual as well as CAD with the facility to choose from Additional problems.

Some problems have been given as home work for practicing at home. HOME WORK

PROBLEMS FOR PRACTICE Additional

problems are given for students to practice and for teachers for setting question papers.

Some useful Indian Standards have been given in Appendix 1. STANDARDS

INTERNET RESOURCES are

given in Appendix 2 to get more information on the topic.

LIST OF IMPORTANT AutoCAD COMMAND are given in Appendix 3.

Contents Preface Acknowledgements First Look

Part A

Drawing Fundamentals

1. Introduction to Drawing 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

1 1

What is a Drawing? 1 Uses of Drawings 2 Elements of Graphics 3 Methods of Expression 3 Methods of Size Description 4 Methods of Preparing Drawings 4 Types of Mechanical Drawings 5 Drawing Standards 7 Theory Questions 7 Fill in the Blanks 7 Multiple Choice Questions 8

2. Introduction to CAD 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

vii xi xii

Introduction to Computer Aided Drafting 9 Advantages of Computer Aided Drafting 10 Starting Autocad Program 10 Autocad Screen 10 Autocad Commands 11 Function Key Assignments 12 Short Cut Key Characters 12 Ucs and Ucsicon 13 Coordinate System 13 Units 13

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2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21

Viewing a Drawing 14 Drawing Aids 14 Object Snap 15 Drawing Basic Entities 17 Correcting Mistakes 24 Object Selection 25 Modify Commands 25 Modify Properties 29 Match Properties 30 Pedit 30 Grips 30 Theory Questions 34 Fill in the Blanks 35 Multiple Choice Questions 36 Problems for Practice 39

3. Drawing Apparatus 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17

Introduction to Drawing Equipment 41 Triangles 42 Ruler 43 Scales 43 Protractor 44 French Curves 44 Instrument Box 44 Stencils 45 Inking Pens 46 Drawing Sheet 47 Types of Paper 50 Drawing Sheet Fasteners 50 Pencils 50 Drawing Ink 51 Eraser 52 Equipment for Computer Aided Drafting (CAD) 52 Computer Software 53 Theory Questions 53 Fill in the Blanks 54 Multiple Choice Questions 55

4. Lines and Freehand Sketching 4.1 4.2

41

Lines 59 Precedence of Lines 59

58

Contents

4.3 4.4 4.5 4.6 4.7

Thickness of a Line 60 Drawing Lines 60 Freehand Sketching 60 Lines using Cad 63 Sketch Command 65 Theory Questions 67 Fill in the Blanks 67 Multiple Choice Questions 68 Assignment on lines, Circles and Arcs Problems for Practice 70

5. Lettering 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

69

71

Introduction 71 Guide Lines 71 Spacing between Lines 72 Width of Characters 72 Line Thickness of Letters 73 Spacing between Letters 73 Inclined Letters 73 Text Command 74 Editing Text 76 Theory Questions 77 Fill in the Blanks 78 Multiple Choice Questions 78 Assignment on Lettering 79 CAD Assignment on Lettering 79

6. Basic Dimensioning 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11

xxi

Introduction 81 Elements of Dimensioning 82 Dimensioning Circular Arcs 86 Dimensioning Diameters 87 Dimensioning Holes 87 Dimensioning Angles 88 Dimensioning Chamfers 88 Dimensioning Tapers 89 Dimensioning Undercuts 89 Dimensioning Repetitive Features 89 Dimensioning Equidistant Features 89

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6.12 6.13 6.14 6.15 6.16 6.17 6.18 6.19

Part B

Dimensioning Threads 89 Dimensioning Curves 89 Dimensioning Methods 90 Placement of Dimensions 91 Sequence of Dimensioning 92 Layers 92 Dimensioning Commands 94 Dimensioning Methods (CAD) 96 Theory Questions 101 Fill in the Blanks 102 Multiple Choice Questions 103 Assignment on Dimensioning 105 CAD Assignment on Dimensioning 105 Problems for Practice 106

Methods of Projection

7. Orthographic Projections107 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17

Introduction 108 What is Projection? 108 Types of Views 108 Principle Picture Planes 110 Methods of Orthographic Projection 110 Hidden Details 112 Preliminary Decisions for Making a Drawing 113 Projecting Side Views 114 Projection of Straight Inclined Face 116 Projection of Circular Boundaries 116 Projection of Curved Boundaries 116 Understanding Orthographic Views (Blue Print Reading) 116 Missing Views 117 Some Drawing Conventions 118 Sequence of Drawing 118 Drawing Orthographic Views 122 Plotting a Drawing 122 Theory Questions 125 Fill in the Blanks 125 Multiple Choice Questions 126 Assignement on Orthographic Views 127

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CAD Assignment on Orthographic Views 128 Homework 128 Problems for Practice 129 Assignment on Missing Views 130 Problems for Practice 131

8. Sectional Views 8.1 8.2 8.3 8.4 8.5

132

Introduction 133 Types of Sections 133 Conventions in Sectioning 136 Section Lines (Hatching) 139 Hatching Using Autocad (Bhatch Command) 145 Theory Questions 147 Fill in the Blanks 147 Multiple Choice Questions 148 Assignment 1 on Sectional Views 149 CAD Assignment on Sectional Views 150 Assignment on Half Sectional Views 151 CAD Assignment on Half Sectional Views 152 Problems for Practice 152 Difficult Problems for Practice 153

9. Auxiliary Views 9.1 9.2 9.3 9.4 9.5 9.6 9.7

157

Introduction 157 Types of Inclined Surfaces 158 Drawing Auxiliary View of an Inclined Surface 158 Drawing Auxiliary View of a Curved Surface 158 Illusions in Auxiliary Views 160 Skew Surfaces 160 Construction Lines 165 Theory Questions 167 Fill in the Blanks 167 Multiple Choice Questions 168 Assignment on Auxiliary Views 168 CAD Assignment on Auxilary Views 169 Problems for Practice 170

10. Axonometric Views and Oblique Views 10.1 10.2

Introduction 172 Types of Pictorial Views

172

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10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 10.13 10.14 10.15 10.16 10.17 10.18 10.19 10.20 10.21

Axonometric Views 173 Isometric Scale 174 Drawing Isometric Views 174 Projections of Non-isometric Lines 175 Isometric View of Angles 176 Isometric Drawing of Circles 176 Isometric Drawing of Arcs 178 Isometric Drawing of Curved Objects 178 Isometric Sections 178 Dimensioning Isometric Drawings 179 Reversed Isometric 179 Dimetric Projections 179 Trimetric Projections 179 Oblique Projection 180 Drawing an Oblique View 180 Cabinet View 182 Isometric Grid 186 Isocircle 187 Viewports Command 187 Theory Questions 190 Fill in the Blanks 191 Multiple Choice Questions 191 Assignment on Isometric and Dimetric Views 193 CAD Assignment on Isometric Views 194 Assignment on Oblique Views 195 CAD Assignment on Oblique Views 196 Homework 196 Problems for Practice 198

11. Perspective Views 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8

Introduction 200 Terminology 200 Factors Affecting Appearance 201 Selection of Parameters 201 Types of Perspective Views 203 Drawing a Perspective View 203 Perspective View of a Cylinder 206 Drawing a Perspective View of a Circle 206

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11.9 Graticulation 208 11.10 Rays Command 212 11.11 Dview Command 212 Theory Questions 214 Fill in the Blanks 215 Multiple Choice Questions 215 Assignment on Perspective Views 216 CAD Assignment on Perspective Views 217 Problems for Practice 217

Part C

Joints and Couplings

12. Riveted Joints 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 12.11 12.12 12.13

219

Introduction 220 Rivets 220 Making a Riveted Joint 221 Classification of Riveted Joints 222 Joint Proportions 225 Applications of Riveted Joints 227 Structural Joints 227 Boiler Joints 231 Light Work Applications 233 Block 235 Creating a Block (Block Command) 235 Retrieving a Block (Insert Command) 236 Inserting a Block at Many Places (Minsert Command) 237 Theory Questions 239 Fill in the Blanks 240 Multiple Choice Questions 240 Assignment on Riveted Joints 242 CAD Assignment on Riveted Joints 242 Homework 243 Problems for Practice 243

13. Threads 13.1 13.2 13.3 13.4

219

Introduction 245 Terminology 246 Classification of Threads 246 Thread Profile 248

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13.5 13.6 13.7 13.8 13.9 13.10

Pitch of Thread 249 Thread Designation 249 Specifications of Threads 251 Thread Representation 251 Internal Threads 253 Array Command 254 Theory Questions 258 Fill in the Blanks 258 Multiple Choice Questions 259 Assignment on Screw Threads 260 CAD Assignment on Screw Threads 260 Homework 261 Problems for Practice 261

14. Bolts and Nuts 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10 14.11 14.12

Introduction 263 Terminology 263 Bolt Proportions 263 Drawing a Bolt/Nut 264 Studs 266 Screws 266 Locking Devices 268 Special Nuts 268 External Locking Devices 271 Spring Washers 272 Bolts and Nuts for Special Applications 272 Wblock Command 274 Theory Questions 276 Fill in the Blanks 277 Multiple Choice Questions 277 Assignment 1 on Bolts and Nuts 279 Assignment 2 on Locking Devices 279 CAD Assignment on Bolts and Nuts 279 Homework 279 Problems for Practice 280

15. Welded Joints 15.1 15.2

262

Introduction 282 Types of Welding Processes 282

281

Contents

15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11 15.12 15.13

Types of Joints 283 Edge Preparation 283 Symbols 284 Specifying a Welded Joint 286 Fillet Welds 290 Groove Welds 291 Spot Welds 292 Seam Welds 293 Plug Welds 293 Surface Welding 294 Graphic Library 297 Theory Questions 298 Fill in the Blanks 298 Multiple Choice Questions 299 Assignment on Welded Joints 300 CAD Assignment on Welded Joints 301 Homework 301 Problems for Practice 302

16. Shafts, Keys, Cotter and Pin Joints 16.1 16.2 16.3 16.4 16.5 16.6 16.7

304

Shafts 305 Keys 306 Types of Keys 307 Splines (IS 2327:1991, IS 3665:1990, IS 13088:1991) 310 Cotter Joints 311 Knuckle Joint 313 Modifying Three-Dimensional Objects 313 Theory Questions 317 Fill in the Blanks 318 Multiple Choice Questions 319 Assignment on Keys and Joints 320 CAD Assignment on Keys and Joints 320 Homework 321 Problems for Practice 321

17. Couplings and Clutches 17.1 17.2

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Couplings 323 Muff Couplings 323

322

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17.3 17.4 17.5 17.6 17.7 17.8 17.9 17.10

Rigid Flange Couplings 324 Flexible Couplings 326 Parallel Coupling (Oldham’s) 327 Universal Coupling 328 Constant Velocity Joint 328 Detachable Couplings 329 Slip Couplings 329 3D Array 332 Theory Questions 335 Fill in the Blanks 336 Multiple Choice Questions 336 Assignment on Couplings and Clutches 337 CAD Assignment on Couplings and Clutches 338 Homework 338

18. Pipe Joints 18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8 18.9 18.10 18.11 18.12 18.13 18.14 18.15 18.16 18.17 18.18 18.19 18.20 18.21 18.22

Pipes 340 Pipe Materials 340 Pipe Designation 340 Pipe Threads 341 Types of Pipe Joints 342 Joints for Cast Iron Pipes 343 Joints for Copper Pipes 344 Joints for Wrought Iron Pipes 344 Joints for Lead Pipes 345 Joints for Hydraulic Pipes 345 Union Joint 346 Expansion Joints 347 Pipe Fittings 348 Cast Iron Fittings 349 Flanged Fittings 350 PVC Fittings 352 Valves 352 Piping Symbols 354 Piping Layouts 356 Pipe Supports 357 Tubes 358 Tube Joints 359

339

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18.23 Multilines (Mline Command) 359 18.24 Creating a New Mline Style (Mlstyle Command) 360 Theory Questions 364 Fill in the Blanks 365 Multiple Choice Questions 366 Assignment on Pipe Joints 367 CAD Assignment on Pipe Joints 368 Homework 368 Problems for Practice 369

Part D

Production Drawings

19. Tolerances, Limits and Fits 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 19.10 19.11 19.12 19.13 19.14 19.15

370

Introduction 371 Terminology 371 Tolerances and Manufacturing Processes 372 International Tolerance Grade (It Grade) 373 Fundamental Tolerances 375 Placing a Dimension with Tolerance 380 Cumulative Tolerances 380 Fits 385 Systems of Fits 385 Specifying a Fit 386 Types of Fits 386 Selection of Fits 387 Fits for Thread Fasteners 393 Gauges 393 Putting Tolerances using CAD 393 Theory Questions 396 Fill in the Blanks 397 Multiple Choice Questions 397 Assignment on Tolerances, Limits and Fits 398 CAD Assignment on Tolerances, Limits and Fits 399 Homework 400 Problems for Practice 400

20. Geometrical Tolerances and Surface Finish 20.1 20.2

370

Introduction 404 Types of Tolerances 404

402

Contents

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20.3 20.4 20.5 20.6 20.7 20.8 20.9 20.10 20.11 20.12 20.13 20.14 20.15 20.16 20.17 20.18 20.19 20.20 20.21 20.22

Terminology 405 Frame 405 Datum 406 Material Condition 407 Tolerance Symbol 408 Tolerance Value 409 Indicating Geometrical Tolerances on Drawings 409 Form Tolerance for Single Features 409 Tolerances on Related Features 411 Run Out 414 Surface Texture 418 Profiles 419 Surface Roughness Number 419 Roughness Symbols 420 Lay 422 Roughness Grade Number and Grade Symbols 422 Roughness with Manufacturing Processes 423 Roughness for Typical Applications 424 Rules for Putting Roughness Symbols 425 Geometric Tolerances 427 Theory Questions 428 Fill in the Blanks 429 Multiple Choice Questions 429 Assignment on Geometric Tolerances and Surface Roughness 431 CAD Assignment on Geometric Tolerances and Surface Roughness 432 Homework 432 Problems for Practice 432

21. Material Specifications 21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.9 21.10

Introduction 436 Types of Engineering Materials 436 Ferrous Metals 437 Designation of Steels [IS 1762–1974 Part 1] 440 Steel Designation According to Chemical Composition [IS 7598–1974] 442 Code Designation for Ferrous Castings [IS 4863–1968] 444 Non-ferrous Metals 444 Plastics 448 Bill of Materials 450

435

Contents

21.11 Table Command 453 21.12 Block Attributes 454 Theory Questions 454 Fill in the Blanks 454 Multiple Choice Questions

455

22. Production Drawings 22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8 22.9 22.10 22.11 22.12 22.13

Part E

457

Introduction 458 Title Block 458 Manufacturing Processes 459 Heat Treatment Processes 462 Tooling 463 Inspection 466 Jigs 466 Fixtures 467 Assembly Drawings 468 Standard Mechanical Components 469 Production Drawing 470 Process Sheet 470 Title Block 472 Theory Questions 473 Fill in the Blanks 474 Multiple Choice Questions 474 Assignment on Production Drawings 476 CAD Assignment on Production Drawings 477 Problems for Practice 478

Machine Parts

23. Springs 23.1 23.2 23.3 23.4 23.5 23.6

xxxi

Introduction 480 Classification 480 Helical Spring 480 Leaf Spring 482 Conventional and Symbolic Representation of Springs 484 Diaphragm Spring 485 Theory Questions 486 Fill in the Blanks 486 Multiple Choice Questions 487

479 479

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xxxii

Assignement on Springs 487 CAD Assignment on Springs 488 Homework 488 Problems for Practice 488

24. Belts and Pulleys 24.1 24.2 24.3 24.4 24.5 24.6 24.7 24.8 24.9 24.10 24.11 24.12 24.13 24.14 24.15 24.16 24.17

Introduction 490 Belts 490 Pulleys 492 Types of Pulleys 492 Flat Belt Pulleys 493 Grooved Pulleys 495 Toothed Pulley 497 Rope Pulley 497 AutoLISP 498 Specifying Variables 501 Extracting Data From List Variable 502 Get Commands 502 Mathematical Operations 502 Angles in AutoLISP 503 Logical Operators 503 Conditional Branching (If Command) 503 Looping a Program 504 Theory Questions 506 Fill in the Blanks 507 Multiple Choice Questions 507 Assignment on Belts and Pulleys 509 CAD Assignment on Belts and Pulleys 509 Homework 509

25. Bearings 25.1 25.2 25.3 25.4 25.5 25.6 25.7 25.8

489

Introduction 511 Classification of Bearings 511 Hydrodynamic Bearings 511 Plain Journal Bearing 512 Plain Journal Bearing Materials 512 Sleeve Bearing Supports 513 Hangers 516 Rolling Bearings 517

510

Contents

xxxiii

25.9 Mounting of Rolling Bearings 521 25.10 Managing Entities 522 Theory Questions 526 Fill in the Blanks 526 Multiple Choice Questions 527 Assignment on Bearings and Supports 529 CAD Assignment on Bearings and Supports 530 Homework 530 Problems for Practice 530

26. Gears 26.1 26.2 26.3 26.4 26.5 26.6 26.7 26.8 26.9 26.10 26.11 26.12 26.13 26.14 26.15

Part F

Introduction 532 Terminology 532 Types of Gears 533 Gear Tooth Calculations 535 Tooth Profiles 535 Base Circle 537 Drawing Approximate Involute Tooth Profile 538 Conventional Representation of Gear Teeth 540 Construction of Gears 540 Spur Gears 541 Helical Gears 542 Bevel Gears 543 Worm and Worm Wheel 544 Rack 546 Cad for Gear 547 Theory Questions 548 Fill in the Blanks 549 Multiple Choice Questions 549 Assignment on Gears 551 CAD Assignment on Gears 551 Problems for Practice 551

Machines

27. Part and Assembly Drawings 27.1 27.2 27.3 27.4

531

Introduction 553 Detail Drawing 553 Assembly Drawings 556 Bill of Materials 558

552 552

Contents

xxxiv

27.5 27.6 27.7

Steps for Creating Assembly Drawings 559 Blue Print Reading 561 Assembly of Parts and Their Details 564 Theory Questions 565 Fill in the Blanks 566 Multiple Choice Questions 566 Assignment on Part and Assembly Drawings 568 CAD Assignment on Part and Assembly Drawings 570 Homework 572 Problems for Practice 573

28. Internal Combustion Engines 28.1 28.2 28.3 28.4 28.5 28.6

Introduction to I.C. Engines 575 Power System 576 Fuel System 595 Ignition System 600 Cooling System 601 Lubrication System 602 Theory Questions 604 Fill in the Blanks 604 Multiple Choice Questions 605 Assignment on I.C. Engines 606 CAD Assignment on I.C. Engines 606 Homework 607 Problems for Practice 607

29. Steam Power Plants 29.1 29.2 29.3

575

Introduction 609 Steam Generator (Boiler) 610 Steam Engine 624 Theory Questions 635 Fill in the Blanks 636 Multiple Choice Questions 636 Assignment on Steam Power Plants 637 CAD Assignment on Steam Power Plants 637 Homework 637 Problems for Practice 638

609

Contents

30. Machine Tools 30.1 30.2 30.3 30.4 30.5

Appendix 1 Appendix 2 Appendix 3 Index

xxxv

639

Introduction and Scope 639 Lathe 640 Shaper 650 Drilling Machine 654 Holding and Clamping Devices 656 Theory Questions 660 Fill in the Blanks 661 Multiple Choice Questions 661 Assignment on Machine Tools 662 CAD Assignment on Machine Tools 662 Homework 662 Problems for Practice 662 Some Useful Indian Standards Some Relevant Internet Sites List of Some Important Autocad Commands

663 664 665 669

PART A Drawing Fundamentals CHAPTER

1

Introduction to Drawing Drawing is a graphic language, which can communicate many things in a compact form without speaking. Drawings are used in many engineering fields like Mechanical drawings, Architectural drawings, Structural drawings, Electrical and Electronics drawings. In addition, they can exhibit data in different forms of charts, graphs, etc. Surveyors use them for maps and topography. Drawings can be prepared manually or using a computer. Engineering drawings can be presented in many forms like pictorial views, detailed views, assembly views, exploded views, etc. Drawings can be prepared free hand, or finished drawing with tools or can be Computer Aided Drawings (CAD). Mechanical drawings can be in many forms like: production drawing, part drawing, assembly drawing, sub-assembly drawing, exploded drawing, installation drawing, tabular drawing or patent drawing. Drawings have to follow standard conventions given by organizations like ISO (International Standard Organization), BIS (Bureau of Indian Standards), ANSI (American National Standards Institute), etc.

1.1 WHAT IS A DRAWING? All school education is in word language. One has to learn it to read, write and speak. In word language, to describe an object for its shape and size, one has to use many sentences for its complete description. Large vocabulary is required to express complete description for communication. The problem is of a higher order when communication is to be done between two persons knowing different languages. For such cases, interpreters who know both the languages are needed to translate. Thus, word language is quite cumbersome, if one tries to use it universally where languages are in dozens. Drawing is a graphic language that can be used to express precisely the shape and size of an object in a compact form without having much vocabulary. Such an expression will be independent of the country and can be understood universally even by illiterate persons to some extent. For example, any illiterate person can identify a chair if he sees its pictorial drawing (Fig. 1.1). An educated person will be able to tell its dimensions mentioned on the drawing. Perhaps one Fig. 1.1 A Chair

Part A – Chapter 1

2

page description will be needed if it has to be described in words. Thus drawing is a silent and compact language through which one can communicate to deaf and dumb individuals as well. Graphic language has the following advantages over word language while describing an object: 1. It is independent of any regions language. 2. It offers compact description. 3. It is a silent language and can be used by even the deaf and the dumb. 4. It gives a clear picture in the mind quickly. 5. Even illiterate persons can follow it to some extent. 6. Complicated machines can be described by different drawings for each part and then an assembly drawing can be made showing the relative position of the different parts.

1.2

USES OF DRAWINGS

In addition to describing an object, drawings can be used for the following important communications. See Fig. 1.2. ∑ It can be used for mathematical conversions in the form of nomograms. For example, to convert degree Centigrade to degree Fahrenheit (Fig. 1.2A). ∑ Dependence of (x, y) variables can be shown graphically by plotting X, Y graphs. For example, a P-V diagram for an engine (Fig. 1.2B). ∑ PI charts can be used to show the percentage of different elements (Fig. 1.2C). ∑ Bar charts can be used to display definite variations of many parameters (Fig. 1.2D). ∑ Electrical/electronics circuitry is drawn showing the various connections of different components (Fig.1.2E). ∑ Structures are shown by structural drawings. ∑ Designs of buildings are shown by architectural drawings. ∑ Surveyors use drawings for maps and topography. Discussion in this book is limited to only the drawing of mechanical engineering components and machines.

Fig. 1.2 Various Applications of Drawings

Introduction to Drawing

1.3

3

ELEMENTS OF GRAPHICS

Drawings are made with lines and arcs. Each line represents an edge of a surface. Curves, arcs and circles define curved objects. Lines are connected according to geometry to represent planes and ultimately the shape of the object. Addition of numerals on the drawing define the object for size. Figure 1.3 shows the drawing of an object for its shape and size. Lines and curves represent its shape and the numbers mentioned are the Fig. 1.3 Graphical Representation of dimensions in mm. When the object is complicated, an Object for Shape and Size multi views are necessary to define it completely.

1.4

METHODS OF EXPRESSION

An object can be expressed by drawing in many ways. One must select the best-suited method to describe the shape. The object can be drawn of the same size if it can be accommodated on the paper. If it is bigger than the paper size, then it has to be reduced and if it is very small, it can be enlarged suitably for easy readability. Two methods of representations used are: (a) Pictorial representation (b) Orthographic projections Pictorial representation is a view which is seen from an angle such that its three faces are visible. Figure 1.4(A) represents a pictorial view of a V block. A pictorial view can be isometric, oblique or perspective. These views are described in detail in Chapters 10 and 11.

(A) Pictorial view

Fig. 1.4

(B) Orthographic projections

A ‘V’ Block

Fig. 1.5 A Sectional View

A majority of engineering drawings use orthographic method of representation. Orthographic views of the block V are shown in Fig. 1.4(B). In this method, the object is placed in such a way that the most representative face is on the front side. The Front view is then drawn while viewing from front. Then what is visible from the top is drawn, and is called the Top view or Plan. Side view is drawn by viewing from either the left or right side, whichever is more informative. Thus generally there are 3 views, viz. Front view, Top view and Side view. For symmetric objects 2 views may be sufficient and for thin objects, even one view may be enough to describe the shape. These views are discussed in detail in Chapter 7. Sometimes sectional views are also drawn to show the internal details; which otherwise would have not been possible in outside views (Chapter 8). Figure 1.5 shows a sectional view of a hollow part. Hatching lines are drawn in the area where the material is cut by the sectioning or cutting plane.

Part A – Chapter 1

4

1.5

METHODS OF SIZE DESCRIPTION

After defining the shape either by pictorial or orthographic views, the second step is the size description for making a complete drawing. Size is given by dimensions for linear distances, radii, diameters, angles, etc. Dimensions given are the actual dimensions of the object and not the scaled dimensions for scaled views. In production drawings, even tolerances are given (Refer Chapters 19 and 22 for more details). These are the maximum possible manufacturing errors that can be tolerated on each dimension. Figure 1.6 shows dimensions with and without tolerances. Units used are mm or metres depending upon the size of the object.

(A) Dimensions without tolerances

(B) Dimensions with tolerances

Fig. 1.6 Specifying Dimensions of Object

1.6

METHODS OF PREPARING DRAWINGS

Drawings can be prepared in three different ways:

1.6.1

Free Hand Sketching

Sketching is done with pencil and paper without any aid of drawing apparatus. It is good in learning process and for preliminary drawings. Lines may not be of the exact length in such a drawing. Once the idea expressed through free hand sketching is finalized, the finished drawing can be made.

1.6.2

Finished Drawings

These drawings are drawn with pencil or ink on paper or special drawing material with the aid of drawing apparatus for good draftsmanship. Straight lines are drawn with T-square or a drafter or set squares (discussed in Chapter 3). Circles and arcs are drawn with compass and angles with protractor. Letters are written using stencils for good presentation. Special apparatus are also used at times to draw curves. The appearance of the drawing depends upon the skill of the draftsman.

1.6.3 Computer Aided Drafting (CAD) This is the latest method of drawing. The drawing is drawn on the screen of the computer using softwares like AutoCAD, CADKEY, etc. Editing becomes quite easy in such drawings. Finally the drawing is saved in the magnetic memory of the computer on a hard disk or on a floppy or a CD. The output is taken with the help of a plotter. Small drawings up to A4 size can be printed by a printer. Large and colored drawings are possible with multipen plotters or by changing pens on a single pen plotter. The quality of such drawings is excellent and does not depend upon the skill of the person. Basics of AutoCAD software are described in Chapter 2.

Introduction to Drawing

1.7

5

TYPES OF MECHANICAL DRAWINGS

1.7.1 Machine Drawings Drawings of machine elements are called machine drawings. They are generally represented with views from different sides like Front view, Top view and Side view. Figure 7. S1 in Chapter 7 shows machine drawing of an object. Dimensions on the views indicate the size. 1.7.2 Production Drawings These drawings are also machine drawings but in addition to dimensions, they furnish tolerances, geometric tolerances (given in boxes), surface finish, heat treatment, etc. Figure 1.7 is a production drawing. Refer Chapters 19 and 22 for details.

Fig. 1.7

1.7.3

A Production Drawing

Assembly Drawings

A machine consists of many parts. Drawing showing the position of each part with respect to each other is called an assembly drawing. Refer the figures in Chapter 27 showing assembly drawings.

1.7.4 Sub Assembly Drawings When a machine is big and has a large number of parts, e.g. a car, it may have sub assemblies like engine, clutch, gearbox, etc. An assembly drawing is an assembly of such sub-assembly drawings. 1.7.5

Exploded Drawings

These drawings give the pictorial views of each component of an assembly and they are arranged in the same sequence in which they are to be assembled. Figure 1.8 is an exploded view with part numbers for an air compressor.

Part A – Chapter 1

6

Fig. 1.8

1.7.6

Exploded Drawing of an Air Compressor

Part Drawings

Detailed drawing of each part of a machine is called a part drawing. Production drawing of a part is also called a part drawing or working drawing.

1.7.7

Installation Drawings

These drawings are supplied by the manufacturers to the client giving the overall and all dimensions of the assembly which may be needed during installation. For example, details of foundation holes.

1.7.8

Tabular Drawings

These drawings are used for parts that have same shape but different dimensions. In that case, the drawing can be dimensioned with Fig. 1.9 A Tabular Drawing sizes as A, B, C, D, etc. and the values of A, B, C and D can be tabulated in a table. Figure 1.9 shows a tabular drawing of a washer.

Introduction to Drawing

1.7.9

7

Patent Drawings

These drawings are used to get a patent of a machine. The drawing could be in orthographic view (Chapter 7) or in pictorial view (Chapters 10 and 11).

1.8

DRAWING STANDARDS

Machine drawing is used to communicate information to industries. To have uniformity in drawings they are required to follow some drawing standard approved by International Standards Organization (ISO). In India, Bureau of Indian Standards (BIS) has been assigned the job of standardizing the items for interchangeability of parts. Standards are available for any machine component as well as for the drawings. Each standard has been assigned a definite number. The original standard for drawing was IS 696 and was formulated in1960. It was revised in 1972. The latest revision was in1988 which is numbered as SP-46. All advanced countries have their own standards. Conventions followed in this book are as per Indian Standards. American National Standards Institute (ANSI) also has standards for drawings.

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

What are the advantages of graphic language over word language? What are the various types of representations that can be made using drawings? Give two important methods of representing a component. What are the various types of machine drawings? Differentiate between an assembly, sub-assembly and part drawing. What is meant by installation drawing? What do you mean by exploded view? Differentiate between a machine drawing and working drawing. Describe the different methods for preparing a drawing. What is a tabular drawing? Where is it used? Explain with an example. What is the importance of the Drawing Standards?

FILL

IN THE

BLANKS

Fill up the blanks by an appropriate word(s) language. 1. Drawing is a . 2. Drawing giving graphical mathematical conversion is called drawings. 3. Design of houses are shown by and 4. Two main methods of representing objects graphically are view show internal details. 5. . 6. Size of an object is shown by drawings. 7. Tolerances on drawings are shown on 8. Pictorial view of each part in sequence is shown by drawing. 9. Objects having same shape but different sizes can be represented by a single

.

drawing.

Part A – Chapter 1

8

10. The job of standardization in India is done by . 11. The latest number of IS standard for Engineering drawing is

.

MULTIPLE CHOICE QUESTIONS Tick the correct answer: 1. Working drawing is used by (a) general public (b) designer (c) salesman (d) manufacturing industry 2. Installation drawing is required by (a) purchase department (b) customer (c) sales engineer (d) production engineer 3. Bar chart is used to (a) give stock position of bars (b) cross-section of steel bars (c) length of bar (d) display data 4. Pi chart is used for (a) converting circumference into diameter (b) evaluating value of Pi (c) displaying percentage of parametric elements (d) none of the above 5. A drawing giving details about size tolerance, heat treatment, etc. is known as (a) production drawing (b) assembly drawing (c) exploded drawing (d) machine drawing 6. The abbreviation CAD stands for (a) Common Application Data (b) Cancel All Drawings (c) Computer Aided Drafting (d) Call A Design 7. The work of standardization in India is done by (a) Government of India (b) National Institutes (c) Indian Institutes of Technology (d) Bureau of Indian Standards 8. The latest number of Indian Standard for Drawing is (a) SP-46 (b) IS 696 (c) ISO 235 (d) ISD 011 9. Tabular drawing is used where parts have (a) rectangular shape (b) same shape and size (c) same shape but sizes are different (d) same size but different shape

ANSWERS to Fill in the Blanks Questions 1. graphic 5. Sectional 9. tabular

2. nomogram 6. dimensions 10. Bureau of Indian Standards

3. architectural 7. production 11. SP: 46-1998

4. orthographic, pictorial 8. exploded

ANSWERS to Multiple Choice Questions 1. (d) 7. (d)

2. (b) 8. (a)

3. (d) 9. (c)

4. (c)

5. (a)

6. (c)

CHAPTER

2

Introduction to CAD Computer Aided Drafting is becoming more and more popular because of its many advantages which are described in this chapter. AutoCAD is one of the most popularly used software for creating drawings using computer. In the beginning of this chapter, a brief description of the screen with AutoCAD 2005 is given. Use of Function keys and Short cut keys is also discussed in this chapter. One can use the World Coordinate System (WCS) or the User Coordinate System (UCS). The coordinate system and the X and Y directions are indicated by the UCS icon on screen. AutoCAD accepts all types of coordinate systems like Cartesian, Cylindrical or Spherical in 2D or 3D format. You can create a drawing of any size and the limit can be set using LIMIT command. Any unit can be set using UNITS command. Drawing aids like GRID, SNAP, ORTHO and OBJECT SNAPS help the user to locate points quickly and exactly. Setting up a drawing environment is explained in example. Drawings can be zoomed to any size using ZOOM command and moved to any location on the screen using PAN command. Draw and Modify toolbars are also described in this chapter. In the Draw toolbar only the most commonly used commands like POINT, LINE, CIRCLE, ARC, RECTANGLE, POLYGON, ELLIPSE, DONUT, HATCH and TEXT are described with the help of solved examples. TEXT command is described in detail in Chapter 5 on Lettering and HATCH in Chapter 8 on Sectional views. The drawing created can be modified at any time. The Modify commands described are: ERASE, COPY, MIRROR, OFFSET, ARRAY, MOVE, ROTATE, SCALE, STRETCH, TRIM, EXTEND, BREAK, CHAMFER, FILLET and EXPLODE. A solved example explains the use of these commands. Dimensioning of a drawing is an important task. This is described in detail in Chapter 6. Production drawings are mentioned with Tolerances and Geometric tolerances also are described in Chapters 19 and 20 respectively. Drawings most commonly used can be saved as a BLOCK if they are to be used in the same drawing or as a WBLOCK if these are to be used in other drawings. These commands are described in Chapters 12 and 14 respectively. ARRAY command is very useful for creating multiple copies in a rectangular fashion. Use of this is described in Chapter 13. This command has a polar option also, the use of which is described in Chapter 17. Some more useful commands are given wherever required in the following chapters of the book.

2.1 INTRODUCTION TO COMPUTER AIDED DRAFTING Computer Aided Drafting (CAD) is becoming a powerful drawing tool even on personal computers now. Many softwares are available for drafting but the most versatile and commonly used software is AutoCAD. It is quite exhaustive and is internationally known for drafting purposes. AutoCAD has been developed in stages. The version AutoCAD 2005 is being described in this book. A minimum requirement for CAD is of Pentium III machine with a minimum 10 GB Hard disk, Floppy and CD ROM drive, true color monitor, 256 MB RAM, 2 or 3 button mouse, a printer or plotter.

Part A – Chapter 2

10

2.2 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

ADVANTAGES OF COMPUTER AIDED DRAFTING Compact storage in a floppy or hard disk. Drawings can be scaled up or down with ease without redrawing. Excellent drawing quality independent of the skill of the user. Editing can be done easily. Colored drawings are possible using multi-pen plotters. Line thickness, type of line and color can be set and constantly maintained. Neat work even without drawing instruments or drafting skills. Commonly used components can be pre-stored in a graphic library. Many hatch patterns that are available can be used wherever desired. Rectangular or polar arrays can be created easily. Dimensioning is very easy as extension lines, dimension lines, arrows and dimension values are put automatically. A multi layer drawing can be created. Each layer acts like a transparency. Three-dimensional drawings can be seen from any viewpoint for better visualization. Information about length, area, perimeter, volume, mass, etc. is easily calculated. Programmable drawings are possible using AutoLISP or C languages.

2.3 STARTING AUTOCAD PROGRAM It is presumed in this book that AutoCAD software is installed on the computer and the user has a basic knowledge of computers. Click the AutoCAD icon on desktop or click START button at the lower left corner of the screen, followed by the sequence of selections given as follows: START

2.4

æÆ Programs

æÆ AutoCAD 2005

æÆ AutoCAD 2005

AUTOCAD SCREEN

The opening main screen of AutoCAD is shown in Fig. 2.1. This screen has the following items from top to bottom: (a) Logo of AutoCAD on the left upper corner followed by a default File name [Drawing1.dwg]. (b) Menu bar showing the menu items like: File, Edit, View, Insert, Format, Tools, Draw, Dimension, Modify, Window, Help. Clicking on any menu item displays a Popup menu displaying many options to choose. (c) Toolbars: A toolbar is a set of small graphical icons assigned to do a specific job. There are 29 toolbars in AutoCAD 2005 and only the required ones should be displayed on the screen. This can be done by clicking View on Menu bar and choosing Toolbars… from the popup menu. From the Toolbars dialog box displayed, click on the box on the left side for the required toolbar and close the dialog box. The required toolbar is displayed on the screen. Bring the mouse in the blue area of the toolbar and dock it on the screen to any desired suitable position. The Standard toolbar contains icons similar to any window application like Open a new file, Open an existing file, Save, Print, Print Preview, Cut, Copy, Paste, etc. Other icons will be described later wherever required. The Draw and Modify toolbar are also shown in Fig. 2.1. It is good to remember the shape of each icon. However, if you forget, just bring the mouse over the icon for a while and a tool tip is displayed showing the job assigned to that icon.

Introduction to CAD

11

Fig. 2.1 AutoCAD Screen

(d) Drawing area: It is the major central area of the screen on which a drawing is created. A cross hair is displayed in this area. When the mouse is moved, this cross hair moves over the screen. (e) UCS icon: It is located at the lower left corner of the drawing area. It displays two arrows showing positive directions of X and Y. Z direction is towards the user from the screen. (f) Layout: It is at the bottom of the screen below the drawing area. You can select a layout here. The default is Model Layout. (g) Command area: It is just below the Layout line. See Fig. 2.1 where “Command” is written. The commands are typed in this area. You must always see this area. All the prompts for the data required are displayed here. Read the prompt, type the data as asked for and then press Enter key (ø). (h) Status line: It is at the bottom most of the screen. On the left-hand corner, it displays X, Y and Z coordinates of the intersection point of the cross hair lines. In the middle, it displays on/off condition of drawing aids such as SNAP, GRID, ORTHO, etc. Select any one by clicking over it. The button is shown pressed when active. (i) Side screen area: It is on the right-hand side of the drawing area. It displays all the options for a command. For example an arc can be made by many options. These options are shown in this area. It is not shown in the Fig. 2.1.

2.5

AUTOCAD COMMANDS

Commands are used to do an activity. For example to draw a line use Line command. First, open a new or an old file. Use any one of the following three methods to issue a command: (a) Click an icon on a toolbar with the left mouse button. (b) Click an option on Menu bar. From the popup menu select a choice by clicking the mouse on it. ø ). (c) At the Command line type a command using the keyboard and then press Enter (ø

Part A – Chapter 2

12

If you can remember the shape of icons, the first method is most convenient. Right mouse button can also work as Enter key. If the same prompt repeats, press Enter to quit. By pressing only Enter at the command prompt, the previously used command is automatically activated. Unless until specified, use only the left mouse button.

Note:

2.6

FUNCTION KEY ASSIGNMENTS

F1 F2

Displays Help Toggles between the text and graphics screen Running Object Snap on/off Tablet (an input device) on/off Switches between the top, front and side views for isometric drawings Coordinates display on/off

F3 F4 F5 F6

2.7

F7 Grid on/off F8 Ortho on/off. If ON, lines are either horizontal or vertical F9 Snap mode on/off F10 Polar option on/off F11 Object Snap Tracking on/off

SHORT CUT KEY CHARACTERS

Shortcut characters given in the first column of Table 2.1 which can be typed at the command prompt instead of a full command name as given in Column 2. Use of the command is shown in Function Column 3. Table 2.1

Commands and their shortcut keys

Shortcut Key

Command name

Function

A C E F H L M O P R S T U V W X Z

ARC CIRCLE ERASE FILLET HATCH LINE MOVE OFFSET PAN REDRAW STRETCH TEXT UNDO VIEW WBLOCK EXPLODE ZOOM

Draws an arc Draws a circle Deletes the selected object(s) Creates a radius at the intersecting lines Displays Boundary Hatch and Fill dialog box Draws a line Moves a drawing from one place to another but drawing coordinates change Draws a parallel line or arc at specified distance Moves drawing on screen without changing its coordinates Redraws the whole drawing Stretches an entity by specified distance Writes Text Undoes the previous action Displays View dialog box Displays Write Block dialog box Breaks a group of entities into separate entities Enlarges or reduces the view

Introduction to CAD

2.8

13

UCS AND UCSICON

AutoCAD uses 3-dimensional coordinate system. The X and the Y axes are considered up to infinity. This coordinate system is called the “World Coordinate System” or in short WCS. Its icon is shown at the lower left corner in the drawing area showing directions of the X and Y axes by arrows and a small square at the intersection (Fig. 2.1). Z direction is taken at right angles to the screen and outwards. There is another coordinate system called the “User Coordinate System” or UCS. Its origin can be anywhere in the WCS or coincide with the WCS. When working with UCS, the square of the icon is not displayed. Its display can be put On or Off or the position can be changed by using the UCSICON command. It further prompts for options. The option between conical brackets < > is default option. Command: UCSICON ø Enter an option [ON/OFF/All/No origin/Origin/Properties] :

Position and Orientation of the UCS origin is controlled by the UCS command. Select any one option by typing a character written in uppercase for the option on the prompt line given as follows: Command: UCS ø Enter an option [New/Move/orthoGraphic/Prev/Restore/Save/Del/Apply/?/World]: :

2.9

COORDINATE SYSTEM

The lower left corner of the screen is taken as the origin and is shown by the location of WCS. Positive X is on the right hand side of the origin and the positive Y is vertically upwards. Relative coordinates with respect to the previous point are prefixed with symbol @. Counter clockwise direction is taken positive for angles. AutoCAD accepts coordinates in any of the systems mentioned with the following syntax: Cartesian X, Y, Z measured from the origin. Relative Cartesian @ x, y, z (x, y and z are distances from a previously defined point). Polar L< q , Z (L is the distance from origin and q is the angle from the X axis and Z is the height). Relative polar coordinate @ L¢¢ < q, Z¢¢ (L¢¢ and q are relative distance and angle from X axis and Z ¢ is relative height from a previously defined point). Spherical coordinate L < q < f (where f is the angle in a vertical plane) AutoCAD accepts coordinates in three dimensions. If only X and Y values are given, it assigns zero value to the Z coordinate automatically. The co-ordinates can be given by typing values for the coordinates from the keyboard or by clicking the left mouse button at the location of the point.

2.10

UNITS

UNIT command is used to set the type of units. Type UN and press Enter key. Command: UN ø The Drawing Units dialog box is displayed. The upper left corner displays the Length tile. In this tile, click the arrow in the combo box and select a Type of length unit: Decimal, Scientific, Engineering, Architectural and Fractions. Similarly, for Angle, choose one of the five options. Decimal Degree is most common.

Part A – Chapter 2

14

Precision is the number of digits after decimal point to appear with the dimension, e.g. 15.1 or 15.13. In the Precision combo box, click the arrow and select the desired accuracy. Similarly, define the precision for angle units. Select appropriate units in Drag and Drop tile combo box. Effect of this setting is displayed in the Sample Output tile at the bottom of the dialog box. The direction from where all the angles are measured is set by clicking the Direction... button at the bottom of the dialog box. The Direction control dialog box is displayed. East direction means horizontal direction. Click on any one of the radio buttons as desired and then click the OK button. You are back to Drawing Units dialog box. Click the OK button to save and exit.

2.11

VIEWING A DRAWING

ZOOM command is used to enlarge or reduce the viewing size of a drawing. ZOOM, PAN and VIEW commands can be used even when another command is in progress. Type Zoom or only Z at the command line and press Enter key. Command: ZOOM ø [All/Center/Dynamic/Extents/Previous/Scale/Window/Object] :

From the options displayed between [ ], choose an option as required by typing the first uppercase character of the option and pressing Enter key. All option fits the whole drawing on the screen and displays the drawing. Window option displays only the drawing which is selected by clicking the mouse on two diagonal corners of the window on the screen. PAN command is used to move the window around the drawing and the coordinates of the objects remain unchanged. In MOVE command, the coordinates of the objects are modified.

2.12

DRAWING AIDS

The following commands help in creating a drawing.

2.12.1

Grid

Grid is a display of small dots on the screen at equally specified distance in the horizontal and vertical direction. The grid can be rotated also. The grid points do not appear while plotting the drawing. A grid can be activated by double clicking the GRID button at the bottom of the screen on Status bar or by pressing Function key F7. Function key is a toggle key. Pressing once puts the grid on, and the second pressing puts it off. Following are the options of this command: Command: GRID ø Specify grid spacing (¥) or [ON/OFF/Snap/Aspect] : Specify a value of grid spacing

2.12.2

Snap

Snap is the smallest invisible distance of increment that can be set for the mouse. If snap is set ON, the mouse moves in steps of the set increment. To select any intermediate point between the dots of the grid, put snap Off by pressing F9 or on Status line click the SNAP button or type the command as under: Command: SNAP ø Specify snap spacing or [ON/OFF/Aspect/Rotate/Style/Type] :

Specify snap increment or put it off

Introduction to CAD

2.12.3

15

Ortho

Ortho is short form of Orthogonal and it means 90° to each other. If this mode is set on, all lines are drawn only along the X and Y directions. On Status bar, click on ORTHO button to put it On or Off or press F8 Function key or type the command at command line as under: Command: ORTHO ø Enter mode [ON/OFF] :

2.13

Type an option: On or Off as desired.

OBJECT SNAP

Object Snap is also called OSNAP in short. Picking a point exactly and quickly with the mouse is quite diffiFig. 2.2 Object Snap Toolbar cult. Object snap allows specifying a point exactly. The Object snap toolbar with 17 icons are shown in Fig. 2.2. The points which can be snapped quickly using Object Snap toolbar are described in the same sequence in which they appear on the toolbar. 1. Tracking To specify relative coordinates in series to locate a point 2. From Snaps a point relative to a base point using relative coordinates 3. Endpoint Selects an end point of a line, an arc or a Pline 4. Midpoint Selects the middle of a line or an arc 5. Intersection Selects the intersection of two entities, e.g. a line with a line or an arc 6. Apparent intersect When entities do not intersect on the screen, but intersection exists beyond the screen limits, it selects that intersection 7. Extension Snaps to an extension 8. Center Selects the center of a circle 9. Quadrant Snaps at 0, 90, 180 and 270 degrees at the circumference of a circle 10. Tangent Snaps to a tangent to an arc or a circle 11. Perpendicular Snaps a point on a line, an arc or a circle that forms a perpendicular from the current point 12. Parallel Draws a vector parallel to an existing object 13. Node Snaps to a point entity 14. Insert Snaps the insertion point of a block or a text 15. Nearest Snaps the nearest point of an object 16. None Ignores the snap option 17. OSnap settings… Displays Drafting Settings dialog box Object snaps can be activated by clicking on an icon of Object Snap toolbar or pressing Shift key and the Right mouse button and selecting an option from the pop-up menu or type DDRMODES and press Enter key. Command: DDRMODES ø Drafting setting dialog box is displayed. Click the right tab for Object Snap and select an option.

When OSNAP is activated, a pick box appears at the cross hair. Click at that time to snap the required point.

16

Part A – Chapter 2

Example 1 Draw a rectangle of size 100 ¥ 80 mm and then draw its center lines, diagonals, etc. as shown in Fig. 2.S1. Save the file with name Example 1. This example demonstrates the following: (a) Starting a new file (b) Use of Setting commands, Grid and Snap (c) Use of LINE command. (d) Saving a file 1. START Æ Programs Æ AutoCAD 2005 Æ AutoCAD 2005

Fig. 2.S1

2. Start a New file by selecting File on Menu bar and choose New from the pull down menu. Select Template dialog box is displayed. Choose acadiso. Click OK button.

Example on use of Grid and Snap

3. At the command prompt, type Limits and press Enter to set the drawing limits as 297, 210 for A4 size sheet. Only the text shown in bold is to be typed by the user. The symbol ‘ø’ represents Enter key. Command: LIMITS ø Specify lower left corner or [ON/OFF] : ø Specify upper right corner : 297,210 ø

4. At the command prompt, type UNITS and press Enter. Drawing Units dialog box is displayed. In the Length tile, click the arrow in the combo box and choose Decimal as Type and Precision as 0.0. Choose the Type and Precision in the Angle tile also as decimal degrees and 0 respectively. Under the Drag and Drop scale combo box select millimeter. Click the OK button. 5. Use GRID command to set grid spacing. Command: GRID ø Specify grid spacing(X) or [ON/OFF/Snap/Aspect] : 10 ø

The grid should now be visible. If not, then press Enter to activate the previous command and type ON and press Enter or press F7. 6. At the command prompt, type SNAP and press Enter to set snap spacing. Command: SNAP ø Specify snap spacing or [ON/OFF/Aspect/Rotate/Style/Type] : 1 0 ø

Check that Snap button of the status bar at the bottom is highlighted. If not, press F9. You can also use the Snap command and type “On” and press Enter. 7. Use Line command to draw the rectangle of Fig. 2.S1. Command: LINE ø Specify first point: Specify next point or [Undo]: Specify next point or [Undo]: Specify next point or [Close/Undo]: Specify next point or [Close/Undo]: C ø

Click on a grid point on lower left corner Click on 10 grid points on right Click on 8 grid points above Click on 10 grid point on left Type C to close the lines

8. Use the LINE command again and make the inner rhombus. When all the four line are drawn press Enter 2 times to quit from the line command. Repeat the same command for diagonals and center lines. 9. On Menu bar, click File. From the pull down menu choose Save option. Save Drawing As dialog box is displayed. In the Save in combo box, click the arrow and choose a folder. At the bottom, in the File name text box type Example 1 and click Save button.

Introduction to CAD

2.14

17

DRAWING BASIC ENTITIES

In AutoCAD, the screen acts as an imaginary drawing sheet. Each drawing is created using entities like lines, circles, arcs, etc. While creating Fig. 2.3 Draw Toolbar a drawing, first the command for the entity is typed, and then the values are specified in the same sequence as prompted by the prompts in the command area. Just watch the prompts and feed the required data. If the prompt repeats, and you want to quit, press Esc key. To create an entity, use any one of the following methods: ∑ On Menu bar, click Draw and from the popup down menu choose an entity or ∑ On Draw toolbar (Fig. 2.3), click the icon of the required entity or ∑ On Command line, type the command for the entity and press Enter

2.14.1

POINT

Point is used to mark a center of an arc or a circle. The command name is POINT. AutoCAD offers 20 styles of points and they can be set using Pdmode system variable at the command prompt. Size of the point can be changed and set by the system variable Pdsize. Both the point style and size can be changed by a single command DDPTYPE. This command displays Point Style dialog box. Click the type of the point required. The size of a point can be entered in absolute units or a percentage of the screen size.

2.14.2

LINE

AutoCAD offers many types of lines given as follows: Line A simple straight continuous line Polyline A wide solid/hollow, parallel/taper line Spline A curved line Multiline Many parallel lines at specified distances Construction line A line from minus infinity to plus infinity for construction purposes Ray A line from a point to infinity for construction purposes To draw a continuous straight line, ∑ On Menu bar, click on Draw and then from the Popup menu click on Line or ∑ On Draw toolbar, click the Line icon shown above or ∑ On Command line, type the command LINE or L and press Enter Command: L ø Specify first point:

Specify a point by mouse or type coordinates through keyboard

Note: The text written above in bold letters is to be typed by the user. The text in italic letters is mentioned as guidance to the user for that prompt. The LINE command can draw one or more than one straight line segments. At the first prompt, enter the coordinates of the starting point of the line either by clicking the mouse at the required location or by entering value of (x1, y1) co-ordinates. Then enter the location of the other end of the line (x2, y2)

Part A – Chapter 2

18

through keyboard or mouse. A line is drawn between the specified points. Keep on entering the points (x3, y3), (x4, y4), etc. and the lines are drawn from the last point to the new specified co-ordinates. Press Enter to end the command. If you want to draw the last line up to the starting point, i.e. (x1, y1), type C to close the polygon made by the lines. After every entry of coordinates at the prompt, do not forget to press Enter key (ø ). Example 2 (after Section 2.14) demonstrates the use of the commands. To draw only horizontal or vertical lines, you can put ORTHO setting ON by clicking ORTHO button on the status line or by pressing F8 key. A simple line can be a dashed line, center line, etc. At the command prompt, type DDLTYPE and press Enter to load different types of lines. Linetype Manager dialog box is displayed. Click on Load… button. Load or Reload Line types dialog box is displayed. Scroll the various line types displayed and choose a line type, e.g. ‘Center’. To choose more than one line type press Ctrl key and then choose another line. Click OK on both the dialog boxes. MODIFY command can modify an existing linetype. On the menu bar, click Modify and then on the Popup menu, click on Properties and select the line to be changed. Then click on the desired type of line. The line is modified to the new desired type. See Chapter 4 on lines for more details.

2.14.3

CIRCLE

To draw a circle, ∑ On Menu bar, click Draw and then choose Circle from the Popup menu or ∑ On Draw toolbar, click the Circle icon shown above or ∑ On Command line, type the command CIRCLE or only C and press Enter. The prompt sequence is: Command: C ø Specify center point for circle or [3P/2P/Ttr (tan tan radius)]:

Circle command offers six options. The default option is Center point and radius. Three more options 3P, 2P and Ttr are displayed at the first prompt. Meaning of each is given as follows: 3P – A circle passing through 3 specified points. 2P – A circle passing through 2 points of a diameter. Ttr – A circle tangent to two specified entities and of specified radius The other two (Tan Tan Tan) and (Center and diameter) are not shown in the options but can be seen in the Popup menu. To choose an option, e.g. 3 points option, type 3P and press Enter key. Then specify coordinates of each point as prompted on the command line.

2.14.4

ARC

Arc command creates an arc from any three specified values from the following list: ∑ Start point ∑ Included angle ∑ Center point ∑ Chord length ∑ End point ∑ Direction at start point ∑ Radius

Introduction to CAD

19

To draw an arc, ∑ On Menu bar, click Draw, and then from the Popup menu click Arc or ∑ On Draw toolbar, click the Arc icon shown above or ∑ On Command line, type the command ARC or A and press Enter The prompt sequence is as follows: Command: ARC ø Specify start point of arc or [CEnter]: Specify second point of arc or [CEnter/End]: CE ø Specify center point of arc: Specify end point of arc or [Angle/chord Length]:

Specify the value asked at the prompt or type the character/s displayed in uppercase to specify other value within [ ] and press Enter key. For example, type CE for specifying [CEnter].

2.14.5

RECTANGLE

Drawing four lines using a LINE command can make a rectangle but RECTANG command can make it by specifying any two diagonal corners. To draw a rectangle: ∑ On Menu bar, click Draw, and then from the Popup menu click Rectangle or ∑ On Draw toolbar, click the Rectangle icon shown above or ∑ On Command line, type the command RECTANG and press Enter The prompt sequence is as follows: Command: RECTANG ø Specify first corner point or [Chamfer/Elevation/Fillet/Thickness/Width]: x1,y1 ø Specify other corner point: x3, y3 ø

(x1, y1) are the coordinates of one corner and (x3, y3) coordinates of the diagonal corner. This command offers many other options within [ ] described as follows. Type the uppercase character of the option and press Enter: Chamfer Creates a rectangle for which each corner is chamfered. First type C to specify the chamfer distances and then specify the imaginary extended diagonal corners. Elevation Creates a rectangle not at z = 0 plane, but at a specified elevation. Fillet Creates a rectangle with rounded corners. First type F to specify the fillet radius and then specify the imaginary extended diagonal corners. Thickness Creates a rectangle of specified thickness. It will be like a box. Width Creates a rectangle with specified line width. It can be combined with chamfer and fillet options also. Specify one option first and then the second option.

2.14.6

POLYGON

A polygon of any number of equal sides can be created by POLYGON command. ∑ On Menu bar, click Draw and then from the Popup menu click Polygon or ∑ On Draw toolbar, click the Polygon icon shown above or ∑ On Command line, type the command POLYGON and press Enter

Part A – Chapter 2

20

The prompt sequence is as follows: Command: POLYGON ø Enter number of sides : Specify center of polygon or [Edge]: Enter an option [Inscribed in circle/Circumscribe] Specify radius of circle:

At the first prompt specify number of sides of polygon and press Enter key. At second prompt there are two options Center and Edge explained as follows: A Center Option Specify the center of a circle. This circle is not visible and is an imaginary circle on which a polygon is drawn. Inscribed option draws polygon inside the circle or Circumscribed option draws polygon outside the circle. Type I for Inscribed or C for Circumscribed.

B Edge Option Do not specify center of polygon at second prompt but type E for Edge option and press Enter key. After this the two end points for any one side of the polygon are to be specified. Further prompts are: Specify first end point of edge: Specify second end point of edge:

Specify coordinates for point 1 and point 2 of an edge. Polygon is drawn in the direction from point 1 to point 2 in an anticlockwise direction.

2.14.7

ELLIPSE

An ellipse or a part of ellipse can be drawn using ELLIPSE command. ∑ On Menu bar, click Draw, and then in the Popup menu click Ellipse or ∑ On Draw toolbar, click the Ellipse icon shown above or ∑ On the command line, type the command ELLIPSE and press Enter The prompt sequence is as follows: Command: ELLIPSE ø Specify axis end point of ellipse or [Arc/Center]: Specify other endpoint of axis: Specify distance to other axis or [Rotation]:

In the default method, specify axis end points (major or minor) at the first and second prompts. Then specify a point on the other axis at right angle to the first axis. If the end points are not known, type C for Center option at first prompt and then specify coordinates of center. At the last prompt, in Rotation option, type R and then specify the rotation angle (angle at which a circle is rotated to look like an ellipse). The Arc option at the first prompt draws an elliptical arc. Type A for this option and press Enter. It further needs the start angle and the end angle measured at the center point of the ellipse from major axis in the anti-clockwise direction. The prompt sequence is as follows: Command: ELLIPSE ø Specify axis end point of ellipse or [Arc/Center]:A ø

Introduction to CAD

21

[Center]: Specify other point of axis: Specify distance to other axis or [Rotation]: Specify start angle or [Parameter]: Specify end angle or [Parameter/Included angle]:

At the last prompt, either specify the included angle or the end angle. Type I to specify the included angle of the arc and then specify the value of the included angle.

Isocircle In isometric views, a circle that appears as an ellipse is known as isocircle. This is very useful for isometric views. See Chapter 10 on isometric views for its use. This option is generally not displayed. It appears only if the Isometric Snap is ON. At the first prompt for the Ellipse, there is an additional option of Isocircle. Use F5 function key for different shapes of isocircle for front, side and top view. 2.14.8

DONUT

DONUT command is used to draw two concentric circles. It is not available on the Draw toolbar. If FILL mode is OFF, only two concentric circles are drawn with radial lines. If Fill mode is ON, a fully darkened circle is drawn. Fill mode can be set by FILL command and then set it on or off. On Menu bar, click Draw and then from the Popup menu choose Donut or type the command as follows: Command: DONUT ø Specify inside diameter : Specify outside diameter: Specify center of donut or : .................. Specify center of donut or : ø

2.14.9

This prompt repeats Press Enter to terminate the command

HATCH

Drawing a regular pattern inside a closed boundary is called hatching. To do hatching, click on the icon shown above or use BHATCH command on the command line. Boundary Hatch and Fill dialog box is displayed. ∑ Click on the Hatch tile. In the Type combo box, select Predefined. ∑ Click the down arrow in the Pattern combo box and choose a pattern from the list. ∑ Click the Pick Points icon on right side. The dialog box disappears and the drawing is displayed. Click anywhere in the area within the boundary. Dashed lines will display the selected boundary. Press Enter key. The Boundary Hatch and Fill dialog box is displayed again. ∑ Click Preview button to see the result. If satisfied, click OK button. This command is described in detail in Chapter 8 on sectional views.

2.14.10

TEXT

Text is used to write title blocks, label parts, write specifications, etc. on a drawing. All text entered uses the current text style, which becomes the default font and format setting. This can be changed to customize the text appearance. The prompt sequence is as under: Command: TEXT ø Specify starting point of text or [justify/Style]: Click at a point from where the text is to start

Part A – Chapter 2

22

Type a number to specify height of letters Specify angle if line of text is to be inclined Type the text and press Enter This prompt repeats Press Enter to quit the command

Specify Height : Specify rotation angle: Enter Text: ................ Enter text: ø

This command is described in detail in Chapter 5 on Lettering. Example 2 Draw all the objects as shown in Fig. 2.S2. This example demonstrates setting, Object Snaps, use of Draw commands LINE, CIRCLE, POLYGON, ARC and ELLIPSE This drawing example has the following entities: 1. A rectangle of size 50 mm ¥ 30 mm 2. An equilateral triangle of side 50 mm created by lines on the top side of the rectangle 3. A hexagon of each edge 30 mm on right vertical side of the rectangle 4. A semicircular arc of radius 15 mm on left vertical side of the rectangle Fig. 2.S2 Example on Draw 5. A TTT (Tangent, Tangent, Tangent) circle, tangent to 3 sides Commands of the triangle 6. A circle of radius 10 mm at the center of the hexagon 7. An ellipse within the rectangle of major axis 50 mm and minor axis 30 mm 8. One horizontal line passing through the centers of the arc and hexagon extreme edge 9. A donut in the center of rectangle 10. One vertical line from the apex of triangle to the base of the rectangle 11. Second vertical line from the top apex of hexagon to its bottom apex

Solution 1. Open a New file and set the Limits and Units as given in Example 1. Set the Grid and Snap spacing as 10 mm. 2. Set the objects snaps. At the command prompt, type DDRMODES and press Enter. Drafting settings dialog box appears. Click the Object Snap tab on the top. In the Object Snap window, click on the squares of Endpoint, Midpoint, Center, Tan Perpendicular and Intersection. A tick (÷ ) sign will appear. If this sign exists, just leave it there. Click OK button. 3. To make a rectangle of size 50 ¥ 30 mm type Rectang and press Enter. Command: RECTANG ø Specify first corner point or [Chamfer/Elevation/Fillet/Thickness/Width]:

Click on the lower left grid point on the screen Specify other corner point or [Dimensions]:

Move the mouse by 5 divisions of the grid horizontally and 3 divisions vertically and click there. This action draws a rectangle of 50 mm width and 30 mm height as grid spacing is set as 10 mm. Alternately at the second prompt you can also specify the relative coordinates as @50,30 and press Enter.

Introduction to CAD

23

4. To draw lines for the triangle use LINE command or L. Command: L ø Specify first point: Specify next point or [Undo]: @507

ZB

100 120

72 36 12

60 30 10

–53

–43

–43

–35

–35

–28

–28

–23

–

–

>7

Lower deviation in microns V X Y Z ZA

380 220 170 120

–41

–34

–34

–28

–28

–23

–23

–19

–19

–14

U

Grades >7 >7

T

360 200 150 100

–9 –17 –26

9 –11 –20 –32

7

–9 –17 –26

–8 –15 –22

–8 –15 –22

–7 –12 –18

–7 –12 –18

–15

–10

>7

S

80

0 ±IT/2 13 18

0 ±IT/2 10 14

7

6

6

6

–6

–8 –12

–4

–6 –10 –15

-4

–2

>7

R

80 100

9

60 30 10

50 25

8 12

8 12

6 10

6

5

3

0

>7

P

65

80

340 190 140 100

320 180 130

8

6 10

5

6

4

0 ±IT/2 10 14

0 ±IT/2

0 ±IT/2

0 ±IT/2

0 ±IT/2

0 ±IT/2

5

2

8

M

65

50 25

40 20

40 20

32 16

32 16

25 13

4

2

7

K

50

80

65

6

20 10

14

7

J

Fundamental deviations for holes

40

310 170 120

300 160 110

65

50

50

40

30

20

6

Grades All All All All All All All

300 160 110

290 150

290 150

270 140

6

3

270 140

3

–

All

Over Up to All

J

JS

Upper deviation in microns C D E F G H

B

A

Diameter in mm

Table 19.5

Tolerances, Limits and Fits 377

All All All

g h

All

js

m

n

p

r

6 10

–340 –190 –140 –100

–360 –200 –150 –100

–380 –220 –170 –120

–410 –240 –180 –120

–460 –260 –200 –145

–520 –280 –210 –145

–580 –310 –230 –145

–660 –340 –240 –170 –100 –50 –15 0 ±IT/2 –13 –21 4 17 31 50 77 122 166 236 284 350 425 520 670 880 1150

–740 –380 –260 –170 –100 –50 –15 0 ±IT/2 –13 –21 4 17 31 50 80 130 180 258 310 385 470 575 740 960 1250

–820 –420 –280 –170 –100 –50 –15 0 ±IT/2 –13 –21 4 17 31 50 84 140 196 584 340 425 520 640 820 1050 1350

–920 –480 –300 –190 –110 –56 –17 0 ±IT/2 –16 –26 4 20 34 56 94 158 218 315 385 475 580 710 920 1200 1550

65 80

80 100

100 120

120 140

140 160

160 180

180 200

200 225

225 250

250 280

9 17 26 34

–9 –15 3 13 23 37 54

–9 –15 3 13 23 37 51

–7 –12 2 11 20 32 43

–7 –12 2 11 20 32 41

–5 –10 2

81 97

88

118 160 218 112 148 200 274

114 136 180 242 325

94

75

87 102 122 144 172 226 300 405

70

68 80

55 64

98 136 188

77 108 150

91 124 146 178 214 258 335 445 585

75 102 120 146 174 210 274 360 480

66

54

60

48

73

60

92 122 170 202 248 300 365 470 620 800

79 104 144 172 210 254 310 400 525 690

71

59

53

43

48

41

– 63

450 500 –1650 –840 –480 –230 –135 –68 –20 0 ±IT/2 –20 –32 5 23 40 68 132 252 360 540 660 820 1000 1250 1600 2100 2600

400 450 –1500 –760 –440 –230 –135 –68 –20 0 ±IT/2 –20 –32 5 23 40 68 126 232 330 490 595 740 920 1100 1450 1850 2400

355 400 –1350 –680 –400 –210 –125 –62 –18 0 ±IT/2 –18 –28 4 21 37 62 114 208 294 435 530 660 820 1000 1300 1650 2100

315 355 –1200 –600 –360 –210 –125 –62 –18 0 ±IT/2 –18 –28 4 21 37 62 108 190 268 390 475 590 730 900 1150 1500 1900

280 315 –1050 –540 –330 –190 –110 –56 –17 0 ±IT/2 –16 –26 4 20 34 56 98 170 240 350 425 525 650 790 1000 1300 1700

–85 –43 –14 0 ±IT/2 –11 –18 3 15 27 43 68 108 146 210 252 310 380 465 600 780 1000

–85 –43 –14 0 ±IT/2 –11 –18 3 15 27 43 65 100 134 190 228 280 340 415 535 700 900

–85 –43 –14 0 ±IT/2 –11 –18 3 15 27 43 63

–72 –36 –12 0 ±IT/2

–72 –36 –12 0 ±IT/2

–60 –30 –10 0 ±IT/2

–60 –30 –10 0 ±IT/2

–50 –25 –9 0 ±IT/2

43

35

47 54

39 45

90 130

97

80

60

–320 –180 –130 –80

9 17 26 34

8 15 22 28

41

33

64

67

50

40

50 65

–8 2

–5 –10 2

–4

–

–

50

52

42

32

40 50

–50 –25 –9 0 ±IT/2

–40 –20 –7 0 ±IT/2

35

28

–

42

35

26

–310 –170 –120 –80

8 15 22 28

7 12 18 23

– 40

–

–

–

–300 –160 –110 –65

–8 2

–6 4

33

– 34

– 28

– 20

All

30 40

–4

–3

–

28

23

18

All

24 30

–40 –20 –7 0 ±IT/2

–32 –16 –6 0 ±IT/2

28

–

–

–

All

zc

–300 –160 –110 –65

7 12 18 23

23

19

14

All

zb

–290 –150 –95 –50

–6 3

6 10 15 19

8 12 15

4

All

za

18 24

–3

–5 2

4

2

Grades All All

z

14 18

–32 –16 –6 0 ±IT/2

–2

–4 1

–4 0

All All

y

–280 –150 –80 –40

–2

–2

All

Lower deviation in microns s t u v x

–290 –150 –95 –50

–25 –13 –5 0 ±IT/2

j k

Grades 5,6 7 4-7 All All All All

j

6 10

–20 –10 –4 0 ±IT/2

–14 –6 –2 0 ±IT/2

All

f

10 14

–270 –140 –70 –30

–270 –140 –60 –20

Grades All

6

All

3

All

3

All

Over Up to

Upper deviation in microns b c d e

–

a

Diameter in mm

Table 19.6 Fundamental deviations for shafts

378 Part D – Chapter 19

Tolerances, Limits and Fits

379

Fig. 19.5 Interpretation of Upper Case Letter Symbols

19.5.2

Letter Symbol for Shafts

Figure 19.6 shows fundamental deviation for for the shafts, which is the mirror image of Fig. 19.4.

