UNIT IV THERMAL METAL REMOVAL PROCESSES I4.1 Electric Discharge Machining (EDM)It is also known as spark erosion machining or spark machining. Material of workpiece removed due to erosion caused by electric spark. Working principle is described below.

Fig.1 Components of EDM

Fig.2 Line Diagram Indicating Working Principle and Process Details of EDM89

4.1.1 Working Principle of Electric Discharge Machining Electric discharge machining process is carried out in presence of dielectric fluid which creates path for discharge. When potential difference is created across the two surfaces of die electric fluid, it gets ionized. An electric spark/discharge is generated across the two terminals. The potential difference is developed by a pulsating direct current power supply connected across the two terminals. One of the terminal is positive terminal given to workpiece and tool is made negative terminal. Two third of the total heat generated is generated at positive terminal so workpiece is generally given positive polarity. The discharge develops at the location where two terminals are very close. So tool helps in focusing the discharge or intensity of generated heat at the point of metal removal. Applications of focused heat raise the temperature of workpiece locally at a point, this way two metal is melted and evaporated. 4.1.2 Electric Discharge Machining Process Details The working principle and process of EDM is explained with the help of line diagram in Figure 1&2 .The process details and components are explained below serially. Base and Container A container of non-conducting, transparent material is used for carrying out EDM. The container is filled with dielectric solution. A base to keep workpiece is installed at the bottom of container. The base is made of conducting material and given positive polarity. Tool Tool is given negative polarity. It is made of electrically conducting material line brass, copper or tungeten. The tool material selected should be easy to machine, high wear resistant. Tool is made slightly under size for inside machining and over sized for cut side machining. Tool is designed and manufactured according to the geometry to be machined. Dielectric Solution Dielectric solution is a liquid which should be electrically conductive. This solution provides two main functions, firstly it drive away the chips and prevents their sticking to workpiece and tool. It enhance the intensity of discharge after getting ionized and so accelerates metal removal rate. Power Supply A DC power supply is used, 50 V to 450 V is applied. Due to ionization of dielectric solution an electrical breakdown occurs. The electric discharge so caused directly impinges on the surface of

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workpiece. It takes only a few micro seconds to complete the cycle and remove the material. The circuit cam be adjusted for auto off after pre-decided time interval.

Tool Feed Mechanism In case of EDM, feeding the tool means controlling gap between workpiece and the tool. This gap is maintained and controlled with the help of servo mechanism. To maintain a constant gap throughout the operation tool is moved towards the machining zone very slowly. The movement speed is towards the machining zone very slowly. The movement speed is maintained by the help of gear and rack and pinion arrangement. The servo system senses the change in gap due to metal removal and immediately corrects it by moving the tool accordingly. The spark gap normally varies from 0.005 mm to 0.50 mm. Workpiece and Machined Geometry The important point for workpiece is that any material which is electrical conductor can be machined through this process, whatever be the hardness of the same. The geometry which is to be machined into the workpiece decides the shape and size of the tool.

4.1.3 Process Parameters The process parameters in EDM are mainly related to the waveform characteristics. Fig. 3 shows a general waveform used in EDM.

Fig.3 Wave form Characteristics

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The waveform is characterized by the The open circuit voltage - Vo The working voltage - Vw The maximum current - Io The pulse on time the duration for which the voltage pulse is applied - ton The pulse off time - toff The gap between the workpiece and the tool spark gap - The polarity straight polarity tool (-ve) The dielectric medium External flushing through the spark gap. 4.1.4 Characteristics of EDM (a) The process can be used to machine any work material if it is electrically conductive (b) Material removal depends on mainly thermal properties of the work material rather than its strength, hardness etc (c) In EDM there is a physical tool and geometry of the tool is the positive impression of the hole or geometric feature machined (d) The tool has to be electrically conductive as well. The tool wear once again depends on the thermal properties of the tool material (e) Though the local temperature rise is rather high, still due to very small pulse on time, there is not enough time for the heat to diffuse and thus almost no increase in bulk temperature takes place. Thus the heat affected zone is limited to 2 4 m of the spark crater (f) However rapid heating and cooling and local high temperature leads to surface hardening which may be desirable in some applications (g) Though there is a possibility of taper cut and overcut in EDM, they can be controlled and compensated 4.1.5 Application of Electric Discharge Machining This process is highly economical for machining of very hard material as tool wear is independent of hardness of workpiece material. It is very useful in tool manufacturing. It is also used for broach making, making holes with straight or curved axes, and for making complicated cavities which cannot be produced by conventional machining operations. EDM is widely used for die making as complex cavities are to be made in the die making. However, it is capable to do all operations that can be done by conventional machining.92

