A worm is a cylindrical, threaded shaft with a helical groove cut into its surface. The worm gear is a toothed wheel that meshes with the worm, converting the rotary motion of the worm into linear motion of the gear. The teeth on the worm gear are cut at an angle that matches the angle of the helical groove on the worm.

In a milling machine, the worm and worm gear are used to control the movement of the milling head or table. The worm is typically driven by a motor, and as it rotates, it engages with the teeth of the worm gear, causing the gear to move. This movement is usually very precise, allowing for accurate positioning of the milling head or table.

One advantage of using a worm and worm gear in milling machines is that it provides a high level of mechanical advantage, allowing for a relatively small motor to drive the worm while still achieving precise movement. Additionally, because the teeth of the worm gear engage with the worm at a shallow angle, there is less friction and wear on the components, resulting in a longer service life for the system.