(no subject)

From: Albin Hastbacka (Sanders  Design International, Inc.)
To: RP-ML
Date: Saturday, April 22, 1995
We here at Sanders Prototype, Inc. have been receiving a variety of three
dimensional STL and DXF files from prospective customers for our Model
Maker and model making services.  These files vary markedly in quality.  In
order to provide guidelines on how to handle STL files on several different
CAD systems, we have put the following information together.  I hope that
you will find it useful.  Let me know if you do.

Regard
Al Hastbacka


CAD Guidelines for Model Preparation for Sanders Model Maker 3D Modeling System

POGO Model-Win

POGO International offers a powerful Windows based pre-processor program
called "Model-Win" for the Model Maker System. It has a graphical user
interface with many convenient features to make the preparation of files
easy and efficient. It imports STL & AutoCAD DXF files, orients parts in
the build volume, moves and scales parts, and  finds voids and open
surfaces. In addition it views 3D shaded images of parts, finds & repairs
bad surface normals, slices & rasterizes models, views binary slice files
graphically, and combines multiple parts for a single build cycle.

Successfully transferring CAD models to the Model Maker requires that the
3D models adhere to some basic guidelines.
 Surfaces must completely enclose a volume
 Surfaces must be trimmed to their intersection boundaries
 Surfaces must have their outer unit normals pointing away from the solid material

POGO supports a variety of file formats for transferring 3D CAD to the
Model Maker. Among these are STL, AutoCAD DXF, and Wavefront OBJ. Virtually
every 3D CAD package is able to produce at least one of these file formats.
Specific guidelines for each CAD system follow.

AutoCAD

AutoCAD files are imported into POGO using the STL file format. Both
surface and solid models can be imported. Before importing an AutoCAD file,
certain entities have to be modified in the AutoCAD drawing. Any Designer
or AME solids model can be imported as well as AutoSurf surface models. Any
surfaces that have been created from AutoCAD primitives are also importable
into POGO.

Most rapid prototype machines require that surfaces be oriented such that
the normal vector to the surface points outward, away from solid material.
This is also true for rendering surfaces in AutoCAD. The rule is, if the
surfaces can be properly rendered  with back faces removed, then the
surfaces can be imported into POGO.

The model will not shade correctly if the outer unit normals are incorrect
and it will appear that the model has missing elements since the shading
algorithm in POGO shades only facets facing the viewer. In some cases, the
facet normals can be repaired using the STL-Tools palette.

DXF and Other Supported File Types

POGO can import AutoCAD DXF files provided they are prepared according to
the guidelines given below. This feature is found under the File menu at
the top of the POGO screen. It is recommended that model files be saved as
STL binary files since DXF files occupy large amounts of disk space by comparison.

Preparing DXF models in AutoCAD

AutoCAD files must be converted to 3DFACE elements before exporting them to
a DXF file. The specific steps needed to do this may vary depending on the
AutoCAD modules used to produce the 3D models. The most common model types
with specific steps for 3DFACE and DXF conversion are:

Designer:

Use the ADMESH command to turn the mesh on and specify the mesh tolerance.
When the model is meshed, save the model in a new temporary drawing file.
Copy the mesh to a clean area of the model space. Explode the copied mesh
twice to reduce it to 3DFACE elements and DXFOUT the result. Discard the
temporary drawing file since it has been permanently altered by the explode
command. In some cases, arrow heads from dimensions are erroneously
exported. If this happens, a workaround in AutoCAD-R12 is to move the mesh
away from the area in which it was created, then DXFOUT entities only.
Select only the mesh using a window or fence.

AME:

Set the SOLWDENS variable to reflect the density of the desired mesh. Use
the SOLMESH command to mesh the model. When the model is meshed, save the
model in a new temporary drawing file.  Explode the mesh twice to reduce it
to 3DFACE elements and DXFOUT the result. Discard the temporary drawing
file since it has been permanently altered by the explode command. Note:
use SOLSTLOUT to produce the STL file directly from AME.

AutoSurf:

Prepare the model by making sure all surface normals are correct and that
the surfaces define a closed volume. Under Surface Display, select Polyface
Mesh and Unhide. Also check the Display Tolerance box and insert a value
small enough to give a smooth mesh. In order to break the AutoSurf model
down into 3DFACE elements, you must use the IGES translator supplied with AutoSurf.

Note: this is not the standard IGES translator that is supplied with
AutoCAD-R12. IGES version 5.2AS cl was used in this example.

