EDM
General Aspects of EDM
1.EDM is a machining method primarily used for hard metals or those that would be very difficult to machine with traditional techniques.
2.EDM typically works with materials that are electrically conductive, although methods for machining insulating ceramics with EDM have been proposed.
3.EDM can cut intricate contours or cavities in hardened steel without the need for heat treatment to soften and re-harden them.
4.This method can be used with any other metal or metal alloy such as titanium, hastelloy, kovar, and inconel.
5.Also, applications of this process to shape polycrystalline diamond tools have been reported.
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TYPES OF EDM(Four types)
1.Sinker EDM:-also called cavity type EDM or volume EDM or die sinker EDM.Electrode is used as per profile shape that to cut.Blind hole and step are cut in this.
§Consists of an electrode and workpiece submerged in an insulating liquid such as oil or other dielectric fluids.
§The electrode and workpiece are connected to a suitable power supply.
§The power supply generates an electrical potential between the two parts.
§As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid, forming a plasma channel, and a small spark jumps.
§These sparks happen in huge numbers at seemingly random locations.
§As the base metal is eroded, and the spark gap subsequently increased, the electrode is lowered automatically so that the process can continue.
§Several hundred thousand sparks occur per second, with the actual duty cycle carefully controlled by the setup parameters.
§These controlling cycles are sometimes known as "on time" and "off time“.
§The on time setting determines the length or duration of the spark.
§Hence, a longer on time produces a deeper cavity for that spark and all subsequent sparks for that cycle.
§This creates rougher finish on the workpiece.
§The reverse is true for a shorter on time.
§Off time is the period of time that one spark is replaced by another.
§A longer off time, for example, allows the flushing of dielectric fluid through a nozzle to clean out the eroded debris, thereby avoiding a short circuit.
§These settings can be maintained in micro seconds.
§The typical part geometry is a complex 3D shape, often with small or odd shaped angles.
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2.Wire EDM:-Wire is used as tool.Through hole is done through it.
§Also known as wire-cut EDM and wire cutting.
§A thin single-strand metal wire (usually brass) is fed through the workpiece submerged in a tank of dielectric fluid (typically deionized water).
§Used to cut plates as thick as 300 mm and to make punches, tools, and dies from hard metals that are difficult to machine with other methods.
§Uses water as its dielectric fluid; its resistivity and other electrical properties are controlled with filters and de-ionizer units.
§The water flushes the cut debris away from the cutting zone.
§Flushing is an important factor in determining the maximum feed rate for a given material thickness.
§Commonly used when low residual stresses are desired, because it does not require high cutting forces for material removal.
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3.Hole EDM(EDM Drilling):-Brass tube is used as tool.This machine purposes is to make hole for wire EDM.
§Uses a tubular tool electrode where the dielectric is flushed.
§When solid rods are used; dielectric is fed to the machining zone by either suction or injection through pre-drilled holes.
§Irregular, tapered, curved, as well as inclined holes can be produced by EDM.
§Creating cooling channels in turbine blades made of hard alloys is a typical application of EDM drilling.
Use of NC system enabled large numbers of holes to be accurately located
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4.EDM Sawing:-
§An EDM variation - Employs either a special steel band or disc.
§Cuts at a rate that is twice that of the conventional abrasive sawing method.
§Cutting of billets and bars - has a smaller kerf & free from burrs.
§Fine finish of 6.3 to 10 μm with a recast layer of 0.025 to 0.130 mm
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5.Rotary EDM :-It is used for machining of spherical shapes in conducting ceramics using the tool and workpiece.
§Shichun and coworkers (1995) used simple tubular electrodes in EDM machining of spheres, to a dimensional accuracy of ±1 μm and Ra < 0.1 μm.
§EDM milling uses standard cylindrical electrodes.
§Simple-shaped electrode (Fig. 1) is rotated at high speeds and follows specified paths in the workpiece like the conventional end mills.
§Very useful and makes EDM very versatile like mechanical milling process.
§Solves the problem of manufacturing accurate and complex-shaped electrodes for die sinking of three-dimensional cavities.
