development of a rapid prototyping design advice system

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Journal of Intelligent Manufacturing (1999) 10, 331–339 Development of a rapid prototyping design advice system RICHARD BIBB,* ZAHARI TAHA,* ROBERT BROWN* and DAVID WRIGHT { *Design Engineering Research Centre, University of Wales Institute, Cardiff, Western Avenue, Cardiff, Wales, CF5 2YB, UK {Department of Design, Brunel University, Englefield Green, Egham, Surrey, TW20 0JZ, UK This paper describes the initial development of a computer based Rapid Prototyping Design Advice System. The system is intended to assist the designer or project manager, particularly those in small and medium sized companies, in planning the prototyping stage of product development. It provides the user with an efficient and user friendly development aid which uses information obtained from the user and Computer Aided Design data to recommend suitable rapid prototyping solutions. Keywords: Rapid prototyping, expert system, knowledge based system, decision support, product development aid 1. Background Rapid prototyping (RP) refers to the technologies that are capable of producing prototypes from Computer aided design (CAD) data. It is increasingly being used by industry as it offers significant advantages in terms of time and cost reduction and improved quality of the final product. However the advent of a variety of rapid prototyping technologies such as Stereolithography, laminated object manufacture, fused deposition modeling and selective laser sintering have put the user in a dilemma as to which technology offers the best solution to their needs. This may be further complicated by the many secondary processes such as vacuum casting which are often used in conjunction with rapid prototyping. Current research in rapid prototyping concentrates mostly on the improvement of existing process and materials development (Rahmati, 1995) and the development of new processes and materials. This type of research is mostly carried out by the manufacturers of rapid prototyping machinery and their partners. Other research is directed at finding ways of manufacturing metal prototype parts and tooling rapidly (Hague, 1995; Iuliano, 1995; Norman, 1995; Spencer, 1995). There is also research being carried out on the use of RP technologies for use in medical applications (Bart Swaelens, 1993; Duffy), stress analysis, flow analysis (Chris Driver, 1996; Calvert, 1995) and automated post finishing of parts to remove the stair stepping effect common to most rapid prototyping processes (Reeves, 1995). It can be seen that due to the rapid development of these RP technologies and the potential benefits they provide there is a need for a computer based system to recommend the best system that suits a user’s requirements. There have been attempts at developing RP selection system which are based on relational databases (Mu ¨ller, 1996; Campbell, 1996). The first approach uses a relational database-management system. In this approach a database of machines and materials as well as calculation of building time and cost are constructed. A user has to specify details of the machine and materials to be used. The user has also to specify the way in which the prototype is to be built. The method of Benefit Value Analysis (Mu ¨ller, 1996) is then used to evaluate various combinations of machine and material against the user’s requirements. 0956-5515 # 1999 Kluwer Academic Publishers

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Page 1: Development of a rapid prototyping design advice system

Journal of Intelligent Manufacturing (1999) 10, 331±339

Development of a rapid prototyping design

advice system

R I C H A R D B I B B , * Z A H A R I TA H A , * RO B E R T B ROW N * and

D AV I D W R I G H T {

*Design Engineering Research Centre, University of Wales Institute, Cardiff, Western Avenue,Cardiff, Wales, CF5 2YB, UK{Department of Design, Brunel University, Engle®eld Green, Egham, Surrey, TW20 0JZ, UK

This paper describes the initial development of a computer based Rapid Prototyping Design Advice

System. The system is intended to assist the designer or project manager, particularly those in small

and medium sized companies, in planning the prototyping stage of product development. It provides

the user with an ef®cient and user friendly development aid which uses information obtained from

the user and Computer Aided Design data to recommend suitable rapid prototyping solutions.

Keywords: Rapid prototyping, expert system, knowledge based system, decision support, product

development aid

1. Background

Rapid prototyping (RP) refers to the technologies that

are capable of producing prototypes from Computer

aided design (CAD) data. It is increasingly being used

by industry as it offers signi®cant advantages in terms

of time and cost reduction and improved quality of the

®nal product. However the advent of a variety of rapid

prototyping technologies such as Stereolithography,

laminated object manufacture, fused deposition

modeling and selective laser sintering have put the

user in a dilemma as to which technology offers the

best solution to their needs. This may be further

complicated by the many secondary processes such as

vacuum casting which are often used in conjunction

with rapid prototyping. Current research in rapid

prototyping concentrates mostly on the improvement

of existing process and materials development

(Rahmati, 1995) and the development of new

processes and materials. This type of research is

mostly carried out by the manufacturers of rapid

prototyping machinery and their partners. Other

research is directed at ®nding ways of manufacturing

metal prototype parts and tooling rapidly (Hague,

1995; Iuliano, 1995; Norman, 1995; Spencer, 1995).

