Abstract:
An electrode-supporting assembly for a contact-start plasma arc torch has an insulator that partially houses an electrode, and employs a spring-loaded plunger to bias the electrode to a forward position. The spring is engaged between the plunger and a contact element attached to the insulator, and may conduct electrical current to the electrode. The plunger, spring, and contact element are retained in the insulator when the torch is opened to replace the electrode, which is a consumable part. The electrode and the plunger have axially-engagable mating surfaces to assure good thermal and electrical conductivity therebetween. Conductivity can be further enhanced by forming the plunger of silver or a silver-bearing alloy. In some embodiments, a passage through the insulator is partitioned into forward and rear chambers, with the plunger, spring, and contact element trapped in the rear chamber.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to contact-start plasma torches, and more particularly to a novel structure for providing electrical connection of a consumable electrode with a power supply. 
       BACKGROUND OF THE INVENTION 
       [0002]    U.S. Pat. No. 4,791,268 teaches a contact-start plasma arc torch where the electrode is biased forward by a plunger which resides in an enclosed structure; this structure is formed such that a significant portion of the electrode is not exposed to gas flow which would enhance cooling, and there appears to be little gas flow past the plunger. Additionally, the contact between the plunger and the electrode is provided only across relatively small planar contact surfaces, which may be susceptible to reduced contact due to any unwanted material interposed between these surfaces. 
         [0003]    U.S. Pat. Nos. 8,035,055 and 8,115,136 teach a variety of electrode configurations for contact-start plasma arc torches, as well as teaching a prior art electrode which employs a spring-loaded contact for the power supply for biasing the electrode toward its forward position. In the prior art device cited in these patents, the electrode and contact remain engaged at all times. In basic embodiments of the invention taught in these patents, a spring is positioned between the electrode and the contact to bias the electrode away from the contact. In these embodiments, the electrode directly engages the contact only when in its rear position, which is the position for sustaining the non-transferred pilot arc and the transferred cutting plasma arc. Unless the electrode includes the spring, the spring may be lost when the torch is opened to change the electrode. 
         [0004]    Perhaps to avoid the possibility of springs being lost when the torch is opened to change the electrode, these patents also teach several embodiments that employ an electrode having a spring-loaded conductive element that is secured to the electrode, trapping the spring. Securing the conductive element and spring to the electrode requires that these components be replaced with the electrode, increasing expense of the electrode, which is a consumable part. 
         [0005]    In still other embodiments, the electrode is installed via a bayonet-style connection where the spring is positioned behind the female section of the bayonet element and thus trapped in the assembly. The electrode is provided with the male portion of the connection and, when inserted and locked in position, this portion contacts the spring. In another embodiment, the spring is retained by a fixed ring which overlaps part of the spring and a pair of prongs are positioned so as to pass though the opening and engage the spring. Such a configuration provides limited contact. While these latter solutions overcome the expense of attaching the spring and a conductive element to the electrode, it complicates the structure of the electrode, again increasing expense of fabrication, and may limit air flow over the spring and the contacting portion of the electrode, thereby reducing cooling. These embodiments also appear to suffer from limited engagement between the spring and the electrode, thus limiting the effectiveness of electrical contact therebetween. These limitations may explain why the electrode currently being commercially offered by the patentee is the embodiment shown in FIGS. 3A and 3B of the &#39;155 and &#39;136 patents, which has a spring and a conductive element secured to the electrode. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is for an electrode-supporting assembly for use in contact-start plasma torches to position and provide electrical contact for an electrode while allowing it to be readily replaced. The assembly includes the structure for providing current to the electrode while allowing it limited longitudinal motion, as discussed below. 
         [0007]    The term “electrode” as used in the present application defines a consumable element of the torch that can be readily be replaced when the nozzle of the torch is removed. 
         [0008]    The plasma torch suitable for incorporating the present invention has a current-carrying cathode that connects to a power supply and terminates in a power transfer surface. The torch has a torch recess for receiving a hollow insulator that slidably engages an electrode and introduces pressurized gas into a chamber defined, in part, by a nozzle element. A retaining element secures the nozzle with respect to the torch recess. The torch is designed to allow the electrode to move between a forward position where it contacts the nozzle element, at which time a current is passed through the electrode to start the torch, and a rear position spaced apart from the nozzle, to which the electrode is blown back by pressure of gas introduced through the insulator, creating the chamber for developing and maintaining plasma. Initially, a pilot arc is maintained from the electrode to the nozzle, developing a pilot plasma arc. When the torch is brought in close proximity to a workpiece to be cut, this non-transferred arc from the electrode to the nozzle element transfers so as to arc from the electrode to the workpiece, thereby establishing a transferred plasma arc. The assembly of the present invention includes an insulator and electrode, as well as related elements to provide more positive electrical contact and improved cooling of the electrode. The related elements allow for simplification of the electrode, allowing it to be easily and inexpensively fabricated. 
         [0009]    The insulator is formed of an electrically non-conductive material and is designed to be slidably inserted into the torch recess of the torch in place of the conventional swirl ring, and is retained therein in the conventional manner. Typically, the insulator is forcibly engaged by the nozzle element which, in turn, is secured by the retaining element. The insulator is provided with gas passages to introduce gas into a region of the torch bounded, in part, by the nozzle element, in the same manner as a conventional swirl ring. This gas applies pressure to drive the electrode to its rear position where it is spaced apart from the nozzle, as well as providing gas to sustain plasma, while the remainder of the gas flows backwards along the electrode, providing cooling. 
