20 TPI threads on Rolling Block?

[KenH]

Hello all – the #1 Rolling Blocks I’ve fooled with (only 3 of them) have 12 TPI square threads. I think that is the standard #1 thread. I’ve got what was sold to me as a Swedish Rolling Block that was in 12.7X44R (I’ve since re-chambered to 50-70). The best I can measure these threads it looks like 20 TPI, but it’s a square thread….. not perfectly square, but more square than Vee.
194805
Has anyone seen a Rolling Block with 20 TPI threads? Is it somewhat normal? Is this anything like normal?

Thanks to all for any info

Ken H>

[country gent]

I know its not the roadrunner hour here but, Maybe an Acme Thread series. If I remember correctly acme thread form was 7* on a side with flat crest and bottom. I would have to dig out the Machinist handbook from my gerstner to know for sure though.

[KenH]

It could well be an acme thread - but even more interesting to me was the 20 TPI count. I've looked thru several sources and seen this in only a few places, and nothing solid about how the 20 TPI came to be.

Ken H>

[country gent]

Not sure why they would do it but look at the end of the threads and see how many "starts" there are, 10 tpi double lead would gage as 20 tpi. Back then in the firearms industry there were few "standards" in place yet.

[KenH]

I hadn't even considered the idea of double lead threads. I'll have to check that next time I've got the barrel removed from action. I've got double lead threads on my desktop CNC machine, but never considered them on a barrel. I don't see much point in that, and wouldn't that be harder to machine than single lead threads?

Ken H>

[enfield]

I have one Husqvarna out of several with "V" threads but even it was 12 tpi I also found that a 43 mauser barrel threaded in the action nicely, just had to turn down some of the knox and run a 12 tpi die down to make it go in the action the rest of the way.

[country gent]

Like I said I'm not sure why they would do it or a good reason for it, but it is a possibility. The only reason I can think off here is to keep thread depth down. But a lot had their ideas back then also.

[KenH]

Well, it was just un-usual to me, so I posted the thread to see if anyone was familiar with the 20TPI thread on the barrel shank.

Thanks to all for comments.

Ken H>

[Fishoot]

I know this is an old thread, but I recently discovered that the 20 tpi barrel I took from a Stockholm marked Swede rolling block has those same funny threads. I presume I can turn 20 tpi 60 degree threads on a blank for a fit? Anyone done this?

[JSnover]

I know this is an old thread, but I recently discovered that the 20 tpi barrel I took from a Stockholm marked Swede rolling block has those same funny threads. I presume I can turn 20 tpi 60 degree threads on a blank for a fit? Anyone done this?
Not on a barrel I haven't but sure, you can turn 20 TPI with a 60 degree tool. Just make sure 60 degrees is the correct angle first.

[KenH]

My threads are not the standard 60⁰ thread. They look more like ACME threads- or something like that. They're flat on top with flats on bottom. I'd grind my threading tool using HSS and make a point that would fit into the "groove" of the thread as close as I could. I'd make a couple of practice threads on a rod to make sure it screwed into action ok. Now, you might use a standard 60⁰ threading tool and it screw up nice 'n tight, but since the face of the threads wouldn't be fitting correct and only touching in small area I wouldn't think it would have full strength.

I didn't cut any of the 20 tpi threads, just had the barrel off action for..... something, don't remember what now. I have threaded a couple of barrels with the 12 tpi square threads, and for those I made a threading tool from HSS.

Good luck and have fun.

[john.k]

Whats needed is a way of comparing the thread and your cutter.....a jewellers eyepiece is probably good enough......square threads are difficult to cut,so the flanks were often angled a bit to provide clearance when cut ......i really doesnt matter what you call the thread,you are needing to cut an accurate match on the new barrel..............or you could use the existing thread as an adaptor.

[country gent]

If you can get to an optical comparator the thread form can easily be measured as to angles shape and or form. At 30X to 100X thread form becomes much more apparent LOL.
If I remember correctly square threads are actually 1 1/2* degrees on a side and the compound is set to this to feed in. Acme threads are 14* and the compound is set to 7 1/2* to feed in. Seeing the difference on the actual threads can be hard with out magnification. Thread pitch gages sometime will show the difference. In this case a machinists handbook (older ones may be better) can be a big help. For descriptions of thread forms, depths, minor dia and major dia.. Another plus is ways to measure when cutting.

Being from a family of tool and die makers I have my grandfathers Hand book ( around 1940 edition) Dads ( around 1960 version) and mine ( 1980s edition) These are a great source of information. the three have many things one or the other dosnt.

[labradigger1]

I know this is an old thread, but I recently discovered that the 20 tpi barrel I took from a Stockholm marked Swede rolling block has those same funny threads. I presume I can turn 20 tpi 60 degree threads on a blank for a fit? Anyone done this?


It is no more work to grind the correct degree bit for single point threading then it is to grind a 60 degree bit. No use in trying to force incorrect thread angles into a receiver. It may go but you may also damage the receiver in trying to crush fit the barrel.
Buy an acme thread gauge and fishtail gauge and research the compound angle, min and max diameters and go to work.
As mentioned above, threads were not standardized then, it could also be square threads.
Good luck with the build.

[kenton]

Just thought I would mention that 20 TPI is .050in. per thread and 1.25mm per thread is .04921in so it may be a metric thread. Which brings me to my normal complaint that if there was a 5/16-20 bolt it would be effectively interchangeable with a 8x1.25mm bolt at least for non critical applications.

[country gent]

It wouldn't be completely interchangeable American vee threads are 60* and metrics are 55*

[kenton]

It wouldn't be completely interchangeable American vee threads are 60* and metrics are 55*

Metric is also a 60 deg vee, Whitworth is the thread with a 55 deg profile. Also maybe British pipe thread?

[Fishoot]

I'll follow the majority opinion and make a tool for the thread I have. Thanks for the advice.:-)

[Red River Rick]

It wouldn't be completely interchangeable American vee threads are 60* and metrics are 55*

CG: You may want to check your info. Metric threads are 60 degree included angle. The only thread that has a 55 degree included thread is the British "Whitworth" thread.

RRR

[M-Tecs]

Acme threads are 29 degree included https://www.engineersedge.com/hardware/external_acme_thread_13360.htm

http://bbs.homeshopmachinist.net/threads/51496-Rolling-Block-Project/page6

per post 59 The Swedish m/1867-68 was built by Husqvarna. At the beginning of production they used Vee threads, later going to the conventional square threads. I was surprised but I had one in my possession.

[KenH]

When you say "Vee threads", do you mean a standard 60⁰ thread? What TPI? Notice the photo above is about 20 TPI, but isn't Vee, nor is it a true square thread like the 12 TPI of most Rolling Blocks. Perhaps more like a ACME thread? I didn't measure the degree since I didn't need to cut any threads of it. Just had it apart for cleaning and took photo.

[M-Tecs]

Most V threads are 60 degree except for the 55 degree Withworth. https://www.practicalmachinist.com/vb/general/55-deg-vs-60-deg-tool-cutting-55-deg-threads-203697/index3.html

Firearm barrels with square threads are shallow square threads

The threads on the pictured barrel shank appear to be damaged. Whatever they are they are not acme or square. Hopefully the threads in the action are better. If not they will need to be recut.

Good overview on threads here http://www.nashua.edu/paradisem1/Machinery's%20Handbook%2027th/27_Thread_09A.pdf

[KenH]

Thanks for the two links. I've downloaded the pdf file to my library. That first link is really interesting - difference between US inch and Enfield inch. If course "everybody" knows the Enfield inch only applies to measures below 2 inches {grinning}