Importance of truing threads?

[Cloudrepair]

What would cause the first few threads to tighten any sooner than the trailing few? The only place I have seen this 4-5 thread nonsense is when studs are tightened up against the end of the threads. Studs should be somewhat floating, like the barrel does with a draw tube setup, or a Savage with a nut.

The first few or last few
The shoulder or the nut.
The thread material distorting when torqued against something

 

[mram10]

Tra, I agree. I don’t see how the torque changes to the first few threads. They are steel 16tpi threads that require a good amount of torque to displace them.

 

[mram10]

If you think about the threads are loose enough to screw on by hand then they hit the shoulder you tourqe it down a bit and it makes sense that the first few threads will start loading up more with the more torque used and the trailing threads will releave slitely or the other way around
Maybe taking a sako with a 1.062-1.5mm pitch and a barrel with 1.062-16 inch thread will balance it all out with some anti sieze.

Cloud, I don’t see how that makes sense. The shoulder acts on the receiver face via torque, but how does that bend the threads? If the threads are true, they will all have equal pressure on the faces....

 

[Cloudrepair]

The first few or last few
The shoulder or the nut.
The thread material distorting when thorqed against something

I don't have the words to express it.
more trying to get someone that does to share.

 

[TRA]

I don't have the words to express it.
more trying to get someone that does to share.

If the threads don't mate up consistently, it's due to the internal and external threads being cut on different equipment. I'm surprised that so many think any old lathe, no matter who made it, or some flimsy chi-com hobby lathe, and bench grinder sharpened tooling will make proper fitting threads. Might just as well use a pipe threader.

We hear so much about the topic being discussed here and about how precise things are indicated to, then posts go on and on that floating a reamer, held with a crescent wrench and cutting out of spec threads doesn't matter. Either it does or it does not.

That couple of thread contact issue is common with taper threaded fittings. Helps keep the Teflon tape business prosperous. At least the Europeans were smart enough to go with straight threads.

 

[alinwa]

I think it should be said...... Several action makers cut a taper into their receiver threads ON PURPOSE.

(I felt that some posts here hinted that "tapered threads" was a defect)

Tapering receiver threads is done to promote more even load-bearing distribution, ie, more threads actually engaged than the typical 3-5 that carry all the load in a parallel threaded joint. With the first thread taking the brunt of the load.

It works.

Kelbly's receivers also have a short threaded engagement, just over a half inch VS some I've seen with an inch and a quarter......

They've won some.

 

[Cloudrepair]

If the threads don't mate up consistently, it's due to the internal and external threads being cut on different equipment. I'm surprised that so many think any old lathe, no matter who made it, or some flimsy chi-com hobby lathe, and bench grinder sharpened tooling will make proper fitting threads. Might just as well use a pipe threader.

We hear so much about the topic being discussed here and about how precise things are indicated to, then posts go on and on that floating a reamer, held with a crescent wrench and cutting out of spec threads doesn't matter. Either it does or it does not.

That couple of thread contact issue is common with taper threaded fittings. Helps keep the Teflon tape business prosperous. At least the Europeans were smart enough to go with straight threads.

Tra i think you could lighten up a bit
I have run many large lathes for many years and I can tell you I do cut correct and accurate threads with my small Taiwan lathe.
I think I will let it go
Like they say it's the Indian not the arrow

 

[TRA]

Tra i think you could lighten up a bit
I have run many large lathes for many years and I can tell you I do cut correct and accurate threads with my small Taiwan lathe.
I think I will let it go

Lighten up? I am being lite. Everyone wants an answer, yet no one wants to hear it. I've run good equipment and bad equipment. Trust me there is a difference no matter what your gauges say. How do you know, if they only mate up with only 4-5 threads? To think that's normal is illogical.

FWIW, I like Taiwan equipment. I own a few made in Taiwan. There's a big difference between high end Taiwan, and low budget chi-com, aka PRC, knockoffs.

When you think the Chinese are not capable of manufacture good quality, you should watch the YouTube video of BMW's engine plant. Pay close attention to the technology and the overall conditions of the plant, workers, and look up a video of an American engine plant.

Here, I'll save you the trouble of searching for it.

The cheap PRC machines and tooling are cheap because the American consumer demands cheap and accepts it's short comings to get it. If you were to get high end machinery out of China you would pay as much or more for it than the other quality machinery available elsewhere.

That's getting off topic, but the fact remains, it does not matter whether it's a Mazak or a China special, they both can cut crappy threads if they have issues.

