Re:[2] CAD software?
From:
Christopher Hicks
Date:
Monday, January 30, 1995
From: Christopher Hicks
To: RP-ML
Date: Monday, January 30, 1995
Subject: Re:[2] CAD software?
>I'd be real interested to know
>if anyone else out there spends as much time staring at individual
>triangles as I do.
>Chuck Kirschman ckirsch@eng.clemson.edu
Chuck,
We design and manufacture orthopedic implants (primarily knee and hip
joints). These parts contain numerous curved surfaces and blended
radii... very similar to turbine blade but with more features.
The curved surfaces look terrible if the meshing is too coarse. In
fact the surfaces can look bad even with a LOT of triangles. I did
some experimenting and came up with a simple explanation as to why the
parts looked so rough, even though when inspected by a CMM they were
within the allotted profile tolerance I included an excerpt below
from an internal report I did a couple of years ago. (sorry if this
is too long, or too basic)
In conclusion, if I have to have smooth curved surfaces (for example,
an RP (HF) master to make a mold) I set the CordHeight to zero during
the tessellation in ProEngineer. ProE will return with an error
saying that the specified CordHeight is too small. ProE will tell you
the minimum ChordHeight permitted. I typically use this or some small
multiple of it to give me the smoothest curve possible.
Excerpted from "Analysis of RP for use in Manufacturing" 9/1/93>>"
3.1 Tessellation inaccuracies
Rapid prototype part accuracy begins with the CAD model. As discussed
in section 1.1.1, the solid model is converted into an STL file. The
STL file format is accepted by all RP systems and is the common format
for data exchange at this time. When a solid model is converted
into an STL file, all surfaces are converted into a collection of
triangles. This process is called tessellation Flat surfaces can be
represented by triangular sections with almost no loss in information.
Curved surfaces however, can only be approximated by a series of
triangles. The way curved surfaces are approximated by triangles is
controlled in ProEngineer through the use of two parameters:
ChordHeight and AngleControl.
AngleControl is important when a surface feature is bounded by a curve
with very small radii relative to its part size, such as a dimple
(small R) on a golf ball (large R). If additional improvement in
AngleControl is not specified, those features will have little
definition in the tessellated output. AngleControl is less important
than ChordHeight for most parts designed at Intermedics Orthopedics.
ChordHeight specifies the maximum distance between a facet edge and
the precise surface of the solid model, as shown in figure 9. Smaller
values of ChordHeight produce more accurate models but increases the
size of the resulting STL file. ProEngineer will impose a lower limit
upon how small cord height can be specified for a given model.
ChordHeight use is analogous to applying a profile tolerance to a
curved surface.
For all the parts built in this accuracy study the lowest permissible
value of ChordHeight was used. For the stem tibia test part discussed
in section 3.1, a ChordHeight value of .0002" was used. An
interesting visual phenomenon occurs as a result of the tessellation
process. For even small values of ChordHeight, the model may appear
to have been coarsely approximated giving the false impression of an
inaccurate part. For example, a typical curved profile used on knee
articulating surfaces has a 1.3" radius. Given the ChordHeight, we
want to calculate the resulting length of the facet edge since the eye
will see the intersections of the facet edges as sharp corners. This
can be solved using basic geometry. {sorry, couldn't include the
figures. Basically, intersect a circle with R=1.3" by a line offset
toward the center of the circle by the distance CordHeight. The
length of the straight line between the two points where it intersects
the circle is the Facet Length.}
ChordHeight Facet length
.010" .321"
.001" .102"
.005" .228"
.0005" .072"
.0001" .032"
.00005" .023"
Thus for even a small ChordHeight of .005" the eye will see facets
which are .228" long. This is in spite of the fact that the part
profile was built to a geometric profile tolerance of +/-.0025".
(This value contains only the variation due to the computer model,
tolerances due to process inaccuracies are not included in this
number.) Facet lengths for other values of ChordHeight are shown in
the table below. Compared to a machined part, the RP facets fool the
eye into believing the RP part is rough because on standard machined
part profiles the surface will gradually vary from the highest spot to
the lowest spot over a large distance.
With tessellated RP parts and tessellated CAD solid models, the part
profile tolerance is being completely used up as the actual surface
zig-zags between the upper and lower profile limits. Parts requiring
smooth visual surfaces will require secondary finishing to fill in the
corners where tessellated surfaces join or remove material until the
surface is level with the intersecting tessellated surfaces.
functionally the tessellation "corners" on a part do not affect
form and fit testing. "<<
Chris
Chris Hicks Intermedics Orthopedics, Inc.
Advanced Manufacturing Development
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