Hi Ed - part of the process of shelling involves generating an offset surface.

An offset surface is generated by a kind of fitting process where it steps along a surface and then tracks a result along the surface normal.

If you have a surface that has a surface normal that kind of flips around wildly from one side of the surface to the other, that will kind of make the offset go crazy as it tries to follow a normal that's bouncing around all over the place.

It's pretty easy to have that kind of wandering surface normal in surfaces that are tightly pinched together or when a surface collapses down to a single point, it's easy for the surface to be kind of tightly folded back and forth over top of itself in these situations.

That's the kind of thing you have here, you can see the start of a tight fold in this region here:



Zooming in:



So note there that the shading indicates a tight curvature in the surface in that area, that's where it starts to fold back over on top of itself, and then you can see a little more to the right it is kind of like a spun towel or something like that.

There isn't much chance for the surface offsetter to be able to follow the surface normal that is varying so tremendously in such a small region of the surface like that.

To get shell/offset friendly surfaces that look like this you're going to generally need to make them starting with a kind of broad sheet like this:



Then produce the sharp outline parts by trimming that large sheet with a profile curve something like this:





That way the underlying surface has a consistent and well formed surface normal along all of it and isn't pinched or folded over top of itself anywhere.

It's hard to produce a surface with a high quality normal if you try to contort the entire surface to be pinched together like the other way.

For other kinds of purposes like just rendering it or something like that, the pinched together surface would probably be fine. But offsetting is much more sensitive to self-intersection.

- Michael