INDUSTRIAL MANUFACTURING PROCESS

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Basic principle of Electro-Discharge Machining (EDM)

The Russians first developed EDM into a manufacturing method and it was the Russian scientists B.R. and N.I. Lazerenko who proposed a thermal theory. Under this theory a charge of electrons jumping from a tool and striking a workpiece causes a sudden rise in the workpiece surface temperature in the immediate area of spark impact sufficient to melt and expel a small globule of molten metal from the surface.

 

This process, first known as spark erosion, may be described as the vaporization of workpiece material by means of high-frequency repetitive electric sparks. The mechanism by which the material is removed is thought to be the combined effect of:-

•  The highly concentrated energy of the spark discharge being released as heat, causing fusion, vaporization and ionisation of minute areas of workpiece material, and also creating highly localized thermal "shock" to accelerate material away from the surfaces.

•  The large current density in the spark area producing electro-magnetic forces which lift the molten particles from the parent stock and create an extremely small crater in the workpiece.

•  The pressure within the spark itself which assists in "blowing" metal out of the workpiece.

The tool is normally made the cathode (negative polarity), and the workpiece the anode (positive polarity) of the electric circuit. It is observed that in general, the erosion rate is greater for the conductor connected to the positive terminal. The reason for this is not known, but it is suspected that the electron bombardment of the positive electrode occurs at an earlier moment than the ion bombardment of the negative electrode, causing the rise in temperature at the anode to be greater than at the cathode.