History of CNC Machining: How the CNC Concept Was Born
Jan 14, 2013Computerized Numerical Control, or CNC as we all now it, came about shortly after WWII as a result of the aircraft industry’s desire to produce more accurate and complex parts. Below is a partial reprint of an article from the August, 1996 issue of American Machinist that explains the history of CNC much better than I ever could:
“Numerical control as a concept developed in the mind of John Parsons as a way to produce integrally stiffened skins for aircraft, and this led to a series of Air Force research projects at the Massachusetts Institute of Technology, beginning in 1949.
“The initial planning-and-study phase was followed by the construction of an experimental milling machine at the Servomechanisms Laboratory at MIT. Prof J.F. Reintjes, director of the lab, James O. McDonough, Richard W. Lawrie, A.K. Susskind, and H.P. Grossimon were the people involved in the research.
“A 28-in. Cincinnati Hydro-Tel verticle-spindle contour milling machine was the starting point. It was extensively modified: all of the table, cross-slide, and head drives and controls were removed, and three variable-speed hydraulic transmissions were installed and connected to leadscrews. Each transmission would produce, through gearing and leadscrew, a 0.0005-in. motion of the table, head, or cross-slide for each electrical pulse received from the director. A feedback system was provided to make sure the machine was doing what it was told. A synchronous motor geared to each motion generated a voltage response to movement; this was sent back to the director and compared with the original command voltage.
“By 1951, the system had been assembled, and application studies were begun. By 1953, enough data had been assembled to indicate practical possibilities that could be developed. A detailed 24-page report on the process that appeared in American Machinist on Oct 25, 1954, started a flurry of further development. […] But it was the initially more awkward, less accurate prototype at MIT, which employed a Flexowriter and its eight-column paper tape, a tape reader, and a vacuum-tube electronic control system that was to become the prototype for the developments that followed.”
The article included a photograph of the prototype machine with the heading “Pioneering setup at Servomechanisms Lab at MIT had control system that surrounded modified milling machine it controlled.” In the photo, you can see the control systems, which consisted of metal cabinets the size of school lockers, extends 12-15 feet (my guess) and is filled with electronics. The controls were larger than the mill itself! A sign on top of these “lockers” is also visible, reading “PILOT SERVO OUTPUT UNITS.” Underneath there are the signs “Table,” “Head,” and “Slide,” each corresponding to a “locker” that the respective sign was above.
The article continues:
“Parsons Corp. [founded by John T. Parsons, who had been introduced earlier in the article] had already developed a system for producing helicopter-blade templates by calculating airfoil coordinates on an IBM 602A multiplier and feeding these data points into a Swiss jig borer—rather than laying out the job manually […] Late in 1948, the Air Force sent a team to the Parsons plant in Traverse City, Mich. There they saw the technique being used in producing templates from data on punched cards.
“For such significant technical contributions to the field of numerical control, John T. Parsons in 1968 became the first recipient of the Numerical Control Society’s Joseph Marie Jacquard Memorial Award. And in 1975, the Society of Manufacturing Engineers awarded him a plaque naming him ‘The Father of the Second Industrial Revolution.’”
What Does a CNC Need to Make it Work? The Theory from a Different Point of View
This was considered the first “true” CNC, but before this there were many incarnations of motion control. Many were considered NC machines and used punched Mylar tape or hydraulic valves and cables and summing levers or cams. Before coming to the CNC world I was—and still am—a licensed aircraft mechanic. I worked for the now-defunct (not my fault) TWA, specializing in the Boeing 747 aircraft. The reason I bring this up is, although the aircraft industry is the motivating factor in new manufacturing technology, they are slow to adapt it into their aircrafts. An example of this is the landing flap control system on the 747. I’ll explain how this works, then explain its function in early motion control systems.
Landing flaps are the large extensions on the rear of the wings. During flight, they are tucked in to make a smooth and aerodynamic wing surface. During takeoff and landing, they are extended to provide more lift at slower speeds. At takeoff, they are extended about halfway to offer lift but without so much drag as to retard the acceleration of the plane. During landing, they are extended all the way to allow lift at slower speeds and to provide the slowest touch down speed possible. This is how they work:
There is a lever in the cockpit connected to a drum, or wheel. On this wheel is a steel cable; this steel cable meanders its way through the fuselage over pulleys, pressure fittings, and guides and finally arrives at the hydraulic control power pack in the aircraft’s wheel well. Here, the cable is connected to a small summing lever. When this lever is moved, the hydraulic fluid will flow at 3000 PSI to a hydraulic motor rotating it forward or reverse depending on the way the lever was moved. As the motor rotates, it turns a ball screw connected to the flaps’ carriage. On the flaps’ carriage there is another cable that comes back to the power packs summing lever, called the null lever. When the flap handle is moved, the hydraulic fluid flows and the flaps move. When the flaps reach the desired position, the null lever chases down the summing lever and stops the fluid flowing—the flaps then stop. So, we have a command lever to start motion and a feedback system to stop motion. This is exactly what happens in a modern CNC machine.
The way it was used in early NC machines was, instead of a pilot moving a lever, the lever would ride on a cam that would move it back and forth, causing the motor to rotate and move the slide of the machine. A special note on this type of system is that, like early CNC systems, if the feedback cable would break, the null lever would not stop the fluid flow and the system would “run away.” The same as if an encoder would fail on an electrical CNC system. Modern CNC systems are too smart for this to happen, but in the early days it was always interesting to see what you would find after a run away!
I’ve been away from the aircraft maintenance industry for nearly 20 years, but the use of electrical motion control was just beginning in the aircraft world back then. The Boeing 767 was fairly new toward the end of my time, and it was using electrical controls for flap and flight controls. This was the so-called “fly by wire” system that got a lot of press at the time. Now it is commonplace in all aircraft. Another note on this system: every control system had two complete and separate motion control systems—if one failed, it had a complete, 100% backup.
CNC: The Essential Systems
What all motion control systems have in common is a command function of some sort, a drive or motion system, and although not necessary, a feedback system. The command function could be digital, analog, a flap handle, a cam follower, or a host of other devices that say “Go.” The motion or drive system could be an electric motor, hydraulic motor, a brake, a clutch, a cylinder, a valve, or any combination thereof. It is the device that makes something move. Lastly, we have (or have not) a feedback device. We are primarily involved with high precision machine tools, so we will always have a feedback device, most commonly an encoder of some sort.
Motion control systems are used in many industries. For example: a rock quarry. At the quarry, an operator pushes a button to start a conveyor. The conveyor is full and has a heavy load. The operator might have a dial to control the speed of the belt. He or she might start the belt very slowly, then increase the speed once the load is moving. This is the command element of the system. The operator then watches the belt to be sure it’s operating at the correct speed—this would be the feedback system. After it’s running, there could also be a sensor that detects that the belt is moving or even how fast it’s going to relieve the operator of the duty of watching the belt. This is also a complete motion control system.