Modify a bcking plate?

[rock_breaker]

I inherited a 3 jaw 5 inch chuck and a backing plate with pre-drilled & tapped holes and had a friend help mount it. Not wanting to drill through the chuck we drilled and tapped new holes in the back of the chuck. This friend is a machinist by trade and has an impressive shop.
My question is can I improve the TIR (-0.003+0.001) by turning the backplate? I have 3 types of cutters that could be used: HSS, braised carbide and carbide inserts. ,and can grind the HSS to where it leaves a good finish. The chuck goes on a Clausing 100 MKIII that has a 1-1/2" x 8 tpi spindle. The spindle register has approximately + 0.0005"-- 0.000 runout. I would start by honing a HSS bit and make a light truing pass on the register with the compound rest cinched down and the cross-slide ways tightened then mount the back plate and make a light cut after adjusting and re-tightening the compound rest Knowing a few things more now than 20 years ago (thanks to this group) I think I can improve the run out but want to be sure I am not overlooking something. Thanks for all help offered.

 

[mikey]

The typical procedure would be to mount the backing plate on the spindle first, being careful to clean everything well before mounting. Then you take a light facing pass on the part of the plate that the back of your chuck will contact. This trues that surface so it runs square to the spindle. From this point, there are two schools of thought.

Some folks prefer to machine the snout that goes into the back recess of the chuck so it is a light press fit, on the order of a thou or so. Theoretically, this is supposed to transfer the accuracy of the spindle to the chuck while also ensuring the chuck cannot move under load.

Other folks feel that it is better to machine the snout about 0.002 - 004" smaller than the chuck recess and then knock the chuck into perfect alignment before tightening the locking screws. Samchully, maker of some of the finest chucks in the industry, is one such company that recommends this method. It is much easier to machine the plate this way as your dimensions are easy to hit.

Keep in mind that you're talking about a 3 jaw chuck, not a collet chuck that requires extreme concentricity. You want it to be very close but for first operations work, close is usually good enough and I would suggest option two above.

 

[benmychree]

That amount of runout is within the realm of what 3 jaw chucks can be expected to attain, especially used ones. They may indicate quite close on one diameter and not so close on other diameters.

 

[rock_breaker]

Thank you Mikey and John. The chuck was new when I received it and the backing plate. The snout is a close fit so I believe it is in the Samchully category. Obviously it is used now and I thought I might improve it's accuracy and as you say John 3 jaw chucks are not totally accurate to start with. I think I will be sure I have a sharp smooth cutting tool then rig a dial indicator so I have some control over depth of cut and try to true up the backing plate somewhat. Thanks again for your help,
Ray

 

[benmychree]

Good plan!