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1193 2002 216:Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture

S H Ahn, S McMains, C H Séquin and P K WrightMechanical implementation services for rapid prototyping

  

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Mechanical implementation services for rapidprototyping

S H Ahn1, S McMains2, C H Se quin3 and P K Wright2*1School of Mechanical and Aerospace Engineering, Gyeongsang National University, Jinju, Korea2Department of Mechanical Engineering, University of California at Berkeley, USA3Department of Computer Science, University of California at Berkeley, USA

Abstract: Inspired by the metal oxide system implementation service (MOSIS) project, CyberCut is anexperimental fabrication testbed for an Internet-accessible, computerized prototyping and machiningservice. Client-designers can create mechanical components, generally using our web-based computeraided design (CAD) system (available at http://cad.berkeley.edu), and submit appropriate ®les to theserver at Berkeley for process planning. CyberCut then utilizes an open-architecture, computernumerical control (CNC) machine tool for fabrication. Rapid tool path planning, novel ®xturingtechniques and sensor-based precision machining techniques allow the designer to take delivery of acomponent machined from high-strength materials with good tolerances, e.g. §0.002 in(0.05 mm).There are also instances where the complex geometry of a component cannot beprototyped on our three-axis machine tool. For these components use is made of solid freeformfabrication (SFF) technologies such as fused deposition modelling (FDM) to build a prototype ofthe design. Based on experience with this testbed, a new characterization of types of relationship, or

‘couplings’, between design and manufacturing has been developed using the three classi®cations‘loose and repetitive’, ‘stiV and one-way’ or ‘strong and bidirectional’. These three couplingsrepresent diVerent trade-oVs between ‘design ¯exibility’ and ‘guaranteed manufacturability’.

Keywords: mechanical implementation service, rapid prototyping, CyberCut, design for manufac-turing

1 INTRODUCTION

During the late 1970s, Mead and Conway [1] created thegroundwork for the fast prototyping of very large scaleintegrated circuits (VLSI). Designers were encouragedto think in terms of ®ve two-dimensional patterns.These patterns de®ned three stacked interconnectionlayers on a metal oxide semiconductor (MOS) waferand their mutual connections through via holes. Thepatterns described the actual geometry of the connectionruns and via holes that would be seen when lookingdown onto the circuit chip, regardless of the exactprocess and number of masking steps that were used toimplement the chip. The system was called the metaloxide system implementation service (MOSIS) [2].Today it provides students at research universities withthe opportunity to obtain prototype chips duringsemester-long computer aided design/manufacture

(CAD/CAM) courses in VLSI design and fabrication.During design, students are obliged to follow therelatively conservative MOSIS layout rules. However,by doing so, they are sure that their chips can be fabri-cated by a number of semiconductor companies thatoVer their services to the MOSIS bureau.

Inspired by the success of the VLSI MOSIS project, USNational Science Foundation (NSF) workshops in theearly 1990s addressed the possibilities of a mechanicalimplementation service (MIS) [3, 4]. However, it wasquickly evident that there was not a perfect analogybetween VLSI and the mechanical domain:

1. The existing MOSIS service focused on just one elec-trical component of a consumer electronic product,namely VLSI logic circuits. To have the same success,an MIS might need to be restricted to one mechanicalcomponent of a consumer electronic product, such asbearings, brackets or casings, rather than completemechanical assemblies.

2. MOSIS initially targeted just one family of manufac-turing processes, namely NMOS planar fabrication.An initial MIS might then focus on just one

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SC02201 IMechE 2002 Proc Instn Mech Engrs Vol 216 Part B: J Engineering Manufacture

The MS was received on 12 July 2001 and was accepted after revision forpublication on 22 April 2002.*Corresponding author: Department of Mechanical Engineering,Collegeof Engineering, University of California at Berkeley, 5133 EtcheverryHall, Berkeley, CA 94720-1740, USA.

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fabrication process such as three-axis millingÐone ofthe more ¯exible of the metal processing fabricationmethods.

