Laser Beam Machining Ppt

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.

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• Laser Beam Machining - Free download as Powerpoint Presentation (.ppt), PDF File (.pdf), Text File (.txt) or view presentation slides online. A brief description on LBM.

Laser Beam Machining - PPT. Presentation Summary: Laser-beam machining is a thermal material-removal. Some materials are also very difficult or impossible to cut. High accuracy parts can be mach.

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 :

Advantages of EBM :

Electron beam machining has the following advantages :

1. It is an excellent process for micro finishing (milligram/ s).
2. Very small holes can be machined in any type of material to high accuracy.
3. Holes of different sizes and shapes can be machined.
4. There is no mechanical contact between the tool and the workpiece.
5. It is a quicker process. Harder materials can also be machined at a faster rate than conventional machining.
6. Electrical conductor materials can be machined
7. The physical and metallurgical damage to the workpiece is very less.
8. This process can be easily automated.
9. Extremely close tolerances are obtained.
10. Brittle and fragile materials can be machined.

Disadvantages of EBM: Limitation of EBM

1. The metal removal rate is very slow.
2. The cost of equipment is very high.
3. It is not suitable for large workpieces.
4. High skilled operators are required to operate this machine.
5. High specific energy consumption.
6. A little taper produced on holes.
7. Vacuum requirements limit the size of the workpiece.
8. It is applicable only for thin materials.
9. 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.
10. It is not suitable for producing perfectly cylindrical deep holes.

Application of EBM :

1. EBM is mainly used for micro-machining operations on thin materials. These operations include drilling, perforating, slotting, and scribing, etc.
2. Drilling of holes in pressure differential devices used in nuclear reactors, aircraft engines, etc.
3. It is used for removing small broken taps from holes.
4. 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.
5. A micromachining technique known as “Electron beam lithography” is being used in the manufacture of field emission cathodes, integrated circuits, and computer memories.
6. 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.

Laser Beam Machining Ppt Design

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

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to melt and displace material in its path. Called PAM, this is a method of cutting metal with a plasma-arc, or tungsten inert-gas-arc, torch. The torch produces a high-velocity jet of high-temperature ionized gas called plasma that cuts by melting and removing material from the workpiece. Temperatures in the plasma zone range from 20,000° to 50,000° F (11,000° to 28,000° C).

This article consists of the Plasma arc machining working principle, plasma arc machining diagram, process, advantages and Disadvantages of plasma arc machining, application of plasma arc machining.

What is Plasma

Solids, liquids, and gases are the three familiar states of matter. In general when solid is heated, it turns to liquids and the liquids eventually become gases. When a gas is heated to sufficiently high temperature, the atoms (molecules) are split into free electrons and ions. The dynamical properties of this gas of free electrons and ions are sufficiently different from the normal unionized gas. So, it can be considered
a fourth state of matter, and is given a new name, PLASMA’. In other words, when the following gas is heated to a sufficiently high temperature of the order of 11,000°C to 28,000°C, it becomes partially ionized and it is known as ‘PLASMA’. This is a mixture of free electrons, positively charged ions, and neutral atoms.

This plasma is used for the metal removal process. The plasma arc machining process is used for cutting alloy steels, stainless steel, cast iron, copper, nickel, titanium, and aluminum, etc.

Working Principle of PAM

In the plasma arc machining process, the material is removed by directing a high-velocity jet of high temperature (11000°C to 28,000°C) ionized gas on the workpiece. This high-temperature plasma jet melts the material of the workpiece.

Process Details of PAM

Details of PAM are described below.

• Plasma Gun
  Gases are used to create plasma-like, nitrogen, argon, hydrogen, or a mixture of these gases. The plasma gun consists of a tungsten electrode fitted in the chamber. The electrode is given negative polarity and the nozzle of the gun is given positive polarity. The supply of gases is maintained into the gun. A strong arc is established between the two terminals anode and cathode. There is a collision between molecules of gas and electrons of the established arc. As a result of this collision, gas molecules get ionized and heat is evolved. This hot and ionized gas called plasma is directed to the workpiece with high velocity. The established arc is controlled by the supply rate of gases.
  
• Power Supply and Terminals
  Power supply (DC) is used to develop two terminals in the plasma gun. A tungsten electrode is inserted to the gun and made cathode and nozzle of the gun is made anode. Heavy potential difference is applied across the electrodes to develop a plasma state of gases.
  
• Cooling Mechanism
  As we know that hot gases continuously come out of nozzle so there are chances of its overheating. A water jacket is used to surround the nozzle to avoid its overheating.
  
• Tooling
  There is no direct visible tool used in PAM. Focused spray of ho0t, plasma state gases works as a cutting tool.
  
• Workpiece
  The workpiece of different materials can be processed by the PAM process. These materials are aluminum, magnesium, stainless steel, and carbon and alloy steels. All those material which can be processed by LBM can also be processed by the PAM process.

