9
Introduction Rapid prototyping (RP) manufacturers are primarily concerned with making and selling machines. However, direct input from 3D computer-aided design (CAD) models is needed in order to sell RP as a fully integrated system. This has caused problems because of the proliferation of CAD systems and also the added complication of supporting more than one release of a particular CAD software. A method of mapping a triangular mesh over a surface or solid model (tessellation) was devised which offered the possibility of an easy universal input from CAD to RP. This approach was taken by more than one RP manufacturer. However, limitations soon became apparent. Manufacturers of cylindri- cal and highly curved objects soon started to request that RP utilize sliced contours which they could produce from their CAD models. In addition, sliced data in the form of copious points in space are produced by the medical industry and by laser and co-ordinate measur- ing machines (CMM) for reverse engineering. The situation appears to be heading towards multiple choice of input according to user needs. However, there is now the possibility of a new product definition software standard which may help to unify some of the input styles. Tessellation Figure 1 shows the principle of tessellation. This involves approximating 3D shapes with a “carpet” of planar triangular patches. Many CAD systems have implemented 3D Systems’ stereolithography (STL) file format[1], which has become the de facto standard. Using this approach, an “accuracy” or offset parameter is input by the designer. This is the acceptable chordal error between the plane of a triangle and the surface it is approx- imating. This value has no relevance for the cube, which is made of planar faces and the concept of tessellating such an object for file transfer to RP is quite efficient. However, the sphere demonstrates the other end of the spectrum, where a high accuracy requirement will result in a very large data file. In reality, of course, most designs will fit somewhere between these two examples and methods can be used such as splitting a model into separate pieces in order to: 4 Direct slicing of CAD models for rapid prototyping Ron Jamieson and Herbert Hacker The authors Ron Jamieson is Senior Research Engineer in the Department of Aerospace Sciences, Cranfield University, Cranfield, Bedfordshire, UK. Herbert Hacker is a student at Cranfield University, Cranfield, Bedfordshire, UK. Abstract The 3D Systems stereolithography file format is a good workhorse for the rapid prototyping (RP) industry. It is supported by all major computer-aided design (CAD) and RP manufacturers and there now exists a selection of third- party software which supports this de facto standard and helps to make it work better. However, input to RP systems is sometimes best suited to the format of sliced contours. These may be produced from a three-dimensional CAD model or via reverse engineering techniques such as laser scans and co-ordinate measuring machines. Other sources include computed tomography and magnetic resonance imaging scans. Takes a brief look at both of the above methods, listing their advantages and disadvantages. Identifies several ways in which sliced data can be used to drive RP processes. Finally, presents in detail a methodolo- gy used to develop a direct and adaptive slicer from a commercial CAD system. Rapid Prototyping Journal Volume 1 · Number 2 · 1995 · pp. 4–12 © MCB University Press · ISSN 1355-2546

Direct slicing of CAD models for rapid prototyping

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Page 1: Direct slicing of CAD models for rapid prototyping

Introduction

Rapid prototyping (RP) manufacturers areprimarily concerned with making and sellingmachines. However, direct input from 3Dcomputer-aided design (CAD) models isneeded in order to sell RP as a fully integratedsystem. This has caused problems because ofthe proliferation of CAD systems and also theadded complication of supporting more thanone release of a particular CAD software. Amethod of mapping a triangular mesh over asurface or solid model (tessellation) wasdevised which offered the possibility of aneasy universal input from CAD to RP. Thisapproach was taken by more than one RPmanufacturer. However, limitations soonbecame apparent. Manufacturers of cylindri-cal and highly curved objects soon started torequest that RP utilize sliced contours whichthey could produce from their CAD models.In addition, sliced data in the form of copiouspoints in space are produced by the medicalindustry and by laser and co-ordinate measur-ing machines (CMM) for reverse engineering.The situation appears to be heading towardsmultiple choice of input according to userneeds. However, there is now the possibility ofa new product definition software standardwhich may help to unify some of the inputstyles.

Tessellation

Figure 1 shows the principle of tessellation.This involves approximating 3D shapes with a“carpet” of planar triangular patches. ManyCAD systems have implemented 3D Systems’stereolithography (STL) file format[1], whichhas become the de facto standard.

