A rationalised approach to theapplication of CAD withinmechanical product designby I. BlackHeriot-Watt University

and H. C. LintonEdwards High Vacuum

This paper reviews the nature of the product design process byfocusing on the types of design that arise throughout the life cycleof an engineering product. It is proposed that considerations of'design by evolution' and "design by innovation' should be carefullyassessed as fundamental criteria when formulating decisions on theadoption and implementation of CAD within mechanical productdesign. It is further suggested that, within the conceptualboundaries of product design, the application of CAD techniquesmay accelerate a product's progression from the state of innovatorydesign towards the evolutionary design condition.

is being offered by a manufacturing organi-sation for its intended markets.

The product design process

Fig. 1 presents a general diagrammaticmodel of the phases associated with the pro-duct design process, as identified by theauthors. Although this may appear to be arelatively simplistic, or even idealistic,model to some, it nevertheless illustrates thebasic flow of information which can occurduring the design of a product. Each iden-

Introduction

A thoughtful analysis of the informationflow required for the design and develop-ment of mechanical products can yieldsignificant results when embarking on pro-grammes that hope to fit computer-aideddesign (CAD) technology to the productdesign activity. However, all too often anarrow view is taken of the product designprocess by engineering management inmanufacturing organisations when imple-menting, or intending to implement, CADtechnology. As a result, the benefits obtain-able from the deployment of CADtechniques are not fully realised.

By recognising the particular require-ments and methods of information flow thatexist within an individual manufacturingorganisation, the authors present a gen-eralised picture of the product design pro-cess, with the intention of (hopefully)stimulating thoughts and ideas that maylead to unique methodologies for the appli-cation of CAD within a company context.Specifically, the authors consider that it isessential to examine the critical role thatconceptual design plays with regard to therational and effective application of CAD.In this connection, a clearer view of thebenefits to be gained from the introductionof CAD methods can be obtained throughunderstanding where a product is situatedwithin its life cycle. This, in turn, reflectson the conceptual 'status' of a product that

a c t i v i t y

specification ofrequirements

concept selection

initiallayout

phase

def i nit i velayout

piece -partdrafting

assemblydrafting

bills of material

specificationor taskclarification

conceptualdesign

(optimisationand refinement)

detaildesign

manufacture

Fig. 1 The product design process

Computer-Aided Engineering Journal October 1987 213

tifiable phase in the product design processis outlined below. It is, however, importantto note that the demarcation between suc-cessive phases is not as rigidly defined asshown — an iterative feedback of informa-tion exists throughout the design cyclewhich fuses these phases together.

If the definition of mechanical productdesign is the 'creation of engineeringproducts to satisfy human needs' (Ref. 1),then it is recognised that the genesis of anyproduct design must encompass therecognition of a market need. Theestablishment of the economic existence ofa need may be the result of market orconsumer studies. In some situations theneed may not be instantly apparent, butthere would exist evidence that the need islatent and that it could be envoked whenthe economic means for its satisfactionbecome available.

Following the identification of the needfor a product there is a stage where the tasksand objectives of the ensuing productdesign process are clearly defined. As partof the specification phase, such factors asoperational requirements, in-servicerequirements, size and weight, mainten-ance, life, reliability, quantity, delivery andoperating costs are considered asthoroughly as possible. Careful assessmentand controlled dissemination of the marketrequirements during this particular phase ofthe design process is the key to a successfulproduct design satisfying the intendedmarket.

A clearer idea of the physical form of theproduct is established during the concep-tual design phase. During conceptualisa-tion, different alternatives for a productconcept should be formulated and assessedfor their feasibility in relation to the interde-pendent factors of function, cost, manu-facture and standards (Ref. 2) — see Fig. 1.

Graphical information plays a major role inthe communication of ideas throughout thisphase.

The result of the conceptual design phaseis, or should be, a definitive product layoutwhich contains a full product definition ingraphical form, and which will provide thenecessary input to the detail design phase.In addition, simulation and analysis tech-niques may be used in order to continuallyoptimise and refine the product layout. Thedefinitive product layout should thereforebe a complete and consistent model of theproposed product design.

Graphical data constitutes the majority ofmanufacturing information produced dur-ing the detail design phase. The manufac-turing instructions are usually in the formof piece-part drawings, bills of materials,process plans, numerical control/computernumerical control tapes and other workshopdocumentation. During the manufacturingphase the instructions emanating from thedesign process are executed.

In most instances pre-production batcheswill help in solving some of the teethingproblems associated with introducing newproducts to a manufacturing environment.Of course, once pre-production batches arecompleted, the product should be subjectto rigorous tests in order to establish its con-formation to the current specification. If itdoes not conform, then either the specifica-tion is modified to reflect the performanceof the product or the product itself under-goes further examination and design changein an attempt to more closely match therequirements of the market. The provenproduct becomes part of the productionrepertoire of the manufacturing company.However, the product's manufacturingprocesses are continually monitored andimproved in an attempt to keep productcosts to a minimum.

