Simulating distortions when chambering?

B

bob3700

Member
OK, there have been discussions concerning the fit of the bbl threads into the action. Some like them very tight and some very loose.

There are engineers out there that will tell you if you fit the threads between the bbl and the action tight, you can alter the dimension of the finished chamber walls. The chamber is said to have it's final dimensions altered by the stress of torquing the bbl to the action. Some smiths will finish reaming the chamber with the action torqued in place to get a perfect chamber.

What if? You manufactured a false receiver ring. Take a piece of steel and drill and cut threads just like a receiver ring. Then mill a couple of flats on it to be able to torque it to the end of the bbl tenion. Install and torque the false receiver ring to the bbl. Now you have pre-stressed the chamber end of the bbl and simulated the action being torqued in place. Now chamber the bbl up.

Would this not provide a chamber of more accurate final dimensions?

The concept is used on racing engine blocks. High performance shops torque thick steel plates to the cylinder block to induce the distortions and stresses that the cylinder heads do. They then bore and hone the block for a more accurate/round/consistent dimensioned hole.

Seems like the same concept being applied. Am I barking at the moon here?

Bob
 
I don't know if you are barking a the moon but I think you are trying to fix something that is not broken...

Threads should not be a tight fit, they should be free enough to thread all the way by hand. Looser is better than tighter. The 60 degree thread will self align when the shoulders butt up and the barrel is tightened.

I have no personal experience of chamber distortion from tightening a barrel on the action.
 
bob3700 said:
OK, there have been discussions concerning the fit of the bbl threads into the action. Some like them very tight and some very loose.

There are engineers out there that will tell you if you fit the threads between the bbl and the action tight, you can alter the dimension of the finished chamber walls. The chamber is said to have it's final dimensions altered by the stress of torquing the bbl to the action. Some smiths will finish reaming the chamber with the action torqued in place to get a perfect chamber.

What if? You manufactured a false receiver ring. Take a piece of steel and drill and cut threads just like a receiver ring. Then mill a couple of flats on it to be able to torque it to the end of the bbl tenion. Install and torque the false receiver ring to the bbl. Now you have pre-stressed the chamber end of the bbl and simulated the action being torqued in place. Now chamber the bbl up.

Would this not provide a chamber of more accurate final dimensions?

The concept is used on racing engine blocks. High performance shops torque thick steel plates to the cylinder block to induce the distortions and stresses that the cylinder heads do. They then bore and hone the block for a more accurate/round/consistent dimensioned hole.

Seems like the same concept being applied. Am I barking at the moon here?

Bob
Click to expand...

Yes, I am afraid you are barking at the moon on this topic.

On this very experienced messageboard, I have never heard anyone make the proposals for the reasons that you have mentioned.

Even if the torqued threads did alter the chamber dimensions it would be so little as to be immeasurable and would most likely be in a symetrical fashion that it would not make an accuracy difference.

Unless you can point to some measurable and recognisable references to this new phenomina, I am not sure that you will find too many takers....................................Don
 
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Distortion

The reason you torque head bolts and main saddle bolts into a cylinder block before final honing is because these parts place uneven distortion on the cylinder.

A round thread will distort the chamber evenly all around.

That is, if you are even worried about such stuff. I put this in the category of a "solution in search of a problem".......jackie
 
Dennis, Don, and Jackie,

Thank you all for your replys. You are all probably right, This is a solution looking for a problem.

One thing that I will say though, is if you base your information/facts solely on what you read on an internet forum, you might possibly be surprised as to it's accuracy.

I can tell you from experience that there are those who feel they have discovered an "Accuracy Advantage" and will poo-poo it on a forum if it surfaces. They will also disseminate false information in an effort to protect any advantage that they think they have.

Please don't think that I feel that way right here. I'm just saying that I have encountered it on other boards and with other topics.

Thanks again for your input.

Bob
 
Why does this remind me of using torque plates for machining engine blocks? That is a proven looser that resurfaces over time.

Seeing as how the tenon and the barrel threads are cut before the reamer or drill (Boring bar) Ever entire the barrel, I don't get it?
 
