When considering the use of SFF, there are many questions a designer might ask. What model orientation should be used, will the model have adequate aesthetic and functional properties, is the STL file suitable for transfer to the SFF machine? These questions could be answered by a comprehensive design support system for SFF. This paper addresses a number of components for such a system that can be met through the use of visualization tools. These include: 1. Visualization of surface roughness 2. Visualization of characteristic features (e.g. surface macro-texture) 3. Visual simulation of fabrication Example applications of these tools are presented together with a status review of their implementation to date. It is envisaged that these tools will be incorporated into an already existing network-based preprocessor used for visualization, repair and slicing of STL files. The direction of future work is also discussed which will include the visual representation of functionally graded materials (connected with FEM results).Mechanical Engineerin

Campbell, R. Ian

Jee, Haeseong J.

Lee, H.S.

English

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Visualization tools for design support in SFFR. Ian Campbell*, Haeseong J. Jee# and H. S. Lee**School of Mech, Mats and Manu Engineering, University of Nottingham, NG7 2RD, UK.#Dept. of Mech Engineering, Hong Ik University, Seoul, South Korea.AbstractWhen considering the use of SFF, there are many questions a designer might ask. What modelorientation should be used, will the model have adequate aesthetic and functional properties, isthe STL file suitable for transfer to the SFF machine?  These questions could be answered by acomprehensive design support system for SFF.  This paper addresses a number of components forsuch a system that can be met through the use of visualization tools.  These include:1. Visualization of surface roughness2. Visualization of characteristic features (e.g. surface macro-texture)3. Visual simulation of fabricationExample applications of these tools are presented together with a status review of theirimplementation to date.  It is envisaged that these tools will be incorporated into an alreadyexisting network-based preprocessor used for visualization, repair and slicing of STL files.  Thedirection of future work is also discussed which will include the visual representation offunctionally graded materials (connected with FEM results).1. IntroductionDesign visualization has become an important part of the computer aided design process.  Itenables designers to produce realistic images of the final product that can be used ascommnuication aids to other personnel, customers and suppliers.  Visualization also has a role toplay in design analysis where the results of a finite element analysis can be shown as shadedstress contours on the computer screen.  Solid freeform fabrication (SFF) has also proved itself asa valuable design tool.  SFF models can be used to verify design concepts, to create workingprototypes and to provide a link into prototype or production tooling.  There are several aspectsof SFF that set it apart from other prototyping and manufacturing processes:(a) it is a material addition process,(b) all commercial SFF techniques are layer-based,(c) the additon of material is usually in the form of discrete volumes e.g. roads in FDM,droplets in ink-jet techniques.These aspects combine to give SFF models some distinctive characteristics.  These include theso-called “staircase” effect, requirement for supports, build imperfections (e.g. air gaps, warping),poor definition of small features and a wide variation in surface roughness.  All of these will havesome effect on the appearance and functionality of the SFF model.This paper endeavours to show that by giving designers the ability to visualize some of thecharacteristics before the SFF model is built, it is possible to make their use of SFF more437effective.  This capability would form a key element of a design support system for SFF thatwould enable designers to answer such questions as what model orientation should be used, willthe model have adequate aesthetic and functional properties, is the STL file suitable for transfer tothe SFF machine?  The authors envisage that such a system could be used over the Internet andthe preliminary development of this aspect is described also.  Finally conclusions about thesignificance of the work are drawn and future research plans are outlined.2. Visualization of design intentVisualization, in general, is a method of extracting meaningful information from complex datasets through the use of interactive graphics and imaging. It provides processes for seeing andunderstanding what is normally unseen, thereby enriching existing scientific methods. Volumevisualization as a subfield of visualization, for example, is a method of interpreting complexvolumetric data. On the other hand, several geometric modeling algorithms have also been usedfor interaction between a virtual and the real world. Called virtual manufacturing, thesealgorithms have become an important part of intelligent machining systems by allowing for theprecise modeling of existing technologies.2.1 Visualization of surface roughnessOne approach for visualizing the physical model frequently used by SFF community has beenapproximating the surface of the object by a collection of surface patches, which results in thedisplay of a smooth, shaded object of the physical model (Salton and Meek, 1995).   When beingfabricated by SFF apparatus, however, an object is built by laying down material layers in agradual, controlled way, which results in the staircase (or laddering) effect on what shouldactually be smooth surfaces.  Traditional geometry-based modelers which display a smooth,shaded object of the physical model hence provide the designer with no information on the actualsurface finish of the object.   