Electron Beam Machining (EBM) – Introduction
Electron beam machining is a thermal process used for metal removal during the machining process. In the electrical beam machining, electrical energy is used to generate the electrons with high energy. In the Electron Beam Machining process, a high velocity focused beam of electrons are used to remove the metal from the workpiece. These electrons are traveling at half the velocity of light i.e., 1.6 x 10∧8 m / s. This process is best suited for the micro-cutting of materials.
In this article we will see the principle of electron beam machining, construction, working, diagram, process parameter, advantages, disadvantages of the EBM process. EBM process has several important applications which explained later on.
Principle of EBM :
When the high-velocity beam of electrons strikes the workpiece, its kinetic energy is converted into heat. This concentrated heat raises the temperature of workpiece material and vaporizes a small amount of it, resulting in the removal of material from the workpiece.
Types of EBM Process :
The following two methods are used in the EBM process.
1. Machining inside the vacuum chamber.
2. Machining outside the vacuum chamber.
Construction and Working of Electron Beam Machining :
(Machining Inside the Vacuum Chamber)
Construction of EBM :
- The schematic arrangement of Electron Beam Machining (EBM) is shown in Fig.
- It consists of an electron gun, diaphragm, focusing lens, deflector coil, work table, etc.
- To avoid collision of accelerated electrons with air molecules, a vacuum is required. So, the entire EBM setup is
enclosed in a vacuum chamber, which carries a vacuum of the order 10-5 to IO-6 mm of mercury. This chamber carries a door, through which the workpiece is placed over the table. The door is then closed and sealed.
- The electron gun is responsible for the emission of electrons, which consists of the following three main parts.
1. Tungsten Filament — which is connected to the negative terminal of the DC power supply and acts as the cathode.
2. Grid cup – which is negatively based concerning the filament.
3. Anode – which is connected to the positive terminal of the DC power supply.
- The focusing lens is used to focus the electrons at a point and reduces the electron beam up to the cross-sectional area of 0.01 to 0.02 mm diameter.
- The electromagnetic deflector coil is used to deflect the electron beam to a different spots on the workpiece. It can also be used to control the path of the cut.
EBM Diagram :
Working of EBM :
- When the high voltage DC source is given to the electron gun, tungsten filament wire gets heated and the temperature raises up to 2500°C.
- Due to this high temperature, electrons are emitted from tungsten filament. These electrons are directed by a grid cup to travel towards downwards and they are attracted by the anode.
- The electrons passing through the anode are accelerated to achieve high velocity as half the velocity of light (i.e., 1.6 x 10 ^8 m /s) by applying 50 to 200 kV at the anode.
- The high velocity of these electrons are maintained until they strike the workpiece. It becomes possible because the electrons
travel through the vacuum.
- This high-velocity electron beam, after leaving the anode, passes through the tungsten diaphragm and then through the electromagnetic focusing lens.
- Focusing lenses are used to focus the electron beam on the desired spot of the workpiece.
- When the electron beam impacts on the workpiece surface, the kinetic energy of high-velocity electrons is immediately converted into the heat energy. This high-intensity heat melts and vaporizes the work material at the spot of beam impact.
- Since the power density is very high (about 6500 billionW/mm ^2), it takes few microseconds to melt and vaporize the material on impact.
- This 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 65,000 microseconds.
- By alternately focusing and turning off the electron beam, the cutting process can be continued as long as it is needed.
- A suitable viewing device is always incorporated with the machine. So, it becomes easy for the operator to observe the progress of the machining operation.
Machining Outside the Vacuum Chamber :
Since the full vacuum system is more costly, the recent development has made it possible to machine outside the vacuum chamber. In this arrangement, the necessary vacuum is maintained within the electron gun and the gases are removed as soon as they enter into the system.
Process Parameters :
The parameters which have a significant influence on the beam intensity and metal removal rate are given below :
1. Control of current. –
2. Control of spot diameter.
3. Control of focal distance of the magnetic lens.
Characteristics Of EBM Processes :
Accelerating voltage : 50 to 200 kV Beam current : 100 to 1000 µA Electron velocity : 1.6 x 10^8 m/s Power density : 6500 billion W/mm^2 Medium : Vacuum (10^-5 to 10^-6 mm of Hg) Workpiece material : All materials Depth of cut : Up to 6.5 mm Material removal rate : Up to 40 mm^3 / s Specific power consumption : 0.5 to 50 kW
Advantages of EBM :
Electron beam machining has the following advantages :
- It is an excellent process for micro finishing (milligram/ s).
- Very small holes can be machined in any type of material to high accuracy.
- Holes of different sizes and shapes can be machined.
- There is no mechanical contact between the tool and the workpiece.
- It is a quicker process. Harder materials can also be machined at a faster rate than conventional machining.
- Electrical conductor materials can be machined
- The physical and metallurgical damage to the workpiece is very less.
- This process can be easily automated.
- Extremely close tolerances are obtained.
- Brittle and fragile materials can be machined.
Disadvantages of EBM: Limitation of EBM
- The metal removal rate is very slow.
- The cost of equipment is very high.
- It is not suitable for large workpieces.
- High skilled operators are required to operate this machine.
- High specific energy consumption.
- A little taper produced on holes.
- Vacuum requirements limit the size of the workpiece.
- It is applicable only for thin materials.
- At the spot where the electron beam strikes the material, a small amount of recasting and metal splash can occur on the surface. It has to be removed afterward by abrasive cleaning.
- It is not suitable for producing perfectly cylindrical deep holes.
Application of EBM :
- EBM is mainly used for micro-machining operations on thin materials. These operations include drilling, perforating, slotting, and scribing, etc.
- Drilling of holes in pressure differential devices used in nuclear reactors, aircraft engines, etc.
- It is used for removing small broken taps from holes.
- Micro-drilling operations (up to 0.002 mm) for thin orifices, dies for wire drawing, parts of electron microscopes, injector nozzles for diesel engines, etc.
- A micromachining technique known as “Electron beam lithography” is being used in the manufacture of field emission cathodes, integrated circuits, and computer memories.
- It is particularly useful for machining of materials of low thermal conductivity and high melting point.
EBM Questions and Answers :
1. State the working principle Of EBM.
When the high-velocity beam of electrons strikes the workpiece, its kinetic energy is converted into heat. This concentrated heat raises the
temperature of work material and vaporizes a small amount of it, resulting in the removal of metal from the workpiece.
2. Explain why the EBM process is performed usually in a vacuum chamber.
1. To avoid collision of accelerated electrons with air molecules.
2. Protect the cathode from chemical contamination and heat losses.
3. The possibility of an arc discharge between the electrons is prevented.
3. Name two methods of focusing the electron beam.
1. Electromagnetic focusing.
2. Electrostatic focusing.
4. 4. Why is the deflection coil provided for electron beam machining?
The electromagnetic deflector coil is used to deflect the electron beam to a different spot on the workpiece. It can also be used to control the path of the cut.
Reference : https://nptel.ac.in/content/storage2/courses/112105127/pdf/LM-40.pdf
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