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Proceedings of the 3rd International Conference on Manufacturing Engineering(ICMEN), 1-3 October 2008, Chalkidiki, GreeceEdited by Prof.K.-D. Bouzakis, Director of the Laboratory for Machine Tools and Manufacturing Engineering (ΕΕΔΜ),

Aristoteles University of Thessaloniki and of the Fraunhofer Project Center Coatings in Manufacturing (PCCa joint initiative by Fraunhofer-Gesellschaft and Centre for Research and Technology Hellas,Published by:ΕΕΔΜ and PCCM

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INTEGRATING DESIGN AND PRODUCTION OF PRECISION

TRADITIONAL CERAMICS

T. Giannakakis 1 , G.-C.Vosniakos 1 , D. Pantelis 2

1. School of Mechanical Engineering, Manufacturing Technology Division2. School of Naval Architecture and Marine Engineering, Shipbuilding Technology Laboratory

National Technical University of Athens, 9 Heroon Polytechniou Ave.,GR-15773, Zografos, GREECE

ABSTRACTThis paper advocates using state-of-the-art engineering tools in design and produc-tion of ceramic parts and the corresponding pressing and casting dies. A feature li-brary is developed for the parametric design of parts with simple geometry, focusingon concave pottery parts. A user-friendly interface is developed, so that parametricdesign can be achieved by technicians with no proficiency in CAD In addition, spe-cial routines were developed for enhancing commercially available functionality in

conversion of point clouds, taken from a laser scanner, into solid models. Last, afuzzy system is developed for the selection of suitable machining operations, ma-chining strategy, cutting tools and parameters, respecting geometry complexity, totalmaterial removal volume and a coupled time-accuracy criterion depending on theuser.

KEYWORDS: Design with features, CAD-CAM, Fuzzy system, Laser scanning

1. INTRODUCTION

In the area of design and production of traditional ceramic parts, new technologies are barelyused, since most of the work is still performed mostly without use of CAD-CAM tools. Artisticmethodologies are usually applied, which rely on the skill of traditional artists. When it comes toreproduction of duplicates of existing ceramic parts, the main problem is accuracy, which for artistic reasons can be dropped to the fairly acceptable levels. This problem is particularly sig-nificant in relation to free-form surfaces, which are often met in traditional pottery. This work ad-vocates using state-of-the-art engineering tools in design and production of ceramic parts andthe corresponding press-forming and casting dies. Feature-based design, point-cloud basedmodelling scanning of artefacts and fuzzy systems as approximate decision making tools in thearea of process planning are the main technologies employed. The scope of this work is to in-troduce these technologies to traditional ceramics technicians, in order to advance their techni-cal level step-wise.

The use of features as a tool for parametric design not only of industrial components /1/, butalso of free-form solids in an artistic and aesthetic manner is widely established /2-3/. Designwith features is widely accepted particularly because they can refer directly to machining opera-tions /4/. Features can be even produced from point clouds /5/, and customized feature librariescan be automatically generated /6/. A feature library is developed in this work for the parametricdesign of parts with simple geometry, focusing on concave pottery parts. A user-friendly inter-face is developed in Visual Basic, so that parametric design can be achieved by technicianswith no proficiency in CAD.

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As far as copying and reconstruction of existing artifacts is concerned, many methods are intro-duced based on registration of point clouds /7-8/, while the large number of points received fromscanners always have to be filtered /9/. In this work, special routines were developed for en-hancing commercially available functionality in conversion of point clouds, taken from a laser scanner, into solid models, with or without surface approximation techniques. Solid models re-sulting from both methods lead to the appropriate die geometry.

Traditional ceramists are by no means experienced machinists, therefore for the actual produc-tion of the die, they need help in deciding how to machine to die. Fuzzy sets have also proved areliable tool for the evaluation and estimation of machining parameters /10-12/. In this work aneasy to use fuzzy system is developed for the selection of suitable machining operations(roughing, semi-finishing, finishing), machining strategy, cutting tools and cutting parameters(scallop height, overlap, stepdown etc.), respecting geometry complexity, total material removalvolume and a coupled time-accuracy criterion depending on the user. The system is developedso as to gradually introduce the appropriate parameters to the user, to allow comprehension of their impact on the process and their mutual interaction and to recommend appropriate valuesfor the relevant parameters.

Each of the three areas of development mentioned above will be presented in some detail in therespective sections 2, 3 and 4 followed by a draw-up of conclusions.