         [0010]    The electrode of the assembly has a longitudinal axis, and a portion of the electrode resides within the insulator when in service. The electrode is movable from the forward position, where it is in contact with the nozzle element of the torch, and the rear position, where the electrode is displaced back from the nozzle element. The forward position serves as a starting position for the torch; when the electrode is so positioned, a current can be passed through the electrode via a resilient element and, as the electrode is withdrawn away from contact with the nozzle element, an arc is generated that initiates the formation of the pilot plasma arc. When the electrode is in this rear position, the pilot arc is maintained between the electrode and the nozzle element with the principal current no longer being provided by the resilient element. When the torch is subsequently brought into close proximity to the workpiece, this non-transferred plasma arc is transferred from the nozzle element to the workpiece. 
         [0011]    The electrode has a distal end, which includes an emissive element, and a proximal end. The proximal end of the electrode preferably terminates in a non-planar electrode rear terminal surface. One such electrode rear terminal surface is a frustoconical protrusion or a frustoconical cup. 
         [0012]    While the discussion below treats the terminal surface in terms of continuous surfaces, it should be appreciated that the surface need not be strictly continuous, and could be quasi-continuous. In either case, a frustoconical surface having an apex angle between 16° and 60° is practical, and a more restricted range of angles from about 40° to 60° is felt to be particularly effective. 
         [0013]    A plunger fabricated from an electrically conductive material also resides within the insulator when in service, positioned rearward of the electrode. The plunger has a front contact surface that is configured so as to releasably mate with the electrode rear terminal surface; these releasable mating surfaces are configured such that they can be brought into mating engagement by translation along a longitudinal axis of the electrode. Thus, when the electrode rear terminal surface is a concave surface, the front contact surface of the plunger is a mating convex surface. Having non-planar mating surfaces such as conical surfaces increases the contact area between the electrode and the plunger to reduce the contact resistance and promote heat transfer, and conical or frustoconical surfaces also provide centering to maintain the electrode and the plunger aligned with each other. The plunger provides a heat sink for extracting heat from the electrode, in part since there is extensive contact between the plunger and the electrode, thereby providing lower operation temperatures for the components during the operation of the torch. The plunger terminates in a rear section terminating in a plunger rear surface. 
         [0014]    Having a plunger that carries current to the electrode provides a benefit in that the electrode employed can have a very simple structure and can be readily replaced without requiring additional parts to be replaced, as is required by several embodiments taught in U.S. Pat. Nos. 8,035,055 and 8,115,136, where a resilient spring is employed to supply current during start-up and this spring is trapped on the electrode by a conductive element. Securing the conductive element and spring to the electrode requires these components to be replaced along with the electrode, increasing expense of the electrode, which is a consumable part. A basic embodiment of these patents lacks a spring-loaded conductive element, and thus does not require that the spring be attached to the electrode. However if not attached to the electrode, the spring is either attached to a cathode of the torch, making replacement of the spring difficult when necessary, or a loose element which may be subject to loss when the electrode is removed for replacement. 
         [0015]    In other embodiments of the &#39;055 and &#39;136 patents where the conductive element is not incorporated into the electrode, such as those shown in  FIGS. 12-15  of these patent in which the electrode is maintained in engagement with the spring by a bayonet coupling, special machining of the proximal end of the electrode is required; this limits the ability to assure good electrical connection therebetween. The bayonet connection may also reduce gas flow past the electrode and thus hamper cooling. 
         [0016]    In another embodiment of the above-referenced patents, shown in  FIG. 16 , posts protruding from the electrode pass through a restricted region of the passage in the insulator. This restriction serves to retain the spring in the insulator; however, it does so by complicating the fabrication of the electrode and reducing the contact area with the spring, which may limit the ability to assure an adequate arc to form a pilot arc. 
         [0017]    In some embodiments of the present invention, the plunger not only provides a large surface for contacting the electrode but also is fabricated from silver or a silver alloy, which offers excellent electrical and thermal conductivity and provides an interface between the plunger and the electrode with low thermal and electrical resistance. The use of silver should also reduce the contact resistance between the spring and the plunger, thereby increasing the heat extraction from the spring in the case where the limiting temperature of the spring results from resistive (FR) heating. 
         [0018]    In some embodiments, the plunger is configured to extend beyond the outer diameter of the proximal end of the electrode so as to provide a general flow of cool gases thereacross. This extension further enhances the cooling of the electrode and thus should extend its useful life. Providing the plunger with enlarged surfaces that are configured to deflect the gas flowing backwards enhances the cooling action of the plunger by increasing flow across the surface of the plunger. Cooling of the plunger can be further enhanced when the plunger rear section has a reduced cross section that results in a stepped profile; this step increases turbulence in the gas flow adjacent the plunger rear section and promotes mixing of the gas to increase cooling. 
         [0019]    A contact element of an electrically conductive material is provided, which is attached to the insulator and configured to engage it in such a manner that the contact element is forcibly engaged against the power transfer surface of the cathode of the torch when the insulator is secured in place by the retaining element. The contact element in the assembly of the present invention has an array of contact gas passages through which gas flowing back along the electrode can pass, and terminates in a contact forward surface and a contact rear surface. In many embodiments, the contact forward surface is configured to mate against at least a portion of the plunger rear surface of the plunger when the electrode is in its rear position. The contact rear surface is engaged against the power transfer surface when the insulator is retained in position. The contact element can be readily secured in position in the insulator by providing a press-fit. 
         [0020]    The resilient element (spring) attaches to the contact element and to the rear section of the plunger. Means for maintaining engagement of the contact element, the plunger, and the spring are provided; this means retains these elements within the insulator when the electrode is replaced, preventing loss. In one embodiment, this means for maintaining contact is provided by frictional engagement between the resilient element, the plunger and the contact element. 
