Cut-surface quality - the pluses and minuses of EDM.
Although EDM is still relatively slow, particularly when a high-quality cut surface is required, the newest equipment is capable of much higher cutting speeds than was possible only a few years ago, with no sacrifice in surface quality. Furthermore, advances such as computer control, sophisticated software, and automation allow untended EDM operations, releasing operators for other duties or automatically machining workpieces overnight or during a weekend.
No machined surface is perfectly smooth. It doesn't matter how the surface was machined. An enlarged workpiece cross-section will show peaks and valleys. These microscopic undulations determine surface quality and whether a component will function as intended or even mate with another component.
Before talking about EDM cut surfaces though, we must reach agreement on what we mean by "high surface quality." Since there are various ways of specifying quality, and since the values can be expressed in either English or metric, we must be careful in evaluating and comparing surface-quality measurements. For example, expressing surface roughness as so many microns is meaningless unless the measurement method also is stated.
One way of expressing surface quality is the root-mean-square (rms) method, which consists of squaring measurements taken over all the peaks and valleys, adding the numbers, and taking the square root of the sum. This sounds cumbersome and it is, but the result indicates surface roughness and can be compared with similarly derived numbers to contrast the quality of one surface against another. Although this method is widely used, it overemphasizes maximum deviations that may only rarely occur across the surface.
Another method gaining acceptance measures the arithmetic mean of all peaks and valleys. This method measures all deviations and derives a mean between the most prominent peaks and deepest valleys. For a given surface, this measurement will be slightly larger than the rms value.
A third view of surface quality is maximum roughness, which measures the distance between the highest peak and the deepest valley. Obviously maximum roughness, R.sub.max in metric or H.sub.max in English, will be considerably higher than the corresponding rms or arithmetic mean measurement. The accompanying chart compares the relative value of various finish measurements.
To understand how EDM affects cut-surface quality and integrity, we must first look at how the process works. Metal is removed by erosion caused by a controlled electrical spark. There is no direct contact between electrode and workpiece. The rate at which metal is removed depends on the electrical conductivity of the workpiece.
One terminal of the power supply is connected to the workpiece, the other to the electrode. The workpiece and electrode are separated by a dielectric fluid--usually deionized water for wire-cutting and a special hydrocarbon for diesinking--which acts as an electrical insulator until the spark occurs. The dielectric also cools the work area after the spark ends, and flushes away metal particles before the next spark occurs.
DC voltage is applied between the electrode and the workpiece. At first, no current flows because the two pieces are insulated by the dielectric fluid, however, an electrical field does build up across the gap. As the electrode approaches the part and the gap narrows, a point is reached where the voltage ionizes the dielectric fluid and a spark jumps the gap.
When the spark first occurs, a large amount of energy is released, vaporizing material from the work surface. As current continues flowing, intense heat melts additional material. When the voltage drops to zero, current stops flowing, and the spark is quenched. At this point material removal ceases.
As soon as the spark is quenched, the vapor bubble begins to collapse. The dielectric cools the area, solidifying the material that was melted. Some of this material is carried away by movement of the dielectric fluid, leaving a small crater at the point where the discharge occurred. Some of the melted material is redeposited into the crater. This recast layer is an important factor in EDM surface quality.
To some extent, this oversimplifies the sequence of events. The accompanying drawings provide more detail. There are two important points concerning EDM that should be emphasized here. First, it isn't simply the current flow that removes material--it is switching the current on and off that actually vaporizes and melts the metal.
Second, the most efficient part of the sequence is the initial discharge when material is vaporized. The longer current flows, the more heat builds up and the more material is melted rather than vaporized. Also, some of the melted material always will be recast into the cavity. There is presently no EDM technology to entirely eliminate the recast layer. Newer power supply designs, however, are effective in minimizing the recast layer as well as any heat-affected material directly below it.
Pulses and quality
Metal removal, as already stated, depends on starting and stopping the spark. The amount of metal removed is a function of the energy released during the spark, which, in turn, is determined by frontal-gap voltage (i.e., voltage between the workpiece and electrode), by the electric current that flows, and by the time (pulse width) current flows before the spark ends.
Older EDM equipment used a bank of capacitors in the power supply to store energy for the spark. This design is known as a "capacitant-discharge" power supply. The capacitors gather and store electrical energy until the equipment senses proper frontal-gap voltage, which creates the spark and releases this energy. After the capacitors "dump" their charge, the spark is extinguished and the capacitors begin a recharge cycle in preparation for the next spark.
