Plasma - the fourth state of matter - is an ionized gas that conducts electricity. Plasma is created by adding energy to an electrically neutral gas. In this case, the gas is compressed air and the energy is electricity. The more electrical energy added, the hotter the plasma arc becomes. Plasma arc cutting machines control this powerful energy by constricting the arc and forcing it through a concentrated area (the nozzle). By increasing air pressure and intensifying the arc with higher amperage, the arc becomes hotter and more capable of blasting through thicker metals and blowing away the cuttings, with minimal cleanup required. It is easy to learn and use the process. First time users of the process can achieve good quality cuts after minutes of practice. Plasma Advantages Plasma provides numerous advantages over other cutting processes. While there are many common methods of cutting metal, the plasma process offers the following advantages:
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• Cuts any type of electrically conductive metals including aluminum, copper, brass and stainless steel
• Cuts faster - up to 130 in. per minute on 1/4 in. steel
• Does not require a pre-heat cycle which saves time and is more convenient
• Produces a small and more precise kerf (width of the cut)-great when precision matters
•Has a smaller heat affected zone which prevents the area around the cut from warping and minimizes paint damage
• Provides gouging and piercing capabilities Sawing or chopping can take a long time and will
typically leave a rough edge - plasma cutting is fast, clean, and leaves a nice straight edge. It is also a less expensive and more convenient method for cutting than many other processes because compressed air is typically available in most applications via shop or portable compressors.
Plasma cutting work by sending an electric arc through a gas as it passes through a constricted opening. The gas can be compressed air, nitrogen, argon, oxygen, etc. The arc elevates the temperature of the gas to the point where it enters the fourth state of matter called plasma. It is the electrical conductivity of the plasma that causes the arc to transfer to the work, while the high current causes the metal to melt. The nozzle's restricted opening causes the gas to squeeze by at a high rate of speed and cut through molten metal. The gas is also directed around the perimeter of the cutting area to shield the cut.
The plasma cutting process, as used in the cutting of electrically conductive metals, utilizes this electrically conductive gas to transfer energy from an electrical power source through a plasma cutting torch to the material being cut.The basic plasma arc cutting system consists of a power supply, an arc starting circuit and a torch. These system components provide the electrical energy, ionization capability and process control that is necessary to produce high quality, highly productive cuts on a variety of different materials.The power supply is a constant current DC power source. The open circuit voltage is typically in the range of 240 to 400 VDC. The output current (amperage) of the power supply determines the speed and cut thickness capability of the system. The main function of the power supply is to provide the correct energy to maintain the plasma arc after ionization.The arc starting circuit is a high frequency generator circuit that produces an AC voltage of 5,000 to 10,000 volts at approximately 2 megahertz. This voltage is used to create a high intensity arc inside the torch to ionize the gas, thereby producing the plasma.The Torch serves as the holder for the consumable nozzle and electrode, and provides cooling (either gas or water) to these parts. The nozzle and electrode constrict and maintain the plasma jet.
Safety precaution is very important to prevent accident before, during and after do the process.Be aware of potential hazards associated with plasma arc cutting. They are: electrical shock, fumes, noise, and radiation Electric Shock Can Kill, operating a plasma cutter completes an electric circuit between the torch and the workpiece. The workpiece and anything touching the workpiece are part of the electrical circuit. Never touch the torch body, workpiece or the water in a water table when the plasma system is operating. The plasma is very hot and you must never place any part of your body near the plasma tip or nozzle, when the power is on. Plasma arc cutting can produce toxic fumes and gases that deplete oxygen and cause serious injury. Keep the cutting area well ventilated or use an approved air-supplied respirator. Do not cut in locations near degreasing, cleaning or spraying operations. The vapors from certain chlorinated solvents decompose to form phosgene gas when exposed to ultraviolet radiation. Noise Levels of Plasma Arc Cutting systems can generate noise levels in excess of 120 dB during high amperage cutting operations. Ear protection should be used when operating or working near plasma arc cutting operations. Wear insulated gloves and boots, and keep your body and clothing dry. Do not stand, sit or lie on or otherwise touch any wet surface when using the plasma cutter system. Insulate yourself from work and ground using dry insulating mats or covers big enough to prevent any physical contact with the work or ground. If you must work in or near a damp area, use extreme caution. So always wear the safety goggles or head shield, make sure a head shield of no less than a #5 lens and gloves. If only wearing tinted goggles, apply UV protective cream or lotion, if operating for a prolonged period of time to protect against burning. Always wear a face shield when grinding. Always turn the power OFF before changing tips, electrode, electrode adapters, baffles, brass or heat shields. Always put your tools back after you set up, practice good housekeeping at all times. If for reason the machine is not operating properly, it must be reported immediately. Turn the power off whenever leaving your workstation. Do the checklist as check the condition of the wire harness. Notify supervisor or lecturer immediately if it is in poor repair.
