DoN. Nichols wrote:

O.K. Did you tighten the screws holding the jaws as you
installed them? Did you press each jaw outward as you tightened the
screws? (Otherwise, they are likely to shift a bit under load.)

They were tight already.

[...]

Hmm ... IIRC, the washer contacts only the hard jaws below the
top (soft) jaws, not the soft jaws so there are no "nubbins" left --
unless you are turning a step to both hold and support a disk-shaped
workpiece.

No. The soft jaws cover the whole thing. There are nubbins...

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".

Not too bad -- depending on the age and quality of the chuck. I
would expect 0.001" or better on a brand new quality chuck (like the
Austrian made chucks for my Compact-5). But those have only one-piece
jaws -- either hardened jaws with steps already made, or soft jaws
which go all the way down to the scroll plate as one piece.

This a is a US-made new Taig chuck.

First thing is whether there are multiple holes for the tommy
bars. If so, try with each one (using only the master jaws to clamp
with at first) and see which one gives the best concentricity. Then
*mark* that tommy-bar hole in the body so you can always use it for
the final tightening. (In the case of chucks which tighten with
keys, if there are three sockets, one should be marked with a "-0-"
or something similar by the manufacturer. Some have only one socket
so there is never any question.

There are three pairs of holes. I did play with them but it did not seem to
make any difference.

But once you have bored the jaws to the proper size for the
current workpiece, you should get well under 0.001". It is only when
you move the jaws to grip a different diameter that you can get
significant change in runout -- especially if the scroll plate is not
truly concentric -- or is loose on the projection of the body so it
can shift from side to side as you tighten.

This is seems to be the recurring motif. I did not appreciate that this is a
feature of scroll chucks.

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.

Make it so it will fit behind the soft jaws in contact only with
the master jaws. You may have to unscrew the chuck from the spindle
to get it into place properly.

Not possible. See above.

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.

Huh? You mean the contact *length* along the faces of the jaws?
Not if you do the clamping only with the master jaws and turn the full
length of the soft jaws. For most things, you *want* the full length
of the soft jaws in contact with the workpiece.

Again, not possible.

And you should have multiple sets of soft jaws. Either buy
more, or machine some aluminum in your small mill (which should be
large enough for this task) to make extra jaws. Make one set for
general purpose -- a stepped set for standard ID gripping, and a
reverse stepped set for larger OD griping. Depending on the
precision you need, you can often get away with turning the existing
jaws end-for-end.

Done that. Mainly on account of being sure that I would roger the first set
of jaws and would need the spare. I was not going to touch anything though
until I understood the current problem.

Whenever you make a set of jaws, use a number stamp set to mark
the jaws for position 1, 2, or 3, so when you put them back on, they
will be on the same master jaws.

Right.


What is the minimum size of the gripping
portion of the jaws to provide secure workholding?

That depends on a lot of things, including the material of the
jaws (aluminum in this case, but hardened steel for hardened top jaws
for larger chucks, or mild steel for soft top jaws for similar sized
chucks. Softer jaws, or softer workpieces need larger contact areas.
And when you are holding a long workpiece and turning some distance
from the chuck jaws, you need the full length of the soft jaws' faces
to minimize tilt in the jaws.

OK. Bigger is better.

There are other ways to hold the jaws preloaded for turning.
Do web searches and find them.

Not too sure that I understand what that means. I shall hit the books.

Thanks,

--
Michael Koblic
Campbell River, BC