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MecSoft Corporation PHONE: (949) 654-8163 18019, Sky Park Circle, Suite K-L FAX: (949) 654-8164 Irvine, CA 92614, USA E-MAIL: [email protected] WWW.MECSOFT.COM Surface Milling Using Faceted Data Abstract Many modern CAM systems use faceted data for 3-axis milling of complex surface data models. This faceted data could be imported, as in STL files, or surface data can be converted to meshes by the CAM system just before creating tool paths. Notable among this trend are products such as VisualMill from MecSoft Corporation [1], Powermill from Delcam International[2] and NX-CAM from UGS. This paper discusses some of the aspects using faceted data for machining complex surfaces. Industry Trends Facet based milling was first pioneered for automotive die making by some of the leading automotive companies in Germany. Internal CAM systems used by these companies gravitated to facet based systems. This was primarily because surface based systems were unable to handle these large part sizes and complexities of the geometry that automotive dies typically entailed. Soon this technology found its way to commercial systems in the 1980s, notable among them are Tebis and WorkNC. After overwhelming success of these systems in the market place, the CAM industry as a whole has migrated to this technology in the 1990s. Today almost all CAM systems use this technology as the fundamental technology for complex milling. When MecSoft Corporation decided to use facet based milling as it’s core technology, the decision was made based on the personal experiences of the several of the system designers at MecSoft Corporation, who were previous employees at various large successful CAD/CAM companies. Our principal system designer was an ex-employee of Unigraphics Corporation and was a key member of the programming team that transitioned the Unigraphics milling system that previously used direct surface milling technology to a facet based milling product. Another of our designers was the team leader who converted the Intergraph EMS milling product MAXMILL to use facets for milling from a direct surface based milling product. The transition was preceded by a long and drawn out struggle in trying to make the direct surface based milling technology work robustly in all situations. This is due to various factors that we will discuss in this paper. Today facet based milling is used in almost all industries including but not limited to mold making [1], tooling/die and aerospace where quality and robustness are of utmost importance. The Accuracy Question An almost immediate question that comes to mind when considering using faceted data for machining is whether it will be accurate enough. Practically speaking, however, this issue is really a non-issue from the machinist's standpoint. There are a couple of reasons for this. • Sooner or later the model will become faceted anyway because all contouring moves in a 3D part program will be composed of linear movements in all but a very few exceptions. • All CAM systems when machining free-form NURBS surfaces have to go to an implicit faceted representation (as opposed to an explicit creation of facets) when computing toolpaths. This happens even in CAM systems that claim to machine surfaces directly. In fact, this was precisely the motivation to go to explicit facet creation in toolpath computation. If you are going to require implicit faceting anyway, why not create them before hand and store them explicitly? One could argue that there is a stacking effect of laying tool path on an approximated model -- an approximation of an approximation so to speak. But one should also take into account that the facets of a tessellated model can be quite small. It's common practice to set faceting tolerances in the range of 0.001 mm (about 0.00004 inch) for machining purposes. Moreover, the tool path processor can apply some relatively simple logic to more closely adhere to the base geometry. For example, on a convex form, the center of a linear path across a facet will obviously come closer to the base geometry, where on a concave form the edges of the facet will come closer, and the tool path processor can take these factors into account [2]. Advantages of using Triangles in Milling Gouge free toolpaths: Because the mathematical representation of the faceted model is quite simple, the algorithms for generating tool path can be much more robust. This robustness directly results in gouge free toolpaths. Conversely, when working from complex surface representations, the tool path algorithms must be able to cope with a much broader set of contingencies embodied in the math and consequently are more complex, more prone to failure and harder to test and maintain. Speed of toolpath Calculation: Calculation of tool path from a faceted model can be much faster than when working directly with complex surfaces. This is again due to the simplicity of the data representation. WYSIWIG Machining: Most CAD and CAM systems can render the input mesh data in shaded mode almost instantly. The fact that the data being machined is the exact data being rendered on the screen also allows the user to check for errors in the data translation very quickly and reliably. Additionally as a general rule of thumb the rendered view of the model, using a flat shading algorithm, will closely resemble the final part shape after machining. A Sort of “what you see is what you get concept” If the mesh data looks faceted and not smooth, that this is what the machine part will look like. Simplified Data Transfer: The key advantage of using the mesh file formats in milling is that it communicates only the data that the NC programmer needs -- the geometry of the part. There is none of the extra information in the file to indicate the structure of the original CAD file. That helps simplify and make error-free the process of transferring work-piece geometry from one system to another. Disadvantages of using Triangles in Milling Most of the disadvantages discussed below apply only when a shop is using mesh file formats for data transfer. CAM systems typically get around this problem by allowing users to transfer surface data files and then perform the faceting of the surfaces just before toolpath computation. Large File Sizes: If a shop is using a mesh file format for data transfer, the size of these files could be an issue. For one thing, it takes a lot of triangles to accurately approximate a 3D form of any complexity. Additionally, some file formats such as the STL file format also contain a great deal of redundant information. Large Data Sets: The file size problem discussed above can be eliminated if the CAM system performs the faceting before creating the cutter paths. Due to the typically small tolerances involved in machining, this does result in large data sets and might be a drain on the memory resources of the computer. However, with today’s powerful computers boasting of large capacity memories, this is also becoming a non-issue very rapidly. Most off the shelf computers today (2005) are more than capable of handling even the most complex models used in the industry today. General rules to follow: 1. The faceting tolerance used to create meshes that are to be used in milling must be at least as tight as the smallest machining tolerance to be used. 2. As a general rule of thumb the rendered view of the model, using a flat shading algorithm, will closely resemble the final part shape after machining. So use this rendering mode to examine the model before starting to machine. 3. It is better to make sure that the triangles or polygons created during the faceting process all have sides that are about the same sizes. Note that this might not always be possible or desirable. In some cases this might cause the triangle/polygon count to go up significantly without a concomitant increase in benefit. However, when possible this is a good rule to follow 4. Experiment, Experiment, Experiment. What works for one application might not work for another. So use trial and error to arrive at the best tolerances to use while faceting. Benchmark To test out the hypothesis that facet based machining affords better surface finish and performance to direct surface based milling, we decided to test our product VisualMill 5.0 against a leading commercially available product that advertises its product as a direct NURBS based system. A simple parallel machining finish toolpath was generated on a part that contained about 100 surfaces was used for the test. Care was taken to make sure that both systems used the same set of parameters for the test. These are given below: Tool Type Ball end mill Tool Size 0.25 inches Machining tolerance 0.001 inches Stepover distance 0.005 inches Cut angle 45 degrees from the X axis A screen shot of the toolpath generated in both systems is shown here. VisualMill 5.0 Direct NURBS machining system Benchmark Results Two areas that were of importance to us were performance and quality of toolpath. The results are summarized below; Performance Similar in performance Quality VisualMill 5.0 was clearly superior We actually were surprised by the results. We had expected VisualMill to be significantly faster than the other product and quality of toolpaths to be roughly the same. We were surprised that the quality of the toolpaths from the other system was not good in transition areas. That is, areas where the cutter makes abrupt changes in direction. VisualMill clearly created superior toolpaths in these areas as the pictures shown below illustrate.