Lean Assembly: The Nuts and Bolts of Making Assembly Operations Flow [1 ed.] 1563272636, 9781563272639

Table of contents :
Title Page
Copyright Page
Dedication Page
Contents
Figures
Tables
A guided tour
Part A Analysis techniques
Chapter 1 Key issues of assembly operations
1.1. What is assembly?
Scope of this book
Assembly = assembly and test.
1.2. Factors in assembly performance
Part supply
Assembly work design
Examples
1.3. Waste in assembly work
Recognizing waste in assembly work
Eliminating assembly waste
CHAPTER 2 Product quantity ana!Jsis
CHAPTER 3 Trend and seasonality ana!Jsis
CHAPTER 4 Takt time and capacity
PART B Assembly concepts
CHAPTER 5 Visualizing the assembly process
CHAPTER 6 The concept ef the assembly line
CHAPTER 7 Collecting assembly time data
CHAPTER 8 Line balancing
PARTC Detailed design
CHAPTER 9 Assembly station sizing
CHAPTER 10 Detailed design if assembly stations
CHAPTER 11 Part presentation
CHAPTER 12 Convryance between stations
CHAPTER 13 Assembly cells
CHAPTER 14 Overall shape if assembly lines
PART D Assembly quality
CHAPTER 15 Preventing picking errors
CHAPTER 16 Inspection, test, and rework operations
Bibliography
Index

Citation preview

LEAN ASSEMBLY The Nuts and Bolts of Making Assembly Operations Flow Michel Baudin

0

CRC Press

Taylor & Francis Group Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

A PRODUCTIVITY PRESS BOOK

CRC Press Taylor & Francis Group, LLC 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2002 by Taylor and Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business

No claim to original U.S. Government works Printed in the United States of American on acid-free paper Version Date: 20140314 International Standard Book Number: 978-1-56327-263-9 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Except as permitted by U.S. Copyright law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfihning, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

Library of Congress Cataloging-in-Publication Data Baudin, Michel. Lean assembly : the nuts and bolts of making assembly operations flow / by Michel Baudin. p. cm. ISBN 1-56327-263-6 1. Production management. 2. Manufacturing processes. 3. Production control. 4. Just-in-time systems. I. Title. TS155 .B34 2002 658.5'1--dc21

Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

To KeiAbe) mentor andfriend

Contents

. . Figures ......................................................................... xi Tables ........................................................................ xvii A guided tour................................................................ 1

PART A

Ana/ysis techniques ................................................ 3

CHAPTER 1 Kry issues of assemb!J operations ........................................... 5 1.1. What is assembly?

............................................................................. 6

Scope of this book........................................................................................ Assembly = assembly and test. .................................... ...... .............. ............

1.2. Factors in assembly performance

6 6

...................................................... 8

Part supply.................................................................................................. 8 Assembly work design.................................................................................. 9 Examples.................................................................................................. 10 1.3. Waste in assembly work ................................................................... 11 Recognizing waste in assembly work........................................................... 11 Eliminating assembly waste .................... ...... ........ ..................................... 13

Lean Assembly

V

CHAPTER 2 Product quantity ana!Jsis..................................................... 15 2.1. Purpose

.......................................................................................... 16

2.2. The concept of P-Q analysis ............................................................ 17 Difference with Group Technology ...... ...... ....... ........ ...... ...... ........ ....... ....... 18 2.3. Bill of materials analysis for mixed-flow lines 2.4. Order profiling for custom assembly

.................................... 20

................................................ 25

CHAPTER 3 Trend and seasonality ana!Jsis ............................................. 27 3.1. Purpose

.......................................................................................... 28

3.2. Responding to demand variability over time 3.3. Data aggregation

..................................... 29

............................................................................. 30

3.4. Making the sales data talk

................................................................ 32

3.5. Demand variability upstream in the supply chain 3.6. Conclusions

.............................. 37

.................................................................................... 39

CHAPTER 4 Takt time and capacity ........................................................ 41 4.1. What is the takt time? ...................................................................... 42 Definition of the term................................................................................ 42 Design takt time and operation takt time................................................... 44 ................................................... 45

4.2. Common mistakes about takt time 4.3. Why takt time matters

..................................................................... 46

4.4. Global and local performance

.......................................................... 48

4.5. Takt time, labor requirements, and line design .................................. 50 The minimum required number of assemblers............................................. 50 Assemblerjob design issues with short takt times....................................... 52 Assemblerjob design issues with long takt times......................................... 55

PARTB

Assemb!JJ concepts ................................................ 59

CHAPTER 5 Visualizing the assemb!J process.......................................... 61 5.1. Needs and evaluation criteria for visualization tools 5.2. The problem with facility blueprints 5.3. Lists and assembly master tables

.......................... 62

................................................. 63

...................................................... 67

5.4. Abstract flow diagrams and their limitations

..................................... 70

S.S. Layout diagrams with flows ............................................................. 75 Two-dimensional diagrams......................................................................... 75 Three-dimensional drawings....... ... ....... ....... ....... .............. .......................... 78

vi

Lean Assembly

Integration

of text and drawings.................................................................

79

5.6. Photographs .................................................................................. 79 Impact of digital photography...................................................................... 79 Shop floor photography guidelines............................................................... 80 5. 7. Video recordings

............................................................................. 81

5.8. Cardboard mock-ups

....................................................................... 81 .............................................................. 82

5.9. Discrete-event simulations

CHAPTER 6 The concept ef the assemb!J line ........................................... 85 6.1. What is an assembly line?

................................................................ 86

6.2. Bench assembly versus the assembly line ......................................... Why assembly lines are still controversial.................................................... Comparing bench assembly with line assembly............................................. Exceptions: where the bench still wins........................................................ 6.3. Assembly lines, assembly cells, and line segments

87

88 89 92

............................. 93

6.4. Assembly and subassembly ............................................................. 97 All assembly work done in one single line................................................... 97 Final assembly line with subassembly feeder lines........................................ 98 Modular assembly...................................................................................... 99 100 Pros and cons of subassembly/feeder lines ................................................ . 101

CHAPTER 7 Collecting assemb!J time data ............................................ 103 7.1. Why this needs attention

................................................................. 103

7.2. Data collection methods

................................................................ 104

7.3. Current status of time and motion studies in manufacturing ............ 105 Predetermined time standards in the automobile industry.......................... 105 MTM and MOST................................................................................. 107 7.4. Time studies with video recordings

................................................ 111

CHAPTER 8 Line balancing .................................................................. 113 8.1. Assembly line balancing

................................................................. 114

8.2. Rebalancing a dedicated line

........................................................... 115

8.3. Multiproduct lines with batch versus leveled sequencing

.................. 119

8.4. Balancing assembly time among products on a mixed-flow line 8.5. Deliberate imbalances

Lean Assembly

....... 121

.................................................................... 122

vii

PARTC

Detailed design .................................................. 125

CHAPTER 9 Assemb!J station sizing .................................................... 127 9.1. Issues with assembly station sizing

................................................. 128

9.2. Assembly stations for small products

............................................. 128

9.3. Assembly stations for large products

.............................................. 132

9.4. Ergonomics and safety .................................................................. 136 Standing versus sitting............................................................................. 136 Work height and assembler height............................................................ 139 9.5. Stations with required dwell times

CHAPTER 10 Detailed design

.................................................. 140

if assemb!J stations ................................

10.1. Issues with assembly station details

143

.............................................. 144

10.2. Assembly fixtures ........................................................................ 144 Fixtures far manual assembly.................................................................. 144 Fixtures far mechanized or automated assembly....................................... 14 9 In-line mechanical automation.................................................................. 151 10.3. Handheld tools ........................................................................... 152 Tools attached to the station and not to the assembler........ ... .................... 152 Tool positioning and orientation............................................................... 154 10.4. Assembly instructions ................................................................. Instruction sheets..................................................................................... Contents of instruction sheets far manual assembly............. ...... .............. ... Content of instruction sheets far mechanized assembly............................... Instruction sheets for mixed-flow assembly................................................ Authoring instruction sheets..................................................................... Use of information technology...................................................................

157

157 158 161 162 163 163

10.5. Visible management .................................................................... 164 Selfexplanatory devices, markings, and color codes................................... 164 Tower lights, stop ropes, and other types of andons.................................... 166 Counters and production monitors................................. ........................... 168

CHAPTER 11 Part presentation ............................................................ 171 11. 1. Scope and purpose ...................................................................... 172 Part presentation requirements................................................................. 172 Controversies about partpresentation....................................................... 175 11.2. Key principles of part presentation ............................................... 176 Removal ofpackaging materials before delivery......................................... 176 Location within arm} reach of the assembler............................................ 178

viii

Lean Assembly

Orientation.............................................................................................. Adjustments to specific part characteristics................................................ Matching quantities................................................................................. Containers with dunnage far counting....................................................... Kitting versus line-side supply................................................................... 11.3. Single-piece presentation and water spiders

179 181 182 184 185

................................... 186

Single-piece presentation ........................................................................... Water spiders and supermarkets...............................................................

187 189

CHAPTER 12 Convryance between stations............................................ 193 12.1. Issues with conveyance systems

.................................................... 194

12.2. Goals for the conveyance system

.................................................. 196 .............................................. 197

12.3. A few types of conveyance systems

Unpowered conveyance.................................. ............................................ Powered conveyance ..................................................................................

197 199

CHAPTER 13 Assemb/y cells ................................................................ 201 .................................................................... 202

13.1. About assembly cells

............................................... 203

13.2. The motivation for cell conversion 13.3. Part supply to assembly cells

........................................................ 206

13.4. Range of applicability of the U-shape

........................................... 208

13.5. Pseudo U-shaped cells

................................................................. 210

CHAPTER 14 Overall shape

if assemb/y lines........................................ 211

14.1. Beyond cells

................................................................................ 211

14.2. Car and related assembly lines 14.3. Airplane assembly lines

PART D

...................................................... 212

................................................................ 218

Assemb!J quali!J ............................................... 221

CHAPTER 15 Preventingpicking errors ................................................ 223 15.1. About this chapter

....................................................................... 224

15.2. Mistake-proofing assembly operations

.......................................... 225

15.3. Approaches to automatic identification

......................................... 226

15.4. Using kit pallets and product fixtures to prevent mistakes 15.5. Mistake-proofing lineside picking

15.6. Mistake-proofing the kitting process 15.7. From stores to the line

Lean Assembly

.............. 228

.................................................. 231 ............................................. 235

................................................................. 237

ix

15.8. Storage and retrieval

.................................................................... 238

15.9. Naming items to avoid confusion 15.10. From the supplier to the dock

................................................. 240 .................................................... 242

CHAPTER 16 Inspection, test, and rework operations............................ 243 16.1. The issues of inspection, test, and rework 16.2. The literature

..................................... 244

.............................................................................. 246

The quality control literature.................................................................... The lean manufacturing literature............................................................ The general literature on assembly............................................................

16.3. 16.4.

16.5. 16.6. 16. 7.

246 246 24 7 Is self-inspection possible? ........................................................... 247 Inspection and test sequencing ..................................................... 249 Sequencing by decreasingjigure of merit and its limitations....................... 250 Sequencing by induction........................................................................... 252 Testing multiple units at once ....................................................... 254 Designing automatic binning operations ....................................... 256 Rework operations ...................................................................... 257 Selfinjlicted rework................................................................................. 258 Rework operations................................................................................... 259

Bibliography ............................................................. 263 0.1. Books in English

........................................................................... 264

0.2. Books in Japanese

......................................................................... 265

0.3. Books in German

.......................................................................... 265

0.4. Books in French

............................................................................ 266

Index ......................................................................... 267

X

Lean Assembly

Figures

FIGURE 1-1. Inspections at Toyota 8 FIGURE2-1. P-QAnalysis 17 FIGURE 2-2. Multi-tiered P-Q analysis 19 FIGURE 2-3. Number of items common to pairs of products 24 FIGURE 2-4. Commonality ratios for a family of products 24 FIGURE 2-5. Order profiling S-curve 26 FIGURE 3-1. Responding to seasonal variations 30 FIGURE 3-2. Raw daily sales data for one product family over two years 33 FIGURE 3-3. Trend and seasonality analysis 34 FIGURE 3-4. Finished goods inventory profile with steady production 34 FIGURE 3-5. Production based on moving average of demand 36 FIGURE 3-6. Finished goods inventory profile with production based on moving average of demand 37 FIGURE 4-1. The chairlift analogy for takt time in a mixed-flow line 44 FIGURE 4-2. Chevys 53-second takt time for tortillas 45 FIGURE 4-3. The ideal of takt-driven production 46

Lean Assembly

xi

FIGURE 4-4. Approximation of takt-driven production 47 FIGURE 4-5. The mixed-flow line as a set of virtual dedicated lines 49 FIGURE 4-6. Possible breakdowns for a 100-minute assembly process 51 FIGURE 4-7. Operators working on batches of parts 53 FIGURE 4-8. Replicated stations around a conveyor 54 FIGURE 4-9. Modules in a tanker hull 56 FIGURE 4-10. Breaking long cycle into sequence of operations 57 FIGURE 4-11. Assembling very large products 58 FIGURE 5-1. Shop floor blueprint 63 FIGURE 5-2. Line design based on available space 64 FIGURE 5-3. Spaghetti map of an assembly process 65 FIGURE 5-4. List of operations versus flow chart 67 FIGURE 5-5. An assembly master table 69 FIGURE 5-6. Flows of materials in a car assembly plant 70 FIGURE 5-7. An assembly cell layout drawing 76 FIGURE 5-8. Three-dimensional cartoon 78 FIGURE 5-9. Photomontage example ofWIP removal 79 FIGURE 5-10. Tips on shop floor photography 81 FIGURE 6-1. The assembly line 86 FIGURE 6-2. Ford Model T magneto assembly 87 FIGURE 6-3. High volume bench assembly versus the assembly line 90 FIGURE 6-4. A cell for cardiac monitor assembly in Japan 94 FIGURE 6-5. One long line or a sequence of cells? 95 FIGURE 6-6. A final assembly line segment at Toyota 96 FIGURE 6-7. Assembly in one single line 97 FIGURE 6-8. Subassembly feeder line 98 FIGURE 6-9. Dashboard assembly for cars 99 FIGURE 6-10. Modular assembly 99 FIGURE 6-11. Modular assembly for the Smart car in Bambach, France 100 FIGURE 7-1. Physical load degree 110 FIGURE 8-1. The principle of assembly line balancing 116

xii

Lean Assembly

FIGURE 8-2. Steps to rebalancing a dedicated line

117

FIGURE 8-3. Parts for old and new version of Zebco 404 fishing reel 122 FIGURE 8-4. Catching up in a deliberately imbalanced line FIGURE 9-1. Large station for small product 129

124

FIGURE 9-2. Critique of the 1913 Ford magneto assembly line 130 FIGURE 9-3. Phonograph assembly at Edison in the 1920s 131 FIGURE 9-4. Assembly of laser printer component at Canon (1990's) 132 FIGURE 9-5. Engine assembly at Toyota 133 FIGURE 9-6. The build saddle concept 134 FIGURE 9-7. Working underneath the product at NUMMI

134

FIGURE 9-8. Access to multiple sections in large product assembly FIGURE 9-9. Sitting operations 136

135

FIGURE 9-10. Raku-raku seat at Toyota 138 FIGURE 9-11. Green corner example 139 FIGURE 9-12. Curing station 141 FIGURE 9-13. Types of oven doors

142

FIGURE 9-14. Oven station example 142 FIGURE 10-1. An example of a pushcart fixture

145

FIGURE 10-2. Welding station in non-rotating fixture 146 FIGURE 10-3. Lazy Susan for flexible computer assembly 147 FIGURE 10-4. Fixture return to starting position in electronics assembly 149 FIGURE 10-5. Orientation of air cylinders 150 FIGURE 10-6. Methods for starting assembly machines FIGURE 10-7. In-line mechanical automation 152

151

FIGURE 10-8. Tools attached to stations versus assemblers FIGURE 10-9. Tool placement 155 FIGURE 10-10. "Spaceship" moving tool fixture 156 FIGURE 10-11. Two approaches to assembler instructions FIGURE 10-12. Instruction sheet concept 159 FIGURE 10-13. Assembly instructions for K'nex Cybots

Lean Assembly

154

158 160

xiii

FIGURE 10-14. Work combination chart for mechanized assembly FIGURE 10-15. Assigned locations and labels 165 FIGURE 10-16. Red bin for defectives and ZD sticker FIGURE 10-17. Effective use of tower lights 166 FIGURE 10-18. Good and bad designs for tower lights FIGURE 10-19. Stop ropes and andon boards FIGURE 10-20. Production monitors 169

162

166 167

168

FIGURE 11-1. The context of part presentation 172 FIGURE 11-2. The assembler as race car driver 173 FIGURE 11-3. Net effect of adding a picker 174 FIGURE 11-4. The value added by preparing parts for use

176

FIGURE 11-5. Printed circuit boards with ESD protection 177 FIGURE 11-6. Separation of unpacking from assembly 178 FIGURE 11-7. Gravity flow racks 179 FIGURE 11-8. Techniques to keep parts within arm's reach

179

FIGURE 11-9. The orientation dilemma 180 FIGURE 11-10. Special presentation devices for large parts

181

FIGURE 11-11. Southworth's PalletPal® 181 FIGURE 11-12. Random replenishment schedule

183

FIGURE 11-13. Divider boxes and item-specific dunnage \ FIGURE 11-14. An example of a kit pallet 186

184

FIGURE 11-16. Single-piece presentation in electronics assembly FIGURE 11-17. Part presentation in FRAPA versus single-piece presentation 189 FIGURE 11-18. Example of water spider operation 191

188

FIGURE 12-1. Spiral conveyor system 194 FIGURE 12-2. Example of part hanging on a chain (Westinghouse, 1904) 195 FIGURE 12-3. Partition blocking the view between two stations 196 FIGURE 12-4. Examples of unpowered conveyance systems 198 FIGURE 12-5. Examples of powered conveyance systems FIGURE 13-1. AU-shaped assembly cell 204 FIGURE 13-4. Staffing flexibility in practice

xiv

199

206

Lean Assembly

FIGURE 13-5. Three ways of supplying parts to an assembly cell FIGURE 13-6. Vertical presentation for long parts 208 FIGURE 13-7. U-shaped cell for long and narrow product FIGURE 13-8. A pseudo cell example 210 FIGURE 14-1. Volkswagen assembly line in 1953 FIGURE 14-2. Snaking assembly line 213 FIGURE 14-3. Reach of each materials service area

207

209

212 214

FIGURE 14-4. Layout of Toyota's Kyushu-Miyata plant 215 FIGURE 14-5. Snaking line with elevators between sections 216 FIGURE 14-6. Tow-cart shuttle 217 FIGURE 14-7. Boeing 717 assembly 218 FIGURE 14-8. B24 assembly line at Willow Run, Michigan during WWII 220 FIGURE 15-1. Auto ID technologies used in manufacturing 227 FIGURE 15-2. Two approaches to detecting mismatch between kit and assembly product 229 FIGURE 15-3. Assembly options tag or build manifest 230 FIGURE 15-4. Mistake-proofing using a flip-lid approach with a limit switch 231 FIGURE 15-5. Mistake-prooding method based on a carrousel 232 FIGURE 15-6. Mistake-proofing using a flip-lid approach 233 FIGURE 15-7. Merry-go-round concept for small parts and multiple products 234 FIGURE 15-8. A vertical carrousel system 235 FIGURE 15-9. Rack with pick-to-light boxes 236 FIGURE 15-10. Flow racks and push versus pull systems 238 FIGURE 15-11. Approaches to warehouse visibility 240 FIGURE 15-12. Using fonts and styles to enhance distinctiveness FIGURE 16-1. Sample pooling for blood testing 255 FIGURE 16-2. Testing multiple electronic assemblies 256 FIGURE 16-3. Automatic test equipment with binning capability FIGURE 16-4. Offline rework station

Lean Assembly

241

257

261

xv

Tables

TABLE 1-1. TABLE 1-2. TABLE 2-1. TABLE 2-2. TABLE 2-3. TABLE 2-4. TABLE 3-1. TABLE 3-2. TABLE 4-1. TABLE 5-1. TABLE 6-1. TABLE 6-2. TABLE 6-3. TABLE 8-1.