For example, a sphere fabricated by SFF apparatus will look like astack of discrete, differently-sized circular disks rather than a simple smoothly skinned crystal-like ball.   Chandru et al (1995) have thus suggested voxel-based modeling for evaluating thegeometrical effect on the surface of a physical part made by SFF techniques.Alternatively, a visualization algorithm of surface roughness based on the slanted angle of STLfacets is proposed in this paper.  The normal vector for each facet in the STL file is analyzed inrespect to the horizontal (x-y) plane.  The angle between the vector and the plane is used todetermine the local surface roughness value.  This is achieved by accessing a database ofempirically measured surface roughness values for several SFF processes (Campbell andMartorelli, 2000).  The surface roughness value is displayed as a particular color shading on thecomputer image.  This gives the designer a visual image of the overall surface roughness and anyproblem areas in the model. An example of an SFF model visualized in this way is shown infigure 1.  The original orientation (on the left) shows an area of poor surface roughness on the lidof the teapot.  If this was deemed unacceptable, the designer would re-orientate the model and re-calculate the surface roughness values.  In this case, the teapot on the right has much bettersurface roughness values on its lid.  An added benefit of the software is that it also shows poorfacetting within an STL file through “speckled” shading.  This is caused by neighbouring facetshaving significantly different normal vector angles.  This is clearly seen around the main body ofthe teapot in figure 1.438Figure 1.  Example of surface roughness visualization images2.2 Visualization of surface textureSurface macro-texture design is an example of an SFF characteristic feature that can be greatlyaided by the visualization technique. Surface macro-texture denotes a set of tiny repetitivegeometric features on an object’s surface. The use of porous macro-texture on the surface of anorthopedic implant to promote bone ingrowth, for example, offers a valuable alternative to acrylicbone cements as a means of fixation.   Fabrication of surface textures using existing technologies,however, is confined to a simple surface texture with little room for varying surface parameters(Bobyn et al., 1980). Designing millimeter or sub-millimeter surface textures for SFF, e.g. for 3DPrinting (Jee, 1996), is difficult due to the complex macro-structure of the tiny texture geometry.It needs to be both compatible with the non-traditional manufacturing method of SFF andrepresentable in an efficiet CAD model structure.  A previous research project proposed a visualsimulation technique for facilitating surface texture designs as shown in figure 2 (Jee and Sachs,2000).Figure 2.  Visualization images for macro-surface texture4392.3 Visual build simulation of SFFIn order to develop a visual simulation tool according to the SFF process rules, manufacturingparameters must be incorporated to properly describe the geometry of the visually simulatedmodel.   In essence, a virtual model can be built purely based on the SFF build file as shown infigure 3. The implemented technique takes into account necessary geometric attributes ofphysical phenomena of the 3D Printing process and hence provides designers with the ability forverifying unseen fabrication capability of the existing prototyping machine in the embodiment oftheir design.   The visualization capability of the proposed method can be demonstrated bycomparing the virtual model of a design with the physical model as shown in figure 4.Depending on the application area, the staircase effect can have a significant effect on partfabrications (Jee, 1996).  Careful examination of the visually simulated  model before the actualfabrication can hence help minimize unwanted design iterations.Figure 3.  Simulated build for SFF process (3D Printing).a. CAD model b. physically fabriacted partFigure 4.  Comparison of build simulation and actual SFF model.In fact, the simulation paradigm can not yet cover all known issues in the SFF community such asmaterial, surface property, and dimensional tolerance.  Instead, it is currently limited to thelowest common denominator for the process capability of all available SFF techniques, i.e. partgeometry.  However, it can still make an important contribution to effective use of SFF by the440designer.   Although the processeses all differ significantly in their process parameters such aspart support, layer thickness, and auxiliary process (post process) steps such as curing orsintering, the underlying theme is the same: all of these processes build a fabricated part one layerat a time.   A common manufacturing capability such as minimum feature size relative to thedimension of one material layer can hence be captured and submitted to a designer so that it willlead only to manufacturable designs.3. Network-based SFF preprocessorNavigating the World Wide Web (WWW) using Internet software has become a highlysuccessful utilization of  communicative networks.  At the same time, rapid advances incomputing and communication technologies are creating a new approach for product design andmanufacturing. It is thought that the lessons learned from the Internet can be applied to SFFtechnologies as well, and a group of researchers and developers has already moved towardcreating an environment for automated SFF capability on the Internet (Bailey, 1995; Luo et al.,1999).  It is also important to devise ways to generate physical prototypes through a cheaper andbetter-amortized process using the Internet.3.1 Development of preprocessorJee and Lee (2000) recently proposed an SFF preprocessor with this functionality for use over theInternet.  Figure 5 shows two different process configurations for SFF usage; the upper one for atraditional process configuration and the lower one for the new configuration supported by theproposed method.Figure 5.  