Electron Beam Machining (EBM) – Process, Diagram, Advantages and Disadvantages

ELECTRON BEAM MACHINING (EBM) – PROCESS, DIAGRAM, ADVANTAGES AND DISADVANTAGES | We, Mechanical Farm provide study materials and the most common Mechanical Engineering Interview Questions and Answers for students and job seekers to help them clear the Exams, interviews and get the dream job.

WHAT IS ELECTRON BEAM MACHINING (EBM)?

Electron beam machining is one of the examples of non-conventional machining under the thermal process category, along with laser beam machining, electric discharge machining, and plasma cutting. This process uses high velocity electrons for material removal, which is done primarily by melting and rapid vaporization due to intense heating by the electrons.

Almost all materials can be handled using electron beam machining, which is one of many benefits coming from this technique. This method is also very fit for micromachining, with the jobs of drilling fine holes, cutting narrow slots, and cutting contours in sheets. However, the high capital cost of equipment and the mandatory requirement of the high-skilled operators of the system should be taken into consideration before installment.

ELECTRON BEAM MACHINING PROCESS

• The electron beam is generated in an electron beam gun.
• Electron beam gun provides high-velocity electrons over a very small spot size.
• Electron Beam Machining is required to be carried out in a vacuum.
• Otherwise, the electrons would interact with the air molecules, thus they would lose their energy and cutting ability.
• Thus the workpiece to be machined is located under the electron beam and held in vacuum chamber.
• The high-energy focused electron beam is made to impinge on the workpiece with a spot size of 10 – 100 μm.
• The kinetic energy of the high-velocity electrons is converted to heat energy as the electrons strike the work material.
• Due to high power density instant melting and vaporization starts and “melt – vaporization” front gradually progresses

• Finally the molten material, if any at the top of the front, is expelled from the cutting zone by the high vapour pressure at the lower part.

• Unlike in Electron Beam Welding, the gun in EBM is used in pulsed mode.
• Holes can be drilled in thin sheets using a single pulse.
• For thicker plates, multiple pulses would be required.
• Electron beam can also be maneuvered using the electromagnetic deflection coils for drilling holes of any shape.

ELECTRON BEAM GUN

• The basic functions of an electron beam gun are to generate free electrons at the cathode, accelerate them to a sufficiently high velocity, and focus them over a small spot size. Further, the beam needs to be maneuvered if required by the gun.
• The cathode is generally made of tungsten or tantalum. Such cathode filaments are heated, often inductively, to a temperature of around 2500⁰C.
• Such heating leads to thermo-ionic emission of electrons, which is further enhanced by maintaining a very low vacuum within the chamber of the electron beam gun.
• Moreover, this cathode cartridge is highly negatively biased so that the thermo-ionic electrons are strongly repelled away from the cathode.
• This cathode is often in the form of a cartridge so that it can be changed very quickly to reduce downtime in case of failure.

• Just after the cathode, there is an annular bias grid. A high negative bias is applied to this grid so that the electrons generated by this cathode do not diverge and approach the next element, the annular anode, in the form of a beam.
• The annular anode now attracts the electron beam and gradually gets accelerated. As they leave the anode section, the electrons may achieve a velocity as high as half the velocity of light.
• The nature of biasing just after the cathode controls the flow of electrons and the biased grid is used as a switch to operate the electron beam gun in pulsed mode.
• After the anode, the electron beam passes through a series of magnetic lenses and apertures. The magnetic lenses shape the beam and try to reduce the divergence.
•  Apertures on the other hand allow only the convergent electrons to pass and capture the divergent low energy electrons from the fringes.
• This way, the aperture and the magnetic lenses improve the quality of the electron beam.
• Then the electron beam passes through the final section of the electromagnetic lens and deflection coil.
• The electromagnetic lens focuses the electron beam to a desired spot.
• The deflection coil can maneuver the electron beam, though by small amount, to improve shape of the machined holes.
• Generally in between the electron beam gun and the work piece, which is also under vacuum, there would be a series of slotted rotating discs. Such discs allow the electron beam to pass and machine materials but helpfully prevent metal fumes and Vapour generated during machining to reach the gun.
• Thus it is essential to synchronize the motion of the rotating disc and pulsing of the electron beam gun.
• Electron beam guns are also provided with illumination facility and a telescope for alignment of the beam with the work piece.
• Work piece is mounted on a CNC table so that holes of any shape can be machined using the CNC control and beam deflection in-built in the gun.
• One of the major requirements of EBM operation of electron beam gun is maintenance of desired vacuum.
• Level of vacuum within the gun is in the order of 10^-4 to 10^-6 Torr. {1 Torr = 1mm of Hg} Maintenance of suitable vacuum is essential so that electrons do not loose their energy and a significant life of the cathode cartridge is obtained.
•  Such vacuum is achieved and maintained using a combination of rotary pump and diffusion pump.

