Abstract:
An electrode for a plasma arc torch is provided with features for improving electrode wear. An emissive insert is received into a cavity formed along one end of the torch body. A portion of the emissive insert is separated from the torch body by a sleeve positioned along the insert near the emission surface of the insert. The sleeve can operate to slow the erosion of the electrode body and thereby improve overall electrode life.

Description:
FIELD OF THE INVENTION 
       [0001]    The subject matter of the present disclosure relates generally to electrodes for plasma arc torches and, more particularly, to the configuration of emissive inserts for such electrodes. 
       BACKGROUND OF THE INVENTION 
       [0002]    The operation of conventional plasma arc torches is well understood by those in the art. The basic components of these torches are a body, an electrode mounted in the body, a nozzle defining an orifice for a plasma arc, a source of ionizable gas, and an electrical supply for producing an arc in the gas. Upon start up, an electrical current is supplied to the electrode (generally a cathode) and a pilot arc is initiated in the ionizable gas typically between the electrode and the nozzle, the nozzle defining an anode. 
         [0003]    A conductive flow of the ionized gas is then generated from the electrode to the work piece, wherein the work piece then defines the anode, and a plasma arc is thus generated from the electrode to the work piece. The ionizable gas can be non-reactive, such as nitrogen, or reactive, such as oxygen or air. 
         [0004]    A longstanding problem with conventional plasma arc torches is the wear of the electrodes. Typically, the electrodes include a hafnium or zirconium insert. These materials are desired for their material properties when cutting with a reactive gas plasma but are extremely costly and require frequent replacement. 
         [0005]    While not intending to be bound by any particular theory, it is believed that multiple factors contribute to electrode wear. For example, during operation of the torch, the insert material becomes extremely hot and enters a molten state as electrons are emitted from the high emissivity material to form the arc. Eventually, a hole or cavity may form at the exposed emission surface of the insert. This cavity, typically concave in shape, is formed due to the ejection of the molten, high emissivity material from the insert during operation. The ejection of material can occur at various times during the cutting process such as e.g., during initial start-up creation of the plasma arc, during cutting operations with the arc, and/or while or after stopping the plasma arc. The ejection of molten material not only provides wear of the insert but can also wear other parts of the torch such as the nozzle. More particularly, the molten material from the insert may be ejected from the electrode to the surrounding nozzle, which in turn can cause the arc to improperly attach to, and thereby damage, the nozzle. 
         [0006]    Accordingly, an electrode having one or more features for improving wear would be useful. More particularly, an electrode that can reduce or minimize the ejection of molten material from the insert would be beneficial. Such an electrode that can also reduce or minimize damage to the portion of the electrode surrounding the insert would also be useful. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention relates to an electrode for a plasma arc torch with features for improving electrode wear. An emissive insert is received into a cavity formed along one end of the torch body. A portion of the emissive insert is separated from the torch body by a sleeve positioned along the insert near the emission surface of the insert. The sleeve can operate to slow the erosion of the electrode body and thereby improve overall electrode life. Additional objects and advantages of the invention will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention. 
         [0008]    In one exemplary embodiment, the present invention provides an electrode for a plasma arc torch. The electrode includes an elongate body defining a longitudinal direction and comprising a high thermal conductivity material. The body has a face at a discharge end of the electrode. The body defines a bore extending along the longitudinal direction. An insert is received into the bore. The insert has an outer portion and an inner portion. The inner portion is in contact with the elongate body and the outer portion has an exposed emission surface that is recessed relative to the face of the elongate body. An annulus is received into the bore adjacent to the insert. The annulus separates the outer portion of the insert from the elongate body. 
         [0009]    In another exemplary embodiment, the present invention provides an electrode for a plasma arc torch. The electrode includes an electrode body comprised of a thermally and electrically conductive metal. The electrode body has a face and a cavity positioned in the face. An insert is mounted in the cavity and comprises an emissive material having a work function less than the work function of the electrode body. The insert is positioned in contact with the electrode body. The insert is recessed relative to the face of the electrode body. A sleeve surrounds the insert and separates a portion of the insert near the face of the electrode body from the electrode body. 
         [0010]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
           [0012]      FIG. 1  provides a schematic view of an exemplary embodiment of plasma arc torch system of the present invention. 
           [0013]      FIG. 2  is a cross-sectional view of an exemplary embodiment of an electrode of the present invention. 
           [0014]      FIG. 3  is a cross-sectional view of another exemplary embodiment of an electrode of the present invention. 
       
    
    
       [0015]    The use of the same or similar reference numerals in the figures denotes the same or similar features. 
       DETAILED DESCRIPTION 
       [0016]    For purposes of describing the invention, reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
         [0017]      FIG. 1  is a simplified schematic view of an exemplary embodiment of a conventional plasma arc torch system  10 . The exemplary embodiment shown in  FIG. 1  is provided by way of example only. Other plasma arc torch systems of different configurations may be used with the present invention as well. 
