I did a retrofit on a Chinese mill/drill. I used 425 in-oz motors with 0.1"
pitch lead screws on the X and Y. I would have preferred even more torque
but the motors were free. The machine worked OK for most small jobs and I
didn't have problems with loosing step except at fast speeds. Step motor
torque goes down fast as the speed is increased. On a Bridgeport I think
the lead screw pitch is larger like 0.2" or 0.25" so I would definitely
recommend a 2:1 reduction on 300 in-oz motors. Most retrofits and even
Bridgeport CNCs have the z axis motor on the quill not the knee. I think
that makes more sense considering things like drilling cycles.

John


Jeridiah wrote in message ...
I am considering doing a CNC conversion on an older Bridgeport. I am
wondering what the torque reqm't would be for stepper drives. I have
some 300 oz steppers, but I don't think they would be big enough.
Maybe ok for the X/Y, but definetly not for the Z(if I move the knee).

Anyone out there have a good number to stick to these? For now, I
plan to use the stock ACME screws, but would like to upgrade to
ballscrews at some point. I know that the ACME screws are going to
have a higher torque reqm't.

JW