Cushman chuck holding stock eccentrically... No more!

Just a 3-jaw Cushman on my 10EE, plenty old but in much better than usual condition. Well, "better" except someone replaced the outer jaws with new ones.

Chuck holds/runs nice and straight, chuck itself showing very little runout, but holds stock consistently .010 eccentric.
I know I shouldn't expect much from a 3-jaw. It just becomes annoying when I flip a part and the chuck repeats that .010 runout. (Again, I know it is a 3 jaw; these aren't precision parts).

At the end of a long Saturday I decided remove & clean jaws, clean scroll and chuck jaw slots, reassemble and measure TIR from a straight shaft chucked. No improvement.

Then I started swapping jaws, keeping track of swaps as I progressed. First switch, no improvement.
Second swap gave .008 repeatably. Third swap, .006 and fourth stayed the same. By this time I was sure that was max improvement.

Went ahead with 5th possible placement of jaws and got .002 repeatable! 6th position got me a consistent .001-.0015! What a pleasant way to end the day.i was absolutely sure the moving of replacement jaws would make no difference.

Now I need to mark each jaw with new numbers, and cross out the original 1,2 and 3 numbers the jaws came with. I guess the previous owner figured installing the numbered jaws in their respective locations was the only right way to install them.

I'm happy with this outcome! No need to buy a better chuck!

My outlook on life has drastically changed!
Every time my skeptical, cautious attitude kicks in, all I have to do is think about the rebirth of my Cushman chuck, and clouds disperse. I am thinking about beautiful flowers, ice cream and brown trout right now, as I write this. WooHoo!

Thanks for sharing. A good reminder not to trust the previous assembler's marks.

When you say repeatability, do you mean repeatability across multiple grabs on the same diameter? Or do you mean accuracy of the chuck across multiple, differing diameters?

I ask partly because I have an 8" 6-jaw Kalamazoo chuck which I bought new. It repeats fine (.0005 or better) on the same diameter, however differing diameters will have up to .010 or .015" of runout. You're making me wonder whether the jaws were assembled in the wrong order at the factory. Being a 2-piece jaw chuck, I've never had reason to remove the master jaws and check.

You've got me planning a disassembly of the chuck next time I'm at my shop.

I need to check different diameters. I do know it has visually been way more stable with larger sizes of bar, but I should actually measure.

Why not go the whole way, Grind the jaws while the chuck is mounted. You need to first remove the jaws, and grind about 1/4" length off the inside end of the jaws surface, this is so you can place a ring of AL or delrin into the ground area of the jaws. Then tighten the chuck against that ring. I you have tool post grinder that has an internal spindle use that, if not by a cheap inline die grinder, check the shaft diameter, likely 25mm, which fits fine into a 1" boring bar holder.

Mount a fine stone of 1" +D, you will want to set the DG at a very slight angle to the bed, so just the end of the stone does the grinding. You want the lathe spindle to turn opposite the grinder spindle, then grind the jaws until each jaws sparks out the same amount.

And yes its a mess, so disassembly of the chuck is needed to clean everything up.
Now the only thing to be out is the scroll fit in the chuck body, and the wear on the threads.
Tom

I remember hearing some say grinding the jaws in the chuck tightened against an inside ring only trues the jaws to that one specific diameter they are set at?? I've wondered if it would be worth doing on another 2 chucks I have with bell-mouthed jaws, or if that is futile?

My two bits worth is that regrinding top jaws can improve the runout of a chuck, but might not do well at curing jaws that are bell mouthed.
Jaws get bell mouthed because of wear in the chuck body slots that locate the jaws. The jaws in effect tilt when closed on the material .That tilt is the result of the clearance between the jaw slot and the chuck body key.
Leading to the jaws spreading wide toward the front.
Fitting a disc to bias the jaws against the scroll is the correct procedure, but bear in mind that the disc is placed at the rear of the jaw and applies no correcting
pressure to take the slack from the wear at the outside of the chuck/jaw....They will still tilt when pressure is applied when clamping material....This effect will likely be reduced through re-grinding, but it won't be eliminated.
Cheers Ross.

TheOldCar said:

I am thinking about beautiful flowers, ice cream and brown trout right now, as I write this. WooHoo!

Click to expand...

Ouch... trout with ice cream? If that is how your tastes in food run together, I am certain we dare not even THINK about your sex life. Inclusive "we". Meaning not you, either.

TheOldCar said:

I remember hearing some say grinding the jaws in the chuck tightened against an inside ring only trues the jaws to that one specific diameter they are set at?? I've wondered if it would be worth doing on another 2 chucks I have with bell-mouthed jaws, or if that is futile?

Click to expand...

Depends on how many different diameters you tested and found able to sustain that low TIR. Where low TIR may be a side-effect of "PBS" AKA Pure Bullshit Luck, and at ONE diameter. Only.