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Electrical Discharge Machining (EDM)
of Metals and Alloys

By
Dr Thoguluva Raghavan Vijayaram PhD
Chartered Engineer (M123412-3, IEI, India)
Senior Lecturer in Manufacturing Engineering and Researcher in Metallurgy, Department of Manufacturing Process and System,
Faculty of Manufacturing Engineering, UTeM, Universiti Teknikal Malaysia Melaka, Ayer Keroh, 75450 Melaka Malaysia. Email: thoguluva@utem.edu.my

EDM is called as Electrical Discharge Machining. Otherwise called as Spark Erosion Machining, Electro-Erosion Machining, Spark Machining, and Electro Discharge Machining. It is an unconventional machining process to machine hard metals and alloys. The principle is based on the erosion of metals by spark discharges. When two current conducting wires are allowed to touch each other, an arc is produced. At the point of contact between the two wires, a small portion of the metal has been eroded away, and leaving a small crater.

The EDM process has become one of the most important and widely used production technologies in metallurgical and manufacturing engineering. It is a process of metal removal based on the principle of metals by an interrupted electric spark discharge between the electrode tool (cathode) and the work piece (anode). The electrodes used are graphite, and copper normally. Fundamentally, the electric erosion effect is understood by the breakdown of electrode material accompanying any form of electric discharge. The discharge is usually through a gas, liquid, or in some cases through solids. The common dielectric fluids used are kerosene, paraffin, and light hydrocarbon oils. A necessary condition for producing a discharge is the ionization of the dielectric fluid and splitting up its molecules into ions and electrons. A schematic sketch of EDM is shown below in Figure-1.

Figure-1 Electrical Discharge Machining

The main components in EDM are the electric power supply, the dielectric medium, the work piece, the tool, and a servo control unit. The work piece and the tool are electrically connected to a DC electric power supply. The current density in the discharge of the channel is of the order of 10000 A / Square centimeter, and power density is nearly 500 MW / Square centimeter. The work piece is connected to the positive terminal of the electric source, so that it becomes the anode and the tool is the cathode.

A gap, known as SPARK GAP in the range, from 0.005 mm to 0.05 mm is maintained between the work piece and the tool, and suitable dielectric slurry, which is non conductor of electricity, is forced through this gap at a pressure of 2 kgf/square centimeter or lesser. When a suitable voltage in the range of 50 to 450 V is applied, the dielectric breaks down and electrons are emitted from the cathode and the gap is ionized. When more electrons collect in the gap, the resistance drops causing electric spark to jump between the work piece surface and the tool. Each electric discharge or spark causes a focused stream of electrons to move with a very high velocity and acceleration from the cathode towards the anode, and ultimately creates compression shock waves on both the electrode surface, particularly at high spots on the work piece surface, which are closest to the tool. The generation of compression shock waves develops a local rise in temperature.

The whole sequence of operation occurs within a few microseconds. The temperature of the spot is hit by the electrons is of the order of 10000 degree centigrade. This temperature is sufficient to melt a part of the metals. The forces of electric and magnetic fields caused by the spark produce a tensile force and tear off particles of molten and softened metal from this spot in the work piece. Thus, the metal is removed in this way from the work piece. The volume of the material removed per spark discharge is typically in the range of (1/1000000) to (1/10000) cubic millimeter. Tolerance value of + or – 0.05 mm could be easily achieved by EDM in normal production. The best surface finish that can be economically achieved on steel is 0.40 micron.

A variation of EDM is wire EDM. Wire EDM is otherwise called as Electrical Discharge Wire Cutting. In this process, a slowly moving wire travels along a prescribed path, cutting the work piece. This process is used to cut plates as thick as 300 mm and to make punches, tools, and dies from hard metals. It can also cut intricate components for the electronics industry. A schematic sketch of wire cut EDM is shown below in Figure-2.

Figure-2 Wire Cut EDM

The wire is usually made up of brass, copper, tungsten, and molybdenum and wire diameter is nearly equal to 0.30 mm for roughing cuts and 0.20 mm for finishing cuts. The wire should have high electrical conductivity and tensile strength and it is used only once, as it is relatively inexpensive compared to the type of operation it performs. It travels at a constant velocity in the range of 0.15 to 9 m / minute, and a constant gap (kerf) is maintained during the cut.

Modern EDM machines are capable of producing three dimensional shapes and are equipped with the features like computer controls for controlling the cutting path of the wire and its angle with respect to the work piece plane, multiheads for cutting two parts at the same time, features such as controls for preventing wire breakage, automatic self threading features in the case of wire breakage, and programmed machining strategies to optimize the operation. The electrical discharge machining is used for the manufacture of tools having complicated profiles and a number of other components. The EDM provides economic advantage for making stamping tools, wire drawing and extrusion dies, header dies, forging dies, and intricate mould cavities. It has been extremely used for machining of exotic materials used in aerospace industries, refractory metals, hard carbides, and hardenable steels.

Typical EDM Applications include:

1. Fine cutting with thread shaped electrode (wire cutting EDM).
2. Drilling of micro-holes.
3. Thread cutting
4. Helical profile milling.
5. Rotary forming.
6. Curved hole drilling.
7. Delicate work piece like copper parts for fitting into the vacuum tubes can be produced by EDM.
8. The process can be applied to all electrically conducting metals and alloys irrespective of their melting points, hardness, toughness, or brittleness.
9. Other applications include deep, small-diameter holes using tungsten wire as the electrode, narrow slots in parts, cooling holes in super alloy turbine blades, and various intricate shapes.
10. Parts should be designed so that the required electrodes can be shaped properly and economically.
11. Deep slots and narrow openings should be avoided.
12. For economic production, the surface finish specified should not be too fine.
13. In order to achieve a high production rate, the bulk of the material removal should be done by conventional processes (roughing cut).
14. Any complicated shape that can be made on the tool can be reproduces on the work piece.
15. Highly complicated shapes can be made by fabricating the tool with split sectioned shapes, by welding, brazing, or by applying quickly setting conductive epoxy adhesives.
16. Time of machining is less than conventional machining processes.
17. EDM can be economically employed for extremely hardened work piece. Hence, the distortion of the work piece arising out of the heat treatment process can be eliminated.
18. No mechanical stress is present in the process. It is due to the fact that the physical contact between the tool and the work piece is eliminated. Thus, fragile and slender work places can be machined without distortion.
19. Cratering type of surface finish automatically creates accommodation for lubricants causing the die life to improve.
20. Hard and corrosion resistant surfaces, essentially needed for die making, can be developed.

About the author
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy, and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

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