I have a question about centering a dead center rest. When I put a rifle barrel on my lathe, dead center in bore and cut from one end to the other I'm about 6-1000th small at the dead center end. Ok, I understand I need to adjust half that away from tool.

Question is.....My dead center can only be adjusted horizontally, bit my problem could actually be vertically couldn't it? I have a reason to know which way it's off other than just getting a straight cut so how do you center the dead rest?

Hope that makes sense, I'm self taught and don't know the terminology of machining.

Vertical center is not adjustable, and can't get out of whack, unless the way's are worn super bad or the base of the tail stock is, not likely. Make sure the way's are clean and lubed with a light machine oil. Use an indicator on a magnetic base to adjust the center. Do you have a live or dead center, and is it true. Chuck up a dowel pin or something with a ground finish and check the head stock too.

I believe the issue you're describing is TIR- Total Indicator Run Out from end to end? One end of the outside diameter is .006 smaller than the other end? Can you install and indicator on your saddle and check the run out. Do you have a morse tapered chuck or are you using a 3-jaw chuck and a tailstock center, if utilizing a 3-jaw chuck you could also turn a new dead center from a piece of scrap.

First off, thanks for your replys. I'll try to answer you both best I can.

I have both, a live and dead center, if by live your saying one that spins. I marked them and put them in the same. I have a 3 and 4 jaw Chuck.
Yes, they are tapered (the chucks and centers) and like I said before, I mark them so I put them in the same. Chucks are key wayed and centers have a flat at end of taper like a screwdriver. I can't see much difference between the the two centers. As for the chucks, the 3 jaw runs out .002.

Yes Eagleye, Chuck end is .006 larger than dead center end. If it shouldn't be off vertically, I'll just bump it away.

Its been a long day ,but as others have said your vertical will be out if you have alot of wear ,but should not cut tapers, you can adjust taper by the tail stock bolts on the sides of your tailstock, theres good you tube videos explaining it, the length your cutting matters to, if your building heat it will cause this over longer cuts. i almost forgot you could have some backlash creep in your cross slide.

Your compensation for the run out is determined by the length of the barrel- ie; to be out .006 on a diameter or .003 per side your margin of error is only .01 degrees over a 24" length. I would make adjustments with gage blocks or you might be chasing this for awhile. Make an adjustment and zero the indicator on the larger dead center end of the barrel- if you can fit a .003 piece of shim stock or feeler gage in between the indicator tip and barrel end opposite the headstock... you're dialing it in.

And if your cutting tool is a little dull, you could end up with both ends near the center/chuck being a little smaller than the middle of the cut due to deflection. And wear on the ways could cause taper as well. Typically more wear near the head stock than 2' away from it.

You could also take a final pass starting at small end and gradually feeding in .006 as you cut to big end. I do this a lot.

My worry is like said above, chasing it. Lol

I'll try just loosening the lock bolt and just bumping it without turning the adjusting bolt.

hippie,
larrywaugh could be correct. We had a rebuilt lathe in the shop where I retired from and the machinist who ran it all the time always had to adjust the tail stock to compensate for the taper it was cutting.

Could be, 99% of the work done on this machine is within a foot of the head so the bed could be worn some there compared to 3' down to bed.

This is an old, large machine and could even be a slight twist in it as we just set it on floor. Been using it for years, just never made long cuts like this. Mostly just used to make small parts, but been doing some barrels the last few years tinkering.

Probably overly complicated for what you want but just to give you an idea.

Thanks Finster, hadn't seen that one. I did look a few up last night.

You can try putting an indicator in your chuck, and indicating the center in the tail stock. this should show you how far off center it is. Depending how much wear is on the machine you may have to adjust your horizontal on the tail stock, and shim the vertical. Older machines are finicky.

Gonna try some things tonight.

I moved tailstock .002 and after thinking about heat someone mentioned, I'm thinking the tool would have to grow the other .001.

I'll have a fresh barrel to try once it gets here. I'm not messing with the other one that's done, that doesn't hurt it any. I just want to get better aligned.

I checked a bull barrel that I bought finished from a popular barrel manufacture and it's off .0045 in 26 inches.

Next project is putting a tapered contour on one!

Bbl OD and axis of the bore being “out “ .003~.005 is common. Install a few threaded muzzle brakes and you’ll see what I mean. Sounds like you need a couple of standard tools for squaring a lathe one a machinist level and a center ground centering bar. In order to get high level precision the lathe ways must be level front to back and from headstock to tailstock. A book on basic lathe operation would be a plus. Brownells has these things. They aren’t cheap but most things worth doing right aren’t either.

I have the axis and od. good, just that I have the slightest taper of the od.

I understood your point from the original question about taper. The solution to that was answered as well as the question about vertical misalignment in previous posts. My comment about OD vs ID was in response to your comment about the finished bbl having.0045 run out. Perhaps I misunderstood your statement about the.0045 and you were talking strictly OD.

Yea, I didn't explain that much.
Yes, I'm talking od being tapered after turned on center. The bought barrel was turned on center by them and is also smaller at muzzle. Bore is centered on both, at both ends.

I didn't check how far off bore was before turning on center, but it had to be a couple thousandth. They say that even the best boring methods have a little drift so raw blanks are probably gonna be off some.