ELECTRON BEAM MACHINING (EBM)
Electron beam machining is a process of metal removal with the use of a high velocity beam of electrons focused magnetically on a very small area to heat, melt and vaporize a small portion of the workpiece material at the point of striking of the beam on the workpiece. The process is best suited for micro-cutting of materials (in mg/s). As the electrons strike the workpiece, their kinetic energy is converted into heat. This concentrated heat raises the temperature of the workpiece material and vaporizes a small amount of it, resulting in the removal of the metal from the workpiece.
The EBM system (Fig.) comprises an electron beam gun with high voltage (150 kV or more) DC power supply, vacuum chamber and vacuum pumping system to achieve vacuum of the order of 10 mm of mercury, a beam aligning system and mechanical tooling and fixtures. When tungsten filament of the electron beam gun is electrically heated in vacuum (to avoid its oxidation) to 2000°C, it emits electrons (carrying -ve charge).
These are repelled by cathode grid cup and are made to pass through a central hole in the anode (+ve charge) in the form of a converging beam. Both the cathode and the anode help in propelling and accelerating the electrons because of the tremendous difference of potential between the cathode and the anode.
Electrons achieve a very high velocity after passing through the anode, about two-thirds of the light. This high velocity of electrons is maintained till such time they strike the workpiece. The electron beam is then focused by means of an electromagnetic focusing coil (lens) which concentrates or spreads the electron beam to the user’s needs. The electron gun may be low voltage or high voltage type. The high voltage electron gun produces hard X-rays also as the electrons bombard the workpiece and hence require extensive screening for the operator’s protection.
The reason for using a vacuum chamber is that, if otherwise, beam electrons will collide with air or gas molecules in the chamber and will scatter.
The high velocity electron stream, after leaving the anode, passes through the positioning tungsten diaphragm and then through the electromagnetic focusing coils (or focusing lens) which manages to position and focus the beam precisely on to the desired spot on the workpiece. The electromagnetic deflector coil then deflects this aligned beam on to the work through which the path of the cut can be controlled. The magnetic deflection coil also makes the electron beam circular to have a cross-sectional diameter of 0.01 to 0.02 mm and is capable of deflecting the beam anywhere on the work.
The power density is very high, about 1.5 billions W/cm³. Pulse frequency may be up to 16,000 Hz. Work table can be adjusted or moved to feed the work to the electron beam as needed. By alternatively focusing and turning off the beam, cutting can be continued as long as needed. Melting and vaporizing of work materials take only a fraction of second but turning off the beam is desirable to conduct away the heat from the workpiece. The cutting operation is seen with a microscope which also helps the operator in locating the beam on the work as required.
The depth-to-diameter ratio in EBM can reach up to 20:1 with multiple pulses. Stock removal rate is about 15 mm³/s with a penetration rate of 0.25 mm/s or more.
EBM is used for drilling and making slots of very small size, drilling of fine gas orifices (less than 0.002 mm) in space nuclear reactors, turbine blade for supersonic acro-engines.
- Making wire drawing dies, light-ray orifices and spinnerets to produce synthetic fibres.
- Making metering holes in injector nozzles for diesel engines.
- Removing small broken taps from holes.
Advantages and limitations:
It is possible to cut any material, metal or non-metal that can exist in vacuum. Since there is no tool pressure during cutting, distortion-free machining is obtained. The equipment cost is very high and only small cuts are possible. The use of vacuum puts limit on the size of the workpiece.
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