Looking at your picture, you have the Z ballscrews bearings bolted to the fixed Z plate, and it looks as if the ballscrew nut will bolt to the moving Z plate. How are you going to adjust the nut or bearing position, or the guide rails, so that everything is nicely aligned? I've been looking at these details on my own design for a while and am looking for inspiration! I have some ideas, but always keen to re-use other people's work...