Electrode for plasma arc torch and method of making the same

Details Electrode for plasma arc torch and method of making the same Electrode for plasma arc torch and method of making the same

2000-10-26

2001-12-11

Walberg, Teresa (Department: 3742)

Electric heating

Metal heating

By arc

C219S121500, C219S121510, C219S121590

Reexamination Certificate

active

06329627

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a plasma arc torch and, more particularly, to a novel electrode for use in a plasma arc torch having an improved service life and a method of making the same.
Commonly used for working of metals, plasma arc torches are used for cutting, welding, surface treatment, melting and annealing. These torches include an electrode that supports an arc that extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex of gas, and in some torch designs, it is conventional to envelope the gas and arc with a swirling jet of water.
The electrode used in a conventional torch of the type described typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive insert embedded therein, which supports the arc. The insert 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, which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the insert is thus capable of readily emitting electrons when an electrical potential is applied thereto, and commonly used insert materials include hafnium, zirconium, and tungsten.
One of the major problems connected with the torches referred to above is the shortness of service life of their electrodes, especially when the torches are used with an oxidizing arc gas, such as oxygen or air. In those torches, the gas appears to rapidly oxidize the copper, and as the copper oxidizes, its work function fails. As a result, the oxidized copper that surrounds the insert begins to support the arc in preference to the insert. After this occurs, the copper melts, thereby causing early destruction and/or failure of the electrode.
U.S. Pat. No. 5,023,425 (Severance, Jr.) which issued on Jun. 11, 1991, and which is incorporated herein by reference, discloses an electrode for a plasma arc torch wherein the electrode includes a copper holder having a lower end which mounts an emissive insert that acts as the cathode terminal for the arc during operation. A sleeve of silver is positioned to surround the insert and forms an annular ring on the lower end surface of the holder to surround the exposed end face of the emissive insert. The annular ring serves to prevent arcing from the copper holder, and maintains the arc on the insert. However, while the silver sleeve of the '425 patent was intended to prolong the life of the copper holder, in practice, this electrode suffers from problems in that the wear does not come from double arcing, but from the hafnium overheating and eroding.
U.S. Pat. No. 3,930,139 (Bykhovsky et al.) which issued on Dec. 30, 1975, and which is incorporated herein by reference, also discloses an electrode for plasma arc working of materials. In the '139 patent, the holder is again formed from copper or copper alloys and an active insert is fastened to the end face of the holder and is in thermal and electrical contact with the holder through a metal distance piece disposed between the active insert and the holder and over the entire contact surface area. The metal distance piece is formed from aluminum or aluminum alloys and the active insert is formed from hafnium or from hafnium with yttrium and neodymium oxides as dopants therein taken separately or in combination. However, while the aluminum sleeve surrounding the active insert in the '139 patent serves to protect the copper holder surrounding the active insert, the aluminum distance piece or sleeve offers no advantages over the silver sleeve of the '425 patent to Severance, Jr.
U.S. Pat. No. 5,676,864 (Walters) which issued on Oct. 14, 1997, which is incorporated herein by reference, discloses an electrode for a plasma arc torch wherein the electrode includes a copper holder having a lower end which mounts an emissive insert that acts as the cathode terminal for the arc during operation. A sleeve of silver is positioned substantially to surround the insert and form an annular ring on the lower end surface of the holder to surround the exposed end face of the emissive insert. The insert assembly further includes an aluminum face plate disposed in the enlarged outer portion of the cavity and which is exposed at the front end of the metallic holder so as to surround a front portion of the sleeve.
U.S. Pat. No. 5,767,478 (Walters) which issued on Jun. 16, 1998, alternatively teaches eliminating the aluminum face plate of U.S. Pat. No. 5,676,864 and instead provides for the front end of the holder to directly contact the emissive insert forming an overlay portion of the holder between the front face thereof and the sleeve, thus protecting the silver sleeve.
SUMMARY OF THE INVENTION
An electrode for supporting an arc in a plasma arc torch, according to the present invention includes, a metallic holder having a front end, and a cavity in the front end. An insert assembly is mounted in the cavity and comprises an emissive insert having an inner face, an outer face, and a bore formed therein. The emissive insert is composed of a metallic material having a relatively low work function. The insert assembly further has a non-emissive core positioned within the bore that acts to draw heat out of the emissive insert. The core has a base portion that substantially covers the inner face of the emissive insert forming an interface between the emissive insert and the metallic holder. This interface aids in transferring heat from the emissive insert to the metallic holder.
The metallic holder is generally tubular and has a front end wall which defines an outer front face, wherein the outer face of the emissive insert and an end portion of the non-emissive insert lie in the plane of the outer front face of the metallic holder.
It is also contemplated that the electrode further comprises a sleeve which surrounds at least a portion of the emissive insert so as to separate the portion of the emissive insert from contact with the holder. The sleeve being composed of a metal which is selected from the group consisting of silver, gold, platinum, rhodium, iridium, palladium, nickel, and alloys thereof.
An embodiment of the invention additionally includes an overlay portion formed in the metallic holder at the front end. The overlay portion directly contacts the emissive insert so that none of the sleeve is exposed at the front end.
It is also contemplated that an embodiment of the present invention includes, a nozzle mounted adjacent the transverse front end wall of the electrode and having a bore therethrough which is aligned with the longitudinal axis. A power supply is also provided, which creates an electrical arc extending from the emissive insert of the electrode through the bore and to a workpiece located adjacent the nozzle. Additionally, gas inlet holes may be provided for generating a vortical flow of a gas between the electrode and the nozzle and so as to create a plasma flow outwardly through the bore and to the workpiece.
The invention also includes a method of fabricating an electrode adapted for supporting an arc in a plasma arc torch, which comprises, creating a bore in a substantially cylindrical emissive material and positioning in the bore, a non-emissive core, having a base, to form an insert assembly. The method further includes preparing a cylindrical metallic blank and forming a first cavity in an end portion of the blank. The insert assembly is then pressed into the first cavity of the blank so that the base of the core interfaces between the cylindrical emissive material and the blank. The blank is swaged so that the insert assembly is bound to the cylindrical metallic blank. Finally, threads are formed on an end distal to the front end, and a second cavity is formed by a boring and grooving process.
The method further con

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