During years, it has been commonly stated that due to the fact that EDM does not require mechanical contact between tool and workpiece, the process can be regarded as “force-free”. However, measurements of EDM discharge forces have shown that their magnitudes induce tool vibration and geometrical inaccuracy of the manufactured cavities, depending on the chosen process parameters. Unexpected failure and plastic deformation of Sinking EDM graphite electrodes during the manufacturing of cavities with an aspect ratio in the order of 35 - 40 have been reported, but current knowledge of EDM does not allow to accurately explain such events. This work addresses the scientific study of the physical causes of discharge forces on Sinking EDM with high aspect ratio electrodes in order to generate a clear relationship between technological process parameters and tool electrode vibration induced by discharge forces.
A method for the measurement and quantitative analysis of EDM single discharge forces based on the simultaneous use of piezoelectric force sensors and high speed camera imaging is presented here. It can be stated that discharge forces do not only depend on the action of the pressure of the plasma and of the gas bubble on the surface of the electrodes. An additional force transmitting phenomenon explained by the action of the expanding shock wave generated at the beginning of the discharge has been analyzed and demonstrated. Dynamical pressure fields acting on the electrode surfaces around the machining area can explain the magnitude and time-based behaviour of the measured discharge forces.
Deflection measurements of slender graphite electrodes as an effect of continuous discharge forces in Sinking EDM are presented within this work. Electrode deflection has been found to depend not only on the magnitude of the discharge forces, but on the frequency of the EDM discharge as well. An analytic model for the dynamic vibration of slender electrodes based on the Timoshenko beam theory has been developed and applied with the use of FEM methods. Analytical results have been validated against experimental measurements and it has been shown that it is possible to find combinations of process parameters that generate the lowest possible deflection amplitude on slender electrodes, in spite of the ever present discharge forces during the process, thus reducing the chance for electrode failure and geometrical inaccuracy.