4.1.6 Advantages of EDM (a) This process is very much economical for machining very hard material. (b) Maintains high degree of dimensional accuracy so it is recommended for tool and die making. (c) Complicated geometries can be produced which are very difficult otherwise. (d) Highly delicate sections and weak materials can also be processed without nay risk of their distortion, because in this process tool never applies direct pressure on the workpiece. (e) Fine holes can be drilled easily and accurately. (f) Appreciably high value of MRRR can be achieved as compared to other non-conventional machining processes. 4.1.7 Disadvantages and Limitations of EDM Process There are some limitations of EDM process as listed below: (a) This process cannot be applied on very large sized workpieces as size of workpiece is constrained by the size of set up. (b) Electrically non-conducting materials cannot be processed by EDM. (c) Due to the application of very high temperature at the machining zone, there are chances of distortion of workpiece in case of this sections. (d) EDM process is not capable to produce sharp corners. (e) MRR achieved in EDM process is considerably lower than the MRR in case of conventional machining process so it cannot be taken as an alternative to conventional machining processes at all.

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4.2 Electrodischarge GrindingElectrodischarge grinding (EDG) removes conductive materials by rapid spark discharges between a rotating tool and workpiece that are separated by a flowing dielectric fluid (Fig.4).

Fig.4 EDG schematic The spark gap is normally held at 0.013 to 0.075 mm by the servomechanism that controls the motion of the workpiece. The dc power source has capabilities ranging from 30 to 100 A, 2 to 500 kHz, and 30 to 400 V. The conductive wheel, usually made of graphite, rotates at 30 to 180 m/min in a dielectric bath of filtered hydrocarbon oil. The workpiece is usually connected to the positive terminal of the dc power supply. As can be seen from Fig.4, the workpiece is machined using a stream of electric sparks. Each spark discharge melts or vaporizes a small amount of metal from the workpiece surface. Higher machining currents produce faster rates of machining, rougher finishes, and a deeper heat-affected zone (HAZ) in the workpiece. Less current is used for the production of smoother and less damaged surfaces. Additionally, higher pulse frequencies make smoother surfaces. Wheel wear ranges from 100:1 to 0.1:1 with an average of 3:1 depending upon the current density, workpiece material, wheel material, dielectric, and sharpness of corner details. Material removal rates range from 0.16 to 2.54 cm3/min. Surface finishes in the range of 1.6 to 3.2 micro m Ra are possible.

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Fig.5 Removal rate and surface roughness in EDG

Fig.6 Elements of EDG95

Fig.5 shows the relationship between removal rate andsurface roughness in EDG. The corner radius depends on the overcut and ranges from 0.013 to 0.130 mm. Greater voltages permit larger gaps, which makes the process suitable for plunge grinding where ease of dielectric flushing is ensured. Tolerances of 0.005 mm are normal with 0.001 mm possible. The surface finish improves with an increase in pulse frequency and is typically 0.4 to 0.8 micro m Ra. These layers must be removed or modified in case of highly stressed applications. Fig.6 shows the main elements of EDG. Abrasive electrodischarge grinding (AEDG) employs the interactive effect of EDE and MA in order to enhance the machining productivity. In the AEDG process the metallic or graphite electrode used in electrodischarge grinding is replaced by a metallic bond grinding wheel. Therefore, electro erosion in addition to the MA action occurs as shown in Figs. 7 and 8. An increase in performance measures of the machining process becomes evident when machining super hard materials (plates with synthetic polycrystalline diamond), engineering ceramics, sintered carbides, and metallic composites.

Fig.7 AEDG machining system components Apart from the above-mentioned effects, the electric discharge causes a considerable decrease in grinding forces, lowers the grinding wheel wear, and provides an effective method for dressing the grinding wheel during the machining process. The relative material removal rate for the EDG and AEDG processes was compared to the material removal rate of the electrodischarge process (under the same conditions). Accordingly, the increase in productivity of the EDG process is attributed to improvements in hydrodynamic conditions of dielectric flow. This improvement results from the

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rotation of the wheel electrode and, partly, by its radial oscillations. Further improvement in EDG performance characteristics can be achieved with the use of vibrating electrodes.

