Ross i did say that the plate could be moved further back. In this design there is no reason why the ballnut could be more central and the spindle moved closer to centre. There is no need to have the nut pushed further away from centre.
Paul I think we must be looking at different drawings

If you look at the Z axis drawing in post #18 there is barely enough room to flip the plate over. if you then centralise the ball screw you will have to move the Z axis assembly further away, making the situation much worse.

As I said there are other more pressing issues with the design but as the parts are bought and he wants that size working envelope then there's not much that can be done other than maximise the other components. Another reason for not moving the ball screw is that despite the large plate on the back the y axis could still twist if Mike decides to cut larger pieces of Ali. with the ball screw in the front it is a simple case of widening the y beam to increase its torsional rigidity.

Anyway I think we will just have to agree to disagree on this one. We all have different ideas and its up to mike to decide as ultimately he knows what is needed from the machine.