Need help with threads

[Turk88]

I am being told that I have to tap the holes in my part for a 8-32 UNJF! I have no idea how to go about figuring what size drill to use and where I can order the tap from!!! I know that the J is a designation for Aerospace but I have no idea ho to tap the hole properly....any ideas???????????

[Turk88]

SOrry...the spec is for a UNJC-3B

[little bubba]

J threads, especially Internal threads are easy. You need to use a standard tap and drill a little oversize. The SAE spec on J threads is like $35. I've got it at work, but since I don't feel like being there right now, I don't have it handy.

On a small thread, under 5/16 or so, the high end of the minor diameter tolerance on a starndard thread will usually put you pretty close to dead in the center of the J thread tolerance. Did that make sense?

Hope that helps, if your still in a quandry, I can look it up tomorrow.

Edit>> a little more info on J-threads since you don't seem sure on what they are. Internal threads, they are just drilled bigger, pitch diameter and all that other stuff is the same, so you just use a standard tap.

External threads, still pretty simple, they just have a modified root radius that is 15-18% of the pitch. The only other difference from a standard UN thread is the minor diameter, which is obviously affected by the root radius.