What causes hourglass shaped threads?

[jwmay]

That’s about all I’ve got. I singlepointed several threads recently, and finished them all with a die. After the die, all of them were full size at both ends, and skinny in the middle. Any idea? I went ahead and used them,because they are of absolutely no importance. But at some point, I’ll have something I care about, and I want to avoid this moving forward.

 

[Bob Korves]

If the shaft is long enough and does not have support from a follow rest, the shaft will flex while you are cutting it from the tool pressure. It will move the material over, and the portion that gets moved most is the one with the least cut. Now, that is completely backwards from what you are describing, skinnier in the middle. Could it be from the ways being worn most in the middle of the cut? No, that would also make the middle fatter. OK, I'm stumped...

 

[kd4gij]

If you can post a pic it would help us help you.
How long was the threads and what size?

 

[kd4gij]

It sounds like the lathe cut a slight different pitch than the die.

 

[jwmay]

It’s 3/4” of thread, 1/2-13. I think it’s the die, but was just hoping someone else has experienced it, and could verify it from personal experience.

@kd4gij , would not having the the toolbit directly on center cause the pitch to be incorrect? I’ll admit I was a bit lackadaisical in setting up for screw cutting this time.

 

[jwmay]

I’ll have to take a picture tomorrow. I’m not nearby to the threads in question.

 

[MarkM]

Hard to say without seeing the operation so here are are a few thoughts regarding your hour glass thread. Cheap die and leading taper out of proportion and possibly the tailstock not used to keep die straight. Why not single point the whole thread?

 

[kd4gij]

It’s 3/4” of thread, 1/2-13. I think it’s the die, but was just hoping someone else has experienced it, and could verify it from personal experience.

@kd4gij , would not having the the toolbit directly on center cause the pitch to be incorrect? I’ll admit I was a bit lackadaisical in setting up for screw cutting this time.

No off center would not change pitch.
What lathe, What material, and what brand die?

 

[jwmay]

Grizzly G4002 12x24, ledloy steel, Mibro carbon steel die.

Why not do the whole thread? I don’t know. I assumed the die was more accurate than I was willing to be. In a hurry I guess.

 

[higgite]

The back end of the thread will be thicker because the die is tapered on the leading side, unless you turned the die around backwards to finish the threads to a shoulder. But, that doesn't explain the front end threads. You would think that everything that passed all the way through the die would come out at the same diameter. Unless.... did you measure the single point thread before hitting it with the die?

Tom