         [0021]    In some embodiments, in addition to the frictional contact surfaces to be engaged by the spring, the plunger and contact element can be threaded together by mating the helix of the spring with mating helical grooves on the contact element and plunger. 
         [0022]    Providing such positive engagement not only assures maintaining these elements in contact during service so as to assure mechanical connection, but also assures good thermal contact between the spring and the plunger. This thermal contact promotes heat transfer from the spring to the elements to which it is connected to enhance dissipation of the heat resulting from resistive heating (I 2 R). In cases where the spring is degraded by overheating due to resistive heating, which can result in either corrosion or tempering of the spring, such frictional contact can result in better heat dissipation and, in this way, reduce the potential for overheating of the spring, which might adversely affect its resiliency. 
         [0023]    In some embodiments the damage to the spring may result from tempering and/or corrosion of the spring caused by environmental heat to which the spring is exposed. The use of a silver plunger may also serve to reduce the temperature fluctuations of the spring. 
         [0024]    In some embodiments, the means for maintaining engagement of the contact element, the plunger, and the spring are provided, at least in part, by the structure of the insulator. In such embodiments, the insulator has a central band of reduced cross-section, providing a passage which is constricted such that the plunger cannot pass therethrough. This constriction provides a bifurcated passage having a passage forward section for receiving the electrode and a passage rear section for constraining the plunger and the spring, although in many embodiments a portion of the plunger protrudes through the constricted band into the passage forward section to engage the electrode. In some embodiments, the central band provides an opening having a cross section sufficiently large as to allow the proximal end of the electrode to enter. However, in all cases the opening must be sufficiently large as to provide a spaced-apart relationship between the electrode and the central band in order to provide open space for air flow. The central band is positioned such that, when the contact element, spring, and plunger are installed in the insulator, the spring (resilient element) is maintained in compression and there is a gap between the plunger and the central band to allow limited gas flow when the electrode is installed in the torch and is in its forward position and in contact with the nozzle of the torch. 
         [0025]    Means for maintaining a consistent rear position of the electrode are provided. Consistent positioning of the electrode when the torch is operating in the plasma generating mode helps to accurately position the electrode with respect to the nozzle element to suit the desired operating conditions, as well as to avoid fluctuation in the volume of the plasma chamber. The details of these means for maintaining a consistent rear position of the electrode are a function of the elements that are employed to provide the conductive path from the contact element to the electrode, discussed below. 
         [0026]    Means for providing a conductive path between the contact element and the electrode when in the forward position and when in the rear position (these positions being discussed above) can be provided by various structures which will, in part, depend on the electrical connection schemes. In all cases where the resilient element is electrically conductive, at least part of the current passes through the resilient element for both positions of the electrode. 
         [0027]    In some embodiments, a conductive stranded element such as a twisted or braided wire or cable (which is at least partially non-resilient) is connected between the contact element and the plunger to pass at least part of the current. In such cases, proper sizing of the resilient element and the conductive stranded element can assure sufficient current to the electrode for operation in both its positions, while allowing sufficient resiliency in the resilient element to assure the smooth transfer between the two limit positions of the electrode when operating the torch. In these cases, the means to maintain a consistent rear position of the electrode can be provided in a variety of ways. In one scheme, the means to maintain a consistent rear position of the electrode is provided by having the resilient element be a compression spring that is sized, relative to the other elements, such that movement of the electrode to its rear position causes the plunger to compress the coils of the resilient element until the coils of the spring are in abutting contact. This stacked configuration of the spring serves as a rigidly non-compressible cylinder for limiting the rearward motion of the electrode. In an alternative embodiment, where the conductive stranded element resides in an envelope defined by the resilient element (the coil), the conductive stranded element folds onto itself and result in a conductive mass residing between the contact element and the plunger. 
         [0028]    For many of the embodiments, current to the electrode when in its rear position is provided, at least in part, through a direct path between the contact element and the plunger. In such cases, abutting contact between the contact forward surface of the contact element and the plunger rear surface of the plunger can also provide the means for maintaining a consistent rear position of the electrode. In a similar scheme, the means for maintaining a consistent rear position for the electrode can be provided by abutting contact between an insulator interposed between the plunger rear surface and the contact forward surface. However, in such cases an alternate conduction path such as a stranded conductor as discussed above may be needed to assure sufficient current flow. The use of an insulator may allow the plunger to be smaller in size, reducing the cost when it is fabricated from silver. The insulator is preferably attached to either the contact element or the plunger, such as by a press-fit or a high-temperature adhesive. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0029]      FIGS. 1-3  are section views illustrating an electrode-supporting assembly for a contact-start plasma arc torch, the assembly forming one embodiment of the present invention.  FIG. 1  illustrates the assembly installed into the torch with an insulator of the assembly secured into a recess in the torch, and the remaining elements of the assembly residing at least partly in the insulator; securing the insulator in position forces a contact element against a current-carrying cathode of the torch.  FIG. 1  shows the assembly when an electrode is in a forward position where it contacts a nozzle element of the torch. 
           [0030]      FIG. 2  is a partially exploded view of the assembly shown in  FIG. 1 , showing the individual components when removed from the torch. The electrode has a proximal end having a rear terminal surface formed as a cavity which mates against a plunger having a matching front contact surface, which in turn is engaged by a resilient element that biases the plunger and the electrode forward relative to the contact element that is affixed to the insulator. 
           [0031]      FIG. 3  shows the assembly shown in  FIGS. 1 and 2  when in service, and when the electrode has been forced by gas pressure to a rear position where it is spaced apart from the nozzle element to generate an arc; the distance between these two positions of the electrode is exaggerated in the figures for purposes of illustration. In this embodiment, current is passed from the contact element to the electrode through the resilient element and the plunger when the electrode is in the forward position shown in  FIG. 1 , and also passes directly from the contact element through the plunger when the electrode is in the rear position shown in  FIG. 3 , where surfaces on the contact element and the plunger are in contact. The contact of these surfaces also serves to provide a consistent rear position of the electrode in this embodiment. 
