Electron Beam Machining EBM Construction, Operating Principle and Working
Components and working of EBM
Electron-beam machining is a kind of machining process by using high-velocity electron beam. It is similar to the electron beam welding machine. In EBM, electrons are accelerated to high velocity as half of the light speed, then it is impinged to work material surface. When the electron beam impact on the surface of work material, the high kinetic energy of the electron converted in to thermal energy, it is then melt and finally evaporate the metal.The above figure shows the schematic diagram of a typical construction of electron beam machining equipment. EBM equipment consists of the following units
Electron Gun: Electron gun generates and directs the electron beam on the material to be machined.
Electronic system to control beam: EBM gun is a powerful system. The electronic system used to control the size, energy and movement of the electron beam. Electron Gun is usually operated at about 12kW. The power supply generates a high voltage (150kv) to accelerate the electrons.
The important process parameters of EBM are the beam current, lens current, beam deflection signals, standoff distance and duration of the pulse.
Vacuum chamber and high vacuum pump system: The generation of the electron beam and its focusing and material removal all takes place in a closed vacuum chamber. The vacuum reduces the energy loss due to electron- air molecule collision and it protect the rapid oxidation (burning) filament. A vacuum chamber should have at least the volume of 1m3 to avoid/minimise the spatter sticking on chamber walls. Normally diffusion pump is used to create high vacuum.
Movable table: Movable table within the vacuum chamber help to mount and positioning of the workpiece.
Working of electron beam gun
The electron beam gun is used to generate the electron beam of the desired shape and then focus it the desired location. EBM gun is worked in pulse mode. A superheated cathode, usually tungsten filament heated up to 2500°C-3000°C to emits electrons and create electron cloud. Due to the high voltage across the cathode and anode, and repulsive force from the cathode, the generated electron is then accelerated at high velocity towards the anode which attracts the electron. The electron beam is passed through the hole in the anode at a velocity of 2/3 of that of light. The beam is then pass through a kind of switch which generate pulses (bias electrode).The electron beam existed through the anode is then refocus by magnetic/ electrostatic lenses. The focused high-velocity electron beam is pointed the surface of workpiece where the hole is desired. The striking of the electron beam to the material cause rapid melting and evaporation of material. The material heating action is due to the conversation of kinetic energy of beam to heat energy. The beam is switched off when the desired depth of cutting is achieved. The diameter of the hole depends on the cross-sectional area of beam, energy of beam and the voltage employed.
There are two types of EBM (i) Thermal and (ii) Non-thermal
Thermal EBM - The electron beam causes melting and evaporation of material. EBM material removal usually is due to evaporation.
Non-Thermal EBM - The electron beam is used to cause the chemical reaction on the material.