In this article we will discuss about the electric discharge machining (EDM):- 1. Introduction to Electric Discharge Machining (EDM) 2. Mechanics of EDM 3. EDM Circuits and Working Principles (With Diagram) 4. Surface Finish and Machining Accuracy 5. Role of Tool Electrode and Dielectric Fluids in EDM 6. Effects of EDM on Metal Surfaces 7. Characteristics.
Contents:
- Introduction to Electric Discharge Machining (EDM)
- Mechanics of EDM
- EDM Circuits and Working Principles (With Diagram)
- Surface Finish and Machining Accuracy of EDM
- Role of Tool Electrode and Dielectric Fluids in EDM
- Effects of EDM on Metal Surfaces
- Characteristics of EDM
1. Introduction to Electric Discharge Machining (EDM):
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The use of a thermoelectric source of energy in developing the non-traditional 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 electric discharge machining, was first started in the USSR around 1943. Then onwards, research and development have brought this process to its present level.
When a discharge takes place between two points of the anode and the cathode, the intense heat generated near the zone melts and evaporates the materials in the sparking zone. For improving the effectiveness, the work piece and the tool are submerged in a dielectric fluid (hydrocarbon or mineral oils). It has been observed that if both the electrodes are made of the same material, the electrode connected to the positive terminal generally erodes at a faster rate. For this reason, the work piece is normally made the anode. A suitable gap, known as the spark gap, is maintained between the tool and the work piece 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 and the work piece surfaces are the closest and since the spot changes after each spark (because of the material removal after each spark), the sparks travel all over the surface. This results in a uniform material removal all over the surface, and finally the work face conforms to the tool surface. Thus, the tool produces the required impression in the work piece.
For maintaining the predetermined spark gap, a servo control unit is generally used. The gap is sensed through the average voltage across it and this voltage is compared with a preset value. The difference is used to control the servomotor. Sometimes, a stepper motor is used instead of a servomotor. Of course, for very primitive operations, a solenoid control is also possible, and with this, the machine becomes extremely inexpensive and simple to construct.
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The spark frequency is normally in the range 200-500,000 Hz, the spark gap being of the order of 0.025-0.05 mm. The peak voltage across the gap is kept in the range 30-250 volts. An mrr up to 300 mm3 / min can be obtained with this process, the specific power being of the order of 10 W / mm3 / min. The efficiency and the accuracy of performance have been found to improve when a forced circulation of the dielectric fluid is provided. The most commonly used dielectric fluid is kerosene. The tool is generally made of brass or a copper Alloy.
2. Mechanics of EDM:
Figure 6.52 shows the details of the electrode surfaces. Though the surfaces may appear smooth, asperities and irregularities are always present, as indicated (in an exaggerated manner, of course). As a result, the local gap varies, and at a given instant, it is minimum at one point (say, A). When a suitable Voltage is built up across the tool and the work piece (the cathode and the anode, respectively), an electrostatic field of sufficient strength is established, causing cold emission of electrons from the cathode at A.
These liberated electrons accelerate towards the anode. After gaining a sufficient velocity, the electrons collide with the molecules of the dielectric fluid, breaking them into electrons and positive ions. The electrons so produced also accelerate and may ultimately dislodge the other electrons from the dielectric fluid molecules. Ultimately, a narrow column of ionized dielectric fluid molecules is established at A connecting the two electrodes (causing an avalanche of electrons, since the conductivity of the ionized column is very large, which is normally seen as a spark).
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As a result of this spark, a compression shock wave is generated and a very high temperature is developed on the electrodes (10,000-12,000°C). This high temperature causes the melting and vaporization of the electrode materials, and the molten metals are evacuated by a mechanical blast, resulting in small craters on both the electrodes at A. As soon as this happens, the gap between the electrodes at A increases and the next location of the shortest gap is somewhere else (say, B).
Therefore, when the cycle is repeated, the next spark takes place at B. In this way, the sparks wander all over the electrode surface and, ultimately, the process results in a uniform gap. So, depending on the negative electrode shape, an impression is created on the other electrode.
Generally, the rate of material removal from the cathode is comparatively less than that from the anode due to the following reasons:
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(i) The momentum with which the stream of electrons strikes the anode is much more than that due to the stream of the positive ions impinging on the cathode though the mass of an individual electron is less than that of the positive ions.
(ii) The pyrolysis of the dielectric fluid (normally a hydrocarbon) creates a thin film of carbon on the cathode.
(iii) A compressive force is developed on the cathode surface. Therefore, normally, the tool is connected to the negative terminal of the dc source.
If the tool is stationary relative to the work piece, the gap increases as the material removal progresses, necessitating an increased voltage to initiate the sparks. To avoid this problem, the tool is fed with the help of a servo drive which senses the magnitude of the average gap and keeps it constant.
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In what follows, we shall attempt a theoretical determination of the material removal rate during electric discharge machining. In so doing, though the quantitative results will not be obtained, many important features will become evident. For now, it would be sufficient to understand the effect of only one spark.
The quantity of material removal due to a single discharge can be determined by considering the diameter of the crater and the depth to which the melting temperature is reached.