           [0032]      FIG. 4  is a section view of an embodiment that is similar to that shown in  FIG. 1 , with the exception that the assembly has a stranded conductor to carry electrical current between the contact element and the plunger to provide greater capacity to carry current to the electrode. In this embodiment, the plunger and the contact element abut when the electrode is in its rear position to assure a consistent rear position of the electrode. 
           [0033]      FIG. 5  is a section view of another embodiment that employs the conductor configuration shown in  FIG. 4 , but differs in that it employs an insulating element interposed between the plunger and the contact element to define the rear position of the electrode and thus the length of the plasma chamber. 
           [0034]      FIG. 6  is a section view of another embodiment similar to that of  FIG. 4 ; however, in this embodiment, the stranded conductor resides within an envelope defined by the coils of the resilient element. 
           [0035]      FIGS. 7-9  are section views illustrating another embodiment, which has many features in common with the embodiment shown in  FIGS. 1-3 , but which eliminates the need for a frictional fit to maintain the elements within the insulator when the electrode is removed.  FIG. 7  illustrates the embodiment before the electrode is brought into contact with the plunger, where the electrode has not been fully inserted into the insulator. The insulator has a central band which partitions a passage into a forward chamber, in which the electrode can be slidably engaged, and a rear chamber which constrains the resilient element and the plunger. The band has an opening of reduced cross section, which is sized to prevent the plunger from passing therethrough, thereby assuring that the resilient element and the plunger remain engaged at all times; the plunger of this embodiment has a protruding collar to assure that it is retained by the central band. This configuration of the insulator and the plunger also enhances cooling by increasing the flow of cooling gases over the surface of the plunger. 
           [0036]      FIG. 8  illustrates the same embodiment as shown in  FIG. 7 ; however, at this position the electrode and the plunger are fully engaged. The reduced cross section of the insulator must be so configured that the plunger cannot pass through it. The size of the reduced cross section region in this embodiment is shaped such the it can accommodate entry of the proximal end of the electrode therethrough as the electrode is blown back by gas pressure. The cross section must also be sufficiently large as to provide a spaced-apart relationship between the electrode and the opening so that, in service, gas flows around the electrode when the electrode is in its rear position. In this embodiment, a portion of the plunger can extend forward of the central band to aid in the alignment of the electrode with the plunger when the electrode is replaced. 
           [0037]      FIG. 9  illustrates the same embodiment as shown in  FIGS. 7 and 8  when the electrode has, in part, passed through the reduced cross section central band of the insulator and the rear surface of the plunger has engaged the contact element. 
           [0038]      FIGS. 10 and 11  illustrate an embodiment similar to that of  FIGS. 7-9 , but where the forward surface of the plunger is extended so as to engage a reduced cross-section region of the insulator without requiring a collar, as employed in the earlier embodiment. This extended surface may provide less obstruction to gas flow through the reduced cross-section region past the plunger. 
           [0039]      FIG. 12  is a section view that illustrates an embodiment similar to that shown in  FIGS. 7-9 , but where the electrode has a non-planar electrode rear terminal surface that is a convex frustoconical surface, and the plunger has a non-planar front contact surface that is formed as a matching concave frustoconical cup. 
           [0040]      FIGS. 13 through 15  illustrate an embodiment similar to that shown in  FIGS. 7-9 , but where the plunger is configured such that it engages the contact element so as to limit rearward travel of the electrode before the proximal end of the electrode reaches the opening between the passage forward section and the passage rear section. This embodiment also illustrates a swirl ring that is formed as a separate piece, rather than as an integral part of the insulator. 
           [0041]      FIGS. 16 and 17  illustrate an embodiment that is similar to that shown in  FIGS. 13-15 , but where the electrode geometry is such that the electrode proximal end has a cross section greater than that of the opening and thus would prevent it from passing into the opening between the passage forward section and the passage rear section independent of its engagement with the plunger. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]      FIGS. 1 through 3  are section views illustrating an electrode-supporting assembly  100  for use in a contact start plasma arc torch  102  (only partially shown). The torch  102  can be similar to those torches taught in U.S. Pat. Nos. 8,035,055 and 8,115,136 with the electrode-supporting assembly  100  replacing the conventional structure for supporting and providing electrical current to an electrode. The torch  102  has a current-carrying cathode  104  (shown in  FIG. 1 ) that connects to a power supply (not shown) and has a power transfer surface  106  for contacting a conventional electrode. The cathode  104  is mounted in a torch body element  108  that is configured with a torch recess  110  for receiving a conventional insulated swirl ring element. A nozzle element  112  can be secured onto the insulator and the torch body element  108  by a retaining element  114  that threadably engages the torch body element  108 . 
         [0043]    The assembly  100  has an insulator  116  that is configured to be slidably installed onto the cathode  104  so as to reside in the torch recess  110  in place of the swirl ring/insulator that is conventionally employed. The insulator  116  is retained in place by engagement with the nozzle element  112  when the retaining element  114  is tightened onto the torch body element  108 . The insulator  116  has an array of swirl gas passages  117 , configured as a conventional insulated swirl ring element, but differs from a conventional swirl ring element in having a contact element recess  118  (best shown in  FIG. 2 ), discussed below. The insulator  116  can be retained in the torch recess  110  by forcible engagement of an O-ring  119 , which is mounted in the insulator  116 , with the cathode  104 . 
