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Monday, December 26, 2011

CNC History (Part 3)

Parsons and Sikorsky

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. At the time, rotors were built in the same fashion as wings, consisting of a long tubular steel spar with stringers (or more accurately ribs) set on them to provide the aerodynamic shape that was then covered with a stressed skin. The stringers for the rotors were built from a design provided by Sikorsky, which was sent to Parsons 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. A wooden jig was built up to form the outside of the outline, and the pieces of wood forming the stringer were placed under pressure against the inside of the jig so they formed the proper curve. A series of trusswork members were then assembled inside this outline to provide strength.
After setting up production at a disused furniture factory and ramping up production, one of the blades failed and it was traced to a problem in 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. The collar was built into the stringer during construction, then slid onto the spar and welded in the proper position. Parsons suggested a new method of attaching the stringers directly to the spar using adhesives, never before tried on an aircraft design.
That development led Parsons to consider the possibility of using stamped metal stringers instead of wood. These would not only be much stronger, but far easier to make as well, as they would eliminate the complex layup and glue and screw fastening on the wood. Duplicating this in a metal punch would require the wooden jig to be replaced by a metal cutting tool made of tool steel. Such a device would not be easy to produce given the complex outline. Looking for ideas, Parsons visited Wright Field to see Frank Stulen, the head of the Propeller Lab Rotary Ring Branch. During their conversation, Stulen concluded that Parsons didn't really know what he was talking about. Parsons realized Stulen had reached this conclusion, and 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 punched 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 punched card machines, he asked Stulen 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 a mill cutting tool. If you cut at each of those points, it would produce a relatively accurate cutout of the stringer. This could cut the tool steel and then easily be filed down to a smooth template for stamping metal stringers.
Stullen had no problem making such a program, and used it to produce 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. For each pair of numbers the operators would move the cutting head to the indicated spot and then lower the tool to make the cut. This was called the "by-the-numbers method", or more technically, "plunge-cutting positioning".

Punch cards and first tries at NC

At that point Parsons conceived of a fully automated machine tool. With enough points on the outline, no manual working would be needed to clean it up. However, 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 Parsons had no funds to develop his ideas.
When one of Parsons's 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-powered designs. He asked if Parsons had anything to help them. Parsons showed Lockheed their idea of an automated mill, but they were uninterested. They decided to use 5-axis template copiers to produce the stringers, cutting from a metal template, and had already ordered the expensive cutting machine. But as Parsons noted:
Now just picture the situation for a minute. Lockheed had contracted to design a machine to make these wings. This machine had five axes of cutter movement, and each of these was tracer controlled using a template. Nobody was using my method of making templates, so just imagine what chance they were going to have of making an accurate airfoil shape with inaccurate templates.
Parson's worries soon came true, and Lockheed's protests that they could fix the problem eventually rang hollow. In 1949 the Air Force arranged funding for Parsons to build his machines on his own. Early work with Snyder Machine & Tool Corp proved the system of directly driving the controls from motors failed to give the accuracy needed to set the machine for a perfectly smooth cut. Since the mechanical controls did not respond in a linear fashion, you couldn't simply drive it with a given amount of power, because the differing forces meant the same amount of power would not always produce the same amount of motion in the controls. No matter how many points you included, the outline would still be rough.

Enter MIT

This was not an impossible problem to solve, but would require some sort of feedback system, like a selsyn, to directly measure how far the controls had actually turned. Faced with the daunting task of building such a system, in the spring of 1949 Parsons turned to Gordon S. Brown's Servomechanisms Laboratory at MIT, which was a world leader in mechanical computing and feedback systems. During the war the Lab had built a number of complex motor-driven devices like the motorized gun turret systems for the Boeing B-29 Superfortress and the automatic tracking system for the SCR-584 radar. They were naturally suited to technological transfer into a prototype of Parsons's automated "by-the-numbers" machine.
The MIT team was led by William Pease assisted by James McDonough. They quickly concluded that Parsons's design could be greatly improved; if the machine did not simply cut at points A and B, but instead moved smoothly between the points, then not only would it make a perfectly smooth cut, but could do so with many fewer points - the mill could cut lines directly instead of having to define a large number of cutting points to "simulate" a line. A three-way agreement was arranged between Parsons, MIT, and the Air Force, and the project officially ran from July 1949 to June 1950. The contract called for the construction of two "Card-a-matic Milling Machines", a prototype and a production system. Both to be handed to Parsons for attachment to one of their mills in order to develop a deliverable system for cutting stringers.
Instead, in 1950 MIT bought a surplus Cincinnati Milling Machine Company "Hydro-Tel" mill of their own and arranged a new contract directly with the Air Force that froze Parsons out of further development. Parsons would later comment that he "never dreamed that anybody as reputable as MIT would deliberately go ahead and take over my project." In spite of the development being handed to MIT, Parsons filed for a patent on "Motor Controlled Apparatus for Positioning Machine Tool" on 5 May 1952, sparking a filing by MIT for a "Numerical Control Servo-System" on 14 August 1952. Parsons received US Patent 2,820,187 on 14 January 1958, and the company sold an exclusive license to Bendix. IBM, Fujitsu and General Electric all took sub-licenses after having already started development of their own devices.

