Dfma handbook

Dfma handbook

(Parte 1 de 7)

CHAPTER 1.1

This is a reference book for those practicing or otherwise having an interest in design for manufacturability (DFM). DFM principles and guidelines are many; no one person should be expected to remember them all nor the detailed information,such as suggested dimensional tolerances,process limits,expected surface finish values,or other details,of each manufacturing process. It is expected that those involved will keep this book handy for reference when needed.

Additionally,this handbook is intended to be an educational tool to assist those who wish to develop their skills in ensuring that products and their components are easily manufactured at minimum cost. Its purpose,further,is to enable designers to take advantage of all the inherent cost and other benefits available in the manufacturing process that will be used.

Like handbooks in other fields,it is a comprehensive summary of information which,piecemeal at least,is known by or available to specialists in the field. Although some material in this handbook has not appeared in print previously,the vast majority of it is a restatement,reorganization,and compilation of data from other published sources.

The subject matter of this book covers the area where product engineering and manufacturing engineering overlap. In addition to being directed to product designers and manufacturing engineers,this book is directed to the following specialists:

Operation sheet writers Value engineers and analysts Tool engineers Process engineers Production engineers Cost-reduction engineers Research and development engineers Drafters

1.4 INTRODUCTION

Industrial engineers Manufacturing supervisors and managers

These specialists and any other individuals whose job responsibilities or interest involve low-cost manufactured products should find this handbook useful.

This book contains summary information about the workings and capabilities of various significant manufacturing processes. The standard format for each chapter involves a clear summary of how each manufacturing process operates to produce its end result. In most cases,for added clarity,a schematic representation of the operation is included so that the reader can see conceptually exactly what actions are involved. In many cases,for further clarification,photographs or drawings of common equipment are presented. The purpose of this brief process explanation is to enable readers to understand the basic principles of the manufacturing process to determine whether it is applicable to production of the particular workpiece they have in mind.

To illustrate further the workings of each manufacturing process from the viewpoint of product engineers,descriptive information on typical parts produced by the process is provided. This book tells readers how large,small,thick,thin,hard,soft, simple,or intricate the typical part will be,what it looks like,and what material it is apt to be made from. Typical parts and applications are illustrated whenever possible so that readers can see by example what can be expected from the manufacturing process in question.

Since so many manufacturing processes are limited in economical application to only one portion of the production-quantity spectrum,this factor is reviewed for each process being covered. We want to help engineers to design a product for a manufacturing process that fits not only the part configuration but the expected manufacturing volume as well. We want to steer them away from a process that,even though it might provide the right size,shape,and accuracy,would not be practical from a cost standpoint.

To aid designers in specifying a material that is most usable in the process,information is provided on suitable materials in each chapter. Emphasis is on materials formulations that give satisfactory functional results and maximum ease of processibility. Where feasible,tables of suitable or commonly used materials are included. The tables usually provide information on other properties of each material variation and remarks on the common applications of each. Where available,processibility ratings are also included. All materials selection is a compromise. Functional considerations— strength, stiffness, corrosion resistance, electrical conducti vity, appearance, and many other factors—as well as initial cost and processing cost,machinability,formability, and so on,must all be considered. When one factor is advantageous,the others may not be. Most of the materials recommendations included in this handbook are for runof-the-mill noncritical applications for which processibility factors can be given greater weight. The purpose is to aid in avoiding overspecifying material when a lower-cost or more processible grade would serve as well. For many applications,of course,grades with greater functional properties must be used,and materials suppliers should be consulted.

The heart of this handbook (in each chapter) is the coverage of recommendations for more economical product design. Providing information to guide designers to configurations that simplify the production process is a prime objective of this handbook.

PURPOSE,CONTENTS,AND USE OF THIS HANDBOOK1.5

Design recommendations are of two kinds:general design considerations and detailed design recommendations. The former cover the major factors that designers should take into consideration to optimize the manufacturability of their designs. Such factors as shrinkage (castings and molded parts),machining allowances,the feasibility of undercuts,and the necessity for fillets and radii are discussed.

Detailed design recommendations include numerous specific tips to aid in developing the most producible designs with each process. Most of these are illustrated and are in the form of “do,don’t,”“this,not this,”or “feasible,preferable”so that both the preferred and less desirable design alternatives are shown. The objective of these subsections is to cover each characteristic having a significant bearing on manufacturability.

Dimensional-tolerance recommendations for parts made with each process are another key element of each chapter. The purpose is to provide a guide for manufacturing engineers so that they know whether a process under consideration is suitable for the part to be produced. Equally important,these recommendations give product designers a basis for providing realistic specifications and for avoiding unnecessarily or unrealistically strict tolerances. The recommended tolerances,of course,are average values. The dimensional capabilities of any manufacturing process will vary depending on the peculiarities of the size,shape,and material of the part being produced and many other factors. The objective in this book has been to provide the best possible data for normal applications.

To give a fuller understanding of these tolerances and the reasons why they are necessary,most chapters include a discussion of the dimensional factors that affect final dimensions.

This handbook helps determine which process to use,but it does not tell how to operate each process,e.g.,what feed,speed,tool angle,tool design,tool material, process temperature,pressure,etc.,to use. These points are valid ones and are important,but of necessity,they are outside the scope of this book. To include them in addition to the prime data would make this handbook too long and unwieldy. This kind of material is also well covered in other publications. The emphasis in this book is on the productrather than the process,although a certain amount of process information is needed to ensure proper product design.

