On 2010-12-20, wrote:

[ ... ]

That is a consideration. I did check with other 9x20 users and some
seem to like 5" chucks better than 4" chucks, however, this may be
related to the quality of the manufacture. None reported any problems.
One even run a 6" 3-jaw.

The main thing to worry about when using a 3-jaw chuck larger
than approximately half the swing of the lathe is that you have to be
*very* careful to always turn the chuck by hand through a full turn
after griping something larger. The back end of the jaws can stick out
far enough to hit the bed ways -- damaging both the jaws and the ways.

Either way the cost of a Polish 5" 3-jaw, back plate and pie jaws is
$500 give or take. Not something to undertake lightly.

Depends on the size of your lathe. With some sizes, that is
beyond dirt cheap. :-)

An interesting point: I see that many manufacturers make the chucks
with plain backs but also with threaded backs. I can see the need to
turn the back plate to fit one's particular lathe to make the chuck
run true but I am not sure how this works with a chuck that has
already been threaded unless it is a "Set-through" chuck which these
are not.

Wrong term there. The usual spelling of one maker's name for
the chuck is "set-tru" which is short for "set true" (*not* through).
There is someone else here who frequently uses that wrong term -- though
usually spelled "adjust-thru" IIRC.

However, if the chuck has already been threaded, it may be that
the jaws have been ground true while mounted in the chuck. The thing
that turning your own backplate does is to compensate for errors in the
spindle of *your* machine -- and make it less accurate on somebody
else's machine. A friend had a 10" lathe with two adjust-tru (another
manufacturer's trademarked name for the same feature) chucks, which ran
fine -- but every time I tried to adapt something else to it it had lots
of runout. It turns out that somewhen in its past, someone had
seriously crashed the machine, and the threaded part of the spindle nose
was bent -- not enough to see easily, but enough to introduce
progressively worse runout as you moved the indicator away from the
headstock.

We eventually got a replacement spindle via eBay.

But -- a pre-threaded chuck is a convenient think -- *if* you
can trust your spindle nose to be true.

Enjoy,
DoN.

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