Stub acme thread for barrel / action mounting

[HarryN]

I have been reading about some of the challenges of matching up rifle threads, actions, and head spacing.

Maybe I am wrong, but it seems like at least some of the challenges might be reduced if the threading was based on a stub acme centering type rather than the more common versions. In theory, this also would reduce stress points. Of course, it is different to cut a stub acme vs. more typical threads.

I was just curious if anyone has played with stub acme centering thread as a means of mounting barrels and actions.

Thanks

Harry

 

[Hired Gun]

Setting head space is very simple with the way we are doing it now and I believe cutting the inside of the action for Stub Acme threads would be a lot harder than fitting a barrel to conventional threads. Wouldn't the original threads need completely removed in order to form Stub Acme threads?

What stress points are you talking about?

Does the stub acme threads have the same clamping force at the same torque as the conventional 16 tpi?

 

[HarryN]

Thanks for the reply.

I could be wrong, but I think it would only make sense to make the thread stub acme if this was the plan for both parts (action and barrel) from the beginning, not as a re-thread.

The typical reasons that acme / stub acme type threads are used are:
- The root of the acme thread is wider, so the individual threads are stronger. On a stub acme, this is even more the case.
- The traditional threads tend to have sharp / narrow edges, which can become stress concentrators. If there is plenty of material it probably does not matter, but if the barrel / action area is getting thinner, this might be a factor.
- Acme are often made to be self centering, which can increase assembly accuracy

There is more information in this Wikipedia article:

Trapezoidal thread forms - Wikipedia, the free encyclopedia


You bring up a good question about clamping force. While these usually will not back drive, a unified thread is probably better in this area for clamping.

Interestingly, the Mark V action with the "9 lugs" looks a great deal like a trapezoidal (stub acme) thread with 3 horizontal slots cut into it.

As a practical matter, I wonder if the equipment that most smiths have is capable of producing a self centering stub acme thread on a barrel ? I am tempted to build up a rifle with this setup as a fun test.

 

[shortgrass]

I've not seen a barrel blown off the receiver. I'm sure it's possible, as anything is. 60* V threads are working just fine. I've personally witnessed them holding over 500,000 lbs. repeatedly. If it ain't broke, don't fix it.

 

[sable tireur]

HarryN,

I'd like to hear about the 'challenges' of the standard thread forms you have mentioned but not expanded on. I have an idea about which you may be referring but I'd like to read it in your words.

The typical reasons that acme / stub acme type threads are used are:
- The root of the acme thread is wider, so the individual threads are stronger. On a stub acme, this is even more the case.
- The traditional threads tend to have sharp / narrow edges, which can become stress concentrators. If there is plenty of material it probably does not matter, but if the barrel / action area is getting thinner, this might be a factor.
- Acme are often made to be self centering, which can increase assembly accuracy

Stronger - How much stronger do the threads need to be?

Sharp, narrow edges, stress risers - We normally solve this by turning a threaded diameter slightly smaller by a few thousandths, leaving a flat on the crest. This accomplishes a no stress alignment by preventing contact with the crest on assembly.

Centering/headspace - With properly turned threads and parallel shoulders, centering is satisfied by the threads and headspace is controlled by the shoulders when both lock up together. The concept of 0.002" crush is a function of stretching the threads which can lead to a change in headspace and deformed threads.

You might want to re-read the article in the link you supplied for us to reference. In it you will find this concept:

The line of General Purpose (GP) Acme threads (ASME/ANSI B1.5-1988) are not designed to sustain external radial loads and both the nut and bolt are, ideally, independently supported, the nut by a linear guide and the screw by shaft bearings. This is due to the need to avoid "wedging" of the thread flanks when subjected to radial loads, which would contribute substantially to friction forces and thread wear.

What this indicates is that this thread form is used in situations where the nut (action or receiver) is supported independent of the screw or thread tenon on the barrel. This is because this thread form is used mostly for motion transferal such as your lathe lead screw. If these two parts are not independently supported but used to 'tighten' in a single form, wedging can occur.

