Screw Threads

[Dick]

It may be worthwhile also listing and discussing barrel and suppressor threads?

I knew what a few were but I found a lot more on the internet. I can publish them but I can't imagine what we can do with them. An irritating point was that if you google barrel threads you get suppressor threads. You have to google action threads. There are some strange ones - CZ is given as 1.1 x 12.70 which can only mean 1.10 inch dia x 12.70TPI. 12.7TPI ??????? That's exactly 2mm. So the dia is an exact inch dia but the pitch metric. Can't be.

[A-R]

I knew what a few were but I found a lot more on the internet. I can publish them but I can't imagine what we can do with them. An irritating point was that if you google barrel threads you get suppressor threads. You have to google action threads. There are some strange ones - CZ is given as 1.1 x 12.70 which can only mean 1.10 inch dia x 12.70TPI. 12.7TPI ??????? That's exactly 2mm. So the dia is an exact inch dia but the pitch metric. Can't be.

I have cut one. Unfortunately I did not think it useful to write it down at the time, but 2mm sounds just right.

Having a database of action and barrel threads may be of use to anyone wanting to plan a project, for example a barrel or action swop. For example, I have a rifle with Lyttleton RSA action, fitted with a barrel from a Musgrave D98 (deluxe version of M98 actioned rifle). I guess the 2 actions have the same threads, but would like to know if my guess is right.
Another example is a project rifle built on a P14 action. The P14 has an ACME thread, so nothing else will fit without machining, or will it?

Suppressor threads are used much more frequently and being able to find the specs may make things easier for those who want to buy a can or have a can and want to know if it will fit on another rifle, etc.

[Dick]

I have cut one. Unfortunately I did not think it useful to write it down at the time, but 2mm sounds just right.

Having a database of action and barrel threads may be of use to anyone wanting to plan a project, for example a barrel or action swop. For example, I have a rifle with Lyttleton RSA action, fitted with a barrel from a Musgrave D98 (deluxe version of M98 actioned rifle). I guess the 2 actions have the same threads, but would like to know if my guess is right.
Another example is a project rifle built on a P14 action. The P14 has an ACME thread, so nothing else will fit without machining, or will it?

Suppressor threads are used much more frequently and being able to find the specs may make things easier for those who want to buy a can or have a can and want to know if it will fit on another rifle, etc.

To start with I'd love to know how you get your DIY rebarelling projects past CFR. Don't need a database because a google search finds most of them. I have no idea what the RSA thread is. Its quite possible that it's the same as the 98 but you can't assume it and the 98 barrel thread might have been re-cut and a chamber reamer run into the resulting short chamber. I'll be very surprised if Musgrave 98s are not standard because there'd be no logical reason to alter them, besides which you'd have to machine out the original thread which would leave the receiver ring wall too thin.

P14 Thread isn't acme it's 1.125 inch dia x 10 TPI square thread. That's what the internet sources say but I have the advantage of having taken one off. The lands and grooves are exactly 0.05 wide and deep. P14 and P17 barrels tend to be tightly fitted and it's quite possible that the square thread is a cause of that. Square threads aren't difficult to cut but there's an important difference in how the barrel fits the receiver. V Threads are self centering for obvious reasons. Square threads are not, and close concentricity is achieved by cutting the barrel threads the same as the receiver. Or preferably very slightly smaller to avoid it refusing to enter. I don't see the need to have both major and minor exactly fitting the action - only one is necessary and the other diameter can be three or four thousandths undersize. I'd also say that the groove in the barrel thread should be both slightly wider and deeper than the nominal 0.05 inch. Male and female screws tighten up against only one shoulder so the land of the barrel thread needs to be slightly smaller than the groove of the action thread for easy fitting without sacrifice of accuracy. If I need to thread a barrel for a P14 I'd cut a dummy first to test what the exact dimensions of the thread should be.