The problem is that the spark is initiated solely on the basis of frontal-gap voltage sufficient to ionize the dielectric. For one spark the capacitors may have become fully charged and will release a maximum amount of energy thereby removing a significant amount of material. For another spark the frontal-gap voltage may be reached before the capacitors achieve full charge. This spark will remove less material. Consequently, metal removal across a workpiece surface may be quite uneven--there will be large craters at some points, small craters at other points.
The pulse-type power supply was developed to eliminate this problem. To control the spark more accurately, engineers designed a power supply where each spark releases the same amount of energy. In addition, each new spark is delayed until all the energy in the previous spark has been used. The energy bursts may now occur at a more random rate, but each time the spark creates an electrical current, the same amount of energy is released and the same amount of material is removed.
The pulse-generator power supply provides more precise control of the spark and enables more efficient use of each spark. This means every eroded cavity will have essentially the same diameter and depth. And, although there always will be some material recast into each cavity, the depth of this recast layer is greatly reduced.
The latest generation of pulse-generator power supplies provides highly refined cut-surface quality. The power supply furnishes a large number of low current, extremely short pulses. Thus each spark is used nearly 100 percent. Because of the low current and short spark duration, material is removed primarily by vaporization. The recast layer and underlying heat-affected zone is now about one-tenth of that generated by a capacitant-discharge power supply. Surface finish quality is such that secondary finishing operations are usually unnecessary. If this recast layer must be removed from the workpiece in subsequent steps, the amount of material will be less than with the irregular cut produced by a capacitant-discharge power-supply system.
Surface quality is especially important when diesinking EDM is used to make injection molds, pressure casting dies, and other components whose cut surfaces must range from smooth-matte to highly-polished. Modern pulse-control power supplies benefit both diesinking and wirecut EDM. With the pulse-generator used in diesinking EDM equipment, a highly-polished surface with an arithmetic mean roughness of 7 to 10 microinches is possible. Such EDM sink-polishing provides a consistent, refined surface which is particularly desirable on delicate materials and highly detailed components that could be damaged or destroyed by traditional polishing methods.
Machining sintered materials
There is no problem with EDM cutting of conventional, homogeneous metals such as tool and high-speed steel, or even titanium. Nonferrous sintered materials, on the other hand, present an entirely different set of problems. A major consideration with EDM cutting of sintered materials is not so much a concern for surface quality as for surface integrity.
These materials include any of the various forms of carbide (i.e., carbides of silicon, tungsten, titanium, tantalum, and chromium). Also, cubic boron nitride (CBN), because it has a hardness approaching diamond and is less costly than diamond, is being used in some critical applications. And then there is the "miracle material of the '80s," polycrystalline diamond.
Many people feel that EDM and sintered materials are incompatible; that carbide and the other materials cannot be successfully machined by this method. Others feel that the materials can be EDMed, but that the necessary allowances, precautions, and limitations create so much uncertainty that the chances for success are slim and unpredictable. Unfortunately, there is some truth in both viewpoints, although certain "wizards" seem to know a few secrets that they haven't shared with the rest of us.
In the case of proprietary materials, such as polycrystalline diamond, material composition and special techniques for successfully EDMing it have been withheld as trade secrets. Only recently has some meager information surfaced.
The problem with electrical discharge machining any of these sintered products is the structure and composition of the material itself. Granules (or crystals) of the primary material are held together by a matrix (or binder). The binder is generally much more electrically conductive than the granules. Thus, during EDM cutting, electrical current from the spark flows through the binder and around the granules, eroding the binder and cutting the material.
The binder, however, does much more than merely hold the granules in place. The sintering process captures the granules within the binder under great tension. When the EDM spark flows through the binder, it reacts electrochemically to the current, releasing some of that tensional force. Following a high-energy EDM pulse, some of the granules are totally free and will simply fall away. Others are only partly held in place and may flake away under moderate pressure.
In studying the effects of EDM cutting on carbide materials, researchers discovered that the shape and duration of the electrical pulse had a marked influence on the surface integrity of EDM-cut carbides. With older power supplies, varying amounts of energy available in the EDM spark had inconsistent effects on the cobalt binder material. The pulse-type spark generators that release an identical amount of energy in each spark limit the electrochemical destruction of the binder, resulting in improved surface integrity.
For more information on EDM equipment, circle E19.
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|Title Annotation:||electrical discharge machining|
|Author:||Bormann, Randall L.|
|Publication:||Tooling & Production|
|Date:||Jan 1, 1985|
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