2. PARAMETRIC DESIGN USING FEATURES

There are many cases where a part has similar geometry to other, already modeled, parts.Many companies offer products, some parts of which are variations of a standard geometricfamily. In other cases, the designer does not know from start the final dimensions of the model,or he/she just needs to experiment with the part’s shape. In any case, the modern CAD systemsprovide the necessary tools to accomplish these goals. This is when parametric design provesindispensable. This approach involves different design techniques which allow the designer todescribe the dimensional parameters of the model not only through numbers, but also throughmathematical equations, which can relate them with variables and other parameters. This al-

lows the model to be defined first in terms of general topology, leaving the definition of the finalgeometry to a subsequent designing stage. This results in faster design and quicker geometrychanges. This design philosophy is accomplished in CAD systems through features.

2.1. Feature Library

A feature library was developed, with the use of which the ceramics designer can easily modelmany common geometries, without having to start from scratch, when it comes to new parts.The library comprises of geometric families often met in the area of ceramics. The library fea-tures fall into certain categories, according to their general geometry and the part of the modelthey belong to. The following general categories are distinguished:

• Bases• Concave walls• Handles• Solid bodies• Tiles

The “bases” category includes features pertaining to the design of the lowest part of any ce-ramic product, e.g. the base of a ceramic plate, an amphora or a statuette. Regardless of thefinal ceramic object as a whole, it is certain that many of them have common bases. As a result,the user can pick from this general category the desired base feature and import it into themodel. Some of the library features included in this category are presented in Figure 1.

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CAD, CAM, CMM, Vitrual Manufacturing 749

(a) (b)

Figure 1: Library features in the “bases” category.

The second category, namely “walls” includes features that represent the main geometry of ausually hollow item, e.g. pots, bottles, vials, amphora, but also plates, etc., while the “handles”category is named after those parts of a ceramic item that are used to lift, hold and move them.It includes many kinds of solids that can be applied usually on a “wall” feature (Figure 2).

(a) (b)

Figure 2: Library features included in the “walls” category.

The last two general categories contain features with solid geometry. The former includes anygeometry that is not hollow, while the latter those that are square or rectangular with a certain

pattern only on the upper surface.In practice, the user can combine features from different categories to build a new model. Sincethere is the possibility of editing a feature before and during its import to the model, there areinfinite combinations that can be acceptable. Feature libraries are useful tools supported fromalmost all modern CAD systems. Its use can dramatically reduce the design time, especiallywhen the desired object can be modeled entirely from features within the library. Use of the fea-ture library is very simple, even to the non-familiar user, because the transition from features tothe final product is achieved through few steps. Every feature has its characteristic name and itis accompanied by a distinguishing thumbnail. Features are inserted in the model by simplydragging & dropping from the library to the design space. They can be left ‘loose’ until the finaldesign stage or they can be bound to the rest of the model from start with specific dimensionsor parametric relations and constrains.

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2.2. Expanding the feature library

One of the most important parameters of the library is its possibility of expansion. It is not nec-essary to import each new model in the library, but when it becomes obvious that the specificgeometry should prove useful in the future, it is better to save it as a new feature for later use.Consequently, it should be named appropriately and located in the right category. Of course,new categories can be created, when needed. The design of features does not differ from anyother kind of modelling, except that they should be saved with the respective extension, accord-ing to the CAD platform used. Special attention should be also given to feature internal relationsand constrains, for optimal control of its behavior during its import to new models.

Usually, features are designed by expert users and designers. In order to make this technologyavailable even to non-familiar users, special software was developed in Visual Basic, adopting avery practical paradigm. According to this, the user should sketch the necessary views of themodel in plain paper and locate the XY coordinates of each view’s characteristic points. Thesecoordinates should be then entered in a text or spreadsheet file, following the X,Y number for-mat. Then, the software developed takes on, by importing these points automatically into theCAD environment. At all times, guidance by the user is absolutely necessary, on how thesepoints are to be connected (using lines, arcs or splines). The user-system interface is friendly,

as presented in Figure 3.

(a) (b)Figure 3: User-system interaction forms (a) for connecting points of a sketch and (b) for

confirmation.

Groups of points can be separately imported, forming separate sketches in the same model.These sketches are then used, to form the final feature geometry, with the automatic executionof the appropriate commands (extrude, revolve, extruded cut etc.), as specified by the user. Heshould always follow the instructions given by the system, to successfully complete the model.