Use of assembler time in an aerospace assembly process 12 Use of assembler time in an electronics assembly process 12 List of procurement codes 24 Items counts by category 24 Part counts by product for a product family. 26 Gozinto matrix sorted by decreasing consumption 27 Weighted aggregate workload calculation 35 Emergence of the bullwhip effect 43 Typical takt times by industry 56 A comparison of material flow mapping symbols 78 Bench vs. line with 100-min. process, takt time = 1 min. 97 Bench vs. line with 100-min. process, takt time= 100 min. 98 Pros and cons of subassembly 108 Batch versus level sequencing in multiproduct lines 126

Lean Assembly

xvii

A guided tour

This book is about the engineering of assembly operations through the following: 1.

Characterizing the demand in terms of volume by product and product family, component consumption, seasonal variability and life cycle.

2.

Matching the physical structure of the shop floor to the demand with the goal of approaching takt-driven production as closely as possible.

3.

Working out the details of assembly tasks station by station, including station sizing, tooling, fixturing, operator instructions, part presentation, conveyance between stations, and the geometry of assembly lines as a whole.

4.

Incorporating mistake-proofing, successive inspection, and test operations for quality assurance.

It is about the technical content of the work done in lean manufacturing to improve existing assembly facilities or design new ones. Its aim is to provide factory personnel engaged in such efforts with ideas, solutions, and analytical tools to assess their relevance in a wide range of applications. Examples are drawn from industries including not only automotive but also aerospace, electronics, household appliances and personal products.

Lean Assembly

1

They include high-volume-low-mix, low-volume-high-mix, and mass customization applications. The techniques described are intended to be used by the actual engineers and managers working in factories today. The readers are assumed to have access to PCs with spreadsheet and graphics software, digital cameras, and video cameras. On the other hand, they are not assumed to have the inclination or the time to become proficient in graduate school level mathematics. This book differs from most others about lean manufacturing is that it is not organized around concepts like standard work or SS but around the needs of assembly. The use of shadow boards and the labeling of tool locations, for example, is discussed as part of assembly station design. This book is about what should be done rather than how to do it. In that sense, it is more consistent with the Japanese than with the American literature on lean manufacturing, which is focused on change management to the point of giving short shrift to technical content. My colleagues and I are implementers, but feel strongly that technical content should drive implementation methods and not the other way around. In every project, the end result determines whose buy-in is needed, who has the required technical knowledge, what it will cost and how long it will take. The ambition of this book is to become a dog-eared and penciled-in resource on every assembly engineer's desk. While we would have like to cover logistics, production control, organization structure and assembly economics, including them would have doubled the size of the book. In the spirit of making ideas flow, we decided to postpone these discussions to a follow-on volume.

2

Lean Assembly

PART A

Lean Assembly

Analysis techniques

3

CHAPTER 1

Key issues of assembly operations

Assemb/y plants put andfit together different parts into subassemblies andfinished products. Assemb/y work is repetitive at the component and subassemb/y level and often, but not always, also repetitive at the finished product level. Because much of the value of the assembler to the customer is the assumption of responsibility far product support, assemb/y plants almost always peiform inspections and tests, which, in some cases, are more expensive than assemb/y itself The two main factors in assemb/y peiformance are part supp/y and the assemb/y work design. Assemblers are prompt to blame all their problems on the supp/y chain, but improvement ofinternal operations must come first if supplier support is to be successful. The value of improving the design of assemb/y work is common/y underestimated ?)I managers who on/y notice that direct labor cost is low compared to materials and overhead. But the design of assemb/y work should command attention as a competitive weapon even if assemblers workedfar free. Assemblers today waste much of their time in activities other than assemb/y and test, where improvement opportunities can be found. The failure to design jobs that keep operators busy causes them to make extra parts. Shortages and imbalances between stations cause operators to wait. The location of stations based on space available multiplies transportation operations. Poor sequencing ofsteps creates extra work. The policy ofassembling product units that are 99% complete causes inventory accumulation. Poor presentation of parts causes multiple handling. Final/y, picking errors and mislabeling create defects.

Lean Assembly

5

Key issues of assembly operations

1.1. What is assembly? Scope of this book When we think of assembly, it is usually as opposed to other activities, such as fabrication or machining. What sets assembly apart is that it consists of putting or fitting together different parts into a product. Assembly in a manufacturing context differs from, for example, the assembly of a piece of furniture by a consumer at home, in that it is conducted as an economic pursuit on an ongoing basis, repetitively for one single product, or for a variety of products. Cars, airplanes, refrigerators, or television sets immediately come to mind as assembled products. On the other hand, we don't usually think of mixing dried food ingredients or compounding cosmetics as assemb!J, even though the issues of picking bags or a spice mix are similar to those of picking and kitting discrete parts, and many of the topics discussed in the following chapters are relevant. The term assemb!J in these industries is usually reserved for the packaging operations.

In fabrication, processes like thin film deposition join different materials. The operation of these processes, however, does not resemble assembly, because the dominant issues are different: process capability, equipment availability, and the sharing of expensive equipment among multiple applications. They are therefore outside the scope of this book. Assembly = assembly and test. Some assembly operations require extensive facilities; some do not. For example, consumers can assemble a PC from a kit of store-bought parts in less than an hour with simple hand tools, but few do. Most are happy to pay $300 over the cost of the components to have somebody else assemble the computer for them. The same consumers generally would not hesitate to spend one hour of their time to save $300. The reason they prefer to pay is not to avoid the work but the responsibility for it. If the computer they assemble them-

6

Lean Assembly

What is assembly?

selves does not work, they have no one to turn to. A third party that put it together for them is liable for its operation. That support is what they are buying. From the manufacturer's perspective, standing behind the product implies being confident in its outgoing quality. Regardless of how well you build quality into your products, inspection and testing activities do not vanish altogether. They may be reduced in scope, but there are few assembled products that are shipped without any testing, and there are few manufacturing processes that do not require some form of inspection somewhere. The lean manufacturing literature may convey the impression that inspection, testing, and rework have been completely eliminated. If, however, we research the issue on the Toyota web site, we find that Toyota itself inspects cars at three different locations, as shown in Figure 1-1. In electronics assembly, testing often takes longer and uses more expensive equipment than assembly itself. While some describe these activities as non-value added, customers disagree: • In defense applications, the government actually demands inspection and test reports to be shipped as part of the product.

• In consumer goods, companies like Hanes or Land's End use the thoroughness of their inspections as an advertising argument. • In computer assembly, testing and burn-in are the key reason there are so few do-it-yourselfers. For all these reasons, we will include inspection, testing, and rework in our discussion of assembly processes.

Lean Assembly

7

Key issues of assembly operations

Body-in -white inspection

Painted body inspection

Fina l inspection

FIGURE 1-1.

Inspections at Toyota

SOURCE: From the Toyota Website (www.toyota.co.jp)

1.2. Factors in assembly performance Part supply "If only we had all the parts we need, we could assemble all we need to do without any problems ... " goes the most common complaint of assemblers, assembly supervisors, and assembly managers. They have a point, in that part supply is the most important factor in assembly productivity, but they tend to use it to drive improvement efforts away from their own activities towards suppliers, and that is not appropriate. Assembly organizations do have control over the last link of the supply chain: how the part physically gets into the assembler's hands. The assembler may grab the part by extending his arm or by walking fifty feet to a shelf, and it is the internal organization of the assembly area that sets which it is.

8

Lean Assembly

Factors in assembly performance

Assembly work design The second major factor in assembly productivity is the design of the assembly work. This includes the following: • The subassembly structure. • The sequencing of assembly steps. • The allocation of assembly steps among assembly stations. • The sizing, fixturing, and tooling of assembly stations. • The design of assembly procedures. • The means of controlling the quality of assembly work. Many assembly managers fail to see this area as a major opportunity. They reason that, since labor is a small fraction of the product costs, their attention should be focused elsewhere, for example on the supply chain, since this is where most of the costs are incurred, or in the administrative area, where wages are higher.

In fact, the design of assembly work would deserve excruciating attention even if assemblers worked for free. That they are a bargain does not diminish their importance to the business. The assemblers should be viewed as a competitive weapon. To take an aviation analogy, the cost of airline pilots is small compared to the total cost of a jumbo jet. Yet, if you are an airline, how pilots fly it is key to the return on this asset.

If assembly work design is neglected, the plant uses more people than needed on direct production tasks, and the following happens: • As in all overstaffed organizations, people fight over the work instead of doing it. Lead times go up and absolute production numbers go down as a result of people being in each other's way. • Quality suffers as a result of diluted responsibilities. Problems are always somebody else's. • The morale of underutilized operators plummets, as they feel insecure in their jobs and humiliated that management feels it can afford to waste their time.

Lean Assembly

9

Key issues of assembly operations

• The only way to obtain price concessions is by pressuring suppliers into giving up some of their own profits. On the other hand, if a company can teach its suppliers how to increase productivity, then suppliers can cut prices and increase profits at the same time, and this effect ripples through the supply chain. Therefore, improvements in assembly work must precede, not follow, improvements in the supply chain. These phenomena can be easily observed in developing countries, where factories are often perceived to be a means of creating jobs for the people. Conversely, the survival of factories in industrialized economies hinges on production work being done by few well paid, highly qualified people. Companies can afford to pay wages that are ten times higher than their competitors, because the assemblers are more than ten times more productive and their output is of superior quality.

Examples Table 1-1 and Table 1-2 are taken from actual examples of videotape analyses of assembly operations conducted years apart in different industries. In both cases, we followed and recorded on videotape the assembly of a product on one bench from beginning to end by one assembler. In both cases, about 40% of the time was spent on tasks that are neither assembly nor test. Not all of the 40% is waste in absolute terms. Much of this work is actually needed, but it is wasteful to have assemblers do it while product units wait. As will be discussed later, the economics of assembly dictate that assemblers should be assembling with both hands at all times. In both cases, the videotapes were reviewed with the assemblers who starred in them. In both cases, they underestimated the amount of time they were frittering away in all non-assembly tasks, and the videotapes were revelations to them.

10

Lean Assembly

Waste in assembly work

TABLE 1-1.

Use of assembler time in an aerospace assembly process

I

Category

I

Time Assembly 151 min. Functional test 60 min. Total assembly and test I 211 min. I Fetching parts 31 min. 22 min. Preparing parts 21 min. Paperwork/administration Cleanup 17 min. Setup 11 min. 10 min. Unpacking/package disposal Stamping serial numbers 9 min. Loss 9 min. Fetching tools 7 min. Outside operations 3 min. Inspection 2 min. Preparing bulk materials 2 min. Total support activities I 143 min. I Grand total I 354 min. I

TABLE 1-2.

Cum 151 min. 211 min. 31 53 74 91 102 112 121 129 136 139 141 143

min. min. min. min. min. min. min. min. min. min. min. min.

I I

I I

I

Comments % 42.7% 17.0% 59.7% 1 8.8% 6.2% Remming \l'lndor hardware, mounting, etc. 5.9% Includes computer transactions 4.8% Returning tools to cabinet, etc. 3.1% Setting up functional test bed 2.7% 2.5% 2.4% Correcting mistakes 1.8% 0.8% Tasks done outside gearbox assembly0.6% 0.6% Mixing sealant, etc. 40.3% 1 I

Use of assembler time in an electronics assembly process Category Final assembly Subassembly Test Subtotal Disass'y/undo Fetching Interruptions Unpack/dispose Mistakes Moving Subtotal Total

Time Percent 15:17 04:15 01:00 62% 20:32 03:21 03:02 02:55 02:03 00:55 00:05 38% 12:21 32:53

1.3. Waste in assembly work Let us review the characteristics of waste in assembly work, as opposed to waste in other types of manufacturing activities. Then we will discuss what is unique about it and its implications for improvement.

Recognizing waste in assembly work 1.

Overproduction. When an operator keeps making more subassemblies even though there are three days' worth accumulated in the output buffer because "It's something to keep me busy," it is a clear case of overproduction. This indicates that the operation has overcapacity and

Lean Assembly

11

Key issues of assembly operations

that management has failed to design a job that keeps the operator busy doing useful tasks. 2.

Waiting. Shortages of parts or subassemblies are the most common reason for assemblers to wait, but there are others. Management's failure to balance the work load of different assemblers who work together causes some of them to wait while the others struggle to finish their tasks. Wherever machines are used, there is always the risk of operators waiting for them to complete automatic cycles. Again, this is due to a failure to plan the use of the operators' time.

3.

Transportation. The main issue is whether moving parts between operations requires an intervention by the materials department. If a forklift must be called, the impact does not change much whether it is to move something 50 feet or 500 feet. There is transportation waste wherever operations that should be arranged to facilitate flow are not.

4.

Process. Process waste is the most difficult to detect, because it requires detailed knowledge of the process. For example, to establish that it is unnecessary to apply sealant on two faying surfaces, one has to know that the spec stipulates that the sealant should be applied only on one. Sometimes assemblers add unnecessary steps to their work, simply because they are embarrassed to have time on their hands while colleagues around them are fully busy. Poor sequencing of steps can also create unnecessary work. In electronics assembly, for example, power supplies have cables running to every module. If the power supply is mounted first, assemblers have to move these cables out of the way to install every other module. This work is eliminated by changing the sequence and installing power supplies last instead of first. Assembly fixtures exist to present workpieces to operators in a convenient location and orientation. If they are missing or inadequate, assemblers cannot apply both hands to assembly work and may have to work in uncomfortable, painful, or dangerous positions.

5.

12

Excess inventory. If no one can explain the purpose of having a particular stack of parts on the shop floor, it is most likely excess inventory and the result of either overproduction or of Materials Management pushing parts on to the production organization.

Lean Assembly

Waste in assembly work

A cause of excess inventory that is particular to assembly is the policy of assembling "cripples" - that is, product units that are 99% complete but can't be shipped for lack of one or two parts. This policy is common among companies with unreliable supply chains as a means of maintaining activity in the face of shortages. This policy is misguided for many reasons. Not only does in inflate work-in-process, but it transforms the installation of the missing parts from an assembly to a repair operation, requiring more work and with a much greater risk of error. 6.

Motion. Motion waste in assembly is easy to observe. Its most common form is multiple handling. After picking parts, instead of assembling them right away, operators set them down on their workstations to pick them up again later. When fixtures are not used, picking up and setting down the product itself can also be a major time drain in highvolume production. Motion waste can also be introduced in picking, by placing bins or racks too far from the workstation or requiring the operator to turn around. Another source is tool handling.

7.

Production of defectives. The most common source of defects in assembly is errors in picking or labeling parts. It is particularly difficult to avoid when making multiple products in the same production lines.

Eliminating assembly waste Assembly waste does not come in lumps that can be easily chopped off. Because it is marbled through the process, a detailed analysis of assembly steps is needed to uncover it. Table 1 shows 31 minutes spent fetching parts. This number is the aggregation of 30 to 40 short trips. Each trip is short, but they add up to 8.8% of the total time. The same is true of the other categories in Tables 1 and 2.

In a long assembly process, examining every operation in excruciating

detail appears to be hopelessly time-consuming. The good news is that there are known ways of designing, organizing, and, running assembly operations that can be applied systematically to avoid or eliminate the waste. The concepts must still be applied at each station, but they do not have to be reinvented. They are the focus of the following chapters.

Lean Assembly

13

CHAPTER 2

Product quantity analysis

Product-quantity, or (PQ), ana!Jsis groups products in volume categories, to which production resources are then allocated. Jypical!J, afew high-volume ?1" items account far 50% to 60% of totalproduction, and each one deserves a dedicated line. Then come "B" items that do not individual!J warrant dedicated lines but can be grouped into productfamilies that do. Final!J, the vast mqjority of the items in the compa1!J} catalog are "C" items, to be made on request using generic resources.

P-Q ana!Jsis is based on numbers of units, not revenue, since the highest volume prod-

ucts (that is those that occupy most if assemb!J} time) mqy not be the highest revenue generators. The approach differs from group technology in that it relegates process similarity to the level of a secondary grouping criterion after volume. In assemb!J, particular!J with mixedflows, P-Q ana!Jsis extends into the bills of materials to classifj components and parts 01 product commonality and consumption volume. The use ofcommon parts is a kry criterionfar grouping B products intofamilies, and the classification ofparts 01 consumption rate is used to plan the logistics ofpart supp!J. Where products are customized in assemb!J, the facus shifts to the smallest subset of purchased components with which 80% of the orders can be shipped complete. Procurement and logistics can then be organized around the need to deliver the mostfrequent!J used items to assemb!J.

Lean Assembly

15

Product quantity analysis

2.1. Purpose Product-quantity, or (P-Q) analysis, is a simple tool that is little known in the United States and underrepresented in the lean manufacturing literature. The idea is to group products in volume categories to serve as a basis for laying out the production floor. Process similarity among products is not ignored, but it is only a secondary criterion, evaluated only among products that are in the same volume category.

P-Q analysis is applicable to processes other than assembly, but in assembly it extends to an analysis of parts and components through the products' bills of materials (BOMs). Part commonality is a criterion in assessing product similarity for grouping in mixed-flow lines, and the aggregate part consumption volume by item serves to tailor the logistics of part supply.

In companies that do not practice P-Q analysis, managers bemoan their inability to take advantage of many tools because "our production is nonrepetitive." They are actually treating all their products as if they were worst cases. P-Q analysis ferrets out the repetitiveness underlying the apparent chaos in the demand and allows the manufacturer to take advantage of it by structuring production lines to match the actual structure of the demand.

16

Lean Assembly

The concept of P-Q analysis

2.2. The concept ofP-Q analysis Product-Quantity (P-Q) analysis is the simplest yet most powerful tool for structuring the demand. A Pareto diagram of volume by product yields an ABC categorization of products as in Figure 2-1.

200,000

100% 'I) ,

>
,

>
50% of all orders •

B-items: The most frequently used items after A's, to the point that 80%

of all orders can be built complete with A and B items. •

C-items: All others.

