In this article we will discuss about:- 1. Purpose of Electro-Discharge Machining (EDM) 2. Machining Accuracy in EDM Process 3. Automation 4. Orbiting Control Facility 5. Advantages 6. Disadvantages 7. Applications.
Purpose of Electro-Discharge Machining (EDM):
In electrical machining processes, electrical energy is used directly to cut the material to final shape and size. Efforts are made to utilise whole of the energy by applying it at the exact spot where the operation is to be carried out.
Another advantage of these methods is that no complicated fixtures are needed for holding the job and even very thin jobs can be machined to the desired dimensions and shape. All the operations are carried out in a single set-up. The process may be applied to machine steels, super alloys, refractories etc.
When a difference of potential is applied between two conductors immersed in a dielectric fluid, the fluid will ionise if the potential difference reaches a high enough value and a spark will occur. If the potential difference is maintained, then the spark will develop into an arc. If the potential difference decreases, the fluid will de-ionise and the discharge will cease.
In electro-discharge machining (EDM) process, the control of erosion of the metal is achieved by the rapidly recurring spark discharges produced between two electrodes, one tool and the other work, and spark impinging against the surface to the workpiece which must be an electrically conducting body.
A suitable gap (0.01 to 0.5 mm) known as spark gap, is maintained between the tool and the work by a servomotor which is actuated by the difference between a reference voltage and the gap breakdown voltage, which feeds the tool downwards towards the workpiece. The metal removal rate depends on the spark gap maintained. If both electrodes are made of same material, it has been found that the greatest erosion takes place upon the positive electrode (anode).
Therefore, in order to remove maximum metal and have minimum wear on the tool, the tool is made cathode and the workpiece as anode. The two electrodes are separated by a dielectric fluid medium such as, paraffin, white spirit or transformer oil which is pumped through the tool or workpiece at a pressure of 2 kg/cm2 or less. The current may vary from 0.5 to 400 ampere at 40—300 V DC. The pulse duration can be varied from 2 to 2000 p sec.
The moment spark occurs, sufficient pressure is developed between work and tool. The repetitive sparks release their energy in the form of local heat, as a result of which, local temperature of the order of 10,000°C is reached at the spot hit by electrons, and at such a high pressure and temperature some metal is melted and eroded.
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Some of it is vaporised and under it fine material particles are carried away by dielectric medium (liquid) circulated around it, forming a character on the workpiece. In this was a true replica of the tool surface is produced on the workpiece. Fig. 10.1 shows the schematic representation of the process illustrating the various components involved in the process.
EDM machine is usually of vertical C type construction, its various parts being base, column, head, table. The head which supports the servo-controlled tool is mounted on the column and the column is fixed to the base. The table is mounted on the base and has a dielectric tank over it.
Workpieces are mounted on the table with magnetic vices, chucks or fixtures. Good circulation and flushing of workpiece by dielectric fluid are ensured. A constant level of dielectric in tank is ensured by an automatic controller and temperature not allowed to exceed a certain limit beyond which the operation is discontinued.
Spark Generator:
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A spark generator performs the important functions of supplying adequate voltage to initiate and maintain the discharge, incorporates provisions for varying current intensity and the discharge duration, and controls the recurring rhythm of the discharge. The simplest system used is the relaxation or R.C. generator which is discussed first in brief.
The circuit (D.C. relaxation circuit) used for getting rapidly recurring discharges is shown in Fig. 10.2 in which D.C. source of supply is used and it, in effect, is a resistance capacitance type of circuit. The range of voltage for the pulse generator is between 40 and 300 V and the frequency of sparks at the rate of 10,000 sparks per sec can be achieved. The system operates on the principle of self-oscillation.
The working of the circuit is explained below:
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Value of U (voltage gradient set up between the tool and the workpiece) depends upon gap between tool and work material and the dielectric media.