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The metallurgy effect of the plasma arc cutting is effect for heat affected zone HAZ, because when material expose to the heated it will produce HAZ. The grain structure can changes in the material depend on the temperature to which the material rises in the HAZ. For example, as the temperature of the cutting and weld area increases, yield strength, elasticity, and thermal conductivity of the steel plate decrease, while thermal expansion and specific heat increase. Plasma arc cutting also creates a large heat-affected zone in the area surrounding the cut. Cutting materials under-water cutting minimizes the size of the heat-affected zone. Dross, the resolidified metal that forms at the bottom of the cut, is a potential issue for processors using plasma cutting since it frequently forms during plasma arc cutting.
Sequence of Operating a Plasma Cutter
The power source and arc starter circuit are connected to the torch via interconnecting leads and cables. These leads and cables supply the proper gas flow, electrical current flow and high frequency to the torch to start and maintain the process.
A start input signal is sent to the power supply. This simultaneously activates the open circuit voltage and the gas flow to the torch (see Figure 2). Open circuit voltage can be measured from the electrode (-) to the nozzle (+). Notice that the nozzle is connected to positive in the power supply through a resistor and a relay (pilot arc relay), while the metal to be cut (workpiece) is connected directly to positive. Gas flows through the nozzle and exits out the orifice. There is no arc at this time as there is no current path for the DC voltage.
After the gas flow stabilizes, the high frequency circuit is activated. The high frequency breaks down between the electrode and nozzle inside the torch in such a way that the gas must pass through this arc before exiting the nozzle. Energy transferred from the high frequency arc to the gas causes the gas to become ionized, therefore electrically conductive. This electrically conductive gas creates a current path between the electrode and the nozzle, and a resulting plasma arc is formed. The flow of the gas forces this arc through the nozzle orifice, creating a pilot arc.
Assuming that the nozzle is within close proximity to the workpiece, the pilot arc will attach to the workpiece, as the current path to positive (at the power supply) is not restricted by a resistance as the positive nozzle connection is. Current flow to the workpiece is sensed electronically at the power supply. As this current flow is sensed, the high frequency is disabled and the pilot arc relay is opened. Gas ionization is maintained with energy from the main DC arc.
The temperature of the plasma arc melts the metal, pierces through the workpiece and the high velocity gas flow removes the molten material from the bottom of the cut kerf. At this time, torch motion is initiated and the cutting process begins.
Oxy fuel cutting
The oxy-fuel process is the most widely applied industrial thermal cutting process because it can cut thicknesses from 0.5mm to 2,500mm, the equipment is low cost and can be used manually or mechanized There are several fuel gas and nozzle design options that can significantly enhance performance in terms of cut quality and cutting speed. The cutting process is illustrated in. Basically, a mixture of oxygen and the fuel gas is used to preheat the metal to its 'ignition' temperature which, for steel, is 700°C - 900°C (bright red heat) but well below its melting point. A jet of pure oxygen is then directed into the preheated area instigating a vigorous exothermic chemical reaction between the oxygen and the metal to form iron oxide or slag. The oxygen jet blows away the slag enabling the jet to pierce through the material and
continue to cut through the material. The equipment oxy fuel cutting is same with oxy fuel welding such as, oxygen cylinder, acetylene cylinder, oxygen hose, acetylene hose, oxygen regulator, acetylene regulator, cylinder pressure gauge, working pressure gauge, oxygen hose connection, acetylene hose connection, tip cleaner, spark lighter and cutting torch. There have 2type torch as welding torch and cutting torch.
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There are four basic requirements for oxy-fuel cutting:
· the ignition temperature of the material must be lower than its melting point otherwise the material would melt and flow away before cutting could take place
· the oxide melting point must be lower than that of the surrounding material so that it can be mechanically blown away by the oxygen jet
· the oxidation reaction between the oxygen jet and the metal must be sufficient to maintain the ignition temperature
· a minimum of gaseous reaction products should be produced so as not to dilute the cutting oxygen
As stainless steel, cast iron and non-ferrous metals form refractory oxides; i. e. the
oxide melting point is higher than the material, powder must be injected into the
flame to form a low melting point, fluid slag.
Purity of oxygen
The cutting speed and cut edge quality are primarily determined by the purity of the oxygen stream. Thus, nozzle design plays a significant role in protecting the oxygen stream from air entrainment. The purity of oxygen should be at least 99.5%. A decrease in purity of 1% will typically reduce the cutting speed by 25% and increase the gas consumption by 25%.
