On Jun 27, 2:11*am, "Michael Koblic" wrote:

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

Most of the time .004 TIR is much better than what is needed.
Consider if you hold something by your four jaw chuck and deliberately
have it not centered by say .100. Now without removing the work from
the chuck, you machine the piece until part of it is .500 dia and
another part is .250. And you part it off. The part you machined is
round and the fact that it was not centered exactly when you started
did not make any difference.

Of course if you have to remove the part and then grip it say from the
other end, well that is a different story. Now you have to mark the
part and one jaw so you can put it back so the TIR does not matter.

I think the washer is the culprit. Although the three points that
touch the washer are three points on a circle, the center of that
circle is not on the axis of rotation.

Note that you can turn something perfectly round whether or not it
started out as round or square and this can be done in either a 4 jaw
or your three jaw chuck. So you can make your own washer. Or use a
bolt and a couple of nuts to hold the washer and turn it to be round.

You can make a washer with a large hole, and hold it with your three
jaw chuck touching the hole, not the outside. Then use a boring bar on
the inside of the jaws. That way you do not have to file the little
nibs off. Then check to see what the TIR is. If it is not as good as
you want, then use the washer again and file the little nibs off.

I personally would not try to get the TIR lower, until you try
checking what the TIR is with different sized round things. The TIR
can be perfect at one diameter and off at another if the scroll in the
three jaw chuck is not perfect.


Dan