Threading conundrum

[hman]

Great that you found it! Unknown errors can really be annoying. And I know that conundrum can be very difficult to machine

 

[epanzella]

Sounds like a large shank Savage. I don't believe Savage uses a standard V thread but rather a squarish thread like an acme. Hard to see with 20tpi. Take a close look at the old barrel if you have it. Harder to tell from the internal threads. In any event a standard thread will work if you fit it on the tight side.

 

[dpb]

Sounds like a large shank Savage. I don't believe Savage uses a standard V thread but rather a squarish thread like an acme. Hard to see with 20tpi. Take a close look at the old barrel if you have it. Harder to tell from the internal threads. In any event a standard thread will work if you fit it on the tight side.

They’re a V thread, at least all the ones I’ve seen (only a half dozen or so).

 

[epanzella]

They’re a V thread, at least all the ones I’ve seen (only a half dozen or so).

I've only built one SML based on a Savage action. The donor rifle was a small shank model 111 in 30-06. A local gun shop owner said that it was NOT a standard V-thread on the Savage so I checked it with a jeweler's loupe. The guy was right! The flat is probably 5 times wider than a V-thread which causes the parts to bind until the tread is inadvertently made too loose. I got mine to fit by machining a few thou off the crests on a practice piece until I got a good fit then did the barrel. It's hard to tell with the naked eye (especially my 70 year old eye) but it's plain as day with a loupe.

 

[derf]

They are all v threads, they just have the crest knocked off. I've been doing Savage prefit barrels for over 20 yrs. The last 17 years, I've been using a geometric die head, with 1-1/16"-20 chasers. I can open it up to do 1-1/8"-20. I also made my own thread pitch diameter gauges to check them with. Pay little attention to the specs that Savage provides, as they are not 100% right, and just because the print says one thing, doesn't mean they actually follow it. I was fortunate enough to have 25-30 take off barrels on hand to measure and get mean dimensions to establish tolerance levels.

 

[epanzella]

They are all v threads, they just have the crest knocked off. I've been doing Savage prefit barrels for over 20 yrs. The last 17 years, I've been using a geometric die head, with 1-1/16"-20 chasers. I can open it up to do 1-1/8"-20. I also made my own thread pitch diameter gauges to check them with. Pay little attention to the specs that Savage provides, as they are not 100% right, and just because the print says one thing, doesn't mean they actually follow it. I was fortunate enough to have 25-30 take off barrels on hand to measure and get mean dimensions to establish tolerance levels.

Thanks for the explanation Derf, I thought I was losing my marbles. The threads looked quite different with those big flats on the crests but I couldn't match they up with any kind of square or ACME thread.

 

[P. Waller]

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.

You did measure the existing thread over wires?

Also the only "incorrect" thread is one that does not fit the mating part.
One may produce any thread desired for an assembly of your own, the ANSI thread standard dimensions as published allow one to use the "ANSI Standard Thread" in advertising if adhered to.

If I were in the firearm manufacturing business I would be tempted to use a proprietary thread for my products, say a 1"- 7 61/64 thread rather then 1"-8, this will surely confuse the buggers.