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17. 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.

18. • 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.

20. Components of Plasma Arc Machining:
1 Plasma Gun:
Different gases like nitrogen, hydrogen, argon or mixture of these gases are used to create plasma. This
plasma gun has a chamber which has a tungsten electrode. This tungsten electrode is connected to the negative
terminal and nozzle of the plasma gun is connected to the positive terminal of the DC power supply. The required
mixture of gas is supplied to the gun. A strong arc is produced between the anode and the cathode.After that, there is
a collision between the electron of the arc and the molecules of the gas and due to this collision, gas molecules get
ionized and heat is generated.
2 Power Supply:
DC Power Supply is used to develop two terminals in the plasma gun. Heavy potential difference is applied
across cathode and anode so that arc produced is strong and is able to ionize the gas mixture and convert it into
plasma.
3 Cooling Mechanism:
A cooling mechanism is added to the plasma gun as heat is produced in it as hot gases continuously pass out
from the nozzle.A water jacket is used to cool the nozzle. The nozzle is surrounded by water jet.
4 Workpiece:
Different materials can be worked using this plasma arc machining. Different metals like aluminum,
magnesium, carbon, stainless steel and alloy steels can be worked using this process.

21. Construction of Plasma Arc Machining:
• The plasma arc cutting torch carries a tungsten electrode fitted in the chamber.
• This tungsten electrode is connected to the negative terminal of the DC power supply.
• For plasma arc machining, a plasma gun is required. This plasma gun has a chamber.
• This plasma gun has a tungsten electrode fitted inside the chamber.
• This tungsten electrode is connected to the negative terminal of DC Power Supply and acts as a
cathode.
• At the bottom of the chamber, there is a copper nozzle that is connected to a positive terminal of
the DC Power Supply and acts as an anode.
• The rest of the chamber is made of insulating material and acts as an insulator.
• Gas enters the chamber through a small passage present at the right side of the chamber.
• The cathode and the anode remain cool despite the hot gases passing through them as they are
water cooled. Water circulation is present around the torch.

22. Working of PAM :
• At first, when a D.C power is supplied to the circuit, a strong arc is produced between the cathode
( electrode ) and the anode (nozzle).
• After that, gas is supplied to the chamber. This gas can be hydrogen, nitrogen, argon or mixture of
these gases chosen according to the metal to be worked.
• The gas used in the process is heated using the arc produced between the cathode and the
anode. This gas is heated to very high temperatures from 11000 °C to 28000 °C.
• As the arc comes into contact with the gas, there is a collision between the electron of the arc and
the molecules of the gas and the molecules of the gas will dissociate into separate atoms.
• Due to the high high temperature generated from the arc, electrons from some atoms will be
displaced and atoms are ionized ( electrically charged ) and the gas turns into plasma. As the gas
is ionized, a large amount of thermal energy is liberated.
• After the gas is ionized, this high temperature ionized gas is directed towards the workpiece with
high velocity.
• The electric arc has some other benefits like it increases the temperature of ionized gas, makes
the beam almost parallel, and increases the velocity of the gas.
• As the plasma jet reaches the workpiece, the plasma melts the workpiece and the high-velocity
gas blows away the molten metal.
• In this way, plasm arc machining is used to remove material from the workpiece.

24. 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.

25. 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.

26. 1.What is plasma?
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.