No way. Cutting tool steel. 8mm rods and nema 23. Small spindel. Be lucky to cut aluminium. I use a Deckel knee mill and it's a bear, even still, tool steel is a stretch with it. Time and patience. Better with a tormach or vmc.
Interesting build! Have you found it stable enough resting on the 20x80 alone? I can imagine doing something similar, but doubling up the Y-axis and spacing the extrusions just far apart enough for two carriages to clear. This would increase stability and rigidity in the X-Y plane without increasing the footprint too much. And increasing the power by dual-driving the Y might help with milling too. I'll be looking forward to seeing it mill aluminum!
I have a design that's on hold for the moment, but is pretty much the same as your M Cubed. I had even been calling it that also. Nice work.