3. MOSIS fabricates 2.5-dimensional VLSI chips, whilemechanical processes can fabricate complex three-dimensional shapes.

4. In digital logic systems it is easy to give up a factor 2±4to achieve modularity and some abstraction andisolation between various subsystems. In mechanicalparts, making a part 2±4 times heavier or larger thanneeded is out of the question.

5. In VLSI, design concerns of device performance,circuit layout and systems architecture can be nicelyseparated and addressed with diVerent representa-tions and abstractions, e.g. the mask geometry, the

‘sticks’ layout and the block diagram respectively.In mechanical systems, the many diVerent concerns,e.g. stability, aesthetic, ergonomics, sound isolationand electrical shielding in the housing of a handdrill, are much more tightly coupled and cannot beeasily separated and assigned to diVerent features.

A consensus thus emerged at these workshops that itwould be a challenge to create an integrated MIS thatcould truly oVer the ¯exibility that most mechanicaldesigners and fabricators require.

2 DESIGN FLEXIBILITY VERSUS GUARANTEEDMANUFACTURABILITY

This short communication reviews some recent progressfor both machining and SFF processing that allows theintroduction of an MIS in the spirit of the originalMOSIS. In comparison with VLSI MOSIS, the mechan-ical domain brings much more complexity to the linkbetween design and manufacturing, thus demanding avariety of approaches to the idea of a fully automatedimplementation service. The philosophical dilemma ofcontinued interest is to balance the following two oppos-ing factors:

(a) the designer’s demand for a ¯exible and extensibleCAD environment,

(b) the inherent limitations of any given manufacturingprocess.

To address this dilemma, it has been found practical tothink in terms of three models of coupling the CAD

tools and the fabrication process:

(a) the traditional, loose, repetitive style,(b) the stiV, one-way mode of VLSI MOSIS,(c) an emerging strong, bidirectional coupling.

In the loosely coupled, repetitive mode, no commitmentto a particular manufacturer need be made in the earlyphases of design. The designer initially works in awide-open design space and eventually transmits designinformation to one or more potential manufacturers.At this point an intense interaction ensues, and thedesign is modi®ed repeatedly on the basis of criticismsand suggestions from the chosen manufacturer.

In the sti‚y coupled, one-way mode, the designerworks in a restricted design space that allows manufac-turing rule checkers based on a particular manufacturer’scapabilities to be easily integrated. Design information,typically in electronic form, is transferred to themanufacturer only once, with no need for feedback anditerative improvements.

In the strongly coupled, bidirectional mode, thedesigner also starts out working in a large design space,but is guided away from unfeasible designs throughrepeated automatic feedback from manufacturabilityprocess planners. The designer commits to a particularmanufacturer or even a particular machine early on togain access to very speci®c feedback. Table 1 shows abrief comparison of the three coupling modes. Thefollowing three sections describe further each mode oflinking designers and fabricators, using illustrativeexamples.

3 THE TRADITIONAL ‘LOOSELY COUPLED’CAD/CAM ENVIRONMENT

Colloquially speaking, this corresponds to ‘over-the-wall’ manufacturing typical of today’s mechanicalCAD/CAM community. The ‘client’ for this loosermodel of CAD/CAM is a designer who uses a ¯exibleCAD tool, who may be only marginally aware of anymanufacturing limitations and who is willing to pay (inboth cost and delays) for the machine shop to redesignthe part so that it can be manufactured.

In industry, such clients might work in an aerospacecompany or a national laboratory, where the ‘voice’ ofthe design community usually dominates over that ofthe manufacturing community. It should nevertheless

Table 1 Comparison of the three coupling modes

Coupling mode Pros Cons

Loose and repetitive Flexible design Cost and delay for redesignStiV and one-way Guaranteed manufacturability Less design freedomStrong and bidirectional Moderately ¯exible design,

guaranteed manufacturabilitySome loss of design freedom

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be emphasized that this mode requires many phone callsor face-to-face meetings between the designer and thefabricators. These are needed to resolve designambiguities and/or manufacturing procedures that willdemand special tooling.