Using this approach, an “accuracy” oroffset parameter is input by the designer. Thisis the acceptable chordal error between theplane of a triangle and the surface it is approx-imating. This value has no relevance for thecube, which is made of planar faces and theconcept of tessellating such an object for filetransfer to RP is quite efficient. However, thesphere demonstrates the other end of thespectrum, where a high accuracy requirementwill result in a very large data file. In reality, ofcourse, most designs will fit somewherebetween these two examples and methods canbe used such as splitting a model into separatepieces in order to:

4

Direct slicing of CADmodels for rapidprototyping

Ron Jamieson and Herbert Hacker

The authorsRon Jamieson is Senior Research Engineer in the Department of Aerospace Sciences, Cranfield University,Cranfield, Bedfordshire, UK.Herbert Hacker is a student at Cranfield University,Cranfield, Bedfordshire, UK.

AbstractThe 3D Systems stereolithography file format is a goodworkhorse for the rapid prototyping (RP) industry. It issupported by all major computer-aided design (CAD) andRP manufacturers and there now exists a selection of third-party software which supports this de facto standard andhelps to make it work better. However, input to RP systemsis sometimes best suited to the format of sliced contours.These may be produced from a three-dimensional CADmodel or via reverse engineering techniques such as laserscans and co-ordinate measuring machines. Other sourcesinclude computed tomography and magnetic resonanceimaging scans. Takes a brief look at both of the abovemethods, listing their advantages and disadvantages.Identifies several ways in which sliced data can be used todrive RP processes. Finally, presents in detail a methodolo-gy used to develop a direct and adaptive slicer from acommercial CAD system.

Rapid Prototyping JournalVolume 1 · Number 2 · 1995 · pp. 4–12© MCB University Press · ISSN 1355-2546

Page 2: Direct slicing of CAD models for rapid prototyping

• Keep file sizes down.• Vary chordal accuracy.• Overcome tessellation problems.• Build in flexibility for multiple models with

minor differences between them.

The first point is very important since it is easyto create extremely large tessellation files. Theworst case so far encountered by the author was55Mb of STL (ASCII) generated from a4.5byte CAD file. Compression tools such as“pkzip” can greatly reduce these files for stor-age and transfer but eventually the file has to besliced, and files of this size have taken up toeight hours for this function to be performed.This does not enhance the concept of “rapid”prototyping as discussed by Jamieson[2].

Splitting a CAD model into parts allows thedesigner to keep a file size down to a manage-able number of triangles by ensuring that onlythose features which are important are finelyfaceted. However, it should be noted thattessellating a model with an acceptable accura-cy can also be misleading. Figure 2 shows whatcan happen. The deviation error in the tessella-tion file is not guaranteed to be the maximum

error of the slice. When a face or facet isinclined to the build direction of the RPprocess, the error can be several times thedeviation error.

Complex models can be difficult to transferthrough some CAD translators. This is espe-cially true for some surface modellers. Howev-er, designers can make success more likely bysplitting complex CAD models into smallerfiles which can later be appended into a singlefile. Unfortunately, not all RP vendors canappend STL files and slice them as a singlefile[3].

Advantages of tessellation include the fol-lowing:• It provides a simple method of representing

3D CAD data.• There is already a de facto standard which

most CAD and RP systems support.• For certain shapes, it can provide small and

accurate files for data transfer.

Disadvantages of tessellation include the fol-lowing:• It creates files many times larger than the

original CAD data file for a given accuracyparameter. The STL file carries a highdegree of redundancy since each triangle isindividually recorded and shared ordinatesare duplicated within a file.

• The implementation of STL translatorswithin CAD systems varies and consistencyof quality is a problem, especially for RPbureaux. This has given rise to “repairsoftware” which slows the production cycletime.

• The subsequent slicing of large STL filescan take many hours and, except for RPprocesses which can slice while they arebuilding the previous layer, this is a severe

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Direct slicing of CAD models for rapid prototyping

Ron Jamieson and Herbert Hacker

Rapid Prototyping Journal

Volume 1 · Number 2 · 1995 · 4–12

Figure 1 The principle of tessellation

Facet 1

Facet 2

Facet 4 Facet 6

Facet 3 Facet 5

Figure 2 The difference between tessellation and slicingerrors

Given errorFaceted surface

Real surfaceReal error

Page 3: Direct slicing of CAD models for rapid prototyping

overhead on time for a so-called rapidprototyping technology.

