The purpose of the Spindle, or Mandrel, is to control the safe rotation of the mower blade during operation. The Mandrel is often made of aluminum on residential mowers and steel on commercial mower decks. Much like a shear pin in other applications, the Mandrel is designed to break if the blade hits a hard object to protect the deck shell from damage. The self-tapping screws required for installation (usually sold separately) will create the threads as they are drilled into the bare holes in the Mandrel.