This is a Cover for the Z axis of your X-Carve that also includes a Knob. The cover prevents dust, fingers, etc. from getting into the GT2 belt that drives the Z axis. The cover extends out to cover your limit switch as well, and includes a slot in the back to run your limit switch wires out of. The included knob fits around the nylock nut on the Z axis allowing you to drive the Z axis up and down without the machine powered on.