• Occasionally, the designer will be unable toget the CAD model through the STLinterface successfully, resulting in remodel-ling.

As stated above, STL file problems have givenrise to a number of software packages which,in effect, produce workable STL and other fileformats from neutral data such as IGES andVDA[4-6]. There are other tessellation algo-rithms which have less redundancy and maybe more efficient, such as the Cubital facet list(CFL) format[7]. The problem with these isthat they are not readily available on majorCAD systems and, even if they were, thechosen RP system may not be able to readsuch formats. Furthermore, no matter howefficient a tessellation file is, eventually itneeds to be sliced to produce boundary con-tours for the RP process to follow.

Direct slicing

An alternative to using an intermediary tessel-lation file is to slice the CAD model directlyand transfer the resultant contours to the RPprocess.

Why use direct slicing?RP models are now commonly used for avariety of applications, including designverification, cold flow testing and direct cast-ing applications. In most cases, the originalCAD model has been generated accurately asa solid or surface model. It is considered ofstrategic importance to many designers thatthe integrity of this model is maintainedthroughout the process from the concept tothe final product. If RP models were onlybeing used for design visualization, designerswould be content with an approximate physi-cal representation of their CAD models.However, with RP models being used increas-ingly for engineering applications, coupledwith the overall requirement for modelintegrity, a more satisfactory method of defin-ing RP data is required. Such a method mustovercome the disadvantages of tessellation aslisted above.

For a particular group of RP users, thoseconcerned with producing large, mostlycylindrical, shapes, the need for direct slicingis urgent. They suffer all the disadvantages oflarge tessellated files, which take a long time

to slice. Also, hand finishing of the models canbe lengthy in order to obtain a smooth surfacefinish.

There are a number of reasons for usingdirect slicing, mainly related to the disadvan-tages of using tessellation:• reduced file size (over-faceted models);• greater model accuracy;• reduced RP machine pre-processing time;• elimination of repair routines (these are an

“unknown” in that they could easily de-tract from model accuracy and evenremove features from the model).

However, there are also potential disadvan-tages of direct slicing which include the fol-lowing:• Supports cannot easily be added to nested

sections.• Ability to reorientate the model is lost.• Beam compensation and offsets still

require processing.• More designer knowledge is required.

It could be argued that most of the disadvan-tages are in fact a benefit. Orientation decidedby the designer is, in the authors’ opinion, agood thing as it requires more dialogue andunderstanding of the process. In the end,designers must design for RP and not forconventional manufacturing if they are torealize the full potential of the technology.This can only be achieved by understandingsuch parameters as:• optimum build orientation for accuracy;• optimum build orientation for least sup-

ports;• optimum build orientation for least cost;• optimum build orientation for accuracy of

critical features;• optimum resin/powder to utilize.

It is unlikely that bureau services would agreewith this but they have different considera-tions, namely competitiveness and through-put. They need the line of least resistance. Inthe longer term though, the industry willbenefit from increased designer knowledge ofwhat is possible and how to obtain the bestresult for a particular design.

Slicing CAD dataThis has always been relatively straightfor-ward. However, there are problems to over-come, especially with surface models where aslice can often be made up of a series of lines,arcs and curves. These may not be contiguous

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Direct slicing of CAD models for rapid prototyping

Ron Jamieson and Herbert Hacker

Rapid Prototyping Journal

Volume 1 · Number 2 · 1995 · 4–12

Page 4: Direct slicing of CAD models for rapid prototyping

and gaps can cause problems which can neces-sitate repair (or fill-in) routines, potentiallydetracting from part accuracy. This set ofcurves then needs to be sorted and convertedto the slice format utilized by a particular RPsystem. However, it is more beneficial to use atechnique which would slice CAD modelsdirectly into the correct format. This approachhas been taken by CENIT, a German softwarehouse which has produced software whichslices CATIA models directly[8].