This categorization is then the basis for separate logistics policies. For example, the components could be managed as follows: • A-items: Deliver to assembly on a replenishment basis. •

B-items: Dedicate a location to each B-item in the warehouse. I!-~

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7:08:28.64 0:00:16.26 7:08:39.08 0:00:10.44 0:07:02.27

10 Install insulator Total

01 :16.5 00:46.4

7:08:12.38 7:08:12.38

Soldering iron

7:06:43.24 0:01 :16.51 7:07:29.65 0:00:46.41

00:28.3 01 :27.1 00:40.4

7:03:19.25 0:00:28.29 7:04:46.36 0:01 :27.11 7:05:26.73 0:00:40.37 Soldering iron

00:30.9

7:02:50.96 0:00:30.87

Phillips screwdriver

00:29.5

In Out

Manual time

7:02:20.09 0:00:29.49

7:01 :36.81 7:01 :50.60 0:00:13.79

Timestamp

Fixture TA1

Wrist strap

Tools

Revision date: 10/16/2001 Department: Firebird

07:47.1 00:09.1

Qty

I

7:07:47.06 7:07:47.06 7:07:56.18 0:00:09.12

L1

724540-333-770 interface OBA GM04A125VA. FM04A2C, FG04A125V1A

Components

T.K.

Assembly Master Table !Author:

board 7 Solder 6 legs on fuse box Install fuses in fuse box 8 Solder 2 legs on L 1 170150-1 9 Unpack LED 1O Trim leads on LED 336437-1 11 Install gasket on LED 12 Assemble and solder board. 13 Remove insulator backing

5 Put fuses into fuse box 6 Install fuse block onto

3 Removing screws from board 4 Assemble coil

2 Form, trim, and cut leads

1 Pick board

Op#

Item#: Frontpanel 301A Cell:Modu/e assembly 1 Comments

Operator removes backing from insulator to expose adhesive surface.

and pliers lying around

Needs fixture for fuse block. Screwdriver

Coil assembly

Picked a board first, then attached strap to wrist. Only did two. One was already preformed.

Takt time: 55 min

Time

Machine

IDaily rate: 15

if

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.

3

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Shipment

72

Lean Assembly

Abstract flow diagrams and their limitations

TABLE 5-1.

A comparison of material flow mapping symbols (cont'd.)

Meaning

Inspection

Delay/waiting

ASME/ JIS-Z8206

Shigeo Shingo

Value Stream Mapping

□

D

V

Machine

~ Inventory

v

Parts

6

& ~ Supermarket

§ Safety stock Outside sources or destinations

CJ

Many corporate lean departments in the US are following suit and mandating that all plants generate current and future value stream maps. While it works technically to describe plant operations at the production control level, it does not show the level of manufacturing engineering detail we are

Lean Assembly

73

Visualizing the assembly process

concerned with in this book. The authors actually recognized this when they came out with another book called "Creating Continuous Flow" 1 . As a tool for analysis and communication in the design of assembly operations, we feel that value stream mapping has the following limitations: • It is complex. The graphic vocabulary of value stream mapping has no less than twenty-five icons, the thirteen in Table 5-1 for material flows and twelve for information flows. Then there are rules that need to be followed to combine these elements in a meaningful fashion. A push arrow, for example, cannot lead to an outside source. This is a whole language that needs to be learned in order to generate or even read these maps.

When the use of such a language is mandated within an organization, the primary result is often that diagrams are reviewed for conformance to standards and not for their meaning. There is a precedent for this in the software industry, with corporate-level software engineering committees mandating the use of formal methodologies for specifying applications. Analysts and programmers comply by generating charts for form's sake, without truly understanding or mastering the tools imposed on them. These charts are then reviewed and approved by committees whose members do not understand the intended applications and can only verify that the analysts' documents have the right kinds of arrows pointing to the right types of bubbles. • It is intended for end-to-end process mapping, from raw materials supplies to shipping of finished goods to customers, at a level of detail that includes supermarkets and operator locations. For anything but the simple processes shown as examples in Learning to See, the value stream map would end up covering entire walls. It would be impossible to do, for example, for the car assembly process shown in Figure 5-6.

• Rother and Shook recommend using pencil and paper, but the icons have shapes that are time-consuming to draw. For the ASME symbols, by contrast, templates can be bought in any stationery store. Since the operation data are entered by hand in the value stream mapping data blocks on paper, they cannot be analyzed using an electronic spreadsheet without retyping. A software tool like Visio, for example, not only

1. Lean Enterprises Inst Inc, (November 2001 ).

74

Lean Assembly

Layout diagrams with flows

supports drawing clean charts from templates, but also lets users upload operation data to Excel or Access. • While the level of detail in value stream maps is high enough to address production control issues, it is too low to support the design of assembly stations or assembly lines. The analysis of production lead time in value stream mapping is strictly between the time the parts spend between operations versus at operations. For our purposes, we need to break down what happens at operations. In an assembly line, conveyance between stations must of course be designed to prevent units accumulating and waiting between stations. This being given, the first focus of an analysis must be what happens within the process time at each station. A 46-second. process time may in fact involve only 16 seconds of assembly and 30 seconds of picking and unpacking parts. The second focus will then be the imbalances in process times among the different stations. This level of detail, which is included in assembly master tables, is missing in these process maps.

5.5. Layout diagrams with flows Two-dimensional diagrams While assembly master tables support work with numbers and lists of parts, they must be translated into a layout with parts being fed to assembly stations and product units moving between stations. As discussed in Section 5.2., we don't want to base this layout on the current facilities blueprint, so the background must be a blank space on which we locate realistically dimensioned pictures of the equipment and people we plan to use. On this tentative layout, we can then map the flows of materials and the movements of assemblers. Figure 5-7 shows an example of such a drawing. It is for a cell conducting sealing and assembly operations on six-foot. long, narrow mating parts that are crated and shipped as a set.

Lean Assembly

75

Visualizing the assembly process

Bushing installation Juke box of side plates

-

Flow r-ack for kits

Upright

fixture - - - - - ~


'

,

Incoming cart with one shipping kit

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FIGURE 5-7.

An assembly cell layout drawing

Everything that is shown in the drawing is intrinsically needed for the process. The key points are as follows: • The cell needs to face a transportation aisle wide enough to accommodate carts with a complete set of product units and to take away full crates. The transportation aisles on both sides are needed to bring components and supplies to all operations. This drawing, on the other hand, shows only the transportation infrastructure, which is needed

76

Lean Assembly

Layout diagrams with flows

regardless of how logistics and production control are organized. It is a complementary tool to value stream mapping: it says nothing about the supermarkets, pull signals, etc. that are the focus of value stream mapping. • The concept shown is to handle the parts vertically through the first operations, so as to use smaller, more concentrated stations and to make the transfers between stations easier. • The dimensions are realistic. The stations are drawn to scale, and the operator symbols show the size of operators with respect to the paths along which they move. • When the data in the assembly master table are combined with this drawing, the performance of this design as well as the economics of implementing it can as well. • The drawing contains no facility-specific features. From a facilities department point of view, if this design is selected, we know that we need a space of 19x25=47 5 square feet facing a wide aisle. • While this drawing looks busy, it is generally understandable by shop floor operators, to the point that they can review it and raise valid issues. From a communications standpoint, this type of two-dimensional drawing can be read easily by anyone trained in reading technical drawings. That it does require map reading skills, however, is a limitation because there are many adults in the workforce who cannot read road maps, let alone a drawing like Figure 5-7. The most elaborate analysis technique to design operator jobs in a lean manufacturing cell is the work combination chart. It helps managers coordinate the activities of people and machines, when each operator services multiple machines and there are manual tasks done on machines while stopped, manual tasks done on machines while running, and automatic machine cycles. Although this situation occasionally occurs in assembly, it is much more common in machining and fabrication, which is why we do not cover it.

Lean Assembly

77

Visualizing the assembly process

Three-dimensional drawings Three-dimensional drawings, like Figure 6-1 on page 86, are an alternative that is heavily used in the Japanese literature on the subject. The perspective used is borrowed from a convention from traditional Japanese painting where the goings-on in a house are shown from an elevated perspective with the roof peeled off, as in the example in Figure 5-8. What such drawings gain in ease of communication with shop floor readers, however, they lose in precision. Exact dimensions, for example, cannot be read from them. In addition, they are difficult to draw, as artists who can do them well are hard to find. This difficulty is evident even in Figure 5-8, which is lifted from a Japanese publication, where the picker in the center of section B looks like a child compared to the assemblers in the back.

FIGURE 5-8.

Three-dimensional cartoon

SOURCE: Courtesy ofSPS consultants, Konryo Seisan system, Kojo Kanri, March 1991. Special Issue, p.127, Nikkan Kogyo Shimbun Sha.

78

Lean Assembly

Photographs

Integration of text and drawings Whether two- or three-dimensional, the illustrations do not stand alone. They have reference information attached, such as author, date, line, and product name. But they are also combined with text and numeric data in various formats.

5. 6. Photographs Impact of digital photography For communication purposes, photographs are easier to generate than three-dimensional drawings and richer in information. But you can only photograph what already exists, which makes the medium usable on improvement projects but not in initial designs. The advent of digital photography has not only made it easy to incorporate photographs in documents and annotate them, but also to use photo fakery as a means of showing a "to-be" situation, as in Figure 5-9. The fakery does not have to be sophisticated, since the purpose is not to deceive but to give an idea of the future state.

Current FIGURE 5-9.

Lean Assembly

To be

Photo montage example of WIP removal

79

Visualizing the assembly process

Shop floor photography guidelines As a tool to both drive and document shop floor improvements, photos are a powerful complement to drawings, charts, and videos. On bulletin board displays of before-and-after pictures, however, viewers commonly encounter the following: • They cannot relate the photographs to the shop floor scene in front of them. • The changes are less than obvious and not pointed out. • The pictures are obviously different, but "after" is not obviously better than "before." • The pictures are so different that they don't appear to show the same operations. • The pictures don't show the relevant features. Photos of cells or workstations should always show people working and be supplemented by layout diagrams showing the flow of work and by callouts indicating where and from which angle the photos were taken. The before and the after pictures, should be on the same scale, with photos taken from the same location and angle, to make the differences stand out and to avoid misleading the viewer as to object sizes. Location and angle can be marked on a map of the shop floor. Alternatively, the location can be directly marked with a circle on the floor and a focusing target on the machine itself. Before-and-after pictures of details, such as machine components are most useful when taken close up, but pictures of cells or line are best taken from a raised position, otherwise the machines in the front block the view to the rear (See Figure 5-10).

80

Lean Assembly

Video recordings

.... ,

Before 1. S hoot from an elevated posit ion, while operators are working .

FIGURE 5-10.

2. Record posit ion an d ang le from wh ic h you took the picture.

Afte1

3. Use cal louts and annotations to make differences stand out.

Tips on shop floor photography

5. 7. Video recordings The use of video recordings as a tool to collect time data and analyze assembly operations is discussed extensively in Chapter 7. It is a powerful tool. Very short operations can be analyzed in slow motion; very long ones, in time-lapse video. The technique has only a few limitations: 1.

A video cannot be posted on a bulletin board. Viewing it requires just enough equipment and setup to preclude casual references to it. You need to arrange a meeting and invite participants, who must be able to free up sufficient blocks of time and have long attention spans.

2.

In terms of persuasion, moving images have in fact less of an impact than still images. As photojournalists know, the public remembers their still images of events long after the full motion video shown on TV is forgotten, and the same is true on the shop floor.

5.8. Cardboard mock-ups A variety of three-dimensional models of lines have been used, some on a reduced scale, as architects do, others full size, with mock-ups of stations and machines. Reduced-size models are primarily used to explain a finished facility to visitors. As a design tool, generating and maintaining them

Lean Assembly

81

Visualizing the assembly process

requires too much work for the additional value they bring compared to drawings. Full-size models are built out of corrugated cardboard and tape. They allow operators to move through the to-be work environment, develop a sense of what the work experience will be, and provide feedback. Full-size models are the ultimate manual simulation tool. The construction of such a model shows a commitment to usability that is appreciated by operators. Some lean manufacturing companies have used this technique for years, using mock-ups detailed enough to actually assemble parts and thereby test and debug the process. The cardboard stations may be replaced several times as they wear out, during an exercise that may last two days. Then the feedback is incorporated in the design and the line is built for real. This approach has been used for reasonably small automobile and aerospace parts, and short assembly processes. It is difficult to imagine how it could be used, for example, in final assembly of cars.

5. 9. Discrete-event simulations Discrete-event simulators are software programs that simulate the state changes of a system as a result of events and planned responses. Originally applied to computer networks, they have also been used during the past fifteen years on manufacturing operations, where the events are, for example, the start of an operation in a machine or the completion of that operation. Their main output is performance measurements on simulated operations that serve to validate a design. Some can also produce animations that are used to show management a proposed line in operation. The software products available for manufacturing simulation include Simul8, Witness, ProModel, AutoMod, and others. These products are based on generic simulation engines and the suppliers claim applications to health care, customer service, call centers, banking,

82

Lean Assembly

Discrete-event simulations

and insurance industries in addition to manufacturing. The implication is that the capabilities of these products are essentially limited to what is common to operations in all these business domains. The simulations of manufacturing are in fact at the level of production control and logistics. Conceptually if not graphically, individual stations are black boxes and the product units are tokens that spend time in each box. It may be possible to coax these products into simulating the impact of vertical versus horizontal orientation for a long and narrow part, or of tool placement on an assembly station, but it is not the kind of capabilities that their suppliers advertise. This focus on the macro level versus the micro level is a reflection not on the software suppliers but on the manufacturing engineering community they serve. Until lean manufacturing started reversing this trend, manufacturing engineers have shied away from and neglected the detailed design of assembly operations in favor of higher-level issues. What lean conversion is causing manufacturers to rediscover, is that the detailed, micro-level is by far the richer mine of improvement opportunities. Over time, this may cause the software industry to shift its focus in this direction, but it has not happened yet. Today's tools are overkill in simple assembly processes, where a full-size mock-up can be built in a fraction of the time it takes to develop a computer simulation. These systems, however, have found a niche in complicated assembly systems involving many products and the coordination of multiple, independently controlled automatic systems, such as the flexible assembly of car bodies with welding robots and automatic guided vehicles (AGVs) for part supply. Companies that have groups specialized in studying multi-million dollar systems using simulation include Honda, Ford, General Motors, HarleyDavidson, Renault, and IBM. The attitudes of individual plant managers vary, but some do withhold approval of projects until validated by simulations. Smaller companies may not be able to afford an internal simulation group. Many suppliers of materials handling systems, however, offersimulation services to customers.

Lean Assembly

83

CHAPTER 6

The concept ofthe assembly line

The assemb!J line is a !)Stem in which product units move through a sequence of stations, each with all the resources neededfar one operation, and at which each unit is processed as itpasses through. Invented at Ford in 1913, the assemb!J line concept hasyet to be implemented !)Stematical!J, in spite ofitsproven superiority over bench assemb!J in all but a few applications like building prototypes. Assemb!J line work is unpopular with operators. It can be made safer, more secure, and less tedious, but not into what children dream of doing as adults. Over decades, thesejobs will be automated, and the challenge is to manage this gradualprocess in a way that assemblers can support, that isincremental enough to work 01 attrition, and that provides assemblers with opportunities far professionalgrowth. An assemb!J cell is a special case of an assemb!J line that can be operated 01 a team of no more than eight to ten operators, and therefore can be laid out and run as a cell. Long assemb!J processes usual!J are not divided into cells, because of the added W1P buffers and transportation. Tqyota, in the 1990s has converted its final assemb!J lines into sequences of 20-station segments, with buffers of up to 5 units in between, to allow operators to stop a segment without idling the entire line. A product can be assembled in a single line, have small components assembled infeeder lines, or have mqjor modulesfad to a shortfinal assemb!J line. Modular assemb!J has been in use in electronics and has recent!J been adopted 01 VW, GM, and Ford, but not Tqyota.

Lean Assembly

85

The concept of the assembly line

6.1. What is an assembly line? An assembly line is an assembly system in which product units move through a sequence of stations, each set up with all the materials, machines, tools, jigs/fixtures, instructions, and operators needed for one operation, and in which each unit is processed as it passes through. All the elements that make up an assembly line are illustrated in Figure 61. Following are key points to keep in mind: •

The flow of product units down the line is one piece at a time, not one batch at a time.

•

Flowing one single-unit kit at a time, however, is consistent with the definition, because all the parts in it eventually go into the same product unit.

•

The direction of flow is not specified: it can be left to right or right to left, or both if work takes place on both sides of the line, as is the case, for example, in the final assembly of cars.

•

Backtracking is not allowed. Product units do not return to stations they have already visited.

•

Some elements, like instructions, are mandatory at every station. Others, like machines or even fixtures, may or may not be needed.

FIGURE 6-1.

86

The assembly line

Lean Assembly

Bench assembly versus the assembly line

6.2. Bench assembly versus the assembly line While it is clearly not simple to implement, the assembly line concept itself is straightforward. Why then did we have to wait until 1913, a good 150 years into the Industrial Revolution, before anybody tried it? It is not as if assembly didn't take place before then: steam engines had already been made by the thousands, railroad cars by the tens of thousands, and sewing machines by the millions. The dominant method was bench assembly, where the product unit does not move, all the equipment and parts are brought to it, usually in kits, and where one assembler or a team of assemblers moves around it. In 1913, inspired by slaughterhouse "disassembly" lines, Ford engineers experimented with the concept of the assembly line for the model T magneto in Highland Park, Michigan, as shown in Figure 6-2.

Bench assem bly FIGURE 6-2.

The wo rld 's fi rst assembly line

Ford Model T magneto assembly

SOURCE: From the collections of Henry Ford Museum & Greenfield Village.

On the first day, the labor required to assemble a magneto went from twenty minutes to thirteen minutes. Within eight months, it was down to four minutes, and it is this success that motivated Ford to apply the concept throughout the plant. While we may never know why the concept was not tried earlier, a more contemporary concern is that there are still many factories where its advantages are not recognized and where complex products are assembled

Lean Assembly

87

The concept of the assembly line

from beginning to end on a single bench or in a single stationary fixture. Industries where this can be seen include the following:

• Aerospace and defense. Jet fighter assembly still takes place in a single start-to-finish fixture. • Semiconductor equipment. Into the 1990's even industry leaders made advanced photolithography or dry etching machines the same way that steam engines were made in the 1890's. • Sports cars. Even in the car industry, birthplace of the assembly line, Volvo attempted a return to the old concept in the 1980's at Uddevalla, Sweden, with 12-member teams assembling cars from start to finish in a single fixture. After several closures and changes of ownership, this plant is still in activity today making the C70 sports car.

Why assembly lines are still controversial The most common reasons given for not using assembly lines are: 1.

The volume is too low.

2.

Each unit is so customized that there is no repetition.

3.

Assembly line work is unbearable for operators.

In almost every case, P-Q analysis refutes points 1 and 2. Even in companies that do extensive custom work, the managers' perception of the extent of customization far exceeds the reality that the numbers reveal: a substantial proportion of the work is in fact repetitive. Once a pilot project reveals to them the advantages of the assembly line, they become converts. Point 3, on the other hand, is a valid concern. Human factors have been the Achilles' heel of the assembly line since its invention. Ford quickly experienced a dilemma that has affected assembly lines ever since. On the one hand, assembly lines beat bench assembly hands down on both productivity and quality. On the other hand, assemblers universally prefer bench assembly. At Ford, they responded to the introduction of assembly lines by leaving the company. Turnover became such a problem that Ford introduced the "$5-day," doubling the market wages of the time as an inducement for workers to stay.