Fig. 19.6

Graphical Illustrations of Tolerance Zones for Shaft

Note the following from this Fig. 19.6. a. Lower-case letters have been used to denote a zone for shafts. b. Maximum negative upper deviation is for letter a. Also see its meaning in Fig. 19.7A. c. Upper deviation is zero for letter h. See Fig. 19.7B also. d. Grade js has variation in both positive and negative side and is equal to ± IT/2. e. Lower deviation goes on increasing for letters m, n….. and is maximum positive for grade zc. Upper deviation for these letters is equal to the lower deviation + IT grade see Fig. 19.7C also.

Fig. 19.7 Interpretation of-lower Case Letter Symbols

Part D – Chapter 19

380

19.6

PLACING A DIMENSION WITH TOLERANCE

Four methods are used for placing dimensions with tolerances:

19.6.1

Basic Size with Deviations

In this method, the basic size followed by its tolerances is placed above the dimension line. Tolerances are prefixed with + and – sign for upper and lower deviations respectively (Fig. 19.8A). Positive tolerance is put above the negative tolerance. These deviations may be equal or unequal. Text size of tolerances numbers is kept smaller than the text size of the basic value. If the deviation is both in + and – side, it is called Bilateral tolerance (Fig. 19.8A). It can be equal or unequal. If one of the deviations is zero it is called Unilateral tolerance.

Fig. 19.8

19.6.2

Placing the Tolerated Dimension

Maximum and Minimum Limits

In this method, instead of placing the deviations, both maximum and minimum limits are placed over the dimension line (Fig. 19.8B). Sometimes maximum value above and minimum value below the dimension line is also placed. The precision of both the tolerance values (places after the decimal point) should be kept same, e.g. 50.1 and 50.025 is not correct. It should be written as 50.100 and 50.025. When expressed in single line, lower limit precedes the higher limit separated by a dash e.g. 49.98 – 50.05. The above two mentioned methods are applicable to angular dimensions also.

19.6.3 General Notes If tolerances are same for all the sizes, a general note can be written as below: UNLESS SPECIFIED, TOLERANCES ARE ± 0.02 mm. If tolerances amre dependent on some range of sizes a tabular Dimension (mm) statement can be made as under: Up to 100 19.6.4

Basic Size with Fundamental Tolerances

100 to 200 200 to 300 300 to 500 More than 500

Tolerance ± 0.01 ± 0.02 ± 0.03 ± 0.05 ± 0.10

In this method, the basic size is followed by the fundamental tolerances Letter symbol and IT grade. It is shown in Fig. 19.7C. For mating parts both hole and shaft dimensions can be together. For example: 50H7/50C9. First value is for hole and second value is for the shaft. Meaning of each character is shown in Fig. 19.9.

19.7

CUMULATIVE TOLERANCES

If the dimensions are given in succession all the tolerances get accumulated to give a large value of tolerance, it is said to be cumulative tolerance (Fig. 19.10A). The overall total positive tolerance

Tolerances, Limits and Fits

381

between the first and the last hole is ±.08. All the individual tolerances have been summed up. This difficulty can be overcome by giving dimensions from a fixed datum (Fig. 19.10B). The tolerance between first and the last hole is now reduced to ±0.2.

Fig. 19.9

Basic Size with Fundamental Tolerances

Fig. 19.10

Cumulative and Non-cumulative Tolerances

Example 1 Figure 19.S1 shows overall limits on length as 100.0 and 99.5. Two holes are drilled at equal distance from center line at distance of 30 mm. Calculate the limits for size A. The same tolerance is applied for the vertical distance between the center lines of the circles. Calculate the limits for size B. Solution Size A is maximum when size 100 is maximum and tolerance on size 30 is minimum. Hence: 2A (Maximum) = 100 – 2 ¥ (29.98) = 100 – 59.96 = 40.04. Hence maximum A = 20.02 Size A is minimum when size 100 is minimum and tolerance on size 30 is maximum. Hence: 2A (Minimum) = 99.5 – 2 ¥ (30.02) = 99.5 – 60.04 = 39.46. Hence minimum A = 19.73 Therefore maximum size of A is 20.02 and minimum size is 19.73. or A = 20 +-00..02 27 Since B = 3A, hence maximum size of B = 60.06 and minimum size = 59.19. or B = 60 +-00..06 81 Example 2 A schematic representation of 80 mm basic size is shown by zero line in Fig. 19.S2. The deviation is shown in microns by shaded area for 5 cases from A to E. Calculate the following for each case: a. Lower deviation b. Upper deviation c. Upper limit size d. Lower limit size

Fig. 19.S1

Fig. 19.S2

Part D – Chapter 19

382 Solution PROBLEM

(A)

(B)

(C)

(D)

(E)

Lower deviation (m)

+ 15

0

–5

– 30

– 40

Upper deviation (m)

+ 30

+ 10

+ 10

0

– 15

Lower limit size (mm)

80.015

80.000

79.995

79.970

79.960

Upper limit size (mm)

80.030

80.010

80.010

80.000

79.985

Example 3 A shaft has basic diameter 60 mm. Its maximum and minimum sizes are 59.8 mm and is 59.7 mm respectively. Find its fundamental deviation zone and IT grade tolerance. Solution Upper deviation = 60 – 59.8 = 0.2 mm = 200 m Lower deviation = 60 – 59.7 = 0.3 mm = 300 m IT grade = Upper deviation – Lower deviation = 300 – 200 = 100 m From Table 19.2 for basic size of 60 mm, the IT grade nearest to 100 is 120, i.e. IT 10. From Table 19.6 for basic size of 60 mm, upper deviation of 200 m is nearest to 190, i..e. zone ‘b’. Hence the dimension can also be given as 60b10. Letter b is fundamental deviation and 10 is IT grade. Example 4

A spindle is specified dimension as 50 ± 0.005. Recommend a suitable manufacturing process.

Solution The deviation is equal both on + and – sides, hence the grade is ‘js’. IT/2 = 0.005. Therefore IT = 0.01 = 10 m From Table 19.2 for size 50 mm the nearest value under js column is 11 i.e. IT5. From Table 19.4 for IT5 grade, the last manufacturing process required is fine grinding or lapping.

Example 5 Calculate maximum and minimum sizes of the holes for the fundamental tolerances given as under: a. 45G7 b. 50H8 c. 80J7 d. 80JS7 e. 90P96 f. 120ZC8 Solution The relevant portions of the Tables 19.2 and 19.5 are being reproduced here for easy reference. Refer Fig. 19.4 also. From Table 19.2 Case

Basic size

Range

IT Grade (ITG)

Tolerance value

A B C D E F

45 50 80 80 90 120

30-50 30-50 50-80 50-80 80-120 80-120

7 8 7 7 6 8

25 39 30 30 22 54

Tolerances, Limits and Fits

383

From Table 19.5 Case A B C D E F

Basic size 45 50 80 80 90 120

Range

Tolerance zone

40-50 40-50 65-80 65-80 80-100 100-120

G H J JS P ZC

Tolerance value in m +9 0 + 18 ± IT/2 – 37 – 690

Upper or lower Lower Lower Lower Lower/Upper Upper Upper

A. 45G7 For basic size 45 mm, the range is 40 to 50 mm under the tolerance Zone G see the lower tolerance value in Table 19.5 for holes as + 9m i.e. 0.009 mm. Therefore minimum size is 45.009 mm. Now find the IT grade values from Table 19.2 For basic size 45 mm (range is 30 to 50 mm), under the IT grade 7, tolerance value is 25m, i.e. 0.025 mm. Therefore maximum size is minimum size + IT grade = 45.034 mm. B. 50H8 For basic size 50 mm, the range is 40 to 50 mm under the tolerance Zone H see the lower tolerance value in Table 19.5 for holes as 0m or 0 mm. Therefore minimum size is 50 mm. Now find the IT grade values from Table 19.2. For basic size 50 mm, (range is 30 to 50 mm) under the IT grade 8, tolerance value is 39m, i.e. 0.039 mm. Therefore maximum size is 50 + 0.039 = 50.039 mm. C. 80J7 For basic size 80 mm, the range is 65-80 mm. Under the tolerance Zone J see the tolerance value in Table 19.4 for holes as +18m, i.e. 0.018 mm. Therefore minimum size is 80.018 mm. For basic size 80 mm, the range is 50 to 80 mm under the IT grade 7, tolerance value 30m i.e. 0.030 mm. Therefore maximum size is minimum size + IT grade = 80.018 + 0.030 = 80.048 mm. D. 80JS7 For basic size 80 mm, the range is 65-80 mm. Under the tolerance Zone JS see the tolerance value in Table 19.5 for holes as ± IT/2. Now find the IT grade values from Table 19.2 For basic size 80 mm (range 50 to 80 mm) under IT grade 7, tolerance value is 30m, i.e. 0.030 mm. Therefore IT/2 = ± 15m = 0.015 mm. Hence answers are: maximum size = 80.015 mm and minimum size = 79.985 mm. E. 90P96 For basic size 90 mm, the range is 80-100 mm under the tolerance Zone P, see the upper tolerance value in Table 19.5 for holes as –37m, i.e. –0.037 mm. Therefore maximum size is 89. 963 mm. For basic size 90 mm (range is 80 to 120 mm), under the IT grade 6, tolerance value is 22m, i.e. 0.022 mm. Therefore minimum size = 89.963–0.022 = 89.941 mm. F. 120ZC8 For basic size 120 mm, the range is 100-120 mm. Under the tolerance Zone ZC see the upper tolerance value in Table 19.5 for holes as – 690 m, i.e. 0.690 mm. Therefore maximum size is 119.310 mm. For basic size 120 mm (range is 80 to 120 mm), under the IT grade 8, tolerance value is 54m i.e. 0.054 mm. Hence minimum size = 119.310 – 0.054 = 119.256 mm.

Part D – Chapter 19

384 Example 6

Calculate the maximum and minimum sizes of the shafts for the fundamental tolerances given

as under: a. 35d7

b. 60h8

c. 85js10

d. 95m6

Solution A. 35d7 From Table 19.6 upper deviation for basic size 35 mm, under tolerance zone d the value is – 80m, i.e. 0.080 mm. Hence maximum size is = 34.92 mm. From Table 19.2 for basic size 35 mm, under IT grade 7 the tolerance value is 25m, i.e. 0.025 mm. Hence minimum size is = 34.92 – 0.025 = 34.895 mm. B. 60h8 From Table 19.6 upper deviation for basic size 60 mm, under tolerance zone h, the value is 0. Hence maximum size = 60.000 mm. From Table 19.2 for basic size 65 mm, under IT grade 8, the tolerance v alue is 46 m , i.e. 0.046 mm. Hence minimum size is = 60.000 – 0.046 = 59.954 mm. C. 85js10 From Table 19.6 upper deviation for basic size 85 mm, under tolerance zone js is ± IT/2. From Table 19.2 for basic size 85 mm, under IT grade 10 the tolerance value is 140 m, i.e. 0.14 mm. IT/2 = 0.07. Hence minimum size is = 85 – 0.07 = 84. 93 mm. Maximum size is = 85 + 0.07 = 85.07 mm. D. 95 m6 From Table 19.6 upper deviation for basic size 95 mm, under tolerance zone m the value is +0.13m, i.e. + 0.013 mm. Hence minimum size is = 95.013 mm. From Table 19.2 for basic size 95 mm, under IT grade 6 the tolerance value is 22m, i.e. 0.022 mm. Hence maximum size = 95.013 + 0.022 = 95.035 mm.

Example 7 A bracket as shown in Fig. 19.S3A is to be sand casted. Give suitable tolerances. Solution Refer Table 19.4. For sand casting process, tolerance grade obtainable is IT16. Note the following values for the sizes required (shown bold for the ranges given in the table) and calculate the tolerances as under:

Fig. 19.S3A A Bracket

Basic size

Range

m) IT16 tolerance (m

Total tolerance (mm)

Symmetric tolerance

10

6-10

900

0.9

± 0.45

25

18-30

1300

1.3

± 0.65

40

30-50

1600

1.6

± 0.8

75

50-80

1900

1.9

± 0.95

200

180-250

2900

2.9

± 1.45

Tolerances, Limits and Fits

385

The tolerances are rounded to one decimal place as sand casting cannot give that much accuracy. These tolerances are indicated on the drawing as under in Fig. 19.S3B:

Fig. 19.S3B Solution to Example 19.S3A Example 8 A stepped shaft is shown in Fig. 19.S4A is to be machined in mass production on a capstan lathe. Give suitable tolerances.

Fig. 19.S4A

A Stepped Shaft

Solution Refer Table 19.4. For turning using capstan lathe, tolerance grade obtainable is IT9. Note the following values for the sizes required (shown bold for the ranges given in the table) and calculate the tolerances as under: m) Basic size Range IT9 tolerance (m 30 18- 30 52 35 30- 50 62 40 30- 50 74 75 80-120 87 The tolerances are indicated on the drawing in Fig. 19.S4B:

Total tolerance (mm) 0.052 0.062 0.074 0.087

Symmetric tolerance ± 0.026 ± 0.031 ± 0.037 ± 0.044

Note: Tolerance value on end diametions have to be changed, if is ends fit in a bearing. See Section 19.8 on fits. Fig. 19.S4B Solution to Example 19.S4A

19.8 FITS The dimensional relation between the mating parts is known as a Fit. It indicates the tightness or looseness of the mating parts. This is very important for many engineering applications for the satisfactory working of parts in movement such as piston in a cylinder, Journal in a bearing, valve in valve guide, gear on splines, wheel on shaft, etc. 19.9 SYSTEMS OF FITS Two systems are used to specify a fit. Any one of the system can be used. ∑ Hole basis – Lower deviation for the hole is zero ∑ Shaft basis – Upper deviation for the shaft is zero.

Part D – Chapter 19

386

19.9.1

Hole Basis

In this system, basic size is taken as the minimum hole size. Hence lower deviation is zero, i.e. fundamental deviation is H. Shaft diameter is calculated by subtracting the desired allowance from the basic hole size. Tolerances are then applied to each part separately (Fig. 19.11).

19.9.2

Fig. 19.11 Hole Basis of Fits

Shaft Basis

In this system, basic size is taken as the maximum shaft diameter. Hence upper deviation is zero, i.e. fundamental deviation is h. Hole diameter is calculated by adding the desired allowance to the basic shaft size. Tolerances are then applied to each part separately (Fig. 19.12).

19.10

SPECIFYING A FIT

Fig. 19.12

Shaft Basis of Fits

A fit is specified by two fundamental tolerance grades on hole and shaft separated by a dash. Some systems use a slash also for separating the tolerances. First tolerance is on hole, and the second tolerance is on of shaft. If it is on the hole basis, tolerance grade H is specified for hole. If it is on shaft basis, tolerance h is specified for the shaft. The Tolerance zones (Tolerance letter and IT grade) get interchanged for both, i.e. the tolerance zone of the hole is applied to the shaft and that of shaft is applied to the hole. For example: H7-d8 It is a fit on hole basis, where H7 is the tolerance zone for the hole and d8 is the tolerance zone for the shaft. The same fit can be specified on shaft basis as under: D8-h7 It is a fit on shaft basis, where D8 is the tolerance zone for the hole and h7 is the tolerance zone for the shaft. Note the difference in both the systems carefully. IT grade 7 for the hole is now for shaft and IT grade 8 of shaft is now for hole. Tolerance letters also get interchanged.

19.11

TYPES OF FITS

Rotating parts require a definite amount of clearance between stationary and moving elements. Stationary mating parts require different type of fit. Mainly there are 3 types of fits as given below: 1. Clearance fit Hole diameter is always bigger than shaft diameter (Fig. 19.13A). Both maximum and minimum clearances are always positive. 2. Transition fit Hole diameter is close to shaft diameter (Fig. 19.13B). Maximum clearance is positive and minimum clearance is negative. Negative clearance is called Interference. 3. Interference fit Hole diameter is always lesser than shaft diameter (Fig. 19.13C). Both maximum and minimum clearances are always negative.

Tolerances, Limits and Fits

Fig. 19.13

19.12

387

Types of Fits

SELECTION OF FITS

Each type of fit is further divided and named to be more precise in defining allowance between internal and external dimensions of hole and shaft respectively. Allowance is the clearance or interference provided intentionally. Various fits and their applications are given in Table 19.7. Table 19.7 Types of fits and their typical uses Hole basis

Shaft basis

Name of fit

Application Clearance Fits

H11-c11

C11-h11

Loose running fit

Commercial rough applications

H9-d9

D9-h9

Free running fit

Large temp. variations, high running speeds and heavy journal pressures

H8-c11

C11-h8

Slack running fit

Oil seals, splined shafts

H8-d9

D9-h8

Easy running fit 1

Slow speed sleeve bearings, plastic bearings

H8-e8

E8-h8

Easy running fit 2

Medium speed sleeve bearings, grease lubricated bearings, sliding blocks, sliding gears on shafts

H8-f7

F7-h8

Close running fit 1

Accurate locations with moderate speeds and loads

H7/f 7

F7-h7

Close running fit 2

A little clearance for main bearings, crank and connecting rod bearings

H7-g6

G6-h7

Sliding fit

Not for running, but to move or turn freely

H7-h6

H6-h7

Locational fit

Locating stationary parts like snug in bearings, cutters on milling mandrels, sliding under manual forces, if lubricated

(Contd.)

Part D – Chapter 19

388

Table 19.7 Hole basis

Shaft basis

(Contd.)

Name of fit

Application Transition Fit

H7-j6

J6-h7

Easy Push fit

Frequently dismantled parts like keys, pulleys, gear trains, hand wheels, Joining with hand pressure

H7-k6

K6-h7

Push fit

Pulleys, gears, bearings on shafts, Easy joining with hammer

H7-m6

M6-h7

Force fit

Inner race of the bearings on shafts, Hard joining with hammer

H7-n6

N6-h7

Light press fit

Worm wheels, big gears with feather keys, Press tools, Rotors on motor shafts Interference Fit

H7-p6

P6-h7

Medium press fit 1

Moving parts with rigidity without much pressure on hub

H7-r6

R6-h7

Medium press fit 2

Valve seats, couplings on shaft ends, Hubs of couplings, Bearing bushes in housing

H7-s6

S6-h7

Heavy press fit

Hollow part is heated and shaft is put in it. On cooling, it gets shrunk fit over the shaft. Shrunk fit on light sections, cylinder liner

H7-u6

U6-h7

Shrunk fit

Shrunk fit for parts requiring heavy forces

Example 9 (FITS) Limits of a shaft and hole are as under: Maximum shaft diameter = 60.00 mm, Minimum shaft diameter = 59.98 mm Maximum hole diameter = 60.005 mm, Minimum hole diameter = 60.002 mm Calculate the following: a. Tolerance on shaft b. Tolerance on hole c. Maximum clearance d. Minimum clearance e. Type of fit Solution

Tolerance on shaft = 60.00 – 59.98 = 0.02 mm Tolerance on hole = 60.005 – 60.002 = 0.003 mm Maximum clearance = Maximum hole diameter – minimum shaft diameter = 60.005 – 59.98 = 0.025 mm Minimum clearance = Minimum hole diameter – maximum shaft diameter = 60.002 – 60.00 = 0.002 mm Since both the clearances are positive hence it is clearance type of fit.

Example 10 Figure 19.S5 below shows schematically tolerances of the shafts by hatched area and on holes by shaded area. Calculate the maximum and minimum clearances and type of fit.

Fig. 19.S5

Tolerances, Limits and Fits

389

Solution (A)

(B)

(C)

(D)

(E)

Maximum clearance

35

31

24

31

–26

Minimum clearance

13

10

–1

–4

–4

Clearance

Transition

Transition

Interference

Type of fit

Clearance

Example 11 Suggest size for a shaft to allow a fit of H8-f8 for a hole of 40 mm. Solution H means lower deviation is zero hence minimum hole diameter is 40 mm. IT grade 8 for basic size 40 mm is 39m or 0.039 mm (Table 19.2) Maximum hole diameter = Minimum hole diameter + IT grade = 40.000 + 0.039 = 40.039 mm. f means upper deviation is – 25m = – 0.025 mm (Table 19.6) Therefore maximum shaft diameter = 40 – 0.025 = 39.975 mm Minimum shaft diameter = Maximum shaft diameter – IT grade tolerance = 39.975 – 0.039 = 39.936 mm In Example 11, Tables 19.2, 19.5 and 19.6 have to be referred to find clearances and limits. Tables 19.8 and 19.9 have been deduced from those tables for the commonly used fits. If the type of fit is same as given in these tables, sizes can be directly found. If not, then the method as explained in Example 11 has to be followed.

Table 19.8

Preferred clearance limits on hole basis

+ Means add to the basic size – Means subtract from the basic size Basic size

Limit

From Up to

Loose running

Free running

H11

c11

Fit

H9

d9

Fit

Close running H8

f7

Fit

Sliding

Locational

H7

g6

Fit

H7

h6

Fit

1

3

Max Min

+60 0

– 60 – 120

+180 +60

+25 0

– 20 – 45

+70 +14 +20 0

–6 –16

+30 +10 +6 0

–2 –8

+18 +10 +2 0

0 +16 –6 0

3

6

Max + 75 Min 0

– 70 – 145

+220 +70

+30 0

– 30 – 60

+90 +18 +30 0

– 10 – 22

+40 +12 – 4 +10 0 – 12

+24 +12 +4 0

0 +20 –8 0

6

10

Max Min

+90 0

– 80 – 170

+260 +80

+36 0

– 40 – 76

+112 +22 +40 0

– 13 – 28

+50 +15 – 9 +13 0 – 14

+29 +15 +5 0

0 +24 –9 0

10

16

Max +110 Min 0

– 95 – 205

+315 +95

+43 0

– 50 – 93

+136 +27 +50 0

– 16 – 34

+61 +18 – 6 +16 0 – 17

+35 +18 0 +29 +6 0 – 11 0

16

30

Max +130 Min 0

– 110 – 240

+370 +110

+52 0

– 65 – 117

+169 +33 +65 0

– 20 – 41

+74 +21 – 7 +20 0 – 20

+41 +21 0 +7 0 – 13

+34 0

30

40

Max +160 Min 0

– 120 – 280

+440 +120

+62 0

– 80 – 142

+204 +39 +80 0

– 25 – 50

+89 +25 – 9 +25 0 – 25

+50 +25 0 +9 0 – 16

41 0

40

50

Max +160 Min 0

– 130 – 290

+450 +130

+62 0

– 80 – 142

+204 +39 +80 0

– 25 – 50

+89 +25 – 9 +25 0 – 25

+50 +25 0 +9 0 – 16

+41 0

(Contd.)

Part D – Chapter 19

390

Table 19.8 Basic size

Limit

From Up to

Loose running H11

c11

Free running

Fit

H9

d9

Fit

(Contd.) Close running H8

f7

Fit

Sliding H7

g6

Locational Fit

H7

h6

Fit

50

60

Max +190 Min 0

– 140 – 330

+520 +140

+74 0

– 100 – 174

+248 +46 +100 0

– 30 – 60

+106 +30 – 10 +30 0 – 29

+59 +30 0 +10 0 – 19

+49 0

60

80

Max +190 Min 0

– 50 – 340

+530 +150

+74 0

– 100 – 174

+248 +46 +100 0

– 30 – 60

+106 +30 – 10 +30 0 – 29

+59 +30 0 +10 0 – 19

+49 0

80 100

Max +220 Min 0

– 170 – 390

+610 +170

+87 0

– 120 – 207

+294 +54 +120 0

– 36 – 71

+125 +35 – 12 +36 0 – 34

+69 +35 0 +12 0 – 22

+57 0

100 120

Max +220 Min 0

– 180 – 400

+620 +180

+87 0

120 – 207

+294 +54 +120 0

– 36 – 71

+125 +35 – 12 +36 0 – 34

+69 +35 0 +12 0 – 22

+57 0

120 160

Max +250 Min 0

– 210 – 460

+710 +100 +210 0

– 145 – 245

+345 +63 +145 0

– 43 – 83

+146 +40 – 14 +43 0 – 39

+79 +40 0 +14 0 – 25

+65 0

160 200

Max +290 Min 0

– 240 – 530

+870 +115 +240 0

– 170 – 285

+400 +72 +170 0

– 50 – 96

+168 +46 – 15 +50 0 – 44

+90 +46 0 +15 0 – 29

+75 0

200 250

Max +290 Min 0

– 280 – 570

+860 +115 +280 0

– 170 – 285

+400 +72 +170 0

– 50 – 96

+168 +46 – 15 +50 0 – 44

+90 +46 0 +15 0 – 29

+75 0

250 300

Max +320 Min 0

– 330 – 650

+970 +130 +330 0

– 190 – 320

+450 +81 – 56 +190 0 – 108

+189 +52 – 17 +101 +52 0 +56 0 – 49 +17 0 – 32

+84 0

300 400

Max +360 Min 0

– 400 +1120 +140 – 760 +400 0

– 210 – 350

+450 +89 – 62 +190 0 – 119

+208 +57 – 18 +111 +57 0 +62 0 – 54 +18 0 – 36

+93 0

400 450

Max +400 Min 0

– 440 +1240 +155 – 840 +440 0

– 230 – 385

+540 +97 – 68 +230 0 – 131

+228 +63 – 20 +123 +63 0 +103 +68 0 – 60 +20 0 – 40 0

450 500

Max +400 Min 0

– 480 +1280 +155 – 880 +480 0

– 230 – 385

+540 +97 – 68 +230 0 – 131

+228 +63 – 20 +123 +63 0 +103 +68 0 – 60 +20 0 – 40 0

Table 19.9

Preferred Transition and Interference limits on hole basis

+ Means add to the basic size – Means subtract from the basic size Basic size

Limit

From Up to

Transition

Transition

Interference

Interference

Interference

Push fit

Light press fit

Medium press fit

Heavy press fit

Shrunk fit

H7

p6

H7

s6

H7

k6

Fit

H7

n6

Fit

Fit

Fit

H7

u6

Fit

1

3

Max Min

+10 0

+6 0

+10 –6

+10 0

+10 +4

+6 +10 – 10 0

+12 +6

+4 +10 – 12 0

+20 +14

– 4 +10 – 20 0

+24 +18

–8 – 24

3

6

Max Min

+12 0

+9 +1

+11 –9

+12 0

+16 +8

+4 +12 – 16 0

+20 +12

0 +12 – 20 0

+27 +19

– 7 +12 – 27 0

+31 +23

– 11 – 31

6

10

Max Min

+15 0

+10 +1

+14 – 10

+15 0

+19 +10

+5 +15 – 19 0

+24 +15

0 +15 – 24 0

+32 +23

– 8 +15 – 32 0

+37 +28

– 13 – 37

10

16

Max Min

+18 0

12 +1

+17 – 12

+18 0

+23 +12

+6 +18 – 23 0

+29 +18

0 +18 – 29 0

+39 +28

– 10 +18 – 39 0

+44 +33

– 15 – 44

(Contd.)

Tolerances, Limits and Fits Table 19.9 Basic size

Limit

Transition

Transition

Push fit From Up to

Light press fit

H7

k6

Fit

H7

n6

Fit

391

(Contd.) Interference

Interference

Medium press fit

Heavy press fit

H7

p6

H7

s6

Fit

Fit

Interference Shrunk fit H7

u6

Fit

16

30

Max Min

+21 0

+15 +2

+19 – 15

+21 0

+28 +15

+6 +21 – 28 0

+35 +22

– 01 +21 – 35 0

+48 +35

– 14 +21 – 48 0

+54 +41

– 20 – 54

30

40

Max Min

+25 0

+18 +2

+23 – 18

+25 0

+33 +17

+8 +25 – 33 0

+42 +26

– 1 +25 – 42 0

+59 +43

– 18 +25 – 59 0

+76 +60

– 35 – 76

40

50

Max Min

+25 0

+18 +2

+23 – 18

+25 0

+33 +17

+8 +25 – 33 0

+42 +26

– 1 +25 – 42 0

+59 +43

– 18 +25 – 59 0

+86 +70

– 45 – 86

50

60

Max Min

+30 0

+21 +2

+28 – 21

+30 0

+39 +20

+10 +30 – 39 0

+51 +32

– 2 +30 – 51 0

+72 +53

– 23 +30 +106 – 57 – 72 0 +87 – 106

60

80

Max Min

+30 0

+21 +2

+28 – 21

+30 0

+39 +20

+10 +30 – 39 0

+51 +32

– 5 +30 – 51 0

+78 +59

– 29 +30 +121 – 72 – 78 0 +102 –121

80 100

Max Min

+35 0

+25 +3

+32 – 25

+35 0

+45 +23

+12 +35 – 45 0

+59 +37

– 2 +35 – 59 0

+93 +71

– 36 +35 +146 – 89 – 93 0 +124 –146

100 120

Max Min

+35 0

+25 +3

+32 – 25

+35 0

+45 +23

+12 +35 – 45 0

+59 +37

– 2 +35 +101 – 44 +35 +166 –4 0 9 – 59 0 +79 – 101 0 +144 –116

120 160

Max Min

+40 0

+28 +3

+37 – 28

+40 0

+52 +27

+13 +40 – 52 0

+68 +43

– 3 +40 +128 – 61 +40 +215 – 150 – 68 0 +100 – 25 0 +190 – 2 1 5

160 200

Max Min

+46 0

+33 +4

+42 – 33

+46 0

+60 +31

+15 +46 – 60 0

+79 +50

– 4 +46 +151 – 76 +46 +265 – 190 – 79 0 +122 – 151 0 +236 – 265

200 250

Max Min

+46 0

+33 +4

+42 – 33

+46 0

+60 +31

+15 +46 – 60 0

+79 +50

– 4 +46 +169 – 94 +46 +313 – 238 – 79 0 +140 – 169 0 +284 – 313

250 300

Max Min

+52 0

+36 +4

+48 – 36

+52 0

+66 +34

+18 +52 – 66 0

+88 +56

– 4 +52 +202 – 118 +52 +382 – 298 – 88 0 +170 – 202 0 +350 – 382

300 400

Max Min

+57 0

+40 +4

+53 – 40

+57 0

+73 +37

+20 +57 – 73 0

+98 +62

– 5 +57 +244 – 151 +57 +471 – 378 – 98 0 +208 – 244 0 +435 – 471

400 450

Max Min

+63 0

+45 +4

+58 – 45

+63 0

+80 +40

+23 +63 +108 – 80 0 +68

– 5 +63 +272 – 189 +63 +530 – 427 – 108 0 +232 – 272 0 +490 – 530

450 500

Max Min

+63 0

+45 +4

+58 – 45

+63 0

+80 +40

+23 +63 +108 – 80 0 +68

– 5 +63 +292 – 219 +63 +580 – 471 – 108 0 +252 – 292 0 +540 – 580

Example 12 A cylinder casing of basic inside side diameter 85 mm is to be shrunk fit with a cylinder liner. Suggest a suitable tolerance for the cylinder casing and liner. What are the maximum and minimum interferences? Solution The basic size is given for cylinder casing and hence hole basis of the fit is taken. From Table 19.7, the suitable fit for cylinder liner is H7-s6. This fit is available in Table 19.9 under column Heavy press fit (Interference). Note the following dimensions from this table for basic size of 85 mm (range of 80-100 mm). Tolerances on hole are + 35 and 0. Hence Cylinder casing max imum inside diameter 85.035 mm Cylinder casing minimum inside diameter 85.000 mm Tolerances for shaft are + 93 and + 71. Hence

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Liner outside maximum diameter = 85.093 mm Liner outside minimum diameter = 85.071 mm Maximum interference = Minimum hole diameter — Maximum shaft diameter = 85 – 85.093 = – 0.093 mm Minimum interference = Maximum hole diameter — Minimum shaft diameter = 85.035 – 85.071 = – 0.036 mm Same values as –36 and –93 can be seen directly from Table 19.9 under the column fit.

Example 13 A hole of 45 mm is dimensioned for maximum and minimum diameters as 45.025 and 45.000 mm respectively. Maximum and minimum shaft diameters are 44.991 and 44.975. Find the following: a. Basis of fit system b. Tolerance grade on hole c. Tolerance grade on shaft d. Type of fit Solution a. Zero lower limit is for the hole. Hence it is based on Hole system. b. Tolerance grade on hole = Difference in maximum and minimum diameters = 0.025 mm, i.e. 25m. From Table 19.2 for 45 mm basic size (range 30-50 mm) value 25 is for IT grade 7. Lower tolerance is zero. From Table 19.5 tolerance letter is H. Hence tolerance grade for the hole is H7. c. Tolerance grade on shaft = Difference in maximum and minimum diameters = 0.016 mm i.e. 16m. From Table 19.2 for 45 mm basic size (range 30-50 mm) value 16 is for IT grade 6. Lower tolerance is 0.009 mm ( 44.991 – 45.000), i.e. – 9m, hence from Table 19.6. This value is for tolerance letter g. Hence tolerance grade for the shaft is g6 d. Therefore type of fit is H7-g6. See same clearances in Table 19.8 also for this fit. Example 14 Specify suitable tolerances, fit and the last manufacturing process for a journal bearing having journal diameter 60 mm such that diameter/clearance ratio remains within 500 to 1000. Solution The clearance is very important for the design of journal bearings and it has to be kept in a narrow range for the satisfactory operation of the bearing. Since the journal diameter is given, hence shaft basis system of fit is selected. Tolerance letter symbol for shaft basis is h. Minimum clearance = 60/1000 = 0.06 mm Maximum clearance = 60/500 = 0.12 mm Hence range of tolerance both for hole and shaft = 0.12 – 0.06 = 0.06 mm Dividing this equally both for shaft and hole, the tolerance for each = 0.06/2 = 0.03 mm Therefore: Maximum shaft diameter = 60.00 mm Minimum shaft diameter = 60.00 – 0.03 = 59.97 mm Minimum hole diameter = Maximum shaft diameter + minimum clearance = 60.03 + 0.03 = 60.06 mm Maximum hole diameter = Minimum hole diameter + tolerance on hole = 60.6 + 0.03 = 60.09 mm Tolerance grade for 60 mm (50 to 80 mm range) for value 0.03 mm (30m) in Table 19.2 is IT7 both for shaft and hole. Hence tolerance for shaft is h7. Tolerance letter for the hole of 60 mm basic size (50 to 65 mm range) for a lower deviation of 60m (0.060 mm) is E from Table 19.5. Hence the fit is E7-h7 To have a tolerance grade of IT7, the process required for the journal and bearing is Precision turning (Table 19.1)

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19.13

393

FITS FOR THREAD FASTENERS

Fit for thread fasteners depends upon tolerance value and positions tolerance for the mating parts. Position tolerance is the distance between the basic size and the nearest end of the tolerance zone called fundamental deviation. For fits of threads, magnitude of tolerance zone is specified by IT grade first and fundamental deviation by letters (Upper case for internal threads and lower case for external threads), e.g. 4H or 8g. Three classes of fits for threads are: Free class fit 7H/8g Medium class fit 6H/6g (This class is most commonly used) Close class fit 5H/4h

19.14

GAUGES

Gauges are used to maintain size of a component within limits while manufacturing. Measuring instruments like vernier caliper, micrometer, angle protractor etc. consume a lot of time in taking reading. These instruments can be used if one or 2 jobs or even for a small batch, but it is highly inconvenient if components are to be manufactured in large numbers. For quality production, to maintain the size within tolerable limits, gauges are used. They are made with high accuracy, are quick and easy to use. Generally, these gauges are of GO and NOT GO type. The gauges decide the limiting dimensions of the object. Sometimes these are marked as HIGH and LOW also or even “H” and “L”. These are made of hardened forged steel and ground to the required limits. Shape of the gauge depends upon the shape of the object and dimension, which is being measured. Different types of gauges are: Plug gauge, Ring gauge, Tapered plug gauge, and Gap gauge; solid or adjustable. A plug gauge is shown in Fig. 19.14. One side is GO gauge and other is NOT GO, i.e. if the dimension of the component is within the tolerable limits then the GO gauge should pass through hole and NOT GO gauge should NOT pass. Tolerance limits for this gauge are shown as 30.000 to 30.021 mm, i.e. the tolerance zone is 0.021 mm. For outside dimension, shape of the gauge is shown in Fig. 19.15 with limits as 29.98 to 29.993 mm.

Fig. 19.14

GO and NOT GO Plug Gauge

Fig. 19.15

GO and NOT GO Gauge

CAD 19.15

PUTTING TOLERANCES USING CAD

Values of tolerances (both upper and lower) are set in the New Dimension Style dialog box shown in Fig. 19.16. To get this dialog box follow the steps given below: ∑ On menu bar, click Dimension and then from the pull down menu, select Style….

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∑ Dimension Style Manager is displayed. Click New… button on right side. ∑ Create New Dimension style dialog box is displayed. Type the name of new style in the text box and click Continue button at the bottom. New Dimension Style dialog box (Fig. 19.16), is displayed. Click the last tab (Tolerances) in this dialog box. Its tolerance format tile has the following combo boxes: A. Method Click the arrow in the Method combo box. It displays five methods of putting tolerances. Click on any one as required. Meaning of each is explained below: (i) None No tolerance is placed. (ii) Symmetrical Adds a plus/minus single tolerance value after the main dimension, e.g. 25 ± 0.1. Enter a tolerance value in the Upper value edit box.

Fig. 19.16

(iii) Limits

(iv) Deviation

(v) Basic

Dimension Style Dialog Box for Tolerances

Actual values of the upper and the lower limits are specified, e.g. 30.2 and 29.9. It displays a maximum and a minimum value, one over the other. The maximum value is the dimension value plus the value entered in the Upper value edit box. The minimum value is the dimension value minus the value entered in the Lower value edit box. Adds different or same plus/minus tolerance values. A (+) symbol precedes the tolerance value entered in the Upper value edit box and a (–) symbol precedes the tolerance value entered in the Lower value edit box. (See Fig. 19.S6) Draws a rectangle around the basic dimension and no tolerance is given.

B. Precision For setting of number of decimal places accuracy for the tolerances, e.g. 0.00 is for 2 decimal places.

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C. Upper value Type here maximum or the upper tolerance value. When the Method selected is “Symmetrical”, AutoCAD uses this value for both the tolerances. D. Lower value

Type here the minimum or the lower tolerance value.

E. Scaling for height Specify a scale factor for the tolerance text to the main dimension text. AutoCAD calculates the tolerance height. F. Vertical position It controls text justification for the symmetrical and the deviation tolerances. Click the arrow in this combo box. Three styles are displayed; Top, Middle and Bottom. Each one is described below: Top Aligns the tolerance text with the top of the main dimension value. Middle Aligns the tolerance text with middle of the main dimension value. Bottom Aligns the tolerance text with the bottom of the main dimension value. Example 15 Figure 19.S6 shows a drawing. Put the dimensions in a style given below: Aim —This tutorial demonstrates the use of dimensions with tolerances. Solution Open a New file with acadiso template. Create a drawing of the size as shown in Fig. 19.S6 at any suitable position with the basic dimensions. Then follow the steps given below for dimensioning. This figure has 4 tolerances styles, i.e. Limits, Deviations, Symmetrical and Basic. So four dimension styles have to be created, named and then used. 1. First set the style of dimensions. Command: DDIM ø Type the command and press Enter. Fig. 19.S6 Tutorial on Dimensions Dimension Style Manager dialog box appears. Click with Tolerances New button. Create New Dimension Style dialog box appears. Assign a style name as Style Limits. Click Continue. New Dimension style dialog box appears (Fig. 19.16). 2. Click on the Tolerance tab. Make the following settings (shown bold) if the default values shown in the dialog box are different from the desired ones (Refer Fig. 19.16): In the Tolerance Format tile set as under: Method: Precision Upper value: Lower value Vertical position

Limits 0.0 0.2 0.1 Middle

Click OK button and then the Close button. The dialog box disappears and the drawing appears again.

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3. Click on the Dimlinear icon (first on the Dimension toolbar) to put Linear dimensions. Click on both the ends of top horizontal line. The dimension is placed as shown in Fig. 19.S6 showing limits. 4. Repeat Steps 1 and 2 and assign style name as Style Dev. Choose the Method as Deviation. Set both the upper and the lower tolerance values as 0.1. 5. Repeat Step 3 and dimension the bottom horizontal line. Note that the tolerances are not in the same format as put in Step 3, but in deviation format. 6. Repeat Steps 1 and 2 and assign style name as “Style Sym”, but choose the Method as Symmetrical and set both the tolerances as 0.1. 7. Click on the Aligned icon (2nd) on the Dimension toolbar to dimension the inclined line. Click on the ends of inclined line and put the dimension at a suitable distance. The dimension is in symmetrical format. 8. Click the Diameter icon (5th) on the Dimension toolbar and put diameter of the circle. 9. Click the Angular icon (6th) on the Dimension toolbar and specify the angle of the inclined line w.r.t. the horizontal line. 10. Repeat Steps 1 and 2 and assign style name as Style Basic. Choose the Method as Basic. 11. Repeat Step 3 and dimension the left vertical line. Note: Following system variables can be used to make the settings for the Method of putting tolerances and tolerance values. These settings can be used at the command prompt instead of the Dimension Style dialog box. Dimlim To put the tolerances on or off. Dimtp To define positive tolerance value. Dimtm To define negative tolerance value. Dimtolj To justify tolerances vertically. Dimtfac To specify ratio of the tolerance text height in relation to the dimension text height.

TIP fi To put different tolerances on the same drawing, any one of the following methods can be followed: a. Set the system variables as mentioned above. b. Create a new Dimension style with different name for a set of tolerances. c. Put a tolerance and then modify by the dimension tolerances. First select the dimension, then click Modify on Menu bar and choose Properties on pull down menu. Scroll the list down to tolerances and modify the values of the tolerances. A box appears around the dimension.

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9.

Differentiate between “Basic size” and “Actual size”. What are the various deviations? Explain them with a neat sketch. Describe at least 6 terms associated with tolerances. How is fundamental deviation specified? Differentiate between symbols A and a, H and h. What is an IT grade? How many grades are there and how are these specified? What are the various methods of specifying tolerances? Describe by examples. Differentiate between hole and shaft basis system of fits. What is meant by the term Fit? What are the various types of fits? Give examples of each type. Differentiate between Tolerance, limit and fit.

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CAD 10. What are the various tolerance methods offered by AutoCAD and how the required one is selected? 11. How are the text size and tolerance values controlled while dimensioning using CAD?

FILL

IN THE

BLANKS

Write the appropriate word(s) in the blank space provided. 1. Difference between basic and actual size is called . 2. Upper deviation is the algebraic difference between and 3. Tolerance value depends upon . 4. Lower the tolerance, is the cost of production. 5. Fundamental tolerance is specified by a and . 6. There are IT grades. 7. Upper case letters are used for specifying tolerance of . 8. A tolerance letter symbol indicates zero value of one limit of hole. 9. A tolerance letter symbol has tolerances ± IT/2 10. Higher the number of tolerance grade more is the . 11. Type of fit having shaft diameter more than a hole size is called fit. . 12. Type of fit used for bearings is

.

CAD 13. Values of tolerances are specified in 14. Method of putting tolerance is selected in

dialog box using combo box.

MULTIPLE CHOICE QUESTIONS Tick the correct answer 1. Difference between basic and actual size is called (a) Fit (b) tolerance (c) deviation (d) gap 2. An allowance is (a) dimensional difference between the maximum limit of mating parts (b) difference between basic and actual size (c) difference between upper and lower deviation (d) difference between basic size of hole and actual size of shaft 3. Fundamental tolerance is specified by (a) a letter symbol (b) a number symbol (c) both by a letter symbol and a number symbol (d) a three digit number

tab.

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4. Upper case letters for tolerance is used for (a) shaft (b) journal (c) hole (d) both for hole and shaft 5. Letters JS as the fundamental tolerance denotes a tolerance value of (a) zero on either side (b) equal on both + and – sides of IT/2 value (c) equal on both + and – sides of any IT value (d) none of these 6. Tolerance value on a drawing depends upon (a) method of manufacturing (b) number of parts to be manufactured (c) skill of operator (d) function of the part 7. Type of fit for a bearing is decided by difference in (a) maximum shaft diameter and maximum hole size (b) minimum shaft diameter and minimum hole size (c) minimum shaft diameter and maximum hole size (d) maximum shaft diameter and minimum hole size 8. Type of fit used for a journal bearing is (a) H6-c8 (b) H6-j8 (c) H6-n8 (d) H6-h6

CAD 9. Method of putting tolerances of a dimension is set in dimension dialog box using (a) Text tab (b) Units tab (c) Fit tab (d) Tolerance tab 10. Tolerances are put on a drawing using AutoCAD by (a) first put basic size and then put upper value and then lower value (b) first put basic size and then put upper and lower values together (c) put basic size, upper and lower values all at one time (d) tolerances cannot be put 11. Text size of tolerance value is (a) of the same size as the basic size (b) 50% more than basic size (c) 125% of basic text size (d) can be set to any size

ANSWERS to Fill in the Blank Questions 1. 5. 8. 12.

tolerance 2. upper limit, basic size letter symbol, number symbol H 9. js or JS sliding fit 13. Dimension, Tolerance

3. 6. 10. 14.

manufacturing process 18 deviation Method

4. higher 7. holes 11. Interference

ANSWERS to Multiple Choice Questions 1. (b) 7. (d)

2. (a) 8. (a)

ASSIGNMENT

3. (c) 9. (d)

ON

4. (c) 10. (c)

TOLERANCES, LIMITS

5. (b) 11. (d)

AND

FITS

1. A hole of 50 mm has tolerance as c10. Calculate its maximum and minimum sizes. 2. A shaft of 80 mm has tolerance as b9. Calculate its maximum and minimum sizes.

6. (d)

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3. A shaft of diameter 30 mm rotates at medium speed in a sleeve bearing. Specify the following: Type of fit required Maximum and minimum shaft diameters Maximum and minimum sleeve inside diameters 4. A gusset plate shown in Fig. 19.P1 is to be cut by gas flame cutting. Specify suitable tolerances. 5. A stepped shaft shown in Fig. 19.P2 is to be produced in large quantity. Specify suitable tolerances for the 55 mm end and for overall Fig. 19.P1 length. 6. A stepped block is shown in Fig. 19.P3. Evaluate tolerance for size A.

Fig. 19.P2 A Stepped Shaft

CAD ASSIGNMENT

ON

Fig. 19.P3

TOLERANCES, LIMITS

AND

A Gusset Plate

A Stepped Block

FITS

7. Create a new dimension style for Method symmetric with tolerances 0.2 mm and Deviation with tolerances of + 0.5 and – 0.3. 8. Draw the following and put the tolerances as shown below in Fig. 19.P4 (A) and (B).

Fig. 19.P4 9. Draw Fig. 19.P5 and put the tolerances as tabulated below: Size Basic size A B C D

20 20 50 30

Upper Lower deviation deviation 0.03 0.02 0.04 0.04

0.01 0.02 0.02 0.02

Method Deviation Symmetrical Limits Deviation

Also calculate the tolerances for the dimensions X and Y.

Fig. 19.P5

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HOMEWORK 10. Suggest suitable fits for the following applications (a) Bush fixed in a housing (b) Locating snug (c) Key in the keyway of a pulley dismantled frequently (d) Ball bearing on a shaft (e) Flywheel shrunk fit over a shaft 11. Name the sizes A, B ... F for a shaft shown in Fig. 19.P6. 12. Calculate maximum and minimum size of holes having fundamental tolerances as (a) 40C11 (b) 65N8 (c) 110Z 13. Calculate limits of the shaft diameter having fundamental tolerance as (a) 50d10 (b) 70g7 (c) 85s9 14. Evaluate sizes and tolerances for dimensions A and B shown in Fig. 19.P7.

Fig. 19.P6

Fig. 19.P7

15. Find maximum and minimum clearance for different fits for the following sizes (a) 55 H11-c11 (b) 65 H7-k6 (c) 75 H7-s6 16. A ring gear having inside diameter 200 mm is shrunk fit over a flywheel of an engine. Specify the limits of the sizes for the inside diameter of ring gear and outside diameter of the flywheel. 17. Find the maximum and minimum clearance for the following fits 55H11-c11, 65 H7-k6 and 75H7-s6.

PROBLEMS

FOR

PRACTICE

18. Limits of size A are 49.45 and 49.20. Limits of size B are 49.96 and 49.70. Find the following: (a) Maximum and minimum clearance between the mating parts. (b) Fundamental deviations of both the parts. (c) Type of fit in terms of fundamental letter and IT grade. 19. A bush is fixed in housing with fit H7-p6 as shown in Fig. 19.P9. A shaft has to rotate in this bush with fit H9-d9. Calculate the following: (a) Shaft diameter limits (b) Bush hole diameter limits (c) Bush outside diameter limits (d) Housing hole diameter limits 20. A knuckle joint shown in Fig. 19.P10 has a pin of 20 mm diameter having fit of H7-h6 in forked ends (A) and (C) and F9-h7 on shaft basis in the eye end (B). Find the following: (a) Limits of holes in A, B and C. Fig. 19.P8 (b) Maximum and minimum size of pin.

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Fig. 19.P9

401

Fig. 19.P10

CHAPTER

20

Geometrical Tolerances and Surface Finish Geometrical Tolerances Permissible variations in the geometrical shapes of individual objects or orientation or position w.r.t other objects are called geometric tolerances. Tolerance zone is an imaginary area/volume within which a component must be contained. Specific portion of a component such as a hole, surface or a profile is called a feature. A datum is a theoretical point, line/axis or a surface from which the dimensions or geometric tolerances are referenced. It is also a feature that has exact form and fixed location. Datum is shown by a right angle triangle as per ASME, equilateral triangle as per ISI and by an arrow line by ANSI. A datum is designated by an uppercase letter such as A, B, etc. in a square box of double the size of the letter height connected with the datum triangle and a leader line. If the datum is an axis it is connected on two extension lines of the part whose axis is being referenced. There can be more than one datum also. Tolerance symbol is a graphical symbolic representation of the tolerated feature. There are 14 tolerance symbols whose shape is standardized. These are classified as Single feature, Related feature and Run out symbols. Single feature symbols are Straightness, Flatness, Circularity, Cylindricity, Profile of a line and Profile of a surface. Related features are Parallelism, Perpendicularity, Angularity, Concentricity, Position and Symmetry. There are two run out symbols; circular and total. Frame is a rectangle having partitions containing tolerance symbol, tolerance value, material condition and a datum letter in a sequence from left to right. The frame is pointed towards the tolerated feature using a leader line. A drawing with geometric tolerances in a frame is called drawing callout. Straightness of a line/axis or of a line on a surface is the perpendicular distance between two parallel lines touching the crests and valleys of the line/surface. Flatness is the distance between two planes enclosing the tested surface at the lowermost and the uppermost positions. Circularity is the difference between maximum and minimum radii of a cylinder at any section. Cylindricity is the tolerance difference in value of radii between two imaginary cylinders enveloping cylinders at outermost and innermost surfaces. Profile of a line is a tolerance zone having a constant height equal to tolerance value normal to the theoretical profile and equally disposed about it. Profile of a surface is the space between two surfaces of the same profile which envelop the highest and lowest points of the surface. Parallelism is the zone between two parallel surfaces in relation to the datum surface enveloping the feature. Perpendicularity is the zone between two planes perpendicular to the datum within which the controlled feature should lie. Angularity is the zone between two parallel planes inclined to the datum plane at the specified angle in which the controlled feature lies.

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Concentricity is the diameter of a circular zone within which the axes of the two cylindrical features may offset from each other. Position tolerance is the variation in position from a theoretical position. Symmetry tolerance is the variation which can be permitted to be unsymmetrical. Run out is the difference in the reading of a dial indicator set on the surface when a circular object is turned one revolution over one cross-section. Total run out is the difference in the reading of a dial indicator set on the surface when a circular object is turned one revolution and is moved over the surface along the axis from the beginning to the last.

Surface Roughness Roughness is the fine irregularity in the surface. For machined surfaces, roughness is caused by the cutting edge of the tool feed. Lesser the feed rate, lesser is the roughness. Roughness width is the distance between two peaks or valleys. Roughness height is the arithmetic average deviation measured from the imaginary center line. It is indicated in microns. Roughness-width cut off is the greatest spacing of repetitive surface irregularities to be included in the measurement of average roughness height. This is indicated in mm. Waviness is the wider spacing than the roughness width cut off. It results from machine or work deflections, vibrations, chatter, heat treatment or warping. Waviness height is the peak to valley distance. Waviness width is the spacing in mm between two successive wave peaks or successive valleys. Lay is the direction of predominant surface pattern determined by the production methods. This is caused due to tool marks. Flaws are the irregularities that occur at a place or places such as scratches, blow holes, cracks, etc. Effect of flaw is not considered in measurement of surface roughness. Actual profile is the actual surface obtained by a manufacturing process. Reference profile passes through the highest point of actual profile. All irregularities are referred to this profile. Datum profile passes through the lowest point of actual profile and is parallel to reference profile. Mean Profile within the sampling length L, is such that the filled up positive and negative areas between this profile and actual profile are equal. Peak to valley height is the distance from reference profile to datum profile. A basic roughness symbol has two unequal legs; long leg at 60° and short leg at 120° joined at bottom end over the surface, for which roughness is being indicated. A horizontal line over the short leg indicates machining required, while a circle between the legs means no machining. Roughness limits, manufacturing method, treatment, lay etc. all are mentioned over the roughness symbol at a fixed location in relation to the long leg. Roughness Grade Numbers are N1 to N12. Roughness of these numbers in microns is: N1 = 0.025, N2 = 0.05, N3 = 0.1, N4 = 0.2, N5 = 0.4, N6 = 0.8, N7 = 1.6, N8 = 3.2, N9 = 6.4, N10 = 12.5, N11 = 25, N12 = 50 Grade numbers N1 to N3 have roughness symbol as four equilateral triangles with apex at the bottom. N4 to N6 three triangles, N7 to N9 have two triangles, N10 and N11 have one triangle and symbol for N12 is a wavy line. Roughness depends upon the manufacturing process. For gas cutting it may be as high as 100 m, while processes like lapping offer 0.12 m. Roughness more than 25 m need not be mentioned. Most commercial applications have roughness between 3 m to 12 m. Good machining can offer 0.8 m to 1.6 m. High quality machining offers 0.2 m to 0.8 m. Roughness less than 0.2 m is very costly.

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CAD To put geometric tolerances, the command is TOL. Geometric tolerance dialog box is displayed with many boxes. When a box in the Sym tile is clicked, it displays symbol dialog box showing 14 geometric tolerance symbols. Choose a symbol from the display and that symbol is placed in the box. Value of the tolerance zone can be specified in the Tolerance 1 text box. Datum letters can be specified in the Datum box 2 or 3. After entering all the values, click OK button.

20.1

INTRODUCTION

Geometric shape of a component is considered exact unless specified. For example, straight line means straightness, circle means that the profile is exactly circular, parallel lines means that these are exactly parallel, lines at right angle to each other implies perpendicularity. If there could be any variation from the exact form, it has to be specified by geometric tolerances. These tolerances should be lesser than the size tolerances. Many times tolerances on size are not sufficient, e.g. a shaft has the size within the specified limits but even then it is not acceptable due to its deformations in geometric shape. Figure 20.1 shows an exact cylinder and some possible shapes other than exact because of which they cannot be accepted. These form variations are called geometric tolerances and these also have to be within specified limits.

20.2

Fig. 20.1

Geometric Shape Variations

TYPES OF TOLERANCES

Tolerances can be of three types:

A Size Tolerances These are given for a size as described in Chapter 19 (Fig. 20.2A). B Form Tolerances In addition to size there could be errors due to form (Fig. 20.2B). Thick line represents the actual shape. Thin lines represent the outermost and innermost radii enveloping the actual shape. C Position Tolerances These are the errors due to position. Figure 20.2C shows two centers; one center which is ideally required and the other is the maximum error which can be tolerated while manufacturing. It is indicated by a thin circle showing the zone within which the actual center of the hole should lie.

Fig. 20.2 Types of Tolerances

Geometrical Tolerances and Surface Finish

20.3

405

TERMINOLOGY

20.3.1

Geometric Tolerance

It is the maximum permissible variation of form, profile, orientation, location and run out specified on a production drawing.

20.3.2

Tolerance Zone

It is an imaginary zone within which a component must be contained (Fig. 20.3). The height of this imaginary zone is the tolerance value.

20.3.3

Feature

Fig. 20.3

Tolerance Zone

Feature is the specified portion of a component such as hole, slot surface or profile. It can have more than one surface but generally it is referred to a single surface.

20.3.4

Axis

Axis is theoretical straight line about which a circular feature revolves, e.g. axis of a straight cylinder.

20.3.5

Median

Median is the center line of a straight or a bent shaft.

20.3.6

Boxed Dimensions

Boxed dimensions are the dimensions subjected only to position tolerance. These dimensions are enclosed in a box as 50 or Ø 80 . Tolerances are not given along with these dimensions.

20.4

FRAME

Frame is a box having some partitions (Fig. 20.4A). The size depends upon height (H) of the datum letter. Each partition contains information about the following and is discussed in subsequent sections: a. Datum letter (Section 20.5) b. Material condition (Section 20.6) c. Tolerance symbol (Section 20.7) and d. Tolerance value (Section 20.8) If two or more frames apply to same feature, they are drawn one over the other with a single leader line as shown in Fig. 20.4B. The frame is put over the feature by any one of the following methods: a. A leader line from the frame to feature (Fig. 20.9A). b. A leader line from the frame to extension line of surface but not in line with dimension line. c. Attaching the side or end of frame to extension line (Fig. 20.9B). d. Locating the frame below the dimension of the feature. A drawing with feature control frame indicating geometric tolerances is called drawing call out.

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Fig. 20.4 A Frame and Its Contents

20.5

DATUM

Datum is a theoretical point, line, or plane from which dimensions are measured and geometric tolerances are referenced. It has an exact form, represents exact and fixed location for measurement.

20.5.1

Datum Feature

It is a feature of component like edge, surface which is taken as the basis for a datum.

20.5.2

Datum Triangle

Datum is shown by a triangle (open or filled) on the datum feature. Figure 20.5 shows three methods of representing a datum along with the proportions of the triangle in terms of text height (H). ASME standard uses a right angled triangle. BIS has also adapted the same. ISO method uses an equilateral triangle and ANSI represents datum only by an arrow line.

Fig. 20.5

20.5.3

Representation of a Datum

Datum Letter

It is an upper case letter enclosed in a box to indicate an arbitrary name of a datum (Fig. 20.6). A leader line is used to connect a frame and the datum triangle. See Fig. 20.7 for its use.

Fig. 20.6

Datum Triangle and Datum Letters

Geometrical Tolerances and Surface Finish

20.5.4

407

Multi-datums

Normally one datum is required for orientation but position tolerance may require two or more datum. These are called primary, secondary and tertiary datum (Fig. 20.7A) which are mutually perpendicular to each other.

Fig. 20.7 Multiple Datums

Multiple datums are referred in separate partitions as shown in Fig. 20.7A. If a single datum is established by two datum features, e.g. a stepped shaft, then the datum letters are placed in the same compartment of the frame with a dash in between (Fig. 20.7B and C).

20.6

MATERIAL CONDITION

There are two material conditions; Maximum and Least. If necessary, one of these conditions is to be specified in the frame by letter M or L in a circle as shown in Fig. 20.4.

20.6.1

Maximum Material Condition (MMC)

It is a condition in which a feature contains maximum material. For example, a shaft having limits as 40.00 and 39.98 will have maximum material when its diameter is 40.00 mm. Similarly a hole having limits as 40.02 and 40.005 will have maximum material when the hole size is 40.005. Clearance fits are generally dimensioned on the basis of Maximum Material condition, in short, known as MMC. If the parts are produced at MMC, the clearance obtained is minimum. It offers an extra advantage that if by mistake the hole is over-size and shaft is under-size, the parts will be still acceptable if their sizes are within tolerances. When a feature is specified by MMC, it is manufactured at MMC size. If the feature departs from MMC size, geometric tolerance increases. This amount of deviation is added to the geometrical tolerance, specified in the control frame. This extra tolerance is called bonus tolerance.

20.6.2

Least Material Condition (LMC)

It is a condition in which a feature contains least material. For example, a shaft having limits as 40.04 and 40.02 will have least material when its diameter is 40.02 mm. Similarly a hole having limits as 40.00 and 40.02 will have least material when the hole size is 40.02. Interference fits are generally dimensioned on the basis of Least Material condition, in short, known as LMC to have minimum interference on the mating parts.

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20.6.3

Regardless of Feature Size (S)

When MMC or LMC is not specified, the tolerance applies regardless of feature size. In short, it is indicated by letter ‘S’. It is used for position tolerances.

20.7

TOLERANCE SYMBOL

It is a graphical representation of a tolerated feature. Fourteen symbols have been standardized and are shown in Fig. 20.8 along with their proportions in terms of size of datum letter H. Use of these symbols is described in subsequent sections. Form tolerances are divided in the following three categories:

A Single Feature Tolerances Straightness, Flatness, Circularity, Cylindricity, Profile of a line and Profile of a surface (Section 20.10). B Related Features Tolerances Parallelism, Perpendicularity, Angularity, Concentricity, Symmetry and Position (Section 20.11).

Fig. 20.8

Geometric Tolerance Symbols and Their Proportions

Geometrical Tolerances and Surface Finish

409

C Runout Tolerances Circular run out and Total run out (Section 20.12). 20.8

TOLERANCE VALUE

This value is indicated in the frame next to the tolerance symbol. It is given in millimeters. Its value is decided from the functional point of view of the part as to how much form variation can be tolerated. However, its value should be lesser than the size tolerances.

20.9

INDICATING GEOMETRICAL TOLERANCES ON DRAWINGS

Geometrical tolerances are put on a drawing in a frame (Fig. 20.4). The frame is connected to the tolerance feature by any of the following methods: a. A leader line terminating with an arrow on the outline of the feature (Fig. 20.9A). b. If the tolerance refers to an axis or median plane of the part, it is terminated on the extension lines (Fig. 20.9B).

Fig. 20.9

Indicating Tolerances on a Drawing

The datum features are indicated by a leader line starting from the frame and terminating by a triangle (open/solid), whose base lies on the outline of the datum feature (Fig. 20.10A). If the tolerance is applied up to a specified length, the length is indicated after the tolerance value, separated by a slash. If the datum is far off from the tolerated feature, datum can be specified separately in a box and the datum letter can be put in the frame (Fig. 20.10B). If the axis of datum feature is to be used then it is shown by extension lines. Tolerance value and datum can be combined (Fig. 20.10C).

Fig. 20.10 Indicating Datum on a Drawing

20.10

FORM TOLERANCE FOR SINGLE FEATURES

20.10.1

Straightness

Straightness of a line/axis or of a line on a surface is the perpendicular distance between two parallel lines touching the crests (the highest point) and the valleys (the lowest point) of the line/surface (Fig. 20.11A). This tolerance zone is indicated in a frame as shown in Fig. 20.11B.

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Fig. 20.11 Indicating Straightness on a Drawing

20.10.2

Flatness

Flatness is the distance between two imaginary planes enclosing the actual surface at the lowermost and uppermost positions (Fig. 20.12A). Symbolically it is shown on a drawing as in Fig. 20.12B.

20.10.3

Circularity

Fig. 20.12

Indicating Flatness on a Drawing

Circularity is also called Roundness. Theoretically, any point on a cylindrical surface from the central axis should be at the same distance. Due to problems in machine tools, it may not be round as shown in Fig. 20.13A. Tolerance value of circularity is the difference between maximum and minimum radii of a cylinder at any section. It can take any form like ellipse, three or four lobed, irregular, etc. The tolerated value is indicated on drawing as shown in Fig. 20.13B.

Fig. 20.13 Indicating Circularity on a Drawing

20.10.4

Cylindricity

Cylindricity is the difference in value of radii between two imaginary cylinders, enveloping cylinders at outermost and innermost surfaces. Figure 20.14A shows the variation in the surface of a cylinder along its axis. The diameter at every cross-section is different and lies in a circular zone. This tolerance is indicated on the drawing as shown in Fig. 20.14B.

Fig. 20.14

Indicating Cylindricity on a Drawing

Geometrical Tolerances and Surface Finish

20.10.5

411

Profile of a Line

Tolerance zone for a profile of a line controls the contour of a curved profile. Figure 20.15A shows the variation in actual top surface. The variation lies between the two curves which envelop the actual curve. Thus the tolerance zone has a constant height equal to tolerance value normal to the theoretical profile and equally disposed about it. Tolerance for profile of line is shown on drawing in Fig. 20.15B.

20.10.6

Profile of a Surface

Tolerance zone for a profile of a surface is the space between two surfaces of same profile which envelop the highest point and the lowest point of the surface keeping the same profile Fig. 20.15 Indicating Profile of a Line on a Drawing (Fig. 20.16A). This tolerance zone is shown on the drawing in Fig. 20.16B.