Even within a university-like prototyping environ-ment, more complex mechanical designs with multiplesubcomponents also demand the ¯exibility of commer-cial CAD systems and the capabilities of a conventionalmachine shop with experienced craftspeople. Forexample, injection moulds and many types of mechanicalpart exhibit the free-form surfaces shown in Fig. 1. This®gure shows an aluminium mould, at the rear of thephotograph, that was used during an injection mouldingprocess to form the plastic casings for the ®ngerprint

recognition devices shown in the foreground. Figure 2shows an artistically driven part with toroidal andspherical surfaces. Our laboratories have routinelycreated and then fabricated such parts that have beendesigned by conventional CAD methods using construc-tive solid geometry (CSG) [5] and parametric [6] and/orconstraint-based design [7]. Commercial packages suchas PTC ProEngineer, the SDRC I-DEAS environment,SolidWorks, AutoCAD and ACIS have been used forthe design phase. For example, tools built on top ofACIS were used to design the parts in Figs 1 and 2.The use of such packages then demands the skill of adownstream craftsperson (usually a senior graduatestudent) on the manufacturing side to carry out severalcrucial steps:

Fig. 1 Machined partÐaluminium mould (back) for the STMicroelectronics ®ngerprint sensor. ABS plastic

parts were made from the mould (front left) and then assembled with a printed circuit board

Fig. 2 FDM and machined partsÐthree-dimensional Yin-Yangs were fabricated by both the FDM process(left) and in aluminium with CyberCut machining (right)

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(a) analysis of the part and its breakdown into millablefeatures,

(b) a decision on the feasible ®xturing methods,(c) the creation of tool paths,(d) the selection of milling conditions.

Despite integrated capabilities in this area, with thedesign and manufacturing engineers working moreclosely than in a standard machine shop, the intensehands-on aspects of manufacturing and the many discus-sions with the original designer move the activity out ofthe realm of a MOSIS-like MIS.

4 EXPERIMENTAL ‘STIFF’ CAD/CAMENVIRONMENT

This environment is modelled after the MOSIS style ofVLSI circuit design. The ‘client’ for this model of CAD/CAM is a designer who is willing to give up a lot ofdesign freedom for the bene®t of guaranteed manufactur-ability, low cost and fast delivery. Such a client might beinterested in quickly obtaining the fused depositionmodelling (FDM) part shown in Figs 3a and b, or themachined metal bracket in Fig. 4 to be used in a piece

of experimental equipment. A testbed, called CyberCut[8], for this ‘stiV’ service has been created at Berkeleyand is now being used by students in the engineeringand CS classes, by colleagues in related research labora-tories and by a limited number of students in collaborat-ing campuses such as Carnegie Mellon University. FDMand machining are the two processes currently beingoVered to these clients. In this ‘stiV service mode’, theclient is obliged to accept the following MOSIS-likelimitations. For FDM parts that are made on theStratasys 1650 machine:

1. Files in STL [9] or SIF [10, 11] format are accepted.2. Build size is limited to parts that are 10 £ 10 £ 10 in in

size.3. Although the Stratasys Quickslice software can

handle much larger ®les, in student class projects itis preferred to limit the number of triangles toabout 40 000.

4. ABS plastic is the work material. Since it is layered,the part strength is less than that of full-strengthABS: on average only 70 per cent in the bestorientation or as poor as 10 per cent in the worstorientation.

5. The accuracy of a mid-size (5 in) part is only §0.03 in.

The FDM process builds a part layer by layer. Therefore,accessibility during fabrication is not an issue, allowingvirtually any complex three-dimensional geometry tobe deposited in a layer-by-layer fashion. Thus, there arenot too many design rules concerning the basic geometryof the part, other than observing a minimum feature sizeof about 0.02 in. However, the selection of the buildorientation and the possible modi®cation of the defaultoverhang angle under which some supporting scaVoldinghas to be built need some consideration.

