Threading conundrum

[dpb]

To preface, I’ve been threading on a lathe for a few years, without any particular difficulties.
I was attempting to copy the threads on a barrel shank, using the pitch diameter as a reference. When cut to an identical pitch diameter, my threads did not fit the receiver. After reducing the pitch diameter an additional .008”, a good fit was achieved. The threads look perfect, the fit feels great, everything appears to be as it should, except, the pitch diameter.
Things I’ve checked:
Compound at 29.5° (the correct 29.5°), confirmed with bevel protractor.
Tool centered for height.
Tool squared to work.
TPI correct.
Different inserts tried, to same effect (full profile, and simple v)

I realize it is possible that the threads I’m copying are incorrect somehow, but I would like to know what I might be doing wrong.

 

[T Bredehoft]

How are you measuring pitch diamteter? Three wire, or thread mikes? Best choice is not worry about the number, but make it fit.

 

[dpb]

Three wire.
1 1/8 x 20tpi, using .029 wires.

 

[dpb]

I’m not good at accepting things I don’t understand, at least in this area.

 

[hman]

SWAG here ... did the threads you made have sharp peaks ... and what about the original barrel shank. Might be measurable as a difference in the OD of the finished threads.

Another SWAG ... are the threads of the original barrel shank at a 60º angle, or possibly something non-standard?

 

[dpb]

My threads do not have sharp peaks, my OD is slightly less than .001” smaller than the original barrel.
It’s a Savage 12, should have, and appears to have, 60° threads.

 

[RJSakowski]

It seems like you are doing everything correctly.

Since different inserts have the same problem, I would look to the receiver threads. Line your threads and the barrel threads side by side and examine under magnification. You may find that the original threads are different. A small variation in the thread angle can have a significant effect on the measured pitch diameter using wires.

The .029" wires come very close to the crest of the 20 tpi thread. If either of the threads isn't properly formed, the wires could be catching the crests of the thread instead of the tops of the wires. I would check using the .032" wires. You would have to run through the math to calculate the pitch diameter but since you are making a measurement relative to the barrel, you can just look at the difference.

As a final comment, are the two o.d.'s the same? If your o.d. was greater than that of the barrel, you could be seeing an interference from the crest of your thread even though the pitch diameters match.

 

[derf]

Savage large shank threads should have a major o.d. of 1.120", but the pitch dia, is the same for a 1.125-20 tpi. If it fits...you're good.

 

[benmychree]

As another responder said, just make it fit. The only times that I use the thread mikes or wires is when I am making a thread plug gage, such as for threading lathe chuck backplates. All other thread cutting is by "cut and try".

 

[dpb]

Solved it.
Under close scrutiny, it looked like the pitch on the first 2-3 threads was slightly off.
Found a small amount of slop in the compound. Adjusted that out, tweaked the cross slide gib, and tried again. Pitch diameter ended up right where I thought it ought to be.