THE history of the EDM process dates back to the days of World Wars I and II. Earlier very few saw the benefits of this process and the popularity of the primitive technology wasd scarce. As much electrode material was removed as that of the work piece and the manual feed mechanism lead to more arcing than sparking. During this time the dither or the vibrator came into the picture and this represented the first attempt towards controlling the spark gap. Vibrating the electrode allowed material removal to be effective. Two Soviet scientists were convinced that many more improvements could be made. Doctors B.R. and N.I. Lazarenko were the people who invented the relaxation circuit, they invented a simple servo controller too that helped maintain the gap width between the tool and the work piece. This reduced arcing and made EDM machining more profitable.
The two principle types of EDM processes are the die sinking and the wire EDM process. The die sinking process was refined as early as the 1940 with the advent of the pulse generators, planetary and orbital motion techniques, CNC and the adaptive control mechanism. From the vacuum tubes, to the transistors to the present day solid state circuits, not only was it possible to control the Pulse on time, but the pause time or the Off time could also be controlled. This made the EDM circuit better, accurate, dependable and EDM industry began to grow.
During the 1960 the CIRP and ISEM conferences were held for the first time in Czechoslovakia which proved to be a driving force in the progress of the EDM process. EDM research was inspired by the old paradigm of technology that each problem in a field must be solved. The founders of this paradigm were Newton and Maxwell.
There were a number of problems faced when mathematical modeling of the ED process was done. The gap pollution, the hydrodynamic and thermodynamic behavior of the working fluid are hard to model. Getting a model in all with practical technological results was difficult. This inability along with the high demand from the market lead to a more pragmatic, application oriented research into the EDM process.
The research going on today aims at a more application oriented field rather than searching for a unified EDM model. Today the EDM market is growing owing to increasing popularity of EDM in the manufacturing market and secondly due to the indirect influence of fundamental and applied EDM R&D, carried out at various labs, industrial ones and at universities.
The evolution of the wire EDM in the 70’s was due to powerful generators, new wire tool electrodes, better mechanical concepts, improved machine intelligence, better flushing. Over the years the speed of wire EDM has gone up 20 times when it was first introduced, machining costs have decreased by atleast 30% over the years. Surface finish has improved by a factor of 15, while discharge current has gone up more than 10 time higher .
THEORIES OF MATERIAL REMOVAL :
The removal of material in electrical discharge machining is based upon the erosion effect of electric sparks occurring between two electrodes. Several theories have been forwarded in attempts to explain the complex phenomenon of "erosive spark". The following are the theories,
1. Electro-mechanical theory
2. Thermo-mechanical theory
3. Thermo-electric theory
Electro-mechanical theory :
This theory suggests that abrasion of material particles takes place as a result of the concentrated electric field. The theory proposes that the electric field separates the material particles of the workpiece as it exceeds the forces of cohesion in the lattice of the material. This theory neglects any thermal effects. Experimental evidence lacks supports for this theory.
Thermo-mechanical theory :
This theory suggests that material removal in EDM operations is attributed to the melting of material caused by "flame jets". These so - called flame jets are formed as a result of various electrical effects of the discharge. However, this theory does not agree with experimental data and fails to give a reasonable explanation of the effect of spark erosion.
Thermo-electric theory :
This theory, best-supported by experimental evidence, suggests that metal removal in EDM operations takes place as a result of the generation of extremely high temperature generated by the high intensity of the discharge current. Although well supported, this theory cannot be considered as definite and complete because of difficulties in interpretation.
It is not absolutely necessary to understand the operating principles of EDM to be a successful machinist. However, an understanding of what is taking place between the electrode and the workpiece can aid the EDMer in several important areas. A basic knowledge of EDM theory can help with troubleshooting, in selecting the proper workmetal/electrode combinations, and in understanding why what is good for one job is not always good for the next.
The following description represents a combination of what is known plus what is theorized about the process.
