Counterweighting the Z axis ?; Power off position holding ?

We are in the process of retrofitting (to cnc) a square (Z)column mill.

We are using a servo/ballscrew drive on each axis, including the Z
axis. The milling head weighs over 150 pounds with the motor, slide,
etc. The ballscrew and ball nut setup is such that with no power to the
motor the milling head will slide down the column as soon as power is
removed from the servo.

I'm interested to hear how people are combating this issue. Should one
counterweight the Z axis ? Even with a counterweight, it is possible it
will move during a tool change. We will be using emc and a yet to be
determined driver to control the mill.

Should we put a slide lock on the Z axis so that it doesn't move when
the power is off ?

--
Kim Lux <lux@...>