Product life cycles

Engineering products have finite lives whentheir contribution to overall company profit-ability is considered. However, although thenotion of limited product life is well knownto marketing specialists, it is often less wellappreciated by engineering management.

Every product goes through a cycle frominception, into an initial growth stage, intoa relatively stable period, and finally intoa phase of decline which may eventuallyend in complete obsolescence (Ref. 3). Fig.2 gives a generalised product life cyclecurve. The plotting of life cycle curves helpsto demonstrate the following importantpoints (Ref. 4):

• The life of a product is never infinite.• Adequate indication is usually availablethat revised or new product designs areneeded.• Failure to develop and launch newproducts in time may result in a temporarydecline in profitability or even the death ofan enterprise.

The form of the life cycle curve will varyaccording to the type of engineeringproduct considered. Some products willexperience a rapid growth phase and thendecline almost immediately. Other productscontinue to exhibit a rising trend evenduring the 'mature' period, followed by agradual or sudden decline.

Spectral analysis of design

A spectrum of product design work can beidentified which contains a varying degreeof original or innovative thought, dependingon which part of the spectrum is being con-sidered. Close to one extreme of this spec-trum, products will originate through

Fig. 2 Product life cycle

214 Computer-Aided Engineering Journal October 1987

Fig. 3 Design spectrum and the product life cycle

'design by innovation' (Ref. 1), where asignificant amount of original thought isinherent in the concept behind a product.However, it can be safely stated that thecondition of total conceptualisation, i.e. the'blank sheet of paper' approach, will veryrarely occur. So a proportion of a productconcept can be attributed to the designengineer's past experience of the 'known'world, and the remainder of the designwork will be innovative.

Near the other extreme of the spectrum,products are evolving. Here a productconcept relies heavily on experience. Therelatively small proportion of originalthought present in such products is a featureof 'design by evolution' (Ref. 1).

In reality, many manufacturing organisa-tions are involvedin designing, developingor changing, to varying degrees, known pro-ducts to satisfy changing marketrequirements. They are operating betweenthe boundaries imposed by evolutionaryand innovative design. The spectral analysisof design is extremely useful when con-sidering the effective deployment of CAD,where the design spectrum may be takento cover the types of design work outlinedbelow. As a result, a manufacturingorganisation striving towards the effectivedeployment of CAD should carefullyexamine the spectrum of design activitiesin relation to its product mix. Broadlyspeaking, product design can be brokendown into four basic categories, all of whichcontain varying degrees of innovation andevolution:

• innovative design, which involveselaborating an original conceptual solutionfor a product with a clearly defined need• adaptive design, which involves adap-ting a known product to a changed need(the overall conceptual solution remainingthe same) — a measure of original designis often required

• variant design, which involves varyingcertain parameters (size, arrangement,materials etc.) of an established product —no significant design problems arise• order execution, which involves com-pleting an order for a product with minimaldesign effort.

It is proposed that each type of design workwithin the indicated spectrum manifestsitself at various stages throughout a pro-duct's life cycle (Fig. 3). In the introductorystage of the product life cycle there is a greatdeal of creative or innovative work. Thisdecreases as the product enters the growthstage, where there may be an emphasis onadapting the product for slightly differentmarket needs. On approaching the maturi-ty stage, the product is arriving at an evolu-tionary state, as small refinements are madein order to develop the product conceptfurther.

As time passes the product conceptbecomes more and more mature, andultimately the market requirement for theproduct becomes established. At this stage,while the product continues to make acontribution to company profitability, it hasreached a 'conceptual plateau' (Ref. 5). Atsome point along this plateau the productenters a stage of decline, where it faceseventual obsolescence.

Product concepts, productdevelopment and CAD application

It has been hinted that the efficient harness-ing of CAD technology to product designis dependent upon the conceptual status ofa product (Refs. 5 and 6). Further, it hasbeen proposed that current product con-cepts are immature, while mature conceptsare those which have reached a conceptualplateau.

The motor car is an example of a product

Computer-Aided Engineering Journal October 1987

that could be considered conceptuallymature. It is also an example of a latentmarket. At its inception the motor car wasconsidered to be a revolutionary step intransportation systems; i.e. the product con-cept was close to an innovatory state. Overthe years, however, while there hasundoubtedly been many specific improve-ments in the motor car, which in themselvesare innovative, there have been no majorinnovations which have completelychanged current perceptions of travel bythis form of road vehicle. All motor carsbasically comprise a powertrain, seats,wheels and a protective enclosure. So wemay say that the motor car is conceptuallymature as the thinking behind this particularproduct has remained relatively fixed sinceinception.

On the other hand, it could be argued thatthe design of spacecraft is conceptuallyimmature as there is still no clear or pre-conceived ideas of their form or functional-ity. They are climbing the introductoryphase of the life cycle curve. So, the authorspropose that engineering products tendthroughout their life towards anevolutionary (mature) state, and it is whenthey are approaching this phase of their lifecycle that the application of CADtechniques becomes effective.

