EDM is a widely used machining method for hard metals and metal alloys, compatible with complex surface geometry, deep ribs (slots) and sharp internal angles in metals. The EDM manufacturing process couples well with other machining methods with similar applications. Today, the modern sinking EDM method incorporates highly efficient automated technology based on extensive knowledge of EDM theory and years of experience.
For a better understanding of the sinking EDM process, it is essential to possess an understanding of the basic aspects of the EDM metal removal theory.
It is known that the geometry and size of an electrode-tool contains all relevant information about the part. The motion of the tool creates an inverted image in the electrode-workpiece. In addition, it is also known that each discharge between the tool and the workpiece leads to the formation of localized craters on the working surface. Servo-controlled superimposition of a large quantity of such craters forms the geometry of the gap between the tool and the workpiece.
Sinking EDM is a stochastic process whereas the conditions of formation and evolution of each discharge depend on the outcome of preceding discharges. There is no known mathematical model for the sinking EDM process due to the lack of a complete definition of interdependent non-linear probabilistic electrodynamic, thermoenergetic, hydrodynamic and thermochemical processes for a single discharge. Consequently, researchers have developed a phenomenological model of the EDM process, based on core, individual aspects:
1. Single Discharge Processes
- Breakdown and Formation of The Discharge Channel
- Energy Balance and Thermal Processes Affecting Electrodes
- Gas Bubble Formation Around the Discharge Channel
- Thermochemical Processes in The Working Fluid
2. Multiple Discharge Processes
- Single Discharge Processes
There is a variety of theories to explain the discharge breakdown in the gap between electrodes. In our opinion, the theory on the impact ionization of liquid between conductive particles leading to the formation of a discharge channel is the most applicable. This theory explains why the EDM process does not occur in a clean dielectric fluid (due to short circuits) and why the gap size strongly depends on the concentration of waste particles within said gap.
There is a high probability of short circuits occurring through the localized waste particles during rough and semi-fine regimes, if the concentration of waste particles is extremely high. It is essential for the generator power at the beginning of a discharge to be sufficient for destruction of conductive by-products of erosion and form a normal discharge channel to stabilize the process on such regimes.
The discharge channel forms after the breakdown between electrodes. The diameter of such a channel grows quickly in the first few microseconds. Afterwards, and as the duration of discharges increases, the speed of channel growth decreases. These speeds depend on the condition of the dielectric fluid – ceteris paribus (temperature, waste particle concentration, etc.).
Plasma, consisting of electrons and ions, forms inside the discharge channel. After a breakdown, in the beginning of a discharge, electrons possess a higher kinetic energy and a higher temperature than the ions. In longer lasting discharges, the temperature of electrons and ions balances out and the plasma of the channel becomes isothermal.
The drop-in voltage inside the discharge channel is virtually proportional to the length of the channel. The rate of increase of the discharge channel diameter depends on the duration and magnitude of the discharge as well as the condition of the above-mentioned dielectric fluid.
It is known that the energy and power of a discharge are distributed across electrodes (cathode and anode) and the discharge channel. The energy is supplied to the electrodes with the help of the following:
- Bombarding with electrons and ions
- Thermal bombarding with hot particles from the discharge channel
- Thermal radiation
- Volumetric sources of heat
Research shows that the main part of the energy supplied to electrodes is determined by electrons and ions. The balance of energy between an anode and a cathode depends on the duration of the discharge – ceteris paribus. If the duration of a discharge is shorter than 10 microseconds, then most of the energy is supplied to the anode by bombarding it with “fast” electrons. If the duration of a discharge is between 100 and 1000 microseconds, then most of the energy is supplied to the cathode with “slow” ions, so that the voltage drops as the anode decreases. The voltage drop is nearly proportional to the current density, which decreases as the diameter of the discharge channel is increased.
Consequently, the removal of metal from the cathode surpasses the removal of metal from the anode with discharges that have a sufficiently long duration. The power at the anode and cathode can be represented as thermal energy. Thermal energy density, similar to current density, depends on the duration and power of a discharge. Thermal energy, that arises within a discharge channel is lost due to the evaporation of dielectric fluid and light emission.
Discharge Thermal Processes
Researchers have been able to identify the following phenomena using classical thermal physics theory:
- For discharges with a duration of less than 10 microseconds, the thermal energy density at the place of contact of a discharge channel and electrodes reaches tens of millions of watts per square centimeter. In this case, electrode particles evaporate immediately, regardless of their thermophysical properties. Simultaneously, very little energy is transferred to the bodies of electrodes. Therefore, a thermally modified layer is practically non-existent after machining with short impulses.