Fig. 20.16

20.11

Indicating Profile of a Surface on a Drawing

TOLERANCES ON RELATED FEATURES

Tolerated feature is assigned a geometric relation with respect to another datum feature for these tolerances. The relation could be parallelism, perpendicularity, angularity, concentricity, symmetry or position.

20.11.1

Parallelism

A surface which is required parallel to a datum may not be exactly parallel. Tolerance on parallelism is the zone between two parallel surfaces enveloping the feature in relation to the datum surface. Fig. 20.17A shows that the top plane which should be parallel to the base (datum) but is inclined to it. Two planes, parallel to the datum shown by dashed lines envelop the actual top surface. Gap between these two enveloping planes is called geometric tolerance on parallelism. This is indicated on drawing as shown in Fig. 20.17B. The base is shown as a datum.

Fig. 20.17

Indicating Parallelism of a Surface on a Drawing

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20.11.2

Perpendicularity

Perpendicularity tolerance is the zone between two perpendicular planes to the datum within which the controlled feature should lie. It is also called tolerance on squareness. It is desired to have vertical face at right angles to the horizontal surface (Fig. 20.18A) but while manufacturing, the end surface of vertical plate can vary in a zone shown by dashed lines. This tolerance zone is shown on the drawing in Fig. 20.18B.

Fig. 20.18 Indicating Perpendicularity of a Surface on Drawing

20.11.3

Angularity

Theoretically, an inclined surface should be at a specified angle. Practically there is some deviation. Tolerance on angularity is the zone between two parallel planes inclined to the datum plane at the specified angle in which the controlled feature lies (Fig. 20.19A). Continuous inclined line falls in this zone and is at angle more than 60°, while the line shown dashed within this zone is at angle lesser than 60°. This tolerance is indicated on the drawing as shown in Fig. 20.19B. The angle 60° in the box is the ideally desired angle. Note that angularity is not defined in terms of angles.

Fig. 20.19 Indicating Angularity of Surface on a Drawing

20.11.4

Concentricity

Theoretically, a perfect concentricity means that the axes of two coaxial cylinders are in a line and coincide. See Fig. 20.20A in which axes of cylinder A and B do not coincide. They are parallel but offset. Even if two axes coincide, center of the cylinder B can be offset at the other end as shown in Fig. 20.20B. This maximum allowable offset in any direction is shown by a small circle showing the tolerance zone. Tolerance on concentricity is the diameter of a circular zone within which the axes of the two cylindrical features may offset from each other. This is indicated on the drawing as shown in Fig. 20.20C.

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Fig. 20.20 Indicating Tolerance for Concentricity on a Drawing

20.11.5

Symmetry

Theoretically, symmetry means the position of a feature is symmetric in relation to datum. Figure 20.21A shows a V block. The inclined faces or top faces are to be symmetrical about the central axis. Practically they may not be symmetrical as shown by dashed lines in Fig. 20.21B. Figure 20.21C shows a symmetric tolerance so that the median plane of the V groove lies within two parallel planes at 0.04 mm apart with respect to its base.

Fig. 20.21

20.11.6

Indicating Tolerance for Symmetry on a Drawing

Position

Theoretical true position of a feature (a hole in this case) is indicated by a boxed dimension (Fig. 20.22A). The actual center of the hole may lie within a tolerance zone indicated by a small circle of diameter 0.1 mm as shown in Fig. 20.22B. This means that a hole having its center anywhere within this small circle is acceptable. This tolerance zone is indicated on the drawing as shown in Fig. 20.22C.

Fig. 20.22

20.11.7

Indicating Tolerance for Position on a Drawing

Position Tolerance for Patterns

When there are many holes arranged in a pattern (in a line, rectangle or a polar array), holes of two similar parts may not match due to distortion of the pattern even if the center distance is exact.

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Fig. 20.23 shows distortion in pattern for two cases. Holes do not match even with exact center distance. A small circle indicates the maximum tolerance that can be tolerated for the pattern.

Fig. 20.23

Tolerance for Pattern

For such situations, two tolerances are specified; one call out is for the pattern and the other for feature location as shown in Fig. 20.24. Feature tolerance has to be lesser than pattern tolerance.

20.12 20.12.1

RUN OUT Circular Run Out

Fig. 20.24

Tolerance for Pattern and Feature

Circular run out is the deviation from an ideal shape when a part is rotated by 360°. It could be radial or axial or both. It is measured by putting a dial indicator over the part at right angles to the surface (Fig. 20.25A). The change in its reading noted for one full turn of the part without changing the axial position of the dial indicator is the circular run out. Its value should not be more than 0.03 mm. It is indicated on the drawing as shown in Fig. 20.25B.

Fig. 20.25

20.12.2

Indicating Circular Run Out on a Drawing

Total Run Out

Total run out is not a circular run out at one particular position but found when the dial indicator is moved axially over the entire surface parallel to the axis of datum while the part is being turned. The difference in the minimum and the maximum Dial Indicator reading form the beginning to the end while rotating the surface is the total run out. Thus it is the space between two concentric cylinders separated by specified tolerances and coaxial tolerance with the datum axis. Its value should not be more than 0.04 mm.

Geometrical Tolerances and Surface Finish

Fig. 20.26

415

Indicating Total Run Out on a Drawing

Example 1 Profile of a surface shown in Fig. 20.S1A is to be specified for geometric tolerances such that: a. Its profile varies within 0.2 mm. b. Its profile and its orientation is 0.3 mm. c. Its profile and its orientation is 0.3 mm and has vertical position also in the middle as 30 mm.

Fig. 20.S1A

Solution a. The required geometric tolerance is shown in Fig. 20.S1B. b. The required geometric tolerance is shown in Fig. 20.S1C. c. The required geometric tolerance is shown in Fig. 20.S1D.

Fig. 20.S1B

Fig. 20.S1C

Fig. 20.S1D

Example 2 A forked support with 20 mm thick forks shown in Fig. 20.S2A has two holes 30 mm below top surface such that their concentricity is within 0.2 mm and position is within 0.05 mm both in X and Y directions at MMC. The axis should be parallel to top surface within 0.1 mm. Show the geometric tolerances. Solution The required geometric tolerances are shown in Fig. 20.S2B.

Fig. 20.S2A

Fig. 20.S2B

Example 3 An inspection gauge is shown in Fig. 20.S3A with its dimensional and geometric tolerances for head and pin for checking coaxiality of part shown in Fig. 20.S3B. Considering feature sizes as tabulated below, complete the table for allowable distances between the two axes.

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Fig. 20.S3 Head size

An Inspection Gauge Pin size

12.0

11.9

11.8

24.8 24.7 24.6 24.5

Solution Ideally the axes should be coaxial, but practically there may be some variation. Coaxiality is similar to concentricity with axes arranged in a straight line. Coaxiality can be controlled by specifying any one of the following four types of geometric tolerances: a. Concentricity tolerance c. Run out tolerance b. Positional tolerance d. Profile tolerance Selection of the method depends upon the function of the part. When surfaces of revolution are cylindrical and control of the axes can be on MMC basis, positional tolerance is recommended. Solution with positional tolerance is shown in drawing call out. The feature control frame shows maximum permissible positional tolerance as 0.2 mm, i.e. the distance between the axes at MMC should not be more than 0.1 (half of the tolerance value). When diameters are at maximum size as 24.S3A for head and 12.0 for pin, the maximum distance between the axes is 0.1. When the diameter of pin is lesser, a greater value of geometrical tolerance can be allowed for distance between the axes. The allowable values are tabulated in Table 20.S1 for different values of pin size.

Table 20.S1

Allowable values of tolerance

Head size

Pin size 12.0

24.8 24.7 24.6 24.5

0.1 0.15 0.2 0.25

11.9 0.15 0.2 0.25 0.3

11.8 0.2 0.25 0.3 0.35

Example 4 An angle block having 30 degrees angle as shown in Fig. 20.S4A has to have angularity within 0.15 mm with its base ‘A’. a. Specify its geometric tolerances. b. If the inclined face also has to maintain squareness with its side within 0.1 mm. Specify its geometric tolerances.

Geometrical Tolerances and Surface Finish Solution

417

a. The drawing callout is shown in Fig. 20.S4B. b. The drawing callout is shown in Fig. 20.S4C.

Fig. 20.S4A

Fig. 20.S4B

Fig. 20.S4C

Example 5 A stepped shaft is shown in Fig. 20.S5A. The middle portion of the shaft has to be concentric with the axis of ends within 0.1 mm. Show the geometric tolerances.

Fig. 20.S5A

Fig. 20.S5B

Solution The drawing callout is shown in Fig. 20.S5B. Example 6 A cylindrical part shown in Fig. 20.S6A has a hole in it such that its concentricity with the outer surface A is within 0.1 mm at MMC. Show the geometrical tolerances on the drawing.

Fig. 20.S6A

Fig. 20.S6B

Solution The drawing callout is shown in Fig. 20.S6B. Example 7

Interpret the six geometrical tolerances shown on drawing of a flange shown in Fig. 20.S7.

Solution Callout 1 – Right side face of the flange is datum A which should have flatness within 0.05. Callout 2 – Hole axis should have perpendicularity to datum A within 0.02 mm and is taken as secondary datum B. Callout 3 – Surface of the boss should not have run out more than 0.1 w.r.t. hole axis. Callout 4 – Outer surface of the flange should be concentric within 0.1 w.r.t. hole axis.

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Callout 5 – Surface of hole should not have total runout more than 0.1 w.r.t. hole axis. Callout 6 – Left face of the boss should be parallel to datum A within 0.04.

Fig. 20.S7

Interpretations of Drawing Callouts

Fig. 20.S8

A Jig Plate

Example 8 A jig plate for drilling two holes is shown in Fig. 20.S8 with the position tolerances. Find the minimum and maximum center distance between the holes and distance A. Solution Maximum distance between centers = 40 + (0.04/2) + (0.02/2) = 40 + 02 + 0.01 = 40.03 Minimum distance between centers = 40 – (0.04/2) – (0.02/2) = 40 – 02 – 0.01 = 39.97 Hence, Maximum distance A will be 40.03 + (20.03/2) + (15.02/2) = 40.03 + 10.015 + 7.51 = 57.555 Minimum distance A will be 39.97 + (20.00/2) + (15.00/2) = 39.97 + 10.000 + 7.500 = 57.470

20.13

SURFACE TEXTURE

Surface texture includes surface roughness, waviness, lays, flaws, etc. No surface is smooth. Every surface has some roughness of microstructure. Even for polished surfaces there are peaks and valleys if seen under a microscope. An actual surface, if exaggerated is as shown in Fig. 20.27. The Fig. 20.27 Surface Texture various terms associated with this are as below: Roughness is the fine irregularity in the surface. For machined surfaces, roughness is caused by the cutting edge of the tool. Lesser the feed rate, lesser is the roughness. Roughness width is the distance between two adjacent peaks or two adjacent valleys. Roughness height is the arithmetic average deviation measured from the center line. It is indicated in microns. Roughness cut off is the width of the greatest spacing of repetitive surface irregularities to be included in the measurement of average roughness height. It is indicated in mm. Standard cut off widths are 0.075, 0.25, 0.75, 2.5, 7.5 and 25 mm. Waviness is the wider spacing than roughness cut-off width. It results from machine/work deflections, vibrations, chatter, heat treatment or warping.

Geometrical Tolerances and Surface Finish

419

Waviness height is the peak to valley distance of the waviness curve. Waviness width is the spacing in mm between two successive wave peaks or successive valleys. Lay is the direction of dominant surface pattern determined by the production methods. This is caused due to tool marks. Flaws are the irregularities that occur at a place or places such as scratches, blow holes, cracks, etc. Effect of flaw is not considered in measurement of surface roughness.

20.14

PROFILES

Refer Fig. 20.28 for the terms described as follows.

20.14.1

Actual Profile

Actual profile is the actual surface obtained by a manufacturing process.

20.14.2

Reference Profile

Reference profile passes through the highest point of the actual profile. All irregularities are referred to this profile.

20.14.3

Datum Profile Fig. 20.28 Profile Definitions Related to a Surface

Datum profile passes through the lowest point of the actual profile and is parallel to reference profile.

20.14.4

Mean Profile

Mean profile is a profile such that within the sampling length (L), the filled up areas between this profile and actual profile is equal to the area of voids between this profile and actual profile.

20.14.5

Peak to Valley Height

Peak to valley height is the distance from reference profile to datum profile.

20.14.6

Mean Roughness Index (Ra)

Mean roughness index is the arithmetic mean of the absolute values of the heights h1 + h2 + h3 + - - - - - - - + hn between the actual and mean profiles (Fig. 20.28). It is given by relation: X= L

Ra = 1/L

z

|hi| dx

(1)

X=0

20.15

SURFACE ROUGHNESS NUMBER

Surface roughness number is the average departure of the surface over a sampling length. This length is generally taken as 0.8 mm, i.e. 800 microns. Measurements are made along a line at right angles to the direction of tool marks on the surface. Surface roughness is generally expressed as Ra value in

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microns. The relation of Ra and heights of actual surface from the mean profile as h1, h2,….hn etc. for n points is given as: h + h 2 + h 3 + � + hn Ra = 1 (2) n

20.16

ROUGHNESS SYMBOLS

Different types of symbols are placed over the line representing the surface roughness. The basic symbol for surface roughness has two legs; one short and other long inclined at 60° to the line representing the surface as shown in Fig. 20.29A. The height of the small leg is taken as 1.5H and long leg as 3H, where H is the height of the text written over these symbols. If a manufacturing process or treatment of the surface is to be indicated, a horizontal line of length 3H is put over the long leg. If the surface is to be machined, a horizontal line is added on RHS of the short leg of the basic symbol (Fig. 20.29B).

Where: a1 – Maximum permissible roughness b – Production method d – Direction of lay

a2 – Minimum permissible roughness c – Sampling length e – Machining allowance

f – Roughness criterion other than Ra

Fig. 20.29

Roughness Symbol with Various Specifications

Various specifications a1, a2, c, d, e and f in microns are added on this symbol as required. These are shown in Table 20.1 and each symbol is marked arbitrarily from A to M. Meaning of each is explained below: a. Basic surface roughness symbol without any specifications. It does not carry any meaning if placed like this. b. If removal of the material is not permitted; a circle is added between the two legs of basic symbol. c. A horizontal line over the short leg means that this surface is required to be machined. d. Specification ‘a’ is the maximum roughness in microns that can be tolerated. e. If a limit of maximum and minimum roughness is imposed, both the values, ‘a1’ and ‘a2’, are to be put over the symbol. Maximum value is put above the minimum value. f. If the sampling length is to be indicated, it is written adjacent to the symbol on right side of long leg near the top. g. If required to indicate lay, it is indicated by a symbol on bottom right side of the long leg of roughness symbol. The symbol shown in table is for perpendicular lay. See Section 20.17 for lay. h. Value of machining allowance ‘e’ is indicated on the left side of short leg of roughness symbol. This value is generally indicated in millimeters.

Geometrical Tolerances and Surface Finish Table 20.1

421

Roughness symbols and their meaning

i. If the final surface is to be produced by a specific production method, it is written above the horizontal line. j. Treatment process like plating, oxidizing, etc. is also indicated on the top of horizontal line. Length of this can be increased if required. Unless stated, the roughness applies to the roughness after treatment or coating. If these are different before and after, both are to be indicated. k. Generally, the roughness value which is indicated is ‘Ra’ value. If some other criterion of specifying roughness is taken, it is indicated below the horizontal line within parentheses. l. If value of roughness is same for all the surfaces as ‘a’, a general note can be written as “All over” and this value be indicated on the surface symbol above the machining line. If it is same for most of the surfaces, except a few, a note can be written as “All over except otherwise stated”. Then wherever it is different, only those surfaces may be indicated for roughness.

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m. A symbol with a set of specifications can be named as X, Y, etc. and only its name can be indicated, whose meaning can be explained separately as shown in the Table 20.1.

20.17

LAY

Symbols used to indicate a lay showing the dominant tool marks are shown in Table 20.2. Tool marks of a shaper are as shown in Parallel lay. If the job is turned by 90°, for the same job the lay appears as perpendicular. If the job is set an angle, a crossed lay may appear. Vertical surface grinding machines produce a multi-directional lay. Facing of a lathe job will produce a circular lay. A suitable symbol can be used as required. No Lay symbol is taken as paralled direction. Table 20.2

20.18

Symbols used for direction of lay

ROUGHNESS GRADE NUMBER AND GRADE SYMBOLS

Roughness can also be indicated by roughness grade numbers from N1 to N12 or by roughness grade symbols shown in Table 20.3.

Geometrical Tolerances and Surface Finish Table 20.3

20.19

423

Roughness grades and grade symbols

ROUGHNESS WITH MANUFACTURING PROCESSES

Surface roughness in microns is given in Table 20.4. The roughness used for average application is mentioned in the right column. This table helps in recommending a suitable process or machine tool on a drawing for a specified roughness or specify roughness for a given process. The processes are arranged in descending order of the maximum roughness for average application range for easy selection. Table 20.4 Range of roughness obtainable with different processes S.No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Process Gas cutting Sand casting Hot rolling Sawing Planning Shaping Forging Turning Milling Filing Drilling Boring Chemical milling Electric discharge machining Electron beam Laser

Minimum– Maximum 6.3 -100 6.3-50 6.3-50 1.6-25 1.6-25 1.6-25 1.6-25 0.32-25 0.32-25 0.25-25 1.6-20 0.2-25 0.8-12.5 0.8-12.5 0.2- 6.3 0.2-6.3

Average application range Minimum–Maximum 12.5-25 12.5-25 12.5-25 1.6-12.5 1.6-12.5 1.6-12.5 3.2-12.5 0.4-6.3 0.8-6.3 3.2-6.3 1.6-6.3 0.4-6.3 1.6-6.3 1.6-6.3 0.8-6.3 0.8-6.3 (Contd.)

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Table 20.4 S.No.

Process

(Contd.)

Minimum–Maximum

Average application range Minimum–Maximum

17

Extruding

0.4 -12.5

0.8-3.2

18

Reaming

0.4 -3.2

0.8-3.2

19

Broaching

0.4-3.2

0.8-3.2

20

Hobbing

0.4-3.2

0.8-3.2

21

Electro-chemical machining

0.05-6.3

0.2-3.2

22

Drawing/Cold rolling

0.2-6.3

0.8-3.2

23

Die casting

0.4-3.2

0.8-1.6

24

Cylindrical grinding

0.05-12.5

0.1-1.6

25

Surface grinding

0.05-12.5

0.1-1.6

26

Burnishing

0.05-0.8

0.1-0.8

27

Honing

0.02-0.8

0.1-0.8

28

Electro polish

0.05-1.6

0.1-0.8

29

Electrolytic grinding

0.1-0.8

0.2–0.6

30

Polishing

0.05-1.6

0.1–0.8

31

Lapping

0.1-0.8

0.05–0.4

32

Super finishing

0.2-0.4

0.1–0.4

33

Plating

Roughness increases with thickness of plating

34

Oxide black coating

20.20

Roughness does not change

ROUGHNESS FOR TYPICAL APPLICATIONS

Table 20.5 can be used as a guideline for specifying a suitable roughness depending upon the application or a suitable manufacturing process can be recommended to achieve that roughness. Table 20.5 Roughness in microns

Applications and suggested roughness Applications

25

Low grade roughness from sand casting, flame cutting, chipping, rough forging. Suitable for un-machined surfaces and is rarely specified.

12.5

Surface resulting from heavy machine cuts, coarse feeds in turning, shaping, milling, boring.

6.3

Used for unfinished clean operations. Resulting from coarse ground surfaces, rough file, commercial turning.

3.2

Roughest surface that can be used for parts subjected to loads, vibrations, high stresses. Medium commercial machine finish on lathe, milling, shaper. Die casting, extrusion and rolled surfaces.

1.6

Good machining finish with high speed and fine feed turning. Used for close fits. Not suitable for high speed shafts having vibrations. (Contd.)

Geometrical Tolerances and Surface Finish Table 20.5 Roughness in microns

425

(Contd.) Applications

0.8

High grade finish by machine tools like centerless, cylindrical and surface grinding machines. Suitable for parts under high stress concentration. Cost is high.

0.4

High quality surface by grinding, honing, lapping specially where smoothness is very important. Suitable for high speed shafts where lubrication is not dependable.

0.2

Fine surface produced by honing, lapping, buffing. Suitable for hydraulic cylinders, high speed shafts where lubrication is not dependable.

0.1

Costly surface produced by honing, lapping, buffing. Suitable for instrument work, chrome plated piston rods.

0.05 and 0.025

Costliest surface produced by super finishing. Used for fine and sensitive instruments, gauge blocks.

20.21

RULES FOR PUTTING ROUGHNESS SYMBOLS

∑ If no surface control is specified, it is presumed that the surface produced by any manufacturing process will be acceptable. If roughness is important, it has to be indicated on a drawing. ∑ The symbol used is a check mark, whose conical point should touch the line indicating the surface. ∑ Wherever the symbol is used with dimension, it Fig. 20.30 Roughness Symbol affects the entire surface defined by dimension. Orientation and ∑ Transition areas such as fillets and chamfers Placement normally have the same roughness as finished area next to them. ∑ If the roughness is indicated for a plated surface, it has to be mentioned whether it is before or after plating or both should be mentioned. ∑ The symbol is to be oriented such that it can be read either from bottom or from right side of the drawing (Fig. 20.30). ∑ The symbol can be connected to the surface by a leader line terminating with an arrow. ∑ It can be put over the extension lines also. ∑ Symbol is to be used only once for a given surface, preferable where the surface is dimensioned. ∑ If same roughness applies to every surface, a note can be written. ∑ If same roughness applies to most of the surfaces except a few, a conditional note can be written. ∑ To avoid clumsiness of specifications over a symbol, a symbol with similar specifications can be named. ∑ For symmetrical surfaces, the symbol is to be put on both the sides. ∑ For cylindrical surfaces, the symbol is to be put only on one side. ∑ All over symbol (L in Table 20.1) can also be used if same roughness applies to all surfaces.

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Example 9 A stepped block shown in Fig. 20.S9 is to be made by sand casting and then machining by the following process. Machining is as per the scheme given below: Surface 1 – Grinding Surface 2 to 5 – Milling 1 Surface 6 – Shaper 2 3 Surface 7 – Unmachined 5 Hole – Drilling Specify roughness for the different faces by: a. Maximum roughness in microns and processes b. Maximum and minimum roughnesses c. Roughness grade numbers by an intermediate value c. Roughness grade symbols

4

6

7

6

Fig. 20.S9A A Stepped Block

Solution From Table 20.4 get the range of average application. Assume a suitable intermediate value. Then choose roughness grade number and roughness grade symbol from Table 20.3. This information is put in the tabular form as shown below:

a. Maximum roughness in microns and processes are shown in Fig. 20.S9B. b. Maximum and minimum roughness are shown in Fig. 20.S9C. c. Roughness grade numbers are shown in Fig. 20.S9D. d. Roughness grade symbols are shown in Fig. 20.S9E.

Fig. 20.S9B

Fig. 20.S9C

Geometrical Tolerances and Surface Finish

Fig. 20.S9D

427

Fig. 20.S9E

CAD 20.22

GEOMETRIC TOLERANCES

The Tolerance icon shown above (11th on Dimension toolbar) is not for dimensional tolerances, but for Geometric tolerances. To put the geometric tolerances use any one of the following methods: ∑ On Menu bar, click Dimension and from the pull down menu, click on Tolerance… ∑ On Dimension toolbar, click Tolerance icon shown above. ∑ On Command line, type TOLERANCE or TOL and press Enter. Type the command or click its icon, a dialog box named Geometric Tolerance (Fig. 20.31) appears. In the Geometric Tolerance dialog box, note that the first and the second rows are identical. Only one row is needed for simple feature control frame. Second row is for creating complex frames. In the Sym tile on left upper corner (Fig. 20.31), click in the upper Fig. 20.31 Geometric Tolerance Dialog Box square box. Symbol dialog box appears with 14 symbols, which are labeled for their use in Fig. 20.32. Choose the required symbol, e.g. the first symbol in this dialog box is for Position. Click on a symbol as required, this dialog box disappears and the selected symbol is placed in the Geometric tolerance dialog box. Select only one symbol at this stage, the selected symbol appears in the upper left corner in the tile labeled Sym. In Tolerance 1 tile, the first square box toggles between a diameter symbol and none by the click of mouse. In Fig. 20.32 Dialog box Symbols the middle text box, type the tolerance

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value. The right square calls for the Material Condition Modifier (MC) dialog box (not shown here). Click on this square and choose M or L or S out of the displayed list. Define the Datum under Datum 1 tile or Datum 2 tile (if more than 1). Click OK button. The dialog box disappears and AutoCAD prompts as: Enter tolerance location Click at the desired location on the screen. Complete frame is placed at the specified position. Example 10 Create a feature control frame as shown in Fig. 20.S10. Solution 1. At the command line, type TOL and press Enter key. Geometric tolerance dialog box appears (Fig. 20.31). 2. In the Sym tile, click in the upper square box. Symbol dialog box apFig. 20.S10 Feature pears. Control 3. Click the Position symbol, (first left on the upper row). This symbol is Frame displayed in the Sym box. 4. In Tolerance 1 tile do the following: a. Click the first square box. It toggles between a diameter symbol and none by click of mouse. Ensure that there is none there. b. In the middle text box type the tolerance value as 0.1. c. Click the right square for the Material Condition Modifier (MC). Material Condition dialog box appears. Choose M out of the displayed list. 5. In Datum 1 tile, under Datum 1 type A in the text box. 6. Click OK button. AutoCAD prompts for tolerance location. 7. Specify the location at AutoCAD prompt, “Enter tolerance location:” 8. Use Leader on Dimension toolbar to point it to the surface and terminate at the frame.

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Differentiate between geometrical tolerance on form and on position. What is meant by datum? What terms are used related to it and how is it shown on drawing? What is a frame? How is it used to show geometrical tolerance? Explain the following terms with a sketch: Circularity, Cylindricity, Angularity and Perpendicularity. What are related features? Explain by examples. What is meant by MMC and LMC? Define the following terms: Reference profile, mean roughness index and roughness number. How is the surface roughness indicated on a drawing? What is meant by N9 in terms of surface roughness? What is direction of lay in surface measurement?

CAD 11. What command is used for putting geometrical tolerances and how are they put? 12. Explain the method to put a geometrical symbol. 13. How are frame and tolerance values indicated on a drawing using AutoCAD?

Geometrical Tolerances and Surface Finish

FILL 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

IN THE

BLANKS

Datum is indicated by a in a box. along with and Geometrical tolerance is written in a Surface variation between two planes is called _____________. is variation in radius at a section. Variation of profile along axis of a cylinder is called . like edge or surface. Position geometrical tolerance is specified from another . Variation in profile of a curve is called symbol. Two oblique lines placed side by side represent . Angularity is specified in relation to other . MMC abbreviation stands for . Average departure from flat surface over a sampling length is called as condition. The condition in which a feature contains least material is called as at regular intervals. Surface roughness number is mean of process. Minimum surface roughness is obtained by symbol. A surface not to be machined is indicated by or by grade symbol. Roughest surface is specified by indicates roughness in microns of the order of A roughness grade symbol like A letter M on right side of roughness symbol indicates laying direction. direction of lay. An equal to sign (=) on roughness symbol indicates triangles. Finest surface is represented by

CAD 21. 22. 23. 24.

The command used to put geometric tolerance is symbols. Symbol dialog box has dialog box. Datum letter is specified in dialog box. Material condition is chosen from

.

MULTIPLE CHOICE QUESTIONS Tick the appropriate answer 1. A datum is represented by a (a) box (b) (c) upper-case letter in a box (d) 2. A frame contains (a) only tolerance value (b) (c) datum letter (d) 3. A parallelogram is a symbol for (a) straightness (b) (c) cylindricity ( d) 4. Position geometrical tolerance is used to specify (a) distance of a feature from an edge (b) (c) X and Y coordinates from a corner (d)

429

upper-case letter upper-case letter in a circle only tolerance symbol all given in a, b and c flatness parallelism variation of distance from an edge all given above in a, b and c

.

.

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5. Straightness geometrical tolerance is used to specify (a) variation along a line (b) variation of a surface about a reference plane (c) parallelism of two surfaces (d) permissible wear along a line 6. Variation in radius along the periphery of a cylinder at a section is called (a) cylindricity (b) circularity (c) straightness (d) profile of a line 7. A number such as N12 specifies (a) roughness number (b) normal at 12 mm distance (c) datum number 12 (d) geometrical tolerance 12 8. Ra in microns is the (a) average height of ‘n’ points (b) r.m.s. value of heights (c) geometric mean of two extreme heights (d) right axial direction of lay

CAD 9. Command for putting geometric tolerances is (a) GEOTOL (b) TOL (c) GTOL (d) GEOMETRYTOL 10. Name of the dialog box displayed by TOL command is (a) Symbol (b) Geometric Symbol (c) Geometric Tolerance (d) Tolerance 11. Geometric Tolerance symbols are selected from (a) Geometric Tolerance dialog box (b) Feature dialog box (c) Tolerance dialog box (d) Symbol dialog box 12. A frame for Geometric Tolerance is created in (a) Geometric Tolerance dialog box (b) Feature dialog box (c) Tolerance dialog box (d) Symbol dialog box 13. To indicate surface roughness symbol following command is used (a) ROUGH (b) ROUGHNESS (c) SURFRUF (d) There is no command

ANSWERS to Fill in the Blank Questions 1. capital letter 2. frame, symbol, TOL. value & datum 3. flatness 4. Circularity 5. cylindricity 6. feature 7. tolerance on profile 8. parallelism 9. related feature 10. Maximum Material Condition 11. Surface roughness number 12. LMC 13. heights

14. lapping

15.

16. N12, ~

17. 12 to 25 21. TOL

18. multi-directional 22. fourteen

19. parallel 23. geometric tolerance

20. four 24. material condition

ANSWERS to Multiple Choice Questions 1. (c) 7. (a) 13. (d)

2. (d) 8. (a)

3. (b) 9. (b)

4. (b) 10. (c)

5. (a) 11. (d)

6. (b) 12. (a)

Geometrical Tolerances and Surface Finish

ASSIGNMENT

ON

GEOMETRIC TOLERANCES

AND

431

SURFACE ROUGHNESS

1. For the isometric view shown in Fig. 20.P1, draw the front view, side view and add the following geometrical tolerances on the appropriate view: ∑ Bottom as primary datum A ∑ Front of 200 mm length as secondary datum B ∑ Right side as tertiary datum C ∑ Bottom has flatness of 0.06 mm ∑ Top has to be parallel with datum A within 0.04 mm ∑ The hole has to be perpendicular to datum A within 0.02 mm and straightness of 0.05 2. A base plate with 3 pins is shown in Fig. 20.P2. Add the following geometrical tolerances to the drawing: ∑ Top surface of the base plate as datum A ∑ Datums B and C as secondary and tertiary for location of pins ∑ Bottom of the base to have parallelism of 0.2 mm and flatness of 0.1 mm with datum A ∑ Location of pins 1, 2 and 3 for position tolerance of 0.1 mm

Fig. 20.P1

Fig. 20.P2

3. Figure 20.P3 shows a support with tolerance on dimensions of ±0.2 mm. Add the following geometrical tolerances: ∑ Bottom as datum A having flatness of 0.01 mm ∑ 20 mm diameter hole is to be parallel with datum A within 0.1 mm ∑ 30 mm hole has to be perpendicular within 0.05 mm with datum A ∑ Slot of 50 mm width is to be perpendicular within 0.15 mm to datum A ∑ Slot has to be symmetrically in the center of length 100 mm within 0.5 mm 4. Draw orthographic views of the slide shown in Fig. 20.P4 and show the following information: ∑ Vertical sides of slot should have maximum roughness of 3.2 mm ∑ Top surface is to be milled having maximum roughness of 0.8 mm ∑ Sides should have maximum roughness of 6.3 mm

Fig. 20.P3

Fig. 20.P4

A Slider

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CAD ASSIGNMENT

ON

GEOMETRIC TOLERANCES ROUGHNESS

AND

SURFACE

5. For the problems Q 1 to Q 3 above, put geometric tolerances using AutoCAD. 6. Create a symbol for surface roughness. Use it for putting surface roughness for question 4 above.

HOMEWORK 7. Figure 20.P5 shows a stepped object with a flat portion on top of length 30 mm. Add geometrical tolerances as under: ∑ Diameter 60 as datum A ∑ End face of diameter 100 as datum B ∑ Width of flat face of 30 mm length as datum C ∑ End face of diameter 20 be flat within 0.2 mm ∑ Center line of diameter 60 as straight within 0.1 RFS ∑ Surface of diameter 20 must be straight Fig. 20.P5 within 0.15 mm 8. Part 1 shown in Fig. 20.P6 has to fit in part 2 such that there is no interference and maximum clearance is not more than 0.01 mm. Add maximum limit for groove of part 2. Put flatness tolerance of 0.02 mm for both the surfaces. 9. For a V block shown in Fig. 20.P7, select suitable machining processes and the roughness for that process. Then specify roughness of different faces by roughness symbols.

Fig. 20.P6

PROBLEMS

Fig. 20.P7 FOR

A V block

PRACTICE

10. Surface A should have straightness tolerance of 0.015 mm and surface B of 0.02 mm. Add the feature control frame on the drawing shown in Fig. 20.P8. 11. Straightness is specified on a part (Fig. 20.P9) as 0.05 mm. What is the maximum permissible deviation from straightness of the center line if width of groove X = 15 ± 0.01? 12. What is the maximum possible deviation from straightness for hole shown in Fig. 20.P10, if perfect form at MMC is required?

Geometrical Tolerances and Surface Finish

Fig. 20.P8

433

Fig. 20.P9

Fig. 20.P10

Fig. 20.P11

13. Show graphically the tolerance zone by thin lines and limits of size for the part shown in Fig. 20.P11. 14. Complete the table for the largest permissible straightness for the feature shown in Fig. 20.P12. Feature diameter in mm

Permissible straightness tolerance

39.5 39.4 39.3

Fig. 20.P12 15. Establish a geometric relationship between pin of diameter 35 mm stepped down to 25 mm with a ring of diameter 70 mm and length 55 mm shown in Fig. 20.P13 with following datum: Datum for pin Datum A – 35 mm diameter Datum B – 25 mm diameter Datum for ring Datum A – 70 mm diameter

Fig. 20.P13

Datum B – 35.5 mm diameter Datum C – 25.5 mm diameter Add geometrical tolerances that will incorporate the following information: ∑ Position of axis of ring must be within 0.01 mm at MMC. ∑ Position of axis of pin must be within 0.015 mm. ∑ Perpendicular of left face of pin should be within 0.02 mm relative to datum B.

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16. Sketch front view of a sand casted bearing support shown in Fig. 20.P14. Show the roughness symbols for different machined surfaces. Use Tables 20.3 and 20.5 for selecting a suitable process and roughness. 17. Sketch two views of a sand casted object shown in Fig. 20.P15. Using Tables 20.3 and 20.5 for selecting a suitable roughness, show the roughness numbers for the following: ∑ Bottom of machine is to be milled ∑ Outside of cylinder is unmachined ∑ Vertical hole on a lathe ∑ Sides on a shaper

Fig. 20.P14

A Bearing Support

Fig. 20.P15

CHAPTER

21

Material Specifications A production drawing of a part specifies the material also. The engineer has to choose a suitable material depending upon many factors like strength, working environment, life, stiffness, etc. Engineering materials are broadly classified as metals and non-metals. Metals are of two types: Ferrous and Non-ferrous. Amongst ferrous metals, wrought iron is the purest form of iron. A little percentage of carbon increases its hardness. Steel up to 0.25% carbon is called low carbon steel. Mild steel is most commonly used having carbon between 0.25-0.6%. Items like tools, etc. are made of high carbon steel with carbon from 0.6 to 1.5%. Metals having 2-3 % of carbon are called Cast iron and between 3-4 % carbon as Grey cast iron. Some alloying elements like Cu, Cr, Pb, Mg, Mn, Mo, NI, P, Si, S are added to improve properties of steels and such steels are called alloy steels. Low and medium alloy steels have these constituents less than 10% while high alloy steel have more than 10%. Steels are designated either on the basis of mechanical properties or chemical composition. For mechanical properties, carbon steels are coded with first letter as Fe followed by codes assigned to specify tensile strength, resistance to brittle failure, surface finish and condition, purity, treatment, weldability, etc. On the basis of chemical composition, these are specified by 100 times % of carbon followed by chemical symbols and their percentage multiplied by some factors for all the alloying elements. High alloy steels are prefixed by a letter X while tool steels with letter XT. Nonferrous metals used for engineering applications are; Sb, Be, Cd, Cr, Cu, Pb, Mg, Mn, Ni, Ag, Sn, Ti, W and Zn. These metals are generally used in the form of alloys. Two commonly used alloys are Copper alloys and Aluminum alloys. Pure copper and aluminum are used for wires due to their high conductivity. Copper alloys are gilding metal, cartridge brass, Standard brass, Muntz metal, Bronzes, Phosphorous bronze, Aluminum bronze and gun metal. These are designated by manufacturing method, chemical composition, alloy index, surface finish and tempering method. Aluminum alloys have constituents like Mn, Mg, Cu. These are coded by a five digit number. Number codes are assigned metals from 1 to 9. First number is code for major alloying element. Second digit is rounded off value multipled by a factor for the next alloying element. Third, fourth and fifth digits are minor alloying elements in descending order. Non metal materials are Plastics which can be molded by heat, pressure or both. Two types of plastics are Thermoplastics and Thermosetting plastics. Thermoplastics are ABS, Acetal Resins, Acrylics, Cellulosics, Fluorocarbons, Nylon, Polycarbonate, Polythene, Polystyrene, Polypropylenes, Vinyls. Thermosetting plastics are Alkyds, Allyics, Aminos, Caseins, Epoxys, Phenolics, Polyesters, Silicons. Elastomers (Rubber) are useful when electric insulation, vibration isolation are to be considered. Thses can be mechanical or cellular. A cellular rubber can be open cell sponge or closed cell sponge. Woods like Teak, Sheesham, Pine, Saal, Rose, Mango, etc. are used for some applications.

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A production drawing contains a Bill of materials also enlisting all the components, their description, material and quantity required.

CAD TABLE command of AutoCAD can create a table for Bill of materials. With this command, Insert Table dialog box is displayed where number of columns and rows, their height, text height, etc. can be specified. Type the text for each cell. Block attributes can also be used for extracting data from a file.

21.1

INTRODUCTION

A production drawing of a part specifies the material also. There is a wide range of materials now available from which a design engineer has to choose according to the requirement for a particular component that performs a specific function. The selection depends upon strength requirement, permissible deflections, mass, life, type of service, working environment, reliability, cost, quantity required, availability, cost and material properties. The various material properties to be considered are as follows: 1. Strength

Bears the stresses coming in the component.

2. Fatigue strength

Ability to bear reversal or change of stresses.

3. Stiffness

Ability to resist bending when loaded.

4. Ductility

Develop significant permanent deformation before break.

5. Brittleness

Break without any appreciable elongation, e.g. glass.

6. Toughness

Ability to withstand shocks.

7. Hardness

Resistance of material to abrasion or indentation.

8. Wear

Progressive loss of material due to rubbing of surfaces.

9. Density

Mass per unit volume. Important for air craft applications.

10. Conductivity

Resistance to flow of current.

11. Thermal conductivity

Ability to conduct heat.

12. Transparency

Ability to pass light.

13. Magnetic properties

Ability to retain magnetic field.

14. Corrosion resistance

Resistance to corrosion.

15. Thermal expansion

Increase in length with temperature.

21.2

TYPES OF ENGINEERING MATERIALS

Materials are classified as metals (ferrous or non-ferrous) and non metals (plastics, rubber, wood, etc.) as shown as below:

Material Specifications

21.3

437

FERROUS METALS

Iron and iron alloys called steels are most commonly used metals. Wrought iron is the purest form of iron and is used for ornamental work, chain links, etc. Most commonly used steel is Mild steel. High carbon steels, Alloy steels, Cast irons increase in carbon percentage as shown in Fig. 21.1.

Fig. 21.1 Name of Ferrous Metals with Percentage of Carbon

21.3.1

Carbon Steels

Carbon steel has small amount of carbon along with other elements like Si, Mg, Cu, S, etc. added to impart certain properties. They can be used for casting/forging/forming or machining. Carbon is the main hardening element in steel. Its percentage more than 0.85, increases hadrness and tensile strength but decreases ductility and weldability. Hot rolled carbon steel sheets are made from heated slabs, which are reduced in thickness by passing through a series of rollers. Cold rolled carbon steel sheets are made from hot rolled coils of flats which are pickled and then clod rolled to reduce thickness. Carbon steel plates of thickness 4 to 6 mm and width 1200 mm are also produced by hot rolling directly from slabs.

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Carbon steel bars are hot rolled from billets in variety of cross-sections like circular, square, etc. Cold finished bars are produced from hot rolled steel by cold finishing process for good surface finish. Steel wires are made from hot rolled rods. Steel pipes are made either by welding from hot/cold rolled flats or seamless pipes by extrusion process. Following are the types of carbon steels:

A Low Carbon Steel Low carbon steel has carbon percentage between 0.06 to 0.25%. It is less tough and has low tensile strength. It is used for chains, rivets, shafts, pressed steel products. Its tensile strength depends upon manufacturing method; Hot rolled-270 N/mm2, Cold drawn-350 N/mm2 and Annealed-295 N/mm2 . B Medium Carbon Steel Medium carbon steel has carbon percentage between 0.25 and 0.6%. It is tough and strong. It has high strength and rigidity. It is used for gears, axles and machine parts. Its tensile strength for Hot rolled is 290 N/mm2, Cold drawn-440 N/mm2 and Tempered-435 to 660 N/mm2 . C High Carbon Steel High carbon steel has carbon percentage between 0.6%. and 1.5%. It is used for cutting tools. Table 21.1 gives the various applications for different carbon steels as the percentage of the carbon is varied from 0.05 to 1.1%. Table 21.1 % Carbon 0.05 0.08-0.15 0.1-0.3 0.25-0.4 0.3-0.45 0.4-0.5 0.55-0.65 0.65-0.75 0.75-0.85 0.85-0.95 0.95-1.1

21.3.2

Applications of plain carbon steels Applications

Sheet, strip, car bodies, wire, rod, tube Sheet, strip, wire, rod, tube, nails, screws, reinforcing bars Plate, sections, structural steel Bright drawn bar Shafts, high tensile tubes Shafts, gears, forgings, castings, springs Forging dies, springs, railway rails Hammers, saws, cylinder liners Chisels, die blocks for forging Punches, shear blades, high tensile wire Knives, axes, taps and dies, milling cutters

Cast Iron

Cast iron is a low cost and easily produced metal. Its application is given in Table 21.2. Rate of cooling of casting can produce different types of cast irons described as follows:

A Ductile Cast Iron Ductile cast iron is also called Nodular iron. It is difficult to control its production. It is used where higher ductility and strength than gray iron is required. It provides good machinability, good fatigue strength, high modulus of elasticity and wear resistance. Its tensile strength varies form 620 to 1035 N/mm2.

Material Specifications

439

B Grey Cast Iron Grey cast iron is supersaturated solution of carbon in iron. Excessive carbon precipitates as graphite flakes. It is fatigue resistant with tensile strength 140-455 N/mm2. C White Cast Iron White Cast Iron is produced by chilling process preventing carbon from precipitating. Gray or ductile iron can be chilled to produce a surface of white iron. Its disadvantage is that it is brittle. It offers excessive hardness, wear and abrasion resistance. D Malleable Cast Iron Malleable Cast Iron is a white iron created in two stages of heat treatment. It is similar to steel for strength and ductility, fatigue and impact properties. It has excellent machining characteristics with tensile strength between 345-725 N/mm2. It is of two types: Ferritic which is more machinable and ductile and Pearlitic which is stronger and harder. Table 21.2

Applications of cast irons

Type

Applications

Ductile cast iron Grey cast iron

Crank shafts, heavy duty gears. Automotive blocks, machine frames, pulleys, big gears, bearing housings, brackets, large size pistons, flywheels, brake disks and drums, machine bases and gears. Mill lines, rail brake shoes, rolling mill rolls, clay or brick equipment, crushers, pulverizers etc. Lathe bed, slideways and guideways.

White cast iron Malleable cast iron

21.3.3

Alloy Steels

Carbon is the main constituent in alloy steel. Alloy steel has minimum of 8% of alloying elements like Ni or Cr to make corrosion resistant or provide strength at temperature above 560°C. It is less tough but more hard. Its tensile strength is: Hot rolled-335 N/mm2, Cold drawn-258 N/mm2, Tempered 470 to 800 N/mm2. It is used for saws, drills, knives, razors, tools musical strings. Table 21.3 gives the alloying elements and the properties imparted by them. Table 21.3

Alloying elements, properties imparted and applications

Alloying element

Properties imparted/Applications

Copper

More than 0.15% improves atmospheric corrosion resistance.

Chromium

Cr 0.3 to 0.5%. It is hard, has great strength and toughness. Used for gears, shafts, bearings, springs, connecting rods.

Chromium-Vanadium

Cr 0.5 to 1.1% and Vd 0.1 to 0.15%. Its important properties are hardness and strength. Used for punches, dies, piston rods, gears, axles, etc.

Lead

Improves machinability.

Magnesium

Lightest metal having density of 1.74 gms/cc with good strength. Gives high strength to weight ratio. (Contd.)

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Table 21.3 Alloying element Manganese

Molybdenum Nickel Phosphorous Silicon Silicon-Manganese Sulphur

21.3.4

(Contd.)

Properties imparted/Applications Mn 1.6 to 1.9% and gives improved surface finish. Does not impart harness or strength but increases rate of carbon penetration during carburizing. Manganese decreases Weldability. Mo 0.15 to 0.3%. It has high strength. Used for axles, forgings, gears, cams, mechanical parts. Adds mechanical properties like corrosion resistance. In large amount, increases strength and hardness but reduces ductility and toughness. Increases strength and hardness but to lesser extent than manganese. It reduces machinability. Si 1.8 to 2.2%. Used where springiness and elasticity is required, e.g. springs. In increased amount reduces transverse ductility, non-impact toughness and weldability. It improves machinability. Used for threads, splines, etc.

Stainless Steels

They have a wide range of alloys having chromium more than 10%. They are classified as follows:

A Austinitic Stainless Steels Austinitic stainless steels have 18% Cr, 8% Ni called as 18/8 steel. They have high resistance to corrosion, good weldability, high toughness at low temperatures, excellent ductility. B Ferritic Stainless Steels Ferritic stainless steels have 16-20% Cr and inferior to Austinitic steels. Used for press work because of high ductility, become brittle at low temperature. They have moderate strength and limited weldability. C Martensitic Stainless Steels Martensitic stainless steels have 12-18% Cr and 1-3% Ni. They have least resistance to corrosion. Not suitable for cold forming or welding, moderate machinability. Table 21.4 Stainless Steel Type Austenitic Ferrite Martensitic

21.4

Applications of stainless steels Application

Window and door frames, chemical plant tanks, domestic hot water pipe lines, spoons, forks, kitchen utensils, nuts, bolts, screws, rivets, coil and leaf springs Car silencers, oil burner sleeve, electric cooker, gas and electric cookers, coinage, spoons, forks, knives, domestic iron soles, driving mirror frames Tools, turbine parts subjected to high temperature, scales, rulers, knives, kitchen appliances, surgical and dental items

DESIGNATION OF STEELS [IS 1762–1974 Part 1]

Steels are designated as follows: a. On the basis of mechanical properties (Section 21.4.1 and Section 21.4.2) b. On the basis of chemical composition (Section 21.5)

Material Specifications

21.4.1

441

Steel Designation According to Mechanical Properties

Steels are designated on the basis of mechanical properties by a code given below. If some of the parameters given are not to be mentioned, they can be omitted. Fe where: Fe E NNN CHS SC AS

21.4.2

E

NNN

CHS

SC

AS

Letter code for Iron is Fe, if strength is based on minimum tensile strength. Letter code E is added after Fe, if strength is based on yield strength. Tensile strength or yield stress in N/mm2. A number 00, if no strength is guaranteed. Chemical symbols for elements present in steel like Cu for Copper. Special characteristics characters like B, D, F, P, Q, S, T, W, etc. See Section 21.4.2 on special characteristics. Application symbol like H suitability for high temperature, L for Low temperature.

Special Characteristics Codes

Code B – Resistance to Brittle fracture Based on results of V notch charpy impact test. Code like B, BO, B2 or B4 are placed. Code D – Formability (for sheets only) No symbol – Commercial quality D1 – Drawing quality D2 – Deep drawing quality D3 – Extra deep drawing quality Code F – Surface Finish (for sheets only) F1 – General purpose finish F4 – Unexposed F7 – Plating finish F10 – Polished and colored blue F13 – Vitreous enamel finish

F2 – Full finish F5 – Matt finish F8 – Unpolished F11 – Polished and colored yellow F14 – Direct annealed finish

Code P – Purity code for steel (for maximum phosphorus or sulphur content) No symbol – 0.005% P25 – 0.025% P35 – 0.035% P50 – 0.05%

F3 – Exposed F6 – Bright finish F9 – Polish finish F12 – Mirror finish

P70 – 0.07%

Code Q – Steel Quality code Q1 – Non aging Q4 – Inclusion controlled

Q2 – Flakes free Q3 – Grain size controlled Q5 – Internal homogeneity guaranteed

Code S – Surface condition code No symbol – As rolled or as forged S3 – Pickled S6 – Bright drawn or cold rolled

S1 – Deseamed or scarfed S4 – Shot/sand blasted S7 – Ground

Code T – Treatment code No symbol – Hot rolled T3 – Normalized

T1 – Shot peened T4 – Controlled rolled

S2 – Descaled S5 – Peeled

T2 – Hard drawn T5 – Annealed

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T6 – Patented T9 – Controlled cooled T12 – Stress relieved

T7 – Solution treated T10 – Bright annealed T13 – Case hardened

Code W – Weldability code W – Fusion Weldable

W1 – Weldable by resistance welding, but not fusion welding

Other character codes R – Rimming (Method of de-oxidation) H – Suitability for high temperature

T8 – Solution treated and aged T11 – Spheroidized T14 – Hardened and tempered

K – Killed (Method of de-oxidation) L – Suitability for low temperature

Examples Fe450CuR Iron, with minimum tensile strength 450 N/mm2, copper alloying element, Rimming quality. FeE500L Iron, with yield strength 500 N/mm2, no alloying element, Suitable for low temperature applications. FeE540F5 Iron, with yield stress 540 N/mm2, no alloying element, with Matt finish. Fe00S7 Iron with no strength guaranteed with ground surface.

21.5 21.5.1

STEEL DESIGNATION ACCORDING TO CHEMICAL COMPOSITION [IS 7598–1974] Unalloyed Steels

Code designation for unalloyed steel is given below. If some code is not to be mentioned, it can be omitted. NN where: NN C T MM G

C or T

MM

G

A number which is 100 times the average percentage of carbon. Letter code for carbon or, Letter code T for tool steels. A number which is 10 times the average percentage of manganese and rounded off. Letter code for guarantee of the special characteristics.

Examples 35C5G Iron with 0.35% carbon and 0.05% manganese guaranteed. 85T12

21.5.2

Tool steel with 0.85% carbon and 1.2 % manganese.

Alloy Steels

Alloy steel can be Low, Medium, High alloy steel or Tool steel.

A Low and Medium Alloy Steels Total alloying elements are less than 10% for this category code. The code is as follows: NN

A1

N1

A2

N2

A3

N3

SC

Material Specifications

where: NN A1 N1

443

A number which is 100 times the average percentage of carbon. Chemical symbol for alloying element 1. Average percentage content of A1 multiplied by a factor as given in table below: Element

Multiplying factor

Co, Cr, Mn, Ni, Si, and W Al, Be, Cu, Mo, Nb, Pb, Ta, Ti, V and Zr P and S A2 N2 A3 N3 SC

4 10 100

Chemical symbol for alloying element 2. Average percentage content of A2, multiplied by a factor as given above in the table. Chemical symbol for alloying element 3. Average percentage content of A3, multiplied by a factor as given above in the table. Special characteristics (See Section 21.4.2).

Example 25Ni8Cr10V2

Alloy steel with 0.25% Carbon, 2% Nickel, 2.5% Chromium and 0.2% Vanadium.

B High Alloy Steels Total alloying elements are more than 10% for this category code. The code is similar for low and medium carbon steels, except a letter X is added in the beginning as follows: X NN where: X

A1

P1

A2

P2 A3

P3 SC

A letter code for high alloy steel.

NN

A number which is 100 times the average percentage of carbon.

A1, A2, A3

Chemical symbol for alloying elements 1, 2 and 3.

P1, P2, P3

Average rounded percentage content of A1, A2, A3 etc. respectively

Example X15Cr10Ni12

High alloy steel with 0.15% Carbon, 10% Chromium and 12% Nickel.

C Tool Steels Steel designation is similar to low, medium and high alloy steels except that letter code XT code is put instead of X. XT

NN

A1

P1

A2

P2

A3

P3

SC

Example XT70W15Ni12V2

Alloy steel with 0.70% Carbon, 15% Tungsten and 12% Nickel and 2% Vanadium.

444

Part D – Chapter 21

21.6

CODE DESIGNATION FOR FERROUS CASTINGS [IS 4863–1968]

These are designated by a code either on the basis of mechanical properties or on the basis of chemical composition. Code on the basis of mechanical properties is: TC

NN

where TC is type of casting with the following code: CS for steel casting FG for Grey iron PM for Malleable Pearlitic iron SG for Spheroidal graphite iron NN is a number representing 10% of tensile strength in N/mm2. Examples FG 15 Grey iron with tensile strength of 150 N/mm2. PM70 Malleable Perlitic iron with tensile strength of 700 N/mm2.

Code on the basis of chemical composition [1762 – 1961] is specified for grey iron where chemical composition is more important than tensile strength. Group symbol is followed by the chemical symbol and their percentages. Its code is given as follows: GS A1 P1 A2 P2 A3 P3 where GS is group symbol, A1, A2, A3 are the chemical symbols for alloying elements and P1, P2, P3 are their percentages respectively. Elements with percentage less than 1 are not indicated. Example CSC16Cr13

21.7

Steel with carbon ranging from 0.12 to 0.2%, Silicon, Manganese, Nickel less than 1%, Molybdenum less than 0.5% and Chromium 11.5 to 15%.

NON-FERROUS METALS

Characteristics and applications of some important metals and alloys are tabulated in Table 21.5. Most commonly used non-ferrous alloys are Copper alloys and Aluminum alloys and are described separately in Sections 21.8.1 and 21.8.3. Table 21.5 Metal/Alloy

Non-Ferrous metals and applications

Characteristics

Applications

Aluminum

Light, corrosion resistive, good electric conductivity

Electric wires

Antimony

Bright lustrous white metal

Alloying element for castings and bearing alloys

Beryllium

Lighter than aluminum, corrosion and heat resistant, good conductor of electricity and nonmagnetic

Nuclear field and electronic tubes

(Contd.)

Material Specifications Table 21.5 Metal/Alloy

445

(Contd.)

Characteristics

Applications

Cadmium

Resistant to saline atmosphere, takes solder easily

Used for plating, electrical storage batteries

Chromium

Anticorrosive

As alloying element and for electroplating

Copper

Good electrical conductivity

Electric wires

Lead

Heavy, soft, ductile, low strength

Chemical equipment, cable sheathing, radiation shields, alloy for solder and bearings

Lead–Tin alloy Tinman’s solder; 1 part lead, 2 parts tin

Electrical jointing, tinplate can sealing

Solder

Plumber’s solder, 2 parts lead, 1 part tin

Lead pipe joints

Magnesium

Very light (¼ of steel)

Alloy element for aircraft and I.C. engine parts, nuclear fuel cans

Manganese

White, hard and brittle

Used in Mn bronze and high Ni alloys to improve corrosion resistance

Nickel

High corrosion resistance

Used for chemical plants, electroplating

Nickel base alloys

Monel: 68% Ni, 30% Cu, 2% Fe

Steam turbine blades, chemical plants

Platinum

Soft, ductile, very high corrosion resistance and chemical attack

Used for electrical contacts, electrodes, resistance wires

Silver

Ductile, malleable, excellent thermal and electrical conductivity

Electrical contacts, plating, bearing lining, as alloying element

Tin

Low melting point, silver color, high corrosion resistance

Used for tin plating, bearing alloys and solder

Titanium

Light metal (4.43 gms/cc), expensive, low density, high strength, excellent corrosion resistance, heat resistant

Aircraft industry, alloying element up to 10%

Tungsten

Melting point very high (3410°C), can be produced by powder metallurgy only

Alloy element for Carbide tools, permanent magnets

White metal

Contains Pb, Sn, Sb and Cu

Bearing linings

Zinc

Low melting point (400 °C) so good for die casting, inexpensive metal, has moderate strength and toughness with outstanding corrosion resistance

Carburetors, fuel pumps, door handles, galvanizing sheet nail, wire and in bronze

21.8.1

Copper Alloys

These are made in rods, sheets, tubes and wire form. Its main use is for electrical conductors due to its high electrical conductivity, corrosion resistance, strength, ease of forming and joining. It is available in a variety of colors. There are many copper alloys such as brasses, leaded bronze, aluminum bronze and silicon bronze. Their applications are given in Table 21.6.

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Table 21.6

Copper and its alloys

Copper alloy and composition

21.8.2

Applications

Pure copper 99.95% Cu

High conductivity electrical applications

Copper 98.85% Cu

Chemical plant, deep drawn and spun items

Gilding metal (Commercial brass) 90% Cu, 10% Zn

Imitation jewellery, decorative works

Cartridge brass 70% Cu, 30% Zn

High ductility for deep drawing

Standard brass 65% Cu, 35% Zn

General cold working alloy

Muntz metal 60% Cu, 40% Zn

Condenser and heat exchanger plates

Monel metal 30% Cu, 1.5% Fe, 1.5% Mn, 67% Ni

High temperature valve seats, chemical engineering equipment

Tin Bronze 89% Cu, 11% Zn

Bearing bushes, pump bodies

Bronzes 95.5% Cu, 3% Sn, 1.5% Zn

Coinage

Phosphorous bronze 10% Sn, 0.03 – 0.25 P, rest Cu

Bushes and bearings

Gun metal 10% Sn, 2% Zn, rest Cu

Pressure tight casings, pump, valve bodies

Aluminum bronze 95% Cu, 5% Al

Condenser tubes, Imitation jewellery

Code Designation for Copper and Its Alloys [IS 2378 – 1974]

These are designated by a group of symbols of Manufacturing Method, Chemical composition, Alloy Index, Surface Finish and Temper in the following order. No code to be used, if any information is not to be specified. MM

CC

AI

SF

T

Codes used for Manufacturing Method (MM)/Method of casting are: G – Sand cast GC – Chill cast GX – Shell/Investment cast h – Hot rolled e – Hot extruded f – Forged No symbol means material is in wrought form. Symbols used for Chemical Composition (CC) are: CATH Cathode copper

GD – Die cast GW – Centrifugally cast d – Cold drawn or extruded

Material Specifications

447

ETP Electrolytic Tough Pitch FRHC Fire Refined High conductivity Copper Alloy Index (AI) is the chemical symbol of Copper followed by next significant element after which elements are stated in order of decreasing percentage. If average is equal, specify in alphabetical order. For alloy element up to 1%, only symbol is used. If percentage is more than 1, then the symbol is followed by rounded off percentage to nearest whole number. Codes for Surface Finish (SF) are: J – Bright drawn or rolled J7 – Bright pickled Codes for temper (T) are: O – Annealed H – Strain hardened

J8 – Bright annealed

MC – Machined

T – Thermally treated to produce temper other than O or H.

Example GD Cu Pb 10 Sn 5 O – It is die casted copper alloy with 10% lead and 5% tin and then annealed.

21.8.3

Aluminum Alloys

Aluminum has density one third of steel. Some of its alloys are stronger than some structural steels. It has high resistance to corrosion. No stains stay due to salts. Their applications are given in Table 21.7. Table 21.7

Aluminum and its alloys

Aluminum alloy

21.8.4

Applications

Pure Aluminum 99%

Sheet, strip, lining for chemical and food plant

1.25% Mn, rest Al

Extruded sections, hollow ware, roofing, paneling, tubes

2% Mg, rest Al

Sheet, plate, tube, extrusions, strong deep drawn articles

4.5% Cu, 0.75% Mg, 0.5% Mn, rest Al

Highly stressed aircraft parts

Code Designation for Aluminum and its Alloys [IS 6051 – 1970]

Aluminum alloys are designated by a 5 digit number.

Codes used for various alloying elements are: 1 – Un-alloyed aluminum 4 – Silicon 7 – Zinc

2 – Copper 5 – Magnesium 8 – Other elements like Ni, Cr

3 – Manganese 6 – Magnesium Silicide 9 – Unassigned

Part D – Chapter 21

448 Example 1

For aluminum alloy with copper 1.7%, Silicon 1% Magnesium 0.8% the code is: 2

4

5

0

No third element Magnesium Silicon Copper percentage rounded to 2% Copper

21.9

PLASTICS

Plastics are non-metallic and can be formed and molded with heat or pressure or both. They are strong, tough, durable, non corrosive, light weight, available in many colors, have good physical properties, suitable for mass production but still cannot replace the metals due to the hardness and rigidity. Plastics can not be used as electric conductors and thermal insulators. They are opaque and are poor in fatigue strength. They can be used as a coating on textiles and papers. They can be used to bind materials as on fiber glass or wood to form boat hulls, airplane wings. They are available in the form of films, sheets, rods and tubes. Practically all types of plastics can be machined with adequate tooling. Plastics are of many types and each has its typical advantages. Broadly they are classified as Thermoplastics and Thermosetting plastics.

21.9.1

Thermoplastics

They soften or liquefy and flow when heated. Removal of heat causes them to set or solidify. They can be reheated, reformed and reused. Their characteristics and applications are tabulated in Table 21.8. Table 21.8 Thermoplastics

Properties and uses of thermoplastics

Characteristics

Applications

ABS

Strong, tough, good electrical properties.

Pipe, wheels, football helmets, battery cases, radio cases, skates for children.

Acetal Resins

Rigid without being brittle, tough, resistant to Automobile instrument clusters, gears, bearings, extreme temperatures, good electrical properties. bushings, door handles, plumbing fixtures, threaded fasteners, cams.

Acrylics

Very clear for light transmission, strong, rigid, resistant to sharp blows, excellent insulators, colorless or full range of translucent or opaque colors.

Cellulosics

Toughest of all plastics, retain lustrous finish Eyeglass frames, toys, lamp shades, combs, shoe under normal wear, good insulators, transparent, heels, steering wheels, radio cases, pipes, tubes, translucent or opaque in variety of colors. playing cards, phone handsets, pens, pencils, flash lights, electrical parts, fabric coating.

Airplane canopies and windows, camera viewing lenses, combs, bowls, trays, lamp bases, auto tail lights.

(Contd.)

Material Specifications Table 21.8 Thermoplastics

449

(Contd.)

Characteristics

Applications

Fluorocarbons

Low coefficient of friction, resistant to extreme Valve seats, gaskets, coatings, linings, tubings. heat, strong, hard and good insulators.

Nylon

Resistant to extreme temperatures, strong and long wearing, range of soft colors, low friction coefficient

Tumblers, gears, bristles of brush, fishing line, small bearings.

Polycarbonate

High impact strength, resistant to weather and transparent.

Aircraft parts, automobiles, business machines, gauges, safety glass lenses.

Polythene

Excellent insulating properties, moistureproof, clear transparent, translucent.

Ice cube trays, tumblers, dishes, bottles, bags, balloons toys.

Polystyrene

Transparent, translucent or opaque. Heat, cold, water and weather resistant. Available in all colors.

Kitchen items, food containers, wall tiles, toys, instrument panels.

Polypropylenes

Good heat resistant, highly resistant to crack, wide range of colors.

Thermal dishware, washing machine agitators, pipe and pipe fittings, wire and cable insulation, packing films and sheets.

Poly-TetraTough and flexible, wide temperature range. Fluoro-Ethylene (PTFE)

Coating on non stick cooking utensils

Vinyls

Rain coats, garment bags, inflatable toys, hose, floor and wall tiles, shower curtains, pipe, panels.

21.9.2

Strong, resistant to abrasion heat, cold and available in wide range of colors.

Thermoset Plastics

They undergo an irreversible chemical change when heated. They become hard, insoluble, infusible and do not soften on reapplication of heat. Their characteristics and applications are given in Table 21.9. Table 21.9 Properties and uses of thermoset plastics Thermoplastics

Characteristics

Applications

Alkyds

Excellent dielectric strength and moisture.

Light switches, insulators, mounting cases. In liquid form used as enamels, lacquers for automobiles and refrigerators.

Allyics

Excellent dielectric strength and insulation resistance. No moisture absorption. Full range of transparent and opaque colors.

Electrical connectors, appliance handles, knobs, coating for plywood, hardboard for moisture protection.

Aminos

Very hard, strong but breakable. Good electrical properties, available in full range of translucent and opaque colors.

Melamine is used for tableware, buttons, table tops, plywood adhesive. Urea is used for radio cabinets, electrical devices, and appliance housings. (Contd.)

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Table 21.9 Thermoplastics

(Contd.)

Characteristics

Applications

Caseins

Excellent surface polish, strong, rigid but affected by humidity and temperature changes, available in wide range of near translucent and opaque colors.

Buttons, buckles, beads, knitting needles, toys and adhesives.

Epoxys

Good electrical properties, water and weather resistant. They firmly bond metals, glass, ceramics, rubber, plastic.

Protective coating for appliances, cans, drums, gym floors, printed circuits, laminated tools jigs, and liquid storage tanks.

Phenolics

Strong and hard, heat and cold resistant, excellent insulators.

Radio and TV cabinets, washing machine agitators, pulleys, electrical insulation.

Polyesters

Strong and tough, bright and pastel colors, high dielectric qualities.

Used to impregnate cloth or mats of glass fiber, paper, cotton and other fibers, reinforcement plastics for use in boats, automobile bodies.

Silicons

Heat resistant with good electrical properties.

Switch parts, insulation for motors and generator coils.

21.9.3

Elastomers (Rubber)

They are useful when considering the properties such as electric insulation, vibration isolation, sealing surfaces, flexibility and chemical resistance. They are of two types; Natural and Synthetic. Rubbers are produced either in mechanical (solid) or cellular form depending upon the desired performance. a. Mechanical rubber is superior to sponge rubber because of its physical properties. It is used in pressure molded, cast or extruded forms. Parts produced by these methods are belts, tires, bumpers. b. Cellular rubber can be produced with Open or Closed cells. Both are available in the form of a block or sheet. ∑ Open cell sponge rubber is made by inclusion of a gas forming chemical in mixture before vulcanization. A gas is formed with the heat of vulcanization which makes a cellular structure. Typical applications are pads and weather stripping. Foam rubber is specialized type of open cell. ∑ Closed cell sponge rubber is made by an inert gas solution that produces many ball shaped cells with continuous walls. When rubber is deformed, cells are displaced rather than deflated. It is very springy when squeezed.

