Machinery's Handbook, 31st Edition
Electro-Thermal Processes 1349 electric arc is generated between the plasma torch and workpiece. The plasma arc melts the material and a high velocity gas jet removes the molten material. A plasma cutting torch consists of an electrode that serves as an attachment point of the arc and a nozzle used to confine the plasma arc and direct the high velocity gas jet toward the workpiece. This gas jet further confines or constricts the arc and protects the nozzle from the high temperature plasma. The core of the plasma arc is made of ionized gas atoms and electrons, which exit the torch nozzle at temperatures near 40,000°F (22,000°C). The plasma arc transfers energy via electrons re-entering the conductive workpiece, thermal conduction, and recombination of the plasma ions. Some plasma torches may use a secondary or shield gas flow to protect the nozzle from spatter, electrically isolate the nozzle from the workpiece, and improve the cutting pro- cess. Other plasma torch enhancements include liquid cooling of torch components, high-precision nozzles to enhance confinement of the plasma jet via additional gases or nozzle geometry, or use of magnetic fields and the resulting Lorentz forces to constrict the arc. Efforts to constrict the arc as much as possible are meant to narrow the plasma jet to produce a narrow kerf, as well as increase the energy density of the plasma to allow a higher range of cutting speeds. Some plasma torch components wear out and need to be replaced periodically to main- tain cut quality and reliable operation. Such consumable items include the electrode, nozzle and shield. Plasma Cutting Performance Characteristics: Plasma cutting is a highly flexible method of cutting metal. Small portable systems are available for large or difficult-to- access workpieces. Some plasma cutting processes can operate on materials submerged in water to reduce noise, fumes, and light emission. Plasma torches also can be used in multiple orientations, enabling three-dimensional cutting. Plasma systems can make both through cuts and non-through gouges. It is possible to cut some materials up to 6 in. (152 mm) thick, but cut quality tends to suffer for thicknesses beyond 2 in. (50.8 mm). Since this is a thermal process, an HAZ will surround each cut. Depending on material and process parameters, an HAZ less than 0.01 in. (0.25 mm) thick is possible with little edge hardening. Cut quality characteristics, including cut smooth- ness, bevel angle, kerf width, and the presence of slag or dross on the bottom of the cut, are highly dependent on material and process parameters. Smoothness of the cut is generally good but can deteriorate at very high speeds or for thicknesses above 2 in. (50.8 mm). A plasma cut usually has a positive bevel angle, where the entrance to the cut is larger than the cut exit. Plasma cuts also have two different bevel angles on the two sides of the cut kerf. This difference is a result of the plasma gas flow swirl component relative to the direction of torch motion. As a result, the plasma torch produces a “good” cut side, with a typical bevel angle of 2 degrees, and a “bad” cut side, with an angle close to 5 degrees. Cutting speed and standoff are the most direct ways to influence bevel angle. High cutting speed and standoff distances increase bevel angle, while lower speed and standoff lower the angle. Other factors include type and pressure of the gas, arc current, and nozzle char- acteristics. Precision systems with the right process settings can improve on this, in some cases, providing a straight cut (0 degree bevel). While kerf width is process and workpiece dependent, it often is estimated as 1.5 times the nozzle orifice diameter. Kerf width is typically larger than 0.02 in. (0.5 mm) and often near 0.1 in. (2.5 mm). High standoff, low speed, and high amperage will widen the kerf. Holes cut with a standard plasma torch should be larger in diameter than 1.5 times the material thickness, and larger than 3/16 in. (4.8 mm). State-of-the-art automated plasma systems can produce holes that are cylindrical and accurate enough to rival drilled holes, with a diameter to thickness ratio as low as 1:1. Some automated table systems also can tilt the torch relative to the workpiece, allowing additional control of the cut bevel angle or produce angled cuts. Dross formation depends primarily on cutting speed. Speeds that are too slow will pro- duce thick dross on the bottom of the cut; speeds that are too fast will produce a fine, beady
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