Computer control programmers and operators use computer numerically controlled (CNC) machines to produce a wide variety of products, from automobile engines to computer keyboards. CNC machines operate by reading the code included in a computer-controlled module, which drives the machine tool and performs the functions of forming and shaping a part formerly done by machine operators. CNC machines include tools such as lathes, laser cutting machines, roll forms, press brakes, and printing presses. CNC machines use the same techniques as many other mechanical manufacturing machines but are controlled by a central computer instead of a human operator or electric switchboard. Many old-fashioned machines can be retrofitted with a computer control, which can greatly improve the productivity of a machine. Computer control programmers and operators normally produce large quantities of one part, although they may produce small batches or one-of-a-kind items. These machines are most commonly used in metalworking industries where precision is imperative, because computers can be more accurate than humans in this work.