If you acquired a backing plate that was already threaded, I'm inclined to suggest that you not touch the existing shoulder, which acts as a register of sorts. The reasoning being that you may or may not achieve a condition that is at a right angle to the pitch line of the thread. The exception would be if the backing plate, when installed on the spindle, runs out. Assuming it is threaded, and you have installed it to see it run, how it runs should answer your questions.Patio wrote:1) Where the back side face, of my backing plate, registers against my spindle, (SB 2-1/8 spindle) Is that surfaced supposed to be faced for proper fit first before facing the backing plate for the chuck? It "looks" like a good fit now.
The best possible scenario is to assemble the chuck with nothing protruding. It's common for a guy to place his hand on the chuck as it slows to a stop. Anything sticking out has the potential to give grief. If the face of the chuck has counterbored holes, you can take advantage of them by using socket head cap screws that fit the tapped holes you'd provide in the backing plate. You can locate the holes reliably by using a transfer punch, then picking up each punch mark using a small drill, no larger than the dimple created. A small drill is flexible enough to pick the punch and drill on location, whereas a larger drill may drill where it is aimed, and not follow the established punch mark. Once you have picked up the punch mark with a small drill, you can then drill the small hole with a larger drill. If your drills are well pointed, you can drill to size. If they have been hand ground, it's not a bad idea to drill slightly undersized, then open the hole with the proper sized drill. By doing so, you insure you don't create an oversized hole for the thread. That's particularly important when machining cast iron, which isn't very strong at the outset, and is easy to cut oversized when using a less than perfect drill.2) My chuck has through holes in it for the mounting bolts! Am I suppose to some how make threaded holes in the backplate for the bolts to thread into?
3) Or do I drill slightly over sized holes in the backing plate and run the bolts all the way through and use nuts on the back side? Being as the bolts just hold the chuck against the backing plate.
Were you to have a functional mill, I'd suggest you rely on trig to locate the tapped holes, assuming the holes in the chuck were established on a bolt circle, which they likely were.
Lots of ways to approach the installation and achieve good results---depending on one's workmanship.