Kawazuki - Thread milling relies on the three axis, X,Y and Z interpolating the thread pitch and diameter of the helix. The cutter, usually a single thread profile, is fed into the helix, and it is assumed that the cutter will cut the entire thread on one pass. This is not a problem, because your spinning that cutter in the thread at 1500-3000 rpm and moving it along the helix as slowly as you want to. Normally, you don't have to run any more than one pass, but as long as you start the helix at the same exact X and Y co-ordinate, it should cut right back into the helix. The problem with thread milling, is that if you want to use a multi tooth form tool, you have to have different form tools for each pitch. Otherwise, if you use a single tooth double angle thread mill, you have to have one small enough to cut inside the diameter of the bore. For external threads, of course, this is not an issue. Otherwise, the problem is that you have to cut the entire helix with a single tooth three or four flute mill, and its very time consuming. Much too time consuming for production work. Using a multi tooth form mill can speed up the time, but there are other problems making sure you move fast enough to clear the chip load, and other issues. Its more complicated than I can fully grasp. That's why the toolmakers usually have their own software to figure out exactly how each of their mills will perform, what pitches and profiles they can cut, and they program the g-code for you. AS LONG as you buy their 175 dollar thread mill.

I'm being facetious really, as Mactec54 has pointed out, threadmills can be had for 30 dollars and up. The problem I have with them is they are so tiny, and I break things.