Bore Centering

[Investigator]

Another late addition to the thread. But what about using a copper wire in the chuck instead of the aluminum. Might make it easier to pivot and not put it in a bind:

 

[Tozguy]

Maybe we are talking about two different animals. I use a "Gritters" type rod with interchangeable bushings in .0002
increments. The rod fits snug but moves within the bore. Move in adjust, move out adjust. Your method is just different than mine. i am trying to indicate the bore in more than one position

I think so. I am very familiar with a range rod but not so much with a Gritters rod.

 

[epanzella]

Never heard of a "gritters" rod but it seems to me if you can slide the rod in and out of the bore you're introducing another source of runout.

 

[Jubil]

I think the right name is Grizzly Rods instead of gritters rods. And I use the copper wire same as Investigator does above.

Chuck
PS: Just for information purposes. Grizzly rods designed by Gordy Gritters of Gordy’s Precision LLC

 

[Tozguy]

Found this video by Gordy Gritters on using the Grizzly rod, interesting!

http://accurateshooter.net/Blog/grizzlychambering.mov

 

[killswitch505]

I like the Gordy/grizzly rod method the best makes the the most sense to me.

 

[Scott-ak]

I am surprised that only one person commented on the aluminum pads under the chuck jaws. The chuck end must be able to pivot else the barrel is being bent and I'd bet that's what you are seeing under power. Use something like investigator shows. I use a ring of large bare copper gound wire from the hardware store. I've seen others that use short pieces of 1/4 dia aluminum rod between the chuck and barrel. What's important is that whatever is used it must be round not flat

 

[Earl Horgeshimer]

I own a relatively inexpensive asian lathe that appears capable, based on results, of producing accurate barrels. No doubt my question, and, possibly my methodology, will underscore my lack of experience in machining and lathe work in particular.

I use the 4-jaw chuck and some aluminum shims to hold the barrel in the head stock. An outboard spider is used to support the muzzle end of the barrel. I have been using PTG range rods with appropriately sized bushings to center the bore prior to cutting and threading the tenon. Starting with a .001 gauge, I adjust both the 4-jaw and outboard spider to obtain the smallest practical runout of the range rod both close to the tenon and at the end of the range rod furthest away from the tenon. After there is less than .001 runout I switch to a .0001 dial indicator. It is a Mitutoyo 2803S-10. This is not a expensive indicator but it is a decent one--I believe. I continue adjusting the 4-jaw and spider until I get less than .0002 of runout close to the tenon and as far away as the rod allows. All of this adjusting is done with the lathe out of gear and spinning the 4-jaw by hand.

Here comes the question part: I expect that when I run the lathe under power that I will get the same runout measurements but, that is never the case. The runout is always much worse under power. Short of buying a better lathe, can anything be done about that and if anyone else is experiencing this, what workarounds are you using that you are willing to share?

I assume this is a result of inexpensive bearings and other mechanically sloppy tolerances. I have 'resolved' or worked around this issue but writing numbers on the tenon with a permanent marker and using my phone to video the lathe in operation in slow motion. I play back the video and make the necessary adjustments to get back to less than .0002 runout under power. I do always wonder, though, which measurement to believe: The ones where I turn the chuck by hand or the one under power. Given that I cut, thread and ream under power, I have chosen that that set of measurements to get 'right.'

Thank you for thoughts,
Henryrifle

I have found that I need to use an indicator tip of at least .080 in order to see steady readings under power. Smaller tips have issues going over the grooves in the barrel and the indicator reading jumps around a lot. I also turn the VFD down to around 15 -20 rpm. I use a Best-Test .00005 indicator with .080 ruby tips for indicating new barrels, I use the Mitutoyo .0001 indicator with a .080 carbide tip for indicating already chambered barrels when I set them back. The Mitutoyo indicator goes right up inside the chamber so I can indicate on the bore rather than indicating on the existing chamber.