There is also research being carried out on the use of

RP technologies for use in medical applications (Bart

Swaelens, 1993; Duffy), stress analysis, ¯ow analysis

(Chris Driver, 1996; Calvert, 1995) and automated

post ®nishing of parts to remove the stair stepping

effect common to most rapid prototyping processes

(Reeves, 1995). It can be seen that due to the rapid

development of these RP technologies and the

potential bene®ts they provide there is a need for a

computer based system to recommend the best system

that suits a user's requirements.

There have been attempts at developing RP

selection system which are based on relational

databases (MuÈller, 1996; Campbell, 1996). The ®rst

approach uses a relational database-management

system. In this approach a database of machines and

materials as well as calculation of building time and

cost are constructed. A user has to specify details of

the machine and materials to be used. The user has

also to specify the way in which the prototype is to be

built. The method of Bene®t Value Analysis (MuÈller,

1996) is then used to evaluate various combinations of

machine and material against the user's requirements.

0956-5515 # 1999 Kluwer Academic Publishers

Page 2: Development of a rapid prototyping design advice system

An example of the evaluation is ``®rst place, 83% of

requirement degree, for the machine ABC with

material XYZ'' (MuÈller, 1996). There are two major

drawbacks to this approach. Firstly the user has to

have a working knowledge of the machines and

materials that they intend to use. This precludes the

use of the system by contract out users who do not

have their own RP system but need to determine the

best system in order to commission a prototype.

Typically these users are in small companies (small to

medium sized enterprises or SMEs). Secondly the

reliability of the results are in question as they are not

veri®ed against the experience of expert users and

therefore have little or no practical value.

The second approach is also developed on a

relational database-management system. The struc-

ture of the database however differs from the ®rst

approach in that individual features are used as RP

benchmarks. These features could be dimensions,

holes, slots or even features which are speci®c to

particular products. This means that RP systems are

selected based on the speci®cations provided by the

user and re®ned through an iterative process. The

system is based on the hypothesis that ``once the

capability of an RP system has been determined for

individual features it's capability for any component

containing these features can be predicted''

(Campbell, 1996). The monumental task of building

a data base that contains the values for every feature

built using every RP system would make the system

unreachable in terms of cost to the casual user such as

small companies.

In this paper we describe the development of a

system that will recommend the most appropriate

route to creating a prototype. The system has the

distinct advantage of being easy to use by minimising

and simplifying input for non-expert users. In addition

the system covers a wider scope of prototyping

technology such as machining as well as pre and post

processing of rapid prototyping technologies such as

Stereolithography.

2. System requirements

In general a system for giving advice must ful®l

several requirements. These are (Swift, 1987)

2.1. Capable of giving useful advice

The advice the system provides should be concise,

relevant and reliable. It should also be presented in a

convenient and familiar format so that it can be

readily assimilated into the existing tasks the user is

undertaking. The resulting advice should therefore be

presented using phrases, terms and units that the user

would already be using.

In reality the same prototyping methods will vary in

cost and lead time depending upon the circumstances

of the service provider and customer therefore

arriving at a precise numerical results may be

misleading. This design advice system should aim to

provide the user with enough information to select a

viable route and seek a detailed quote from a service

provider. The results of the system should be in the

form of generalized ``advice'' or ``guidance'' as

opposed to de®nitive numerical results. Calculations

that are essentially numerical will obviously produce

results as real numbers and in such cases these should

be rounded to comply with the overall aims of the

system.

2.2. Quick enough so that it does not signi®cantlyoccupy a designers time

The system should enable the required advice to be

obtained quicker than the existing methods of

seeking advice. In addition the system should not

be laborious to use. The user input procedure

especially should be kept to a minimum. The

system should not require input information that

the user could not be expected to have readily to

hand. The calculation time should also be a quick as

is reasonably possible as the user is effectively idle

whilst waiting for the system to produce the resulting

advice.

