Total Guide to CNC Jigs, Fixtures, and Workholding ...
In CNC, a 4th Axis is commonly a rotary axis that is aligned to spin along an axis parallel to one of the machine''s other 3 axes. On Vertical Mills, the 4th Axis is frequently parallel to X or Y, and is laid down. On a Horizontal Mill, the 4th Axis is also parallel to X or Y, but it is standing up.