Al,

I take it you feel that torque plates used to bore or hone an engine block are a waste of time and money! Ever use a dial bore gauge in a cylinder and measure the distortion in the bore before and after the installation of torque plates. That is all it takes to see the difference.

True, the chamber is cut after the tenion is turned and threaded. But the bbl and action are then torqued onto each other. The stress of torquing on the threads is said to distort the chamber end of the bbl and slightly change the dimensions of the chamber. Some smiths actually finish the chambering of a bbl with the action installed so that the finished chamber dimensions are what the reamer was designed to cut. At least that is their theory.

Bob
 
The only way we will know is if somebody has access to a Mitutoyo roundness tester, a Talyor Hobson "tally Round" or some other machine that can check the size, roundness, and cylindricity.
 
Chamber a barrel and screw a custom nut on the tenon and indicate it. That will tell you if it happens. A motor has 5-6 headbolts around the hole with a 60deg gap or a 72deg gap between them.. A rifle bore has a head nut all around it with no gap.
Butch
 
Big Al said:
Why does this remind me of using torque plates for machining engine blocks? That is a proven looser that resurfaces over time.
Click to expand...

Cummins would be surprised to hear that. Last I knew they do it routinely in their factory on at least the 5.9L engines.

Fitch
 
All the engine manufactures use torque plates. The way car companies count pennies it has to make a difference for them to add another step to the manufacturing. On a engine when the bolts are tightened the pressure on the threads tries to spread the hole. The head bolts being next to the cylinder caused a bulge in the cylinder in that area. With a tenion the action would be squeezing all the way around so it may want to make the chamber smaller. If it did make it smaller one or two tenths I don't think it would mater.
 
Dave,

I think that you are right about the very small squeezing of the chamber. Many on this website will expound on how they are so very precise on their setups. They make sure the bore of the bbl is indicated to within .0002" and won't accept anything less. They check the chambering job many times during the process again targeting the magical .0002" runout as their goal.

If they take the bbl and chamber that is manufactured to this very low runout/distortion standard and then torque the bbl into the action. They may be causing dimensional variations that are as great or greater than the low standard they set during the chambering job and dismissing those torque induced changes.

I do think that each gunsmith believes that they are doing the best job possible and that they have arrived at their techniques thru many years of experience. The proof will be on the target for sure.

This thread was started just to get people to think and question what is or may be a standard practice. Are the steps we take to chamber a rifle bbl the best or are we doing the same thing cause someone else said that is the way it is done.

Really the best way would be to actually measure the chamber before and after torquing it into an action or using the false receiver ring. Sounds like an interesting research project.

Bob
 
I'll bet that the variation in reamer diameter dimensions and the actual diameter variations of finished chamber are several times the amount of compression from 60 to 100#' of torque on the joint.
If you believe that it makes a difference, you should take measures to minimize that affect. Many 600/1000 yard shooters are now using powder scales that show four significant digits. Some sort bullets by weight, internal concentricity, base to ogive length, boattail to ogive length, diameter of altered meplats and probably other things that they feel important. Brass is another complete area of deep concerns.
 
If there is such a thing as chamber compression(which I doubt) then equal or near equal barrel torque equals same or similar chamber compression. Much to do about nothing.

Dave
 
There can be some distortion of the chamber if the action threads are not round and if the barrel is screwed in very tightly. In addition, I'm sure there is a certain amount of compression if the walls are thin and torque values high.
The only serious distortion of a chamber I've seen was on Winchester Model 70's (post-64). One these actions, the locking lug raceways run right through the threads so the thread is interrupted. If the thread fit is a bit sloppy and the barrel torqued up tight, the chamber will be oval. This was especially problematic on those rifles chambered in the WSM cartridges but also showed up occasionally on belted magnums. Never really noticed it on those chambered for standard (.470 head dia) cartridges. I rethread all new M70's to 1 1/16x16 tpi when I build rifles based on them. The problem with the oval chambers was the reason Winchester made those late actions with the 28tpi thread (1 1/16 diameter, I think.)
In the old days, it was common practice to finish ream the chamber with the receiver on but I doubt that chamber distortion was the reason.
Speaking of chamber distortion. A friend of mine once put a rifle together only to find he had cut the headspace a little short. He removed the barrel and decided to just ream a bit by hand since he only needed a couple thou. He left the barrel in the barrel vice to do this and was puzzled when the reamer seemed to have to cut quite a bit before he hit the shoulder. Of course, the barrel vice was squeezing the chamber enogh that the reamer was cutting top and bottom all the way in; or at least for the 1 1/2 inches clamped in the vice. The chamber ended up about .002 out of round. Nothing to do with the threads of course.
The square threads as used on Springfileds and Enfields shouldn't compress the tenon at all and Acme threads would also produce less compressive load as would buttress threads. The much touted (by some) Spiraloc threads would be worse than conventional vee threads. Regards, Bill.
 
