Engineering question:
- Thread starter Pete Wass
- Start date
In a commercial production environment thread tolerances are not nearly as tight as a well fit barrel threading into a receiver.
High strength threaded fasteners (for holding things that could turn into real hazards or disasters) are a very different world.
There are numerous safety rated fasteners on any vehicle that are allowed a SINGLE USE only.
They should be replaced after being torqued to a stated value once and then removed for any reason.
There are plenty of other fasteners that should be replaced but it is not really required to replace them.
Rifle barrel threads directly affect headspace.
Barrels for Panda and other actions that may be easily changed out have square shoulders to provide a solid stop to barrel position.
Other actions can (and do) relly on tapered areas to provide additional locking of threads.
Some military rifles use threads that are 'short' in various ways (female threads not as long as the male part, or threads that are larger
in diameter than the female side) to create an interference fit and thoroughly lock the parts together.
Very large torques are used to deform the threads upon mating.
They are designed to be permanent.
Final headspace may even be set after mating barrel to action with a finish cut chamber reamer.
It takes very precise machining to easily interchange barrels.
Even then a safety check is then required.
One last inspection step to make sure everything is correct.
Think of AR-15 (and M-16) barrels.
Even the M-2 machine gun.
When swapped correctly they SHOULD be correct.
But a check is still a good idea.
Torques are set to ensure correct thread engagement and bolt stretch on things like engine head bolts.
While you might get away with re-using head bolts it is not the best idea.
They are NOT going to produce the same loading the second time around.
The bolt steel has already been stretched once.
Applies to studs also.
'Prevailing torque' fasteners have had the female side made slightly elliptical to lock them tightly.
There is often also a tightening torque specified (with lubricated threads) for fastener stretch.
See "Prevailing torque fastener US 5302067 A" for the details.
The fasteners can be reused in some applications but far from all.
Sometimes the criteria is the torque required to install the nut.
If it is high enough the assembly is acceptable.
Not high enough and both parts must be replaced.
On aircraft the prevailing method is one time use.
With the correct choice of rifling hand and barrel tenon thread hand the action of firing tightens the barrel anyway.
A very good choice.
If is use a torque wrench to tighten the barrel on my Panda varmint rifle the shift in POI among the calibers is very repeatable with my preferred loads in each caliber.
I have been using 80 ft-lb and had excellent repeatability.
So much I no longer bother checking headspace.
Unscrew one caliber, screw on another, torque, adjust scope, return to shooting.
The only thing I added was a faint witness line on each barrel and the action as a quick check.
High strength threaded fasteners (for holding things that could turn into real hazards or disasters) are a very different world.
There are numerous safety rated fasteners on any vehicle that are allowed a SINGLE USE only.
They should be replaced after being torqued to a stated value once and then removed for any reason.
There are plenty of other fasteners that should be replaced but it is not really required to replace them.
Rifle barrel threads directly affect headspace.
Barrels for Panda and other actions that may be easily changed out have square shoulders to provide a solid stop to barrel position.
Other actions can (and do) relly on tapered areas to provide additional locking of threads.
Some military rifles use threads that are 'short' in various ways (female threads not as long as the male part, or threads that are larger
in diameter than the female side) to create an interference fit and thoroughly lock the parts together.
Very large torques are used to deform the threads upon mating.
They are designed to be permanent.
Final headspace may even be set after mating barrel to action with a finish cut chamber reamer.
It takes very precise machining to easily interchange barrels.
Even then a safety check is then required.
One last inspection step to make sure everything is correct.
Think of AR-15 (and M-16) barrels.
Even the M-2 machine gun.
When swapped correctly they SHOULD be correct.
But a check is still a good idea.
Torques are set to ensure correct thread engagement and bolt stretch on things like engine head bolts.
While you might get away with re-using head bolts it is not the best idea.
They are NOT going to produce the same loading the second time around.
The bolt steel has already been stretched once.
Applies to studs also.
'Prevailing torque' fasteners have had the female side made slightly elliptical to lock them tightly.
There is often also a tightening torque specified (with lubricated threads) for fastener stretch.
See "Prevailing torque fastener US 5302067 A" for the details.
The fasteners can be reused in some applications but far from all.
Sometimes the criteria is the torque required to install the nut.
If it is high enough the assembly is acceptable.
Not high enough and both parts must be replaced.
On aircraft the prevailing method is one time use.
With the correct choice of rifling hand and barrel tenon thread hand the action of firing tightens the barrel anyway.
A very good choice.
If is use a torque wrench to tighten the barrel on my Panda varmint rifle the shift in POI among the calibers is very repeatable with my preferred loads in each caliber.
I have been using 80 ft-lb and had excellent repeatability.
So much I no longer bother checking headspace.
Unscrew one caliber, screw on another, torque, adjust scope, return to shooting.
The only thing I added was a faint witness line on each barrel and the action as a quick check.
OK so heres a question.
