I have never been quite happy with Gordy's method of indicating in a barrel so I tried something a bit different yesterday. First I indicate the outside of the barrel at the chuck jaws and at the spider end (muzzle) of the barrel. Once I get it running true there I switch to indicating using the range rods with the reamer bushings and once again indicate the barrel true at the chuck jaws. Then I would withdraw the rod and indicate again near the end of the barrel. This frequently gave me odd readings and TIRs that seemed too far off to be only 2 inches from the last reading. Then I got to thinking that being the end where the button or rifling cutter was started it was likely that the beginning part of the barrel might not be good enough to indicate off of and may actually give bad readings. Yesterday instead of indicating off this section of the bore I pushed the rod FURTHER into the bore and took an indicator reading about 2 inches further into the bore instead of at the end. I got a much more consistent reading off the range rod than I was accustomed to getting at the end of it. So good in fact that I didn't do any further adjusting at the spider end. Anyone else tried indicating farther up the bore rather than at the end of it?
Clowdis,
Firstly, if you can, cut about 1" off the breech end of the barrel before doing anything else. Deburr it, etc. . I find with alot of button rifled barrels that they are tight at the very end, then loosen up (from honing and lapping) about an inch or so into the barrel bore.
Secondly, erratic readings can be due to having a too tight pilot on the 'gordy range rod', a too loose pilot, or mis-formed lands (more common on buttoned barrels). Or else your headstock bearings are just not up to the job, and causing in-consistent readings. Remedy for that is try tightening up the main spindle bearings, or get a new/better lathe.
Thirdly, if you are using the 'gordy/grizzly' type range rod that is loosely supported in the drill chuck, they are about 12" long. When you are checking the very breech end of the bore, and you have the DTI stem right up close to just behind the bushing, you will be pretty much reading about what the movement of the pilot bushing is. BUT, when you push it in 2 inches into the bore the readings on your DTI will not be 'true', you will be reading about 10/12ths of the actual movement of the pilot bushing. Go in another inch you will be reading 9/12ths of the actual movement. Therefore, the further in you go, the less run-out you 'perceive' you are getting.
That is assuming I have interpretted what you are saying correctly???
Cheers,
Dean.