I'm considering building a large format router/mill to carve wood and stone, and would like to beef it up as much as I can, and am curious to know if it's within all my powers to even make it make cuts into steel. I want to make it about 2 by 3 meters, and put a 4th axis headstock and spindle on it.

At this point I'd like to discuss the build just one aspect at a time. Let's assume I'm building it into a perfect rectangle that I've carved out of pre-cambrian rock mountainside, and that the gantry will be made of pure magic. Magic that weighs as much as required to get the job done. And assume that there is a machine shop around the corner that can lathe and outside diameter hone heavy gauge steel pipes to accurate dimensions.

Here is an idea for the rails I'd like to get feedback on.

The rails would be made from high carbon steel tubes filled with epoxy granite. Perhaps 5 inches outside diameter. A slot would be milled in them and two would be joined in parallel with a 12 inch wide epoxy-granite plate. The two joined rails should therefore be quite stiff and act as a frame - supporting each other.

They would have pillow blocks riding at the top and bottom, connecting to the gantry. The four pillow block bearings would be wide and use filled teflon for better damping than roller bearings. Pillow blocks should lower the cost over roller bearings as well. The idea is to make rails with better characteristics for the project than are commercially available, and at a better price. Epoxy-granite filled tubes combined with filled teflon pillow blocks seem like a good design for that. The added friction would be overcome by using heavier stepper motors.

The rails would attach to the sides of the concrete table frame through bolts through the rails epoxy-granite middle connecting slab. Careful attention would be paid to the attaching bolts, perhaps using Moglice to take out any play. The table would have 6 or more legs, so the rails would be supported in at least three places.

For joining the parallel rails with the epoxy plate, a casting form could be built with the rails in place, allowing for precice placement of the rails during the casting process. The rails would be cast together laying on a perfectly flat expoxy-granite slab.

In order to make cuts into steel, what would be the required dimensions of a well made rail as described, if it were to have only three supports?