Assembly Process

(1)

(2)

THE HANDBOOK OF

MANUFACTURING

ENGINEERING

Second Edition

Assembly

Processes

Finishing, Packaging,

and Automation

(3)

(4)

A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.

Boca Raton London New York

THE HANDBOOK OF

MANUFACTURING

ENGINEERING

Second Edition

E

DITED BY

Richard Crowson

Assembly

Processes

Finishing, Packaging,

and Automation

(5)

Published in 2006 by Taylor & Francis Group 270 Madison Avenue New York, NY 10016

Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1

International Standard Book Number-10: 0-8493-5565-6 (Hardcover) International Standard Book Number-13: 978-0-8493-5565-3 (Hardcover) Library of Congress Card Number 2005020353

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.

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, microfilming, 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

Crowson, Richard.

Assembly processes : finishing, packaging, and automation / Richard Crowson and Jack Walker. p. cm.

Includes bibliographical references and index. ISBN-13: 978-0-8493-5565-3 (alk. paper)

1. Assembly-line methods. 2. Production planning. I. Walker, Jack M., 1924- II. Title. TS178.4.C76 2005

670.42'7--dc22 2005020353

Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com

Taylor & Francis Group is the Academic Division of T&F Informa plc.

(6)

Preface

Handbooks are generally considered to be concise references for speciﬁc subjects. Today’s fast-paced manufacturing culture demands that such reference books provide the reader with how-to information with no frills. Some use handbooks to impart buzzwords on a particular technical subject that will allow the uninitiated to gain cred-ibility when discussing a technical situation with more experienced practitioners.

The second edition of Handbook of Manufacturing Engineering was written to equip executives, manufacturing professionals, and shop personnel with enough infor-mation to function at a certain level on a variety of subjects. This level is determined by the reader.

The final book, Volume IV, deals with the finishing of the product. Packaging and automation are also discussed. The selection of the assembly process and the influence of production rate and quality of the product must be considered by the manufacturing engineer as the productivity of the facility and workers is balanced.

Jack M. Walker, who was unable to participate in the editing of this book, but who contributed greatly in the last few months of his life, was a pioneer in new ways of solving old problems.

Jack loved the advent of rapid prototyping. He spent many hours sharing how rapid prototyping had applications in choosing methods of manufacture or in selecting materials that could not be selected by mathematics alone. Jack as the manufactur-ing engineer loved to place prototypes before the persons responsible for makmanufactur-ing the ﬁnal decision in new products. He often called this “touchy, feely” time the point at which a person would love or hate the design.

Some products lend themselves to hands-on evaluation, and the finish, appear-ance, and feel are very important in the final choice of a material in this case. But, as nanometer-level technology develops, the issues of finish and assembly become much more critical. An engineering science called tribology deals with the inter-activity of miniscule particles of materials as they come in contact with each other.

Manufacturing engineers must think in terms of this area of assembly and ﬁnishing and ways to relate experience with larger components to the micron- and nanometer-sized components used in newer technologies today. Thus, this book was edited to provide the background and working knowledge for the manufacturing professionals of the next decade.

Richard D. Crowson SET, CMfgT, CMfgE

(7)

(8)

Editor

Richard D. Crowson

Richard Crowson is currently a mechanical engineer at Controlled Semiconductor, Inc., in Orlando, Florida. He has worked in the ﬁeld of engineering, especially in the area of lasers and in the development of semiconductor manufacturing equipment, for over 25 years. He has experience leading multidisciplinary engineering product development groups for several Fortune 500 companies as well as small and start-up companies specializing in laser integration and semiconductor equipment manufac-ture.

Crowson’s formal engineering training includes academic undergraduate and graduate studies at major universities including the University of Alabama at Bir-mingham, University of Alabama in Huntsville, and Florida Institute of Technology. He presented and published technical papers at Display Works and SemiCon in San Jose, California.

He has served on numerous SEMI task forces and committees as a voting mem-ber. His past achievements include participating in writing the SEMI S2 speciﬁca-tion, consulting for the 9th Circuit Court as an expert in laser welding, and sitting on the ANSI Z136 main committee that regulates laser safety in the United States.

(9)

(10)

Contributors

† Deceased.

Frank Altmayer

Scientiﬁc Control Laboratories, Inc. Chicago, Illinois

Shrikar Bhagath

Delco Electronics Corporation Geoffrey Boothroyd

Boothroyd Dewhurst, Inc. Wakeﬁeld, Rhode Island Robert S. Busk

International Magnesium Consultants, Inc.

Hilton Head, South Carolina Greg Chandler

Manufacturing Engineering, Hubbell Premise Wiring, Inc.

Wilmington, North Carolina Stephen C. Cimorelli Learjet, Inc.

Wichita, Kansas Richard D. Crowson Melbourne, Florida Denise Burkus Harris

Mechanical Design and Developmental Engineering Department,

Westinghouse Corporation Baltimore, Maryland Alexander Houtzeel

Houtzeel Manufacturing Systems Software, Inc.

Waltham, Massachusetts

Aravinda Kar

University of Central Florida Orlando, Florida

Robert L. Lints

Quality Assurance Systems St. Louis, Missouri

John F Maguire

Materials and Structures Division, Southwest Research Institute San Antonio, Texas

Timothy L. Murphy McDonnell Douglas Corp. Titusville, Florida Clyde S. Mutter Titusville, Florida Michael Pecht

CALC Electronics Packaging Research Center (EPRC),

University of Maryland College Park, Maryland Robert E. Persson EG&G

Cape Canaveral, Florida Allen E. Plogstedt

McDonnell Douglas Aerospace East Titusville, Florida

Marc Plogstedt† ITEC

(11)

Lawrence J. Rhoades Extrude Hone Corporation Irwin, Pennsylvania Paul R. Riedel Rockledge, Florida Thomas J. Rose

Advance Processing Technology/ Applied Polymer Technology, Inc. Norman, Oklahoma

Vijay S. Sheth

McDonnell Douglas Corporation Titusville, Florida

John P. Tanner Tanner and Associates Orlando, Florida V. M. Torbilo

Ben-Gurion University of the Negev Beer-Sheva, Israel

Jeffery W. Vincoli

J. W. Vincoli and Associates Titusville, Florida

Jack M. Walker† Merritt Island, Florida William L. Walker

National High Magnetic Field Laboratory, Florida State University Tallahassee, Florida

Don Weed

Southwest Research Institute San Antonio, Texas

Bruce Wendle

Boeing Commercial Airplane Company Seattle, Washington

Kjell Zandin

H. P. Maynard & Company, Inc. Pittsburgh, Pennsylvania

(12)

Manual Assembly

John P. Tanner

and

Jack M. Walker

1.0 INTRODUCTION TO MANUAL ASSEMBLY

In today’s complex manufacturing world, it is sometimes difﬁcult to remember what the real purpose of manufacturing a product is—and what the total elements of the process consist of. If we assume that whatever we manufacture, we will insist on good quality products, on-time delivery, and complete customer satisfaction, then we should concentrate on the most economical method to achieve these results. We should make our manufacturing decisions based on cost. Of course, cost is not a simple thing to determine. There are a lot of different ways of looking at cost.

