So on the outside rails you could use heavier material with some support. Ground and polished would make for nice outer rails although I would imagine its quite expensive. However I believe Ryan is correct here that you don’t want to use heavy material like that for your inner x, y, and z axis. Keep them lighter so they are easier to move around. Now if you intend to always move the machine and cut very slow then maybe it doesnt matter. And depending on how long your x and y are will depend on the deflection. But solid bar does bend under its own weight. Just cause its solid doesnt mean it will bend less. Its a function of size vs weight x length vs type of material. If you dont believe it go to the hardware store with some rebar and pick up a piece by one end. It will bend like crazy under its own weight. Then pick up a piece of conduit the same way. Much less bend. But it still bends or bows.