Hi Adam,

I think you're probably about right - I'm planning something similar, myself. No reason for not using bigger rails (25mm?) on the long axis, as they're not moving, but lighter for the gantry, of course; supported: definitely - can't see the point of doing it any other way - and why not just make them structural as well?

As for ball-screws, thinner limits the revs for whip, but thicker makes for greater inertia, i.e. limits acceleration, so it's a compromise (there was a discussion about this - with diameters/lengths and revs., on another thread, somewhere). But it all wants to be as concentric as possible, no obvious run-out or mis-alignment, good bearings; and well-balanced. Vibration analysis would be a Good Thing, if you can find someone to do it... (optional !-) Only just spotted your query, I'm new, too. Hope it helps.