For a method of arriving at the lowest cost while maintaining quality, delivery, and happy customers, perhaps we should examine a one-person business operation. A few hundred years ago, there were a lot of them—and even today, there are more than most of us realize. The nation’s small businesses, those with one to ten employees, grew in numbers during recession-plagued 1991, resisting the downturn experienced by medium and larger companies, according to a U.S. Census Bureau report. The number of small businesses increased up to 1% per year between 1987 and 1991. Larger busi-nesses increased up to 3% per year through 1990, then declined in 1991; those with 10 to 100 employees were down 0.2% in 1991, while those with more than 100 fell 1.7%. Businesses with more than 100 employees are generally concentrated in the manufac-turing sector, which as recently as 1970 accounted for 35% of the workforce. By 1991, manufacturing workers made up less that 20% of the workforce.

In 1991 there were more than 4 million establishments in the United States with fewer than 10 employees, about 1.5 million with 10–99 employees, and 134,000 companies with more than 100 employees (a total of 6,199,339 establishments). See Figure 1.1 for a breakdown of the manufacturing industries.

Now let’s get back to our one-person factory. Upon receipt of an order, the owner makes each of the parts, assembles them, and does the ﬁnish painting, packing, and delivery. The difference between his total income and the amount of money he spent during the month is his salary, or proﬁt. Of course, he may be paying rent on the build-ing, buying raw materials and supplies, and even making payments on his equipment

(15)

2 Assembly Process: Finishing, Packaging, and Automation

and machines. Let’s deduct these, and now we have his proﬁt. Whoops—he probably pays for heating, lighting, insurance of some type—and taxes—and the remainder was his proﬁt. We can see that even in the one-person factory, real cost is not so easy to determine.

If business improves, one person may not be able to do everything by working at a faster pace, or working longer hours—and at some point the owner will have take some action in order to continue on-time delivery, et cetera. He might decide to buy the parts and just perform the assembly operation (or vice versa). Another option might be to add helpers and continue to perform all the operations in-house. In most cases, a growing company will probably elect to continue to perform the assembly function in order to have better control of quality, finish, delivery time, and so forth. It will still be “their” product as far as the company’s customers are concerned, which will keep the customers satisfied and give the company the opportunity to add additional sales. As the company continues to grow, the owner might reconsider his make-or-buy decisions and perhaps add equipment to fabricate critical parts in-house. In the assembly area, the first step might be to add automated screwdrivers, nut runners, riveters, spot-welding heads, and perhaps pick-and-place mechanisms. To move parts from the fabrication or receiving department to the assembly stations, some type of transfer device might be a logical improvement. The same applies for the transfer of parts and assemblies down the assembly line. There are an infinite number of options, including redesigning the

U.S. MANUFACTURING INDUSTRIES

(X 100) 1963 1967 1972 1977 1982 1987 1990 1995 Total Establishments 312 311 321 360 358 369 UNDER 20 EMPLOYEES 20–99 " 100–249 " 250–999 " 1000 & OVER " (X MILLION) TOTAL EMPLOYEES PRODUCTION WORKERS 207 199 203 237 230 238 70 18 10 3 3 3 2 2 2 74 20 11 1121 1222 2111 2211 76 78 84 86 17 17.3 19.314.4 13.519 19.613.7 19.112.4 12.2 12.1 11.518.9 18.8 18.1 PERCENTAGE OF PRODUCTION WORKERS IN ASSEMBLY TASKS: (1967)

MOTOR VEHICLES 45.6

AIRCRAFT 25.6

TELEPHONE, ETC. 58.9

FARM MACHINERY 20.1

HOME REFRIG. & FREEZERS 32.0

OFFICE EQUIPMENT 35.9

HOME COOKING EQUIP. 38.1

MOTORCYCLES, BICYCLES, ETC. 26.3 CENSUS OF MANUFACTURERS

U.S. BUREAU OF CENSUS

(16)

Manual Assembly 3

product to reduce the number of parts required to be fabricated and thereby simplifying the assembly process.

In the end, the assembly process may well become the key to the owner’s continu-ing success. The small ﬁrm that started with a one-person assembly operation has now grown to a multiemployee company, probably using many of the same techniques that were successful in the beginning of the one-person shop. With more orders, and probably more diverse products, it is decision time again. This is the subject of this chapter.

There are as many factors inﬂuencing the assembly process decisions as there are products, customers, and factory managers. Geoffery Boothroyd, in Assembly Automation and Product Design, quotes Henry Ford’s principles of assembly as follows:

First, place the tools and then the men in the sequence of the operations so that each part shall travel the least distance whilst in the process of ﬁnishing. Second, use work slides or some other form of carrier so that when a workman completes his operation he drops the part always in the same place which must always be the most convenient place to his hand and if possible have gravity carry the part to the next workman. Third, use sliding assembly lines by which parts to be assembled are delivered at convenient intervals, spaced to make it easier to work on them.

Assembly operations can be performed manually, automatically, or integrated in some manner using a combination of systems. If manual assembly is employed, an operator can adapt to changing conditions such as those brought about by part variation, mislocation, and product model mix. An operator can compensate for these changing conditions and, as a result, may not require elaborate tools and ﬁxtures to perform the assembly tasks. However, operator error and fatigue can result in quality problems.

When production volumes are high enough, some assembly operations can be performed automatically with special-purpose machines. These automatic assem-bly machines consist of workstations grouped along some type of transfer system for part conveyance. Each station performs one task with the aid of dedicated sta-tion equipment, jigs, and fixtures. Part variasta-tion, misalignment, and product mix are not readily adapted to, because sensors cannot always be employed efficiently or economically to guide or monitor the assembly process. Therefore, part varia-tions and slight misalignments can result in jamming, incomplete operavaria-tions, and excessive machine downtime. However, automation can still be justified when the production volumes are high, product life is long, and assembly tasks are simple. For an assembly operation to be performed successfully on a repetitive basis, it is absolutely essential that part variation and location be minimized and consistency in dimensions and location be maximized. To achieve this in a mass production environment requires elaborate and costly tooling, fixtures, and the employment of expensive production controls. Therefore, many assembly operations are performed manually to resolve some of the problems in mating parts with variations or mislo-cations, which may result in increased assembly costs and lower productivity.

