I know this has been answered before but I can't find a step by step process for doing this. After getting great advice from Harold Vordos I think that my jaws are sprung and I am going to pick up a new 8" Bison Adjust Tru 3 jaw with a partially finished backplate. I do not want to destroy my new backplate so I am going to outline what I THINK I should do to fit this backplate.
1) Put the backplate onto the spindle and take a facing cut on the nose of the backplate.
2)Put backplate onto spindle backwards with a spacer ring to protect the spindle nose. Bore the register for the shoulder of the spindle. Question: What should I use as a spacer washer and what should the shoulder register be turned to tolerance wise?
3) Turn the backplate around the right way and take another facing cut across the nose.
4) Againg turn the backplate around backwards and take a facing cut acrossed the back of the back plate.
5) Turn the backplate around and turn the register to mount the chuck onto. Question: How much over should the register be turned? .001?
6) Mark, drill, and tap mounting holes in backplate.
7) Turn down the backplate to the proper diameter.
8) Mount the chuck and pray that you didn't screw anything up.
I'm going to figure that by 'nose' you mean the front, or the large diameter side. I'm not sure step (1) will really be needed - and you might make things worse because the existing front surface is probably pretty square with the original bore on the backplate before they threaded it. When you put the backplate on the spindle the regular way, there will be no register diameter so the threads in the backplate will bottom onto the thread runout of the spindle, and may c*ck a bit.
So I would probably omit this step for that reason. Others may suggest why it's truly imperative.
Basically just a spacer washer that has the thickness controlled to a thou or better, and with the ID five or ten thou over the spindle major diameter, so it slips on nicely. Thick enough that the threads in the backplate don't contact the incomplete threads at the base of the spindle.
The deal is to get the threads a fairly generous loose fit, but the register bore in the backplate as tight a fit as you can to the unthreaded register diameter on the spindle. If you can get down below a half thousanth that's OK. Tighter than a thousanth is good. Basically you just get as close as you can before it drops in. The trouble is at some point the reproducibility of mounting the backplate/spacer on the spindle is not good enough, so when you want to take off that teeny bit more, it winds up coming off of only one side of the bore. :(
You can make a plug gage: a piece of stock that measures the same as the register diameter, and use that, so you don't need to unmount the backplate to check it.
Again, I would definitely omit this step, it serves no function.
You want to do this (4) in the *same* setup as the final bore of the register diameter, so the axis of the bore, and the surface of the rear face of the backplate, are truly square. This is why I would omit step three.
Here of course, you need to face the working side of the backing plate as well, as this has not been done yet, along with turning the OD. If you are getting an adjust-true then you will need to be sure to get the center boss the correct size. There will probably be a spec sheet with the chuck.
These should be quite generous for clearance.
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