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View Full Version : Dialing in a Barrel Bore



parshal
07-28-2016, 08:13 PM
I just ordered a reamer so I've got plenty of time to figure out how to properly set up a barrel for chambering. I've made a spider out of steel with two sets of four bolts to hold the barrel and my steady rest is around that. I can dial in the spider as well as the outside of the barrel to less than .001". When I use a dial indicator inside the bore I get nearly .003" of runout measuring the grooves. The strange thing is that three of the grooves are within .001" but the other two are .002 to .0025" off the others. I've not cut any barrel off the chamber side so I'm wondering if the discrepancy is due to the rifling process and that this is not uncommon. This is a Bartlein barrel and I'm using a Mitutoyo 513-504 inside the bore so I can only reach in so far.

Mike Bryant
07-28-2016, 08:28 PM
Indicate in on the bore. The outside of the barrel is irrelevant. However, you do the indication, you can only indicate in a barrel at two points. Where you choose to indicate in the bore is the million dollar question. Just about everyone on this board that indicates a bore will indicate at the projected throat at the chamber. Where you choose to indicate for the other point can be either at the muzzle end getting the throat running true and the muzzle running true or you can indicate at the projected throat and at the start of the chamber letting the muzzle swing out where ever it does. Lots of good shooting barrels have been done either way. As well as lots of good shooting barrels have been chambered never using an indicator, chambering between centers using a steady rest. So, pick how you want to do it and see how it shoots. Next barrel use the other method and see how it shoots. Not all methods work well in all lathes. Shorter or longer headstocks, shorter on longer lathe beds can determine how you go about it. You just have to figure what method works the best in your lathe and get after it.

CMaier
07-28-2016, 09:03 PM
did you read the stickie's on chambering ??

parshal
07-28-2016, 09:13 PM
Thanks Mike. I guess I should have provided some more details. I'm using a SB 10K so I don't have a large enough spindle bore so this is between centers. I initially put the barrel between centers and cut the muzzle end down to 5/8" and flipped barrel around with the muzzle end in a collet in the headstock. So, I'm hoping that would put the muzzle end concentric but I have no real way to tell since I can put it through the headstock and measure it on the opposite side.

When I set it up with the muzzle in the headstock I put on the spider and a live center in the chamber end. I indicated in the spider to try to get it concentric (good practice even if it's not necessary). I set up the steady before removing the live center. I then dialed in the outside of the barrel before the bore so I could at least get it close. Using the test indicator as far as it will reach (it's not terribly long) I can get the grooves within .0015" of each other. Note, I tried it as far is it would go after I started this thread.

Since I can't indicate in the muzzle and I can't reach far enough to dial in the throat I'm kind of stuck with where I can measure and that's only about 3/4" into the breech. Am I missing something obvious?

parshal
07-28-2016, 09:14 PM
did you read the stickie's on chambering ??

Yes, I did. I've read so much I've probably forgotten most of it! I no longer see the sticky at the top of this forum, though.

CMaier
07-28-2016, 09:42 PM
i think mike said to indicate off the BORE, not the lands.
see if that is more consistent.

parshal
07-28-2016, 10:02 PM
That's where I'm indicating, the bore, not the lands.

jackie schmidt
07-28-2016, 11:38 PM
Thanks Mike. I guess I should have provided some more details. I'm using a SB 10K so I don't have a large enough spindle bore so this is between centers. I initially put the barrel between centers and cut the muzzle end down to 5/8" and flipped barrel around with the muzzle end in a collet in the headstock. So, I'm hoping that would put the muzzle end concentric but I have no real way to tell since I can put it through the headstock and measure it on the opposite side.

When I set it up with the muzzle in the headstock I put on the spider and a live center in the chamber end. I indicated in the spider to try to get it concentric (good practice even if it's not necessary). I set up the steady before removing the live center. I then dialed in the outside of the barrel before the bore so I could at least get it close. Using the test indicator as far as it will reach (it's not terribly long) I can get the grooves within .0015" of each other. Note, I tried it as far is it would go after I started this thread.