         [0018]    Plasma arc torch system  10  includes a plasma arc torch  11  that has a basic body, generally indicated as  12 . Body  12  includes a torch supply tube  34  defining a supply chamber  36  that is supplied with a source of pressurized ionizable gas from gas supply  24  through gas supply line  26 . A remotely actuated valve, such as solenoid valve  28 , is disposed in line between supply tube  34  and gas source  24  to shut off the supply of gas to torch  10  upon actuation of the valve. As is appreciated by those skilled in the art, the plasma gas may be non-reactive, such as nitrogen, or reactive, such as oxygen or air. 
         [0019]    Torch body  12  includes an elongate electrode body  46 , typically formed from e.g., copper. An electrode insert or element  50  is fitted into the lower end of electrode body  46 —exemplary embodiments of which will be more fully described below. Element  50  is typically formed of hafnium or zirconium, particularly when a reactive gas is used as the plasma gas. 
         [0020]    An insulating body  38  generally surrounds the supply tube  34  and electrode body  46 . A cathode body  40  is disposed generally surrounding supply tube  34  and an anode body  42  is disposed surrounding insulating body  38 . A nozzle  16  is disposed at the forward end of electrode body  46  and defines an arc passageway  52  aligned with electrode insert  50 . A swirl ring  44  is disposed around the electrode body  46  and has holes defined therein to induce a swirling component to plasma gas entering plasma gas chamber  14 , as will be discussed in greater detail below. 
         [0021]    A power supply  18  is provided to supply electrical current to electrode body  46  and electrode element  50 . A negative power lead  20  is in electrical communication with supply tube  34  and cathode body  40 . In a pilot arc mode, a positive power lead  22  is in electrical communication with anode body  42  through switch  23 . Insulating body  38  electrically isolates anode body  42  from cathode body  40 . Positive power lead  22  is also connectable to a work piece  54  that is to be cut by the plasma torch once switch  23  is opened. Power supply  18  may constitute any conventional DC power supply sufficient to provide current to the torch at an appropriate voltage to initiate the pilot arc and then maintain the arc in the operational cutting mode of the torch. 
         [0022]    In operation, plasma gas flows from source  24 , through supply line  26  and shut off valve  28  into chamber  36  of supply tube  34 , as generally indicated by the arrows. The plasma gas flows downward in chamber  36  through orifices in the cathode body and orifices in swirl ring  44  before entering the lower plasma gas chamber  14 . It should be understood that lower plasma gas chamber  14  is in pneumatic communication with the entirety of the supply chamber  36  of supply tube  34  so that a change in pressure anywhere within the system will effect a change in pressure within lower plasma gas chamber  14 . In operation, a differential pressure exists between supply chamber  36  and lower plasma chamber  14  so that the plasma gas flows from supply chamber  36 , through swirl ring  44 , and out nozzle  16  with a swirling component induced thereto. 
         [0023]    In the pilot arc mode of torch  10 , switch  23  is closed so that the positive lead is connected to anode body  42 . Power supply  18  provides current at the appropriate voltage to initiate the pilot arc between electrode element  50  and nozzle  16 . A desired plasma gas flow and pressure are set by the operator for initiating the pilot arc. The pilot arc is started by a spark or other means, such as a contact starting technique, all of which are known in the art. 
         [0024]    The plasma gas flow during the pilot arc mode is from supply  24 , through supply line  26  and solenoid valve  28 , into supply chamber  36 , through orifices in cathode body  40 , through the holes in swirl ring  44 , into lower plasma chamber  14 , and out through arc passageway  52  of nozzle  16 . The swirling flow generated by swirl ring  44  is desired as a means for stabilizing the arc in the operational cutting mode so that the arc does not impinge on and damage the nozzle. 
         [0025]    In order to transfer torch  10  to the cutting mode, the torch is brought close to work piece  54  so that the arc transfers to the work piece  54  as switch  23  opens so that positive power is fed only to work piece  54 . The current is increased to a desired level for cutting such that a plasma arc  56  is generated which extends through arc passageway  52  to work piece  54 . The operational current levels depend on the type of torch and application desired. For example, the operational current levels can range from about 20 to about 400 amps. 
         [0026]    As the operational current is increased during the start of the cutting process, the plasma gas within lower plasma chamber  14  heats up and a decrease in plasma gas flow out of nozzle  16  results. In order to sustain sufficient plasma gas flow through nozzle  16  to sustain the plasma arc  56 , the pressure of the plasma gas being supplied must be increased with the increase of current. Conversely, towards the end of the cutting process, reduction of the level of current and plasma gas flow can be carefully coordinated to e.g., prevent damage to the electrode. 
