In a great many machines, such as looms, sewing machines, printing presses, punch presses, automobile engines, etc., it is often necessary to give to each machine a motion peculiar to itself. In one machine it may be necessary to change circular or rotary motion into back-and-forth or reciprocating motion at definite times during the working of the machine. In another machine the opposite effect may be desired. A curved plate or groove, called a cam, is used for producing such irregular motion. Cams are constructed in various shapes and dimensions. They may consist of simply a wheel, a projecting part of a wheel, or a revolving piece. The nature of the motion given by the cam to a machine is determined by the shape of the cam.