Originally posted by Fred in NC
<br />Rich, cutting outside threads on a metal lathe is not a problem.
The inside threads are more difficult, because you need a boring bar type of tool, and you cannot see what you are doing.
If you can cut the outside threads with the lathe, the same setup should be able to thread a piece of drill rod. All you need is to cut some flutes on it, and you have a tap.
I will look into it. Maybe I can make these taps.
Quite so [
] You can buy a bit that is ground for cutting inside threads. (I have one bought from littlemachineshop.com but can't find it in their current lineup.) Yes, you can make your own tap, to tap soft materials like plastic and wood. I have read of people just grinding 3 flats on a bolt. I have made such taps as you describe - took a Dremel and cut a bunch of flutes into essentially a bolt. Not pretty - but worked for me. The thing is I believe ideally you want a positive rake at the cutting teeth, otherwise you get a bite but not a good thread. I don't think you need to worry about hardening. At least not in something for your own use [
] What I think you can do is cut the male thread, then mill or grind flutes. If that leaves a burr in the wrong place (depending on how you cut the flutes) you can clean that up with a chaser die maybe.
The point is to cut a thread for fastening things is easy, and the simple tricks you read about in the metalworking groups are serviceable. But for a pen cap, you need much better than that - getting a grip is not good enough. It has to feel good also
Not to discourage you in the least! I'm just giving you some things to think about from the start to get to the desired results faster, I've gone through the thought process
With a metal lathe, the thread I would think about is a 36 TPI triple-lead thread.
BTW I'm with you - there is a certain amount of angst even thinking about interior threads, not seeing what you're doing. I think that will be overcome with a little practice. (Anyway I hope so!!!)