The present invention relates to plasma arc cutting, and more particularly to plasma arc cutting torches.
The art of plasma arc cutting is well known for cutting materials such as steel at very high temperatures using a tightly spun jet of ionized electrically conductive gas (known as a plasma arc). As shown in FIG. 1, the plasma arc 16 is generated by a torch 10, and is directed at a workpiece 32. The workpiece 32 functions as a conductor through which the plasma arc 16 completes a circuit. The torch body 12 includes the electrical, gas, and cooling connections for transferring the plasma arc 16 to the workpiece 32. A nozzle 14 is attached at the end of the torch 12 over a cathode. The nozzle 14 provides a chamber for ionizing a jet of gas, and focuses the resulting plasma arc 16 through an exit orifice.
The cutting torch nozzle is a consumable fabricated of a relatively inexpensive material such as copper or brass. It is common to replace the nozzle every few hours of cutting time, the length of time between replacements being at least partially dependent on the power of the plasma arc. The primary function of the nozzle is to focus the plasma arc 16 through the relatively small exit orifice. Precise focus is important to provide adequate cutting power. If the nozzle 14 is incapable of focusing the plasma into a tightly spun jet, the resulting plasma arc 16 may not have the power to cut a desired workpiece 32. The inexpensive materials used for fabricating the nozzles have relatively low melting temperatures. Consequently, small changes in the width of the plasma arc, caused, for instance, by erosion of the cathode over time, can change the path of the plasma arc, causing it to melt a portion the nozzle. This in turn leads to further deformed arc patterns.
In the ideal and classic plasma cutting situation, the workpiece is placed directly in line with the nozzle exit orifice. However, it is often the case that a workpiece is presented close to a nozzle without being directly under the nozzle orifice, causing the plasma arc to reach off to one side in search of a completed electrical circuit through the workpiece. This can bring the arc into close proximity or contact with the copper nozzle, melting away some of the copper material and preventing the nozzle from focusing the plasma arc. This failure mode is shown in FIG. 2, wherein a workpiece 32 is positioned so that is it not directly in front of the exit orifice 22, and the normally cylindrical orifice 22 has a portion 22′ melted away. Once the nozzle has been damaged, it is no longer capable of focusing the plasma arc properly and must be replaced.
During cutting, molten pieces of the workpiece are sprayed in many directions. The molten pieces, known as slag, are hot enough to melt the outer surface of the nozzle and adhere to the nozzle surface, further deforming the nozzle and significantly shortening the nozzle's useful life.
Prior artisans have attempted to reduce nozzle wear by adding a heat resistant insulating cap, such as a ceramic, to the end of the nozzle. The high temperature qualities of these caps provide some protection from slag, and the insulating qualities act to reduce the tendency of the arc to stray in search of a conductor. Unfortunately, such caps are not completely effective at preventing stray arcs in the presence of a large conductor such as a workpiece, and they provide no protection for the inner surface of the nozzle orifice in these situations.
In a different plasma arc field, plasma spray technology is used to spray a coating onto the surface of another material. The plasma spray torch provides a lower power plasma arc that uses the surface of the nozzle as an anode. This is known as a non-transferred plasma arc. When the non-transferred arc is starting or ending it engages the inner surface of the nozzle orifice and causes metal loss in the inside of the nozzle orifice. Accordingly, it is known in plasma spray applications to provide a high-temperature insert, such as tungsten, in the nozzle opening to reduce nozzle wear and therefore to increase nozzle life. An example of such an insert is illustrated in U.S. Pat. No. 5,897,059 to Müller.