ELECTRO-DISCHARGE MACHINING (EDM)

ELECTRO-DISCHARGE MACHINING (EDM)

Electro-discharge machining, commonly known as EDM or spark erosion involves a controlled erosion of electrically conductive materials by the initiation of rapid and repetitive spark discharge between the elec­trode tool (usually cathode) and work piece (anode) separated by a small gap of about 0.01 to 0.5mm known as spark gap.

EDM electrodes may be made from metal or carbon (graphite) and shaped by moulding or machining to the desired geometry (shape of the work piece).

DIELECTRIC FLUID:-

The spark gap is either flooded or immersed under the dielectric fluid. This dielectric fluid may be a light lubricat­ing oil or kerosene and this fluid should be a poor or non­conductor of electricity

The spark discharge is produced by the controlled pulsing of the direct current between the work piece and the tool. The fluid in the spark gap is ionized under the pulsed application of the direct current, thus enabling a spark discharge to pass between the tool and the work piece. Each spark produces enough heat to melt and vapourise a tiny volume of work piece material, leaving a small crater on its surface.

                          Basic scheme of electrical discharge machining

The workpiece and the tool are electrically connected to a dc electric power. The workpiece is connected to the positive terminal of the electric source, so that it becomes the anode. The tool is the cathode. A gap. known as 'spark gap ' in the ranges of 0.005 to 0.05 mm is maintained between the workpiece and the tool, and suitable dielectric slurry, which is non conductor of electricity is forced through this gap at a pressure of 2 kgf/cm²or less. 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. In fact, a small ionized fluid column is formed owing to formation. of an avalanche , of electrons in the spark gap where the process of ionization collision takes place. When more electrons collect in the gap the resistance drops causing electric spark to jump between the workpiece 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 workpiece 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. However, the temperature of spot hit by the electrons is of the order of 10,000°. 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 on the workpiece. A part of the metal may vaporize and fill up the gap. The metal is thus removed in this way from the workpiece.

The electric and magnetic fields on the heated metal cause a compressive force to act on the cathode tool so that metal removal from the tool is at a slower rate than that from the workpiece.  Hence, the work piece is connected to the positive terminal and tool to the negative terminal. The current density in the discharge of channel is of the order of 10000 A/cm²; the power density, of the order of 500M/cm².

Electro hydraulic servo control It is usually preferred. The servo gets its input signal from the difference between a selected reference voltage and the actual voltage across the gap. The signal is amplified and the tool, as it wears a little, is advanced by hydraulic control; A short circuit across the gap causes the servo to -reverse the motion of-the tool until the correct gap is established.

Spark generator: The spark generating circuit may be one of the following types :

a.   Relaxation                 b. Pulse-generator

The spark generator supplies current to a condenser, the discharge from which produces the spark.   The work piece alternatively becomes a positive electrode (anode) or negative electrode (cathode) respectively. On each reversal of polarity the tool is eroded more than the work piece. Hence, the tool wear is greater with this type of arrangement.

The introduction of pulse generators has overcome the drawbacks of relaxation generators. Pulse generators are available, fitted with transistorized pulse-generator circuits in which reverse pulses are eliminated. These generators consist of electronic switching units which let the current pass periodically. Modem pulse generators possess the means of accurate control over discharge duration, pause time and the current. These factors determine the overcut and hence the accuracy and surface finish. The tool wear is also greatly reduced.

Overcut : The shape of the area of the cavity produced in the workpiece should theoretically be the same as that of the tool. This, however, is not exactly true because of the overcut.

Overcut is the distance the spark will penetrate the work piece from- the tool and remove metal from the workpiece. Theoretically, it is slightly larger than the gap between the end of the tool and the workpiece. The overcut is generally 0.025 to 0.2 mm, on all surfaces. Over-cut causes internal comers on the workpiece to have fillets With radii equal to the overcut. Another effect of overcut is to cause the radius of the cavity in the workpiece slightly larger than the corresponding radius of the tool nose and also to cause the radius of projection on the workpiece to be slightly lesser than the radius of the cavity of the tool.This overcut is a function of the voltage of the spark. The overcut increases with higher current and decreases with higher frequency.

The electrode (tool): The shape of the tool will be basically the same as that of the product desired except that an allowance is made for side clearance and overcut. For broaching small holes, solid rods may be used but for larger ones, hollow tools are preferred. Dielectric may then be pumped through hollow tools. If ah object" is having a geometric shape or is having some symmetry about some axis, a tool equal to only a part of a object will be sufficient for complete machining of the object. Such segmented tools are specially useful for machining complex shapes that do not require close accuracy, ft may sometimes be convenient to use a series of simpler tools rather than a complex single tool to produce a particular cavity,

The-material used for the tool influences the tool wear and the side clearance and hence, in turn, it has considerable influence on the rate of metal removal, finish obtained, and the production rate. Copper, yellow brass, zinc, graphite and some other materials are used for tools. Low wearing tools include silver-tungsten, , copper tungsten, and metallized graphite.    For commercial applications, copper is best suited for fine machining, aluminium is used for die-sinking, and cast Iron for rough machining. One of the advantages of EDM is due to the fact that a tool made of a material softer than the workpiece material and which is a good conductor of electricity can be used to machine a material of any hardness.