All answers are critical to the procedure and should be precise and logical.

3. SCANNING AND MODELLING OF PARTS WITH FREE-FORM SURFACES

Before one decides to scan an object, one must be sure that it cannot be designed by anymeans in a CAD system. Scanning is a time-consuming operation, therefore it should be usedonly when it is absolutely necessary. This usually happens when dealing with free-form or sculptured surfaces, often met in traditional ceramic objects.

All 3D scanners actually digitize the scanned surfaces and usually have two exporting options.They either export a text file containing a point cloud, or an STL model of the scanned object,which results after the system has applied a triangulation technique. The latter is useful only if one needs an exact copy of the object. STL files can be opened by nearly all commercial CADplatforms. However, if editing of the file is needed, e.g. a copy of the original object in a differentscale, the first option should be preferred. The point cloud contained in the exported text file,

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can be easily read by CAD systems. If not, it is still possible to develop a tool in this direction, asshown in this work.

In any case, a large number of points is necessary during the scan, in order to acquire the sur-face precision needed. On the other hand, a large point cloud is hard to manipulate, thus high-end computers are necessary. To overcome this problem, the user has to achieve different localpoint densities, according to the demanded accuracy and to the shape of the scanned surface.For example, areas with intense changes in the radius of curvature need a far more dense pointcloud, than areas that are almost flat. One solution is to apply different resolutions in differentareas of the part during a scan, which can prove difficult and time-consuming. Some CAD sys-tems provide clever algorithms for local point cloud filtering, which can be used after scan.Which of the two methods is to be used, relies upon the types of surfaces found on the objectand on the experience of the user. Some of the point clouds scanned in this work appear in fig-ure 4. Both objects have actually two surfaces: one flat and one free-form. Scanning was per-formed with uniform resolution and cloud filtering was applied aftewrwards.

(a) (b)

Figure 4: Point clouds achieved with a laser scanner, using uniform scanning resolution: (a)246.000 points and (b) 763.000 points.

(a) (b)

Figure 5: Solid models achieved after surface reconstruction.

Surface fitting tools provided by the CAD system were applied. The point clouds presented infigure 4 were fitted with a 2 nd grade spline surface, resulting in maximum deviation of 0.35mmand average deviation of far less than 0.1mm, which was deemed acceptable. After thoroughchecking of the fitted surfaces in the CAD system, it was found that deviations over 0.1 mmwere obtained in areas where lack of points was observed, i.e. in close neighborhood of edges

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of the object, as clearly presented in both point clouds of figure 4. The surfaces reconstructedby these algorithms appear in figure 5.

The surface in figure 5b is more complex than the one presented in figure 5a, consequently acomparatively less aggressive point cloud filtering algorithm was applied. Furthermore, other algorithms, provided by the CAD system, were also used. The steps followed for every objectscanned were the following:

• Point cloud filtering• Triangulation and mesh creation• Hole filling of the mesh• Surface fitting• Solid model creation

Special routines were developed for this purpose, to help users handle the creation and holefilling methods of the mesh. These routines are actually scripts that, when activated by the user inside the CAD environment, start an interactive user-system procedure. The system triggerssequentially the necessary commands and the user has to select the appropriate geometries inthe design area, or enter the appropriate values asked for. Explanatory messages pop-up in

every step to inform the user of the procedure he is engaged in and its particulars.The first script has a point cloud as an input, and performs all necessary actions to create aproper, closed mesh. However, in some cases the object is divided into areas each of which isscanned separately with possible overlaps of multiple point clouds, which have to be initiallyfitted together. The fitting is performed by the second script, which actually joins the pointclouds. The newly created point cloud can then be processed by the first script.

4. PRODUCTION OF THE CASTING DIE

After creating the solid model of an object, it is easy to create the corresponding die, using Boo-

lean commands on solids. The problem that arises is the clay shrinkage during casting, whichhas to be taken into account on the solid model before the die design. Clay shrinkage rangesbetween 5% and 20%, depending on the material and the object’s cross-section. On this level of progress of the work, a uniform shrinkage was applied (10%). This is not realistic, of course,thus research is focused on an algorithm that can predict local shrinkage, taking into account allrelevant parameters. A uniform shrinkage can be achieved in the corresponding surfaces of themodel with the “surface offset” command. Using this methodology, the dies resulting from themodels shown in figure 5 are presented in figure 6.