[Dick]

I said that I'd ask my instrument maker neighbour how he cuts small threads. For him 3mm is big. His answer was disappointing and reminds me how professional instrument makers, toolmakers and gunsmiths are generally not helpful. When I referred to single point lathe cutting he looked at me is if I was joking and asked why on earth I'd do that when it's quicker with a die. His argument was that its cheaper to buy a die than spend the time lathe cutting. Naturally I pointed out that that might be true for him in the workplace but its not true for the amateur, for whom one of the benefits of a Myford lathe is its ability to cut any thread to any diameter. But it was apparent that he hasn't lathe cut a thread since he was an apprentice.

He said something else that was disappointing and surprising. His response to the technique of drilling a tapped hole slightly bigger to achieve less than 100% engagement was amazement at the very idea. He told me that he drills 5mm for an M6 tap. There's no problem with it he said. I can assure you guys that it takes MUCH more torque than a 5.3mm hole for zero benefit in terms of good fit or anything else. But it was worse - he said that an oversize hole will result in a loose male-female fit. No it won't because the ODs are the same and the flanks will still engage closely. A few years ago I machined an M16 thread about 200mm long for a machine rest. It is used with two hardware store M16 nuts. I cut the thread so that the nuts were a free running but close fit without shake. There was no practical benefit - I did it for the hell of it. But just to test the partial engagement proposition I bought a third nut and bored it out to more or less 50% engagement. It fits as perfectly as the others. The partial engagement principle is of crucial importance to amateurs. I'll continue to go with Tubal Cain after having found what's in his book to be 100% correct.

Finally, this shows the value of exchange of information. I had to learn a lot of things from first principles, and this episode is by no means the first time I've found professionals to be of no use. Another example is the bluing info published by Frank Brownell. Pages of polishing, all by machine. Guys, trust me, machine polishing takes a lot of practice to avoid rounded corners and dished screw holes. Hand polishing is slow but it's all but impossible to make a serious mistake simply because it takes a lot of polishing to take off a small amount of metal. This is all simple stuff tested and proven in practice.

[Sean KZN]

Finally, this shows the value of exchange of information.

Indeed it does. Mistakes are a learning curve, though it does make the learning easier if you have a foundation to build on.

A friend of mine did 2 trades, the first was a tool and die maker. He has a lot of gear in his garage, 3 lathes, milling machine, band saws and drill presses although machining is not his income earner, trade 2 does that although trade 1 does compliment trade 2.

I'm more of a chainsaw artist than machinist, my lathe is a 1940's vintage South Bend less some gears but functional for me.

On the other hand, my friend has a pretty talented ability, he's single point cut an 1 1/2'' X 8 internal thread for me on my 2 chuck mounting plates, material was bright mild which is hardly kind to work with.

[A-R]

To start with I'd love to know how you get your DIY rebarelling projects past CFR...

It's you who made the assumption here...

No, I did build a rifle or 2 way back before the FCA. All that is missing is the SABS proof mark, which is apparently not a thing any more.

Now I hold a few competencies besides the normal 4, but I am not actively involved in the trade at present.

[Dick]

I thought again about that CZ 1.10 inch x 12.70TPI barrel thread. As we said 12.70TPI is exactly 2.00mm. 1.10 inch is VERY slightly less than 28mm, and as it is not unusual for threads to be slightly smaller than nominal OD and having regard to 2mm being expressed as 12.70TPI, I reckon that 1.10 inch is actually 28. Thus 28 x 2mm.

These are some barrel/receiver threads I found. I know some to be correct but not sure about all of them, especially considering the very annoying American habit of expressing metric as inches.

