Cushman chuck issue

[Jim F]

I have a 3 jaw Cushman chuck on my SB 9C, I have gotten the TIR to within ,010",
Anything short of re-grinding to try ?

 

[mikey]

I suggest you chuck up a 2-3" long piece of mild steel or aluminum stock and turn the OD down 0.010", then skim it at 0.002", then see if that piece runs out. Use a sharp HSS tool if you can. This first turning is called a first operation and is what a 3 jaw chuck is intended for. It should read zero run out on a first operation test cut. If it does, the chuck is fine.

 

[Tozguy]

I have gotten the TIR to within ,010",

What have you done to reach that point? Just so we don't suggest things you have already done.

 

[Jim F]

What have you done to reach that point? Just so we don't suggest things you have already done.

Re-clocked the backplate.

 

[Tozguy]

Not sure that I understand what you mean by re-clocking the backplate. Did you check run out on the face of the backplate?

First I would disassemble it completely, clean, inspect and lube the chuck.
Then, if there is more than one pinion to turn the scroll, try each pinion to identify the one that gives the least run out. Only use that one from then on.
Test for run out with different diameters, say 1/2'', 3/4 and 1'' to see how they compare. Repeat at least twice to see if results are repeatable.
Take the diameter with the least run out and try the tap in method to reduce run out.

All this will give you an idea of what the limits of the chuck are and how to get the best from it.

Re grinding the jaws will in principle only fix run out at the position the jaws are holding during grinding. Change the size of the jaw opening and all bets are off.

 

[Jim F]

Not sure that I understand what you mean by re-clocking the backplate. Did you check run out on the face of the backplate?

First I would disassemble it completely, clean, inspect and lube the chuck.
Then, if there is more than one pinion to turn the scroll, try each pinion to identify the one that gives the least run out. Only use that one from then on.
Test for run out with different diameters, say 1/2'', 3/4 and 1'' to see how they compare. Repeat at least twice to see if results are repeatable.
Take the diameter with the least run out and try the tap in method to reduce run out.

All this will give you an idea of what the limits of the chuck are and how to get the best from it.

Re grinding the jaws will in principle only fix run out at the position the jaws are holding during grinding. Change the size of the jaw opening and all bets are off.

Yes, the backplate has 0 runout.
I measured the inside faces of the jaws tonight.
1 has .0085 taper.
2 and 3 have .005 taper.

I am thinking I should have the jaws surface ground on the 60 deg. side and then grind the inside face flats in my lathe.
Ideas ?

 

[Tozguy]

It seems to me that just grinding the jaws on the lathe would suffice.

 

[Jim F]

It seems to me that just grinding the jaws on the lathe would suffice.

That is what I am doing.
They are better, but have a couple more passes to do.