On Jun 27, 2:36*pm, john wrote:
Michael Koblic wrote:

Unless the shaft you are chucking is the same diameter as the washer you
used to cut the jaws, you will get an error caused by the scroll that
moves the jaws in and out.

John

You could stuff the test bar in so it contacts only as much as the
washer did.

Personally the way I use a 3-jaw, all that matters is that the jaws
are parallel when tightened, so the work doesn't wobble. And if it
does anyway, like the stamped head of a bolt, I'll support the end
with the tailstock. You've probably achieved parallelism already
unless the jaws tilted. They shouldn't have if the washer was back
near the scroll.

Plan the job so you can make the finish cuts on all surfaces without
loosening the chuck. It doesn't matter for roughing as long as you
leave an allowance larger than the runout.

I doubt you will ever get the 3-jaw to run true enough that you can
reverse the work and make the cuts from both ends meet invisibly.
That's difficult even with a Set-Tru, 4-jaw or collets, and a good
reason to turn between centers. You could make a gnomon with extra
metal in the ends for the center holes and then part them off later in
the 3-jaw. Or turn to a step or groove from both ends, a little runout
won't show across it.

jsw