Re: Re: Threading on CNC lathe

>
> From: Jon Elson <jmelson@...>
> > > 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

Yeah, I guess thread design and tolerance is based on allowance for
such errors.
Maybe I overestimated how clode the tolerances are.

> 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,

The following error, and the fact these are done in a lot of light passes,
as in 10-20 on 1/2" bolts, are 2 factors I didn't count on.

The errors, although there due to acceleration of the tools, and lag, are
consistent, that explains a lot.

The practice of a lot of light cuts, which would not slow the spindle as
much would reduce the error I envisioned there as well.

Thanks all.

Marc