@box said:
The main problem is your hole isn't a hole.
Should it go all the way through or is it a cavity?
Is the size critical, as in, can it be slightly narrower so that it hits the flat bottom correctly?
Thanks for drawing my attention to that.
I looked at the drawing in Freecad and it appeared to look OK - until I read your response and looked at it again in wireframe. That showed strange extra faces, but I never thought it was to do with the hole being of larger diameter than the bottom face.
I reduced the hole size and now it exports fine.
Looks like I will have to rethink the design as the hole size needed is for tapping, and re-drilling before tapping the 3D printed output may not be satisfactory. We'll see.
Thanks again