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6.EDM milling :-
§EDM milling enhances dielectric flushing due to high-speed electrode rotation.
§Electrode wear can be optimized due to its rotational and contouring motions.
§Main limitation in EDM milling - Complex shapes with sharp corners cannot be machined because of the rotating tool electrode.
§EDM milling replaces conventional die making that requires variety of machines such as milling, wire cutting, and EDM die sinking machines.
The main components in EDM:-
1.Electric power supply
2.Dielectric medium
3.Work piece & tool
4.Servo control unit.
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POINT TO BE NOTE:-
§The work piece and tool are electrically connected to a DC power supply.
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§The current density in the discharge of the channel is of the order of 10000 A/cm2 and power density is nearly 500 MW/cm2.
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§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.
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§Dielectric slurry is forced through this gap at a pressure of 2 kgf/cm2 or lesser.
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EDM – Working Principle
§It is a process of metal removal based on the principle of material removal by an interrupted electric spark discharge between the electrode tool and the work piece.
§In EDM, a potential difference is applied between the tool and workpiece.
§Essential - Both tool and work material are to be conductors.
§The tool and work material are immersed in a dielectric medium.
§Generally kerosene or deionised water is used as the dielectric medium.
§A gap is maintained between the tool and the workpiece.
§Depending upon the applied potential difference (50 to 450 V) and the gap between the tool and workpiece, an electric field would be established.
§Generally the tool is connected to the negative terminal (cathode) of the generator and the workpiece is connected to positive terminal (anode).
§As the electric field is established between the tool and the job, the free electrons on the tool are subjected to electrostatic forces.
§If the bonding energy of the electrons is less, electrons would be emitted from the tool.
§Such emission of electrons are called or termed as ‘cold emission’.
§The “cold emitted” electrons are then accelerated towards the job through the dielectric medium.
§As they gain velocity and energy, and start moving towards the job, there would be collisions between the electrons and dielectric molecules.
§Such collision may result in ionization of the dielectric molecule.
§Ionization depends on the ionization energy of the dielectric molecule and the energy of the electron.
§As the electrons get accelerated, more positive ions and electrons would get generated due to collisions.
§This cyclic process would increase the concentration of electrons and ions in the dielectric medium between the tool and the job at the spark gap.
§The concentration would be so high that the matter existing in that channel could be characterised as “plasma”.
§The electrical resistance of such plasma channel would be very less.
§Thus all of a sudden, a large number of electrons will flow from tool to job and ions from job to tool.
§This is called avalanche motion of electrons.
§Such movement of electrons and ions can be visually seen as a spark.
Thus the electrical energy is dissipated as the thermal energy of the spark
§The high speed electrons then impinge on the job and ions on the tool.
§The kinetic energy of the electrons and ions on impact with the surface of the job and tool respectively would be converted into thermal energy or heat flux.
§Such intense localized heat flux leads to extreme instantaneous confined rise in temperature which would be in excess of 10,000oC.
§Such localized extreme rise in temperature leads to material removal.
§Material removal occurs due to instant vaporization of the material as well as due to melting.
§The molten metal is not removed completely but only partially.
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§Thus to summarise, the material removal in EDM mainly occurs due to formation of shock waves as the plasma channel collapse owing to discontinuation of applied potential difference
§Generally the workpiece is made positive and the tool negative.
§Hence, the electrons strike the job leading to crater formation due to high temperature and melting and material removal.
§Similarly, the positive ions impinge on the tool leading to tool wear.
§In EDM, the generator is used to apply voltage pulses between the tool and job.
§A constant voltage is not applied. Only sparking is desired rather than arcing.
§Arcing leads to localized material removal at a particular point whereas sparks get distributed all over the tool surface leading to uniform material removal.
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EDM – Electrode Material
§Electrode material should be such that it would not undergo much tool wear when it is impinged by positive ions.
§Thus the localised temperature rise has to be less by properly choosing its properties or even when temperature increases, there would be less melting.
§Further, the tool should be easily workable as intricate shaped geometric features are machined in EDM.
