I was having a problem where the driver ICs were overheating when doing slow moves. Solution? Cooling fans liberated from Pentium heatsinks and a little G02 action on the top of the stepper driver box.

The airflow has eliminated my driver chip overheating problems.

They look 'cool' as well.

To recap the setup so far, there are 55 oz-in motors on all axii. I run X and Y with a rapid of 15 ipm, Z set slightly slower. When the mill is adjusted and lubricated properly, it will reliably travel faster. However, 15 ipm is a conservative setting to allow for a 'safety margin'.

X and Y share a 24V power supply, Z has two supplies for a total of 41 volts (the maximum for the Allegro SLA7042 driver IC is 46V).

Driver for all three axii is a microstepping, chopping unipolar-style (not to be confused with those cheap 4-transistor unipolar drivers).

For some Z-axis experimentation, I got a Gecko G201 bipolar driver, a 48V switching supply, and a low-inductance 3A Vexta motor.

The motor is small, about the same size as the 55 oz-in motor, but square instead of round.

After removing the handleweel (it was making the whole mill shake), I was able to achive 60 ipm rapid on Z! Yes, 1200 rpm. Very fast!

And there was still some torque at that speed. I couldn't stall the motor by pressing down on the headstock as it was rising.

If I keep this on Z, I will probably limit it to 28-30 ipm, both to have a safety margin, and also so the mill doesn't shake itself apart.