To do this, we shall make the following assumptions:
(i) The spark is a uniform circular heat source on the electrode surface and the diameter (=2a) of this circular source remains constant.
(ii) The electrode surface is a semi-infinite region.
(iii) Except for the portion of the heat source, the electrode surface is insulated.
(iv) The rate of heat input remains constant throughout the discharge duration.
(v) The properties of the electrode material do not change with the temperature.
(vi) The vaporization of the electrode material is neglected.
Figure 6.53 shows the details of the idealized heat source. In our analysis, H- amount of heat input (cal), θ = temperature (°C), t = time (sec), k=thermal conductivity (cal/cm-sec-°C), α = thermal diffusivity (cm2/sec), td = discharge duration (sec), and θm = melting temperature (°C).
Because of circular symmetry, the temperature at any point depends on r and z. The equation for heat conduction is –
Since, intuitively, it can be seen that the depth to which the melting temperature is reached is maximum at the centre, our interest lies in the solution at r = 0. The temperature at a point on the axis at the end of the discharge (assuming that the maximum temperature is reached at t = td as the heat input stops at this instant) is given by –
So, it is clear that Z gives an indication of the volume of material removed by each spark. Figure 6.54a shows the theoretical values of Z for a given spark energy and a constant spark diameter for Cu, Al, and Zn as the electrode materials. Figure 6.54b depicts the actual nature of variation of the crater volume with td for different spark energies. The trends are quite similar.
One important feature which becomes evident from these results is that the material removal is very low for a small discharge time and increases with td. Then, reaching a peak value, it suddenly drops to zero. Also, it has been established that the material removed per discharge strongly depends on the melting point of the material.
The effect of cavitation in the mechanical removal process is also important. The mrr during a single spark plotted against time is as shown in Fig. 6.55. Clearly, the mrr is maximum when the pressure is below atmospheric, showing the importance of cavitation.
For arriving at a rough estimate, empirical relationships have been developed for the material removal rate during EDM. Since the size of the crater depends on the spark energy (assuming all other conditions remain unchanged), the depth and diameter of the crater are given by –
In this relation, we have assumed average sparking condition.
The mrr also depends strongly on the circulation of the dielectric fluid. Without a forced circulation, the wear particles repeatedly melt and reunite with the electrode. Figure 6.56 shows the nature of the mrr characteristics without and with a forced circulation of the dielectric.
After the discharge is completed, the dielectric medium around the last spark should be allowed to deionize. For this, the voltage across the gap must be kept below the discharge voltage until deionization is complete; otherwise the current again starts flowing through the gap at the location of the preceding discharge. The time required for a complete deionization depends on the energy released by the preceding discharge. A larger energy release results in a longer deionization time.
3. EDM Circuits and Working Principles (With Diagram):
Several basically different electric circuits are available to provide the pulsating dc across the work-tool gap. Though the operational characteristics are different, in almost all such circuits a capacitor is used for storing the electric charge before the discharge takes place across the gap. The suitability of a circuit depends on the machining conditions and requirements.
The commonly-used principles for supplying the pulsating dc can be classified into the following three groups:
(i) Resistance-capacitance relaxation circuit with a constant dc source.
(ii) Rotary impulse generator.
(iii) Controlled pulse circuit.
(i) Resistance-Capacitance Relaxation Circuit:
The resistance-capacitance relaxation circuit was used when the electric discharge machines were first developed. Figure 6.57a shows a simple RC circuit. As is clear from this figure, the capacitor C (which can be varied) is charged through a variable resistance R by the dc source of voltage V0.
The voltage across the gap (which is almost the same as that across the capacitor) V varies with time according to the relation where t denotes the time starting at the instant V0 is applied.
So, V will approach V0 asymptotically, as shown in Fig. 6.57b, if allowed to do so. If the tool-work gap and the dielectric fluid are such that a spark can take place when the voltage across the gap reaches a value Vd (commonly known as the discharge voltage), a spark will occur, discharging the capacitor completely whenever the voltage across the tool-work gap (V) reaches Vd.
The discharge time is much smaller (about 10%) than the charging time and the frequency of sparking (v) is approximately given by the following equation (since the time required for deionization is also very small under normal circumstances) –
Thus, for maximum power delivery, the discharge voltage should be 72% of the supply voltage V0.
If we assume the material removed per spark to be proportional to the energy released per spark, then the mrr can be expressed as –
(ii) Rotary Impulse Generator:
The relaxation circuit for spark generation, though simple, has certain disadvantages. Of these, an important disadvantage is that the mrr is not high. For increasing the removal rate, an impulse generator is used for spark generation. Figure 6.59 shows the schematic diagram of such a system. The capacitor is charged through the diode during the first half cycle. During the following half cycle, the sum of the voltages generated by the generator and the charged capacitor is applied to the work-tool gap.
The operating frequency is the frequency of the sine wave generation which depends on the motor speed. Though the mrr is higher, such a system does not produce a good surface finish.
