In mill terms, the Z axis is your limiting factor. The Onefinity makes an awesome CNC router, but the harsh reality is that it simply doesn’t have the Z axis backbone to make a decent mill.
It might be ok for aluminum, but I wouldn’t attempt steel on it, unless it was a one-time thing. You’re looking at 5" from the tip of the cutter to actual support. On a mill, this distance is much shorter and much beefier. The end mill will most likely chatter with this relatively little amount of support. That equals bad finish & short end mill life. You could definitely limp by a steel mill job on it, but not a good idea to repetitively use it to mill steel.