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Lean Assembly

Bench assembly versus the assembly line

Even as, over the decades, the $5-day turned into the $20-hour, this dilemma has not been resolved to this day. Even operators who "like putting things together" dislike doing it in an assembly line. They acquiesce because they can find no other job with comparable pay at their level of skills. Lean manufacturing makes the work safer, more secure, and less boring, but it falls far short of making assembly the sort of work children dream of doing when they grow up. While paying assemblers twice as much to be four times more productive is affordable, it should not be viewed as a permanent solution. Fundamentally, assembly line work is not something human beings should be asked to do, and it will eventually be automated away. What concerns us today is what to do in the meantime. Today and for decades to come, this is work that will still be done by people. Much of the lean approach to human resources and automation aims to manage the reduction in manual assembly work in a way that assemblers can support. The shift toward automation must be incremental enough to be covered by attrition, while providing assemblers with opportunities for professional growth. When productivity improvements make it possible to pull one person away from the line, it is the best and most versatile worker who is chosen, to be moved into a more rewarding role. The details of how this is done are in the chapters on organization.

Comparing bench assembly with line assembly Figure 4-6 on page 51 showed several possible ways to break down a 100minute assembly process in order to produce to a takt time of 1 minute. The option of having 100-minute, start-to-finish operations at each station with 100 stations running in parallel is what we call bench assembly, and the alternative of having 100 stations in series with one minute of work assigned to each is the assembly line, as illustrated in Figure 6-3:

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The concept of the assembly line

Assembly benches

Assembly line Stations smalllerthan benches

□ FIGURE 6-3.

Single fixture Tools and gauges for operation Operation instructons

High volume bench assembly versus the assembly line

Table 6-1 summarizes the implications of the two choices. Division of labor is usually the first advantage of assembly lines that comes to mind, but, as Table 6-1 makes clear, there are many others, equally compelling. In fact, the only area in which the bench wins over the assembly line is job satisfaction. Let us now consider what happens if the demand for the product drops by a factor of 100. The takt time is now 100 minutes, production requires only one operator, and division of labor is off the table. In the assembly line, the same operator successively attends to all the stations in the line, and derives the same satisfaction from the job as on the bench. Table 6-2 shows how the two approaches stack up in this context::

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Lean Assembly

Bench assembly versus the assembly line

TABLE 6-1. Bench versus line with 100-minunte process, takt time= 1 minute

Bench assembly

Assembly line

Following the takt time

100 benches with start times staggered one minute apart. Speed differences among operators, however, would prevent finished product units coming out exactly one minute apart.

Product unit transfers between operations are paced to occur at every beat. It is possible to ensure that finished product units come out exactly one minute apart.

Assembler skills and division of labor

On each shift, 100 operators would have to be crosstrained on all the steps of a 100-minute assembly process.

Each operator only needs to learn how to do one minute of work.

Resources

Each bench needs a full set of fixtures, calibrated tools and gauges, and up-to-date instructions.

Each station only needs fixtures, tools, gauges, and instructions for a one minute operation.

Setups

100 minutes of work on a single bench usually requires multiple setups.

Each station is permanently set up for a I-minute operation.

Logistics

Each component must somehow be delivered to 100 locations.

Each component is delivered to only one location.

Quality

In a bench assembly line running three shifts per day, the same task will be routinely performed by 300 operators, making it impossible to guarantee consistency

In a bench assembly line running three shifts per day with job rotation within teams, the same task will be routinely performed by 10 to 15 operators.

Operator job satisfaction

Feeling of accomplishment from assembling a complete product from scratch.

Alienation and boredom make operators dislike assembly line work.

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The concept of the assembly line

TABLE 6-2. Bench versus line with 100-minute process, takt time= 100 min.

Bench assembly

Assembly line

Resources

The full set of fixtures, tools, gauges, and instructions must be available at single bench

Each station only needs fixtures, tools, gauges, and instructions for a process segment.

Setups

l 00 minutes of work on a single bench usually requires multiple setups.

Each station is permanently set up for its process segment.

Logistics

There may not be enough shelf space at the bench for all required items. If there is, it is crowded and picking is error-prone.

Only the subset of parts needed for each process segment is presented at each station. Similar parts are assigned to different stations to prevent errors.

Quality

The long cycle job at the bench is difficult to keep track of.

Moving between stations helps assemblers remember where they are in the process.

Floor space

The generic bench is larger than each station, and needs space around it for multiple setups.

There are multiple stations, but each is small and needs no setup. A transportation aisle is needed for part supply.

If division oflabor were the main advantage of assembly lines, the concept would not be beneficial at low volume. The other advantages, however, still apply at low volume. If we make the same product at a takt time of 100 minutes, we only need one operator, on a bench or an assembly line. The bench will be set up many times. In the assembly line, the operator moves the product on a pushcart between stations that are permanently set up with the fixtures, tooling, instructions, and parts needed for one step.

Exceptions: where the bench still wins In both high- and low-volume production, it is almost universal that the assembly line outperforms the bench, yet there are exceptional circum-

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Lean Assembly

Assembly lines, assembly cells, and line segments

stances that negate the advantages of assembly lines. Following are a few examples: • The degenerate case where the assembly process is so simple that the assembly line only has one station and is indistinguishable from a bench. This happens, for example, in low-volume assembly of simple hand tools, with fewer than ten parts. • The product unit is too large to move. While it is made of sections that do move, a tanker hull is assembled in one dry dock and moves only once it can float. • The unit is a prototype or a one-of-a-kind system with no process similarity with anything else the company makes. This is the residual activity that remains after P-Q analysis has separated out repetitive production. Not only is there no takt time, but the assembly process itself has to be customized. Planning mistakes are common, resulting in components being disassembled and reassembled several times.

6.3. Assembly lines, assembly cells, and line segments An assembly eel/is a special case of an assembly line that can be operated by a team of no more than eight operators, and therefore can be laid out and run based on the concepts of cellular manufacturing as shown in the example of Figure 6-4. The stations are arranged in a U-shape with the operators working inside and parts fed from the outside For reasons discussed in detail in Chapter 13, this is a desirable mode of operation because there is no interference between part supply and production, and because there is staffing flexibility when volume changes, and easy comunication among operators for training and support. There are, however, many assembly processes that do not fit this pattern, the best known being final assembly of cars, which requires approximately 200 stations, and engine assembly, which takes about 50 stations.

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The concept of the assembly line

Raw Finished Materials Goods

~

q

s:::

In-process in spection

::::,

Assembly

Electrical un its (From H. Hi rands JIT Fac!;ory Revolution)

FIGURE 6-4.

A cell for cardiac monitor assembly in Japan

In principle, the same long assembly process could be implemented as a

series of cells or as a single line, as shown in Figure 6-5. A series of cells is between two transportation aisles. The left-side aisle is used to deliver parts; the right-side aisle, to transfer the output of each cell to the input buffer of the next one. Cell leaders take care of moving the parts from the transportation system interface to the assembly stations of the cells.

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Lean Assembly

Assembly lines, assembly cells, and line segments

IV1i~~m~ t2~ .i_ Queueing and and batching between cells

Wat er spider

-

Ba lance between st ations difficult

FIGURE 6-5.

One long line or a sequence of cells?

Alternatively, we can do the same work in one single, straight line, avoiding the buffers and transportation between cells but losing the advantages of U-shaped cells and cell teams. There is only one transportation aisle, for part deliveries to flow racks and similar devices bringing parts within arm's reach of assemblers. The movement of the product itself is taken care of by the assembly line's conveyance mechanism (see Figure 6-6). Traditionally, the long assembly line has prevailed, and cells have been used mostly when they can encompass a complete assembly or subassembly process. But the vulnerability of a long assembly line to disruptions is also an issue, and it has caused Toyota to rethink its car final assembly process in the early 1990s and redesign it as a sequence of five to twelve segments of twenty stations each, separated by buffers of up to five units, which, in the worst case, adds sixty cars to the work-in-process.

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The concept of the assembly line

General grou p lead er

Team leader+ 4 team members

FIGURE 6-6.

,,~ A~

d88~

Qua lity c heck station

A final assembly line segment at Toyota

Since the Toyota production system is not known for adding unnecessary WIP, there have to be compelling reasons for this change. Takahiro Fujimoto 1 gives the following, which do not appear strong enough to support such a massive change: •

Facilities design is easier with line segments.

•

Related assembly tasks are grouped by segments, making the jobs more meaningful for assemblers.

•

The group leader has more management autonomy.

Toyota assembly lines have stop ropes to allow assemblers to stop the line, a device that has been adopted by many other companies. Pulling that rope in a traditional final assembly line for cars means idling hundreds of colleagues as well as a system that generates more than $10,000 of revenues every minute. It is in fact such a daunting prospect for assemblers that

1. T. Fujimoto, The evolution of a manufacturing system at Toyota, Oxford University Press, 1999.

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Lean Assembly

Assembly and subassembly

they practically never do it, occasionally resulting in the very situation that stop ropes are intended to avoid: product units moving forward in spite of defects or missing parts, that are then replaced or added in repair mode at the end of the line. Breaking the line into segments enables assemblers to stop that segment only for a few minutes without affecting the output of the plant. The presence of a quality check station at the end of each module also allows any needed repair to be performed within the segment in which the need arose, by assemblers from this group, thus providing faster and better feedback. The corresponding reduction in the number of units that need repair at the end of the line may cause the buffers between segments to actually decrease the total amount of WIP in the system.

6.4. Assembly and subassembly There are many ways of breaking a product down into subassemblies, and the one in the MRP system's bill of materials may be irrelevant to manufacturing because it only pertains to purchasing.

All assembly work done in one single line For manufacturing, the simplest design has no subassemblies at all. All the work is done in one sequence, and all that needs to be done to coordinate and pace the work is to make sure that all product units proceed to the next station at takt intervals, as shown in Figure 6-7. Parts/components

FIGURE 6-7.

Lean Assembly

Assembly in one single line

97

The concept of the assembly line

Final assembly line with subassembly feeder lines Within that single sequence of operations, however, there are subsequences of steps affecting only one small part of the product while the rest of the product waits, serving only as a fixture. In addition, the product may be an inadequate fixture, providing inconvenient access to the parts being worked on. It is typically much larger than a dedicated fixture would be, and it is not designed to orient this subset in a way that is convenient for assembly. Where this occurs, it's worthwhile to consider making that subset a subassembly and putting it together on a feeder line with a fixture designed specifically for it. This allows work to proceed in parallel on the subassembly and on the rest of the product, as shown in Figure 6-8. Feed er line

FIGURE 6-8.

Subassembly feeder line

Dashboard assembly in cars is a case in point, as shown in Figure 6-9. Traditionally, it was done inside the car body by an assembler working in a succession of uncomfortable positions. Several car makers, including Toyota and Honda, have now designed their products so that dashboards assembled off-line are now simply "plugged in" by a robot.

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Lean Assembly

Assembly and subassembly

The old way: squatting inside car FIGURE 6-9.

The new way: in an asse mbly line

Dashboard assembly for cars

SOURCE: ©1997 Springer Verlag from Tranforming Automobile Assembly

We pay for this, however, by having to coordinate the feeder line and the main line. Typically, the feeder line has an output buffer, from which subassemblies are pulled for use on the main line. As feeder lines proliferate, this coordination problem grows.

Modular assembly A recent trend of the automobile industry is to go for modular assembly. This means having suppliers put together major subsystems of cars and cutting down the amount of work in final assembly by about 90%. This is not a Toyota concept and has not been endorsed by Toyota. The concept has been heavily promoted by Jose Ignacio Lopez at GM and VW, and applied in France, Spain, and Brazil. Large-scale projects are underway to apply it in the United States (See Figure 6-10). Modu le lines

~

FinalAssembly

~

FIGURE 6-10.

Lean Assembly

Modular assembly

99

The concept of the assembly line

While new to the car industry, the concept of modular assembly is a common practice in electronics. Final assembly of PCs, for example, is simple because most of the complexity is in such modules as motherboards, processor chips, and disk drives. The plant built to assemble the Smart car in Bambach, France, is an example of modular assembly, as shown in Figure 6-11.

TNT log istics

Suriema paintshop

Salesstcre

411 I-

Krupp Hoesch rear powertrain

ssembly plant for the sma rt Dyna mit Nobel plastic panels

Bosch front powertra in

Uniport doors

FIGURE 6-11.

100

Modular assembly for the Smart car in Hambach, France

Lean Assembly

Assembly and subassembly

Pros and cons of subassembly/feeder lines There is no single, universal answer to the question of how much subassembly is warranted for making a given product at a given takt time. The pros and cons of every subassembly opportunity must be weighed one by one by line designers. Table 6-3 summarizes the issues they need to address. TABLE 6-3.

Pros and cons of subassembly Subassembly/main line Asynchronous operation Specific fixtures and tools for subassembly work

All assembly in one line Synchronous operation Common fixture

Parallel work Cell opportunity

Work in series Too large for a cell

WIP buffers

No buffers between operations

Requires transportation system

All conveyance within one line

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CHAPTER 7

Collecting assembly time data

Collecting time data is aprerequisite to improving assemb!J operations, but it is more difficult in practice than one would expect. In mostfactories, it is either not done at all or done with methods that do notprovide accurate results. Operator interviews or retrieval of numbers of unknown originfrom ERP databases are not appropriate methods. Predetermined time standards methods like MTM or MOST should be applied on!J in new line projects, where there there are no operations that can be measured. Otherwise, time studies should be conducted, with stopwatches or through video recordings, after explaining the purpose of the studies to the affected operators, securing their consent before taking measurements, involving them in the ana!Jsis afterwards. Tqyota} TVAL is a new approach that entails not on!J measuring the time neededfar an assemb!Jjob but also quantifies the fatigue itgenerates as afunction ofposture, weight carried, and time. Reducing the TVAL rating of each job then becomes an improvementgoal.

7.1. Why this needs attention We cannot design or run assembly operations without good data about how long it takes to assemble. And, in most assembly plants, not only is

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Collecting assembly time data

this data not readily available, but its acquisition is a major undertaking, both in terms of preparing the organization for it and in terms of actually doing it. It is one thing to take measurements in a small cell; another, to take them on every one of fifty stations in an assembly line. It is sensitive information for the people working on the line and must be handled accordingly. If there is an immediate link between these times and pay rates, or if the assemblers perceive that the purpose of the measurements is to extract more physical effort out of them, they can and will prevent the collection of realistic data.

7.2. Data collection methods There are many methods for collecting assembly time data. Some of the most common are descrubed in the following list:

• Official numbers usedfar MRP. These numbers are easy to get but unreliable. In one place within the plant, they may be identical to the actual times, and elsewhere inflated by a factor of two to three. • Operator interviews. A form is handed out to operators for them to fill out about how long each task takes. Among three operators doing the same work, the answers may vary by a factor of two to three, and will be round numbers. • Predetermined time standards. These methods rely on a detailed analysis of the work content of operators, and they assign a standard time to each element of motion. They are labor intensive, and used in the automobile industry, particularly to staff new assembly lines. The time estimates from these methods typically are within ±30% of the actual times. • Stopwatch studies. Stopwatch studies can be conducted, but the operators being measured must have been briefed beforehand as to the purpose of the study and know how the data will be used. It helps if the stopwatch is held by a colleague rather than an outsider. Direct stopwatch measurements are appropriate for operations lasting a few minutes with extensive manual intervention.

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Current status of time and motion studies in manufacturing

• Videotaping. Videotaping works on the same type of tasks as stopwatch measurements but requires more equipment. It has, however, many advantages over the stopwatch. The team can review the videotape off line with the operator in it, and incorporate his or her comments in the analysis. In addition, seeing themselves on tape is often a revelation for operators and makes them realize how much of their effort is wasted. • Manual/y timestamping machining operations. For long machining operations, operators can keep a ledger of start and end times, and managers can then use this to calculate the assembly times. Under pressure, however, operators are likely to forget to maintain the ledger in real time and either leave missing data or enter approximate values later. The quality of this data is not significantly improved by replacing the paper ledger with a computer terminal. • Automatic timestampingfar long machining operations. A supervisory control system can track operation start and completion signals from a CNC and provide accurate actual operation times across a multiplicity of media and tools.

7.3. Current status oftime and motion studies in manufacturing Predetermined time standards in the automobile industry In the United States, the automobile industry is the only one routinely conducting time and motion studies. The most common pattern is for a car company to have a department of time specialists trained in a predetermined time standards method, usually MTM or MOST, which are both discussed in more detail later in this section. Technically, using such methods to analyze existing lines is incorrect, because you can get better data faster by direct observation. This misuse, however, is widespread.

These time specialists generally do nothing other than calculating standard times and reassigning tasks among stations to balance the standard times. They do not attempt to improve processes and generally do not understand them. For example, when faced with a fastening task, they may issue

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Collecting assembly time data

standard times for running down a nut and for applying torque with a torque-wrench. But they are most likely unaware that these two operations can be combined into one by using a DC nutrunner. Picking parts from a pallet on the floor is ergonomically inconsistent and takes varying amounts of time depending on where in the pallet load the operator picks from. It obviously takes less time to pick from the nearest corner of a full pallet than from the far corner of a nearly empty one. What this situation would benefit from is reengineering the part supply method to provide the parts always in the same location to the operator, but the time specialist will not get involved in this. He or she will simply record the maximum time needed. Another source of inaccuracy is the method's failure to take into account the effect of repetitions within a job. If, for example, an operator fastens a row of ten identical bolts on each workpiece, the standard time for this operation would be determined by the time specialist to be ten times what it takes to fasten one bolt. In fact, the operator's actual time per bolt can be up to 50% less for ten bolts than for one bolt. Repetitive stress and fatigue are also ignored. Repetitive stress cannot be seen from observing a job performed a few times, but over time it makes it impossible for an operator to continue doing the job. Allowances for fatigue are built into the standards, but not the fact that when you perform the same tasks 400 times in a shift, your performance at the end of the shift may not be the same as at the beginning. Within lean manufacturing, repetitive stress is addressed, albeit imperfectly, by job rotation within teams. Fatigue is adressed by using an analysis of muscle load ratio in operator job design. The modern Toyota approach to ergonomics is described later in this section. There are several conclusions to draw from our discussion of time standards: •

To work in the automobile industry, you need to interface with their current work time collection tools, at least MTM and MOST.

•

Most other industries do not have this requirement. In every industry except automotive, these tools are little used and little known.

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Lean Assembly

Current status of time and motion studies in manufacturing

•

Technically speaking, process engineering, ergonomics, and time measurement are all different facets of operator job design and should be considered jointly, by the same people, not by different departments acting independently.