Remington 700 1.0625" (1 & 1/16) x 16TPI
Winchester 70 1.00 x 16TPI
Ruger 77 1.00 x 16
CZ550 26 x 2mm
CZ Magnum 28 x 2mm
1903 Springfield 1.04" x 10TPI Square
P14 1.125 x 10TPI Square
Sako Unknown
Tikka T3 1" x 16TPI available info insists that it is an inch thread which makes no sense for a European rifle
Zastava Mini Mauser 22 x 1.5mm
Weatherby Mk V 1.0625" (1 & 1/16) x 16TPI
Ruger N0 1 Unknown
SMLE 1" x 14TPI
Mauser 98 1.10" x 12TPI Whitworth

I wish I could say that it is safe to assume that rifles originating in the US will have the typical US 60 degree flank angle, but the modern UNC and UNF threads were standardised after WW2 so it is not impossible that an early rifle like the 03 Springfield might have Whitworth threads. The US habit of never mentioning the difference between US and British inch threads makes it impossible to know. I can only say that I wouldn't fit a barrel to any rifle without measuring the receiver thread.

We can't even assume that British rifles are Whitworth because I've seen info that the SMLE has a different flank angle than the Whitworth 55 degree. Furthermore, that the No1 and No 4 are different. I can't confirm either of those things. The Mauser 98 is of course pure Whitworth as I have mentioned before.

[Dick]

In my previous post I said that the 03 Springfield might have a Whitworth thread. No, of course not - that was a general comment made without thinkin - the 03 Springfield has a square thread like the P14.

But what I want to mention is after-market barrels which is a big business in the US. While I can't be sure, I have the impression that they are 60 deg flank angle even if intended for the 55 deg M98. That's terribly sloppy, especially when its such a big business that cutting them 55 deg shouldn't be an economic problem. I also recall Brownells selling a tap to recut the M98 to 60 deg. To start with, why do that when its so easy to cut the barrel tenon correctly in the first place. But in addition, 98 receivers are case hardened only a few thousandths deep. That's why removing metal like lapping locking lugs must be done carefully, and recutting the receiver thread will cut through the case hardening. Maybe I'm too picky but I believe in zero error when it comes to firearms.

[Dick]

In an earlier post I said that my instrument maker neighbour is scornful of single point lathe screw cutting. I have a little Unimat 3 miniature lathe, which has 14 x 1 threads on the spindle nose and tailstock ram, and a 10 x 1 lead screw, neither of which are standard or even metric fine. 14 x 1 at the back end of the spindle too, where the drive belt pulley fits. It is useful to be able to make items like different faceplates, sacrificial faceplates and different pulleys - in fact I'll need to make two or three pulleys to work with a new motor that runs at different speeds from the old one. Therefore I have a 14 x 1 tap which I bought from FEW many years ago. A tap is a lot easier than internal threading on the lathe, and fortunately most of us can limit tapping to three or four diameters so the cost of a few taps isn't high. External threads are another thing. I said earlier that single point lathe threading is done a lot less often than tapping, but the few I've done have been several different diameters, which would have meant several dies most of which would have been used only once. That's the advantage of the screw cutting lathe, especially for amateurs who can't waste money.

[Dick]

There's always something new to learn, isn't there? I just got roped in to assist with tapping a lot of holes. The first batch will be 300 workpieces. Each will need eight holes to be tapped M3 (the holes will already be there) one hole to be drilled and tapped M3 and four 6mm holes to be drilled but not tapped. 300 x 9 = 2700 tapped threads. If the person who ordered the work successfully sells the items of which these pieces are a component, it could continue indefinitely. We surely can't tap 2700 holes by hand, so we bought a tapping head for use in a drill press. If you google "tapping head" there are several videos, the best being the demonstration of a Tapmatic tapping head in which the demonstrator says that a test run worked out at five seconds per hole.

While speed is the purpose of tapping heads, they are also designed to function in a way that minimises tap breakage. Yesterday I ran my first attempt and successfully tapped a dozen holes in a couple of minutes. That was slow because it was my first trial, but it tended to confirm that five seconds is realistic. It's not difficult but one needs to develop the right feel for the tap being used, so although my first threads are OK I'll need to tap 50 or 100 as a learning curve before attempting the real work. Its very interesting.