Long internal threads on a lathe

[Camputer]

Is there a trick to doing long internal threads on a lathe? I have a 3/4 OD part that has a 1/2-20 thread all the way through the part which is 4 inches long. The idea is to run them consecutively out of barstock so it would basically be a blind hole. The material is hardened 15-5.

 

[Oldwrench]

The material is hardened 15-5.

Cut off parts, tap through using a long reduced-shank gun tap...in SOFT material. I don't think it's even possible in hard 15-5.

 

[Bobw]

Cut off parts, tap through using a long reduced-shank gun tap...in SOFT material. I don't think it's even possible in hard 15-5.

15-5 is basically 17-4 that costs more. "Soft" is about a 35C in the solution heated condition, what some call annealed, but its not "annealed", annealed is soft forever, until its solution treated, like the difference between in 6061 between A condition "0" and a condition "T0" they are not the same thing.

Yet when heat treated, you can end up anywhere between a 28 and a 46. I haven't messed with 15-5 too much, but in 17-4, even well into the 40's it machines better than it does in the mid 30's when its in the solution treated condition. It loses a lot of its "Stainless-y" machining properties once its heat treated and behaves much like a hardened alloy steel.

As for on a lathe, you aren't going to single point it. So you have to tap it. I wouldn't want to do it, especially blind.

Hopefully its a J thread so you can open up the minor, but your lathe need to be dead nuts square to try it.

At least its a fine thread, forces will be a lot lower than a course thread.

How many are we talking here. 500 or 20? I'd probably run them individually, maybe bar feed, add a back side chamfer into an undersized hole, and part. Then flip and tap as deep as I could with a standard tap, then finish by hand with the expensive long tap. But I wouldn't do that for 500 of them.

 

[Ox]

I'm no help on that material, but I completely agree that you aint gunna tap that before cut-off.
At least not many times...


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Think Snow Eh!
Ox

 

[gregormarwick]

LMT-Fette make screw on head form taps. You might talk to them about the possibility of one of those on an extended shank.

Not much experience with 15-5, but 17-4 form taps very nicely even at H900, so I'd be optimistic about it.

Page 40 of the linked pdf, there is a 1/2-20 with a standard shank of 100mm, so nearly there off the shelf. Not what they're designed for of course, but might be workable.

Internal Threading Technology

 

[Camputer]

Thanks for the tips guys, I'll keep this all in mind if I end up having to do them. I think I'm gonna have a chat with the boss to see if we can maybe skip this one. The shop we're getting them from currently does a pretty nice job and the part has some other features that aren't really suited for us.

Btw, we cut a lot of 15-5 and I'd prefer it over any other stainless if I had my choice. I just don't think I'd have much fun putting an 8xD tap into anything.