"Michael Koblic" wrote in message
...
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
Check runout before removing the washer, should be dead nuts on. Open and
close jaws back on the washer, recheck. The washer has nothing to do with
your runout. The three contact points is true, (as someone else said) no
matter how much runout you have before cutting. When all three jaws are
trimmed, should be .000 runout.