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Technical Articles
Home >> Technical Articles >>An Overview of Electrical Discharge Machine
An Overview of Electrical Discharge Machine
Time: 2009-01-15
The origin of electrical discharge machining goes back to 1770, when English scientist Joseph Priestly discovered the erosive effect of electrical discharges. In 1943, Soviet scientists B. Lazarenko and N. Lazarenko had the idea of exploiting the destructive effect of an electrical discharge and developing a controlled process for machining materials that are conductors of electricity.
With that idea, the EDM process was born. The Lazarenkos perfected the electrical discharge process, which consisted of a succession of discharges made to take place between two conductors separated from each other by a film of non-conducting liquid, called a dielectric. The Lazarenkos achieved a form of immortality with this circuit, which today bears their name. Today, many EDMs use an advanced version of the Lazarenko circuit.
How It Works
During the EDM process, a series of non-stationary, timed electrical pulses remove material from a workpiece. The electrode and the workpiece are held by the machine tool, which also contains the dielectric. A power supply controls the timing and intensity of the electrical charges and the movement of the electrode in relation to the workpiece.
At the spot where the electric field is strongest, a discharge is initiated. Under the effect of this field, electrons and positive free ions are accelerated to high velocities and rapidly form an ionized channel that conducts electricity. At this stage current can flow and the spark forms between the electrode and workpiece, causing a great number of collisions between the particles.
During this process a bubble of gas develops and its pressure rises very steadily until a plasma zone is formed. The plasma zone quickly reaches very high temperatures, in the region of 8,000 to 12,000 [degrees] Centigrade, due to the effect of the ever-increasing number of collisions. This causes instantaneous local melting of a certain amount of the material at the surface of the two conductors.
When the current is cut off, the sudden reduction in temperature causes the bubble to implode, which projects the melted material away from the workpiece, leaving a tiny crater. The eroded material then resolidifies in the dielectric in the form of small spheres and is removed by the dielectric.
Growth of EDM
EDM has rapidly earned its place alongside milling and grinding equipment as a proactive, mainstream technology. EDM is best known for its ability to machine complex shapes in very hard metals. The most common use of EDM is machining dies, tools and molds made of hardened steel, tungsten carbide, high-speed steel and other workpiece materials that are difficult to machine by "traditional" methods.
The process has also solved a number of problems related to the machining of "exotic" materials such as Hastelloy, Nitralloy, Waspaloy and Nimonic, which are used on a large scale in the aeronautical and aerospace industries.
Because of technical advances in electrode wear, accuracies and surface speed, EDM has replaced many of the traditional processes in some applications. Another factor contributing to the growing use of EDM is the expansion of the work envelope, particularly when it comes to heights and tapers. Standard wire EDMs can cut parts 16 inches tall with a straightness of [+ or -] 0.0005 inch per side!
With the reduction in electrode wear and increased sophistication of EDM controls in rams, new EDM processes use simple-shaped electrodes to 3D mill complex shapes. EDM also is being used for polishing small, intricate surfaces.
Since EDM does not involve workpiece/tool forces like a mill or grinder, it is possible to EDM shapes that would break conventional cutting tools or be broken by them.
The unattended nature of EDM also makes it a cost-effective process for a three-shift-per-day operation - without adding manpower. Because of the resulting fast turnaround time in EDM for small lots of parts, the process also helps shops reduce inventory and shorten deliveries - which contributes to improved cash flow and reduced operating expenses.
When Should You EDM?
Whether you're a mold shop owner looking to replace contour form grinding for core and cavity details or a process engineer who wants to explore how EDM can streamline design and production capabilities, what criteria should you use to determine "When to EDM?"
This question was posed to manufacturing engineers, shop owners and our own technical support specialists. They identified a range of appropriate workpiece materials and geometries, plus the processes EDM can replace. For this article, our recommendations are sorted based on three physical characteristics of the EDM process:
* No force between tool and workpiece.
* Workpiece is vaporized not cut.
* No rotation of tool or workpiece.
No Force
Because EDM is a no-contact and no-force process, it is well suited for making frail or fragile parts that cannot take the stress of machining. The type of parts that fit this profile includes printer hammers in dot matrix printers, graphite electrodes and any part that features tough-to-machine honeycomb shapes.
Grinders and cutting tools have trouble with thin walls. EDM, on the other hand, is ideal for this application because the process does not involve force, contact or deformation. Accordingly, a wire EDM is capable of making walls as thin as 0.005 inch. A ram EDM can produce walls as thin as 0.002 inch. EDMs designed for making very small holes can create walls as thin as 0.0002 inch. Examples of thin-walled parts produced by EDM include surgical tools, microwave horns and the satellite structural components shown in Figures 1 and 2.
Consider using EDM when parts have high ratios of cavity depth to width, such as slots and ribs. Since the EDM process does not require force, you can use very long electrodes to make extremely intricate ribs. Wire and ram EDMs are used to make fixtures, collets, jet engine blade slots, mold cooling ribs and reinforcing ribs.
If you have a difficult recessed cut to make, you'll probably need to use a ram EDM. In many cases, traditional cutting tools cannot reach cutting areas and apply the required force.
Why is EDM the preferred process for tough materials such as Inconel, Monel, Hastelloy, Nitralloy, Waspaloy, Nimonic and Udimet? Since the electrode does not come in contact with the material, there's no adhesion of the workpiece to the tool. This fact makes wire and ram EDMs ideal for making magnetic reader heads for missiles, artificial joints, turbine blades and car engine prototypes.
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