How Hydrogen Dielectric Strength Forces the Work Voltage in the Electric Discharge Machining

Abstract

An electro-thermal model based on the Joule heating effect is proposed to simulate a single discharge in an electric discharge machining process. Normally, the dielectric strength of the hydrocarbons oil is approximately 20 MV/m, but it varies with both the thickness of the film and its decomposition. After the breakdown, the hydrocarbon oil has an average dielectric strength value of 2 MV/m. This value is close to the dielectric strength of the hydrogen, which is the main gas that results from the hydrocarbon oil decomposition, at temperatures between 6000 K and 9000 K. Therefore, the electric discharge occurs in a hydrogen atmosphere that imposes both the discharge gap and the work voltage. A 200 V voltage is associated to a 100 μm discharge gap, leading to a 20 V work voltage. Therefore, the 3 V work voltage control corresponds to approximately 15 μm. In other words, the increase of the discharge gap originates other discharge during the discharge pulse. The work voltage control, together with the multiple discharge method, is taken into account. The 100 μm discharge gap corresponds to the higher value of the transitory discharge gap that over evaluates the material removal and the tool wear rates. The results of the numerical simulations are validated with experimental data.

Modeling of Material Removal Rate and Surface Roughness Generated during Electro-Discharge Machining

“…Compared to the standard FEM model, the FEM model with instantaneous evaporation had 1.5% better agreement with the experimental values. Figure 10 shows a comparison of the model with numerical and experimental results from Almacihna et al [22]. For discharge energies of 420, 660, and 1340 mJ, Almacinha provided two sets of data; lower values of MRR correspond to experiments without flushing while higher values were experiments when flushing was activated.…”

“…The deviation between our model and Salonitis model and experimental values were 6.4% and 7.5%, respectively. Another comparison of surface roughness obtained from the model with experimental values [22] are shown in Table 3. The model predicts the increasing trend of surface roughness with an increase in discharge energy.…”

“…Shankar et al [21] numerically analyzed the EDM process by solving for the electrical potential and heat transfer in the EDM process. Electro-thermal models based on the Joule effect were developed by Marofona et al and Almacinha et al [20,22]. Kansal et al [23] performed simulations on powder-mixed EDM using Ansys finite element software.…”

“…Required time for breakdown of dielectric fluid was considered equal to 500 ns as suggested by Almacinha et al [22] The only distinction in assumptions of both set of simulations was that the calculation of the surface roughness considered the residual heat transfer from previous discharges in the formation of the next crater.…”

“…Performance parameters of MRR simulation. Comparison of MRR obtained from our numerical model with experimental values from Almacinha[22] (discharge voltage 20 V).…”