Importance of truing threads?

[gunsandgunsmithing]

When I was turning blanks for core bits we used tapered shoulders and although they may have helped center the joint I was told it was to help lock it together.
They where multiple entry threads for quick attaching and the taper would lock them up so they wouldn't come apart when pulling out of the hole.

I believe that is coorect

 

[Dans40X]

Thread Fit-
60 degree class3
Lets say for conversation that you have a 12" piece of thick wall seamless tubing(pick a schedule) that is bored & threaded it's entire internal diameter to the nearest .0001" indicated.

Lets say again that you have a matched ground threaded rod of the same thread pitch/class but is 2" longer than the above piece of tubing.

You lube/thread the threaded rod into said round stock that has(pick a number)said .001" pitch clearance on both pieces it's entire length.

Now lets install a classed NUT to tighten against 1 end of said tubing.

How many threads are doing ALL the work & how many threads are for ALIGNMENT purposes ONLY?

More flyers are induced from improper thread class that your single granule of powder.

 

[Will Henry]

A tapered shoulder or barrel seat should provide a stronger, more rigid connection but whether or not this would necessarily lead to greater precision is hard to say. I used an angled (30 degree) seat to fit pistons to hydraulic cylinder rods where there was an issue with breakage and it ended the breakage but that ain't gun smithing and likely bears no relationship to barrel fitting.
I like a barrel to just thread in by hand if using vee threads but I like them a bit tight with square threads. On one lathe I have, the vee thread is definitely what centers the chuck up and it's generously loose.
When I re-thread a Model 70 to 1 1/16", I bore a relief about 3/16 long and turn a corresponding shoulder on the barrel tenon. Since it is done in the same set-up as the threads it should be concentric to the thread axis. I try for .001 or less tolerance on the diameter. I doubt that it makes any difference but it makes me feel like I've done something. WH

 

[Dans40X]

Yes, there are more ways of engaging more than the first few threads.
But
Once again,you have chosen to avoid the question.

How many threads are under compression & how many are for alignment purposes?

 

[johnfred1965]

Yes, there are more ways of engaging more than the first few threads.
But
Once again,you have chosen to avoid the question.

How many threads are under compression & how many are for alignment purposes?

Vaughn, chapter 6...Have you ever TIGed a barrel to a receiver to improve accuracy?

 

[mram10]

I’ll bite! With .0001 as our variance and .001 clearance, I say all of them. Not sure how much crush moves the thread faces together along a foot, but on roughly .750, it seems like a lot

 

[GenePoole]

Have any of you guys ever heard of Joseph Whitworth?

 

[rangertim]

SDS. Good post.

Sleeving bolts would be much less time consuming if we could buy premade split sleeves.

You could at one time from Brownells !

 

[Will Henry]

I never sleeve bolts anymore. WH

 

[TRA]

The reason excess clearance is necessary when threading a barrel is due to the inability of old well used equipment not being able to cut threads that will mate up properly to threads formed on modern precision equipment, and especially the difference in tooling. All inserts are not equal, and shop ground tooling is even worse. Before you think it's all about "CNC", even a CNC lathe will not cut perfect, consistent rather, threads with a basic single point tool.

 

[Dans40X]

I don't know. How many? I have not seen a tenon 12" long yet, so to date, I have only concerned myself with making sure Im engaging barrel tenon threads as much as possible by analyzing contact patterns.

Do you relieve the first 1.5 threads at action face & Bbl tenon during your process?
If not,when bubba over torques a barrel 1 time, thread galling will occur at both thread sections of the fitment.

Fit a receiver/barrel tenon to the micron clearance & ELIMINATE harmonics/vibration issues frequently observed as flyers.

 

[TRA]

Not sure I can agree. I have thread mics and can measure my threads across the whole tenon. I have found a lot of tapered threads in custom actions as well as different PDs in the same model actions. Thats not knocking custom actions. They are putting out great products but its still production.

The PD may be consistent but the pitch and the thread profile will vary. The work around is to keep cutting until they fit.

 

[butchlambert]

No!

Care to elaborate on this?

 

[STS]

Not sure I can agree. I have thread mics and can measure my threads across the whole tenon. I have found a lot of tapered threads in custom actions as well as different PDs in the same model actions. Thats not knocking custom actions. They are putting out great products but its still production.

