Truing barrel in CNC chamber Setup

jackie schmidt

jackie schmidt

New member
Much of the discussion in chambering barrels centers around the specific points that are indicated true so subsequent machining operations will run truly straight with these points.

In a CNC Setup, how is this accomplished?
 
I use an adjustable 16C collet chuck, same as I would a 6 jaw set-tru on a manual. Because the muzzle doesn't exit the spindle, the other end, I'm a bit more tight-lipped about due to the amount of time I have into developing it. It serves to center the bore and also an attachment for my through-coolant. It's very simple in principal.
 
jackie schmidt said:
Much of the discussion in chambering barrels centers around the specific points that are indicated true so subsequent machining operations will run truly straight with these points.

In a CNC Setup, how is this accomplished?
Click to expand...

Nowadays some guys use one of the adjustable chucks or a double spider and indicate with the "Gordy Gritters" Method. A bit more old school is to set up the blank between centers and take a light truing cut on the tenon, then use a freshly trued soft jaw chuck to chuck on the true tenon surface. Since most NC lathes have long headstocks it usually takes a little ingenuity, the method you have described using in earlier posts would be perfectly serviceable. What I'm getting at is that the setup and indicating is pretty much the same as for a manual machine and is tailored to the machine you have.
 
jackie schmidt said:
It appears they are setting barrels up in a similiar way I do in my long headstock Pratt & Whitney.
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I'm not sure your method, but if a spindle can spin at 6k RPM vibration free, I'm thinking it's probably pretty concentric inside. Probably a decent place to use as a center 2'+ from the chamber...
 
Rubicon Prec. said:
I'm not sure your method, but if a spindle can spin at 6k RPM vibration free, I'm thinking it's probably pretty concentric inside. Probably a decent place to use as a center 2'+ from the chamber...
Click to expand...

I like the idea of an adjustable dead center combined with through coolant. The guys I know using CNC to do barrels generally cut the tenon, thread, pre drill, and rough the chamber with an NC cycle and then finish ream manually feeding the reamer with the tailstock. One guy adapted a small tailstock from a small manual lathe to mount on the saddle of his HAAS TL for finish reaming. The standard tailstock is optional on the HAAS toolroom lathes and is so large that there is little "feel" with it. Another guy mounts the reamer in a tool holder on the tool post, indicates it straight and true and feeds with the saddle. This method has absolutely no tactile feedback because the handwheel merely operates a servo.

The cycle runs only a few minutes and it is possible to rough the entire chamber including the neck and shoulder so that only a very small amount is left to finish ream. Some rimfire smiths finish the chamber with a boring bar and don't need a reamer.

It is possible to do very high quality fitting and chambering with CNC and for shops with batch orders it seems to pay off. The HAAS tool room models run about $25000 to $75000 depending on options, so it takes quite a few barrels to pay for one.
 
I'm asking this for a friend. He has a small Romi 420. He had a contract making hydraulic fittings and small high pressure valve parts, and he lost it. He was thinking about getting into the AR barrel game, he shoots AR's quite a bit, and knows that crowd around Houston.

His Romi has a independent 4- jaw chuck on it. The spindle hole is a tad over 2 inches, and as far as he can tell, it runs true with the spindle.

He has watched me do a barrel in my Pratt & Whitney manual Lathe, which has a long headstock. He figured if he set the barrels up the way I do, it would take longer to set the darn thing up than the whole machining operation.

His idea is to do all of the tenon work, rough drill the chamber, bore it dead true, and use Carbide chambering Reamers to finish the chamber.

I think the manual tailstock will let it take about 36" between centers.
 
The CNC lathe I use for my barrels has a 2-1/16" spindle bore. I built several delrin bushings that are a few thou smaller than the bore and about 3" long. It has a 1/8" o-ring about 1/2" from either end. I have the center step bored in a way that the bushing will make contact near its center and lock on to the outer barrel taper near the muzzle end.

I made a spider like piece that can clamp in a 3-jaw chuck with four cross bolts for centering the chamber end of the barrel. I use pieces of copper pipe and saw a slot on a band saw to make a split bushing. I expand that over the barrel for the bolts to make contact with.

I first use rags and a cleaning rod to clean the spindle bore. Then wedge the bushing on the muzzle end. Slide it into the spindle bore through the chuck. Slide the spider over the chamber end of the barrel and clamp in the chuck. I roughly indicate the bore and lock it in. I rough turn the end and over the threads and rough drill the chamber. Then I finish indicate at the throat. Then finish turn, thread and bore.

I had to do a barrel once without much taper on the muzzle end for the bushing to lock on to. I put an internal o-ring in my delrin bushing after boring it close to the barrel diameter and it worked very well.
 
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