Solid Modeling, CAM and CNC: Problem Detection & Troubleshooting

Don LaCourse, 3DCADTips.com
Robert Byrnes, VX Corporation
December 10, 2004
reprinted by permission from

Problems can and will arise that can bring a machine shop to a halt. During such times quick reaction times are critical to minimize down time. If you suspect that the problem is geometry related, here are some helpful detection and troubleshooting tips.

• Test for Hidden Problems
  When problems are encountered during NC programming, their cause must be determined. If the quality of the part's underlying geometry is suspect there are a number of tests that can be performed within the 3D CAD software. The results of these tests may indicate problems which otherwise might pass through to other applications. Finding problems early in the design phase will prevent a possible cascade of downstream difficulties.

• Display Tolerance
  Try decreasing the display tolerance. If allowed, this can be a simple and cost effective test. Look for surface display curves (edges and isolines) which appear strange. The edges may not meet up or they may overlap in the corners. This may be a bad surface, or it could simply be a characteristic of the modeler's display utilities. If all of your surfaces share this, you probably should not bother using this as an indicator.

CAUTION:
After inspection do not forget to reset your display tolerance to its previous value.

• Display Curves (Isolines)
  Try increasing the number of display curves (isolines). This ensures that you are seeing a more comprehensive evaluation of the surface and can reveal abnormal surface areas.

• Outside Offset
  Try offsetting the solid toward its outer side. (Surfaces may also be offset individually if only a few faces are suspect.) With large offsets you are likely to see strange results. A small but visually significant offset should be used. You are only interested in seeing that the offset geometry is smooth and predictable. There should be no sudden, unexpected, or large variations in the display curves, nor there be "ripple effects" along the edges. If the amount of offset is greater than the curvature of any concave regions, the geometry may fold. This is a reasonable result in many modeling applications which can be avoided with a smaller offset.

• Shading or Rendering
  Try shading or rendering the part. This can also reveal problem geometry. Surfaces that should appear smooth or change curvature in a uniform manner should not show sudden shading differences or isolated visual features. Shading can be useful because it is generally quick (compared to other evaluation techniques) and also uses the surface normal information. Decreasing the display tolerance will result in a more comprehensive test.

WARNING:
This can consume memory rapidly. Try to avoid using display tolerances so small that the test cannot be completed.

CAUTION:
Do not forget to reset the display tolerance to its previous value.

The simpler the shading utility the better. Just as some modeling and NC software compensate for anomalies in the geometry, so do some rendering software.

• Undetected Problems
  If a part does contain problematic geometry, the NC application may handle it masterfully or tool path generation may come to a halt. This is also an invitation for the NC operator to fit new geometry that has behavior which is more acceptable to the NC software, but does not necessarily fulfill the design intent.

About the Authors

Robert Byrnes is Vice President of VX Corporation and a pioneer in the integration of CAM and CNC with solid modeling applications. He currently heads the VX CAM development group at VX Corporation.

Don LaCourse is Editor and Founder of 3DCADTips.com, Principal Partner of eDocHelp, and Contributing Editor for Cadalyst Magazine. Don has over 25 years of experience in design, documentation and writing about CAD/CAM/CAE issues.

3DCADTips.com is a free, independent and objective information resource for 3D CAD users, administrators, and managers developed exclusively by eDocHelp and in partnership with Cadalyst Magazine, TenLinks.com and many other 3D CAD sites.

Its goal is simple but challenging - to continuously provide a wide range of valuable tips on 3D CAD, CAM and CAE and to bring all application tips on the web under one roof where they can be contents, indexed, searched and browsed by you.

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