Fig.8 Material removal mechanisms in AEDG Introducing mechanical effects into the AEDG process leads to a further increase in the metal removal rate by about 5 times that of the EDM process and about twice that of the EDG process. As the number of wheel revolutions increases, the effect of abrasive action is also increased. This may be evidence of better utilization of electrical discharge energy.

4.2.1 Applications

EDG and AEDG can be used on 1. Steel and carbide at the same time without wheel loading 2. Thin sections on which abrasive wheel pressures might cause distortion 3. Brittle materials or fragile parts on which abrasive materials might cause fracturing 4. Form tools and tungsten carbide throw away bits for which diamond wheel costs would be excessive

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4.3 Electric Discharge Wire Cutting ProcessWire Electric Discharge Machining (Wire EDM) is a special form of EDM that uses a small diameter wire as the electrode to cut a narrow kerf in the work. Wire EDM is illustrated in the figure:

Fig.9 The setup of Wire Electric Discharge Machining (WEDM) process

The workpiece is fed continuously and slowly past the wire in order to achieve the desired cutting path. Numerical control is used to control the work-part motions during cutting. As it cuts, the wire is continuously advanced between a supply spool and a take-up spool to present a fresh electrode of constant diameter to the work. This helps to maintain a constant kerf width during cutting. As in EDM, wire EDM must be carried out in the presence of a dielectric. This is applied by nozzles directed at the tool-work interface as in the figure, or the workpart is submerged in a dielectric bath.

Wire diameters range from 0.08 to 0.30 mm, depending on required kerf width. Materials used for the wire include brass, copper, tungsten, and molybdenum. Dielectric fluids include deionized water or oil. As in EDM, an overcut in the range from 0.02 to 0.05 mm exists in wire EDM that makes the kerf larger than the wire diameter. This process is well suited to production of dies for sheet metalworking, cams, etc. Since the kerf is so narrow, it is often possible to fabricate punch and die in a single cut, as illustrated in the figure:

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Fig.10 Punch and die fabricated in a single cut by WEDM

4.4 Power generator Circuits of EDM Fig.11 depicted general nature of voltage pulses used in electro-discharge machining. Different power generators are used in EDM and some are listed below: Resistance-capacitance type (RC type) Relaxation generator Rotary impulse type generator Electronic pulse generator Hybrid EDM generator

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Fig. 11 Basic circuits for different types of EDM generators 4.5 Mechanism of Material Removal In EDM, the removal of material is based upon the electrodischarge erosion (EDE) effect of electric sparks occurring between two electrodes that are separated by a dielectric liquid as shown in Fig.12.

Fig.12 EDM components100

Metal removaltakes place as a result of the generation of extremely high temperatures generated by the high-intensity discharges that melt and evaporate the two electrodes.

Fig.13 Typical EDM pulse current train for controlled pulse generator A series of voltage pulses (Fig.13) of magnitude about 20 to 120 V and frequency on the order of 5 kHz is applied between the two electrodes, which are separated by a small gap, typically 0.01 to 0.5 mm. When using RC generators, the voltage pulses, shown in Fig.14, are responsible for material removal.

Fig.14 Variation of voltage with time using an RC generator 4.6 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 tailoring or 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:101

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 the same heat load and same tool wear by weight there would be less volume removal or tool wear and thus less dimensional loss or inaccuracy 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 4.7 Dielectric Fluid In EDM, as has been discussed earlier, 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 ionise 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. 4.8 Accuracy and Surface Integrity The spark-machined surface consists of a multitude of overlapping crates that are formed by the action of microsecond-duration spark discharges. These craters depend on the physical and the mechanical properties of the material and the composition of the machining medium as well as on the discharge energy and duration. The integral effect of many thousands of discharges per second leads to the formation of the corresponding workpiece profile with a specified accuracy and surface finish. The102

depth of the resulting craters usually represents the peak to valley (maximum) surface roughness Rt. The maximum depth of the damaged layer can be taken as 2.5 times the average surface roughness Ra. According to Delpreti (1977) and Motoki and Lee (1968) the maximum peak to valley height, Rt, was considered to be 10 times Ra. The average roughness can be expressed in terms of pulse current ip and pulse duration tp (s) by

Jeswani added also that as the pulse energy is decreased, the surface finish improves and consequently, the depth at which all formed craters disappear from the machined surface (free polishing depth) is reduced. This depth was found to lie between 3 to 6 times Hrms and is important when polishing dies and molds and when the residual stresses are to be removed from the machined surfaces. Masuzawa and Saki (1978) adopted a quick electrochemical finishing technique, using a mate electrode. Accordingly, a reduction of surface roughness from 22 m Rmax to 8 m has been reported together with the removal of the heat-affected layer. In contrast, the matte appearance of the machined surfaces has been found satisfactory in some applications of electro discharge texturing (EDT).

