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thousands of pounds of flushing pressure separating the electrode and a large workpiece.

Refined power supplies and better electrodes and dielectrics have made it possible to control metal removal from more than 100 cubic inches an hour to as little as 0.001 cubic inch an hour. The quick extension of EDM's capability makes its application economically possible for more and more diemaking and production processes.

USE OF EDM IN MICHIGAN

So long as metal removal rates were low, and its principal customer used dies made from standard metals, the Michigan tooling industry could ignore EDM. Now, with the intense competition forcing the reduction of lead time, the elimination of laborious hand finishing, and the improvement of dimensional accuracy, the automotive industry has been rushing the refinement of EDM. The captive shops of Ford, General Motors, and Chrysler are already employing EDM for sinking and finishing dies to be used in forming sheet metal. The recent increases in metal removal rates have prompted one automotive manufacturer to form a special task force to adapt EDM more rapidly to toolmaking. The company's goal is to convert the machining of most blanking, trimming, flanging, and piercing dies to EDM. Another company has already achieved this goal and is now stressing the finishing of surface dies by EDM.

In the past, automobile design was sometimes limited by the difficulty of economically machining and finishing intricate curves in dies. Since the electrode does not touch the workpiece in EDM, clearance for abrasive cutting and hand finishing is no longer a concern. The greater freedom in die design allows more one-piece dies and eases component mating for better die performance.

The cost of mating blank and trim dies by hand is now cut by using the punch configuration to produce the electrode or even using it as an electrode. The female die is mounted on a special EDM machine, or usually a spotting press retrofitted with a dielectric tank and electrical equipment. Meanwhile carbon is cemented and cut to conform to the finished edge of the punch and mounted on the machine. The tank is filled and the punch lowered under a servomechanism control onto the opposite element. At the required gap, sparks erode away the metal until the properly dimensioned female die is finished. Dies processed in this way are more accurate than those manufactured by the old method. The problems resulting from annealing and heat-treating are avoided. The dies hold their shape better, require almost no hand finishing, and have longer, more productive lives.

Surface dies are made in essentially the same way. The only difference, aside from the obvious need for stronger power supplies, larger machines, and better flushing, is in the preparation of the electrode. In surface work

the electrode is cut from a block of carbon on a numerically controlled milling machine with coordinate information from programmed body engineering and styling information. This process is relatively simple compared to milling a casting because carbon is so soft it can be easily finished by hand. Now that electrode wear can be controlled, the same electrode is used to machine more than one die. One captive shop has machined a set of five dies for a hood panel with no appreciable wear on the electrode.

Other rewards from the "no wear" advancement were described recently by engineers of the Ford Motor Company. Ford electrically machined a small die for hot-forging stabilizer bar eyes, using reverse polarity. The die was roughed and finished to 500 rms with no measurable wear on the carbon electrode and no costly hand finishing. The cost of producing the die with EDM was half that of the conventional method. Later, when the die needed redressing, it was refinished with the same electrode, again with no hand work and at only one-fourth the cost of the conventional method.

An important first step in the use of EDM in toolmaking is to break with traditions and outdated conventions. The new process is so different that to use it as a supplement for older processes too often involves an overlap of work. Recently, many reviews of the total manufacturing process have been conducted to find the appropriate uses of EDM specially teamed with numerical control, and to assure the elimination of traditional but now superfluous operations. A reduced need for models and checking fixtures is already apparent.

The economic success of EDM in diemaking has encouraged auto makers to investigate its potential on special production jobs. EDM drills holes that are burr-free and more accurate both in internal dimensions and in spacing than is possible with conventional methods; in drilling holes for carburetor components, for instance, where a single burr can easily plug one of the narrow fuel and air flow channels, EDM may be a better process to use.

Auto makers are certain EDM will improve quality and allow more distinctive styles. As the process spreads through the industry, the independent toolers should get some relief from the shortage of skilled workers and should also improve the prospects for product diversification. Reductions in blanking, die fit-up, and die surface barbering lower skill requirements. EDM does not need a full-time operator; a semiskilled attendant occasionally monitoring the machine is sufficient.

The technological progress of EDM is a good example of how important advancements-better electrode materials and power supplies, and reversed polarity-can alter the economic competitiveness of a tool

1 James Dillon and Fred L. King, "EDM in Automotive Body Die Manufacturing," Frontiers in Manufacturing Technology, Vol. III, The University of Michigan, 1968.

making process. The 16 surveyed firms using EDM for primarily exotic materials or parts of intricate shapes can now apply the process profitably to many other jobs. Those few firms that found that EDM helped them to diversify into aerospace markets will now have a head start in the application of EDM to conventional toolmaking.

The popularity of EDM will be influenced by its relatively low cost. One survey respondent who owned only conventional equipment and was slipping behind the competition felt that EDM was the only new technology he could afford. Many of the EDM machines covered by the survey were conventional knee-on-column milling machines retrofitted by the regular staff with advice from power supply manufacturers. The power supply and associated electrical apparatus accounted for most of the capital expense.

The advancement that made EDM profitable for conventional tooling occurred so recently that our survey was unable to sufficiently analyze EDM's impact in the toolmaking industry. As this report is published, it appears that EDM may have an impact on the industry comparable to that of numerical control.

Chapter Six

FEATURES OF NUMERICAL CONTROL

Like many previous metalworking innovations, numerical control began in Europe and was subsequently refined by American mechanics and engineers. During World War II the U.S. Government was informed that the Germans had conceived a technique for the automatic control of general-purpose machine tools. These concepts remained speculative until 1948 when John Parsons of Traverse City, Michigan, began to experiment with computer and servo-control technologies for the precise manufacture of complex aircraft parts. In 1949, Parsons successfully demonstrated the technique to the Air Force. The development and implementation followed at an unprecedented rate.

Within three years the laboratory experiment had been successfully demonstrated. The Federal government then placed the first order for numerically controlled machine tools with funds made available by Congress to provide a stimulus for the slumping American machine tool industry. The Government's initial order for 105 numerically controlled machines impelled several reluctant machine tool builders to quickly assimilate the laboratory knowledge necessary for production.

THE RAPID SPREAD OF

NUMERICALLY CONTROLLED EQUIPMENT

Only 400 numerically controlled machine tools were sold during the 1950's and just a few of these were for toolmaking. The United States Department of Commerce statistics show that 60 companies built and shipped 2,583 numerically controlled machine tools between 1954 and the close of 1963, over 80% during the last four years of that period. Less than 1% of all machine tools in the United States today are numerically controlled, but the machine tool industry's 1965 backlog, 26% of the unfilled orders for metal cutting machine tools, called for numerical control. The rapidly expanding proportion of machine tools being shipped with numerical controls is shown in Figure VI-1.

1 A numerically controlled machine tool produces a part in response to a series of commands that specify the coordinates of points the cutting tool must pass over to produce a desired part configuration. Ancillary commands on the tape also direct spindle speed, cutter feedrate, tool changes, coolant flow, etc. The set of commands may constitute drilling operations at specified locations, or may direct the cutting tool to machine in a continuous path mode, such as in a milling or turning operation.

[blocks in formation]

1954 58 1959 1960 1961 1962 1963 1964 1965 1966

[graphic]

FIGURE VI-1. Numerically Controlled Machine Tools As a Percent of Metal Cutting Machines. Source: Numerical Control Society, NC: A Vehicle for Progress, p. 5.

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