ELECTRON BEAM PROCESS PARAMETERS

The process parameters, which directly affect the machining characteristics in Electron Beam Machining, are:

• The accelerating voltage
• The beam current
• Pulse duration
• Energy per pulse
• Power per pulse
• Lens current
• Spot size
• Power density

ELECTRON BEAM PROCESS CAPABILITY

• EBM can provide holes of diameter in the range of 100 μm to 2 mm with a depth up to 15 mm, i.e., with a l/d ratio of around 10.
• A wide range of materials such as steel, stainless steel, Ti and Ni super-alloys, aluminum as well as plastics, ceramics, leathers can be machined successfully using electron beam.
•  As the mechanism of material removal is thermal in nature as for example in electro-discharge machining, there would be thermal damages associated with EBM.
•  However, the heat-affected zone is rather narrow due to shorter pulse duration in EBM. Typically the heat-affected zone is around 20 to 30 μm.
• Some of the materials like Al and Ti alloys are more readily machined compared to steel.
• Number of holes drilled per second depends on the hole diameter, power density and depth of the hole as well as material type.

ELECTRON BEAM MACHINING APPLICATIONS

• With its high penetration depth, high impulse frequency, and fast beam deflection, electron beam machining has been widely used in many fields other than manufacturing, for instance in aerospace, food and chemical, insulation, and clothing industries.
• In the manufacturing field itself, electron beam machining is mainly applied for micromachining, including drilling fine holes, cutting narrow slots, and cutting contours in sheets.

ELECTRON BEAM MACHINING ADVANTAGES AND DISADVANTAGES:

ADVANTAGES OF ELECTRON BEAM MACHINING

• EBM provides very high drilling rates when small holes with a large aspect ratio are to be drilled.
• Moreover, it can machine almost any material irrespective of its mechanical properties. As it applies no mechanical cutting force, work holding and fixturing cost is very less.
• Further for the same reason fragile and brittle materials can also be processed. The heat-affected zone in EBM is rather less due to shorter pulses.
• EBM can provide holes of any shape by combining beam deflection using electromagnetic coils and the CNC table with high accuracy.

LIMITATIONS OF ELECTRON BEAM MACHINING

• However, EBM has its own share of limitations.
• The primary limitations are the high capital cost of the equipment and necessary regular maintenance applicable for any equipment using a vacuum system.
• Moreover, in EBM, there is a significant amount of non-productive pump down period for attaining the desired vacuum.
• However, this can be reduced to some extent using vacuum load locks.
• Though heat-affected zone is rather less in EBM recast layer formation cannot be avoided.

ELECTRON BEAM MACHINING PPT

Also Read: Mechanical Engineering Interview Questions

REFERENCES:

• Crawford, C., 1962, Introduction to Electron Beam Technology, John Wiley & Sons, New York.
• Grote, K., Antonsson, E., 2009, Handbook of Mechanical Engineering, 10th edn, Pringer, Berlin.
• McGeough, J.A., 1988, Advanced Methods of Machining, Chapman and Hall, London.
• Moarrefzadeh, A., 2011, ‘Finite-Element Simulation of Electron Beam Machining (EBM) Process’, International Journal of Multidisciplinary Sciences and Engineering, vol. 2, no. 6, pp 51-56, viewed 29 December 2012. < http://www.ijmse.org/Volume2/Issue6/paper10.pdf

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