On 2009-05-14, Charles U Farley wrote:
Don Foreman wrote in
:

A 4-jaw will enable more precise centering and is more versatile,
3-jaw is quicker and more convenient. With a bit of practice it only
takes a minute or two to get something centered in a 4J to under a
thou.

Time spent indicating work in a four jaw chuck on a dividing head will be
quite a bit longer than on a lathe. 40:1 turns ratio makes for 20 handle
turns every time you adjust a jaw and check with an indicator. Repeat a
few, maybe three times for jaws one and three, three more for jaws two and
four.

I don't know about your dividing head, but mine had a knurled
thumbscrew which will disengage the worm from the spindle, allowing you
to turn the chuck much more quickly by hand. It also has a ring behind
the chuck with a lever-controlled pin for simple dividing
(2/3/4/6/8/etc) so you don't even need the plate for simple dividing.

My past experience has been that a three jaw is adequate for the class of
work done on items that require plain indexing cuts. For more complex and
higher tolerance dividing work like cutting gears the job was placed
between centers and driven with a dog. I've not had a project that
required the hold of a four jaw or accuracy exceeding a 3 jaw with a bit of
shim to correct chuck runout.

And -- you can also set things up on an arbor between centers to
get plenty of accuracy.

I'd vote for a 3 jaw for starters. Get the 4 jaw later if you have a
project that can't be gripped in the 3 jaw.

I don't even (yet) have a 4-jaw for my index head. Between the
3-jaw and the between centers I've been able to do what I need to do.
But a 4-jaw would be nice, and if I stumble across one which fits it,
I'll pick it up.

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