Hi Linker, yeah unfortunately Shell is pretty much the least reliable function in MoI in general... But it also has a tough job to do.

In this case it looks like there is a difficulty in processing the juncture between your top sweep lip and the bottom part.

Even though you have carefully drawn those profile curves so that they are tangent pointing straight downward, the actual surface that is generated there does not have that same direction through its entire bottom edge.

That's because of the different lengths of and shapes of the rails - as those profiles slide along those rails they slide at slightly different speeds along each one, this causes a kind of twisting or torquing of the profile as it slides along, which affects the tangent direction of the surface.

So when the sheller tries to process that spot, it sees it as a sharp edge, not as a tangent edge. That means it tries to extend each surface, then offset them, then tries to intersect those. It's having problems calculating the intersection there, it can be difficult to calculate clean intersections when things are intersecting one another at a very shallow angle.

This can be a problem when things are nearly tangent but just not quite.

One method that I used that worked was to do one big sweep around the whole circumference, and then trim off a small bit off the top of the bottom part and then put in a blend between the sweep and the bottom - that then shelled well because the pieces were all tangent.

If you separate stuff into individual surfaces and then offset or shell those, that is working since there isn't any extension and intersection between extended surfaces happening in those cases.

I'll see if I can think of any other tips. In general if you can kind of weld your pieces together using either blends or fillets it can help to ensure tangency and make shelling work better...

- Michael