It is also one of the thermal machining processes. Here the method of heat generation is different than EDM and LBM. Working Principle of PAM In this process gases are heated and charged to plasma state. Plasma state is the superheated and electrically ionized gases at approximately 5000oC. These gases are directed on the workpiece in the form of high velocity stream. Working principle and process details are shown in Figure 5.7. Gases are used to create plasma like, nitrogen, argon, hydrogen or mixture of these gases. The plasma gun consists of a tungsten electrode fitted in the chamber. The electrode is given negative polarity and nozzle of the gun is given positive polarity. 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.

1. UNIT-5
Plasma Arc Machining

2. Definition
• A gas molecule at room temperature consists of
two or more atoms.
• When such a gas is heated to a high temperature
of order 2000˚C or so, the molecule separates
out as atoms.
• If the temperature is raised to 3000˚C, the
electrons from some of the atoms dissociate and
the gas becomes ionized consisting of ions and
electrons.
• This state of gas is known as “PLASMA”.

3. • Thus, Plasma is glowing , ionized gas that
results from heating of a material to extremely
high temperature.
• Plasma gas is neutral. It consists of equal
amount of ions and electrons.
• Thus the plasma gas becomes electrically
conductive as well as responsive to
magnetism.
• Because of such behaviour, plasma is known
as a fourth state of matter.

4. • The temperature of plasma can be of the
order 33,000˚C.
• When such a high temperature source reacts
with work material, the work material melts
out and may even vaporize, and finally is cut
into pieces.
• Many materials like Al, stainless steel.. Have
good thermal conductivity, large heat capacity,
and good oxidation resistance. As a result they
cannot be cut by conventional technique like
oxy-fuel cutting. But these materials can be
easily cut by Plasma Arc Cutting (PAC).

5. Plasma Arc Machining
2% thoriated tungsten

6. PAC System
• PAC system uses DC power source.
• PAC system operate on either
1. Non- Transferred arc mode or
2. Transferred arc mode

7. PAC System

8. Elements of PAC system
• Power source
• Gas supply
• Cooling water system
• Plasma torch.

9. DC power source
Can deliver up to 1000A
at about 200V

10. Gas supply system
• The gases used should be inert(non- oxidising)
• High thermal conductivity, to transfer more
heat.
• Generally used plasma gas is Nitrogen.
• Shielding Gases: Carbon dioxide & Argon-
Hydrogen….)

11. Types of Plasma torches
• There are mainly two types of torches
1. Non- transferred arc torches
2. Transferred arc torches
a. Turbulent mode flame torch
b. Laminar mode flame torch
c. High power torches

12. Plasma Torch

13. 1. Non- Transferred arc torch
Useful for spraying, ceramic working & chemical synthesis

14. Types of Transferred arc Plasma
torches
1. Transferred arc torches
a. Turbulent mode flame torch
b. Laminar mode flame torch

15. b. Laminar mode flame has low velocity,
lengths as long as 1m flame is emitted
It is used for spheroidizing & melting ceramics.
High carbon steels
Types of Transferred arc Plasma
torches

16. Types of Transferred arc Plasma
torches
a. Cutting torches
b. Welding torches
c. Micro torches or needle torches

17. 2. Transferred arc torch
a. Cutting torches
i. Air plasma flow torch
ii. Oxygen injected torch
iii. Dual flow torch
iv. Multiport nozzle torch
v. Water injected torch

18. Air plasma flow torch
•Tapered machined
surface
•Double arcing.
•Zirconium electrode
life 2hr ( resistance to
oxidation)

19. Dual flow torch
•Plasma gas: nitrogen
•Shielding gas: oxygen, co2,argon – hydrogen etc.
•Sharp corners on the top side of cutting edges

20. Oxygen injected torch
• To avoid oxidation of electrode or
life of the electrode.
• Nitrogen is the plasma gas. oxygen is
injected down stream of the
electrode.
• MRR
• Poor squareness of the cut edges.

21. Multiport torch

22. Water injected torch
•Water pressure 1.2MPa
•Plasma gas Nitrogen at 1MPa
•To avoid double arcing
•Swirl motion of the water
results in one edge being cut
is straight

24. General considerations
• Large current requires large orifice diameter
cathode, orifice length & electrode gap.
• For larger arc voltage increase the angle of taper
at the cathode tip.
• For avoiding turbulence the edges of construction
are rounded off & smaller cathode diameters are
used.
• Non- transferred arc modes use long throat
lengths, while for transferred arc mode , the
minimum arc lengths are used.
• High quality Insulation is used & the insulator
portion should be away from the arc zone.

25. Accuracy and surface finish

26. Effect of cutting voltage

27. Accuracy and surface finish
• If the speed is the upper edge of the cut
reaches too far. i.e., the kerf is too wide
forming a V- shaped c/s.
• speed also results in kerf.
• The optimum cutting speed is achieved by
advancing the torch at a rate of distribution of
heat flow from plasma in to material is
uniform throughout the thickness of the
material.

28. Accuracy and surface finish
• For example kerf width on a plasma cut in
25mm stainless steel is about 5mm.
• A plasma cut in 175mm thick stainless steel
produces 30mm kerf width.
• Thickness of plate kerf width
• Plasma arc cuts are dross free if proper cutting
conditions and the appropriate cutting gases
are used.

29. Accuracy and surface finish
“Dross “ is the name given to metal that melts
During cutting but adheres to the bottom edge
of the cue face.

30. Applications
• Plasma arc welding
• Plasma arc cutting
• Plasma arc surfacing
• Plasma arc spraying