Re: [CAD_CAM_EDM_DRO] Z-Axis CNC Control of Grizzly Minimill

brucen@... wrote:

> I understand it is not straightforward to convert the quill feed of
> the Grizzly Minimill to CNC. Could someone please explain in more
> detail the reason for this? Is it because the whole motor moves up
> and down and has to be counter balanced with springs? Why can't you
> just hook the stepper motor up to the original quill handle shaft
> with a timing belt like the X and Y axes?

There are several reasons for this, on almost all machines with quill
feed handles like a drill press. These use a rack and pinion to drive either
the quill or the entire head up and down. Although rack and pinion drives can be
made with minimal backlash, it is not that easy to do, and the coarse
racks in quill feeds have notorious backlash, often .050" when new.
This assures they won't bind up, but makes precision machining
impossible. Attempting to bias the quill feed against one side of the
gear teeth is a very iffy proposition, and can lead to excitement when
the cutting forces suddenly pull the quill or head to the other side of the
backlash.

The second reason is that the low precision an loose fit of the rack and pinion
are very unlikely to to provide linear motion. By that I mean that there
is a cyclical error as each gear tooth engages, sweeps through its arc and
disengages. This can cause positioning errors of several thousandths of
an inch in Z axis movements.

Really, the only workable solution is to bypass the quill or head rack, and
install a tight screw, preferably a ballscrew that drives the quill through a
straight linkage. This requires the screw to be as close to the side of the
quill as possible to reduce bending moments on the linkage.

Jon