Electron Beam Machining – Parts, Working, Advantages, Disadvantages and Applications

Electron Beam Machining (EBM) is a machining process in which material is removed from the workpiece by focusing a beam of high-velocity electrons towards the workpiece.
The source of energy in electron beam machining is high velocity electrons. A high voltage DC power supply is used to generate electrons with high energy in this process.
It is a non-traditional machining process for material removal. Generally, electron beam machining is carried out in a vacuum chamber for removing unnecessary scattering of electrons and for making the process more efficient.
This process is best suited for the micro-cutting of metals. It can also be used for boring of wide range of metals.

Working Principle of EBM:

The EBM works on the principle that when a high beam of electron strikes a workpiece, the kinetic energy of the electrons is converted into heat energy and this concentrated heat energy raises the temperature of the part of the workpiece at which the electron beam strikes and that part of the workpiece are vaporized and removed from the workpiece. In this way, the material is removed from using the EBM process.

Electron Beam Machining
Electron Beam Machining

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Parts of Electron Beam Machining:

Electron gun:
It is the main part of electron beam machining. It generates the beam of an electron which is further used to remove material from the workpiece. This electron gun contains tungsten or tantalum filament which acts as cathode.

Bias Grid:
This bias grid is used to control the flow of electrons which is generated by an electron gun.

Anode:
This anode is used to accelerate the electrons to very high velocity.

Magnetic Lens:
This magnetic lens is made up of the magnet. The main function of this magnetic lens is the same as that of an optical lens that is to concentrate the beam of electrons.

Aperture:
This aperture is similar to the aperture of the camera but the purpose of this aperture is a little different from the aperture of the camera.This aperture is used to capture the stray electrons so that only focused and concentrated beam of electrons beam passes through the aperture.

Electromagnetic Lens:
The electromagnetic lens is finally used to focus the electrons beam on the workpiece.

Diffusion Pump:
The diffusion pump is used for maintaining the vacuum within the electron beam chamber. The level of vacuum in this chamber is from 10 to the power minus 4 to 10 to the power minus 6 torque.

Deflector Coil:
This deflector coil is used to deflect the electron beam by a small amount in case a proper hole is not being created by an electron beam.

Optical Viewing System:
Optical Viewing System is used by the operator to check whether the process is under control or not.
This optical viewing system consists of a telescope and illumination system.

Slotted Disc:
This spotted disc is used to remove the vapour and fumes created while machining the workpiece using electron beam machining so that this vapour and fumes do not obstruct the optical windows if the electron beam gun. This slotted disc is synchronized with the electron beam.

Working of Electron Beam Machining:

  • At first, a vacuum is created and maintained using the diffusion pump within the electron beam chamber.
  • After that, a potential difference is applied across the tungsten filament so that the filament temperature goes around 2500°C. This tungsten filament acts as a cathode as the negative terminal of the DC power supply is connected to it.
  • At such high temperature of 2500 °C and under vacuum, there would be the emission of thermo-ionic electrons from the tungsten filament. As the cathode is negatively biased, the electron emitted from the cathode is
  • repelled and move away from it.
  • After the emission of electrons from the electron gun, the electrons passed through the bias grid which controls the flow of the electrons. This bias grid is also negatively biased so that the electrons do no get collected on it.
  • After the electrons pass the bias grid, the electrons reach the anode and as the electrons are negatively charged and there is a potential difference between the anode and the cathode, the electrons are accelerated as they pass the anode. When the electrons pass through this anode section, the electrons attain a velocity which is nearly half the velocity of light.
  • After passing the anode the electrons pass through the magnetic lens. As the beam passes through the magnetic lens, the beam of electrons gets much more concentrated and focused than before.
  • Some of the electrons diverge from their way and the rest of the electrons move toward the workpiece as a beam. The electrons which are diverged in the way are called stray electrons.
  • After passing through the magnetic lens, the electron beam passes through the aperture. Whenever the stray electrons are available near the aperture, the aperture captures those stray electrons and only focused and concentrated beam of electrons passes through this aperture. As the beam emerges from this section, the beam becomes very concentrated and focused and there are no stray electrons.
  • After that, an electromagnetic lens finally focuses the electron beam on the workpiece.
  • Before reaching the workpiece, the electron passes through deflector coil which can deflect the beam by a small amount if needed. This deflector coil is only used when we are not getting a proper hole or to improve the shape of the machined holes.
  • The highly focused electron beam is made to impinge on the workpiece with a spot size of 10-100 μm.
  • A telescope and illumination system is used to check whether the beam is rightly placed or not and if not, the deflector coil is used to properly align the electron beam.
  • As the electrons strike the workpiece high velocity or kinetic energy of electrons is converted into heat energy and due to high power density, the workpiece starts melting and vaporizing instantly.
  • In electron beam gun is operated in pulse mode. A single pulse is needed to drill a hole in a thin sheet. For thicker sheets, multiple pulses would be required.
  • If the vacuum is not used in this process than the electrons would not emit from the cathode and even if the emission happens the electron would not achieve that acceleration due to collision with the air molecules.

Process Parameters:

  • Accelerating Voltage (Va) – 100 KV
  • Beam Current (Ib) – 250 μA to 1A.
  • Pulse duration (ton) – 50 μs to 50 ms.
  • Energy per pulse – 100 J/Pulse
  • Spot Size – 10 μm to 100 μm.

Advantages:

1 This process provides very high drilling rates when small holes with large ratios are to be drilled.
2) It can be used to machine almost any material irrespective of their mechanical properties.
3) The heat-affected zone is very less in this process due to shorter pulses.
4) As in this process, no mechanical cutting force applies therefore holding and fixing cost is very less.
5) This process can be used to create holes of any shape with high accuracy by combining it with the CNC table.
6) This process can be automated easily.

Disadvantages:

1) The primary limitations are the high capital cost of the equipment.
2) It requires regular maintenance of the equipment using a vacuum system.
3) Racast layer formation takes place in this process.
4) Many non-productive pumps are used for attaining the desired vacuum.
5) If not properly handled the bottom of through hole would become cone-shaped.
6) The power consumption of this process is very high.
7) The material removal rate of this process is very low.
8) Skilled operators are required to operate this machine.

Applications:

1) EBM process is used to remove small broken taps from holes.
2) It is also used for making fine gas orifices in space nuclear reactors.
3) Used for making turbine blades for supersonic aero engines.
4) It is also used to manufacture field emission cathodes, integrated circuits, and computer memories.
5) Used for making drawing dies and flow orifices.
6) Used for frilling synthetic jewels in the watch industry.
7) Used for micro-machining of thin materials. drilling, perforating, slotting and scribing are some of the micro-machining operations that can be done using ultrasonic machining.
8) Used for welding small pieces of highly reactive and refractory metals.

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