Jim Wilkins wrote:
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.


Thank you and all the others.

The critical bit of information seems to be that 3-jaw scroll chucks do not
necessarily hold true at all diameters thus trying to improve on the 4 thou
of runout would probably put me on the flat portion of the diminishing
return curve.

I am not set up for turning between centres yet - that comes next and will
invove cobbling some sort of centre for the headstock and a dog (apparently
the cognoscenti like a hose clip for this). This will invove turning things
to a point so a compound slide will be in order. Another issue is facing off
longish cylinders (the inner diameter of the spindle is only 5/16") and I
shall be needing a steady rest.

But all of that had to be put on hold as I have not had a decent arrangement
to grind lathe bits. I finally finished it today.

This is worse than having a baby.

I wonder at what point one returns to some semblance of productive work
rather just continue making tools for tools...:-) It has become a running
joke in the family.

--
Michael Koblic
Campbell River, BC