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.