MTMandMOST MTM (Methods-Time-Measurement) dates back to 1948. See http:// www.aviation.uiuc.edu/institute/acadProg/ epjp/MTA html for a critical discussion of what it is and its limitations. There is an MTM association, the Methods-Time Management Association for Standards and Research, 16-01 Broadway, Fair Lawn, New Jersey 07410, http:/ /www.mtm.org/. MTM establishes time standards by "recognizing, classifying, and describing the motions used or required to perform a given operation and assigning predetermined time standards to these motions." The MTM association supplies the "MTM software family" consisting of MTM-LINK, ADAM, ERGOPlan, Time Ladders, and PC Graphics, http:/ /www.mtm.org/htdocs/software/, and they also publish the MTM

Journal ofMethods-Time Management.

MOST stands for "Maynard Operation Sequence Technique," and was developed by Kjell Zandin in Sweden, and released in 1975. 1 The reference to Maynard suggests that MOST is a successor or an upgrade to MTM, which MTM adherents disagree with. They simply view MOST as a competitor. While the building blocks of motion analysis in MTM are at the level of Reach, Move, Turn, Grasp, Position, Release load, Disengage, Walk, etc., MOST is based on the idea that the same sequences of such elementary motions keep recurring in manufacturing operations and that the motion analysis can be made an order of magnitude faster on the basis of these standard sequences.

1. Kjell Zandin, MOST Work Measurement Systems (New York: Marcel Dekker, 1990).

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Collecting assembly time data

MOST worksheets have these sequences preprinted, and the analyst's job is reduced to: •

Describing each sequence using simple clauses with verbs that are MOST keywords, such as "Get lift straps."

•

Parameterizing the standard sequence, so that " A B G A B P _N_' becomes ''A 10B 6 G 3A 10B 0P 0A 0 ." In this sequence, ''_N_' means reach, "B," body motion, "G," gain control, and so on.

MTM and MOST use the same time unit, the TMU, where 1 TMU 0.00001 hour, and 1 second = 27.8 TMU.

=

A MOST Computer System has been available since the late 1970s. It includes a line balancing program, which supports the reassignment of suboperations between stations and operators.

Toyota's TVAL TVAL (Toyota Verification of Assembly Line) is a process that drives targeted ergonomic improvements in an assembly line. If some of the tasks in it are so tiring that assemblers finish their shifts exhausted, there is no way that the tasks will be performed at the same level of speed and quality at the end of the shift as at the beginning. Toyota's stated motivation was not for TVAL to improve line balancing but to make assembly line work more attractive to a new generation of Japanese, fewer in number and more demanding in comfort than their elders. The impact of fatigue on line balancing is not discussed at all in the literature, but seems obvious. TVAL was used for the first time in the plant in established in Miyata, Kyushu in 1992. The same concepts were later applied in Motomachi for RAV4 assembly and in Georgetown, Kentucky's #2 line in 1994. TVAL is still a work in progress and its publicly available documentation is incomplete. The idea ofTVAL is to measure a "physical load degree" or "TVAL value" associated with each assembly task, which is a function of the following:

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Current status of time and motion studies in manufacturing

• The duration of the task. • The posture in which the operator has to perform it. • The amount of weight carried through it. This metric is then used to determine where improvements are most needed. It is based on endurance studies conducted with a bicycle ergometer to establish human limits. Having reached one's own limit of endurance is a subjective assessment, and that limit will clearly not be the same for trained athletes as it will be for the cross-section of people who work on an assembly line. Toyota's goal was to ensure that all the assembly jobs could be performed comfortably by men and women up to retirement age with no special physical strength. The general formula for physical load degree is as follows: Physical load degree

= 25.51

X log (Duration)

+ 117.6 Xlog (Force) - 162

where Duration is in seconds and Force in Newtons. The significance of this formula is shown in Figure 7-1. It established an equivalence between carrying a heavy load for a short time and a light load for a long time. All the combinations of duration and force that give you the same physical load fall on a straight line on a log-log plot of duration versus farce. From the point of view of impact on the human body, applying ten pounds of force for 3 seconds is equivalent to applying four pounds for 200 seconds.

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Collecting assembly time data

Load-time combinations with constant physical load degree 10000 -

-----------------------~

Time (seconds)

Physica l load degree= 50

50N

3

10

~ 10 lbs. 100

Load (newtons)

FIGURE 7-1.

Physical load degree

However, because applying force to put components together is done mostly by power tools, the main exertion for assemblers is from lifting, carrying and positioning components that may be heavy. To get the TVAL value, the Toyota engineers replaced Force in the equation with a function of the assembler posture and the weight of the part, for which they developed a lookup table. Their final formula is therefore: TVALvalue 162

= 25.51

Xlog (Duration)+ 117.6 Xlogif{Posture, Weight}) -

where duration is in seconds, posture is one value out of a finite set of options, as follows:

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Lean Assembly

Time studies with video recordings

Posture

=

Standing Forwardbend Deepforwardbend Squatting

and weight is in Kg. This number is then calculated for all the tasks of each operation, and the high TVAL values point out the targets for improvement. At Toyota's Kyushu Miyata plant, this analysis drove the following kinds of improvements: • Height-adjustable devices to improve posture. • Floor conveyors moving with the line to maintain relative position of assembler and workpiece. • "Raku-raku" seats mounted on a moving arm to support work done inside cabins. • "Nagara" fixtures, traveling alongside the line ("pirate ships") or hanging above ("spaceships") and carrying parts and tools. • Power assists to reduce weight carried by operators. • Miscellaneous improvements to the work environment in lighting, air conditioning, and noise prevention on tools and conveyors.

7.4. Time studies with video recordings Both predetermined time standards and stopwatch time study methods require the analyst to explicitly break every operation down into minuscule steps. A reference video can make this tedious work unnecessary. There are many uses of video recordings in conducting time studies and doing process improvement that do not involve a "reference video." A reference video is a recording of an assembly process qfterimprovement and performed at a normal pace. It should not reveal any obvious improve-

Lean Assembly

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Collecting assembly time data

ment, because those have been done, but it should serve as the most precise documentation of the process. Digital video recordings can be stored on computer hard disks in the MPEG format. In the form of MPEG files, the reference videos could also be stored in the process definition database. Particularly in the type of line rebalancing effort described in Chapter 8, the reference video can save most of the analysis labor by eliminating the need to identify elementary steps. Within the constraints of technical feasibility, we can balance operations by cutting and pasting video segments of appropriate lengths in appropriate operations. The resulting spliced video can then be used to communicate and review the proposed changes with affected assemblers and identify the changes needed to the layouts of both assembly stations. Once the changes are implemented, new reference videos would be shot for both operations.

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Lean Assembly

CHAPTER 8

Line balancing

Balancing an assemb!J line is feasible because unlike other manufacturing operations, assemb!J tasks can be broken down into arbitrari!J small increments. These smaller tasks can then be moved between stations, until the work loads of all assemblers are within ± 5% of their average and are all close the line} takt time. Within dedicated lines, rebalancing is routine!J undertaken to improve productivity or to respond to changes in demand. Depending on the size and complexity of the line, it may be doable in afew minutes 01 direct observation, or it may be aproject involving a team of engineersfar several monthsjust to do the required ana!Jsis. In lines used to assemble multipleproducts, theproblem is handled diffirent!J, depending on whether the line is short enough to be flushed between products. In this case balancing is needed between stationsfar each product, but not at each station between products. Otherwise, differentproducts have to coexist on the line and must move farward at the samepace, which requires them to have matching assemb!J times at each station. This is accomplished either 01 automating the process or 01 have more work done in subassemb!J lines far some products than far others. To decide which products should be assembled on the same lines, ana!Jtical tools based on electronic spreadsheets can be applied to the products' bills ofmaterials to assess their similarity in complexity and theirparts commonality.

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Line balancing

8.1. Assembly line balancing The ideal situation is to have at all times the work content of each operator exactly match the takt time at all times.. This ideal situation is actually assumed in economic calculations about the value of assembler time. Each assembler is assumed to be a bottleneck, so that a second added to his or her workload delays all the other assemblers up and down the line by the same amount, which makes assembler time an expensive commodity. Perfect balance is also ideal in machining or fabrication, but the need to coordinate people and machines makes it impossible to even come close. Assembly work, particularly manual assembly, is different: it is almost infinitely divisible. Assembly tasks can be broken down into minuscule increments that can be reassigned among different assemblers until they have nearly identical workloads, closely approaching the takt time. It is this activity that is called assembly line balancing. There are many assembly lines with different characteristics, each of whose needs should be addressed as well, such as:

• Dedicated lines making one product, possibly quite complex, for months

or even years, during which volume is adjusted up and down numerous times. Whenever major rate changes occur, groups of engineers spend weeks or months rebalancing the line, which means calculating how many operators need to be added or pulled out.

• Mixedjlow lines alternating between products one piece at a time. This

requires engineering the assembly times to be as close as possible for the different products, which is done by doing more subassembly work or more automation on some products than on others.

• Batch assemb!J lines that alternate between products one batch at a time. This is much more common than true mixed-flow lines due to the widespread belief among assembly managers that it is more efficient. If at all true, this is only if you fail to consider the implications on the logistics of parts supply.

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Rebalancing a dedicated line

8.2. Rebalancing a dedicated line Rebalancing a dedicated assembly line is undertaken in three types of circumstances: • To improve productivity. Even without any change in speed, rebalancing a line of 200 operators that has been put together in a hurry can easily yield a 10% to 20% increase in productivity, due not only to improvements in the balance of the work content but also to improvements in the design of the jobs themselves. • To cope with a major increase in production rate. • To cope with a major drop in production rate. As shown in Figure 8-1, the principle ofline balancing is deceptively simple. The work content of each station is represented by a vertical bar. All the bars must fit below the takt time. If one bar pierces it, you must shorten it by improving the operation, by reassigning tasks upstream or downstream, or both. Then, even when all bars fit underneath the takt time line, you still need to make them as equal as possible and as close to the takt time as possible, to increase productivity and avoid the humanly untenable situation of having two operators side by side with one visibly busier than the other. The simplicity of the chart is deceptive because the amount of engineering work needed to do this on a line with tens or hundreds of stations involves tens of engineers for several months, for the following reasons: • The sequencing flexibility of assembly operations is broad but not absolute. Some tasks are technically constrained to be done before others. You can't mount a gasket after bolting the parts together. Tasks cannot be moved as easily as the arrow in Figure 8-1 suggests. • The bars represent times that must be measured. Furthermore, to reassign segments of these bars, we need to be able to break them down into smaller pieces and have times for those pieces as well. Few United States assembly plants have usable time measurements, and the old fashioned industrial engineering approach of using time studies to set pay rates or extract more effort from workers has poisoned the well for this activity, compounding the technical difficulty of doing it right with the social difficulty of doing it tactfully.

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• Every station is under a microscope during such a project, and it reveals all sorts of issues, from poor ergonomics to lights located behind the operator instead of over the work, and multiple handling of parts that need to be promptly corrected. • Moving a task between stations means moving tooling and instructions and retraining the operators. • Moving a task also implies rerouting supplies, which involves the materials management organization. How such an effort proceeds is shown in Figure 8-2. There is no point in rebalancing poorly defined jobs, and therefore the improvement of targeted individual jobs is the first priority. Then comes improvement of the jobs of the other operators on the same team, and finally a rebalancing of these jobs to pull out at least one operator. There is less work involved in improving and rebalancing jobs within one team than across multiple

116

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Rebalancing a dedicated line

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Line balancing

teams. We will therefore improve and rebalance the individual teams. When we are done with this, we can restructure the line's team breakdown, which itself may have been thrown out of balance by our efforts. On a long line making a complicated product, rebalancing can be a dauntingly big job. Conversely, in small, simple lines, line balancing can be done informally, by direct observation. Frying pan assembly, for example, involves the following operations: 1.

Riveting a handle to a pan.

2.

Inserting the pan into a plastic bag.

3.

Taping it shut, boxing it, and closing the box.

This is done on a twenty-second takt time. It doesn't take long to notice that the riveter has less work that the packager. It is easy to see, because he is able to work ahead and accumulate WIP. If you prevent him from overproducing, he has to wait during each cycle. So you ask him to insert the pan into the plastic bag after riveting, thus improving the line balance by relieving the packager. It doesn't need much analysis. It doesn't need a time study. Large lines often have subassembly feeder lines that are small and simple, and in which supervisors can work directly with operators as described here. The Japanese factory management monthly Kojo Kanri 1 reports an adaptation of this on-the-fly balancing approach to long assembly lines, by having one observer standing behind each team of four to six operators. The observers work in parallel and intervene in the manner described above for frying pan assembly. Whenever the work can be improved simply by direct observation, it should be. The main precondition is for labor-management relations to be such that it is possible. The changes made in this form must be documented and posted for communication across shifts and to prevent backsliding. It is quite possible that, coming back from a break or the following

1. Factory Management, Vol. 48, Nr. 6, pp. 43-44

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Lean Assembly

Multiproduct lines with batch versus leveled sequencing

day, operators will return to the old way, until their hands have memorized the new pattern.

8.3. Multiproduct lines with batch versus leveled sequencing There are fewer true mixed-flow lines in operation than one would expect. True mixed-flow lines - that is, with leveled sequencing- are in most circumstances the best approach. Most assembly managers don't understand the value of leveled sequencing and believe that running multiple products in batches is more efficient. The lean manufacturing literature does a poor job of explaining the advantages ofleveled sequencing, and, in addition, the main benefits of smoothing parts supply to assembly is for subassembly feeder lines, logistics and suppliers, most of which final assembly managers are not in charge of. Even when they understand what leveled sequencing does, they perceive it as making their own lives more complicated to make other people's easier. The issues are summarized in Table 8-1. The first has to do with disruptions. Sometimes, you are not able to run a complete batch through assembly, but parts have been supplied to all stations for a complete batch, which means that the leftovers need to be managed, and brought back to the line when the stragglers finally make it. By contrast, in leveled sequencing, a one piece anomaly has almost no impact on part supply. The advantages of steady part supply are substantial but can be difficult to realize in particular industries. Prestige cosmetics is one of them, because of the seasonality of the demand, combined with long lead times on bottles and jars, the intrinsically batch nature of the compounding process, and the wide range of sizes that need to be supported in filling lines.

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Line balancing

TABLE 8-1.

Batch versus level sequencing in multiproduct lines

Batch sequencing

Leveled sequencing

Straggler management. Parts leftover from incomplete batches must be managed.

Single unit changes in sequence. A single unit missing in the sequence has a negligible impact on part supply.

Inability to make exact required quantity (multiples of batch size only).

Exact required quantity per time period.

Need for finished goods inventory.

No finished goods inventory. Sales may maintain an inventory of finished goods, but manufacturing doesn't have to.

Uneven part supply:

Steady part supply:

• Stop and go on feeder lines.

• Feeder lines on takt time.

• Variations in need for subassembly labor. • Need for raw materials/component stocks.

• Constant need for subassembly labor. • Small raw materials/component stocks.

Management of differences in assembly times between products. If the line is short enough to be flushed empty during changeovers, it can work at different speeds for different products.

Need to engineer away differences in assembly times between products at every operation.

Low risk of picking errors.

High risk of picking errors.

In addition, cosmetics packaging lines are so short that you can flush them empty as you change them over from one product to the next, which allows them to run at different speeds for different products. By contrast, if you run batches of 20 cars through a line with 200 stations, 90% of the line is occupied by different models, so that they must all proceed at the same speed. Table 8-1 highlights the two major technical challenges of leveled sequencing: engineering away the differences in assembly time between products

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Lean Assembly

Balancing assembly time among products on a mixed-flow line

at every operation, which is another form of assembly line balancing, and preventing picking errors.

8.4. Balancing assembly time among products on a mixed-flow line The mixed-flow line situation poses the additional challenge of balancing the assembly time at each station among multiple products. This is simply because the one product-station with the highest work content constrains the pace of the entire line. Given that different products running on the same line are usually similar, they normally have many common components and work contents that differ little at most operations but perhaps substantially at a few. For example: • For car engines with different numbers of cylinders, the differences will be focused on the assembly steps that deal with the cylinders. • In rack mounted servers, the chassis, disk stacks, and memory configurations may be identical, while the number of processors varies. As stated earlier, the most successful approach to equalizing the assembly times on different products is to off-load more of the assembly work on the more complex products to subassembly feeder lines. The limit to this approach is that not every assembly task lends itself to separation into a subassembly. If, on the other hand, you have a rich subassembly structure and many feeder lines, it is often technically possible to transfer the subassembly work to the main line. You are more likely to be able to equalize the work content of two products at a station by enriching the work content for the simpler product than by farming out some of it for the more complex one, provided of course that the takt time permits it. If that is not possible, the alternative strategy of differential automation remains: you bring down the work content on the more complex product by applying more automation to it.

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Line balancing

The most powerful strategy for reducing the risk of picking errors is to increase the commonality of parts among the different products. This is generally viewed as a problem of DFMA (design for manufacturability and assembly). DFMA must be addressed in the product design stage and is difficult to fix afterwards. Achieving a high level of common parts is difficult, and the current offerings of the software industry make the situation worse rather than better. Engineers have CAD tools to support the proliferation of different parts, but no intelligent search tools to identify the existing parts that are closest to their requirements and for the reuse of which they might make design trade-offs. Technology for this exists, but it is not in common use in manufacturing companies. Figure 8-3 shows the parts used to make two successive versions of the same product, the Zebco 404 fishing reel. In this example there is only one common part between the old and the new.

Old model New model FIGURE 8-3.

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8. 5. Deliberate imbalances When an assembly line is paced by a powered conveyor, all stations see the product for the same amount of time, and balancing the work among stations is simply a matter of making the best use of that time. The issues are different in the following cases: • The movement of units between stations is triggered or carried out by operators. There is no mechanical pacing device. • The line is made up of segments, each of which may have a powered conveyor, but transfers between segments are manually controlled.

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There will then be variability in the times product units spend at each station or within each segment. If all the stations in all the segments are perfectly balancd , any delay at any station - whether due to a machine microstoppage or a tired operator - will result in lost production. One way this is handled is by reducing the workload slightly as you go upstream along the assembly line, giving the upstream stations slack to ensure that the last station, just before shipping, never runs out of work. Figure 8-4 shows how this works in an exaggerated example, where the

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first station has 10% less work than the last, and where one unit is accidentally delayed fifty seconds at station 1. The slack provided is sufficient for this part to catch up by the time it reaches Station 10, and the line does not miss a beat. Where that concept is actually used, the amount of slack added to upstream operations typically does not exceed 3% to 5% of the takt time on a line of 50 to 100 stations, and it is barely perceptible. At downstream stations, there is no opportunity to catch up, and it is therefore essential that no delay be incurred there, and any machine used there must have near perfect uptime.

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PART C

Detailed

design

Lean Assembly

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CHAPTER 9

Assembly station .. szzzng

For smallproducts, assemblers work on!J on one side of the station and pick parts across the productfrom the other side. Part replenishmentfrom the back does not interfere with assemb!J. The stations do not have to be wider than operators and can be arranged into U-shaped cells with the assembler work area inside. The products are often light enough to be passed between assemblers 1:ry hand, without convryors. Largeproducts move on convryors or carts and are usual!J worked onfrom both sides of the station, with parts delivered behind or to the side of the assemblers. Presenting the point of assemb!J to the assembler is a challenge with large products, which is sometimes met through fixtures, as described in chapter 10, and sometimes through cantilevered "raku-raku" seats. The norm is far assemb!J work to be peiformed standing, and moving or rotating between stations as needed. Green corners are provided near the line far assemb!J teams to sit during breaks and hold meetings. The work height needs to be uniform among stations, and platforms cannot be used, as thry restrict assembler mobility. This makes providing the work at an ergonomical!J appropriate height a challenge when the workfarce includes both short and tallpeople. The height of kitchen counters in the local area ofthe plant is a starting point, and some limited adjustability can be provided in the fixtures.

