Thread relief groove when threading barrel.

Sierevello · Nov 22, 2008

Can anyone comment on whether a thread relief groove hurts accuracy on a rifle if you use one cutting the threads on a new barrel blank?

Thanks, Steve

Ben Hooper · Nov 22, 2008

A relief groove is required with some recievers. I do not see how it could have anything to do with accuracy, but someone will probably say it does.

There is a lot of black art with these things, with no real mechanism that could affect accuracy.

rws · Nov 22, 2008

There are probably more top smiths that cut a groove than you think. There really isn't anything wrong with it as long as it's only as deep as the threads. It is used as a "safe" way to thread up to a shoulder for a stopping point.

It is however more or less looked down upon as not the most professional means of threading.

Get yourself a 2" travel dial and a mag-mount. Set your tool at the end of the thread against the shoulder and set the dial to zero. Then just watch the dial when threading, pull the half nuts and back out the cross feed when it reaches zero. It's not hard to do at all.

Wayne Shaw

Ben Hooper · Nov 22, 2008

RWS,

How do you fit a Mauser barrel without a relief groove?

Ben

rws · Nov 22, 2008

99.99% of the barrels I do are benchrest grade barrels on benchrest actions. They don't need a relief groove. However, as you say, there are some that need it, and one that comes to mind is sleeved Remington where there is no recoil lug. I prefer not to cut a relief inside the action, so a groove is cut. It is kept as small and shallow as possible. But I do not cut a groove if there is a relief in the action.

adamsgt · Nov 22, 2008

I bought four cast off benchrest barrels to use for machining practice and was surprised at the width of the groove on some of the barrels. They were done by different smiths. When I went to gunsmithing school 25 years ago, we threaded right up to the shoulder. quickly learned the coordination of disengaging the half nuts and backing out the carriage. Now I have to relearn that process. Hope my 25 years older hand - eye coordination is still up to the task.

deltaenterprizes · Nov 22, 2008

For those that say a relief groove hurts accuracy, what does it do to cause inaccuracy?

Jay Idaho · Nov 23, 2008

No groove needed... invert the threading tool, set compound to the left (29.5º or 30º) instead of to the right, run lathe CW instead of CCW, set carriage stop or use a dial indicator to insure the came starting point of threads and thread away.
With the carriage moving away from the headstock, you don't have to be concerned about reaching the end of the thread, it is no longer cutting anything.
Only problem is, if you have a threaded spindle on the lathe, you might unscrew your chuck.

Butch Lambert · Nov 23, 2008

For those that are worried about backing out quickly, why not watch your indicator-Trav-a-Dial and just disengage the half nut. Then you can slowly back out the tool. It will leave an almost invisible thread relief. As long as the shoulder is square to the bore a thread relief has no bearing on accuracy. A 3/8" thread relief won't hurt you either. I thread to the shoulder because I like the look and my Buds can see that I can do it.
Butch

Ben Hooper · Nov 23, 2008

A vfd makes it real easy...I wish I had one at home....

cheechako · Nov 23, 2008

I have cut a relief groove only where it was necessary. Most custom actions are relieved and you don't need one on a 700 Rem, but there are times when it's the easiest way. I used to be ashamed of my barrels that had a groove until I started noticing that some better smiths than me had them too. After that I stopped trying to hide them. I have even set a grooved barrel back a few threads to cut a new chamber and the old groove really makes it look shoddy but the barrel didn't seem to mind. It still shot better than I did. And I've seen other Benchrest barrels made by masters that have an extra groove.

I would not worry about it. You can always tell other shooters that you did it that way after a lot of testing proved that it was better.

JMHO

Ray

garyphansen · Nov 23, 2008

"You can always tell other shooters that you did it that way after a lot of testing proved that it was better."

My guess is if there was a lot of testing done, it would prove that it is better or at least that there is no difference. Gary P. Hansen

300sniper · Nov 23, 2008

i am not sure how a thread relief can be any weaker than fully threaded if it is not cut deeper than the thread root.

garyphansen · Nov 23, 2008

Ideally the thread relief groove should be shaped like an U and not a V and not deeper than the root of the thread. If it is done that way it should prove stronger than if it was threaded right to the shoulder. At least that is what I remember from my industrial materials class playing around with tinsel strength testing machines. Gary P. Hansen

300sniper · Nov 23, 2008

Jay said:
No groove needed... invert the threading tool, set compound to the left (29.5º or 30º) instead of to the right, run lathe CW instead of CCW, set carriage stop or use a dial indicator to insure the came starting point of threads and thread away.
With the carriage moving away from the headstock, you don't have to be concerned about reaching the end of the thread, it is no longer cutting anything.
Only problem is, if you have a threaded spindle on the lathe, you might unscrew your chuck.

hey jay,

i had thought about this method before but was worried about feeding the compound into the threads while the machine is running. it seemed like a lot of coordination to engage the half nut and accurately feed into the part at the exact time the thread dial is at the correct position. i can see it being much easier even with a small relief so you can have the tool already at the correct depth before engaging the half nut.

then again, i am still learning and am barely coordinated enough to make a nice thread no where near a shoulder.

one thought about inverting the threading tool, if using an indexable holder it would put the cutting point away from the shoulder. i was going to use an internal threading tool on the back side of the part and spindle turning in reverse while making very light cuts.

Ben Hooper · Nov 24, 2008

garyphansen said:
"You can always tell other shooters that you did it that way after a lot of testing proved that it was better."

My guess is if there was a lot of testing done, it would prove that it is better or at least that there is no difference. Gary P. Hansen

agreed

It is better since it has less risk of damaging the shoulder.

A U-shaped groove is stronger because there is no stress riser with the radius....but....think about it, there is already a V shaped groove, 1/16" inch back...The thread!

So, you might as well cut the groove with a threading tool from a strength standpoint.

Ben

garyphansen · Nov 24, 2008

Ben because the threads spiral down the barrel they do not set up the same type stress point as a groove that goes all the way around the barrel. However, with that said most likely the extra strength provided from a U groove over a V groove will never be needed. Gary P. Hansen

300sniper · Nov 24, 2008

garyphansen said:
Ben because the threads spiral down the barrel they do not set up the same type stress point as a groove that goes all the way around the barrel. However, with that said most likely the extra strength provided from a U groove over a V groove will never be needed. Gary P. Hansen

in my opinion, a radius from the root diameter to the shoulder would technically be the strongest but it is a moot point in the way the barrel will be stressed. i have not taken any industrial materials classes (i really want to though) but i am guessing a threaded rod would not be much, if any stronger than the same rod turned to the root diameter. it seems like the v groove of the thread would be one continuous stress riser and may actually be worse than a straight rod. again. i have no formal training and have just read a bit on the subject here and there. please correct me if i am wrong.

Jaycee · Nov 24, 2008

I'm in total aggrement with garyphansen, Far too many thread relief grooves are cut with
a parting tool, (sharp corners). A groove with a full radius or at least radiused corners
should be used, no deeper than the root diameter of the thread to provide a strong stress free requirement.
As for accuracy, as long as the strength of the material or joint is not comprimised,
neither should the accuracy.

Fussy1 · Nov 24, 2008

At last, cutting through the relief-groove BS! ;~) I suspect that the threading to the shoulder was originally done to save a step in mass production. It's a nice way to show off your lathing skills, though.
F

PS: Over-stressed/faulty barrels blow radially much more often than longitudinally.

Thread relief groove when threading barrel.

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