Knee or Quill Drive?

Hi Group,

I am in the process of converting an Enco 9x42 mill to CNC. X & Y
axis are working fine and I recently connected drive to the knee. I
am using 600oz-in peak servos with G320s, all axis are geared down
2:1. Using Master5 X and Y feed limits at 46 in/min, Z loses track at
3 ins/min. The torque measured to lift the Z axis is 40lb-in (belt
off). The measured motor current (lifting)is 8 amps, measured across
a 0.1ohm resistor with a 'scope, ripple across the 37 VDC supply is
2.5Vp-p. From the limited data on the motors the peak current rating
is 12 amps max, nominal voltage is 38 volts, RPM is 900.

The question I am now facing, and asking the groups advice on, is it
best to carry on working on the knee drive or would I be better off
somehow driving the quill?

To date I have not replaced the Z acme screw, how much improvement
would a ball-screw give and would it add further complications like
the knee dropping if the drive kicked out?

How large a motor do I need for the measured torque? From the
measurements and talk with Marriss I could and should improve the
supply with additional capacitance and the supply voltage could be
increased some, but the fundemental problem seems that the motor is
too small. Does anyone have a source for reasonably priced brush
servos?

What strategies do you recommend for driving the quill? I've looked
at Jon's method of driving a dog through the stop mechanism in the
quill and also seen a pulley on the quill coarse feed lever. Any
others?

All comments / suggestions gratefully received.

Bernard Randall