Hi Barry

I dont think you would have any problems with racking if you have a single drive on the x axis. Your gantry is identical to mine and I get no problems even on aggressive cuts.

I dont see any advantage of a 10mm pitch on the z screw. x and y speed are important but how often do you need to plunge into the workpiece at high speed? With a 5mm pitch you will not need a brake (or a spanner).

It looks good to me, but then I'm biased :)