My first posting here. I do have some formal machinist training under my belt-but thought I would throw this out for conceptual ideas.
I picked up an old enco 9 x 20 lathe a few weeks ago.just beginning to get it set up to do regular work. I had been retrofitted from the origional 4 inch chuck to what looks to be a bison three jaw with reversible jaws. But the guy who owned it first seems to have been a bit amatuerish. there is as much as 10 thou run out in it with ground steel test bar,even right next to the jaws. None of the jaws are labled in any way except for a serial number that is the same on all. Switching them does not seem to help.
I was a bit puzzled by this, as the bison is supposed to be a fairly good make of chuck. I figure I am going to have to do this to fix it: make a spanner wrench to fit the spindle and knock the chuck loose, it's seized. Take the chuck off of the backing plate. Check the step in backplate for size. See if he made it too loose. If the size seems tight and concentric.....the issue is with the chuck. Chuck jaws run out from the back refference.
In which case, the back inside of the chuck will have to be machined concentric. I will use one of the old three jaws to hold a 1.5 inch bar first, tighten it down good. leave about two inches sticking out, and machine that with to smooth finish back to the jaws of the smaller chuck. Which will ensure that refference runs true.
Clamp the larger chuck onto the bar, jaws facing jaws...and test the back inside for concentricity with my dial indicator.. If I find runout there, bore it out just enough to clean it up.
Unchuck it, pop off the small chuck......put the five inch backing plate back on, and face it back enough to cut a new step . Return the step to actually run true, rebolt it. I had considered just running a grinder through the insides of the jaws to make it run true, but that would not help it when I turned the jaws for larger work.
Also thought of putting in setscrews to adjust it true-but those always work loose.
Am I on the right track with this fix, or does anyone have better ideas to get this thing right?
You seem to know what steps to take to find and cure the problem. Let us know how you do.
What type mount is it? A, L, D or ??