Build rules for planning a successful FDM fabrication(for example, orienting the largest face down on themachine table) are available from references [12] and[13]. For faster FDM builds of large solid parts, thetechnique described in reference [14] is used. Optimizing

Fig. 3 FDM partÐpackage of GPS module for PDA device

Fig. 4 L-bracket machined by the CyberCut system

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builds for solid freeform fabrication (SFF) technologiesin general is discussed in references [15] to [18].

For machined parts that are made on the Haas VF0three-axis CNC milling machine, the following limita-tions apply:

1. Build-size is limited to parts that are 36 £ 15 £ 12 inin size but, for student class projects, typically a12 £ 4 £ 4 in limitation is set.

2. Steel, aluminium and ABS plastic are the availablework materials.

3. The accuracy of the resulting part is as good as§0.002 in.

For the fully automated, MIS testbed for milling, theclient designer must use the WebCAD [19] front-end asthe design software. In addition to enforcing the limita-tions on build size and material, this software imposes astrict destructive solid geometry (DSG) design philosophyand checks machining rules [8]. To guarantee manufactur-ability, the designer begins with a cuboid stock andremoves features that can be milled or drilled. Limits onthe corner radii of inside corners and limitations onfeature placement are shown to the designer during thegraphical editing process. Part designs are output fromthis environment in SIF-DSG format [20] and submittedto the server, where they are processed automatically toproduce G-code tool paths for the milling machine.

Consequently, at the time of writing, the componentsbeing made in this fully automated but ‘stiV’ machiningtestbed are single subcomponents that exhibit relativelysimple 2.5-dimensional geometries composed of milledpockets and drilled holes (Fig. 4). Turner and Andersondescribe such a machining feature based approach todesign [21]; Cutkosky and Tenenbaum described theimplementation of another such stiV environment thatsupported concurrent process planning [22]. Thedesigner must use a special-purpose CAD environmentonly allowing design features that can be automaticallymapped onto machining features. Such an environmentalso makes it feasible to add the built-in rule checkingcapabilities that are available. The evidence so far isthat a fully constrained, or stiV, design environment, inwhich MOSIS-like rules and limitations are strictlyimposed, is acceptable to only a small subset of alldesigners.

5 AN EMERGING ‘STRONGLY COUPLED,BIDIRECTIONAL’ CAD/CAM ENVIRONMENT

At the time of writing, the focus is on an alternativemodel oVering the best of the two previous models.This third approach is bidirectionally coupled. From hisor her local workstation, the CAD designer can access,over the Internet, certain software agents that describethe limitations of downstream manufacturing andallow process planning routines to be run on an emerging

part design. The goal is to urge the designer to considermanufacturability limitations (CAM) at critical junc-tures during design (CAD), thereby insuring thatmanufacturing can more readily take place on a speci®cmachine at a chosen subcontractor.

The intended ‘client’ for this coupled model of CAD/CAM is a designer who wants to use a ¯exible, commer-cially oriented design system but who is willing to checkits manufacturability, from time to time, as the designemerges on the CAD screen. This requires extra eVortand some compromises on the part of the designer.However, it is anticipated to be a useful mode of attackthat will avoid many (but not all) of the ‘downstream’manufacturability problems.

The current experimental environment for this thirdtype of implementation service for milling begins withany CAD system such as SolidWorks, ProEngineer orSDRC I-DEAS. After some initial design work hasbeen done to create a new component, the user outputsthe CAD ®le in a neutral format (currently, STEP [23]or the ACIS .sat format [24] is used). The neutral part®le is then ftp-ed to the remote server at the manu-facturing site, where it is converted to ACIS .sat ifnecessary, so that it can be used as input to the suite ofautomated process planning and analysis tools. Thecurrent modules include:

(a) feature recognition,(b) feature sequencing and ®xture planning routines,(c) feature decomposition and tool path planning

routines,(d) special-purpose tool path routines to minimize

deleterious burrs on a part,(e) cost estimations for producing the features so far

speci®ed.

