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Encyclopedia > Plasma cutting

Plasma cutting is a process that is 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. The plasma is sufficiently hot to melt the metal being cut and moves sufficiently fast to blow molten metal away from the cut. Plasma can also be used for plasma arc welding and other things. For other uses, see Steel (disambiguation). ... This article is about metallic materials. ... For other uses, see Plasma. ... Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). ...

Contents

Process

Freehand cut of heavy metal
Freehand cut of heavy metal

The HF Contact type typically found in budget machines uses a high frequency high voltage spark to ionise the air through the torch head and initiate an arc. The arc can only be formed if the torch is in contact with the job material. HF Contact type machines are not suitable for applications involving CNC cutting. Image File history File links Metadata Size of this preview: 800 × 533 pixelsFull resolution (2400 × 1600 pixel, file size: 617 KB, MIME type: image/jpeg) Heavy metal cut with a plasma cutter freehand. ... Image File history File links Metadata Size of this preview: 800 × 533 pixelsFull resolution (2400 × 1600 pixel, file size: 617 KB, MIME type: image/jpeg) Heavy metal cut with a plasma cutter freehand. ...


The Pilot Arc type 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 pilot arc has a return electrical path built into the torch head. The pilot arc will maintain until it is brought into proximity of the workpiece where it ignites the main plasma cutting arc. Plasma arcs are extremely hot and are in the range of 15,000 degrees Celsius. Josephson junction array chip developed by NIST as a standard volt. ... This article does not cite any references or sources. ...


Plasma is an effective means of cutting thin and thick materials alike. Hand held torches can usually cut up to 2 in (48 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. For other uses, see Steel (disambiguation). ...


Since plasma cutters produce a very hot and very localized 'cone' to cut with, they are extremely useful for cutting sheet metal in curved or angled shapes.


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. It has been suggested that this article or section be merged with Radio waves. ... For other uses, see CNC (disambiguation). ... Diagram of a copper cathode in a Daniells cell. ... Diagram of a zinc anode in a galvanic cell. ... A Silicon Controlled Rectifier (or semiconductor controlled rectifier) is a 4-layer solid state device that controls current flow. ...


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 a MOSFET. IGBT transistors operate independently of one another whilst MOSFET transistors need to be paralleled. With paralleled MOSFET transistors if one of the transistors activates prematurely it can lead to a cascading failure of one quarter of the inverter. IGBT transistors do not have this problem as they are independent however are very expensive to manufacture and generally used in high current machines where it is not possible to parallel sufficient MOSFET transistors. The transistors are switched at thousands of Hertz, which greatly reduces the magnetic flux in the step down transformer, and therefore the size of the transformer is reduced accordingly. 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. The Insulated Gate Bipolar Transistor combines the simple gate drive characteristics of the MOSFET with the high current and low saturation voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device. ... The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is by far the most common field-effect transistor in both digital and analog circuits. ... A switched-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator - an internal control circuit that switches the load current rapidly on and off in order to stabilise the output voltage. ... Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor (p. ...


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 configuration (torch nozzles and gas diffusers are usually different), and a longer 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.


CNC cutting methods

Plasma cutters have also been used in CNC machinery. Manufacturers build CNC cutting tables, some with the cutter built in to the table. The idea behind CNC tables is to allow a computer to control the torch head making clean sharp cuts. Modern CNC plasma equipment is capable of multi-axis cutting of thick material, allowing opportunities for complex welding seams on CNC welding equipment that is not possible otherwise. For thinner material cutting, plasma cutting is being progressively replaced by laser cutting, due mainly to the laser cutter's superior hole-cutting abilities. Laser cutting is a technology which uses a laser to cut materials, and is usually used in industrial manufacturing. ...


A specialized use of CNC controlled Plasma Cutters has been in the HVAC industry. Software will process information on ductwork and create flat patterns to be cut on the cutting table by the plasma torch. This technology has enormously increased productivity within the industry since its introduction in the early 1980s. HVAC may also stand for High-voltage alternating current HVAC systems use ventilation air ducts installed throughout a building that supply conditioned air to a room through rectangular or round outlet vents, called diffusers; and ducts that remove air from return-air grilles Fire-resistance rated mechanical shaft with HVAC... A round duct connecting to a typical diffuser Ducts are used in heating, ventilation, and air conditioning (HVAC) to carry air - these include conditioned, fresh, stale, foul (toilet extract), and contaminated air (flue exhaust, fume extraction) - from place to place. ...


New technology

In the past decade plasma torch manufacturers have engineered new models with a smaller nozzle and a thinner plasma arc. This allows near laser quality on plasma cut edges. Several manufacturers have combined precision CNC control with these torches to allow fabricators to produce parts that require little or no finishing.


Costs

Plasma torches were once 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. USD redirects here. ... For other uses, see Inverter (logic gate) and Inverter. ...


See also

Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). ... A diagram of a water jet cutter A water jet cutter is a tool capable of slicing into metal or other materials using a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance. ...

External links

  • Alternative to the cut puts molding on (waterjet cutting)
  • HowStuffWorks on plasma cutting
  • Plasma cutting survey

  Results from FactBites:
 
Plasma Cutting: Determining if it's Right for You and What to Look for in a Machine | Lincoln Electric (3019 words)
In simplest terms, plasma cutting is a process that uses a high velocity jet of ionized gas that is delivered from a constricting orifice.
Plasma cutting is typically easier for the novice to master, and on thinner materials, plasma cutting is much faster than oxyfuel cutting.
The plasma cutting machines are typically more expensive than oxyacetylene, and also, oxyacetylene does not require access to electrical power or compressed air which may make it a more convenient method for some users.
Stainless Plate Products - White Paper, Plasma Cutting (2449 words)
A plasma jet can either be operated in the transferred mode, where the power supply is connected between the electrode and the workpiece, or in the nontransferred mode where the power supply is connected between the electrode and the nozzle.
Although a stream of hot plasma emerges from the nozzle in both modes of operation, the transferred mode is always used in plasma cutting because the usable heat input is most efficiently applied when the arc is in electrical contact with the workpiece.
An important property of these cuts is that when viewed in the direction of the cut, as shown in Figure 6, the right side of the kerf is square and the left side of the kerf is slightly beveled.
  More results at FactBites »

 
 

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