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Plasma Arc Machining Advantages, Disadvantages, Types of Torches, Use of Plasma and Safety precautions.

Plasma Arc Machining-

The process of Plasma Arc Machining is mentioned in the previous posts. Now we are talking about process parameters, types of torches used, uses of PAM and precautions.


Process Parameters-

The process parameters which govern the performance of PAM can be categorized into three groups. They are as follows-
1. The terms which are related to the design and operation of torch.
2. The terms which are related to the physical configuration of setup.
3. The location at which the work is performed.
The torches are designed such that they provide the maximum thermal output and higher efficiency. The efficiency of torches increases the electrodes life by attaining the maximum temperature of gas and decreasing the electrode losses. In addition to this, the minimum rate of erosion of electrode is to be considered in the design of torches.
While designing the torches, the following terms are to be considered which influences the performance of the torch.
 
1. The size of the cathode
2. Taper section of the cathode
3. Convergence of the nozzle.

Non-transferred Arc Torches-

 
Turbulent Mode Flame Torch-

Turbulent mode flame torch uses cathode rod which is having nearly 25 mm nozzle throat length. The orifice diameter of nozzle can be varied up to a wide range. It has 15 cm length of flame at 400 amperes of current. Turbulent mode flame possesses very high velocities and is emitted with characteristics of “shriek” or “hiss”. These type of torches are more useful in spraying, insulator working and chemical synthesis applications.

 
Laminar Mode name Torch-
 

Laminar mode flame torches are employed for spherodizing and ceramic melting applications. This type of torches generally possess low The flames have maximum length of 1 meter and emitted with a soothing hum. Laminar mode flame uses minimum diameter of cathode and maximum length of nozzle throat up to 125 mm.

High Power Torch-

This type of torches are used for tunnels, having maximum arc temperature. High power torches are operated at very high currents i.e. ( 2,2000 amperes ). The electrode erosion in the torches can be protected by using special magnetic confining fields at fie nozzle section.

 
 General Guidelines-

 
1. If the higher amount of current is used then the large orifice diameter of cathode, orifice length and electrode gap is needed.
2. When torches are operated at higher arc voltages, it is better to enlarge taper angle at the cathode tip. When torches are operated in the mode of transfer Fe4 arc anode (high arc voltage), flat face cathodes are suitable to use.
3. The cathodes having rounded off sections and smaller in size are used in order to prevent the of turbulence, higher ratio of orifice (length/diameter) is used for balancing the laminar now modes in the design of tortes.
4. For any torch, it is necessary to have a better coordination between the electrodes.
5. A non-transferred arc should have large orifice throat section whereas a transferred are mode should have small length of orifice
6. The insulator section of torch should be placed far from the arc zone so that the proper protection can done.
7. The optimum cooling of cathode is to be provided for less uniform erosion without breaking the cathode and collapsing the maximum stability.
8. High frequency voltage is used for the ignition. Hence, high quality insulation must be used in other sections of the torch.
9. The use of oxygen or compounds of Oxygen in plasma gas are harmful to cathode and there must be no water leakage into tie plasma chamber.


Characteristics of Plasma Arc Machining-

The characteristics of PAM are summarized as follows:
1. In plasma arc machining, the cutting rates obtained are 2500 to 1700 mm / min and it depends upon the type of work material to be used and its thickness.
2. The planes that are machined by using plasma torch are smoother than the planes machined by oxy-acetylene flame.
3. However, the ends of the surfaces are rounded in the shape.
4. The walls of the cut have a v-shape which has an inclination of 5 to 10 degrees. The nozzle having more than one port operating under perfect condition will decrease this angle to 20 degrees. It has kerf width which varies from 2.5 to 9 mm.
5. The accuracy of width of slots and diameter of holes vary for different thickness of plates, such as 0.8 mm for 6 to 30 mm + 0.3 mm for 100 to 150 mm thick plates.
In PAM, the deepness Of heat affected zone is generally depends upon the following:
(i) Material of the work piece
(ii) Thickness of work piece
(iii) Cutting speed.

 
Merits-

 
1. The main advantage of PAM is that it uniformly effects the material irrespective of its hardness or refractory nature since there is no connection between the tool and work piece.
2. The cutting rates in this method are high enough to assist the PAM technique on all the materials.
 

De-merits-

 
1. Surfaces contain metallurgical alterations,
2. It requires secondary machining on the surfaces.
3. Operators require eye shielding and noise protection.

 
Uses of Plasma in Manufacturing Industries-

The uses of plasma in the various fields of manufacturing industries are as follows:
1. The most important application of plasma arc machining is the profile cutting of metals, specifically steel and aluminum.
2. By using plasma arc machining, the profile cutting of metals such as mild steel alloy steel and titanium can be done instantaneously and clearly.
3. Plasma arc machining increases the rate of production in various manufacturing industries by incorporating the computer numerical control table.
4. Plasma Arc Machining is employed for rough turning of medium size bar stocks. It is also used for turning of hardened  shafts so that it can be utilized for making grooves on various pulleys.
5. In normal turning and milling machines, plasma is used for machining of hard materials.
6. Plasma arc machining involves some other applications such as:
 
A. Plasma arc surfacing, used for facing of hard alloys
B. It is used for generating surface coating of refractory materials. It is also used for the fabrication of free standing shapes.

Safety Precautions –

 
The safety precautions that are to be taken while performing plasma arc machining are as follows:
1. It is essential to protect the eyes from the ultraviolet and infrared radiations. These radiations are generated from the plasma flame and served in large quantity, it will be harmful to the eye. Hence, more care must be taken while working with plasma.
2. The over-ex in of ultraviolet and infrared radiation causes the reddening and due to loss of sleep a gritty feeling will be observed by an operator.
3. The overexposing of ultraviolet rays may lead to painful skin burns and cancer in most of the cases.
4. It is very essential that before going near to the torch, the worker should wear proper glasses and dresses.
5. The worker should cover the most of the body and quality of glasses must be good, so that it can protect from ultraviolet and infrared rays.
6. The torches are required to be operated in an airy room so that toxic gases such as NO, O2 etc., can be synthesized in the atmosphere.
7. While operating the torch, it may be noticed that the noise levels are very high. Hence, ear plugs should be used to protect the ears.
8. While operating the hand torches, the worker should wear asbestos gloves having leather as inner layer.
9. Every operator seek a consultation by a health physician depending upon the hours limit in operating the plasma torch. 

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