(17)

1.1 ASSEMBLY WORK INSTRUCTIONS

“Assembly processing” is another way of saying “assembly methods.” Assembly methods sheets, or work instructions, must describe clearly what is to be done, in what sequence, and with what tools and materials. Assembly methods sheets should minimize operator learning time and must be economical to prepare, reproduce, dis-tribute, and change. Assembly process planning should include an assembly process summary or process routing, detailed work instructions for each operation called out in the summary, an operations parts list for each operation, process sketches or visual aids, and a workplace layout for each operation. The work instructions should call out all tools necessary to perform the operation, and there should be a standard time on the process summary for each operation called out, broken down to the level of setup and run times.

In process planning for fabrication, whether for machining or forming, the skilled machinist or sheet metal mechanic could work to what amounts to an out-line process routing supplemented by the engineering drawing of the piece part. As a skilled worker, he can set up the machine and perform the work with a minimum of written work instructions. Such is not the case with assembly operations. The work must be totally and carefully planned by the manufacturing engineer, and complete work instructions prepared. These are the two extremes. In most manu-facturing plants today, process planning will fall somewhere in between.

If the plant is a high-volume producer of a single product line, then detailed assembly work instructions may be unnecessary. Once operators are trained to per-form a short-cycle assembly operation, little else is needed except possibly some clear, concise visual aids showing the critical details of the operation in pictorial, or exploded view, form. However, the manufacturing engineer must plan such produc-tion down to the most detailed level. He or she must prepare a layout of the assem-bly lines, show each and every workstation in plan view, plan the assemassem-bly tools required, and write a complete description of the work performed at each station on the line. The manufacturing engineer must establish standard times and decide where visual aids are needed and prepare them, and then ﬁne tune or balance the line, assist in training the operators, and ﬁnally shake down or debug the line.

All of the above documentation is necessary when assembly lines are initially established or set up, and to train the operators. Once the line is ﬂowing smoothly and the operators are trained, there will be less and less reliance on written work instructions and even visual aids. This initial planning documentation should always be available for ready reference and should be kept up to date by the manufacturing engineer.

If the company manufactures a variety of different product lines in medium to high volumes, or sets up and produces to a job-order-type system, or does both, assembly process documentation that is complete and to the greatest level of detail is especially important. It is a proven fact that good assembly process planning and documentation signiﬁcantly reduce operator learning (and relearning) time. This is especially important when the production run is relatively short. It also teaches the correct methods to operators and thereby reduces costs of assembly labor.

(18)

Manual Assembly 5

The assembly process documentation package is essential to the operation of an ongoing production-control and time-keeping system. The assembly process routing provides the steps or sequences that materials, parts, assemblies, and work in process must follow to build the product. It provides the time standards for each operation, and the assembly parts list for each operation provides the information needed by production control to pull and kit material for production.

In a small plant, where production runs may be small to nonexistent, assembly process planning with only minimum documentation is required and can be justiﬁed for the reasons mentioned earlier. Even in the case where no formal production con-trol system exists and the production supervisor draws material from the stockroom in one batch issue for the entire job, pictorial visual aids, workstation layouts, a tool list, and an assembly process routing should be provided.

1.2 ASSEMBLY OPERATION SEQUENCES

In assembly process planning, operation sequences usually parallel the indented parts list or engineering tree chart, because it should represent how the product goes together or is assembled. This initial assembly process sequence plan should define an assembly operation for each major and minor subassembly and for the final assembly (Figure 1.2). It should be emphasized that this is an initial breakdown and normally will be followed by a more thorough analysis of the steps required to assemble the various subassemblies and the final assembly. This detailed analysis is normally done in the preproduction planning phase in the form of an operation

Material Feeding Into The Process Purchased Material Purchased Material Material On Which Work Is Performed Material On Which Work Is Performed

Material On Which Work Is Performed

Purchased Material Purchased Material Purchased Material Subassembly Or P art

Which Joins Another Subassembly Or P

art

Subassembly Or P

art

Which Joins Another Subassembly Or P

art

Subassembly Or P

art

Steps Of Process In Sequential Order

Part Or T

op Assembly On Which Most Operations

Are Performed

(19)

process chart. Figure 1.3 shows an example of an operation process chart for a Coast Guard radio receiver.

The assembly process may include soldering, wiring, press ﬁtting, brazing, shrink ﬁtting, welding, adhesive bonding, riveting, and mechanical fastening. Within each of these assembly processes a series of sequences is required to accomplish the

(20)

Manual Assembly 7

process, without regard to the product conﬁguration, material, or quantity to be pro-duced, or the rate of production. For example, many of the steps in creating a circuit card assembly, a wire harness, the frame of a truck, or in the installation of ﬁttings on a sailboat are essentially the same. The detailed instructions for the sequence should spell out the differences peculiar to the product at hand.

Often in assembly work, standard sequences or operations are possible for any product where these processes of assembly are used. The result is a considerable saving in manufacturing engineering time and in the elapsed time required to prepare and release an assembly process plan to the shop. Such standard processes enable preprinted process planning documentation, which may only require a part number and quantity to be entered before it is ready for release. A good example of this is in the manufacture of cables for electrical or electronic equipment, where diagrams are preprinted of the various electrical connector pin conﬁgurations, requiring only that the manufacturing engineer sketch in the wires terminated to the pins for the speciﬁc cable application. Computer-aided process planning (CAPP), covers the use of a database to accomplish this task.

1.2.1 Routings, Work Instructions, and Visual Aids

Assembly process routings, such as the one shown in Figure 1.4, list the operations in the sequence in which they must occur to assemble the item or product called out in the heading. In addition to listing the operations in their proper sequence, it lists the standard times for each operation, the performing department, and the latest revision level of the process instruction sheets. The issue or revision level of the process information establishes configuration control of the product on the shop floor, because normally the assembly department does not work to engineering drawings. The importance of this cannot be emphasized enough. It is the responsibility of the manufacturing engineer always to have the latest revisions to the engineering drawings incorporated into the assembly process documentation, especially when the job is active on the production floor. In many companies the inspection department uses the process documentation to perform in-process inspections of the product. This is especially true where detailed process work instructions are used, and the process documentation is also used for shop configuration control.

As indicated earlier, the assembly process summary or routing is used by production control to move material or work in process to the next operation or sequence. A copy of the assembly process summary travels with each batch of parts and material and in effect becomes a routing sheet or shop traveler. For this to happen, the sequences must be stamped off, either by inspection or by the operator, as they are completed. If line production is involved, such a routing or traveler is unnecessary, as the progression on the assembly line is the routing followed by the assemblies. The process routing is an especially valuable tool in the job shop, where the shop is laid out by function or pro-cess, and does not follow the product ﬂow.