When voltage is applied across C, first it keeps or rising and when it is U. i.e. sufficient enough to breakdown the dielectric medium due to development of a strong electrostatic field between the electrodes, medium between tool and work is ionised and spark takes place. Millions of electrons are developed in each spark. During sparking period voltage immediately falls and it again starts rising as shown in Fig. 10.3.
Here U = 150 volts, C = 120 Micro-farads.
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Maximum current = 40 A, Sparking period = 100 micro-seconds.
The circuit shown in Fig. 10.2 is also known as relaxation circuit. In this circuit metal removal rate depends largely on high amperage and capacitance than on an increase in the sparking frequency, because the higher energy sparks cause larger over cuts and thus produce larger chips.
To maintain longer gaps, which are necessary to remove the wear products, a higher voltage is necessary. This type of circuit is simple in construction, rugged, reliable and cheap. It is best suited for large amount of metal removal where critical surface finish is not desired. It has been found that with this circuit, tool is eroded more than the workpiece.
This circuit consists of three parts:
(i) Charging circuit,
(ii) Discharging circuit,
(iii) Sparking portion.
For the charging circuit, i.e. when condenser C is getting charged from the e.m.f. E applied across it.
For machining steels by this process; MRR (mm3/min) = 27.5 W1.55 (W is power input in kW).
Main difficulty in this process is that as machining continues, metal keeps on chipping off and gap between tool and work increases and thus value of U is changed.
Ratio of U/E should be between 0.7 to 0.9 for best results.
Thus to overcome this defect, i.e. to keep U/E nearly constant, some suitable feedback circuit is employed so that the tool also moves as the metal is removed. Further in this process, whatever form is required on the work, tool also should be given same shape.
Figs. 10.4 to 10.7 show how the MRR varies with various parameters in the case of relaxation circuit in EDM process.
This method is used for cutting very hard materials which are difficult to be machined otherwise. Under this process, material comes out in the form of fine particles due to electron bombardment by erosion of material, thus it is also known as spark erosion process.
Relaxation circuit was employed in old EDM machines. Though the discharge current in a relaxation circuit reaches a high value, but it is of very short duration, preventing the full erosion effect of the high temperature attained. Other drawbacks are; Current discharge duration and energy are interdependent and can be chosen to suit requirements. Their values vary from cycle to cycle and thus the machining process is unpredictable.
The use of high frequencies is limited, since the time for charging the capacitor is high. Modern machines employ transistorised pulse generator circuit. In it reverse pulses are eliminated. It is possible to have closer control of size and frequency of the electric current which governs the spark gap and the rate of cutting, i.e. roughing or finishing. Tool wear is greatly reduced.
These use electronic switching units and produce square form of current pulses. Two types of pulse generators are in use. In one the pulse trains are generated independent of machining performance. In other case, the pulse time is adjusted by means of a feedback device which monitors the start of the spark and thereby ensures a constant energy discharge per cycle.
It is possible to control important parameters like discharge duration, pulse time and current in the modern pulse generators.
Fig. 10.9 shows a schematic representation of a constant energy pulse generator using electronic switching units. It is possible to adjust precisely the value of discharge duration time (td), current time and pause time to control the overcut and surface finish.
The accuracy and surface finish which are dependent on the overcut produced can be easily controlled by varying the frequency and current. The overcut is increased by increasing current and by decreasing frequency.
For optimum metal removal and better surface finish, high frequency and maximum possible current is used. For roughing operation, low frequency and high current are used and for finishing application, high frequency and low current settings are used.
The best use of power supply can be made by employing multi lead application so that same source feeds many electrodes, thereby increasing productivity.
Machining Accuracy in EDM Process:
(i) In this process, taper effect is produced while intending to drill a straight hole. This is due to the presence of frontal spark accompanied by side spark in the midst of suspended particles.
Usual value of taper is 0.005 to 0.05 mm/10 mm depth. After penetration of about 75 mm, taper effect is zero. It can be eliminated by using suction flushing.
