Before I rush off and redesign the frame again I'd best ask a few questions :-).

What is the maximum length length I could reasonably use RM1610 up to and still get a good feed rate? I see people talking about 7 and 8m/min as the top speed they run their machine at so 6m/min doesn't seem so bad. I suspect though that 6m/min on this length is like getting 100mph out of a Nissan Micra - possible, but not advisable.

Lets say for a moment that I find a few more pennies down the back of the sofa and I can afford RM2510 screws and Nema 34 steppers. Would I then have to beef up all the electronics as well? The 34's seem to be about four times the price of 23's (on Zapp) if that's also the case with the electronics I think it puts it out of reach. Could I use 23's on Y and Z or would they have to be 34's as well?

Am I right in thinking that the problem with the screw having a bend is a reduction in the critical speed? If I understand correctly the critical speed is the point at which the screw starts whipping around, yes?

Yes, the bed will be fixed for rigidity. On another thread someone suggested that for machining aluminium I'd be best off raising the piece up on a stand to reduce the Z-axis extension which seems like a simpler solution than trying to make the bed move up. I want to work aluminium but I don't see myself machining large pieces so a stand is a pragmatic solution.

The side sheets were there mainly to prevent chips flying everywhere but I'll add some additional brackets to the design so they act as bracing.

Cheers.