There's been a generous amount of discussion about the importance of thread fit for as long as I've been pursuing rifle precision. About 45 years worth at least. Logically, it would seem that the better the thread fit the better the rifle would shoot but it certainly doesn't seem to work that way in the real world. There have simply been too many barrels that were record setting or multiple match winning barrels that have absolutely horrible threads. Alex provided us with an excellent example of that. I saw one barrel that a budding new shooter threaded himself on his new lathe and he had the compound set a 45 degrees which most of you gentlemen know that would yield a thread with a stair step profile. He just kept threading deeper until that barrel would screw on the action. That poor barrel was competitive. I have also seen taper in custom actions and at one time I got all in a lather about that kind of flaw but it really doesn't seem to matter. What matters more is having correct clearances and having things concentric.

 

[Macropod]

I think one of the most consistant barrel tenon designs, but more work, to fit is in the CG Inch action. Double register and double shoulder should theoretically be the best self centering system around. One of my rifles using this action shows very consistant zeros with barrel removal and replacing.

Coincidence? Maybe, but my gut instinct tells me that the tenon design with a very closely toleranced barrel to action fit works well in this case.

 

[Shawnba67]

No matter how well your threads fit, upon torqueing doesnt the rear edge of the "V" pull away creating clearance?
I would love to discuss more about how that affects vibration.

 

[carlsbad]

Threads have very little to do with accuracy. Once tightened, they need to hold the barrel stable against the action. If the threads and shoulder aren't square, the rifle will not be aligned with the action so the scope may not have enough travel. The barrel may not be centered in the stock. But accuracy will suffer little. (It will likely suffer a little because the bolt face won't be square to the chamber.) The threads DO locate the chamber in the center of the action threads. But that just happens. All the lathe operator needs to do in thread and chamber using the same (correct) setup.

Good threads are required for the following:

1. Safety. Too loose threads would be a safety hazard. The joint would be centered but they could pull out.
2. Tightness: Bad thread finish can cause false tightness. A joint is tightened, after cyclic fatigue (firing multiple rounds) the high points settle and the joint loosens up. A loose joint can crack and fail. If you have a bolt failing in a machine, the most likely cause is that it is loosening up and then experience fatigue cracking. Extremely bad thread fit or finish can result in galling.
3. Locating: Tapered thread by design center the male and female threads. We discussed this in this thread. Using a tight clearance between the bolt nose and the barrel requires good centering. If the bolt nose contacts the barrel tenon, accuracy will suffer.

That's about it. Nice thread finish provides some nice features that aren't necessarily a requirement. For example repeatability. I had a barrel that wasn't shooting. I set it back and rechambered. Reassembled and took it to the range. The POI changed 3/4" at 100 yards. A well threaded barrel when removed and re-installed should not change much-important for switchbarrel applications.

Vibration is probably unaffected but if your tightness is questionable, vibration nodes/frequency could be affected and that could affect load development.

But the beautiful threads we admire photos of here are mostly pride in workmanship--which is also nice. The smith is proud to provide them and the buyer is proud to own them. it is just one indication that the work is likely done right. Because there are a lot of other, subtler things, that if not done right do affect accuracy. You want someone building your rifle that takes time and has the knowledge, experience, and ability to do it right.

--Jerry

 

[mram10]

Jerry, just read about your technique of screwing the action together multiple times prior to final fitment. I’ll be doing that today. I came down with pneumonia so the wife won’t let me go to the range to test my builds, so I’ll try your technique to verify the barrels and actions mate up well.

 

[carlsbad]

Mram,
We used that technique to solve a problem with mounting bolt failure on a rotating baffle on a critical pump . the bolts were beautiful and perfect and you would think their fit was perfect as well. But when we started torquing and loosening them several times before final assembly, failures went away.

I'm not saying barrel joints will fail, but repeated torquing and loosening (with proper lubricant) will improve the thread mating.

--Jerry

 

[okie]

Mram,
We used that technique to solve a problem with mounting bolt failure on a rotating baffle on a critical pump . the bolts were beautiful and perfect and you would think their fit was perfect as well. But when we started torquing and loosening them several times before final assembly, failures went away.

I'm not saying barrel joints will fail, but repeated torquing and loosening (with proper lubricant) will improve the thread mating.

--Jerry

interesting--never thought about that...