Driving the gantry on Z axis?

[Robin Hewitt]

Sounds needlessly complicated. Could it be you have no clear idea of what this proposed machine will be designed to cut so you are trying to make it universal?

[Skipsoaring]

Thanks Robin. Possibly true, although I do know that I want to create dished moulds in Aluminium (possibly stainless if it's achievable) block up to 2000mm long and no more than 1000mm wide. The depth of cut will be no more than 100mm.
I believe that ideally I would want the spindle stationalry and move the bed so was thinking that removing one axis movement from the spindle would help with strength.
I'm not sure I would have the space to accommodate a 4m+ machine (moveable bed).
I suppose one thing in my favour is that I don't want much width with the length?