Micrometer adjustment nut
On a manual milling machine, the micrometer adjustment nut limits the depth to which the cutting tool may plunge into the workpiece.
The nut is located on a threaded rod on the mill head. The machine operator moves it up or down by rotating it clockwise or counter-clockwise. Moving the nut down increases the depth to which the cutting tool may plunge into the workpiece. Moving the nut up reduces the depth to which the cutting tool may plunge into the workpiece.