The product development curve, asshown in Fig. 4, gives further clues as towhen it is probably most opportune toapply CAD techniques (Ref. 7). Returningto the quoted product examples, the motorcar is approaching a point on the curvewhere the rate of innovation is minimal, i.e.a conceptual plateau. However, the designof spacecraft undoubtedly still lies on thesteep initial portion of the product develop-ment curve. If it can be established wherethe rate of innovation (effectively thegradient of this curve) begins to level out,then this may well be the time in productdevelopment where the application of CAD

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methods will prove most effective. Hencethe authors propose that to optimise theeffectiveness of CAD, application should bemade to a product design that has attained,or is approaching, conceptual maturity, i.e.an evolutionary state.

However, the authors recognise that theadvocated CAD application rationale mayhave significant repercussions for innovativeproduct design. It is suggested that designtasks which are evolutionary in nature willhave a better chance of effectively utilisingCAD methods. Evolutionary design workrelies mainly on established product designsand design methods, together with theutilisation of families of parts. CAD involvesthe establishment and growth of a centraldatabase of product information, and thisdatabase will gradually accumulate suchevolutionary product data (for example, pastdesign forms, modular product definitionsetc.).

The notion of an integrated centraldatabase is that a single unifying source ofproduct data is available for re-use by allactivities associated with design andmanufacture of a product). But it has beensuggested in some quarters that thecondition of database integration may forcea design engineer into accepting limitedoptions available in the central productdatabase (Ref. 2). The consequences of thissituation could be that a degree of constraintis placed on the amount of innovation possi-

216

ble in product design utilising CAD tech-niques, and that product concepts maytherefore be accelerated more quicklytowards evolutionary status.

In response to this, the authors stipulatethat complete reliance by the designengineer on information held within a cen-tral product database during conceptualisa-tion should be avoided, especially if anyoriginal design work is being undertaken.

However, for evolutionary product designsan integrated central product database willprobably increase overall design effective-ness, provided that a high degree of integrityis inherent in the stored product data.

Concluding remarks

It must be stressed that the hypothesesproposed in this article serve only as guide-lines for recognising the suitability of CADapplication within the product design activi-ty. However, as the process of productdesign becomes more dependent upon con-vention or product-line precedents, themore evolutionary becomes the designactivity itself. This particular situation maywell be suitable for the effective applicationof CAD technology. With designs based onless mature concepts the converse could betrue (Ref. 5).

In the wake of what has been discussed,it is reasonable to assume that the primeachievement of future CAD technologywould be to move the position at whichCAD techniques can be effectively appliedto the initial phase of the product life cycle.In other words, what may be desired is CADapplications software with an inherentability to innovate. In this connection,expert systems have been hailed as thefuture CAD technique for mechanicalproduct design (Refs. 8 and 9), but, sincetheir operation is also based on priorknowledge and experience, the authorsconsider that this emergent technology willnot be instrumental in achieving the (ques-tionable) condition of 'ideal' CAD appli-cation.

It is the authors' belief that the productdesign process needs to remain firmly in thecontrol of the design engineer, where CADtechnology is used as a tool, rather than anend in itself.

References

1 ASIMOW, M.: 'Introduction to design' (Prentice-Hall, 1962)2 BLACK, I.: 'Applications and influences of interactive computer graphics within mechanical product

design: a critical appraisal and evaluation'. M.Sc. Thesis, Heriot-Watt University, Edinburgh,Scotland, Dec. 1986

3 DIETER, G. E.: 'Engineering design' (McGraw-Hill, 1983)4 OAKLEY, M.: 'Managing product design' (Weidenfield & Nicolson, 1984)5 PUGH, S.: 'The application of CAD in relation to dynamic/static product concepts'. Proceedings

of International Conference on Engineering Design (ICED 83), Copenhagen, Denmark, 1983,Vol. 2, pp. 564-571

6 PUGH, S.: 'Further development of the hypothesis of static/dynamic concepts in product design'.Proceedings of ISDS Conference, Tokyo, Japan, 1984, pp. 216-221

7 UTTERBACK, J. M., and ABERNATHY, W. J.: 'A dynamic model of process and product innova-tion', OMEGA, International Journal of Management Science, 1975, 3, (6), pp. 639-657

8 SIMMONS, M. K.: 'Artificial intelligence for engineering design', Computer-Aided EngineeringJournal, 1984, 1, (3), pp. 75-83

9 DIXON, J. R., and SIMMONS, M. K.: 'Computers that design: expert systems for mechanicalengineers', Computers in Mechanical Engineering, 1983, 2, Nov., pp. 10-18

I. Black is a Lecturer with the Department of Mechanical Engineering, Heriot-Watt University, RiccartonCampus, Edinburgh EH14 4AS, Scotland, and H. C. Linton, formerly with the Department ofMechanical Engineering, Heriot-Watt University, is a Manufacturing Technologist with Edwards HighVacuum, Dolphin Road, Shoreham-by-Sea, Sussex BN4 6PY, England.

Computer-Aided Engineering Journal October 1987