(iii) Controlled Pulse Circuits:
In the two systems we have discussed, there is no provision for an automatic prevention of the current flow when a short circuit is developed. To achieve such an automatic control, a vacuum tube (or a transistor) is used as the switching device. This system is known as a controlled pulse circuit. Figure 6.60 schematically shows such a system. During sparking, the current which flows through the gap comes from the capacitor.
When the current flows through the gap, the Valve Tube (VT) is biased to cut off and behaves like an infinite resistance. The bias control is done through an Electronic Control (EC). As soon as the current in the gap ceases, the conductivity of the tube increases, allowing the flow of current to charge the capacitor for the next cycle.
The circuit can be simplified and the operating stability improved if the flow of current is allowed cyclically with an imposed frequency. This can be done by controlling the bias with the help of an oscillator. In this case, the capacitor is not needed. Figure 6.61 shows such a circuit, using a transistor.
4. Surface Finish and Machining Accuracy of EDM:
Since the material removal in EDM is achieved through the formation of craters due to the sparks, it is obvious that large crater sizes (especially the depth) result in a rough surface. So, the crater size, which depends mainly on the energy / spark, controls the quality of the surface. Figure 6.62 shows how Hrms (root mean square value of the surface unevenness) depends on C and V0.
The crater depth (hc) can be approximately expressed in terms of the energy released per spark (E) as –
The dependence of surface finish on the pulse energy E and the comparison of surface finish with that obtained by the conventional processes are indicated in Fig. 6.63. A lot of effort has been spent on determining a suitable relationship between the rate of material removal and the quality of surface finish. But a very dependable relation of general applicability is yet to emerge. However, the mrr and the surface unevenness, when machining steel under normal conditions, are approximately related as –
Where Hrms is the root mean square of the surface unevenness in microns and Q is the material removal rate in mm3 / min.
The forced circulation of the dielectric has been found to generally improve the surface finish. The cross-sections of the brass electrode surface produced by EDM with and without the forced circulation (voltage 40 V, current 0.2 A, frequency 1.12 kHz) are shown in Fig. 6.64. It is clear that the forced circulation leads to a significant improvement in the surface finish.
5. Role of Tool Electrode and Dielectric Fluids in EDM:
The electrodes play an extremely important role in the EDM operation, and therefore certain aspects of the tool electrode should be kept in mind to achieve better results.
a. Tool Electrode Wear:
During the EDM operation, the electrode (i.e., the tool), as already mentioned, also gets eroded due to the sparking action. The materials having good electrode wear characteristics are the same as those that are generally difficult to machine. One of the principal materials used for the tool is graphite which goes directly to the vapour phase without melting. The wear ratio (rQ), defined by the ratio of the material removed from the work to the material removed from the tool, is found to be related to rθ (=melting point of the work / melting point of the tool) as –
b. Electrode Material:
The selection of the electrode material depends on the:
(i) Material removal rate,
(ii) Wear ratio,
(iii) Ease of shaping the electrode,
(iv) Cost.
The most commonly used electrode materials are brass, copper, graphite, A1 alloys, copper-tungsten alloys, and silver-tungsten alloys.
The methods used for making the electrodes are:
(i) Conventional machining (used for copper, brass, Cu-W alloys, Ag-W alloys, and graphite),
(ii) Casting (used for Zn base die casting alloy, Zn-Sn alloys, and Al alloys),
(iii) Metal spraying,
(iv) Press forming.
Flow holes are normally provided for the circulation of the dielectric, and these holes should be as large as possible for rough cuts to allow large flow rates at a low pressure.
c. Dielectric Fluids:
The basic requirements of an ideal dielectric fluid are:
(i) Low viscosity,
(ii) Absence of toxic vapours,
(iii) Chemical neutrality,
(iv) Absence of inflaming tendency,
(v) Low cost.
The ordinary water possesses almost all these properties, but since it causes rusting in the work and the machine, it is not used. Another reason why water is not recommended is as follows. The electrodes are constantly under some potential difference, and due to the good conductivity of water, the ECM process starts distorting the work piece. Also, power is wasted. However, in some cases, deionized water is used.
The most commonly used type of fluid is hydrocarbon (petroleum) oil. Kerosene, liquid paraffin, and silicon oils are also used as dielectric fluids.
6. Effects of EDM on Metal Surfaces:
The high temperature generated by the sparks causes the melting and vaporization of the metal and, obviously, this high temperature affects the properties of the shallow layers (2.5-150μm) of the surface machined.
The outermost layer is rapidly chilled, and it is therefore very hard. The layer directly below this is in a somewhat tempered condition. Figure 6.67 shows the variation of hardness with depth for both rough and finish EDM operations on steel. It is clear that in finish machining such a hardening is not prominent. However, the outer layer is tempered and the hardness is low.
The hardening of the surface layer during the EDM operation imparts a better wear resistance characteristic. However, the fatigue strength reduces due to the micro cracks that develop in the surface layer during chilling. Figure 6.68 shows the comparison between the fatigue strength of identical parts produced by conventional milling and EDM. The properties of the thin surface layers do not have much effect on the tensile strength. Their structure gets transformed and, due to the sparks, their chemical composition alters to some extent. These generally reduce the erosion resistance.
7. Characteristics of EDM:
