Abstract:
An electrode having a ribbed configuration providing a large surface area for cooling the electrode. The electrode includes an elongated electrode body having a first end and a second end. The electrode also includes a shoulder having an enlarged diameter body integral with the electrode body. The shoulder has an imperforate face toward the first end and at least one rib extending aft of the face towards the second end of the electrode body.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of plasma arc torches and systems. In particular, the invention relates to an electrode for use in a plasma arc torch having an enhanced cooling configuration. 
     BACKGROUND OF THE INVENTION 
     Plasma arc torches are widely used in the processing (e.g., cutting and marking) of metallic materials. A plasma arch torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. The gas can be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air. 
     In process of plasma arc cutting or marking a metallic workpiece, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). The pilot arc ionizes gas passing through the nozzle exit orifice. After the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc then transfers from the nozzle to the workpiece. The torch is operated in this transferred plasma arc mode, characterized by the conductive flow of ionized gas from the electrode to the workpiece, for the cutting or marking the workpiece. 
     U.S. Pat. No. 4,902,871, assigned to Hyperthemi, Inc. describes and claims an apparatus and method for cooling a “spiral groove” electrode in a contact start torch. A gas flow passage, preferably a spiral fin machined on the outer side surface of the shoulder portion, diverts a portion of the gas flow from the plasma chamber to a region above the electrode where it is vented to atmosphere. The fin is machined to form a spiral groove that is sufficiently constricted that a substantial pressure drop appears along the path, while allowing a sufficient gas flow to produce the desired cooling. The adjacent portions of the spiral fin are preferably closely spaced to enhance the surface area of the electrode in a heat transfer relationship with the cooling gas flow. 
     While spiral groove electrodes operate as intended, applicants have perceived the need for an alternative form of the electrode which is simpler to manufacture, but still provides the same benefits as the spiral groove electrode. 
     SUMMARY OF THE INVENTION 
     The present invention resides in the recognition that an electrode having a ribbed configuration is easy to manufacture and provides a large surface area for cooling the electrode. The ribbed configuration provides for a plurality of independent cooling passages that extend from a first (front) end to a second (aft) end of the electrode. In one embodiment, the electrode includes an elongated electrode body having a first end and a second end. The electrode also includes a shoulder having an enlarged diameter body integral with the electrode body. The shoulder has an imperforate face toward the first end and at least one rib extending aft of the face towards the second end of the electrode body. 
     The at least one rib has a varying height forming at least one groove in the shoulder body of varying depth. In one embodiment, the depth of each groove is greater toward the second end of the electrode than toward the first end. The at least one rib has an orientation between limits of being longitudinally aligned and substantially circumferentially disposed relative to the electrode body. As stated previously, these grooves act as independent, parallel cooling passages that provide a large surface area and facilitate substantial cooling of the electrode. 
     In a detailed embodiment, the electrode can comprise a high thermal conductivity material (e.g., copper) and can have an insert disposed in a bore formed in at least one of the first end and the second end. The insert can comprise a high thermionic emissivity material (e.g., hafnium or zirconium), and the shoulder can have an enlarged body of constant diameter that includes a plurality of ribs (and grooves). 
     The present invention also features a method of cooling an electrode in a torch body of a plasma arc torch. The torch includes a nozzle disposed relative to the electrode and a swirl ring to define a plasma chamber. The electrode is provided comprising an elongated electrode body having a first end and a second end. The electrode also includes a shoulder having an enlarged diameter body integral with the electrode body. The shoulder has an imperforate face toward the first end and a plurality of ribs extending aft of the face toward the second end of the electrode. A flow of pressurized gas is directed to the plasma chamber via the swirl ring. A portion of the pressurized plasma gas is directed through the plurality of grooves between the ribs to a rear chamber. The grooves act as parallel, independent cooling paths to cool the electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will become apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being place on illustrating the principles of the present invention. 