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Assembly station sizing

Stations at which glue must dry or sealant cure holdproductsfar much longer than the takt time. Thry are often flow-through tunnels, which must be laid outperpendicular to the line to allow product units to come out next to theirpoint of entry and avoid separating upstream and downstream stations.

9.1. Issues with assembly station sizing The dimensions of the product set lower bounds for assembly station sizing, but the line designer then has latitude in deciding how much wider and deeper than the product the station should be and how high the work surface will be. These decisions, in turn, have a great impact on the flow of materials and on ergonomics.

9.2. Assembly stations for small products Small in this context means small enough for the assembler to pick parts across the station. Products in this category include most manufactured goods for consumers - such as electrical razors or VCRs - as well as subassemblies or spare parts for larger products, ranging from shock absorbers for cars to cockpit switches for airliners. The small product size creates opportunities in station design that are not available for larger products, such as the following: • Since work takes place on one side only, such stations can easily be arranged into U-shaped cells with the assembler work area inside. • Part deliveries do not interfere with the work, and stations can be arranged to face transportation aisles. • Many small products are light enough to be handled manually. Most manufacturers fail to exploit these opportunities today. The most common concern is station size. You do not need a 60"-by-30" bench to work on a product whose longest dimension is six inches, and using these unnecessarily large stations causes the following problems:

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Lean Assembly

Assembly stations for small products

• Neighboring assemblers are too far apart. Their communications are reduced, they are unable to relieve each other, and one assembler cannot conveniently work at more than one station when volume drops. • The excessive distance between stations complicates conveyance and hinders the product flow. • Just because it is there, the extra bench space fills up with unnecessary WIP, tools, fixtures, and other items unrelated to the operation at hand. • The floor space used by a line of these large stations is twice or three times more than it should be. Conveyor abuse is also common. Conveyors are commonly used to move parts that are small and light enough to be passed around by hand, for the sole purpose of enforcing pace. The assembly station is then frequently off the conveyor, as in Figure 4-8 on page 54. In short operation cycles, the additional handling this generates can account for 40% of the assemblers' time. The example in Figure 9-1 summarizes these issues. Even though the assembler picks parts across the station, a closed wall not only prevents replenishment from behind, but also blocks the view to the next station.

Products move back and forth t o conveyor

FIGURE 9-1.

Lean Assembly

Wa ll prevents replen ish ment fro m beh ind and blocks t he view

Large station for small product

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Assembly station sizing

Such benches are used today, not because they meet the needs of assembly, but because they are available off the shelf from multiple suppliers. By contrast, let us go back to the Ford magneto assembly line of (see Figure 9-2). Since it is the first assembly line ever built, it is clearly not made from off-the-shelf modules but custom-designed. Each station is not much wider than the product itself. The magneto assembly line, however, had many questionable features that we would not design into a modern line. In 1913, of course, the designers had the excuse that they were breaking new ground. The most obvious points are as follows: • The assemblers' head gear suggests that the shop floor is not properly heated. • Placing fasteners under the work is ergonomically inadequate, forcing the assembler to pick in an uncomfortable position. • This layout is also prone to parts contamination, since fasteners can easily drop from assemblers' hands into the wrong bins.

over

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FIGURE 9-2.

Critique of the 1913 Ford magneto assembly line

SOURCE: From the collections of Henry Ford Museum & Greenfield Village.

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Lean Assembly

Assembly stations for small products

As shown in Figure 9-3, it didn't take long before some companies started addressing these issues. The exact date of the picture is unknown, but the workers' clothing and hair styling points to the 1920s. In this picture, assemblers pick parts across the conveyor, at a comfortable height and without risk of contamination. This approach is still appropriate today, whenever the size of the product makes it feasible.

FIGURE 9-3.

Phonograph assembly at Edison in the 1920s

Many lamps hang from the ceiling. This indicates that the stations are well lit, but the windows are behind the assemblers, so that the assemblers cast their own shadows over the work, and artificial light may be needed even in broad daylight. We may also note that the space above the stations is not put to any use other than lighting. In a modern line, powered screwdrivers would be hanging on balancers above the stations. Figure 9-4 shows a contemporary descendant of the Edison line. This station is used to assemble a component of a laser printer. It has two levels of components to pick across the product, and a screwdriver hanging on a

Lean Assembly

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Assembly station sizing

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balancer. There is no conveyor, and the assemblers pass the units to each other through single-unit buffers. Screwdriver on balancer

Station bu ilt with erector set

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FIGURE 9-4.

Assembly oflaser printer component at Canon (1990s)

SOURCE: Courtesy of Canon, Kajo Kanri, Vol. 45, No. 7, 1999, Nikkan Kogyo Shimbun-sha.

The station itself is built out of plastic-coated metal tubes fastened with metal fittings. Unlike its ancestors in Figure 9-2 and Figure 9-3, it can easily be taken apart and reconfigured.

9.3. Assembly stations for large products There is a product size beyond which picking across an assembly station is no longer an option. This limit is not expressible as an exact number, but it is easy to see, for example, that VCR assembly is on one side of the limit and car engine assembly on the other. The part presentation challenges associated with large products are discussed in detail in chapter 11.

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Assembly stations for large products

With large products, it is also possible and frequently advisable to have assembly operations proceed on both sides of the conveyor, as shown in Figure 9-5.

FIGURE 9-5. SOURCE:

Engine assembly at Toyota

Toyota web site (www.toyota.co.jpNirtual_Factory/as/as.html).

In this case, the conveyor prevents assemblers who work at the same station from collaborating with or relieving each other. This conveyor is in fact as formidable a barrier as a freeway or railroad tracks can be in a city, and the assemblers on both sides are organized in different teams. The situation is different, however, when build saddles are used instead of a conveyor, as is done, for example, in the assembly of automatic transmissions. Build saddles are mounted on poles that are pulled forward by a drive mechanism located under the floor, as shown in Figure 9-6. These devices not only allow the assemblers to flexibly orient the unit, but also to circulate around it.

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Assembly station sizing

FIGURE 9-6.

The build saddle concept

With large products, presenting a particular assembly spot to the assembler is also a challenge. Sometimes, assembly work must even proceed underneath the product, as in Figure 9-7. Whenever possible, this should be avoided by providing fixtures that rotate the product to present the target assembly areas to the assemblers. This also happens, for example, when attaching wings to the fuselage of an airliner. The assembly of the wing itself is done with it held upright, but its connection with the fuselage can only be made in its final orientation, which requires assemblers to stand underneath the work.

FIGURE 9-7.

134

Working underneath the product at NUMMI

Lean Assembly

Assembly stations for large products

Another aspect of large product assembly on a moving line is that the assembler sees different sections of the unit as it proceeds through. As shown in Figure 9-8 for car assembly, the front of the car first appears in the rear of the station, giving the assembler access to the front of the car. Then the movement of the line places the midsection of the car in the middle of the station, and finally the rear section in the front of the station. The larger the product, the more pronounced this effect is. Some aerospace assembly operations involve mounting bracketry and cables onto a workpiece that may be fifty feet long. In this case, the primary reason for moving the unit past the station is to provide the assemblers with the opportunity to perform similar work on multiple sections.

Flow

Front parts

Center pa rts

FIGURE 9-8.

Access to multiple sections in large product assembly

Some assembly processes, on the other hand, consist of filling a frame with components bottom to top. This is the case, for example, in the assembly of semiconductor production equipment or large office copiers. The challenge then is to provide assemblers with a uniform work height.

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Assembly station sizing

One solution is to mount the units on scissorlifts that start high and are lowered from station to station as the unit moves forward.

9. 4. Ergonomics and safety Standing versus sitting One of the most controversial aspects of the lean approach to assembly stations is the requirement they be chair-free - that the assemblers stand rather than sit. It is already a common practice in some industries, such as automotive or food packing, but not in others, such as aerospace or defense electronics. Given the choice between standing and sitting most people prefer to sit. Observation of factories where assemblers have been sitting at the same stations for years shows, however, that the short-term comfort of sitting does not translate to long-term well-being but leads instead to weight and back problems. Doing assembly work sitting doesn't mean having the freedom to sit, but being constrained to have most of your body motionless for an entire shift, in positions such as those shown in Figure 9-9. This is different from the experience of office workers at desks, who are not subjected to the discipline of an assembly line and can, at their discretion, move, stand up, and walk to the water cooler.

FIGURE 9-9.

136

Sitting operations

Lean Assembly

Ergonomics and safety

From a strictly ergonomic standpoint, sitting all the time is in fact no better than standing all the time. What would be ergonomically optimal would be jobs with alternations of sitting and standing, which assembly operations unfortunately cannot be designed around.

In an environment in which assemblers need to be mobile and able to attend to more than one station when the line works at reduced speeds, sitting operations have the following drawbacks: • Chairs are api?Jsical obstacle. To move between stations, assemblers not only need to stand up and sit down again, but also to move around the chairs. A chair-free environment, by contrast, has no impediment to motion. • Chairs are P!Jchological anchors. Assemblers who sit at a station develop a sense of ownership of it and soon begin to personalize it by bringing cushions from home and decorating it with family pictures. Moving between stations then means having to work around pictures of other people's children. • Sitting reduces the visibility ofimbalances between stations. Balancing work content among assembly stations is much easier with the active help of assemblers than without it, and assemblers who are underutilized in standing operations are more likely to report it than in sitting operations, simply because sitting makes waiting more bearable. There are exceptions to the systematic preference for standing operation. When Toyota redesigned its final assembly lines in the 1990's, they introduced the raku-raku seats that are cantilevered inside car bodies to allow assemblers to perform jobs that previously required to crouch in uncomfortable positions (see Figure 9-10).

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Assembly station sizing

FIGURE 9-10. SOURCE:

Raku-raku seat at Toyota

Courtesy of Toyota Motor Corp.

In addition, there are specific processes that are commonly thought to require sitting. The main one is sewing, but there have been recent reports of factories with sewing operations performed standing. Soldering in electronics assembly has also been hotly debated. It is a sitting operation in most factories but has been done successfully standing, particularly in such high-volume applications as loudspeaker assembly. Standing does not mean standing on hard concrete. Rubber floor mats can be used to cushion assemblers' feet. The mats, however, should cover not only each station but the spaces in between, lest they become another territory marker discouraging assemblers from moving between stations. Human beings need a rounded mix of activities, which assembly line work rarely provides. Employees who work sitting need to stretch their legs; those who work standing, to sit down; those who do physical work, to apply their minds. To meet the needs of assemblers, who stand and do physical work, the plant needs to provide each team with a "green corner" near the line or cell to sit during breaks, hold start-of-shift meetings, or carry out small group improvement activities. Figure 9-11 shows an example of a green corner.

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Lean Assembly

Ergonomics and safety

FIGURE 9-11. SOURCE:

Green corner example

Courtesy ofKojo Kanri.

Green corners are rest areas on the shop floor for cell or assembly line teams, located as close to the work area as possible. Their intent is to provide assemblers who work standing with a place to sit during breaks without having to hike to a distant cafeteria. Green corners are not for assemblers who work sitting. Standing is almost never raised as a problem by assemblers when it is in place, but the transition from sitting to standing is difficult, especially for individuals whose bodies have adapted to sitting over many years.

Work height and assembler height Assembly is physical work, and it is legitimate to require physical conditions as qualifications for it. Just as firefighters must be able to carry people out of burning buildings, assemblers must have the motor skills and strength required for the jobs. Some may be better suited for arc-welding ship sections; others for soldering printed circuit boards. As long as the assignments are based on ability and not on gender, race, ethnicity, or any other group affiliation unrelated to the work content, this is not discrimination. There is no legal or moral requirement on the employer to make all jobs available to all employees regardless of physical ability.

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Assembly station sizing

It is, however, in the manufacturer's best interest to have the broadest possible labor pool available for each job. If, for example, a job requires heavy lifting, it not only excludes most women as well as many men, but results in fatigue-induced losses in productivity and quality even for those who can handle it. Redesigning the job to eliminate the lifting, or investing in lifting devices, both improves the job and makes it accessible to a much broader population. Doing exactly this is the purpose of the TVAL analysis discussed in chapter 7. Ergonomics requires the work height to be between hip and shoulder. But this range will obviously be different for someone who is five feet tall than it will be for someone who is seven feet tall. Adjustable station heights would solve this problem but create another, since work station heights need to be uniform along an assembly line to facilitate the flow of product units. Adjustable platforms would be compatible with a uniform station height, but they would also be an impediment to assembler movement. A better approach, sometimes feasible, is to provide adjustability in the fixture. In most cases, however, we must make do without adjustability, and the question then is what the station height should be. Factories draw their work forces from the population near them. The work height of assembly stations in the plant should therefore cater to the broadest possible range of adult members of that population. The makers of kitchen counters face the same challenge, and provide them at different heights in Sweden, Korea, or Mexico. The easiest approach for assembly stations is to borrow their solutions and design them at the same work heights as kitchen counters in the local area. A workforce of 500 assemblers will include tens of left handers who will struggle to follow processes defined for right handers. To the extent possible, assembly stations should be made ambidextrous.

9. 5. Stations with required dwell times Many assembly process include operations where parts must remain in one place, to cool off, until glue is dry, or until sealant is cured, for a period

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that exceeds the takt time. The challenge then is to design stations through which the units flow, but where they come out right next to where they go in, so as to allow one operator to handle both loading and unloading while possibly taking care of upstream and downstream stations as well. As shown in Figure 9-12, tunnel ovens support first-in-first-out but they keep assembly stations apart. A shuttle oven, on the other hand, keeps the stations close together but is a last-in-first-out device.

Out

~ X Tunnel oven separates X S huttle oven r un upstrea m and downstrea m st ations.

FIGURE 9-12.

last-in -first-out.

X Tunnel oven with return fl ow rack mainta ins first-in -fi rst-out and keeps upstrea m an d downstrea m stations contig uous.

Curing station

In addition, off-the-shelf ovens frequently have swing out doors that force their whole span to be kept empty (see Figure 9-13). Horizontal sliding doors don't interfere with the loading area but still force the oven apart from upstream and downstream stations. Only the vertical sliding door is free of interference. Valid alternatives include roll-up doors or even curtains if the environment conditions inside the oven allow it.

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X Swing-out door

interferes with next station and with loading.

FIGURE 9-13.

X Horizonta l sliding

door interferes wit h next station.

✓Vertical sliding door does not interfere.

Types of oven doors

Figure 9-14 shows an example from Korry Electronics where all of these issues were properly addressed.

FIGURE 9-14. SOURCE:

142

Oven station example

Courtesy of Korry Electronics.

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Detailed design of assembly stations

The ideal assemb/yfixtures let the assembler work with both hands, present allpoints of assemb/y in an ergonomical/y appropriate orientation, dock as needed in automatic equipment, double as a kitpalletfor product-specificparts, and ifpossible can hold more than one product. Fixtures far manual assemb/y travel through multiple stations, and need to return afterward to the first station at which thry are used, traveling above or below the assemb/y line. Fixtures far mechanized assemb/y holdparts with clamps orpneumatic rylinders and must be orientedfar ea!} loading and unloading. The machine must be preventedfrom starting while the operator} hands are inside, but the most common approach - dual finger switches - is aproductivity drag in short rycle jobs; therefore, equal/y safe but more productive alternatives need to be sought on a case-01-case basis. In-line mechanical automation is a alternative to offline robotics that is cheaper in high-volume applications and relies on jigs and locating pins to align components. Hand tools must be attached to the station and not to the assembler, positioned within arm} reach, orientedfar immediate use, and as much as possible fitted with a mechanism to return them automatical/y after use. On large products, "nagara"fixtures are used to keep tools aligned with movingproduct. Assemb/y instructions are provided in 11-inch 01 17-inch sheets posted at rye level at each station, rather than in binders. These sheets contain a drawing orphotograph, sur-

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rounded with blocks of text with sequences of operations, safety rules, quality checks, and other infarmation as needed. Digitalphotograpi?J andpage layout software are routine/y used to generate instruction sheets, and aJew companies use PDM (Product Documentation Management) software far revision control. Computer screens, however, are notyet a practical alternative to paper to display instructions at assemb/y stations. Instruction sheets are supplemented ?)I visible management, in the farm of the use of self explanatory devices, markings and labels, tower lights and andon boards, andproduction monitors.

10.1. Issues with assembly station details The assembly station is the point where materials, fixtures, tools, machines, gauges, methods, and assemblers converge to make one assembly operation happen.

10.2. Assembly fixtures Fixtures for manual assembly Now we shift our focus to the fixtures holding the product units on the assembly line. The purposes of these fixtures, while self-explanatory, are often forgotten or neglected. They are as follows: •

Allow assemblers to work with both hands.

•

Present the work zone to the assembler in ergonomically appropriate orientation.

•

Provide access to all required zones of the workpiece.

•

Dock into any automatic assembly equipment.

•

Double as a kit pallet where needed.

•

Support multiple products if needed and possible.

Manual assembly fixtures differ from machining fixtures in several ways:

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Assembly fixtures

•

They are not subjected to the forces of cutting tools.

•

Whereas machining fixtures mostly circulate between a rack and one machine, manual assembly fixtures often carry a unit of product through multiple stations.

•

They must be people-friendly and must not interfere with assembler movements and in particular be free of sharp edges and trip hazards.

Figure 10-1 shows an example of a fixture holding a product about thirty inches long and on which rotation around a single axis is sufficient to grant access to all assembly work zones. The fist version is a plain "rotisserie" fixture, but it needs a number of refinements and details to become fully functional and safe. Some thinking is needed, for example, regarding which casters swivel. The cart is easiest to steer if only the front casters swivel, but having all four casters swivel enables it to go around tighter corners. A foot brake is a good idea to keep the unit in position while working on it. Grips for steering from the back

Ant itrip bars (ca n a lso be used to steer t he cart)

~

/

Legs that impede operation movement and are a trip hazard

FIGURE 10-1.

An example of a pushcart fixture

In addition, rotation in a rotisserie fixture should never be completely free, since the workpiece must be held firmly in place while assembly is taking place. But it should not be allowed to be clamped in any random orientation either. For process consistency, it must only be allowed to lock into a finite set of positions, and there is more to the rotating mechanism than meets the eye. The effect of overlooking this kind of detail is shown in Figure 10-2. To mount brackets onto the I-beam, the welder has to assume an uncomfort-

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Detailed design of assembly stations

able and even painful position, which could be avoided simply by rotating the I-beam.

FIGURE 10-2.Welding station in non-rotating fixture

The most effective fixture is not always the most elaborate. In low-volume-high-mix computer assembly, for example, the simple Lazy Susan fixture of Figure 10-3 was all that was needed. For all models, the assembly process consists of populating a chassis from the top with such parts as power supplies, disk stacks, printed circuit boards, and plug-in components and connecting them with cables. The lazy Susan allows the assemblers to always have the point of assembly within easy reach.

