After adjusting the backlash, cleaning, oiling, greasing and and putting the mill back together, I decided to install my new latest-and-greatest revision of the stepper motor driver and give it a try. Now my backlash was reduced to 0.001-0.002 " in Y direction, and 0.003-0.004" in X. I am not sure why I failed to reduce the X backlash further. Maybe I can play with it a little more some other time, but for now it is "good enough". I did not play with Z due to lack of time. Besides, there's a pretty heavy motor with headstock to minimize any adverse effect of backlash in Z.
I was experimenting with X axis, running a 2-stack 208 oz*in pacific scientific motor on it. The controller was set to provide 4.60A @ 48 volts, which is the maximum recommended current, but only 2/3 of nominal voltage for the motor. So, I expect to get good torque at low RPM, but performance to be sub-par at higher RPM. I used mach2 software for this experiment.
If I set acceleration to the maximum 31.25(?) in/sec^2, I can jog at up to 110 IPM without loosing steps. At that point I start noticing lost steps at the right end of the X axis. Reducing acceleration to 5 in/sec^2 allows me to overcome that problem and reliably operate at 120 IPM. I cannot stall the X axis by hand no matter how hard I try at this point. Binding problem reappears in the same region at 135 IPM with the same acceleration. If I stay away from the problematic region, I can increase the speed a little further, just short of 150 IPM, at which point I can stall the table by hand.
Based on the results of that experiment, I set the table to move at 90 IPM in X and Y and 30 IPM in Z (it is too scary to jog Z faster -- it is too close to the table for comfort). It looks like that should be a safe setting, and should work well. Time will tell if I am right...
