Hi,
I made a small mill, 200mm x 200mm x 200mm machining volume, on cast iron beds with 15mm Rexroth rails, two per axis.
They are sufficiently rigid for my machine to mill steel.
My new project, a bigger version of the same thing, 350mm x 350mm x 350mm machining volume, cast iron beds, 32mm C5 ground
ballscrews with pairs of 20mm THK HSR rails and 750W Delta B2 AC servos. I anticipate the 20mm rails will be adequate.
In lots of respects the linear rails are easy, you can buy whatever size you need. Finding good ballscrews (C5 or better) is a much, much harder
battle, yet the ballscrews really set the tone of the machine rigidity and accuracy. Rather, finding good ballscrews is easy IF you pay big bucks for new
but trying to find GOOD second hand is much harder. I discounted ALL rolled C7 ballscrews as being inadequately accurate.
Craig