DI-SPARK LTD PART FOUR OF OUR GUIDE TO EDM WIRE EDM AND Spark EDM Basic Theory
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5. BASIC EDM THEORY – Wire EDM & Spark EDM
Although the EDM process has been in use for decades, it is still widely mis-understood. EDM is a metal removal process where two electrodes are used to produce a spark, one electrode being the work-piece itself, the other being either a formed tool or a wire. The two electrodes should never come into contact with each other. A small gap is maintained between them at all times. The process involves a spark that is generated by a pulsed electrical current being discharged through an insulating dielectric fluid (water or oil) across the very small spark gap between the electrodes. Material is removed by the thermal energy of repetitive sparks. The spark is reported to be in the range of 8,000 to 12,000 degrees Centigrade and it vaporises and melts the work-piece material. The process can be used when the work-piece material is too hard, or the shape or location of the detail cannot easily be conventionally machined.
EDM was first implemented over 40 years ago, and used primarily to remove broken taps and drills from expensive parts. The early machines were quite crude in construction with hand-fed electrodes. During World War II, two Russian scientists, B.R. and N.I. Lazarenko, adapted the first servo-system to an EDM machine. This offered some semblance of the control that is required for efficient but safe EDM machining today.
Through the years, machines have improved drastically – progressing from RC (resistor capacitance or relaxation circuit) power supplies and vacuum tubes to solid-state transistors with nanosecond pulsing. From hand-fed electrodes to modern CNC machines with controlled simultaneous multi- axes machining. Now with the development of wire EDM the process will revolutionise many industries and change long-standing methods of manufacturing.
5.1 THEORY – Spark EDM & Wire EDM
Assuming that the electrode is positively charged and the work-piece is negatively charged (or vice-versa), the electrode is advanced into the work-piece through an insulating liquid, or dielectric fluid. This is usually hydrocarbon based oil for spark EDM machines, and de-ionised water for wire EDM machines. The dielectric fluid is integral to the process as it provides insulation against premature discharging, cools the machined area and flushes away the debris.
As the electrode, charged with a high-voltage potential, nears the work-piece, an intense electromagnetic flux is formed and eventually breaks down the insulating properties of the dielectric fluid. Picture the ends of two bar magnets with the north and south poles held apart. If one were to lay a piece of white paper over the magnets and sprinkle fine iron filings onto it, the filings would be caught in the magnetic flux and become aligned.
When the ions in the dielectric obtain polar alignment the resistivity of the fluid is at its lowest. Electrical discharges are able to flow through the ionised “flux tube” and strike the work-piece. The voltage drops as current is produced, and the spark vaporises anything in contact with it, including the dielectric fluid, encasing the spark in a sheath of gasses composed of hydrogen, carbon, and various oxides. The area struck by the spark will be vaporised and melted, resulting in a small crater on the workpiece surface being formed. Due to the heat of the spark and the contaminates being produced by the work-piece, electrode and the dielectric fluid, the field of ionised particles is disrupted, and resistivity increases rapidly. Voltage will rise as resistivity increases and the current will drop, as the dielectric can no longer sustain a stable spark. At this point, the current must be switched off.
During the time current is flowing through the spark gap, the plasma-hot area will rapidly expand away from the heat source – the spark. When the current is switched off, there is no more heat source and the sheath of vapour that was around the spark implodes. Its collapse creates a void or vacuum and draws in fresh dielectric fluid to flush away swarf and cool the area. This off period allows the re-ionisation process of the dielectric fluid to be completed and provides favourable conditions for the next spark. The duration of the off-time must be sufficient enough to flush away the spark debris and damaged dielectric, or stability will be difficult to maintain, resulting in arcing or a broken wire. This briefly describes one EDM cycle; it must be repeated over and over again, switching on and off thousands of times per second for successful machining to occur.
The on and off pulses comprise a single cycle of electrical discharge machining. The length and duration of these parameters will depend upon the work-piece material, electrode material, flushing, metal removal rate and the surface finish. Generally speaking, low frequencies are used for rough machining and high frequencies are used for finish machining. Some materials due to density, conductivity and melting temperature must be machined with higher frequencies even during roughing operations (e.g. titanium, carbide, copper). This will result in improved finishes and surface integrity, but with substantially increased electrode wear.
The relationship of the on time to the off time is known as the duty cycle. It is calculated by adding the on-time and the off-time together and dividing this total into the on time. Multiply this quotient by 100 will give the percentage of efficiency, or duty cycle.
Duty cycle = (on-time / (total cycle time)) x 100
Obviously, it would be desirable to reduce the off-time to the smallest possible increment, but many variables such as flushing conditions, electrode material, work-piece material, dielectric condition, etc., can drastically affect the ability to maintain concurrent efficiency and stability.
Many of the modern power supplies have the ability to monitor and change conditions and duration of the spark by using adaptive controls. This ability can compensate for marginal cutting conditions automatically, allowing unattended operations.
Next post: Part 5, Machining Parameters
