The tool holder for a small bit is probably the best way to go as you don't
need a large tool tip to do 40tpi threads. You DO need a fair bit of relief
of the tip in order to stay away from the cut surface. If you can't fit
your tip into a properly finished thread at the proper angles, you need to
do more relief of the tip.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!