Another slight drawback is the number of threads per inch given our current standard diameters used for thread tenons and receiver threads. By diameter the chart lists 5 TPI as being maximum for a diameter of 1" and 1-1/4" where 1-1/16" falls. With thread tenons running from about 3/4" to just over 1.2" in length, this leaves darn few threads to work with.

The obvious solution is to make both the action and the thread tenon or barrel larger in diameter to accommodate more threads per inch. My receivers have gone from 1.4" in diameter to 2.1" in diameter with a cost increase of a couple of hundred dollars per receiver while maintaining the same standard diameters for threads. Additional increase in size will lead to excessive material and machining charges. This is greatly simplified of course but I really don't want to write a dissertation.

Just a thought or two... not criticism.

Regards.

 

[J E Custom]

I've not seen a barrel blown off the receiver. I'm sure it's possible, as anything is. 60* V threads are working just fine. I've personally witnessed them holding over 500,000 lbs. repeatedly. If it ain't broke, don't fix it.

+1

There are lots of reasons to use other types of threads and for some applications Stub or acme threads are preferred.

There are actions that have almost any type of threads and some for a very specific reason.
Example the Acme threads are used on lots of military rifles because they can be mass produced
and even with some error they will still work/be assembled because the only load one surface
so the thread fit is not that much of an issue.

The 60o thread can be fit very precise and has more threads per inch. A good fitting 60o thread will make up hard with only 5 to 10o of rotation where as the Acme threads on military rifles normally
take the recommended 1/4 turn to make up at recommended barrel torque.

I, like others don't understand the problems with 60o threads. If they are done correctly, they are as near as a perfect fit as possible and still have enough clearance for assembly.

As others have said, all different types of threads have there place. I have barreled up rifles and cut these types of threads and one is no harder than the other they just require different tools and set ups.

J E CUSTOM

 

[shortgrass]

I believe the threads in 03 & 03A3 Springfields and 1914/1917 Model rifles is a true square thread. Acme has some relief angles . At the time, they probably choose square for many of the same reasons Mauser choose a buttress for their bolt shroud. The availability of modern alloy steels has negated the use of these 'special' threads for barrel tenons and bolt shrouds. The Acme threads are commonly used for transmitting power/motion, although there are better screw systems available today, like ball screws. I've never worked with any foreign military rifles other than the Mausers, and as is well known, they have a 55* Whitworth thread. like I said,, if it ain't broke don't fix it!

 

[Deleted member 46119]

I have a bench rest target rifle that is a switch barrel. The action has an Acme thread insert Locking cog, head space adjusters, left hand thread locking ring/dust cover.

It has 20 rounds of 6BR on one barrel and 1 round of 17 Remington on the other.

so..

I don't know how it shoots.

For short range BR it better get down into the .2s or less.

Problems, of course. All designs have "problems". Every design is a compromise. Even down to things like thread choices.

This rifle, because of the size of the Acme thread will be limited to BR and smaller, might be able to reach 6.5x47L but definitely not magnums. Not even a 284 based case.

I believe there is no engineering or technical reason not to use an Acme thread.

but...

There is no technical or engineering reason to switch and the cost of change will prevent adoption.

 

[shortgrass]

I have a bench rest target rifle that is a switch barrel. The action has an Acme thread insert Locking cog, head space adjusters, left hand thread locking ring/dust cover.

It has 20 rounds of 6BR on one barrel and 1 round of 17 Remington on the other.

so..

I don't know how it shoots.

For short range BR it better get down into the .2s or less.

Problems, of course. All designs have "problems". Every design is a compromise. Even down to things like thread choices.

This rifle, because of the size of the Acme thread will be limited to BR and smaller, might be able to reach 6.5x47L but definitely not magnums. Not even a 284 based case.

