What is your setup tolerance to bore?

[iraiam]

I was wondering what the average tolerance is when dialing in on the bore of a barrel for cutting crowns and other work on a lathe.

I have been using 10um (.00039") as a tolerance to stay inside of, this is about as accurate as I can get on my equipment, I tried 7um (.00027") once but I spent quite a bit of time setting it up, I think I can build a better barrel holder and get is dialed in there.

I'm using micrometers because that is the most accurate dial indicator that I have, I have one that the resolution is .0005", but I find it easier to use the finer resolution to set up because the gauge moves more.

 

[HiBC]

I use a 4 jaw chuck,with a spider on the back end of the spindle.I put a best fit pin gage in the bore,and rest the contact tip of a Browne and Sharpe test indicator on the pin.The indicator has .0005 grads.

I indicate in both ends of the barrel.I have some practice at doing it,its not hard to get a TIR (total indicated runout,maximum + to maximum -) inside .0005.

Getting the last .0002 or .0003,well,depends on how much time you want to spend.Around there I say "Close enough"

 

[Bart B.]

Any setup that makes the muzzle turn on bore center; exactly.

That's most easily done by fitting a centering pin in the bore with its diameter exactly what the bore diameter is, putting that in the tailstock center, then turning the outside diameter so its surface is equidistant from bore center. Then put the muzzle in a 3-point steady rest so it turns exactly on bore center to face and crown it.

HiBC's approach works. As does others depending on their preference.

 

[Clark]

I buy between sizes pin gauges that fit in the bore from MSC.

I make a gimbal from heavy Copper wire in the 4 jaw.
I adjust the spider where ever it needs to be to get the pin [spud] that is just to the right of the gimbal to be concentric and parallel with the spindle.

I do the same for the crown and the chamber.

Here are the two pin gauges I use for .223 and 6mm. They are ~ $3 each.

With an Intrepid .0001" test indicator I can get the bore very close, but I no longer believe that concentric chambers have anywhere near the importance of concentric ammo. Ammo can be chambered with any rotational orientation, but the chamber error is always the same rotational orientation. I chambered a 7mmRM .005" off center and it shoots great.

 

[iraiam]

I use ground pins that fit the bore to dial indicate to, my dial indicator is a Mitutoyo 2109S-11, accurate to .004 mm (.00015"), although I think actually indicating to within .00015" is probably beyond the tolerance of my machinery.

I either use a 4 jaw chuck, or I made a barrel jig that goes on in place of the chuck and has 2 sets of 4-way brass tipped bolts as adjustments. I was thinking of re-making the barrel jig with fine thread bolts, to hopefully make it easier to dial in closer.