There was some discussion about this, but it was on another thread about an older machine, so I thought I would put it out here for Mill Turn owners only.
The standard configuration for the Mill Turn is a 4-1 reduction from the motor to the drive pulleys, so the speeds are set at about 15 IPM. That is more to do with the RPM of the motor than the torque, because steppers reach a point in RPM where the torque drops off very quickly. I figured I would try to find that sweet spot where the torque is at its best and try to increase the rapids. With some measuring, I found that I could replace the 18 tooth motor drive pulley with a 24 tooth and still use the original mount and belt, so just a pulley swap was all that was necessary. Going to a 24 tooth pulley changes the ratio to 3-1, so theoretically it increases the speed by a factor of 1.33, boosting the speed from the 15 range up to about 20. But as a practical matter, because the motor speed is reduced, it seems closer to that max torque point, and I could actually get 28 IPM. That was the limit for the upstroke lifting the mill head casting , motor etc. which probably weighs 150 #. On the down stroke, the IPM could run at 60 IPM easily until it reached the RPM limit, so I figured that with some lift assistance on the weight of the mill head, the rates could probably be somewhere in between 28 and 60. I had a pair of old gas struts that came off a hatchback and they had a 12" stroke. For a super scientific rating, I just placed one end on a bathroom scale and pushed down on the other watching the scale until the strut started to compress. That was at about 80#, so I figured 2 of them would give me 160# of uplift on the mill head. Just to test my idea, I simply propped each strut up under the mill head on each side of the spindle cartridge. Right away I could see an improvement- I kept upping the speed in the motor tuning file until I reached about 45 IPM where it began to stall on the uplift, on the downstroke it would go a bit higher. So I set my speed to 40 IPM and ran it through a bunch of cycles with no stalling. Mounting the struts took some figuring, because you can't mount them in the center of the mill head, and on each end you cannot put them in the center either. Finally I decided to try one strut on the tailstock end just behind the metal shield and the other on the headstock end coming up from between the lathe motor and casting. This gave me equal pressure on both ends and diagonally across. The struts have nice male ball joins on each end with 8mm threads, so I just had to drill and tap the casting for the lower mounts and then made some simple angle iron brackets for the mill head mounting. Its all up and working, and 40 IPM is a big improvement for the rapids. I think that I could go to even a bigger motor pulley like a 36 and get to a 2-1 ratio and maybe get even better speed, but that would require changing the motor mount, cutting away the shield cover and a longer belt.