Outstanding. Thanks very much.
Question: On many of your filenames you have 100MM listed. At 2.54 cm/inch, I get 3.937007874 inches.
What am I missing here? I'm sorry if this is a dumb question.
There are 3.937007874 inches in 100mm. You can change what is displayed by changing Preferences->Generel->Units. The model always stays the same.
To get a nice start of the groove cut I just added another turn to the helices and moved them 1 turn down. This is "dd.ddOD * pi = -108.1242mm" value in the helix pitch and helix attachment Z. This is in the original file with the nub. Then in order to remove the nub I just moved the helices up so their Z offset became -100mm instead of -108.1242. And that is why I named the files that way to easy distinguish them.