(a) (b)

Figure 6: Respective dies of the models presented in figure 5.

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As far as the actual production of the die is concerned, it comes to a multi-parametric processplanning problem. Usually, in these kind of dies, three sequential machining operations are ap-plied: roughing, semi-finishing and finishing. The second one is not always necessary, depend-ing mostly on the surface complexity of the die, the tools used in the other two processes andthe precision demanded. At the same time, in every process different cutting strategies can beapplied, and for every strategy, different parameters can be used. Table 1 presents the parame-ters involved in every cutting strategy, according to the operation used. Furthermore, independ-

ently of all other parameters, the surface offset, the tool diameter and the cutting conditions(e.g. feed per tooth and surface speed) should be taken into account.

Table 1: Parameters involved in every cutting strategy and machining operation used.

Cutting Strategies Paremeters involvedRoughing

Hatch Overlap, stepdownContour Overlap, stepdown

Semi-FinishingLinear Stepdown, stepover, scallop, max.stepover Circular pocket Stepover Constant Z Stepdown, scallop, max.stepover

FinishingLinear Stepdown, stepover, scallop, max.stepover Circular pocket Stepover Constant Z Stepdown, scallop, max.stepover

In this work, all machining operations were studied, while every strategy was simulated with dif-ferent combinations of parameters, for two different dies shown in figure 6. This led to 188 sce-narios in total, simulated in a commercial CAM platform. This way, valuable trends were discov-ered on how all the above mentioned parameters affect the time needed for the die productionand the quality achieved. The evaluated results were used for the development of a fuzzy sys-

tem for the selection of suitable machining operations, machining strategy, cutting tools and cut-ting parameters.

Fuzzy system

The developed fuzzy system is addressed to those users that are unfamiliar with machining op-erations. It constitutes a good tool for preliminary definition of the machining processes for theconstruction of the die, recommending the tool diameter, the cutting strategy needed and theappropriate cutting parameters. It comprises of 6 sub-systems which are executed sequentially:

• Roughing Tool• Finishing Tool• Semi-finishing• Rough Strategy• SemiFinish Strategy• Finish Strategy

The name of each sub-system evidently explains its purpose. Generally, for every parameter involved in the system, a fuzzy membership function was created, and parameter values wereassociated according to the results of the cases studied experimented with in CAM. The outputof each rule is a fuzzy set; fuzzy sets are aggregated into a single output fuzzy set. Finally theresulting set is defuzzified, or resolved to a single number.

The first two sub-systems are independent, but have to be both executed before going on to the

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third. The decision over the roughing tool diameter depends on two parameters: the total vol-ume of the material to be removed, and the “initial to final surface ratio”. This ratio, representingthe ratio of corresponding areas, is an index of the complexity of the final surface of the die. Therelationship of these two parameters is presented in figure 8. The decision over the finishing tooldiameter accordingly depends on the minimum radius of curvature of the final die surface andon the “Time-Quality” criterion, set by the user. The latter has to be chosen by the user, as aninteger between one and five, with five meaning that the target is die quality, without any con-

cern about time (figure 7). Both these first sub-systems give a result that is not necessarily aninteger. It is obvious that the user should finally choose the closest available tool diameter.

Figure 7: The Time-Quality membership function used in the fuzzy system.

When both roughing and finishing tool diameters are chosen, the semi-finishing sub-systemshould be activated, for a conclusion to be drawn over the necessity of such an operation. Thisrelies on three fuzzy membership functions, which describe the “initial to final surface ratio”, thetime-quality criterion, as well as the roughing tool diameter, as already described. Combinationof these three parameters leads to conclusions on the necessity of a semi-finishing operation(figure 8).

(a) (b) (c)

Figure 8: Surfaces expressing relationships between input and output parameters of thefuzzy sub-systems, where “Tot_Vol” the total volume to be removed, “In-FSR” theinitial to final surface ratio and “TTQ” the target time-quality parameter. (a)

Roughing Tool (b & c) Semi-finishing for different pairs of input parameters.

