Set up a dial indicator on the lathe carriage so you know when to disengage the halfnuts. If you run in slowest backgear, you can stop the carriage within a thou or two of the same place every time, even with a fairly coarse thread.
Or run the lathe in reverse and thread it coming out. You'll have to put in enough of a runout groove in the bottom of the hole to start the threading tool in.
Try to make a living, not a killing. -- Utah Phillips
Don't believe everything you know. -- Bumper sticker
Everybody is ignorant, only on different subjects. -- Will Rogers
There are lots of people who mistake their imagination for their memory. - Josh Billings
Law of Logical Argument - Anything is possible if you don't know what you are talking about.