While several theories of how EDM works have been advanced over the years, most of the evidence supports the thermoelectric model. The following nine illustrations show step-by-step what is believed to happen during an EDM cycle. The graphs below the illustrations show the relative values of voltage and current at the point depicted
A charged electrode is brought near the workplace. Between them is insulating oil, known in EDM as dielectric fluid. Even though a dielectric fluid is a good insulator, a large enough electrical potential can cause the fluid to break down into ionic (charged) fragments, allowing an electrical current to pass from electrode to workpiece. The presence of graphite and metallic particles suspended in the fluid can aid this electrical transfer in two ways : the particles(electrical conductors) aid in ionizing the dielectric oil and can carry the charge directly; and the particles can catalyze the electrical breakdown of the fluid.
The electrical field is strongest at the point where the distance between the electrode and workpiece is least, such as the high point shown. The graph in the illustration shows that the potential(voltage) is increasing, but current is zero.
As the number of ionic(charged) particles increases, the insulating properties of the dielectric fluid begin to decrease along a narrow channel centered in the strongest part of the field. Voltage has reached its peak, but current is still zero.
A current is established as the fluid becomes less of an insulator. Voltage begins to decrease.
Heat builds up rapidly as current increases, and the voltage continues to drop. The heat vaporizes some of the fluid, workpiece, and electrode, and a discharge channel begins to form between the electrode and workpiece.
A vapour bubble tries to expand outward, but its expansion is limited by a rush of ions towards the discharge channel. These ions are attracted by the extremely intense electro-magnetic field that has built up. Current continues to rise, voltage drops.
Near the end of the on-time, current and voltage have stabilized, heat and pressure within the vapour bubble have reached their maximum, and some metal is being removed. The layer of metal directly under the discharge column is in molten state, but is held in place by the pressure of the vapour bubble. The discharge channel consists now of a superheated plasma made up of vaporized metal, dielectric oil, and carbon with an intense current passing through it.
At the beginning of the off-time, current and voltage drop to zero. The temperature decreases rapidly, collapsing the vapor bubble and causing the molten metal to be expelled from the workpiece.
Fresh dielectric fluid rushes in, flushing the debris away and quenching the surface of the workpiece. Unexpelled molten metal solidifies to form what is known as the recast layer.
The expelled metal solidifies into tiny spheres dispersed in the dielectric oil along with bits of carbon from the electrode. The remaining vapor rises to the surface. Without a sufficient off-time, debris would collect making the spark unstable. This situation could create a DC arc which can damage the electrode and the workpiece.
This on/off sequence represents one EDM cycle that can repeat up to 250,000 times per second. There can be only one cycle occuring at any given time. Once this cycle is understood we can start to control the duration and intensity of the on/off pulses to make EDM work for us.
Analysis of the Pulses used in the EDM Process
Performance measures such as MRR, tool wear, and surface finish for the same energy depend on the shape of the current pulses. Depending upon the situation in the gap which separates both electrodes, principally four different electrical pulses may be distinguished:
a) Open circuit or open voltage
b) Effective discharges or real Sparks
c) Arcs and
d) Short circuits
They are usually defined on the basis of time evolution of discharge voltage and (or) discharge current (Fig). Their effect upon material removal and tool wear may differ quite significantly. Open voltages, occurring when the distance between both electrodes is too large, obviously do not contribute to any material removal or electrode wear.
When contact between tool and workpiece takes place, a short circuit occurs which also does not contribute to material removal.The range of the electrode distances in between these two extreme cases can be considered to be a practical working gap yielding actual discharges, i.e., sparks and arcs. Both pulse types do show a characteristic voltage drop across the gap during a pulse. The difference between sparks and arcs is quite difficult to establish. It is believed that arcs occur in the same spot, or on the electrode surface and may therefore severely damage tool and workpiece.It is assumed that arcs occur when the plasma channel of the previous pulse is not fully deionized; the current during the following pulse will flow by preference along the same current path. Therefore, in such a case, no time is required to form a new gaseous current path. The formation of the gaseous channel is normally considered to be necessary to initiate a new spark breakdown. This peculiarity of EDM arcs is often proposed as a discrimination characteristic with respect to effective discharges or real sparks.It is believed that only "sparks" really contribute to material removal in a desired mode. Until now it remains an open question how much arcs contribute in terms of material removal and tool wear.
