EDM is a modern machining method that uses thermal energy to remove material from a part. This material is removed by locally melting or vaporizing small areas on the surface of the part being machined.
This is done by a series of repeated sparks between the tool (called an electrode) and the workpiece in the presence of a medium fluid.
EDM also has several names, they are: spark erosion, spark machining, wire burning, wire burning, die, or wire burning.
Working Principle of EDM Machining
EDM applies a potential difference, between the tool and the work material. Both the tool and the workpiece must be electrical conductors. The tool and the workpiece are recessed in the medium. Kerosene or deionized water is generally used as the dielectric fluid. Keep a gap between the tool and the workpiece. The establishment of the electric field depends on the applied potential difference and the gap between the tool and the working material.
Usually, the tool takes the negative pole of the power supply, and the working material takes the positive pole of the power supply. When the electric field begins to build up, the free electrons on the tool are subjected to electrostatic forces. If there is a smaller work function or smaller electron bonding energy, the emission of electrons will come from the tool (assuming it is connected to the negative electrode). This electron emission is called cold emission.
Through the dielectric medium, the cold emitted electrons are then accelerated towards the working material. As they gain speed and energy and start moving towards work, collisions occur between electrons and dielectric molecules. This collision may end in the ionization of the dielectric molecules, which depends on the work function or ionization energy of the dielectric molecules, and therefore on the energy of the electrons. Because the electrons are accelerated, more positive ions and electrons are created as a result of the collision.
This cycling process increases the electron and ion concentrations in the dielectric fluid. This happens where there is a spark gap between the tool and the work material. The concentration would be so high that the material present in the channel might be described as "plasma". The resistance of such a plasma channel will be very small. So a lot of electrons will flow from the tool to the workpiece, and ions will suddenly flow from the workpiece to the tool. This movement of electrons is called an avalanche.
This movement of electrons and ions is often seen visually as sparks. So the electricity is dissipated like the thermal energy of the spark. The high-speed electrons then hit the working material and the tool on the ions. The k.e of electrons and ions when they collide with the workpiece surface and the gear surface, respectively, are converted into heat energy or heat flow. This extreme localized heat flow results in an instantaneous limited temperature rise, possibly in excess of 10,000°C.
This localized extreme temperature rise can lead to material removal. Material removal occurs because of instantaneous evaporation of the fabric, but also because of melting. The molten metal is not removed completely, but only partially. When the potential is withdrawn, the plasma channel is no longer sustained. The plasma channel generates pressure or shock waves as it collapses. This will evacuate the molten material to form a crater that clears the material around the site of sparks.
Thus, the formation of shock waves facilitates the removal of EDM fabrics when the plasma channel collapses due to disruption of the applied potential. Generally speaking, work materials are positive and tools are negative. The electrons strike the workpiece, causing a pit to form due to heating and melting and material removal. Also, positive ions hitting the tool cause tool wear.
In EDM, a generator uses voltage pulses between the tool and the workpiece. No constant voltage is applied. In EDM only the spark and not the arc are needed. The arc causes localized material removal at specific points and causes sparks to be distributed across the tool surface. This in turn results in an even distribution of material removal under the tool.