MIT's machine

MIT fit gears to the various handwheel inputs and drove them with roller chains connected to motors, one for each of the machine's three axes (X, Y, and Z). The associated controller consisted of five refrigerator-sized cabinets that, together, were almost as large as the mill they were connected to. Three of the cabinets contained the motor controllers, one controller for each motor, the other two the digital reading system.
Unlike Parsons's original punched card design, the MIT design used standard 7-track punch tape for input. Three of the tracks were used to control the different axes of the machine, while the other four encoded various control information. The tape was read in a cabinet that also housed six relay-based hardware registers, two for each axis. With every read operation the previously read point was copied into the "starting point" register, and the newly read one into the "ending point" register. The tape was read continually and the number in the registers incremented with each hole encountered in their control track until a "stop" instruction was encountered, four holes in a line.
The final cabinet held a clock that sent pulses through the registers, compared them, and generated output pulses that interpolated between the points. For instance, if the points were far apart the output would have pulses with every clock cycle, whereas closely spaced points would only generate pulses after multiple clock cycles. The pulses are sent into a summing register in the motor controllers, counting up by the number of pulses every time they were received. The summing registers were connected to a digital to analog converter that increased power to the motors as the count in the registers increased, making the controls move faster.[
The registers were decremented by encoders attached to the motors and the mill itself, which would reduce the count by one for every one degree of rotation. Once the second point was reached the counter would hold a zero, the pulses from the clock would stop, and the motors would stop turning. Each 1 degree rotation of the controls produced a 0.0005 inch movement of the cutting head. The programmer could control the speed of the cut by selecting points that were closer together for slow movements, or further apart for rapid ones.
The system was publicly demonstrated in September 1952, appearing in that month's Scientific American. MIT's system was an outstanding success by any technical measure, quickly making any complex cut with extremely high accuracy that could not easily be duplicated by hand. However, the system was terribly complex, including 250 vacuum tubes, 175 relays and numerous moving parts, reducing its reliability in a production environment. It was also very expensive, the total bill presented to the Air Force was $360,000.14 ($2,641,727.63 in 2005 dollars). Between 1952 and 1956 the system was used to mill a number of one-off designs for various aviation firms, in order to study their potential economic impact.

Proliferation of NC

The Air Force funding for the project ran out in 1953, but development was picked up by the Giddings and Lewis Machine Tool Co. In 1955 many of the MIT team left to form Concord Controls, a commercial NC company with Giddings' backing, producing the Numericord controller. Numericord was similar to the MIT design, but replaced the punch tape with a magnetic tape reader that General Electric was working on. The tape contained a number of signals of different phases, which directly encoded the angle of the various controls. The tape was played at a constant speed in the controller, which set its half of the selsyn to the encoded angles while the remote side was attached to the machine controls. Designs were still encoded on paper tape, but the tapes were transferred to a reader/writer that converted them into magnetic form. The magtapes could then be used on any of the machines on the floor, where the controllers were greatly reduced in complexity. Developed to produce highly accurate dies for an aircraft skinning press, the Numericord "NC5" went into operation at G&L's plant at Fond du Lac, WI in 1955.
Monarch Machine Tool also developed a numerical controlled lathe, starting in 1952. They demonstrated their machine at the 1955 Chicago Machine Tool Show (predecessor of today's IMTS), along with a number of other vendors with punched card or paper tape machines that were either fully developed or in prototype form. These included Kearney & Trecker’s Milwaukee-Matic II that could change its cutting tool under numerical control, a common feature on modern machines.
A Boeing report noted that "numerical control has proved it can reduce costs, reduce lead times, improve quality, reduce tooling and increase productivity.” In spite of these developments, and glowing reviews from the few users, uptake of NC was relatively slow. As Parsons later noted:
The NC concept was so strange to manufacturers, and so slow to catch on, that the US Army itself finally had to build 120 NC machines and lease them to various manufacturers to begin popularizing its use.
In 1958 MIT published its report on the economics of NC. They concluded that the tools were competitive with human operators, but simply moved the time from the machining to the creation of the tapes. In Forces of Production, Noble claims that this was the whole point as far as the Air Force was concerned; moving the process off of the highly unionized factory floor and into the un-unionized white collar design office. The cultural context of the early 1950s, a second Red Scare with a widespread fear of a bomber gap and of domestic subversion, sheds light on this interpretation. It was strongly feared that the West would lose the defense production race to the Communists, and that syndicalist power was a path toward losing, either by "getting too soft" (less output, greater unit expense) or even by Communist sympathy and subversion within unions (arising from their common theme of empowering the working class).

To be continued
Source : wikipedia

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