I just got a 3-jaw chuck fo my Taig lathe. It comes with two Tommy (?) bars,
a hex key, a washer and a set of instructions. The jaws are aluminium and
unfinished, the instructions tell you how to turn the jaws true.

Before I started I measured the runout using a 1/2" drill rod. It came to
0.006".

I did as the instructions told me. I used the washer provided, chucked it up
at the back of the jaws, made sure it was nice and flat and carefully turned
0.002" off the jaws. The I filed down the little nubbins at the back of the
jaws where the washer was being held during the procedure. I re-measured the
runout: This time it was 0.004". I inspected the jaws: There was evidence of
"clean-up" on all three, the filing seemed satisfactory (I touched up one of
the jaws just to make sure).

I was puzzled by this poor result. I could not think of an explanation. Then
I measured the washer and it turns out to be out of round by 0.004".

I have a number of questions:

1) Is 0.004" TIR satisfactory for a small 3-jaw chuck? I suspect that far
from it but I do not want to be unreasonable.
2) Is the washer the most likely culprit?
3) How to rescue the situation? The obvious solution (assuming the washer is
the culprit) is to find something tthat is perfectly round, chuck it up
again and repeat the procedure. Presumably the object will have to be
thicker than the filed-down portion of the jaws so it is held by the turned
down portion but I may be wrong.
4) What object to use for that purpose? The best I can think right now is to
get a piece of aluminium bar and turn and face it in my 4-jaw chuck and then
part it off at the correct thickness.
5) If one used an object that is too thick or repeated the procedure a few
times the turned down (and hopefully true) portion of the jaws will become
smaller and smaller. What is the minimum size of the gripping portion of the
jaws to provide secure workholding?

Thanks,

--
Michael Koblic
Campbell River, BC