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Assembly fixtures

Disks ~

Board';i( Cables~

~

G ~I

~ I ~ I

~

~I

I

Multiple model

FIGURE 10-3.

Lazy Susan for flexible computer assembly

It is often not possible for a single fixture to serve from the beginning to the end of the line, and product units typically have to be transferred between fixtures two or three times along the way. For each fixture used through multiple stations, a means must be provided for it to return from the last station to the first station where it is used. In a U-shaped assembly cell, light fixtures can be hand-carried from the last station to the first, and heavier fixtures moved on a cart. This wouldn't work, however, at takt times on the order of fifteen to thirty seconds. The example in Figure 10-4 is of an electronics assembly line running at a twenty-seconds. takt time and using two fixtures: one to present the top of the unit to the assembler, the other one to present the bottom. In this situation, not one but two fixtures must be returned to their starting points. One possible way to avoid this situation would be to design one single fixture able to hold the product in both positions. Short of doing this, we need two return paths. The first version of the design involved closing off two sublines with return flow racks, which violated the principle of keeping

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Detailed design of assembly stations

one point of entry or exit open at all times. To enter or leave the work area, the assemblers would have had to lift a section of the rack, hoping that it would be free of returning fixtures at that time. The more costly but inevitable alternative is to use overhead return paths. This is an issue that the line designer ignores at his or her peril. In the line shown in Figure 10-4, parts are not fed from behind the line, as they should be for products of that size. The assemblers face instead a partition wall. The reason for this is that the conveyor is shaped as a horizontal loop and the area behind the wall is used to return empty fixtures to the head of the line. Fixture return should be set up underneath or above the line, but not behind it, so as not to interfere with part flows.

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Assembly fixtures

Shelves for sma ll it ems

In

Out

-.Q.x.w 'W 'W 'W 'W

9 Subline 2 9~

'W 'W 'W -..Q;-.Q.x.w ~ bline1

'CY' 'CY' 'CY' 'CY'

[""'Ill 0

Manua l t ransfer~

~

'Ci' 'Ci'~~

I I Pa llet retu rn

First attempt Operator ent ry/ex it

9

~

o)

'-W '-W '-W

Single assembly line

. ~ rc5" rc5" rc5"

Overhead pa llet return Refinement FIGURE 10-4.

Fixture return to starting position in electronics assembly

Fixtures for mechanized or automated assembly Fixtures used for pressing together or welding parts do not travel, and they hold the parts in place with clamps or pneumatic cylinders. The station design issues with this type of fixtures are to make manual loading and unloading easy, fast, and safe. If, for example, a small cylindrical part is held in place by two perpendicular pneumatic cylinders, it will be easier to load and unload if the cylinders are at a 45-degree angle to the assembler station than if they face it straight on, as shown in Figure 10-5

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Detailed design of assembly stations

Air cy linder interferes with operat or act ion FIGURE 10-5.

Ea sier loading with f ixtu re rotated 45 °

Orientation of air cylinders

When a fixtures holds a part for an assembly operation done by a machine, the signal to start the machine must be given in a way that ensures the assembler's hands are out of the way first. The most common approach is to require two finger-switches to be activated at the same time. In short cycle jobs, however, the hand motions required to activate the finger switches can take 10% or 15% of the takt time and it makes sense to look for an equally safe but faster alternative. The key safety issue is to ensure that the assembler's hands are out of the machine's enclosure, not that they are in a specific place outside of it. If the machine has, or can be fitted with, a door with a limit switch that keeps the machine from starting unless it is closed, then the controller can be programmed to start the machine whenever the door closes. Doors are often replaced by light curtains, which can be used likewise to start the machines. After loading the part, the assembler pulls back his or her arms from the machine enclosure and closes the light curtain. This event can serve as a safe start signal, but in the United States, this method is often disapproved by the Occupational Safety and Health Administration (OSHA). These concepts are illustrated in Figure 10-6.

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Assembly fixtures

Assembly machine

Finger switches FIGURE 10-6.

Lim it switch

Safety light curta in

Methods for starting assembly machines

In-line mechanical automation In-line mechanical automation is a term used by T. Fujimoto in The Evolution of a Manufacturing System at Tqyota (pp. 229-234) to describe an approach that relies on jigs to automate repetitive assembly tasks on a moving assembly line. This is cheaper than taking the product offline and using robots and is applicable wherever the robots' flexibility is not needed. Robot-based assembly automation is mostly done offline. The product units are taken off the assembly lines and are assembled behind safety fences by sophisticated robots, guided by vision systems and able to control the application of force to a fraction of a pound. By contrast, in-line mechanical automation leaves the product on the assembly line and relies on jigs with locating pins that move with the line to line up parts properly for assembly. Figure 10-7 is based on an example published by Toyota in 1994 and reproduced by Fujimoto.

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Flow

FIGURE 10-7.

In-line mechanical automation

10.3. Handheld tools Common hand tools used in assembly include the following: • Pneumatic or electric screwdrivers and nut runners. • Wrenches and torque wrenches. • Fluid dispensers: cleanser, adhesive, sealant, or flux. • Soldering irons. • Pliers and crimping tools. • Cotton swabs and brushes. • Mallets and hammers.

Tools attached to the station and not to the assembler The first priority with hand tools is that they be attached to the station rather than assigned to the assembler. The main reason for this is the need

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for process consistency. If each assembler uses private tools, these is no way to ensure that the job will be done the same way every time. In addition, hand tools attached to the station can be tethered to power sources and electronic controls. Private tool boxes in sizes ranging from shoe boxes to wardrobes on casters are a common sight in assembly shops, primarily in the aerospace industry. On some car assembly lines, assemblers wear tool belts, which are less bulky than tool boxes, provide easier access to the tools, and are much less likely to become repositories of personal items unrelated to the work. From the point of view of process consistency, however, tool belts are no better than tool boxes, since they also carry private tools. The use of private tool boxes reflects a confusion between the needs of maintenance technicians and production operators. Maintenance technicians gear up for contingencies. When they open a machine, even for routine maintenance, they need to be ready for whatever they may find. In additions, the tools they use are not needed at each machine on a routine basis. It therefore makes sense for them to take a toolbox around. Production operators, on the other hand, execute the same routine with the same few tools over and over again at the same locations. Extraneous tools are not wanted, because they may interfere or be confused with the right ones. Assemblers should be able to approach an assembly station with nothing more than clothing and safety gear on their person, and find all the tools they need at the station (See Figure 10-8).

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Detailed design of assembly stations

X With t ools attached t o assemblers, process consist ency cannot be guaranteed.

FIGURE 10-s.

✓Attach too ls t o st ations, not assemblers.

Tools attached to stations versus assemblers

Tool positioning and orientation The location of hand tools is an area in which the assemblers themselves have the most expertise. They should always be consulted, and whenever possible, involved in making decisions about it. While assemblers are generally not that concerned about such issues as plantwide material flows, tool location affects their immediate daily work experience. Locating a tool means positioning it within arm's reach of the assembler - with the most frequently used tools the closest - and orienting it in such a way that the assembler does not have to turn the tool 180 degrees or even 90 degrees prior to using it. Screwdrivers used for vertical screws are pencil-shaped and hang vertically on balancers; those meant for horizontal screws, are gun shaped and hang horizontally on balancers.

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Handheld tools

Figure 10-9 shows an example of tool placement on a station for small product assembly

Doub le arm to prevent to rque on operator's hand

FIGURE 10-9.

Cable spool

Tool placement

For large products, particularly on moving conveyors, the challenge of attaching tools to stations is met by mounting the tools on moving or nagara fixtures. that follow the product through the station and automatically return to the starting position afterward. Figure 10-10 shows an example of such a fixture, nicknamed "spaceship," and following a moving cart through a station

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Detailed design of assembly stations

Counterweight

Product

FIGURE 10-10.

"Spaceship" moving tool fixture

Tools that are always used together should hang or be stored together. This includes, for example, dispensers for the two components of an epoxy glue, or an oil dispenser and the marker used to identify those units that have been filled with oil. To accommodate the minority of left handers, workstations should be made ambidextrous whenever possible, for example, by putting balancers on arms that each assembler can move to the left or the right. Holders should be tool-specific to prevent mistakes but should not hold the tools too snugly, so that assemblers do not have to pull hard to remove them. To minimize the damage done when assemblers accidentally drop tools, the heaviest tools should be stored at the bottom and the lightest ones at the top.

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Assembly instructions

To support the assemblers, the tools should be kept out in the open where they can be seen and retrieved easily. The idea of tools out in the open, however, raises two main objections:

• There are too ma1!J tools to fit within the space available. In reality, there should not be so many as to require the capacity of a stack of drawers. The job at a station must be engineered not to require a huge array of hand tools. • The tools will 'walk away' if not kept under lock and kry. Tool theft is usually not as widespread as initially feared. In assembly cells, theft is discouraged by the vigilance of the cell team, the single point of entry and exit to the cell, and markings on tools identifying them as belonging to the cell. Tools disappear from a station much more frequently because someone borrows and forgets to return them than as a result of intentional theft. Engraving the company name on the tools discourages assemblers from taking them home; engraving the name of the line and station, from taking them to other lines or stations.

10.4. Assembly instructions Instruction sheets What is the purpose of assembly instructions? If they are on a shelf in a binder that has been gathering dust for two years, their content is most likely obsolete and their only purpose can be to show outsiders that they exist. A telltale sign that these Victorian novels are not being used by assemblers is that the workplace only has furniture to hold them closed. There would otherwise be some form of music stand by the assembler to hold them open. An alternative format for instructions is an 11-inch by 17inch sheet of paper at eye level above the station. The two approaches are contrasted in Figure 10-11.

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Detailed design of assembly stations

Red tape across binders ensures they are all securely in place.

a. Instruct ions sheet at eye level FIGURE 10-11.

b. Binders for externa l aud itors

Two approaches to assembler instructions

The content of the instruction sheets varies. The easiest approach is to start by considering that 11 inches by 17 inches is the area available to communicate with the assembler and then to populate that area with the most useful information possible. The instructions in binders are often extraordinarily verbose. Once stripped of boilerplate elements and lengthy explanations that would only be useful to a beginner - but that no beginner would have the patience to read - their true information content usually fits in the 11-inch by 17-inch sheet with room to spare.

Contents of instruction sheets for manual assembly In manual assembly, a common pattern is to show, in landscape mode, a drawing or a photograph of the product, with parts assembled at the station highlighted in one color and parts assembled at the previous station in another. Around the picture are callouts or blocks of text including the following: • Sequences of operations. • Key quality issues and quality claims traced back to the station. • Housekeeping instructions. • Required safety precautions.

158

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Assembly instructions

These sheets are supposed to be revised every few months, and one text block, usually in the bottom right-hand corner, carries the revision level and approval signatures. Figure 10-12 shows this concept in a simplified example. Parts checked by touch ing

Parts assembled

Houeekeepi~ Ghift :,tm:t:· Clean toole>

Gbifteod

Assembly steps

Revision

t,,,,: 1 ,-F-,c-k-bra 'i - ck_et _ e,_ ~15 - ,.e-c~----t Level: 15

Return too ls> @Mount brad::etg 2 1 MC 1',ppr:Jved: to board t - = - - - - - + - - - - + - - - - t Date lf' 0/02

FIGURE 10-12.

Instruction sheet concept

Some consumer goods, especially toys, can be a source of ideas on how to generate assembly instructions. Figure 10-13 covers steps 19 and 20 from a 38-step assembly process to be carried out by an eight-year old child without special training. The instructions are entirely graphic. Not a word of text is involved, but the drawing follows many self-explanatory conventions: • Red arrows indicate how assembly is done. • The assembly as of the end of step 18 is shown in dimmed colors. • The assembly as of the end of step 20 is shown in a separate drawing to the right.

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Detailed design of assembly stations

This drawing is of course simpler than those needed in most manufacturing situations, because it involves a small set of common parts and no tools or fixtures. Parts from the previous station are highlighted for successive inspection: each assembler checks the work of the previous one by touching each of these parts. The text in these instruction sheets is limited to simple imperative sentences, in the plainest language possible. If the workforce is multilingual or of limited literacy, pictograms and cartoons are used as much as possible to ensure communication, as, in the "Safety" block of Figure 10-12. The original purpose of this type of instruction sheets when Taiichi Ohno introduced their ancestors at Toyota, was to allow supervisors to easily verify that assembly operations were performed correctly and consistently. During normal operations, you do not see assemblers checking the instruction sheets, but if they deviate from it, the supervisor can.

FIGURE 10-13.

160

Assembly instructions for K'nex Cybots

Lean Assembly

Assembly instructions

If an assembler's deviation concerns the process itself, for example using the wrong screw or applying more sealant than called for, then, the supervisor's intervention must be to enforce the standard. If, on the other hand, the deviation is about methods, then it may be that the assembler has found a better way to perform the process. In this case, the better way should become the new standard and be documented in an update of the instruction sheet.

Content of instruction sheets for mechanized assembly If the assembly work is performed by machines, the assembler's task is to load, unload, and start the machines; perform quality checks on the units; and move them between machines. The focus of the job design is then to avoid keeping the assembler waiting for a machine or a machine waiting for the assembler. The picture of the product in the instruction sheets is then replaced by a work combination chart. Figure 10-14 shows such a chart in an instruction sheet for a job involving stacking three flat parts for simultaneous pressing of the left-hand and right-hand assembly in a press. The key points are to minimize the number of trips the assembler takes between the left and the right sides, and to take advantage of the automatic press cycle to do other manual work, such as visual inspection of the previous pair of parts. The supervisor seeing both the assembler at work and this chart displayed at the station can verify that the assembler does the work in the prescribed sequence and in the time allocated for it. Again, if one assembler finds a better way to sequence the work, then it should make its way into an update of the chart to become the new standard.

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Detailed design of assembly stations

l Manual work /VVVVVV

Walk\ Automatic cycle c::==:::::J Waiting -

7 7 3

@ Pick rig ht stack @ Start press @ Inspect previous pair

10

20

8 3 Total (seconds) 38

20

@ Unload press @ Stack new pair at output

FIGURE 10-14.

rH=BJJ

Manual In Out Mach.Walko

CD Pick left stack

3 1 3 1 1 4 12

10

20

Time in seconds

30

40

Takt tim e 50

Work combination chart for mechanized assembly

Instruction sheets for mixed-flow assembly Instruction sheets are a special challenge in lines that make more than one product at a time. If the line is flushed and changed over between products, then instruction sheets for each product can be spiral-bound, and flipping to the right one can be part of the changeover procedure. On the other hand, true mixed-flow assembly involves leveled sequencing, and the product may therefore change with every unit. In this case, the assemblers don't have the time to flip a book of instruction sheets. Only one instruction sheet is used, and its focus is on information that is relevant to al/the products in the mix. Product-specific information then travels with the product units, either explicitly through such devices as the build manifests (see Figure 15-3 on page 230) that carry options information, or implicitly through self-identification devices such as RFID (RadioFrequency ID) tags.

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Assembly instructions

Authoring instruction sheets It is a challenge in many manufacturing organizations to find authors for instruction sheets. The required qualifications are as follows:

• They must be proficient in writing and enjoy it. This excludes most production assemblers. • They must be able to express themselves in a language that assemblers understand, which excludes most engineers. In many organizations, technicians who have come up from the shop floor into the engineering offices and meet these requirements.

Use of information technology Computer technology helps generate instruction sheets and manage the revision and approval process. The value of digital cameras and page layout software is immediately obvious: it is cheap, and its use is on an individual basis - that is, it doesn't require close group collaboration. One author may draft an instruction sheet, and others review it, but that can be done in hard copy. The only computer skill required is for each author to use the software individually. The product documentation management (PDJ\1) software that can be use for revision control is a different story. It has to be used by a community of people, following such disciplines as not modifying a document unless they have checked it out of the system, promptly reviewing the updates submitted for their approval, and posting on the floor the revisions that have been approved. In addition, when such a piece of software is implemented, the user organization needs a way to convert and integrate all the preexisting documentation. For all these reasons, even though it is clearly useful, few manufacturing organizations are willing to commit to its implementation based on internal needs. The spread of this type of software is mostly driven by external mandates like ISO-900x certification. Some companies have also tried to replace hard copy instruction sheets with computer screens, which automatically display the last approved ver-

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Detailed design of assembly stations

sion of the instructions and can instantly change for different products. This practice, however, is not widespread, for the following reasons:

• Cost. The cost of the computer network needed to provide online instruction sheets at hundreds of locations is difficult to justify. • Alternative uses of technology. Investments in assembly technology gravitate more toward the full or partial automation of the process itself rather than the online display of information to support manual assembly. • Bulk. Compared to paper, computer monitors are bulky, and finding a place for them in an otherwise crowded assembly station is a challenge. Flat panel displays that are sufficiently large - that is more than 19 inches wide - are still prohibitively expensive as of 2002 and have a narrow viewing angle. • Human interaction. Unlike paper, computer screens are still an intimidating medium for many assemblers.

10. 5. Visible management Self-explanatory devices, markings, and color codes By complementing the instructions, markings and the use of self-explanatory devices at the assembly station make the instruction sheets easier to generate and reduce the time and effort needed to train new assemblers as well as the frequency of human error. A powered screwdriver hanging on a balancer is a self-explanatory device: an assembler who has never seen one immediately understands how to make it work and notices that it returns to its starting position automatically when released. A DC nutrunner is slightly more complex, in that a green light comes on when the required torque is reached, but that is also self-explanatory for most people, because the use of a green light is consistent with cultural conventions. While self-explanatory devices are obviously preferred, there is information that can be conveyed only by explicit markings. Unlike tools that are

164

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tethered to a location, many devices like jars, tweezers, and spray cans can be located in a number of different places. But consistency across multiple assemblers on multiple shifts requires that they have an assigned location and that the discipline of always returning them to that location after use is enforced. Location markings, shadows on shadow boards, and labels are indispensable for this purpose (see Figure 10-15).

Assigned location --allllo.!.

Labels

FIGURE 10-15. SOURCE:

Assigned locations and labels

Courtesy of Karry Electronics Co.

Sometimes, special stickers are used to identify a category of objects, such as mistake-proofing devices with "ZD," or "Zero-Defect" stickers. Scrap bins need to be provided wherever parts are checked and may be scrapped. Scrap bins must all be the same color, usually red, and easily recognizable. These bins need to be frequently checked by the quality department (see Figure 10-16). The use of markings looks easier than it is. Taping and labeling an area at a station may be easy to do, but it is also easy to ignore. Before doing it, an engineer must make sure that the assemblers will consent to follow it and that the supervisor will support it. The line organization must be committed not only to follow it but to maintain it so that it stays current with

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Detailed design of assembly stations

changes made to the operation. Otherwise it becomes a management directive that is routinely ignored and thereby undermines the credibility of all other directives.

~J I

Sticker identifying can as mistake-proof device

• FIGURE 10-16.