TOOL WEAR AND RECTANGULAR AND NONRECTANGULAR CURRENT PULSES:
The advent of the rectangular pulse generator in EDM has resulted in improved metal removal rates and reduced electrode wear compared to the performance of the former relaxation-type generator. Nevertheless, it is a fact that the problem of electrode wear still persists and that it is particularly important during finishing operations. During finishing operations higher MRR, though preferable, is not very important. Accuracy is the key factor. As a solution to the tool wear problem, besides parameter selection, non-rectangular current pulses, comb current, and other types of current pulses have been applied successfully. The state-of-art electrical generators are designed to produce both rectangular and non-rectangular current pulses; Therefore, it has become possible to use different types of pulses and observe their effect on machining performance. Previous investigations indicate that considerable tool wear reduction is possible when sloped current pulses are used. However, the full effects of pulse on-time, which for a given average current level has a considerable influence in determining both tool wear and material removal rate, were not taken into account. In the study reported in an attempt made was to find the region in which non-rectangular pulses have a clear advantage over rectangular pulses. Due to machine limitations, a maximum peak current of 16 amperes was tried, so the results of this study cannot be considered as valid for all settings as they are from a small experimental range.One other study reported in made use of trapezoidal pulses in rough machining. A maximum average pulse current of 22 amps was used and only pure trapezoidal current pulses were employed. Also, a lift cycle for the tool was used to get rectangular current pulses was not used. Using the lift cycle for the tool normally does not give precise results but this probably could have been a machine constraint. The removal of material in electro-discharge machining is based primarily on the conversion of electric energy into thermal energy. Accordingly, the distribution of the thermal energy on the electrode surface is of considerable importance. The plasma channel is believed to expand throughout the pulse duration approximately with the relation
r = sqrt(t')
where 0<=t'<=t(on time)
r is the radius of the plasma in microm, t(on time) is pulse on-time in microSec and t' is time in pSec
The heat density on the electrode surfaces decreases by the same degree when the instantaneous discharge energy is constant. Therefore, at the beginning of a discharge with a small diameter of the discharge channel, maximum heat density exists. As the channel diameter increases the heat admitting surface expands and the proportion of energy dissipated by radiation, convection, and conduction rises markedly. If these physical mechanisms are true, the supplied discharge energy is not optimally utilized with the rectangular current pulse, because at the beginning of the discharge the current density is so high that the material to be removed is heated far beyond the required melting temperature. The current density varying during a discharge must be regarded as the criterion for the electrode erosion because it is argued that at the beginning of the discharge, only the anode is thermally affected . In this phase the electrons in the plasma channel are accelerated towards the anode and transfer their energy of motion to the anode by impact. At that time the plasma channel rapidly expands but its diameter is still small. The energy imposed on the anode decreases after a few microseconds because of the decrease in energy density on the anode, along with further expansion of the plasma channel. Thermal modelling of the process also shows that the input power and tool wear are related by thermal conductivity of the tool and pulse on-time. Therefore, an adaptation of the supplied power input will result in a reduction of the tool.
Another factor that influences tool wear is the pulse interval time. It is known that with the diminishing of the pulse interval to the limit at which the discharges deteriorate into stationary arcs leads to the generation of socket discharges by favoring deposition of graphite protective film. This increases the resistance to electrical erosion of the copper tool electrode.Inversely, an increase of pause interval determines both an increase of tool wear and the diminishing of graphite film deposition. Thus, the adequate conditions for the deposition of the graphite film and the diminishing of current at the beginning of discharge, causes the decrease of tool wear.
DIELECTRIC FLUID :
The EDM setup consists of a power supply whose one lead is connected to the workpiece immersed in a tank having dielectric coil. The tank is connected to a pump, oil reservoir, and a filter system. The pump provides pressure for flushing the work area and moving the oil while the filter system removes and traps the debris in the oil. The oil reservoir restores the surplus oil and provides a container for draining the oil between the operations.
