2. Need for PAM
• Drilling, sawing, machining, punching or cutting of thick metals is very
difficult by traditional machining processes
• Plasma arc cutting is used in place of traditional machining methods,
since it can be used to cut through a wide variety of metals of varying
thickness at high speeds
• It provides the greatest economical advantage, speed and quality on
carbon steels, aluminum, stainless steels etc., when compared to
traditional machining methods
3. Principle of PAM
• Plasma arc machining or plasma arc cutting (PAC) is a thermal
machining process used for cutting thick sections of electrically
conductive materials
• The principle of the process is based on utilizing a high-temperature
plasma resulting from heating of gases to elevated temperatures (to
a few thousand degrees)
• The plasma in this state is composed of free electrons that have
become disassociated from the main gas atoms, positive ions and
neutral atoms
• The temperature of the plasma can be as high as 30,000°C, which
means that, it can be utilized to cut rapidly any type of material
5. PAM Operation
• In operation, a strong arc is struck between the tungsten
electrode (cathode) and the nozzle (anode), and meanwhile,
a suitable gas/gas mixture is forced into the chamber
• As the gas molecules collide with the high velocity electrons
of the arc, the gas get ionized and a very large amount of heat
energy is evolved
• This high velocity stream of hot ionized gas is called plasma
7. • The maximum velocity of the jet is
around 550 m/sec and the temperature
is as high as 28,000°C
• The arc is maintained stable, so that it
heats the flowing gas and maintains it in
the plasma state
• The high-velocity jet of high-
temperature gas is then directed on to
the work piece, which cuts by melting
and removing the material from the
work piece
8. PAM Equipment:
The equipment consists of a,
Power supply,
Gas supply unit,
Cooling water system,
Control elements and
A plasma torch
9. 1. Plasma torch
• The torch carries a 2 % thoriated
tungsten electrode connected to the
negative terminal of a DC power
supply
• The tungsten electrode thus acts as
a cathode in the circuit of the
equipment
• A converging nozzle with a suitable
orifice encloses the tungsten
electrode
10. • The nozzle is connected to the positive terminal of the power supply
through a suitable Resistor to limit the current through the nozzle to
about 50 Amp
• The work piece metal to be machined is then connected directly to the
positive terminal of the power supply
11. 2. Gas supply and cooling water system
• On one side of the torch is provided a
passage for the supply of gas into the
chamber
• The type of gas used depends on the
type of work piece material being
machined
• A provision for circulating the water
around the torch is provided to cool
the electrode and the nozzle during
the operation
12. Commonly used gas mixture for different work piece
materials in PAM:
SI.
No.
Material to be cut Gas /Gas mixture to be used
1. Aluminum and Magnesium Nitrogen, nitrogen-hydrogen mixture, argon-
hydrogen mixture.
2. Stainless steel and non-ferrous metals Nitrogen-hydrogen mixture, argon-hydrogen
mixture.
3, Carbon and alloy steels, Cast iron Nitrogen-hydrogen mixture and compressed air.
13. Process Parameters of PAM:
The parameters which affect the metal removal rate, accuracy and surface
finish include:
• Voltage and current
• Torch-work distance and
• Gas flow rate
The above parameters vary for different materials and with different
thickness
14. 1. Gas flow rate
• The gas flow rate depends on the type of gas being used
• For argon gas, the flow rate is in the range of 2270 - 3700 liters/hr
• The thicker the work piece material, the greater should be the gas
flow rate
15. PAM parameters:
SI.
No. Material Thickness (mm) Torch-work
distance (mm)
Current
(A)
1.