     FIG. 1 is a perspective view of a conventional plasma arc cutting torch having an electrode with a spiral groove; 
     FIG. 2A is a perspective view of an electrode having a shoulder with a plurality of ribs incorporating the principles of the present invention; 
     FIG. 2B is a top view of the electrode of FIG. 2A; 
     FIG. 2C is a bottom view of the electrode of FIG. 2A; 
     FIG. 3 is cross-sectional view of the electrode along axes A—A of FIG. 2C and; 
     FIG. 4 is a perspective view of a conventional plasma arc cutting torch having an electrode with a ribbed configuration. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a plasma arc torch  10  of the type described and claimed in U.S. Pat. No. 4,902,871, the specification of which is hereby incorporated by reference. As shown, the torch  10  has a torch body  12  with an inner component  12   a  and an outer component  12   b , a plunger  14  and a spring  16  that drives the plunger downwardly, as shown. Consumable parts of the torch  10  include a swirl ring  18  secured to the lower end of the body component  12   a , a nozzle  20  with a central plasma arc exit orifice  20   a , an electrode  22 , and a retaining cap  24  threaded onto the body component  12   b  at its lower end. The cap  24  captures the nozzle and holds it in place. The electrode  22  is slidable axially (shown in the vertical direction) within the swirl ring  18 . In a starting position, the lower end face  22   a  of the electrode  22  closes off the exit orifice  20   a . In the operating position, an upper surface  22   a ″ of the body portion of the electrode either abuts or is near the lower end of the body component  12   a  and the nozzle exit orifice  20   a  is open. The movement of the electrode  22  is accomplished using fluid forces. 
     A pressurized plasma gas flow  26  enters the torch via passage  28 , port or ports  30 , an annular passage  32  and canted ports  34  in the swirl ring  18 , finally entering a plasma chamber  36  defined by the electrode, the swirl ring and the nozzle. The plasma gas flow  26 , except for a portion  26   b  that exits the cap through the holes  44 , passes through the canted ports  34  to enter the plasma chamber  36  which pressurizes the chamber to create a fluid lifting force acting on the lower surfaces of the electrode. This force overcomes the spring force causing the electrode to move upwardly to its operating position. The pilot arc produced as the electrode breaks electrical connection with the anode initiates a plasma arc, which exits the torch through the orifice  20   a  and attaches to a workpiece to be cut or marked. When the electrode is raised, the main gas flow  26   c  in the plasma chamber  36  has a swirling motion about the lower electrode body portion  22   a . The flow  26   b  through the cap holes  44  serves to cool torch parts other than t he electrode. 
     As shown, a gas flow passage  48  formed in the electrode extends from a first end  48   a  in fluid communication with the plasma chamber  36  and a second end  48   b  in fluid communication with the region above the electrode  46 . The passage  48  is a spiral groove formed in the outer side wall of the shoulder portion  22   b  of the electrode. The passage  48  acts as a serial cooling path for a cooling gas flow  26   d . The cross-sectional dimensions, the length, and the configuration of the passage are such that the cooling gas flow  26   d  travels up the passage to the region above the electrode  46 , but the passage is sufficiently restrictive to the flow that there is substantial pressure drop along the passage. 
     FIGS. 2A-2C illustrate an embodiment of an electrode of the present invention. The electrode of the present invention can replace the electrode  22  of FIG. 1 (see FIG.  4 ). In FIG. 2A the electrode  122  has an elongated electrode body portion  122   a  and a shoulder portion  122   b  having an enlarged substantially constant diameter integral with the electrode body portion  122   a . The shoulder  122   b  can have a substantially constant diameter. The elongated electrode body portion  122   a  has a first end  122   d  and a second end  122   e . The electrode  122  has multiple ribs  122   c  and corresponding grooves  148  formed in the shoulder  122   b  portion of the electrode  122 . The ribs  122   c  are disposed aft of an imperforate face  122   f  and extend toward the second end  112   e  of the electrode body portion  122   a . The imperforate face  122   f  of electrode  122  can be substantially flat to increase the “blow back” of the electrode  122  when the plasma arc is started. 