2.3 Easy to understand and use

The workings of the system should be as simple to

operate and understand as possible. This includes how

information is input in to the system, how the system

is controlled and how the resulting advice is

presented. The input information required by the

system should be in a form the user is familiar with.

Unlike the systems described previously, it would not

assume previous user knowledge of rapid prototyping.

The system though designed for a target user should

332 Bibb et al.

Page 3: Development of a rapid prototyping design advice system

be clear enough to be used and understood by

anybody.

2.4. Able to explain the resulting advice

If the user is to have a high degree of con®dence in the

resulting advice the system should be able to indicate

to the user in simple, understandable terms how or

why a certain result was obtained. This should be in

part indicated by the presentation of the resulting

advice but more detailed information should be

available to the user if requested.

2.5. Easily updated as new knowledge becomesavailable

The system should be able to use the most up to date

information possible to provide the optimum advice.

The system should therefore have facilities built into

it that will allow the information stored within it to be

updated regularly and easily with the minimum

hindrance to the user. Some of this information may

be in a form that user can control but some may need

to be controlled by the system authors.

2.6. EducationalÐusage of the system reduces thenumber of crucial errors made by the designer

The system should aim to educate the user in both

the use of the system and the subject of the system.

The system should increase the users knowledge and

furnish them with enough information as to have

con®dence in the resulting advice. The system

should enable the user to be in a position to discuss

a proposed prototyping route with a service

provider.

2.7. Forgiving of non-crucial errors on the part ofthe designer

The system should be constructed in such a way as to

tolerate errors or omissions in non-crucial areas

without hindrance to the user. Where crucial errors

are made the system should indicate not only the error

but potential default values or limits which will enable

the user to continue using the system and help to

eliminate similar errors in the future.

2.8. Easily accessible

To be successful the system must be accessible to the

target user. In computing terms this will involve

considerations of computer type and operating

system. Cost will also be signi®cant. The cost of

implementing the system should be within the

economic reach of the target users.

3. The structure of the system

Traditionally the structure of an expert system uses a

knowledge base and an inference engine or inter-

preter. The rules are often conditional expressions

such as IF, THEN, ELSE routines. In rule based

applications these conditional statements are

expressed as a pattern rather than a simple Boolean

expression. Another characteristic is that the ¯ow

control is determined by the interpreter rather than

moving from one rule to the next in a lexical sequence

(Ringland, 1988)

In this case this traditional structure was not used

although in essence similar tasks are undertaken by

the software. As with other rule based systems the

tasks are broken down into subroutines. Some of these

subroutines make decisions based on the input data

and other subroutines contain rules for checking and

either implementing or rejecting the decision. The

method of working is similar to traditional rule based

systems although all the subroutines are held in the

same programming environment.

This was possible due to the nature of the subject.

The criteria upon which the decisions are made can be

prioritized and this can give an overall order to the

subroutines. For example the ®rst consideration would

be the ``Number of parts required'' as this has a

greater effect on the subsequent decisions than the

other inputs. Therefore it can be used as an initial

selection of a method, subsequently the other inputs

are checked against this rule according to their

priority.

The use of software authoring tools enabled

graphical user interface design and program structure

to be devised relatively quickly and simply. When

tasks were required that the authoring package could

not handle other applications were automatically

called from the main program to accomplish them

and return the results. This eliminated a great deal of

Rapid prototyping design advice system 333

Page 4: Development of a rapid prototyping design advice system

programming and allowed more time to be spent on

the content of the system rather than coding.

A major aim of the system was a simple user

interface. To this end the number of user inputs was

minimized. This involved the interpretation of three

dimensional Computer Aided Design data (CAD).

This was accomplished by having another program

called from the main program to read the CAD data

and calculate properties and ``infer'' some character-

istics from it. These properties are then passed to the

main program as numerical input data.

4. Linking to CAD data

The main problem with using CAD data is obtaining

the speci®c characteristics of an object from the CAD

description. Simple properties such as overall size and

volume can be obtained quite easily but the

recognition of actual features is problematic at best.

(Swift, 1987) This can be seen by the fact that early

CAD integrated expert systems used relatively simple

two dimensional or two and a half dimensional

objects. For example plates with holes drilled through

them.

