Plasma Arc MACHINING

1. Plasma Arc Machining

2. Plasma
When a flowing gas is heated to a sufficiently high temperature to become partially
ionized, it is known as ‘plasma’. This is virtually a mixture of free electrons, positively
charged ions and neutral atoms.
PAM is a material removal process in which the material is removed by ionized gas of
high temperature (11,000 to 30,000 C) which applied by a high-velocity jet on the
workpiece.

3. Plasma Arc Machining Setup

4. WorkingPrinciple
The principle of plasma arc machining is in a plasma torch, known as the gun or plasma-
Tron, a volume of gas such as H2, N2, O2, etc. is passed through a small chamber in which
a high-frequency spark (arc) is maintained between the tungsten electrode (cathode) and
the copper nozzle (anode), both of which are water-cooled.
 When DC power is given to the circuit, a strong arc forms between the cathode
(electrodes) and anode (nozzle).
 Following that, the chambers are filled with gas. This gas may be a mixture of hydrogen,
nitrogen, argon, or other gases selected based on the metal being worked.
 The gas used in the process is heated by an arc formed between the cathode and the
anode. This gas heats at extremely high temperatures ranging from 11000 ° C to 28000 ° C.

5.  When the arc makes contact with the gas, the electrons of the arc collide with the
molecules of the gas, causing the gas molecules to separate into different atoms.
 Because of the high temperatures produced by the arc, electrons from some atoms are
displaced, the atoms are ionized (electrically charged), and the gas is converted into
plasma. A significant amount of thermal energy is released as the gas is ionized.
 After the gas has been ionized, the high temperature ionized gas is directed with high
velocity towards the workpiece.
 As the plasma jet approaches the workpiece, the plasma melts it and the molten metal
is blown away by the high-velocity gas.

6. Components Of PAM
Plasma Gun
 Gases, like plasma, are used to make nitrogen, argon, hydrogen, or a mixture of these gases.
 The plasma gun consists of a tungstens electrode that is fitted into the chamber.
 The electrodes are given negative polarity, and the gun nozzle is given positive polarity.
 The supply of gases remains in the gun. A strong arc is established between two terminals, anodes, and
cathode. There is a collision between the molecules of the gas and electrons of the established arc.
Power Supply and Terminals
 DC is used to develop two terminals in a plasma gun.
 A tungsten electrode is inserted into the gun, and a cathode is made, and the gun nozzle is the anode.
 A massive potential difference is applied to the electrodes to develop a plasma state of gases.

7. Cooling Mechanism
Because hot gases continuously exit the nozzle, there is a risk of overheating. To prevent the nozzle from
overheating, it is surrounded by a water jacket.
Tooling
In PAM, there is no visible tool. A focused spray of hot, plasma-state gases is used as a cutting tool.
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. The optimum stand-off distance depends on the thickness of
the metal being machined and varies from 6 to 10 mm.

8. Advantages
 In plasma arc machining, hard and brittle metals can be made easily.
 It can be used to cut any metal.
 Computer numerical controlled PBM is used for profile cutting of metals that are difficult to tackle by
oxyacetylene gas technique such as stainless steel and aluminum.
 There being no contact between the tool and work piece, only a simply supported work piece structure is
enough.
 Due to the higher heat production and the plasma jet allows faster travel speeds
 It leaves narrower kerf

9. Limitations
 Plasma arc machining equipment setup is expensive
 Metallurgical changes occur on the surface of the workpiece.
 Inert gas consumption is high.
 Shielding is required as oxidation and scale formation occur.
 Safety precautions are necessary for the operator and those in near by areas.

10. Applications
 It is mostly used for cryogenic, high-temperature corrosion-resistant alloys.
 It is also used in the case of titanium plates up to 8 mm thickness.
 PAM is used in nuclear submarine pipe systems and welding steel rocket motor cases.
 PAM is a staple for applications related to stainless tubes and tube mills.

11. Differences between transferred and non-transferred arc plasma torch

13. Laser Beam Machining

14. Material Removal Mechanism
Laser Beam Machining (LBM) is a form of machining process in which laser beam is used for the
machining of metallic and non-metallic materials. In this process, a laser beam of high energy is made
to strike on the workpiece, the thermal energy of the laser gets transferred to the surface of the w/p
(workpiece). The heat so produced at the surface heats, melts and vaporizes the materials from the
w/p. Light amplification by stimulated emission of radiation is called LASER.