     In one embodiment, the ribs  122   c  and grooves  148  can be longitudinally aligned relative to a central axis (CA) (FIG. 3) extending through the body. In another embodiment, the ribs  122   c  and grooves  148  can be substantially circumferentially disposed relative to the electrode body. In other embodiments, the ribs  122   c  and grooves  148  can be aligned anywhere between longitudinally aligned or circumferentially disposed relative to the electrode body. In addition, the ribs (and grooves) can have a constant or varying thickness. 
     The electrode  122  can be manufactured from of a high thermal conductivity material. The high thermal conductivity material can be copper, silver, gold, platinum, or any other high thermal conductivity material with a high melting and boiling point and which is chemically inert in a reactive environment A high thermal conductivity can be any metal or alloy having a thermal conductivity greater than 40 Btu/hr ft ° F. 
     The grooves  148  can be formed using a key-cutter sawing operation, or by any other method known to those skilled in the art. 
     FIG. 3 is a cross-sectional view along section A—A of FIG. 2C of the electrode  122 . As shown, the depth of the grooves  148  increases from the first end  122   d  toward the second end  122   e  of the electrode  122 . The electrode  122  has a bore  150  formed in the first end  122   d  of the electrode  122 . The bore  150  can be formed by drilling into the electrode body  122   a  along a central axis (CA) extending longitudinally through the body. An insert  152  comprising high thermionic emissivity material (e.g., hafnium or zirconium) is press fit in the bore  150 . A high thermionic emissivity can be defined as a relatively low work function, in a range between about 2.7 to 4.2 eV. The insert  152  includes a closed end  152   a  which defines an emission surface. The emission surface  152   a  is exposable to plasma gas in the torch body. 
     FIG. 4 shows electrode  122  installed in a plasma arc torch  10 . In FIG. 4, like parts are identified with the same reference number as used in FIG. 1. A principal feature of the invention is the plurality of grooves  148  which form multiple, parallel, independent gas flow passages in the electrode  122  from the imperforate face  122   f . The cross-sectional dimensions, the length, and the orientation of the grooves  148  are configured such that cooling gas flows  126   d  travel through each groove  148  to the region  46  aft of the electrode  122 . The grooves  148  are dimensioned to produce a substantial pressure drop in the gas flow passing through the groove passages. The velocity of the cooling gas flows  126   d  decreases as the gas flows into grooves  148  past the ribs  122   c  toward the second end of the electrode  122   e.    
     The plurality of ribs  122   c  act as heat transfer surfaces for cooling the electrode  122 . As such, an increased the surface area of the electrode is exposed to the cooling gas flows  126   d  resulting in more effective cooling of the electrode  122 . The plurality of grooves  148  allow multiple cooling gas flows  126   d  to flow through the shoulder  122   b  of the electrode  122 . 
     Because there is a substantial pressure drop through the grooves  148 , and because of the large surface area of the imperforate face  122   f , the gas flow  26   c  pressurizes the chamber  36  rapidly with only a small pressure acting on the opposite surfaces of the electrode in the region above the electrode  46 . This pressurization “blows back” the electrode against the force of the spring  16  allowing the flow  26   c  in the plasma chamber to assume an unrestricted swirling pattern, which is conducive to the formation of a stable plasma arc. The electrode  22  of the present invention therefore provides both an effective cooling process as well as reliable contact starting. 
     While the invention has been described with respect to its preferred embodiments, it will be understood that various modifications and alterations will occur to those skilled in the art from the foregoing detailed description and the accompanying drawings. For example, while the invention has been described with respect to an electrode that moves axially for contact starting, the features of the present invention could be applied to a stationary electrode. Further, while the electrode has been described as moving within a swirl ring as a guide and support element, it will be understood that it could be mounted to move within the torch body or some other replaceable torch component. Therefore, as used herein, “torch body” should be interpreted to include the swirl ring or other component acting as a guide and support for the electrode. These and other modifications and variations are intended to fall within the scope of the pending claims.