         [0027]      FIG. 2  provides a cross-sectional, side view of another exemplary embodiment of the elongate electrode body  46 . Electrode body  46  defines a longitudinal direction L and has a face  60  positioned at discharge end  62 . Electrode body  46  is constructed from a material that is highly conductive thermally and highly conductive electrically. For example, electrode body  46  may be constructed from copper or silver. Electrode body  46  may be constructed with various features for attaching body  46  to plasma arc torch  11 . As shown, the exemplary embodiment of  FIG. 2  includes threads  64  for complementary receipt into torch  11 . Other configurations may also be used. Electrode body  46  also includes a chamber  58  that can be provided with e.g., a heat transfer fluid to help cool electrode body  46  during cutting operations. 
         [0028]    Electrode body  46  defines a cavity or bore  66  that extends along longitudinal direction L from face  60 . For this exemplary embodiment of electrode body  46 , an insert  68  is received into bore  66 . Insert  68  is constructed from a highly emissive material having a low electron work function such as e.g., hafnium, zirconium, tungsten, and alloys thereof. As such, insert  68  will readily emit electrons from emission surface  72  upon e.g., application of a sufficient electrical potential difference between insert  68  and an adjacent work piece. Notably, the electron work function of insert  68  is less the electron work function of electrode body  46  such that the plasma arc is generated at emission surface  72 . 
         [0029]    Insert  68  includes two portions, namely, an outer portion  76  that includes emission surface  72  and an inner portion  78  that is concealed within electrode body  46 . Inner portion  78  is in contact with electrode body  46 . Such contact provides an electrical connection through which current may pass to generate the plasma arc at emission surface  72 . Additionally, contact between inner portion  78  and electrode body  46  also provides for heat transfer away from the emissive insert  68 . 
         [0030]    Outer portion  76  provides the emission surface  72  where the plasma arc is preferably generated during operation of the torch system  10 . As shown, outer portion  76  is separated from contact with electrode body  46  by a sleeve or annulus  70 . More specifically, both insert  68  and annulus  70  are received into bore  66  of electrode body  46 . However, outer portion  76  of insert  68  is enclosed within annulus  70  so that the end of insert  68  providing emission surface  72  is isolated from electrode body  62 . For this exemplary embodiment, the exposed end of annulus  70  is also provided with a chamfered surface  74 . Additionally, as shown, the emission surface  72  of outer portion  76  is recessed relative to the face  60  of electrode body  46 . 
         [0031]    Without being bound to any particular theory of operation, the inventors believe that by providing annulus  70  around the outer portion  76  of insert  68  while recessing insert  68  relative to face  60 , annulus  70  provides a material that isolates insert  68  and acts differently than insert  68  during operation of plasma arc torch system  10 . More specifically, without annulus  70 , it is believed that material from recessed insert  68  will wet the exposed circumferential surface (see, e.g., surface  75  in  FIG. 3 ) of bore  66  near face  60  to provide limited protection of electrode body  46  from wear. However, as the insert  68  wears, eventually emissive material from insert  68  no longer wets the exposed circumferential surface of bore  66  and the electrode body  46  will wear undesirably. Yet, the inventors have determined that by positioning annulus  70  around the recessed outer portion  76  of insert  68 , the material of annulus  70  operates as a refractory to further shield the electrode body  46  and provide additional improvement in electrode wear. Chamfered edge  74  on annulus  70  can also further minimize wear of electrode body  46 . 
         [0032]    Additionally, in one exemplary embodiment of the invention, the material used for annulus  70  may comprise the same material used for insert  68 . For example, both annulus  70  and insert  68  may be constructed of hafnium. Thus, even when annulus  70  and insert  68  are made of the same material, improvements in electrode wear may be had as annulus  70  acts to isolate insert  68  thermally and acts a refractory relative to the electrode body. 
         [0033]    In other embodiments of the invention, annulus  70  is constructed from a different material than insert  68  and has a higher electron work function, a higher melting point temperature, or both, relative to the material used for insert  68 . In still other embodiments of the invention, annulus  70  comprises an electrical and thermal insulator. For example, a ceramic material such as e.g., aluminum oxide, silicon carbide, and/or tungsten carbide may be used for annulus  70  to improve its ability to act as a refractory material. 
         [0034]      FIG. 3  provides another exemplary embodiment of the present invention similar to the embodiment of  FIG. 2  except for the position of surface  74  of annulus  70  relative to face  60  of electrode body  46 . More particularly, for this exemplary embodiment, both annulus  70  and insert  68  are recessed within bore  66  of electrode body  46 . For this exemplary embodiment, it is believed annulus  70  still operates as a refractory to help isolate insert  68  from electrode body  46  as described for the embodiment of  FIG. 2 . The materials used for construction of annulus  70  and insert  68  are similar to that described for the exemplary embodiment of  FIG. 2 . In still other embodiments of the invention, annulus  70  may be recessed with respect to face  60  but is not flush with the emission surface  72  of insert  68 . 
         [0035]    While the present subject matter has been described in detail with respect to specific exemplary embodiments and methods thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art using the teachings disclosed herein.