Red bin for defectives and ZD sticker

Tower lights, stop ropes, and other types of andons Tower lights are now found in many factories but are mostly misused for lack of a clear understanding of their purpose. When multiple small assembly lines or cells are operating in parallel, one tower light for each cell, placed above the last station along the transportation aisle, allows managers and supervisors at a glance to see which lines are operating, which are idle, and which are down. Since an assembly line is either on or off as a unit, tower lights are not needed at every station.

FIGURE 10-17.

166

Effective use of tower lights

Lean Assembly

Visible management

Green, yellow and red tower lights are a simple and effective communications tool, which suppliers and users are conspiring to make complicated and unintelligible by adding more colors of lights, allowing combinations of lights with slow blinks, fast blinks, and no-blink patterns, and assigning to these combinations meanings that vary with each station (see Figure 10-18). Red = Down

Red= Down

Yellow = Idle

Yellow = Idle

Green = Runn ing

Green = Running Bl ue = ? White = ? Solid white wit h blinking blue =? Fast bl inki ng yellow with slow blinking yellow= ?

X

S im ple and effective

FIGURE 10-18.

✓i nco mprehens i b l e

Good and bad designs for tower lights

The rules for proper usage are as follows:

• Green, yellow, and red on/y. The same three colors used in traffic lights should be the only ones allowed. There should be no white or blue light. • One and on/y one color solid/y lit at a time. Blinking and combinations of lights are not only unnecessary, but harmful since no one in management is able to tell the meaning of "solid white with blinking blue." • Consistent meaning. The meaning of the colors must be consistent throughout the plant as well as with cultural conventions. Green may mean the line is running, yellow that it is idle, and red that it is down. If that is the convention, it must apply to every tower light in the plant. Green must not be used to indicate that a line is down or stopped, as this would be inconsistent with the cultural conventions established by traffic signals. More elaborate communications require different devices. Large, moving assembly lines are fitted with stop ropes that allow assemblers at any station and anywhere within that station to stop the line. Pulling the rope is

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Detailed design of assembly stations

effective at keeping the line from producing defective units but by itself, does not help bring the line back up. For this purpose, the maintenance department needs to know on which of the possibly 200 stations on the line to intervene. This is done by a combination of visible and audible devices, as shown in Figure 10-19. The visible part is a panel displayed in a central location with lights showing the section of the line the stoppage originated in and the status of the response. The audible part is a section-specific tune played on the public address system. For example, the William Tell overture may point to a particular section of the trim line, and the Blue Danube to powertrain. Stop rope stops line, Start s section -specif ic music, light s Andon board

1A

2B

3

4

7

8

9

10

16

17

18

19

QC 10

11

12

13 I

QC

Extention for short operators FIGURE 10-19. SOURCE:

Blinking amber for department notif ied

15

MT

Sol id amber for action in progress

Stop ropes and andon boards

Courtesy of Toyota Motor Corp.

Counters and production monitors While moving conveyors enforce the takt time, they are not present at every assembly operation. Where they are not present, the assemblers need other means of knowing whether they are working too slow, too fast, or at just the right pace. Production monitors such as the ones shown in Figure 10-20 serve this purpose in many assembly cells and short lines.

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Lean Assembly

Visible management

15 -inch mon itor over assembly line -133 l'roducr.;

Goal:

Shift plan Plan upto now

Actual:

Actual

Difference:

Difference

a. Production mon it or commercially ava ilable in Japan (Model II DE4300/600 fro m Heretu K.K.) FIGURE 10-20. SOURCE:

120 120 10

b. Production mon itor int erna lly developed with obsolete PCs

Production monitors

Reprinted courtesy of Korry Electronics Co.

The displays are located above the last station of the line, and typically show cumulative numbers from the start of the shift, including how many units should have been completed so far, how many actually have been, and the difference between the two. One key engineering issue is how the production monitor is told that one more unit of product has been completed. Asking the assembler to press a button is simple but adds work and is error-prone. Arranging for finished units to pass by a photo-eye on the way out is a better way. Monitors such as the one from Herutu shown in Figure 10-20a have been available in Japan for more than ten years, and are now available from several U.S. vendors under names like "industrial scoreboard," "production scoreboard," or "production pace timers." Rather than using wireless communication, many are connected by a wired network to a programmable logic controller (PLC). Figure 10-206 shows an alternative implemented in-house at Korry Electronics, a manufacturer of cockpit switches in Seattle, Washington. Their first attempt at production monitoring was a white board manually

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Detailed design of assembly stations

updated every 30 minutes, which failed to keep the cell running at the right pace. The Korry engineers then recovered surplus i486 computers and 15-inch monitors that the company was about to throw away and programmed them into the production monitors shown here, with daily summaries for the supervisors. These have been successful, as evidenced by daily summaries that show actual production hugging the plan curve closely.

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Lean Assembly

CHAPTER 11

Part presentation

The way parts are presented to operators impacts both productivity and quality. Assembler time is expensive because operators work in series on the line, but materials handlers, on the other hand, work in parallel. The materialsgroup is the pit crew to the assembler} race car driver, and shouldfacus first on providing effective support to assemb!J and second on doing it efficient!J. Regardless of what managers may believe about "non-value added" activities, the number ofpeople handling and preparingparts is often increased in order to reduce the total amount of labor spent on assemb!J. Parts should be presented to assemblers unpacked, within arm} reach, with their smallest dimensionsfacing out, and orientedfar ea.ry installation. Deliveries to the line should be at fixed intervals, in quantities matching the consumption rate, and in returnable containers with dividers orpart-specific dunnage, to facilitate counting and prevent errors. The picking strategies of kitting and lineside supp!J should not be viewed as mutual!J exclusive but used in combination. When used, kitting should be far single product units. It should be done just before assemb!J, and into trays with kit-specific dunnage. Single-piece presentation and water spiders are further techniques used in particular to allow assemblers of large products to pick parts without turning around in "washing machine" movements.

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Part presentation

11.1. Scope and purpose Part presentation requirements This chapter is about the activities that take place behind or in front of an assembly station and make parts available for operators to pick and assemble, as shown in Figure 11-1.

The supply chain

Parts delivered in front of operator -,i,---1+--+--l-l► Product

'-OJ

Assembly station for small products

Product f low

f low Parts delivered beh ind the operator

--+::::::::::1:::::::::_!! -:_-:_-:_-:_-:_-:_-:_-:_+-•►► Assembly station for large products

FIGURE 11-1.

The context of part presentation

Part presentation is the last time each part is handled before being mounted on the product, and management often underestimates its impact on productivity and quality. Present the parts too far from the assembly station and in the wrong orientation, and you add seconds of walking and handling time to the assembler's job that constrict the capacity of the entire assembly line and needlessly increase fatigue at the end of the

172

Lean Assembly

Scope and purpose

shift. Make the parts easy to confuse and they will be, resulting in defective products. Assembly line work has special dynamics that are not found in many human activities. By doing a thorough job of balancing the work among stations, we have in fact made every operator a bottleneck. Let us assume we have a balanced final assembly line for cars, using 500 operators to make 500 cars per shift and we add one second to the job of one operator. Then all other 499 operators must wait one second for each of the 500 cars they are assembling in a shift. The added labor time is therefore: 1 sec x 500 operators x 500 cars= 4,167 min> 10 operator-shifts

Adding one second to the job of one operator costs more than ten operator-shifts per day. Assembler time is expensive because operators work in series on the line. The same logic, however, does not apply to materials handlers, because they work in parallel, and adding one second to the job of one materials handler has no impact on the others. Another way to think about this is shown in Figure 11-2. The assembler is the race car driver responsible for getting the product to the finish line. The members of the materials supply organization should perceive themselves as the pit crew supporting the assemblers.

The assembler FIGURE 11-2.

Th e part supply pit crew

The assembler as race car driver

Because the assemblers' time is so extraordinarily valuable, we want to relieve them of any work that is not directly assembly, and that can be done by someone who works in parallel with the assemblers. This is why

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Part presentation

we must be adamant about supplying parts to assembly unpacked, within arm's reach, and properly oriented. Conversely, for transporters and materials handlers, the quality of service they provide to assembly is much more important than the efficiency with which they provide it. The way it works out in practice is illustrated in Figure 11-3, which shows the net effect of adding one picker. The numbers in Figure 11-3 are actually conservative. In an actual example, we have seen two assembly lines of five assemblers each go down to three assemblers each with one full-time picker for both, bringing the total number of operators down from ten to seven, or a 30% net productivity gain. Before

10 operators assembling 75% of the takt time and spending the remaini ng 25% in 11 11 washing mach ine movements to pick parts.

_l_ 5ft

T

After

Wide flow rack used to del iver kits of matching parts Narrow fl ow ra ck used to del iver sing le units

FIGURE 11-3.

174

8 operators+ 1ful l time picker. Operators assembly 95% and pick 5% of the takt time. Picking operations are designed to need less picker time per part than when assemblers pick.

Net effect of adding a picker

Lean Assembly

Scope and purpose

In Figure 11-3, the product is too large for parts to be picked across the conveyor. The changes made between the before and the after diagrams of Figure 11-3 are as follows:

• Parts are fed down narrower flow racks, just wide enough for one unit of each item, pointed directly at a workstation. Since the same part is always delivered at the exact same location, operators can pick them without turning around in the "washing machine" movement shown in the before picture. • Where appropriate, the picker organizes parts into kits on kit pallets that flow down larger racks right up to the assembly station. The runway separating the assembly stations from the picking face in the upper part has been eliminated by attention to details in assembler job design, and no operator needs to follow product units down the line to finish his or her job. • With narrower racks, it has also been possible to reduce the station width from five to three feet, supporting a higher work density.

Controversies about part presentation Most manufacturing managers are unreceptive to this message. They perceive all materials handling work to be non-value added, and reinforcing that function is the last thing on their minds. Operators pushing carts of parts or transferring parts between containers are typically viewed as doing non-value added work, because they do not transform the parts in anyway that would contribute to what customers want. In daily life, however, we do not hesitate to pay higher prices for retail than for wholesale, and by doing so, we recognize the retail markup as the value added to products by moving them and transferring them from pallets and crates to display cases and shelves, as shown in Figure 11-4. It is inconsistent to recognize that this work adds value in the grocery store while denying it on the shop floor.

In most plants, the materials manager is measured more on the efficiency of the department as a standalone unit than on its effectiveness in support of production. Changing this around is one of the key motivations for

Lean Assembly

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Part presentation

product alignment in the organization structure. In a lean production organization a product line manager has authority over not only the production line itself but its support structure as well.

---~~-~---~---~~:---------~ ~~le

FIGURE 11-4.

~✓ "~v

~

m

~~

The value added by preparing parts for use

11.2. Key principles ofpart presentation Removal of packaging materials before delivery Many parts arrive at the assembly plant protected by packaging that needs to be removed prior to installing the part in a product. Somewhere between the receiving dock and the assembly line, this work must be done. There are many possible ways to organize it, which we will discuss in the section on logistics. Disposal of the packaging materials is easiest if it is done at the receiving dock, but containers have to be provided to hold the parts from there to the line side. On the other hand, if it is done just behind the line, the collection of used packaging materials needs to be organized plantwide. The one person who should not be unpacking, however, is the assembler.

176

Lean Assembly

Key principles of part presentation

While the assembler fusses with packaging materials, the product waits, and it shouldn't. Asking the assembler to do this is like asking a race car driver to change tires or pump gas. Regardless of where and how it is accomplished, when parts are presented to assemblers on the line, they should be in such a condition that they can be picked up and installed in the product directly. Finding the way to do this is sometimes challenging with parts that are easily damaged, such as windshields in cars or electronic components. Brittle glass parts or scratch-prone painted parts need foam-padded racks and separators. Printed circuit boards and integrated circuits need special dunnage for protection against electrostatic discharge (ESD), or need to be kept in a Faraday cage. These frequently cited reasons for keeping parts packaged until used can be overcome in two ways: • By using special containers or bins designed specifically for this purpose. These are known problems, to which many suppliers offer solutions. Figure 11-5 shows the example of a printed circuit board.

X

No! Requires assembler t o unpack.

FIGURE 11-5.

✓ Yes! Pick and assemble.

Printed circuit boards with ESD protection

• By limiting the exposure time. Parts need to be out of their protective packages when assembled anyway, and our only constraint is that the assembler not be burdened with unpacking. If they are unpacked just behind the line a few minutes before they are used, the opportunity for damage is limited in space and time and is essentially the same as if the

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Part presentation

assembler did the unpacking. Figure 11-6 shows an example of how this might be organized in some cases.

µ~~

Packagirgi

□ materials Cfer;;itor

attendsto mJtiple stations

t

line

ti 0

Flcwr;;ick

1o□ aEno1 □

hseml:iy

Unpacl

~hsembler

/assembes

parts

Parts in packages

FIGURE 11-6.

Separation of unpacking from assembly

Location within arm's reach of the assembler What many companies call point-of-use storage is in fact tens of feet away from assembly stations and requires operators to walk back and forth. In others, parts are presented as in the before picture of Figure 11-3, just far enough to require the operator to turn around for picking in the washing maching movement. Assemblers then naturally try to make the most of each turn by picking several items each time, with which they then do one of the following: • Keep them in one hand while assembling with the other. This slows them down, as they should be able to assemble with both hands. • Place them in their pockets or on the assembly station, which results in multiple handling. To avoid this, we must find a way to present parts within arm's reach of the assembler. The following methods can be used for this purpose: • Have parts move down a flow rack to the station, as shown in Figure 11-7.

178

Lean Assembly

Key principles of part presentation

Bins roll down by gravity

FIGURE 11-7.

Gravity flow racks

• Make assembly fixtures double as kit pallets and have some parts travel along with the product units down the assembly line. • Use a moving parts tray (see Figure 11-8). This approach is best for small parts going into large products.

P arl:€> mounwd on assemb ly

fixture

fruck chassis

(upside down) FIGURE 11-s.

Techniques to keep parts within arm's reach

Orientation The attention warranted by the orientation of a part depends on its size, shape, weight, and frequency of use. Following are a few extreme examples of parts with different orientation requirements: • The balls that go into computer mice or bearings are orientation-free. More generally, the more symmetries a part has, the less of an issue orientation is. • For screws and other fasteners, volume is the key factor. They are most commonly provided randomly oriented in bins, but in high-volume

Lean Assembly

179

Part presentation

operations, automatic feeders are used to deliver them in a controlled orientation. • Airliner bulkheads are tens of feet long and wide, but less than one foot thick and weigh thousands of pounds. Even at takt times measured in days, you want to present them to assembly in such a way that you just have to slide them in. Orientation plays a role in both picking and installing a part, and the objectives for both these activities are not necessarily in harmony. In picking, you want to maximize the number of different items available to the assembler. To achieve this, you orient each part so that its smallest dimensions appear on the picking face. To make it easy to install, on the other hand, you present it so as to minimize the required motion by the operator. This dilemma is illustrated in Figure 11-9.

Orientat ion for picking FIGURE 11-9.

Ori ent ation fo r inst alling

The orientation dilemma

Which objective prevails depends on the specifics of each station. If the number of items needed allows it, then you present parts can be presented in the orientation that is best suited for installing. Orientation priority may also be given to installing for the most frequently used parts and to picking for the least frequently used. Factors other than just operator time may also have an impact. For example, rotating long and narrow parts increases space requirements and may be a safety hazard.

180

Lean Assembly

Key principles of part presentation

Adjustments to specific part characteristics Presentation methods must always be adapted to the physical characteristics of the parts. In the orientation section, we have already discussed the case of long and narrow parts. Other categories needing special treatment include flexible parts like wire harnesses and tangle-prone parts like springs,as shown in Figure 11-10.

Swiveling bumper carrier FIGURE 11-10.

Wire ha rness ca rrouse l

Special presentation devices for large parts

Pallets are in general inappropriate on the line side, because picking from the near corner on a full pallet does not take the same amount of time as walking to the far corner, squatting, and picking from a near-empty pallet. The problem of picking from pallets can be alleviated by using "pallet pals," (See Figure 11-11) rotating tables mounted on a scissorlift with springs. Pallet pals allow operators to turn the pallet to the current picking position, and the springs push the pallet pal higher as the load becomes lighter.

FIGURE 11-11.

Southworth's PalletPal®

These are an adequate solution when, for example, one pallet load barely represents thirty minutes of production on items that are easy to carry.

Lean Assembly

181

Part presentation

This situation occurs in cookware assembly. Tangle-prone parts like small springs can be fed down tubes or attached to tacky boards.

Matching quantities In many plants, the materials organization delivers items to the line in quantities that occupy the same amount of space rather than quantities that will support assembly for the same amount of time. Whether it is because the materials uses forklifts, and drivers won't waste their time on partial pallets, or because the automatic storage and retrieval system (AS/ RS) is not programmed to handle partial pallets, the result is that full pallets arrive at the line side, whether one pallet will cover demand for thirty minutes or a week. If one unit of item X has the volume of five units of item Y, then item Xis delivered in quantities that are one-fifth of those of item Y, regardless of the numbers consumed. The main consequences are as follows: • Without knowing the consumption rates of hundreds of items, a supervisor or an operator cannot see whether a half-empty rack signals an imminent shortage. • The different replenishment periods for hundreds or thousands of items combine into a workload for transporters that is characterized by random peaks separated by intervals of idleness, as shown in Figure 11-12.

182

Lean Assembly

Key principles of part presentation

Line s ide quantities

I

I I

I I I

I

I I ~

I

"' FIGURE 11-12.

De livery schedule

Time

Random replenishment schedule

The variability in the aggregate delivery schedule in Figure 11-12 is selfinflicted, because it occurs in spite of constant consumption rates for all items. Its cause is the policy of using forklifts to bring full, single-item pallets to the line. By forcing the materials orgainization to cope with artificial peaks and valleys of activity, this policy works against its own goal of efficient use of transportation resources. The quantities delivered to the line should instead be based on consumption rates and calculated to support the line for the same amount of time, or occasionally for small multiples of this time. This avoids imbalances in line-side supplies and allows the line to be supported by regular, periodic deliveries of multiple items in matching quantities. This is called the milk run system. The delivery vehicles used in implementing that policy tend to be pushcarts or trains of towcarts rather than forklifts. There are items, typically nuts, bolts, washers, or screws, for which one small bin is sufficient to support the line through multiple delivery periods.

Lean Assembly

183

Part presentation

But the delivery period should be set so that all major components are delivered in matching quantities each time.

Containers with dunnage for counting On many assembly lines, supervisors are first aware of shortages when they actually occur. There are many causes for this, some of which are related to part presentation. Mismatched delivery quantities, as discussed in the previous section, is one of them, but lack of part count visibility on the line side are another. Shortages are intolerable for obvious reasons, but overages are also undesirable because excess parts require attention. When parts are heaped in bins or in hoppers, it is practically impossible to know how many are there. When spot-checking part count on assembly lines, it is common to find that a box supposed to hold ten parts actually holds nine or twelve.

Divider box makes part count visible FIGURE 11-13.

It em- specif ic dunnage prevents mi stakes

Divider boxes and item-specific dunnage\

Item specific dunnage offers cut-outs in the shape of the part, which prevents accidentally loading the bin with the wrong item.

184

Lean Assembly

Key principles of part presentation

Kitting versus line-side supply I