BP knee stepper selection

[Sparky_NY]

I could use some input on stepper motor selection for the knee of my manual bridgeport retrofit. I am having a tough time deciding which motor to use.

First, the mill is already retrofitted and been running nicely with steppers for the last year so I am going to stick with steppers, servo drive for the knee is not a option. Second, the knee is going to be used only for tool lenght offsets at tool change time, the quill is under cnc control presently and used for all the Z moves while cutting. Third, I installed a set of gas springs on the knee to lighten the load required by the motor.

I rough calculated the force for the knee by using a fish scale on the handle and doing the calculations. Before the gas struts the knee required about 10lbs force on the handle, with the gas struts in place it is now 2-3lbs. Calculating by the crank length etc that works out to about 350 oz in of torque.

Naturally, I would like a reasonable speed on the knee and using the original drive mechanics in the knee of .1 per revolution it would take 500 rpm at the crank to yeild 50 IPM of the knee. I'd be very happy with 50ipm if its possible.

This is where the fun starts! It appears both the 1800oz and 1200 oz motors drop to about the same output torque at 500rpm. IF I am interpreting the charts right. For that reason I am looking at 1:1 belt drive ratio between the motor and crank to keep the motor rpms down where reasonable torque can be obtained. I am also considering the 1200 oz motor with maybe a slight gear reduction to the knee, something like 1.5:1 to increase the torque which might be a option with the flatter torque curve of the smaller motor. Yet another option would be the larger motor and a belt drive increase around 1.5:1 that would lower the motor speed into a better area of its torque curve and make use of the larger motors higher torque at maybe 275 rpm or there abouts.

So, am I reading the charts right? And any input on which of the two motors might be more suitable would be especially appreciated. I am running gecko drives at 63 volts by the way.

Thanks much for any suggestions!