Chamber compression is very common. It is caused by using "V" threads, as Bill notes above. The vector resolution component of the V thread forces are half towards pulling the action face and barrel together and the other half is towards compressing the chamber body. A 16 pitch thread exerts about 100 times the force of the torque (neglecting friction), so 100 ft pounds torque has a compressive vector force of 1000 pounds or so.

This effect was first brought to my attention when working with Smith and Wesson revolvers. When lapping the barrel for use with cast slugs there is usually a significant constriction in the barrel where it joins the frame. This needs to be removed in order for the revolver to reach its best accuracy. This constriction is significant enough that it takes quite a bit of lapping to remove - i never put gauges to it, but estimate it is .0003-.0005".

Rifle barrels are much heavier with coarser thread pitches, but still show compression due to the torque - one of my reasons for not using any more torque than needed to positively secure the barrel. If you doubt the compression occurs, you don't need laboratory grade measuring equipment to detect it - just chamber a barrel, torque it fimly in place and then re-insert the same chamber reamer and give it a turn. The new chips on the reamer will pin point the compression area (they won't be on the reamer shoulder unless that is where the threads lie).

All that being said, I'm not sure that a tenth or two of compression in the chamber has any effect on accuracy. It will affect barrel dimensions however, and devices that thread/clamp on the barrel need to be considered carefully.

Scott
 
frwillia said:
Cummins would be surprised to hear that. Last I knew they do it routinely in their factory on at least the 5.9L engines.

Fitch
Click to expand...


I don't have a clue about the 5.9L however every big bore diesel I have ever dealt with uses sleeves. Both Cummings, Cat, Walkeshau, Detroit Diesel. But then I've never dealt with small engines in car applications, just vary large stationary engines for power generation up to and including turbines.:D
 
Al,

The reason smaller engines use the torque plates for honing is that they DON"T have sleeves. Sleeves isolate the cylinder bore from the block. On the type of diesels you are talking about, you can replace sleeves, pistons, etc. without ever taking the engine out of the vehicle. Some earlier commercial gasoline engines (like Chrysler 318s and 392 Hemi's) used sleeves so that the engine block would have a long life. That is the expensive part you don't want to have to throw away because it is worn out!

With a cast iron block, no sleeves, the walls of the block are distorted by the head bolts and the cylinder bore can easily be pulled out-of-round and the ring seal suffers. For most applications a little blow-by may not be an issue. For a racing engine, 100 percent ring seal ( or as close as you can get to 100 percent) is mandatory if you want to win the race.

That is why they hone the blocks with simulated heads torques in place.

RGDS

Bob
 
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Bad Chambers and threads

Chambering a barrel which is not aligned properly, will certainly
give you an out of round chamber, it will also be oversize.
This can make you believe that you have distortion. Threads
that need to be forced all the way to seat a shoulder will
produce galled threads and a non shooting rifle. A small
amount of compression at the shoulder reduces Headspace
and must be accounted for when holding , or matching
headspace dimensions on additional barrels. This compression
typically amounts to .001, maybe .0015. Barrel tenons do
stretch as any bolt does, when tensioned. Given crooked
threads and or action face, you may also get a curvature
in a chamber.
 
Bob,
I have tried on purpose to chamber an out of round chamber and have not had any success. How can you do it with out have a tool holder that operates as a cam and goes in and out while boring. Or you have a tailstock that sways back and forth in time with the barrel while running the reamer in.
I think that it may be oversize, but not out of round.
Butch
 
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