Titanium 13 6 400
25 10 550
2. Copper
Copper / Nickel
13 6 400
25 10 550
3. Cast iron 16 6 400
With increase in thickness of a particular work piece material, the
current and torch-work distance also increases
16. Advantages of PAM:
• Any material, regardless of its hardness and refractory nature
can be efficiently and economically machined
• Faster cutting speeds due to high velocity and high
temperature of cutting gas
• Requires minimal operator training
• Process variables such as type of gas, power, cutting speed
etc., can be adjusted for each metal type
17. Limitations of PAM:
• The high-temperature, high-velocity impinging gas causes
metallurgical alterations in the work piece material
• Shielding and noise protection adds additional equipments,
and also burden on the operator's precautions
• High equipment cost
18. • Manufacturers of transportation and agricultural equipments, heavy
machinery, aircraft components, and many other products have
discovered the benefits of plasma arc machining
• The process is used for sawing, drilling, machining, punching and
cutting operations
• In transportation industries, plasma arc machining is used to form the
outer skins of tractor-trailers, buses and agricultural equipment
Applications of PAM:
19. • They are also used in the heating, ventilating and air conditioning
industries to cut complex duct work
• Makers of large construction machinery use PAM to produce
cranes, bulldozers and other large equipments
• Plasma torches placed on robots are being used increasingly for
contour cutting of pipes and vessels, and also cutting of formed
shapes, angles and curves in various work parts
20. Mechanism of metal removal:
• The metal removal in PAM is
basically due to the high
temperature produced
• The heating of the work piece
is, as a result of anode heating,
due to the electron
bombardment plus convection
heating from the high
temperature plasma that
accompanies the arc
21. • The heat produced is sufficient to raise the work piece temperature above
its melting point and the high velocity gas stream effectively blows the
molten metal away
22. • Under optimum conditions, up to approximately 45% of the electrical
power delivered to the torch is used to remove metal from the work
piece
• The arc heat is concentrated on a localized area of the work piece and
it raises it to its melting point
23. Modes of operation of D.C. plasma torches:
Two modes of electrical connection in the torch are used
Non-transferred arc mode:
The d.c. power source is connected directly across the cathode
and the nozzle, so that the cathode and nozzle carry same
current
In such cases, the plasma is in the form of a flame, useful for
spraying, ceramic working and chemical synthesis
The hottest portion does not appear outside the nozzle. The
anode dissipation is lost in useless heating of the nozzle
24. Transferred arc mode:
In this mode of operation, the cathode is connected directly to the
negative of the d.c. source, while the anode nozzle is connected to
the positive of the supply through a suitable resistor to limit the
current through the nozzle to about 50 amp
The metal work piece to be processed is then connected directly to
the positive of the supply
25. When ignited, a pilot plasma flame is established between
the cathode and nozzle which provides a conducting path for
a high current constricted arc between the cathode and work
piece
Once this arc is struck the pilot flame circuit is
disconnected.
This method is limited to cutting, welding and hard
surfacing of metals
The electro thermal efficiency of transferred arc mode is 20-
30% more than non-transferred arc mode.
26. Non- thermal generation of plasma:
• The method of heating the gases by first ionizing them is one of the most
popular methods of generating hot plasma
• This can be done either by applying a suitable electric field across the gas
column or by exposing the gas to ionizing radiation
27. Process Characteristics in PAM
1. Gas for plasma generation:
• Practically, any gas that does not attack the tungsten electrodes or
the work piece can be use as the plasma gas
• For cutting of carbon and alloy steels, a mixture of nitrogen and
hydrogen or with compressed air is used
• For cutting of stainless steel, aluminium and other non-ferrous
metals mixtures of argon-hydrogen or nitrogen – hydrogen are used
29. Typical gas flow rates are 2 to 10 m3/hour
Direct current rated at about 400 V and 200KW output is generally required
Arc current ranges between 150 and 1000A for a cutting rate of 250-1700mm/min
Conditions for Plasma Arc Cutting of Stainless Steel
30. Cutting rates:
• The cutting rates possible in plasma arc machining are in the range of 250
to 1700 mm/min, which depends mainly on the thickness of the metal
being cut
• Sometimes water is injected into the jet which helps in confining the arc,
in blasting away the scale and smoke reduction
• Water injected plasma can increase the cutting rates by about 40 to 50%
• A 5mm thick carbon steel plate can be cut at 6000mm/min with water
injected plasma
31. Surface finish
• Plasma arc cutting can yield better surface finishes than oxy-
acetylene gas cutting
• However, the cut edges are round, with a corner radius of
about 4mm. there is also problem of taper (about 2-5°).
• The cut width is around is 2.5 to 8mm
32. Accuracy:
• Accuracy of the width of slots and diameter of holes is in the range
of ±0.8mm on 5 to 30mm plates and it is about ±3.0mm on 100 to
150mm thick plates
• The depth of heat affected zone is very high and depends on the
work material, its thickness and cutting speeds
• About 4mm of heat affected zone is common on 25mm thick
plates, which can reduced considerably by increasing the cutting
speed
33. Types of plasma torch:
Air plasma torch
• Air plasma cutting torch uses air as the
gas that is ionized and performs the
cutting
• The constricting nozzle (made of metal)
is directly exposed to the work piece
• With this design, it is possible for
electricity to arc from the electrode to
the nozzle and then from the nozzle to
the work piece
• This undesirable arcing is known as
double arcing and is the number one
cause of premature nozzle
34. • The presence of oxygen in the air acts to
support the exothermic burning of the metal
and provides for very fast cuts in oxidizable
materials such as steel
• Other material such as aluminium, can be
cut by this technique, but the cut edges are
left with severe oxidation
35. • Because of the elevated temperature and the presence of oxygen, a
tungsten electrode would last only a matter of seconds in the air plasma
torch and hence tungsten electrodes cannot be used with air plasma
cutting
• Instead, zirconium or hafnium electrodes are used because of their
superior oxidation resistance
36. Oxygen-injected torch
An oxygen-injected plasma torch eliminates the electrode oxidation problems
encountered with air plasma torches by using nitrogen as the plasma gas and
injecting the oxygen well downstream of the electrode
37. Oxygen-injected torch
• Although electrode life is greatly
extended, nozzle life is relatively short
• Oxygen-injected plasma systems are
used almost exclusively for mild-steel
plate cutting.
