ballscrew question,

hi, do ballscrews have to be supported at both ends? Is there a
certain length or rule of thumb? I was thinking of the z-axis for a
homemade cnc router and the whole travel is only about 6 inches.
what about a 20" travel, the most the screw would be hanging out at
the end is 10". support it? I was looking at a mill/drill plan and
I did not see pillow blocks on the free end. Thanks Mark.