One problem to address is getting the exact center of the backing plate to exactly line up with the exact center of the chuck jaws when they are clamped on something tight. This might be a few thousands or more, off of the center of the hole in the back of the chuck, depending on the quality of the chuck machining.?
Two choices:
You could try mounting the backing plate accurately to the machined back of the chuck, and then carefully check and machine/grind the jaw gripping points to make them grip exactly on the center of a metal rod.
Alternately you can center a pin in a rotary table or mount a rotating pin on a mill table centered under the mill chuck. Flip over the new chuck and grip the pin tightly as you would if you were mounting a rod in the chuck on a lathe. This will remove any slop and give you the exact center of the chuck when holding a round object. Move the table over to the radius of the listed bolt circle and as you spin the new chuck you can determine if the chuck jaws are really centered when under pressure. If it is off, you can see that the bolt holes will be off a little one way vs. the circle when the new chuck is rotated. You could take a dial indicator in the mill chuck and line it up with a machined surface on the new chuck to see how much off center it is and how much adjustment in the position of the bolt circle on the backing plate will be necessary to get as close as possible to the center point of the jaws when the new chuck and mounting plate are assembled an chucked in a lathe. So a slightly offset bolt circle on the mounting plate will correct the position of the jaws center position.? If it is a minor offset, a few thousands of correction might be possible with just clearance in the bolt holes.
Point here is that if you line up the bolt holes perfectly with the chuck center line, mount the chuck in the lathe and chuck a machined rod in the jaws and tighten, there may be a slight offset to the chucked rod as it spins. If it is a lot, it might be hard to adjust out. Good to know this before you get too far machining the face plate. You will also want to mount the face plate in the lathe and machine the back and then the front to make sure the face plate runs true before drilling the holes and before the chuck is mounted. Worst case the back of the chuck might need a slight smoothing if it seems slightly off of true when mounted to the backing plate.? The farther the chuck jaws are from the pressure edge of the spindle the more any run-out is amplified.?
You probably already know this but this thread might be read in the future by a person new to setting up a new chuck with a separate mounting plate. I have done a couple of these and it takes patience to assure the chuck runs true when finished mounting to the lathe.