         [0044]    The insulator  116  serves as part of the structure for positioning and supplying electrical power to an electrode  120  having a longitudinal axis  122 , formed of a conductive material such as copper. The electrode  120  is movable with respect to the insulator  116  between a forward position (shown in  FIG. 1 ) and a rear position (shown in  FIG. 3 ). The electrode  120  terminates at a distal end  124  (labeled in  FIG. 2 ) having an emissive element  126  embedded therein, and a proximal end  128  that terminates at a non-planar electrode rear terminal surface  130 . In this embodiment, the electrode  120  has a spiral fin  132  that aids in extracting heat to cool the electrode  120 . This electrode is a free standing element and unencumbered by attachment to other elements, so that it can be readily removed once the retaining element  114  and the nozzle element  112  are removed from the torch body element  108 . The removal of the electrode  120  can be done while leaving the rest of the structure of the assembly  100  intact. 
         [0045]    The electrode rear terminal surface  130  of the electrode  120  is a concave surface, forming a cavity. In this embodiment, the electrode rear terminal surface  130  is symmetrically disposed about the longitudinal axis  122  and terminates at the proximal end  128 . The electrode rear terminal surface  130  increases in cross section as it approaches the proximal end  128  of the electrode. In the assembly  100 , the electrode rear terminal surface  130  is frustoconical, providing a continuous surface. The cone section defining the electrode rear terminal surface  130  can be formed by rotation about the longitudinal axis  122  of a line segment inclined with respect to the axis  122  so as to define a cone having an apex angle α measuring between about 16° and 60°, and more preferably between about 40° and 60°. An apex angle of about 50° is felt to provide a desirable area of contact while maintaining the angle α sufficiently large as to reduce the tolerances required to provide accurate longitudinal positioning of an element mated against the electrode rear terminal surface  130 , as discussed below. 
         [0046]    The electrode-supporting assembly  100  also has a plunger  136 , which is again fabricated from a highly conductive material. The plunger  136  for this embodiment can be fabricated from silver or silver-based alloy, resulting in high thermal and electrical conduction across the interfaces between the plunger  136  and adjacent elements. The plunger  136  has a plunger front section  138  (again, labeled in  FIG. 2 ) having a non-planar plunger front contact surface  139 , and a plunger rear section  140  (best shown in  FIG. 2 ) that terminates in a plunger rear surface  142 . The plunger front contact surface  139  is configured to mateably engage the electrode rear terminal surface  130 , and thus is convex and frustoconical in this embodiment. While frustoconical surfaces ( 130 ,  139 ) are shown, it should be appreciated that other surface shapes could be employed. These non-planar mating surfaces should enhance thermal and electrical conduction compared to the use of planar surfaces due to the increased contacting surface area. To allow the electrode  120  to be brought into mated engagement with the plunger  136 , the mating surfaces should be configured so as to avoid any obstructions that would prevent them from being brought into engagement by translation along the longitudinal axis  122 . For frustoconical surfaces ( 130 ,  139 ), defining the cone to have an apex angle α of at least about 16° will prevent binding, allowing the surfaces ( 130 ,  139 ) to provide non-planar releasably mateable surfaces that can be readily released from engagement when the electrode  120  is replaced. 
         [0047]    In cases where trapped air is likely to be a concern, a plunger passage  144  such as a drilled hole can be provided, which passes from the plunger front section  138  through the plunger rear surface  142 , to allow escape of air trapped between the plunger front contact surface  139  and the electrode rear terminal surface  130 . Alternative structures to provide a path for escape of trapped air, such as one or more grooves in one or both of the mating surfaces ( 130 ,  139 ) could be employed. 
         [0048]    A contact element  146  formed of an electrically conductive material attaches to the insulator  116  so as to reside in the contact element recess  118 ; preferably, the contact element  146  is press-fitted into the insulator  116 . 
         [0049]    The contact element  146  has an array of gas passages  148  therethrough, and has a contact forward surface  150  and a contact rear surface  152 . The contact element  146  is configured such that, when the insulator  116  is secured in the torch recess  110  by the nozzle element  112  and the retaining element  114 , the contact rear surface  152  is forcibly engaged against the power transfer surface  106  of the cathode  104 . This forcible engagement provides a more positive contact with the cathode  104  than in many of the embodiments taught in the &#39;055 and &#39;136 patents, which rely on spring pressure to provide such contact. The contact forward surface  150  is provided on a contact forward section  154  of the contact element  146 . 
         [0050]    A resilient element  156  connects between the plunger  136  and the contact element  146 , and in this embodiment the resilient element  156  is a coiled compression spring. The resilient element  156  engages the plunger rear section  140  and the contact forward section  154 , serving to bias the plunger  136  into contact with the electrode rear terminal surface  130  when the electrode  120  resides in the torch  102  and is constrained therein by the nozzle element  112 , thereby biasing the electrode  120  to its forward position shown in  FIG. 1 . In this position, contact of the electrode  120  with the nozzle element  112  allows current passed through the electrode  120  to the nozzle element  112  to complete a circuit. Gas is subsequently introduced through the insulator  116  and pressurizes the region bounded by the nozzle element  112  and the distal end  124  of the electrode  120 ; this pressure forces the electrode  120  back against the bias of the resilient element  156  to its rear position, shown in  FIG. 3  (the distance traversed by the electrode  120  is exaggerated in the figures for purposes of illustration). As the electrode  120  is moved away from the nozzle element  112 , the current results in an arc forming therebetween, this arc heating the gas in the bounded region to generate a plasma, the bounded region surrounding the distal end  124  serving as a plasma chamber  158 . The rearward movement of the electrode  120  moves the plunger  136 , which is engaged with the electrode  120 , forcing the plunger  136  to move towards the contact element  146 , compressing the resilient element  156 . Means for maintaining engagement of the plunger  136 , the resilient element  156 , and the contact element  146  with each other are provided in this embodiment by configuring the plunger rear section  140  and the contact forward section  154  such that they are frictionally engaged by the resilient element  156 . That is the resilient element  156  is frictionally engaged with both the plunger  136  and the contact element  146 . The frictional force is sufficient that the plunger  136  remains in place within the insulator  116  and thus within the torch recess  110  when the torch  102  is opened and the electrode  120  is removed. 
