"Bill Janssen" wrote in message
...

Ok Thanks. My lathe spindle is 1 1/2 X 8 and the chuck (without
backing plate) has a 1 1/2 hole. If the chuck had a larger bore
then I would not have been concerned. I have to get a backing plate
and machine it to fit. All seems well except I was wondering if
the 1 1/2 inch bore was sufficient

Bill,

You're not only confusing yourself over the hole size in the chuck, but
you're confusing everyone with your descriptions of the spindle.

To be clear, your Jet 9x20 Lathe has an external 1-1/2" x 8 TPI threaded
spindle mount. (The spindle internal through-hole is 25/32".)

As for the hole in the chuck, it has nothing to do with the spindle mount.
When you make a threaded backplate adapter for your chuck, the threaded part
of your spindle will be contained completely within the backplate. In other
words, if you were to mount the backplate on the spindle (without the chuck)
the end of the spindle should be less than flush with the face of the
backplate (the mounting surface for your chuck). To get a better idea how
the backplate adapter should be machined, take a look at the backplate on
your 3-jaw scroll chuck. But note that most 4-jaw independent chucks do not
use a full diameter backplate. It's more like a "plug" that fits into a
recess in the back of the chuck. So your 6" dia chuck will probably require
a max backplate diameter of less than 4". (As an example, my Bison 160mm
[6.3"] 4-jaw independent plain-back chuck required a 82.55mm [3.25"]
diameter backplate adapter.)

If you don't want to make a backplate adapter from scratch, I suggest you
try one of these semi-machined backplate adapters from LittleMachineShop.com
(size depending on your chuck requirements):

4" Dia:
http://www.littlemachineshop.com/pro...ProductID=1792

6" Dia:
http://www.littlemachineshop.com/pro...ProductID=1919


Regards,
Michael