My take on a chamber/barrel fixture - pics

AnimalMother224

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For what its worth, new guy here but Ive been lurking for a long time. Pretty active on the "other" site for a long time

Since getting into PRS Ive been doing some of my own work in my shop. I was using a shop built chamber fixture. Basic steel heavy wall tube bored out, 4 jaw style. Since my 14x40 lathe head stock is pretty long, getting anything short like an AR barrel in there proves difficult for using the rear spider and the four jaw.

I havnt seen any kind of this fixture on line, albeit I didnt look real hard. I was growing tired of loading the fixture into the 4 jaw, indicating it in, then loading the pipe im working on and spending 45 mins to indicate the bore in. So I removed one of those steps

I picked up a D1-4 steel backplate, turned it down, cut a weld prep on my bar stock and cooked in a nasty hot bead. I then turned the fixture down to my wanted dimensions. Went to the mill to get some flats, and a series of 3/8X24 holes for my jack bolts. (brass tipped set screws)

Lining up my witness marks on my single and the backplate/fixture Im getting repeatable <.001 runout on the fixture bore itself

Pros

Fast ... cam lock it on and im ready to indicate in my bore

Allows me to "8 point" indicate parts that are too short to run through my headstock (rear spider and front 4 jaw chuck)
its short, and tight up to the spindle to maintain rigiity. Anytime I was using my traditional fixture in my 4 or 6 jaw, it would end up hanging almost 30 inches out from the spindle after considering the depth of whatever chuck it was in ..

adding another set of jack screws I can indicate pistol barrels now

Cons

cost - back plate was close to $200

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Praeger

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This type of barrel fixture has been around a while. I'm sure many gunsmiths made their own version before Viper and Grizzly began to produce them. Your's is the first I've seen welded to a back plate rather than chucking in a 4 jaw.
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Easy_E

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Been planning to do the same for a while but plan to install the D1-4 pins on the steel tube so theres no seam . Not sure if it makes a difference other than seeing my welds .
 

FisherT&C

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If a barrel is too short to use the spider and the muzzle is threaded you can make an internally threaded bushing with a concentric O.D. to match your spindle bore. Vice versa with the tenon threads if there are any. Just another way to skin the cat.
 

dustingaunder

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If a barrel is too short to use the spider and the muzzle is threaded you can make an internally threaded bushing with a concentric O.D. to match your spindle bore. Vice versa with the tenon threads if there are any. Just another way to skin the cat.
This is what I do. My headstock needs about 28-30” with my 4 jaw. I have a spider chuck that works with a few less inches. I prefer the 4 jaw with a ring of 8-10ga copper wire so I just make more extensions. For a new blank I can thread the muzzle end between centers since I’m cutting it off before I’m done anyway. Only need 4-5 threads to hold the extension in the outboard spider while you work on the chamber end. After the barrrel is fit I use another extension on the chamber end and cut a proper thread on the muzzle.
 

AnimalMother224

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I thought about the thread and bushing the muzzle end for doing chamber work, but when I start with a short blank im still back to square one. I was using a shop built fixture Praeger posted for a while. Now its just REALLY nice to slap in the D1-4, crank the cams and load the barrel. So far ive cut a number of barrels and havnt had a single issue.

The only thing I might change is I might try to run my knurler tool to give the backplate some texture so when im rotating by hand its easier to grab. the surface finish I left on the back plate is glass.
 

mcfred

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Looks good, thanks for sharing. I had a similar approach for my older, larger machine, I just don't have a welder. It's done pretty well for me too. If it doesn't work then it'll work real well as a boat anchor. :) If needed I'll add more screws for things like pistol barrel work later.


 

mcfred

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Borris the Bullet Dodger said:
Weight is a sign of reliability. If it doesn't work, then you can always throw it at them.


Larger OD's are more rigid, so there's less tool deflection. Mass helps surface finishes too... ;D