Best Method for Allowing an Octagon Barrel to Pivot in 4-Jaw

hepburnman · Mar 19, 2014

I'm working up to chambering a barrel through the spindle and I'd like to ask what is the best way to chuck an octagon-shaped barrel in a 4-jaw chuck so that it can pivot without bending when the spider at the muzzel-end is adjusted? I am planning right now to use a ring of heavy copper wire between the barrel and the jaws of a 4-jaw chuck. Some others here use a split-ring of aluminum but this is usually used on/made for round barrels. I'd like to know if the ring of copper wire will be as good as the split-ring method for an octagon barrel? [the theory is that you want to not grip the barrel at the breech-end so rigidly that the barrel bends, rather than pivots, when you adjust the spider at the muzzle-end when you indicate and center a short-length of the throat-area of the bore to run concentric with the spindle. Also, this copper-ring/pivot-point on the barrel should be in-line with the forward-most throat-area in the bore that you are indicating (with a long-point DTI) so that this concentric "circle" does not shift position when the spider is adjusted - sorry if this is old-hash for everyone!].

Artv · Mar 19, 2014

Use a hefty rubber "O" ring.

Art

hepburnman · Mar 19, 2014

Artv- I respect your suggestion but I would be hesitant to use rubber between the barrel and jaws of a chuck. This rubber would have to be pretty hard to give me confidence that the threading operation, at least, would not deflect the barrel into the rubber due to the pressure of the threading tool. I might have less concern with rubber during chambering as this operation develops axial forces rather than radial, I would think. Can you suggest a particular o-ring part number or durometer? It would take me some time to warm up to this idea, perhaps.

Artv · Mar 19, 2014

hepburnman said:
It would take me some time to warm up to this idea, perhaps.

Then don't use it. It is what I used to do, now I handle it differently.

Art

MilGunsmith · Mar 20, 2014

Use the copper wire. Or just make 4 L shaped pieces to fit between jaws and barrel.

MetalCarnage · Mar 20, 2014

FWIW - In similar situations I've been known to wrap work pieces in a few layers of brass shim stock. It prevents the jaws from gouging and the multiple layers (2, 3, or 4 depending on the shim material) will allow "slippage" between the layers.

-Ron

hepburnman · Mar 20, 2014

MetalCarnage said:
FWIW - In similar situations I've been known to wrap work pieces in a few layers of brass shim stock. It prevents the jaws from gouging and the multiple layers (2, 3, or 4 depending on the shim material) will allow "slippage" between the layers.

-Ron

This is a good idea because I was wondering if we would really get the expected slippage between the barrel and the copper ring when adjusting the out-board spider. For the barrel to be adjusted off-center at the muzzle the areas of the barrel in contact with the copper ring near the 4-jaw (or opposite sides of the barrel) would have to slip in different directions equally for the center of the barrel at this point to remain stationary (axially).

hepburnman · Mar 20, 2014

While probably really not necessary, a ball and socket type device would be the thing to produce a true pivot for the barrel, without requiring any axial motion of the barrel, at the breech-end of the barrel.

GGaskill · Mar 20, 2014

The copper wire technique is what I use along with not really torquing down the jaws/screws at either end until I am basically where I want to be. Takes a little longer but eliminates the sliding issue as much as possible.

Sea Sick Steve · Mar 22, 2014

hepburnman said:
While probably really not necessary, a ball and socket type device would be the thing to produce a true pivot for the barrel, without requiring any axial motion of the barrel, at the breech-end of the barrel.

Here you go
Straight Shot Gunsmithing

manualmachinist · Mar 22, 2014

Cut the "legs" off of a large cotter pin and cut them into pieces. put the radiused side of each piece against the barrel.

ditchdigger · Mar 22, 2014

hepburnman said:
While probably really not necessary, a ball and socket type device would be the thing to produce a true pivot for the barrel, without requiring any axial motion of the barrel, at the breech-end of the barrel.

