Electric heating – Metal heating – By arc
Reexamination Certificate
2001-02-27
2004-03-09
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121540, C219S121520, C219S121590
Reexamination Certificate
active
06703581
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to plasma arc torches, and more particularly to a contact start plasma arc torch.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting, welding, and spray bonding metal workpieces. Such torches typically operate by directing a plasma consisting of ionized gas particles toward the workpiece. In general, a pressurized gas to be ionized is directed through the torch to flow past an electrode before exiting the torch through an orifice in the torch tip. The electrode has a relatively negative potential and operates as a cathode. The torch tip, which is adjacent to the end of the electrode at the front end of the torch, constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the torch, an arc is established across the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as a flame extending externally from the tip. As the torch head or front end is positioned close to the workpiece, the arc transfers between the electrode and the workpiece because the impedance of the workpiece to negative potential is typically lower than the impedance of the torch tip to negative potential. During this “transferred arc” operation, the workpiece serves as the anode.
Plasma arc torches may be found in both “non-contact start” and “contact start” varieties. In non-contact start torches, the tip and electrode are normally maintained at a fixed physical separation in the torch head. Typically, a high voltage high frequency signal is applied to the electrode (relative to the tip) to establish a pilot arc between the electrode and the tip. As mentioned above, when the torch head is moved toward the workpiece, the arc transfers to the workpiece. By way of contrast, in conventional contact start torches, the tip and/or the electrode make electrical contact with each other generally at the bottom of the electrode. For example, a spring or other mechanical means biases the tip and/or electrode longitudinally such that the tip and electrode are biased into electrical contact to provide an electrically conductive path between the positive and negative sides of the power supply. When the operator squeezes the torch trigger, a voltage is applied to the electrode and pressurized gas flows through the torch to the exit orifice of the torch tip. The gas causes the tip and/or the electrode to overcome the bias and physically separate. As the tip and electrode separate, a pilot arc established therebetween is blown by the gas toward the exit orifice of the tip.
One disadvantage associated with the conventional contact start plasma torch described above is that repeated axial movement of the electrode, the tip or both can result in axial misalignment between the electrode and tip. Also, by establishing the pilot arc between the electrode and the tip at the bottom of the electrode, damage is caused to the tip adjacent the central exit orifice of the tip. Axial misalignment of the electrode and tip, as well as any damage to the tip, can result in decreased torch performance and/or cut quality. Consequently, frequent replacement of the tip is required. For conventional contact start torches in which the tip is movable for establishing electrical contact with the electrode, the tip is in different longitudinal positions in the on and off modes of the torch, making it cumbersome for an operator to control the relative position of the tip with respect to a workpiece being cut. It is also difficult to conduct drag cutting of a workpiece, where the tip is set down onto the workpiece during cutting, because the tip would be undesirably moved into contact with the electrode upon being set down onto the workpiece.
SUMMARY OF THE INVENTION
Among the several objects and features of the present invention is the provision of a contact start plasma torch and method of operating such a torch which reduces the frequency of torch tip replacement; the provision of such a torch and method which reduces the risk of axial misalignment between the electrode and the tip; the provision of such a torch which reduces the risk of tip damage adjacent the central exit orifice of the tip; and the provision of such a torch and method which eliminates the need for axial movement of the electrode and/or the tip to generate a pilot arc.
In general, a contact start plasma torch of the present invention comprises a cathode body adapted for electrical communication with the negative side of a power supply and an anode body adapted for electrical communication with the positive side of the power supply. A primary gas flow path directs working gas from a source of working gas through the torch. A conductive element of the torch is constructed of an electrically conductive material and is free from fixed connection with the cathode body and the anode body. The torch is operable between an idle mode in which the conductive element provides an electrically conductive path between the cathode body and the anode body and a pilot mode in which a pilot arc formed between the conductive element and at least one of said cathode body and said anode body is adapted for initiating operation of the torch by exhausting working gas in the primary gas flow path from the torch in the form of an ionized plasma.
Another embodiment of the present invention is directed to a contact start plasma torch of the type having a primary gas flow path for directing a working gas through the torch whereby the working gas is exhausted from the torch in the form of an ionized plasma. The torch of this embodiment generally comprises an electrode having a longitudinally extending side surface and a bottom surface. A tip surrounds the electrode in spaced relationship therewith to at least partially define the primary gas flow path of the torch for directing a working gas through the torch in a downstream direction. The tip has a central exit orifice in fluid communication with the primary gas flow path for exhausting working gas from the torch. The bottom surface of the electrode is in longitudinally opposed relationship with the central exit orifice of the tip. Opposed contact surfaces are disposed in the torch, with at least one of the contact surfaces being movable relative to the other one of the contact surfaces. The torch is operable between an idle mode in which the contact surfaces are positioned relative to each other to provide an electrically conductive path therebetween and a pilot mode in which the contact surfaces are in spaced relationship with each other whereby a pilot arc is formed between the contact surfaces. The contact surfaces are disposed in the torch upstream from the bottom surface of the electrode whereby the pilot arc is formed generally within the primary gas flow path upstream from the bottom surface of the electrode and is blown by working gas in the primary gas flow path toward the central exit orifice of the tip for exhausting working gas from the tip in the form of an ionized plasma.
A conductive element of the present invention is adapted for use in a contact start plasma torch of the type having an electrode in electrical communication with the negative side of a power supply and a tip surrounding the electrode in spaced relationship therewith to at least partially define a primary gas flow path of the torch, the tip being in electrical communication with the positive side of the power supply and having a central exit orifice in fluid communication with the primary gas flow path for exhausting working gas from the tip in the form of an ionized plasma. The conductive element generally comprises a generally cup-shaped body constructed of an electrically conductive material. The conductive element is adapted for movement relative to the electrode and the tip between a first position is corresponding to an idle mode of the torch in which the conductive element provides an electrically conductive path between the positive
Hewett Roger W.
Horner-Richardson Kevin D.
Jones Joseph P.
Small David A.
Harness & Dickey & Pierce P.L.C.
Paschall Mark
Thermal Dynamics Corporation
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