THanks Michael.
Hopefully I have uploaded it right. Didnt work the first time.

I want to remove the red shapes from the cyan cone.

Sime.

Hi Simon, the problem was that shell made a bad result, the inner shell surface is kind of mangled.

Instead of shell you'll get a better result in this case making a profile curve like this and then revolving it:



That's what I did with the attached 3DM file which should now boolean ok.

I am planning on overhauling Shell to use the ACIS library which should make it a lot more reliable.

- Michael

Thank you Michael.
That works.

Simon.