Here the brake rotor is mounted to the brake lathe and the lathe turns the rotor and shaves a thin layer of metal from the rotor. The result is a smooth and even surface on the rotor like this one.
My finger is pointing to the lathe bits that shave the metal from the rotor. You can see metal shavings all around the machine.
The metal shavings from the rotor are collected in a tray located underneath the lathe.
