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CAD Data Repair

Butlin, Geoffrey and Clive Stops

5th International Meshing Roundtable, Sandia National Laboratories, pp.7-12, October 1996

INTERNATIONAL
MESHING
ROUNTABLE

Abstract
Using CAD data for analysis is becoming more common, but is still fraught with dirty geometry problems such as slivers, crossovers, minute edge lengths, stray points 'on the moon', wonderful clean-as-a-whistle models that are useless for meshing, patchworks of faces that attract unnecessary elements, birds nests of draughting geometry stuck in a corner, etc. Even if binary, native CAD files are received, the engineering analyst is at the mercy of the idiosyncrasies of the CAD operator and the CAD software.

Data exchange through standards or native formats is only part of the solution. Indeed if IGES is used, the problems are often exacerbated. Data transfer only provides the exchange of geometric entities, 'as-is'. A CAD model usually requires significant modification, or transformation to get it into a form suitable for engineering analysis.

While the transfer function can be fully automatic with IGES, STEP or native formats, the transformation process is typically subjective, application specific and requires engineering judgement, and is therefore not readily automated.

FEGS CADfix software provides data transfer through IGES, STEP and native formats interactive repair tools for cleaning, and healing interactive de-featuring tools for merging edges and faces and removing slivers and tangencies interactive editing tools for splitting up solid models into hex-meshable and CFD block-meshable regions.

Following the repair, smoothing, simplification, splitting up etc, the model can then be exported directly into the geometric modellers in analysis code pre-processors, such as ANSYS and Patran.

The typical target for CAD data transfer is auto tet-meshing. Such automatic algorithms are unfortunately at the mercy of idiosyncrasies of an apparently valid Brep model. Minute slivers, fine tangencies, narrow ,necks' and face/edge sloppiness etc, can challenge even the most robust tet-mesher. A solution to this problem is proposed in the form of a batch macro of CADfix procedure calls that put the solid model 'through the laundry', washing out all minute features and ironing out all edge and face coincidences, discontinuities and discrepancies, according to a specified resolution. Such a model 'laundry' or super-cleaner, when invoked as a batch pre-processor promises to significantly raise the robustness of auto tet-meshing.

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