how to center a bore in a lathe?

[king nero]

if you throw the barrel in the jaws of a lathe, how do you know that the bore itself sits in the true center?
Jaws are selfcentering, allright, and if the barrel is a perfect circle (which they usually are), it's perfectly centered.

But if the bore is just a bit of the true center (I mean, bore and barrel not perfectly concentric), how to check for this?
Do you put some gauge in the bore, and go by this? Or how to make such a precise set-up?

thx in advance...

[savageT]

if you throw the barrel in the jaws of a lathe, how do you know that the bore itself sits in the true center?
Jaws are selfcentering, allright, and if the barrel is a perfect circle (which they usually are), it's perfectly centered.

But if the bore is just a bit of the true center (I mean, bore and barrel not perfectly concentric), how to check for this?
Do you put some gauge in the bore, and go by this? Or how to make such a precise set-up?

thx in advance...

king nero,
 I think I can help...........To begin with, you said that if you place a barrel in a lathe's jaws they are selfcentering.  This statement is not necessarily correct.  A three-jaw chuck is geared together so all jaws are self-centering (more or less).  However, most machinists will use a 4-jaw chuck.  These have independant jaws so that when you rotate the barrel you place a dial indicator (that's a device to measure the run-out of concentricity.  It's mounted on the toolrest, and has a finger that rests against the barrel surface).  With a four-jaw chuck, you get a much better rotation alignment.  Now if the bore is truely lined up or concentric with the outside of the barrel, you can next place the dial indicator on a bore spindle that fits tightly to the bore and check the run-out for the bore.  The reason for the spindle is because the bore has rifling and would cause the dial indicator to jump around when you rotate the barrel.

Hope I answered your qustion......Jim

[king nero]

Ah, all right.
Yes, I was talking about three-jaw chucks. And I know about that dial indicator, so far so good.

I'd like to know more about that spindle you put in the bore...
Is that a special gunsmith tool? Or something I could easily make myself?

I have access to lathes with both three-jaw and four-jaw chucks, and am happily willing to make such a thing, I suspect it's nothing but a (brass?) rod with the correct outside diameter to fit snugly in the bore?

thanks in advance,
and pls. forgive any spelling/grammatical errors as for tech talk, I'm not quite knowledgeable... dutch is my mother tongue...

edit/ It's meant for a .22RF Cal. rifle, dunno whether or not that's worth measuring out as the bore diamater will be very small,
and compared to the barrel diamater it'll most likely be perfectly centered... any opinions/advice on this are welcome!

[savageT]

No actually.........You're doing fine.  About the spindle; a brass spindle a bit undersize on one end and a bit oversize on the other with a slight taper would allow you to place it in the bore tightly.  If you are concerned that the bore is out of alignment to the O.D. of the barrel, you should be able to detect that with your eye initially.

Since this spindle is something you'll need to make (without damaging the rifling) I would suggest checking with Brownells @ www.brownells.com   1-800-741-0015 to see if they make a bore spindle for that purpose that you might rent.  I know there are such things as expandable reamers available, but again you need to be careful about damage to the bore.

Can you share with us why it is you want to check run out of the barrel bore?

Jim

[gunnut69]

I most often chuck a dead center in the chuck(4 jaw) and with the indicator set it to run true. The barrel is mounted between the dead center in the chuck and a live center in the tailstock. Use a lathe dog to pull the barrel and make a trueing cut on the tailstock end(breach end) and install a steady rest.  The barrel can then be checked but if you've done everything correct, it is running true..  Since the truing cut made the outside of the breach end of the barrel true it can be swapped end for end and just chucked in the 4 jaw and centered with the dial indicator while the muzzle end is set up in the live center.  The barrel can then be contoured.  Remember to use a spring loaded live center or to simply true a spot on the barrel and use the steady rest again. It is very easy to warp a barrel with the heat generated by the lathe tool and the barrel between centers.

[savageT]

gunnut69,
Thanks allot for the approved method for turning a long piece like the barrel.  It's been about 50 years since I played on one of those lathes.  At that time in industrial highschool I believe we were making a drill press and recall trying to turn the cast iron center column shaft like that.

Jim

[king nero]

OK, thx very much for the explanation.
I' asking this because I'm playing with the idea of making a rifle starting from scratch, with only a new barrel as the only part I can't make on my own. But I'd like to recrown it with a match crown, using a lathe. Hence the bore should be perfectly centered in the lathe.

I've drawed a trigger/hammer mechansim unlike most fabricants use (doubtful that it's ever been used before), and as I'm doing mechanical engineering, I have the opportunity of actually realising my drawings.

The setup is really simple, and initiially it's going to be a break-open gun. I've been playing with a loading mechanism of a pistol type, which (when manually operated: you pull the slide going backwards) is also a very doable project. We'll see if my idea works in the real world (drawings are always nice to see, aren't they )

Thx for the advice!

[gunnut69]

Sounds like fun!!  Check back once in a while to let us know how your doing..