Electron Beam Machining ( EBM )-
What is Electron Beam Machining? What is its working principle? What are its merits and demerits?
The principle elements of electron beam machining is shown in figure. In this process, machining is performed with the help of a high velocity electron beam. This electron beam is allowed to focus on a work piece that is held inside a vacuum chamber (i.e. consists vacuum of the order 10-5 mm of Hg). Vacuum chamber is employed to prevent from scattering when an electron collides with the gas molecules. The chamber comprises a door to place the work piece on the table and for safety.
A tram of electrons is emitted from an electron gun which is categorized into three components,
explained as follows:
1 Tungsten filament
A tungsten filament connected to negative terminal of D.C power supply acts as cathode. This filament wire is heated to a temperature of about 2500 degrees Centigrade in vacuum which emits a high negative potential electrons (i.e. , a cloud of electrons)
2 Grid Cup
A grid cup is at a negative potential with respect to filament. It the cloud of electrons to travel downwards.
The anode is connected to the positive terminal of the power supply. The electrons are attached and accelerated obtain high velocity i.e., around two-third of light. This velocity is attained until the electron beam strikes the work piece.
After leaving from anode, a high velocity of electron beam passes through tungsten diaphragm and focusing lens.
Finally, an electron beam passes a deflector coil, where it deflects the beam on to the work piece so as to perform cutting operation. The kinetic energy produced by an electron beam is released and gets converted into heat energy. This high intensity of “heat results in melting and vaporising the work piece material at the spot of beam impingement. Thus, the operation continues by alternative focusing and turning off the beam to obtain the desired geometry of work piece.
Theory of Electron Beam Machining-
It is a metal removing process, in which a high velocity electron beam is used for machining. The machining is processed by converting the kinetic energy of electrons to heat energy. When the beam strikes the component which is placed in vacuum, then the material gets melted and vapourized due to heat. The diameter of the electron beam should be slightly smaller than the required diameter of hole. An organic backing plate is placed on the exit side of the hole.
After the through hole, the beam slightly penetrates the backing plate. A small hole is also produced in the plate material which gets melted and vaporizes due to high pressure. The metal is expelled from the hole along with the vaporized metal. This machining process is commonly used for different shaped holes in metals, ceramics, plastics, etc. Electrically good and bad conducting materials can also be processed. The process can also be automated by the use of computer controls.
Process Parameters in Electron Beam Machining –
The important process parameters involved in Electron Beam Machining are as follows:
1 Beam current
2 Pulse duration
3 Lens current
4 Shape of beam
1 Beam Current-
The pulse or energy supplied to the component is regulated by-the beam current. The machining time for drilling hole mainly depends upon the energy or pulse supplied. The beam current for an Electron Beam Machining process varies from 100 micro A to I A.
2 Pulse Duration –
Duration of pulse to be supplied depends upon the depth and diameter of hole to be produced. Deep and wider holes can be drilled using long pulse duration. In an EBM process, it varies from 0.05 micro seconds to 10 micro seconds.
3 Lens Current-
The distance between the focal point and the electron beam gun is determined by the lens current. It can also be used for determining the diameter of focal beam.
4 Shape of Beam-
The shape of the beam usually depends upon the type of hole (straight, tapered, etc.,) to be drilled. Desired shape of the hole is obtained by proper positioning of the focal point below the top surface of the component.
Characteristics or EBM Process-
The significant characteristics of an EBM process are as follows:
1 Electrically conducting and non-conducting materials can be easily machined.
Examples: Aluminum, copper, nickel, ceramics, plastics, leather, etc.
2 Thin, brittle and low strength component materials can also be machined.
3 The physical, mechanical and metallurgical properties of components do not affect the metal removal rate.
4 Relatively very small diameter holes with large depth can be obtained.
5 Inclined or tapered holes can also be produced.
6 Since there are no mechanical forces, the component is free from distortion.
7 The thermal properties of component and pulse energy determines the surface finish of the edge produced
8 The required voltage for an EBM process is 150 kV.
9 The power density required for the process is 6500 GW/mm^2.
10 The specific power consumed by the equipment is 500 W/mm^3/min.
11 A high vacuum medium is required of about 10^-5 mm of Hg.
Merits of Electron Beam Machining (EBM) –
1 Heat impingement can be focused on the specified spot where material has to be removed.
2 It provides excellent techniques for micro machining.
3 There is no physical contact-between the fool and the work piece.
4 Dimensional accuracy is good.
5 Tool wear is non-existent.
6 No physical or metallurgical damage to the work piece.
7 Machining of any material is possible.
Demerits of Electron Beam Machining (EBM)-
1 Initial investment cost is high.
2 Highly experienced and’ skilled operators are needed to perform the operation.
3 This process is not applicable for producing perfect cylindrical deep holes.
4 Metal removal rate is very low.
5 Power consumption is high.
6 Limited size of work piece, due to lack of space in vacuum chamber.
7 This process is limited to small and fine cuts.
8 Difficulty in producing slots and holes of very close dimensional accuracy.