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Plasma cutting is a process that is used to cut steel and other metals of different thicknesses (or sometimes other materials) using a plasma torch. In this process, a gas (oxygen, air, inert and others dependant on material) 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 hot enough to melt the metal being cut and moves fast enough to blow molten metal away from the cut.
A plasma torch requires you to start an arc between the electrode in the torch and the work you intend to cut. To start this arc there are various methods used, commonly High Frequency or Blow Back. HF (High Frequency) is used in most modern industrial plasma systems and in many older systems. The second common method is known as blow back, or short circuit start.
In a high frequency plasma system there are no moving parts in the torch. The electrode is connected to the power sources negative output and the work connected to the positive. The electrode is a the conductor from which the arc starts and travels to the work piece. To start the arc the plasma initially connects the nozzle to positive. The nozzle is responsible for directing the gas flow, it wraps around the electrode and has a small output hole in which the gas flow and the plasma arc is directed. A DC potential between the nozzle an electrode is established and the HF circuit is turned on. The high frequency, high voltage causes a small low current arc to transfer between the nozzle and electrode in the torch. The low HF current creates a path of ionized gas allowing the lower voltage DC output to conduct. The current established between the electrode and nozzle in the torch is known as a Pilot Arc.
In the blow back method the arc is started with the plasma torch's electrode and nozzle initially touching. The power source draws a current from the nozzle to the electrode. After the current is established the power source will turn on the gas flow. Due to the design of the torch when gas begins to flow the electrode will pull away from the nozzle. As the electrode retracts the current draw between it and the nozzle will create a spark. With this ionized path of gas, lower voltage DC current is allowed flow and once again have a Pilot Arc.
In both methods above you achieve a pilot arc, which is an ionized path of gas between the electrode and nozzle in the torch. Once this pilot arc is brought close to the work, which is at the same potential as the nozzle, current will transfer directly from the electrode to the work. The plasma source will detect the current into the work and disconnect the nozzle (in most cases) allowing full current flow from the electrode to nozzle.
Plasma is an effective means of cutting thin and thick materials alike. Hand-held torches can usually cut up to 38mm thick steel plate, and stronger computer-controlled torches can cut steel up to 150 mm thick. 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.
Plasma cutting grew out of plasma welding in the 1960s, and emerged as a very productive way to cut sheet metal and plate in the 1980s. It had the advantages over traditional "metal against metal" cutting of producing no metal chips, giving accurate cuts, and producing a cleaner edge than oxy-fuel cutting. Early plasma cutters were large, somewhat slow and expensive and, therefore, tended to be dedicated to repeating cutting patterns in a "mass production" mode.
As with other machine tools, CNC (computer numerical control) technology was applied to plasma cutting machines in the late 1980s into the 1990s, giving plasma cutting machines greater flexibility to cut diverse shapes "on demand" based on a set of instructions that were programmed into the machine's numerical control. These CNC plasma cutting machines were, however, generally limited to cutting patterns and parts in flat sheets of steel, using only two axes of motion (referred to as X Y cutting).
Proper eye protection (but not gas welding goggles as these do not give UV protection) and face shields are needed to prevent eye damage called arc eye as well as damage from debris, as per Arc Welding. It is recommended to use green lens shade #8 or #9 safety glasses for cutting to prevent the retinas from being "flashed" or burned. OSHA recommends a shade 8 for Arc Current less than 300, but notes that "These values apply where the actual arc is clearly seen. Experience has shown that lighter filters may be used when the arc is hidden by the workpiece." Lincoln Electric, a manufacturer of plasma cutting equipment, says, "Typically a darkness shade of #7 to #9 is acceptable." Longevity Global, Inc., another manufacturer, offers this more specific table for Eye Protection for Plasma Arc Cutting at lower amperages :
|Current Level in Amps||Minimum Shade Number|
Leather gloves, apron and jacket are also recommended to prevent burns from sparks and debris.
Plasma cutters use a number of methods to start the arc. In some units, the arc is created by putting the torch in contact with the work piece. Some 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, spark gap maintenance, and the large amount of radio frequency emissions. Plasma cutters working near sensitive electronics, such as CNC hardware or computers, start the pilot arc by other means. 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 supply to DC, which is fed into a high-frequency transistor inverter between 10 kHz to about 200 kHz. Higher switching frequencies give greater efficiencies in the transformer, allowing its size and weight to be reduced.
The transistors used were initially MOSFETs, but are now increasingly using IGBTs. With paralleled MOSFETs, if one of the transistors activates prematurely it can lead to a cascading failure of one quarter of the inverter. A later invention, IGBTs, are not as subject to this failure mode. IGBTs can be generally found in high current machines where it is not possible to parallel sufficient MOSFET transistors.
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 (that means manufacturer of the inverter unit forbids doing so; it is only valid for small, light portable generators). However newer models have internal circuitry that allow units without power factor correction to run on light power generators.
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 or heavy insulation.
CNC cutting methods
Some plasma cutter manufacturers build CNC cutting tables, and some have the cutter built into the table. CNC tables allow a computer to control the torch head producing clean sharp cuts. Modern CNC plasma equipment is capable of multi-axis cutting of thick material, allowing opportunities for complex welding seams that are not possible otherwise. For thinner material, plasma cutting is being progressively replaced by laser cutting, due mainly to the laser cutter's superior hole-cutting abilities.
A specialized use of CNC Plasma Cutters has been in the HVAC industry. Software processes information on ductwork and creates 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.
In recent years there has been even more development. Traditionally the machines' cutting tables were horizontal, but now vertical CNC plasma cutting machines are available, providing for a smaller footprint, increased flexibility, optimum safety and faster operation.
In the past decade plasma torch manufacturers have engineered new models with a smaller nozzle and a thinner plasma arc. This allows near-laser precision 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.
Plasma torches were once quite expensive. 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 little, yet equal or exceed the capacities of older ones.
- Laser cutting
- Air carbon arc cutting
- Plasma arc welding
- Water jet cutter
- List of plasma (physics) articles
- The Life and Times of Plasma Cutting - How The Technology Got Where It Is Today by Thierry Renault and Nakhleh.
- Making Plasma Cutting Easier - Using CNC Automation Technology by Brad Thompson and Kris Hanchette, The Fabricator, August 2003.
- Sacks, Raymond; Bohnart, E. (2005). "17". Welding Principles and Practices (Third ed.). New York: McGraw_Hill. p. 597. ISBN 978-0-07-825060-6.
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