Due to these problems the generic STL ®le was

used. The STL ®le format is a triangular faceted

model of the object and is available as an output from

almost all three dimensional CAD systems (STL ®le

format). Originally developed by 3D System Inc. for

use with stereolithography it has subsequently

become a standard input format for all rapid

prototyping systems. This would mean some of the

characteristics would be absolute (such as volume)

but others would be approximations or inferred

properties. These properties although invisible and

meaningless to the user enable specially written rules

to make decisions based upon assumptions made from

these properties. These may not be as accurate as

absolute data but allowed enough information for the

operation of the system within the limits de®ned.

This proved much more satisfactory than the

alternative method of manually de®ning the object

as an assembly of primitive shapes, as has been done

with other software solutions, as this would take an

unreasonably long time for the user to manually

interpret and input the object description whilst still

remaining more approximate than the STL ®le route.

5. Basic rule structure

In this system there are basically two sorts of rules:

(a) Decision rules.

(b) Calculation rules.

The decision rules use the input data to select

possible methods for a solution. These will often be in

the form of either IF, THEN, ELSE statements or Case

Statements. The calculation rules will perform

calculations to create the results of the system.

These will often be mathematical in nature.

The input data consists of manually input data and

inferred data. The manually input data is in the form

of real numbers, integers and selections from pick

lists. These will de®ne the users requirements for the

prototype part. The other data consists of an object

description. This data is inferred or calculated

automatically from the STL ®le of the object.

The result of the system is an Rapid Prototyping &

Tooling (RP&T) ``method''. This method will be a

collection of activities that will enable the production

of the required number of prototypes with the required

physical properties. Each RP&T method will have a

limit on the number of parts it can reliably produce. If

each method has a rule or subroutine written for it

they can be assembled in order of increasing

production limit. This limit can be directly compared

to the user input ``Number of parts required''.

Therefore the ®rst rule encountered in the system

will move down the list of subroutines until it reaches

the ®rst subroutine that satis®es the number required.

Another important requisite of the method will be

whether the physical properties of the parts produced

by this method will satisfy the users speci®cation. To

simplify matters this is de®ned by categories and the

user picks a category. There is a relationship between

the number of parts a particular tooling process is

capable of producing and the physical properties of

the parts they produce. As the process approaches

volume production methods and capabilities the

number of parts possible increases. Therefore the

subroutines for the methods can be grouped into the

categories whilst still being listed overall in order of

increasing number possible. Thus a second sorting

rule can use this selected category to ``jump'' down

the list until it meets a method capable of producing

parts with the required physical properties.

If no physical properties are speci®ed by the user

the initial selection will be governed by the ``Number

334 Bibb et al.

Page 5: Development of a rapid prototyping design advice system

of parts required''. If a physical property is de®ned by

picking the appropriate category the selection will be

performed ®rstly on the ``Number of parts required''.

This initial selection will then be checked against

which category of physical properties it can produce.

If this is correct the selection takes place, if not the

system will jump down the list until the condition is

satis®ed. This is the most basic rule in the system and

can be seen in schematic form in Fig. 1. The basic

structure of the code is indicated in Fig. 2.

Once a method (subroutine) has been initially

selected other rules within that subroutine will then

check whether the other aspects of the input data can

be satis®ed by the selected method. These rules will

decide whether the method will be:

(a) Capable. If not the system will jump to the

next subroutine.

(b) Suitable. If it is capable what additional

processes (if necessary) will be

required to use the method success-

fully. Are there more suitable

alternatives?

If all of the input data is satis®ed the subroutine will

follow another set of rules which will calculate and

display the resulting advice, the costs and lead times

and prepare the explanation material.

The methods for producing signi®cant numbers of

prototype parts are most often secondary tooling

processes which use rapid prototyping (RP) to provide

the necessary master pattern. While the secondary

tooling method is being selected other rules will be

running to select which RP method would be best

suited to the production of the original master pattern.

This is often done independently of the secondary

tooling decisions although there may be some cases

where these two sets of rules affect each other.

For example, the selection of Selective Laser

Sintered tools. These are built directly on RP

machines and therefore no master pattern is required.

