Re: Z axis on the knee, how practical????

Nice work Jon. I had the "pleasure" of installing one of the first
ever Z axis mods, let out of captivity by BP for an EZ-track. Rigid
was a word that was never used anywere near the thing.
They didn't use your setup the first time, instead a bracket was
installed on the nose of the spindle and reached off the side a bit
to the ballscrew.
From a rigidity perspective moving the knee is better.
Deckel and Makino are two examples of "real" CNC kneemills that use
the knee as the Z axis.

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> aaalfano wrote:
>
> > Jon,
> > How did you convert the quill on the Bridgeport with ball screw
& ball nut?
>
> I managed to obtain a surplus ballscrew of high quality. I looked
at some
> other retrofits, including Bridgeport's own Ez-Trak, the Anilam
and such.
> These all drive the quill on a standard J head from the hole that
the stop
> ring normally mounts to. So, I decided to duplicate the same
setup with
> the screw that I had. See http://pico-systems.com/zaxis.html for
> some pictures, etc. The tough part of this is that the rounded
shape
> of the J-head and especially the belt housing were impossible to
> measure accurately, and when I got the whole thing built, the
encoder
> on top had about 1/2" of interference with the belt housing. I
should have
> cut cardboard templates and test-fitted them before cutting
metal. So,
> I had to slot the two mounting holes and move the whole assembly
about
> 1/2" farther out from the quill so the encoder would clear. I had
not cut the
> link that attached to the quill yet, so I didn't have to patch
anything
> there. I had made it extra long in case such a situation came up.
>
> This contraption seems to have about .0012" backlash, due to flex
in
> the parts. Most of that is rocking of the link. the ballscrew
doesn't
> prevent the link from rocking a little, and the small area of
contact on
> the quill doesn't, either. I don't think it can be done much
better than
> this with this arrangement. So, I guess that is why the old BOSS
> machines used a coaxial ballscrew drive for the quill.
>
> Jon