A Primer on the Taguchi Method

A PRIMER ON THE TAGUCHI METHOD SECOND EDITION Ranjit K. Roy Copyright © 2010 Society of Manufacturing Engineers 987654321 All rights reserved, including those of translation. This book, or parts thereof, may not be reproduced by any means, including photocopying, recording or microfilming, or by any information storage and retrieval system, without permission in writing of the copyright owners. No liability is assumed by the publisher with respect to use of information contained herein. While every precaution has been taken in the preparation of this book, the publisher assumes no responsibility for errors or omissions. Publication of any data in this book does not constitute a recommendation or endorsement of any patent, proprietary right, or product that may be involved. Library of Congress Control Number: 2009942461 International Standard Book Number: 0-87263-864-2, ISBN 13: 978-0-87263-864-8 Additional copies may be obtained by contacting: Society of Manufacturing Engineers Customer Service One SME Drive, P.O. Box 930 Dearborn, Michigan 48121 1-800-733-4763 www.sme.org/store SME staff who participated in producing this book: Kris Nasiatka, Manager, Certification, Books & Video Ellen J. Kehoe, Senior Editor Rosemary Csizmadia, Senior Production Editor Frances Kania, Production Assistant Printed in the United States of America Preface My exposure to the Taguchi methods began in the early 1980s when I was employed with General Motors Corporation at its Technical Center in Warren, Mich. At that time, manufacturing industries as a whole in the Western world, in particular the automotive industry, were starving for practical techniques to improve quality and reliability. Having perfected his concepts in Japan and a few places elsewhere during the late 1940s, Dr. Taguchi introduced his quality improvement methods to the United States in the early 80s. His focus was to optimize performance and make design robust by use of the statistical technique known as design of experiments (DOE), which was originally introduced by R.A. Fisher in England in the 1920s. To make the technique more effective and easy to use, Dr. Ta- guchi recommended a standardized version of DOE and devised ways for practical application and analysis of results. For quality improvement champions, this was an attractive tool. Manufac- turing organizations of all kinds readily learned and applied the methods to benefit the design of numerous products and processes. Remarkable progress was made by many throughout the 1980s. For automotive manufacturers, the process of catching up in quality with foreign manufacturers was launched. Those of us who were actively promoting the quality improvement effort within our organizations and had the responsibilities to implement the technique were challenged to find better ways to teach and apply the technique. In the early 80s, there were only a handful of books and hardly any documented application examples. The primary reference in the market was Dr. Taguchi’s ix x A Primer on the Taguchi Method own book, System of Experimental Design (Quality Resources, 1987). That book and related training materials were my first resources to explain the Taguchi technique to others in a simpler form. A credit to the effectiveness and acceptance of the Taguchi technique is that there now are more than a dozen textbooks and thousands of published reports available to practitioners. Following the overwhelming surge of interest in learning and implementing the technique by manufacturing companies of all kinds, the focus shifted in the mid 1990s. Introduction of general and company-wide quality improvement disciplines such as ISO/ QS-9000 and Six Sigma unintentionally diminished the priority and funding for activities that required special techniques. These general quality systems, which were value-added and beneficial to most businesses, also required use of statistical techniques like DOE, Taguchi methods, statistical process control (SPC), and so on, but for most companies these were among the last few things to do and easily postponed. The turn of the 21st century saw the beginning of a downturn in the economy. Auto companies were downsizing; most didn’t have much money to spend on training as business survival over- shadowed quality improvement concerns. Now as the economy is starting to show signs of recovery, manufacturing companies can refocus on implementing statistical techniques. While teaching, training, and practicing in the 1990s, I found greater demand for a reference on the application of the technique rather than on the theory. Thus, the first edition of A Primer on the Taguchi Method (Society of Manufacturing Engineers, 1990) introduced basic concepts through application examples and case studies. This led to my second book, Design of Experiments Using the Taguchi Approach (John Wiley & Sons, 2001), which covered a minimal amount of theory while describing many more applica- tions in detail. Both books were received favorably by readers in academia, business, and industry. HIGHLIGHTS OF THE SECOND EDITION The following are highlights of additions and changes in this second edition of the Primer: Preface xi • Chapters 1–4: Minor changes in clarity and in the defini- tions of quality • Chapter 5: Taguchi robust design strategy and the two-step optimization technique added • Chapter 6: Advanced analysis of multiple-sample results described • Chapter 7: Relationship of the loss function to other performance and capability statistics defined • Chapter 8: Comprehensive experiment planning discussions included, and transformation to overall evaluation criteria revised and expanded • Chapter 9: An example depicting current practices in application to production problem solving added ACKNOWLEDGMENTS This book would not be in circulation today without the Society of Manufacturing Engineers undertaking continued printing of the first edition after the original publisher folded technical publish- ing activities. For this second edition, I am indebted to Rosemary Csizmadia of SME, who was relentless in her conviction of market demand for the book even in difficult economic times. I would also like to thank Ellen Kehoe of SME for her grasp of language and insight into the technology in editing the manuscript. I am grateful to my professional and business associates, Larry Smith, Mike Comerford, Greg Adams, Larry Tracey, Dave White, and Jay Chandra, for their trust in my ability to support their clients seeking training and application of the Taguchi technique. I would like to express my sincere thanks to Larry Smith, in particular, who spent an extraordinary amount of time on a meticulous review, and to Andrea Stamps, Kush Shah, Side Zhao, Fred Schenkelberg, and Pradeep Kumar for their detailed and constructive suggestions. Finally, I thank my wife, Krishna, who has put up with my dedication to the life of an independent consultant, trainer, and author for more than two decades. Ranjit K. Roy January 2010 Contents Preface .......................................................................................................................... ix Abbreviations and Symbols ........................................................................... xii 1 Quality Through Product and Process Optimization BACKGROUND ..................................................................................................................... 1 DESIGN OF EXPERIMENTS—THE CONVENTIONAL APPROACH ......................................... 1 DESIGN OF EXPERIMENTS—THE TAGUCHI APPROACH ..................................................... 5 EXERCISES ..................................................................................................................... 6 2 Taguchi Approach to Quality and Cost Improvement BACKGROUND ..................................................................................................................... 9 TAGUCHI PHILOSOPHY ...................................................................................................... 10 CONCEPT OF THE LOSS FUNCTION ................................................................................... 13 EXPERIMENT DESIGN STRATEGY ...................................................................................... 18 ANALYSIS OF RESULTS ...................................................................................................... 19 AREAS OF APPLICATION ................................................................................................... 21 THE NEW APPROACH—ITS APPEAL AND LIMITATIONS .................................................. 22 EXERCISES ................................................................................................................... 23 3 Measurement of Quality THE QUALITY CHARACTERISTIC ........................................................................................ 25 VARIATION AS A QUALITY YARDSTICK .............................................................................. 26 COST OF VARIATION .......................................................................................................... 27 QUALITY AND VARIATION ................................................................................................. 27 THE QUALITY WE ARE AFTER ............................................................................................ 29 v vi A Primer on the Taguchi Method TAGUCHI QUALITY STRATEGY ........................................................................................... 30 SELECTING

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