Electric heating – Metal heating – By arc
Reexamination Certificate
1999-04-29
2001-01-23
Walberg, Teresa (Department: 3742)
Electric heating
Metal heating
By arc
C219S121590
Reexamination Certificate
active
06177647
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a plasma arc electrode for use in plasma arc torches and, in particular, to an electrode which can be readily fabricated and provides a long service life, as well as a method for making the same.
BACKGROUND OF THE INVENTION
Plasma arc torches are commonly used when fabricating metal structures. They are frequently employed in operations such as cutting, welding, treating surfaces, and annealing. These torches include an electrode which supports an arc which is struck between the electrode and the workpiece. The arc is sustained therebetween when the torch is in the arc transfer mode of operation. The electrodes currently used in many plasma arc torches have an electrode tip body fabricated from a high conductivity material such as copper or a copper alloy. An emissive insert is placed in the tip of the electrode. These emissive inserts are fabricated from materials with a low work function, and are frequently made from hafnium, zirconium, or tungsten. Further discussion of these electrodes is found in U.S. Pat. No. 5,023,425, which also notes that such electrodes have short lives, since the arc may become supported from the tip body of the electrode, rather than from the emissive insert, causing deterioration of the tip body. To extend the life of such electrodes, the '425 patent teaches providing a sleeve having a radial thickness of at least about 0.01 inches positioned between the emissive insert and the copper or copper alloy tip body of the electrode. This sleeve is fabricated from a metallic material having a work function which is greater than that of the material of the emissive insert, and maintains the arc being supported by the emissive insert. The '425 patent suggests the use of a variety of noble metals for the sleeve, and provides examples of electrodes employing silver sleeves.
The '425 patent teaches a method for fabricating such an electrode which includes the following steps. A first blank of copper or copper alloy is provided, having a front face. A cavity is bored in the front face. A second blank of silver is formed and is metallurgically bonded into the cavity of the first blank by use of a brazing material. An opening is then drilled into the second blank, and an emissive insert is force fitted into the drilled opening. The assembly is then machined to provide a smooth front face for the assembly.
U.S. Pat. No. 5,200,594 teaches an alternative solution to the problem of short electrode life by employing a plated emissive insert. The emissive insert is first plated with nickel and thereafter plated with a noble metal.
While the solutions of both of the above mentioned patents provide an electrode with a longer life, they complicate the process of fabricating the electrode. Furthermore, the electrodes of the '425 patent require substantial quantities of silver to fabricate, much of which is machined away during the manufacturing of the electrode. The method of the '594 patent reduces the quantity of the high work function metal which is required; however, it requires a double plating process where an intermediate layer of nickel is deposited onto the insert before plating with a noble metal. Furthermore, the '594 patent teaches that the use of a nickel plate is required to assure adhesion of plated layers during the subsequent processing. The adhesion is reported to be important to maintain a good thermal path for dissipating the heat generated in use, thereby extending the life of the electrode.
Thus, there is a need for an electrode which can be readily fabricated with little waste and which will provide a long service life.
SUMMARY OF THE INVENTION
The present invention relates to an electrode for use in plasma arc torches and a method for making the same. These electrodes have a tip body which is typically fabricated from copper or a copper alloy. The tip body is symmetrically disposed about a tip central axis and terminates in a torch engaging end and a free terminating surface. The free terminating surface is substantially normal to the tip central axis. The tip body is configured with a tip cavity, having a cavity sidewall which extends inward from the free terminating surface and is symmetrically disposed about the tip central axis. An emissive insert having an insert sidewall which is symmetrically disposed about an insert axis resides in the tip cavity and is positioned such that the insert axis and the tip central axis are substantially coincident.
The method of fabricating an electrode of the present invention is initiated by providing a tip body blank, which is typically either copper or a copper alloy. The tip body blank has a blank central axis and a free terminating surface which is substantially normal to the blank central axis.
A tip cavity is formed in the free terminating surface of the tip body blank. The tip cavity is configured such that it has a cavity sidewall which is symmetrically disposed about the blank central axis of the tip body blank.
An emissive insert is provided, having a textured insert sidewall symmetrically disposed about an insert axis. The emissive insert is configured to be loosely insertable into the tip cavity of the tip body blank, such that the insert axis is substantially coincident with the central axis of the tip body blank when the emissive insert is so inserted. The insert sidewall has a textured surface with protuberances incorporated therein to increase the effective surface area of the insert sidewall. Preferably, the protuberances are sufficient in size and number to increase the effective surface area by at least about 30%. Such a textured surface can typically be provided by grooves or by etching the surface of the insert sidewall.
A noble metal foil is selected, the foil having an insert-contacting surface and a cavity-contacting surface which are spaced apart by a thickness T of less than about 0.01 inches. While a variety of noble metal foils can be employed, including silver, gold, platinum, and rhodium, it is preferred that the foil be silver and it is further preferred that the silver foil have a purity of at least 99.5% by weight, and more preferably 99.7% by weight.
The emissive insert and the noble metal foil are placed into the tip cavity of the tip body blank, arranged such that the insert-contacting surface of the foil faces the insert sidewall of the emissive insert, while the cavity-contacting surface of the foil faces the cavity sidewall. The foil is of sufficient size to substantially surround the insert sidewall, providing a foil-wrapped insert.
When the emissive insert has a diameter of greater than about 0.06 inches, the insert-contacting surface of the noble metal foil is preferably wrapped around the insert sidewall to provide a foil-wrapped insert prior to insertion into the tip cavity. The tip cavity, the thickness T of the foil, and the emissive insert are sized such that the foil-wrapped insert can be readily inserted into the tip cavity. Since there is an elastic component to the deformation of the foil when it is bent around the emissive insert, it is frequently necessary to maintain the foil in position around the insert sidewall until the foil-wrapped insert is placed into the tip cavity.
Alternatively, when the emissive insert has a diameter of less than about 0.06 inches, the foil is preferably rolled, with the cavity-contacting surface facing outwards, and inserted into the tip cavity prior to inserting the emissive insert. Again, the elastic component of the deformation of the foil provides a spring like action. When the rolled foil is inserted into the tip cavity and released, the elasticity of the foil forces the cavity-contacting surface of the coiled foil into contact with the cavity sidewall. Again, the tip cavity, the thickness T, and the emissive insert are sized such that there is sufficient clearance to allow the insertion of the emissive insert into the foil-lined tip cavity to provide a foil-wrapped insert.
In either case, after the emissive insert and the foil have been plac
Semprebon Jeffrey E.
Tatras, Inc.
Van Quang
Walberg Teresa
Weins Michael J.
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