21.9.4

Wood

It is also used in engineering applications, e.g. patterns are made of wood. Teak, Sheesham, Pine, Saal, Rose, Mango, etc are commonly used. Some CNC machines also use a soft material similar to wood to do testing operation.

21.10

BILL OF MATERIALS

A production drawing contains a bill of materials (items list), enlisting all the components. From this bill, the workers cut the material to length and number of pieces shown on the bill and send to shop

Material Specifications

451

floor for manufacturing. Sometimes it may be necessary to figure the mass of material also. For heavy items, it may be used to avoid overloading of the crane. This table appears on a production drawing at a suitable place. A sample bill of materials is given in Table 21.10. Table 21.10

Bill of materials

S.No.

Item

Description

Material

Quantity

1

SHAFT

Ø 30 ¥ 300

35C5G

1

2

KEY

SQUARE 8 ¥ 8

M.S.

1

3

BEARING

BALL BEARING

SKF # 3200

2

4

BOLTS

HEXAGONAL Ø 15 ¥ 30

M.S.

4

5

NUTS

HEXAGONAL Ø 15

M.S.

4

6

BASE

PATTERN # B312

CAST IRON

1

7

COVER

PATTERN # C12

CAST IRON

1

Example 1 Left hand column of Table 21.S1 lists some items. Write suitable materials for these components on the right hand side. Solution The suitable materials are written in the right column of Table 21.S1. Table 21.S1

Materials for some items

Item Axle

Suitable material Mild Steel, Molybdenum steel

Bearings

Phosphorous bronze, Gun metal, Babbitt, Chromium steel

Bush

Nylon, gun metal

Cam

Molybdenum steel

Car body

Low carbon steel

Carburetor

Aluminum alloy

Chain links

Low carbon steel

Chisels

High speed steel

Coin

Bronzes

Connecting rod

Chromium steel

Cutter for milling

High carbon steel, High Speed Steel (HSS)

Dies

High carbon steel, Cr–Vd steel

Drill

High carbon steel

Gasket

Fluorocarbon

Gear

Mild steel, Molybdenum steel, Chromium steel

Knife

High carbon steel (Contd.)

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Table 21.S1 (Contd.) Item

Example 2

Suitable material

Machine parts

Mild steel

Nails

Mild steel

Nuts

Mild steel

Pipes

Low carbon steel, Copper, Lead, PVC, Stainless steel

Piston

Cast iron, Aluminum alloy

Rivet

Low carbon steel

Punch

Cr–Vd steel

Saw

High carbon steel

Screw

Mild steel

Shaft

Low carbon steel, Chromium steel

Structural steel

Mild steel

Surgical equipment

Ni–Cr steel

Springs

Si–Mn steel, Cr–Vd steel, Si–Cr steel

Taps

High speed steel

Tubes

Mild steel, Copper, Aluminum

Write the code for the materials given in the left column of Table 21.S2.

Solution The codes for the materials are written in the right column of Table 21.S2. Table 21.S2 Codes of some materials Material details

Code

Killed steel with minimum tensile strength 420 N/mm2 with Copper as alloying element 2

Fe 420 Cu K

Steel with minimum 550 N/mm yield strength with maximum Sulphur and Phosphorous 0.05%

FeE 550 P50

Steel with minimum tensile strength 450 N/mm2 weldable by resistance welding

Fe 450 W1

2

Steel with minimum tensile strength 500 N/mm and resistant to brittle fracture of grade B

Fe 500 B

Steel with 0.25% carbon with 1% Manganese and Guaranteed hardenability

25C10G

Alloy steel with 0.3% carbon, 1.5% Ni, Chromium 2.5%, 0.1% Vanadium

30Ni 6Cr10V1

High alloy steel with 0.2% carbon, 10% Chromium, 8% Nickel

X20Cr10Ni8

Tool steel with 0.75% carbon, 10% Tungsten, 8% Ni, 2% Vanadium

XT 75W10Ni8V2

2

Cast iron with minimum tensile strength 400 N/mm

FG 40

Steel with carbon 0.1% carbon, Silicon 1%, Chromium 10%

C S Cr 10

Sand cast Copper alloy with 8% Zinc 4% Tin bright annealed

G Cu Zn 8 Sn 4 J8

Aluminum alloy with Si 5.6%

4300

Material Specifications

453

CAD 21.11

TABLE COMMAND

To create a table using AutoCAD, TABLE command can be used. Type Table at the command prompt and press Enter key. Insert Table dialog box is displayed as shown in Fig. 21.2. Specify number of columns and their width, number of rows and their height in the text boxes either by using arrow keys or by typing directly. Click OK button. A prompt appears as: Command: TABLE ø Specify insertion point:

Click on the screen wherever the left upper corner of the table is desired. An empty table of specified rows and columns is placed on the screen along with Text formatting dialog box as shown in Fig. 21.3 having gray area at its bottom. Start typing the text. Click OK to place the text on the table. Bring cursor in any cell and type the relevant text.

Fig. 21.2 Insert Table Dialog Box

Gray area

Fig. 21.3 Text Formatting Dialog Box

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21.12

BLOCK ATTRIBUTES

Extracting attribute information is an easy way to produce bill of materials directly from the drawing. For example, a drawing might contain blocks representing an equipment. If each block has attributes identifying the details, a report can be generated that estimates the cost of the equipment. The Attribute Extraction wizard guides through selecting drawings block instances and attributes. If attribute data is extracted to a table, the table is inserted in the current drawing and current space. By updating the table, the attribute information is extracted again and data rows in the table are replaced.

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Write the various parameters to be considered while selecting a suitable material for a part. Classify the materials used for engineering applications. What are the various types of carbon steels? Write the difference and typical applications for each type. What are the various types of cast irons? How do they differ from steels? Write their typical applications. What are the main alloying elements and the properties which they impart? What are the uses of stainless steels? How many types of stainless steels are there? What are special characteristic codes? Write a few about surface finish, purity and treatment. How is a steel designated on the basis of mechanical properties? Write the code for designating an unalloyed steel. Differentiate between low, medium and high carbon steels. How are they designated? What is the code designation for the ferrous castings? Write names and typical characteristics of any main ten non-ferrous metals. What are the various copper alloys? Write the uses of each. How are the copper alloys designated? How are Aluminum alloys designated? Write typical uses of aluminum alloys. Differentiate between the various types of plastics? Write names of any five thermoplastics and their applications. Write names of any five thermosetting plastics and their applications. Differentiate between mechanical and cellular elastomers. Write their uses. What is meant by Bill of materials? Where is it given and what are its contents?

CAD 21. Describe the use of TABLE command. 22. How is a table of 4 columns and 6 rows created on a drawing?

FILL 1. 2. 3. 4.

IN THE

BLANKS

The most important parameter to be considered for material of a job is its form of the iron. Wrought iron is the types of plain carbon steels. There are mainly %. Percentage of carbon in mild steel is between

.

Material Specifications 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Tools are generally made of . %. Percentage of carbon in cast iron is more than certain properties. Alloying elements are added to iron to . A letter E in code of steels indicates that strength is based on and on the basis of . Steels are designated on the basis of times the average percentage of carbon. First number in steel designation is a number is put first for designating high alloy steels. A letter A letter ‘XT’ in the beginning of high alloy steel indicates steel. . Pure copper is used for digit code. Aluminum alloys are coded by a . Plastics which undergo an irreversible chemical change with heat are called rubber is made by an inert gas solution.

CAD 17. The command to create a table in AutoCAD is

.

MULTIPLE CHOICE QUESTIONS 1. High carbon steel has percentage of carbon more than (a) 0.02 (b) 0.25 (c) 0.5 (d) 0.6 2. Bolts are made of (a) Wrought iron (b) Low carbon steel (c) Mild steel (d) High carbon steel 3. Surgical equipment is made of (a) cast iron (b) stainless steel (c) Aluminum (d) Copper alloy 4. First letter in designation of the code of steel on the basis of chemical composition indicates (a) tensile strength (b) yield strength (c) percentage of carbon (d) a number 100 times the percentage of carbon 5. A letter X in the designation of alloy steels indicates (a) no alloying element (b) Low alloy steel (c) Medium alloy steel (d) High alloy steel 6. Solder material is an alloy of (a) Lead and Tin (b) Lead and Magnesium (c) Manganese and Tin (d) Lead and Nickel 7. Standard brass has percentage of copper and zinc in the ratio of (a) 60:40 (b) 65:35 (c) 55:45 (d) 50:50 8. Gun metal is used for (a) guns (b) coins (c) valve bodies (d) condenser tubes

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9. Big size of pistons are made of (a) mild steel (b) cast iron (c) stainless steel (d) bronze 10. Plastics which can be reheated and reformed are called as (a) Thermoplastics (b) thermosetting plastics (c) Elastomer (d) Polymer 11. Thermosetting plastics (a) undergo irreversible chemical change (b) do not soften when reheated (c) are infusible with reheat (d) all given in a, b and c 12. Belts are made of (a) mechanical rubber (b) cellular rubber (c) open cell sponge (d) closed cell sponge

CAD 13. TABLE command displays (a) a predefined table (c) Insert Text dialog box

(b) Insert Table dialog box (d) all objects in a table

ANSWERS to Fill in the Blank Questions 1. 5. 9. 13. 17.

strength High carbon steel mechanical properties, chemical composition Closed cell sponge

2. 6. 10. 14. 18.

purest three hundred electric wires TABLE

3. 7. 11. 15.

three improve X five

4. 8. 12. 16.

0.25 to 0.6 yield stress tool Thermosetting plastic

ANSWERS to Multiple Choice Questions 1. (d) 7. (b) 13. (b)

2. (c) 8. (c)

3. (b) 9. (b)

4. (d) 10. (a)

5. (d) 11. (d)

6. (a) 12. (a)

CHAPTER

22

Production Drawings Production drawing is a part drawing indicating its size, tolerances, geometric tolerances, surface finish, fit if required, material, manufacturing method, heat treatment if required, tools and gauges to be used, jigs or fixtures if required. Size, tolerances, geometric tolerances and surface finish are mentioned on the drawing itself, while other details are written in a table called process sheet. Title block used for production drawings is more detailed. Name of the person who draws, checks and approves are also mentioned along with date for accountability purposes. It also contains information about part number, material specifications and surface roughness also if it is to be specified. The various manufacturing methods which are used are: Casting processes: Sand casting and Die casting. Forming processes: Forging, Rolling, Drawing and Extrusion. Joining processes: Arc welding, Gas welding, Spot welding, Seam welding, Brazing, Soldering and Riveting. Material removal processes: Turning, Shaping, Milling, Drilling, Planning, Hobbing and Broaching. Chemical processes: Electric Discharge Machine, Electro-Chemical Machine, Etching and Electroplating. Surface finishing processes: Surface grinding, Cylindrical grinding, Lapping, Honing and Painting. CNC machines: Computerized lathe and Milling. Heat treatment is done to make the job of required hardness. In this process, a job is heated to a temperature depending upon carbon percentage and then cooled at a specified rate. Hardening requires quick cooling in water or oil. Tempering is done to reduce harness of a hard material up to required hardness. Annealing is done by cooling a hot job at a very slow temperature in furnace it self. Normalizing is done to make the metal machineable by cooling hot job in still air. Tools used for material removal processes are made of High Carbon Steel, High Speed Steel or Tungsten Carbide. Tool angles like top rake, side rake, front and side clearance are provided so that the tool cuts effectively. Surface finish is best if the material is cut at the correct cutting speed. Soft metals require higher cutting speeds and vice versa. Tools used for lathe are single cutting point. These are turning tool, grooving tool, facing tool, threading tool, knurling tool, form tool, drill and boring tool. Shaper uses a side cutting tool and grooving tool. They are similar to lathe tools but more strong to bear impact loads coming over them. Milling machines use multi-cutting point rotary tools like Slitting saw, Angle cutter, Gear cutter, Form cutter and Slab cutter. Vertical milling uses End mill cutters, T- slot cutters and Face cutters. Drilling machine uses drills; parallel or taper shank. Slotting machine uses similar to lathe tools but the cutting edge is at right angles to the axis of the tool. Broaching machine is used for making keyways and splines, etc. Hob cutters are used for generating gears in mass production.

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Planning machine uses tools similar to that of shaper. Inspection is not done by measuring tools like vernier, etc. in production but by Go-Not Go gauges, pneumatic gauges or comparators. Jigs are used to hold and support a job firmly and guide the tool for its correct locations. Fixtures do not guide the tool, but only help in supporting the job on machine. Assembly drawings have many parts. Each part is indicated by a reference number written in a circle. These circles are connected to the part by a leader line. Standard components are specified by the code as per IS in the process sheet. Production drawing has all the above-mentioned information as required along with a process sheet. Process sheet is a table that indicates sequence of operations along with machines, tools, gauge required and time to complete an operation. Each operation is numbered.

CAD MVSETUP command can be used to start a drawing with border, centering marks and grid reference numbers.

22.1

INTRODUCTION

A production drawing is an authorized document to produce a part in the workshop. Mentioning tolerances, geometric tolerances, fits, surface finish and materials are also a part of production drawing. They also provide following information, which is not mentioned in class room drawings: ∑ Manufacturing tolerances (Chapter 19) ∑ Type of fit; if 2 parts are mating with each other (Chapter 19) ∑ Geometric tolerances (Chapter 20) ∑ Surface finish (Chapter 20) ∑ Material specifications (Chapter 21) ∑ Title block (Section 22.2) ∑ Manufacturing methods (Section 22.3) ∑ Heat treatment if required (Section 22.4) ∑ Tooling details (Section 22.5) ∑ Inspection details (Section 22.6) ∑ Jigs and fixtures (Sections 22.7 and 22.8) ∑ Assembly drawing (Section 22.9), if more number of parts ∑ Standard component representation (Section 22.10) ∑ Production drawing (Section 22.11) ∑ Process sheet (Section 22.12)

22.2

TITLE BLOCK

Title block used in industry differs from classroom work. It also provides the information about the name of the company, part identification number, drawing number, tolerances on size, type of finish, material to be used and name of the part. Name of the person, who creates the drawing, checks the

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459

drawing and approves the drawing also appear with date for the purpose of accountability. The location of the information in the title block and its size may differ for different industries but the information more or less remains similar. A sample title block is shown in Fig. 22.1. PRECISION BEARINGS COMPANY IDENTIFICATION NO. BRG/237

DRAWING NO. 1410

BEARING BRASSES MATERIAL BRONZE

SCALE FULL SIZE SURFACE FINISH

25

DRAWN BY

HARDEEP MANKU

DATE 05/06/2007

CHECKED BY

ANAND VASAGIRI

DATE 10/06/2007

APPROVED BY

NARINDER PAL

DATE 12/06/2007

Fig. 22.1

22.3

SHEET NO.1

TOLERANCE 0.04

A Title Block used in Industry

MANUFACTURING PROCESSES

A production drawing gives information about manufacturing process also in a process sheet (Section 22.12). Aim of this chapter is not to describe manufacturing processes in detail but provide the relevant information in brief. Mainly manufacturing processes are of following types: a. Casting processes Sand casting and Die casting (Section 22.3.1) b. Forming processes Forging, Rolling, Drawing and Extrusion (Section 22.3.2) c. Joining processes Arc welding, Gas welding, Spot welding, Seam welding, Brazing, Soldering and Riveting. (Section 22.3.3) d. Material removal processes Turning, Shaping, Milling, Drilling, Planing, Hobbing, and Broaching (Section 22.3.4) e. Chemical processes Electric discharge machine, Electro-chemical machine, Etching and Electroplating (Section 22.3.5) f. Surface finishing processes Surface grinding, Cylindrical grinding, Lapping, Honing and Painting (Section 22.3.6) g. CNC machines Computerized lathe and Milling (Section 22.3.7)

22.3.1

Casting Processes

A Sand Casting Casting needs a pattern of the same shape as the job. It is generally made of wood with some pattern allowances. If more number of parts are to be produced, then it may be of aluminum. The pattern drawing should be inclusive of all pattern allowances like shrinkage allowance, draft allowance, machining allowance, etc. Sand casting is done for ferrous and non-ferrous materials. Surface quality and tolerances are poor, suitable for small and big jobs. It is relatively cheaper in comparison to material removal process. It is not suitable for mass production. B Die Casting Metallic dies are prepared and the molten metal is injected in the die with gravity pressure or by a pump. Metals of low melting points can be casted by this process. Surface quality is superior to sand casting and close tolerances can be met. It is suitable for mass production.

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22.3.2

Forming Processes

Forming is done in cold or hot condition. Cold working leaves work stresses, while in hot working the grain structure is again modified and stresses do not remain.

A Forging Forging process is making a job by applying forces manually or in a die in hot or cold condition. Force application can be by hammer or a press. Surface roughness is poor in hot working. B Rolling Rolling slabs of metals are passed through a pair of rollers to form sheets in successive passes. In every pass, the thickness is reduced. This process can produce plates, sheets, rods, wires, etc. The shape depends upon the shape of rollers. All ductile metals and thermoplastics can be rolled. It is economical for rolling long jobs. Dimension stability is high. Cold rolling gives good surface finish but with hot rolling, the surface is poor due to oxidation at high temperature. It is suitable for mass production with close tolerances. C Drawing Drawing is a process, in which the material is pulled through a die. The cross-section takes the same shape as that of the die. Rods are used to reduce in size to make wires. Sheets can be used for deep drawing using die and punch to make parts like tumblers. The material has to be soft and ductile for this process. Initial cost of the equipment is high but is very suitable for production with low cost. Close tolerances are possible with good surface quality. D Extrusion Extrusion is a process in which the material is forced to flow through a die opening. Large forces are required to force the metals and hence more suitable for soft metals or non-ferrous alloys. Aluminum window frames and other various sections are easily produced by changing the die of different crosssection. It is suitable for mass production, with good surface quality, close tolerances and low running cost. The initial cost of equipment is high. 22.3.3

Joining Processes

Welding is done to join two parts. Welding is of many types but only the main types are described.

A Arc Welding Arc welding is suitable for welding thick metal sheets. A step down transformer is used to decrease AC 440 or 220V AC to a low voltage (50-80 V) to increase current. Two cables are used as output from this transformer. The earth cable is connected to the parts to be welded. The other cable has an electrode holder at its free end, in which an electrode coated with a flux is fixed. When the electrode is brought near the part, an arc is generated. The heat of the arc melts the material and the metals in fusion state become one, when cooled. Edge joints are generally prepared to form a groove. B Gas Welding Gas welding is suitable for thin sheets. It uses two cylinders; oxygen and acetylene. A welding torch having hose connections from these cylinders is used to regulate flow of each gas separately. When a spark is shown to the mixture of gases, acetylene starts burning by getting oxygen for burning from the other hose. Joining parts are heated and fused with a filler rod.

Production Drawings

461

C Spot Welding Spot welding is done by passing heavy current at low voltage through electrodes and the plates to be joined pressed between them. Local heating takes place due to contact resistance. Metals melt locally and fuse to each other to form a spot weld. D Seam Welding Seam welding is similar to spot welding except that the electrodes are in the form of rollers and the plates are passed between them to form a continuous seam of weld line. E Brazing Brazing uses a solder material of copper alloy having melting point 650°C to 700°C. Its strength is not as that of welding but stronger than soldering. Parts are cleaned, heated and then solder material is filled. Steel tubes, bicycle frames use this method. Brass is also used as filler material. F Soldering Soldering uses solder material having low melting point 180°C to 210°C. A soldering iron is used to melt the solder and put on the heated parts. It is widely used for making electric connections of electronic components on Printed Circuit Boards (PCB). G Riveting Riveting is done to make permanent joints using rivets. Plates to be joined are drilled with holes and rivet is inserted and the head is formed on the other side. It is used for making boiler shells and structural member joints. Small rivets are used for cooking utensils and fabrication work. 22.3.4

Material Removal Processes

A product is made by removing material from the areas that are not required. Various machines are used for this purpose and each does a specific type of job which is given as follows: a. Lathe – Cylindrical or taper turning, thread cutting, drilling, boring, knurling and grooving b. Shaper – Short flat surfaces, inclined surfaces and grooves. Surface finish is not good. c. Milling – Flat surfaces grooves, gears and splines with good surface finish d. Vertical milling – Profile milling of any contour, die making and surface milling e. Drilling machine – Holes from 3 to 15 mm on pedeshtal drill machines and bigger holes on radial drill machines f . Slotting machine – Keyways g. Broaching machine – Keyways and splines h. Hobbing machine – Gear cutting i. Planing machine – For making long flat surfaces and long grooves like lathe beds

22.3.5

Chemical Processes

A Electric Discharge Machine (EDM) Electric Discharge Machine (EDM) uses a shaped tool of copper or wire electrode. The job is immersed in a tank containing dielectric fluid (kerosene) which is circulated to flush away debris. It works by removing eroding material in the path of electric discharges that form an arc between an electrode tool and work piece. It is an accurate process and affordable.

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B Electro-Chemical Machining (ECM) Electro-Chemical machining (ECM) is based on a controlled anodic electro-chemical dissolution process of work piece (anode) with tool (cathode) in an electrolytic cell during an electrolytic process. C Etching Etching is done to remove material chemically. It is generally used for making Printed Circuit Boards for mounting of electronic components. A bakelite sheet with thin copper sheet over it called copper clad is put with a protective coating photographically of the same shape as required. Then it is etched in Ferric Chloride solution to remove copper from the places which are not protected. D Electroplating Electroplating is done to provide a coating on a surface to protect it from weather conditions. Some types of plating are Nickel plating, Chromium plating, Cadmium plating, Silver plating and Zinc plating (also called Galvanizing). Plating is done by passing high current between an electrode of the metal to be plated to the job through electrolyte. Anode material is transferred to job electrolytically. 22.3.6

Surface Finishing Processes

When the surface smoothness required is more than what conventional machines can give, grinding process is used. These machines offer roughness from 0.06 to 5 microns. Grinders are also used for tool grinding. Grinding machines are of following types: a. Surface grinding machine is used to make flat surfaces smooth. b. Cylindrical grinding machine is used to make cylindrical surfaces smooth. c. Centerless grinding is used for high production jobs of circular cross section. d. Internal grinder is used to grind internal holes. e. Honing is done by rubbing grinding sticks over the surface. Roughness varies between 0.025m to 0.4m. For example, cylinder liners. f. Lapping is used to give better surface finish (by using abrasive particles) than honing (0.12m to 0.16m). g. Painting is done to protect the surface and not to improve surface roughness. It is done by using paint with the help of a brush or with a spray gun, like car bodies.

22.3.7

CNC Machines

CNC stands for Computerized Numerically Controlled machines. The machines, either lathe or a milling is connected to a computer through an interface, which transfers the digital data coming from computer in the form of NG code to the machines. Stepper motors that rotate 1/400 or 1/200 of revolution per pulse are used to move the job through screws. The spindle offers stepless speed variation for correct cutting speed. These are useful when the same job is to be made again and again. The programs are saved in computer and a job can be made according to the program called for.

22.4

HEAT TREATMENT PROCESSES

Heat treatment process is done by heating a job up to specified temperature. This temperature depends upon percentage of carbon and type of heat treatment process. It is then cooled at a specific rate of

Production Drawings

463

cooling depending upon the process. Each process is to attain desired hardness and certain other properties. These processes are described as follows.

A Hardening It is done to make a job hard. For very low carbon (0.1%), steel is heated between 900 to 980°C. This temperature range decreases linearly to 700°C to 780°C as carbon percentage increases up to 0.85. For all carbon percentage more than 0.85, heating is done between 700°C to 780°C. Then the hot job is quenched in water or oil. On cooling the job becomes hard. B Tempering Quenched carbon steels by hardening process are very brittle and not suitable for immediate use. Therefore tempering is done to increase toughness at the expense of some hardness. Tempering is done by re-heating a hardened carbon steel to temperature between 220°C to 300°C depending upon the component, and then again quenched in oil or water. Typically edge tools are heated up to 200°C, twist drills 240°C, taps 250°C, cold chisels 280°C and springs 300°C. Medium carbon steels are heated between 450°C to 600°C. C Annealing Purpose of annealing is to soften plain carbon steels that have been quench hardened. It is also used to impart ductility for severe cold working. Depending upon percentage of carbon, the metal is heated at same temperature as required for hardening. The metal is then allowed to cool very slowly in the furnace with power shut off. It imparts softness, ductility and good grain growth. D Normalizing After annealing, the material is very soft, ductile and has large grain structure. This is ideal for cold working but unsuitable for machining. Annealed steels tend to tear and leave very poor surface finish when machined. To make it suitable for machining, normalization is done. In this process, steel is heated according to its carbon content as for hardening, then taken out of furnace and cooled in still air away from any draft. This results in a tough steel with fine grain structure and gives good surface finish while machining. 22.5

TOOLING

Tools are required for metal removal processes. There are many types of tools. Each machine tool uses a specific type of tool. The tool to be used is mentioned in the process sheet (Section 22.12).

22.5.1

Classification of Tools

According to tool material – High Carbon Steel (HCS), High Speed Steel (HSS), Tungsten carbide (WC). According to cutting points – Single point, dual point (like drill), multi-point like milling cutters, hacksaw blades, saw, etc. According to cutting action – Linear like hacksaw, broach and rotary like milling cutters.

22.5.2

Tool Angles

Tools angles are provided with sharp edges to minimize cutting forces and improve surface finish. Single point tools are provided with angles like top rake, side rake, front and side clearance, etc. to

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make them cut effectively (Fig. 22.2). Carbide tools use negative top rake angle and offer higher cutting speeds.

22.5.3

Cutting Speed

Cutting speed is the relative velocity between tool and work piece. It depends upon material of tool and work piece. Soft materials require higher cutting speed and greater top rake angle.

22.5.4

Fig. 22.2

Lathe Tool Cutting Angles

Tools Used for Different Machines

A Lathe Uses single cutting point turning tool, grooving tool, facing tool, threading tool, knurling tool, form tool, drill, boring tool (Fig. 22.3). B Shaper Uses side cutting tool, grooving tool. They are similar to lathe tools but stronger to bear impact loads coming over them. C Milling Milling machines use multi-cutting point rotary tools. These are slitting saw, angle cutter, gear cutter, slab cutter (Fig. 22.4).

Fig. 22.4

Fig. 22.3

Lathe Tools

Milling Cutter

D Vertical Milling End mill cutters, T- slot cutters and Face cutters (Fig. 22.5). E Drilling Machine Drills have two cutting points. The upper plain cylindrical portion of the drill is called shank. Up to 15 mm size, they are made parallel shank (Fig. 22.6A) but drills bigger than this size are normally

Production Drawings

465

taper shank drills which fit in the sleeve with Morse taper and run with friction only (Fig. 22.6B). A center drill is a combination of two small drills (Fig. 22.6C). The front end is of about 3 mm while the rear portion is about 8 mm. This is used to put center on lathe, to accommodate dead center at the free end of the job on lathe. Helix angle Parallel shank

Lip

11 8

Flute Neck (A) Parallel shank drill

Morse taper

Taper shank

(B) Taper shank drill

(C) Center drill

Fig. 22.5

Milling Cutters

Fig. 22.6 Types of Drills

F Slotting Machine It uses grooving tool which is similar to lathe tools but the cutting edge is at right angles to the axis of the tool. G Broaching Machine Broaching is used for making keyways, splines, etc. Broaching tool has teeth of increasing height (Fig. 22.7). The front pilot is pushed into the hole first and then pulled by the broaching machine. Every teeth cuts a small material. Initial teeth called roughing teeth have greater cut while finishing teeth have less cut.

Fig. 22.7

Broaching Tool

H Hobbing Machine Hob cutters are used for gear cutting in mass production (Fig. 22.8). It is a rotary cutter having teeth along a helical path like that of a worm. The gear blank is brought into contact with the hob. The involute profile is generated by this cutter as the hob is fed radially inwards towards the gear. I Planing Machine Uses single point side cutting tool, grooving tool similar to lathe/shaper.

Fig. 22.8

Hob Cutter

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22.6

INSPECTION

Raw material before manufacturing is inspected first visually and then checked for size, shape, composition, specifications, etc. After manufacturing the finished product, it is checked for size tolerances, geometrical tolerances, surface finish, etc. Critical dimensions are specified on the drawing with tolerances such as 30 ± 0.02. Less critical tolerances can be given by a general note in the title block such as “All tolerances ±0.04 mm.” (Refer Fig. 22.1). Inspection method depends upon the value of the size tolerance [IS 3455 – 1971]. Vernier caliper or micrometer is not convenient to use for large production, as these are time consuming. Hence gauges are used for jobs produced in mass production. The type of inspection gauge to be used is mentioned on production drawing in the process sheet (Section 22.12). Gauges have no scale to give actual size, but only check whether the size is acceptable or not. Go and Not-Go gauges are very common. For a size to be acceptable, it should pass through the Go gauge and should not pass through Not-Go gauge. These gauges have ends which are hardened so that the size does not change due to continuous use causing wear. They have high surface finish of the order of 0.02 to 0.1 micron. Gauges are made with close tolerances which are generally 5 to 10% of the work piece tolerance. Different types of Go and Not-Go gauges are used depending upon what to check. For example, Plug gauge is to check hole size tolerances, Ring gauge is to check outside diameter of cylindrical jobs, Snap gauge has both Go and Not-Go gauges combined into one (See Chapter 19 for details). Pneumatic gauges are also used. The indication is by the resistance offered to air pressure by the job. If the clearance between job and gauge is less, resistance to flow of air is more. So a higher pressure is indicated on a pressure gauge. If size is less, more gap, hence less pressure is indicated. The pressure gauge is marked for two extreme sizes which can be accepted. Comparators use a dial indicator for comparison purpose. When a job is placed under dial indicator, the size variation is indicated on the dial. Limits of the sizes are set on the dial indicator.

22.7

JIGS

Interchangeability of parts in mass production is a prime requirement for assembly. To produce identical parts, jigs are used for guiding tools while manufacturing. Jigs not only increase the speed of production and accuracy, but also reduce production cost and time. Jigs perform two functions: a. Support and Hold a job securely b. Guide the location of a cutting tool For example, a chain link has two holes, which should be exactly at same center distance for all the links. Figure 22.9 shows a jig for drilling two holes in the chain link. The jig supports chain link in the cavity of the supporting device. After putting the link in the cavity, a cover is put over it. The cover is hinged at one end and the other end has a slot to clamp it. A clamping lever, when lifted up, rides over the cover to keep it tight over the link. Two bushings in the cover guide the drill for keeping the same distance for all links. After drilling the holes, the clamping device is pulled out of the slot to release the cover. The cover is lifted up about the hinge. An ejecting lever when pressed, pushes the link out of the cavity.

Production Drawings

Fig. 22.9

467

A Jig for Drilling Holes in a Chain Link

The jig is drawn in continuous lines while the job is usually shown by chain dotted lines for differentiation purpose. Type of jig required for each job depends upon its shape and hence has to be suitably designed and details are given on the production drawing. The jig described above facilitates in locating the centers quickly and exactly. Sometimes it is not possible to drill a hole without a suitable jig. Example 1 shows such a jig. Example 1

Design a jig to drill a hole in a rod at angle of 60° with its axis.

Solution Figure 22.10 shows such a jig. The locator supports the work piece at the required angle. A bolt on the top is used to hold the work piece so that the job does not move. The guide bush guides the drill to drill hole at an angle which otherwise would have been very difficult as the drill will have a tendency to slip over the inclined surface.

Fig. 22.10 A Jig for Drilling Holes at an Angle

22.8

FIXTURES

Fixture is a device to locate and hold the work securely in a definite position. It only holds the work and does NOT guide any tool. Generally it is fitted on the machine. Fixtures are also drawn by chain dotted lines in the drawing. Figure 22.11 shows a fixture to hold a block on the face plate of lathe.

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Fig. 22.11

22.9

A Fixture on a Lathe

ASSEMBLY DRAWINGS

A machine is constructed by assembling many individual parts. Part drawings are prepared for manufacturing each part. An assembly drawing is also necessary to indicate the relative position of each part for the functioning of the machine. It need not contain all details but only main dimensions such as overall size, centerline to centerline distances, etc. are given. Figure 22.12 shows an assembly drawing for a drill vice.

Fig. 22.12

An Assembly Drawing with Part Numbers

Table 22.1

Part list for the drill vice

Part No.

Part Name

Material

Quantity

1

Body with fixed jaw

Cast iron

1

2

Jaws

High carbon steel

2

3

Moveable jaw

Cast iron

1

4

Screw

Mild steel

1

5

Handle

Mild steel

2

6

Handle stop

Mild steel

2

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469

Each part is assigned a reference number and labeled on assembly drawing (Fig. 22.12). Reference numbers are assigned in a sequential order of some letter height and are enclosed in a circle. These numbers are placed outside the drawing. These are connected to the part by a leader line. Leader line should be inclined to the part. A part list is prepared (Table 22.1) giving part number, its name, material and quantity.

22.10

STANDARD MECHANICAL COMPONENTS

A machine generally contains standard components like bolts, nuts, pins, oil seals, bearings, etc. These are designated in the process sheet as described in Table 22.2. IS number is also sometimes mentioned at the end of designation to avoid any ambiguity. Table 22.2 Standard component Hexagonal Bolt

IS number IS 1363 – 1967

Designation of standard components Designation

Meaning

Hex Bolt M 10 ¥ 40

Hexagonal bolt with metric threads of diameter 10 mm and length 40 mm

Hexagonal Bolt with nut IS 1363 – 1967

Hex Bolt M 10 ¥ 40N

Hexagonal bolt with metric threads of diameter 10 mm and length 40 mm along with nut

Hexagonal Bolt with nut and lock nut

IS 1363 – 1967

Hex Bolt M 10 ¥ 40NL

Hexagonal bolt with metric threads of diameter 10 mm and length 40 mm along with nut and lock nut

Castle nut

IS 2232 – 1967

Castle nut M20

Castle nut with metric threads for 20 mm bolt size

Plain washers A – for Hex. bolts B – for cheese head screws

IS 2016 – 1967

Machine washer A 20

Washer for 20 mm bolt

Precision bolts

IS 1364 – 1967

Hex Bolt M 20 ¥ 1.5 ¥ 60NL

Hexagonal bolt with metric threads of diameter 20 mm, pitch 1.5 mm and length 60 mm along with nut and lock nut

Square bolts

IS 2585 – 1968

Sq. bolt M 16 ¥ 70N

Square head bolt of diameter 16 mm, metric threads length 70 mm with nut

Allen head screws

IS 2269 – 1967

Hex. Socket head M 10 ¥ 30

Hex. Socket head screw 10 mm diameter metric thread and 30 mm length

Counter sunk screw

IS 1365 – 1968

Counter sunk screw M 8 ¥ 20

Counter sunk screw 8 mm diameter metric thread and 20 mm length

Cheese head screw A – fully threaded B – half threaded

IS 1366 – 1968

Cheese head screw A M 6 ¥ 25

Fully threaded Cheese head screw with 6mm diameter metric threads and 25 mm length

Studs A – for steel use B – for Cast iron C – for Aluminum

IS 1862 – 1975

Stud B M 10 ¥ 40

Stud for cast iron of 10 mm diameter metric threads and 40 mm length

(Contd.)

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Table 22.2 Standard component Taper key Splines Cylindrical pin Taper pin Solid or Split

(Contd.)

IS number

Designation

IS 2292 – 1974

Taper key 15 ¥ 10 ¥ 70

A taper key with 15 mm width, 10 mm height and 70 mm length IS 2327 Splines 6 ¥ 22 ¥ 25 6 splines with nominal diameter 25 mm and root diameter 22 mm IS 2393 – 1980 Cylindrical pin 12h8 ¥ 25 Cylindrical pin of diameter 12 mm with tolerance h8, length 25 mm IS 6688 – 1972 Solid taper pin 10 ¥ 50 Solid taper pin of 10 mm diameter and 50 mm length

Rivet

IS 2155

Snap head rivet 8 ¥ 30

Oil seal A – Rubber cased B – Metal cased C – Built up Ball bearing Extra light 100 Light 200 Medium 300 Heavy 400

IS 5129

Oil seal A 25 ¥ 35 ¥ 6

22.11

Meaning

Ball bearing 305

Snap head rivet of 8 mm diameter and 30 mm length Oil seal, Rubber cased for shaft diameter 25 mm, housing diameter 35 mm and width 6 mm Medium duty ball bearing for 25 mm diameter (5 ¥ 5 = 25)

PRODUCTION DRAWING

A production contains the elements as required and mentioned in Section 22.1. Figure 22.13 shows a production drawing of a V grooved pulley. Note the following characteristics in this drawing: ∑ Border is drawn on all sides with more margin on left hand side for filing purpose. ∑ Centering marks are indicated on all the four sides along with their numbers. ∑ Title block is different than classroom title with details of the persons and dates. ∑ The drawing contains size, limits and geometric tolerance with a datum wherever required. ∑ Surface finish of the surfaces indicated and also a general note about it in the title block.

22.12

PROCESS SHEET

A process sheet is a table that indicates sequence of operations along with machines, tools/gauge required and time to complete an operation. Each operation is numbered as 1, 2, 3, etc. or 5, 10, 15, etc. at intervals of 5 so that some more operations could be inserted if required. Following are the contents of a process sheet: 1. Job description - Name of the part 7. Operation number 2. Code number of part or part number 8. Machine on which it is to be produced 3. Required material size and specifications 9. Type of operation 4. Number of parts required 10. Tool required 5. Cycle time in minutes 11. Inspection gauge required 6. Initial inspection or setting time 12. Time for each operation

Production Drawings

471

Fig. 22.13 Production Drawing of a V Grooved Pulley

In any industry, there are many sections like machine shop, welding shop, forging shop, inspection section, etc. These are called work centers. Each work center may have many machines. For example, machine shop may have some lathes which are numbered as L1, L2, L3, … etc. drill machines as D1, D2….etc., shaper machines as S1, S2, S3, …. etc. These are called work places or machines which are mentioned in the process sheet. Example 2 Create a process sheet to make a grooved pulley for single V belt of bore 25 mm and outside diameter 90 mm as shown in Fig. 22.13. Solution The process sheet is shown in Table 22.3. Table 22.3 Process sheet for a V groove pulley Description of work Drawing Number Material

: Pulley : 123 : Cast iron

Operation No.

Machine

05 10 15 20

– L1 L1 L1

Number required Cycle time Pulley diameter Operation Inspect the casting Turn outside Ø90 mm Face one side Set compound slide

: 15 : 81 Minutes : 90 mm

Tool/gauge

Time—minutes

Visual Turning tool/Gauge Facing tool –

2 6 10 2 (Contd.)

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Table 22.3

(Contd.)

Operation No.

Machine

Operation

Tool/gauge

Time—minutes

25

L1

Cut groove one side

Grooving tool

5

30

L1

Set compound slide

–

2

35

L1

Cut groove other side

Grooving tool

5

40

L1

Drill

Drill f10, f15, f20

45

L1

Bore Ø24 mm

Boring tool

8

50

L1

Set for other face

–

3

55

L1

Face other face

Facing tool

60

–

Ream Ø25 mm

Reamer

7

65

Broaching

Keyway

Broach

4

70

–

Inspect pulley

Bore – Plug gauge

1

16

10

CAD 22.13

TITLE BLOCK

AutoCAD can facilitate work of making borders, centering marks and title block very quickly (Fig. 22.14). On Standard toolbar, click File and then from pull down menus, choose New. It displays Select Template dialog box showing many options in a window to choose. Let the Look in combo box remain as Template. Scroll down and select ISO A3 – Named Plot Styles. Click Open button at right hand bottom corner. A sheet with border, centering marks and grid reference numbers and a default title block is displayed on the screen. Alternately, MVSETUP command can also be used as follows: Command: MVSETUP ø Enable paper space? [No/Yes] : ø Enter an option [Align/Create/Scale viewports/Options/Title block/Undo]: T ø Enter title block option [Delete objects/Origin/Undo/Insert] : I ø

Available title blocks are: 0: None

1: ISO A4 Size (mm)

2: ISO A3 Size (mm)

3: ISO A2 Size (mm)

4: ISO A1 Size (mm)

5: ISO A0 Size (mm)

6: ANSI-V Size (in)

7: ANSI-A Size (in)

8: ANSI-B Size (in)

9: ANSI-C Size (in)

10: ANSI-D Size (in)

11: ANSI-E Size (in)

12: Arch/Engineering (24 ¥ 36 in)

13: Generic D size Sheet (24 ¥ 36 in)

Choose a size from the window, say ISO A3. The screen looks as shown in Fig. 22.14.

Production Drawings

473

Fig. 22.14 ISO A3 Template

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

How does a production drawing differ from classroom drawing? What are the contents of a title block of a production drawing? What are the various types of manufacturing processes? Compare sand and die casting. Describe in brief the various forming processes. What are the types of welding processes? Describe arc and gas welding. Differentiate between brazing and soldering and their typical use. List the various machines used for material removal processes and the type of work which a machine can perform. How is metal removed in chemical processes? Describe chemical etching process and its use. Explain the various types of surface finishing processes. Differentiate between hardening, tempering, annealing and normalizing. What is meant by CNC machine? How does it work? Describe various tool angles with the help of a neat sketch. Name the various tools used for a lathe and their function. Differentiate between lathe and milling tools. Describe various types of milling cutters used on horizontal milling machine. What types of tools are used for vertical milling machine. What types of works can be performed on this machine? What are the various types of drills? Write their typical use. Sketch a broaching tool and explain its working. What types of works can it perform? What is a hob cutter? Describe its use. How is a part inspected in a shop floor? Describe main types of gauges used. Explain use of a jig with the help of a sketch.

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21. Differentiate between jig and fixture by an example. 22. Describe the salient features of an assembly drawing. 23. Differentiate between process sheet and production drawing.

CAD 24. Describe use of MVSETUP command. What for it is used?

FILL 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

IN THE

BLANKS

Name of the approving authority of a drawing is given in . . Decreasing size of material by passing through rollers is called process uses a die through which material is pulled. point cutting tool and is made of . Milling cutter is cutting points and the angle between them is A drill has . Zinc plating is also called Hob cutter is used to cut in mass production. process of surface finishing uses stone sticks. . Annealing is done to make the metal or welding. Thin plates can be joined without gas by process. Printed Circuit Boards are made by . Tool angles are given so that a tool cuts machine. A center drill is used on machine. End mill cutter is used on cutter. Splines are cut by a cutter is used to cut gears in mass production. size of a part. Comparators are used for . A tool is guided in a Irregular jobs can be fixed on a machine by . drawing. Parts are numbered on . Cycle time is mentioned over a

CAD 22. A drawing with frame and title block can be inserted using

command.

MULTIPLE CHOICE QUESTIONS 1. Name of the person who draws a drawing is written in (a) process sheet (b) title block (c) border area (d) none of the above 2. Die casting is a (a) drawing process (b) material forming process (c) heat treatment process (d) casting process

.

Production Drawings

475

3. Hobbing is a (a) casting process (b) chemical process (c) material removal process (d) forming process 4. An electrolyte to transfer metal is used in (a) galvanizing (b) etching (c) Electric discharge machining (d) Electro-chemical machining 5. Thick sections are welded by (a) gas welding (b) arc welding (c) spot welding (d) brazing 6. Long flat surfaces can be produced by (a) shaper (b) horizontal milling machine (c) broaching machine (d) planner 7. Splines can be cut easily on (a) lathe (b) hobbing machine (c) broaching machine (d) shaper 8. Ferric chloride is used in (a) electroplating (b) etching (c) electro-chemical machining (d) EDM 9. Normalizing is done to make the job (a) less hard (b) machinable (c) soft (d) strong 10. Slab cutter is used on (a) lathe (b) shaper (c) horizontal milling machine (d) slotter 11. Which of the following is not used for checking size in mass production? (a) pneumatic gauge (b) snap gauge (c) vernier (d) comparator 12. A jig is used for (a) supporting the job (b) holding the job tightly (c) guide the tool (d) all given in a, b and c 13. Assembly drawings mention (a) fits and tolerances (b) surface finish (c) overall and center to center distances (d) part details 14. A process sheet tabulates (a) machine details (b) machining operation (c) tools/inspection gauges (d) all given in a, b and c 15. A production drawing has (a) size tolerances (b) geometric tolerances (c) surface finish (d) all given in a, b and c

ANSWERS to Fill in the Blank Questions 1. 5. 9. 13. 17. 21.

Title block two, 118° soft Lathe checking process sheet

2. 6. 10. 14. 18. 22.

Rolling galvanizing spot, seam vertical milling jig MVSETUP

3. 7. 11. 15. 19.

Drawing gears Chemical etching broach fixture

4. 8. 12. 16. 20.

multi, HSS Honing effectively Hob assembly

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ANSWERS to Multiple Choice Questions 1. (b) 7. (c) 13. (c)

2. (d) 8. (b) 14. (d)

3. (c) 9. (b) 15. (d)

ASSIGNMENT

ON

4. (a) 10. (c)

5. (b) 11. (c)

6. (d) 12. (d)

PRODUCTION DRAWINGS

1. Figure 22.P1 shows a flange of a coupling. Draw its production drawing along with a process sheet. Specify suitably the tolerance of hole, geometric tolerances for concentricity of outside diameter and hole, perpendicularity of face and hole axis, surface finish and material.

Fig. 22.P1 Flange of a Coupling

Fig. 22.P2

Half Sleeve Bearing

2. Half sleeve bearing is shown in Fig. 22.P2. Draw its production drawing with specifications of material, surface finish tolerances on hole and outside diameters. Specify suitable geometric tolerances and surface finish. Suggest a process sheet with cycle time. 3. A liner is to be fitted in a cylinder as shown in Fig. 22.P3. Suggest a suitable fit between them along with limits of their sizes. Mention these tolerances on the drawing along with surface finish and geometrical tolerances. Draw production drawing of each part separately.

Fig. 22.P3

Cylinder and Liner

Production Drawings

CAD ASSIGNMENT

ON

477

PRODUCTION DRAWINGS

4. A hydraulic cylinder is shown with a piston inside in Fig. 22.P4. Recommend a suitable fit between these parts. Draw both the parts separately for their production drawing giving tolerances and geometric tolerances required and surface finish. Prepare a process sheet also to make the cylinder.

Fig. 22.P4

Cylinder and Piston

5. A taper sleeve shown in Fig. 22.P5 is to be made in mass production. Draw its part drawing giving all the production details and prepare a process sheet.

Fig. 22.P5

Taper Sleeve

6. A bearing block shown in Fig. 22.P6 is to be made. Suggest a suitable fixture to drill its hole for the bearing. Give the required tolerances and geometric tolerances along with roughness and prepare a process sheet.

Fig. 22.P6

Bearing Block

478

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PROBLEMS

FOR

PRACTICE

7. Figure 22.P7 shows a flange fitted on a shaft with a key in between. Suggest a suitable fit between keyway and key, and key and flange hole. Specify the tolerances for the shaft and hole. Draw production drawings for the shaft and the key separately with all the tolerances and finish symbols. Specify suitable material also for all the three parts.

Fig. 22.P7

Shaft and Flange

8. A piston with gudgeon pin to fix connecting rod is shown in Fig. 22.P8. Specify the type of fit between the mating components. Draw production drawing of piston only with tolerances, geometrical tolerances, surface finish. Suggest a suitable gauge to check grooves of piston rings. Specify material and surface finish. Create a process sheet to manufacture it.

Fig. 22.P8

Piston and Gudgeon Pin

PART E Machine Parts CHAPTER

23

Springs Springs store energy when deflected and come back to original shape releasing energy when the load is removed. Springs can be compression, tension or torsion type. A coil spring consists of many coils of wire (generally circular) in a helical fashion. A compression spring has gap between the coils and the wire is inclined at a helix angle. The length of spring without any load is called free length. When it is fully compressed its length decreases to solid length and each coil touches each other. A conical helical spring is one in which the coil diameter goes on decreasing along its length. It offers variable stiffness. Ends of a compression springs can be open, closed or square. A coil type tension spring has coils touching each other. Its ends can be full loop or half loop. Its length increases when the load is applied. A helical torsion spring is closed wound and may or may not have gap between oils. These are used to take torsional loads. The ends may be straight or circular or one straight and one circular. Some torsion springs are dual type; half having LHS turns and half RHS turns with a radial extension in the middle. A leaf spring is a set of many leaves of varying length from increasing length to decreasing lengths kept in position by clamps and a center bolt. The topmost leaf is called the master leaf and can have eye ends also. The leaves are of rectangular cross-section. Its semi-elliptical shape is most common. A full elliptical spring can be considered as two semi-elliptical springs joined at ends. It is more flexible than semi-elliptical. Quarter elliptical spring is half of elliptical spring having eye end on one end and clamped at the other end. Three quarter elliptical is a combination of half elliptical and quarter elliptical springs. A torsional spring is a thin steel strip wound as a spiral. Its one end is curved to take the reaction and the central end is straight to which the load is applied. A diaphragm spring is made of thin spring steel disk having conical shape. It can be without slots or can have radial slots to increase flexibility. When load is applied axially, it is deflected towards flat shape. It offers variable stiffness and is used for clutches of automobiles.

CAD CAD can be quite helpful in drawing springs as many useful tools are available which makes the drawing very easy. An example to draw a helical spring is demonstrated. HELIX command can be used to draw a 3D spring.

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23.1

INTRODUCTION

Spring is an elastic machine element to store energy when deflected and releases energy when it comes to its original position. They are used for example in automobiles to absorb the road shocks. Spring is used in a spring balance to measure the load. It can also be used to keep any machine element pressed or stretched and many other applications. Springs are available in many shapes and each has a typical application.

23.2

CLASSIFICATION

A According to Tension spring Compression spring Spiral spring

Deflection due to Load It gets elongated when load is applied. It gets compressed when load is applied. It gets twisted when load is applied.

B According to Geometrical Shape Helical spring The wire is wound in a helical fashion (Fig. 23.1). Leaf spring Rectangular strips of steel are bent in the shape of an arc and joined together (Fig. 23.8). Torsion spring A thin steel strip is wound in a spiral form (Fig. 23.9). Torsion bar A long rod is fixed at one end and twisted at the other end. Diaphragm spring A steel disk bent in the shape of a saucer (Fig. 23.11). 23.3

HELICAL SPRING

Helical springs are made of steel wire (generally circular in cross-section) bent in a helical form. The different terms used with these springs are (Refer Fig. 23.1): Wire diameter (d) Diameter of the wire with which a spring is made. Coil diameter (D) Mean diameter of the helical coil. Outside diameter Coil diameter plus wire diameter. Inside diameter Coil diameter minus wire diameter. Number of turns (n) Number of complete turn of the coils in a spring. Pitch (p) Axial distance between centers of two adjacent coils. Helix angle (a) Angle which the coil makes with a line normal to axis of spring. Free length (FL) Total axial length between two extreme coils without load. Solid length (SL) Total axial length between two extreme coils when fully compressed. Maximum deflection Maximum displacement which a spring can take (FL – SL). Stiffness Load per unit deflection. Shape of ends The ends may be gaped, closed or ground (Fig. 23.2).

23.3.1

Compression Spring

A compression spring is just a coil wound in helical fashion (Fig. 23.1A). The visible side of the wire is shown at the helix angle (a). The rear portion of the coil is visible partially, as it is covered by front coils. The rear coils are also inclined at same helix angle but are opposite to the front coils. If the spring wire is circular, it is visible as semicircular at sides and flat, if the wire is rectangular. To draw a compression spring of circular wire, draw a center line and then draw semicircles (having curvature opposite to center line) of given number of turns (n) and radius (d/2) at a distance of coil

Springs

481

radius (D/2) from the center line, spaced at a distance of (p). Draw another set of semicircles facing opposite to the previously drawn semicircles but displaced axially by a distance (p/2). Join the semicircular arcs by inclined lines . Then draw the lines for the rear side of the spring as shown in Fig. 23.1. The ends may be terminated at the center line after desired number of coils. The other view of the spring is just two circles of outside diameter = D + d and inside radius = D – d. If the number of turns are large, drawing of spring may be time consuming and laborious. Drawing of such a spring can be represented by a few turns at the ends and then the middle turns can be just by straight dashed lines.

Fig. 23.1

A Helical Compression Spring

Fig. 23.2

Ends of a Compression Spring

If the outside diameter of all coils remains constant, it is called a cylindrical helix or generally only helical spring. If the diameter changes along the axis, it is called conical helix. It is shown in Fig. 23.1B. It offers variable stiffness during deflection; less stiffness at small deflections and more at large deflections. Ends of compression spring may have any of the following shapes: a. Open Gap between the last and adjacent coil remains the same as the pitch (Fig. 23.2A) b. Closed The extreme ends are pressed to touch the adjacent coil so that there is no gap there (Fig. 23.2B). c. Ground The ends are ground on a grinding machine to make them square with its axis (Fig. 23.2C).

23.3.2

Tension Spring

These are drawn in the same way as the compression spring, but the pitch of the coil is same as wire diameter, i.e. p = d and hence all the coils touch each other on sides. Since there is no gap between the coils hence the rear portion of the spring is not visible (Fig. 23.3)

Fig. 23.3

A Tension Spring (Full Loop End)

Fig. 23.4

Ends of a Tension Spring (Half Loop End)

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Type of ends used for tension spring are different than compression spring. Two types of ends are used for these springs: full loop and half loop. A full loop end is circular (Fig. 23.3) while half loop is semicircular with smooth curvature with the coils (Fig. 23.4)

23.3.3

Torsion Spring

These springs are wound either with closed coils, i.e. no gap between the coils similar to tension springs or with some gap also. Ends of the coils may be either straight or circular or one straight and other circular as shown in Fig. 23.5. Load is applied at right angles to the axis so as to twist the coil. For these springs, stiffness is given by torque/degree of rotation angle. Maximum deflection is also given in terms of degrees from its free position. Some of the torsion springs are made dual, i.e. left half has LHS coils and right half has RHS coils. In addition to two ends, there is a third radial extension of the spring (Fig. 23.6). The torque is applied in the center while the extreme free ends take the reaction.

Fig. 23.5 A Torsion Spring

23.4

Fig. 23.6

A Dual Torsion Spring

LEAF SPRING

A leaf spring is made of rectangular steel strips of different lengths (called leaves) clamped together in lengths of descending order; longest strip at the top and the smallest at the bottom. Each leaf is given a curvature. The length, cross-section and number of strips depend upon the load to be supported. The topmost leaf has the ends to form a circular eye (Fig. 23.7) for fixing purpose; however, the ends can be straight also for some applications. Leaves are kept in their position by a center bolt and clamps along the length to keep leaves below each other and do not allow the leaves to rotate.

23.4.1

Shapes of Leaf Springs

A leaf spring can have many shapes like quarter elliptical, semi-elliptical, three quarter elliptical and full elliptical

A Semi Elliptical This shape is most generally used. It is so called because its shape is like a semi-ellipse (Fig. 23.8A). Ends are supported and load is applied at the center. Its use can be seen in most of the heavy duty vehicles for rear suspension of the wheels. B Elliptical Two semi-elliptical springs are joined together end to end to form an elliptical shape (Fig. 23.8B). This combination offers more deflection than semi-elliptical for the same load.

Springs

Fig. 23.7

483

Components of a Leaf Spring

C Quarter Elliptical This shape is just half of the semi-elliptical spring. That is why it is called quarter elliptical spring. One end has an eye end and the leaves are clamped in the center (Fig. 23.8C). D Three Quarter Elliptical It is a combination of semi-elliptical and quarter elliptical to form a three quarter elliptical spring. It is shown in (Fig. 23.8D).

Fig. 23.8

Shapes of Leaf Spring

E Torsional Spring These springs are used to store torsional energy, e.g. winding of a watch or clock. A steel strip is wound in the form of a spiral (Fig. 23.9). One end is curved to take the reaction and torque is applied at the central end which is straight. 23.4.2

Drawing a Leaf Spring

Following dimensions should be known for drawing a leaf spring: ∑ Radius of curvature

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∑ ∑ ∑ ∑ ∑ ∑

Number of leaves Length of each leaf Width and thickness of leaves Number of clamps Shape of ends If ends are circular then center to center distance between the eyes and eye diameter

To draw a leaf spring, adopt the following method: 1. Draw center line of the spring and mark centers of the eye ends at right angles to this axis. 2. Draw a circle at the center of the eye end of eye diameter. Make another concentric circle for the eye ends at distance equal to the Fig. 23.9 Torsional thickness of the leaf. Spring 3. Open compass equal to radius of curvature and set its center on the center line such that it is tangential to the circles of the eye. 4. With the same center but radius increased by leaf thickness every time, draw arcs of decreasing lengths for given number of leaves. 5. Draw the center bolt and clamps at the appropriate places.

23.5

CONVENTIONAL AND SYMBOLIC REPRESENTATION OF SPRINGS

On drawings where the number of springs is large, drawing a spring in its actual shape is too much time consuming and laborious. Hence springs are shown either conventionally or symbolically. The wire is represented just be a line inclined at helix angle. Conventions followed to draw a compression spring (a), a tension spring (b), torsion spring (c) and a leaf spring (d) are shown in Fig. 23.10.

Fig. 23.10

Symbolic Representations of Springs

Springs

23.6

485

DIAPHRAGM SPRING

These are made of thin steel disk having conical shape. It is also called Belleville spring. It stores energy when pressed axially. It may be an annular disk as shown in Fig. 23.11A or with radial slots (Fig. 23.11B). The radial slots increase its flexibility. Its important dimensions are outside diameter, inside diameter, cone angle and thickness of the disk.

Fig. 23.11 Diaphragm Springs

CAD AutoCAD can be helpful in drawing a spring. Draw only one coil of a helical spring and rest can be copied as a rectangular array using ARRAY command as demonstrated in Example 1. Example 1 Draw a helical coil compression spring of 8 turns for a wire diameter of 5 mm and coil diameter 30 mm. Gap between the coils is 5 mm. Solution Wire diameter is 5 mm and gap between coils is 5 hence pitch = 5 + 5 = 10 mm. 1. Draw a vertical center line of length 80 mm as length of spring (8 ¥ 10). 2. Use OFFSET command and draw two more lines at distance of 15 mm (half of the coil diameter) on each side of the center line. (Fig. 23.S1A) 3. At the lower end of the left line, draw a circle of 2.5 mm radius (half of the wire diameter). 4. Copy this circle at two more points using COPY command. Use the base point of circle as center of the circle and second point as @30,5 and for the other circle as @0,10. 5. Using Tangent object snap draw lines 1-3, 2-4, 3-5 and 4-6 (Fig. 23.S1A) Fig. 23.S1 Drawing a Helical Spring Using CAD 6. Using TRIM command trim these circles. Select the boundary for trimming as these inclined lines and object to trim as the circle on the inner side within the lines as shown in Fig. 23.S1B. 7. Trim the lines 3-5 and 4-6 also using TRIM command. 8. Using ERASE command, delete the arc 5-6.

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9. Use ARRAY command to complete the spring. Specify the data as follows: Type of array — Click on Rectangular radio button Number of rows — 8 Number of columns — 1 Row offset — 10 (5 mm gap between wires and 5 mm as wire diameter) Click preview button. If satisfied, click OK button. Figure 23.S1C is displayed as a coil spring. * HELIX command has come recently in AutoCAD 2007, which can draw a 3D spring directly. (See Chapter 30).

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8.

What is the function of a spring? Classify springs. Differentiate between conventional and symbolic representation of a spring. What are the various types of coil springs? Differentiate between each type. What is meant by ends of springs? Describe the various ends used for compression type coil spring with sketches. Differentiate between a coil and a leaf spring. What are the parameters deciding the stiffness of a leaf spring? What is a torsion spring? Give two examples of any type by sketches. Differentiate between coil and diaphragm spring.

CAD 9. Describe the method to draw a compression type helical spring using CAD. 10. How can a leaf spring be drawn using CAD?

FILL

IN THE

BLANKS

Fill up the blanks by appropriate word(s) when load is applied. 1. A tension spring gets spring. 2. A spring with many leaves of reducing lengths is called 3. A diaphragm spring is of the shape of a . 4. If coil diameter of a helical spring reduces, such as spring is called as 5. Ground end of a helical spring make degrees with its axis. 6. Gap between coils of a tension helical spring without load is . symbol. 7. A compression type helical spring is represented by 8. in a leaf spring are used to keep the leaves aligned at their position.

CAD 9. ___________ can be used to copy many coils of a helical spring. 10. In ARRAY command, row offset is _______________.

spring.

Springs

487

MULTIPLE CHOICE QUESTIONS 1. A spring is used to (a) as energy reservoir (b) support vibrating parts (c) to take load (d) for all mentioned in a, b and c 2. Material for a leaf spring is (a) cast iron (b) mild steel (c) spring steel (d) aluminum 3. Stiffness of a spring remains (a) always constant (b) always varies (c) depends upon type of spring (d) constant up to certain deflection then varies 4. Cross-section of material for a leaf spring is (a) rectangular (b) circular (c) square (d) thin plate 5. Shape of a diaphragm spring is like (a) circular disk (b) saucer (c) spiral (d) helical 6. Generally gap between coil of a tension type coil spring is (a) equal to wire diameter (b) two times the wire diameter (c) 1.5 times the wire diameter (d) no gap 7. Solid length of a helical compression spring is when (a) wire used for spring is solid circular (b) gap between two coils is zero (c) inside diameter is zero (d) there is no load on the spring

ANSWERS to Fill in the Blank Questions 1. elongated 5. 90 9. ARRAY

2. leaf 3. saucer 6. zero 7. 10. distance between two rows

4. conical 8. clamps

ANSWERS to Multiple Choice Questions 1. (d) 7. (b)

2. (c)

3. (c)

ASSIGNMENT

4. (a)

ON

5. (b)

6. (d)

SPRINGS

1. Draw two views of a compression type coil spring with the following data: Wire diameter = 8 mm Coil diameter = 80 mm Pitch = 15 mm Number of turns = 7 Type of ends is square 2. A semi-elliptical spring has 6 leaves of size 100 ¥ 6 mm cross-section. Its master leaf has circular eye ends of radius 15 mm. Center to center distance between the eyes is 1000 mm and radius of curvature is 1200 mm. All leaves are joined with a center bolt of 15 mm diameter of a suitable length. Four clamps are used (2 on each side) at equal distance made of 50 ¥ 3 mm flat. Draw the assembly to a suitable scale.

488

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CAD ASSIGNMENT

ON

SPRINGS

3. Draw two views of a tension type coil spring with the following data Wire diameter = 5 mm Coil diameter = 60 mm Number of turns = 12 Type of ends as full loop 4. Draw a full elliptical spring for the data given in question 2 above. Hint: Draw only 1 leaf and then use OFFSET command to draw other leaves. Use TRIM command to set their lengths. Use MIRROR command to copy semi-elliptical to full elliptical spring.

HOMEWORK 5. Draw a torsion leaf spring with the following data Strip size 25 ¥ 1 mm Number of turns 10 Starting radius 20 mm Ending radius 120 mm Central end straight of straight length 30 mm and Last turn with an eye end of radius 5 mm 6. Draw a diaphragm spring of outside diameter 200 mm, inside diameter 80 mm, thickness 1.5 mm, 8 radial slots of width 10 mm and cone angle 160 degrees. Assume a suitable length of T.

PROBLEMS

FOR

PRACTICE

7. Draw two views of a conical helical compression type coil spring with the following data Wire diameter = 10 mm Coil diameter at base = 100 mm Coil diameter at top = 50 mm Pitch = 18 mm Number of turns = 12 Type of ends is ground 8. Draw a conical helical spring with material as flat of cross section 1 ¥ 10 mm (10 mm along its axis) with the following data Coil diameter at bottom 80 mm Coil diameter at top 40 mm Pitch 12 mm Number of turns 8 Type of ends open

CHAPTER

24

Belts and Pulleys Belts are used to transmit power from one pulley to another pulley. Belts are put as open belt if direction of rotation of driver pulley is same as that of driven pulley. Sometimes an idler pulley is provided as belt tensioner. If the direction of rotation of driven is opposite to driver, then the crossed belt arrangement is used. Quarter twist belt is used for shafts, whose axes are at 90° to each other. Belts are of many types. Flat belts can run on small pulleys. They can be rectangular, grooved or toothed for non-slip drive. V belts are generally used in industry. They are available in large variety of standard sizes, offer more speed ratios than flat belts, have long life, offer quiet operation and low maintenance. They are designated by its cross-section letter A, B, C, D and E followed by inside length. Angle of all belts is same as 40°, only the width and height varies. Ropes of circular cross-sections were used in olden days but now replaced by flat and V belts. Pulleys for flat belts have slight camber on the outer periphery to keep the belt aligned centrally over the pulley. Small pulleys are made solid while medium sized pulleys are webbed between the rim and hub. The web is sometimes provided with holes to make it light and save material. Larger sized pulleys have arms between rim and hub, which can be straight radial or curved. Very large pulleys are built up in two halves and then joined together. Stepped pulleys are used where different speed of the driven shaft in 3 or 4 steps is required. Fast and loose pulleys are used when it is desired to disconnect power, whenever desired. V belts use grooved pulleys. Number of grooves can be increased up to 8, if power transmission is more. The cross-section depends upon the type of V belt (A, B, C, etc.). Grooved stepped pulleys are also used for the same purpose as for flat stepped pulleys. Toothed pulleys are to be used along with toothed belt for non-slip transmission. Smaller pulley is provided with collar on sides so that it does not slip on sides while bigger pulley need not have collars. Pulleys for ropes are of large size and have one or more grooves of 45°. These are of radial arm construction.

CAD Parametric drawings use AutoLISP programming language. This is useful where the shape is same but the size changes for different drawings, e.g. belts, rolling bearings. In this language, program starts with an opening parenthesis followed by Defun and then name of function. The program ends with a closing parenthesis. Each instruction is written within parentheses. Comment line starts with a semicolon (;). User is prompted for input by Prompt command. Setq command is for assigning a value to a variable. Getpoint is used to get coordinates of a point clicked by mouse. AutoCAD command is executed by Command keyword. Mathematical operations are done by first writing the symbol (+, –, *, /) followed by the numbers with spaces. Program can be branched by If command. If instructions for If are more than one, Progn function can be used.

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24.1

INTRODUCTION

In machines, mechanical power is transmitted from one component to another depending upon the requirement. Various types of power transmission systems are available. Important ones are belt and pulleys, gears (Chapter 26), etc.

