Thanks for the suggestions Jonathan. I've redrawn the x axis screw so it is as close to the supported rail as possible. This has bought the ball nut mount up by 60cm.

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Regards the z axis. I had no idea I had to consider the height of profiled slides when I bought them of eBay. Lesson learnt, and if as you say I machine about 10mm grove out of both z axis plates, I still need 10mm. Say I grove out 10mm from the front plate, then sink the ball nut even more, up to a point where the screw shaft just clears the front plate, then I might just get away with that last ten. I'll draw this later to see if it is feasible.

I would definitely consider outsourcing to one of you guys to machine these plates as it will make a much sleeker z axis and also bring the spindle cutting point to within the x axis bearing blocks.