On Mon, 25 Aug 2003 06:12:59 GMT, "DeepDiver"
wrote:

snip
Some argue that the threads on threaded spindles are also partly responsible
for keeping the adapter (and chuck) located with respect to the spindle, so
they should also be well-finished and of close tolerance. You should make a
mock spindle plug that is an exact copy of the threaded portion of your
spindle nose, including the locating shoulder. That way you can test the fit
of your adapter while you are machining it (you obviously do not want to be
taking it off during the machining process to test against your actual
spindle nose).

The fitting of a chuck backplate to a threaded spindle has been
discussed here a number of times. Close fitting threads (short of any
binding) are good but the critical point of fit is where the back
plate goes over the unthreaded portion of the spindle just past the
end of the thread (the counter? I can't seem to come up with the right
word) and then contacts the face of the shoulder behind the thread.
Having about .0005 or .001 clearance there and full contact with the
shoulder provides very positive and repeatable locating.

If you have another pretty good chuck, a good way to fit a back plate
is to mount the rough casting facing backwards in that chuck (3 or 4
jaw, don't unchuck it for any reason until finished). Then bore it,
cut the thread, face the back shoulder perpendicular to the thread,
and chamfer the shoulder (both inside and outside edges).

Then start relieving the thread to the depth (a little extra is OK on
the depth to avoid thread bind) and diameter (critical!) that will fit
the counter. When you get mega close, simply take the chuck off and
turn it around to test the fit.

When the counter recess in the back plate will wipe blueing off the
counter shoulder, quit cutting, and polish it well. With any luck it
will wipe the blueing on both the counter O.D. and the shoulder face
and you're done with the critical part.

You may also want to take clean up cuts on the outside of the part
that goes over the spindle and the back of the plate. If you see
small holes or pockets don't try to go deep enough to get a clean
surface, you may run out of material first.

When you can wring the backplate down against the shoulder by hand you
are ready to turn it to the needed O.D. and clean up the face. That
can be a challenging cut, you may have to change speeds part way
through the cut to reduce the surface speed if it starts chattering.

Dirty work? You have not seen dirty work on a lathe until you have
machined a rough casting. Especially the cheap imports. There are no
chips, just powdered graphite and dirt.

For the Bison with the back recess, fit the O.D. to the recess and
chamfer the edge to avoid bind. Maybe even cut a very slight taper
towards the front so that the back plate is pulled into the recess and
flush against the chuck by the chuck mounting screws. Steal the
mounting bolt layout with a close fitting center punch through the
bolt holes.

You'll feel good about yourself every time you mount a chuck with a
well fitted backplate that you did yourself. And it will repay the
time spent almost every time you use it.



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