Solid Modeling, CAM and CNC: Revealing Geometry Gaps

Don LaCourse, 3DCADTips.com
Robert Byrnes, VX Corporation
December 15, 2004
reprinted by permission from

The process of manufacturing a part must adhere to a more exact description of a part than is required for display the part and in some cases even modeling this part. Gaps between surfaces can and will occur. Here are some tips on how to define, investigate and locate geometry gaps that can cause problems during tool path generation.

• Gap Tolerance
  Gaps are voids, or spaces, between surfaces. No data defines the part shape in these regions. These regions should be small, being about the size of the tolerance your modeler uses for solid or surface intersections. In places where there is no data, the NC software must make assumptions about how to control machining.

If the gaps are small the NC software will probably perform well. If the gaps are large enough (with respect to the tool path tolerance and tool size) the resultant tool path may gouge the part. Gaps can appear between faces of a solid model as the result of the following types of modeling operations.

1. Solid creation through sewing surfaces together
2. Adding, intersecting or subtracting solids
3. Adding features
4. Filleting operations

Some gaps are created as the result of software errors and others are the unavoidable results of the algorithms used by the solid modeling system. It is this latter case we address here. These gaps should always be less than a user specified tolerance.

Beware that when solids are displayed on the screen, gaps that exist between edges may be invisible. Rather than display all edges of all faces in a solid, when edges are shared only one is usually displayed. No amount of "ZOOMing" in will show more than a single edge. However, depending on your manufacturing tolerances that invisible gap may be significant.

• Investigative Methods

1. Investigate commands which will allow measurement of distances between edges in the solid.
2. Explode faces from the solid and then measure distances between surface edges.
3. Try cutting sections and measure distances between resulting curves. A section of a solid should form one or more closed loops. These loops will consist of one or more curves. Adjacent curves will overlap, intersect or have gaps between them. The distances involved should be acceptably small.

• Gaps may be Masked

Compensating software within the solid modeling system may successfully mask many gap problems. One example is the displaying of only one member of a set of shared edges within a solid as mentioned above, another example is rendering software which can compensate for gaps and provide attractive displays. Unfortunately, the requirements for producing good tool paths are often more difficult to satisfy than those for display.

• Translate Out and Back

Try writing the part out through a translator (such as IGES or STEP) and reading it back into your 3D CAD system. If the geometry will be sent to NC through such a translator, this is the most accurate way to inspect the geometry. Simply processing the data in this way may reveal problem areas. Also, translated data is often simpler (consisting of curves and surfaces) and may therefore be less difficult to inspect.

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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