Assembly work instructions are the heart of the manufacturing engineering documentation package, and explain how the product is to be assembled in production. The assembly work instructions should be available at the operator workstation,

(21)

FIGURE 1.4 Assembly process summary for wiring harness subassembly.

preferably on an easy-to-see holder mounted on the workbench. The assembly pro-cess routing lists all of the operations for assembly, in the sequence that they must be performed; the assembly work instructions or assembly methods sheets for each listed operation explain in detail exactly how to perform the operations. Figure 1.5 is an example of an assembly methods sheet for the wiring of a connector that becomes part of a wiring harness in a marine short-wave radio receiver.

(22)

Manual Assembly 9

An assembly operation parts list should be included with the assembly work instruc-tions or assembly methods sheets for each operation. This tells production control and the operator what parts and materials are required to perform the assembly operation for one unit. Figure 1.6 shows one version of such a parts list. In addition, there must also be a list of standard and special design assembly tools needed to perform the operation. In this example it is included in the operation parts list. It can also appear as a separate call-out on the work instruction or methods sheets.

(23)

An extremely important part of the assembly process documentation is visual aids. Visual aids can be anything from an actual mockup of the product to a black and white or color photograph, to a three-dimensional isometric or exploded-view drawing, to a simple sketch, or to a tracing lifted directly from the engineering drawing. Figure 1.7 shows a light table being used by a manufacturing engineer to trace parts of the engi-neering drawing in order to make up a visual aid. If regular ofﬁce copy machines are

(24)

Manual Assembly 11

used, visual aids can be constructed using cut-and-paste methods as shown in Figure 1.8. Illustrations may also be done by graphic artists or illustrators as shown in Figure 1.9. It should be kept in mind that all process documentation costs money, and consideration should be given to the length of the production run, anticipated changes during produc-tion, and what is really needed to instruct a particular group of assembly operators. One other very important consideration is that a visual aid supplemented by minimal notes and instructions is far superior to lengthy written work instructions. As explained earlier, visual aids that highlight key assembly details are all that are used in many companies.

1.3 WORKSTATION AND LINE LAYOUT

Workstation layouts are important from the standpoint of assembly operator methods. They tell the assembly supervisor how to set up and configure the individual workstations for optimum productivity and flow of work. Workstation layouts are usually in the form of a plan view of the workstation and show where tooling, fixturing, and part bins should be placed, and include work instructions, tote pans for staging completed work and for placing incoming work, and any other information pertinent to the opera-tion and the setup of the workstaopera-tion. Figure 1.10 shows a workstaopera-tion layout used in a small job order shop. Figure 1.11 shows how this layout might look set up in the shop.

Line layouts are used where progressive assembly lines are to be used to build the product, and the plant layout drawings do not show which workstation goes where. Again, these are used by supervision in setting up the line to conform to the assembly process ﬂow and to ensure optimum methods and work ﬂow. See Figure 1.12 for an example.

FIGURE 1.7 Manufacturing engineer using a light table to make a tracing

(25)

1.4 MANUFACTURING METHODS ANALYSIS

In developing the manufacturing process, whether for fabrication, machining, forming, finishing, or assembly, the manufacturing engineer must specify the most economical methods for the job and for the work at hand. In order to do this he or she must under-stand and be able to apply the fundamental techniques of methods analysis, motion economy, and work simplification. The presumption of good methods and the ability of the manufacturing engineer in methods analysis is so basic that not to be proficient in this art is tantamount to being incompetent as a manufacturing engineer. The purpose and intent of this section is to provide the basic information needed by the manufactur-ing engineer to gain a degree of proficiency in manufacturmanufactur-ing methods analysis.

Even the best and most thorough process planning will sometimes overlook details or specify methods that can be improved upon later, after the product is in

(26)

Manual Assembly 13

FIGURE 1.9 Visual aid made using formal artwork.

ADAPTER HALVES NEW SLING STRAIGHT HEADED PIN ADAPTER ADAPTER ADAPTER INSUL SLEEVING TOP VIEW SIDE VIEW SLING-HOOK FORWARD SLING RETAINER

(27)

FIGURE 1.11 Assembly workbench

arrange-ment as it would appear on the shop ﬂoor.

FIGURE 1.12 Manual line assembly with manual transfer of the workpieces (a) in a line

(28)

Manual Assembly 15

production. Part of the job of the manufacturing engineer is to be alert for these opportunities to improve the process and the ﬂow.

1.4.1 Work Simpliﬁcation

Work simplification can be defined as the organized application of common sense to find easier and simpler ways of doing work. Work simplification provides a sys-tematic, common-sense approach to make work easier and at the same time to lower costs. The basic premise of work simplification is that there is always a better way to do any task. The work simplification pattern includes five basic steps:

1. Selection of a job to be improved 2. Recording of the job details 3. Analysis of the job details

4. Development of the improvements 5. Installation of the improvements

Selecting the job to be improved requires careful consideration and study. Efforts expended for improvement should be made ﬁrst where the returns will be the greatest. Priority should be given to bottlenecks, choke points, trouble spots, jobs that require excessive amounts of time, or where generally unsatisfactory conditions exist. The following list provides assistance in making this selection:

1. Greatest cost: work that involves the greatest expenditure of funds, labor hours, or use of equipment

2. Greatest workload: the largest volume of work being performed by the activity

3. Number of persons assigned: work that requires large numbers of people to perform similar tasks

4. Walking: jobs that require a lot of walking around 5. Bottlenecks: work not ﬂowing smoothly

6. Schedules not met: failure to meet deadlines, resulting in work backlogs or overtime

7. Excessive waste: work that results in wasted materials, in scrap, and in rework

8. Excessive fatigue: work that requires great physical effort or that is being done with frequent rest periods

9. Unsafe or unpleasant work practices: work that results in numerous acci-dents or is undesirable because of extreme conditions such as dust, noise, fumes, vapors, or extremes of temperature

Every job is made up of three parts:

1. Make ready: the time and effort put into the setup, or getting ready to work 2. Do: the actual work accomplished

(29)

3. Put away: the time and effort put into cleaning up after the “do” part of the job

For example, if we have a carpenter making a wooden box, we might expect the job to break down as follows:

1. Make ready: open bin, pick up nails, close bin, pick up hammer, lay nails on the bench

2. Do: hammer nails

3. Put away: put hammer aside, pick up box, put box aside

Anything that reduces the time required for the “make ready” and “put away” parts of the job reduces the nonproductive time associated with the job.