24.2 BELTS Belts are used for power transmission from one pulley to another. If the direction of rotation of the driver and driven is same, the open belt arrangement is used (Fig. 24.1A). One side of the belt is called tight side while the other is called slack side. Tight side should be kept at bottom. Sometimes an idler is used inside or outside the belt to keep the belt tight over the pulleys (Fig. 24.1B). If the direction of rotation of the driven is opposite to the driver, then crossed belt arrangement is used (Fig. 24.1C). If the axes of the driver and driven are at 90°, the arrangement is shown in Fig. 24.1D. Center distance between the pulleys has to be long for this arrangement.

Fig. 24.1

Belt Arrangements

Belts with rectangular cross-section are called Flat belts (Section 24.2.1). V shaped belts are called V belts (Section 24.2.2). In olden days a rope of circular cross-section was also used (Section 24.2.3).

24.2.1 Flat Belts These are low cost and can run on small pulleys. They have a tendency to slip and hence need high tension resulting in high bearing loads. They are noisier than other belts and have low efficiency at moderate speeds. These are made of leather, fabric, rubberized fabric, non-reinforced rubber/plastic, reinforced leather, etc. Flat belts are of three types: A Plain Flat Plain Flat belts are of rectangular cross-section with no teeth or groove (Fig. 24.2A). B Grooved or Serrated Grooved or Serrated belts have longitudinal grooves of V section adjacent to each other made on the inside periphery of the belt (Fig. 24.2B). The power transmission capacity increases due to wedge formed by V groove. Sometimes they are also called poly V belts.

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491

C Toothed Belts Toothed belts have teeth on the internal periphery of the belt along length (Fig. 24.2C) which fit into the similar size of teeth on the pulley. They are used for positive drive, e.g. cam shaft of an engine.

Fig. 24.2

Types of Flat Belts

24.2.2 V Belts [IS 2494 (1974)] V belts are most commonly used in industry. These are available in large variety of standard sizes and types. They offer more speed ratios than flat belts, have long life (3-5 years), offer quiet operation and low maintenance. Best speed for operation for belts is between 8 to 30 m/s. Ideal speed for standard belt is 23 m/s and for narrow belts 50 m/s. V belts are made in two sizes; conventional and narrow. Conventional belts are designated as A, B, C, D and E. Width and thickness of these belts is shown in Fig. 24.3 while the angle for all is 40°. More than one belts are used to increase power transmission capacity. Number of belts on one pulley should not be more than eight. If the number is more than eight, then larger section should be chosen. Narrow belts are designated as 3V, 5V and 8V.

Fig. 24.3

V Belt Cross-Sections

V belts are designated by its cross-section letter A, B, etc. followed by inside length. For example A1262 means a belt with cross-section as A and inside length 1262. Table 24.1 shows the sizes of various belts and the range of inside length (minimum to maximum). Table 24.1 Width Height Min. length Max. length

24.2.3

5 3 154 860

6 4 212 1262

8 5 296 1916

10 6 420 2820

Size of V belts 13 8 585 4245

17 11 832 6332

22 14 1650 14050

32 19 2303 18063

38 23 3230 18080

Ropes

Ropes of circular cross-section were used in olden days but have been replaced by flat and V belts.

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24.3 PULLEYS They are used to transmit power with help of belts. A pulley is a circular machine element having a hub in the centre with a keyway that fits on the shaft. The outer rim is plain or arced for flat belts (Fig. 24.4A). For a grooved pulley, the number of grooves can be one or more than one (Fig. 24.4B). The belt allows a certain amount of slip and hence they do not transmit positive power (no slip). Toothed belts use a pulley (Fig. 24.4C) that have teeth on its outside periphery to fit in the teeth of the belt for positive power transmission.

Fig. 24.4

Various Types of Pulleys

Small pulleys can be made of forged steel, but large pulleys are made of Cast Iron. Aluminum pulleys are used where light construction is required. The rim is connected to the hub either by a solid disk or spokes for large diameter pulley. The solid disk is sometimes provided with a web between hub and rim with holes to reduce weight of the pulley.

24.4

TYPES OF PULLEYS

Pulleys are classified in many ways as follows: A. According to shape: ∑ Flat (Section 24.5) ∑ Grooved (Section 24.6) ∑ Stepped (Section 24.5.5 and 24.6.3 ) B. According to driving belt: ∑ Flat belt – The belt is of rectangular section. ∑ Single V belt – A single belt having 40° angle called as V belt is used (Section 24.6.1). ∑ Multi V belt – Many V belts are used in parallel for more power transmission (Section 24.6.2). ∑ Toothed belt – It has teeth on the inside periphery and is used for positive drive (Section 24.7). ∑ Rope – A circular rope is used (Section 24.8). C. According to construction: ∑ Solid pulley – Pulley is one solid casting or forging (Section 24.5.1). ∑ Webbed pulley – A disk is provided between hub and rim (Section 24.5.2). ∑ Armed pulley – It has rim for belt, hub in the center and arms joining hub and rim (Section 24.5.3). ∑ Built up pulley – Large pulleys are fabricated by joining two or more pieces (Section 24.5.4). D. According to use: ∑ Fast and Loose pulley – Fast pulley is keyed to a shaft while loose pulley has no key (Section 24.5.6). ∑ Fast pulley – Fast pulley is keyed to a shaft (Sections 24.5 and 24.6). ∑ Idler pulley – Does not transmit power. Generally used for providing belt tension.

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24.5 FLAT BELT PULLEYS Flat belt pulley has its outer periphery almost flat. A slight camber is given to make the outer periphery slightly convex (Fig. 24.4A). This helps in keeping the belt positioned centrally on the pulley. Flat pulleys are of many types, e.g. solid, webbed, straight armed, curved armed, built up, stepped, fast and loose. Each type is described in the following sections. 24.5.1 Solid Pulley Small pulleys are made solid as shown in Fig. 24.4 and its sectional view in Fig. 24.5. The hole is of the shaft size according to the fit required and has a keyway or a collar to fit by a screw. 24.5.2 Webbed Pulley Medium sized pulleys are made webbed to reduce weight and save material (Fig. 24.6). Sometimes the web is provided with holes to decrease weight and save material.

Fig. 24.5

Solid Pulley

Fig. 24.6

Webbed Pulley

24.5.3 Armed Pulley Large sized pulleys have a rim and arms joining the rim and central portion called the hub. The arms can be straight (Fig. 24.7A) or curved (Fig. 24.7B). Number of arms can be 4, 6 or more depending

Fig. 24.7A Straight Arms

Fig. 24.7B

Curved Arms

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upon the size of pulley. Cross-section of the arm is generally elliptical but can be circular. The hub has hole and keyway to suit the shaft size.

24.5.4 Built Up Pulley Very big pulleys are made as built up type. Both hub and rim are made in two halves and then joined together with bolts and nuts (Fig. 24.8). Hub and rim are provided with radial holes. Arms are of circular cross-section with outer end having a collar and step at one end and inner end is stepped. Inner ends of the rods are shrunk fit into holes of the hub. The rim is supported by the collar at outer end of the rod and the stepped portion of rod is riveted to the rim. The rim halves are joined by a curved strap on the inside periphery of the rim. The strap is riveted to one side of the rim using counter sunk rivets and bolted by a counter sunk bolt at the other side. The two halves of hub are bolted on shaft with a key in between.

Fig. 24.8

Built up Pulley

24.5.5 Stepped Pulley With stepped pulleys, speed of the driven pulley can be changed by changing the position of belt from one step to another step. The steps are generally three or four (Fig. 24.9). More than four are not commonly used. The pulleys are so arranged that the belt at smallest step of the driver aligns with biggest step of driven pulley. Their sizes are so adjusted that the belt length required remains same. 24.5.6 Fast and Loose Pulley Sometimes two pulleys are adjacent to each other. One of the pulleys is keyed to the shaft called fast pulley, while the other is without key called loose pulley. Loose pulley runs freely on a bush without transmitting power to shaft. This arrangement is known as Fast and Loose pulley (Fig. 24.10). Whenever the power transmission is not required, belt is made to slip from fast to loose pulley. Some machine tools use this type of power transmission to connect and disconnect power as required.

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Fig. 24.9 Stepped Pulley

Fig. 24.10

24.6

Fast and Loose Pulley

GROOVED PULLEYS

Grooved pulleys use V belts. These belts have a standard cross-section A, B, C, D and E. The section to be selected depends upon the power to be transmitted. Section A is for less power and E for larger power. If the power to be transmitted is more than the capacity of one belt, two or more belts can be used in parallel. Table 24.2 gives minimum size and preferred diameter of the pulley required for different size of belts. Table 24.2 Cross-section A B C D E

Belt cross-sections and Pulley diameters (mm) Minimum pulley diameter 75 140 225 325 525

Preferred diameter 85 185 325 425 700

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Although the angle of the belt is 40°, but the angle of pulley groove is made lesser than 40°, so that the wedging action causes more gripping force between pulley and belt. Driver pulleys (generally smaller) have included angle between 32° and 34° and large pulley 36° and 38°. Depth of the groove is also kept about 10% more than the actual depth of belt so that when the belt gets compressed due to continuous use, it may not touch the bottom surface of the groove, else the wedging action will not take place. Grooved pulleys can have one groove or more than one if more power is to be transmitted.

24.6.1

Single Belt Grooved Pulley

Only one V belt is used (Fig. 24.11). It may be solid or webbed. The slip of the belt over the pulley is lesser than flat belts due to wedging action of the belt into the groove.

24.6.2

Multi Belt Grooved Pulley

Many V grooves are cut adjacent to each other (Fig. 24.12). Wall thickness between the groove varies from 3 to 5 mm. Outer wall thickness is kept slightly more than inner wall thickness. The gap between grooves depends upon the material also. Larger size of pulleys are webbed and holes are also made to make them light.

24.6.3

Fig. 24.11 Single Belt Groove Pulley

Stepped Grooved Pulley

Purpose of stepped grooved pulley is same as that of stepped pulley for flat belt. These pulleys are also used in pair, aligned such that belt of smallest pulley aligns with biggest pulley. Angle of groove is kept between 34° and 38° (Fig. 24.13). The groove is kept deeper than the height of belt. Size of groove depends upon cross-section of belt A, B or C. The pulley shown in Fig. 24.13 is for B belt.

Fig. 24.12

Multi Belt Groove Pulley

Belts and Pulleys

Fig. 24.13

24.7

497

Stepped Pulley for V Belts

TOOTHED PULLEY

Their use is limited to applications where slip may affect the performance of the machine. The outer periphery has teeth that fit into the grooves of the toothed belt to check slip. Such a pulley is shown in Fig. 24.14. Their use can be seen for cam shaft drive in I.C. engines. A collar is provided on the sides of driver to keep the belt within the width of the pulley, which may shift on sides due to slight misalignment of shafts.

Fig. 24.14

24.8

Toothed Pulley

ROPE PULLEY

They are not commonly used. Very large pulleys may use this type of drive. A rope of circular crosssection is used over the groove of pulley. Groove has 45° included angle. Figure 24.15 shows a pulley for rope drive. Two grooves are adjacent to each other for more power transmission.

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Fig. 24.15 Rope Pulley

CAD 24.9

AUTOLISP

It is a programming language which comes with AutoCAD. It can create drawings using a program programming written in this language. It is similar to any other language and can perform conditional branching using IF command. It can perform repeated looping actions like any other language. Aim of this section is not to teach this language but just appraise the powers of AutoCAD using this language. It is very useful where the shape remains same and only the size changes. For such situations, the variables can be assigned to varying dimensions and then changing the values of these variables will automatically create a drawing according to new dimensions. An idea of AutoLISP is given in Example 1.

24.9.1

Creating an AutoLISP Program

This is demonstrated by Example 1. Example 1 Write a program to draw two circles; one of red color with radius 40 mm and other of blue color of radius 55 mm with center (100,180), one square of 40 ¥ 40 in center of circle, a line joining diagonal corners and some text. Solution 1. AutoLISP program is written in windows notepad and NOT in MS word. To open note pad, click Start at left bottom corner of screen and then click in the following sequence.

Start

Programs

Accessories

Notepad

2. Start typing the program exactly as shown below. This program is to give results as shown in Fig. 24.S1A. Fig. 24.S1B shows the various names of variables used in the program. Be very careful about syntax errors. AutoLISP has a poor debugger and if any mistake is committed, it will be difficult to debug. Explanation of the program is given at the end of the program.

Belts and Pulleys

Fig. 24.S1A

499

Fig. 24.S1B

(defun C:C1() ; This function draws two circles one rectangle, one line and a line of text in different colors (graphscr) ;feed input data (setq x 100) (setq y 80) (setq r 40) (setq cen (list x y)) (setq cr1 '(80 60)) (setq cr2 '(120 100)) ; ;Use AutoCAD commands ;Draw circles with Circle command (command "color" "Red") (command "circle" cen r) (setq r (+ r 15)) (command "color" 4 ) (command "circle" cen r) (command "color" 5) ; ;Using Rectangle command (command "Rectangle" cr1 cr2) ; ;Using LIne command (command "Line" cr1 cr2 "") ; ;Using text command (setq h 6) (setq ang 0) (setq sp '(55 140)) (setq txt "My First Program") (command "Text" sp h ang txt) ; (prompt "Congratulations")

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(terpri) (prompt "for your first program\n") (prompt "Thanks"\n) )

3. Save the program with the name Tut1.lsp in any folder. Explanation of the program It may be noted that each instruction is written within parenthesis. The number of opening parentheses has to be equal to closing parentheses. If there is a difference of one, an error message 1> appears while executing the program. Meaning of each line is given on RHS in italics. (defun C1()

A function starts with an opening parenthesis ( Defun command defines a function whose name is given asC1. The parentheses ( ) having nothing in between indicate that all the required data for this function is available within the function.

; This function draws two circles one rectangle, one line and a line of text in different colors

;feed input data (graphscr) (setq x 100)

(setq y 100) (setq r 40) (setq cen (list x y)) (setq cr1 '(80 60))

A line starting with semicolon (;) is a comment line and does not take any part in execution of the program. It helps to give relevant information about the program. Comment line for information to programmer. This command starts a graphic screen. Setq command is used to assign a value to a variable. Here a value 100 is assigned to a variable x, which is the x coordinate of the center point of the circle. Assigns y coordinate of the center point. Assigns 40 to radius variable r. Combines x and y coordinates using list command to specify a center point to the variable name cen. Another way of assigning coordinate directly with apostrophes mark. Assigns (80,60) to variable cr1. Assigns (120,100) to variable cr2. A semicolon leaves a blank line for clarity of program.

(setq cr2 '(120 100)) ; ;Use AutoCAD commands ;Draw circles with Circle command (command "color" "red") Any AutoCAD command can be used in the program and it has to be en-

(command "circle" cen r) (setq r (+ r 15)) (command "color" 4) (command "circle" cen r) ; ;Using Rectangle command (command "color" 5)

closed in double quotes. Here color command is used to specify current color as red. Uses Circle command of AutoCAD to draw a circle with center cen and radius r. Adds 15 to the value of radius r for the second circle. Now r = 55. Sets color number 4. Another way of specifying color. Draws another circle of radius 55 mm.

Sets color number 5.

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501

(command "Rectangle" cr1 cr2) ; ;Using LIne command

Draws a rectangle with diagonal corners cr1 and cr2.

(command "Line" cr1 cr2 "")

Draws a line with points cr1 and cr2. Command is terminated by Enter key which is given by two double quotes.

; ;Using text command (setq (setq (setq (setq

h 6) ang 0) sp '(55 140)) txt "My First Program")

(command "Text" sp h ang txt) (prompt "Congratulations") (terpri)

Assigns height of text as 6 to variable h. Assigns angle of text as 0 to variable ang. Assign starting point of text as (55,140) to variable sp. Assign the string “My first program” to variable txt. Uses AutoCAD Text command with all required data.. Prompt command is used to display message at the command line. The message is specified within double quotes. This command is a short form of Terminate Print. Its effect is to transfer control to the next line.

(prompt "for your first program\n")

(prompt "Thanks") )

24.9.2

This message now appears on the next line. Command “\n” also moves to the next line and is same as teripri. It has to be used within double quotes. Displays message as “Thanks” on the next line. A closing parenthesis specifies end of the program.

Loading an AutoLISP Program

∑ Start AutoCAD program. ∑ On the menu bar, click Tools and in the pull down menu click on Load Application…. Load/ Unload dialog box appears. ∑ In the Look in combo box, click the arrow on RHS and select the folder where the Lisp program is saved. A list of programs is shown in the window below it. Choose the name of the program. Name of the file appears in the File name text box. Click Load button. ∑ Click Close button at the bottom of the dialog box. A message appears at the command line that the program is successfully loaded.

24.9.3

Executing the Program

At the command line type the name of the function. Do not confuse with file name and function name. The function name in Example 1 is C1 while file name is Tut1.lsp. Type C1 at the command line and the drawing is created by the program automatically as shown in Fig. 24.S1A.

24.10

SPECIFYING VARIABLES

In Example 1, all variables have been defined in the program itself using Setq command. To make it user interactive, these have to be supplied at the prompts to make it interactive. Autolisp permits different types of variables, e.g. integer variable, real variables, and string variables. It also

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502

uses List variables, which is like a subscripted array. For example: (setq (setq (setq (setq

a b c d

24.11

Assigns an integer value 23 to variable a Assigns a real value (with decimal point) 23.4 to variable b Assigns a string value (text) to variable c Assigns four values to a list variable d. Note the parentheses.

23) 23.4) “John”) (list 20 30 50 60))

EXTRACTING DATA FROM LIST VARIABLE

List function contains many values in it. These values can be extracted by the Car, Cadr, Caddr and Cdr commands. There are more commands but only important ones are described here. Use of these commands is shown by the following example: (setq (setq (setq (setq

e p q r

(list 5 6 7 8 9)) Car (list e)) Cadr (list e)) Caddr (list e))

(setq s Cdr (list e))

24.12

Assigns five values to a list variable e Extracts first item of list and assigns its value to p, i.e. p = 5 Extracts second item of list and assigns its value to q, i.e. q = 6 Extracts third item of list and assigns its value to r, i.e. r = 7 Extracts all items from the list except the first item and assign its value to another list s, i.e. s = (6 7 8 9)

GET COMMANDS

There are many GET commands like getangle, getcorner, getint, getpoint, getreal, etc. to get values from the user interactively either from screen by mouse or from the command line using key board. Only a few are described below: Getpoint is used to get the coordinates of click of mouse on the screen. The prompt within quotes guides the user. Its use is shown below, in which coordinates of a variable P is assigned same wherever the mouse is clicked. (Setq P (getpoint "Pick a point on screen

"))

Getdist command is used to get a value from the command line. Example below shows to feed the data through command line. User is prompted by the prompt within quotes. A value specified at the command line is assigned to variable d in the following expression: (setq d (getdist "Specify distance

24.13

"))

MATHEMATICAL OPERATIONS

Mathematical operations like addition, subtraction, multiplication, division, etc. can be done on variables tabulated as follows: Operation Addition Subtraction Multiplication Division

Use of operator (Setq a (+ n1 n2 n3)) (Setq b (– n1 n2)) (Setq c (* n1 n2)) (Setq d (/ n1 n2))

What it does (n1 + n2+n3) value is assigned to a variable a (n1– n2) value is assigned to a variable b (n1* n2) value is assigned to a variable c (n1/ n2) value is assigned to a variable d

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503

Operations like sqrt, log, exp (raise to power), cos, sin, tan, abs, etc. can also be performed. Refer an AutoLISP book for details of these operators.

24.14

ANGLES IN AUTOLISP

Angles used by AutoLISP have to be in radians only, while AutoCAD by default takes in degrees. Angle command can evaluate the angle between two specified points. For example: (Setq theta (angle p1 p2))

Angle in radians between the line joining points p1 and p2 with horizontal line is assigned to variable theta.

If working frequently with angles, then a small function as given below can be defined to convert degrees to radians. Name of the function is dtr. A value supplied through parentheses will be converted to degrees. (defun dtr (a) (* pi (/ a 180.0) )

Polar command is used to get relative coordinates of a point p2 for a given angle and distance from another point p1. Its syntax is: (setq p2 (polar p1 ang1 dist1))

The above instruction calculates and assigns coordinates of p2 with respect to p1 for given angle stored in variable name ang1 and distance in variable name dist1.

24.15

LOGICAL OPERATORS

Variables can be compared by logical operators like equal to (=), greater than (>), less than (=), lesser than and equal to ( x 50)) ( setq z 60) ) (if (or (< x y) (= x 50)) ( setq z 70) )

This means if x < y and x> 50 then z = 60 This means if x < y or x = 50 then z = 70

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504

Progn command is to be used if expressions are more than 1. Number of opening and closing parentheses should be kept equal. Indenting of the program is done for clarity to check the parentheses. The last parenthesis is to end the If condition statement. Example (If (condition) (progn (Expression (Expression ) (progn (Expression (Expression ) )

24.17 24.17.1

1) 2)

3) 4)

LOOPING A PROGRAM Repeat

Repeat is used to tell how many times a loop is to be repeated. Example of Repeat command is as follows: (Repeat 6 (Expression (Expression (Expression (Expression )

1) 2) 3) 4)

Expressions 1, 2, 3 and 4 will be executed 6 times by this loop.

24.17.2

While

While loop is useful when the number of times the loop is to be repeated is not known. The loop is controlled by comparing value of a variable at the end of the loop. Example below shows a function to create a table of 5. When the value reaches 50 the program is terminated. When value of variable b1 = 50, a is assigned nil and the loop terminates. (Defun t5 ( ) (setq b 5) (setq n 1) (setq a "T") (while a (setq b1 (* b n)) (print b1) (setq n (+ n 1)) (if (= b1 50) (setq a nil)) ) )

Belts and Pulleys Example 2

505

Write a program to draw cross-section of a belt for given section A, B, C, etc.

Solution Sn is the variable name given to section A, B, etc. p1, p2, p3 and p4 are the corners of the belt section as shown in Fig. 24.S2A. p5 and p6 are the points for indicating location of dimension. The user is prompted to type letter for the section and then in the next prompt the location of point p1 on the screen by mouse click. The output of the program is shown in Fig. 24.S2B. Belt angle is taken as 40°.

Fig. 24.S2A

Fig. 24.S2B

The program is given as follows. Type it in note pad and save it with the name Belt in any folder. (defun C:blt() (graphscr) (Setq sn (getstring "Specify cross-section of belt in upper case A, B, C, D or E (terpri) ;Selecting width w and height h of the belt for any size of belt (if (or (= sn "A") (= sn "a")) (setq w 13)) (if (or (= sn "A") (= sn "a")) (setq h 8)) ; (if (or(= sn "B") (= sn "b")) (setq w 17)) (if (or(= sn "B") (= sn "b")) (setq h 11))

"))

; (if (or(= sn "C") (= sn "c")) (setq w 22)) (if (or(= sn "C") (= sn "c")) (setq h 14)) ; (if (or(= sn "D") (= sn "d")) (setq w 32)) (if (or(= sn "D") (= sn "d")) (setq h 19)) ; ; Alternate way of using Progn function (if (or(= sn "E") (= sn "e")) (Progn (setq w 38) (setq h 23) ) ) ; p1 upper left corner, p2 lower left corner, p3 lower right corner and p4 upper right corner (Setq p1 (getpoint "Specify location of left upper corner of belt ")) (setq (setq (setq (setq

x1 (car p1)) dx1 (* h 0.36397)) dx2 (* dx1 2)) dx3 (- w dx1))

(setq x2 (+ x1 dx1)) (setq x3 (+ x1 dx3))

Part E – Chapter 24

506 (setq x4 (+ x1 w)) (setq x5 (+ x1 dx2)) (setq x6 (- x1 5)) ; (setq y1 (cadr p1)) (setq y2 (- y1 h)) (setq y5 (+ y1 5)) (setq y6 (- y1 5)) (setq p2 (list x2 y2)) (setq p3 (list x3 y2)) (setq p4 (list x4 y1)) (setq p5 (list x5 y5)) (setq p6 (list x6 y6)) ; ;Drawing belt cross-section (setvar "Osmode" 0) (command "Line" p1 p2 p3 p4 p1 "") (command "Dimlinear" p1 p4 p5) (command "Dimlinear" p1 p2 p6) )

Load the program as mentioned in Section 24.9.2. Execute the program by typing blt at command line. Then specify the section letter and at the next prompt, click a location where the drawing is required. If the drawing is very small use Zoom command with All option.

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Describe the different belt arrangements over pulleys. What are the various types of belts? Describe them by sketches. What are the various standard cross-sections of V belts? Classify pulleys. Sketch any three types. Differentiate between solid and webbed pulley. What are the advantages of webbed pulley over solid pulley? Describe the construction of a built up pulley with a sketch. What are the various types of armed pulleys? Sketch any two types. What is the use of a stepped pulley? Sketch a stepped pulley and describe its construction. Describe the application of a fast and loose pulley. How do they work? Differentiate between a flat and grooved pulley. Sketch a stepped grooved pulley and describe its use and applications. Why is multi grooved pulley used? Sketch a pulley for 3 V belts of B size. What is the application of a toothed belt? What are its advantages over other types? Sketch a rope pulley and describe its construction.

CAD 15. Describe the method to write, save, load and execute an AutoLISP program. 16. What is “Setq” command? Describe its use giving at least 5 examples.

Belts and Pulleys 17. 18. 19. 20. 21. 22.

Describe the various “Get” commands by giving examples. What is a list command? What are the commands to extract an item from a list variable? Explain the use of any four mathematical operators by giving examples. What is meant by logical operators? How are they used in IF command? Explain by examples. Discuss the utility of IF command and “Progn” command. Differentiate between “Repeat” and “while” loops. Describe the use of each.

FILL 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

507

IN THE

BLANKS

For a quarter twist belt, axes of the shaft are at angle. . An idler pulley is used as a to each other. Direction of rotation of shaft for cross belts is side of the belt. Toothed belt has teeth on the section of the belt is of the biggest size. over the shaft. Loose pulley uses a than solid pulley. For the same diameter, webbed pulley is one part. Built up pulley is made in or . Arms of an armed pulley can be Fast pulley uses a between hub and shaft. pulleys. Slight camber is given on than belt angle. Angle of groove of belt is kept slightly pulley. Generally a collar is provided on a degrees. Groove angle for rope pulley is

CAD (AutoLISP) AutoLISP program is created in software. . All instructions are written within . AutoCAD command is used between two . A comment line starts with a command is used to assign values to a variable. on screen. Getpoint command is used to get coordinates of Polar command is used to find coordinates of a point relative to other point for given . 22. command extracts the second item from a list variable. instructions are to be used. 23. Progn command is used if

15. 16. 17. 18. 19. 20. 21.

MULTIPLE CHOICE QUESTIONS 1. Direction of rotation of pulleys for a cross belt is (a) opposite to each other (b) in same direction (c) in same direction but axes at right angles (d) none of above

and

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2. Serrated belt has grooves (a) along its length (b) (c) inclined at 40° (d) 3. Biggest cross section of V belt is (a) C (b) (c) 3V (d) 4. Webbed pulley is used (a) to reduce weight and cost of pulley (b) (c) for better performance (d) 5. Number of arms in an armed pulley depends upon (a) cost considerations (b) (c) diameter of pulley (d) 6. Stepped pulleys set offers (a) step less variable speed (b) (c) speed in any number but more than 5 steps (d) 7. Multi groove pulley is used for (a) more power transmission (b) (c) small sized pulley (d) 8. A toothed belt (a) has square cross-section (b) (c) grooves inclined at 40° to axis (d)

along width inclined at 30° E 8V to reduce manufacturing time for better appearance power to be transmitted material used speed in 3 or 4 steps a compact arrangement more strength of pulley light construction grooves along its length used for positive power transmission

CAD 9. An AutoLISP program starts with (a) Alisp (b) Command (c) Function (d) Parentheses 10. AutoCAD commands are used within (a) parentheses (b) double quotes (c) apostrophes (d) curly brackets 11. To extract second item from a list variable is (a) Car (b) Cadr (c) Caddr (d) Cdr 12. Getpoint command is used to (a) specify coordinates of a point by click of mouse on screen (b) specify coordinates of a point relative to a point for a given angle and distance (c) get coordinates from the end of an entity (d) none of above 13. Polar command is used to (a) set polar coordinate system (b) get relative coordinates of a point for an x and y distances from reference point (c) get relative coordinates of a point for a given angle and distance from a reference point (d) none of above 14. “IF” command is used to test (a) one condition (b) two conditions together (c) any one of the two conditions (d) all given in a, b and c

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15. “While” command is used for (a) conditional branching (b) looping with test condition in beginning of a loop (c) looping with test condition in the end of a loop (d) looping without any test

ANSWERS to Fill in the Blank Questions 1. 5. 9. 13. 17. 21.

90° E straight, curved toothed Double quotes angle, distance

2. 6. 10. 14. 18. 22.

tensioner bush key 45 semicolon Cadr

opposite lighter flat Notepad setq more than one

3. 7. 11. 15. 19. 23.

4. 8. 12. 16. 20.

inner more than lesser parenthesis mouse click

ANSWERS to Multiple Choice Questions 1. (a) 7. (a) 13. (c)

2. (a) 8. (d) 14. (d)

3. (b) 9. (d) 15. (c)

ASSIGNMENT 1. 2. 3. 4.

ON

4. (a) 10. (b)

BELTS

AND

5. (c) 11. (b)

6. (b) 12. (a)

PULLEYS

Draw half sectional front view and side view of a webbed pulley shown in Fig. 24.6. Draw full sectional front view and side view of a stepped pulley shown in Fig. 24.9. Draw half sectional front view and side view of fast and loose pulleys as shown in Fig. 24.10. Draw a stepped pulley for a V belt of cross-section (similar to Fig. 24.13) with three steps of outside diameter 140 mm, 170 mm and 200 mm. Take shaft diameter as 30 mm.

CAD ASSIGNMENT

ON

BELTS

AND

PULLEYS

5. Draw an armed pulley with straight arms and sectional front view for a shaft diameter of 50 mm. Use the dimensions given in Fig. 24.7A and 24.7B. 6. Draw a multi belt grooved pulley in full section as shown in Fig. 24.12. 7. Write an AutoLISP program to draw a flat belt pulley (without shaft) as shown in Fig. 24.5. Input data should be: Hole diameter (d1), outside diameter (d2), Width of pulley (w), key size (d1/4 ¥ d1/4) and camber 2% of outside diameter.

HOMEWORK 8. Draw A, B, C, D and E cross-sections of V belts in double scale. 9. Sketch a built up pulley as shown in Fig. 24.8. 10. Draw a single belt V groove pulley as shown in Fig. 24.11.

CHAPTER

25

Bearings Bearings are used to support a shaft. They take radial, axial or both types of loads. Mainly there are three types of bearings; Plain Journal or Sleeve Bearing (also called as hydrodynamic bearings), Rolling bearing (also called as Anti-friction bearings) and Hydrostatic bearings that use external air or oil under pressure to support the shaft. Portion of the shaft inside the bearing is called Journal. Clearance between sleeve and journal is very crucial. More clearance does not create enough pressure in the bearing to support load on the shaft and less clearance causes direct metal to metal contact due to surface roughness. Materials used for sleeve bearings are: Babbitt, Bronzes of copper, Aluminum, Phosphorous, Lead, etc. Porous sintered materials are used for small size bearings. Plastics like PTFE, Nylon, Teflon, etc. are becoming popular due to their low friction property. Full circular sleeve is used for small bearings and for large bearings it is cut in two halves along the axis. Rotation of the sleeve is prevented either by a snug at the bottom half or by changing the shape of the outer surface of the sleeve, like square, hexagonal or octagonal. Sleeve is supported either on a simple support or in Plummer block that houses in two halves. The lower and upper halves are bolted together after placing the sleeve in between. If the height of shaft is more from ground level, pedestal supports are used. Line shafts which run parallel to the wall at a height, use wall brackets to support the shaft. Foot step bearing support is used for vertical shafts. Rolling bearings offer less running friction than sleeve bearings. They do not need much attention for lubrication. There is a rolling element in the form of a ball, roller or needle. The important parts are outer race, inner race, rolling element and cage. The cage keeps all the rolling elements at their relative positions. These are available in variety of types and sizes. Ball bearing can be deep groove, angular contact, double row, self aligning, thrust or self aligning thrust bearing. Roller bearings can be cylindrical roller, taper roller, spherical roller, needle roller or thrust bearing. Roller bearings are specified by a two digit code. First number (1 to 6) is for width and second number is for outside diameter. There are many methods to mount rolling bearings. The simplest is using end covers on housing. Sometimes one bearing is kept floating and the bearings are fitted on shaft with a nut. An oil seal on both the sides of the bearing is sometimes provided to check leakage of the lubricant.

CAD The drawings created in AutoCAD are stored in a database in a coded format. To do any operation on these entities, these have to be extracted from the data base. Entsel command is to select an entity and add to selection set. Ssget command can pick up many entities in a window and add to the selection set. Sslength command helps in finding out the number of entities in the selection set. Ssname command is used to extract the name of entity by using an index number which starts from 0. Example on a ball bearing demonstrates the use of these commands for extracting the entities from database and perform operations like Trimming, polar array, etc.

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25.1 INTRODUCTION A rotating shaft has to be supported so that it could rotate at its position and transmit power with minimum friction. Bearings permit low friction movement between the bearing and shaft surfaces. Friction is reduced either by providing a thin film of oil or by providing rolling elements like balls or rollers. Supports are used to provide support to the bearings which carry the load. 25.2

CLASSIFICATION OF BEARINGS

Bearings can be classified in many ways as given below: A. According to load bearing media ∑ Hydrodynamic – Working fluid (generally oil) is supplied at atmospheric pressure. ∑ Hydrostatic – Working fluid oil or air is supplied at high pressure. ∑ Rolling – Rolling elements like balls/rollers/needles are provided. (Anti-friction bearings). B. According to type of load ∑ Radial – Axis of load is radial (90° to axis). ∑ Axial – Axis of load is along the shaft axis. ∑ Radial and axial – Load is radial and also along shaft axis. C. According to material used for bearing ∑ Cast iron ∑ Aluminum ∑ Brass or Bronze ∑ Teflon/Nylon ∑ Babbit (Tin and lead base alloys) D. According to relative movement between bearing and shaft ∑ Rotating – Majority of the shafts rotate in the bearings. ∑ Oscillating – Shaft oscillates in the bearing like small end of connecting rod of an engine. ∑ Sliding – Movement is linear along an axis, e.g. carriage of type writer, CNC M/C slides.

25.3

HYDRODYNAMIC BEARINGS

These bearings are often referred as Sleeve bearings, if the type of load is radial. If the load is axial, they are called as Thrust bearings. The portion of the shaft which remains inside the bearing is called Journal. An oil film is maintained in the clearance space between the bearing and journal. Lubrication of these bearings is very crucial for satisfactory operation. The clearance between sleeve and journal is very important and has to be specified in close tolerances. A big clearance does not allow building up pressure and shaft touches the bearing causing metal to metal contact and hence bearing is destroyed very quickly. Too little clearance of the size of surface roughness can also ruin the bearing due to contact of the peaks of the mating surfaces. Design charts can be used or design calculations can be done to calculate clearance (refer any book on machine design). The clearance depends upon load, length of sleeve (L), journal diameter (D), rotational speed, and oil viscosity. As rough rule, journal diameter to clearance ratio is about 500 to 1000. L/D ratio is kept between 0.5 to 1. Clearance is shown enlarged for demonstration purpose in Fig. 25.1. When the shaft does not rotate, it touches the bottom of the bearing (Fig. 25.2A). As the shaft rotates, the oil due to its viscosity also starts rotating with it in the converging wedge formed between bearing and journal. This increases the pressure at the bottom and the shaft is lifted up (Fig. 25.1B).

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Part E – Chapter 25

Fig. 25.1

25.4

Hydrodynamic Bearing

PLAIN JOURNAL BEARING

Plain bearings are also called as bush or sleeve bearings, because their shape is like a sleeve. The bearing sleeve is also called bearing shell. Small bearings are in one piece as completely circular (Fig. 25.2A), while larger bearings are made in two halves (Fig. 25.2B), each of semi-circular shape. Sometimes it is provided with a collar to check its axial movement in its support. An oil hole on the top of the shell is provided to put oil. A circumferential groove is also sometimes provided for better supply of oil and cooling (Fig. 25.2C). Small bearings use sintered metal that contains a lubricant in its pores and does not need lubrication frequently. A heavily loaded bearing is provided with some arrangement to introduce oil under pressure using a pump. Such bearings are called Hydrostatic bearings.

25.5

Fig. 25.2

Sleeve or Bush Beating Shells

PLAIN JOURNAL BEARING MATERIALS

A Babbitt Sleeve bearings use tin and lead base alloys called Babbitt. They are popular due to their ability to embed dirt, excellent properties even when oil film is very less. For auto and small applications Babbitt is used as a thin coating over steel strip. For heavy duty bearings, thick Babbitt is put on cast iron or steel backing. B Bronzes Many copper alloys like aluminum bronze, tin bronze, phosphorous bronze, lead bronze, etc. are used. C Aluminum Alloys They offer high load bearing capacity, thermal conductivity, fatigue and wear resistance. Their application is for main and big end bearings of connecting rod, steel mills and reciprocating compressors. D Porous Metals Porous bearings are made of powdered metal by sintering. They are cheap and used for small motors and light applications. Oil is impregnated in the pores and supply can be made with oil cup, felt pad or wick. The oil moves by capillary action.

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E Plastics Plastic bearings are becoming popular due to high strength of modern plastics. Some materials like Teflon, Nylon, etc. offer excellent low friction property. 25.6

SLEEVE BEARING SUPPORTS

The sleeve bearing has to be supported on some support. Various types of supports are used depending upon the application. These are: a. Simple bearing support b. Plummer block c. Pedestal support d. Wall bracket e. Foot step bearing

25.6.1 Simple Bearing Support For shafts of small diameter, the sleeve is in one piece and such a support can be used for full circular bearing. The support consists of a flat base with a semi-circular shape in the center (Fig. 25.3). The base has holes, so that it could be grouted on foundation at the base level with foundation bolts or on machine structure. Shape of holes is made as a slot with semi-circular ends. This is done to account for slight misalignment, which could occur while grouting the bolts. Sometimes a circular hole of slightly bigger diameter than the foundation bolt is also used to account for grouting inaccuracies. A circular bush is pushed into the central hole of the support. Outside diameter of the bush is same as the inside diameter of hole in the support. A screw is then fixed from one side in such a way that half side of the screw is in the bush and remaining half in the support. This is done to check the relative movement of the sleeve with respect to the support. An oil hole at the top aligns with the oil hole of the sleeve and is used for lubrication. Its Fig. 25.3 Isometric View of a Simple Bearing sectional front view and top view are shown in Fig. 25.4. Support

Fig. 25.4 Sectional View of a Simple Bearing Support

25.6.2 Plummer Block This type of support is in two parts. Its lower part is provided with a base having holes for the foundation bolts. Sometimes the base is made hollow partially to accommodate the surface waviness of the foundation (See Figs 25.4 and 25.6). It has a semi-circular cavity in the center, in which the lower bearing shell is placed.

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Rotation of the bearing shell can be prevented by many methods. These are: a. Provide a snug (circular or rectangular) at the bottom of lower sleeve, which fits into the cavity of the block (Fig. 25.5A). b. Provide an axial flange on the sides of the bearing shell (Fig. 25.5B). c. Making the outer side of shell as square (Fig. 25.5C), hexagonal (Fig. 25.5D) or octagonal so that it does not rotate (Fig. 25.5E).

Fig. 25.5 Sleeve Motion Restriction Methods

Upper sleeve is placed in the upper half of the support and is tightened with the help of studs or bolts and nuts (Fig. 25.6). If bolts are used, their head is made square that fits into the square cavity at the bottom of support, so that the bolt does not rotate while tightening or opening the nut over it. An oil hole is provided in top portion through which oil is admitted. It aligns with the oil hole of the top bearing shell. Oil hole is sometimes plugged to save the bearing from getting dust through this hole.

25.6.3

Pedestal Support

Fig. 25.6 Plummer Block Plummer block holds the bearing shell but its center line is very near its base. Sometimes center line of the journal is required at a higher level than the base level. Pedestal bearings are used for such situations. Figure 25.7 shows the center line of sleeve at much higher level than the Plummer block.

25.6.4

Wall Bracket Support

Line shafts are generally not kept at ground level as they occupy a lot of space. They are mounted on the side of wall near the ceiling to save ground space. At ground level (Fig. 25.8) is a wall bracket is grouted with the wall and the bearing sleeve is bolted on the bracket.

25.6.5

Foot Step Support

Supports discussed above are used for horizontal shafts. For vertical shafts a foot step bearing is used. This support has to take not only vertical load but radial load also. The former is taken by a thrust pad which is simply in the form of a circular disk placed at the bottom of the shaft and for radial loads a sleeve is provided. Both these bearings are placed in housing as shown in Fig. 25.9. The base of the support has four holes for fixing the support using foundation bolts. Shape of the top of bearing is to provide an annular space which works as an oil reservoir.

Bearings

Fig. 25.7

Fig. 25.8

A Pedestal Support

A Wall Bracket Support

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Fig. 25.9

25.7

A Foot Step Bearing Support

HANGERS

Hangers are used to support shaft by fixing the base to the ceiling and the shaft support hangs from there. There are two types of hangers; U hanger and J hanger.

25.7.1 U Hanger It is so called as its shape is like letter ‘U’ (Fig. 25.10). The hanging arms are made of T-section to make it light and rigid. The bearing support is in two halves. Lower half is integral part of the hanging arms. After placing the bush bearing, the upper half is clamped on the top by studs and nuts. (Not Shown)

Fig. 25.10

U Hanger

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517

25.7.2 J Hanger It is also of the hanging type and its shape is like letter J (Fig. 25.11). The top face is bolted to the ceiling while the lower portion supports the bush. The hanging structure is made of I section for lightness and rigidity. After putting the bush, the upper cover is fixed by studs and nuts. (Not Shown)

Fig. 25.11 J Hanger

25.8 ROLLING BEARINGS Rolling bearings transmit load by using a rolling element to roll between the outer and inner race rather than slide. In these bearings, starting friction is about two times more than running friction but still lesser than the starting friction of sleeve bearings. Rolling bearings are also called Anti-friction bearings. These bearings are manufactured for following types of loads: ∑ Pure radial load ∑ Pure axial load ∑ Combined radial and axial load 25.8.1 Nomenclature The terms used for a rolling bearing are shown in Fig. 25.12. It consists of an outer ring known as outer race having a groove in the inner side periphery. Outside diameter is the maximum outside diameter of the outer race (D). Width of bearing (W) is the axial width of the outer race. Shoulder is the area on the side of groove. Inner race is placed concentrically with the outer race. This race has a circumferential groove at its outer periphery to accommodate rolling elements. Bore is the size of hole in the inner race. Corner radius is the radius (r) at the inside diameter of inner race. Rolling element is the main rolling part of the bearing. It can be in the shape of ball, cylinder, needle, etc. There can be two rows of these elements for increasing load capacity. Figure 25.12 shows balls as the rolling element. Balls are inserted in the grooves formed by these races.

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Cage keeps the rolling elements equally spaced. Cage is made in two parts. Rivets are used to join the two halves after putting the rolling elements.

Fig. 25.12

A Rolling Ball Bearing

Rolling bearings are manufactured in a large variety. Selection depends upon the application. Ball bearings are discussed in Section 25.8.2 and roller bearings in Section 25.8.3.

25.8.2

Ball Bearings

These bearings use spherical steel balls as rolling elements. The outer and inner race shape is different for different types of bearings (Fig. 25.13).

Fig. 25.13 Types of Ball Bearings

Deep groove

It is a general purpose ball bearing and is mostly used. In addition to radial load, it can take some percentage of axial loads also.

Angular contact

In addition to radial load, it also takes axial load in one direction only.

Double row

There are two rows of balls placed side by side. This increases the load carrying capacity of the bearing.

Self aligning

Inner surface of outer race is modified as circular along the axis. It can align automatically if there is any mis-alignment between bearing and shaft axis.

Thrust

It is designed for axial loads only.

Self aligning thrust

It is an axial thrust bearing provided with a spherical seat for self aligning.

Bearings

25.8.3

519

Roller Bearings

Ball bearing has a point contact with the races. In case of roller bearings, it is a line contact and hence these bearings can carry more load than ball bearings. Various types of roller bearings are shown in Fig. 25.14 and their use is explained below:

Fig. 25.14

Cylindrical roller Taper roller Spherical roller Needle roller Thrust bearing

25.8.4

Types of Roller Bearings

It uses a cylindrical roller as rolling element and hence increases load capacity than ball bearing. Roller is in the form of a frustum of a cone. It can take heavy radial as well as axial loads. It is useful when heavy load is there and mis-alignment could also occur. Roller of small diameter is called a needle. It is used when there is less space. One of the cylindrical surface of the part acts as a race. It is meant for heavy axial loads only.

Bearing Specifications

Anti-Friction Bearing Manufacturing Association (AFBMA) has recommended two digit code to specify bearing size. First number of code is width series (0, 1, 2, 3, 4, 5 and 6) and second number is outside diameter series (8, 9,..., 1, 2, 3, 4). Figure 25.15 shows sizes of the different series of bearings. Series 1, 2 and 3 are most commonly used. Important dimensions of 02 and 03 series bearings are tabulated in Tables 25.1, 25.2 for ball bearings and Table 25.3 for roller bearings.

Fig. 25.15

Specifications of Rolling Bearings

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520 Table 25.1 Bore 10 12 15 17 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Outside diameter 30 32 35 40 47 52 62 72 80 85 90 100 110 120 124 130 140 150 160 170

Table 25.2 Bore 10 12 15 17 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Outside diameter 35 37 42 47 52 62 72 80 90 100 110 120 130 140 150 160 170 180 190 200

Ball bearing dimensions in mm of series 02 Width 9 10 11 12 14 15 16 17 18 19 20 21 22 23 24 24 26 28 30 32

Shoulder diameter 1 12.5 14.5 17.5 19.5 24 30 35 41 46 52 56 63 70 74 79 86 93 99 104 110

Shoulder diameter 2 27 28 31 34 41 47 55 65 72 77 82 90 109 114 119 119 127 136 146 156

Fillet radius 0.6 0.6 0.6 0.6 0.6 0.6 1.0 1.0 1.0 1.0 1.0 1.5 1.5 1.5 1.5 1.5 2.0 2.0 2.0 2.0

Ball bearing dimensions in mm of series 03 Width

Shoulder diameter 1

Shoulder diameter 2

11 12 13 14 15 17 19 21 23 24 27 29 31 33 35 37 39 41 43 45

12.5 16 19 21 24 31 37 43 49 54 62 70 75 81 87 93 99 106 111 117

31 32 37 41 45 55 65 70 80 89 97 106 116 124 134 144 153 161 170 179

Fillet radius 0.6 1.0 1.0 1.0 1.0 1.0 1.0 1.5 1.5 1.5 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.5 2.5 2.5

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Table 25.3 Roller bearing dimensions in mm of series 02, 03 Bore 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 110 120 130 140

25.9

02 Series Outside diameter

Width

Bore

52 62 72 80 85 90 100 110 120 124 130 140 150 160 170 180 200 215 230 240

15 16 17 18 19 20 21 22 23 24 24 26 28 30 32 36 38 40 40 42

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 110 120 130 140

03 Series Outside diameter 62 72 80 90 100 110 120 130 140 150 160 170 180 190 200 215 240 260 280 300

Width 17 19 21 23 24 27 29 31 33 35 37 39 41 43 45 47 50 55 58 62

MOUNTING OF ROLLING BEARINGS

There are many methods of mounting anti-friction bearings, but only a few are commonly used and are described below. Housing bore and shaft diameter must be manufactured in close limits for a good mounting. While mounting ball bearings, the revolving race is kept press fit on the shaft or its mating surface and stationary race is kept in transitional fit (Refer Chapter 19 for fits). Theoretically these bearings do not need any lubrication, but to protect them from corrosion and ensure smooth running, these are lubricated, generally with grease. Generally there are two bearings, one at each end of the shaft. Inner race is backed up against the step of the shaft (Fig. 25.16). Outer race is backed up against the housing shoulder and is kept pressed from both sides by end cover plates.

Fig. 25.16 Mounting of Ball Bearings in a Housing

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If there are any chances of expansion of shaft, the other side for outer race for the second bearing may be kept floating (Fig. 25.17). The inner race is kept in position by nuts on threaded shaft.

Fig. 25.17

Alternative Mounting of Ball Bearings in a Housing

While mounting bearings, it should be ensured that the lubricant does not leak. Its leakage is checked by providing lip seals (Fig. 25.18).

Fig. 25.18 Mounting of Ball Bearings in a Housing with Seals

CAD 25.10

MANAGING ENTITIES

Drawing created in AutoCAD is stored in its database. Each entity has information about its end points, color, linetype, layer etc. This section informs how to select an entity in the program, which is already drawn. The information varies for each entity. Each entity has a number. A. Entsel command is used to pick only one entity at a time. Use of this command is shown below: (setq n1 (entsel)) AutoCAD prompts to select one entity on the screen by mouse click over it. The selected entity is added to selection set n1. B. Ssget command is to select many entities in a window. Use of this command is shown below: (setq n2 (ssget “w” p1 p2)) AutoCAD prompts to select objects on screen. Create a window by diagonal points p1 and p2 around the objects to be selected. The selected entities are added to selection set n2. C. Ssname command helps in extracting name of the entity from database, which is in the form of a hexa-decimal number and not a text-like line or circle etc. For example:

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523

n1 is the selection set and 0 is the index number which starts from 0. The entity name and index number are assigned to variable name b. D. Sslength command finds the number of entities in a specific selection set. Its use is shown below: (setq c (sslength n2)) n2 is the name of selection set. Number of entities in that set is assigned to variable c. (setq b (ssname n1 0))

Example 1 Write a program to draw a ball bearing as shown in Fig. 25.S1with the following input data. ∑ Center of bearing ∑ Inside diameter of inner race ∑ Outside diameter of outer race Solution 1. Open Microsoft Notepad (Not Word) and start typing the program as given below: ; This function draws a Ball bearing (defun C:B2() (graphscr) (setq cen (getpoint "Click to Specify center point of Bearing at center\n" )) (setq d1 (getdist "specify bore diameter " )) (setq d4 (getdist "specify outside diameter " )) (setq xbrg (car cen)) (setq ybrg (cadr cen)) (Setq gap (- d4 d1)) (Setq Th (/ gap 3)) (Setq d3 (- d4 Th)) (setq d2 (+ d1 Th)) (setq r1 (/ d1 2)) (setq r2 (/ d2 2)) (setq r3 (/ d3 2)) (setq r4 (/ d4 2)) (setq pi (/ 22 7.0)) (setq pi2 (/ pi 2.0)) (setq pi23(* pi2 3.)) Fig. 25.S1 A Ball Bearing ;Draw inner race (command "color" 3) (Command "CIRCLE" cen r1) (Command "CIRCLE" cen r2) ;Draw outer race (command "color" 4) (Command "CIRCLE" cen r3) (Command "CIRCLE" cen r4) ;Calculate number of balls (setq d2d3 (+ d3 d2))

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524 (setq (Setq (Setq (Setq (Setq (setq (setq (Setq (Setq

pcr (/ d2d3 4.0)) circum (* pcr 3.14)) racegap (- r3 r2)) Th2 (/ Th 3)) Balldia (+ racegap Th2)) numballs (/ circum Balldia)) n (fix numballs)) bc (polar cen 0 pcr)) Ballrad (/ Balldia 2))

;Draw ball (command "color" 5) (Command "CIRCLE" bc Ballrad) ;Drawing cage (setq cageoffset (/ racegap 4.)) (setq cagerad1 (- pcr cageoffset)) (setq cagerad2 (+ pcr cageoffset)) (command "color" 6) (Command "CIRCLE" cen cagerad1) (Command "CIRCLE" cen cagerad2) ;Defining points for trimming (setq eb1 (- pcr Ballrad)) (setq eb2 (+ pcr Ballrad)) (setq p1 (polar cen 0 r2)) (setq p2 (polar cen 0 cagerad1)) (setq p3 (polar cen 0 cagerad2)) (setq p4 (polar cen 0 r3)) (setq p5 (polar cen 0 eb1)) (setq p6 (polar bc pi2 Ballrad)) (setq p7 (polar bc pi23 Ballrad)) (setq p8 (polar cen 0 eb2)) ;Extracting entities from the database (setq a (ssget p1)) (setq b (ssget p2)) (setq c (ssget p3)) (setq d (ssget p4)) (setq (setq (setq (setq (setq

xc (car bc)) yc (cadr bc)) wx1 (— xc Ballrad)) wx2 (+ xc BALLrad)) wy1 (+ yc Ballrad))

Bearings (setq wy2 (— yc Ballrad)) (setq pw1 (list wx1 wy1)) (setq pw2 (list wx2 wy2)) ;Naming the entities for trimming (setq na (ssname a 0)) (setq nb (ssname b 0)) (setq nc (ssname c 0)) (setq nd (ssname d 0)) ;Triming the ball (setvar "Osmode" 0) (command "trim" na "" p5 "") (command "trim" nb nc "" p6 "") (command "trim" nb nc "" p7 "") (command "trim" nd "" p8 "") ;Creating a polar array (setq ne (ssget "w" pw1 pw2)) (setq nw1 (ssname ne 0)) (setq nw2 (ssname ne 1)) (setq nw3 (ssname ne 2)) (setq nw4 (ssname ne 3)) (setq af 360) (setq yn "_y") (command "_array" nw1 nw2 nw3 nw4 "" "_P" cen n af yn) ;Calculating angle for position of rivet (setq a1 (/ pi 180.0)) (setq rivetangd (/ 180.0 n)) (setq angr (* a1 rivetangd )) (setq rc (polar cen angr pcr)) ;Calculate Rivet diameter (setq rivetdia (/ cageoffset 3.0)) ;Draw rivet (command "color" 6) (Command "CIRCLE" rc rivetdia) ;Draw polar array of rivet (setq p9 (polar rc 0 rivetdia)) (setq rd (ssget p9)) (setq nr (ssname rd 0)) (command "_array" nr "" "_P" cen n af yn) )

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2. Save the file as Bearing.lsp in any folder. For quick access, let it be in the root directory. 3. On Menu bar click Tools and on the pull down menu click Load Application…. In the Load/Unload dialog box displayed, choose the folder in the Look in combo box. Select the file Bearing.lsp in the window below it. Click Load button and then Close button at the bottom. 4. To execute the file in AutoCAD, at the command prompt type the function name B2 as under: Command: B2 ø

5. Program starts and prompts for the input data. Specify the values as prompted, i.e. center point by click of mouse, inside and outside diameters through key board. The bearing of the desired values is created. 6. Re run the program by step 4 only and feed different data this time and see that the drawing is created with new values.

THEORY QUESTIONS 1. Classify different types of bearings. 2. Describe different materials used for making a sleeve. 3. Differentiate between plummer block, pedestal support and wall bracket for supporting bearings. 4. For which application is a foot step bearing used? 5. Why is the base of bearing supports made hollow and holes as non-circular? 6. What types of loads can be taken by taper roller bearing? 7. Suggest a suitable rolling bearing for the following applications: (a) Heavy radial load

(b) Medium load with slight mis-alignment

(c) Only axial load

(d) Radial load with limited space

8. Show by a sketch cage, shoulder and races of a ball bearing. 9. List the various types of rolling elements. 10. How do you specify a rolling bearing? 11. Sketch any one type of mounting a shaft using rolling bearing.

CAD 12. Describe use of the commands to select entity/entities from the database of AutoCAD by examples. 13. Discuss the use of “Ssname” command by an example.

FILL

IN THE

1. Two main types of bearings are 2. Some materials used for bush bearings are

BLANKS

and

. ,

,

.

Bearings 3. A part supporting a journal bearing is called

527 . above base level.

4. Pedestal supports are used where shaft axis is are fitted on wall to support a bearing.

5.

support.

6. Vertical shafts are supported by

7. The part of a rolling bearing fitted on shaft is called as

.

of a rolling bearing generally does not rotate.

8.

9. An element keeping the rolling elements equally spaced is called

.

bearings are used where there is some misalignment.

10.

11. A thrust bearing takes

load.

bearings are used for heavy loads.

12.

13. A roller bearing which takes axial and radial load also is called

. bearing.

14. The bearing with very small diameter rollers is called 15. Heavy loads with misalignment can be taken by

roller bearings.

is used to check leakage of oil.

16.

CAD 17.

command is used to select one entity from the AutoCAD database.

18. Many entities can be selected from database by

command.

19. Number of entities in a selection set are found by 20.

command.

command is used to extract name of entity from database.

MULTIPLE CHOICE QUESTIONS 1. Friction is minimum while using (a) sleeve bearing in two halves

(b) rolling bearing

(c) ball bearing

(d) circular bush bearing

2. A plummer block is used to support (a) heavy blocks

(b) bearings

(c) vertical shafts

(d) only roller bearings

3. A footstep bearing is used for (a) only radial loads

(b) axial loads

(c) variable loads

(d) very heavy radial loads

4. A needle bearing is used where (a) shaft is conical

(b) space is less

(c) shaft is of small diameter

(d) load is less

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5. A double row ball bearing with cylindrical outer race offers (a) higher radial load capacity

(b) increased only axial load capacity

(c) reduced size of balls

(d) self aligning

6. A ball thrust bearing has grooves (a) centrally on periphery

(b) eccentrically on periphery

(c) on one face only

(d) on adjacent faces

7. Type of groove in outer race of self aligning ball bearing is (a) straight

(b) circular

(c) trapezoidal

(d) elliptical

8. Rolling bearings are designated by (a) bore size only

(b) bore and outside diameter

(c) bore and width

(d) width and outside diameter series

9. A stepped shaft is used to (a) reduce weight

(b) accommodate in bearing support

(c) check axial sliding of shaft

(d) reduce stress concentration

10. Oil seals are used with bearings (a) to lubricate bearings

(b) to check leakage of lubricant

(c) for smooth operation

(d) to increase load capacity

CAD 11. The command to select many entities from the data base is (a) Entsel

(b) Ssname

(c) Sslength

(d) Ssget

12. Ssname command is used with (a) name of selection and index number

(b) name of entity and its length

(c) only name of selection set

(d) only index number

ANSWERS to Fill in the Blank Questions 1. 5. 9. 13. 17.

journal, rolling Wall brackets cage taper roller Entsel

2. 6. 10. 14. 18.

Bronze, white metal, PTFE foot step self aligning needle Ssget

3. 7. 11. 15. 19.

plummer block inner race axial spherical Sslength

4. 8. 12. 16. 20.

high Outer race roller oil seal Ssname

ANSWERS to Multiple Choice Questions 1. (c)

2. (b)

3. (b)

4. (b)

5. (a)

6. (d)

7. (b)

8. (d)

9. (c)

10. (b)

11. (d)

12. (a)

Bearings

ASSIGNMENT

ON

BEARINGS

529 AND

SUPPORTS

1. Draw half-sectional front view, half-sectional side view and half-sectional top view of a plummer block shown in Fig. 25.P1.

Fig. 25.P1 Plummer Block 2. Draw half-sectional front view, half-sectional side view and top view of the foot step bearing shown in Fig. 25.9. 3. Draw half-sectional front view and side view of the bearing mounting shown in Fig. 25.16 4. Draw a ball bearing of bore size 50 mm series 02. Take the dimensions from Table 25.1. Other dimensions may be assumed suitably.

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CAD ASSIGNMENT

ON

BEARINGS

AND

SUPPORTS

5. Write a program in AutoLISP to draw front view of a simple bush bearing support shown in Fig. 25.P2. The data to be specified is as under. Variable names which can be used are given in parentheses. Base length (BL) Base thickness (BT) Center of bush from base (C) Inside diameter of bush (D1) Outside diameter of bush (D2) Outside radius of support (R) Test the program in AutoCAD by feeding suitable values of the variables as given below: BL = 140, BT = 10, C = 40, D1 = 40, D2 = 50 and R = 40.

Fig. 25.P2

Simple Bush Bearing Support

HOMEWORK 6. 7. 8. 9.

Sketch outside front view and top view of a pedestal support shown in Fig. 25.7. Sketch sectional view of various types of ball bearings shown in Fig. 25.13. Sketch a floating bearing arrangement for mounting roller bearings shown in Fig. 25.17. Sketch front view of a U hanger shown in Fig. 25.10.

PROBLEMS 10. 11. 12. 13.

FOR

PRACTICE

Draw a sectional front view and side view of the wall bracket shown in Fig. 25.8. Sketch Front view of a J hanger shown in Fig. 25.11. Sketch sectional view of different types of roller bearings shown in Fig. 25.14. Sketch mounting of a roller bearing with oil seals shown in Fig. 25.18.

CHAPTER

26

Gears Gears are used for positive power transmission for increasing or decreasing speed. Gear ratio is generally not more than 5. For high gear ratios, worm and worm wheels are used. Small gear is called pinion. Rack is a gear with infinite diameter which means it is straight. Some terms used with gears are: Pitch circle Circular pitch (CP) Diametrical pitch (DP) Module (m) Addendum (a) Dedendum (d) Whole depth Working depth Clearance Face width (W) Tooth thickness Root radius Base circle Pressure angle (f)

An imaginary circle on which rolling takes place without slip (PCD). Arctual distance between two adjacent teeth from the center to center at PCD. Ratio of the number of teeth to pitch circle diameter. Ratio of PCD to number of teeth. Radial distance from tip of tooth to PCD. Radial distance from PCD to root of the tooth. Length of tooth (addendum + dedendum). Distance by which tooth mates with another gear and is equal to 2a. Difference between whole depth and working depth and is equal to (d – a) Axial width of gear. Thickness of tooth at pitch circle diameter (CP/2). Radius at the bottom of tooth with material at the bottom. An imaginary circle on which the tooth profile (involute) is generated. Angle between line of action (tangent to the base circles) and the common tangent to the pitch circle at line of centers. Helix angle (a) Angle made by the gear tooth with the axis of the gear. Gears are of many types. Spur gears have straight teeth parallel to the axis. Load comes all of a sudden when a tooth meshes with another gear. Helical gears have inclined teeth cut in helix form at a helix angle and hence the engagement of teeth with another gear is gradual. Therefore, they can take more loads and offer quieter operation than spur gears. Both spur and helical gears are used for shafts having their axes parallel to each other. Bevel gears are used when shaft axes are at an angle. The axes may be intersecting or non-intersecting. The teeth are cut on periphery of a frustum of a cone and their cross-section decreases towards the center. The teeth can be straight, or curved for spiral bevel gears. Bevel gears for non-intersecting axes are called hypoid gears. Worm is in the form of a screw thread of trapezoidal section. It engages with a worm wheel having teeth cut at the helix angle same as that of worm. Outer periphery is rounded to envelop the threads of worm. Face width of gear is kept 3 times the circular pitch (CP). Gears up to 100 mm are made solid, while 100 to 250 mm diameters are webbed. If radial length of web is more, holes are also drilled to make it light. Thickness of web is taken equal to CP. Gears diameters more than 250 mm are made with a rim integral with hub by 6 or more radial arms. The solid rim thickness for all gears is taken equal to whole depth of teeth. Hub is made 1.2 to 1.4 times longer than face width of the gear.

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Teeth on gears are cut in involute of profile as these gears can bear some tolerance in the center distance between two gears. The gear tooth of cycloid profile needs exact center distance for satisfactory working. Drawing involute profile is time consuming and laborious. Approximate methods are used to approximate this curve by an arc of suitable radius. The methods differ slightly if number of teeth is more or less than 30 teeth. In drawings having many gears, these are drawn in a conventional method by showing outside diameter

and pitch circle radius in center line.

CAD AutoCAD is very useful in drawing gears. Only one tooth is created and then rest of the teeth can be easily copied by ARRAY command with Polar option.

26.1

INTRODUCTION

When two wheels, A and B, mounted on parallel shafts are made to touch, then if A is rotated in anti-clockwise direction, wheel B starts rotating by friction at their periphery in clockwise direction (Fig. 26.1A). If slight load is put on the driven shaft, the wheel starts slipping. To avoid this, slipping teeth are cut on their periphery and such a wheel is called gear (Fig. 26.1B). Speed can be increased or decreased by selecting the diameters suitably. Gear ratios (Driver to driven speed ratio) up to 5 are common. Gear is never used alone. It is always used in pair. Pinion is a small gear. Rack is a gear of infinite diameter, i.e. it is straight. If speed ratio is more than 5, then either gear trains (gear set in series) or worm gears are used.

Fig. 26.1

26.2

Concept of Gear

TERMINOLOGY

Following terms are used for a gear and shown in Fig. 26.2: Pitch circle Circular pitch (CP) Diametrical pitch (DP) Module (m) Addendum (a) Dedendum (d) Whole depth Working depth

An imaginary circle on which rolling takes place without slip. This diameter is called Pitch Circle Diameter (PCD). Arctual distance between two adjacent teeth from the center to center at PCD. Ratio of the number of teeth to pitch circle diameter. Ratio of PCD to number of teeth. Radial distance from tip of tooth to PCD. Radial distance from PCD to root of tooth. Length of tooth (addendum + dedendum). Distance by which tooth mates with another gear and is equal to 2a.

Gears

Clearance Face width (W) Tooth thickness Crest Face Flank Root radius (r) Addendum circle (Do) Dedendum circle (Dd) Base circle Pressure angle (f) Helix angle (a)

Difference between whole depth and working depth. (d – a) Axial width of the gear. Thickness of tooth at pitch circle diameter (CP/2). Outermost periphery of the tooth. Curved surface of tooth above the pitch circle up to its tip. Curved surface of tooth below the pitch circle up to root. Radius at the bottom of tooth with material at the bottom. Circle of outside diameter Do = PCD + 2a. Circle passing through the roots of teeth Dd = PCD – 2d. Imaginary circle on which the tooth profile (involute) is generated. Angle between line of action (tangent to the base circles) and the common tangent to the pitch circle at line of centers. Angle made by the gear tooth with the axis of the gear.

Fig. 26.2

26.3

533

Terms used for Gears

TYPES OF GEARS

Gears are of many types. They are classified according to the inclination of their teeth as follows:

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Spur Gears Spur Gears are the simplest type of gears and are most commonly used. They have straight teeth whose axis is parallel to the axis of the gear (Fig. 26.3). Helical Gears Helical Gears have their teeth inclined at helix angle a(Fig. 26.4). Due to this inclination these gears give rise to axial thrust. This thrust can be cancelled by using double helical gears called Herringbone gears (Fig. 26.4B).

Fig. 26.3

Spur Gear

Fig. 26.4

Helical Gears

Bevel Gears Bevel Gears are used where power is to be transmitted at an angle. The gear blank takes the form of a frustum of a cone instead of a cylinder (Fig. 26.5). Teeth also decrease in thickness and height towards center. All teeth point towards the center. Worm and Worm Wheel Fig. 26.5 Bevel Gear Worm and Worm wheel are used when large speed reduction is required. Ratio up to 20 or even more can be got in one stage only. Worm is in the form of a threaded screw of trapezoidal section with large pitch that matches with the worm wheel having mating grooves of the same profile (Fig. 26.6). Rack Rack is a gear of infinite diameter and hence all its teeth are in a straight line (Fig. 26.7). The teeth may be at right angles to the axis or inclined. It meshes with a pinion.