Thousands of minimum dimention, concentric chambers get cut straight out of a four jaw chuck/tail spider set-up. All the gadgets in the world won't make up for lack of skill and experience. There's just no voodoo or black magic to it, just straight forward technique aquired by doing. The machinists that were in their mid to late 40s, 50s and 60s, who were my mentors/trainers in the mid 1970s would have no trouble deciding how they would make a set-up to chamber a rifle barrel to the highest standards without all the gadgets many seem to think are necessities, today. My partner in the job shop, who is 12yrs. younger than I and never chambered a rifle barrel, would have no trouble with the task because he has the needed experience as a machinist. We come up with ways to do/complete seemingly complex jobs on a daily/ weekly basis without spending a gagillion dollars to do it.

Chuck Kiegh · Mar 22, 2014

We were using copper wire and copper pennies at Trinidad.... takes time to dial in but we were holding 2/10 ths. All done on ancient South Bend lathes.

hepburnman · Mar 22, 2014

Sea Sick Steve said:
Here you go
Straight Shot Gunsmithing

Very nice system that basically moves the whole chuck both radially and angularly. At $1300 though, without the chuck, this may be something only for those that do a lot of barrel work and really want to cut down on the time required. This system also appears to make the spider unnecessary.

hepburnman · Mar 22, 2014

ditchdigger said:
Thousands of minimum dimention, concentric chambers get cut straight out of a four jaw chuck/tail spider set-up. All the gadgets in the world won't make up for lack of skill and experience. There's just no voodoo or black magic to it, just straight forward technique aquired by doing. The machinists that were in their mid to late 40s, 50s and 60s, who were my mentors/trainers in the mid 1970s would have no trouble deciding how they would make a set-up to chamber a rifle barrel to the highest standards without all the gadgets many seem to think are necessities, today. My partner in the job shop, who is 12yrs. younger than I and never chambered a rifle barrel, would have no trouble with the task because he has the needed experience as a machinist. We come up with ways to do/complete seemingly complex jobs on a daily/ weekly basis without spending a gagillion dollars to do it.

Unless you try different things (they dont necessarily need to cost a lot) how would you know if an improvement is possible. No question that older and maybe traditional ways work well but newer/different ways may may also do well and maybe offer an improvement. There's always room for improvements and new techniques/experiments are what push the technologies.

hepburnman · Mar 22, 2014

Chuck Kiegh said:
We were using copper wire and copper pennies at Trinidad.... takes time to dial in but we were holding 2/10 ths. All done on ancient South Bend lathes.

Good to hear because that is what I have/will be using!

hepburnman · Mar 22, 2014

manualmachinist said:
Cut the "legs" off of a large cotter pin and cut them into pieces. put the radiused side of each piece against the barrel.

Pieces of large cotter pins is a good idea too but I would have to this more of this to understand if there would be an improvement over the "ring of copper wire" idea.

tdmidget · Mar 22, 2014

Chuck Kiegh said:
We were using copper wire and copper pennies at Trinidad.... takes time to dial in but we were holding 2/10 ths. All done on ancient South Bend lathes.

Careful with pennies. New ones are zinc and will deform and turn loose of the part. Had a piece come out of a chuck due to that. We used the old ones to protect the table on a jig grinder.

trevj · Mar 22, 2014

hepburnman said:
Unless you try different things (they dont necessarily need to cost a lot) how would you know if an improvement is possible. No question that older and maybe traditional ways work well but newer/different ways may may also do well and maybe offer an improvement. There's always room for improvements and new techniques/experiments are what push the technologies.

That's really funny, considering the source! Y'know, since you are still waffling and working your way towards your first barrel and all.

Cheers
Trev

hepburnman · Mar 23, 2014

trevj said:
That's really funny, considering the source! Y'know, since you are still waffling and working your way towards your first barrel and all.

Cheers
Trev

Working, yes, waffling, no. Lot's of good stuff going on working up to the real thing!

Best Method for Allowing an Octagon Barrel to Pivot in 4-Jaw

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