(ii) Further in this process, overacts are produced in the workpieces because of the presence of side sparks. The overcut depends on the gap length and crater size. An overcut of 5—100 pm is produced depending upon the finishing or roughing operation.
Corner radii equal to the spark gap are also produced.
(iii) The surface produced by this process consists of microscopic craters and the quantity of the machined surface mainly depends on the energy per pulse. It the energy content per pulse is high then the depth of crater will increase causing a poor surface finish and vice versa [Refer Fig. 10.12].
The surface roughness is inversely proportional to the frequency of cutting which is equal to (1 / (parking time + charging time)) (sparking time being very small compared to charging time) it could be said that frequency of cutting = (1 / charging time).
The surface finish depends on the amperage, frequency and finish of the electrodes (Refer Fig. 10.14 and 10.15). Larger crater sizes (depth in particular) result in rough surface.
Fig. 10.16 shows how surface roughness in the case of relaxation circuit in EDM process varies with change in value of capacitance and voltage U0.
Depth of crater dc = K1 E0.33 mm, Where E = spark energy in joules.
Surface unevenness in microns is = 1.11 (MRR)0.384, where MRR is metal removal rate in mm3/min.
The surface finish of the order of 0.4 pm CLA is possible. However, the surface finish produced has a matt appearance and is particularly suitable for subsequent polishing. The work surface may be damaged mechanically and thermally.
(iv) Heat Affected Zone:
The melted material is not completely removed but part of it is resolidified on the machined surface to form a hard skin about 2—10 µm deep. Thermal stresses, plastic deformation and fine cracks form in this grain boundary.
Volume of crater made by an electric spark = π/6 x depth of crater [3/4 (Dia of crater)2 + (depth of crater)2] mm3 [Refer Fig. 10.13]
For copper electrode and kerosene dielectric, Vol. of crater = 1.42 CU2 and Metal removal rate = Vol. of crater x Frequency of charging.
The volume of crater can also be related to the mean current setting (Im) by the relation:
Metal removal rate is expressed as volume of metal removed in unit time. Sometimes it is defined as the volume of metal removed per unit time per ampere. During roughing cut of steel with graphite electrode, about 400 mm3/min material can be removed with 50 A generator and 4800 mm3/ min with 400 A generator. With low current and high frequency as low as 2 mm3/min of material rate can be attained.
Metal Removal Rate:
The amount of metal removed by a single discharge is proportional to the diameter of crater and depth of which the melting temperature is reached. The spark is considered as a uniform circular heat source on the electrode surface and the diameter of this circular source remains constant. Further the rate of heat input remains constant during the period of discharge duration.
Fig. 10.17 shows how the crater depth varies with discharge time for copper and aluminium. Fig. 10.18 shows how material removal rate varies with spark duration time td for different spark energies. It would be seen that MRR increases with increase in td upto a certain limit and then after reaching a peak value, it suddenly drops to zero.
MRR also depends on the melting point of the material. Cavitation also plays an important role in mechanical removal process because it is observed that MRR is maximum when the pressure is below atmospheric.
High metal removal rates lead to poor surface finish and the selection of parameters is usually a compromise between these conflicting requirements. Metal removal rates upto 80 mm3/sec can be achieved, or surface finishes of 0.25 pm can be obtained at very low cutting rates. For high metal removal rate with good surface finish; roughing and finishing cuts with two electrodes are used.
Dielectric Fluid:
The dielectric fluid has the following functions:
(i) It helps in initiating discharge by serving as a conducting medium when ionised, and conveys the spark. It concentrates the energy to a very narrow region.
(ii) It maintains value of U.
(iii) It helps in quenching the spark, cooling the work, tool electrode and enables arcing to be prevented.
(iv) It carries away the eroded metal along with it.
(v) It acts as a coolant in quenching the spark.
The electrode wear rate, metal removal rate and other operational characteristics are also influenced by the dielectric fluid.
