ELECTRO-DISCHARGE MACHINING (E.D.M.)
38. This type of machining removes metal from th workpiece by converting the kinetic energy of electric sparks into heat as the sparks strike the workpiece.
39. An electric spark results when an electric potential between two conducting surfaces reaches the point at which the accumulation of electrons has acquired sufficient energy to bridge the gap between the two surfaces and complete the circuit. At this point, electrons break through the dielectric medium between the conducting surfaces and, moving from negative (the tool electrode) to positive (the workpiece), strike the latter surface with great energy; fig, 22-13 illustrates a typical spark erosion circuit.
40. When the sparks strike the workpiece, the heat is so intense that the metal to be removed is instan- taneously vaporized with explosive results. Away from the actual centre of the explosion, the metal is torn into fragments which may themselves be melted by the intense heat. The dielectric medium, usually paraffin oil. pumped into the gap between the tool electrode and the workpiece, has the tendency to quench the explosion and to sweep away metallic vapour and molten particles.
41. The amount of work that can be effected in the system is a function of the energy of the individual sparks and the frequency at which they occur. 42. The shape of the tool electrode is a mirror image of the passage to be machined in the workpiece and, to maintain a constant work gap, the electrode is fed into the workpiece as erosion is effected.