I did have the figures, but if you look at Workholding in the lathe by Tubal Cain, he goes into the errors in 3 jaws.
I think, for a standard quality 3 jaw, using the preferred pinion, about 5 thou is all that they guarantee new, at a distance form hte face. See the appropriate DIN standard which many work to..
If you were .020 out then either the chuck is not set properly in its backplate (unlikely in view of its other results) or possibly you had a flake of swarf in somewhere – possibly even internally. Or maybe you didn’t use the preferred key.
Could one expect repeatability of .002 every time (using any key?) No. (unless you were lucky)
I have a brand new Pratt Super Precision chuck. The error on that varies between 1.27 thou to about. .0004″ using the preferred pinion on bars. About 1/2 thou on 100 mm rings on the same preferred pinion.
Thats a new instrument of a grade somewhat higher than the standard chuck (and it had to be reworked twice to get it into the SP tolerance so it took some weeks to be delivered)
Would you do better with a new chuck from RDG and ARc euro. Possilby – BUT, what are you to gain? No one in their right mind takes a bar and puts it into a 3 jaw and regards it as true. If you need a bore concentric with a circumfenrence, you either turn them at the same setting, or you reset in a Griptru, collet or 4 jaw.
For rough work, yes you can turn end for end or reset, but if it matters, as in an axle tube with bearings at each end and you want to be able to slide and axle through, then no you most certainly cannot, not even with an SP chuck, unless you are very lucky, because the error is greater than the reaming allowance oversize.
Edited By mgj on 29/05/2011 22:08:22