§Thus the basic characteristics of electrode materials are:
§High electrical conductivity – electrons are cold emitted more easily and there is less bulk electrical heating
§High thermal conductivity – for the same heat load, the local temperature rise would be less due to faster heat conducted to the bulk of the tool and thus less tool wear.
§Higher density – for less tool wear and thus less dimensional loss or inaccuracy of tool
§High melting point – high melting point leads to less tool wear due to less tool material melting for the same heat load
§Easy manufacturability
§Cost – cheap
§The followings are the different electrode materials which are used commonly in the industry:
§Graphite
§Electrolytic oxygen free copper
§Tellurium copper – 99% Cu + 0.5% tellurium
§Brass
§Graphite (most common) - has fair wear characteristics, easily machinable.
§Small flush holes can be drilled into graphite electrodes.
§Copper has good EDM wear and better conductivity.
§It is generally used for better finishes in the range of Ra = 0.5 μm.
§Copper tungsten and silver tungsten are used for making deep slots under poor flushing conditions especially in tungsten carbides.
§It offers high machining rates as well as low electrode wear.
§Copper graphite is good for cross-sectional electrodes.
§It has better electrical conductivity than graphite while the corner wear is higher.
§Brass ensures stable sparking conditions and is normally used for specialized applications such as drilling of small holes where the high electrode wear is acceptable.
§In addition to the servo-controlled feed, the tool electrode may have an additional rotary or orbiting motion.
§Electrode rotation helps to solve the flushing difficulty encountered when machining small holes with EDM.
§In addition to the increase in cutting speed, the quality of the hole produced is superior to that obtained using a stationary electrode.
§Electrode orbiting produces cavities having the shape of the electrode.
§The size of the electrode and the radius of the orbit (2.54 mm maximum) determine the size of the cavities.
§Electrode orbiting improves flushing by creating a pumping effect of the dielectric liquid through the gap.
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EDM – Electrode Wear
§The melting point is the most important factor in determining the tool wear.
§Electrode wear ratios are expressed as end wear, side wear, corner wear, and volume wear.
§“No wear EDM” - when the electrode-to-workpiece wear ratio is 1 % or less.
§Electrode wear depends on a number of factors associated with the EDM, like voltage, current, electrode material, and polarity.
§The change in shape of the tool electrode due to the electrode wear causes defects in the workpiece shape.
§Electrode wear has even more pronounced effects when it comes to micromachining applications.
§The corner wear ratio depends on the type of electrode.
§The low melting point of aluminum is associated with the highest wear ratio.
§Graphite has shown a low tendency to wear and has the possibility of being molded or machined into complicated electrode shapes.
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EDM – Dielectric
§In EDM, material removal mainly occurs due to thermal evaporation and melting.
§As thermal processing is required to be carried out in absence of oxygen so that the process can be controlled and oxidation avoided.
§Oxidation often leads to poor surface conductivity (electrical) of the workpiece hindering further machining.
§Hence, dielectric fluid should provide an oxygen free machining environment.
§Further it should have enough strong dielectric resistance so that it does not breakdown electrically too easily.
§But at the same time, it should ionize when electrons collide with its molecule.
§Moreover, during sparking it should be thermally resistant as well.
§Generally kerosene and deionised water is used as dielectric fluid in EDM.
§Tap water cannot be used as it ionises too early and thus breakdown due to presence of salts as impurities occur.
§Dielectric medium is generally flushed around the spark zone.
§It is also applied through the tool to achieve efficient removal of molten material.
§Three important functions of a dielectric medium in EDM:
1.Insulates the gap between the tool and work, thus preventing a spark to form until the gap voltage are correct.
2.Cools the electrode, workpiece and solidifies the molten metal particles.
3.Flushes the metal particles out of the working gap to maintain ideal cutting conditions, increase metal removal rate.
§It must be filtered and circulated at constant pressure.
§The main requirements of the EDM dielectric fluids are adequate viscosity, high flash point, good oxidation stability, minimum odor, low cost, and good electrical discharge efficiency.