This approach could be followed for eachand every CAD system on the market. How-ever, the range of hardware platforms, operat-ing systems and continued CAD evolution(new releases generally on an annual basis) istoo great for the industry to support commer-cially at this time. As an alternative, userscould write their own machine drivers. This isan option for machine owners only. Materi-alise has taken this approach[4]. It has pro-duced a range of software which cannot onlycreate sliced data in different formats, but alsoconvert between them. In this way, vendor-specific data can be produced from a moreaccessible format such as the Common LayerInterface (CLI)[9]. Recent comments by otherusers would indicate that Materialise is notalone in writing its own machine drivers.

Two recent studies have indicated thatadaptive slicing can actually increase accuracyof a part while reducing the number of slicesat the same time. This requires an awarenessof the accuracy needed and the ability toconcentrate slices in important areas such ashighly curved regions. At the same time, areasof constant section, i.e. no change in shape,would not suffer from thicker slicing for agiven build orientation. In all slicing routinesit is important to detect both upward- anddownward-facing flat areas. Discussions onthis, the effects on cusp height and compara-tive data of adaptive and regular spaced slicingare given by Dolenc and Makela[10] and Suhand Wozny[11]. The mechanics of slicing andadaptive slicing of solid models will be dis-cussed later.

Utilizing an accepted standardHewlett-Packard Graphics Language (HPGL)is an accepted plotter driver standard. ManyCAD packages support the standard and thisfact has been exploited by RP manufacturersFockele and Schwarze (F&S) and D-MEC.

The F&S approach is outlined in the paper“New methods and dimensions in rapid

prototyping”[12]. The approach, as seenfrom a designer’s point of view, would be toautomate a slicing routine which generates asection slice, invokes the plotter routine toproduce a plotter output file and then loopsback to repeat the process. This could beautomated using macros or a simple program.One disadvantage is that the files would notbe appended, potentially leaving hundreds ofsmall files needing to be given logical namesand then transferred. Also, all the supportstructures required are generated in the CADsystem and sliced in the same way. Parameterssuch as laser spot correction are applied bythe F&S software.

The Standard for the Exchange of ProductData (STEP) is an ISO standard (ISO10303), and work on this standard began inJuly 1984. The first parts of STEP were dueto have undergone full international approvalready for release in the latter part of 1994.STEP has the support of many of the majorCAD vendors. A useful introduction to STEPis given by Owen[13].

One of the recent initiatives set up by theEuropean Action on Rapid Prototyping(EARP) was a study into STEP as a tool fordata exchange to current RP systems[14]. RPvendors were canvassed for their views ondata exchange and several points were raised:• It can be expensive for RP vendors to

support exclusive formats.• Any replacement of the STL format should

be more concise and include informationon topology.

• RP machine vendors are either planning orare already able to accept a slice format.

The study also identified possible methods ofusing STEP to represent both faceted andsliced data.

Utilizing planar data obtained via medical scanningSlicing is not new to the medical industry.Computed tomography (CT) and magneticresonance imaging (MRI) scanners are regu-larly used to help visualize internal conditions.The techniques produce copious data in aplanar form. A number of companies nowspecialize in handling these data. In Japan,Nakai and Marutani[15] have developed atechnique which reconstructs 3D data fromMRI, CT or a mixture or both, and inter-polates by means of third-order splines utilized radially around the data. These data

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Direct slicing of CAD models for rapid prototyping

Ron Jamieson and Herbert Hacker

Rapid Prototyping Journal

Volume 1 · Number 2 · 1995 · 4–12

Page 5: Direct slicing of CAD models for rapid prototyping

are then finely sliced for direct input to thesolid object ultraviolet laser plotter(SOUP/RP) system. In Europe, Materialisehas also developed software which allowsscanned data to be produced on the stere-olithography system[4]. In the USA, at leastone major CAD vendor, Intergraph, is mar-keting software to both visualize and produceIGES data from CT scans[16]. This can thenbe read into a CAD system.