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4.9 Characteristics of Spark Eroded Surface According to McGeough, the application of voltage pulses, causes electrical breakdown to the dielectric in a channel of radius 10 m. The breakdown arises from the acceleration toward the anode of both electrons emitted from the cathode by the applied field and the stray electrons present in the gap. These electrons collide with neutral atoms of the dielectric, thereby creating positive ions and further electrons, which in turn are accelerated respectively toward the cathode and anode.

Fig.15 EDM spark description When the electrons and the positive ions reach the anode and cathode, they give up their kinetic energy in the form of heat. Temperatures of about 8000 to 12,000C and heat fluxes up to 1017 W/m2 are attained. With a very short duration spark of typically between 0.1 to 2000 s the temperature of the electrodes can be raised locally to more than theirnormal boiling points. Owing to the evaporation of the dielectric, the pressure on the plasma channel rises rapidly to values as high as 200 atmospheres. Such great pressures prevent the evaporation of the superheated metal. At the end of the pulse, the pressure drops suddenly and the superheated metal evaporates explosively. Metal is thus removed from the electrodes as shown in Fig.15.

4.10

Machine Tool Selection of EDM

Material - Metals with a high melting point and good electrical conductivity are usually chosen as tool materials for EDM. Graphite is the most common electrode material since it has fair wear characteristics and is easily machinable and 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 0.5m Ra. Copper tungsten and silver tungsten are used for making deep slots under poor flushing104

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.

Movements - 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.

Polarity - Electrode polarity depends on both the workpiece and electrode materials. Table 1 shows the possible electrode polarity for different workpiece and tool combinations.

TABLE 1 Electrode Polarities for Different Workpiece Materials

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4.11 Wire Cut Electric Discharge Machining (WCEDM)This is a special type of electric discharge machining that uses a small diameter wire as a cutting tool on the work. Working a principle of wire cut electric discharge machining is same as that of electric discharge machining. Process Principle of WCEDM Process details of WCEDM are almost similar to EDM with slight difference. The details of the process are indicated in the line diagram shown in Fig.16. Its major difference of process details with EDM process details are described below.

Tool Details The tool used in WCEDM process is a small diameter wire as the electrode to cut narrow kerf in the workpiece. During the process of cutting the wire is continuously advanced between a supply spoil and wire collector. This continuous feeding of wire makes the machined geometry insensitive to distortion of tool due to its erosion. Material of wire can be brass, copper, tungsten or any other suitable material to make EDM tool. Normally, wire diameter ranges from 0.076 to 0.30 mm depending upon the width of kerf.

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Machine portion Fig.16 Line Diagram for Process Details of Working of Wire Cut Electric Discharge Machining

Tool Feed Mechanism Two type of movements are generally given to the total (wire). One is continuous feed from wire supply spoal to wire collector. Other is movement of the whole wire feeding system, and wire along the kerf to be cut into the workpiece. Both movements are accomplished with ultra accuracy and predetermined speed with the help of numerical control mechanism. Dielectric Fluid and Spray Mechanism Like EDM process dielectric fluid is continuously sprayed to the machining zone. This fluid is applied by nozzles directed at the tool work interface or workpiece is submerged in the dielectric fluid container. Rest of the process details in case of WCEDM process is same as that in case of EDM process. 4.11.1 Application of WCEDM WCEDM is similar to hand saw operation in applications with good precision. It is used to make narrow kerf with sharp corners. It does not impose any force to workpiece so used for very delicated and thin workpieces. It is considered ideal for making components for stamping dies. It is also used to make intricate shapes in punch, dies and other tools.

Advantages of WCEDM Advantages are listed below: (a) Accuracy and precision of dimensions are of very good quality. (b) No force is experienced by the workpiece.

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(c) Hardness and toughness of workpiece do not create problems in machining operation.

Disadvantages and Limitations of WCEDM The major disadvantages of this process are that only electrically conducting materials can machined. This process is costly so recommended for use specifically at limited operations.

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