The dielectric fluid chosen should not evolve toxic vapours or gases during operation, must be inflammable (high flash point), chemically inert with respect to tool material, work material, etc. It’s physical properties and other characteristics should be such as to cause de-ionisation of the medium very quickly after discharging and make it effective insulating medium during the next charging operation of the condenser.
It must have sufficient and stable dielectric strength to serve as an insulation between the electrode and the tool. The viscosity should also be optimum so that the erosion particles are carried out as soon as produced and good wetting capacity is obtained. All these properties should be maintained under all working conditions of varying temperatures, contamination, etc. Lastly the dielectric fluid should be easily available at reasonable price.
The dielectric fluids generally used are transformer on silicon oil, white spirit (or kerosene) or paraffin. These dielectrics have the essential dielectric properties, they de-ionise rapidly and do not vaporise excessively. It may be noted that the rate of de-ionization determines the maximum rate of sparking which in turn limits the rate of metal removal.
The choice of any dielectric fluid depends on the workpiece size, type of shape, tolerance, surface finish and metal removal rate. White spirit is best suited for machining tungsten carbide and where intricate details and good surface finish are destroyed in small parts.
For better finish, the viscosity of dielectric fluid should be less. The dielectric fluid should not be changed frequently on a machine, and thus chosen according to the most frequent application to be carried out.
Rough estimate of MRR in EDM process can be had in terms of melting point of workpiece material by following relation (assuming average sparking conditions).
MRR = 4 x 104 x θm-123 mm3/Amp. min.
where θm = melting temperature (°C)
The MRR also depends on the circulation of dielectric fluid because without forced dielectric circulation, the wear particles repeatedly melt and reunite with the electrode.
The correct circulation of dielectric fluid between the electrode and the workpiece (flushing) in EDM is an important aspect as the efficient machining has direct bearing on the correct layout and adjustment of the flushing system. The eroded particles should be flushed out at the earliest as these reduce the further metal removal rate.
The various methods of flushing are pressure flushing, suction flushing, side flushing and pulsed flushing synchronised with electrode movement. In the last method, dielectric injection is synchronised wish the upward movement of the electrode so that the spark gap is under dielectric pressure only during that part of the cycle when no machining takes place. This is done so because wear increases with machining under pressure than suction flushing.
Automatic Feedback Circuit:
The description of the working of the feedback circuit is as follows:
As gap between tool and work increases, value of U across condenser goes on increasing, and therefore, voltage across R falls down. When voltage across R is same as reference voltage, no current flows and the servomotor does not work.
When voltage across R falls, then imbalance between R and reference voltage causes the resultant current to operate the servomotor which adjusts the gap between tool and work and keeps it constant.
A short circuit across the gap causes the servo to reverse the motion of the tool till the correct gap is ensured.
If distance between tool and work is less, then voltage across C will be less and voltage across R rise, which sends signal (error voltage) and operates servomotor to lift the tool up. Electrohydraulic servo control is usually employed due to its fast response of the order of milli-second.
The frequency of operation, i.e. charging and discharging in this process varies from 1000 to 2000 c/s and the surface finish is obtained upto 12 R.M.S. value.
The electrode tool material has to convey electrical machining pulses to erode the workpiece and no erosion occurring on itself.
Automation of EDM and Arc Monitor:
Automation of EDM process results in more reliable operation in terms of job achievement, geometry and details, finish and accuracy. Programmable systems in which chosen EDM conditions, i.e. operational steps and parameter changes can be entered via keyboard are available. The servo-system controls the progress of operation as per programme.
All this will hold good and work nicely only if arc is detected and corrected continuously. If arc is not detected, it may occur in non- predictable way, cavity be penetrated and electrode becoming more ‘engulfed’, and situation deteriorate. Whenever arcing has occurred, sub-surface damage to the tool, die or component usually takes place. This is important when fine surface finish is required. Thus for seeking and maintaining efficient EDM conditions, which safeguard the work and electrode, installation of an arc monitor is essential.