Utilizing data obtained via reverse engineering techniquesCo-ordinate measuring machines have beenutilized on many occasions to inspect finalform. The output from these machines isoften written to a file which is then comparedwith a similar file generated from the CADdata. Dimension errors are then easily high-lighted. These same files can be used to gen-erate splines and curves which are in turnused to generate a surface within a CADmodel. It is then an easy step either to gener-ate an STL file or to utilize a direct slicingroutine. More recently, laser scanners havebeen used to measure objects. EOSCAN byEOS (Electro-Optical Systems) GmbH[17]uses Moire fringe techniques. The fringepattern corresponds to height contours, andby means of image processing and laser trian-gulation the absolute co-ordinates can bemeasured. Cyber_Site Europe markets a 3Ddigitizer which captures complex images andcan reverse engineer them into a number ofdifferent software packages. Its software hasbeen used to digitize human heads to assist inmaking individual helmets[18].

Direct slicing of CAD data – the Cranfieldapproach

At Cranfield University (CU), a variety ofCAD software is used including Parasolid.Parasolid is the solid modelling kernel ofUnigraphics and can be accessed directly by alanguage such as C to access its underlyingprograms. In this way it is easy to slice a solidusing software calls. Initially it was intendedto convert the Parasolid slices to:• CLI;• HPGL; and• SLC (the 3D Systems contour algorithm).

EOS GmbH, a German RP machine manu-facturer, acted as a partner and the CLI format was obtained together with an agree-

ment to manufacture a part from the CUdata. Fockele and Schwarze were also helpfulin outlining their process of handling HPGLdata. 3D Systems (USA) was more difficult todeal with and, although the SLC format isnow available, its availability is currently“logged”, and individuals must apply directlyfor their code which may then be granted. Atthe time of this work, however, the above wasnot possible and so only the CLI and HPGLformats were used. Although HPGL supportsarcs and lines, the current version of CLI onlyallows for straight polylines which immediate-ly dashed one of the “perceived” advantagesof direct slicing, i.e. not having to use facetedrepresentation of curves.

Boundary representation in ParasolidParasolid is a boundary representation (b-rep)solid modeller. In a b-rep modeller, the solidbody is described by the faces which deter-mine the border between the body and thevoid space which surrounds it. A b-rep bodyin Parasolid is described by its faces, edgesand fins as illustrated in Figure 3. Faces arethe main descriptive element for the body.The face is limited by an anti-clockwise loopof fins. Wherever a face connects with anedge, the edge has a fin. Edges have curves astheir underlying geometry. A fin can be con-sidered as the side of an edge on the connect-ing face.

A special kind of body, which is mentionedhere because of its significance within thiswork, is the sheet solid. A sheet solid is a solidwith zero thickness as illustrated in Figure 4.It consists of at least one closed ring of edgeswith two faces connected to them. The two

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Direct slicing of CAD models for rapid prototyping

Ron Jamieson and Herbert Hacker

Rapid Prototyping Journal

Volume 1 · Number 2 · 1995 · 4–12

Figure 3 Representation of a solid body in Parasolid

Face Loop of fins

Edge

Face

Face normal

Page 6: Direct slicing of CAD models for rapid prototyping

faces have identical geometry but differentface normals, so that one can be said to be oneither side of the sheet. Every hole in the sheetneeds an additional loop of edges to surroundit.

A problem that can occur in b-rep mod-ellers is the so-called “non-manifold body”.In b-rep models, a body cannot be allowed tointersect itself or even touch itself at a point ora line. Bodies which do so are called non-manifold. Parasolid does not allow this. How-ever, this state can be accidentally achievedvery easily. Consider a block with a hole in it,as shown in Figure 5. When this block is cutin two by a section plane tangential to thehole, the part of it where the hole remainswould touch itself in a line in the sectionplane. This is illegal and the sectioning willnot take place. This case can occur in theslicing procedure used in this work as it isbased on sectioning the solid.

The solid body is not the top-level dataitem in Parasolid. Bodies, together with otherentities, can be grouped together into anassembly. This capability is used by the slicingprocedure to group several sections together.

Implementation of direct slicingSeveral procedures can be used to slice asolid. The one used at CU sections the solidusing planes, lists the faces created duringsectioning and creates, as a first step, sheetsolids from the faces. The loops and fins ofthese sheets are later investigated for theirgeometry to build the output files.

