On the Woes of CAD File Conversion

It can take many different CAD and 3D fabrication packages to make a project like Divining Rods successful.  Some, like Rhino, are great for flexible surface modeling to create great looking forms.  Others like Solidworks or Autodesk Fusion work well for engineering features like screw bosses or creating tool paths for machining.

The problem arises when you try to take a model from one and import it in another…

Here we see a sharp ridge produces a strange folding loop when the surface is offset (light yellow):

sharp_ridge

Looking into what might be going on, we see that the artifact doesn’t run the whole ridge, so it’s probably not a problem with the offset tool’s algorithm.  When looking closely at the ridge in cross-section we see that the ridge is actually looped through itself:

 

It’s an in-depth discussion into the mechanics behind what’s going on during these conversions, but without going into too much detail, some of the problems arise between the different purposes for which these programs were created.  Surface modelers are a more flexible tool for creating complex curved geometry by forming and bending surfaces using a variety of tools.  Solid modelers by contrast do a lot of the same things, but they have the added constraint that they do not allow geometry that cannot exist physically in the real world like the ridge example above.

Another problem arose from the way the mesh line curves forming the surface are defined.  In the surface modeler, a sphere shape was manipulated to create the Diving Rod form.  The curve lines defining the sphere had form similar to what’s seen on a globe.  Where all the lines come together at the “poles” very sharp angles are formed.  In the transfer to the solid modeler, the program had trouble with areas:

In one more example, the program simply rejected the offending surface panel during the file conversion and left the model missing a couple surface “panels”.  The gray zones ahow the interior of this shelled form:

missing_surface

Not all transfers where failures however.  One solution was to remove the surfaces areas that contained the sharp angled “polar” surface panels and replace it with a simple flat surface panel.  The result transferred easily allowing further work to be done in the solid modeling program.  In this successful file conversion example, the model has been split into halves, shelled, and screw bosses added.  This model will be 3D printed and electronic boards inserted.  The screw bosses will allow the two shelled halves to be attached back together:

shelled-with-bosses

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