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Electrical-Dischargefor Cutting and Machining.SOLUTIONS with Effective, Practical Advice
Electrical-Discharge Cutting is an alternative cutting process, sometimes used successfully by welders. As a manufacturing process permitting to shape metallic objects it is called ED Machining or Material Removal, and also Spark Erosion. Electrical-Discharge profiling is applied in the production of complex shapes like dies in hardened tool steels. For cutting purposes it can be compared in yield and costs to abrasive water jet cutting, laser cutting, plasma arc cutting and other cutting processes. Its advantages are remarkable when dealing with hard or tough materials, where conventional mechanical chip removal processes are at loss. The material removal is the product of successive Electrical Discharge pulses occurring when two metallic objects, called electrodes, separated by a dielectric liquid, are connected to a sufficiently high voltage difference provided by a Direct Current source. One of the electrodes is obviously the workpiece whose final shape and tolerances define the operation requirements. The other electrode, made of a copper alloy, has to be continuously replaced as it becomes locally damaged beyond usefulness.
In the Electrical Discharge or spark that follows contact closure, through the breakdown of the dielectric liquid, electric current is converted into heat. A tiny area on the material surface, bordering with the discharge channel, is heated to melting temperature and beyond.
The Electrical-Discharge therefore produces the explosive evaporation of minute amounts of metal from both electrodes leaving minute craters as the workpiece surface is continuously eroded. The recurring formation of sparks at the right location helps in achieving the required operation, be it cutting along a predetermined line, or machining to the wanted shape. The practical application of the above principles confronts the necessity to check, after each spark, the removal of debris, the restored presence of uncontaminated dielectric liquid, the proper inter-electrode distance or spark gap, and the correct shape of the tool electrode. Automated machines are built to work independently of human intervention, so that a single worker can supervise a number of EDM (Electrical Discharge Machines) operating concurrently on different jobs. One useful development of these Electrical-Discharge machines consists in substituting the tool with a continuous loop of copper alloy wire, whose relative position to the object is kept so that cutting progresses along the wanted cutting line. This arrangement presents some advantages relative to a static cutting electrode, making this a preferred solution for many situations, including that of piercing two dimensional irregular holes in a plate, starting with a small hole needed to pass the wire through. One important consideration that may affect applications is the fact that the new cut surface is quite rough, being composed of a recast layer, due to the local melting intrinsic to the process. If this layer is considered objectionable to the further service of the item manufactured, it must be removed by suitable means. In conclusion, the chance that Electrical-Discharge Cutting could be the most cost effective solution for any real case should not be dismissed without checking.
Look for an interesting article An Article on Capacitor Discharge Welding published (2) in Issue 139 of Practical Welding Letter for March 2015, reports on a process called Electro Spark Deposition (ESD), described as EDM in reverse. An Article on Traceability in Automated Cutting was published (2) in Issue 146 of Practical Welding Letter for October 2015.
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