Methods for truing up jaws in a lathe chuck

HMF

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You can restore the jaws to quite respectable condition:

Open the jaws to a given diameter, large enough for a grinding wheel to enter the opening. It need not be a large wheel, a mounted point (3/4" or so) will suffice. When you have an opening that is acceptable, use either a washer that is an acceptable size, or make a thin disc of the proper diameter that can be chucked at the very deepest part of the jaws. It should be round, not irregular in shape. Make certain that the disk runs true (no flutter side to side) when you tighten the jaws, so the pressure is at right angles to the spindle. Allowing it to lean will generally yield a poor job because the jaws will not be loaded properly against the slides. You'll grind up to the disc, then kill the miniscule area after grinding.

An older chuck has usually experienced considerable springing, so the jaws grip only at the rear. By loading the jaws as suggested, the jaws will spread to the sprung condition, leaving the jaws out of parallel. Your grinding operation will then make them parallel, so they grip front to back in use. You load the jaws as suggested so they aren't free to move about while you're grinding them. The disc will load the jaws such that they will be held captive, generally in the general position they occupy in use. That provides for a correction of eccentricity as well as correcting the angle of the jaws.

It is recommended that you seek the best of the three holes in your chuck, then mark it. By using that hole when tightening the chuck, you'll minimize the amount of error in chucking. Don't expect an old chuck to repeat within anything less than .003" as you move through a range of sizes, but it should grip squarely after you've ground the jaws.

It's a pretty tall order to expect tooling to stand up to the rigors of the interrupted cut and the hardened condition of the jaws. Even if you could get the jaws to machine, you'd be hard pressed to get them machined from front to back without considerable grief. You'll be far better served to try grinding them. If you don't have access to a tool post grinder, you might investigate the possibility of using an air or electric grinder firmly clamped to the toolpost by some means. I've done that with reasonable results, even though they don't have high precision bearings. As long as you're not trying to hold tenths, you might be pleasantly surprised at the results.

Other methods?
 
Something else to look for is a warped backplate. I have a Bison 6 1/4" 3 jaw that came on my 10K when i got it 6 years ago. It was so so on how true it has run. But lately it is wobbling real bad and i can see the line where the chuck body bolts to the backplate moving. So today i couldn't stand it anymore so i unbolted the chuck and took 3 nice cuts off the backplate. I also removed the jaws and cleaned the crud off the scroll and oiled everything (and yes i runied another tee shirt) I rebolted the chuck to the backplate and the wobble was gone. The parting line runs true. Someday i will prob grind the jaws a little. But for now its pretty good...Bob
 
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