ELECTRON BEAM MACHINING (EBM)

ELECTRON BEAM MACHINING

In electron beam machining (EBM), electrons emitted by a hot surface and accelerated by a voltage of 10 to 50 kV are focused to a very small areas on the workpiece. This stream of high energy electrons possess a very high energy density (of the order of 10M kW/mm2) and when this narrow stream strikes the workpiece (by impact), the kinetic energy of the electrons is converted to powerful heat energy which is quite sufficient to melt and vaporize any material. Even though, the electrons can penetrate metals to a depth of only a few atomic layers the electron beam can melt metal to a depth of 25 mm or more.  The electron beam which travels at about half to three-fourth -the velocity of sound is focused on the workpiece by electro-static or electro-magnetic lenses. EBM is done in a high vacuum chamber to eliminate the scattering of the electron beam as it contacts the gas molecules on the workpiece. Figure below shows the schematic view of EBM.

Since   a continuous beam loses considerable   heat   by conduction through the workpiece, a pulsed beam at a frequency of less than 100 cps is used in electron beam machining. This consists of repeatedly striking the electron beam on the workpiece for a few milli-seconds and then turning it off for a certain period of time.

Recent developments have made it possible to machine outside the vacuum chamber. In this arrangement, the necessary vacuum is maintained within the electron gun proper by removing gases as soon as they enter. The fully vacuum system is more costly, but it has the advantage that no contaminating gases are present and the electron gun can be located at a considerable distance from the workpiece. The most commonly used electron-beam power are 1 to 25 kW, current =0.001 A.

EBM  Applications: Electron beam machining   is   mainly used   for micromachining operations on thin materials. These operations include: drilling, perforating, slotting, scribing and engraving. A micromachining technique is called "Electron-beam lithography" is being used in the manufacture of field emission cathodes, integrated circuits and computer memories.

Holes as small, as 0.03 mm (with surface finish of 0.4 to 0.6 microns m) can be drilled with ease.

EBM is being used to drill holes for the following products :

Thousands of holes in turbine blades for transpiration cooling, holes in mixer plates, combustion chamber rings, metering or flow ortifices. EBM is particularly useful for materials with high melting points and low thermal conductivity.

Limitations:-

1. Holes produced in materials of thickness greater than about 0.13 mm, are tapered, with a side wall taper of about 1 to 2 deg, the minimum diameter being at the exit of the hole.
2. The method is quite cumbersome and the equipment is very costly.
3. The metal removal rate is very low.                      
4. At the spot where the electron beam strikes the material, a small amount or recasting and metal spatter can occur. This has to be removed afterwards by abrasive cleaning.