Originally Posted by
awerby
Steel box tubes are stiffer than aluminum extrusions, and cost less. If you build a machine with big enough pieces of box tube and pay attention to details, you might end up with a machine that will do an acceptable job of machining aluminum. Steel, not so much - take a look at mills with that part envelope designed for cutting steel to get an idea of the mass and rigidity required.
Sag in your gantry is proportional to the height and thickness of the tube selected; if you use a massive enough tube, sag won't be much of an issue, although inertia may be.