I believe there is no engineering or technical reason not to use an Acme thread.

but...

There is no technical or engineering reason to switch and the cost of change will prevent adoption.

So,,, who's the manufacturer of the action?

 

[Deleted member 46119]

So,,, who's the manufacturer of the action?

The action is an AMT right bolt, left port. The stub insert was made by the gunsmith, retired or passed. I don't remember his name. I think I can find out from my buddy who has the twin to this rifle.

Mine is apart right now, I'm stripping and redoing the stock.

I'll try to get some pics. I'm pretty bad about that lately.

 

[HarryN]

Thanks for all of the very useful replies. I don't claim expertise, I am just opening up an idea to see if it is useful.

Perhaps it would help if I explain a few things more clearly about why I am considering trapezoidal threads, even when the existing thread concepts are working fine.

a) There is a lot of pressure on smaller barrel makers to move to smaller diameter blanks. Unless there is a shift, 1 inch diameter could be as large of blank as the "sporting rifle market" can get ahold of, unless they are large enough, or willing to group together, to buy a batch of steel.

b) My interest is primarily on the .416 and .378 weatherby rounds and trying to build a sort of double or vierling type rifle, but using some of the technology discussed here on long range hunting to substantially improve the experience. The diameter requirement of a belted magnum and pressures keep things interesting. It isn't worse than some other similar setups.

c) Multiple barrels adds some interesting challenges to making an accurate setup. I don't expect to achieve 1000 meter results, but I am trying to break the mold and move well beyond the 50 meter level of most double rifles. In addition to the normal complications, it is ideal to use one load in all barrels, so it is sort of like making 4 rifles that match their load results.


You might ask "why" and the answer is mostly "just for fun". I don't have the mobility anymore to really go on serious hunts and with a rebuilt shoulder, routinely using a .416 wby is unlikely. Maybe with sufficient recoil reduction built in, some use will be possible.

I am pretty much set on these calibers as I really like the belted magnum concept. Maybe I am wrong, but that is where my dead is. It also will make extraction a lot easier for this type of arrangement.

The net effect of this project is that barrel diameter will be pressured to be smaller and the thread depth will be a consideration.

Coming back to the "centering stub acme thread" itself.

I am a big fan of Wikipedia, but some articles have more depth than others. That particular article is a little bit light.

There is a substantial difference in use and results between:
- Standard acme
- Stub acme
- Centering styles vs non centering styles
- General vs precision thread

What they do have in common though:
- Typically considered much easier to machine that V notch threads
- Typically cause less tool wear
- There is fairly wide spread use in other industries, such as machine tools, oil pipe connections, etc.
- For applications where the tube thickness is limited and a thread is needed, a stub acme can provide quite a strong, precise connection

Anyway, that is the project and goals, so it might not apply at all to other rifle projects.

Take care,

Harry

 

[Canadian Bushman]

I work in the oil field and have cut literally hundreds of acme, stub acme, and mod acme threads from pitch 4-26.

The #1 reason acme threads are used, they fuction properly when filled with crap. This is mainly because the tolerance they are spec'd to. Usually S.A and acme are cut sloppy as all hell. This is how they are designed. They can be cut to very specific tolerances for a tight fit but so can every other thread.

The thing about any thread cut on a helix is that it only really touches the mating part on the pitch diameter. Therefore strength is determined by how closely they fit and how thick the thread is. A 16tpi S.A. thread may be stronger than a 16tpi V thread but drop the pitch down to a 12 or 10 and now the V thread is just as strong.

You would acomplish much more by cutting a fine thread with inteference and lapping it in as far as precision and strength go than you would changing design, and since strength is relevant to size and pitch thats how its controlled. Since V threads are easily cut and everyone is already geared up for them, we all just keep on using them.

I personally think a square thread would provide the most surface area with the least amount of work. And since the sufaces are perpindicular they would bode the best for alignment. But when held close and fit properly i really dont think it matters.