In this post, you learn what is electron beam machining, how it’s done, Its working principle, advantages, accuracy and limitations.
Electron Beam Machining
Electron beam machining is the metal removal process by a high-velocity focussed stream of electrons which heats, melts and vaporizes the work material at the point of bombardment.
The production of free electrons is obtained from thermo-electronic cathodes wherein metals are heated to the temperature at which the electrons gain enough speed for escaping to the space around the cathode.
The acceleration of the electrons is carried by an electric field while the focussing and concentration are done by controlled magnetic fields.
The kinetic energy of a beam of free electrons is transformed into heat energy as a result of the interaction of the electrons with the workpiece material. EMB is, therefore, a thermo-electric process.
Working Principle of Electron Beam Machining
The figure shows the principle of operation of electron-beam machining. A beam of electrons is emitted from the electron gun which is a triod consisting of:
- A cathode is a hot tungsten filament (2500°C) emitting high -ve potential electrons.
- The grid cup, negatively based on the Filament.
- An anode which is kept at ground potential, and through which the high-velocity electrons pass.
- A gun is provided with an electric current from a high voltage dc source.
- The flow of electrons is regulated by the -ve bias applied to the grid cup.
- Electrons passing through the anode are accelerated to two- thirds of the velocity of light by applying 50 to 150 kV at the anode, and this speed is maintained till they strike the workpiece.
- Due to the pattern of the electrostatic field produced by the grid cup, the electrons are focussed and made to flow in the form of a converging beam through a hole in the anode.
- A magnetic deflection coil is used to make the electron beam circular having a cross-sectional diameter of 0.01 to 0.02 mm and deflect it anywhere.
- The built-in microscope with a magnification of 40 on the workpiece enables the operator to accurately locate the beam impact and observe the actual machining operation.
- As the beam impacts on the workpiece surface. the kinetic energy of high-velocity electrons is immediately converted into the thermal energy and it vaporizes the material at the spot of its impact.
- Power density is very high (about 1.5 billion W/cm’) it takes a few microseconds to melt and vaporize the material on impact.
- The process is carried out in repeated pulses of short duration.
- The pulse frequency may range from 1 to 16,000 Hz and duration may range from 4 to 64,000 microseconds.
Tolerances are about 10% of hole diameter or slot width. The taper of about 4° included angle is present in slots and holes and this limits the depth-to-width ratio. A depth to diameter ratio can reach 20:1 with multiple pulses.
Heat affected zones of up to 0.03 mm deep have been observed. The stock removal rate is generally in the region of 1.5 mm/s with a penetration rate of about 0.25 mm/s or faster.
Design Consideration of Electron Beam Machining
- EBM can produce holes of diameter in the range of 100 μm to 2mm with a depth up to 15mm, i.e., with length/diameter ratio of around 10.
- Burr formation doesn’t occur in electron beam machining
- The heat-affected zone is narrow due to the shorter pulse duration in EBM.
- Many holes drilled per second depending on the diameter of the hole, power density and depth of the hole. as well as material type.
- Electron beam machining does not apply cutting force on the workpiece.
Applications of EBM
Some typical applications of the process are:
- Drill fine gas orifices, less than 0.002 mm, in space nuclear reactors, turbine blades for supersonic aero-engines.
- To produce wire drawing dies, light-ray orifices and spinnerets to produce synthetic fibres.
- Produce metering holes in injector nozzles in diesel engines, etc.
- To scribe thin films.
- To remove small broken taps from holes.
Advantages and limitations of EBM
- EBM is an excellent method for micro-finishing. It can drill holes or cut slots which otherwise cannot be made.
- It is possible to cut any known material, metal or nonmetal that can exist in a vacuum.
- No cutting tool pressure or wear. Distortion-free machining having precise dimensions can be achieved.
- The biggest disadvantage is the high equipment cost and employment of high skill operator.
- Besides, only small cuts are possible. Further, the need of vacuum restricts the size of specimens that can be machined.
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