Fig. 26.6 Worm and Worm Wheel

Fig. 26.7

Rack and Pinion

Gears

26.4

535

GEAR TOOTH CALCULATIONS

26.4.1 Types of Pitches Pitch is given in different ways as Circular Pitch (CP), Diametral Pitch (DP) and Module (m). If PCD is pitch circle diameter and Z is number of teeth, these are related to each other as under: CP = (p ¥ PCD)/Z DP = Z/PCD m = PCD/Z These pitches are related to each other as given below. If one is known, the other can be calculated. CP = p m = p/DP DP = 1/m = p/CP CP ¥ DP = p 26.4.2 Gear and Tooth Proportions Proportions of gear tooth are given in terms of module as follows: Addendum (a) = m Dedendum (d) = 1.157 m Whole depth = 2.157 m Clearance = 0.157 m Circular pitch = pm Outside gear diameter = PCD + 2 m Root diameter = PCD – 2.314 m = Do – 4.314m Base circle diameter = PCD ¥ cos (f) where f is Pressure angle Angle between two adjacent teeth = 360/Z The module of gear can be selected from the list of standard modules as follows: [IS 2535:1978 Revised 1991]: 1 1.125 1.25 1.375 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5 3.75 4 4.5 5 5.5 6 6.5 7 8 9 10 11 12 14 16 18 20 26.5

TOOTH PROFILES

There are two standard tooth profiles; Cycloid and Involute.

26.5.1

Cycloid Profile

When a circular wheel rolls on a flat surface, the locus generated by a point on the circumference of the wheel is called a Cycloid. The locus shown in Fig. 26.8 is for a point “0”on the wheel. Line 0-0 is the circumference of the wheel. Points 1, 2, 3, etc are 8 segments of circumference. Points 1¢, 2¢, 3¢, etc are the centers of the wheel when it rotates 1/8th of the revolution. Points 1≤, 2≤ are the points where the arc of radius of wheel cuts the vertical heights projected from each position. The curve joining the points 1≤, 2≤, 3≤, etc. is a cycloid. When the bottom surface is convex instead of a straight surface, the path generated is Epi-cycloid. It is drawn in a similar way as the cycloid, except that the horizontal lines are replaced by arcs parallel to the bottom surface and vertical lines are replaced by radial lines (Fig. 26.9A).

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Fig. 26.8 Cycloid Profile

Fig. 26.9

Epi-cycloid and Hypo-cycloid Profiles

For a hypo-cycloid, the bottom surface is concave and circular, hence center of the bottom surface and center of the rolling cylinder both are above the curved surface. Rest of the procedure is same as that for Epi-cycloid. It is shown in Fig. 26.9B. A cycloid tooth is epi-cycloid above pitch circle and its flank is hypo-cycloid below pitch circle (Fig. 26.9C). This profile requires exact center distance between shafts, which is difficult to produce; hence involute profile is generally used.

26.5.2 Involute Profile When a string wound around a cylinder is unwound keeping the end stretched, the locus of free end of string is involute. This curve forms flank and face of the gear tooth. If the direction of wound of string is changed from clockwise to anti-clockwise, the other side of the tooth can be generated. To draw the Involute profile for a cylinder, follow the procedure given below: ∑ Draw a circle of the base diameter with O as center (Fig. 26.10). ∑ Divide the circumference into small equal parts. Mark them as 1, 2, ...7. ∑ Draw tangents at every point. These will be right angles to the radial lines. ∑ With 1 as center and radius of arctual length P-1, cut an arc on the tangential line drawn at this point. Fig. 26.10 Drawing Involute Profile Mark it as 1¢.

Gears

537

∑ Lines 1-1¢, 2-2¢, 3-3¢, etc. are equal to arctual length between P and tangents to points 1, 2, 3, etc. respectively. ∑ Starting from point P, join all the points 1¢, 2¢, 3¢...7¢ to give the Involute profile. This profile is only for left side of the tooth. Right face is the mirror image of this profile about a radial line passing through the center of tooth thickness.

26.6

BASE CIRCLE

Base circle is a circle from which involute profile is generated. Its center is same as pitch circle. Fig. 26.11 shows two mating gears with their pitch circles touching each other at common point P called as Pitch point. Line AB is a common tangent to both the circles at this point. A line is drawn at an angle f with the common tangent AB. This line is called line of action. Radial lines O1C and O2D are drawn from the centers O1 and O2 which are perpendicular to the line of action. Circles drawn tangent to the line of action are called base circles. These circles pass through points C and D respectively. Value of the Fig. 26.11 Pressure Angle and Line of Action radius of Base Circle (O1C) can be calculated by formula, O1C = R1 Cos f, and O2C = R2 Cos f, where R1 and R2 are half of Pitch Circle Diameter (PCD) for the respective gears. Pressure angle is chosen according to the load coming on gear tooth. For light loads, pressure angle is 14.5°, for medium loads 20°, and for heavy loads 25°.

Example 1

Draw involute profile for a gear tooth with 18 teeth, 20° pressure angle and module 10.

Solution From the gear tooth proportion formulas, calculate the following: PCD = m ¥ Z = 10 ¥ 18 = 180 mm Base circle diameter = PCD cos f = 180 ¥ cos 20 = 169 mm Base circle radius = Base circle diameter/2 = 84.5 mm. Addendum = m = 10 mm Dedendum = 1.157 m = 11.6 mm approx. Outside diameter = PCD + 2 m = 180 + 2 ¥ 10 = 200 mm; Alternately outside diameter = (Z + 2) ¥ m = 20 ¥ 10 = 200 mm Root diameter = PCD – 2.314 m = 180 – 23 = 157 mm approx. Circular pitch = pm = 3.14 ¥ 10 = 31.4 mm Fillet radius = Circular pitch /8 = 31.4 / 8 = 4 mm approx. Angle between two teeth = 360/Z = 360 / 18 = 20°. Tooth thickness lies within = 20°/2 = 10°

538

Part E – Chapter 26

Fig. 26.S1

Drawing Involute Gear Tooth

Follow the steps given below: 1. Draw center lines to locate the center of the gear as O as shown in Fig. 26.S1. 2. Draw outside diameter circle of 200 mm, PCD of 180 mm, base circle diameter of 169 mm and root diameter or dedendum diameter of 157 mm as calculated above. 3. Draw radial lines at angle of 5° and extend them up to base circle and mark as 1, 2, 3, 4 and 5. Start the radial line at 5° as OP. 4. Draw tangents to base circle from these points. 5. With 1 as center and radius of length P-1, cut an arc on the tangential line drawn at point 1¢. Cut these tangential lines for all the points and mark them as 2¢, 3¢, 4¢, etc. 5. Join the points 1¢ through 7¢ to form an involute profile for one face. 6. Provide a fillet radius of 4 mm between radial line below the base circle and root circle. 7. Cut a card board of this profile and then reverse the card board to draw other flank of the tooth as mirror image about center line of tooth.

26.7

DRAWING APPROXIMATE INVOLUTE TOOTH PROFILE

Drawing tooth profile by the method given in Section 26.5.2 is quite laborious and time consuming without much advantage. Hence, tooth profile can be approximated by an arc of appropriate radius. The method to draw such an arc for teeth more than 30 is described in Section 26.7.1. The method for teeth less than 30 is slightly different and is described in Section 26.7.2.

26.7.1

Tooth Profile for Teeth More than 30

1. With O as center, draw outside diameter circle (addendum circle), pitch circle diameter and root diameter (dedendum circle). Mark point P on the PCD (Fig. 26.12). 2. Draw a semi-circle with OP as diameter which is PCD/2, i.e. its radius is PCD/4. Its center C1 is shown in Fig. 26.12. 3. With P as center and radius as (PCD/8), draw an arc cutting the semi-circle at X. 4. With O as center, draw another circle passing through X. Centers of arcs for tooth profile lie on this circle (circle for centers).

Gears

539

5. Mark points 1, 2, 3 and 4 on PCD at distance (CP/2). With radius R = PCD/8, draw arcs which pass through these points and its center is on the circle for centers. These arcs form the tooth flanks. 6. Fillet the flank and dedendum circle by fillet radius CP/8.

Fig. 26.12

26.7.2

Drawing Approximate Tooth Profile for Teeth Less than 30

Tooth Profile for Teeth Less than 30

1. Repeat Steps 1 to 5 as given in Section 26.7.1. 2. From the circle for centers, draw radial lines tangent to these arcs up to root circle (Fig. 26.13). 3. Fillet the radial line and root diameter circle (dedendum circle) by fillet radius CP/8.

Fig. 26.13

Drawing Approximate Tooth Profile for Teeth Less than 30

Note the difference in tooth shape that there are no radial lines in the profile for teeth more than 30.

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26.8

CONVENTIONAL REPRESENTATION OF GEAR TEETH

In a drawing where many gears are to be shown, to draw even an approximate profile will be time consuming. Hence, teeth are represented by only outside diameter and pitch circle (Fig. 26.14).

Fig. 26.14

26.9

Conventional Representation of Gear Teeth

CONSTRUCTION OF GEARS

Construction of gear depends upon its size as given below: Solid Webbed Armed

Small size spur gears are made solid (Fig. 26.15A). Gears more than 100 mm diameter are generally made webbed (Fig. 26.15B). The web can be solid or made light by holes in it (Fig. 26.15C). Gears bigger than 250 mm size are provided with arms (Fig. 26.15D). Number of arms depends upon diameter. Arms generally taper from hub to rim.

Fig. 26.15

Construction of Gears

Gears

541

Face width can be approximated as three times the circular pitch, i.e. W = 3CP. Web Thickness (Tw) can be taken equal to circular pitch, i.e. Tw = CP. Radial thickness (Tr) of the solid rim as a thumb rule can be taken as whole depth of the tooth. Hub diameter (Dh) can be taken as 1.5 to 2 times the hole diameter for shaft. Hub length (L) can be taken as 1.2 times the face width of gear, i.e. L = 1.2W.

26.10

SPUR GEARS

In spur gears, tooth contact is over one straight line over the tooth face at one instant and then it changes from one tooth to next tooth abruptly. Therefore, gear loads are applied suddenly developing high impact stresses and noise. These are easy to manufacture due to straight teeth. Example 2

Draw a webbed gear of outside diameter 240 mm and number of teeth 28 for a shaft of 30 mm.

Solution Number of teeth is given, i.e. Z = 28 Module = Outside diameter/(Z + 2) = 240 /30 = 8 Pitch circle diameter = Outside diameter – 2m = 240 – 16 = 224 mm Root diameter = Pitch circle diameter – 2.314 m = 224 – 18.4 = 205.6 mm Addendum (a) = m = 8 Dedendum (d) = 1.157 m = 9.2 mm Whole depth = a + d = 8 + 9.2 = 17.2 mm Circular pitch = p m = 3.14 ¥ 8 = 25.12 mm Face width W = 3 ¥ CP = 3 ¥ 25.12 = 75.36 say 75 mm (Only non-important dimensions can be approximated) Web thickness Tw = CP = 25 mm Hub diameter Dh = 1.75 ¥ hole diameter = 1.75 ¥ 30 = 52 mm Hub length L = 1.2 W = 1.2 ¥ 75 = 90 mm Solid radial thickness of rim Tr = Whole depth = 17.2 say 17 mm approx. After the calculations are done, the gear is drawn as shown in Fig. 26.S2. Right hand side is the drawing in conventional view.

Fig. 26.S2

Drawing a Spur Webbed Gear

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26.11

HELICAL GEARS

In helical gears, teeth are cut in the form of a helix on their pitch cylinder at an angle called helix angle (a). Gradual engagement of teeth due to helical profile results in less stresses and hence they can take greater loads than spur gears and offer silent operation. Like spur gears these are also used for transmitting power between two parallel shafts. Further, due to inclination of teeth, the gear causes axial loads and hence these gears need thrust bearings to take these loads. Double helical gears called Herringbone gears do not cause these thrusts as half of the gear is in opposite direction of teeth. Helix angle (a) varies from 15° to 30° for helical gear and 23° to 30° for Herringbone gears. Pitch circle diameter of a helical gear depends upon helix angle also. Addendum, dedendum and other proportions remain same as that for spur gears. Following are some terms related to helical gear: Normal Pitch Pn = CP ¥ cos a

Pitch circle diameter = Z ¥ Pn/(p ¥ cos a)

When helical gears mesh, if one gear has right hand helix, then its counter mating gear should have left hand helix as shown in Fig. 26.16.

Fig. 26.16 Helix Angles in Helical Gearing Example 3 Draw a helical gear in conventional representation with 30 teeth, 20° helix angle, hole diameter 30 mm and module 4. Solution Circular pitch = CP = pm = 3.14 ¥ 4 = 12.56 mm Normal pitch = Pn = CP ¥ cos a = 12.56 ¥ cos 20 = 12.56 ¥ .94 = 11.8 mm Pitch circle diameter = Z ¥ Pn/(p ¥ cos a) = 30 ¥ 11.8/(3.14 ¥ 0.94) = 354/2.9516 = 120 mm Addendum circle = PCD + 2 m = 120 + 8 = 128 mm Dedendum circle diameter = PCD – 2.314 m = 120 – 9.26 = 110.74 mm Dedendum = 1.157 m = 4.63 mm Gear width W = 3 ¥ CP = 3 ¥ 12.56 = 38 mm approx. Hub diameter Dh = 1.75 D = 1.75 ¥ 30 = 52.5 mm Fig. 26.S3 Hub length L = 1.2W = 45 mm approx. Web thickness Tw = CP = 12.5 mm Whole depth = 2.157 m = 2.157 ¥ 4 = 8.6 mm Solid rim thickness = Whole depth = 8.6 mm The conventional representation of the gear is shown in Fig. 26.S3.

Conventional Representation of Helical Gear

Gears

26.12

543

BEVEL GEARS

In bevel gears, teeth are cut on a conical surface. The apex of the cone lies on the shaft axis and is known as Vertex. These are used for transmitting power between intersecting shafts. Generally the angle between the shafts is 90° but they can transmit power at any angle. The cross-section of the tooth decreases from outside radius towards apex side. Bevel gears are of three types: a. Straight teeth bevel gears – Straight teeth converge towards the apex of the cone. b. Spiral bevel gears – Teeth are curved. Axes of the shafts intersect. c. Hypoid bevel gears – Teeth are curved but the axes of the shafts do not intersect. Only straight bevel gears are described. Figure 26.17A illustrates various terms used for bevel gears.

Fig. 26.17

Pitch line Pitch cone radius Pitch angle Addendum Dedendum Face width

Bevel Gear Nomenclature

A straight line joining the vertex and a point on outer edge of pitch circle diameter Length of pitch line (Rc). It is related with PCD as Rc = PCD/(2 sin a) Angle between axis of gear and pitch line (a) Height of the tooth from pitch line to outer edge Depth of the tooth below pitch line at the outer edge Length of the tooth approximated as W = Rc/3.

When two bevel gears are in mesh, it is known as bevel gear meshing (Fig. 27.17B). If size of the bevel pinion is small, the bevel gear is made integral with its shaft as shown in the figure. The pitch cone angles for bevel pinion and bevel gear are different and are found from the angle between the shaft axes and their pitch circle diameters. Sum of these angles is the angle between the shafts axes.

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Example 4 Draw sectional front view and side view of a bevel gear with 36 teeth, pitch angle 45°, hole diameter 40 mm and module 5. Solution Pitch circle diameter = Z ¥ m = 36 ¥ 5 = 180 mm Rc = PCD/(2 sin a) = 180/(2 sin a) = 90/ 0.707 = 127 mm Circular pitch CP = pm = 3.14 ¥ 5 = 15.7 mm Face width W = Rc/3 = 127/3 = 42 mm approx. Addendum = m = 5 Dedendum = 1.157 m = 5.78 mm Whole depth = 2.157 ¥ m = 10.78 mm Hub diameter varies from 1.5D to 2D assumed as 2D. Dh = 80 mm Hub length L = 1.2W = 50 mm approximately Web thickness Tw = CP = 15.7 mm say 16 approximately Solid rim thickness is taken equal to whole depth Tr = 10.8 mm approximately The view of the bevel gear with the above calculated dimensions is shown in Fig. 26.S4.

26.13

WORM AND WORM WHEEL

Fig. 26.S4 Bevel Gear with Straight Teeth

Worm and worm wheels are useful when high reduction in speed is desired. Power generally flows from the worm to worm wheel. If the gear ratio is high, it is an irreversible type of transmission, i.e. power flows from worm to worm wheel and cannot be from worm wheel to worm. It can be made reversible if the helix angle is taken of high value and less gear ratio. Worm is in the form of a screw of trapezoidal threads with large pitch that matches with the worm wheel having mating grooves of the same profile (Fig. 26.6). The threads can be left hand or right hand. Single start threads are most common but multi start threads can also be used. Driven member called worm wheel has inclined teeth at the same angle as the helix angle of worm. The teeth are cut on the concave shape at the outer periphery of the worm wheel. Radius of the top concave surface is according to the root diameter of the worm. The top threaded portion of the worm wheel envelopes the worm teeth. Diameter of the worm is based on strength calculations to transmit the required torque. Some terms used with worm and worm wheel are explained below and shown in Fig. 26.18. Axial pitch Lead y) Helix angle (y

The distance between two adjacent threads at the pitch circle diameter of worm. It is axial pitch multiplied by number of start Angle of threads of worm at pitch circle y = tan–1 (Lead/PCD of worm).

Length of worm (Lw) – Threaded length of worm along its axis Lw = ( Do 2 – PCD 2 ) Pressure angle f = 14.5° for single and double start worms and, f = 20° for triple and quadruple start worms. Face width W = 2.38 CP + 6 mm for single and double start worms and, W = 2.15CP + 5 for triple and quadruple start worms.

Gears

545

Gear ratio, G = Number of teeth on worm wheel/Number of start of worm. Throat diameter is the outside diameter at the centerline of the worm wheel. Dt = PCD + 2m Outside diameter at side of worm wheel = PCD + 0.9549 ¥ CP Web thickness, Tw = CP Solid rim thickness = whole depth = 2.157 ¥ m Hub diameter, Dh = 2 ¥ Hole diameter Hub length, L = 1.2 to 1.4 times the face width (W)

Fig. 26.18 Worm and Worm Wheel Nomenclature Example 5 Draw a sectional view of worm and worm wheel in engaged condition with the following data: Module 3 mm Gear ratio G = 30 Worm wheel hole diameter 35 mm Worm is double start with root diameter of 40 mm Solution Worm wheel size calculations: Number of teeth on worm wheel = 30 ¥ 2 = 60 PCD of worm wheel = 60 ¥ 3 = 180 mm Addendum (a) = m = 3 mm Dedendum (d) = 1.157 m = 3.5 mm Whole depth = Addendum + Dedendum = 3 + 3.5 = 6.5 mm

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Throat diameter (Dt) = PCD + 2m = 180 + 6 = 186 mm Root diameter (Dr) = PCD – 2 ¥ d = 180 – 7 = 173 mm Circular pitch (CP) = pm = 3.14 ¥ 3 = 9.42 mm Outside diameter (Do) = PCD + 0.9549 ¥ CP = 180 + 0.9549 ¥ 9.42 = 180 + 9 = 189 mm Face width (W) = 2.38 ¥ CP + 6 = 2.38 ¥ 9.42 + 6 = 28.4 say 28 mm Pressure angle for double start is 14.5° Thickness of web (Tw) = CP = 9.42 say 10 mm Thickness of rim (Tr) = 2.157 m = 6.5 mm Hub diameter (Dh) = twice the hole diameter 70 mm Hub length (L) = 1.4 ¥ W = 1.4 ¥ 28 = 39.2 say 40 mm Worm size calculations: Root diameter of worm = 40 mm (Given) PCD = Root diameter + 2d = 40 + 2 ¥ 3.5 = 47 mm Outside diameter of worm = PCD + 2a = 47 + 2 ¥ 3 = 47 + 6 = 53 mm Axial pitch = CP = 9.42 mm Lead = Axial pitch x number of start = 9.42 ¥ 2 = 18.84 mm Length of worm (Lw) = (Do2 – PCD2 ) = (1892 – 1802 ) = 57.6 say 58 mm The drawing as per the sizes calculated is shown in Fig. 26.S5.

Fig. 26.S5

26.14

Worm and Worm Wheel Gearing

RACK

A gear of infinite diameter is called a rack. Thus all circular dimensions become linear. To draw a rack, module, pressure angle and length of rack should be known.

Gears Example 6

547

Draw a rack of 10 teeth, module 10 with 20° pressure angle. Solid thickness equal to whole depth.

Solution

Addendum (a) = m = 10 mm Dedendum (d) = 1.157 m = 11.57 mm Whole depth a + d = 21.57 mm Circular pitch (CP) = pm = 3.14 ¥ 10 = 31.4 Tooth thickness = CP/2 = 15.7 mm Solid thickness = whole depth = 21.57 mm Length of rack = 10 ¥ CP = 314 mm The rack of the required size is shown in Fig. 26.S6.

Fig. 26.S6 A Rack

CAD 26.15

CAD FOR GEAR

AutoCAD can be of great help in drawing gear tooth, complete gear and pair of gears in meshed condition. The method can be mentioned in brief as under:

A Draw Involute Profile Draw involute profile by the method given in Section 26.5.2. Draw base circle, only one radial line and one tangent line (Refer Fig. 26.10). Use ARRAY command with Polar option to create many radial and tangent lines. Calculate the arc length for each point and draw circles with centers at 1, 2, 3, etc. Use SPLINE command to join the points of intersection of tangential lines and circles. B Draw Profile by Approximate Method Method given in Section 26.7.1 or 26.7.2 as required can be used to draw one side of the tooth. Draw pitch circle and then circle for centers (Refer Fig. 26.12 and 26.13). Draw addendum and dedendum circles. Find the arc radius graphically as shown in these figures. Draw radial lines with angle between than equal to 90/Z as shown in Fig. 26.19. With arc radius as PCD/8 and center on circle of centers draw an arc forming one face of the tooth. Draw another arc using MIRROR command about the adjacent radial line drawn at angle 90/Z. Draw radial lines between base circle and dedendum circle if number of teeth is less than 30. Fillet the flank of the tooth with root diameter using FILLET command and root radius as CP/8. C Drawing Gear Complete one tooth with tooth thickness as CP/2 within the radial lines between 360/Z angle by any one method given above in A or B. Use ARRAY command with Polar option to draw all other teeth. Select one complete tooth as objects to array, center of array as point O, number of items equal to number of teeth (Z) and angle to fill as 360°. The complete gear is drawn.

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Fig. 26.19

Drawing a Tooth with Approximate Method

Note: You can use EXTRUDE command to convert a 2D gear into 3D gear.

THEORY QUESTIONS 1. Define the terms: addendum, dedendum, whole depth, working depth and clearance. 2. Differentiate between module, circular pitch and diametrical pitch. How are they related to each other? 3. Sketch a gear to indicate the following parts of gear tooth: crest, face, flank, root radius, tooth thickness and face width. 4. Define base circle, pressure angle and their standard values. 5. Define addendum circle, pitch circle, dedendum circle and base circle with a sketch. 6. Compare the type of teeth for spur, helical and herringbone gear. 7. Define cycloid, epi-cycloid and hypo-cycloid curves. Sketch a gear tooth with these profiles. 8. What is an involute profile? Show by a sketch, how is this curve generated? 9. Describe the method to draw an approximate involute profile as an arc for 40 teeth gear. How does the method differ if there are 20 teeth? 10. What is the importance of conventional representation? Explain by sketches the conventions for spur, helical and bevel gears. 11. Describe the various construction shapes with sketches for spur gears of various diameters. 12. What is helix angle and normal pitch? How is the pitch of a helical gear calculated? What are the advantages of helical gear over spur gear? 13. Describe the construction and use of bevel gears with a sketch. 14. Explain the construction of worm and worm wheel. How can they be made reversible? 15. Explain the method to draw a rack of given module and pressure angle with a sketch.

CAD 16. Write how AutoCAD can be used to draw a gear. 17. If one tooth of the gear is drawn, how can a gear be drawn with all teeth? 18. How can a cycloid be drawn using AutoCAD?

Gears

FILL 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

IN THE

549

BLANKS

Circular pitch is the distance between two at pitch circle. . Radial distance between outside diameter and pitch circle is called Herringbone gear is a special type of gear. is preferred for speed reduction where gear ratio is high. Generally the profile of gear tooth is . For a cycloid tooth profile, face is of curve and flank is of An involute profile is generated on circle. Angle between common tangent at pitch circle and line of action is called Arc radius for approximate tooth profile is . A gear is made webbed if its radius is more than mm and less than Radial thickness of the solid portion of the gear rim is equal to . Web thickness is taken equal to . is the angle between pitch line and axis of the bevel gear. Radius of worm wheel at the outermost circle at the center line is called

curve. . mm.

.

CAD 15. If one face of the tooth is drawn, the other face can be drawn by using command. command with option can be used to copy all the teeth if one tooth of the gear 16. is drawn.

MULTIPLE CHOICE QUESTIONS 1. Proportion of addendum in terms of module (m) is (a) m (b) 1.25 m (c) 1.5 m (d) 0.8 m 2. Base circle of an involute gear lies (a) always between PCD and root diameter (b) between PCD and outside diameter (c) below root diameter (d) depends upon other dimension parameters 3. A higher value of pressure angle makes the gear tooth (a) weak (b) wear resistant (c) strong (d) less noisy 4. A bevel gear has tooth thickness (a) constant for whole length (b) depends upon radius (c) depends upon cone angle (d) depends upon tooth depth 5. A worm and worm wheel transmission is used when (a) speed reduction is high (b) less friction is required (c) there are space restrictions (d) no arrangement for cooling gears 6. A worm and worm wheel offers (a) always reversible transmission (b) always irreversible transmission (c) depends upon on gear ratio (a) depends upon tooth profile 7. A helical gear is used for power transmission where (a) power is to be transferred at angle (b) less noise is required (c) less friction is required (d) lubrication can not be provided

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8. Pressure angle for heavy load is kept (a) 14.5° (b) (c) 25° (d) 9. Part of the tooth just below pitch circle is called (a) face (b) (c) crest (d) 10. Worm has thread cross-section as (a) Involute (b) (c) Hypo-cycloid (d) 11. Angle between two axes of bevel gears is (a) always 90° (b) (c) 135° (d) 12. A tooth profile requiring exact center distance is (a) Involute (b) (c) cycloid (d) 13. Approximate radius of arc for tooth profile is (a) PCD/4 (b) (c) CP/4 (d) 14. A gear of 350 mm outside diameter is normally (a) solid (b) (c) webbed with holes (d) 15. Hub length is taken equal to (a) hole diameter (b) (c) face width (d)

20° 45° flank root Epi-cycloid trapezoidal 45° any angle straight circular PCD/8 CP/8 webbed armed 1.2 to 1.4 face width hub diameter

CAD 16. A gear is made using AutoCAD by (a) GEAR command (b) CYCLOID command (c) INVOLUTE command (d) None of a, b, c 17. Procedure to draw a gear is to draw (a) one tooth and use ARRAY command with polar option (b) radial lines and fit every tooth within these lines (c) one face of the tooth and use MIRROR command (d) quarter gear and use MIRROR command about X and Y axes two times

ANSWERS to Fill in the Blank Questions 1. 5. 9. 13.

adjacent teeth involute PCD/8 Pitch angle

2. 6. 10. 14.

addendum 3. helical epi-cycloid, hypo-cycloid 7. base 100, 250 11. whole depth Throat radius 15. MIRROR

4. 8. 12. 16.

worm and worm wheel pressure angle Circular pitch ARRAY, Polar

ANSWERS to Multiple Choice Questions 1. (a) 7. (b) 13. (b)

2. (d) 8. (c) 14. (d)

3. (c) 9. (b) 15. (b)

4. (b) 10. (d) 16. (d)

5. (a) 11. (d) 17. (a)

6. (c) 12. (c)

Gears

ASSIGNMENT

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ON

GEARS

1. Draw at least two involute teeth for 24 teeth spur gear, 20° pressure angle and module 8. Name important parts of the tooth. 2. Calculate the sizes required for a helical gear of outside diameter 400 mm and 38 teeth and helix angle 20°. Draw the same. 3. Draw cycloid profile for a wheel of 50 mm diameter. 4. A 200 mm PCD involute spur gear meshes with a pinion of 150 mm PCD. The teeth have module 10 and pressure angle 14.5°. Draw addendum, dedendum, pitch circle and base circles. Show at least two teeth in mesh. 5. Draw a bevel gear of 30 teeth by conventional method for pitch angle 40° and module 6. 6. Draw a single start worm for axial pitch 15 mm and outside diameter 55 mm.

CAD ASSIGNMENT

ON

GEARS

7. Draw a spur gear with 28 teeth, module 6 with approximate profile. 8. Draw a spur gear of 40 teeth with module 6. Copy the gear drawn in Q 7 and rotate it suitable to show both these gears in meshed condition. 9. Draw epi-cycloid profile for a wheel of 50 mm diameter rolling over a curved surface of radius 250 mm. 10. Draw a rack of 14 teeth, pressure angle 20°, module 8 meshed with a pinion having 12 teeth.

PROBLEMS

FOR

PRACTICE

11. Draw two gear teeth of involute profile with pitch circle diameter as 192 mm and number of teeth as 16. 12. Make a drawing showing a pinion and a gear of PCD 100 and 300 mm with number of teeth 20 and 60 respectively, face width 30 mm and hole diameters 35 mm. 13. With the same dimensions as in Q 12 and draw a helical gear with helix angle of 20°. 14. Draw a bevel gear with pitch circle diameter as 150 mm, face width 30 mm and number of teeth as 15 to mate another bevel gear with 90° axes and 30 teeth. 15. Draw a rack showing 6 teeth and pitch of 30 mm and pressure angle 14.5°. 16. Draw a single start worm and worm wheel with the following data: Worm PCD 40 mm Axial pitch 10 mm Shaft diameter 25 mm Worm length 60 mm Worm wheel Number of teeth 40 Hole diameter 35 mm Face width 30 mm 17. Draw a hypocycloid profile of concave surface of diameter 300 mm with a wheel of 40 mm rolling over it.

PART F Machines CHAPTER

27

Part and Assembly Drawings A machine comprises of many parts joined together. A working drawing supplies information and instructions for manufacturing of machines. Working drawings can be classified as Detail drawing and Assembly drawing. Detail drawing provides information about shape with enough number of views, size with tolerances, geometrical tolerances, datum, specifications about surface texture, heat treatment and additional information like part number, drawing number, scale, method of projection symbol and names of drafter, checker and approver with dates in the title block. Selection of paper size for a part drawing is decided by number of views, overall size of each view and scale chosen. Size of text should be easily readable. Drawings with all the dimensions but without other details like tolerances, etc. are sometimes called as Part drawings. As an additional precaution a check list can be prepared to check the drawing for all the points. If a part is completed by many processes, then multiple drawings are to be prepared, one for each process. While modifying processes, revisions are made and it is not felt necessary to change the drawing, then only the revisions are mentioned on a Revision table showing revision number, description of the dimension being revised, date of revision and signature of the person who approves it. Assembly drawing is prepared from the part drawings and informs relative position of parts. A drawing showing all the parts in assembled state is called Assembly drawing. They usually indicate overall dimensions, center to center dimensions, dimension between parts, operating instructions, etc. There are many types of assembly drawings and each has a different purpose. Design assembly drawings are prepared at the design stage while developing a machine. Each part is given a part number. These numbers are placed over the assembly drawing enclosed in a circle with a leader line pointing towards the part. Detail assembly drawings are made for simple machines having less number of parts. Each part is dimensioned for fabrication on the assembly drawing itself. Sub-assembly drawings are drawn when a machine has large number of parts and it is not possible to give all the dimensions in one drawing. Hence, an assembly drawing is split in sub-assembly drawings and sub-subassembly drawings. Installation assembly drawings give dimensions important from installation point of view. Catalog assembly drawings are specially prepared for catalog of a company to create interest in the product by the potential buyer. Important dimensions are given in letters like A, B, C, etc. on drawing with a table showing size for these letters for different range of sizes/models.

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Exploded assembly drawings help a reader in clearly visualizing each part. Exploded drawing can be pictorial or orthographic. Bill of materials (BOM) is a table in assembly drawing giving list of all the components. It is also called item list. This list of items is used by the purchase department to procure the right type of material in required quantity. Standard components like bolts, nuts, etc. should also have a part number and mentioned in the list. Blue print reading is the art of engineers to interpret the drawings and know the operations required for manufacturing. Blue print is a copy taken on ammonia paper from the drawing on tracing paper. Blue print reading enables to understand shape, size and manufacturing operations to produce it from a part drawing. Blue print reading of assembly drawing is to understand function, size, position, quantity, material and relative positions of each part. Now blue print is obsolete however some industries still follow this method of duplicating a drawing.

CAD To create an assembly drawing, WBLOCK can be used to create blocks of different parts and then joined together using INSERT command. To create part drawing from assembly drawing, select the required part completely by windowing and then copy elsewhere with COPY command. Delete the extra selected lines by selecting them and using Del key or ERASE command.

27.1

INTRODUCTION

A machine comprises of many parts joined together in relation to each other. Every part has a role to play. A working drawing supplies information and instructions for manufacturing or construction of machines. These drawings may be sent to another company to make assembly of parts and hence drawing should be according to the standard conventions of their country. Working drawings can be classified in two categories:

A Detail Drawing It provides information about size and tolerances for the manufacture of the parts. A dimensioned drawing of a part without details of tolerances, surface finish and other details is sometimes called Part drawing. B Assembly Drawing It informs about the relative position of parts for assembling. It is prepared from the part drawings. 27.2

DETAIL DRAWING

Selection of paper size for a detail drawing is decided by number of views, overall size of each view and scale chosen. Size of text, etc. should be carefully selected so that the information provided on the drawing is easily readable.

27.2.1

Requirements of Detail Drawing

Detail drawing must have information in following four categories:

A Shape Description There should be enough number of views to describe the shape of a part. Generally orthographic views are drawn. Sectional views, auxiliary views and enlarged views may be added if outside views are not able to show the shape clearly. B Size Description The part should be fully dimensioned. Duplicate dimensions should be avoided. Tolerances, geometrical tolerances and datum features are to be indicated on the drawing as these affect the manufacturing

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process selection. Dimension should be so selected that it does not require any calculation, e.g. for cylindrical objects give diameter and not radius. Specifying radius will require calculation. Dimension should be given in a way in which it is to be checked. Fillet should be given by its radius and not diameter.

C Specifications General notes about material, heat treatment, general surface finish, general tolerances are indicated on or near the title block. D Additional Information This includes name of part, drawing number, scale, method of projection symbol and names of drafter, checker and approver with dates. All this information is given in the title block. 27.2.2

Checklist for a Detail Drawing

As an additional precaution some companies provide check list for the person who checks the drawing. A sample check list is as under: 1. Completeness – Do all the views indicate complete shape description? 2. Dimensions – Are all dimensions given and placed at proper position? 3. Tolerances – Are the tolerances specified for linear and angular dimensions? 4. Surface texture – Is the surface roughness specified for the different surfaces or by a general note? 5. Material – Has the material been specified? 6. Standards – Does the drawing confirm the standard selected? 7. Scale – Is the drawing according to scale shown in the title block?

27.2.3

Multiple Drawings

Sometimes a part is completed not by one process but many processes are required to complete it. For example a part made by casting has to have a pattern first which should include all the pattern allowances like draft allowance, shrinkage allowance and machining allowance. Then it is casted and finally machined on regular machines or specialized machines for required surface texture, etc. Several drawings are prepared for the same part for each process. For a forged part, one drawing can be of forging (Fig. 27.1A) and other of finished part (Fig. 27.1B). Note the difference in sizes for the areas to be machined and also the tolerances on each drawing. A fabricated part will have the drawing of each portion and then location and size of welding to join different portions.

Fig. 27.1A A Part Drawing of Forged Connecting Rod

Part and Assembly Drawings

Fig. 27.1B

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Part Drawing for Forged Part Shown in Fig. 27.1A

Multiple part drawings can be drawn on separate sheets for each process or they can be grouped on one large sheet. Generally the grouping is done according to the department in which it is to be manufactured. These parts are made in different shops and then sent to assembly shop. For easy assembly each part is given an identification number by which these are referred in assembly drawing. If parts are less in number, assembly drawing can also be given on the same sheet.

27.2.4

Drawing Revisions

A company always tries to reduce cost and improve quality by modifying processes. Revision drawings are made when it is not felt necessary to change the whole part drawing. Only the revisions made are mentioned on a revision table as shown in Table 27.1. The table shows revision number, description of the dimension being revised, date when revision is made and signature of the person who approves it. Table 27.1

Revision table

Revisions Rev. No.

Description

Date

1

Diameter changed from 25 to 20 mm

5/12/2006

2

Fillet added of radius 5 mm

23/5/2007

Approval

If the revisions are too many, it is recommended that a new part drawing be made and mention a note as REVISED. The revision number can be indicated in the title block with the new date.

Example 1

Figure 27.S1A shows an assembly drawing of screw jack. Draw its part drawings.

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Fig. 27.S1A

Assembly Drawing of Screw Jack

Fig. 27.S1B Part Drawing of Screw Jack

Solution The solution is shown in Fig. 27.S1B

27.3

ASSEMBLY DRAWINGS

All machines are built of many parts. A drawing showing all the parts in combined state is called Assembly drawing. They usually indicate overall dimensions, center to center dimensions, dimension between parts, operating instructions, etc. These are used for different purposes and their name is given accordingly. Following are the types of assembly drawings:

27.3.1

Design Assembly Drawings

These drawings are prepared at design stage while developing a machine. It helps to clearly visualize performance, shape and clearance of various parts. From this drawing, detailed drawings are made and each part is given a part number. These numbers are placed over the assembly drawing enclosed in a circle (circle is optional) and point towards the part with a leader line (Fig. 27.2). Part numbers should not be identical on different parts when several items are listed. These drawings are drawn at a larger scale for clarity of each part. Modifications for any change thought by the designer are made for the functional operation or aesthetic appearance.

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Part list of lathe is given in Chapter 30 on Machine tools in Section 30.2

Fig. 27.2

27.3.2

Design Assembly Drawing with Part Numbers

Detail Assembly Drawings

These are made for simple machines having less number of parts. Each part is dimensioned for fabrication on assembly drawing itself. Figure 27.3 shows a working drawing for a machine vice with dimensions and part numbers.

Fig. 27.3 Working Assembly Drawing

27.3.3

Sub Assembly Drawings

When a machine is large having many parts it is not possible to give all the dimensions in one drawing. Hence, an assembly drawing is split in sub-assembly drawings. For example a car will have a sub assembly drawing of an engine, power transmission system, Braking system, steering system, suspension system, body, etc. Then each sub assembly can be further split in sub-sub assembly drawings. For example power transmission system may have sub-sub assembly drawings of clutch, gear box, differential, axles, etc.

27.3.4

Installation Assembly Drawings

These drawings give relation between different units of machine giving location and dimensions of important parts from installation point of view. For example for installation of a machine, center distances between the foundation bolts will be of more importance. These are mentioned in this type of drawing. Overall length, height and width are important for selecting a suitable location for installation.

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27.3.5

Catalog Assembly Drawings

These are specially prepared for catalog of the company to create interest in the product by the potential buyer. Important dimensions are given in letters like A, B, C, etc. with a chart showing the meaning of these letters for different range of sizes/models (Table 27.2). Figure 27.4 shows a catalog drawing for a lathe machine. Overall dimensions and other specifications like length of bed, swing and maximum length of the job are indicated as letters. Sizes of different models are tabulated in the Table 27.2. Table 27.2 Models of lathe and their relevant dimensions Model No.

A

B

C

D

E

LB – 900

150

1200

1500

900

900

LB – 1200

200

1500

1850

900

1200

LB – 1500

225

1850

2500

950

1500

LB – 1800

250

2200

3000

950

1800

D

E

B

C

Fig. 27.4

27.3.6

Shows a Catalog Drawing

Exploded Assembly Drawing

It is difficult for an inexperienced person to read and interpret engineering drawing correctly. Exploded drawings help the reader in clearly visualizing each part and their relative position. Exploded drawing can be pictorial or orthographic. Figure 27.5 shows an exploded pictorial assembly drawing and Fig. 27.6 exploded orthographic drawing.

27.4

BILL OF MATERIALS

A table giving all the components is shown on assembly drawing called item list or Bill of Materials (BOM). It can be prepared separately also for the ease of handling and duplicating. This list of items is used by the purchase department to procure the right material and required quantity, hence it should show the name of raw material size rather than finished size. Standard components like bolts, nuts, washers, bearings, belts should have a part number and mentioned on the list. Information given should not have any ambiguity while purchasing these items. If this table is put at the top of drawing, its sequence should be from top to bottom. If the table is put at the bottom of drawing, its sequence should be from bottom to top. This practice permits easy additions at any stage. A typical list is shown in Table 27.3.

Part and Assembly Drawings

Fig. 27.5 Exploded Pictorial Assembly Drawing Table 27.3 Part No. 1 2 3 4 5 6 7 8

27.5

Item Body Cap Bearings Shaft Key Set screw Bolts and nuts Spring washers

559

Fig. 27.6

Exploded Orthographic Assembly Drawing

Bill of materials for a bearing block Description Casting Casting Radial ball bearing 5200 Ø 25 ¥ 200 Square Allen head Hexagonal M15 ¥ 30 Helical

Material C.I. C.I. SKF M.S. M.S. M.S. M.S. Spring steel

Quantity 1 1 2 1 1 1 4 4

STEPS FOR CREATING ASSEMBLY DRAWINGS

Following steps can help to create assembly drawing from the part drawings. Scope of the work is limited to only simple machines for classroom work. 1. Understand function, working principle and field of application for the machine. 2. Try to understand function of each part separately.

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3. Study external and internal features of each part. 4. Sketch free hand a block diagram of each part at their relative positions and estimate the overall size of the view. 5. Decide the number of views which may be required to give all details. 6. Estimate the overall size of each view. 7. Estimate the overall size of the whole drawing after making some allowance for the space between views and for dimensions. 8. Draw axes for each view and first start drawing the front view with the main biggest part. Then go on adding the small parts by seeing the dimension and place of each part. 9. Project other views from the front view. 10. Put the overall dimensions. 11. Label the parts by numbers enclosed in circles and leaders. 12. Prepare a table of parts giving part number, its name, material and quantity. Example 2

Figure 27.S2A shows parts of a crane hook. Draw its assembly drawing with the main dimensions.

Fig. 27.S2A Part Drawing of a Crane Hook

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Table 27.4

Bill of materials for a Crane hook

Part No.

Item

1 2 3 4 5 6 7 8

Side plate Pulley Pulley pin Nut Nut lock pin Slotted nut Hook trunion Hook

Material

Quantity

M.S. C.I. M.S. M.S. M.S. M.S. M.S. Forged steel

2 2 2 6 1 1 1 1

Solution The Assembly drawing of crane hook is shown in Fig. 27.S2B. Bill of materials is in Table 27.3.

27.6

BLUE PRINT READING

Blue print is a copy taken on ammonia paper from a drawing on tracing paper. Color of this copy is slightly bluish, therefore it is called Blue Print. Nowadays with the development of new reproduction methods like Xeroxing, plotters, etc. blue print is obsolete. However, some industries still follow this method. Blue print reading has nothing to do with the blue print but the name still continues. It is the art of engineers to interpret the drawings and know the Fig. 27.S2B Assembly Drawing of operations required for manufacturing, etc. Crane Hook Blue print reading enables: a. Understand shape and size of a component from a detail drawing along with manufacturing operations required to produce it. b. Understand relative positions and function of each part from the assembly drawing.

27.6.1

Reading Detail Drawings

Try to deduce the following by reading a detail drawing: ∑ Study method of projection from the relative position of the views or from the projection method symbol in the title block. ∑ Study the views and try to imagine physical shape of the object. ∑ Study maximum length, width and height of the part. ∑ Get the dimensions from different views. ∑ Study the tolerances and geometric tolerances provided. ∑ Study any surface roughness specifications. ∑ Read the material specifications.

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∑ Study the manufacturing process used to make that part. ∑ Read any special treatment if required, e.g. heat treatment, electroplating, etc. ∑ Note the type of threads, pitch, length and diameter, etc. ∑ Study the size, number and types of holes like blind, through, countersunk, etc. ∑ Identify if any special jigs and fixtures are required to produce that part. ∑ Note if there is any mention about the inspection method. ∑ Read notes on lot size or number of parts to be produced. Example 3 (Blue print reading – Part drawing) Figure 27.S3 shows a link supporting two ends; one hexagonal and other square. Interpret this drawing. Solution Interpretation of the drawing is given as follows: Views: The job is complicated and hence requires three views for better understanding. Material – Cast iron Manufacturing: Overall size of the part is 400 mm long, 188 mm high and 124 mm thick. Sand casting is to be done to give shape to the Fig. 27.S4 job. Hence material is cost Iran. The curved surface of ellipse does not need machining for its function and hence will remain as such. The web in the middle is only for strength and does not need any machining. Diameters 160, 121 and 78 are to be prepared on lathe machine. For small number, it can be done by holding in 4 jaw chuck. For large numbers, a fixture will be better. Hexagonal and square holes are to be made on slotting machine. Part description Overall shape is elliptical It has two holes; one hexagonal with distance across corners 82 mm and other square with diagonal distance 60 mm. Center distance between two holes is 220 mm. Shape in the middle of the part is of I section with flange width 60 mm and web in the center is 32 mm thick. Surface roughness is not mentioned it is presumed the maximum obtainable by lathe, i.e. 25 microns.

27.6.2

Reading Assembly Drawings

Assembly drawings show how and where the parts are to be fitted. There is no standard practice for dimensioning an assembly drawing. Normally it should contain overall dimensions, center line to center line distances, main dimensions, labeled part numbers, part list, etc. Detailed information about a part

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can be obtained from the part drawings. Interpretation and understanding method of assembly drawing is given as follows: (a) Study the part list to get total number of parts, their part numbers, material of each and quantity required. (b) Study the purpose of the machine, for which it is being used. (c) Study the machine description as under: ∑ Study each part individually ∑ Try to understand the purpose of that part ∑ Get the size of every part ∑ Find how a part is fixed? ∑ What is the relative position of each part with respect to the other? ∑ How many holes, their size and other information like threaded/unthreaded lengths, blind or full, countersunk at the top or not? ∑ How many bolts, nuts, their type of head, length and diameter, left hand or right hand, number of start. ∑ Distance between center lines ∑ Material of each part ∑ Tolerances, if any ∑ General notes specifications Example 4 (Blue print reading – Assembly drawing) Figure 27.S4 shows a trolley for a tower crane. Interpret this drawing.

Fig. 27.S4

Assembly Drawing of a Trolley for a Tower Crane

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Solution Interpretation of the drawing is given as follows: Application: The figure is for a tower crane trolley, which is used to roll over the radial arm of the tower crane on four wheels; two on each side. The lowermost hole in the frame is used to hang a hook there. Views: Two views are shown with the part numbers, name, material and quantity required. Materials used – Cast iron, Cast steel and M.S. Manufacturing – Each part is to be made as follows: Part 1 – Trolley frame is made by sand casting and then machined for the faces for the nut, bush, etc. on lathe machine with a suitable fixture as this can not be fixed directly on the machines. Part 2 – Trolley wheels are sand casted and then machined on lathe. Part 3 – Wheel bush is made by simple turning on lathe Part 4 – Wheel support is made on lathe by turning outside, then drilling and boring and finally threading on lathe. A keyway is provided so that it does not rotate with the wheel by milling machine. Part 5 – Bolt is turned and then threads are cut on lathe. Hexagonal head can be made by milling. Part 6 – Woodruff key can be made by cutting and then filing.

27.7

ASSEMBLY OF PARTS AND THEIR DETAILS

1. The trolley frame is a U shaped casting and has two holes on both the sides, each of 35 mm diameter with center to center distance 150 mm. The casting is 10 mm thick while the bosses are 8 mm thick on outside and 4 mm on inside with diameter 54 mm. Both the faces of the bosses are machined. The bottom leg at the bottom is 20 mm thick with a hole of 15 mm diameter. A key way of 6 mm width and 3 mm depth is cut on the top inside the hole. 2. Wheels have maximum diameter 80 mm with a boss on one side of diameter 54 mm and thickness 4 mm. Then there is a collar of radius 2 mm. A taper of 5° is given on its outer periphery which helps in keeping the wheels of both the sides centrally. Width of the wheel is 33 mm. 3. Wheel bush has outer diameter 35 mm, inside diameter 29 mm and length same as wheel, i.e. 33 mm. Bush is inserted in the hole of the wheel. 4. Wheel support has a collar of 45 mm diameter and length 6 mm followed by a step of 33 length and diameter 29 mm. A step of 16 mm length and 23 mm diameter is made again. A semi-circular groove of 6 mm width and radius 6 mm is made to accommodate key in this step. 5. Bolt of hexagonal head has body diameter 23 mm and length 8 mm. The threaded portion is of 12 mm diameter with standard coarse threads of length 30 mm. It is tightened from outside in the support. 6. Woodruff key is 6 mm wide and 6 mm radius of semicircular shape is fitted between frame and support.

CAD 27.7.1

Creating Assembly Drawings from Part Drawings

WBLOCK command discussed in Chapter 14 on Bolts and nuts is a very useful tool for assembly drawings. Create a part in AutoCAD then use WBLOCK command. In the Write Block dialog box,

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select the complete part as a block. Selection of the base point is important. Generally a point on center line at top or bottom is found more convenient. Study of assembly drawing can guide where to locate the base point. Save it in any folder but remember the location of folder. Specify its name, which should be representative of the part name for convenience. Save all the parts by the method described above. For creating assembly drawing, open a new file. Start inserting parts created as blocks one by one. First start inserting with the biggest part. Use INSERT command, Insert dialog box is displayed. In this dialog box, click Browse… button and scroll the list in the path defined earlier and click on the file required. Click Open button. Set X, Y and Z scales as 1. Then specify the insertion point. Click OK button. The part is inserted in the drawing along with dimensions. Go on inserting each part one by one at the required locations. Some of the dimensions may interfere with each other. Shift their location by clicking on them and move by using grips (Blue squares).

27.7.2

Creating Part Drawings from Assembly Drawings

If assembly drawing is available, first identify all the parts in the drawing. Use COPY command and then make a window to completely enclose one part. Window should be created from left corner to right side diagonal corner. This will not select the parts which are partially cut by the window. On the other hand, if a crossing window (first corner of window on right side and second corner on left diagonal side) is used for selecting a part, then all the lines, which are even partially coming in the window will also be selected. So, it is recommended that use only simple window for selection and NOT crossing window. Press Enter key after selection is complete. Entities can be selected one by one also in the selection set. Specify any suitable base point and when prompted for second point of displacement, click where the part is to be placed. Selected entity can be cancelled by pressing shift key and then clicking over it. Use Del key or ERASE command to remove the unwanted lines for that part. Use the above mentioned method for every part and place them with suitable spaces between the parts.

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

What are the two main types of drawings? Differentiate between each type by examples. Write the requirements of a detailed drawing. What is a check list for a detailed drawing? Write the points to be checked. When are multiple drawings prepared? Explain by example. Describe how revisions are made in a drawing? What are the various types of assembly drawings? Write the typical use of each. Describe a detailed assembly drawing with an example. How does a design assembly drawing differ from a detailed assembly drawing? Differentiate between Catalog assembly drawing and Installation assembly drawing. What are sub and sub-sub assembly drawings? Explain their uses by an example. What is the use of an exploded assembly drawing? What are the different types? Write a note on Bill of Materials. What are the details extracted while reading a blue print of a detailed drawing? How does blue print reading of an assembly drawing differ from blue print reading of a detailed drawing?

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CAD 15. 16. 17. 18.

Write the method to create an assembly drawing using AutoCAD. How is a part drawing saved, if it is to be used as drawing of another assembly drawing? Describe the method to create an assembly drawing from part drawings. Explain the method to make part drawings from Assembly drawings.

FILL 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

IN THE

BLANKS

Drawings are classified mainly as and drawings. drawing. Type of drawing, which gives tolerance is called to finish a job. Many drawings of the same part are made when there are many drawing. Revision table is put on a numbers are put on a design assembly drawing. number of parts. Sub assembly drawings are used when machine has . Foundation details and over all dimensions are given in has main dimensions in terms of letters. Exploded assembly drawings are used for visualization. . A table containing list of items, material and quantity is called . A copy of drawing taken on ammonia paper is called Blue print reading means a drawing.

CAD command is used to create detailed drawings of different parts to be used for assembly drawing. command. 14. Saved parts can be put together using 13.

MULTIPLE CHOICE QUESTIONS 1. A drawing indicated all tolerances and other information is called (a) detailed drawing (b) assembly drawing (c) auxiliary drawing (d) blue print 2. If a part is produced by different processes, the drawings to be used are (a) detailed (b) production (c) multiple (d) process 3. A slight modification in a drawing is done by (a) multiple drawing (b) revision table (c) revised new drawing (d) detailed drawing 4. Part numbers are given on (a) design assembly drawing (b) Installation assembly drawing (c) catalog assembly drawing (d) detailed assembly drawing

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5. A drawing with complete dimension of every part in an assembly drawing is called (a) design assembly drawing (b) detailed assembly drawing (c) Sub assembly drawing (d) working drawing 6. A machine having many systems and each having many parts will require (a) detailed assembly drawing (b) sub assembly drawing (c) sub-sub assembly drawing (d) layout drawing 7. Drawing having list of products with sizes in terms of letters A, B, C, etc. is called (a) catalog assembly drawing (b) Installation assembly drawing (c) design assembly drawing (d) model assembly drawing 8. An assembly drawing showing parts in pictorial views at specific locations is called (a) detailed assembly drawing (b) catalog assembly drawing (c) exploded assembly drawing (d) pictorial drawing 9. Bill of materials contains (a) total cost of material (b) part-wise cost of material (c) part-wise and total cost of material (d) Name of part with its description, material and quantity 10. A copy of drawing on ammonia paper is called (a) Xerox copy (b) plotter drawing (c) blue print (d) yellow print 11. Blue print reading means (a) reading text on the drawing (b) getting size of a part (c) getting overall size of a part (d) visualize shape and extracting all information

CAD 12. Command to save a drawing to be available in other drawing is (a) PART

(b) BLOCK

(c) WBLOCK

(d) SAVE

13. Command to copy a saved block in a new drawing is (a) COPY

(b) COPYCLIP

(c) PASTE

(d) INSERT

ANSWERS to Fill in the Blank Questions 1. Detailed, Assembly 5. Part

2. Detailed 6. large

9. better

4. revised

7. Installation drawing

10. Bill of materials

13. WBLOCK

3. processes

8. Catalog drawing

11. Blue print

12. interpreting

14. INSERT

ANSWERS to Multiple Choice Questions 1. (a)

2. (c)

3. (b)

4. (a)

5. (b)

6. (c)

7. (a)

8. (c)

9. (d)

10. (c)

11. (d)

12. (c)

13. (d)

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ASSIGNMENT

ON

PART

AND

ASSEMBLY DRAWINGS

1. Figure 27.P1 shows an assembly drawing of a swivel bearing. Draw its part drawings.

Fig. 27.P1 Assembly Drawing of a Swivel Bearing

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569

2. Figure 27.P2 shows parts of worm and worm wheel type of reduction gear box. Draw its assembly drawing.

Fig. 27.P2 Part Drawing of Reduction Gear Box

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CAD ASSIGNMENT

ON

PART

AND

ASSEMBLY DRAWINGS

3. Figure 27.P3 shows an assembly drawing of a caster wheel. Draw its part drawings.

Fig. 27.P3

Assembly Drawing of Caster Wheel

Part and Assembly Drawings 4. Figure 27.P4 shows parts of V belt drive. Draw its assembly drawing.

Fig. 27.P4

Assembly Drawing of V Belt Drive

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Part F – Chapter 27

572

HOMEWORK

5

5. Figure 27.P5 shows a part drawing of a flange coupling. Draw its assembly drawing. Ø60

2

28

Ø18

30

25

3

Ø12

30

Ø18 M10

PCD 90 Ø128

3

4

28

Ø12

R5

R1

5

M10

Ø18 4 Ø30 PCD 90

4

SQ8

4

Qty. 1 1 4 4 4 2

4

55

6

8 C.I. C.I. M.S. Rubber M.S. M.S.

Material

Part name Flange 1 Flange 2 Bolt Bush Nut Key 8×8

1

No. 1 2 3 4 5 6

Ø60

Key way 8 4

30

R2

25

Part List

Ø10

15

Ø128

Fig. 27.P5

Part Drawing Flange Coupling

3

Part and Assembly Drawings

PROBLEMS

FOR

PRACTICE

6. Figure 27.P6 shows an assembly drawing of an automatic clutch. Draw its part drawings.

Fig. 27.P6 Assembly Drawing of an Automatic Clutch

573

574

Part F – Chapter 27

Fig. 27.P7

Assembly Drawing of Friction Clutch

7. Figure 27.P7 shows an assembly drawing of a friction clutch. Draw its part drawings.

CHAPTER

28

Internal Combustion Engines Petrol and diesel engines are called Internal Combustion engines because the combustion of the fuel takes place inside the cylinder. Fuel and air mixture (called charge) is sucked inside the cylinder in suction stroke for petrol engines (only air for diesel engines) and compressed in compression stroke to increase pressure and temperature at the end of compression stroke. In petrol engines, fuel is ignited by a spark plug, while in diesel engines; the injected fuel ignites by itself due to high temperature. The high pressure of burnt gases pushes the piston downwards and this is called power stroke. These burnt gases are then turned out in exhaust stroke. This cycle of four strokes continues in every two revolutions. Such engines are called four stroke engines. Medium and large engines work on four stroke cycle. Small engines are generally two stroke engines, in which bottom part of the piston is also used to do a part of suction and compression work and the cycle is completed in two strokes only i.e. one revolution. An I.C. engine consists of many systems described as follows: Power system transfers energy developed in the cylinder to the crank shaft. It consists of cylinder, cylinder liner, piston, piston rings, connecting rod and crank. Cams on cam shaft control inlet and exhaust valves. In overhead cam shaft engines, cams are on two cam shafts, which directly push the valves to open. In an alternate arrangement a single cam shaft is used. Cam followers push a push rod, which in turn transfer motion to the valve through rocker arms supported on a rocker shaft. Cams are of two types, radial and cylindrical. A follower rests over the cam, which could be knife edge, round or roller type. Movement of the follower may have constant velocity, constant acceleration or simple harmonic motion. Fuel system takes fuel from fuel tank through filter with the help of a pump and sends to the carburetor for petrol engines. A high pressure fuel pump is used for diesel engines, which sends fuel to an injector mounted in cylinder head. Ignition system takes current from battery and gives high voltage to spark plug mounted in cylinder head for igniting fuel by giving high voltage spark at the right moment. The voltage is increased by using an ignition coil and contact breaker. The distributor distributes the spark in multi- cylinder engines in a firing order. Cooling system keeps the engine at optimum temperature so that the parts do not get distorted due to high temperatures. Water is circulated in the jackets of cylinder and cylinder head by a water pump. The hot water is cooled in a radiator and sent back to the pump. Air cooled engines use fins on cylinder for increasing heat transfer. Lubrication system helps in lubricating different parts having relative motion. The oil is sucked from oil sump by a gear pump and supplied at high pressure to different parts through oil galleries or holes or through external pipes.

28.1

INTRODUCTION TO I.C. ENGINES

Petrol and diesel engines are called Internal Combustion engines or in short as I.C. engines. They are so called because combustion of the fuel takes place inside the cylinder. An I.C. engine operates on the

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principle of four stroke or two stroke cycle. Majority of the engines work on four stroke cycle. These four strokes are:

A Suction Stroke Air for diesel engines or mixture of air and fuel called charge for petrol engines is sucked in the cylinder through inlet valve due to the vacuum created by the piston while moving away from cylinder head. B Compression Stroke Piston moves towards the cylinder head to compress the air/charge sucked inside the cylinder. This increases pressure and hence temperature. A diesel engine uses higher compression ratio than petrol engine. C Power Stroke Fuel is burnt in this stroke either with the help of a spark in petrol engines or by injecting high pressure diesel for diesel engines. The burnt gases push the piston down and transmit power to the crank shaft through connecting rod. D Exhaust Stroke Burnt gases are expelled out through the exhaust valve while the piston moves towards cylinder head. This cycle of four strokes is repeated after every two revolutions in a four-stroke engine. In a twostroke engine all the four operations are completed in one revolution only. Lower part of the piston is used for suction and partial compression and the charge is transferred through transfer ports above the piston for rest of the operations. An engine consists of five main systems given as follows: ∑ Power system (Section 28.2) ∑ Fuel system (Section 28.3) ∑ Ignition system (Section 28.4) ∑ Cooling system (Section 28.5) ∑ Lubrication system (Section 28.6)

28.2

POWER SYSTEM

An internal combustion engine is shown schematically in Fig. 28.1. It consists of a cylinder block containing cylinders, in which a piston reciprocates and water jackets for cooling. It is covered by a cylinder head containing inlet and exhaust valves. Piston rings on the piston are used to check the leakage of high pressure gases through the clearance between cylinder and piston. Oil ring at the bottom part of piston scraps lubricating oil. Gudgeon pin is used to connect piston with the small end of the connecting rod. Other end of the connecting rod called big end is fixed over the crank shaft. This system converts reciprocating motion of the piston into rotary motion of crank shaft. Lubricating oil is filled in the oil sump at the bottom for lubricating various rubbing parts. Two cam shafts driven by timing chain rotate at half the speed of crank shaft. They open the inlet and exhaust valves using cams. Valves are closed due to spring between cylinder head and spring retainer washer. Some engines have only one cam shaft with two cams per cylinder and is driven by timing gears.

Internal Combustion Engines

577

Followers above the cams operate valves in the cylinder head through a valve gear mechanism comprising, push rod, Rocker arm and rocker shaft (Fig. 28.17). It also operates a fuel pump. There are no valves and timing gears in two stroke engines.

Fig. 28.1

28.2.1

Schematic Diagram of an I.C. Engine

Cylinder

It is the most important part of an engine. A multi-cylinder engine has many cylinders arranged either in one line or in V shape or radial fashion. Temperature of combustion reaches very high in cylinder and hence needs cooling so that the temperature does not exceed beyond certain limits otherwise, it is not possible to maintain its straightness and rigidity. A cylinder for a single cylinder engine is shown with wet liner in Fig. 28.2A. In this cylinder outside of cylinder comes in contact with cooling water and hence called wet liner. In alternative type, a separate liner called dry liner is shrunk fit in the cylinder block (Fig. 28.2B). This can be replaced if the liner wears out.

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Surface of cylinder is finished by honing process. A worn out cylinder appears more smooth than a new finished surface due to honing marks. Average surface roughness (Ra) is of the order of 0.3 to 0.6 microns. The honing process leaves cross hatch pattern on the surface which should be about 45° to each other and about 22° to 32° to the horizontal deck surface. Too steep angle can pump excessive oil and too shallow can have a ratcheting effect when ring passes over the valley. The bore geometry is equally important for a cylinder. Flatness of the top surface for most of the push rod type engines varies from 0.075 to 0.1 mm for cast iron blocks. For aluminum heads, it should not be more than 0.05 mm in any direction. Circularity of the cylinder bore is recommended less than 0.001 mm. Taper of the bore should be zero or limited to 0.0002 mm. Cooling can be done by water or by air. If it is water cooled, water is circulated in the jackets around the cylinder and cylinder head. In an air-cooled cylinder, there are circumferential fins (Fig. 28.2C) or axial fins to increase the surface area for heat transfer. It has ports on the bottom side. Charge enters from inlet port, through a reed valve bolted at the bottom of cylinder. It allows charge to enter crank case but closes automatically when there is any compression at bottom of piston. Charge goes from crank case to cylinder above the piston through transfer ports and is then exhausted from the exhaust port after power stroke.

Internal Combustion Engines

579

Fig. 28.2 Types of Cylinders

28.2.2

Piston

It is a cylindrical part closed at one end which reciprocates in the cylinder. It is made as light as possible to reduce inertia effects. It has to withstand high temperatures occurring in the combustion space. It has circumferential grooves on the periphery to accommodate piston rings. First piston was used by N.A. Otto in 1866. Since then, there had been lot of modifications in its design. Modern pistons are designed to withstand high temperatures, reduce pollution, run for long life (200,000 km), without scuffing and light weight. The data given in this chapter is mostly for the parts used for cars. Top surface of the piston is called crown (Fig. 28.3A). The crown is provided with ribs below it to impart strength. Most critical area for heat management is the top ring area called land below the crown. Its width varies from 7 to 8 mm but modern pistons have decreased up to 3 mm. Upper land is made slightly smaller in diameter to allow more thermal expansion. Crown of the piston used to be flat which has been now replaced by dished piston, domed piston, piston with intricate contours to swirl the fuel mixture for better combustion and attenuator grooves to enhance the valve relief. Typical width of ring grooves is 1.2 mm for top compression ring, 1.5 mm for second ring and 3 mm for oil ring. Some engines have used compression rings as thin as 1 mm and oil ring as 2 mm. Thinner rings offer less friction losses but they reduce the heat transfer rate and run hotter. Central portion of piston contains bosses to fit a pin called gudgeon pin. Distance from center of gudgeon pin to its crown is called compression height which varies from 35 to 42 mm. It has been reduced to 30 mm also. Gudgeon pin (also called as piston pin) rather being round and straight is taking

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Part F – Chapter 28

new shapes. Some are oval and some trumpet shaped, flaring out toward the inside edges of the bosses to accommodate pin bending. These modifications measure in microns but extend piston life.

Fig. 28.3

Pistons

Internal Combustion Engines

581

The portion of the piston below the gudgeon pin is called skirt, which provides the contact area to take side thrust arising due to inclination of the connecting rod. Long skirts of size more than 60 mm sometimes crack and hence new pistons have mini-skirts as low as 37 mm, which offer less weight. Continuous effort is being made to reduce the weight of the piston and it has come down to 600 gm by reducing length of piston. Clearance between cylinder and piston is typically between 0.012 to 0.025 mm to reduce piston slap by the skirt over the cylinder surface. Material used for the pistons is alloy of aluminum and silicon, which improves heat strength and reduces coefficient of expansion. These are as follows: Hypoeutectic aluminum alloy, which contains 8.5% to 10.5% silicon, Eutectic aluminum alloy, which contains 11% to 12% silicon, Hypereutectic aluminum alloy, which contains 12.5% to 16% silicon, Cast steel or cast iron pistons are also used. Coatings: Applying a low friction coating to the sides of the pistons, protects against scuffing. Coatings used are of Moly-disulfide. Anodizing and/or use of ceramic coatings on the tops of the piston and upper ring groove, makes a piston heat resistant and may allow top ring with no end gap. Figure 28.3A shows a piston of petrol engine. Piston of a diesel engine (Fig. 28.3B) may contain a combustion chamber also. Figure 28.3C is a piston of two stroke engine with a deflector on its top to avoid mixing of charge coming from transfer ports and the exhaust gases passing from the other side.

