CAD software for creating STL.

From: Jon Hunwick (Delcam  International PLC)
To: RP-ML
Date: Tuesday, January 31, 1995
Subject: CAD software for creating STL.
Why Solid models are not necessarily better than Surface models
===============================================================

There has been a fair amount of discussion in this group lately
regarding CAD systems capable of generating good quality STL files.

DUCT was born from a research programme at the University of Cambridge
in the UK, and became a commercially viable product in the early 1980s.
It is a **surface** modeller, with exceptional abilities in the areas of
defining, and machining, complex 3D forms such as are found in most
plastic moulded parts. For this reason it has become the No. 1 system
for toolmaking in the UK, and has a rapidly growing reputation
internationally as well.

Over recent years holoforming has become increasingly important, and
many users now need to produce holoformed parts for examination by
stylists etc. before cutting metal. Most people assume that it is
**much** easier to generate STL data from solid models, as they
contain all the necessary topological information to allow the meshing
algorithm to work out where to go next. This is not necessarily the
case, and DUCT is among the few surface modellers that produce
high quality STL files with ease. The user has two lines of attack open.

The first approach is to assemble a set of trimmed surfaces into a
pseudo-solid (which we term a Shell). This Shell may be manifold or
non-manifold, and can be subjected to Boolean operations to complete
the part. Once the part is finished, what remains is (in effect) a
single topologically sound solid, which can be Meshed in the usual
way, and the Mesh written to disk.

The second approach is to ensure that the model is complete, and
closed, but not bother with defining the topology. The individual
surfaces which make up the part are Triangulated, and the resulting
Triangles stored to disk.

To generate STL data, a second program is used. This can read and
write DUCT format Mesh files, and binary STL files. The program can
detect errors or problems with the mesh, and repair them. In addition
the program can 'chop' an STL file into manageable chunks for
holoforming machines which are too small to build the part. Another
useful feature is that STL files may be converted from one format to
another - e.g. from binary to ASCII (though not the other way at
present), or into DUCT readable Triangles. DUCT can then take these
Triangles and generate NC code to machine them.

If anyone out there would like to know where DUCT can be obtained,
please e-mail to Clive Martell <cpm@delcam.co.uk> who is Marketing
Director, and he'll put you in touch with your friendly local agent.

|Jon Hunwick    			| Mail:jph@delcam.co.uk		|
|Delcam International PLC, Talbot Way,	| Tel: +44 21 766 5544		|
|Small Heath, Birmingham,		| Fax: +44 21 766 5511		|
|England B10 0HJ.		        |