CNCing the knee axis...

Hi people.

Has anyone ever CNC'd a BP clone by controlling the knee instead of the
quill ?

We are about to retrofit a BP clone and are thinking of controlling the
knee. We've already CNC'd a square column milling machine with a heavy
head. We counterweighted the head to make it lighter. It works well.
We are moving it at 100 IPM with ballscrews and Ametek motors, direct
drive.

Our CNC retrofit would replace the acme screw and 90 degree drive with a
ballscrew and belt drive. We'd like a 2:1 ratio (motor speed to screw
speed) to retain some Z axis speed.

I've got two concerns with controlling the knee:

a) does the knee remain square to the machine when not locked ? Ie is
the knee designed to be an axis feed, ie moved up and down without being
locked ? (my friend tells me no...) Our mill does not have a gib
adjustment on the knee.

b) how would one counterweight it ? We used actual weights and a cable
system to counterweight the head on the square column , but the knee is
going to be heavier than that head was. I'm thinking of constant
pressure air cylinders ?

Thanks for your thoughts and advice.


--
Kim Lux <lux@...>