Recording of the job details can best be accomplished through the use of process chart techniques, specifically the flow process chart and the flow diagram. The flow process chart is a device for recording each step of a job in a compact manner, as a means of better understanding it and improving it. The chart represents graphically the separate steps of the events that occur during the performance of work, or during a series of actions. The process chart may be used to record the flow within a unit, a section, a department, or between departments. The flow process chart has no bounds.

No matter how complicated or intricate the series of operations may be, a flow process chart can be constructed if you take one step at a time. The flow process chart, however, like other methods of graphic representation, may need to be modified to meet the requirements of a particular situation. For example, it may show in sequence the total activity of a production operator, or it may show in sequence the steps that the worker, part, or material goes through. The chart could be either the operator type or the material type, and the two types should not be combined.

A careful study and analysis of such a chart, giving a graphic picture of every step in the process, is almost certain to suggest improvements. It is not uncommon to ﬁnd that some operations can be eliminated or that a part of an operation can be eliminated, that one operation can be combined with another, that better routes for the parts can be found, more economical machines can be used, delays between operations can be eliminated, and other improvements can be made, all of which go to produce a better product at a lower cost.

To make a ﬂow process chart requires careful adherence to the following rules: 1. State the activity being studied. Make certain you are really naming the

activity you have chosen to study.

2. Choose the subject to follow. Decide on a person or a material, and follow him or it through the entire process. When you have picked a subject, stick with it.

3. Pick a starting and ending point. This is to make certain you will cover all the steps you wish to cover, but no more or less.

4. Write a brief description of each detail. Step by step, no matter how short or temporary, describe each detail.

(30)

Manual Assembly 17

5. Apply the symbols. The description determines each symbol. Draw a con-necting line between each of the proper symbols.

6. Black in the “do” operation. Shade in the symbols for those operations that you decide are the “do” operations. This will help you later in your analysis, when you begin challenging.

7. Enter distances. Whenever there is a transportation, enter the distance traveled.

8. Enter time if required. This is often not necessary. However, if it will help, note the time required or elapsed.

9. Summarize. Add up all of the facts and put them in the summary block. The summary should indicate the total number of operations, transporta-tions, inspectransporta-tions, delays, storages, and distance that is traveled.

Figure 1.13 shows a flow process chart that is completely filled in, with circled numbers used to illustrate each of the rules listed above. The elements of the flow chart are as follows; please see Figure 1.13 or their corresponding symbols:

O Operation. An operation occurs when an object is intentionally changed in any of its physical or chemical characteristics, is assembled or disassem-bled from another object, or is arranged or prepared for another operation, transportation, inspection, or storage. An operation also occurs when infor-mation is given or received, or when planning or calculating takes place. ⇒ Transportation. A transportation occurs when a person moves from one

workplace to another or when an object is moved, except when such movements are part of the operation or are caused by the operator at the workstation during an operation or an inspection.

Inspection. An inspection occurs when an object is examined for identiﬁcation or is veriﬁed for quality, quantity, or any of its characteristics.

D Delay. A delay occurs to an object when conditions, except those that inten-tionally change the physical or chemical characteristics of the object, do not permit or require immediate performance of the next planned action. ∇ Storage. A storage occurs when an object is kept and protected against

unauthorized removal.

It is sometimes helpful to supplement a flow process chart with a flow diagram. A flow diagram is simply a layout of the area involved in the job being studied, over which you indicate by a line the path of the object or person followed in the flow process chart. It is often desirable to indicate the action taking place by using the same symbols as on the chart. They may, if desired, be keyed to each other by item numbers. Figure 1.14 shows a flow diagram to accompany the flow process chart in Figure 1.13.

The third step of the work simpliﬁcation pattern involves questioning every part, aspect, or detail of the job. Here we examine each operation and ask some very pointed questions. Don’t be satisﬁed until you have asked all possible questions and

(31)

(32)

Manual Assembly 19

received the related answers. The ﬁrst thing to do is to question the entire job being studied. Why is it done? Is the job really necessary? If it is, then question each “do” operation. If you can eliminate a “do” operation, you also eliminate the “make ready” and “put away” operations that go with it. The “do” operations are those that add value to the product or process being studied.

There is always a better way, and our task is to ﬁnd it.

Why? The overriding question; it establishes the reason for the job. The answer deﬁnes and justiﬁes the purpose of the job.

What? What is done? What are the steps? What does each step do? What makes the step necessary?

FIGURE 1.14 Flow diagram example.

Operations: Subject Charted:

Pricing and Posting Orders Unpriced Orders

PRESENT METHOD LAYOUT Time stamp PBX A _{Wooden Box} ACCOUNTING OFFICES Files _Files C B

Clerk A – Receptionist and PBX Operator Clerk B – Pricing Clerk

(33)

Where? Where should this step be done? Can it be done more easily? Can it be done using less time, energy, or transportation, or by changing the location of employees or equipment?

When? When should this step be done? Is it done in the right place in the sequence? Can the job be simpliﬁed by moving this step ahead or back? Who? Who should do the job? Is the right person handling it? Would it be

more logical to give the job to someone else?

How? How is the job being done? Can it be made easier? Can the job be done better with different equipment or a different layout?

The following question-and-answer approach will suggest improved methods:

Why and what lead to elimination.

Where, when, and who lead to combination or sequence change.

How leads to simplicity.

Careful consideration of the possibilities presented in looking for ways to eliminate, combine, change sequence, and simplify in the questioning approach brings us ﬁnally to the better method and provides us with the answer to “How should the job be done?” The simplest way is the best way. Figure 1.15 shows a proposed improvement ﬂow process chart for the pricing and posting of orders in Figure 1.14.

1.5 PRINCIPLES OF MOTION ECONOMY

The following discussion explores the rules or principles of motion economy that have been and are now being used successfully in manufacturing methods studies. These principles form a basis, code, or body of rules that, if applied correctly, make it possible to greatly increase the output of manual factory labor with a minimum of fatigue. These principles will be examined under the subdivisions of operator tasks and the workplace, and as applied to tools and equipment.

1.5.1 Operator Tasks

The principles of motion economy as related to the tasks of the operator are as follows: 1. The two hands should begin as well as complete their motions at the

same time.

2. The two hands should not be idle at the same time except during rest periods.

3. Motions of the arms should be made in opposite and symmetrical directions and should be made simultaneously.

These three principles are closely related and should be considered together. It seems natural for most people to work productively with one hand while holding the object

(34)

Manual Assembly 21

being worked on with the other hand. This is extremely undesirable and should be avoided. The two hands should work together, each beginning a motion and complet-ing a motion at the same time. Motions of the two hands should be simultaneous and symmetrical.