The rules may still select the best method for

producing a master pattern but the subsequent tooling

method could either eliminate it or adjust the results to

indicate that although possible it is not necessary to

produce a master pattern.

Selection of the most suitable RP method will

identify characteristics of the object and use them

initially to eliminate the methods which are not

capable. Of the remaining methods some may be

unsuitable and these too will be selected out. Rather

than iterating until only one method remains if a

number of methods are all suitable they will all be

displayed and their relative merits described in the

explanation material. The user can then make an

informed decision of which method to choose based

on that explanation.

As before, some of these rules will be more

important than others. For example ``producible wall

thickness'' will be crucial in selecting an appropriate

method. Equally important may be accuracy.

However, build size constraints are less important. If

a part is too big for the RP machine it can simply be

built in parts and assembled. The rules in that situation

would just adjust the method to account for the

separate builds and the assembly. As before once the

method is established other rules calculate the results,

costs, lead times and prepare the explanation material.

6. Updating the system

The subroutines for the secondary tooling methods are

self contained. In the event of a change either the

subroutine can be replaced or a new one can be

inserted in a suitable place in the order according to

tool life.

For the rules which select rapid prototypingFig. 1. Basic rule structure.

Rapid prototyping design advice system 335

Page 6: Development of a rapid prototyping design advice system

methods for producing small numbers of parts or

master patterns the processes have similar properties

because they essentially operate the in the same

manner (layer manufacturing). The properties such as

accuracy are all de®ned as limits on variables, and

these could be modi®ed by a simple ®nd and replace

routine. If an entirely new method became available

it's properties would need to be inserted and

compared to the existing methods to ®nd where it

should be placed with respect to the initial selection

rule.

7. Worked example

Figure 3 shows the input screen after the STL ®le has

been read in. The object is a part of a ratchet spooling

device for headphones. Some of the properties of the

STL ®le are shown to the user in this screen when the

results are read in. A three dimensional view of the ®le

is shown in another window to the right of the input

window. The user then speci®es the number of

prototypes required, the minimum wall thickness,

maximum aspect ratio and accuracy. The user then

proceeds to the next screen, shown in Fig. 4 to specify

the use required for the prototype and the intended

material.

Once the user has selected a category for the

prototype use and material and any special features

that may be required they proceed to the results

window shown in Fig. 5.

The results are shown broken down into the tasks

required for rapid prototyping of the master pattern,

®nishing the master pattern, and the secondary

tooling process. An estimate for cost and lead time

is indicated for each stage. Clicking the ``More

Info'' buttons opens a windows containing a

description of the processes including diagrams

and photos where necessary. These can be seen in

Fig. 6.

8. Conclusion

The use of this rule structure has enabled a

working system with an excellent user interface

Fig. 2. Basic code structure.

336 Bibb et al.

Page 7: Development of a rapid prototyping design advice system

to be constructed very quickly and simply without

resorting to high level programming environments

or powerful computer hardware. The system will,

within the de®ned limits, recommend a viable rapid

prototyping and tooling method given user inputs

and an object description in the form of an STL

®le. Now a working system can be used in trials

there is scope to incrementally improve the system

according to the results of the trials.

9. Further work

Once the basic structure and interfaces are established

the system can be incrementally improved. This will

be necessary from the point of view of incorporating

developing RP&T technology but also in improving

the level of interpretation of the object to be produced

for example identifying draught angles could be an

important factor. Some of this may be quite

Fig. 3. The ®rst user input window.

Fig. 4. The second user input window.

Rapid prototyping design advice system 337

Page 8: Development of a rapid prototyping design advice system

Fig. 5. The results window.

Fig. 6. The ``more info'' windows.

338 Bibb et al.

Page 9: Development of a rapid prototyping design advice system

demanding in terms of coding and computing

resources and these should always be balanced against

the requirements of the system and the target users to

ensure the system remains fast, ef®cient and simple to

use.

Acknowledgments

The authors would like to thank Jon Ward and Paul

Freeman for their invaluable help in creating this

system.

References

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Ed.

Hague, R. and Dickens, P. M. (1995) In Stresses causedin ceramic shells using QuickCast models, University

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Iuliano, L., Gatto, A. and De Filippi, A. (1995) Metallizationand Rapid Tooling, Politicnico Di Torino, Italy,

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Rapid prototyping design advice system 339