Thanks for the comments, Eddy - I guess that my ideas are sufficiently unexceptional that no-one else has commented. Not surprising - I think I've nicked most of them from this forum anyway!

I agree that there's some bracing and triangulation missing - across the closed end of the bed (can't put anything across the open end as I want the spindle to be able to cut off the bed - although I guess I could extend the top rails a bit further still and then put in a cross-bar) and a bit of triangulation on the bed wouldn't do any harm.

I'm a bit nervous about starting a build, ordering ballscrews (and to a lesser extent rails), and then finding that they don't quite fit - seen lots of comments about not knowing exactly what design of support blocks, etc, are going to arrive - so next step will be to do a bit more detail on the drawings, work out exactly what I think I shall need, order the bits, and then tweak the drawings to suit and then start building. Going to be head-down in the 3D CAD for a bit...

Back of the envelope sums seem to say 2010 for the X axis ballscrews - gives .0125mm resolution with 400 microsteps and 1:2 belt drive from motor to screws, and 6m/min is 600RPM which looks to be well under the critical speed for that length screw. Might as well make Y the same pitch so that would be 1610, and 1605 for Z. Should be fine for wood. I'll probably go for 20mm X rails and 15mm for Y/Z as much as anything because that feels about right (based on zero experience of using them, of course...).

Back to the drawing board with a bit more focus on detail.