         [0051]    In a similar embodiment to that shown in  FIGS. 1-3 , the plunger and the contact elements have threads configured to threadably engage the resilient element with the plunger and the contact element. 
         [0052]    To stabilize the volume of the plasma chamber  158  when the torch  102  is operating in either a non-transferred arc or transferred arc mode, means for providing a consistent rear position of the electrode  120  are provided. In this embodiment, the plunger  136  and the contact element  146  are configured such that the plunger rear surface  142  of the plunger  136  engages the contact forward surface  150  of the contact element  146  when the electrode  120  is moved backwards to the rear position. It should be noted that this position is maintained not only during the transferred arc mode of operation, but is needed to maintain a stable non-transferred arc mode. 
         [0053]    The contact element  146  is provided with a vent groove  160  across the contact forward surface  150 , positioned to communicate with the plunger passage  144  to provide a path for escape of heated air when the plunger  136  and the contact element  146  are in abutting contact. It should be appreciated that such a vent groove could alternatively be provided on the plunger  136 . 
         [0054]    Means for providing a conductive path between the contact element  146  and the electrode  120  when in the forward position and when in the rear position are provided to carry electrical current from the power supply of the torch  102  to the electrode  120 . In the electrode-supporting assembly  100 , the means for providing a conductive path include the resilient element  156 , which conducts all the current to the electrode  120  when the electrode  120  is in the forward position shown in  FIG. 1  (where there is contact between the electrode  120  and the nozzle element  112 ), and includes the engagement of the plunger rear surface  142  and the contact forward surface  150  when the electrode  120  is in its rear position. It should be noted that this rear position is the dominant position for the electrode, and is even the position for the stabilized pilot arc mode. In both positions, the current is conducted from the plunger  136  to the electrode  120  via the contact between the plunger front contact surface  139  of the plunger  136  and the electrode rear terminal surface  130  of the electrode  120 . Thus, in the pilot or transferred arc mode of operation where the electrode  120  is in its rear position, the engagement of the plunger  136  and the contact element  146  provides both stabilization of the rear position of the electrode  120  and a conductive path from the contact element  146  to the plunger  136 , which in turn conducts current to the electrode  120 . Due to the relatively large contact surfaces ( 142 ,  150 ), the current passed to the electrode  120  in the rear position through the latter path is considerably greater than the current supplied via the resilient element  156 , and the reduced current as well as heat transfer to the plunger  136  and the contact element  146  protects the resilient element  156  from overheating that might otherwise damage its resilient character. 
         [0055]    In addition to the tempering problems degrading the resiliency of the spring  156  by resistive heating of the spring, the spring  156  is subject to heating through conduction of heat from the electrode  120  through the plunger  136  to the spring  156 . Further heating may be caused by the gas passing over the spring  156  may be sufficiently hot to result in similar deterioration of performance with use. Also, the gas passing over the spring  156  may degrade the spring  156  by corrosion if the gas is sufficiently heated. 
         [0056]      FIG. 4  is a section view of an electrode-supporting assembly  200  that is similar to the electrode-supporting assembly  100  shown in  FIG. 1 , having a insulator  202 , an electrode  204 , a plunger  206 , a contact element  208 , and a resilient element  210 . In this embodiment, a supplemental conductor  212  is provided that connects directly between the contact element  208  and the plunger  206 . The supplemental conductor  212  is preferably a stranded cable to provide a high degree of flexibility. The supplemental conductor  212  can provide the means for providing a conductive path between the contact element  208  and the electrode  204  via the plunger  206  when the electrode  204  is in the forward position and in the rear position, either alone or in combination with the resilient element  210 . 
         [0057]    Since the supplemental conductor  212 , either alone or in combination with the resilient element  210 , can carry all the current from the contact element  208  to the plunger  206  when operating in either mode, the plunger  206  and the contact element  208  need not be configured to engage when the electrode  204  is in the rear position. However, the contact scheme illustrated the embodiment shown in  FIG. 4  does not in and of itself assure that the electrode  204  is stabilized when a torch in which the assembly  200  is employed is operating in plasma-generating mode. Alternate structure for providing means for providing a consistent rear position of the electrode may be needed. One such means could be provided by configuring the resilient element  210  such that its coils are completely collapsed and contact each other when the resilient element  210  is compressed as the electrode  204  moves to its rear position. Such a scheme may be more practical when the resilient element is formed by machining away a tubular element, in which case the resilient element may be formed integrally with the contact element. However, the requirement of the resilient element  210  may increase its cost of fabrication and may provide only limited stability. 
         [0058]      FIG. 5  is a section view of another embodiment of the present invention, an electrode-supporting assembly  250 , which again has a insulator  252 , an electrode  254 , a plunger  256 , a contact element  258 , a resilient element  260 , and a supplemental conductor  262 . However, the assembly  250  differs from the electrode-supporting assembly  200  in the structure that is employed to provide means for providing a consistent rear position of the electrode  254 . In the electrode-supporting assembly  250 , an insulator  264  is interposed between the plunger  256  and the contact element  258 . The insulator  264  is attached to either a rear section  266  of the plunger  256 , as shown in  FIG. 5  and discussed below, or to a contact forward surface  268  of the contact element  258 . The insulator  264  can be formed of a suitably rigid, non-conductive material such as Vespel® plastic, and can be attached to the plunger  256  by a friction fit or a high-temperature adhesive such as Loctite® Super Glue ULTRA Gel Control. Alternatively, a non-conductive material can be deposited onto a rear surface of the rear section  266  of the plunger  256  to form the insulator  264 . Similar techniques can be employed when the insulator  264  is to be attached to the contact element  258  rather than to the plunger  256 . 
