VAULT // 1958
Computer Control of Machine Tools
Is there a computer in the house? Your production machines may be automatic, but can they think? Are they self-acting and self-regulating? A look at what was new computer control in 1958.
Written by George M. Reynolds
THE IDEA OF AUTOMATIC CONTROL of automatic control of production machines is not a new one. Tools are already in use which take raw materials, stampings, and castings and turn them into finished products without the touch of human hands. Highly competitive mass-producing industries require procurement and maintenance of production machines at the lowest possible cost. Speed of operation and high quality are necessities; but another desirable characteristic of machine tools has become more prevalent in recent years—versatility!
Still, Sammie was a vast improvement for HF engineers, who had typically interpreted anthropometric tables themselves. The tables, which consisted of measurements of standard anthropometric variables derived from representative samples of human populations, defined anthropometry in terms of percentile information. (For example, a 95th-percentile man would be taller, stronger, or more intelligent, say, than all but 5 percent of the population.) With this information in hand, HF engineers would draw reach and visual envelopes or even construct two-dimensional cardboard manikins to replicate the seated, standing, and crawling movements of product users.
Computer control applied to machine tools means production machines which are self-acting and self-regulating. Instructions to these machines are recorded on tape, and the taped data interpreted into specific instructions to the production machine by a machine-control unit.
Preparation of Instructions
The first step in utilizing computer control for the manufacture of a part is the designing and drafting of the part’s three orthogonal projections.
Instead of dimensioning this part by lengths and radii, it must be specified in terms of significant points relative to a system of axes. The origin for these axes is quite arbitrary, except that for some systems their origin must not be within the workpiece so that negative values will be avoided.
The points to be specified are the points of change from one curve to the next. Straight lines require detail of their end points only, circles and arcs, their radii and extent of arc. These co-ordinates are recorded on a planning sheet in the order which coincides with the cutter-tool path.
The designer can call up any ellipse, hyperbola, straight line, or parabola. A curve which does not have these characteristics can be expressed as ordinates, and if they are inserted in the planning sheet with appropriate instructions to the computer, a smooth curve can be interpolated through these points.
Also recorded on the planning sheet, alongside the appropriate point intervals, are the cutter speed and feed changes. These are necessary for the best finish possible and at the same time complete the operation in the shortest possible interval.
The next step is to feed this information into a computer. Before this can be done, the planning sheet must be “encoded” by a device similar to a typewriter, sometimes called a flexowriter or teleprinter. While making typed copy of the planning sheet, this device also simultaneously punches the data, in highly condensed form, on process tape. The process tape is then inserted into the computer.
The computer carries out the detailed process calculations. It performs the necessary mathematical manipulations to describe the path of the center of the cutting tool, and makes allowances for the diameter and cutter wear. As the computer reaches its solutions, it records them on another tape, known as the control tape. This is then inserted in the machine-control unit.
Source: Getty Images
The Machine-Control Unit
Associated with each production machine is a console containing equipment for reading the control tape. As the machine-control unit interprets the control tape, it sends instructions to the production machine. These instructions are in terms of controlled power for servomechanism drives.
For each axis of tool or table movement, one servomotor is required. For a two-dimensional profile, two servomotors are required. A three-dimensional system would need three servomotors.
For one of the special machines which Kearney and Trecker are building, Bendix hydraulic servos were selected. Using a variable-speed positive-displacement output can be varied from 0 to 6 bhp. Two stage valves, electronically directed by the machine-control unit, governed these servomotors.
Also under consideration for use on these machines were electrical servomotors built by Farranti Limited, of England. These servos are 3-phase, 40-c/ps, induction motors with an output of 0.5 to 2.0 bhp. Coupled with this motor is a 1000-to-12 speed reducer, bringing the 10,000 rpm down to a usable 120.
It is necessary, in order to maintain the accuracy of the system, that all gearing and lead screws involved in the movement of cutter tool, or table, contain anti-backlash design.
Feedback Systems
Feedback is used in order that the machine-control unit can know what the production machine is doing, and how well it is carrying out the control tape’s instructions. For each axis, there must be one feedback signal.
The Bendix system employs electromagnetic pulse generators. Each pulse corresponds to 0.0002 in. of tool or table movement along one of the axes.
An interesting feedback mechanism used by Farranti applies a fundamental principle of physics. When a diffraction grating whose set-let lines are orientated in one direction is placed directly over another grating whose lines are at a slight angle to those underneath, alternate light and dark bands appear. These are known as Moiré fringe bands.
Moving one grating with respect to the other, these bands move at right angles to the grating at right angles to the direction of the top grating’s movement. For each grating line of displacement, one dark and light band will pass any one point on the top grating, and if there are 1000 lines per in. of grating length, each dark-band interval represents 0.001 in. relative displacement; 500 lines per in. of grating length, and each dark-band interval represents 0.0001 in. relative displacement. Photocells placed under this double grating send signals back to the machine-control unit, and we have the exact duplications of the Farranti feedback principle.
By attaching the long grating to a moving table and maintaining the counting device stationary, the table movement will be accurately interpreted by the machine control unit.
A man working on a section of the electronic brain designed by the Manchester firm Ferranti in 1955. Source: Getty Images
Application of Computer Control
In testing the Farranti system, a standard vertical-milling machine was modified to accommodate the components. The machine control unit is housed in a cabinet with a tape mounted in the upper chamber.
The milling of aircraft parts is another for much of the acceleration in the application of computer control to machine tools. Missiles and aircraft must be built with maximum strength and minimum weight to withstand the dynamic forces of supersonic flight. Reinforced sections and structural members subject to strong vibrations have led the aircraft industry to demand tools which will machine whole sections of plates and missiles—eliminating riveted, welded, or, in general, fabricated parts.
Machines such as these are the types under development. One is a gantry-type vertical-milling machine measuring 12 by 24 ft in table size. It will machine from ingots of aluminum, titanium, or alloys. Anyone who has seen the wing of an airplane with its fuselage removed can appreciate the complexity of this operation.
Construction had not yet been completed when this paper was prepared; however, another machine recently was completed and is now in operation.
A horizontal milling machine of smaller proportions than the gantry type, it is designed to mill aileron linkages, wheel supports, or, in general, smaller aircraft parts of complicated and intricate shape, from forgings, castings, or solid blocks of material.
Other Applications
At present, the application of computer control on machine tools in industries other than aircraft is limited for a number of reasons. One is, that it is not yet economically feasible to equip a machine producing parts for a competitive market. The outlay in capital is large, and the industry is not yet in such a position to warrant it, in spite of the tremendous man-hours of labor which can be saved by its use.
In the field of computer-controlled machines, large government expenditures are now being made through the weapons industry, and a major portion of the controlled machines was on its way before the government’s investigation, but the new funds made available have stimulated and multiplied its development many times.
Initially, controlled machines will be a boon to industries where the markets for any one item are insufficient to justify the outlay for elaborate machinery. Eventually they will be applied to all or nearly all industries. In the years to come, they will be a common sight in all plants engaged in the machining of metals.
George M. Reynolds was a student at Northwestern University, receiving the “Old Guard Prize” for this piece from among 12 Regional Student Conference winners, at ASME’s Semi-Annual Meeting in June 1957.
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