Barrel Threads

[MOO1072]

When a thread is cut on a new barrel, is there any allowance for an interference fit? If so, by how much?

 

[sjk]

Threads,,and this includes all ) come in classes of fit, from very sloppy to a class 5 interference fit. Whichever one you want, check it on the way down using three wire method or thread mics. You will have to have the pitch dia. of the threads you are mating to or to use the mating part be careful as you may go on by VERY easily. Wires and a plug gage can be used to check this . Now that you have both pitch dia's you can cut any class you want

 

[Rustystud]

Most rifle manufactures recomend a 2B fit. If you truly want to know what a 2B fit is, I would recomend reading the MAHINIST HANDBOOK, it give the most detail of any refrence material I know. If you don't have a Machinist hand book they are easily found at the library, most trade and engineering schools. There are literally hundreds of used one available on e-bay, and other book outlets. If you are not doing CNC machining a 1950s/1960s edition will tell you more than you need to know. Great reference book for every shop. A new edition is about $90.00.
Rustystud

 

[sjk]

Rusty's right. There's a wealth of information out there at anyone's disposal. These clearances can be found in charts generated by ones a lot more knowledgable than me. Here's a except for MACHINERY'S HANDBOOK that throws some light on threa
ding, and it's only the tip of a large threading iceberg



Interference fit of threads. Thread classes are distinguished from each other by the amount of tolerance and allowance between the A ,external) and B,internal) thread. Interference fit ,sometimes known as a crush fit) is when the externally threaded member is larger than the internal threaded member when both members are in a free state, which when assembled, both become the same size and develop a holding torque through elastic compression, plastic movement of metal, or both. By custom, these threads are designated Class 5

But what the books do not tell you is that when removed a few times the interference fit is swaged to a different size due to metal dispalcement and holding torque is diminished

 

[MOO1072]

Thanks for the info. My shootin buddy is a qualified machinist so it will make a lot of sense to him.

 

[rednek]

Your threads actually are not really all that critical. It is much more important that the shoulder is square and true to the bore and threads. The shoulder is the actual "crush" surface and not the threads.

 

[JDJones]

I don't know about the rest of the gun builders here but I like my threads as tight as I can get them to screw together at room temp. I personaly hold all my tolerances to .0002" max run out and I'll be damnd if I'll put some sloppy threads on their and allow for any miss alignement and a 2B fit has more than .0002" slop!!

I'm not saying that accurate rifles can't have crappy threads, buit I havenever seen a BR quality gun with loose threads. If your gonna do a job you might as well do it right the whole way out.

 

[Rustystud]

JDJONES:

Not arguing with you. I would love to see your threads with ".0002" yes you said tenths slop screw together and apart again. If you can get them started they will be smoking when you get the shoulder to meet up with the action face. I will stick with a smooth with slight resitence 2B fit as called for by most action makers. I don't like stress in my action or barrel. I have seen actions twisted and bent from over tight threads. Screw threads are rated on the amount of surface contact they have. Witout looking it up and relying on memory I believe a 2B thread has between 50 and 60 percent contact between the male and female thread. That is measured from half way between the crest and bottom of the thread. Basically following the measurement standard established by the three wire method. It has been a while since I went to machinist school. If one where to take a Savage action because it has 20TPI X 1.0625,easy to figure)with a hypothetical 1 inch tenon you would have 21.250" inches of thread surface contact. Even with moly grease, I think with a .0002" fit the threads will be gaulded by the time the lug meets the shoulder. I am sure there are some quicker minds than mine who can can chime in and set me straight.
Rustystud

 

[JDJones]

I diden't meen that I like a .0002" fit, I ment that I like a nice tight slick fit so that their is very minimal movement to impact the close tolerance fit that everything else has had done. I always start with my threads slightly over sized so that the action won't get started then make .001" cuts fit checking each time to finalize the fit. I agree that you dan't want any stress in the action especialy from the barrel shank thats seeing stress from the rounds pressure. I did have a barrel and action gall once both were SS and the fit was to tight, I guess when I wiped off the barrel shank I did to good of a job and diden't leave any oil to prevent galling, that was an expensive lesson !!

 

[Eddie Harren]

Jones, what you said was, "I hold my threads to .0002" total runout". I don't think anyone likes "sloppy" threads but, I have to agree with Rusty that .0002" interference threads would gaul for sure. I've seen shooters at BR matches screw barrels on by hand and snap them tight by hand, shoot the match and have to use a wrench to remove the barrel.

 

[edpmedic]

http://www.engineersedge.com/thread_strength/thread_classes.htm

 

[gunsandgunsmithing]

Stainless on stainless with a very tight thread fit can get you in trouble in a hurry. A 2B fit will have a little felt slop at the end of a 20"-30" rifle barrel. It's not a problem. I strive for the very best in every aspect when building a gun too, but sometimes less is more. How do you keep .0002" runout the same while fitting a barrel to a glued in action and removing .001" then checking the fit till it's right?---Mike