Re: [CAD_CAM_EDM_DRO] SeriesII Quill Zaxis CNC addon ideas: Like Jon E's?

Marv Frankel wrote:

>Tim,
> Thanks for pointing out the screw wear problem to me. I hadn't even
>considered it, but it makes sense if you think about the constant
>up-and-down in almost the same area on the screw. I was pointed to some
>pictures the other day, at http://www.motovidia.com/MILL%20pics/, which
>showed a Z axis knee, with a ballscrew, and to keep the knee from falling
>through gravity, he had to mount a 150 lb. gas spring from the knee to the
>base, to balance it. I wonder if the tendency of the knee to fall, can be
>overcome by using a ball screw with a greater number of TPI?
>
>

Not reliably. You will find that ballscrews finer than 5 TPI have to
have smaller and smaller
balls, which limits the load-bearing capacity. I have a 10 TPI
ballscrew on my quill Z axis,
but I sure wouldn't recommend that for the knee. If you don't want the
gas spring, then you
need a fail-safe brake on the motor. This is a pretty common
arrangement on CNC systems,
anyway.

Jon