Many kinds of work can be accomplished better using both hands than by using one hand. For most manufacturing assembly operations, it is advantageous to arrange similar work on the left- and right-hand sides of the workplace, thus enabling the left and right hands to move together, each performing the same motions. The symmetrical movements of the arms tend to balance each other, reducing the shock and jar on the body and enabling the operator to perform the

(35)

task with less mental and physical effort. There is apparently less body strain when the hands move symmetrically than when they make nonsymmetrical motions, because of issues of balance.

The fourth principle of motion economy states that hand and body motions should be conﬁned to the lowest classiﬁcation with which it is possible to perform the work satisfactorily.

1.5.2 Classes of Hand Motions

The five general classes of hand motions emphasize that material and tools should be located as close as possible to the point of use. The motions of the hands should be as short as the work permits. In the listing of classifications shown below, the one requiring the least amount of time and effort is shown first:

1. Finger motions

2. Motions involving ﬁngers and wrist

3. Motions involving ﬁngers, wrist, and forearm

4. Motions involving ﬁngers, wrist, forearm, and upper arm

5. Motions involving ﬁngers, wrist, forearm, upper arm, and shoulder (causes posture change)

It should be pointed out that ﬁnger motions have been found to be less accurate, slower, and more fatiguing than motions of the forearm. Evidence seems to indicate that the forearm is the most desirable member for performing light work. In highly repetitive work, motions about the wrist and elbow are superior to those of the ﬁngers or shoulders.

The ﬁfth principle of motion economy states that momentum should be employed to assist the worker wherever possible, and it should be reduced to a minimum if it must be overcome by muscular effort. The momentum of an object is deﬁned as its mass multiplied by its velocity. In the factory environment, the total weight moved by the operator may consist of the weight of the material moved, the weight of the tools moved, and the weight of the part of the body moved. It should be a real possibility to employ momentum to advantage when a forcible blow or stroke is required. The motions of the worker should be so arranged that the blow is delivered when it reaches its greatest momentum.

The sixth principle of motion economy states that smooth, continuous, curved motions of the hands are preferable to straight-line motions involving sudden and sharp changes in direction. Abrupt changes in direction are not only time-consuming but also fatiguing to the operator.

The seventh principle of motion economy states that ballistic motions are faster, easier, and more accurate than restricted or controlled movements. Ballistic move-ments are fast, easy motions caused by a single contraction of a positive muscle group, with no antagonistic muscle group contracting to oppose it. A ballistic stroke may be terminated by the contraction of opposing muscles, by an obstacle, or by dissipation of the momentum of the movement, as in swinging a sledge hammer.

(36)

Manual Assembly 23

Ballistic movements are preferable to restricted or controlled movements and should be used whenever possible.

The eighth principle of motion economy states that work should be arranged to permit an easy and natural rhythm wherever possible. Rhythm is essential to the smooth and automatic performance of any operation. Rhythm, as in a regular sequence of uni-form motions, aids the operator in peruni-forming work. A uniuni-form, easy, and even rate of work is aided by proper arrangement of the workplace, tools, and materials. Proper motion sequences help the operator to establish a rhythm that helps make the work a series of automatic motions where the work is performed without mental effort.

The ninth principle of motion economy states that eye ﬁxations should be as few and as close together as possible. Where visual perception is required, it is desirable to arrange the task so that the eyes can direct the work effectively. The workplace should be laid out so that the eye ﬁxations are as few and as close together as possible.

The Workplace

The first principle of motion economy related to the workplace states that there should be a definite and fixed place for all tools and materials. The operator should always have tools and materials in the same location, and finished parts and assembled units should be placed in fixed positions or locations. For example, in the assembly of mechanical hardware, the hand should move without mental direction to the bin containing flat washers, then to the bin containing lock washers, then to the bin containing bolts, and finally to the bin containing hex nuts. There should be no thinking required on the part of the operator to do any of this.

The second principle of motion economy related to the workplace states that tools, materials, and controls should be located close to the point of use. In the hori-zontal plane, there is a deﬁnite and somewhat limited area that the worker can use with a normal expenditure of effort. This includes a normal working area for the right hand and one for the left hand for each working separately, and another for both hands working together. Figure 1.16 shows this and the dimensions of normal and maximum working areas in the horizontal and vertical planes. Both the standing and sitting positions are included. It also shows normal bench work surface heights, which can have a signiﬁcant adverse effect if they are not correct.

Figure 1.17 shows in greater detail the areas of easiest reach for the left and right hands, for both hands working together, and the area in which small objects can be most easily picked up.

The third principle of motion economy related to the workplace states that gravity-feed bins and containers should be used to deliver the material close to the point of use. This can sometimes be accomplished by using parts bins with slop-ing bottoms that feed parts by gravity to the front of the bin, eliminatslop-ing the need for the assembly operator to reach down into the bin to grasp parts.

The fourth principle of motion economy related to the workplace states that drop deliveries should be used wherever possible. This requires conﬁguring the work-place, for example, so that ﬁnished units may be disposed of by releasing them in the position in which they are completed, delivering them to their next destination by

(37)

gravity. Besides the savings in time, this frees the two hands so that they may begin the next cycle immediately without breaking the rhythm.

Other principles of motion economy related to the workplace include the following:

Materials and tools should be located to permit the best sequence of motions. Provision should be made for adequate lighting.

FIGURE 1.16 Normal and maximum working areas and heights.

Total height Eye level Shoulder Elbow Seat Knee Normal working height 28" 14" 15" 10" 2" 9½" 20 " 29 " 39" 52 " 58 " 63 " 23.5 " 14" 14" 40" 58" 2" Work position Maximum work area Edge of work height Normal work area 37 " 9½ "

(38)

Manual Assembly 25

The height of the workplace and chair should be arranged so that alternate sitting and standing at work are easily possible.

A chair of the type and height to permit good posture should be provided for the operator.

1.5.3 Tools and Equipment

Principles of motion economy as related to the design of tools and equipment include the following:

1. The hands should be relieved of all work that can be done more effectively by a jig, ﬁxture, or foot-operated device.

2. Two or more tools should be combined wherever possible. 3. Tools and materials should be prepositioned whenever possible.

FIGURE 1.17 Horizontal view of areas of easiest reach for each hand and for both hands working together. (a) Maximum areas of reach for left and right arms. (Broken lines indicate enclosed area covered by hands when forearm is pivoted on the bent elbow. (b) Area inside in which small objects are most easily picked up. (c) Area in which the eye can follow both hands working simultaneously and symmetrically.

LEFT HAND RIGHT HAND

(a) (b) (c) A C D E E E Y X W Z F F G G D D A A B

(39)

4. Where each finger performs some specific movement, the load should be distributed in accordance with the inherent finger capacities (arrangement of typewriter keys).

5. Levers, crossbars, and hand wheels should be located in such positions that the operator can operate them with the least change in body position, and with the greatest mechanical advantage.

1.6 STANDARD MANUFACTURING PROCESSES

In addition to operator work instructions, process routings, visual aids, and operation parts lists, a key ingredient of the documentation package that should be provided by manufacturing engineering includes standard manufacturing processes. Standard manufacturing processes include workmanship standards, equipment operating pro-cedures, and standard repair procedures. These standard processes are common to all products manufactured in a plant and therefore are either referenced in the product work instructions, as in “Assemble per standard manufacturing process 367,” or the appropriate text or illustrations, or both, are copied directly into the product instruc-tions, thereby eliminating the requirement for the production operator to leave the work position and look up information in a separate book or document. Obviously, the latter method is preferred.

1.6.1 Workmanship Standards

Workmanship standards tell the operator what is acceptable work or practice and what is not. These standards can be in the form line drawings supplemented with narrative text, color photographs of acceptable and unacceptable work supple-mented by narrative text, or actual models or prototype units, known as stan-dards. Figure 1.18 shows an example of a line-drawing workmanship standard for taping the core of an iron-core transformer in the magnetic components industry. It should be noted that instructions are superimposed where needed, pointing out the important points to watch for when tape-wrapping the core. Also, it should be noted that this workmanship standard tells only the requirements for making an acceptable tape wrap and that it is necessary to refer to a product speciﬁcation to determine the tape material and number of layers required.

1.6.2 Equipment Procedures

Equipment procedures are start-up, operating, and shut-down instructions for a machine or piece of production equipment. They should be posted at or near the machine or equipment and are usually prepared by the manufacturing engineer from the manual that is prepared by the maker of the machine or equipment. These procedures are especially valuable where process variables controlled by machine or equipment settings can be critical. Equipment procedures ensure the proper training of new operators and serve as reminders to the experienced operator. Equipment procedures should be prepared for everything from vapor degreasers to heat-treat ovens.

(40)

Manual Assembly 27

1.6.3 Standard Repairs

Standard repair procedures provide ready-made work instructions to be used any time certain types of repair work must be done to product parts, subassemblies, or assem-blies. These procedures or instructions have been approved in advance by inspection or quality control, and if necessary, by design engineering and the customer. They not only tell manufacturing how to repair certain kinds of product defects, but they also eliminate the need for formal rejection by inspection before the repair procedure can be implemented.

For example, the standard repair for a mislocated or design-changed hole in a metal part might read as follows:

1. The maximum number of holes to be plugged or welded in any one part by this procedure is 20% of the total holes in the part, or eight (8) holes, whichever is less.

2. The preferred method for repair shall be welding, except where heat would cause distortion.

3. In applications where surface heat due to welding will cause distortion, a press ﬁt plug or a formed ﬂat head rivet are acceptable alternatives.

FIGURE 1.18 Workmanship standard for taping iron-core transformers.

ONE HALF OVERLAP ON INSIDE

LESS THAN ONE HALF OVERLAP ON OUTSIDE NO EXCESSIVE BUILD UP OF TAPE ON INSIDE CORNERS OF COIL *LEADS *LEADS

A. WIND TAPE UP CLOSE TO LEAD B. FOLD LEAD OVER TAPE C. CONTINUE WINDING TAPE

SECURE START AND FINISH WITH MYLAR TAPE IF NECESSARY TAPE MATERIAL AND NUMBER OF LAYERS TO BE SPECIFIED ON D-SPEC.

(41)

4. The holes to be ﬁlled by weld material shall be prepared by chamfering both sides of the hole. The chamfer shall be sufﬁcient to insure weld penetration. 5. Surfaces to be welded shall be cleaned in accordance with established

welding practices for the specific type of material involved. 6. The holes shall be completely filled with weld material. 7. Welding shall be accomplished by a certified welder.

8. The surfaces shall be ground or otherwise made ﬂush to eliminate weld buildup.

9. Holes to be ﬁlled by plugging shall be prepared by the incorporation of a chamfer or lead on the side to accept the plug.

10. The plug shall be of such diameter that a press ﬁt is ensured.

11. After plug insertion, the surfaces shall be ground or otherwise made ﬂush to eliminate excess material.

12. The plug shall be staked to the part per approved practices.

13. Holes to be plugged with a rivet shall be countersunk on both sides to accept a ﬂat head rivet.

14. The rivet shall be formed and ground or otherwise made ﬂush with the mating services.

15. Rivets shall not be loose after forming.

1.7 SPECIAL MANUFACTURING INSTRUCTIONS

Frequently it becomes necessary for the manufacturing engineer to look beyond the operations in his or her own plant and to make certain that material and parts received from outside suppliers come into the plant and the manufacturing operations ready to become part of the product being produced. In the majority of cases, if the vendor meets the requirements of the purchase order and the engineering drawings (if applicable), the parts or material will be ready for processing when they arrive at the receiving dock. In some instances, however, it may be required that the vendor only partially complete a part or subassembly, or that the vendor produce a certain part on the low side of the drawing tolerance if the parts are to ﬁt together in assembly.

To ensure that this information is correctly transmitted to the vendor, the manu-facturing engineer may prepare special manumanu-facturing instructions that are called out on the purchase order along with engineering drawings and speciﬁcations. The special manufacturing instructions along with the drawings and speciﬁcations then become the acceptance criteria for the parts or material when they go through receiving inspection.

Special manufacturing instructions are prepared by the engineer at the time the make-or-buy determination is made and before the detailed internal work instruction documentation is prepared. These instructions can be in the form of simple handwrit-ten information given to the buyer to incorporate directly into the purchase order, or a formal document that becomes an attachment to the purchase order. The important point to remember is that the manufacturing engineer needs to review the items to be

(42)

Manual Assembly 29

purchased and make certain that if these items are made exactly as the drawings and speciﬁcations say, they can enter directly into production without any problems or unplanned processing steps.

REFERENCES

Boothroyd, G., Assembly Automation and Product Design, Marcel Dekker, New York, 1992. Tanner, J. P., Manufacturing Engineering, Marcel Dekker, New York, 1991.