28.2.3

Piston Rings

Piston rings are used to check leakage through the clearance space between cylinder and piston. There are two types of rings; compression rings (Fig. 28.4A) to check the leakage and oil rings (Fig. 28.4B) to scrap the lubricating oil from the cylinder walls back to oil sump. Steel compression rings have been used for 30 years in heavy duty trucks but now getting changed to grey cast iron for most cars. If ring thickness is less than 1.5 mm C.I. can break and hence steel is still used. Steel rings are coated by gas nitriding to a depth of about 0.02 mm, to improve its resistance to wear. Ceramic faced carbide rings using Moly Cermet (80% moly and 20% chromium) have also been developed for racing cars. Free gap of the ring is the gap between its open ends when it is outside the cylinder. End gap of the compression ring is important while putting a new ring. It is measured by putting the ring in the cylinder at the top and bottom of the cylinder bore. Use the bottom position to set the end gap. If the cylinder wear is more than 0.075 to 0.0125 mm of taper, cylinder should be re-bored. Table 28.1 shows the recommendations for the minimum end gap in terms of bore diameter. Table 28.1 Ring

Ordinary engines

Minimum end gap High performance engines

Nitrous blown engines

Minimum end gap

Minimum end gap

Minimum end gap

Top compression ring

0.004 times the bore

0.005 times the bore

0.006 times the bore

Second compression ring

0.005 times the bore

0.006 times the bore

0.007 times the bore

Oil ring

0.4 mm for any bore

0.4 mm for any bore

0.4 mm for any bore

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Oil rings have many radial slots for the oil to pass through (Fig. 28.4B). It could be in one piece (Fig. 28.4C-1) or in two pieces with a spring on the back side (Fig. 28.4C-2), or in three pieces; the third piece is an expander between them (Fig. 28.4C-3). It can be multi rails with spacer between top and bottom rails (Fig. 28.4C-4).

Fig. 28.4 Piston Rings

28.2.4

Connecting Rod

Connecting rod connects piston and crankshaft. It is a taper rod of generally I section to make it light and has two ends; small end and big end (Fig. 28.5). Small end is fixed in piston using gudgeon pin in the bosses of piston and is kept in position by two circlips inserted in the grooves in the boss (Fig. 28.3B). In alternate arrangement it can be tightened with a set screw (Fig. 28.3A and C). The big end is mounted on the crank pin of the crank shaft. It has two halves with metal lining as bearing material. Some connecting rods have the parting line of the two halves at 45º to reduce the width of the big end (Fig. 28.P1 at the end of Chapter).

Internal Combustion Engines

Fig. 28.5

583

Assembly Drawing of a Connecting Rod

A radial engine has axes of cylinders arranged in a radial form. Connecting rods of the pistons called articulated rods are connected to one master connecting rod at its periphery. Figure 28.6 shows a master connecting rod and four articulated rods for a 5 cylinder radial engine. The connecting rod bolts bear more fatigue stress than any other bolt and hence special bolts to withstand fatigue loads are used to avoid stress concentration. Connecting rods are made either by hot forging or powder forging. In hot forging process, a billet of calculated quantity is taken, heated and forged in the dies for its shape. Holes are created by piercing. Shot peening is used for surface hardening to improve resistance to fatigue stress, corrosion, cracking and galling. Rod is then machined for holes and sides, followed by grinding of the bores. No heating operations should be permitted after shot peening. C-70 steel has been introduced by Europe as material for connecting rod. In powder forging process, pre-blended is compacted and sintered followed by Shot peening and machining. Most forging grade alloy powders use carbon 0.5 to 0.57%, manganese 0.3% to enhance iron hardenability, sulphur 0.12%, copper 3%. Some use Nickel and Molybdenum also as alloying elements. Cast-Steel rods are poorly suited to any type of serious performance use. Original-equipment forged steel rods are the next step up the strength and reliability. Attention to detail and better parent material are the main attractions offered by after-market forged steel. Most after-market forged rods benefit from extra care during the critical machining operations. True billet steel rods are fairly uncommon in today’s marketplace. Aluminum rods are manufactured by the forging process, or they can be cut from a sheet of aluminum plate, billet-style. Aluminum rods are 25 percent lighter than steel rods, and for this reason they are popular with racing cars looking to shed mass from the reciprocating assembly.

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Fig. 28.6

Connecting Rod of a 5 Cylinder Radial Engine

Titanium rods offer the highest strength-to-mass ratio of them all. A well-designed titanium rod is about 20 percent lighter than a comparable steel rod. The most common alloy has 6 percent aluminum and 4 percent vanadium to improve machineability. Materials of high performance engine bearings is in 4 layers. Function of each layer is given below: (a) Steel as the main backing material (b) Copper lead lining for high fatigue strength (c) Nickel barrier to prevent tin from migrating to the underlying copper lead lining. (d) Lead/Tin/Copper Babbitt overlay of 0.013 mm for improved fatigue performance.

Internal Combustion Engines

585

Radial clearance between rod and bearing is generally 0.001 mm for every mm of pin size. Generally for small end, it is of the order of 0.004 to 0.008 mm and for big end 0.02 to 0.05 mm. For high performance applications an additional clearance of 0.013 mm is given. Aluminum connecting rods should have clearances reduced by 0.013 mm to compensate for increased expansion in operation. Side bearing clearance used for connecting rod is of the order of 0.12 to 0.25 mm. End play on thrust faces of 0.12 to 0.15 mm is generally adequate for most applications. Parallelism/twist between bore and gudgeon pin hole is 0.025 mm in 150 mm length. The two bores of small and big end have to be exactly parallel to avoid any twist but it should not be more than 0.03 mm. The axes of bores have to be at right angle to the axis of the rod otherwise may result in overheating of the pin. Bore surface finish should be within 1.5 to 2.3 microns. Out of round of bore should not exceed 0.025 mm. Taper/Hourglass/Barrel permitted is 0.003 mm up to 25 mm, 0.005 mm from 25 to 50 mm and 0.008 mm for length more than 50 mm. The rods have to be symmetrically balanced about central axis. For a multi-cylinder engine, weight difference of each rod should not be more than 2 gm. Some bearings provide features of ¾ grooving in the middle, for increased oil supply to the highly loaded bearings. Example 1 Draw sectional front view of assembly drawing from part drawings of cylinder, piston, piston rings, oil rings and connecting rod shown in Figs 28.2A, 28.3A, 28.4A, 28.4B1 and 28.5 respectively. Use a gudgeon pin of hole diameter as half of the outside. Solution Assembly drawing of a piston, compression rings, oil ring, gudgeon pin and connecting rod with major dimensions is shown in Fig. 28.S1.

28.2.5

Crank Shaft

Crank shaft converts reciprocating motion of the piston into rotary motion. It is supported on main bearings and has crank pins revolving eccentrically about the axis of the main bearings. Crank shafts are classified as; built up type for two stroke engines and solid type for four stroke engines. Built up crank shafts are made by shrink fitting journals to the crank webs. Crank webs are made of cast or forged steel while journals are of forged steel.

Fig. 28.S1 Assembly Drawing of Cylinder, Piston, Rings and Connecting Rod

Solid type crank shafts are press formed from steel ingots for medium and high speed engines with cylinder bores up to 600 mm. An important requirement of the crank shaft is the high strength. Modern materials are able to offer strength up to 950 M Pa. Significant improvement in fatigue strength can be

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obtained by cold rolling the fillets causing work hardening and compressive residual stresses on material surface. Figure 28.7 shows a crank shaft for a single cylinder engine. Eccentricity of the pin from its axis is half of the stroke of the piston. Crank pins and main journals of the crankshaft are joined with two crank disks (Fig. 28.7) or crank webs (Fig. 28.8). All parts are made with steel forging, followed by machining and grinding. A balance weight to balance the eccentric mass of the crank pin, some portion of big end and crank web is placed opposite to the crank pin on crank webs. This weight may be integral for small crankshafts or may be bolted separately for large-sized crank shafts. For a two stroke engine most of the crank case volume is occupied by the crank disks for better compression below the piston.

Fig. 28.7

Crank Shaft of a Single Cylinder Two Stroke Engine

A multi cylinder engine could be In-line (all cylinders in one line), or V (cylinder axes arranged at angle to form a V) or radial (cylinder axes arranged radially). For an inline engine, number of crank pins is equal to the number of cylinders. A V engine has two connecting rods on one crank pin. A radial engine has only one crank pin and all other connecting rods are mounted on the master rod as shown in Fig. 28.6. Figure 28.8 shows a crankshaft for four-cylinder inline petrol engine. Balance weights are integral with the crank web. The crank pins are arranged in the firing order 1-3-4-2 or 1-2-4-3. Main bearings are provided after two crank pins.

Fig. 28.8

Crank Shaft of a Four Cylinder Petrol Engine

Internal Combustion Engines

587

Fig. 28.9 Crank Shaft of a Six Cylinder Diesel Engine

In petrol engines, pressure in the cylinder is lesser than the pressure in the cylinder of diesel engine. Hence main journals can be alternate in a petrol engines, but for diesel engines, main journals are provided after every crank pin. Figure 28.9 shows a crankshaft of a six cylinder diesel engine. It has six crank pins and 7 main bearings. Crank pins are arranged at 120° in the firing order 1-5-3-6-2-4 or 1-4-2-6-3-5. Tolerances and Geometric tolerances for the main bearing and crank pin journals are as under: Diametrical clearance for crank journals and big end journals is 0.013 up to 38 mm and 0.025 mm for more than 38 mm diameter. Surface finish of journal should be within 0.25 microns and of thrust faces 0.38 microns or better. Out of round of bore should not exceed 0.005 mm. Taper/Hourglass/Barrel permitted is 0.003 mm up to 25 mm, 0.005 mm from 25 to 50 mm and 0.008 mm for length more than 50 mm. Alignment on adjacent journals 0.013 mm. Maximum overall Alignment should be within 0.025 mm. Parallelism between main journals and crank pin should be within 0.013 mm.

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28.2.6

Cams and Followers

A Cams Cam is a machine element, which when rotated gives motion to its follower, in a desired manner. The motion may be linear or non-linear. There are many types of cams and cam followers. The type of cam and its follower depends upon the requirement. In I.C. engines, cams are used to operate inlet valve, exhaust valve and fuel pump. Following terms relate to cam and follower (Fig. 28.10A): Follower Face Nose Nose radius Base radius Flank Lift Rise period Fall period Dwell period

A machine element, which follows the profile of the cam. Circumferential area of the cam, on which the follower moves. Curved surface at the maximum radius of the cam. Radius of the curved surface at the nose. Minimum radius of the cam. Portion of the cam which lifts the follower from its minimum position to maximum position and returns back. Linear distance moved by the follower. Angle in degrees of cam rotation during which the follower moves upwards. Angle in degrees of cam rotation during which the follower moves downwards. Angle in degrees of cam rotation during which follower does not move.

Cams can be classified according to the shape, and type of motion given by the cam. Various shapes of the cams are shown in Fig. 28.10. According to the shape, there are two main types: radial and cylindrical

Fig. 28.10 Types of Cams

In radial type of cam, (Fig. 28.10A and B) follower moves perpendicular to the axis of rotation. Radius changes with change in angle. Peripheral face is used to drive the follower. Figure 28.11 shows a generalized radial cam with the various periods of motion for the follower. At minimum radius (base circle) the follower is at the lowermost position. When the cam rotates, it moves the follower radially outward during lift period. Then the follower remains at its highest position for the dwell period. It then descends during the fall period back to the base circle and remains at this position for the other dwell period. A cam can have any value of these periods to give the desired type of motion.

Internal Combustion Engines

589

In cylindrical type of cam, (Fig. 28.10C and D) follower moves parallel to the axis of the cam. A groove is cut on the periphery of a cylinder as shown in Fig. 28.10D. The follower in this groove follows the profile of the groove. A cylindrical cam can have one rotation cycle or more than one also.

Fig. 28.11

A Radial Cam

Fig. 28.12 Shapes of Radial Cam

Velocity during lift and fall period depends upon the shape of the flank. Various shapes that are commonly used are: Tangent cam: Circular arc cam: Circular cam: S.H.M. cam: Constant acceleration cam:

Flanks of the cam are straight surfaces, which are tangent to the base circle and nose (Fig. 28.12A). Flanks of the cam are arcs of some circle of given radii (Fig. 28.12B). Cam is completely circular, but its hole for the drive shaft is made eccentric. Thus the lift of the cam is twice the eccentricity (Fig. 28.10B). Movement of the follower from its base radius follows simple harmonic motion (Fig. 28.12C). Movement of the follower from its base is at constant acceleration.

B Followers Followers are used to follow the contour formed by flank of cam. Various types of followers used are knife edge, round, flat, and roller types (Fig. 28.13). A knife edge follower (Fig. 28.13A) follows the profile of cam most faithfully, but spoils the surface due to its sharp edge and hence not used practically. A round shape (Fig. 28.13B) increases the area of contact and hence the surface of the cam is not damaged as in knife edge follower. Flat follower is just like a circular disk (Fig. 28.13C). If its diameter is large, then the radial lift given by the cam and distance moved by the follower may be different, because the edge of the follower may not touch the cam at the center line, but somewhere else. Flat follower is mounted slightly eccentric so that the friction between cam and follower gives a slight rotation to the follower. This makes the follower to wear uniformly. This type is widely used in I.C. engines. A roller follower (Fig. 28.13D) has the advantage of low friction as rolling friction is lesser than sliding friction. A roller follower also modifies the lift of the cam, but lesser than flat follower. A roller follower is also used for cylindrical cams as shown in (Fig. 28.13E).

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590

Fig. 28.13 Types of Followers

A cam with all the above mentioned followers can only lift the follower. They are brought back during fall period by the help of external force of spring or dead weight etc.

C Lift Diagrams Before drawing a cam profile, it is necessary to draw the lift diagram for the rise and fall period. It is nothing but X-Y plot showing rotation angle on X-axis and movement of the follower on Y-axis. It is needed both for radial and cylindrical cam. These diagrams are generally made full size, so that the lift could be directly measured and transferred to the drawing of the cam profile drawing. To draw the lift diagram, draw the abscissa OX as the period of rise or fall in degrees to any suitable scale. Draw a vertical line at point X equal to the lift above the OX line, preferably to full scale (Fig. 28.14). Divide the base line OX in 6 equal parts and number them as 1, 2 etc. Number of points can be increased, if base diameter is large or if more accuracy is desired. Construction for three types of motions, i.e. constant velocity, Simple Harmonic motion and constant acceleration is shown in Fig. 28.14. Left side of the vertical line at 0 of Fig. B and C shows the construction required for S.H.M. and constant acceleration respectively. Transfer the lifts by drawing horizontal lines from 1¢, 2¢, 3¢, 4¢, 5¢ and 6¢ on the vertical lines drawn at points 1, 2, 3, 4, 5 and 6 on the line OX. Draw a smooth curve (called lift diagram) passing through these points.

Fig. 28.14

Construction for Different Lift Diagrams

Internal Combustion Engines

591

D Drawing a Radial Cam Following information is necessary before drawing a cam profile: ∑ Lift ∑ Rise period in degrees ∑ Type of motion like constant velocity or constant acceleration etc. during rise period ∑ Fall period in degrees ∑ Type of motion during fall period ∑ Dwell period in degrees at base circle and nose If both the rise and fall periods and type of motion are same, only one lift diagram is enough, but if they are different, then two lift diagrams are required, one for each period. There can be two or more rise periods and falls in one rotation for special cams. The dwell periods may be zero in a typical cam. But this does not change the method of the drawing. Draw center lines and then a circle of given base circle diameter. Mark the angles in sequence, opposite to the direction of rotation for the following (Refer Fig. 28.S2B): a. Rise period c. Fall period b. Dwell period at full lift d. Dwell period at base circle A most general case is being discussed here. Divide the rise and fall period in equal parts of the same number as used for the lift diagram and draw radial lines. Mark these radial lines 1, 2, ...6 for rise period and as 6, 5, 4, ...1 in reverse order for fall period. Measure the lift from the lift diagram by a divider and transfer these distances on the radial lines from the base circle and mark these points. Draw a smooth curve passing through these points. For dwell period at nose, draw circular arc with center as the center of the cam and radius as that at full lift. Dwell period at the fall period is the base circle itself. Example 2 Draw a radial cam profile having base circle of diameter 50 mm and lift 30 mm with knife edge follower. It should offer constant acceleration for 80° and deceleration for 80° with total rise period for 160°, dwell at full lift for 40°, fall period simple harmonic for 120°. Rest of the motion for 40° is dwell at the base circle. How will the cam profile change with: a. Flat follower of diameter 20 mm and b. Roller follower of diameter 16 mm? Solution First draw the lift diagrams for rise period and fall period as shown in Fig. 28.S2A. OX can be any distance. Note the difference between construction of acceleration and deceleration lift diagrams. Draw the base circle and radial lines at 20° interval. Transfer the vertical distances of the lift diagram on the respective radial lines for lift and fall periods. Then draw arcs of constant radius for the dwell periods as shown in Fig. 28.S2B. Join all points with a smooth curve. Cam profile with flat follower is shown in Fig. 28.S2C. A line of follower size i.e. 20 mm is drawn symmetrically at right angle to every point i.e. 1¢, 2¢ etc. and center on the radial lines. Then a curve is drawn passing from the extreme points of the follower. For rise period it is touching the right end, while for the fall period it touches the left end of the follower.

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Fig. 28.S2A Lift Diagrams

Fig. 28.S2B Cam with Knife Edge Follower Figure 28.S2D shows cam profile with roller follower. Circles are drawn of roller diameter 16 mm with center on radial line and passing through each lift point 1¢, 2¢ etc. A curve is drawn touching the circumference of the roller. Note the difference in each profile of Fig. 28.S2C and 28.S2D due to change in the follower.

Fig. 28.S2C Cam with Flat Follower

Fig. 28.S2D

Cam with Roller Follower

Internal Combustion Engines

28.2.7

593

Cam Shaft

It is a shaft containing many cams. Some engines use two cam shafts having one cam for every cylinder on each shaft. Number of cams depends upon the number of cylinders. If single cam shaft is used, two cams are provided for each cylinder; one to actuate the inlet valve and other for the exhaust valve. It is shown in Fig. 28.15. An extra cam is provided for a fuel pump to suck petrol from petrol tank and send it to carburetor. In diesel engines, a fuel pump is driven by a cam to provide high pressure diesel fuel to the injector. Cams are forged integral with the shaft. Their lobed portion is fixed at predetermined orientation to actuate the valves at the desired timings. Note the orientations of various cams in the side view. The cams used for cam shaft have a nose radius and flanks are circular arc. Some engines use tangent cams. Cam for the fuel pump is of circular type. At one end of the shaft, there is a keyway to mount the driving gear or sprocket with a key. For cam shaft bearings, heavy duty aluminum materials are available. These bearings can take heavier loads than lead based Babbitt materials. Minimum clearance is of 0.038 mm for heavy duty bearings.

Fig. 28.15

28.2.8

A Cam Shaft

Valves

Valves are located in the cylinder head, which open or close an opening formed by valve seat at the right moment (Fig. 28.16). A pair of valves is used in each cylinder; one controls the opening of air/charge and other exhaust gases. The type of the valve used is puppet type (5, 6). It is kept tight against its seat (8, 9) with the help of a helical spring (3). A retainer washer (2) provided at the top of the valve stem, transfers the spring force to its stem through a lock (1). Exhaust valve is hotter than inlet valve, as it handles hot gases. It is sometimes made hollow and filled with sodium to increase heat transfer rate. Inlet valve is made bigger than exhaust valve to increase volumetric efficiency of the engine and hence more charge in cylinder for more power. Inlet valves operate around 450°C to 550°C while exhaust valves between 650°C to 800°C. Hence different materials are used for inlet valves and exhaust valves. Inlet valves are made of carbon steel with Manganese to improve corrosion resistance and chromium for high strength. For high performance inlet valves, Sichrome is used with 8.5% chromium. Exhaust valves are made from martenstic steel with Chrome and silicon alloys or in two pieces (stem separate) head of stainless steel and stem of Martensitic steel. Inconel trade mark high strength austenitic nickel-chromium iron alloy is used for

594

Part F – Chapter 28

high performance valves. Titanium is the ultimate valve alloy, because of its lightness (40% of steel). They are coated with moly or another friction coating to reduce risk of stem galling.

Fig. 28.16

Valve Arrangement

Generally valves are operated by a valve gear mechanism (Fig. 28.17A). It consists of a cam follower, which pushes the push rod and then this pushes a rocker arm supported on rocker shaft. The rocker arm pushes the valves down and the valves close automatically by spring action. In overhead Camshaft engines, valves are actuated by cam, directly over them (Fig. 28.17B). Some clearance called tappet clearance is necessary between valve and rocker arm so that valve does not open by itself due to increase in length due to temperature when it becomes hot. An adjusting screw is provided between push rod and rocker arm to adjust this tappet clearance. Since exhaust valves run hotter than inlet valves, more clearance is provided for them.

Internal Combustion Engines

(A) Conventional valve gear mechanism

Fig. 28.17

28.3

595

(B) Over head cam shaft

Valve Gear Mechanisms

FUEL SYSTEM

In the fuel system for petrol engine, fuel is taken from the fuel tank using a fuel pump and is sent to the carburetor where it is atomized, vaporized and mixed with air in the right required portion. In a diesel engine, fuel is sent to high pressure fuel pump, which sends fuel to an injector mounted in the cylinder head. Due to high pressure, the fuel is atomized and it starts burning, when it comes in contact with high pressure and high temperature air in a cylinder.

28.3.1

Carburetor

Carburetor is a device, which mixes air and fuel in proper proportions for all loads and speeds according to the requirement. Carburetors can be broadly classified as: (a) Fixed venturi, and (b) Variable venturi.

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In a fixed venturi carburetor, a venturi is provided in the middle of a barrel. Fuel is sucked through main jet in this venturi due to vacuum created due to flow of air in the barrel. Other jets like idling, slow running, compensating, acceleration jet, etc. adjust the fuel according to the speed and loads on the engine. The other side of the barrel has a choke which closes the passage of barrel to reduce amount of air and supplies rich mixture at the time of cold starting.

Fig. 28.18 A Variable Venturi Carburetor

In a variable venturi carburetor, the venturi size automatically adjusts according to the load and speed of the engine (Fig. 28.18). It consists of a body (7), joining the passage from air filter to the inlet manifold. On the top of the body, a cylinder (2) is fixed with a cover (1). A piston (3) slides in this cylinder up and down and controls the venturi size in the barrel. Position of the piston is balanced by the vacuum communicated to the cylinder (through a hole in piston) above the piston and the downward force by the spring (4). An oil damper with cylinder and piston (5 and 6 respectively) damps the sudden

Internal Combustion Engines

597

movements of the piston. Quantity of the fuel is controlled by a tapered needle valve (11). The needle valve is locked to the piston by a set screw (17). The needle valve moves up and down in a jet head (14) at the bottom of the barrel. Jet head can be adjusted by an adjusting screw (12). It can be locked by lock nut (13). The fuel is supplied from a float chamber (not shown in the Figure), which maintains a constant level of the fuel using a float valve. Fuel is sent to the carburetor through a fuel feed tube (16). Quantity of charge (air/fuel mixture) is controlled by a butterfly valve (8) mounted on a spindle (9) in the barrel and locked by a split pin (10).

28.3.2

Diesel Fuel Pump

In diesel engines, fuel is supplied at high pressure and quantity is regulated according to the load and speed of the engine. It has a precisely ground helical grooved plunger (4) reciprocating in a ground barrel (2) with the help of a cam (Fig. 29.19A). The plunger can be rotated in the barrel with the help of a rack (9) and pinion (3) arrangement, to change the effective stroke of the plunger due to its helical groove. Both rack and pinion are positioned in the body (1). The plunger is reciprocated by a cam follower (11) operated by a cam on the cam shaft and is returned by a return spring (10). Details of parts are shown in Fig. 28.19B.

Fig. 28.19A

Assembly Drawings of a Diesel Fuel Pump

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598

Fuel is supplied to the fuel pump through a banjo bolt (15) screwed in the body. Any air trapped in the inlet system can be bled off by the bleed screw (13). When the plunger moves up, a delivery valve (6) positioned above valve seat (5) opens against the force of spring (7), when pressure inside the cylinder is more than the set pressure (120 to 200 bars). The plunger returns back due to force of spring (10).

Fig. 28.19B

Part Drawings of a Diesel Fuel Pump...Contd.

Internal Combustion Engines

Fig. 28.19B

28.3.3

599

Part Drawings of a Diesel Fuel Pump

Injector

Injector is connected to the diesel fuel pump with high pressure tube using flare joints (Chapter 18 Section 18.21). Assembly of an injector is shown in Fig. 28.20A. It has a needle valve (2), which gets lifted in its body (1) due to high pressure fuel coming from fuel pump. The needle valve is kept pressed by a spring (8) resting between the spring seats (7) through a spindle (6). The nozzle body (3) is tightened on the body with nut (4). The fuel is atomized and sprayed through the fine holes of the nozzle. Pressure of the spring can be adjusted by an adjusting screw (10) and locked by nut (11). Cap nut (13) is used to cover the adjusting screw. Air trapped in the system can be removed using a bleed screw (12). Details of the parts are shown in Fig. 28.20B.

Part F – Chapter 28

600

Fig. 28.20A

28.4

Assembly Drawing of an Injector

Fig. 28.20B

Diesel Fuel Injector Parts

IGNITION SYSTEM

This system takes current from a 12 V or 24 V battery and voltage is increased in the range 10,000 V to 30,000 V using an induction coil and contact breaker arrangement. This high voltage is supplied to a spark plug, which is fitted in the cylinder head to give spark at the right time decided by the contact breaker in the distributor. In multi-cylinder engine, distributor distributes the spark to the required spark plug in a firing order. Diesel engines do not need spark plug as the combustion takes place automatically due to high temperature inside the cylinder after high compression.

Internal Combustion Engines

601

Fig. 28.21 Assembly Drawing of a Spark Plug

A spark plug is shown in Fig. 28.21. A brass terminal (1) is put at the top of central electrode for High Tension connection. High voltage reaches the central electrode (8), which is kept insulated by porcelain insulator (2) from the bent electrode (9). This bent or side electrode is a part of the shell (4), which is screwed in the cylinder head. Two sealing gaskets (3) and (5) make the joint leak-proof from insulator. A carbon resistor (6) is put in between the central electrode and terminal to reduce television and radio interference (static) from the high voltage surges in the ignition circuitry. It is also called Radio Frequency Interference (RFI). It is sealed on both sides.

28.5

COOLING SYSTEM

Due to combustion in cylinder, very high temperatures are obtained, which can distort the cylinder, if not cooled. Cooling system keeps the engine at the optimum temperature by circulating water around the cylinder in the water jackets provided for this purpose. A water pump circulates water through cylinder and then to the cylinder head. The hot water coming out of cylinder head is cooled in a radiator and sent back to the inlet of the water pump. A water pump (Fig. 28.22A) consists of a C.I. body (1) having volute casing. It is bolted to the cylinder block at the front end. An impeller (2) mounted tight fit on the Fig. 28.22A Assembly Drawing of a Water Pump

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602

shaft (3) rotates by getting power from the crank shaft using a belt and pulley. The shaft is supported on bush bearing (5). The leakage of water from the bush is checked by a seal (4). Due to rotation of impeller, water pressure is slightly increased and it enters the cylinder block through the volute casing of the body, which helps in increasing its pressure. Details of parts are shown in Fig. 28.22B.

Fig. 28.22B

28.6

Part Drawings of a Water Pump

LUBRICATION SYSTEM

This system lubricates all the rubbing parts. It uses oil stored in the oil sump in the crank case below the crank shaft (Refer Fig. 28.1). The oil is sucked through an oil strainer by a gear pump and then sent through filter to various parts using oil gallery or holes or external pipes. Assembly of a gear pump used for this purpose is shown in Fig. 28.23A and its part drawings in Fig. 28.23B. A gear pump consists of a C.I. body (1) with two equal size spur gears (5) and (6) made of cast steel. One gear (6) fixed by a woodruff key (8) is driven by a shaft (3) getting power from the crank shaft. The other gear (5) is driven by the first gear and is supported on a bush (7) on idler shaft (4). The body is covered by a cover (2).

Internal Combustion Engines

Fig. 28.23A Assembly Drawing of a Gear Pump

Fig. 28.23B

Part Drawings of a Gear Pump

603

604

Part F – Chapter 28

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Name the important parts of an I.C. engine and their function. What is meant by two stroke and four stroke engine? Differentiate between them. Which are components the fitted on Piston and connected to it? Write materials used for it. What are the various types of pistons? Sketch them. Describe the materials and coatings used for pistons. Describe names of various portions of piston and their proportions for an auto engine piston. How does a cylinder of a two stroke engine differ from four stroke engine cylinder? Write the tolerances and geometrical tolerances used for cylinders. Explain the different types of engine cylinders and their construction. Sketch the piston rings. Describe their construction and use. What do you mean by small and big end of connecting rod? Describe its construction. Differentiate between connecting rod of an in-line and radial engine with a sketch. Write suitable tolerances and geometric tolerances for a connecting rod. What is the function of a crank shaft. Explain construction of different types of crank shafts. Sketch a camshaft and write its function. Describe the construction of S.H.M. and constant acceleration lift diagrams with sketch. What is the function of a valve in an engine? How is it actuated from the crankshaft? What are the various types of cams? Describe with a neat sketch. Describe the construction and working of a diesel fuel pump. What is the function of an injector in a diesel engine? Explain its working. What is the function of a spark plug? How a spark is created? Describe its construction. Describe the construction of a water pump with sketch for cooling a water cooled cylinder. What is a gear pump? Where is it used? Describe its construction with a neat sketch.

FILL 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

IN THE

BLANKS

In a two stroke engine, a cycle of all strokes is completed in revolution. pin. Piston and connected rod are connected by system of engine. Spark plug is a part of . Gear pump takes oil from for cooling. A water cooled cylinder is provided with are provided on air cooled engine for cooling. . Piston rings are provided on piston to of the connecting rod is mounted on crank pin. on crank web. Eccentric mass of the crank pin is balanced by speed of crank shaft. For a four stroke engine, cam shaft rotates at . Valves are driven by Diesel fuel pump supplies fuel to . . Air and fuel mixture for a petrol engine is prepared by . A helical plunger is used in

Internal Combustion Engines 15. 16. 17. 18. 19. 20.

is used to ignite fuel in a petrol engine. to increase pressure. Water pump uses to increase pressure. Lubricating oil pump uses . Lift diagram is used to draw Dwell period of a cam is the period in which the follower process. Surface of the cylinder liner is finished by

605

.

MULTIPLE CHOICE QUESTIONS 1. Correct sequence of strokes in a four stroke engine is (a) suction, compression, power and exhaust (b) compression, suction, power and exhaust (c) suction, exhaust, compression and power (d) power, suction, exhaust and compression 2. An air cooled engine is used where (a) there is no water (b) water is rarely available (c) not much heat is to be dissipated (d) light construction is required 3. Bosses in a piston are provided to fix (a) big end of connecting rod (b) small end of connecting rod (c) gudgeon pin (d) piston rings 4. Skirt of a piston is its (a) central portion (b) lower portion (c) top portion (d) inside area 5. Balance weights on a crank shaft are provided to (a) balance the eccentric mass of crank pin (b) balance the crank webs (c) to increase mass for inertia (d) balance weight of piston 6. A cam shaft can NOT be driven by (a) gears (b) V belt (c) Chain and sprockets (d) Toothed belt 7. Dwell period of a cam is (a) when follower moves fast (b) when follower moves slow (c) when follower does not move (d) none of these 8. Pressure generated in a diesel fuel pump is of the order of (a) less than 20 bar (b) 21-50 bar (c) 51-100 bar (d) more than 100 bar 9. An injector is used to (a) atomize the fuel in small droplets (b) suck fuel from fuel tank (c) mix fuel with air in proper proportion (d) supply air and fuel mixture to cylinder 10. The voltage used in spark plug is of the order of (a) 12 V (b) 24 V (c) 440 V (d) more than 10,000 V 11. An impeller is used in (a) gear pump (b) fuel pump (c) water pump (d) electric motor

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606

12. Pressure in lubricating oil pump is increased by using (a) gear (b) (c) impeller (d) 13. A resistance in a spark plug is used to (a) reduce voltage (b) (c) reduce radio interference (d) 14. Radius of a cam during dwell period (a) increases (b) (c) first increases then decreases (d)

pulley injector increase voltage safeguard from high voltage spikes decreases remains same

ANSWERS to Fill in the Blank Questions 1. 5. 9. 13.

one water jackets balance weights carburetor

2. 6. 10. 14.

17. gears

gudgeon Fins half diesel fuel pump

18. cam profile

3. 7. 11. 15.

Ignition check leakage cam shaft Spark plug

4. 8. 12. 16.

19. does not move

oil sump Big end injector impeller

20. honing

ANSWERS to Multiple Choice Questions 1. (a) 7. (c) 13. (c)

2. (d) 8. (d) 14. (d)

3. (c) 9. (a)

ASSIGNMENT

4. (b) 10. (d)

ON

5. (a) 11. (c)

6. (b) 12. (a)

I.C. ENGINES

1. Figure 28.6 shows part drawings of a connecting rod of a radial engine. Draw its assembly drawing. 2. Figure 28.19B shows part drawings of a diesel fuel pump. Draw half sectional front view of the assembly, side view and top view. 3. Figure 28.22B shows part drawings of a water pump. Draw sectional front view and side view of the assembly. 4. Figure 28.23B shows part drawings of a gear pump. Draw sectional front view of the assembly drawing and side view.

CAD ASSIGNMENT 5. 6. 7. 8.

ON

I.C. ENGINES

Figure 28.1 shows a schematic diagram of an I.C. engine. Draw it in any suitable size. Figure 28.20B shows part drawings of an injector. Draw a sectional front view of the assembly drawing. Draw the part drawings of a spark plug shown in Fig. 28.21. Draw a cam profile with rise and fall period both of 140° with S.H.M during each period. Dwell at top 30°. Assume a base circle of 40 mm and roller follower of diameter 20 mm.

Internal Combustion Engines

607

HOMEWORK 9. Sketch a water cooled cylinder shown in Fig. 28.2B. 10. Sketch a piston for a diesel engine shown in Fig. 28.3B. 11. Sketch a crank shaft for a four cylinder petrol engine as shown in Fig. 28.8. 12. Draw a tangent cam with base diameter 30 mm, lift 10 mm and nose radius 5 mm with angle of operation 110°. Assume a roller follower of diameter 15 mm. 13. Sketch a piston shown in Fig. 28.3C. 14. Sketch oil rings as shown in Fig. 28.4C. 15. Sketch a crank shaft for a single cylinder diesel engine as shown in Fig. 28.7. 16. Sketch a valve gear mechanism as shown in Fig. 28.17A. 17. Sketch a crank shaft for a 6 cylinder in line engine. 18. Sketch various types of cams and followers to a suitable scale as shown in Fig. 29.19. 19. Draw part drawings of a variable venturi carburetor as shown in Fig. 28.18.

PROBLEMS

FOR

PRACTICE

20. Draw a sectional front view and side view of the connecting rod shown in Fig. 28.P1. Specify the geometric tolerances.

Fig. 28.P1

A Connecting Rod

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608

21. Draw assembly drawing from the part drawing of a gear pump shown in Fig. 28.P2.

Fig. 28.P2

Part Drawing of a Gear Pump

CHAPTER

29

Steam Power Plants Steam power plant is one of the many types of the plants used for electric power generation. Mainly this plant has a boiler for generating steam and a steam turbine or steam engine to convert heat energy into mechanical power, which is ultimately converted to electric power. Only boiler accessories and steam engine are described in this chapter. Boiler is a high pressure vessel used to generate steam. The pressure inside the boiler is high, hence a feed pump is used to feed water at high pressure in it. A steam injector, which does not need any mechanical power, can also be used in place of feed pump. The injector uses energy of steam by passing it through nozzle to increase pressure of water. A feed check valve controls flow rate of water from feed pump to boiler and also acts as one way valve, which allows water to flow in one direction and stops automatically in the other direction. Water level in the boiler is monitored by watching its level through a glass tube of water level indicator mounted at front end of the boiler. Steam flow to a turbine or engine is controlled by a stop valve mounted between the boiler and steam engine or turbine pipe line. For safety of the boiler, a safety valve is provided, which blows off steam automatically, if the pressure exceeds beyond a set limit. Safety valves are of two types: Dead load type and spring loaded type. A drain plug at the bottom is used for removing sediments or to drain water, if boiler is to be emptied. Steam engine is a steam driven prime mover. A piston with rings over it reciprocates in a cylinder. Since both the sides of piston are used, one side of the cylinder is provided with a stuffing box through which the piston rod passes. Stuffing box is used to check the leakage of steam along the piston rod. A cross head is used in between piston rod and connecting rod to take the transverse thrust, which comes due to obliquity of connecting rod. Crank shaft converts reciprocating motion of the piston into rotary motion. An eccentric converts rotary motion into reciprocating motion. This is used to move a D-slide valve or a piston valve on the top of the cylinder to control steam inlet and outlet openings at the required timings.

29.1

INTRODUCTION

Power plants are set up for electrical power generation. There are many types of power plants like nuclear, hydro, diesel, steam power plants, etc. Only steam power plants are being described here. Although there are uncountable components in a power plant, only a few are being described, which need attention from the drawing students. Figure 29.1 is a schematic diagram of a steam power plant. A small power plant may use steam engine instead of steam turbine. Two main types of equipment used in these plants are: ∑ Boiler and its accessories (Section 29.2) ∑ Steam turbine or Steam engine (Section 29.3)

Part F – Chapter 29

610

Fig. 29.1 Schematic Diagram of a Steam Power Plant

29.2

STEAM GENERATOR (BOILER)

Some important components used in a boiler are given below along with their function: 1. Feed pump (Section 29.2.1) or injector (Section 29.2.2) to supply soft water to the boiler. 2. Feed check valve allows water to flow only in one direction, i.e. from feed pump to boiler. 3. 4. 5. 6.

One way valve controls only the direction and not the flow rate (Section 29.2.3). Water level indicator indicates level of water in a boiler (Section 29.2.4). Stop valve controls amount of steam to be supplied to steam turbine or engine (Section 29.2.5). Safety valve automatically blows off and releases excessive steam pressure to ensure safety of boiler, if the pressure increases beyond certain set pressure (Section 29.2.6). Drain plug at the bottom of the boiler is used to drain water from the boiler (Section 29.2.7).

29.2.1

Feed Pump

It is a positive displacement type of a reciprocating pump also called as ram pump. Its assembly is shown in Fig. 29.2A. Details of the parts are shown in Fig. 29.2B. Its main part is a C.I. body (1) in which a C.I. plunger (2) reciprocates. A barrel liner (3) made of brass is put inside the barrel. A C.I. cover (5) having cover liner (4) covers the barrel and is fixed to it using four bolts and nuts. The barrel is connected to a valve chest (7) using a flange joint. Two identical valves (9); one inlet (9A) and other the outlet valve (9B) are placed over the tight fit valve seats (8) in the valve chest. An air vessel (10) is bolted to the top of chest to maintain uniform flow of water. A bracket (11) is put on top of each valve to limit the lift of the valve with a screw (12). The brackets (11) and the chest covers (6) are bolted together to the valve chest. When the plunger moves upwards in the barrel, suction is created in the valve chest, which opens inlet valve (9A) and closes delivery valve (9B). When the plunger moves downwards, inlet valve closes and the delivery valve opens due to pressure developed in the chest. Water is forced through delivery valve, till the plunger moves downwards. It is driven by electric motor or by steam engine.

Steam Power Plants

Fig. 29.2A

Assembly of Feed Pump and Part List

611

612

Part F – Chapter 29

Fig. 29.2B

Part Details of a Feed Pump

Fig. 29.3 Steam Injector

Steam Power Plants

29.2.2

613

Injector

A feed pump uses mechanical energy to supply high pressure water to the boiler. An injector can also be used in place of feed pump for the same purpose. It has the advantage that it has no moving parts. Steam when passed through a convergent nozzle (Fig. 29.3) located centrally in the body, attains a high velocity, but its pressure gets reduced (because the total head has to remain the same). Due to this reduction in pressure, water is sucked in and mixed with the incoming steam in the mixing nozzle. Then it is passed through a divergent nozzle (Diffuser) to change it from low pressure to high pressure. Here the velocity reduces and pressure increases.

29.2.3

Feed Check Valve

A One Way Valve It is used in between the pipe line from feed pump to boiler to prevent back flow of water, when pump is stopped. Water enters from one side of the valve. A valve is located in the body with a pin joint over an inclined valve seat. The valve opens automatically with the flow of water. Lift of the valve is limited by an adjusting screw. Water after passing from the valve, leaves from the other side of the valve. An opening at the top is used to fix the valve, which is closed by a nut.

Fig. 29.4

One Way Valve

When water is supplied by the feed pump, the valve is lifted up due to pressure of water and it enters the boiler. When pump stops, pressure on pump side becomes low and then high pressure of boiler side water closes the valve automatically.

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Part F – Chapter 29

B Feed Check Valve One way valve controls only the direction of flow, while feed check valve controls flow rate as well as direction. It has two valves; check valve and feed valve. The check valve closes automatically, if the water tries to flow in the other direction. Feed valve can be moved up and down by a hand wheel to control flow rate. It consists of a check valve body (1) with two flanges (Fig. 29.5A). The bottom flange is bolted to flange of the pipe coming from the feed pump and the other side with the feed valve body (2). A check (5) is placed centrally in the valve seat (3). Movement of this valve is limited by the feed valve (6) above it. Feed valve body and check valve bodies are bolted together with bolts and nuts (19). Another valve seat (4) made of gun metal (89-91% copper, 8-11% tin and 1-2% zinc) is tight fit in the feed valve body (2), in which a valve (6) is put. The valve is fitted to a gun metal (G.M.) spindle (8) using a valve nut (7). The body is covered by a cover (9) through which the spindle passes. Top portion of the cover acts as a stuffing box in which a gland (10) is fixed. The gland is tightened with nuts over studs. (21) to prevent leakage. The spindle is attached to a threaded spindle (14) by putting a pin (15). The spindle passes through a square threaded bridge (13) supported by two pillars (11). These pillars are screwed in the cover and the bridge is bolted on their top. Top end of the spindle has a key way on which a hand wheel (16) is put with a key (17) and tightened with a nut (18).

Fig. 29.5A Assembly Drawing of a Feed Check Valve

Steam Power Plants

Fig. 29.5B

29.2.4

615

Part Drawing of a Feed Check Valve

Water Level Indicator

Principle of water level indicator is that fluids always maintain a constant level under same pressure. Two cock bodies (1) and (2) are connected to a thick walled glass tube (8) to show the level of the water inside the boiler (Fig. 29.6). Its upper cock (1) is connected to the steam space and bottom cock (2) in water space of the boiler. Stop cocks (3) are provided in these bodies to stop water/steam to the tube in emergency. Glass tube is connected to cock body using a gland (6) and a nut (7) over it to check

616

Part F – Chapter 29

the leakage along the tube. If glass tube breaks, high pressure steam and water may cause injury to the operator and hence balls (17) are provided in the passages, which move automatically forward and block the passage of water/steam. In some indicators, the glass tube is further covered with another thick glass plates (not shown here) for further safety. A cap (4) is provided on top to close the opening. Side bolts (5) are screwed at the front of the cock bodies for cleaning, the passages. Nuts (11), (12), (13) and (14) keep the cocks in position in the cock bodies. A drain cock (9) is connected to the bottom cock body (2). Drain pipe (15) is fitted to the drain cock body using nut (16).

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Fig. 29.6 Water Level Indicator

29.2.5 Stop Valve Stop valve is used to control or completely stop the steam supply from the boiler when not required. Normally lift of this valve is less for quick opening and closing. It is mounted in the steam pipe line near to engine. It has a gun metal body (1) with three flanges (Fig. 29.7). Left and right flanges are for inlet and outlet respectively, while top flange is to fit a cover (2) using a threaded joint.

618

Part F – Chapter 29

Fig. 29.7

Stop Valve

Bottom part of the cover has threads for the spindle, while its top part forms a stuffing box, in which packing is tightened between a gland (3) and nut (4). A valve seat (5) is tight fit in the body. It supports a valve (6) on its top face. The valve is attached to a threaded spindle (7) by putting a split pin (9) in the collar of the valve. When the spindle is rotated, the end of the spindle can rotate in the groove provided in the valve but valve has to move axially. Top end of the spindle has square end on which a hand wheel (8) is put and tightened with a washer (10) and bolt (11). Example 1 (Stop valve) Figure 29.S1A shows part drawings of a stop way valve. Write its function, construction, part list and draw a sectional front view. Solution Function Stop valve is used to control steam flow rate to any prime mover in the pipe line carrying steam from boiler. Construction It has a body made of C.I. (1) with three flanges at 90° to each other. Left hand side flange is for the inlet and the right hand flange is connected to outlet pipe. Top flange is used to fix a cover (2) over it using studs (15) and nuts (16). A neck bush (5) is placed at the bottom of cover in the hole, while a gland (3) is bolted on top of cover using studs (17) to form a stuffing box. Valve seat (7) is screwed in the body over which a valve (6) is guided in the central boss supported by the radial ribs in the valve seat. The valve is attached to the spindle (4) by a collar (8) with pin (9). A bridge (10) supported on two pillars (12) has threads in the center to form a nut for the spindle. The pillars are screwed on the top of cover (2). Top of the spindle is fixed with a hand wheel (11) using a key (18) and nut (14).

Steam Power Plants

Fig. 29.S1A

Part Drawings of a Stop Valve

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Assembly drawing of the stop valve with major dimensions and part list is shown in Fig. 29.S1B.

Fig. 29.S1B

29.2.6

Assembly Drawing of a Stop Valve

Safety Valve

Safety valves are so called because they are provided for the safety of the boiler. These valves normally remain closed, if the pressure inside the boiler is less than the set pressure. If the pressure increases beyond this set limit, it releases the pressure and thus keeps the boiler in the safe limits. Safety valves are of two types; Dead weight and Spring loaded type and are described as follows:

A Dead Weight Type Safety Valve Dead weight type of safety valve was used in olden days. It has a C.I. body (1), (Fig. 29.8A) over which a casing (2) is fitted. A valve seat (6) is screwed at the top of the casing (2) by three bolts (14). A valve (8) rests over the seat (6) and guided by guide (7). The valve is loaded through a spindle (9) by putting dead weight (4) over the dead weight carrier (3). The spindle is guided by a bush (10). The weights are covered

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by a hollow cylindrical cover (5), which is fitted over the spindle. Two set screws (15) are fitted in the casing to limit the lift of valve. A cap (11) is fixed on top of the spindle. Details of the parts are shown in Fig. 29.8B. The pressure of the steam is balanced by the dead weight on the valve. Excessive steam pressure lifts the valve along with dead weight and the steam is released through a pipe fitted on flange of casing.

Fig. 29.8A

Part Drawing of a Dead Weight Type of Safety Valve

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Fig. 29.8B

Assembly Drawing of a Dead Weight Type of Safety Valve

B Spring Loaded Safety Valve Due to increased steam pressures in the modern steam plants, the dead weight required is very heavy and hence becomes bulky. A spring loaded safety valve has replaced the dead weight type. Figure 29.9 shows such a safety valve. It consists of a U shaped housing (1). Two valve seats (3) made of G.M. are screwed on the top of this housing. Two identical valves (2) of G.M. rest over each valve seats. Valves have a conical top, over which conical pivots (4) fitted on a lever (5) rest. One pivot is built in with the lever while the other is put on the free end of the lever using a forked pivot (4) with the help of a pin (12). The lever is loaded by a tension spring (10) placed centrally between the housing. Spring is designed for the required stiffness according to the set pressure. One end of the spring is fitted in the hole of the lever while the other to a shackle (7) fitted on the housing. A safety link (6) is placed centrally inside the spring. It has a slot at one end to limit the movement of the lever. When the force on valves by pressure of the steam exceeds the force exerted by the spring, valves are pushed up and the steam leaks. Since these valves normally do not release steam, there are chances that they may get stuck at their seats. This sticking is not desirable and hence it is checked by lever. When lever is pressed down, one valve should release steam and the other when lever is pushed up. Safety valve shown in Fig. 29.9 is generally used in steam locomotives.

Steam Power Plants

Fig. 29.9 Spring Loaded Safety Valve

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C Lever Type Safety Valve Some safety valves use dead weight over a lever. This reduces the dead weight required due to lever action. 29.2.7 Drain Plug It is very simple type of valve. It is provided at the bottom of the boiler to drain the sediments periodically which may get accumulated or drain water completely for cleaning from inside. It has a body (1) having a tapered hole, in which a G.M. cock (4) fits with corresponding taper (Fig. 29.10). It is covered by a cover (2) and fixed with studs (5) and nuts (6). The cock has a rectangular slot aligned diametrically. Normally cock remains in closed position. In this position the cock blocks the outlet passage. When the cock is rotated by 90°, the slot aligns with the opening of the valve and the water is drained. Top of the cover has a stuffing box, in which a gland (3) is tightened with studs (7) and nuts (8) to prevent leakage of water from top. The gland has a hole on the top surface, through which a handle can be put on the top of square end of the cock to rotate it. A base screw (9) is fitted at the bottom of the body with lock nut (10). 29.3

STEAM ENGINE

Steam engine is a steam driven prime mover. It converts energy of the steam into mechanical power. A schematic diagram of a steam engine is shown in Fig. 29.11. It has following main components: 1. Cylinder where the power is generated. It is closed at both ends by cylinder covers (Section 29.3.1). 2. Piston reciprocates in the cylinder (Section 29.3.2). 3. Piston rings are put over the piston to check leakage of steam from one side of the piston to the other side (Section 29.3.3). 4. Piston rod connects piston with cross head (Section 29.3.4). 5. Stuffing box is provided at one end of the cylinder head to check leakage of steam along piston rod (Section 29.3.5). 6. Cross head is provided to connect piston rod and connecting rod with a pin joint and takes the lateral thrust due to obliquity of the connecting rod (Section 29.3.6). 7. Connecting rod connects cross head and crank (Section 29.3.7). 8. Crank shaft converts reciprocating motion of cross head into rotary motion (Section 29.3.8). 9. Eccentric is driven by crank shaft to convert rotary motion into reciprocating motion to be given to the valves (Section 29.3.9). 10. D slide valve or piston valve controls entry of steam to the cylinder and exit from it (Section 29.3.10).

29.3.1

Cylinder and Covers

Cylinder is made of cast iron and closed at the ends by cylinder covers. Cylinder of a steam engine has ports on both the sides, through which the steam enters and goes out alternately. A cylinder, piston with rings, cylinder heads, stuffing box with packing and gland and are shown in assembled condition in Fig. 29.12. Cylinder covers are bolted to the cylinder by bolts/studs and nuts. Sometimes a spigot is provided in the center, that aligns the cover centrally, automatically and exactly. A flat gasket is placed between the flanges of the cylinder and cover is placed to check the leakage of steam from the joint. The gasket material is asbestos base and can withstand temperature of the steam. Center of the cover sometimes protrudes outside to accommodate for the piston nut.

Steam Power Plants

Fig. 29.10 Drain Plug

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626

Fig. 29.11

Fig. 29.12

Schematic Diagram of a Steam Engine

Assembly of Cylinder, Piston, Rings and Stuffing Box

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Cylinder cover for the other end of the cylinder is provided with a stuffing box and is described in Section 29.3.5.

29.3.2

Pistons

Piston is a cylindrical part, which reciprocates in the cylinder by the pressure of the steam. Shape of piston used for steam engine is quite different from that used for I.C. Engines. Pistons are made slightly smaller in diameter than cylinder bore so that they can slide freely. It has grooves over the periphery for the piston rings to check the leakage of steam from one side of the piston to the other side. Solid pistons are used for small engines. For bigger engines, they are made in different cross sections (Fig. 29.13A and B) or hollow to reduce the weight as shown in Fig. 29.13C. Hollow pistons are plugged with a threaded plug to close the other side of the piston. Three piece pistons are also sometimes used, in which the central ring is sandwiched between two disks (Fig. 29.13D). A tapered hole is provided at its center, in which the piston rod is fixed. Tapered hole helps the piston rod in self-centering. Pistons are generally made of Cast iron. Steel pistons are also used for big size engines as they offer lighter and stronger construction than C.I. pistons.

Fig. 29.13

29.3.3

Types of Pistons

Piston Rings

Piston rings are made slightly bigger in diameter than the cylinder bore. A small piece is then cut at an angle about 45° to create a free end gap between the piston ring ends (Fig. 29.14). It is then expanded and put in the piston grooves. When piston with rings is inserted in the cylinder, rings get compressed and the end gap becomes negligible. Gaps of adjacent rings are kept far from each other. Piston rings are also made of cast iron.

29.3.4

Piston Rod

Pistons are attached to piston rod by different methods. For small engines, the piston rod is screwed in the central threaded hole in the piston (Fig. 29.15). The end is riveted so that the rod does not

Fig. 29.14

Pistons Rings

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628

get loosened. Alternately, hole in the piston is tapered, in which a tapered end of the piston rod is inserted. The free end of the rod is tightened with a locking device like castle nut and a pin. Taper varies from 1 in 6 to 1 in 16 on diameter.

Fig. 29.15

Piston End of Piston Rod

Fig. 29.16 Other End of Piston Rod

Other end of the piston rod is connected to cross head (Section 29.3.6) by different methods as shown in Fig. 29.16. In one method, rod is flared and fixed to cross head using bolts and nuts (Fig. 29.16A). In some rods the end is forged rectangular and a rectangular slot is made. A gib and cotter are then used to fix with cross head. (Fig. 29.16B and C)

29.3.5

Stuffing Box

Steam engine is double acting and hence the other end of the cylinder is also covered by a cylinder cover. But this cover has to have a hole through which the piston rod should pass. Stuffing box is used to check leakage of steam along the piston rod. It is cast integral with the cover having a hole bigger than piston rod diameter to accommodate for a neck bush (Fig. 29.17). Asbestos fiber packing is put in the annular space around the piston rod. The packing is compressed with gland using studs and nuts. Ends of neck bush and gland are sloped at 30° angle, which help in pressing the packing tightly. It allows the piston rod to reciprocate, but does not allow the steam to leak. Sometimes the gland is lined with a brass liner so that only the liner could be replaced when worn out. Vertical engines provide an oil cup on the top of gland for lubrication (Fig. 29.17). Flanges of the gland are oval shaped for small engines. For bigger engines these can be triangular or circular also using 3 or 4 studs. For a very small engine, complete gland can be made of brass and tightened directly with a screwed joint over the cover as shown in Fig. 29.7.

Fig. 29.17 Stuffing Box

Steam Power Plants

29.3.6

629

Cross Head

Steam engine is a double acting engine and hence other side of the cylinder is also to be closed. This does not allow use of connecting rod directly, due to obliquity of the connecting rod, hence a cross head (Fig. 29.18) is provided to take the lateral thrust. Cross head is a rectangular block and forms a pin joint between piston rod on one side and connecting rod on the other side. Piston rod is attached to it in various forms, e.g. threaded joint or by 2 bolts or by a cotter as shown in Fig. 29.16. Small end of connecting rod is connected to cross head with the help of a gudgeon pin to form a pin joint. Construction of cross head is of many types as follows: (a) Solid – The body is solid and small end of connecting rod is forked (Fig. 29.18A). (b) Hollow – It is hollow in the center, where an eye end of the connecting rod is fixed (Fig. 29.18B) (c) Single shoe – There is only one shoe, which slides in the guides (Fig. 29.18A) (d) Double shoe – There are two shoes; one on each side to slide in the guides (Fig. 29.18C) (e) Fixed shoes – The shoes are integral with the body (Fig. 29.19) (f) Detachable shoes – The shoes are separate and fixed to the body (Fig. 29.20).

Fig. 29.18

Types of Cross Heads

Details of a solid cross head with single fixed shoe are shown in Fig. 29.19. The gudgeon pin passes through the brasses in two pieces and joined together by a cover plate using bolts and nuts. Figure 28.20 shows a cross head with double detachable shoes. A wedge block and a bolt are used to fix the two halves of the bearing. It slides between two guides (Fig. 29.18C) to constrain its motion in a straight line and take the side thrust. The sliding surfaces are sometimes provided with separate liners, which can be replaced, when worn out.

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Fig. 29.19

29.3.7

Cross Head with Single Fixed Shoe

Connecting Rod

Connecting rod connects cross head and crank pin of a steam engine. It is made of forged steel. Rod of steam engines may be of circular or rectangular cross-section. Both the ends are enlarged. One end called small end is connected with cross head using a gudgeon pin. Figure 29.21A shows an isometric view of small end of connecting rod, in which the brasses are tightened in position by a gib and cotter joint (Fig. 29.20B). A lock screw on the side locks the cotter so that it may not slip due to vibrations.

Steam Power Plants

Fig. 29.20 Cross Head with Double Detachable Shoes

Fig. 29.21A Isometric View of Small End of Connecting Rod

Fig. 29.21B Half Sectional Views of Small End of Connecting Rod

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The other end, called big end is fixed over the crank pin. Two brasses having semicircular hole are joined to form a full circular hole (Fig. 29.22). The hole is lined with white metal, which has excellent properties required for a bearing. Both the brasses are sandwiched between rod end and cap with bolts and nuts. A thin spacer is put between the brasses which can be reduced or removed when the brasses wear out. The brasses are kept in position by a cap with bolts and nuts.

29.3.8

Fig. 29.22

Big End of Connecting Rod

Crank Shaft

crank shaft is used in a steam engine to convert reciprocating motion to rotary motion. Distance between center of crank pin and axis of crank shaft is half the stroke of piston. Flywheel is provided at one end to smoothen out the fluctuations in the torque. Cranks of large engines can be built up, in which webs and crank pin are made separately and then joined firmly (Fig. 29.23). Small engines have one piece forged crank shaft (Fig. 29.24).

Fig. 29.23 Built up Crank Shaft

Fig. 29.24

Forged Crank Shaft

Crank shaft may be over hung (Fig. 29.25), in which there is only one crank web and the crank pin overhangs. The web has two bosses. One boss has crank pin riveted to the web and other hub contains the shaft; both are shrunk fit in the bosses. The crank pin can be fixed in a taper hole also with a threaded joint (Fig. 29.26). A disk crank (Fig. 29.26) is another form of overhung crank. A segment of a disk is made heavier than other side to balance the weight of the crank pin and some weight of big end of connecting rod.

Steam Power Plants

Fig. 29.25

Over Hung Crank Shaft

Fig. 29.27

Fig. 29.26

633

Disk Crank Shaft

Eccentric with Single Sheave

Part F – Chapter 29

634

Fig. 29.28 Eccentric with Double Sheave

29.3.9

Eccentric

Eccentric is provided to drive the valves. It is mounted on the crank shaft and converts rotary motion to reciprocating motion. It consists of an eccentric disk called sheave, which is mounted on the crank shaft with a taper sunk key. The sheave can be cast in one piece; in that case it is mounted on crank shaft from its end only (Fig. 29.27). Therefore it is generally made in two halves so that it could be fitted anywhere on the crank shaft easily (Fig. 29.28). Both the halves when joined together with bolts and nuts form a stepped rim. Two shims are used between the two halves during machining. While assembling, this shim is removed to make a tight joint. A strap in two halves is mounted over the sheave. These are of semicircular shape having circumferential groove on the inner surface to suit the stepped rim of the sheave. Sometimes a reversed arrangement is also used in which, groove is cut in the sheave and corresponding projection in the strap. The strap halves are held together by strap bolts or by a cotter joint. A shim is placed between two halves of straps. Later this shim can be removed to decrease the gap between sheave and strap, if the strap wears out. One of the strap is connected to the eccentric rod with bolts. Both sheave and strap are made of cast iron.

29.3.10

Slide Valves

D-slide valve or piston valve are the two types of valves, which are used to control the ports of steam entry to cylinder and exit from the cylinder. In steam engine, the port through which the steam enters the cylinder, is used for the exit also. In case of D-slide valve (Fig. 29.29), steam enters from outside the valves and is received back from inside the valve, which is connected to the outlet pipe. The valve is

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driven by an eccentric rod in phase difference to the piston movement. The valve is attached to valve rod by nuts and lock nuts on both the sides of the valve for adjustment. The valve rod passes through a stuffing box on the steam chest, to check leakage along the valve rod. The valve is guided between the guide ways provided in the steam chest.

Fig. 29.29

D-Slide Valve

THEORY QUESTIONS Name the important boiler mountings and functions of each. Differentiate between a feed pump and injector used for boilers. Describe the construction of a water level indicator with a sketch. Explain the construction of a stop valve. What are the different types of safety valves? Describe a spring loaded safety valve. What is the limitation of a dead load safety valve? Describe the function, construction and working of a single sheave solid cross head used in steam engine. 8. What is an eccentric? Where is it used in steam engine and how does it work? 9. Explain the function of a D-slide valve in a steam engine. 1. 2. 3. 4. 5. 6. 7.

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FILL 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

IN THE

BLANKS

is used to supply high pressure water in boiler. Injector creates high pressure of feed water using . Level of water can be seen inside the boiler using . is provided to blow off extra steam, if pressure is more than set limit. Dead weights are used for the safety of boiler in . Quantity of steam is controlled by . Water is drained from boiler using . Steam engine is acting engine. is placed between piston rod and connecting rod. Valve in a steam engine is driven by . In steam engine, inlet and outlet ports are controlled by . Eccentric converts motion into motion.

MULTIPLE CHOICE QUESTIONS 1. An injector is used in a boiler to (a) inject water softening chemicals (b) supply water at high pressure (c) supply diesel (d) to eject dirty water 2. Water level indicator in a boiler is connected to (a) water storage tank and boiler (b) condenser and boiler (c) top and bottom of boiler drum (d) stop valve and boiler 3. One way valve is used to (a) control steam flow rate (b) control water flow rate (c) direction of steam (d) direction of water 4. To control the steam flow rate from boiler to steam engine, the valve used is (a) drain plug (b) stop valve (c) one way valve (d) safety valve 5. A safety valve is used on a boiler for the safety of (a) boiler operator (b) boiler itself (c) persons in nearby area (d) all mentioned above 6. A dead weight safety valve uses a dead weight equal to (a) weight of boiler (b) weight of steam in boiler (c) force applied by steam below valve (d) 20% of steam force 7. Function of stuffing box is to (a) check steam leakage around piston rod

(b) keep the cylinder full of steam

(c) to provide additional capacity for cylinder

(d) cover the cylinder ends

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8. A cross head is placed between (a) piston and crank shaft

(b) piston and connecting rod

(c) connecting rod and piston rod

(d) crank shaft and valve

ANSWERS to Fill in the Blank Questions 1. feed pump

2. steam

3. water level indicator

4. safety valve

5. dead weight safety valve

6. stop valve

7. drain plug

8. double

9. cross head

10. eccentric

11. D-slide valve

12. rotary, reciprocating

ANSWERS to Multiple Choice Questions 1. (b)

2. (c)

7. (a)

8. (c)

3. (d)

ASSIGNMENT

ON

4. (b)

5. (d)

6. (c)

STEAM POWER PLANTS

1. Part drawings of a feed check valve are shown in Fig. 29.5B. Draw its assembly drawing in half sectional front view and side view from outside. 2. Part drawing of a stop valve is shown in Fig. 29.7. Draw its assembly in half sectional front view and side view. 3. Figure 29.8 shows assembly and part drawing of a dead weight type safety valve. Draw its part drawings in half sectional views and top view of each part. 4. Figure 29.20B shows assembly drawing of a cross head. Draw its part drawings.

CAD ASSIGNMENT

ON

STEAM POWER PLANTS

5. Assembly drawing of piston and other components is shown in Fig. 29.12. Draw the parts given as follows: A. Piston in half sectional front view B . Two views of piston rings C . Piston rod D. Stuffing box with cover in half sectional front view 6. Part drawings of a spring loaded safety valve are shown in Fig. 29.9. Draw its assembly drawing in full section front view and side view.

HOMEWORK 7. Draw a sectional view of steam injector shown in Fig. 29.3. 8. From the assembly drawing shown in Fig. 29.17, draw the part drawings of stuffing box.

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9. Draw the part drawing for assembly drawing of disk type crank shown in Fig. 29.26. 10. From part drawings of a drain plug shown in Fig. 29.10, draw its assembly drawing in full section. 11. Sketch a D-slide valve as shown in Fig. 29.29.

PROBLEMS 12. 13. 14. 15. 16. 17. 18. 19.

FOR

PRACTICE

Draw sectional front view and top view of a feed pump shown in Fig. 29.2A. An assembly drawing of a connecting rod is shown in Fig. 29.21B, draw its part drawings. Draw sectional assembly drawings from part drawing of water level indicator shown in Fig. 29.6. Figure 29.28 is assembly drawing of an eccentric. Draw its part drawing with lower half in section. Figure 29.4 shows an assembly drawing of a one way valve. Draw its part drawings. Sketch various types of pistons used for steam engine as shown in Fig. 29.13. Draw full sectional Front View, half sectional side view and top view of cross head shown in Fig. 29.19. Draw assembly drawing of single sheave eccentric from the part drawings shown in Fig. 29.27. Draw sectional Front View, Left Side View & Top View.

CHAPTER

30

Machine Tools There are many machines in workshop but only some parts of machines are discussed in this Chapter, which are of interest for classroom teaching of machine drawing. A lathe is used for making cylindrical objects. Its main parts are head stock, chuck, bed, saddle and tail stock. Head stock is located on left side of the lathe and provides different speeds and power to feed rod and lead screw for automatic feed and thread cutting. Its spindle supports the chuck on its right side. Chucks are of two types; three jaw and four jaw. In a three jaw chuck all the jaws move together radially inwards or outwards with a scroll plate and hence a circular job is centered automatically. In four jaw chuck each jaw moves independently and hence can be used for irregular shapes. It offers more gripping power than three jaw chuck. A tool post is fitted over the compound slide to hold the tool. It can hold one or more than one tools also. A long job is supported at the other free end by dead center or revolving center mounted on tail stock on right side of the lathe. Shaper is used to make flat surfaces and grooves, etc. A ram reciprocates horizontally, which is driven by quick return mechanism, moving a tool slow while cutting in forward stroke and returns fast during its return stroke. A tool slide is provided at the front end of the ram to give feed to tool. A clapper box is fitted on the tool slide, which holds the tool. This box permits slight lift of the tool during return stroke, so that the tool does not scratch a line on the job. Drill machines are of many types. A hand drill is useful for small holes in wood working or where there is no power. A drill chuck mounted at the bottom of driven spindle has three jaws and is of self centering type. Clamping devices are used to hold the job firmly while working over them. Pipe vice is useful to hold circular jobs. Pipe wrench can help tightening of circular items where spanners will not work. There are different types of spanners like Double ended, Ring, Pipe, Box spanner, etc. Use of each depends upon space available for their use. An adjustable spanner can be used for any size.