This book also does not contain very much functional design information. There is little material on strength of components,wear resistance,structural rigidity,thermal expansion,coefficient of friction,etc. It may be argued that these kinds of data are essential to proper design and that consideration of design only from a manufacturability standpoint is one-sided. It cannot be denied that functional design considerations are essential to product design. However,these factors are covered extensively and well in innumerable handbooks and other references,and it would be neither economically feasible nor practicable to include them in this book. This handbook is to be used in conjunction with such references. The subjects of functional design and design for manufacturability are complementary aspects of the same basic subject matter. In this respect,DFM is no different from industrial design,which deals with product appearance,or reliability design or anticorrosion design,to cite some examples of subsidiary design engineering disciplines that have been the subject of separate handbooks.

The responsibilities of design engineers encompass all aspects of design. Although functional design is of paramount importance,a design is not complete if it is functional but not easily manufactured,or if it is functional but not reliable,or if it has a good appearance but poor reliability. Design engineers have the broad responsibility to

1.6 INTRODUCTION produce a design that meets all its objectives:function,durability,appearance,and cost. A design engineer cannot say,“I designed it. Now it’s the manufacturing engineer’s job to figure out how to make it at a reasonable cost.”The functional design and the production design are too closely interrelated to be handled separately.

Product designers must consider the conditions under which manufacturing will take place,since these conditions affect production capability and costs. Such factors as production quantity,labor,and materials costs are vital.

Designers also should visualize how each part is made. If they do not or cannot, their designs may not be satisfactory or even feasible from the production standpoint. One purpose of this handbook is to give designers sufficient information about manufacturing processes so that they can design intelligently from a producibility standpoint.

Manufacturing engineers have a dual responsibility. Primarily,they provide the tooling,equipment,operation sequence,and other technical wherewithal to enable a product to be manufactured. Secondarily,they have a responsibility to ensure that the design provided to the manufacturing organization is satisfactory from a manufacturability standpoint.

It is to the latter function that this handbook is most directly aimed. In the well-run product design and manufacturing organization,a team approach is used,and the product engineer and manufacturing engineer work together to ensure that the product design provides the best manufacturability.

Another function of manufacturing engineers,cost reduction,deserves separate comment. Manufacturing and industrial engineers and others involved in manufacturing under industrial conditions have,since the process began,made a practice of whittling away at the costs involved in manufacturing a product. Fortunes have been spent (and made) in such activities,and no aspect of manufacturing costs has been spared. No avenue for cost reduction has been ignored. In my experience,by far the most lucrative avenue is the one in which the product design is analyzed for lower-cost alternatives (value analysis). This approach has proved to provide a larger return (greater cost reduction) per unit of effort and per unit of investment than other approaches,including mechanization,automation,wage incentives,and the like.

This book can be used with any of three methods of reference:(1) by process,(2) by design characteristic,and (3) by material. Readers will use the first approach when they have a specific production process in mind and wish to obtain further information about the process,its capabilities,and how to develop a product design to take best advantage of it. Most of the handbook’s chapters are concerned with a particular process,e.g.,surface grinding,injection molding,forging,etc.,and it is a simple matter to locate the applicable section from the Contents or Index.

The problem with the process-oriented book layout is that it is not adapted to designers (or manufacturing engineers) who are concerned with a particular product

PURPOSE,CONTENTS,AND USE OF THIS HANDBOOK1.7 characteristic and do not really know the best way to produce it. For example,designers having the problem of making a nonround hole in a hardened-steel part may not be aware of the best process to use or even of all processes that should be considered. This is the kind of problem for which this handbook is intended to provide assistance. There are three avenues that readers can use to obtain assistance in answering their questions:

1.The handbook chapters,as much as possible,are aimed at a workpiece characteristic,e.g.,“ground surfaces flat,”rather than a process,e.g.,“surface grinding.”

2.The Index has numerous cross-references under product characteristics such as “holes,nonround”or “surfaces,flat.”It provides page listings for various methods of making such holes,e.g.,electrical-discharge machining (EDM),electrochemical machining (ECM), broaching, etc.

3.This a chapter entitled,“Quick References”(Chap. 1.4),where readers can obtain comparative process-capability data for a variety of common workpiece characteristics such as round holes,nonround holes,flat surfaces,contoured surfaces,etc. A full listing of quick-reference subjects can be found in the Contents.

To aid readers interested in obtaining information about the manufacturability of particular materials,there is a section entitled,“Economical Use of Raw Materials” (Sec. 2),that summarizes applications of common metallic and nonmetallic materials and recommends certain material formulations or alloys for easy processibility with common manufacturing methods.

This handbook can be used for reference at the following stages in the design and manufacture of a product:

1.When a new product is in the concept stage of product development,to point out,at the outset,potentially low-manufacturing-cost approaches. This is by far the best time to optimize manufacturability.

2.During the design stage,when prototypes are built and when final drawings are being prepared,particularly to ensure that dimensional tolerances are realistic.

3.During the manufacturability-review stage,to assist manufacturing engineers in ascertaining that the design is suitable for economical production.

4.At the production-planning stage,when manufacturing operations are being chosen and their sequence is being decided on.

5.For guidance of value-analysis activities after the product has gone into production and as production quantities and cost levels for materials and labor change,providing a potential for cost improvements.

6.When redesigning a product as part of any product improvement or upgrading.

7.When replacing existing tooling that has worn beyond the point of economical use.

At this time it usually pays to reexamine the basic design of the product to take advantage of manufacturing economies and other improvements that may become evident.

Most dimensional data in this handbook are expressed in both metric and U.S. customary units. Metric units are based on the SI system (International System of Units). In some cases,data have been rounded off to convenient values instead of following exact equivalents. This was done with design and tolerance recommendations when it was felt that easily remembered order-of-magnitude values were more important than precise conversions.

(Parte 1 de 7)

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