Arc monitor provides an early warning of spark-gap pollution, which, if uncorrected can rapidly degenerate into a damaging arc. Arc monitor measures the extent of arcing. This enables the operator to ‘tune’ to good and efficient machining and thereby optimise his conditions. The arc monitor thus relieves the operator from the need of continually attending the machine.
Tool Material:
Rate of material removal is very slow. Tool material used is any conducting material generally brass or copper or alloy of copper or cast iron. These materials can be easily shaped to the required profile. Where high accuracy and long electrode life are required higher melting point materials such as copper-tungsten, graphite or tungsten carbide are used.
As the tool does not come into contact with the work, life of tool is long and less wear and tear takes place.
The selection of proper tool material is influenced by:
(i) Size of electrode and volume of material to be removed,
(ii) Surface finish required,
(iii) Tolerances desired,
(iv) Nature of coolant application etc.
It has been experienced that certain materials are more suitable than others depending upon the materials to be machined and the type of generator used.
The characteristic of tool material should be such that the wear ratio, i.e. ratio of tool wear rate/metal removal rate is much less than unity and its hardness does not allow any deformation of the tool during the machining process since in that case the machined surface shape will be damaged.
It is interesting to note that the wear ratio for brass work is 0.5, for hardened plain carbon steel work 1.0 and for tungsten carbide work is 3.0. Wear ratio has been reduced to 0.1 by using graphite anode with a pulse generator machine.
The performance of electrode materials can be gauged by its material removal rate, low wear, ability to be accurately machined or formed.
The tool electrode for EDM constitutes the most important part and accounts for major cost. Commercially EDM tool electrodes are made of any of the following three categories of materials, viz., metallic (electrolytic copper, tellurium or chromium copper, copper tungsten, brass, tungsten carbide, aluminium, etc.), non-metallic (graphite), and combination of metallic and non-metallic (copper graphite).
The three most commonly used materials are:
(i) Graphite:
Fine grained graphite having isotope properties is usually used for EDM tool electrodes, though these are costlier than copper. A wide range of grades are available in graphite and these are used for variety of applications. A big advantage of graphite, though abrasive, is its ease of forming by several methods like machining, moulding, copy milling, grinding etc. It is low in cost and has exceptional wear resistance.
Dust nuisance is a problem which can be easily tackled. Graphite can generally achieve better metal removal rates than metallic tool electrodes with less wear, and fine surface finishes close to those obtainable with copper electrode can be achieved. Graphite has a greater propensity to arc than metallic electrodes and as such anti- arcing devices need to be incorporated.
(ii) Copper:
This is the second choice as EDM tool electrode. It can be produced by casting or machining. Copper electrodes with very complex features are formed by chemical etching or electro-forming.
(iii) Copper-Tungsten:
It has very low wear ratio and this aspect is of particular importance when detailed features have to be transferred. It is a difficult to machine material and has low metal removal rate. Though it has high cost, it is best suited for ED machining of carbides.
Usually some model is required for producing a particular EDM tool electrode shape but by computer aided design (CAD) technique any complicated shape in 3-dimension can be easily produced, thus dispensing with the need or a model. Metal spraying, press forming, and electroplating techniques are also used for making electrodes for EDM.
Electrode wear is dependent of factors like polarity, thermal conductivity, melting point of electrode, duration and intensity of spark discharges, types of power supplies used, the type of work material used in relation to the tool material, dielectric flow in the machining zones.
Electrode holders should be such that these enable quick and accurate loading of the electrodes on the machine to increase productivity.
Orbiting Control Facility for EDM:
Orbiting facility could either be provided as a retrofit unit to ED machine or be supplied as built-in orbital system with some degree of integration with the normal EDM controls. In orbiting, the electrode does not rotate, but each point on it moves in a small circular orbit in relation to the workpiece.
This action enlarges the size of tool slightly and if a properly shaped tool is used for roughing operations, same tool can be used for finishing due to above action. Tool electrode is driven by servo control in all the three directions.