Fuel gases are characterized by their
· flame temperature - the hottest part of the flame is at the tip of the primary flame (inner
· fuel gas to oxygen ratio - the amount of fuel gas required for combustion but this will vary
according to whether the flame is neutral, oxidizing or reducing
· heat of combustion - heat of combustion is greater in the outer part of the flame
The five most commonly used fuel gases are acetylene, propane, MAPP (methylacetylene-
propadiene), propylene and natural gas. The relative performance of the fuel gases in terms of pierce time, cutting speed and cut edge quality, is determined by the flame temperature
and heat distribution within the inner and out flame cones.
Table show content of gas
Oxyfuel Welding Setup
The procedures of oxy fuel cutting is select an appropriate tip for cutting the particular metal thickness and hold it in the torch. Adjust acetylene regulator to the working pressure recommended for the tip size selected .Close acetylene valve on the torch which was made open earlier before step to purge the lines of any air. Open cutting oxygen valve on the torch. Adjust oxygen regulator to working pressure recommended for the selected size of the tip. Close the cutting oxygen valve on the cutting torch. Open acetylene valve on the torch and ignite the gas with a spark lighter. Open the oxygen valve (and not the cutting oxygen valve) on the torch gradually to obtain a neutral preheat flame. (vi)Position the cutting torch such that the cutting tip is kept per pendicular to the surface for plate thicknesses 13 mm or more. For thin plates, the tip can be tilted in the direction of cut. Tilting increases cutting speed and helps prevent slag from freezing gap cross the kerfs. Place the preheat flames halfway over the edge of the plate to be cut, holding the end of the flame tones 1.5 to 3 mm above the work surface. The tip axis should be aligned with the plate edge. Note as the top corner of the plate reaches a reddish yellow, color. Put the tip entirely over the material to be cut. The preheat flame is moved back and forth a short distance along the line of cut until it reaches ignition temperature. Then the tip is brought to the starting point ,and The high pressure cutting oxygen valve is opened gradually to start the cutting operation. Close acetylene valve first to shut off the torch. After doing so close the oxygen valve too.
Safety precaution is very important to prevent accident, before, during and after do the process, such as use personal protective equipment (PPE) for example, wear the safety goggles to protect the eyes against glare and flying sparks, wear heavy flame work clothing with long sleeves to protect arms. Make sure sleeves are secured around wrist. Always wear a face shield when grinding. Always put your tools back after you set up, practice good housekeeping at all times. For safety fire protection, do not use oxy-fuel equipment near oil or grease containers,
Keep oil grease and combustible dust away from all oxygen equipment. For safety workplace is do not ventilate with oxygen from tanks. . Fireproof any surface used as a worktable. Fire brick is recommended because it is inexpensive, easy to find, and long-lasting. Secure fuel and oxygen cylinders by chaining to wall or bench, or use cylinder cart. Do not lay cylinder down on floor.
The metallurgy effect of the oxy fuel cutting is also effect for heat affected zone HAZ, because when material expose to the heated it will produce HAZ. Oxy fuel cutting are easy to spot since they display a large heat-affected zone. When a processor is looking to be able to hold tight tolerances on the cut, they will more likely select plasma, water jet, or the laser cutting processes over oxy fuel cutting.
Comparing between plasma arc cutting and oxy fuel cutting
Compared to oxyfuel, plasma offers many benefits that are worth considering; faster cut speeds, better cut quality, less rework, higher productivity, more material flexibility, increased efficiency and lower total costs of operation. Plasma cutting can be performed on any type of conductive metal such as mild steel, aluminum and stainless are some examples. Plasma machine is easy to set up, often including pre-set gas pressure controls. The plasma machine can cut faster, does not require a pre-heat cycle and arc plasma torches give the highest temperature available from many practicable sources. Then it produces a small and more precise kerfs width, and has a smaller heat-affected zone, which prevents the surrounding area from warping or damaging the paint. The Plasma process also cuts any type of electrically conductive metal (the Oxyfuel process cannot cut stainless steel or aluminum). Plasma cutting is a cleaner, less expensive and more convenient method of metal cutting because clean, dry air is used for most plasma cutting applications.
Oxyfuel cuts by burning, or oxidizing, the metal it is severing. It is therefore limited to steel and other ferrous metals which support the oxidizing process. Metals like aluminum and stainless steel form an oxide that inhibits further oxidization, making conventional oxyfuel cutting impossible. Plasma cutting, however, does not rely on oxidation to work, and thus it can cut aluminum, stainless and any other conductive material. Special arrangements are necessary to transport the gases which also add to the costs. Plasma not need to add the extra costs, since it does not require an open flame or any flammable gas. If a user only cuts very thick mild steel, or frequently needs to heat metal for shaping or bending it, oxyfuel is clearly a good choice.