 

[TRA]

If you thread two parts together and only a few threads mate up, then they yield and are deformed. That sure would make disassembling and reassembling that connection, a mess. Wouldn't be long and you'd have a barrel stuck into an action, as after a few times of deforming the threads, you'd end up with one component trying to straighten the deformed threads. If your threads are that far out where 4-5 have to deform in order to tighten up, you better be rethinking your threading operations.

 

[Tucker65]

I'm fairly new to gunsmithing but have been doing machine and fabrication work for awhile. There has been much talk about the threads centering up the barrel and action connection with some agreeing it does and some saying it doesn't. Also with the talk of only the first 4-5 threads doing all the work why isn't the barrel to receiver connection something more like a shoulder bolt. Wouldn't that solve both issues, if they are even an issue at all?

John

 

[Cloudrepair]

I don't think there any permanent deformation in the threaded joint do to the size of threads and amount of torque it would take.
Now if your a six hundred pound gorrila and you gave six hundred pounds of torque then the barrel threads would be the ones to fail first.
All the more reason to go with a custom barrel. And the proper amount of torque.

 

[TRA]

A google search will explain to you how and why the first few threads take most of the load guys.

What's happening here is most are confusing what carries all the tensile load and you're right it's the area under the first few threads. That is not to mean that only the first few threads are engaged, it's only that the bolt will fail in that area long before the threads fail totally. Bolts will generally fail in the area right at the end of the last 2 threads as that is the weakest area on the bolt. Diesel engines break head bolts at a common rate and it's got nothing to do with the lack of thread engagement. If it were due to the threads yielding and deforming, you would have to re tap the block every time you replace a head bolt. 99% of the time you can remove the broken off threaded section from the block with your fingers or a long screwdriver without removing the head. This is a case where a common occurrence is being used as an excuse for poor thread fit. Old John Deere 2 cylinders had studs that needed serious force to remove and reinstall their studs, as they were used to seal out water and oil, and were purposely made with an interference fit.

Notice these threads are all in engagement. The bolt if it breaks, it will in the area of the first 2 threads. Does not mean 25% thread engagement is acceptable. This is why I never thread without a proper thread relief.

 

[Roeder]

Nope, not intended to mean that at all. But when one action has taper and the next of the same model doesnt that is a defect. Plus I like to be the one cutting the taper

Just to make things clear: Jerry Stiller used a slight taper in his early actions intentionally. It worked very well if the threads were cut correctly, but he had so many complaints from gunsmiths that didn't understand the concept that he discontinued the practice. If a later Stiller Viper is fitted with a minimum clearance thread fit to the tenon, that barrel will not screw all the way on to an early Viper. A lot of matches have been won with early and late Stiller actions, so it didn't seem to make a significant difference in the grand scheme of things, just caused some headaches.

 

[carlsbad]

Can someone provide a written reference, on a manufacturer's website, stating that they use tapered threads?

I've found very slight tapers in many actions and measuring pitch diameter on barrels coming out of custom builds I've found minor taper. None of what I've seen would be considered "on purpose".

However, slightly tighter threads deeper in the action, which can be the natural result of tool dynamics, will help to spread the load away from the first few threads.

Now having the first few threads take all the load doesn't bother me a bit. 35 years working in a power plant, with hundreds of thousands of threaded joints, and I've never seen that fact to cause a problem. One rule of thumb I've seen over the years is that a length of threads .6 x D will provide a full strength joint.

--Jerry

 

[TRA]

Can someone provide a written reference, on a manufacturer's website, stating that they use tapered threads?

I've found very slight tapers in many actions and measuring pitch diameter on barrels coming out of custom builds I've found minor taper. None of what I've seen would be considered "on purpose".

However, slightly tighter threads deeper in the action, which can be the natural result of tool dynamics, will help to spread the load away from the first few threads.

Now having the first few threads take all the load doesn't bother me a bit. 35 years working in a power plant, with hundreds of thousands of threaded joints, and I've never seen that fact to cause a problem. One rule of thumb I've seen over the years is that a length of threads .6 x D will provide a full strength joint.

--Jerry

But just to point out, that .6 X D only pertains to tensile strength, with regards to threaded fasteners. Threads in many instances serve functions other than just tensile strength. Such is the case with firearms like those being discussed here. Someone could make the argument that tensile strength of the threads alone is sufficient to cancel out the other forces present, but I don't buy it.

FWIW, all these references to thread engagement or lack of and how many threads are necessary are straight out of the fastener textbook. Nuts and bolts are hardly a good comparison when the discussion is about high precision firearms.

