Threading question - 1-72 UNF

Machine Shop
1

I’m threading some brass rod but results aren’t what I expected. The major diameter of the thread is supposed to be about 0.070 - 0.073" but I’m consistently getting about 0.059" - 0.060".

Is there anything in technique (AKA operator error) that could account for such a big discrepancy?

Thread: 1-72 UNF
Set-up: Lathe with ww collet, 1/72 UNF die, tailstock die holder
Not threading under power

Since this is suspiciously like the major diameter of a 0-80 UNF, I guess one possibility is that my cheap Chinese die is mis-labeled. I’ve never used it before.

IMG_0677x1
IMG_0677x11200×921 215 KB

IMG_0677b_800px
IMG_0677b_800px800×600 106 KB

2

If it was anyone but you I’d suggest deflection in such small stock.

Measure your die and see if that is what’s up?

3

Is your die clamped too tight? You can adjust the dies

4

It’s possible I suppose, but I wouldn’t think it’s deflection. The rod is only protruding 1/4" from the collet. The first couple attempts it was only protruding about 1/8"

I don’t really know how to measure it.

5

Maybe gauge pins? If you’re careful not to scratch them up you could at least see how far until you hit whatever diameter.

6

If you have a tap that is the same size as the die, it should fit inside, the threads of the tap should be slightly smaller than the die. If it doesn’t go in, then the die is being squeezed. Short of a calibration G/NG gage that would be my method, or use a manufactured thread to see if it fits.

7

That’s a good suggestion and it was worth a try. I got a couple more thousandths out of it, but no where near the specified major diameter.

8

It doesn’t go in.

However, my 0-80 die threads easily onto the rod I just threaded with the alleged 1-72 die.

9

So we have a die lie…

3 Likes