Electron Beam Machining (EBM) Principles

 

The source of energy in electron beam machining is high velocity electrons, which strikes the surface of the workpiece and generate heat. Electrons escapes from the hot surface and a voltage of 50 to 200 kV helps to accelerate them. These high energy electrons possess high energy density generally in the order of 10ˆ4 kW/mm² . Thin and high energy stream strikes the workpiece . As a result the kinetic energy of the electrons are converted to heat energy. This heat energy is more than sufficient to melt and even vaporize any material. Electrons can penetrate only a few atomic layers of the metals and can melt metal up to a depth of 25 mm.  The electron beamtraveling at a speed of ¾ of the velocity of the sound is focused on the material to be machined. To focus the electron beams electro-static or electro magnetic lenses are used. Generally electron beam machining is done in a high vacuum chamber to avoid the unnecessary scattering of the electrons . The following figure shows the schematic diagram of electron beam machining process.

Electron Beam Machining (EBM) Set-up Schematic

 

Schematic illustration of the electron-beam machining process.  Unlike LBM, this process requires a vacuum, so workpiece size is limited to the size of the vacuum chamber.

 

For observing the process of machining an optical viewing system consisting of lens and prism is also incorporated. The beam can be controlled very accurately and focused on a width as small as 0.002 mm. The electrons on impingement over the workpiece heat it up and raise its temperature to a value as high as 5000°C. Due to this the material melts and vaporizes locally.

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.

Advantages

·         Very hard, heat resistant materials could be machined or welded easily

·         No physical or metallurgical damage results in the workpiece.

·         Close dimensional tolerance could be achieved since there is no cutting toolwear.

·         In electron beam welding there is virtually no contamination and close control of penetration is possible.

·         Holes as small as 0.002 mm diameter could be drilled.

 

Disadvantages

·         The equipment costs high and operator of high skill is required for carrying out operations.

·         The power consumption is exceedingly high

·         It is not very suitable for sinking deep holes, if the sides must be parallel. In other words, it is not possible to have perfectly cylindrical deep holes by this method.

·         Unless special care is taken the bottom of a thorough hole would become cone-shaped.

·         It is most suitable for machining operation where much less material is to be removed. The material removal rate being of the order of a fraction of a milligram per sec.

·         The electron beam operation can be carried out only in vacuum.

 

Applications

·         It is used for drilling synthetic jewels in the watch industry.

·         Holes as small as 0.002 mm diameter can be produced in hard synthetic sapphires.

·         Electron beam can be suitably used for welding small pieces of highly reactive and refractory metals.

·         For making fine gas orifices in space nuclear reactors and turbine blades for supersonic aero engines, it is used

·         Wire drawing dies, flow orifices could be produced by this process.

·         Fine copper wire can be welded to in transistors.