How Plasma ARC Cutting Works
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Background
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Plasma-arc welding was originally developed as a means of welding together aircraft materials during World War II (WWII). As the technology was improved, it was discovered that the electrical arc combined with high-pressure plasma (ionized gas), could be used to cut metal in addition to welding it. Plasma-arc cutting is widely seen as an alternative to oxyfuel cutting methods. Oxyfuel methods use oxidation to cut the metal, but many metals including stainless steel, aluminum and alloys will not oxidize, or they oxidize poorly. Plasma-arc welding does not rely on oxidation and is often used to cut nonoxidizing metal.
Fundamentals
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Most plasma-arc cutting systems follow a design and method similar to what follows. At the core of the system is a tungsten electrode. A chamber will surround the electrode and be filled with pressurized gas/plasma. Inert gases (argon and nitrogen, for example) are typically used to create the plasma, but systems have been developed that employ regular air. Such systems require the use of more expensive electrodes made of hafnium or zirconium, which have copper holders. Many systems use water cooling around the plasma chamber to keep the nozzle unit, usually a small-bore copper nozzle that encases the electrode plasma chamber, cooled. Alternative systems make use of water injection or dual gas systems to create better cuts through plasma constriction. Water shrouding systems have been used to decrease the noise levels and fumes generated during the process.
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Cutting
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To cut using plasma-arc systems requires that an arc is created between the workpiece and the nozzle. While typically only 50 to 60 volts are needed to maintain the arc, it can require up to 400 volts to create the initial arcing process between the nozzle and the workpiece. To accomplish this, the electrode is negatively charged and the workpiece is positively charged. This allows the ionized gas to carry the arc or electron stream between the electrode/nozzle and the metal to be cut. The plasma itself, due in large part to the high pressure, is extremely hot and may reach temperatures of over 20,000 degrees Celsius. The arc itself actually melts the metal, while the pressurized plasma removes loose detritus from the cutting path. The nature of the melting cut can make the process inappropriate for very thick metals because it tends to round the upper edges of the metal. This cutting process tends to work best with metals less than two inches thick.
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