I use the "thread from left to right method, with a square plunge cut a few thousandths deeper than the minor diameter of the desired thread to give the tool bit a starting point. I have used the inverted tool bit/reversed spindle method, but the lathe I'm currently using has a threaded-on chuck that's not too friendly about cutting threads with the spindle running backwards- - - -it tries to unscrew the chuck! With a pin-drive or key-drive chuck and a spanner nut on the spindle, that's not a problem. One really fun thread that I don't like to try to turn is for a 1903 Springfield rifle barrel- - - -it's a 10 TPI SQUARE thread, (Not ACME) with a thread depth of .050"! That leaves NO room for error whatsoever as it's machined- - - -the thread spacing and depth must be absolutely perfect if the barrel is going to fit the receiver properly! A Springfield also has a coned breech with a counterbore for the boltface to fit into, but very few gunsmiths, me included, try to duplicate that feature on a replacement barrel!
When I was teaching auto mechanics at a Tennessee state trade school, one of the projects the students across the hall in the machine shop were required to complete was a stainless steel shaft for a plumbing gate valve with a 1 1/4" diameter 8 TPI left hand ACME thread. They used the upside down tool bit, reversed spindle method to do that project. Quite a skill test, there!
Jerry