After all decisions concerning the machining operations and the corresponding tool diametersare made, the last three sub-systems activate sequentially. The cutting strategies depend onthe tool diameter and the values of the respective cutting parameters, as shown in Table 1.However, in this case, instead of having the cutting parameters as an output, the time-qualitycriterion is used. Treating the tool diameter as a constant, since it is already chosen, the user has to properly adjust the cutting parameters so as to achieve the initially set value of the time-quality criterion, see figure 9. The reason for this inversion is simple: instead of executing thesystem as a black-box, the user can now visualize the effect of each parameter, being graduallyintroduced to the involved parameters and comprehending both their impact on the process and

R o u g

h T o o

l D i a

T_V In-FSR

R o u g

h T o o

l D i a

T_V In-FSR In-FSR

S e m

i F i n i s h i n g

TTQ

Time Quality

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Figure 9: Structure of the fuzzy sub-system for the selection of the finishing strategy.

Figure 10: The appropriate value for the time-quality criterion in the last column can be ap-proached by adjusting the values of all input parameters.

their mutual interaction. Adjustment of the parameters is achieved by the user-friendly interfaceof the fuzzy system in the Matlab environment, see figure 10.

5. CONCLUSIONS

In this work, product engineering technologies such as laser surface scanning, feature-basedCAD modelling and CAM techniques are integrated for next generation design and productionof precision traditional ceramics and the corresponding press-forming and casting dies.Since ceramic technicians or artists are not familiar with these technologies, all tools were de-veloped in such a manner, so as to provide a comfortable, explanatory and user-friendly envi-ronment.The feature library was developed for particular classes of shapes and their function on the ce-ramic artefact, but it can equally well be expanded towards other geometries. In addition, a low-

level, practical methodology was adopted for guided step-by-step conversion of 2D sketches onpaper to 3D model in CAD systems.In laser scanning of artefacts with a view to transferring their models into CAD environments, itwas realised that some steps of the process needed enhancement, so special scripts were alsowritten to sew different point clouds together and to repair unwanted holes in fitted meshes.Solid model offsetting is used to create an expanded die cavity to account for shrinkage, isautomatically offset, and subtracted to result in the model of the die, taking into account the clayshrinkage.Help in deciding machining processes, tool diameters and cutting parameters is certainly nec-essary for a ceramics technician to enter the machinists world, therefore a fuzzy system proto-type was developed.

FinishToolDia

Stepover

Scallop

MaxStepover

FinishStrategy

FinishStrategySelection

Target Time-Quality

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Improvements can be achieved by expanding the fuzzy system with more parameters, in order to increase results’ reliability. Furthermore, the clay shrinkage is to be studied thoroughly, so asto provide a local prediction algorithm and incorporate it into a variable offset methodology.

ACKNOWLEDGEMENT

This work has been funded by ELKEA SA.

6. REFERENCES

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2. Stamati V., Fudos I., A parametric feature-based CAD system for reproducing traditionalpierced jewellery, Computer Aided Design, 37 (2005) 431-449.

3. Pernot J.-P. et al., Incorporated free-form features in aesthetic and engineering product de-sign: State-of-the-art report, Computers in Industry, 59 (2008) 626-637.

4. Sun G. et al., Operation decomposition for free-form surface features in process planning,Computer-Aided Design, 33 (2001) 621-636.

5. Ke Y., Fan S. et al., Feature-based reverse modeling strategies, Computer-Aided Design,38 (2006) 485-506.

6. Qamhiyah A.Z., A strategy for the construction of customized design libraries for CAD,Computer-Aided Design, 30 (1998) 897-904.

7. Benko P., Martin R.R., Varady T., Algorithms for reverse engineering boundary representa-tion models, Computer Aided Design, 33 (2001) 839-851.

8. Zhang L.Y., Zhou R.R., Model reconstruction from cloud data, J. Mat. Processing Technol-ogy, 138 (2003) 494-498.

9. Huang J., Menq C.H., Combinatorial manifold mesh reconstruction and optimization from

unorganized clouds with arbitrary topology, Computer Aided Design, 34 (2002) 149-165.10. Ip R.W.L. et al., An economical sculptured surface machining approach using fuzzy modelsand ball-nosed cutters, J. Mat. Processing Technology, 138 (2003) 579-585.

11. Hashmi K., El Baradie M.A., Ryan M., Fuzzy-logic based intelligent selection of machiningparameters, J. Mat. Processing Technology, 94 (1999) 94-111.

12. Wong S.V., Hamouda A.M.S., El Baradie M. A., Generalized fuzzy model for metal cuttingdata selection, J. Mat. Processing Technology, 89-90 (1999) 310-317.

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