 

[carlsbad]

Most actions are designed with about 1 x D threads on tenon. But I have seen less. If you put a relief groove behind the threads on an action with a shoulder to center the recoil lug, you'll end up with less then 1 D of threads--but well over .6D.

I would never shorten the tenon. As you point out, it performs a lot of other functions.

The rule of thumb about .6D was just meant to make people feel better about the first few threads taking the load.

-Jerry

 

[TRA]

Lets also keep this in perspective, A threaded fastener of the most common size rifle tenon would be under up to 1000 pf of torque, depending on bolt classification. There is no rifle barrel torqued to anywhere near thread deforming or tenon stretch, as seen in threaded fasteners. The only way to lose all but 3-5 threads being in contact is either the pitch is wrong or the bolt, or tenon in this case, stretched. I don't see the latter being the case.

 

[carlsbad]

TRA,
In order to keep a threaded joint tight (without a lockwasher or similar) you really need to stretch the fastener a little. In this case, it would be the tenon that stretches. I'll calculate the stretching stress . For the stretching force, the formula T=nFD

T= torque in Foot lbs
n=friction factor. For good lubricated threads use .15
F= stretching force in lbs
D= Diameter in feet . For a R 700 use 1.060/12

I calculate the force is about 7500 lbs for 100 ft-lbs torque.

Then the stress from torquing is the F calculated above divided by the stressed area--An annulus between the threads and the cartridge.

Area at root of threads is .787 in sq. (from internet calculator)

Area of 308 chamber is .176 So stress area is .611 in sq.

7500/.611= 12.3KSI.

This is enough stress to stretch steel and well below yield strength.

Jerry

 

[TRA]

TRA,
In order to keep a threaded joint tight (without a lockwasher or similar) you really need to stretch the fastener a little. In this case, it would be the tenon that stretches. I'll calculate the stretching stress . For the stretching force, the formula T=nFD

T= torque in Foot lbs
n=friction factor. For good lubricated threads use .15
F= stretching force in lbs
D= Diameter in feet . For a R 700 use 1.060/12

I calculate the force is about 7500 lbs for 100 ft-lbs torque.

Then the stress from torquing is the F calculated above divided by the stressed area--An annulus between the threads and the cartridge.

Area at root of threads is .787 in sq. (from internet calculator)

Area of 308 chamber is .176 So stress area is .611 in sq.

7500/.611= 12.3KSI.

This is enough stress to stretch steel and well below yield strength.

Jerry

So how much did the tenon stretch and where did it yield? Did the first threads hold the load and the tenon stretched in the middle or the area next to the shoulder? How much total. 100 pf of torque can be had with precision fitted pieces by not much more force than slamming the well lubed action against the shoulder. I f the threads are yielding or the tenon is stretching at the forces involved, 650-950 pf would cause serious damage to that action and barrel. Lugs on my GMC are torqued at ~145 PF. Takes no effort to achieve torque. Taking them off is the fun part. When tightened the lug nuts run up straight to 145pf without any mushy feeling while reaching recommended torque.

I'd entertain the test where a barrel tenon stub were installed into an action, or a fixture with the action internal threads. Drill all but the last .100 thick at the bolt face for a chamber. Torque the stub at 50 and measure with a depth gauge, torque 100 pf and measure again. Do this over and over until you get up to 6-800 pf and measure the tenon with the depth gauge. At least we would have some hard numbers verifying the thread contact or tenon is actually stretching. I really doubt a ~100 pf archived with just a hard twist and a touch up with a strength would cause any distortion. Anything we'd do would be much wiser than screwing a barrel in hand tight and tighten up a couple of setscrews.

 

[carlsbad]

TRA,
it strained, it didn't yield. Strain is the change in length as the result of a stress below yield strength. Young's modulus (Y) is the ratio of the stress over the strain. For steel Y is about 30E6. DeltaL = SL/E (S=stress) So we have to determine how long a piece of the tenon is being stretch. Easier for a bolt than for the tenon. Lets just say 1/2" is being stretched.

deltaL= 1.2E4 x .5 / 30e6 = .0002"

When you loosen it, everything goes back to where it was. Yield occurs when you get stresses up to yield strength. Assuming the barrel is made of good steel with a yield strength of 100,000 psi or so, we calculated above that the stress is about 1/8 of yield. if I were designing a bolted fixture just to be a bolted fixture, I'd probably use a slightly higher stress to ensure that the fixture never relaxed from cyclic stress. But we have a lot of experimental data to tell us this design works.

The professor is headed to the range.

--Jerry