On Oct 1, 1:28 am, "Michael Koblic" wrote:
"DoN. Nichols" wrote in message
What sort of improvement do you expect going to 4-jaw chuck?
...I would expect perhaps to be able to tune to 0.0001"
When you are
mounting a chuck, are most of the mounts self-centering ?
Can you center the chuck by using the indicator
on the periphery of the chuck?
You can't depend on the chuck's body to be concentric with its
grip. ...
or do you need to chuck something demonstrably
round and center on that?...
Note that there are chucks available with a feature called
"set-tru" by one manufacturer,...
Also -- you can use two-piece jaws in your larger chucks, and
bolt soft jaws onto the master jaws and turn them to perfectly fit your
workpiece

Thank you. I just knew it was going to be simple...:-)

So the bottom line is you need a set of perfect cylinders of varying
diameters to get your chuck right on. This made perfect common sense to me
but oddly enough I could not find any reference to this in any literature I
perused. Are such sets being sold for this very purpose?
Michael Koblic,

You are asking the wrong questions. The blank doesn't have to be
accurately centered in the chuck. As long as it is large enough to
clean up all the way around the cutting tool will generate a true
concentric cylinder. When you plan the job, assume that you will lose
some concentricity if you remove the work from the chuck no matter
what type of chuck the lathe has, even collets. If you need to remove
and replace it, turn it between centers.

Otherwise turn all surfaces that -must- be concentric at one chucking.
Plan how you will chuck it when reversed to turn the other end, and if
necessary leave extra material with enough room to hold it in a 4-jaw
and indicate it true enough.

You can rough the part close to size all over in a 3-jaw chuck, hone
the tool and finish the non-critical surfaces, then change to another
more accurate setup and dial it in carefully to finish the critical
dimensions without unclamping it.

Here's a good example. I just made a 17mm mandrel with center holes to
fit the 6203Z bearings in the HF 1 Ton I-beam trolley so I could turn
down the rollers to fit into 3 [ 5 channel iron.
http://www.harborfreight.com/cpi/cta...emnumber=97392

I chucked the rollers by the outer rim in a 3-jaw to rough them close
to size, then put them on the mandrel between centers and drove them
with a hose clamp to finish the rolling surfaces true with the bores
of the bearings. They were out about 0.020" - 0.035" from the first
chucking, mostly because the outer rims weren't machined true with the
bearing recess. I suspect the castings were machined on the two sides
in two different operations.

The 3-jaw is a new Bison Set-Tru good within about 0.001". I don't
know how they hold up in production but for hobby use it's a very good
chuck.

The roughing setup was solid and I could cut 0.025" deep without
chatter from that particular round-nosed HSS bit. The finish setup was
rather fragile, driven by a long 1/4" bolt pushing the hose clamp
screw, and only allowed 0.005" per pass. This is typical of ad hoc
faceplate driving schemes, so plan ahead.

When I spin the rollers only the rim that was about 0.035" off wobbles
visibly. The ~ 0.020" ones look OK. That should give you an idea of
how much accuracy you really need for art work.

As for the perfect setting cylinders, just use the part itself. Turn
the end slightly oversize with a good finish, reverse it and adjust it
to run true. Once you have a lathe the paradigm shifts from buying to
making the parts you need.

Jim Wilkins