Thanks Michael!

Here's the final version of my thread making script.

I'm not so confident with the UI though. It starts to behave strange when I run the script more than once i.e. the UI dissapears after some time.
Also the transition from the thread part to the unthreaded part is a cheat. I simply join the parts using a dowel. The correct way would be one cylinder and a thread that tapers to zero using a tapered helix but that turned out to be too fragile to work reliable.

It would be nice if there where an updated tapered helix function that allows to set the start of the taper to any point between start and end of the helix e.g. for a 50 mm long helix a setting of zero would give the tapered helix we already know while a setting of 25 would produce a helix that runs with the start radius straight up to the middle and then starts to taper. This would be better than having to join two helix curves get different tangents and problems with the Sweep function.

EDIT: I deleted the attachement and attached the newest version of this script to the first post of this thread.