§For most EDM operations kerosene is used with certain additives that prevent gas bubbles and de-odoring.
§Silicon fluids and a mixture of these fluids with petroleum oils have given excellent results.
§Other dielectric fluids with a varying degree of success include aqueous solutions of ethylene glycol, water in emulsions, and distilled water.
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EDM – Flushing
§One of the important factors in a successful EDM operation is the removal of debris (chips) from the working gap.
§Flushing these particles out of the working gap is very important, to prevent them from forming bridges that cause short circuits.
§EDMs have a built-in power adaptive control system that increases the pulse spacing as soon as this happens and reduces or shuts off the power supply.
§Flushing – process of introducing clean filtered dielectric fluid into spark gap.
§If flushing is applied incorrectly, it can result in erratic cutting and poor machining conditions.
§Flushing of dielectric plays a major role in the maintenance of stable machining and the achievement of close tolerance and high surface quality.
§Inadequate flushing can result in arcing, decreased electrode life, and increased production time.
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FLUSH FLOW TYPES(4)
1.Normal flow (Majority)
§Dielectric is introduced, under pressure, through one or more passages in the tool and is forced to flow through the gap between tool and work.
§Flushing holes are generally placed in areas where the cuts are deepest.
§Normal flow is sometimes undesirable because it produces a tapered opening in the workpiece.
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2.Reverse flow
§Particularly useful in machining deep cavity dies, where the taper produced using the normal flow mode can be reduced.
§The gap is submerged in filtered dielectric, and instead of pressure being applied at the source a vacuum is used.
§With clean fluid flowing between the workpiece and the tool, there is no side sparking and, therefore, no taper is produced.
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3.Jet flushing
§In many instances, the desired machining can be achieved by using a spray or jet of fluid directed against the machining gap.
§Machining time is always longer with jet flushing than with the normal and reverse flow modes.
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4.Immersion flushing
§For many shallow cuts or perforations of thin sections, simple immersion of the discharge gap is sufficient.
§Cooling and debris removal can be enhanced during immersion cutting by providing relative motion between the tool and workpiece.
§Vibration or cycle interruption comprises periodic reciprocation of the tool relative to the workpiece to effect a pumping action of the dielectric.
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§Synchronized, pulsed flushing is also available on some machines.
§With this method, flushing occurs only during the non-machining time as the electrode is retracted slightly to enlarge the gap.
§Increased electrode life has been reported with this system.
§Innovative techniques such as ultrasonic vibrations coupled with mechanical pulse EDM, jet flushing with sweeping nozzles, and electrode pulsing are investigated by Masuzawa (1990).
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§For proper flushing conditions, Metals Handbook (1989) recommends:
1.Flushing through the tool is more preferred than side flushing.
2.Many small flushing holes are better than a few large ones.
3.Steady dielectric flow on the entire workpiece-electrode interface is desirable.
4.Dead spots created by pressure flushing, from opposite sides of the workpiece, should be avoided.
5.A vent hole should be provided for any upwardly concave part of the tool-electrode to prevent accumulation of explosive gases.
6.A flush box is useful if there is a hole in the cavity.
Advantages
Some of the advantages of EDM include machining of:
§Complex shapes that would otherwise be difficult to produce with conventional cutting tools.
§Extremely hard material to very close tolerances.
§Very small work pieces where conventional cutting tools may damage the part from excess cutting tool pressure.
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§There is no direct contact between tool and work piece. Therefore delicate sections and weak materials can be machined without any distortion.
§A good surface finish can be obtained.
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Disadvantages
Some of the disadvantages of EDM include:
§The slow rate of material removal.
§For economic production, the surface finish specified should not be too fine.
§The additional time and cost used for creating electrodes for ram/sinker EDM.
§Reproducing sharp corners on the workpiece is difficult due to electrode wear.
§Specific power consumption is very high.
§Power consumption is high.
§"Overcut" is formed.
§Excessive tool wear occurs during machining.
§Electrically non-conductive materials can be machined only with specific set-up of the process
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