Ignoramus3498 wrote:

On 2011-02-24, Pete C. wrote:

Ignoramus3498 wrote:

On 2011-02-24, Karl Townsend wrote:
I'm taking the knee apart on my CNC mill and sending out for
rebuild...

I have an extra ball screw on hand that is perfect for replacing the
Acme hand knee lift screw. I'd then put a servo motor on the knee.

I did some preliminary layout for initial design of how to do this. it
will work best to rigidly mount the ball screw and spin the ball nut
with the nut located right at the bottom of the knee sitting on a
taper bearing.

Anyway, with this layout it would be all but impossible to change out
a timing belt. You'd have to take the knee apart. (There's always a
design weakness for maintenance)

Anyway, a #35 roller chain would make for a far easier and less
expensive installation. Given that the knee will only move slowly at
tool changes and always have downward force; is this causing another
problem that I don't see?

Karl

A good timing belt should last a very long time under these
conditions. I would expect the condition of the 1,000 lbs knee pushing
on the ball screw all the time, to be quite punishing for the ball
screw. If you have pneumatic support, it would help.

But please keep us posted, this sounds like a very interesting plan.

i

Second the recommendation for a pneumatic or hydraulic counterbalance
and an axis brake. Pretty much all commercial CNCs with heavy Z axes
have such a counterbalance, as well as a mechanical fail-safe brake to
prevent the axis from dropping when the servo is not powered.

Mine does have a pneumatic counterbalance.

As for coupling the servo, I'd go with an oversized timing belt.
Something like a 2" wide belt could be reasonably expected to outlive
you as long as the pulleys are properly aligned.

Those timing belts live just about forever. They look like new on my
mill.

The failsafe brake should work even if the timing belt breaks,
though. (iow, it should not be on the motor). Not a big deal, but
would be preferable.

It is a great project, take some pictures and keep us posted.

i

Most commercial CNCs don't use timing belts to couple the servo, so the
brake is usually inline. An oversized timing belt shouldn't brake in our
lifetimes so we don't need to worry. Even if it did, the table would
lower at a moderate rate, since there are losses in back driving a
ballscrew. If you have flow restrictors on the counterbalance cylinders,
they will limit the drop rate also.