I'm trying to design a screw thread more suited to 3D printing than standard threads.
As a trial run I drew a 8mm standard thread profile, swept it around a helix and unioned it with a core - no problem

Then I drew a modified screw profile and repeated that procedure but couldn't get the core and swept solid to union. I've checked everything I can think of, and started from the beginning several times but am getting nowhere.
Along the way I noticed that I couldn't Boolean difference the swept profile (the object 'coil') with a straight line to section it so it looks like I've stuffed the helix or sweep step.

The attached file contains the screw profile (which is a closed curve), the helix (spiral), the swept solid (coil) and the core.

I think that I'm doing something wrong at the helix step...

Any pointers greatly appreciated.