Could barrel to action torque have an effect on barrel harmonics and ultimately accuracy?
I dont suppose any of use would expect much from a hand tightened barrel on a match rifle so could there be an optimum torque?
Or more likely could there be a minimum that we need to exceed and after that it makes no difference?
I guess like many things regarding rifles and accuracy it is hard to test for and so far nothing has been proven one way or another. I headspace my chambers to hand tight plus around 0.125" rotation, it seems to work fine so Ive never really looked any deeper.
Could barrel to action torque have an effect on barrel harmonics and ultimately accuracy?
I dont suppose any of use would expect much from a hand tightened barrel on a match rifle so could there be an optimum torque?
Or more likely could there be a minimum that we need to exceed and after that it makes no difference?
I guess like many things regarding rifles and accuracy it is hard to test for and so far nothing has been proven one way or another. I headspace my chambers to hand tight plus around 0.125" rotation, it seems to work fine so Ive never really looked any deeper.
Last edited:
JerrySharrett
Senile Member
As an engineer, a Senior Manufacturing Engineer of 45 years, a guy who designs the manufacturing process, and IMO, there probably isn't a magic torque value when installing a benchrest barrel to a benchrest quality action.
Consider-the mating shoulders of the barrel and the action need to be true surfaces and exactly perpendicular to their rotational axis. Their mating threads need to be of the exact same pitch and profile. i.e. most all rifle actions today have 60 degree Vee threads. These need to be of exactly 60 degrees and their pitch (TPI-threads per inch) need to match exactly. (On some CNC one could, for example, program a 18.05 TPI instead of an 18 TPI.
If these mating surfaces, shoulders and thread profile and pitch, match exactly and are lubed before installation, probably as little as 25 LB/FT would suffice. As to too much torque, I doubt if, in hand tightening, there could be too much. Note too, these mating surfaces need to be properly lubed before the installation is undertaken. Greg Tannel of Gretan Gunsmithing keeps a lathe set up to "uninstall' barrels that were not lubed and those mating surfaces galled and stuck.
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Consider-the mating shoulders of the barrel and the action need to be true surfaces and exactly perpendicular to their rotational axis. Their mating threads need to be of the exact same pitch and profile. i.e. most all rifle actions today have 60 degree Vee threads. These need to be of exactly 60 degrees and their pitch (TPI-threads per inch) need to match exactly. (On some CNC one could, for example, program a 18.05 TPI instead of an 18 TPI.
If these mating surfaces, shoulders and thread profile and pitch, match exactly and are lubed before installation, probably as little as 25 LB/FT would suffice. As to too much torque, I doubt if, in hand tightening, there could be too much. Note too, these mating surfaces need to be properly lubed before the installation is undertaken. Greg Tannel of Gretan Gunsmithing keeps a lathe set up to "uninstall' barrels that were not lubed and those mating surfaces galled and stuck.
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All of this.
'Fit' is used to describe how threads mate.
It is a whole set of code letters and numbers for machinists to work to.
It is more applicable in a mass production environment.
Never noticed detailed call out on barrel shank thread drawing and manufacturers are notably reluctant to reveal their manufacturing drawings.
A lower value may be adequate but something a little higher to ensure nothing shifts appreciably on firing is likely worthwhile.
The setup is designed to tighten the barrel upon firing (bullet rams into rifling and turns barrel tighter).
After a decent number of shots I have measured torque to break loose and found numbers only slightly greater than I installed at on my Panda action.
The surfaces are likely VERY square.
As long as the threads are tight the effect on vibration will be minimal.
No relative movement is no movement.
I made a pass at computing the torque once years ago.
Not enough data was available on bullet engraving force to get very accurate numbers.
I tried to measure the force but could never obtain consistent numbers doing it slowly with hydraulics.
Excessive skip-stick behavior.
Pretty simple isn't it Wilbur, just a slight bump.....done. Same place, time after time.
M
mks
Guest
Yes, I think you are correct. An engineer would calculate the stretch in the barrel and in the receiver when the maximum chamber pressure is applied to the chamber and the bolt face. VarmintAl may have an example on his website. One could include the case, as well, but neglecting it provides a worst case. If the stretch results in a gap in the joint (or just too little compression), then it might settle to a different position after each shot. With enough torque, the joint remains stationary. This perceived problem caused a discussion of tapered barrel/receiver joints a while back. Not a bad idea, really.
Cheers,
Keith
Boyd Allen
Active member
If I remember correctly, Vaughn also came up with an improved barrel to action joint design. It has yet to be commercially produced. I will have to dig out my copy of his book and review the matter. He also said that the barrel joint issues that he discussed were not likely to occur in a short range benchrest rifle.
mwezell
Mike Ezell
On topic and out of curiosity, what is the science and/or formula for why the bore will get larger instead of smaller, when over tightened? I know that I run into this occasionally while making my wind flags. The soft plastic that is the pivot material that the pinwheel spins, on its axle..if over tightened, gets tighter on the axle and the hole must be reamed. As I read here, that should not be the case, but just the opposite. There may well be more at play than first meets the eye here, but at first glance it appears very much like the barrel scenario being discussed. Bottom line, with the threaded plastic and a nylon nut, the ID gets smaller and too tight on the pin. Why is this different?
