Newbie getting in over my head?

[Harold_V]

What might help make up your mind would be to do the math on the bolt circle to see what the overall diameter over the counterbores would be. If they don't break out on the perimeter, the smaller one might serve perfectly well. If not, I suspect that they are made of either gray iron, or "semi-steel", both of which are easily machined, so it then gets down to a matter of economics. Cutting down an 8" plate shouldn't be all that difficult, especially if you can use a grade 2 carbide (such as GE's 883). On that diameter, assuming you have the power, it would do a great job.

I would be more inclined to spin polish the chuck body than to bead blast. By using a large flat stone, you can achieve decent results and restore the chuck to what would be its original appearance without rounding edges. That said, so long as the body is blasted while the chuck is not assembled, no big deal. All depends on what you hope to have it look like.

I don't know the level of your ability, so please accept the following in the spirit in which it is offered.

Measuring the diameter of the short depth of the counterbore that accepts the spigot you must machine on the backing plate would be difficult at best, and calipers are NOT to be trusted to yield an accurate reading. If you intend to use a caliper, do so ONLY to get close to size (no closer than ten thou). Do NOT rely on the caliper for the desired fit. That's almost a guarantee of not hitting size.

I'd like to caution you that the spigot that locates the backing plate in the chuck body is critical. It should be dead concentric with the spindle and a snug fit in the chuck. If it is a poor fit, you risk having the chuck body move about on the spigot and it won't register the chuck as it should.

Before mounting the new plate, run the spindle at high speed and lay a flat stone on each of the critical faces to ensure that there are no dings that will alter the desired relationship between the plate and spindle. Anything that stands proud will be readily disclosed and removed. Also, check the plate for any dings BEFORE mounting for machining. If any are found, be sure they are FULLY removed so they don't alter the desired running condition.

Regardless of the amount of time it takes to fit the plate, once you are close to size, take one thou passes and check for fit with each pass by taking a shallow cut and checking the fit before completing the cut. If the chuck doesn't slip on, take the cut to depth and repeat. Do NOT change the dial setting until each pass is complete, otherwise the resulting size may not be reflected in the finish pass due to cutting pressure variations. With a sharp tool, a thou cut in this material isn't difficult. Remember, what work you do now will be reflected in the performance of the chuck until the day you no longer operate the machine. It's worth taking the time to ensure the proper outcome.

H

[nessism]

I understand how to machine the backplate.

I tried to clean up the surface finish, but the pitting is too deep. Oh, well. At least it's functional.