The process has been explained by different theories, being the thermo-electric model the one that best fits the practical experiments [1]. The next figures represent the different steps described by the mentioned theory for a spark cycle Werner [6], Houman [7] y Poco Graphite [8].

The charge loaded electrode approaches the surface of the component, which is laded with the opposite charge. In between both electrodes there is an isolating fluid, referred as dielectric fluid. Despite being an electric insulator, a large voltage difference can produce the dielectric breakage, producing ionic fragments that make possible the electric current to jump between the electrode and the workpiece. The presence of metallic particles suspended in the dielectric fluid can be good for the electricity transfer in two different ways: on one side, the particles are good to ionise the dielectric and, what is more, they can provide the electric charge; on the other side, the particles can catalyze the dielectric breakage. For that reason, the electric field is larger in that position in which the electrode and the workpiece are closer. In this process stage the situation can be represented in fig. 3.

In the next stage (figure 4), as the number of ionic particles in the dielectric increases, the isolating capabilities of the dielectric fluid drop in a narrow channel that appears in that position in which the electric field is larger. At the same time, the voltage difference presents the highest value, being the current still zero.

Figure 4. 2nd stage of the EDM cycle

Just as represented in fig. 5, the current passes by the dielectric fluid that doesn’t play as insulator. As the current passes, the voltage decays.

Figure 5. 3rd phase of the EDM cycle

Fig. 6 depicts the next stage, the heat produced in the area increases fast as the current increases and the voltage difference decreases. The generated heat ablates part of the dielectric fluid, the part and the electrode, creating a discharge channel between the electrode and the part.

A vapour bubble (fig. 7) is produced and expands against the ions entering the discharge channel. The ions are attracted by the intense electromagnetic field arising during the discharge. At the same time, the current increases as the voltage drops.

By the end of the pulse (fig. 8), the electric current and the voltage achieve an equilibrium, while the produced heat and pressure reach their maximum value. At the same time, the part material is removed. The material layer just under the discharge is melted but remains in the same position due to the bubble pressure. The discharge channel consists of a plasma formed out of part, dielectric and electrode material.

--{PS..41}--> Figure 8. 6th phase of the EDM cycle

When the pause time between consecutive discharges starts (fig. 9), the electric current and the voltage drop to zero. The temperature decreases fast, collapsing the vapour bubble and producing the ejection of the melted material.

Figure 9. 7th phase of the EDM cycle

In the next phase (fig. 10), new dielectric flushes into the area, cleaning and cooling the part surface. The material which was melted but was not ejected, is solidified producing a recast layer.

Figure 10. 8th phase of the EDM cycle

In the last phase (fig. 11), the ejected material creates small spheres dispersed in the dielectric, some electrode particles and vapour that goes to the dielectric surface. For a short pause time the melted material and electrode would accumulate making the spark to become unstable and producing electric arcs that would damage the electrode and the part. Concerning the dimensions of the produced particles, the experimental studies indicate that they follow a normal statistical distribution, with values fitting the theoretical model proposed by Rajurkar [9].

Some other jobs, like Schumacher [10], propose that gap contamination has influence on the process ignition, spark location and gap width.

All the sequence is repeated at a rate circa 250000 times per second but, in an instance only one cycle can be present.

Figure 11. 9th phase of the EDM cycle

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