Re: Sherline Spindle Pulse disk for threading

--- In CAD_CAM_EDM_DRO@yahoogroups.com, John Dammeyer <johnd@...> wrote:

>
> Hi,
>
> > The comment about the spindle slowing is the exact
> > reason why
> > >threading with one pulse per rev "shouldn't work well".
> >
> > But it does :)
>
> Yes. As long as the spindle speed remains constant. If it varies the
> thread may end up looking pretty awful. But here's the rub. If the
> spindle is under powered and slows down under slight loads the issue
> isn't with the one pulse per spindle rev but with the speed control on
> the motor which changes the SFM and can also screw up surface finish.

I have made a few threads on Sherline lathes. The Sherline has some
factors that affect the threading capability.

1) 1/3 horsepower spindle max. It's not going to change in this size
machine, without an investment in a spindle and controller that costs
more than the whole machine. The controller has load feedback and will
maintain speed under load as much as possible.

2)With a 2.3-3.1 inch chuck it is very easy to combine tools and
material in a depth of cut that grossly exceeds the spindle capability.

3) Threading with carbide is diffucult at best with a low hp spindle,
so most users stick with HSS, which permits wide variations in speed
without chipping, galling, or affecting size and finish. This helps
with those conditions where the spindle actually stops turning under
light cutting conditions. Carbide almost always chips immediately if
stalled.

4) The Sherline lathes with stepper systems typically have a maximum
cutting feedrate of about 20 ipm. This yields a max RPM of 400 RPM on
a 1/4-20 thread and around 260 rpm on a 1/2-13. A 32 tpi thread is
around 650 rpm. A stepper system can usually handle a 10-32, but a 1/4-
20 is a challenge. That's about the best the machine can do with
steppers. Our servos on the other hand typically run at 45 ipm, which
yields a spindle speed closer to the cutting speeds of Steel(up to
about a 14 TPI). Softer materials can be cut at 3-5 times the cutting
speeds of steel, which really limits the stepper systems.


We use a 500 line quadrature encoder with index pulse. This permits
the Sherline lathe to stall under a heavy cut, and yet by turning the
spindle, the 2000 counts per revolution keeps the threading tool
synchronized to the spindle. It also permits the spindle to lug down
to whatever speed it can maintain under the given cutting conditios and
still maintainthe tool position. Simply forgetting to oil the thread
between passes can sometimes cause enough load to stall or severely
slow the spindle.

Here is a link to some steel threads that were cut on a Sherline:

http://www.imsrv.com/sherlinemachines/#lathe

This describes how the threading process is done:

http://www.imsrv.com/deskcnc/lathe/desktopcncthreading.pdf

Fred Smith - IMService
http://www.cadcamcadcam.com/hobby