oh I don't know, alot of the gantry machines are Aluminum, I've done some hybrids, either epoxycrete/al or steel/modular al superstructures, depends on where your needs fall. 99% of my customers (signshops) don't need sub thou accuracy. Heck, some machinist I've talked to will actually admit (reluctantly) to the hollowness of the .0001 accuracy of their machines in non temp. controlled shops, w/o coolant fed tools, leadscrews, etc. I've worked on some big, big iron (Okuma, Makino) but in reality outside of heavy, production manufacturing is it really needed? Can you hold tenths without temperature control and aggresive tooling and workpiece cooling?
(in my experience failure to account for coeff. of expansion errors swamp machine errors, by order of magnitudes)