I don't know if this is "runout", but my 4-jaw chuck doesn't hold stock
straight. When stock is centered at the jaws, it isn't centered away
from the jaws . It's eccentric.
It took a piece of 1" round and checked it between centers for straight
and centered. I then chucked it to within .001 at the jaw ends. 3 1/2"
away it was .035 eccentric!
This means that the jaw faces aren't parallel to the spindle, right? Is
there anything that I can do to fix it?
BTW - I checked the face of the check, to see if it wasn't seated on the
spindle right. It indicated +-.001 (6" chuck).
Thanks for the replies. I have checked the things you suggested.
OK - some more facts:
- the chuck doesn't have a mounting plate. The body is tapped for the
- the chuck came with the (used) lathe and this is the first that I've
- the brand is "Union" (USA), unknown age.
- the jaws are not numbered.
- there aren't any nuts on the back.
- the +-.001 I had measured was on the outside edge of the face (3" from
the spindle axis). It is also +-.001 on the circumference of the chuck
(front and back).
- when I reverse the jaws it's even worse: .055 in 4".
- with nothing clamped in the chuck, I indicated the jaws axially. That
is, perpendicular to the chuck face. With my fingers, I moved the jaws
.001, .002 (twice) and .003. This is the play in the jaw grooves. But
don't the jaws rise to the bottom of the tongue?
- the chuck body is off .001 in 3". If that were the source of the
stock eccentricity, the stock would be off the same proportion (.001 in
3"). But it's off .017 in 3 1/2" (about .015 in 3).