Electron Beam Machining
Electron beam machining is a thermal process used for the metal removal during the machining process. In the electrical beam machining, electrical energy is used to generate the electrons with high energy.
Process of Electron Beam Machining Process:Advertisement:
In the electron beam gun the electric beam is generated. Electron beam consists of a small spot size, from which it provides the high velocity electrons. The electron beam machining process is carried out in the vacuum. This is due to the electrons present in the process react with the air molecules so they lose the energy and ability of cutting. The work piece material must be placed under the electron beam, and where the equipment is placed under the vacuum. With the spot size of 10 to 100 , the high energy absorbed electron beam is ready to show impact on the work piece material. The high velocity electron consists of kinetic energy, the energy is converted into the heat energy, where the electrons strikes the work material. Because of the high energy present in the electrons it starts to melting and vaporization of the work piece material. The process is done from top to the bottom of the work piece material. In the electrical beam machining the gun is used in the pulsed mode. By using the single pulse holes, and is drilled on the thin sheets. Multiple passes are required for the thicker plates.
Equipment used in the Electron Beam Machining:
- Cathode, Bias grid, anode, electromagnetic lens, electromagnetic coils, deflector coils, telescope, vacuum gauge, throttle valve, diffusion pump.
- The electron beam machine consists of an electron beam gun used to produce free electrons at the cathode. The high velocity particles are moving through the small spot size. The cathode (tool) is made of tantalum or tungsten material. The cathode filaments are heated to a temperature of 2500to 3000and the heating leads to thermo – ionic emission of electrons. The magnitude varies from the 25 mA to 100 mA. The solidities lies between 5 Ac to 15 Ac .The emission current is influenced by the voltage that is nearly 150kV, and the current is applied between the anode and cathode to release the electrons in the direction of work piece.
- Bias grid: It is also known as grid cup. The grid cup is a negative that is subjected with respect to the filament. So, the electrons generated with the help of the cathode will directly flow towards the anode. During the flow of the electrodes no diversions are seen. The anode attracts the electrons and gets accelerated; the electrons will gain a high velocity.
- The cathode controls the flow of the electrons, and the grid cup used to operate the gun in pulsed mode only.
- After the anode the electron beam passing through the magnetic lens and the apertures are connected in series. The magnetic lens is used to shape the electron beam and reduce the diversion factor.
- The apertures allow the convergent electrons to permit and caught the low energy divergent electrons from the fringes.
- Finally the electron beam passes through the electromagnetic lens and deflection coil. Then the deflection coil sends the electron beam through the hole, to improve the shape to machine a hole.
- The vacuum is created between the work piece and the electron beam gun, and there is a series of rotating disc with slots.
- The disc allows the electron beam to pass over the material for machining, and it prevents from the fumes and vapors generated during the machining.
- Work piece is placed on the CNC bench. Then holes of any shape are made on the work piece material. In the gun beam flection and CNC control are used to shape.
- Vacuum is maintained in gun, and the vacuum ranges from Suitable vacuum is maintained because the electron as it does not lose their energy, and where the life of the cathode is obtained. By using the diffusion pump and rotary pump the vacuum is maintained.
- Diffusion pump should act as an oil heater. If the oil is heated then the oil vapor rushes upwards. The nozzle changes the direction of the oil vapor and starts moving in the downward direction at high velocity. The oil vapors are reduces in the diffusion pump; this is because of the presence of the cooling water cover.
Parameters in the Electron discharge machining:
- We already know that the electron gun is works within the pulse mode. The bias grid is located after the cathode. Then pulse is given to the grid cup, where the pulse duration ranges from 50to 15 ms.
- Beam current is related to the electrons that are emitted from the cathode or available in the beam. Beam current is ranges from the 200micro amps to 1 amp. If the beam current increases, simultaneously there is also an increase in the energy per pulse. High pulse energy is used to machine thicker plates and make the holes larger.
- The power and energy density is ruled by the energy per pulse and the nozzle spot size. With the help of the electromagnetic lens the spot size is controlled. For lower spot size they require a high energy density. The metal removal must be high; this is when compared to the holes size where the hole must be similar.
- The plane of focusing must be above or below the surface of the work piece material.
Capability of Electron Beam machining process
- Electrical beam machining makes a hole ranges from 100 to 2 mm.
- The depth of cut must be 15 mm with a length to diameter ratio of nearly 10.
- Holes can be elongated along with the barrel shape or depth.
- Reverse tapper can also be performed below the surface of the work piece material.
- In the electron discharge machining Cut formation is not observed
- With the help of the electron discharge machining we can machine the wide range of materials like stainless steel, aluminum, steel, plastics, ceramics etc.
- In EBM the heat affected zone is narrow; this is because of the short pulse occurrence. The heat affected zone is nearly 20 to 30
- Compares to the steels aluminum and titanium is freely machined.
- Based upon the type of the material, power density, depth of cut holes diameter, which are the reasons for the number of holes drilled per second on the material.
- The EBM does not apply any cutting forces on the material.
- During the process very simple investment is required for work
- EBM process allows machining of brittle and fragile materials.
- Holes are drilled at an angle of 20 to 30
Advantages of Electron beam machining
- There is no contact between the tool material and work piece material
- Very small holes are also machined on different type of work piece materials with high accuracy
- Drilling is also done on the work piece material with a diameter of nearly 0.002 inches
- Drilling parameters are changed automatically during the machining
- Distortions are not observed to the work piece material
- This process is proficient in attaining high accuracy along with repeatability.
- Compare with the other process, formation of holes is easy with the other process.
- The cost of the equipment is very high
- Metal removal rate during the process is low
- Small cut operations are performed on the work piece material with the help of EBM machine
- Vacuum requirements boundaries the dimensions of the work piece material
- Need for secondary backing materials.
Applications of EBM:
With the help of the different pressure, holes are drilled in the air craft engine, nuclear reactor etc.
We can see small cross sectional area in the wire drawing die-machines.
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