英语翻译The History of NC and CNC DevelopmentNumerical Control (

英语翻译
The History of NC and CNC Development
Numerical Control (NC) is any machining process in which the operations are executed automatically in sequences as specified by the program that contains the information for the tool movements.The NC concept was proposed in the late 1940s by John Parsons of Traverse City,Michigan.Parsons recommended a method of automatic machine control that would guide a milling cutter to produce a "thru-axis curve" in order to generate smooth profiles on work pieces.
In1949,The U.S.Air Force awarded Parsons a contract to develop a new type of machine tool that would be able to speed up production mathods.Parsons commissioned the Massachusetts Institute of Technology (M.I.T.) to develop a practical implementation of his concept.Scientists and engineers at M.I.T.built a control system for a two-axis milling machine that used a perforated paper tape as the input media.In a short period of time,all major machine tool manufaturers were producing some machines with NC,but it was not until the late 1970s that computer-based NC became widely used.NC matured as an automation technology when inexpensive and powerful microprocessors replaced hard-wire logic-making computer-based NC systems.
when Numerical Control is performed under computer supervision,it is called Computer Numerical Control (CNC).Computers are the control units of CNC machines,they are built in or linked to the machines via communications channels.when a programmer input some information in the program by tape and so on,the computer calculates all necessary data to get the job done.
On the first Numerically Controlled (NC) machines were controlled by tape,and because of that the NC systems were known as tape-controlled machines.They were able to control a single operation entered into the machine by punched or magnetic tape.There was no possibility of editing the program on the machine.To change the program,a new tape had to be made.
Today's systems have computers to control data; they are called Computer Numerically Controlled (CNC) machines.For both NC and CNC systems,work principles are the same.Only the way in which the execution is controlled is different.Normally,new systems are faster,more powerful,and more versatile.