The main functions of the dielectric fluid are:
- To flush the eroded particles produced during machining, from the discharge gap and remove the particles from the oil to pass through a filter system.
- To provide insulation in the gap between the electrode and the workpiece.
- To cool the section that was heated by the discharge machining.
The two most commonly used fluids are petroleum based hydrocarbon mineral oils and de-ionized water. The oils should have a high density and a high viscosity. These oils have the proper effects of concentrating the discharge channel and discharge energy but they might have a difficulty in flushing the discharge products.
For most EDM operations kerosene is the common die electric used with certain additives, that prevent gas bubbles and de-odoring. Silicon fluids and mixture of these fluids with petroleum oils have excellent results.
High removal rates, less tool wear, better surface finish have been obtained in the machining of titanium alloys. Other dielectric fluids with a varying degree of success include polar compounds such as aqueous solutions of ethylene glycol, water in emulsions and distilled water.
The main things that the EDM user should be concerned with are:
Flash point: This is the temperature at which the vapors of the fluid will ignite. This explanation is a little simplistic as conditions for testing are more involved but for the sake of discussion and safety’s sake, the higher this number, the better. Unless you are doing extremely small, low power cavity work or drilling the tinist of holes, be especially concerned with anything on a spec sheet or MSDS rated lower than 180 degrees Fahrenheit.
Dielectric strength: This is the ability of the fluid to maintain high resistivity before spark discharge and in turn the ability to recover rapidly with a minima amount of OFF time. An oil with a high dielectric strength will offer a finer degree of control throughout the range of frequencies used, especially those used when machining with high duty cyles or poor flushing conditions. This will provide for better cutting efficiency coupled with a reduced potential arcing.
Viscosity: The lower the viscosity of the fluid the better is the accuracy and finishes that can be obtained . In mirror finishing or close tolerance operations, spark gaps can be as small as 0.005 or less. With such tight, physical restrictions such as this, it is much easier to flush small spark gaps with lighter and thinner oil. Good finishing EDM oils are on the thin side. In the EDMing operations requiring moderate finishes like in the forging dies, high MRR, high current values, heavier oils can be used. Viscosity in such conditions can be high because of larger spark gaps and this will also prevent the excess loss os fluid through vaporization.
Specific gravity: Often confused with viscosity, this is the “weight” of a substance measure by a hydrometer. The “lighter” the oil or lower its specific gravity, faster the heavier particles (chips) settle down. This reduces the gap contamination and possibilities of secondary discharge and/or arcing.
Color: All dielectric oils will eventually darken with use, but it seems only logical to start with a liquid that is as clear as possible to allow viewing of the submerged part. Clear or “water-white” should be your choice, because any fluid that is not clear when brand new certainly contains undesirable or dangerous contaminants.
Odor: Besides for the obvious reasons for aesthetic of choosing a fluid with no discernable odor, the oils that have a strong odor give an indication for the presence of sulfur which is undesirable in the EDM process.
Depending on the use of the oil and maintenance the oils can last several years. Regularly filtered oil prevention of water contamination will extend its useful life considerably.
Water contamination cannot be eliminated completely as condensation will occur on the electrode surface when the surface heats up. Graphite electrodes will contribute more to the condensation than the metallic electrodes as they have a porous structure and absorb moisture from the air. That is the reason why the graphite electrodes should be stored in dry areas. Some shops will keep the electrodes in dry ovens the night before they are used. A less obtrusive method to keep humidity and moisture absorption to a minimum, would be to allow a 60 watt bulb remain to remain lit within the strong cabinet.
The color of the oil is not necessarily an indication for oil replacement. All oils, no matter how clear they are when new, will darken in shades from amber to brown with use and age, because these products will break down when exposed to high heat. Obviously, sustained high amperage machining will breakdown the oil more rapidly. Tars, resins and hydrocarbons are generated when this occurs and this is what “stains” the oil. No amount of filtration will remove this discoloration, so don’t mistake “colored oil” for “dirty oil”.
Dirty oil can be judged by the following factors:
- Pressure gauge readings as described in the machines maintenance manual.
