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CATEGORIES (articles) > Chassis & Bodywork Construction > Metal working > Plasma cutting, how it works

Plasma cutting, how it works

Plasma cutting is a process used to cut steel and other metals (or sometimes other materials) using a plasma torch. In this process, an inert gas (in some units, compressed air) is blown at high speed out of a nozzle; at the same time an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. This plasma is sufficiently hot to melt the metal and moving sufficiently fast to blow molten metal away from the cut. The result is very much like cutting butter with a hot jet of air.

The torch uses a two cycle approach to producing plasma. First, a high-voltage, low current circuit is used to initialize a very small high intensity spark within the torch body, thereby generating a small pocket of plasma gas. This is referred to as the pilot arc. The now conductive plasma contacts the workpiece, which is the anode. The plasma completes the circuit between the electrode and the workpiece, and the low voltage, high current now conducts. If the plasma cutter uses a high frequency/high voltage starting circuit, the circuit is usually turned off to avoid excessive consumable wear. The plasma, which is maintained between the workpiece and electrode, travels at over 15,000 km/h (over twelve times the speed of sound of the ambient air).

Plasma is an effective means of cutting thin and thick alike. Handheld torches can usually cut up to 1/2 in (13 mm) thick steel plate, and stronger computer-controlled torches can pierce and cut steel up to 12 inches (300 mm) thick. Formerly, plasma cutters could only work on conductive materials, however new technologies allow the plasma ignition arc to be enclosed within the nozzle thus allowing the cutter to be used for non-conductive workpieces.

Plasma cutters produce a very hot and very localised 'cone' to cut with. Because of this, they are extremely useful for cutting sheet metal in curved or angled shapes.

Plasma torches were quite expensive, usually at least a thousand U.S. dollars. For this reason they were usually only found in professional welding shops and very well-stocked private garages and shops. However, modern plasma torches are becoming cheaper, and now are within the price range of many hobbyists. Older units may be very heavy, but still portable, while some newer ones with inverter technology weigh only a few pounds yet equal or exceed the capacities of older ones.Also available are computer-controlled plasma cutters that take a CAD file and use it to cut a path in a piece of sheetmetal with great accuracy.

CNC Cutting Methods

Plasma cutters have also been used in CNC machinery. Many manufacturers build CNC cutting tables, some with the cutter built in to the table(Retro Systems). Or more economical table such as PlasmaCAM tables. The ides behind CNC tables is to allow a computer to control the torch head making clean sharp cuts.

Starting methods

Plasma cutters use a number of methods to start the pilot arc, depending on the environment the unit is to be used in and its age. Older cutters use a high voltage, high frequency circuit to start the arc. This method has a number of disadvantages, including risk of electrocution, difficulty of repair, sparkgap maintenance, and the large amount of radio frequency emissions. Plasma cutters working near sensitive electronics, such as CNC hardware or computers, use the contact start method. The nozzle and electrode are in contact. The nozzle is the cathode, and the electrode is the anode. When the plasma gas begins to flow, the nozzle is blown forward. A third, less common method is capacitive discharge into the primary circuit via a Silicon Controlled Rectifier.

Inverter plasma cutters

Analog plasma cutters, typically requiring more than 2 kilowatts, use a heavy mains frequency transformer. Inverter plasma cutters rectify the mains voltage into DC, which is fed into either an IGBT or, more popularly, a MOSFET. The transistors are switched at thousands of hertz, which greatly reduces the magnetic flux, and therefore the size of the transformer. The switch mode topology is referred to as a dual transistor off-line forward converter. Although lighter and more powerful, some inverter plasma cutters, especially those without power factor correction, cannot be run from a generator.

Plasma Gouging

Plasma gouging is a related process, typically performed on the same equipment as plasma cutting. Instead of cutting the material, plasma gouging uses a different torch configeration (torch nozzles and gas diffusers are usually different), and a further torch-to-workpiece distance, to blow away metal. Plasma gouging can be used in a variety of applications, including removing a weld for rework. The additional sparks generated by the process requires the operator to wear a leather shield protecting their hand and forearm. Torch leads also can be protected by a leather sheath.

Note: Plasma torches exist in the real world and are used in the manufacturing industries. They are not a sort of plasma beam weapon (plasma rifle, etc): such weapons are still fictional, as explained in the article Directed-energy weapon.

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