ELECTRIC DISCHARGE MACHINING (EDM) SNS COLLEGE OF ENGINEERING, COIMBATORE- 641 107.   DEPARTMENT OF MECHANICAL ENGINEERING ELECTRIC DISCHARGE MACHINING (EDM) Prepared by B.BALAMURALI AP/MECH ME6004 - UMP/Topic - 17,18

CONTENTS . INTRODUCTION . PRINCIPLE . CONSTRUCTION . TOOL MATERIAL . FLUID PROPERTIES . WORKING . EFFECTS OF EDM ON METAL SURFACE . ADVANTAGES . DISADVANTAGES . APPLICATIONS ME6004 - UMP/Topic - 17,18

INTRODUCTION EDM is a machining method typically used for hard metals which are electrically conductive. It makes possible to work with metals for which traditional machining techniques are ineffective. The use of a thermoelectric source of energy in developing the nontraditional techniques has greatly helped in achieving an economic machining of the extremely low machinability materials and difficult jobs. The process of material removal by a controlled erosion through a series of electric sparks, commonly known as EDM, was first started in the USSR around 1943. Then onwards, research and development have brought this process to its present level. ME6004 - UMP/Topic - 17,18

PRINCIPLE The principle of EDM is also called electro discharge or spark erosion or electro erosion, is based on erosion of metals by spark discharges. Since, when two current carrying conductors/wires are allowed to touch each other, an arc is produced.If we look closely at the point of contact b/w the two wires, we note that a small portion of metal has been eroded away, leaving a small crater. ME6004 - UMP/Topic - 17,18

. In EDM,when a discharge takes place b/w two points of the anode and the cathode, the intense heat generated near the zone melts & evaporates the materials in the sparking zone. For improving effectiveness, the w/p & tool both are submerged in a dielectric fluid (hydrocarbon or mineral oils). It has been observed that, If both the electrodes are made of same materials, the electrode connected to +ve terminal generally erodes at faster rate. For this reason, the w/p is normally made the anode. ME6004 - UMP/Topic - 17,18

. . A suitable gap, known as the spark gap, is maintained b/w the tool & the w/p surfaces. . The sparks are made to discharge at a high frequency with a suitable source. . Since the spark occurs at the spot where the tool & the w/p surfaces are the closest and since the spot changes after each spark(because of material removal after each spark), the spark travels all over the surface. . This results in a uniform material removal all over the surface, and finally the work face confirms to the tool surface. . Thus, the tool produces the required impression in the workpiece. ME6004 - UMP/Topic - 17,18

CONSTRUCTION ME6004 - UMP/Topic - 17,18

The main components of EDM are- -> Power Supply -> Tool (Electrode) -> Spark Generator -> Di-electric fluid Power Supply: Powersupply converts AC into pulsed DC, used to produce spark b/w tool and w/p. This high power pulsed output responsible for generating spark between the electrodes. ME6004 - UMP/Topic - 17,18

Solid tool(rod) may be used but for larger ones hollow tools are used. TOOL (ELECTRODE) : The shape of the tool should be replica of the desired product, except that an allowance is made for side clearance, over cuts for broaching small holes. Solid tool(rod) may be used but for larger ones hollow tools are used. If object have geometrical shape having symmetry about some axis, tool symmetric to only a part of the object will be sufficient for complete machining of the object. SPARK GENERATOR : The spark generator performs the important function of supplying sufficient voltage to initiate & maintain the discharge. It also absorbs the supply of varying current intensity and the discharge duration and controll the recurring rhythmof the discharge. ME6004 - UMP/Topic - 17,18

. DI-ELECTRIC FLUID : In EDM, the dielectric fluid is flushed through the spark gap. It is supplied either to a hole in the tool or from external jet. This fluid works as the spark conductor. The widely used dielectric fluid is kerosene; others used are deionized water, Si oils, ethylene glycol etc. The fluid must be hydrocarbon becoz deionization occurs due to the involvment of hydrogen in hydrocarbon. ME6004 - UMP/Topic - 17,18

TOOL MATERIALS The selection for tool material is depends upon various factors. The main factor is, the tool should have electrical conductive. Other factors are, volume of material to be removed, required surface finish, desired tolerance, wear ratio & many more. The various materials which can be used for making tool may be of bross, copper, graphite. ME6004 - UMP/Topic - 17,18

FLUID PROPERTIES The dielectric fluid used in EDM should have following properties- It should have sufficient dielectric strength. It should have low viscosity & good wetting capacity. It shoud be chemically neutral. It should not evolve toxic vapors during entire operation range. The flash point should be high. It should provide sufficient cooling medium. It should be less costly & easily available. It should flush the debris produced during operation. ME6004 - UMP/Topic - 17,18

WORKING The EDM system consist of a shaped tool(electrode) & w/p, which are connected to the dc power supply and placed in electrically non conducting dielectric fluid. When a voltage is applied to the tool, a magnetic field causes suspended particles in the dielectric fluid to concentrate, eventually forming a bridge for current to flow to the w/p. An intense arc is then generated, causing sufficient heating to melt a portion of w/p & usually some of the tooling material also. In addition, the dielectric fluid heated rapidly, causing evaporation of the fluid in the arc gap; this evaporation increases resistance of interface until the arc can no longer maintained. ME6004 - UMP/Topic - 17,18

. Once arc is interrupted, heat is removed from the gas bubble by surrounding dielectric fluid, and the bubbles collapses(cavitates). The gap b/w the tool & w/p called overcut is critical; thus the downward feed of the tool is controlled by a servomechanism, which automatically maintains a constant gap. Hence, the machining of material is done by EDM technique. ME6004 - UMP/Topic - 17,18

EFFECTS OF EDM ON METAL SURFACE The high temperature generated by sparks causes the melting & vapourization of the metal & obviously this high temperature affects the properties of shallow layers of the surface to be machined. The outer most layer is rapidly chilled, and therefore becomes very hard. The hardening of the surface layerduring the EDM imparts a better wear resistance characteristic. The fatigue strength reduces due to the micro cracks devoloped in the surface layer during chilling. ME6004 - UMP/Topic - 17,18

ADVANTAGES Hard & tough metals can be machined with better surface finish. Complex & internal shapes can be machined. No cutting force due to no contact b/w metal and w/p. The surface finish is non-directional. No effects of material hardness and tool wear. Tolerance upto 0.4 micro meter is achieved. Secondary finishing operation can be neglect. No need of Heat treatment before machining. ME6004 - UMP/Topic - 17,18

DISADVANTAGES 1) Low MRR. 2) The w/p must be electric conductor. 3) The surface after machining is highly stressed. 4) Machining time is too long. 5) Specific power conjumption is too high. 6) Excessive tool wear. ME6004 - UMP/Topic - 17,18

APPLICATIONS It is used in the production of die for fabrication of all components. EDM can be used in manufacturing of hydraulic valve spools, gear wheels, fine holes or slots in hard blade materials used in gas turbines, compressors, diesel engines etc. It is used in fabrication, extrusion, blanking, stamping and embossing dies. It is also used for removal of drilland tapes from components. ME6004 - UMP/Topic - 17,18