- For discharges with a duration in the range of 100-1000 microseconds, the thermal energy density is reduced drastically. Most of the metal from the discharge crater is removed in the melted state as opposed to the gaseous state. At the same time, a significant portion of thermal energy is transferred to the bodies of electrodes. This leads to the formation of a thermally modified layer with a depth of 0.01-0.2 millimeters.
That being said, it is possible to use the differences in thermophysical properties between electrodes for drastic reduction of wear of a tool. For example, the temperature of graphite sublimation is around 3,500 °C; the thermal conductivity of graphite or steel is only a fraction of the thermal conductivity of copper; tungsten possesses a relatively high thermal conductivity and a melting point of 3,367 °C. Consequently, the above-mentioned materials are widely used as tool-electrodes for EDM.
There is no exact known model of the EDM process. However, there are known existing models of thermal processes in electrodes which allow us to estimate the roughness of the surface and the depth of the affected zone after EDM processing. Based on these models, it is possible to closely determine the metal removal rate and the extent of tool wear.
Gas Bubble Dynamics Surrounding the Discharge Channel
At the beginning of the discharge, the channel is compressed as the dielectric fluid in which it forms is practically incompressible. Therefore, at the beginning moment of a discharge formation an “impact wave” occurs. Such an impact wave has most of its pressure at the front, whereas pressure depends on the growth rate of the electrical current. Then a gas bubble, filled with hot gas, forms and expands around the discharge channel. The dimensions of the bubble depend on the parameters of the discharge, the degree of interdependency of the discharge, and the properties of the dielectric fluid. The gas bubble expands to its maximum volume and bursts. The duration of the bubble is much longer than the duration of the discharge.
Evacuation of Metal from a Crater
Melted particles and vapors of metal fly out of a crater, formed in the body of the electrode by a discharge, at a high speed that can reach 100 meters per second. This happens partially during the discharge due to a fluctuation in pressure at the border between the discharge and the metal. However, most of the metal is evacuated at the melted and gaseous state at the moment of a drop of electrical current at the end of the discharge, which is related to the abrupt drop in pressure in the discharge channel. This leads to partial evaporation of already melted metal and its explosive removal from the crater. The shorter the pulse fall time, the more intensive the evacuation of metal from a crater.
Thermochemical Processes in the Dielectric Fluid
EDM dielectric fluids usually contain carbon. Thermal cracking by-products – asphaltic resinous compounds and particles of pyro graphite – emerge during the heating and evaporation of fluid during a discharge. Dielectric fluid evaporates not only around the discharge, but also around the very hot zones resulting from other discharges. These processes consume 10-15% of overall energy supplied by a generator. Thermal cracking by-products are conductive, so their concentration determines the size of the gap between the electrodes – ceteris paribus.
Multiple Discharge Processes
A series of discharges influences the conditions for breakdown and formation of subsequent discharges even if pauses between discharges are much longer than the duration of discharges. In rough and semi-fine EDM regimes, normally selected pause duration is much shorter than the duration of a discharge. Consequently, the interdependence of discharges that are initiated in the limited space between the electrodes have utmost significance in dispersion of discharges across the work surface. Besides the rate of metal removal and tool wear, the depth of a thermally modified layer and the uniformity of a work surface’s roughness after processing also depends on the dispersion of discharges.
The interdependence of discharges leads to their grouping in localized regions of the work zone as the conditions for initiation of subsequent discharges are better in the places where discharges already took place.
The superposition of temperatures in localized regions of group discharge formations leads to the overheating and evaporation of dielectric fluid in those regions. Vapor leaves the work zone, carrying erosion by-products with it. If the volume of vapor exceeds the volume of a work zone, the process of evacuation of erosion by-products terminates. Discharges start to occur between the electrodes and the eroded by-product particles, leading to their bake-on adhesion and formation of slag. The goal of the EDM process control is to prevent slag formation.
Due to the superposition of temperatures, the depth of the thermally modified layer created by groups of discharges is always greater than the depth of the thermally modified layer created by a single discharge. Both electrodes accumulate thermal energy, which is next transferred to the dielectric fluid and ultimately leaves the work zone. Up to 80% of the energy supplied by a generator may be lost as heat is transferred to the electrodes and the dielectric fluid.
There is yet another process related to the grouping of discharges. A thin layer of pyro graphite adheres to the surface of an anode where groups of discharge emerge in hot localized regions. Under certain conditions, this layer may drastically reduce the tool wear, especially if the tool is made of copper and graphite. This explains the importance of the initiation of a series of discharges in such a way that prevents slag formation.
We hope that this brief overview of the EDM process is helpful in understanding our work and the capabilities of our products.