/diy/ - Do It Yourself

For anyone using inventor (and maybe other 3D modeling software too) I think I have a good explanation about the underlying cause of an error I sometimes get when adding a fillet to an object:

part.ipt: Errors occurred during update
Fillet: Could not build this Fillet
The attempted operation encountered a non-manifold body due to multiple model edges at an intersection. Retry operation with different inputs.

I now think I understand both why a non-manifold body is so unacceptable to inventor that it simply wont allow you to do something that would create one, and also what exactly makes a body non-manifold. I got this from this website, but I think I've generalized it even further. www.sculpteo.com/en/3d-learning-hub/create-3d-file/fix-non-manifold-geometry

First: Why are non-manifold bodies illegal in inventor?
In short, because such bodies *cannot* physically exist in real space, and inventor is intended to model things that *do* exist in real space.

Next: Why *can't* non-manifold bodies exist in real space?
At first glance, it looks to me that if a non-manifold body *did* exist in real space, it would require some or all of it to be infinitely thin, ie have 0 thickness. Even "2D" materials like graphene have SOME thickness, even if its only the thickness of a single atom.

Finally: What makes a model non-manifold?
The website I linked above gave several different conditions that would make a body non-manifold, but I think it can be generalized into 2 rules.
1) Every vertex must touch exactly 3 edges, no more, no less.
2) Every edge must touch exactly 2 faces, no more, no less.
If either of these conditions are not met, the body becomes non-manifold. (the reason *why* this is the case is illustrated quite well on the linked website, but they didn't actually provide these conditions)

>>tldr: I think I found an explanation for a common error in inventor, am I right? (also read if you want to know too)