         [0059]    The insulator  264  has a rearward-facing insulator bearing surface  270 . When the electrode  254  is moved from its forward position to its rear position, the insulator bearing surface  270  is brought into engagement with the contact forward surface  268 , and the engagement of these surfaces ( 268 ,  270 ) provides stabilization of the plunger  256  and the electrode  254  in a manner similar to that of the surfaces ( 142 ,  150 ) of the electrode-supporting assembly  100  discussed above. 
         [0060]    The attachment of the insulator  264  to the plunger  256  may block a plunger passage  272  extending through the plunger  256 . To extend the plunger passage  272 , the insulator  264  is provided with an insulator passage  274 . 
         [0061]    While the embodiment shown in  FIG. 5  employs an insulator interposed between the plunger and the contact element to limit rearward position of the electrode, it should be appreciated that alternate structures for physically limiting the rearward motion of the electrode without requiring direct contact between the plunger and the contact element could be employed, particularly when a supplemental conductor is provided. For example, the insulator could be provided with projections that are configured to be engaged by the electrode and/or the plunger to block further rearward motion once the electrode has reached its specified rear position. 
         [0062]      FIG. 6  is a section view of an electrode-supporting assembly  300  which is again similar to the electrode-supporting assembly  200  shown in  FIG. 4 , but again differing in the means for providing a consistent rear position of an electrode  302 . Again, a plunger  304  and a contact element  306  engage a resilient element  308  and are also connected together by a supplemental conductor  310 , these elements all residing within a insulator  312 . In the electrode-supporting assembly  300 , the supplemental conductor  310  resides within a cylindrical envelope defined by the resilient element  308 . 
         [0063]      FIGS. 7-9  are section views illustrating an electrode-supporting assembly  350 , which forms another embodiment of the present invention, having many features in common with the electrode-supporting assembly  100  shown in  FIGS. 1-3 . The electrode-supporting assembly  350  again has an electrode  352  engaged by a plunger  354  which in turn is engaged by a resilient element  356 , which in the case is a conductive spring. With the assistance of the resilient element  356 , the plunger serves to bias the electrode  352  forward as well as provide an electrical current path to the electrode  352 . This embodiment eliminates the need for a frictional fit of the plunger  354  with a resilient element  356 , as well as a frictional fit between the resilient element  356  and a contact element  358  to maintain the plunger  354  and the resilient element  356  in place in an insulator  360  when the electrode  352  is removed. While a binding fit is not required to retain the plunger  354 , it may still be desirable to assure electrical contact between these elements. In this embodiment, the insulator  360  incorporates a swirl ring and has a passage  362  therethrough, which traverses the length of the insulator  360 . 
         [0064]      FIG. 7  illustrates the electrode-supporting assembly  350  when the electrode  352  is removed from contact with the plunger  354 . The passage  362  through the insulator  360  is provided with a band  364  having a reduced cross section that forms a band opening  366 , which partitions the passage  362  into a forward chamber  368  and a rear chamber  370 . The forward chamber  368  has a cross section such that the electrode  352  can be slidably engaged therein. The plunger  354  in turn is provided with a collar  372  that is sized larger than the band opening  366 ; this sizing arrangement assures that the movement of the plunger  354  is restrained such that the collar  372  and resilient element  356  are confined to the rear chamber  370 . Thus, the resilient element  356  biases the plunger  354  so as to forcibly engage the collar  372  against the band  364 , as shown in  FIG. 7 . 
         [0065]    The collar  372  is positioned rearward of a front contact surface  374  of the plunger  354 , which is configured to mateably engage an electrode rear terminal surface  376  in a proximal end  378  of the electrode  352  when the electrode  352  is installed so as to reside partially within the insulator  360 , as shown in  FIGS. 8 and 9 . While the collar  372  is prevented from passing through the opening  366  of the band  364 , the front contact surface  374  must extend forward sufficiently to allow the plunger  354  to bias the electrode  352  against a nozzle (not shown) of the torch into which the assembly  350  is installed. The insulator  360  and the plunger  354  should be configured such that, when the electrode is installed in a torch and a nozzle is in place, a gap (a) is provided between the band  364  and the plunger  354 , as illustrated in  FIG. 8 . This gap (a) should be made sufficient in size to avoid restricting the backward flow of cooling gas that passes a spiral fin  380  of the electrode  352 . 
         [0066]    The proximal end  378  of the electrode  352  in this embodiment is sized such that, when the electrode  352  is blown back to its rear position where the proximal end  378  passes into or at least partly through the band opening  366 , the electrode  352  and the band  364  remain in a spaced apart relationship to leave a gap (b) therebetween as shown in  FIG. 9 . This gap (b) is made sufficient to maintain free flow of gas between the band  364  and the electrode  352  when the electrode  352  has moved to its rear position. In this embodiment, a portion of the front contact surface  374  of the plunger  354  extends through the band  364  when the collar  372  engages the reduced cross section band  364 , this forward-extending portion of the front contact surface  374  serving to aid in bringing the plunger  354  into axial alignment with the electrode  352  so that the front contact surface  374  of the plunger  354  becomes properly engaged with the electrode rear terminal surface  376  when the electrode  352  is installed. 