(43)

(44)

31

Assembly Automation

Jack M. Walker

and

Vijay Sheth

2.1 INTRODUCTION TO ASSEMBLY AUTOMATION

In this space age, when society regards everything as possible, there are many mis-conceptions about automation. Automated technology has been prescribed as the ultimate cure-all for all technical and social problems. The disinclination of workers to perform dull, repetitive tasks, considerations of worker health and safety, dramatic increases in labor rates and fringe beneﬁts, and the increased ability of tool engineers to devise sophisticated machinery have combined to create a dramatic increase in automated equipment during the past several decades. Computer-controlled machine tools now make complex machined parts, electronic components are installed auto-matically in circuit boards, machines inspect detail parts on a 100% basis, and parts are positioned, assembled, and checked out automatically in the creation of assem-blies—untouched by human hands. Robots of all descriptions have become part of the industrial scene.

It is true that most businesses automate primarily to reduce costs and thereby improve their competitive position in the market. However, the real objective of this investment is to make money, not just save money. When compared to manual assembly operations, the beneﬁts usually derived from automated assembly include the following:

Reduced unit costs Consistent high quality

Elimination of hazardous manual operations Increased production standby capacity

The aerospace industry, because of its low production quantities and low-rate production, has been relatively untouched by automation except in the areas of complex machined parts and in the assembly of some types of electronic devices, when the needs for precision and accuracy justify the cost and time of automation. However, the production quantities of some of the more complex weapons are now adequate to consider automation, especially where a number of individual parts are identical on each ﬁnal assembly. Examples of such weapons are the Dragon rocket

(45)

motors and ﬁring circuit boards, all of which have multiple uses on each Dragon missile, designed and built by McDonnell Douglas.

Semiautomated operations are those in which the worker plays a substantial role in the activity. The worker’s role exceeds that of supplying the automated equipment with parts or materials or removing ﬁnished parts from the work area. To date the preponderance of factory operations have actually been semiautomated rather than fully automated because the worker–machine combination is often the most efﬁcient and effective in involved tasks.

2.2 ASSEMBLY MACHINES IN THE FACTORY

When they see the term automated factory, many people think of a Japanese factory containing hundreds of robots. But Japan is not alone in this ﬁeld; many U.S. corpora-tions are making similarly impressive strides in boosting factory productivity and efﬁ-ciency. And robotics, though critical to some manufacturers, represents only a small part of the overall automation scene. Of major importance are the computer and communica-tions networks that manage and analyze the information relevant to factory operation. In addition, every manufacturing facility has its own unique needs. The fabrication and assembly equipment and the processes that can serve them best can vary considerably.

Although lower product unit costs (and increased overall company earnings) are the prime drivers in selecting an automated process, the manufacturing process may be automated in areas where the work is dangerous or monotonous for people, or where machines can substantially improve the quality of the products or increase the production rate. Moreover, the products themselves should be designed to facilitate automated manufacturing, and changes in designs should be easily communicated to the shop ﬂoor. Many existing designs cannot be effectively automated if they were designed for manual assembly.

Equipment for the factory of the future will reduce the need for some of the skilled trades as we know them today. The motive on our part will not be to cope with the shortage of skilled trades, although that already exists in some companies and in some geographical areas. It will not be to solve the problem of getting young individuals, who can enter other areas of work for more money, to go into relatively low-paying jobs in manufacturing. The motive will be the old one: productivity and bottom-line cost.

Automated technology can achieve greater productivity, better quality, reduced costs, and increased proﬁts—if it is properly applied. However, machinery installa-tions or business systems may operate as designed but may not solve the ultimate problem they were created to overcome.

Small details are the stumbling blocks to efﬁcient automatic processing. This follows from the need for precise control of the movement of the various parts going into the assembly. There is no single, simple technique for obtaining control.

2.3 BASIC AUTOMATION CONCEPTS

One key rule of automation is that part design must be compatible with the needs of automatic feeding. Parts are usually introduced to an assembly machine as bulk

(46)

Assembly Automation 33

components. They are placed in a hopper and tracked to a loading station. Whether the hopper is vibratory, rotary, or oscillatory, it relies on gravity or friction, or both, for part movement. Some sort of gating or orientation device allows only those parts in the proper attitude or position to enter the track.

Efﬁciency is affected by the system used; nonvibratory feeders, for example, are limited in the number of orientations they can perform. Part geometry is a critical factor. Soft parts may tangle in the hopper. Bowl driving forces may distort parts to the point where orienting in the bowl is impossible. Distortion of parts due to stock-ing and handlstock-ing can also cause serious difﬁculties.

Parting line ﬂash from cast and molded parts is an example of a problem that never appears on a part drawing. The sensitivity of the part to moisture, static elec-tricity, and residual magnetism may not become apparent until mechanical handling is attempted. Sometimes a particular surface may be declared critical and must be protected for subsequent operations. This situation may preclude or restrict use of some of the automatic feeding methods.

Incomplete molded or broken parts reduce feeding efﬁciency. Foreign material in the hopper is certain to alter the level of performance. This includes not only the presence of material due to machine environment, but contamination of the parts from previous processes.

Turning to the main part, or body, that will receive the oriented parts, several questions arise. For example, the body must be strong enough to withstand assem-bly tooling forces. Typically, forces for assemassem-bly are not as high as for machining, but pressing, sizing, or machining operations might be required on the assembly machine. High production rates may increase these forces.

The main part must be placed accurately on the assembly machine. This calls for fixturing, flat locating surfaces, precision locating holes, etcetera. Parts are most effi-ciently placed in the main body with simple, short, straight-line movement. Grasping a part may be necessary. This requires some type of actuating force with its atten-dant timing and control elements. If spearing or grasping the part is impractical, a vacuum force may be used in transferring parts. Magnetic force is used occasionally in handling parts, but this is not recommended, because the attraction of metal chips and dirt degrades equipment performance rapidly. Also, residual magnetism often cannot be tolerated in the final assembly. The author experienced problems with the Dragon rocket motor assembly machine due to the cast retainers occasionally becoming magnetized due to improper heat treatment of the stainless steel. We had to degauss them all to achieve good hopper feed.

2.4 TYPES OF AUTOMATED ASSEMBLY MACHINES 2.4.1 Standard Machine Bases

There are essentially four types of standard base assembly machines: Dial indexing machines

Assembly Process

THE HANDBOOK OF

MANUFACTURING

ENGINEERING

Second Edition

Assembly

Processes

Finishing, Packaging,

and Automation

THE HANDBOOK OF

MANUFACTURING

ENGINEERING

Second Edition

E

Richard Crowson

Assembly

Processes

Finishing, Packaging,

and Automation

Preface

Editor

Richard D. Crowson

Contributors

Contents

Manual Assembly

John P. Tanner

Jack M. Walker

Assembly Automation

Jack M. Walker

Vijay Sheth