The procedure is to export the model as an IGES file, and then read it back
in again. This breaks the individual surface meshes out of their AutoSurf
format. In the IGES translator dialog, click on Edit Options, and then
click on IDP Mappings. Check the Polyface mesh option on. Start a new
AutoCAD drawing, and use the IGES translator to re-enter the IGES model.
This time, on the IGES dialog, make sure the Use AutoSurf Mappings item is
NOT checked. Re-read the mesh into the AutoCAD drawing and use the copy
command to make a copy of the mesh. Explode this mesh and then DXFOUT the model.

Dealing with small Radii and Fillets

For parts that have extremely small fillets and radii, many small triangles
are generated around the radius. In some cases, the size of the triangles
approach the numerical precision of the STL file format creating invalid
triangles. To avoid this issue, ensure that the smallest features on the
part are not excessively small (less than 0.002 inches) and that the STL
mesh tolerance is not set smaller than 0.001 inch. Do not generate support
facets manually since this is done automatically by the POGO program. Note
that the Model Maker System has the ability to consistently produce models
with layer thicknesses to 0.0005 thousandths of an inch.

Pro-Engineer and IDEAS

In general, Pro-Engineer and IDEAS provide excellent STL files. There are
two areas to be aware of:
(1) Be sure individual objects are merged together. This operation trims
intersecting surfaces and removes redundant surfaces from inside the part.
(2) Observe the recommendations above in dealing with small radii and fillets.

Intergraph I/Design and EMS

These two products provide excellent STL files. In I/Design, be sure that
individual surfaces are trimmed to their intersecting boundaries. Also
ensure that the surfaces match at their common boundries to within 0.0001
inches. If there is a large gap, POGO will attempt to close it.

In EMS, be sure individual objects are boolean UNIONED together. This
operation trims intersecting surfaces and removes redundant surfaces from
inside the part. Any overlapping solid must be unioned. Solids that are
disjoint (separated in space) do not have to be unioned. Observe the
recommendations above in dealing with small radii and fillets.

Alias and Wavefront

In general, Alias provides acceptable STL files and also provides OBJ and
DXF files. In Alias: (1) Ensure the surfaces are trimmed to intersections.
(2) Force all normals to point outward from the model.
(3) The recommended file format is STL and OBJ. Observe the recommendations
above in dealing with small radii
and fillets.

3DStudio and Simply3D

These products are capable of exporting 3DFACE elements directly. Be sure
to verify that the models render correctly with back-face rendering turned
off. This will ensure that unit normals are oriented correctly.

Unigraphics

STL files are produced from Unigraphics if the model was built with solids.
To ensure redundant faces are not surfaced, the model must not contain
multiple solids that share the same face(s) or interfere with each other.
If multiple solids are used to construct a model, ensure they are merged
into one solid entity. Use the solid to surface function to create a set of
surface entities that accurately define the boundary of the model. Use
these surfaces in the STL module to create a clean STL file.

CADKEY and Picture-It

Excellent STL files can be generated in Picture-It and in CADKEY designs of
solid models. There are three areas to be aware of:
(1) Be sure individual objects are merged together. This operation trims
intersecting surfaces and removes redundant surfaces from inside the part.
(2) Observe the recommendations above in dealing with small radii and fillets.
(3) In general, if you can view the STL file correctly in Picture-It, the
STL file will work in the Sandres Model Maker System.
Note: CADKEY's advanced modeler program is not recommended for use as an
STL file generator for the Sanders Model Maker System.

CATIA

The STL translator provided by CENIT-GMBH allows excellent STL files to be
produced from CATIA formatted files. There are two areas to be aware of:
(1) Be sure individual objects are merged together. This operation trims
intersecting surfaces and removes redundant surfaces from inside the part.
(2) Observe the recommendations above in dealing with small radii and fillets.

Notice:  Model Maker, 3D Plotting, and Variable Slice Thickness are
trademarks of Sanders Prototype, Inc; Windows is a trademark of Microsoft
Corp; Model-Win is a trademark of POGO, Int'l; AutoCAD, DXF, AutoSurf, AME,
Designer, 3D-Studio, are all trademarks of Autodesk, Inc; Simply-3D is a
trademark of Visual Software, Inc; I/Design and EMS are trademarks of
Intergraph, Inc; Alias and Wavefront are trademarks of Silicon Graphics
Inc; Picture-It and CADKEY are trademarks of CADKEY, Inc; Pro-Engineer is a
trademark of Parametric Technologies Corp; Unigraphics is a trademark of
EDS, Inc; and IDEAS is a trademark of SDRC. CATIA is a trademark of
Daussault Systems.


'Nuff Said.