On 12/18/24 15:22, Guy Edkins wrote:
?
?Hi all,
I am in the process of rehabbing a 3995 lathe. It was free
and maybe I now know why. The runout on the spindle inner
and outer diameter is really good. Like .0013. Screw on
the chuck and chuck up a mandrel and it all goes bad,
really bad. You can see it, let alone measure it. The
chuck is a Cushing 3 jaw.
So is it worth buying a new chuck, i.e. will it mitigate
much of the issue, or is there something else at play here
(see what I did there) that I have missed? I am new at
lathe details and the ins and outs, but an engineer by
background.
Well, there are several possibilities.? First, see if there
is wobble on the outside of the chuck body.? If not, then
the problem is in the jaws.? If the outside of the chuck has
wobble or runout, then the backplate needs to be faced
true.? Possibly the register on the chuck side of the
backplate also needs to be trued.? If you do that, then
there will be looseness between the register and the chuck.?
You might be able to put a shim in there, or make the "poor
man's adjustable chuck".? Leave the chuck mounting bolts
slightly loose, indicate a bar in the chuck, bump the chuck
body until the bar runs true and then tighten the bolts.
For severe runout, the chuck jaws may have been inserted
into the wrong slots.? The jaws are numbered to match
numbers on the slots.? If you put the wrong jaws in the
wrong slots you will have huge runout.? If that is not the
issue, the jaws may be "bell-mouthed", meaning worn more at
the tips than farther back. This makes it hard to hold long
stock securely.? The fix is to grind the inner faces of the
jaws.? Usually some sort of "spider", putty or rubber bands
are used to hold the jaws against the scroll face as it
would be if holding a piece of work.
One other gotcha is the possibility of a loose scroll.? Some
chucks have a circular shim that centers the scroll in the
body. If that had been lost, then the scroll will move
around and the jaws won't hold anything on center.
And, finally, if you can't find the source of this issue,
then the chuck may have been crashed, and the grooves for
the jaws may be bent.? There's really no way to fix that.
Jon