CAD HELIX command is used to draw cylindrical or conical helix. A spiral can be drawn if height of helix is specified as zero.

30.1

INTRODUCTION AND SCOPE

Machine tool is a subject by itself in Production Engineering. Machines are of so many types, but only a few mentioned here have been selected for this chapter to limit the size of the chapter. ∑ Lathe is used for making cylindrical jobs (Section 30.2) ∑ Shaper is used for making flat surfaces (Section 30.3) ∑ Drilling machine is used to drill holes (Section 30.4) ∑ Miscellaneous clamping devices and hand tools (Section 30.5)

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30.2

LATHE

Lathe is a very commonly used machine for cylindrical machining (Fig. 30.1). It has main components as head stock, bed, saddle and tail stock. Head stock is placed on left hand side of the machine and gives power to the main spindle of the machine. It has either a gear box or stepped pulleys to rotate spindle at different speeds depending upon the diameter of the work piece and material to be cut. The spindle is provided with a chuck to hold the job firmly. Chucks are of two types; three jaw and four jaw chuck. These are described as follows:

Fig. 30.1

30.2.1

Lathe

Three Jaw Chuck

Main components of a 3 jaw chuck are a scroll plate, jaws, bevel gears and body (Fig. 30.2A). Scroll plate (3) is a circular plate having a spiral groove of square cross-section on one side and a bevel gear on the other side. Part drawings are shown in Fig. 30.2B. A jaw (5) is a stepped component with grooves on its rear side to match with the grooves of the scroll plate. The jaws slide in the T slots provided in the body (1), radially at 120° degrees to each other. The jaws are numbered as 1, 2 and 3. While assembling, precaution has to be taken that each number is fitted in the groove of same number; otherwise the job will not be gripped centrally. Steps of the jaws can be used to hold a hollow job of various diameters. Three small bevel pinions (4) mesh with bevel gear at back side of the scroll plate. A square cavity is provided on top of bevel pinion so as to rotate it with the help of a handle. After placing the scroll plate and bevel pinions in position in the body, a back plate (2) is fixed on back side using three bolts (7). The whole assembly is bolted to the flange (6) using bolts (8). When the bevel pinion is rotated, it rotates the scroll plate, which slides all the three jaws simultaneously radially inward or outward, depending upon the direction of rotation. Therefore it can hold only cylindrical jobs or triangular or hexagonal shapes. Separate set of jaws is to be used if the jaws are to be inverted to hold a big diameter job.

Machine Tools

Fig. 30.2A

Assembly of Three Jaw Chuck

Fig. 30.2B (Contd...)

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Fig. 30.2B

30.2.2

Part Drawings of Three Jaw Chuck

Four Jaw Chuck

In outer appearance, the difference between a three and four jaw chuck is only in the number of visible jaws, but internally the construction is quite different (Fig. 30.3). There is no scroll plate, but square threaded bolts (3) are provided in the body (1) to slide the jaws (2). Each jaw can be adjusted to any radial position independently. Any jaw can be placed in any slot, even the jaws can be inverted to hold large components. In the T slots at front of the body, Jaws with threads on back side like half nut to suit the threads of the jaw bolts are put. A half collar pin (4) is put in the hole of the body over the screws from back side, to fit in the groove of jaw bolt and kept in position by circlips (5). Three dowel pins (6) are put in the holes from back side of the body and locked into position by screws (7). Half of the screw is in the threads of pin and half in the threads of the body to check rotation and axial movement of the dowel pin. The chuck can be fixed on the spindle of head stock using threads (Fig. 30.2) or by using dowel pins (Fig. 30.3). The assembly is fitted on the spindle flange (8) using dowel pins (6). The pins are locked in position by flange pins (9). These pins have a square hole at top by which these can be rotated. These pins are kept in position by position pins (10) put from the side hole and then plugged by the flange screws (11). When the flange pin is rotated, it locks the dowel pin.

Machine Tools

Fig. 30.3 (Contd...)

643

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Fig. 30.3 Four Jaw Chuck

30.2.3

Tool Post

The cutting tool is held firmly in a tool post on the compound slide. For a good surface finish, it is necessary that the tool is adjusted at the center line of the job. Hence arrangement is provided to adjust the tip of tool at center of job. In one arrangement a spherical seat as a ring (5) is provided (Fig. 30.4) so as to swivel tilting block (3) in vertical plane or rotate the tool in horizontal plane at any angle in the base (4). The tool is fixed in the slot provided in the body (1) and is tightened with a bolt (2) at the top.

Fig. 30.4 (Contd...)

Machine Tools

645

Fig. 30.4 Assembly and Part Drawings of a Tool Post

Fig. 30.5A

Assembly of Square Tool Post

In the other arrangement, tool is adjusted by putting thin steel strips at the bottom of the cutting tool. In some machines, tool post can accommodate four tools (Fig. 30.5A), right angles to each other. It has a square tool holder (1) having rectangular grooves on all the four sides to accommodate tool. It is pivoted centrally on the base plate (2) with the help of a stud (6). It is clamped with the base plate using

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646

clamp nut (3) and handle (4) with knob (5). Tool is tightened with three screws on top of to provide a good grip for the tool with the tool holder. A spring (9) and a ball (8) are put in its bottom hole before fixing on the base plate. The ball fits into the notch and keeps the holder at square position with the compound slide. The base is fixed with the compound slide using four screws (10).

Fig. 30.5B Square Tool Post

30.2.4

Tail Stock

If the work piece is long, it needs support at the other end. This support is provided with the help of a tail stock (Figs 30.6 and 30.S1). A conical center called as dead center is fitted at the front end of the tail stock. It fits into a counter-sunk hole provided in the work piece at the center. A revolving center (Fig. 30.7) can also be used in place of dead center.

Machine Tools

647

Tail stock consists of a C.I. body (1), in which a barrel (2) moves axially with the help of a spindle (3) and hand wheel (4). Axial position of the spindle is constrained by a thrust bearing (7) and washer (8) and a cover plate (16) fixed to the body with four Allen screws (18). The spindle is provided with a Morse taper at its front end, which matches with the taper of the dead center. The handle is locked with spindle by a woodruff key (19) and a screw (6). This screw has two diametrical blind holes for tightening. The hand wheel (4) has a handle (5) for easy rotation. Tail stock can be locked at any position over the bed with the help of a lever (10). This lever rotates an eccentric on cam shaft (13) which lifts the cam link (17) upwards. This link is connected to a clamping plate (9) below the bed with the help of a bolt (12), which clamps it with the bed when the link is moved upwards. Some designs use a bolt and nut also to clamp with bed.

Fig. 30.6 (Contd…..)

Part F – Chapter 30 3

648

15

5

25

72

Ø3 Ø30

24

18 Enlarged

13

Ø18

M8

16

6 M8

Ø52

Ø58 Ø20

40

Ø20

19

Enlarged

12 48

5 5

M8

8

8

0

25

2

0

27

R1

R1

4

Enlarged

17

18

35 25 8

7

Ø8

13

36 36

8

R4

90

Ø14

15

15 15

70

Ø20 12

70

88

Ø25 9

Enlarged

12

°

Ø23

80

94

14

1 R1

10 M12

13

10 M10

Ø15

50

20

Fig. 30.6

28 Ø23

Ø23

Ø20

18

1.5

110

135

11

Ø15 23

37 20

130

Ø20

Ø23

1.5

28

1 R1

75

M12

104

M12

M12

M10

Part Drawings of Tail Stock

After setting the dead center in the end of the job, barrel can also be locked with another lever (11). This lever also rotates a barrel lock cam (14), which pushes the feather key (15) upwards to lock the barrel. It also prevents rotation of the barrel when spindle is rotated. The conical point of the dead center and the axis of the chuck have to be exactly aligned, otherwise it may result in taper turning. Sometimes axis of the tail stock is set deliberately eccentric in horizontal plane to make small tapers on the component. This is done by providing a transverse slide on the base of the tail stock (not shown in this design), and the spindle can be offset by screws provided on the side.

Example 1 Part drawings of tail stock are shown in Fig. 30.6. Draw full sectional front view of its assembly drawing and create a part list. Solution Assembly drawing and part list of tail stock are shown in Fig. 30.S1. Dimensions should be taken from part drawings and place on the drawing to complete the solution.

Machine Tools

649

Fig. 30.S1 Assembly of Tail Stock Spacer ring

Ball bearing

Body

Ø60

Ø3 Deep 3 PCD 32

9

12

11

12

Morse taper shank 20

5

Ø8

M8

Ø24

M40

60°

Dead center

R 2 77

19 6

14

16

14

5

1.5

21

Fig. 30.7

Revolving Center

8

Ø19

Needle bearing Cover plate

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30.2.5

Revolving Center

A dead center offers sliding friction at the conical contacting surfaces between the job and center. Hence has to be kept lubricated with grease. A revolving center rotates with the job and hence does not need any lubrication at the center. It uses two antifriction bearings to take radial and thrust loads and to support the center. Since axial load is not much, even a deep groove ball bearing can take the axial load. A spacer is placed between the two bearings. The bearings are covered with a nut at its front end, which can be tightened by two diametrically drilled holes. A threaded hole is made at the rear end of the taper shank to put a long bolt to remove the bearings from the body.

30.3

SHAPER

A lathe is used for turning of circular components while shaper is used to make flat machined surfaces. It consists of a box like column to support the components (Fig. 30.8). An electric motor is used to give power to a ram through pulleys/gears to reduce speed. The ram is driven by a quick return mechanism (Section 30.3.1) and can be locked at any position by a ram lock lever. Front end of ram is bolted with a swivel base, which can be locked at any angle shown on a calibrated dial. A tool slide slides over the guideways of the swivel base. A clapper box with a tool holder can be swiveled over the tool slide and locked by a clamping bolt. The tool can be moved with a hand wheel on the top of tool slide.

Fig. 30.8

Schematic Diagram of a Shaper

The job is fixed on a table or on a vice mounted on table. The table can be moved across the ram axis over a cross slide. The table can also be moved vertically by elevating screw to adjust height according to the job. For inclined surfaces, the tool slide is to be set at the required angle and feed is to be given to the tool post. Stroke of the ram is adjusted by rotating a shaft with bevel gear (Fig. 30.9), which changes the radius of crank (inside the box column). A feed disk with a diametric slot to adjust crank radius is used for automatic feed with pawl arrangement on the gear mounted on cross slide screw.

Machine Tools

30.3.1

651

Quick Return Mechanism

The ram is driven by a special link mechanism called “Quick return mechanism” (Refer Fig. 30.9). It is so called, because the time taken in reverse stroke is lesser than the time taken while cutting in forward direction.

Fig. 30.9

Quick Return Mechanism

Power from a driving pinion is given to a big bull gear. Due to size difference of these gears, the speed reduces. A slide is bolted to the bull gear which carries a sliding block into which a crank pin is fixed. The block can be moved radially by a set of bevel pinions and a lead screw. This movement is used to adjust the stroke of the ram. Sliding block mounted on the crank pin is fitted in the slotted lever. This lever is pivoted at the base with a pivot pin. Upper end of the slotted lever is either forked or has another link to join with the ram using pin joints. Ram can be moved manually for adjustment along its axis by a screw shaft and ram nut using two bevel gears and with a hand wheel. It can be clamped to the drive at any suitable position with the help of a clamping lever.

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Part F – Chapter 30

Fig. 30.10 Tool Slide

Machine Tools

30.3.2

653

Tool Slide

Tool slide is used to feed the tool against the job. Swivel base is fixed at the front face of the ram (Fig. 30.8). A tool slide (1) is put over this base to slide over its dovetailed slide guideways (Fig. 30.10 and 30.S2). A handle bar (3) is fixed at the top of the screw with a woodruff key (13) and nut (10). A handle (4) is screwed on the handle bar for easy rotation. The slide can be moved up or down with the help of a slide screw (2).

30.3.3

Clapper Box

Clapper box (5) is a special tool holder, which allows the tool to be lifted slightly up by a pin joint provided at top (Fig. 30.10), so that it does not scratch the surface during return stroke. This box gives the sound of clapping in each stroke and hence called as clapper box. It is fixed at the front of the slide by a clamping bolt (7) and swivel pin (12). This box can be rotated about swivel pin to rotate the tool at any desired angle. A clapper plate (11) is fitted in this box with the help of pin (9). A tool holder (6) has a diametrical slot in which the tool is put and tightened with the help of bolt (8). Example 2 Parts of shaper tool slide of a shaper are shown in Fig. 30.10. Draw a sectional front view of its assembly drawing and create a part list with material and quantity. Solution Assembly drawing of the shaper tool slide and part list are shown in Fig. 30.S2. Dimension the figure for complete solution.

Fig. 30.S2

Shaper Tool Slide

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30.4

DRILLING MACHINE

This machine is used to drill holes from size as small as 0.5 mm to 50 mm. Portable machines can be taken in hand to drill holes at any place and at any angle. Bench drilling machines are table mounted for small holes, while Pillar and Radial drilling machines are for medium and large size holes respectively. Holes of large diameters are made on Boring machine.

Fig. 30.11 Hand Drilling Machine

Machine Tools

30.4.1

655

Hand Drilling Machine

A hand drilling machine is shown in Fig. 30.11 and in 30.S3. It uses the manual power to rotate the chuck holding the drill bit. It consists of one driving bevel gear (9) and two bevel pinions (10 and 11). Pinion (10) is idler and is supported on shaft (4). These gears are made of alloy cast iron. Bevel pinion (11) is driven and is fixed with its shaft (17) using a pin (22). The bevel gear is supported on a pin (12) and driven by a crank (7) fixed to the gear with screw (8). A handle (5) with its ferrule (16) is fixed to the crank by crank pin (13). The pinions and gears are supported in the body (6). A handle (1) with its ferrule (3) is fixed to the body using a handle shaft (2) and pin (22). A lateral knob (14) with its ferrule (15) is put on the pin (12) for support. At the free end of the shaft (17), a chuck is screwed. It has a back plate (18) and a cap (21) in which three jaws (19) can be tightened by rotating the cap. The jaws move radially inwards when they are tightened in a conical hole. The jaws are kept apart using three coil springs (20) mounted in an arctual fashion and supported in their holes on the sides. Example 3 Parts of a hand drill are shown in Fig. 30.11. Draw most informative half sectional view of its assembly drawing. Solution Assembly drawing of the hand drill is shown in Fig. 30.S3. Complete the solution by putting dimensions.

Fig. 30.S3 Assembly of Hand Drilling Machine

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30.5

HOLDING AND CLAMPING DEVICES

30.5.1

Pipe Vice

It is used to hold pipes for cutting or threading operations. It has a V shaped base housing with serrations in V groove for a good grip (Fig. 30.12). The housing has a threaded hole on the top through which a screw passes. Bottom of the screw is attached with a movable inverted V shaped jaw with serrations on inclined surfaces. The jaw is fixed on screw with the help of a set screw. The main screw is fitted with a handle bar to rotate it. Handle nuts are fitted on this bar at each end so that the handle does not come out of the hole. The pipe is put between the jaws and clamped by rotating the screw from the top.

Fig. 30.12

30.5.2

Pipe Vice

Pipe Wrench

It is used for plumbing work to lay pipe lines. It is an adjustable tool whose one of the jaw can be adjusted to suit a pipe diameter. It has a fixed jaw with serrations and a long handle to apply force

Machine Tools

657

(Fig. 30.13). The other movable jaw with serrations has a screw integral with it. The screw passes through a nut. When nut is rotated, the movable jaw slides along the handle axis. Pipes bigger than 65 mm diameter, are handled more conveniently by a chain wrench.

Fig. 30.13 Pipe Wrench

30.5.3

Spanners

Various types of spanners are used to tighten or open hexagonal bolts and nuts. They are available in a set of 8 spanners having smallest slot size as 6 mm at one end and 7 mm at the other. Other sizes are 8-9, 10-11, 12-13, 14-15, 16-17, 18-19 and 20-21. Another set is available for sizes greater than 21 mm. Double End spanner (Fig. 30.14A) is most commonly used. It has two slotted ends of different sizes to suit a head size. The ends are inclined at 15 degrees to its axis for a better approach in different positions at restricted spaces. If a hexagonal head is damaged due to use of improper spanners/tools, double end spanner is not much useful. Ring spanner (Fig. 30.14B) offers a better grip by enclosing the head by a ring having 12 sided hole to fit the head in 12 different positions. Box spanners (Fig. 30.14C) are conical plugs having a hexagonal hole at one end and a square hole at the other. The hexagonal side is fixed over the bolt head and a handle with square end is fixed on the other side to rotate the plug. Pipe spanner (Fig. 30.14D) is useful when there is no free space near the bolt head, e.g. spark plug on an engine. It has hexagonal ends with diametrical holes on each side. A round rod is put in this hole to rotate the pipe spanner. An Allen key is a hardened hexagonal rod bent in L shape (Fig. 30.14E). One side is longer than the other. It is used for Allen head bolts having hexagonal cavity in the circular head. Shorter side is inserted first to apply more torque. Once the head is loosened the longer side can be inserted for quick rotation.

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An adjustable spanner (Fig. 30.14F) can be used for any size of the head. It has a fixed jaw and the other movable jaw, which can be moved by a nut on its threaded end. Since it does not give a rigid grip, its use should be avoided and restricted only when proper spanner is not available.

Fig. 30.14

Various Types of Tightening Tools

CAD HELIX command has been recently included in AutoCAD 2007 and does not exist in versions earlier than this. It is very useful for making cylindrical or conical helical features in 3D. This command can be accessed by any one of the following methods:

Machine Tools

659

∑ Click on Draw at the menu bar and select Helix from the pull down menu. ∑ At the Command line type HELIX and press Enter key. The prompt sequence is in the following section. Input parameters are the diameter at base and top, number of turns, direction of helix and height of complete helix or the axial distance between two adjacent turns known as pitch. It draws a helix of default number of turns (3), which can be changed at the last prompt. The helix can be clockwise (CW) or counter-clockwise (CCW). This also can be set at the last prompt. By default it takes CCW. The axis of the helix can be set inclined also by first typing A and then specifying other end point of the axis. If the top and base dimensions are same a cylindrical helix is drawn. If these are different a conical helix is drawn. If height is specified as zero, the helix is nothing but a spiral. This can be used very conveniently to draw back side of scroll plate of a 3 jaw chuck, which has a spiral on one face. Use of this command is shown in Example 4. Example 4 turns.

Draw a spiral starting with radius 80 mm and ending at radius 160 mm with eight number of

Solution

Type the command and press Enter key. Default values or the previous set values are displayed. Number of turns = 3.0000 Twist=CCW Ignore this display. Specify center point of base: 0,0 ø Click at center point of the base of helix or enter coordinates at the key board. Specify base radius or [Diameter] : 40 ø Type value of base radius and press Enter. Specify top radius or [Diameter] : 8 0 ø Type value of top radius and press Enter. Command: HELIX ø

Specify helix height or [Axis endpoint/Turns/turn Height/tWist] : T ø

Enter number of turns : 8 ø

At this prompt type T to specify number of turns and press Enter. Specify desired number of turns here and press Enter.

Specify helix height or [Axis endpoint/Turns/turn Height/tWist] : W ø

If helix is desired not in CCW direction, type W and press Enter. Enter twist direction of helix [CW/CCW] : CW ø

Type CW for Clockwise direction and press Enter. Specify helix height or [Axis endpoint/Turns/turn Height/tWist] : 0 ø

If pitch is known then type H to specify height between turns else specify height of complete helix with all turns. Value of height as zero draws a spiral.

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The display is shown in Fig. 30.S4A.

(A) Spiral

(B) Scroll Plate

Fig. 30.S4

Spiral using AutoCAD

Note: OFFSET command will not work with the object created with HELIX command because it is 3D object. Pitch for the spiral is (80 – 40)/8 = 5 mm. Hence to draw a double lined spiral as required for a scroll plate, offset would be (5/2 = 2.5). For the second spiral increase the top and bottom radii by the required offset distance and specify radii as 42.5 and 82.5 (Fig. 30.S4B).

THEORY QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

What are the types of works that can be done on lathe? Write names of its important parts. What is the function of a chuck? Differentiate between a three jaw and a four jaw chuck. What is a scroll plate? Where is it used? Describe its construction. Sketch a tail stock and name its main parts. What is the purpose of a tool post? Describe any two types of tool posts used on lathe. Sketch a shaper machine and label its main parts. What is quick return mechanism? How does it work and what are its advantages? Describe the function of a clapper box. Sketch a pipe vice and describe its construction. Describe the construction and use of a pipe wrench. Sketch the various types of spanners and write the typical use of each.

CAD 12. Describe the method to draw a cylindrical helix with base diameter 40 mm and height 80 mm with distance between turns 10 mm. 13. How can HELIX command be used to draw a conical helix of base radius 60 mm top radius 30 mm and height of spring 70 mm with 7 number of turns. 14. Explain the method to draw a spiral with innermost radius 30 mm and outermost radius 100 mm and 7 turns. 15. How can HELIX command be used to draw a scroll plate.

Machine Tools

FILL 1. 2. 3. 4. 5. 6. 7. 8.

IN THE

661

BLANKS

Cylindrical turning is done on machine. Name of self centering chuck is . Irregular shapes of the job can be fixed on chuck. Free end of long job is supported on lathe by . Time taken by tool on shaper to move forward is then its return. allows to lift the tool slightly on return stroke. Pipe vice is used to hold . Plumbing work is done using for tightening pipes.

CAD 9. To draw a helical spring the command used is . command and specifying 10. Spiral can be drawn by using

MULTIPLE CHOICE QUESTIONS 1. Cylindrical jobs are machined on (a) lathe (c) milling machine

(b) shaper (d) press

2. Irregular cross sections can be fixed on a lathe using (a) Tail stock (b) live center (c) four jaw chuck 3. Scroll plate is used in (a) three jaw chuck (c) face plate 4. Clapper box is used in (a) press (c) drill machine 5. In a hand drill, the driver gear is (a) one helical gear (c) two bevel gears

(d) self centering chuck (b) four jaw chuck (d) machine vice (b) shaper (d) horizontal milling machine (b) one bevel gear (d) three bevel gears

CAD 6. A scroll plate can be drawn using command (a) SCROLL (b) HELIX (c) SPIRAL (d) None of these

as zero.

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ANSWERS to Fill in the Blank Questions 1. lathe 5. more 9. HELIX

2. three jaw chuck 6. Clapper box 10. HELIX, height

3. four jaw 7. circular objects

4. tail stock 8. pipe wrench

ANSWERS to Multiple Choice Questions 1. (a)

2. (c)

3. (a)

ASSIGNMENT

4. (b)

ON

5. (b)

6. (b)

MACHINE TOOLS

1. Figure 30.3 shows a part drawing of a four jaw chuck. Draw half sectional front view and side view of its assembly. 2. Figure 30.4 shows a part drawing of a tool post. Draw its assembly drawing in full section. 3. Figure 30.6 shows a part drawing of a tail stock. Draw the following views: a. Full sectional front view b. Side view c. Top view 4. Figure 30.10 shows a tool slide with details of its parts. Draw front view, side view and top view of its assembly drawing.

CAD ASSIGNMENT

ON

MACHINE TOOLS

5. Draw the following parts of a three jaw chuck, shown in Fig. 30.1. a. Scroll plate b. Any one jaw 6. Draw part drawings of a tool post shown in assembly drawing in Fig. 30.5.

HOMEWORK 7. Draw a ring spanner as shown in Fig. 30.14B. 8. Figure 30.2 shows part of 3 jaw chuck. Draw its half sectional front view and top view of its assembly drawing. 9. From assembly drawing of a pipe wrench shown in Fig. 30.13, draw its part drawings. 10. From assembly of a pipe vice shown in Fig. 30.12, draw its part drawings.

PROBLEMS 11. 12. 13. 14.

FOR

PRACTICE

Draw part drawings of a revolving center shown in Fig. 30.7. Draw front and side view of the assembly for a hand drill from the parts shown in Fig. 30.11. Draw different types of spanners as shown in Fig. 30.14 (A to E). Draw an adjustable spanner as shown in Fig. 30.14(F).

APPENDIX

1

Some Useful Indian Standards Alloys-Steels Belts-V Bolts and nuts – Black – Precision Castings-Ferrous Gears Hexagonal bolts and nuts Keys and Key ways-Taper keys – Woodruff – Gib head – Feather Materials-Aluminum and its alloys – Chemical composition – Copper and copper alloys – Copper sheets Nuts-Castle Oil seals Pins-Cylindrical – Taper PVC-Pipe fittings Rivet-Snap head – Material Screws-Allen head – Slotted countersunk – Slotted cheese head Splines Studs Tolerance grades Tolerance on forms Washers-Plain

IS 7598–1974 IS 2494–1974 IS 2585–1968 IS 1363–1967 IS 1364–1967 IS 4843–1968 IS 2535–1978 (1991) IS 3640–1967 IS 2292–1974 IS 2294-1986 IS 2293–1974 IS 2048–1963 IS 6051–1970 IS 1367–1967 IS 2378–1974 IS 288–1960 IS 2232–1967 IS 5129 IS 2393–1980 IS 6688–1972 IS 7834–1975 IS 2155 IS 1148/49 IS 2269–1967 IS 1365–1968 IS 1366–1968 IS 2327–1991, 3665–1990, 13088–1991 IS 1862–1975 IS 919–1963 IS 800–1976 IS 2017–1967

APPENDIX

Some Relevant Internet Sites 3D solid modeling 3D solid modeling Adhesives ANSI drafting standard ASME standard Bearings Bearings Bearings Belt drives Cad software Cad software Cad software Cam design Canadian standard Careers in drafting Components for jigs Computers Couplings, chains, flexible shafts Developments Draft reference guide Drafting certification and jobs Drafting equipment Drafting office Drill bushings for jigs Gear drives Gear drives Keys, splines, pins, springs, rivets Precision bearings V belts Valves Welded joints

http://www.proe-design.com http://www3.autodesk.com http://www.3m.com http://www.ansi.org http://www.asme.org http://www.honeywell.com http://www.skf.com http://www.timken.com http://www.gates.com http://www.ptc.com http://www.cadkey.com http://www.autodesk.com http://www.saltire.com/cams.html http://www.cas.ca http://stats.bls.gov/ocohome.htm http://www.arriane.com http://www.hewlett-packard.com http://www.moresecontrols.com/products http://www.thesheetmetalshop.com http://www.adda.org http://www.adda.org http://www.staedtler.com http://www.mayline.com http://www.americandrillbushing.com http://www.datadynamics.com http://www.bostgear.com http://www.machinedesign.com http://www.torrington.com http://www.dayco.com http://www.cranevalve.com http://www.studweldmfl.com

2

APPENDIX

3

List of Some Important Autocad Commands COMMAND

FUNCTION

3DARRAY

Creates a three dimensional array in rows, columns and levels

ARC

Draws an arc

AREA

Calculates area and perimeter

ARRAY

Draws multiple copies in rectangular or circular pattern

BHATCH

Fills a closed area with associative hatch pattern

BLOCK

Groups objects into a single object

BOX

Creates a 3D solid box

BREAK

Breaks object in to parts

CAL

Starts AutoCAD built-in calculator

CAMERA

Sets a different camera and target location

CHAMFER

Chamfers corners of lines or surfaces

CHANGE

Modifies the properties of an existing object

CHPROP

Modifies the common properties of the object

CIRCLE

Draws circle

COLOR

Sets default color for new objects

CONE

Draws a solid cone with circular or elliptical base

COPY

Copies objects

CYLINDER

Draws a solid cylinder with circular or elliptical base

DDEDIT

Displays dialog box to edit text

DDIM

Controls dimensioning settings

DDINSERT

Displays dialog box for the INSERT operation

DDLTYPE

Displays dialog box for loading line types

DDMODIFY

Displays dialog box for editing properties of the objects

DDOSNAP

Displays dialog box for Osnap settings

DDPTYPE

Displays dialog box for choosing point style

DDRMODES

Displays Drafting Settings dialog box

666

Part F – Appendix 3

DIM

Starts dimensioning mode

DIM1

To use a single dimension command

DIMALIGNED

Dimensions as aligned linear

DIMANGULAR

Dimensions an angle

DIMBASELINE

Dimensions from a base line

DIMCENTER

Draws a center mark for an arc or a circle

DIMCONTINUE

Dimensions in continuation to a linear dimension

DIMDIAMETER

Dimensions diameter of a circle

DIMEDIT

Move/Rotate the dimensions text

DIMLINEAR

Draws horizontal, vertical or rotated linear dimensions

DIMORDINATE

Draws ordinate point type of dimensions

DIMOVERRIDE

Overrides the dimension system variable

DIMRADIUS

Dimensions a radius of an arc or a circle

DIMSCALE

Controls scale of linetype and dimensions

DIMSTYLE

Creates and modifies dimension style

DIST

Displays the distance between two specified points

DIVIDE

Marks points at regular equal segments

DONUT

Draws a circle with a wide line

ELLIPSE

Draws an ellipse

ERASE

Deletes the selected objects

EXPLODE

Breaks a group of objects in separate entities

EXTEND

Extends an entity up to the specified boundary

EXTRUDE

Draws a solid by extruding a 2D object

FILL

Controls visibility of filled objects

FILLET

Rounds off the corners or adjacent edges

FILTER

Allows to select object by their properties

FIND

Finds, replaces, selects, or zooms to specified text

FOG

Provides visual cues for the apparent distance of objects

GRAPHSCR

Toggles from text screen to graphic mode

GRID

Sets grid settings

HATCH

Fills a closed area with non-associative hatch pattern

HELP

Informs about the commands and operations

INSERT

Inserts a file or a block

INTERFERE

Creates a composite 3D solid from the common volume of two or more solids

LAYER

Offers layer control at command line

LEADER

Draws an arrow

LENGTHEN

Modifies length of objects and included angle of arc

List of Some Important Autocad Commands

667

LIMITS

Sets the drawing limits

LINE

Draws lines

LINETYPE

Controls line-type settings

LINEWEIGHT

Controls line weight to objects and layers

LIST

Displays object properties

LWEIGHT

Sets the current line weight, line weight display options, and line weight units

MATCHPROP

Copies the properties from one object to one or more objects

MEASURE

Puts mark at regular intervals

MINSERT

Inserts blocks in a rectangular array

MIRROR

Creates a mirror copy of the selected object

MIRROR3D

Creates a mirror image of objects about a plane

MLEDIT

Edits multi-lines

MLINE

Draws multi-lines

MLSTYLE

Defines style for multiline

MOVE

Moves a selected object

MTEXT

Write a para of text

OFFSET

Draws parallel copies of object

OOPS

Undoes the last erasure

ORTHO

Forces the lines to be horizontal or vertical

OSNAP

Permits exact selection of the geometry of an object

PAN

Moves the current display

PLINE

Draws a 2D polyline

PLOT

Plots a drawing file

POINT

Draws a point of the selected point style

POLYGON

Draws a polygon of the specified number of sides and size

PROPERTIES

Displays Properties dialog box

QUIT

Exits AutoCAD

RAY

Draws a semi-infinite line from a specified point

RECTANG

Draws a rectangle

REDO

Reverses the last undo command

REDRAW

Refreshes the display

REVOLVE

Creates a solid object by rotating a 2D object about an axis

REVSURF

Creates a surface by revolving an object about an axis

ROTATE

Rotates an object about a specified point

ROTATE3D

Rotates a 3D object about a specified axis in space

RULESURF

Draws a 3D mesh between two curves

SAVE

Saves a drawing

668

Part F – Appendix 3

SCALE

Modifies the size of object

SHADE

Shades 3D models

SKETCH

Allows sketching

SLICE

Slices a solid at the specified plane

SPHERE

Draws a 3D solid sphere

STRETCH

Stretches an object from the vertices

SUBTRACT

Creates a new solid by subtracting a solid object from other solid object

TOLERANCE

Puts geometric tolerances

TORUS

Creates a 3D solid of the shape of a car tube

TRIM

Trims the objects extending beyond a specified boundary

U

Undoes one command at a time

UCS

Controls User Coordinate System

UCSICON

Controls User Coordinate System Icon

UNDO

Undoes commands in reverse order

UNION

Creates a new solid by adding solid objects

UNITS

Selects the unit style

VPOINT

Defines a view point to view 3D objects

VPORTS

Divides screen in vports in Model Space

WBLOCK

Saves a block which can be used by other drawing files

WEDGE

Draws a solid wedge tapering along X axis

XLINE

Draws a line from minus infinity to plus infinity through a specified point

ZOOM

Increases or decreases the apparent size of objects in the current view port

Index A Adhesive Tape 50 Advantages of Computer Aided Drafting 10 Aligned 97 Alloy Steels 439, 442 High Alloy Steels 443 Low and Medium 442 Tool Steels 443 Alloying elements 439 Aluminum Alloys 447 Code Designation 447 American National Standards Institute 7 ANGLES IN AUTOLISP 503 Polar 503 Angular 97 Applications of plain carbon steels 438 Applications of Riveted Joints 227 Applications and Suggested Roughness 424, 425 Light work applications 227 Pressure vessels 227 Structural joints 227 ARC 18 architectural drawings 2 ARRAY 26 Array Command 254 Polar Array 255 Accept 256 Center point 255 Modify 256 More 256 Preview 256 Rotate items 256 Select objects 255 Rectangular Array 255 Accept 255

Angle of array 255 Column offset 255 columns 255 Modify 255 Preview 255 Row offset 255 Rows 255 Array command 485 Arrowheads 85 Assembly Drawings 5, 468, 553, 556 Catalog Assembly Drawings 558 Design Assembly Drawings 556 Detail Assembly Drawings 557 Exploded Assembly Drawing 558 Installation Assembly Drawings 557 Sub Assembly Drawings 557 Autocad Commands 11 Screen 10 Autolisp 498 Creating an Autolisp Program 498 Executing the Program 501 Loading an Autolisp Program 501 Auxiliary 82 Views 157 Axonometric Views 173 foreshortening 173 Isometric Scale 174 Axonometric Views and Oblique Views 171

B Bar charts 2 BASE CIRCLE 537 Base Line 98 BEARING MATERIALS 512 Aluminum Alloys 512 Babbitt 512

670 Bronzes 512 Plastics 513 Porous Metals 512 Bearings 510 Belts 490 crossed belt 490 idler is 490 open belt 490 Quarter twist belt 490 Belts and Pulleys 489 Bend 348 Types of Dimensioning 82 Fits 386 Clearance Fit 386 Bevel Gears 543 Hypoid bevel gears 543 Spiral bevel gears 543 Straight teeth bevel gears 543 Vertex 543 Bhatch Command 145 Angle 145 Boundary 145 Select Object 145 Pick Point 145 Boundary Hatch and Fill dialog box 145 Hatch Pattern 145 Palette 145 Preview 145 Scale 145 Bhatch command 21 Big Compass 44 Divider 45 Bill of Materials 450, 558 Steps for Creating Assembly Drawings 559 Blind Rivets 234 breaking 234 non-breaking 234 Block 235 insertion base point 235 Block Attributes 454 Blue Print Reading 117, 561 Reading Assembly Drawings 562 Reading Detail Drawings 561 Body 264 Bolt Specification 264 DRAWING A BOLT/NUT 264

Index Boiler Joints 231 Axial Joint 232 Bolt 245 Nut 245 Screws 266 Cap Screw 267 Machine Screw 267 Set Screw 267 Shapes of Screw Head 267 Types of Screw Ends 268 Bolt Proportions 263 Bolt Head 263 Head thickness 263 Size across flats 263 Bolts and Nuts 262 bolt 263 head 263 nut 263 threads 263 washer 263 Bolts and Nuts for Special Applications 272 Eye Bolt 272 Foundation Bolts 274 High Strength Bolts 273 J Bolt 272 Thumb Screw 273 Turn Buckle 273 U Bolt 272 Various Types of Bolts 273 Wing Nut 273 Yoke Bolt 272 Border and Frame 48 Break 28 Bureau of Indian Standards 7

C Cabinet View 182 Cavalier and Cabinet views 182 CAD FOR GEAR 547 Approximate Method 547 Draw Involute Profile 547 Drawing Gear 547 Cams 588 cylindrical type 589 radial type 588 Shapes 589

Index Cam Shaft 593 Carbon Steels 437 High Carbon Steel 438 Low Carbon Steel 438 Medium Carbon Steel 438 Carburetor 595 fixed venturi 596 variable venturi 596 Cast Iron 438 Applications of cast irons 439 Ductile Cast Iron 438 Grey Cast Iron 439 Malleable Cast Iron 439 Ferritic 439, 440 Pearlitic 439 White Cast Iron 439 CAST IRON FITTINGS 349 Sizes of cast iron pipe fittings 349, 350 Casting Processes 459 Die Casting 459 Sand Casting 459 Center Lines 61 Mark 99 Option 20 Centering Marks 48 CHAMFER 19, 28 Chain Dimensioning 90 Chemical Processes 461 Electric Discharge Machine (EDM) 461 Electro-Chemical Machining (ECM) 462 Electroplating 462 Etching 462 CIRCLE 18, 61 Circle Stencil 45 CLASSIFICATION 480 Compression spring 480 Diaphragm spring 480 Helical spring 480 Leaf spring 480 Spiral spring 480 Tension spring 480 Torsion bar 480 Torsion spring 480 CLASSIFICATION OF BEARINGS 511 Axial 511 Hydrodynamic 511

671

Hydrostatic 511 Radial 511 Radial and axial 511 Rolling 511 CLASSIFICATION OF RIVETED JOINTS 222 Arrangement of Plates 223 Butt joint 223 Lap joint 223 Arrangement of Rivets 224 chain riveting 224 zigzag riveting 224 Diamond Riveting 224 Number of Cover Plates 223 double cover plates 223 single cover plate 223 Number of Rows of Rivets 223 double riveted joint 223 triple riveted joint 223 CLASSIFICATION OF THREADS 246 Acme threads 247 B.S.W. threads 247 Buttress threads 247 Double start 247 External 246 Internal 246 Knuckle threads 247 Left hand 247 Quadruple start 247 Right hand 247 Single start 247 Square threads 247 Triple start 247 V threads 247 Classification of Tools 463 Clutches 322 Chetch Pencil 51 CNC Machines 462 Code Designation 444 Ferrous Castings 444 Color 64 Combined Dimensioning 91 Command area 11 line 11 Components of a Leaf Spring 483 Compression Spring 480 Closed 481

672

Index

Ends of a Compression Spring 481 Ground 481 Open 481 Computer 52 COMPUTER AIDED DRAFTING 4, 9 COMPUTER SOFTWARE 53 CONDITIONAL BRANCHING (IF COMMAND) 503 IF 503 Progn 504 Conical Pen 47 Connecting Rod 582, 584 big end 582 Materials 584 Radial clearance 585 Radial Engine 584 Constant Velocity Joint 328 Construction Lines 165 of Gears 540 Continue 98 Conventional 484 Representation of Gear Teeth 540 Conventions in Sectioning 136 of Breaking in Sectioning 138 Hidden edges 136 Preferred features in section 138 View 138 Section Lines 136 Sizes of Objects 137 Thin parts 137 Hatching of Thin Parts 137 Threads in section 138 Cooling System 601 water pump 601 Coordinate System 13 Copper Alloys 445 Alloy Index 447 Code Designation 446 Copy 26 Correcting Mistakes 24 Cotter Joints 311 Gib and Cotter Joint 312 Socket and Spigot Joint 311 Couplings 322, 323 Detachable 323 Flexible 323

Parallel 323 Rigid 323 Slip 323 Universal 323 Crank Shaft 585 Cylinder Diesel Engine 587 Four Cylinder Petrol Engine 586 Tolerances and Geometric tolerances 587 Two Stroke Engine 586 Creating a Block 235 a New Layer 93 Assembly Drawings from Part Drawings 564 Part Drawings from Assembly Drawings 565 Cumulative Tolerances 380 Cutting Speed 464 Cycloid Profile 535 Cylinder 577 bore geometry 578 Surface 578

D Datum 406 Feature 406 Letter 406 Triangle 406 Multi-datums 407 datum edges 90 DDEDIT command 76 DDIM 94 DDINSERT 236 Explode 237 Insertion point 236 Scale 236 factors 237 DDLMODES 92 DDLTYPE 18 DDLTYPE command 63 DDPTYPE 17 DDRMODES 15 Dead Ends 349 Deleting a Layer 93 Detachable Couplings 329 Detail Drawing 553 World Coordinate System 13

Index Worm and Worm Wheel 544 Axial Pitch 544 Face Width 544 Gear Ratio 545 Helix Angle 544 Hub Diameter 545 Length 545 Lead 544 Length of Worm 544 Outside Diameter 545 Pressure Angle 544 Solid Rim Thickness 545 Throat Diameter 545 Web Thickness 545 Diagonals 61 Diameter 97 Diaphragm Spring 485 Diesel Fuel Pump 597 Digital Camera 53 Dimension Line 83 Style 94 Style Manager 94 Dimensional Values 86 Dimensioning Angles 88 Chamfers 88 Circular Arcs 86 Commands 94 Curves 89 Diameters 87 Equidistant Features 89 Holes 87 Isometric Drawings 179 Methods 90, 96 Repetitive Features 89 Tapers 89 Threads 89 Toolbar 94 Undercuts 89 Dimetric Projections 179 DIMSTYLE 94 distances and offsets 90 Donut 21 Drafter 42 Drafting Machines 42 Draw Toolbar 17 Drawing a Bolt/Nut 264

673

Alternate Method for Drawing Head/Nut 3 Face View 265 Direct Method 264 Square Head 266 Drawing a Perspective View 203 Angular Perspective View (2 Vanishing Points) 205 Parallel Perspective View (One Vanishing Point) 204 Drawing a Perspective View of a Circle 206 Drawing a Radial Cam 591 Drawing Aids 14 Drawing an Oblique View 180 Choice of axes 181 Drawing Approximate Involute Tooth Profile 538 Tooth Profile for Teeth Less than 30 539 Tooth Profile for Teeth More than 30 538 Drawing area 11 Drawing Auxiliary View of a Curved Surface 158 Auxiliary View from Actual Dimensions 159 Main Views from Auxiliary View 160 Two Auxiliary Views 159 Drawing Auxiliary View of an Inclined Surface 158 Drawing Basic Entities 17 Board 42 Clip 50 Equipment 41 Freehand Lines 60 Horizontal and Vertical lines 60 Ink 51 Involute Gear Tooth 538 Isometric Views 174 Lines 60 Orthographic Views 122 Pin 50 Revisions 555 Sheet 47 Sheet Fasteners 50 Standards 7 Dual Torsion Spring 482 DVIEW Command 212

E Edge View 161 Sleeve Bearing Supports 513 Foot Step Supports 514

674 Pedestal Support 514 Plummer Block 513 Sleeve Motion Restriction Methods 514 Simple Bearing Support 513 Edge Option 20 Preparation 283 Types of Grooves For Welds 284 View 161 Editing Text 76 Effects 75 Elements of Dimensioning 82 of Graphics 3 Elevation 19 Ellipse 20 Stencil 45 Ends of a Compression Spring 481 Enter 12 Equipment for Computer Aided Drafting 52 Erase 25 Eraser 52 Expansion Joints 347 Bellows Type 347 Loop Type 347 Stuffing Box Type 347 Explode 29 Exploded Drawings 5 Explosive Rivets 235 Extend 28 Extension Bar 44 Line 82 External Locking Devices 271 Full Locking Plate 271 Half Locking Plate 271 Screw 271 Split Pin 271 Extracting Data from List Variable 502

F Factors Affecting Appearance 201 Effect of Distance 201 Orientation of Object 201 Ferrous Metals 437 Wrought iron 437 Fillet 19, 29 Welds 290

Index Dimensioning Fillet Welds 290 Intermittent Staggered Welds 291 Finished Drawings 4 FITS 370, 385 Fixing a Drawing Sheet 50 Fixtures 467 Flanged Fittings 350 Sizes of flanged pipe fittings 350, 351 Flare Joint 359 Compression Joint 359 flared joint 359 inverted flare 359 Spigot Joint 359 Flat Belt Pulleys 493 Armed Pulley 493 Built Up Pulley 494 Fast and Loose Pulley 494 Solid Pulley 493 Stepped Pulley 494 Webbed Pulley 493 Flat Belts 490 Grooved or Serrated 490 Plain Flat 490 Toothed Belts 491 Flexible Couplings 326 Flexicurve 44 Flush Joints 234 breaking 234 counter sunk rivet 234 non-breaking 234 Folding a Sheet 49 Font name 75 style 75 Form Tolerance for Single Features 409 Circularity 410 Cylindricity 410 Flatness 410 Profile of a Line 411 Profile of a Surface 411 Straightness 409 Forming Processes 460 Drawing 460 Extrusion 460 Forging 460 Rolling 460

Index Frame 405 Free Hand Pen 46 Sketching 4, 60 French Curves 44 Fuel System 595 carburetor 595 fuel pump 595 Full Loop End 481 Function Key Assignments 12 Functional 82 Fundamental Tolerances 375 Letter Symbol 375 Letter Symbol for Holes 376 Letter Symbol for Shafts 379 Number Symbol 376

G Gauges 393 Gear Tooth Calculations 535 Gear and Tooth Proportions 535 Types of Pitches 535 Gears 531 General Purpose Joints Types of Small Rivets 233 Geometric Tolerance Symbols and Their Proportions 408 datum features 409 Indicating Datum on a Drawing 409 leader line 409 Proportions 408 Symbols 408 Geometric Tolerances 427 Dialog box Symbols 427 Geometric Tolerance Dialog Box 427 Material Condition Modifier 428 Tol 427 Tolerance 427 Geometrical Tolerances and Surface Finish 402 Get Commands 502 Getdist 502 Getpoint 502 Graphic language 1 Library 297 Graticulation 208 Grid 14 Reference Numbers 48

Grips 30 Groove Welds 291 angle 292 size 291 Root opening 291 Specifying Groove Welds 291 Grooved Pulleys 495 Belt cross-sections (mm) 495 Multi Belt Grooved Pulley 496 Single Belt Grooved Pulley 496 Stepped Grooved Pulley 496 Guide Lines 71

H Half Loop End 481 Terminology 200, 246, 263, 371, 405 Actual Size 371 Allowance 372 Axis 405 Basic Size 371 Bolt Length 263 Boxed Dimensions 405 Cone Angle 200 Conical Threads 246 Crest 246 Depth of Thread 246 Deviation 372 Lower Deviation 372 Upper Deviation 372 Distance Across Corners 263 Flats 263 Elevation Angle 200 Feature 405 Flank 246 Fundamental Deviation 372 Geometric Tolerance 405 Ground Line (GL) 200 Plane 200 Plane (Gp) 200 Hand of Helix 246 Head Thickness 263 Helix Angle 246 Horizon 200 Horizontal Plane (HP) 200 Lateral Angle 200 Lead 246

675

676 Limits 372 Major Diameter 246 Median 405 Minor Diameter 246 Picture Plane (PP) 200 Pitch Diameter 246 of Thread 246 Root 246 Standard Size 371 Start of Thread 246 Station Point (SP) 200 Terminology 371 Thread Angle 246 Threads Per Inch 246 Tolerance Zone 405 Tolerances 372 Hangers 516 J Hanger 517 U HANGER 516 Hatch 21 Pattern 145 Hatching Using Autocad 145 Heat Treatment Processes 462 Annealing 463 Hardening 463 Normalizing 463 Tempering 463 Height 75 Helical Gears 542 Helix angle 542 Helical Spring 480 Coil diameter (D) 480 Free length 480 Helix angle 480 Number of turns (n) 480 Outside diameter 480 Pitch (p) 480 Solid length 480 Stiffness 480 Wire diameter (d) 480 Helix angle 480 Hexagon Stencil 46 Hidden Details 112 Hollow Key 307 Hydrodynamic Bearings 511

Index

I I.C. Engines 575 Compression Stroke 576 Exhaust Stroke 576 Power Stroke 576 Suction Stroke 576 Ignition System 600 spark plug 600 Illusions in Auxiliary Views 160 Inclined Letters 73 Indicating Geometrical Tolerances on Drawings 409 Injector 599 Inking Pens 46 Inlet Valves 593 Insert 236 Inspection 466 Comparators 466 Plug gauge 466 Pneumatic gauges 466 Ring gauge 466 Snap gauge 466 Installation Drawings 6 Instrument Box 44 Interference Fit 386 Transition Fit 386 Types of Gears 533 Bevel Gears 534 Helical Gears 534 Rack 534 Internal Threads International Standards Organization 7 International Tolerance Grade (It Grade) 373 empirical relation 374 IT grades of various manufacturing processes 375 Involute Profile 536 Isocircle 21, 187 Shapes of Isocircle 187 Isometric Drawing of Arcs 178 Circles 176 Four Centers Method 177 Offset Method 176 Isometric Drawing of Curved Objects 178 Isometric Grid 186 Drafting Settings Dialog Box 186 Grid On (F7) 186

Index Y spacing 186 Isometric Grid and Cursor Shapes 186 snap 186 Isocircles in Front View 188 Isocircles in Top View 189 Isometric Sections 178 View of Angles 176

J JIGS 466 Joining Processes 460 Arc Welding 460 Brazing 461 Gas Welding 460 Riveting 461 Seam Welding 461 Soldering 461 Spot Welding 461 Joint Proportions 225 Diagonal Pitch 226 Margin 225 Pitch 225 Row Pitch 225 Joints for Cast Iron Pipes 343 Cast Iron Pipes 343 Joints for Copper Pipes 344 Copper Pipes 344 Joints for Hydraulic Pipes 345 Hydraulic Pipes 345 Joints for Lead Pipes 345 Lead Pipes 345 Joints for Wrought Iron Pipes 344 Wrought Iron Pipes 344 Justification 75 Justify 74

K Keys 306 Keyway 306 Keyway and Their Shapes 306 Knuckle Joint 313

L Lateral Angle 201 Lay 422 Symbols Used for Direction of Lay 422

Layer Properties Manager 92 Layers 92 Toolbar 92 Layout 11 Leader 99 Line 83 Leaf Spring 482 Components of a Leaf Spring 483 Drawing a Leaf Spring 483 Elliptical 482 Quarter Elliptical 483 Semi Elliptical 482 Shapes of Leaf Spring 483 Three Quarter Elliptical 483 Torsional Spring 483 Lengthen 27 Lettering Stencil 46 Lift Diagrams 590 Light Work Applications 233 General Purpose Joints 233 Limits 370 Line 17 Thickness of Letters 73 Thicknesses with Pencil and Ink 60 Linear Dimension 96 Liner 45 Lines 59 Linetype 64 Manager 18 Lineweight 64, 65 Settings Dialog Box 65 Locking Devices 268 External Devices to Hold the Nut 268 Special Nuts 268 Washers 268 Logical Operators 503 And 503 Or 503 Looping a Program 504 Repeat 504 While 504 Lubrication System 602 Gear Pump 602

M Machine Drawings 5 Making a Layer Current 93

677

678 Making a Riveted Joint 221 Caulking 222 Fullering 222 Manufacturing Processes 459 Maps 2 Match Properties 30 Material Condition 407 Least Material Condition (LMC) 407 Maximum Material Condition (MMC) 407 Regardless of Feature Size (S) 408 Material Removal Processes 461 Material Specifications 435 Material Properties 436 Material used for Bearing 511 Mathematical Operations 502 Addition 502 Division 502 Multiplication 502 Subtraction 502 Maximum and Minimum Limits 380 Menu Bar 10, 11 Methods of Orthographic Projection 110 Comparison of First Angle and Third Angle Projection 111 First Angle Method 110 Third Angle Method 111 Methods of Preparing Drawings 4 Minsert Command 237 Mirror 26 Missing Views 117 Box Method 118 Projection Line Technique 118 Mistakes in Drawing Arrows 85 Putting Dimension Lines 84 Mline Command 359 Elements 360 Justification 360 Bottom 360 Top 360 Zero 360 Offset 360 Scale 360 MLSTYLE COMMAND 360 Element Properties Dialog Box 360 Multilines Styles Dialog Box 360

Index Modify Commands 25 Properties 29 Properties Box 64 Toolbar 25 Modifying a Layer 93 Three-dimensional Objects 313 3d Mirror 314 Chamfer 314 Fillet 313 Rotate 3d 314 Solidedit 314 Move 27 Muff Couplings 323 Full Muff Coupling 323 Half Lap Muff Coupling 324 Split Muff Coupling 323 Multiple Drawings 554 Sheet Drawings 49 Mvsetup Command 472 Available Title Blocks 472 Iso A3 Template 473

N New Dimension Style Dialog Box 95 Nomograms 2 Non Functional 82 Non-ferrous Metals 444 Applications 444

O Object Orientation 201 Object Properties 64 Selection 25 Snap 15 Snap Toolbar 15 Oblique Angle 75 Projection 180 Cabinet View 180 Cavalier View 180 Oblique Surface 158 Oblique Views 172 Perspective Views 172 Types of Pipe Joints 342 Screwed Flange Joint 342

Index Screwed Socket Joint 342 Socket and Spigot Joint 343 Offset 26 Oil Rings 582 Oldham’s Coupling 327 Oops 25 Ortho 15 Orthogonal 110 Orthographic Projections 3, 107 Orthographic Views 109 Plane of Projection 110 Projectors 110

P Parallel Sided 310 Parallel Coupling (Oldham’s) 327 Rubber Disk Type Coupling 327 Parallel Dimensioning 90 Keys 308 Double Head Key 309 Feather Key 309 Peg Key 309 Single Head Screw Key 309 Part Drawings 6 Patent Drawings 7 Pattern 21 Pdmode 17 Pdsize 17 Pedit 30 Pencil Sharpening Machine 51 Pencils 50 Perpendicularity 412 Perspective Projection 200 Vanishing Point 200 Perspective View of a Cylinder 206 Views 199 Pi Charts 2 Pick Points 21 Pictorial Representation 3 Views 109 Axonometric View 109 Isometric View 109 Oblique View 109 Parallel 109 Perspective View 109

Pipe Fittings 348 Branching 348 Cross 349 Lateral 349 Tee 349 Types of Bends 348 45° Elbow 348 Bend 348 Elbow 348 Pipe Joints 339 Materials 340 Cast Iron 340 Pipes 340 Galvanized Iron 340 Lead Lined Pipes 340 Pipe Designation 340 Sizes 341 Threads 341 Plastic Pipes 340 Stainless Steel Pipes 340 Wrought Iron Pipes 340 Pipe Supports 357 Ceiling Pipe Roll 357 Support 357 Floor Pipe Roll 357 Wall Bracket Support 357 Well Support 357 Pipes 340 Piping Layouts 356 Double Line Layout 356 Single Line Layout 356 Piping Symbols 354 Fitting Symbols 354 Fluid Symbols 356 Joint Symbols 354 Valve Symbols 356 Piston 579 Coatings 581 Material 581 Piston Rings 581 End Gap 581 Free Gap 581 Pitch of Section Lines 139 Pitch of Thread 249 Coarse Pitch 249

679

680

Index

Fine Pitch 249 Medium Pitch 249 Placement of Dimensions 91 Dimensions in Restricted Area 84 Numerals 86 Placing a Dimension with Tolerance 380 Basic Size Deviations Bilateral Tolerance 380 Basic Size with Deviations 380 Unilateral Tolerance 380 Basic Size with Fundamental Tolerances 380 General Notes 380 Plain Journal Bearing 512 Plastic Films 50 Plastics 448 Elastomers (Rubber) 450 Cellular Rubber 450 Mechanical Rubber 450 Thermoplastics 448 Plotting Devices 122 Plug Welds 293 Plug Valve 352 Sizes of Valves 353 Valves 593 Exhaust Valves 593 Point 17 Style 17 Polar Array 27, 255 Polygon 19 Position 413 Position Tolerances 404 Position Tolerance for Patterns 413 Power System 576 Precedence of Lines 59 Preliminary Decisions for Making a Drawing 113 Number of Views 113 Object Orientation 113 Selecting Views 113 Selection of Scale 114 Spacing of Views 114 Preview 21 Principle Picture Planes 110 Printer/Plotter 53 Process Sheet 470 Production Drawings 5, 458, 470 Profiles 419

Actual Profile 419 Datum Profile 419 Mean Profile 419 Mean Roughness Index (Ra) 419 Peak to Valley Height 419 Projection Scheme 108 Projection of Circular Boundaries 116 Curved Boundaries 116 Straight Inclined Face 116 Non-isometric Lines 175 Properties Toolbar 64 Proportioning Sizes 62 Proportions of Letters 72 Protractor 44 Pulleys 492 Grooved Pulley 492 Flat Pulley 492 PVC Fittings 352

Q Quick 98

R Rack 546 Radii and Centers 89 Radius 97 Range of Roughness Obtainable with Different Processes 423 Rays Command 212 Reading Directions for Aligned Dimensioning 86 Rectangle 19 Rectangular Array 26 Coordinates Dimensioning 90 Redo 24 Reference Profile 419 Projecting Side Views 114 By Measuring 115 Using a Compass 115 Divider 115 Removed Section 135 Revolved Section 135 Types of Tolerances 404 Form Tolerances 404 Representation of Springs 484 Requirements of Detail Drawing 553

Index Additional Information 554 Checklist for a Detail Drawing 554 Shape Description 553 Size Description 553 Retrieving a Block (Insert Command) 236 Reversed Isometric 179 Right Mouse Button 12 Rigid Flange Couplings 324 Marine Coupling 324 Protected Flange Coupling 325 Rigid Flange Couplings 324 Riveted Joints 219 Rivets 220 Rivet Dimensions 221 Shank Diameter 220 Rope Pulley 497 Ropes 491 Rotate 27 Roughness 418 Flaws 419 Lay 419 Roughness Cut off 418 Height 418 Width 418 Waviness 418 Height 419 Width 419 Roughness for Typical Applications 424 Roughness Grade Number and Grade Symbols 422 Symbols 420 Symbols and Their Meaning 421 with Manufacturing Processes 423 Round Key 308 Ruler 43 Rules for Putting Roughness Symbols 425 Ruling Pen 46 Run Out 414 Circular Run Out 414

S Saddle Keys 307 Scale 27, 43 Scanner 53 Screw Thread 245 Setting the Plotting Device 122 Plotting a Drawing 122

Seam Welds 293 Section Lines Angle of Section Lines 139 Drawing Section Lines 139 Section Lines (Hatching) 139 Sectional Views 132, 133 Selection of Fits 387 Parameters 201 Effect of Position of Object in Relation to Horizon 202 Location of Eye (Elevation Angle) 202 Station Point (Lateral Angle) 201 Sequence of Dimensioning 92 Drawing 118 Setting a New Dimension Style 95 Shafts 305 Axle 305 Dead Axle 305 Hollow Shaft 305 Journal 305 Line Shaft 305 Live Axle 305 Representation of Long Shafts 306 Rod 305 Spindle 305 Standard Shaft Diameters 306 Stepped Shaft 305 Shaft Basis 385, 386 Shafts, Keys, Cotter and Pin Joints 304 Shape Description 553 Short Cut Key Characters 12 Side Screen Area 11 Simplified 251 Single Feature Tolerances 408 Size Description 4 Size of Letters (H) 72 Size Tolerances 404 Sizes of Iso Drawing Sheets 47 Valves 353 Size of V Belts 491 V Belt Cross-sections 491 Valves 352 Butterfly Valve 353 Check Valve 353 Gate Valve 352 Globe Valve 352

681

682 Sketch Command 65 Sketching Pictorial Views 62 Skew Surfaces 160 Drawing Skew Surfaces 160 Skew 158 Types of Joints 283 Types of Keys 307 Flat Key 307 Slotted Nut 269 Slotting Machine 465 Vertical Milling 464 Tooth Profiles 535 Small Compass 44 Divider 45 Snap 14 Some Drawing Conventions 118 Spacing Between Letters 73 Lines 72 Spark Plug 600 Special Nuts 268 Castle Nut 269 Lock Nut 268 Miscellaneous Lock Nuts 270 Oddie Nut 270 Philidas Nut 270 Simmonds Nut 271 Pinned Nut 268 Ring and Groove Nut 270 Sawn Nut 270 Specifications 554 Specifications of Threads 251 Specifying a Fit 386 Specifying a Welded Joint 286 Arrow Line 287 Blank Length 289 Length of Weld 289 Reference Line 287 Side of the Joint 287 Spacing of Welds 289 Weld Size 288 Specifying Groove Welds 291 Specifying Variables 501 Splines 310 Involute 310 Spot Welds 292 Spring Washers 272

Index Springs 479 Spur Gears 534, 541 Stainless Steels 440 Austinitic 440 Ferritic 440 Martensitic 440 Square Thread 249 Square Key 307 Standard 10 Mechanical Components 469 Sizes of Drawing Sheets 48 Start Point 74 Starting Autocad Program 10 Status Line 11 Steel Designation 442 Chemical Composition 442 Stencils 45 Stepped 492 Webbed Pulley 492 Types of Sections 133 Assembly Section 136 Auxiliary Section 136 Broken or Partial Section 135 Full Section 134 Inclined and Offset Cutting Planes 134 Inclined Cutting Planes 134 Offset Cutting Planes 134 Straight Cutting Plane 134 Half Section 134 Stretch 27 Structural Drawings 2 Joints 228 Angle Joint 229 Build Up Girder 230 Dimensioning of Structural Joints 232 Gusset Plates 231 Mating Beams 231 Rolled Steel Section Designations 228 Seated Connections 230 Structural Sections 228 Studs 266 Style 74 Sub Assembly Drawings 5 Sunk Keys 307 Gib Head Key 308 Proportions of Taper Sunk Keys 308 Rectangular Key 307

Index Super-imposed Running Dimensioning 90 Surface Finish Symbols 285 Weld Symbols 284 Systems of Fits 385 Hole Basis 385, 386 Surface Finishing Processes 462 Roughness Number 419 Texture 418 Welding 294 Symbol for Projection Method 112 Symbolic Representations of Springs 484 Symbols 284 Additional Welding Symbols 286 Contour Symbols 285 Symmetry 413

T T-square 42 Table Command 453 Tabular Dimensioning 91 Tabular Drawings 6 Tension Spring 481 Tappet Clearance 594 Viewing a Drawing 14 Viewports Command 187 Text 21 Command 74 Style 75 Thermoset Plastics 449 Plotting 122 Centering the Plot 123 Choosing Paper Size 122 Defining Plot Area 123 Extents 123 Limits 123 Window 123 Difining Plot Area Display 123 Drawing Orientation 123 Previewing and Plotting 123 Thickness of a Line 60 Thread Designation 249 Dimensions for Metric Threads 250 Profile 248 Acme Thread 249 British Standard Whitworth 248

Buttress Thread 249 Knuckle Thread 249 Method 253 Threads 244 on a Pipe 342 Title Block 49, 458, 472 Tolerance Symbol 408 Related Features Tolerances 408 Runout Tolerances 409 Tolerance Value 409 Tolerances 370 and Manufacturing Processes 372 with Different Diameters 373 on Related Features 411 Angularity 412 Concentricity 412 Parallelism 411 Total Run Out 414 Tooth Thickness 533 Toothed Pulley 492, 497 Topography 2 Putting Tolerances using Cad 393 Lower Value 395 Method 394 Basic 394 Deviation 394 Limits 394 None 394 Symmetrical 394 Precision 394 Scaling for Height 395 Upper Value 395 Vertical Position Bottom 395 Middle 395 Top 395 Tool Tip 10 Toolbars 10, 11 Tooling 463 Tool Angles 463 Tools Used for Different Machines 464 Broaching Machine 465 Drilling Machine 464 Hobbing Machine 465 Lathe 464 Milling 464

683

684 Planing Machine 465 Shaper 464 Torsion Spring 482 Tracing Cloth 50 Paper 50 Triangles 42 Trim 28 Trimetric Projections 179 Triple Start 247 Ttr 18 Tube Joints 359 Tubes 340, 358 Tubular Pen 47 Types of Couplings 348 Straight Couplings 348 Nipple 348 Socket 348 Types of Bends 348 Types of Inclined Surfaces 158 Types of Lines and Their Application 59 Mechanical Drawings 5 Paper 50 Types of Perspective Views 203 Angular Projection 203 Oblique Projection 203 Parallel Projection 203 Types of Views 108 Types of Welding Processes 282 Arc Welding 282 Fusion Welding 282 Gas Welding 282 Pressure Welding 282 Seam Welding 282 Spot 282 Welding 282 Types of Projection 113 Types of Shafts 305

U Ucs and Ucsicon 13 Ucs Icon 11 Unalloyed Steels 442 Steel Designation 441 Mechanical Properties 441 Other Character Codes 442 Purity Code 441

Index Special Characteristics Codes 441 Treatment Code 441 Weldability Code 442 Undo 24 Union Joint 346 Units 13 Universal Coupling 328 Unwin’s Formula 220 Use of Leaders 84 Modify Commands 31 User Coordinate System 13 Uses of Drawings 2 Use of Xline Command 166 Using a Miter Line at 45° 115

V Vanishing Points in Perspective Views 203 Types of Pictorial Views 172 Axonometric Views 172 Vanishing Point (VP) 200 Vertical Position 395 V Belts 491 Vision Axis (VA) 200 Terms Used for Gears 533 Addendum (A) 532 Circle 533 Base Circle 533 Circular Pitch (CP) 532 Crest 533 Dedendum (D) 532 Circle 533 Diametrical Pitch (DP) 532 Face 533 Width 533 Flank 533 Helix Angle 533 Module (M) 532 Pitch Circle 532 Pressure Angle 533 Root Radius 533 V Thread 248 Thread Representation 251 Conventional 251 Representation 253 Schematic 251 Representation 252

Index

W Wall Bracket Support 514 Slip Couplings 329 Cone Clutch 329, 331 Multi-plate Clutch 329, 331 Single Plate Clutch 329, 330 Wblock Command 274 Weights and Thickness of Drawing Sheets 47 Welded Flange 343 Pipes 343 Types of Pulleys 492 Armed Pulley 492 Built Up Pulley 492 Fast and Loose Pulley 492 Fast Pulley 492 Flat 492 Belt 492 Grooved 492 Idler Pulley 492

685

Multi V Belt 492 Single V Belt 492 Solid Pulley 492 Weld Symbol 289 Welded Joints 281 What is a Drawing? 1 What is Projection? 108 Whole Depth 532 Width Factor 75 of Characters 72 Wooden Screw 267 Wood 450 Woodruff Key 309 Word Language 1 Working Drawing 553 Worm and Worm Wheel 534

X Xline Command

165