indefinite future, and one thought it had future value. The remaining firms thought it had no future value in their operations.

Three survey firms indicated an intent to purchase an electrochemical milling machine in the immediate future; two others had indefinite purchase plans. Only a small fraction understood the process and saw a future for it in their operations. Sixty-one percent understood little or nothing of the fundamentals of this process.

These technologies, with the exception of EDM and numerical control, have limited application in Michigan because they remove metal slowly. Except for ECM, these processes are slow at metal removal but highly effective for use in cutting exotic metals. In regions where the tool and die industry supplies aerospace firms that use such hard metals, there is broader acceptance.

As long as the current market structure, management attitudes, and scale of operations continue in Michigan, EDM and numerical control will become increasingly important. Perhaps the manager of a 75-man plant best summarized the relative importance of these innovations when he said, "The new techniques (electrical discharge machining and numerical control) are the outstanding accomplishments of the last 35 to 40 years in the industry."

The techniques and implications of each process will be considered in the next three chapters; the problems which tool and die firms face because of the spreading use of numerical control will be considered in the last section of this report.

Chapter Five

ELECTRICAL DISCHARGE MACHINING IN TOOL
AND DIE MANUFACTURE

Electrical discharge machining (EDM) has progressed faster during this project than any of the toolmaking technologies investigated. At the beginning of the study, EDM was limited to parts of complex design or to exotic metals; by the fall of 1967, some large body dies for 1969 cars were being rerouted from numerically controlled milling machines. to more efficient EDM equipment. Metal removal rates were greatly increased and electrode wear was almost eliminated. New, more efficient EDM equipment demonstrated early in 1967 has shown that radical changes in die and mold making will result. By contrast, gage, jig and fixture manufacture should be only slightly affected.

THE DEVELOPMENT OF EDM

For the past two decades EDM has been thought of as an aerospace-era process. The demand for products that can continue operating for hours at high speeds with intense heat has forced aerospace manufacturers to use high-alloy and refractory metals too hard for machining economically with standard chip-cutting methods. The machining time and tooling costs for processing such metals conventionally are so extravagant that they have been called "the exotic metals."

During the Korean War, when the Government demanded lighter, more powerful jet engines, despite the increased cost several research projects were funded to discover more economical ways of machining the hard metals. While expensive carbide and ceramic tools could chip the high-alloys fast enough for production schedules, the tool wear at these speeds was so quick that tools had to be replaced frequently— sometimes in the middle of short cuts- adding to the difficulty of maintaining dimensional requirements. The high tooling costs and the disproportionate number of scrap parts, especially with those complex parts having strict dimensional requirements, stimulated further research.

EDM was one of the new methods investigated. It had been in use since the early days of World War II for removing taps and other steel tool components which were broken off in the workpiece. Removing these broken pieces of tooling by conventional means was often impossible before EDM because the salvage process sometimes distorted the prescribed dimensions of the workpiece.

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PROCESS FUNDAMENTALS

EDM is basically the process of shaping metal by a succession of controlled electrical discharges (arcs) from the electrode tool to the workpiece, evaporating or eroding the metal. The electrode is wired to a generator which controls the power and frequency of the electric discharge. Both electrode and workpiece are immersed in a flowing dielectric fluid-e.g., hydrocarbon oil-which increases the electrical potential in the gap and carries away debris. The complete machining system is shown in Figure V-1.

As the electrode advances through the dielectric fluid and nears the workpiece, the voltage across the gap builds until an adequate electrical field develops between the closest points. At about 70 volts, the dielectric fluid ionizes, giving way to the arc; microscopic foreign particles in the dielectric fluid assist in the ionization. The energy in the arc vaporizes the surrounding dielectric fluid in an enlarging column, admitting an increase of current. The voltage in the gap drops to about 20 volts. As a result of the heat generated by kinetic resistance in the arc, the temperatures on the surfaces of the tool and the workpiece rise above the melting point. A minute portion of each is liquefied and vaporized. Energy released by the liquefaction forces the liquid metal away from its parent. When the current shuts off, the eroded metal cools and solidifies. The dielectric fluid flowing into the gap flushes it away. This process is repeated at a consistent frequency based on the durability of the metal being formed, the resistance to wear of the electrode, the capacity of the dielectric fluid, and the surface finish requirements.

As the metal is removed the span of the gap increases. This information, measured in terms of the voltage required to force the arc, is constantly being fed into the servomechanism which controls the tool's rate of penetration. If the gap between the electrode and the workpiece is too large, ionization will not occur; but if it is too small, there will either be a short causing the servomechanism to retract the electrode or the electrode and the workpiece will weld themselves together.

Although the use of EDM in production was initially limited to the machining of extraordinarily hard metals, the new process can be applied profitably to the machining of any metal which has been hardened and received the minor distortions and possible cracking or decarbonization which accompany heat-treating. EDM can also be applied profitably to delicate or complex machining jobs which are either impossible with abrasive tooling or require that the workpiece be sectioned, machined, and fused. Now holes of any shape, free from troublesome, expensive burrs, can be cut through a workpiece at perfectly consistent intervals. Such precision in complex machining jobs result, for example, in better control of air, water, and fuel as it is supplied to an engine.

At first, crusts and low metal removal rates-affected by the con

ductivity of the electrode, tool wear, power supplies, and flushing-restricted the extension of EDM to other processes. Manufacturers found that EDM left a thick crust of resolidified, unexpelled metal on the machined surface, which required long and expensive bench finishing to remove. Some of the early customers were disturbed by the discoloration and wondered, since the crust was harder than the metal underneath, if the surface would wear evenly. To send the workpiece back for more heat-treating negated one of the advantages of EDM. Experience subsequently showed that crusted surfaces were better.

There were many experiments in the early applications of EDM with different types of electrodes to get higher metal removal rates with lower wear. The effectiveness of EDM was severely limited because the electrode wore out too quickly at the high metal removal rates in standard production. Experiments proved that the properties of graphite, or carbon, and to a lesser degree of copper as electrode material exceeded the potential of the power supplies. The use of graphite also helped reduce the cost of electrode preparation. Previously, copper or brass electrodes often cost as much to prepare as the savings in machining of "one-of-a-kind" dies.

One of the initial advantages of EDM was that by reducing the power and frequency of the spark to a removal rate of 0.0001 cubic inch an hour, a surface could be finished to about 5 microinches of tolerance. Unfortunately, an increase in the power and frequency of the discharge also meant an increase in tool wear and a loss of dimensional control. On the machining of surfaces, the peak economic removal rate was, at first, 3; later, 12; and, later still, 100 cubic inches an hour. Recent experiments have shown that increasing the duration of the discharge gives higher metal removal rates with greatly reduced electrode wear.

It was also discovered recently that with pulse-type power supplies by reversing the polarity of the electrode and the workpiece—that is, by making the electrode the positive pole-electrode wear could be almost eliminated. This removes one of the economic restrictions to higher metal removal rates. With reverse polarity there is a smaller gap and a higher temperature on the tool surface. Together, they allow metal expelled from the workpiece to adhere to the face of the tool. This metal-iron carbide from steel does not adversely affect conductivity, and the rate of transfer can be controlled so that it is possible to compensate completely for tool wear.

With possible metal removal rates of 100 cubic inches an hour, refined flushing techniques were needed to dispose quickly enough of the gas, heat, and metal refuse. The breathing holes in large surface dies are used in EDM for flushing. When the dielectric is flushed through under 40 pounds of pressure and the workpiece has, e.g., 3,000 square inches of surface to be machined, the separating force may be close to 100,000 pounds. Special heavy-duty machines had to be designed to endure the