Traditional and proposed process configurations for SFF usage.ClientSTLRP part RP machineServicebureauClient  ServerRP machineVisualImage2D slicedlayers (www.CyberRP.com)RP part InternetSTLSTEPServicebureau441The proposed preprocessor can be realized by developing a client/server model on the networkand directly providing users with a convenient tool, a preprocessor for SFF over the Internet, sothat it can quickly fill the gap between various CAD systems and distributed SFF machines.  Inthis client/server model the server program continuously listens on one end of the channel, andthe client program periodically connects with the server to exchange data. Also, all the distributedSFF machines are connected to a server that will manage the process schedule of these machines.The server mainly manages a database for handling STL files so that a client can access the serverin order to search appropriate STL files.  The client can also directly upload and register its ownSTL files to the server for SFF processing.  The server will first read and translate the importedSTL file into an analytic model for visualization, and the resulting image data will be sent to theclient so that it can be displayed on the client terminal.  The server also provides clients withfunctionality of cleaning/repairing ill-defined CAD files, converting STL file into data recognizedby an actual SFF machine, and visually simulating fabrication (Jee and Sachs, 2000) as desiredby the SFF industry.  Each client can therefore order an appropriate slice thickness for the STLfile over the network, and an image generated from the slicing can be sent to the client for displayon the client terminal.  The client can also order a request for visual simulation functionality onthe server before the real fabrication of the STL file using SFF machines. The server will thencreate a virtual model and send an image data to the client for display.3.2. Example use of preprocessorUsers see the familiar browser interface at their client computers, and server computers supplysimple Web-like pages or do complex data processing in response to the user input.  Figure 6shows a visualization image of an example STL file, spider, displayed over the network.   Figure7 shows a visually simulated SFF part image using the virtual prototyping (visual simulation ofthe SFF part) option over the network.  If the resulting image looks acceptable, users can assign aworking order for SFF fabrication to the server.  The server can then search an appropriate SFFservice bureau and pass the order in order to execute the fabrication.  Working orders, on theother hand, could possibly be assigned directly to a specific service bureau depending on users’preferences.Figure 6.  Example of normal STL file visualization.442Figure 7.  Example of build visualization showing layer simulation.As part of future work, we will also add a functionality of a Remote Search and Evaluation (RSE)tool to the proposed preprocessor. Using the proposed RSE functionality on the Internet, users’attention can quickly be switched to a specific SFF process in which they might be interested.Figure 8 shows an example of an RSE browser tool which will be added to the SFF preprocessorfor searching and evaluating different SFF machines distributed across the Internet.  Eventually,this will enable direct tele-manufacturing using tele-control of SFF machines over the network.Figure 8.  RSE browser tool interface.4434. Conclusions and future workSeveral modules for design visualization of SFF models have been developed.  They enable adesigner to visualize surface roughness, surface features and build geometry.  The framework foran Internet-based SFF preprocessor has also been developed to help end-users have moreeffective communication with SFF providers.  The future aim of the research is to incorporate thevisualization modules into the preprocessor to create a comprehensive design support system forSFF.  Additional functionality will be added to the system including orientation optimization,improved layer simulation and visual representation of functionally graded materials (determinedusing FEM or similar).AcknowledgementThe authors wish to acknowledge the financial support provided by the Royal Society for DrJee’s  study to visit to the UK.ReferencesBailey, J.M. (1995) Tele-Manufacturing: Rapid Prototyping on the Internet. IEEE ComputerGraphics & Applications, 15 (6), 20-26.Bobyn, J.D., R.M. Pilliar, H.U. Cameron and G.C. Weatherly. (1980) The Optimum PoreSize for the Fixation of Porous-Surfaced Metal Implants by the Ingrowth of Bone. Clin. Orthop.,150: pp. 263-270.Campbell, R.I and Martorelli, M. (2000) Surface Roughness Performance of Rapid PrototypingProcesses – A Comparative Analysis, Proceedings of the 9th Europen Conference on RapidPrototyping and Manufacturing, Athens, Greece, pp. 37-44.Chandru, V., S. Manohar and C. Edmond Prakash (1995) Voxel-Based Modeling forLayered Manufacturing. IEEE Computer Graphics and Applications, November, pp. 42-47.Jee, H. (1996) Computer-Aided Design of Surface Macro-Texture for Three DimensionalPrinting. Ph.D. Thesis, Mechanical Engineering Dept. MIT, Cambridge, MA. USA.Jee, H. and Sachs, E. (2000) A Visual Simulation Technique for 3D Printing. Advances inEngineering Software, 31 (2), pp. 97-106.Jee, H. and Lee, S. (2000) An RP Preprocessor Over the Internet, Proceedings of the 9thEuropen Conference on Rapid Prototyping and Manufacturing, Athens, Greece, pp. 65-77.Luo, C. R. et. al (1999) Tele-Control of Rapid Prototyping Machine Via Internet for AutomatedTele-Manufacturing. IEEE International Conference on Robotics and Automation, 3, pp. 2203-8.Salton, D.J. and D.S. Meek (1995) Point Normal Interpolation for Stereolithography Modeling.Comput. & Graphics, Vol. 19, No. 3, Pergamon, pp. 345-353.444

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Visualization Tools for Design Support in SFF 437

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