The wear of the tool in the EDM process due to electron bombardment is inevitable. The tool wear rates determine the machining accuracy, tool movement, and the-too! consumption. The tool wear is a function of the rate of metal removal, material of the workpiece, current setting, machining area, gap between the tool, and the workpiece and the polarity of the tool. It has been found that the higher the too! material melting point, the less the tool wear. Wear is best defined as :

Wear ratio = Volume of work material removed/ Volume of electrode consumed

This is often simplified to :

Wear ratio = Depth of cut / Decrease in usable length of electrode

The wear ratio of carbon electrodes is upto 100 :1. Other wear ratios (for cutting steel) are copper 2 : 1; brass 1 : 1; copper tungsten 8:1. Thus, a piece of copper cutting 25 mm deep into steel will wear 12.5 mm. These ratios are approximate and will vary considerably.

Dielectric fluids : the essential requirements of a dielectric fluid to be used in EDM process are that they should :

1.     Remain electrically nonconductive until the required breakdown voltage is reached, i.e., they should have high dielectric strength.

2.    Breakdown electrically in the shortest possible time once the breakdown voltage has been reached.

3.  Rapidly quench the spark or deionize the spark gap after the discharges have occured.

4.    Provide an effective cooling medium..

5.    Be capable of carrying away the swarf particles, in suspension, away from the working gap.

6.     Have a good degree of fluidity.

7.     Be cheap and easily available.

Light hydrocarbon oils seem to satisfy these requirements best of all. The common dielectrics used are kerosene, paraffin, transformer oil or their mixture and certain aqueous solutions. Water being an electrical conductor, gives a metal removal rate of only about 40 percent of that obtained when using paraffin as a dielectric.

The dielectric should be filtered before reuse so that chip contamination of the fluid will not affect machining accuracy. This is usually accomplished by filtration.

Metal removal rate (MRR) : The metal removal rate is generally described as the volume of metal removed per unit time. The machining rate during roughing of steel with a graphite electrode and 50A generator is about 400 mm/min and with a 400A generator it is about 4800 mm/min. For precision machining with low amperage and high frequency the material removal rate is as low as 2 mm^/min. It is, therefore, evident that the MRR- is proportional to the-working current value.

The material being cut will affect the MRR. Experiments indicate that the MRR varies inversely as the melting point of the metal. The approximate value is :

MRR = 2.4/(melting point, deg.C)¹-²⁵ Thus EDM will cut Aluminium much faster than steel.

Accuracy : Tolerance value of + or - 0.05 mm could be easily achieved by EDM in normal production. However, by close control of the several variables a tolerance of ±0.003 mm could be achieved, A typical taper value is about 0.005 to 0.05 mm per 100mm depth. The taper effect decreases substantially to zero after about 75 mm penetration. An overcut of 5 to 100 micron is produced, depending upon finishing or roughing.  The best surface finish that can be economically achieved on steel is 0.4 micron. In 'no wear" machining, using graphite electrode a surface finish within 3.2micron can be achieved.

Applications of EDM :  The electrical discharge machining is used for the manufacture of tools having complicated profiles and a number of other components. The decision to use EDM process for either of these broad applications is usuallybased on one or more of the basic characteristics inherent in the process. The EDM provides economic advantage for making stamping tools, wire drawing and extrusion dies, header dies, forging dies, intricate mould cavities, etc. It has been extremely used for machining of exotic materials used in acrospace industries, refractory metals, hard carbides, and hardenable steels.Delicate workpiece like copper parts for fitting into the vacuum tubes can be produced by this method. The workpiece in this case is fragile to withstand the cutting tool load during conventional machining.

Advantages:-

Extremely high popularity of the EDM  process is due to the following advantages :

1. The process can be applied to all the electrically conducting metals and alloys irrespective of their melting points, hardness, toughness or brittleness.
2. Any complicated shape that can be made on the tool can be reproduced on the workpiece.
3. Highly complicated shapes can be made by fabricating the tool with split sectioned shapes, by welding, brazing or by applying quick setting conductive epoxy adhesives.
4. Time for machining is less than conventional machining process.
5. EDM can be employed for extremely hardened workpiece- Hence, the distortion of the workpiece arising out of the heat treatment process can be eliminated.
6. No mechanical stress is present in the process. It is due to the fact that the physical contact between the tool and the workpiece is eliminated. Thus,, fragile and slender workpieces can be machined without distortion.
7. Cratering type of surface finish automatically creates accommodation for lubricants causing the  die life to improve.
8. Hard and corrosion resistant surfaces, essentially needed for die making, can be developed.

Disadvantages:-

The following disadvantages of the process limit its application:

1. Profile machining of complex contours is not possible at required tolerances.
2. Machining times are too long.
3. Machining heats the workpiece considerably and hence causes change in surface and metallurgical properties.
4. Excessive tool wear.
5. High specific power consumption