The slicing algorithm starts with theinquiry about the part’s highest and lowestpoints. The slicing direction is the Z direction,so the top is the highest and the bottom is thelowest Z co-ordinate of the part. The slicing ofthe part is achieved by consecutively callingthe function for one slice and putting the tagsof the slice together in an assembly.

An optional adaptive slicing procedure isincluded in the program. When the section ofthe part does not change in a large interval,the layer thickness is increased up to a maxi-mum layer thickness which is given as one ofthe input parameters. This parameter is disre-garded if the adaptive slicing option is notselected.

When a section taken at the standard layerthickness has the same geometry as the previ-ous section it is disregarded and sectioning atthe maximum layer thickness is attempted. Ifthe section at maximum layer thickness isequal to the previous one it is added to theassembly of sheet solids. If not equal, thesectioning takes place halfway between thestandard layer thickness and the maximumlayer thickness to produce a middle section. Ifthe middle section is equal to the previoussection, a section halfway between the middleone and the maximum layer thickness is takenand checked for equality. If not equal, a sec-tion halfway between the middle one and thestandard layer thickness is taken.

The process of halving between two previ-ous sections is continued until the differencebetween two subsequent sections is lowerthan a certain tolerance value that is set in thesource code. After this process, the highestsection equal to the previous one is added tothe assembly of sheet solids. This may soundvery tedious, but one has to bear in mind that,by continually dividing the layer difference bytwo, it becomes very small very quickly. Afteronly ten iterations the difference is reduced toone thousandth of its initial value.

When the adaptive slicing option is chosen,an additional function to compare two slices isneeded. This works by translating a copy ofthe newly created slice to the height of the

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Direct slicing of CAD models for rapid prototyping

Ron Jamieson and Herbert Hacker

Rapid Prototyping Journal

Volume 1 · Number 2 · 1995 · 4–12

Figure 4 A sheet solid

Figure 5 Creation of a non-manifold body by sectioning

Touching lineSectioning plane

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previous slice and comparing the two. Com-parisons have to be undertaken edge by edge,where every edge of one slice has to be com-pared with every edge of the other slice untilan equal edge is found. Trying to comparefaces, which would promise to be a muchquicker method, was not possible. There is nodirect way in Parasolid to compare faces. Theunderlying geometry of the faces in this appli-cation, which could be compared, consistedof unbound planes only.

As explained earlier (see Figure 5), non-manifold bodies can occur during sectioningoperations. If this happens during the slicingprocess, the modeller is set back to its statebefore the slicing using a rollback facility, theZ co-ordinate of the slice is decreased by asmall amount, the part sliced, and the result-ing slice is translated upwards by the sameamount. The distance is currently set to0.001mm but can be altered in the sourcecode of the program. In case the result is stillnon-manifold, the procedure of decreasingthe slicing height would be repeated until amanifold result can be obtained.

Use of the Common Layer InterfaceAs discussed earlier, the output format imple-mented was the CLI. It has been developed aspart of the BRITE-EURAM project RapidPrototyping Techniques[9]. The CLI formatis meant as a vendor-independent format forlayer-by-layer manufacturing technologies. Inthis format, a part is built by a succession oflayer descriptions. The CLI file can be inbinary or ASCII format. Only the ASCIIformat has been implemented in this work.

The geometry part of the file is organizedin layers in ascending order. Every layer isstarted by a layer command, giving the heightof the layer. The layers consist of a series ofgeometric commands. The first layer is a“blind layer”, which does not include anygeometry. It is only included to provide infor-mation about the lowest point in the part,because the layer height is meant to be the topof the layer.

The CLI format only has the capability ofproducing polylines of the outline of the slice.This has to be seen as a major drawback, asone of the great advantages of direct slicing isthat the real, often curved, outline of the partcan be obtained. By reducing the curve tosegments of straight lines, an advantage overthe STL file format is lost. But, as was seenearlier (see Figure 2), the confidence in terms

of not exceeding the expected error is stillgreater with the combination of direct slicingand CLI.

The polylines are closed, which means thatthey have a unique sense, either clockwise oranti-clockwise. This sense is used in the CLIformat to state whether a polyline is on theoutside of the part or surrounding a hole inthe part. Counter-clockwise polylines sur-round the part, whereas clockwise polylinessurround holes. This allows correct directionsfor beam offset. Note the similarity with thedirection of fins in Parasolid. In both cases thepart is “to the left” of the entity. The informa-tion about whether the polyline is external orinternal is also explicitly given in the polylinestatement, where a sense flag is used to statethe direction.

A second geometric entity in the CLIformat is the hatching to distinguish betweenthe inside and outside of the part. As thisinformation is already present in the directionof the polyline, and hatching takes up consid-erable file space, hatches have not beenincluded into the output of this program.

TestingTo ensure the usability of the program, anumber of tests were carried out. The slicingprocedure was tested using several differentparts. The parts were sliced and then dis-played in a Parasolid viewer to ensure theywere correct. Features that were tested in theparts included:• horizontal holes in parts (Figure 5), making

sure the slicing takes place exactly on thebottom or the top of the hole withoutprogram failure;

• parts with multiple section results at a givenheight – this creates the need for severalsheets to be created for one layer (seeFigure 6).

Almost all the parts contained height intervalswhere the sliced outline stays constant. Thesewere used to check the adaptive slicing routine.

The final test for the program was actuallyto produce a part from one of its output files.The Munich-based company EOS offered todo so on one of its Stereos machines able toprocess CLI files. As the RP machine is cur-rently unable to produce adaptive sliced partsof variable thickness, the part was producedwith constant slice thickness. The part isshown in the photograph. For the Stereosprocess, supports had to be added. These arenot shown here.

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Direct slicing of CAD models for rapid prototyping

Ron Jamieson and Herbert Hacker

Rapid Prototyping Journal

Volume 1 · Number 2 · 1995 · 4–12

Page 8: Direct slicing of CAD models for rapid prototyping

Slicing the part with the parameters given inTable I took 90 minutes. Manufacturing itfrom the resulting files did not present anyproblems and took approximately threehours. For this part, the size of the CLI filewas 91kb. An STL file produced with compa-rable parameters had 170kb.

Conclusions

There is a need for direct slicing of CADmodels. An open format like the CLI coupledwith use of software such as that marketed byMaterialise enables a change of format to thatof a specific RP system. In addition, the oldarguments that sliced data are more difficultto manipulate and create supports for are nolonger valid.

It is clear that RP vendors see a need forboth tessellated and sliced data from CADmodels to be input into RP machines. STEPoffers the chance to accomplish both usingone internationally accepted data exchangestandard. In addition, it promises the possibil-ity of a more compact tessellation algorithm.

Input via reverse engineering, CT andMRI scanners is more specialized. Already,choices in software exist on more than one

continent and it is unlikely that there will bethe same need to house these in a standardsuch as STEP.

The work started at Cranfield has provedsuccessful and will continue. It has beenshown that direct slicing can be beneficial interms of file size and in cutting out the need toslice a tessellated equivalent model. Accuracycan be enhanced, especially on rounded ortubular designs, which also benefit fromreduced processing time before the build canstart. The ability to perform direct slicing on aparticular CAD system has an appeal tocertain industry sectors.

Finally, it is quite likely that all the majorCAD system vendors will develop direct andadaptive slicers in the future since they are auseful tool for supporting RP and no one willwish to be seen as trailing behind their com-petitors in this growing field.

Notes and references

1 Further details of 3D Systems’ StereolithographyInterface Specifications are available from 3D Sys-tems, 26081 Avenue Hall, Valencia, CA 91355, USA.

2 Jamieson, R., “CAD methods in rapid prototyping”, inDickens, P.M. (Ed.), Proceedings of the 3rd EuropeanConference on Rapid Prototyping and Manufacturing,The University of Nottingham, Nottingham, July 1994.

3 Jamieson, R. and Hammond, J., “Rapid wind tunnelprototype using stereolithography and equivalenttechnologies”, Proceedings of 18th Congress of theInternational Council of the Aeronautical Sciences,International Council of the Aeronautical Sciences(ICAS), September 1992.

