Mechanical CAD Translation Notes

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Mechanical CAD Geometry Translation

Author: Gary Clough (formerly of JPL)

There are a number of different file formats which can be used to transfer data between Mechanical CAD systems (e.g.: STEP, IGES, DXF, etc.) Most file formats can be produced in a variety of "flavors". Before generating any type of interchange data, the following items must be addressed:

  1. What is the current representation of the data being transferred?
  2. Can the receiving system handle the current data representation?
  3. What functionality is needed after translation?
  4. What information actually needs to be transferred?

Another very important thing to remember is that this type of translation does not provide bi-directional integration of applications. Yes, these translations can be performed in both directions, but in most cases data is lost or filtered during each translation. For example, at this time there is no file format supported to transfer parametric data contained in a parametric solid. This means, if a parametric solid model is translated into a STEP file and then read directly back into the Parametric CAD Solid Modeler, all parametric information is lost. [Editor's note: extensions to STEP to support parametric data are being developed, but will most likely not appear in COTS STEP translators until 1999 at the earliest. --Steve W.]

Answering the above questions will help in determining what file format to use and how the data should to be represented in the interchange file. Generate and transfer only the data necessary and keep it as simple as possible. It should also be noted, that any translation of geometry between two CAD systems, whether using STEP, IGES, etc., requires testing to develop the right combination of parameters for each translator. A STEP file suitable for importing into CAD Vendor A may not import properly into CAD Vendor B.

The following is a list of CAD representations and some associated file formats to support them:

Representation File Formats
Parametric Solid Models Not supported at this time
Solid Models STEP (Some IGES translators)
Surface Models STEP, IGES
Wireframe Models STEP, IGES, DXF
Tessellated Models VRML, STL

The following is a list of common functionality needed after translation and the associated CAD representations which support them:

Functionality Needed Representations Supporting It
Mass Properties Solid Models
Detail Design Solid Models, Surface Models, Wireframe Models
Manufacturing Solid Models, Surface Models, Wireframe Models
Assembly Integration Solid and Surface Models
Meshing Solid and Surface Models
3D Shaded images Solid, Surface and Tessellated Models
Dimension Query Solid, Surface, Wireframe and Tessellated Models*
WEB Access Tessellated Models, Raster images, Vector images

*Note: Dimensions can be queried from Tessellated Models, but accuracy becomes an issue due to the approximation of the actual model. Also, certain features such as rounds are converted and radius informaion is not accessible.

Obviously, the best way to share data is by using the same CAD application. If this is not possible, solid models via STEP provide the most functionality after translation of CAD data. If either system does not support solid models, the data must be "filtered" which reduces the amount of functionality and data is lost.

Understanding what the data will be used for, the capabilities of the CAD systems and the limitations introduced from the data translation are the key ingredients for a successful transfer. If mass properties must be calculated using imported data, then transfer of solid models is the only way to satisfy this requirement without having to rebuild geometry. Consideration must be placed on what functionality is required with the imported data and the amount of rework that is necessary to support it.

(HTML Translation by Steve Waterbury)

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