Hi JTB, can you please post a .3dm model file with those curves in it so I can try the shell over here?

But it is not really surprising to me that you would get a strange result with a shell or offset on such a twisty object with tight curvature in it like that.

In general an offset type procedure becomes difficult when you ask for an offset distance that is greater than the curvature of the surface or curve. It creates a kind of bunching in the generated offset, here is an attempt to illustrate that:



Generally to get a nice result for offset, shelling, or fillets, the offset distance/shell thickness/fillet radius that you use should be smaller than the tightest radius of curvature in the curve or surface to avoid that kind of bunching.

Instead of using shell you may get a better result in this case by creating a smaller circle and doing a second extrusion with it. That will not create something with a totally constant thickness, but it will be difficult to create such constant thickness in your shape there due to its bendy-ness anyway.

- Michael