15. Laser Beam Machining setup

16. Stimulated Emission
When the electrons in the excited state do not jumps back to the ground state by its own. This situation is called
meta-stable state. When a photon is fired to the meta- stable state of atoms, this stimulates an electron at excited
state and it jumps back to its ground state giving of two photons (one photon that we fired and other produced by
the electron). These two photons stimulate other atoms electrons and produces more photons- a chain reactions
starts and number of photon increases. This process is called stimulated emission.

17. Types of Laser
1. Gas Lasers: In these types of laser, gases are used as the medium to produce lasers. The commonly used gases are He-
Ne, argon and Co2.
2. Solid State Lasers: The media of the solid state lasers are produced by doping a rare element into a host material.
Ruby laser is an example of solid state laser in which ruby crystal is used as medium for the generation of laser beam.
The other media used in the solid state lasers are
(i) YAG: For yttrium aluminum garnet which a type of crystal.
(ii) Nd:YAG – Refers to neodymium-doped yttrium aluminum garnet crystals

18. Working Principle
 In this process, the Laser Beam is called monochromatic light, which is made to focus on the workpiece to be
machined by a lens to give extremely high energy density to melt and vaporize any material.
 The Laser Crystal (Ruby) is in the form of a cylinder as shown in the above figure or Diagram with flat reflecting ends
which are placed in a flash lamp coil of about 1000W.
 The Flash is simulated with the high-intensity white light from Xenon. The Crystal gets excited and emits the laser
beam which is focused on the workpiece by using the lens.
 The beam produced is extremely narrow and can be focused to a pinpoint area with a power density of 1000
kW/cm2. Which produces high heat and the portion of the metal is melted and vapourises.

19. Power Supply:
The electric current or power is supplied to the system. A high voltage power system is used in laser beam machining. It
will give initial power to the system after that reaction starts in a laser that will machine the material. There is a high
voltage supply so that pulses can be initiated easily.
Capacitor:
During the major portion of the cycle, a capacitor bank charges and releases the energy during the flashing process. The
capacitor is used for the pulsed mode for charging and discharging.
Flash Lamps:
It is the electric arc lamp that is used to produce extremely intense production of white light which is a coherent high-
intensity beam. It is filled with gases that ionize to form great energy that will melt and vaporizes the material of the
workpiece.

20. Reflecting Mirror:
Reflecting Mirror are two main types of internal and external. Internal mirrors also called a resonator that is
used to generate maintain and amplify the laser beam. It is used to direct the laser beam towards the
workpiece.
Laser Light Beam:
It is the beam of radiation produced by the laser through the process of optical amplification based on the coherence of
light created by the bombarding of active material.
Ruby Crystal:
Ruby laser produces a series of coherent pulses which is deep red in color. It achieves by the concept of population
inversion. It is a three-level solid-state laser.
Lens:
Lenses are used to focus the laser beam onto the workpiece. First laser light will enter into the expanding lens and then
into the collimating lens which makes the light rays parallel and the expanding lens expands the laser beams to the
desired size.

21. Applications of Laser Beam Machining
 Laser Machining is used for making very small holes, Welding non-conductive and refractory material.
 It is best suited for brittle material with low conductivity and Ceramic, Cloth, and Wood.
 Laser Machining is also used in surgery, micro-drilling operations.
 Spectroscopic Science and Photography in medical science.
 It is also used in mass macro machining production.
 Cutting complex profiles for both thin and hard materials.
 It is used to make tiny holes. Example: Nipples of the baby feeder.

22. Advantages:
 In Laser Beam Machining any material including non-metal also can be machined.
 Extremely small holes with good accuracy can be machined.
 The tool wear rate is very low.
 There is no mechanical force on the work.
 Soft materials like plastic, rubber can be machined easily.
 It is a very flexible and easily automated machine.
 The heat-affected zone is very small.
 Laser Machining gives a very good surface finish.
 Heat treated and magnetic materials can be welded, without losing their properties.
 The precise location can be ensured on the workpiece.

23. Disadvantages:
 Laser Machining cannot be used to produce a blind hole and also not able to drill too deep holes.
 The machined holes are not round and straight.
 The capital and maintenance cost is high.
 There is a problem with safety hazards.
 The overall efficiency of the Laser beam machining is low.
 It is limited to thin sheets.
 The metal-removing rate is also low.
 The flash lamp life is short.
 There is a limited amount of metal removing during the process.