The 3-D capability increases efficiency as well as range of use of EDM. It is also possible to create a draw angle effect by combining linear Z-axis feed motion with a progressively decreasing orbit. Not only draw angle effect, but a large number of forming possibilities can be realised by varying orbital radius with depth in some predetermined manner. Many more forms can be created by exercising controls over X and Y translation motions.
The action for orbiting which results in relatively large instantaneous electrode spacing also provides an excellent ‘flushing’ action. Setting of the orbiting controls can be achieved manually, the available controls depending on the orbiting device design.
Setting of the device can also be achieved from an electronic programmer and, therefore, be incorporated as part of an automatic machining system. With arc monitor, attempts are being made to computerise operation with on-line optimisation of the process.
Advantages of EDM:
(1) It can be used for any hard material and even in heat treated condition. It is very difficult to machine carbide but by this process it can be machined easily. It can also machine ceramic carbide materials and other hard materials.
(2) Surface finish obtained by this process is very good. As tool and work do not come in contact so no cutting forces act on the work and consequent error due to elastic deformation is eliminated.
(3) By this process very thin sections can also be machined.
(4) Tool material need not to be harder than work material, therefore, that material must be used which can be easily shaped. It is due to this reason that any complicated shape that can be made on the tool can be reproduced on the workpiece.
(5) Tolerances upto 0.04 pm can be achieved.
Disadvantages of EDM:
Metal removal rate is slow, therefore, this process should only be used where other conventional machining processes are not suitable as machining costs are very high. Power requirement is very heavy. Reproduction of sharp corners is the limitation of the process.
Surface cracking may take place in some materials owing to their affinity to become brittle at room temperature especially when higher energy per pulse is used. Also the distortion of surface microstructure by this process is detrimental to some cases and necessitates subsequent etching. Workpiece metal must be an electrical conductor.
Applications of EDM:
Applications of EDM are as follows:
i. This process is used for shaping alloy steel and tungsten carbide dies, used for moulding, forging, extrusion, wire drawing or suitable mould cavities, press tools and to give any intricate shape or profile.
ii. Fine slits can be made by using a wire electrode.
iii. By this method a hole as small as 0.1 mm in diameter can be made.
iv. This process is very useful for making hole of nozzles, other holes, shapes, profiles and embossing, engraving operations on harder materials.
v. Internal threads and internal helical gears can be cut in hardened materials by using a rotary spindle and suitable attachments.
vi. It is also used for production work for special applications where the oil retention properties of the work surface are important.
vii. Accuracy upto 0.005 mm can be obtained.
viii. Maximum metal removal rate in rough machining is 520 mm3/mt for hardened too steel materials and for good surface finish 15 mm3/min or less than this.
ix. If gap between work and tool is more, value of U is more and, therefore, metal removal rate will be more. Hence, by adjusting value of U, i.e. adjusting value of resistance R we can adjust the gap between work and tool, and thus obtain different machining rates. A number of accessories are available which enhance the applications of EDM.
Example 1:
It is required to drill a hole of 1.69 mm diameter to a depth of 7.32 mm in steel sheet using R-C circuit by electro-discharge machining method using a brass electrode. The surface finish required is to be 20 micron. Determine the source voltage to be set up for a condenser and resistance setting of 120 micro-farads and 100 ohms respectively. Also find out the time required for drilling. Assume sparking period = 100 µ-sec.
Given that:
(i) surface roughness = K1((1/2) CU2) K2tp K3 microns where K1, K2 and K3 are constants and their values for brass electrode and steel workpiece are 13, 0.45 and 0.22 respectively.
C = capacitance of the condenser in farads,
U = voltage across the capacitor
Tp = pulse duration in µ-sec.
(ii) Metal removal rate = 1.42 tcU2mm3/sec (tc = charging time).
Solution:
Characteristics of EDM Process:
