Finally mated the back plate I made to the ER40 Collet Chuck.
First thing I did was make the recess deeper on the ER chuck using the 4J. Originally I was going to use a precision bar held in an ER collet in the spindle and attach the collet chuck to that. Two problems with the idea was that I only have one collet per size and the collet that fits in the spindle needs a draw bar. Since my ground piece was to long so that wouldn't work. Looking back I could of just turned a piece of stock down. I don't think it needed to be a precision rod, as long as I turned it down I think that is what would matter. Anyway, didn't think of that at the time.
Now I faced the back plate with the correct register. The chuck fit on really nice and tight, had to tap it on with a dead blow.
At this point something went wrong I believe. I had to take the back plate off to drill and tap for the chuck, removing all the camlocks. I made a witness mark previous to this so the back plate went on the same way.
Got everything back together and run out was unacceptable .005" - .009" depending on how I mounted the back plate.
Took off the chuck and checked the face of the back plate, same thing.
I took a clean up cut across the back plate and also took some off the register diameter.
Mounted the chuck but had to tap out the run out. I got it down to ~ .0004". I'm satisfied with that.