Hi Martin,

> Is there a way Michael of making the Offset/Shell command stay planar

Well, the edge of the generated offset is planar in your case here, it's just on a different plane than the input surface.

There is no way in the geometry library to generate a constrained partial offset like I think you're asking about - the offset that is generated is always a full offset of the input surface, if the surface normals of the input surface flare upwards or downwards then the offset surface will track along those as well.

If you want to make a partial offset, you can get that by trimming the offset with 2 horizontal lines to cut away the area that you don't want. That's normal that you might need to use more than one tool to finish a particular job - it's not really feasible in general for every command to try and build in too many options for trying to handle special cases, each command would become very bloated and complex.

There is no restriction for offset to only work on things that start out with planar ends on them, it can work on any kind of surface.

For something like you've got there it might be more convenient to work more at the curve level - generate an offset of the profile curve, trim that and then revolve both pieces to make your desired shape. Sometimes working more at the curve level can let you do a bit more localized stuff.

- Michael