Once the analysis has been completed, the designer isinformed whether a suitable process plan was foundand what the incremental costs are. Because a detailedprocess plan has been produced, a reliable estimate ofthe cost can be made without relying on heuristics. ACNC ®le can also be generated and sent back on request,if the part is indeed manufacturable.

As an alternative implementation, such softwareagents as direct ‘plug-ins’ to the CAD designer’s desktophave also been con®gured. This con®guration allows thedesigner immediately to run ‘virtual process plans’ on theemerging part design on the local workstation for fasterfeedback. Implementing multiple plug-ins in a researchlaboratory is generally not feasible, however, as theymust be con®gured to the speci®c APIs of each individualcommercial CAD system, a time consuming and non-platform independent task [25].

The process planning work in these systems builds onmany years of related research in feature recognition.Popular approaches include graph matching, rule-based volume decomposition and knowledge-basedtechniques [26, 27]. Surveys of feature recognition and

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automatic manufacturability analysis can be found inreferences [28] and [29]. Analysing manufacturabilityand evaluating process plans are discussed in references[30] and [31]. Recent work by Han et al. uses featurerecognition for cost estimation [32].

6 DISCUSSION

As a ®nal note, it is worth posing the following question:what factors might encourage an industrial designer togive up the familiarity and ¯exibility of one of the well-known commercial CAD systems in favour of the ‘stiV,one-way’ mode of a MOSIS-like MIS? In today’s indus-trial settings the answer to this question is usually drivenby cost considerations. If there is a major cost saving thatcan be obtained as a result of the ‘guaranteed manufac-turability and fast turnaround’ of the CyberCut MIS,then an industrial designer will be obliged to consider itmore seriously and forego some of the creative shapesthat are possible on a CAD system. Of course, if thedomain of parts that can be designed with WebCADcan continue to be expanded and ‘guaranteed manufac-turability’ can still be maintained, then so much thebetter.

Also, recall that mechanical devices usually consist ofmany subcomponents. Do all of these subcomponentsneed to exhibit graceful complex curves? Probably not:subcomponents that are destined to be buried deepinside a washing machine or an automobile can morereadily be designed and fabricated with the ‘stiV’ modeof CyberCut. By contrast, the outer bodies of a car ora consumer productÐthose needing ‘eye-catching’graceful linesÐmight be the subcomponents that needto be designed with the full ¯exibility of a mature CADsystem, where full design creativity is desirable andpotential manufacturing di�culties are tolerated. Thiscan be achieved in either the traditional loosely coupledmode or in the newer, strongly coupled bidirectionalmode; but the latter is expected to lead to much shortercompletion times, in most cases.

A valuable insight gained in the CyberCut andMOSIS++ projects was the identi®cation of thesethree existing modes of interaction between designersand fabricators, and realizing that all three of themmay have their advantages for some particular designsituation. All three diVerent types of test bed have beenimplemented, and students and clients have been allowedto explore them.

7 CONCLUSIONS

1. Inspired by VLSI MOSIS, an experimental mechanicalimplementation service (MIS) has been developed thatemploys prototyping by layered manufacturing(FDM) and three-axis milling.

2. Each manufacturing process plays a vital role atdiVerent points in a product development cycle.Layered manufacturing will be more important atthe start of a product development cycle, whilemachining becomes most appropriate at a laterstage to create highly accurate moulds. Finally,plastic injection moulding will dominate in the ®nal,high-volume production phase of a consumerproduct.

3. New interchange formats for SFF processes (SIF)and for machining (SIF-DSG) have been developedwhich are designed to serve as the link betweendesigners and fabricators in an MIS.

4. We can characterize the coupling between design andmanufacturing as either ‘loose and repetitive’, ‘stiVand one-way’ or ‘strong and bidirectional’, choosingdiVerent points in the trade-oV between ‘design¯exibility’ and ‘guaranteed manufacturability’. Thedecision as to which approach to use depends onthe designer’s preference, on cost considerations andon the type of parts to be fabricated.

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