Hi Lars - in this case it looks like there is a kind of messed up edge in this area:



The trim or boolean did not do a great job of calculating that bottom edge there.

To clean this up, I used the ShrinkTrimmedSrf command (it is a special command you can run by a keyboard shortcut) and then selected the trim curves and used delete to "untrim" the surface and that gets a nice trim boundary on it.

That version is attached here as shell2_fixedcorner.zip


With that tuned version, shell or offset should now work on this case, previously it would shoot a strange little bit off to the side right in that area where that wiggly edge was at.


But it sounds like you may not want a "true" shell in this case? The offset and shell command will create an outside surface that is a constant distance away from the original. So if the surface is twisting, the offset will also twist as it must in order to keep a constant distance.

If I understand you correctly, it seems like you may want a shape that does not have a constant thickness to it? To do that you will need to construct the surface manually using something like sweeping, I'll see if I can give you an example.

- Michael