Graham
You've seen Clyde's approach to a torsion box right?
http://cnczone.com/forums/showthread...ht=torsion+box
You could build it to support the HMS Ark Royal if needed! Or use it as a vacuum platform as well as a base for the machine. It could be Ali and rivetted or otherwise mechanically fastened - or built of composites and glued up. Another member used recycled tyres (well here thery were -ires) to add some dampening. I suppose that could add some dampening and still allow you to pull a vacuum thru the base and elminate the need for building a T-slot base at the same time.
Mass is always good - till ya gotta move it! Its worrying the mechanical stability, resonance, cost, availability, and fabrication that gets added to the seemingly simple choices -- eh?
Cheers - Jim
Experience is the BEST Teacher. Is that why it usually arrives in a shower of sparks, flash of light, loud bang, a cloud of smoke, AND -- a BILL to pay? You usually get it -- just after you need it.