RE: RP CAD

From: Dick Newton (Ford  Motor Company)
To: Elaine Hunt (Clemson  University), Ian Gibson (University of Hong Kong)
Cc: RP-ML
Date: Monday, April 3, 1995
Subject: RE: RP CAD
> >****** I have never heard a discussion of what different preparations are
> >necessary for using the same STL file on different RP systems. Decided
> >to highlight that incase someone wishes  to talk.  Just what file
> >verification processes are available on other RP systems?

On a helisys machine you need to bolean (or trim) everything together
if you want a single part out. If you had two intersecting cubes and
you didn't join those together, you would get three separate pieces out
of the machine. The machine will cut along the edge of each box as
shown in the simple diagram of a single slice of two intersecting
cubes. Every where there is a line, the laser will cut.

          NOT JOINED
           +---------+
           |         | 
           |         |
      ++++++++++     |
      |    |   |     |
      |    +---+-----+
      |        |
      |        |
      |        |
      ++++++++++


The following is the profile of a slice of two cubes that have
been joined, which is what you'd like for a helisys part to cut
correctly

           JOINED
           +---------+
           |         | 
           |         |
      ++++++         |
      |              |
      |        +-----+
      |        |
      |        |
      |        |
      ++++++++++

Either of these parts would build as expected on an SLA or DTM machine
because it wouldn't matter much if the machine double cured/sintered
the area common to each cube. In the end, they would still be joined
together.

These same unjoined cubes would also cause problems on the Stratasys
machine since it generates "tool" paths based cross sections of the stl
file. Stratasys' software would catch this type of error but if it
didn't the machine would try to extrude overlaping roads of material in
the same spot, kind of like trying to pump 20mm^3 of material into a
10mm^3 box.  Stratasys allows you to fix this type of problem by hand
by going through on a layer by layer basis and correcting the slice
profile.

As for file verification, the Helisys machine's software checks for
psuedo (zero area) triangles and it also generates a connectitivity
list. Any edge that is not shared (connected) by exactly two triangles
will flag a trouble and could potentially cause problems during a
build. Helisys ignores normal information for the most part.

3D systems Software will check and correct for
Positive Cad Space
Degenerate (zero area triangles)
Gaps (narrow and wide)
Triangle normal orientation
And other problems.

Stratasys checks on a per slice basis for intersecting profiles and
open profiles

Dick Newton