Construction Of Plasma arc Machining :

• The schematic arrangement of plasma arc machining is shown in Fig.
• The plasma arc cutting torch carries a tungsten electrode fitted in a small chamber.
• This electrode is connected to the negative terminal of a DC power supply. So it acts as a cathode.
• The positive terminal of a D.C power supply is connected to the nozzle formed near the bottom of the chamber. So, nozzle acts as an anode.
• A small passage is provided on one side of the torch for supplying gas into the chamber.
• Since there is a water circulation around the torch, the electrode and the nozzle remains water-cooled.

Working of PAM :

• When a D.C power is given to the circuit, a strong arc is produced between the electrode (cathode) and the nozzle (anode).
• A gas usually hydrogen (H2) or Nitrogen (N2) is passed into the chamber.
• This gas is heated to a sufficiently high temperature of the order of 11,000°C to 28,000°C by using an electric arc produced between the electrode and the nozzle.
• In this high temperature, the gases are ionized and a large amount of thermal energy is liberated.
• This high velocity and high-temperature ionized gas (plasma) is directed on the workpiece surface through the nozzle.
• This plasma jet melts the metal of the workpiece and the high-velocity gas stream effectively blows the molten metal away.
• The heating of workpiece material is not due to any chemical reaction, but due. to the continuous attack of plasma on the workpiece material. So, it can be safely used for machining of any metal including those which can be subjected to the chemical reaction.

ACCURACY

• Plasma arc machining is a roughing operation to an accuracy of around 1.4 mm with the corresponding surface finish. Accuracy on
the width of slots and the diameter of holes is ordinarily from ± 4 mm on 100 to 150 mm thick plates.

GASES USED IN PAM

The selection of a particular gas for use in this process mainly depends on the expected quality of surface finish on the work material and economic consideration. The gases used in this process, should not affect the electrode or the workpiece to be machined. The commonly used gases and gas mixtures are given in the following table.

STAND OFF DISTANCE

Stand-off distance is the distance between the nozzle tip and the workpiece. When the stand-off distance increases, the depth of penetration is reduced. With an excessive reduction of the stand-off distance, the plasma torch can be damaged by the metal spatter. The optimum
stand-off distance depends on the thickness of the metal being machined and varies from 6 to 10 mm.

ADVANTAGES OF PAM

1. It can be used to cut any metal.
2. The cutting rate is high.
3. As compared to the ordinary flame cutting process, it can cut plain carbon steel four times faster.
4. It is used for rough turning of very difficult materials.
5. Due to the high speed of cutting, the deformation of sheet metal is reduced while the width of the cut is minimum and the surface quality is high.

DISADVANTAGES OF PAM

1. It produces a tapered surface.
2. The protection of noise is necessary.
3. The equipment cost is high.
4. Protection of eyes is necessary for the operator and persons working in nearby areas.
5. Oxidation and scale formation takes place. So, it requires shielding.
6. The work surface may undergo metallurgical changes.

APPLICATIONS

1. It is used for cutting alloy steels, stainless steel, cast iron, copper, nickel, titanium, aluminum, and alloy of copper and nickel, etc.
2. It is used for profile cutting.
3. It is successfully used for turning and milling of hard to machine materials.
4. It can be used for stack cutting, shape cutting, piercing, and underwater cutting.
5. Uniform thin film spraying of refractory materials on different metals, plastics, ceramics are also done by plasma arcs.

CHARACTERISTICS OF PAM

• Metal removal technique: Heating, melting, and vaporizing by using plasma.
• Work material .-All materials that conduct electricity.
• Tool: Plasma jet
• Velocity of plasma jet: 500 m /s
• Power range: 2 to 220 kW
• Current: As high as 600 amp.
• Voltage : 40 – 250 V
• Cutting speed: 0. 1 to 7 m / min
• Metal removal rate: 145 cm3 /min

Some Questions and Answers Related to PAM

Laser Assisted Machining

1. What is plasma?

Laser Beam Machining Ppt Examples

When a flowing gas is heated to a sufficiently high temperature of the order of 1 l,000°C to 28,000°C, it becomes partially ionized and it is known as plasma.

2. What is transferred arc type plasma?

In the transferred arc type process, the electrode is connected to the negative terminal of the D.C power supply, and the workpiece is connected to the positive terminal of a D.C power supply. So, more electrical energy is transferred to the work, thus giving more heat to the work.

3. What is non-transferred arc type plasma?

In non-transferred arc type, the electrode is connected to the negative terminal of a D.C power supply and the nozzle is connected to the positive terminal of a D.C power supply. When the working gas passing through the nozzle, a part of the working gas becomes heated, ionized, and emerges from the torch as the plasma jet. This plasma feeds the heat to the workpiece. This type of torches are used for non- conducting materials.