That said, on my few RF builds, I use 30-35ft/lbs on most of them. On CF, I use approximately 80-90ft/lbs. If the joint is tight enough to prevent unloading under fire, I assume it to be good. As to how unloading would affect harmonics, I know of no such test but am sure that it must have an effect. Of course, the barrel will tighten with firing.
As for the torque to yield bolts that have been mentioned, I don't see that being an apples to apples analogy. They are designed to do exactly as their name implies, which is to be torqued to a value that is BEYOND their state of elasticity and to intentionally deform at such loads. I don't think we are remotely approaching that point with rifle barrels with 1"+ barrel tenons. That is not at all saying that the bore can't be changed at our respective torque values, but that we are nowhere near reaching the point where the barrel is permanently changed, due to to fact that to do so, it would have to be tightened beyond it's YIELD point.
That said, on my few RF builds, I use 30-35ft/lbs on most of them. On CF, I use approximately 80-90ft/lbs. If the joint is tight enough to prevent unloading under fire, I assume it to be good. As to how unloading would affect harmonics, I know of no such test but am sure that it must have an effect. Of course, the barrel will tighten with firing.
As for the torque to yield bolts that have been mentioned, I don't see that being an apples to apples analogy. They are designed to do exactly as their name implies, which is to be torqued to a value that is BEYOND their state of elasticity and to intentionally deform at such loads. I don't think we are remotely approaching that point with rifle barrels with 1"+ barrel tenons. That is not at all saying that the bore can't be changed at our respective torque values, but that we are nowhere near reaching the point where the barrel is permanently changed, due to to fact that to do so, it would have to be tightened beyond it's YIELD point.
M
mks
Guest
Mike,
Under what conditions have you seen the bore get larger? Most materials, including steel and plastics, have a positive Poisson's ratio, which means that when it is stretched in one direction, it gets smaller in perpendicular directions. https://en.wikipedia.org/wiki/Poisson's_ratio
The V thread of your wind flag stem also applies some inward pressure locally, which makes the pin hole smaller in the vicinity of the nut.
Best,
Keith
mwezell
Mike Ezell
Thanks Keith! The reason I said what I did is that seems to be the consensus on some of these forums as to what happens to the bore when over tightened...that the bore gets larger. My example using plastic is just the opposite, which is what I'd expect to see, be it steel or plastic.
I appreciate your response and I knew you'd know the answer. Hope to see you soon.
never heard of the bore being affected by the fit of the tenon into the action, but the chamber area yes. A slight growth in bore diameter due to machining of the outside of the barrel yes- primarly related to button rifled barrels because of the induced stresses caused by that type of rifling and that any contouring and such has to be done after the rifling is completed releasing some of those stresses. Unlike a cut rifled barrel which is/can be externally modified prior to rifling and does not suffer anywhere near the stress factor in the rifling process as compared to button rifling.
Pete Wass
Well-known member
It would seem to me
that if one stretches a piece of anything it gets smaller in diamater, not bigger and fatter. I think the same would apply to a hole through whatever the materiel is. If one tightens a bolt, it grows longer and smaller in diameter. It likely follows that a hole through it would become smaller as well, in the area of the stretch, i.e. a chamber would get smaller if it were stretched by torquing the threads around it.
BC use to claim he could feel the stamped writing on a barrel by slugging the bore. Some sense of feel, eh!!! I struggle to find the tight spot in a barrel, never mind the impression of lightly stamped lettering.
Pete
that if one stretches a piece of anything it gets smaller in diamater, not bigger and fatter. I think the same would apply to a hole through whatever the materiel is. If one tightens a bolt, it grows longer and smaller in diameter. It likely follows that a hole through it would become smaller as well, in the area of the stretch, i.e. a chamber would get smaller if it were stretched by torquing the threads around it.
BC use to claim he could feel the stamped writing on a barrel by slugging the bore. Some sense of feel, eh!!! I struggle to find the tight spot in a barrel, never mind the impression of lightly stamped lettering.
Pete
The 'general rule' for most (if not all) materials is that holes in a uniform material move the same way as a solid piece of the missing material would under similar loading.
Things can get seriously weird in materials that have large variation in material properties with direction.
Welcome to the incredibly nasty world of 'tensor analysis.'
Strains (movement) vary depending on the direction of the stress (force applied) in the material.
If you try to apply detailed engineering to composite materials it gets really bad really fast.
Things can get seriously weird in materials that have large variation in material properties with direction.
Welcome to the incredibly nasty world of 'tensor analysis.'
Strains (movement) vary depending on the direction of the stress (force applied) in the material.
If you try to apply detailed engineering to composite materials it gets really bad really fast.