i own 3x tapmatic tappings head, but since switching to thread mills ive not used the tapping heads once.

thread mills are ace, the best thing ever.

granted i use m2 and m3 threadmills in ally, but i always get good results.

what you could try is altering the dia of the threadmill in the coding setup, so do a few passes getting deeper (or wider, till you reach the tools actual dia) and deeper till you have taken the correct amount of material off. the good thing with thread mills is you dont have to take the full depth of cut in one go.

hope this helps