Since I can't indicate in the muzzle and I can't reach far enough to dial in the throat I'm kind of stuck with where I can measure and that's only about 3/4" into the breech. Am I missing something obvious?

It's not uncommon to find spots in the ID of a barrel that will be as much as .001 out from a spot just a inch or so away.that is what you are experiencing.

When you indicate the spider on center, that means it runs true with that one spot. Farther up in the barrel will not be true.

Many of us have stated many times that bores of barrels are not straight with their own selves. That is why many of us strive to indicate two chosen points that we believe are the most important points, and then single point rough bore the chamber to those two points.

Good machine shop practice dictates that when dealing with a a piece that is not straight, you can only indicate two points true. To get anther point true, you must single point it.

Treat your steady rest spider like a chuck. Dial in the barrel as far in as you can reach in, then rough drill it so you can reach in with your indicator and redial in the throat area. Then you can single point the body of the chamber with you compound offset the proper amount to accept the reamer about 1/2 way in. Then finish ream the chamber.

I hope this helps.

alinwa
07-29-2016, 12:23 AM
It's not uncommon to find spots in the ID of a barrel that will be as much as .001 out from a spot just a inch or so away.that is what you are experiencing.

When you indicate the spider on center, that means it runs true with that one spot. Farther up in the barrel will not be true.

Many of us have stated many times that bores of barrels are not straight with their own selves. That is why many of us strive to indicate two chosen points that we believe are the most important points, and then single point rough bore the chamber to those two points.

Good machine shop practice dictates that when dealing with a a piece that is not straight, you can only indicate two points true. To get anther point true, you must single point it.

Treat your steady rest spider like a chuck. Dial in the barrel as far in as you can reach in, then rough drill it so you can reach in with your indicator and redial in the throat area. Then you can single point the body of the chamber with you compound offset the proper amount to accept the reamer about 1/2 way in. Then finish ream the chamber.

I hope this helps.


WELL SAID!

as usual

tx

parshal
07-29-2016, 10:22 AM
...Then you can single point the body of the chamber with you compound offset the proper amount...

Do you mean offset so as to create a taper in the chamber?

jackie schmidt
07-29-2016, 11:23 AM
Do you mean offset so as to create a taper in the chamber?

Yes. On most Benchrest cases, it's a tad over 1/2 degree on the compound on my Pratt & Whitney. I established this years ago and put a permanent mark on the degree scale.

Here is a picture of the drill I use to rough out the chamber, (.405 diameter), and the boring bar I use to single point bore it. Note the 30degree angle.

http://benchrest.com/attachment.php?attachmentid=18084&stc=1&d=1469806285

parshal
07-29-2016, 11:31 AM
Thank you everyone!

Mike Bryant
07-29-2016, 08:36 PM
The most important thing when chambering in a steady rest is to have a good true center. I've been told to take a 60 degree center reamer and run it in to get a center started, then push the end of non piloted side of the center reamer over to the side. Not sure how your spider is set up on your steady rest, but pretty sure that with a steady rest you can adjust the points on the steady rest where your barrel is running concentric but be off of the centerline of the lathe. The spider would have to be swept with a dial indicator mounted in the headstock to make sure that the spider is concentric with the lathe centerline. I guess that you have a steady rest with bearing mounted in a homemade type steady rest that has a a spider built into the bearing mechanism. With this kind of mechanism, it would be the same as indicating a barrel with a spider mounted on the front and/ or rear of the headstock. Just takes a little time to get the spider adjusted to get the bore running true. Sometimes a spider can be persnickety. You make an adjustment and it moves farther than you want. Really hard to say for sure without seeing your setup. Like is said in previous posts, no one way fits all you just have to figure out a method that works with your lathe and set up. Any of us on here will be glad to help you out anyway we can.