A woodruff key, also known as a half?moon key, is a semi-circular machine shaft key that prevents gears, hubs, or other components from moving independently of a rotating shaft or spindle. Woodruff keys are inserted into a key seat created by a woodruff key cutter or arbor cutter. A gear or other component with a woodruff keyway is then pressed onto the shaft until the woodruff key interfaces with its keyway.