Hi Keith,

> Even if I could achieve this goal it still isn't clear to me how
> I could chop the top off.

Well, if you can get a curve that is projected on to the outer surface of that shape, then you could delete the other surfaces and then trim the outer surface with the curve to slice off the top part, then use Shell on it to thicken that trimmed surface into a solid.

But it may be easier to try and construct a cutting surface.

You mentioned using rail revolve, but instead you may want to use Loft on a radial pattern of lines. For example here I used Transform > Array > Circular to duplicate a line in a radial pattern, then moved some of the lines up and then used Loft (with the closed option checked) to build a surface through them.

Once you have that surface created, you can then use it as the cutting object in Boolean difference to slice your solid into 2 pieces and delete the top part.













- Michael