Electric heating – Metal heating – By arc
Reexamination Certificate
2001-01-30
2002-03-26
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S075000, C219S121510
Reexamination Certificate
active
06362450
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to plasma arc torches and, more particularly, to a method and apparatus for supplying a gas flow for supporting an electric arc in a plasma arc torch.
BACKGROUND OF THE INVENTION
Plasma arc torches are commonly used for the working of metal, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to a workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc in a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein, which supports the arc. The emissive element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of this low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and alloys thereof. A nozzle surrounds the discharge end of the electrode and provides a pathway for directing the arc towards the workpiece.
A problem associated with torches of the type described above is the short service life of the electrode, particularly when the torch is used with an oxidizing gas, such as oxygen or air. More particularly, the emissive elements of these torches often erode below the surface of the copper holder at the discharge end. Additionally, the gas tends to rapidly oxidize the copper of the electrode that surrounds the emissive element and, as the copper oxidizes, its work function decreases. As a result, a point is reached at which the oxidized copper surrounding the emissive element begins to support the arc, rather than the emissive element. When this happens, the copper oxide and the supporting copper melt, resulting in early destruction and failure of the electrode.
In order to prevent or reduce the formation of oxidized copper surrounding the emissive element, particularly for air cooled plasma arc torches, the air is circulated rapidly about the electrode to improve heat transfer from the arc away from the electrode. A conventional method for the air to be distributed in an air cooled plasma arc torch is for the air to first be used in some fashion to cool the electrode and then to be split into separate primary and secondary flows. Typically, this is accomplished by means of a gas baffle positioned between the nozzle and the electrode for splitting the flow into the primary or cutting gas flow and the secondary or shield gas flow, which helps maintain the position of the arc. More specifically, the primary flow of the gas passes through holes in the gas baffle into a chamber defined by a primary nozzle and the electrode and is ejected by the primary nozzle, while the rest of the gas is directed out a secondary nozzle so as to surround the primary gas flow. Disadvantageously, the baffle splits the gas into the primary flow and secondary flow before the nozzle chamber, which limits the ability of the torch to transfer heat from the electrode and can reduce the speed of the torch, as discussed below.
Baffles also add to the cost and complexity of manufacturing and assembling the torch. More specifically, baffles are subject to failure and can occasionally be inadvertently omitted by an operator during assembly of the torch. Furthermore, baffles tend to become brittle over time and eventually develop cracks, which often lead to catastrophic failure unless the baffles are frequently replaced. Even when replaced on a regular preventative maintenance cycle, which adds further cost to the torch, human error may lead to the baffles being left out during assembly of the torch, which can damage the torch or cause the torch to operate incorrectly. In addition, baffles can also permit the arc to “jump” or track across the baffle, which can also damage the torch. Specifically, the use of baffles can result in a convoluted set of passages in and around the electrode through which air can pass, which can lead to migration of the arc through the passages. Although attempts have been made to insulate the labyrinth of passages through the torch, arcing through the often damp air in the passages has been a problem with conventional torches.
Another problem with conventional torches is the lack of cooling achieved by the gas due to splitting the gas into different flows before the gas has circulated along substantially the entire length of the electrode. In particular, many torches split the gas into the primary and secondary flows at a location intermediate the opposite ends of the electrode. This is considered necessary in order to limit the pressure realized in the nozzle chamber while providing adequate flow for cooling. In order to cool the torch while avoiding failure of the torch due to excessive nozzle pressure, often as much as 70-90% of the total gas supplied to a conventional torch is diverted away from the nozzle chamber to other outlets, which direct the secondary flow. As a result, only a portion of the total gas supplied to the torch is available for cooling the electrode along substantially the entire length of the electrode, and even less gas pressure than is optimal may be available at the exit end of the nozzle as a primary gas flow. Accordingly, conventional torches have limited cutting speeds, which adds time and expense to the torch operation. It is desirable to provide a greater nozzle chamber pressure so that higher cutting speeds can be realized. This is a difficult proposition, however, due to the limitations of conventional torches as described, and for the fact that most manufacturing locations and welding shops use standard “shop” air pressure that cannot be increased in order to increase the gas pressure in the nozzle chamber.
Several patents discuss plasma arc torches having various flow patterns. For example, U.S. Pat. No. 5,726,415 to Luo et al. discloses a plasma are torch with an electrode having an metallic holder with an emissive element positioned at a discharge end thereof. The torch also includes a nozzle, which in combination with the holder defines an annular gas chamber therebetween for directing a cooling gas about the electrode. The nozzle also defines a cylindrical exhaust port for directing a primary gas flow towards a workpiece, and bleed ports positioned in the rear portion of the nozzle for venting a majority of the gas through bores for use as a shield or secondary gas flow. In operation, the bleed ports bleed approximately 90% of the gas, thus leaving 10% of the gas to cool the full length of the electrode and exit the cylindrical exhaust port as the primary gas flow towards the workpiece. Thus, only a fraction of the gas entering the torch travels substantially the length of the electrode, which decreases the cooling capability of the gas.
U.S. Pat. No. 4,558,201 to Hatch discloses a plasma arc torch having a reversible electrode that has both a forward insert and a rearward insert positioned at opposing ends thereof. The electrode defines a plurality of passageways for directing the gas towards a workpiece. In particular, gas is directed through channels around the exterior of the electrode as well as through a central passage extending along the longitudinal axis of the electrode. As the gas reaches the midpoint of the electrode, however, the gas is split into a primary flow and a secondary flow, wherein the secondary flow is directed away from the electrode around an insulator to a front portion of a chamber defined
Alston & Bird LLP
Paschall Mark
The ESAB Group, Inc.
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