I have been working on the Z axis drive. My original slap together conversion of the X2 used the rotating nut, fixed screw design used by Daniel Kemp. I decided to look at other designs since I had the chance to start over. The fixed screw, rotating nut designs are much simpler to execute, but they all have what I consider a serious fault. The screw extends into the swarf generating area. The lubrication on the screw would retain metal particles. The Asian 1605 series ball screws that I am using do have a wiper of sorts, but they aren’t very effective. The X and Y axis are mostly protected by the saddle and table, but the Z axis is not easily covered. The rotating nut is more complicated to execute, but the screw stays above the head and therefore out of the primary swarf cloud.
The axle and bearing stack that carries the drive gear and the ball nut has a problem with backlash in the Kemp design. In the cross section drawing, red represents the axle, blue the retainer plate, and green the thrust bearings. The only way to adjust the pre-load, or lack of, is to change the length of the axle by putting shims between the axle and the retainer plate. That is a bit inconvenient when trying for minimum end play (backlash) and adjusting for wear.
A better way to handle fixing the axle would be using the same way the X and Y axis do.A single nut with set screw or even a double nut that would allow adjustment to eliminate backlash. The top of the axle where the gear and screw will mount is 2.5, the axle is 1.25”, and the threaded portion is 1-14. I didn’t have a nice piece of 2.5” round bar laying around, so I shrunk fit the axle to the top. I heated the top to 275F and froze the axle for an hour. It dropped in a little over half way before seizing up. Being prepared for such an event, I whacked it twice with a #2 hammer to seat it.