I know this thread was recently brought back from the grave, but for what it's worth -
For the X and Y - For X and Y I used size 42 steppers through a 1.5:1 ratio with 5TPI ballscrews on my BP retrofit and I can reach 135+ IPM using 48 volts (I have never measured the actual Amps) to the steppers, but I de-tuned the motors to achieve around 90 IPM to guarantee there were no lost steps during acceleration and rapids. Even with the gib locks snugged down, this arrangement is powerful enough to move the table and saddle. If the gib locks are really cranked down, the motors will stall out. 90 IPM is fast enough for me for the table and saddle. I am not 100% sure, but I believe the steppers I used are around 800 - 900 OzIn and again, I went through a 1.5:1 reduction.
For Z axis - I chose to drive the quill through the original, manual handle axle. There is a fair amount of backlash but I have found ways to get around it. I am using a small, NEMA 23 stepper (276 OzIn I believe) on the quill drive through two, 6:1 reductions - one on each side of the head to provide a resultant 36:1 drive to the original quill handle which has a slightly defeating ratio. This screwed up arrangement proved to be ridiculously fast - I had to de-tune that also. It is now more than fast enough for peck drilling and such, but it does not have enough power to push a 3/8" drill through aluminum at a normal feedrate. I am likely going to replace the small stepper or increase the reduction further. In fact, I am headed to Automation Direct right now!
I think driving the knee is not a really good idea for a Z axis on a BP retrofit. The knee was not really intended for that purpose - that's why the quill has 5" of stroke. I evaluated it when I was designing my retrofit and eventaully decided against it - even counterweighted, it is cumbersome. Driving the quill, in the opinion of many, is a better solution and there are several ways to accomplish this.
Scott
Consistency is a good thing....unless you're consistently an idiot.