Re: Attn. Pro/Engineer users!! Attn. Pro/Engineer users!!

Date: Sat Nov 25 1995 - 18:12:05 EET

Lee Humphrey in
Subject: Attn. Pro/Engineer users!!

> We have recently converted from CATIA to ProE. Normal ProE translations are
> several levels above the best of CATIA models. I have not had a bad ProE
> translation in the 6 months since the changeover.
> There are several things you can do to make the ProE translations better. First
> is increase accuracy during setup and also during translation set the chord
> height to 0. The system then comes back with the minimum chord height allowable
> within the accuracy parameter of setup. Also radius chord to 0. Be prepared
> for files with many triangles. 50-100k triangles are usual for complex curved> surfaces

I would like to bring this point up for discussion.

I have seen files with over 100k triangles that were completely useless. The
STL converter, when setting up the smallest tolerance which is typically
machine precision, could take a perfectly planar surface and tessellate it
in many hundreds or thousands of triangles when just a few should have been
necessary. Machine precision would create holes, disjointed triangles, and
all the problems associated with such a model. Furthermore, it took too much
space to store, and an inordinate amount of time to slice.

The user should use common sense and understand what the translator does. Such
problems occur because of the inadequacy of some translators. A common
mistake done by many users is to try use the maximum capabilities of the
software. Double precision is used, minimum chordal tolerance is used
without too much concern over what happens next with such files.

In some cases, these are the necessary parameters, but must they always be
the rule?

One solution is to have better STL translators. Another is to get rid of such
translators and directly slice the CAD model. This would require the RP
manufacturers to release their proprietory file slice formats, or the CAD
vendors to write translators to each RP format. A third method would consist
in deriving from the RP machine parameters the minimum tesselation
parameters for the models.

Comments? discussion?
Some of these issues are presented in a paper that will be posted at the
Internet conference next month.

Georges M. Fadel 
Associate Professor
Mechanical Engineering Department			Tel: (803) 656-5620
Clemson University					Fax: (803) 656-4435
Clemson SC 29634-0921 USA

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