• While the squareness of the cut edges
is rather poor, the cutting rates can be
quite high with this method
38. Dual gas torch
• Dual gas plasma cutting system uses
nitrogen as the primary gas for both
generating the plasma and protecting
the electrode and a variety of
secondary shielding gases depending
upon the material being cut
• Typical applications may use such
secondary gases as oxygen for steel,
and an argon-hydrogen mixture for
cutting aluminium.
39. Dual gas plasma torch
One of the advantages to using the
dual gas system is that the nozzle can
be recessed inside of a ceramic cup to
eliminate double arcing
The relatively cool shield gas
protects the ceramic cup from thermal
damage
Another advantage to the dual gas
system is that sharp corners can be
maintained on the top side of cut
edges
40. Water injected plasma torch
• Utilizing a plasma torch with water
injection results in the higher
quality cuts available by PAC
• With this system, water is injected
either radially or as a swirling
vortical cone to constrict the
plasma
41. As the water impinges on the plasma, about 10% of the water vaporizes
producing a thin layer of steam which constricts the plasma
This thin boundary layer is known as the leidenfrost layer and acts to insulate
the nozzle the same way as the vapour layer which causes a drop of water to
glide over the surface of a very hot cooking pan without immediately vaporizing
42. • Water-injected systems use nitrogen as the plasma gas at a pressure of 1
MPa and at flow rate of 28 L3/min
• Water is used at a pressure of 1.2 MPa and at a rate of 1.9 L3/min to
constrict the plasma
• Because water is being used for constriction, the lower portion of the
nozzle can be be made of ceramic to eliminate double-arcing problems
• The benefits of water constriction include a reduction in smoke, smaller
heat-affected zone, increased nozzle life, and limited formation of oxides
on the cut edges of the work piece
43. PAM parameters:
The parameter the govern the performance of PAM can be divided three
categories:
• Those associated with the design and operation of torch
• Those associated with the physical configuration of the set up
• Environment in which the work is performed
44. Design of DC plasma Torches:
• The plasma torch is designed to obtain maximum thermal output
• The increase in efficiency only helps in achieving the better heating of the
gas but also in reducing the electrode loses and there by increase in life of
electrode
• The design also ensures that the erosion rate of the electrode are kept to
minimum
45. The parameters which affect the performance of the torch are,
•
1. cathode size,
2. its taper near the gap,
3. convergence of the nozzle,
4. nozzle orifice diameter and orifice length,
5. electrode gap
6. and cooling of the electrode
46. Physical configuration
• The variables such as stand-off distance, angle, depth of cut, feed and
speed of the work towards the torch are playing an important role in
plasma arc cutting
• The feed and depth of cut determine the volume of metal removal
• Figure shows the metal removal as a function of the torch angle
47. Work environment:
The environment group of variable for PAM includes,
• any cooling that is done on the cathode,
• any protective type of atmosphere used to reduce the oxidation of the
exposed high temperature machined surface
• any means that might utilized to spread out or deflect the and Plasma
striking area
• But in this field very little work has been done to investigate the effect
of these techniques on operation
48. Safety Precautions:
The plasma flame emits a high intense beam, particularly strong in
ultraviolet and infrared radiations
These radiation if taken in large doses might cause permanent damage
eye in the form of cataract
Overexposure results in gritty feeling in the eyes due to the loss of sleep
Over exposure to ultraviolet rays may also cause painful skin burns and in
extreme cases, these leads to cancer
49. Though these radiations are very intense in all the plasma flames, the
laminar plasma flame is much more dangerous
Hence it is very important that proper glasses and proper dresses are
worn before going close to the torch
The glasses should be good at ultraviolet and infrared cut off and most
of the body should be covered