- Increased occurrence of DC arcing or pitting with settings that were previously successful. (Assuming of course that no other changes have been made such as the grade of the electrode material, flushing pressures etc.)
- Longer cutting cycles and / or degradation of finishes.
- A visual inspection of the oil. To do this, fill the work tank but do not machine. Collect a sample of oil in a clear container. Visually check the sample immediately for any cloudiness and again after several hours, to check for sediments on the bottom or color striations in the oil itself. Dirty oil is usually tinted gray or black and this coloration will dissipate if the oil is allowed to circulate through the filtration system. Replacing the filters is the least expensive ans most likely remedy to the above symptoms.
Some of EDM usage
Micro EDM :
Micro EDM, similar to conventional macro EDM, is an erosion process where the material is removed by electrical discharges generated at the gap between two electrically conductive electrodes. Micro EDM is used to machine micro holes , channels and 3D micro cavities in electrically conductive materials including super alloy such as tungsten carbide and stainless steel. Micro EDM has been used to drill not only circular holes but also holes with irregular cross sections. The shape and size of the the micro hole made by micro EDM is determined by the electrode prepared by the WEDG.
The micro EDM machine used is panasonic MG-ED72W , This machine includes MG-ED71 ( Standard NC Boring machine ) + WEDG Unit ( Micro Electrode Tooling unit )
Recently conducted projects include Integration of Uniform Wear Method with CAD/CAM and Machining of Microhole with high aspect ratio and non circular blind micro hole.
EDM Drilling :
Once relegated to a last resort method of drilling holes, fast hole EDM drilling is now used for production work. Drilling speeds have been achieved of up to 2 ipm. Holes can be drilled in any electrically conductive material, whether hard or soft, including carbide. Fast hole EDM drilling is used for putting holes in turbine blades, fuel injectors, cutting tool coolant holes, hardened punch ejector holes, plastic mold vent holes, wire-EDM starter holes, and other operations. The term fast hole EDM drilling is used because conventional ram EDM can also be used for drilling. However, ram EDM hole drilling is much slower than machines specifically designed for EDM drilling. Fast hole EDM drilling uses the same principles as ram EDM. A spark jumps across a gap and erodes the workpiece material. A servodrive maintains a gap between the electrode and the workpiece. If the electrode touches the workpiece, a short occurs. In such situations, the servodrive retracts the electrode. At that point the servomotor retraces its path and resumes the EDM process.
Recent research investigates the influence of process prameters on the surface integrity of the electrodischarge drilling process.
Wire EDM is an electrical discharge machining process with a continuously moving conductive wire as tool electrode. The mechanism of metal removal in wire electrical discharge machining (WEDM) involves the complex erosion effect of electric sparks generated by a pulsating direct current power supply between two closely spaced electrodes in dielectric liquid. The high energy density erodes material from both the wire and workpiece by local melting and vaporizing. Because the new wire keeps feeding to the machining area, the material is removed from the workpiece with the moving of wire electrode. Eventually, a cutting shape is formed on the workpiece by the programmed moving trajectory of wire electrode. The equipment is extensively used in making dies and molds.
The related research projects include:
Avoidance of wire breakage, development of monitoring and control system, database, machining of advanced materials,comparison of different wire performance and thermal as well as vibration modeling.
Abrasive Electro Discharge Grinding ( AEDG )
AEDG is a hybrid process, which combines EDM and grinding. In AEDG mechanical abrasion of a metal bonded diamond wheel is combined with the electro-erosion of electrodischarge machining (EDM). The removal of conductive or partially conductive material is by a combination of rapid, repetitive spark discharges between workpiece and rotating tool, separated by a flowing dielectric fluid and also by a mechanical action of irregularly shaped abrasive particles on the periphery of the wheel.
The recent research includes monitoring and control, new power generator, 2-axis NC wheel dressing unit , environmental performance of different dielectric fluids. The current research involves strategy for optimizing neural network modeling, the study of self dressing characteristics and sequence of operations and using neural networks for controls in AEDG.