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Short notes for GATE Mechanical Engineering - Electron Beam Machining

Continuing our series of short notes on Unconventional Machining for GATE Mechanical Engineering, here is the second process Electron Beam Machining (EBM).

Electron Beam Machining (EBM):

EBM is a process for machining materials with the use of high velocity beam of electrons. The workpiece is held in a vacuum chamber and the electron beam is focussed on to it magnetically. As the electrons strike the workpiece, their KE is converted into heat. This concentrated heat raises the temperature of workpiece and vaporises a small amount of its material. The reason for using a vacuum chamber is that, if otherwise the beam electrons will collide with gas molecules and will scatter. 

The complete EBM setup is enclosed in a vacuum chamber, which carries vacuum of the order of 10-5 mm of Hg. This chamber has a door, through which workpiece can be placed on the table. The door is then closed and sealed. The electron gun, which is mainly responsible for emission of electrons, consists of three main parts:

  1. A tungsten filament
  2. The grid cup
  3. The anode

The filament is connected to the negative terminal of the DC Power Supply which acts as cathode. The filament wire is heated to a temperature of about 2500 degrees in the vacuum. With the result, a cloud of electrons is emitted by the filament, which is directed by the grid cup to travel downwards. As the electrons are attracted by the anode, they pass through it aperture in the form of a controlled beam without colliding with it. 

A potential difference of 50 to 150 kV is maintained between the filament and the anode. As such, the electrons passing through the anode are accelerated to achieve as high a velocity as around two third of light. 

This high velocity electron stream, after leaving the anode, passes through the tungsten diaphragm and then through the electromagnetic focusing coils. The electromagnetic deflector coil the deflects the aligned beam on-to the work, through which the path of cut can be controlled. The high velocity beam impinges on the workpiece, where its KE is released and gets converted into heat energy. The high intensity heat so produced, melts and the vaporises the work material at the spot of beam impingement. Adequate vacuum is needed to be maintained inside the chamber so that the electrons can travel from cathode to anode without any hindrance. 

Advantages of EBM:

  • Any material can be machined
  • Workpiece is not subjected to any physical or metallurgical damage
  • Problem of tool wear is non existent. So, close dimensional tolerances can be achieved
  • Heat can be concentrated on a particular spot
  • An excellent technique for micro machining
  • There is no contact between the work and tool

Disadvantages of EBM:

  • High initial investment needed
  • High skill level of operator needed
  • Not suitable for machining perfectly cylindrical holes
  • It is suitable for small and fine cuts only
  • Workpiece size is limited due to vacuum requirement in the chamber
  • Material removal rate is low
  • High power consumption
  • It is difficult to produce slots and holes of uniform shapes and dimensions

Applications of EBM:

  • Very effective for machining of materials of low heat conductivity and high melting point
  • Micro machining operations on work piece of thin sections
  • Micro drilling operations (upto 0.002 mm) for thin orifices, dies for wire drawing, parts of electron microscopes, firbre spinners, injector nozzles for diesel engines etc.

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