         [0067]    As with the assembly  100  shown in  FIGS. 1-3 , the plunger  354 , the resilient element  356 , and the contact element  358  are designed to allow the plunger  354  to be forced by the electrode  352  (which is being driven rearward by the gas pressure being introduced through the swirl ring) against the bias of the resilient element  356  until the plunger  354  engages the contact element  358 , as shown in  FIG. 9 . The engagement of the plunger  354  and the contact element  358  defines the rear position of the electrode  352 , and this engagement again provides both means for providing a consistent rear position of the electrode  352  and means for providing a conductive path between the contact element  358  and the electrode  352  when in the rear position. Means for providing a conductive path when the electrode  352  is in the forward position are provided by the resilient element  356 , but could include a supplemental conductor such as those discussed above. 
         [0068]    An additional benefit of the collar  372  of the plunger  354  is that it should act to deflect the rearward flow of cooling gas that has passed through the gap (b) between the electrode  352  and the band  364 . This deflection should increase the flow of cool gas across the surfaces of the plunger  354 , thereby enhancing its ability to act as a heat sink to aid in cooling the electrode  352 , with which the plunger  354  is in thermal contact. The collar  372  may further enhance cooling by providing a shoulder over which the gas flows, thereby increasing the turbulence of the flow over the rear portion of the plunger  354  to promote mixing of the gas as it flows past the plunger  354 . 
         [0069]      FIGS. 10 and 11  illustrate an electrode-supporting assembly  350 ′ that is similar to the electrode-supporting assembly  350  shown in  FIGS. 7-9 , but where the plunger  354 ′ lacks a collar  372 . In this embodiment, the front contact surface  374 ′ of the plunger  354 ′ is extended, and the opening  366 ′ of the reduced cross-section band  364 ′ of the insulator  360 ′ is configured to be engaged by the front contact surface  374 ′ to limit forward motion of the plunger  354 ′, while allowing a portion of the front contact surface  374 ′ to pass through the opening  366 ′ of the reduced-cross section band  364 ′ for engagement by the electrode  352 ′. When the electrode  352 ′ is engaged with the plunger  354 ′ and the nozzle of the torch (not illustrated) is in place such that the nozzle engages the electrode  352 ′, the gap (a′) exists between the plunger  354 ′ and the band  364 ′ so as to allow gas to flow therethrough. The electrode  352 ′ has also been sized such that, when it is blown back to its rear position where it passes into the opening  366 ′, the resulting gap (b′) is sufficient for gas to flow therethrough. 
         [0070]      FIG. 12  illustrates an electrode-supporting assembly  400  that shares many features in common with the electrode-supporting assembly  350  discussed above. Again, the assembly has a insulator  402 , an electrode  404 , a plunger  406 , a resilient element  408 , and a contact element  410 , and the insulator  402  is formed with a band  412  to provide an opening  414  having a reduced cross section. 
         [0071]    In the assembly  400 , the electrode  404  has a proximal end  416  that is tapered to form a convex frusto-conical electrode rear terminal surface  418 . The plunger  406  has a plunger front contact surface  420  that is formed as a frustoconical cavity, shaped to mateably receive the electrode rear terminal surface  418 . The electrode  404  is configured relative to the band  412  so as to be insertable into engagement with the plunger front contact surface  420 . 
         [0072]      FIGS. 13-15  illustrate an electrode-supporting assembly  450  that forms another embodiment of the present invention, and which has many features in common with the electrode-supporting assembly  350  shown in  FIGS. 7-9 . The assembly  450  again has an electrode  452  that engages a plunger  454  which in turn engages a resilient element  456  that connects to a power contact element  458 , these elements ( 454 ,  456 ,  458 ) serving to provide electrical power to the electrode  452  when biased to the electrode&#39;s forward position by the resilient element  456 . The electrode  452  mates with the plunger  454  as is the situation with the earlier embodiments. An insulator  460  is provided, having a passage  462  therethrough. The passage  462  has a band  464  which partitions the passage  462  into a forward chamber  466  and a rear chamber  468 . Again, the plunger  454  and the resilient element  456  are trapped in the rear chamber  468 . 
         [0073]    This embodiment differs from the earlier electrode-supporting assembly  350  in that the plunger  454  has a cylindrical extension  470  positioned between a frustoconical plunger front contact surface  472  and a collar  474 . This cylindrical extension  470  has a length L (labeled in  FIGS. 14 and 15 ) that is chosen to be sufficiently long as to prevent the electrode  452  from entering an opening  476  (labeled in  FIG. 13 ) defined by the band  464 . This length L assures that a gap (a) remains free when the torch is operating, as illustrated in  FIG. 14 . The extension  470  must also have the length L sufficient that a gap (b) is present when the electrode  452  is in its rear position, as shown in  FIG. 15 . These conditions assure flow of gas past the plunger  454  when gas is introduced into the passage  462 . 
         [0074]    The insulator  460  of this embodiment does not include an integral swirl ring, but rather has an insulator stepped forward edge  478  that stabilizes a separate swirl ring  480 , as best shown in  FIG. 13 . 
         [0075]      FIGS. 16 and 17  illustrate an electrode-supporting assembly  450 ′ which has all the limitations of the electrode-supporting assembly  450  illustrated in  FIG. 13-15 , but differs from the earlier embodiment in that the electrode  452 ′ has a proximal end region  482  having an electrode diameter D E  which is greater than an opening diameter D O  of the opening  476 ′; this geometry should increase the turbulence of the flow of the air over the plunger  454 ′ and thus should enhance the heat transfer between the air flow and the plunger  454 ′. 
         [0076]    While the novel features of the present invention have been described in terms of particular embodiments and preferred applications, it should be appreciated by one skilled in the art that substitution of materials and modification of details can be made without departing from the spirit of the invention.