Re: Threading on CNC lathe

>
> > As a second question, how good is the accuracy of threading on
> > systems with encoders on the spindle or chuck?
> >
> > Clearly it must work, but I still have doubts considering the
> > rotational speeds of the work, and the inertia of it, and the tools
> > themselves.
> >
> > I'm talking single point on CNC lathes in particular.
> >
> > Is it really possible with any accuracy, and can it be
> > done in multiple passes and pick up at the correct location?

On a good, tight CNC lathe, with a good ballscrew, it gives BETTER
results
than a conventional lathe. The gear trains on the average screw cutting
lathe are
not the best, and each gear contributes its own cyclical error. Add up
the errors
from about 8 gears in the train at any particular setting, and you can
accumulate
quite a bit of error. Also, the split nuts and worn leadscrew on an
average lathe
can deflect the carriage, causing cyclical fluctuations in the diameter
of the part.
(These are small, but I can see it as a pattern when using the tool post
grinder.)

Of course, it can pick up at exactly the right position. It would be
inadvisable
to change spindle speeds in the middle of a threading job, because that
would
change the following error, but as long as you run at the same spindle
speed,
it can pick up right in the center of the last pass, plus or minus about
.0002"
or so.

Jon