David,
You have done the job a bit A over T. But easily able to salvage.
BTW, you are doing the friction turning all wrong with regards to chuck jaws. I will explain later, no time to do it now.
Get the plate somehow mounted or clamped down onto your mill, if you use the original centre drilling you have in there, to hold the plate down while clamping you won't be far out. Use a fixed or rotating centre in the quill of your mill. Mount it over one of the table t-slots. Then drill and ream your centre hole.
Getting a fair sized lump of barstock into your lathe chuck, above 2" in size, to give your backplate a bit of support while machining. Turn a spigot on the end of it for a nice tight fit in the hole, but not quite as long as the plate is thick. Try to get the spigot as close to the chuck jaws as you can. Without removing the bar from the chuck, drill and tap it for the largest sized bolt that will go in the end of the spigot, without weakening the spigot too much, say if you have a 10mm spigot, use an 8mm bolt. Mount up the plate onto the spigot using some hi strength loctite on the flange part of the spigot you have just cut, then using a bolt and a good strong large washer, clamp it all up.
You should then be able to do all your facing cuts if you keep your speed down and cuts light.
Don't worry about being able to face all the way across, just get it as close to the holding washer as possible. The recess in the back of the chuck will be deeper than the spigot that you will be cutting for mounting the chuck on, so the unmachined bit will be hidden under the chuck.
If you haven't got the loctite everywhere, you should be able to remove the centre bolt and check your chuck for fit. You might have to do that a few dozen times when you are getting to spigot size.
Once you have your chuck and spigot matching nicely, take the lot out of the chuck, apply a little heat to the chucking spigot you made, leave for a couple of minutes, then you should find they will come apart.
Your main problem will be turning up your RT lining up bit. If you just turn up a bar to fit, then hold it in the chuck to turn the spigot that goes into your backplate, you can guarantee it won't be concentric, and your chuck will be off centre to the RT.
So, turn up a short blank that is a nice snug fit in your RT centre, that is going to be your master.
Get a piece of stock that is slightly larger than the required size, mount into your chuck and turn down to the exact size you have on your master blank. Once you have done that, turn the end down to the size you have in your chuck backplate, then just part it off. You will now have a perfectly concentric aligning slug, and if you mount it gently into your chuck jaws, you can turn the ends down to the correct length.
Clamp the backplate upside down onto the RT using your new alignment slug, and drill all your mounting holes for the t-bolts. For the chuck bolts, which will need to be recessed, only drill part way thru the backplate (otherwise you end up with nice holes in your RT face), then cut the recesses with a slot drill. Finish off the thru holes by hand after you have taken it off the RT. Make a bit of a wiggle factor on your mounting holes, say 0.5mm oversized, to make sure you have no binding bolts due to drilling errors. Your spigots and slugs will align everything up, the bolts are only there to hold all the bits together.
If you can't understand all this, give me a shout, and I will do a C-o-C, but I might not get to it until tomorrow, as I have just come out of a sleep, and am now going back for another one.
John
