Gunsmithing Indicating rods or between centers

[Stokes]

Hey Guys,

This is probably just over thinking, but I'm hoping one of you can tell me I'm just being dumb about this.

This is specific to cutting threads on the muzzle end, when the barrel isn't run through the head stock.


Why (or would) using an indicating rod be preferable to just turning them between centers?
My thoughts are that if the bore is perpendicular to the crown, it just wouldn't matter, and if it wasn't perpendicular, then indicating rods wouldn't do anything to help that anyways.

The biggest advantages to turning between centers would seem to be speed, not having to buy lots of different rods, and not inserting anything into the bore.

 

[E. Bryant]

Turning between centers does not guarantee that you will get the crown perpendicular to the bore, as the bore may very well be curved as it passes the length of the barrel.

 

[Killswitch Engage]

im not a gunsmith but have threaded my share of barrels. i think threading between centers and driving with a dog of some sort would be fine provided your gentle on the crown with saver of some sort and find a good way to pick up in the chamber end like with a piece of brass with no primer in it. i prefer indicating off the bore but i have a short headstock and have no issues doing this. you would obviously need a different route if you were need to shorten and recrown the barrel as well. you may be able to achieve good results with a 4j chuck and steady rest indicating off the bore. theres plenty of ways to skin the cat and the more i machine things the more i am figuring this out.

 

[Stokes]

Turning between centers does not guarantee that you will get the crown perpendicular to the bore, as the bore may very well be curved as it passes the length of the barrel.

If the crown isn't perpendicular to the bore, then the difference between turning between centers or indicting off a rod is meaningless. Your problems are much bigger.

 

[Stokes]

im not a gunsmith but have threaded my share of barrels. i think threading between centers and driving with a dog of some sort would be fine provided your gentle on the crown with saver of some sort and find a good way to pick up in the chamber end like with a piece of brass with no primer in it. i prefer indicating off the bore but i have a short headstock and have no issues doing this. you would obviously need a different route if you were need to shorten and recrown the barrel as well. you may be able to achieve good results with a 4j chuck and steady rest indicating off the bore. theres plenty of ways to skin the cat and the more i machine things the more i am figuring this out.

This what I'm thinking. I think a couple of savers on both ends and drive it with a dog, and I should be right there. Cut and crown in a different operation if needed.

 

[WhiteDingo]

While having done both in the past, I've come to much prefer indicating the bore directly vs. just running between centers. As others have pointed out, you're relying on on the bore being straight inside the barrel and the crown being perpendicular to the line of bore.
If you're reasonably certain the bore is straight and the crown is true, then by all means turn between centers. You'll just have to be careful to not ding up the crown or chamber ends. If you're crowning the barrel first, it makes sense to me to have a setup where you can do both the crown and threading without having to re-fixture the barrel. If you'd prefer to do that with a 4 jaw and a steady rest, I've done that too and it worked just fine for a brake install

 

[E. Bryant]

If the crown isn't perpendicular to the bore, then the difference between turning between centers or indicting off a rod is meaningless. Your problems are much bigger.

Err, no - actually, your problems are much bigger. I wouldn't be the one with the non-perpendicular crown

Let me try to explain this another way, after reading your original post again.

Even assuming that the barrel is already properly crowned and that threads are to be cut, simply spinning between centers will still not ensure that the threads are perpendicular to the crown. This is because the two faces at each end are not necessarily parallel to one another (at least not if the bore is bent and each end of the barrel was machined properly).

I'm simply spit-balling here since I'm attempting to procrastinate on some stuff for the day job, but it seems that what someone could do in this case would be to support the muzzle with a live center (using a "crown saver", of course) and the chamber end in a spider chuck or 4-jaw (the latter probably requiring a strip of wire or similar means to allow it to pivot within the chuck jaws). Dialing in the chamber end would be accomplished by measuring the run-out of the crown; if it's truly perpendicular to the bore, then the proper set-up would be achieved upon measuring effectively zero axial run-out. This setup really makes very little sense (especially because the muzzle diameter isn't so large as to give me much confidence in my measurement of its runout), but I guess it could be done.

 

[Lucky Jack]

I've done this hundreds of times and have never, not one time, had a problem. I get people who walk into the shop or who mail us rifles and barrels (or other guns) their gunsmith either wouldn't, or couldn't thread. SBR's, Ruger pistols, Handi-rifles, Ak's etc that really can only be chucked up center-to-center or you'll need to refinish them. Now, if it's your Surgeon Scalpel I would recommend not using the center-to-center method but otherwise it's gets the job done, within the .001-.003 "gunsmithing tolerance" and it enables you to cut threads in about 30 minutes. If you stack a few of these jobs up you can have a very profitable afternoon.

If it is a barrel that you need to shorten, re-crown then thread and it won't fit through the head stock you can go center-to-center with a steady rest already set up but loose. Bring the portion you are threading to diameter then use the steady rest while you cut the crown. After that you release the steady rest and cut the threads center-to-center. If you keep your lathe straight you can reduce your set up time and tool change time by a lot and increase the number of barrels you can do in a work day.

And, yes if something on the barrel doesn't look right or if you think something is off then don't do it. Pretty simple.

 

[jake6547]

between centers will work if its just for a brake but if you want to ever run a can I promise this is not the best way, indicating the bore does take longer but is the perfect way to do it. I setup every crown/ thread job the same way I setup to fit a barrel to a action. And if the barrel is to short to fit between the spiders i make an extension. Take the time to do it right it it will be worth it.