ELECTRICAL DISCHARGE MACHINING (EDM)
Electrical Discharge Machining (EDM) is a non traditional machining and electro thermal process in which material from the workpiece is removed by using electrical discharges (sparks). It was first observed in 1770 by Joseph Priestley. He was an English physicist. In EDM machine the material is removed by rapidly recurring (repeating) discharges of current in between the electrodes. The electrodes are separated by dielectric liquid and a high voltage is applied across it. It is used to machine those materials which are difficult to machine and have high strength temperature resistance. EDM can be used to machine only electrically conductive materials. Otherwise it cannot be used. One of the electrodes is called as tool and other is called as workpiece. Here the tool is connected with the negative terminal of the power supply and the workpiece is connected with the positive terminal.
In Electrical discharge machining; a potential difference is applied across the tool and w/p in pulse form. The tool and workpiece must be electrically conductive and a small gap is maintained in between them. The tool and workpiece is immersed in a dielectric medium (kerosene or deionized water). As the potential difference is applied, electrons from the tool start to move towards the workpiece. Here the tool is negative and w/p is positive. The electrons moving from the tool to the w/p collide with the molecules of dielectric medium. Due to the collision of electrons with the molecule, it gets converted into ions. This increases the concentration of electrons and ions in the gap between the tool and w/p. The electron moves towards the w/p and ions towards the tool. An electric current is set up in between the tool and w/p and called as plasma. As the electrons and ions strikes the w/p and tool, its kinetic energy changes to heat energy. The temperature of the heat produced is about 10000 degree Celsius. This heat vaporizes and melts the material from the workpiece. As voltage is break down, the current stops to flow between the tool and w/p. And the molten material in the w/p is flushed by circulating dielectric medium leaving behind a crater. The spark generation is not continuous because constant voltage is not applied across the electrodes. The voltage is applied in pulse form.
Equipment The various equipment used in Electrical Discharge Machining are 1. Dielectric Reservoir, Pump and Circulating system Pump is used to circulate the dielectric medium between the two electrodes ( tool and workpiece). Kerosene or deionized water is used as dielectric medium.
2. Power Generator and Control Unit Generator is used to apply potential difference. The voltage used in this machining process is not constant but it is applied in pulse form. A control unit is used to control the different operation during machining process.
3. Working Tank with Work Holding Devices It has working tank with a work holding device. The workpiece is hold in the work holding devices. The tank contains dielectric medium.
4. Tool Holder It is used to hold the tool.
5. Servo System to Move the Tool A servo system is used to control the tool. It maintains the necessary gap between the electrodes ( tool and workpiece).
Fig: Sample of electrical discharge machining process
Electrical Discharge Machining vs. Other Machining Processes
“The biggest point in favor of piece part manufacturing with wire EDM is when you get into part detail and you’re trying to do it with a slitting saw or a grinder, or any type of friction machining, which is going to create a burr,” said Langenhorst. “In that case, you have to have a secondary process for burr removal, whether it’s vibratory or having a bunch of workers scratching the burrs off with hand tools. On the other hand, when you take a part off an EDM machine, there is no burr: it’s perfect.” Pfluger pointed out another advantage to using electrical discharge machining for parts with small or complex features: “The EDM process becomes more attractive as workpiece materials become harder, and as the part geometry becomes smaller and deeper,” he said. “Unlike conventional milling, EDM does not encounter limitations with L:D as internal radii features become smaller and workpiece thickness increases. In general, the EDM process is reserved for smaller part feature sizes and higher accuracy requirement applications (+/- 0.0005” or +/- 0.012mm or finer accuracy).” Moreover, since EDM is a non-contact machining process, the fixturing requirements for cutting small parts are considerably less onerous compared to those of a standard CNC mill. “There’s no cutting pressure, so if you’re working with little tiny pieces, you don’t need much of a fixture to hold them,” said Langenhorst. “If you tried to mill them, have you to have it held tight enough that your machining process won’t pick it up or bend it. For example, if you’re doing core pins for molds and you try to grind them, they’ll move all over the place. You can wire cut those with just a 90-degree flip fixture and they come out great.”