The first step in the design was to select a set of ball screws and motor mounts. Like so many before me, I thought that I wanted to keep the hand wheels on each axis so I could still use the system as a manual mill. Also like most others who make this choice I find that I will probably never use the system manually again. It is just too easy to write G Code to do the same thing but better than I cold by hand. Anyhow, I selected the conversion kit from CNCConversionKit.com. I selected the NEMA 34 motor mounts and double ball nuts for each axis. I think this was a good choice. My backlash on X and Y is less than .002".
There are several things about this kit that I would do differently if I could.
1. The ball screws have been turned down at the end to accommodate a coupler to mate with the motor shaft. The shaft diameter where the coupler mounts is only 8mm in diameter. Also there was no key or flat milled into the shaft. With my NEMA 34 servo's the provided couplers almost immediately spun on the screw shaft. No amount of tightening the set screw on the coupler would keep it from slipping. To remedy this, I had to replace the flexible couplers with Spider couplers that clamp onto the shaft instead of using a set screw. This solved the problem, but I am still only able to set the maximum allowed torque of the servo's to 50%. On my next build the screw shaft diameter will be as large as possible, and definitely no less than 12mm.
2. Optimally the axis screws need to be setup so that there are opposing ball bearings at the end of the screw to zero out the backlash. This kit, although there is a bearing at each end, they are not setup so that you can adjust the runout of the shaft to zero the backlash. I had to remake the mount for the Z and Y so I could add an additional bearing.
3. The finish of the parts is not the best. Most of the parts I put into the mill and removed a few thousandths to clean them up.