Hi Andy,
I was also wondering about the double Z axis arrangement, thanks for explaining.

Ross,
Yes, there are some risks from bearings on ali section. My machine is still some way from running yet so I don't know how it will cope with bearing surface wear. I picked some reasonably sized bearings to reduce the surface stress. The bearings are on axles which can be adjusted in slots, so I can add a running surface at a later date.
Since I'm only machining balsa and liteply, and all 2D (formers, ribs etc.) , my requirements are more simple than some and I think this will be OK for my needs. I know what I'd really like to do, but it's all a question of finance !

Barry