[Psychology] Mechanical Assemblies Phần 1 pps

Mechanical Assemblies

Their Design, Manufacture, and Role in Product Development

Daniel E. Whitney

Massachusetts Institute of Technology

New York Oxford

OXFORD UNIVERSITY PRESS

2004

MECHANICAL ASSEMBLIES

Their Design, Manufacture, and

Role in Product Development

Oxford University Press

Oxford New York

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Copyright © 2004 by Oxford University Press, Inc.

Published by Oxford University Press, Inc.

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All rights reserved. No part of this publication may be reproduced,

stored in a retrieval system, or transmitted, in any form or by any means,

electronic, mechanical, photocopying, recording, or otherwise,

without the prior permission of Oxford University Press.

The information, methods, and any software or algorithms in this book

and on the accompanying CD-ROM are believed to be accurate but are

presented for the purpose of education only and should not be relied on

for engineering calculations for any specific design or product. The author

and publisher make no warranty of any kind, express or implied, with

regard to the contents of this book. If expert advice is needed, the services

of a competent professional should be obtained.

Library of Congress Cataloging-in-Publication Data

Whitney, Daniel E.

Mechanical assemblies: their design, manufacture, and role in product development/by

Daniel E. Whitney.

p. cm.

Includes bibliographical references and index.

ISBN 0-19-515782-6

1. Production engineering. 2. Design, Industrial. I. Title.

TS171.4.W48 2004

658.5'752-dc22

Printing number: 98765432 1

Printed in the United States of America

on acid-free paper

2003066170

PREFACE

AIMS OF THIS BOOK

The overt aim of this book is to present a systematic

approach to the design and production of mechanical

assemblies. It should be of interest to engineering pro￾fessionals in the manufacturing industries as well as to

post-baccalaureate students of mechanical, manufactur￾ing, and industrial engineering. Readers who are interested

in logistical issues, supply chain management, product

architecture, mass customization, management of vari￾ety, and product family strategies should find value here

because these strategies are enabled during assembly

design and are implemented on the assembly floor.

The approach is grounded in the fundamental engineer￾ing sciences, including statics, kinematics, geometry, and

statistics. These principles are applied to realistic exam￾ples from industrial practice and my professional experi￾ence as well as examples drawn from student projects.1

It treats assembly on two levels. Assembly in the small

deals with putting two parts together. These are the basic

processes of assembly, much as raising a chip is a funda￾mental process of machining. Assembly in the large deals

with design of assemblies so that they deliver their re￾quired performance, as well as design and evaluation of

assembly processes, workstations, and systems.

The sequence of chapters follows the three themes

in the book's title: design of assemblies, manufacture of

assemblies, and the larger role of assemblies in product

development.

Assembly is the capstone process in discrete parts prod￾uct manufacturing. Yet there is no book that covers these

themes. This is very surprising because there are many

books about the design and manufacture of machine ele￾ments like shafts and gears. But these items do not do any￾thing by themselves. Only assemblies of parts actually do

anything, except for a few one-part products like baseball

bats and beer can openers. Assemblies are really the things

that are manufactured, not parts. Customers appreciate the

things products do, not the parts they are made of.

The lack of books on assemblies is reflected in many

companies where it is easy to find job descriptions corre￾sponding to the design of individual parts but hard to find

job descriptions corresponding to design of assemblies.

As one engineer told me, "The customer looks at the gap

between the door and the fender. But it's an empty space

and we don't assign anyone to manage empty spaces."

There are also many books about tolerances and sta￾tistical process control for the manufacture of individual

parts, but little or nothing about assembly process capa￾bility or the design of assembly equipment to meet a par￾ticular level of capability, however it is defined. There are,

in addition, many fine books about balancing assembly

lines and predicting their throughput, given that there is a

competently designed assembly ready to be assembled.

But what is a competently designed assembly and how

would we know one if we saw one? This book is directed

at that question.

A deeper aim of the book is to show how to apply prin￾ciples from system engineering to design of assemblies.

This is done by exploiting the many similarities between

systems in general and assemblies in particular. Students

who learn about parts but not about assemblies never get

XIX

1Many of my curious experiences in professional practice are in￾cluded in footnotes or used as quotes at the beginning of many

chapters.

XX PREFACE

a high-level view of how parts work together to create

function, and thus they do not know how to design parts

that are intended to contribute to a function in conjunction

with other parts. For this reason, they design parts as in￾dividual items and are satisfied when they think they have

done their individual job well. They are as disconnected

from the product they are designing as is the assembly line

worker who installs the same part for thirty years without

knowing what product is being produced. Products and

companies can fail for lack of anyone who understands

how everything is supposed to work together.

The systems focus of the book is part of a trend at

MIT to complement traditional engineering science with

integrative themes that unite engineering with economic,

managerial, and social topics.

OUTLINE OF THIS BOOK

Chapter 1 provides a discussion about what an assembly

is and why it is important. Chapters 2 through 8 deal with

the design of assemblies, including

a requirements-driven approach to designing assem￾blies that is based on mathematical and engineering

principles,

a theory of kinematic assemblies2

that shows how to

specify and tolerance assemblies so that they deliver

geometrically defined customer requirements,

the method of key characteristics for defining the

important dimensions of an assembly, and

the datum flow chain technique for designing assem￾blies to achieve their key characteristics.

Chapters 9 through 11 deal with the basic processes of

assembly, including

how to describe the motions that parts undergo during

assembly operations and

what the conditions are under which a part mating

attempt will or will not be successful.

Chapters 12 through 18 extend the scope of inquiry to

include manufacturing methods and systems and the role

of assembly in product development. Important topics in

2As explained more completely in Chapter 4, a kinematic assembly

is one that can be assembled without applying force or storing energy

in the parts.

these chapters include

assembly in the large, a view of how product function

and business issues each can be viewed through the

prism of assembly,

how to analyze an existing assembly and perform a

design for assembly (DFA) analysis,

an exploration of product architecture, including

its relationships to business strategy and design for

assembly,

design of assembly systems and workstations, and

economic analysis of assembly systems.

A compact disc accompanies this book. The CD-ROM

contains an additional chapter, Chapter 19, which is a com￾plete case study that applies the book's methods to an air￾craft structural subassembly. In addition, the CD-ROM

contains supporting material such as chapter appendixes,

student class project reports, a professional consulting re￾port, software, and MATLAB routines that duplicate ex￾amples and methods in Chapters 3, 4, 5, 6, 16, 18, and 19.

HOW THE COURSE HAS BEEN TAUGHT

The material in the book has been presented to MIT grad￾uate students for several years. The explicit prerequisites

include linear algebra (to help the students with the matrix

math) and applied mechanics (to provide a background in

statics and statically determinate structures). There is no

prerequisite for a knowledge of probability and statistics,

even though the treatment of tolerancing makes use of

those ideas and presents the basics in passing. Neverthe￾less, one student emphasized to me the huge paradigmatic

difference between the usual way of teaching design (there

is one answer) and the fact that we live in a stochastic

world where designs and objects are really members of

histograms. Until he took this course, he had seen only

the former, never the latter.

Implicit prerequisites that make it easier for students to

grasp the concepts include some experience in mechanical

design, some work in industry, and an ability to make

reasonably realistic perspective or isometric sketches of

mechanical parts and simple assemblies.

Raw ability to manipulate equations or computer

simulations will not be enough to either teach or learn

this material.

The class taught by me meets twice a week for 1.5 hours,

for a total of 25 class sessions. Each session focuses on