On 2010-12-22, wrote:
On 21 Dec 2010 05:19:41 GMT, "DoN. Nichols"
wrote:

[ ... ]

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.

I thought there was an issue with the bearings and the size of the
spindle.

A consideration -- especially with a small diameter spindle
which is easy to bend. But that is something which either is or is not
a problem to be determined once. The jaws hitting the bed are something
to be considered every time you use the chuck.

I've got a 10" 4-jaw on my 12x24" Clausing, but only a 6-1/4"
3-jaw.

Also the biggies take longer to stop. The chatter issue
should be alleviated by the soft pie jaws which would be the point of
getting a new chuck in the first place.

The chatter might be damped by the greater mass, (and certainly
by the pie jaws), but you still have the problem of flexing of a skinny
spindle.

Craftsman has sold (in the past) two flavors of 6" lathe. The
better one is the one made by Atlas, with (IIRC) a 1"-10 spindle nose
thread, while they also had one made by Ann Arbor machine tools which
only had a 1/2"-20 spindle nose -- and those were notorious for getting
bent the first time you took a heavy cut.

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. :-)

No. Depends on the size of your bank account.

"Dirt cheap" in terms of what is available to fit your lathe.
There are lathes which have chucks 40" diameter or larger.

[ ... ]

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

Also much more expensive. OTOH others tell me that they had problems
with some of the back plates.

The pre-threaded ones, or the ones which they had to machine to
fit their lathe?

My Clausing came with a 2-1/4x8 spindle nose, and I had to make
some backplates to fit (sometimes enlarging the hole in existing
backplates which had been used on 1-1/2x8 spindles or the like.
Overall, I had no problems with those. The trick for turning the bore
to clear the register was to mount the backplate backwards with a spacer
ring to hold it far enough out so I could bore the threads off at that
end.

Enjoy,
DoN.

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