Numerical control ( NC ) refers to the automation of machine tools that are operated by abstractly programmed commands encoded on a storage medium, as opposed to manually controlled via handwheels or levers, or mechanically automated via cams alone. The first NC machines were built in the 1940s and '50s, based on existing tools that were modified with motors that moved the controls to follow points fed into the system on paper tape. These early servomechanisms were rapidly augmented with analog and digital computers, creating the modern computer numerical controlled ( CNC ) machine tools that have revolutionized the design process.
In modern CNC systems, end-to-end component design is highly automated using CAD/CAM programs. The programs produce a computer file that is interpreted to extract the commands needed to operate a particular machine, and then loaded into the CNC machines for production. Since any particular component might require the use of a number of different tools—drills, saws, etc.—modern machines often combine multiple tools into a single "cell". In other cases, a number of different machines are used with an external controller and human or robotic operators that move the component from machine to machine. In either case, the complex series of steps needed to produce any part is highly automated and produces a part that closely matches the original CAD design.
History
Earlier forms of automation
Cams
The automation of machine tool control began in the 1800s with cams that "played" a machine tool in the way that cams had long been playing musical boxes or operating elaborate cuckoo clocks. Thomas Blanchard built his gun-stock-copying lathes (1820s-30s), and the work of people such as Christopher Miner Spencer developed the turret lathe into the screw machine (1870s). Cam-based automation had already reached a highly advanced state by World War I (1910s).
However, automation via cams is fundamentally different from numerical control because it cannot be abstractly programmed. There is no direct connection between the design being produced and the machining steps needed to create it. Cams can encode information, but getting the information from the abstract level of an engineering drawing into the cam is a manual process that requires sculpting and/or machining and filing. At least two forms of abstractly programmable control had existed during the 1800s: those of the Jacquard loom and of mechanical computers pioneered by Charles Babbage and others. These developments had the potential for convergence with the automation of machine tool control starting in that century, but the convergence did not happen until many decades later.
Tracer control
The application of hydraulics to cam-based automation resulted in tracing machines that used a stylus to trace a template, such as the enormous Pratt & Whitney "Keller Machine", which could copy templates several feet across. Another approach was "record and playback", pioneered at General Motors (GM) in the 1950s, which used a storage system to record the movements of a human machinist, and then play them back on demand. Analogous systems are common even today, notably the "teaching lathe" which gives new machinists a hands-on feel for the process. None of these were numerically programmable, however, and required a master machinist at some point in the process, because the "programming" was physical rather than numerical.
Servos and selsyns
One barrier to complete automation was the required tolerances of the machining process, which are routinely on the order of thousandths of an inch. Although it would be relatively easy to connect some sort of control to a storage device like punch cards, ensuring that the controls were moved to the correct position with the required accuracy was another issue. The movement of the tool resulted in varying forces on the controls that would mean a linear output would not result in linear motion of the tool. The key development in this area was the introduction of the servo, which produced highly accurate measurement information. Attaching two servos together produced a selsyn, where a remote servo's motions was accurately matched by another. Using a variety of mechanical or electrical systems, the output of the selsyns could be read to ensure proper movement had occurred.
The first serious suggestion that selsyns could be used for machining control was made by Ernst F. W. Alexanderson, a Swedish immigrant to the U.S. working at General Electric (GE). Alexanderson had worked on the problem of torque amplification that allowed the small output of a mechanical computer to drive very large motors, which GE used as part of a larger gun laying system for US Navy ships. Like machining, gun laying requires very high accuracies, less than a degree, and the motion of the gun turrets was non-linear. In November 1931 Alexanderson suggested to the Industrial Engineering Department that the same systems could be used to drive the inputs of machine tools, allowing it to follow the outline of a template without the strong physical contact needed by existing tools like the Keller Machine. He stated that it was a "matter of straight engineering development." However, the concept was ahead of its time from a business development perspective, and GE did not take the matter seriously until years later, when others had pioneered the field.
Parsons and the invention of NC
The birth of NC is generally credited to John T. Parsons, a machinist and salesman at his father's machining company, Parsons Corp. In 1942 he was told that helicopters were going to be the "next big thing" by the former head of Ford Trimotor production, Bill Stout. He called Sikorsky Aircraft to inquire about possible work, and soon got a contract to build the wooden stringers in the rotor blades. After setting up production at a disused furniture factory and ramping up production, one of the blades failed and it was traced to the spar. As at least some of the problem appeared to stem from spot welding a metal collar on the stringer to the metal spar, so Parsons suggested a new method of attaching the stringers to the spar using adhesives, never before tried on an aircraft design.
But that development led Parsons to wonder about the possibility of using stamped metal stringers instead of wood, which would be much easier to make and stronger too. The stringers for the rotors were built to a design provided by Sikorsky, which was sent to them as a series of 17 points defining the outline. Parsons then had to "fill in" the dots with a french curve to generate an outline they could use as a template to build the jigs for the wooden versions. But how to make a tool able to cut metal with that shape was a much harder problem. Parsons went to visit Wright Field to see Frank Stulen, who was the head of the Rotary Ring Branch at the Propeller lab. Stulen concluded that Parsons didn't really know what he was talking about, and realizing this, Parsons hired him on the spot. Stulen started work on 1 April 1946 and hired three new engineers to join him.
Stulen's brother worked at Curtis Wright Propeller, and mentioned that they were using punch card calculators for engineering calculations. Stulen decided to adopt the idea to run stress calculations on the rotors, the first detailed automated calculations on helicopter rotors. When Parsons saw what Stulen was doing with the punch card machines, he asked him if they could be used to generate an outline with 200 points instead of the 17 they were given, and offset each point by the radius of the cutting tool on a mill. If you cut at each of those points, it would produce a relatively accurate cutout of the stringer even in hard steel, and it could easily be filed down to a smooth shape. The resulting tool would be useful as a template for stamping metal stringers. Stullen had no problem doing this, and used the points to make large tables of numbers that would be taken onto the machine floor. Here, one operator read the numbers off the charts to two other operators, one on each of the X- and Y- axes, and they would move the cutting head to that point and make a cut. This was called the "by-the-numbers method".
At that point Parsons conceived of a fully automated tool. With enough points no manual working would be needed at all, but with manual operation the time saved by having the part more closely match the outline was offset by the time needed to move the controls. If the machine's inputs were attached directly to the card reader this delay, and any associated manual errors, would be removed and the number of points could be dramatically increased. Such a machine could repeatedly punch out perfectly accurate templates on command. But at the time he had no funds to develop these ideas.
When one of Parsons' salesmen was on a visit to Wright Field, he was told of the problems the newly-formed US Air Force was having with new jet designs. He asked if Parsons had anything to help them. Parsons showed Lockheed their idea of an automated mill, but they were uninterested. They had already decided to use 5-axis template copiers to produce the stringers, cutting from a metal template, and had ordered the expensive cutting machine already. But as Parsons noted:
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