4 Materialise, various software solutions, Kapeldreef 60,3001 Leuven, Belgium.

5 Brockware, Rapid Prototyping/StereolithographyInterfaces, Brock Rooney & Associates Inc., Birmingham, MI, 1994 version.

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Direct slicing of CAD models for rapid prototyping

Ron Jamieson and Herbert Hacker

Rapid Prototyping Journal

Volume 1 · Number 2 · 1995 · 4–12

Figure 6 Part with two results obtained from one section

h

Two sheets for layer at height h

Plate 1 Part constructed from direct slicing data

Table I Parameters of part used for testing

Parameter Value

Layer thickness 0.25mmMaximum layer thickness (Continuous slicing)Maximum chordal error 1mmMaximum number of points 5,000Journal file NoneOutput ASCIIFile accuracy 5 (0.01mm)Rollback size 3,000,000 bytes

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6 Deskartes Portfolio of Rapid Prototyping, HelsinkiUniversity, Helsinki, 1994 version.

7 Further details of the Cubital facet list can be obtainedfrom Cubital Ltd, 13 Hasadna Street, Industrial ZoneNorth, Raanana 43650, Israel.

8 Details of this software can be obtained from CENITGmbH, Schulze-Delitzsch-Strasse 50, 70565 Stuttgart,Germany.

9 BRITE-EURAM, Common Layer Interface (CLI), Version1.31, BRITE-EURAM Rapid Prototyping Techniquesproject, Project No. BE5278, 1994.

10 Dolenc, A. and Makela, I., “Slicing procedures forlayered manufacturing techniques”, Computer-AidedDesign, Vol. 26 No. 2, February 1994.

11 Suh, Y.S. and Wozny, M.J., “Adaptive slicing of solidfreeform fabrication process”, in Marcus, H.L.,Beaman, J.L., Bourell, D.L. and Crawford, R.H. (Eds),Proceedings of Solid Freeform Fabrication Symposium,University of Texas at Austin, TX, August 1994.

12 Fockele, M. and Schwarze, D., “New methods anddimensions in rapid prototyping”, 27th ISATA Pro-ceedings, Fockele u. Schwarze BbR, Barchen-Alfen,Germany, 1994.

13 Owen, J., STEP: An Introduction, Information Geometers Ltd, Winchester, 1993.

14 Bloor, S., Brown, J., Dolenc, A., Owen, J. and Steger,W., “Data exchange for rapid prototyping”, summaryof EARP investigation presented at the Rapid Prototyping and Manufacturing Research Forum,University of Warwick, Coventry, October 1994.

15 Nakai, T. and Marutani, Y., “Applications of UV laserfabrication to organ models interpolated from CT and

MRI images”, Applied Optics, Vol. 31 No. 25, September 1992.

16 Surgicad package, Intergraph Corporation, Huntsville,AL, 1994 version.

17 Further details about EOSCAN can be obtained fromEOS GmbH, D-8033 Planegge, Pasinger Str. 2, Munich,Germany.

18 For further information regarding this 3D digitizer andsoftware contact Cyber_Site Europe, CyberwareDigitizer Sales, Murray Engineering Co., 38A StationRoad, North Haddon, Middlesex, UK.

Further reading

Armit, A.P., “Curve and surface design using multipatchand multiobject design systems”, Computer AidedDesign, Vol. 25 No. 4, April 1993.

Barequet, G. and Sharir, M., “Piecewise-linear interpola-tion between polygonal slices”, School of Mathe-matical Sciences, Tel-Aviv University, Tel-Aviv, Israel.

Crawford, R., “Computer aspects of solid freeform fabrica-tion”, Department of Mechanical Engineering,University of Texas, USA.

Guduri, S., Crawford, R. and Beaman, J., “A method togenerate exact contour files for solid freeformfabrication”, Department of Mechanical Engineer-ing, University of Texas, USA.

Kirschman, C. and Jara-Almonte, C., “A parellel slicingalgorithm for solid freeform fabrication processes”,Centre for Advanced Manufacturing, MechanicalEngineering Department, Clemson University,Clemson, SC, USA.

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Direct slicing of CAD models for rapid prototyping

Ron Jamieson and Herbert Hacker

Rapid Prototyping Journal

Volume 1 · Number 2 · 1995 · 4–12