Patent Publication Number: US-6703581-B2

Title: Contact start plasma torch

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to plasma arc torches, and more particularly to a contact start plasma arc torch. 
     Plasma arc torches, also known as electric arc torches, are commonly used for cutting, welding, and spray bonding metal workpieces. Such torches typically operate by directing a plasma consisting of ionized gas particles toward the workpiece. In general, a pressurized gas to be ionized is directed through the torch to flow past an electrode before exiting the torch through an orifice in the torch tip. The electrode has a relatively negative potential and operates as a cathode. The torch tip, which is adjacent to the end of the electrode at the front end of the torch, constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the torch, an arc is established across the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as a flame extending externally from the tip. As the torch head or front end is positioned close to the workpiece, the arc transfers between the electrode and the workpiece because the impedance of the workpiece to negative potential is typically lower than the impedance of the torch tip to negative potential. During this “transferred arc” operation, the workpiece serves as the anode. 
     Plasma arc torches may be found in both “non-contact start” and “contact start” varieties. In non-contact start torches, the tip and electrode are normally maintained at a fixed physical separation in the torch head. Typically, a high voltage high frequency signal is applied to the electrode (relative to the tip) to establish a pilot arc between the electrode and the tip. As mentioned above, when the torch head is moved toward the workpiece, the arc transfers to the workpiece. By way of contrast, in conventional contact start torches, the tip and/or the electrode make electrical contact with each other generally at the bottom of the electrode. For example, a spring or other mechanical means biases the tip and/or electrode longitudinally such that the tip and electrode are biased into electrical contact to provide an electrically conductive path between the positive and negative sides of the power supply. When the operator squeezes the torch trigger, a voltage is applied to the electrode and pressurized gas flows through the torch to the exit orifice of the torch tip. The gas causes the tip and/or the electrode to overcome the bias and physically separate. As the tip and electrode separate, a pilot arc established therebetween is blown by the gas toward the exit orifice of the tip. 
     One disadvantage associated with the conventional contact start plasma torch described above is that repeated axial movement of the electrode, the tip or both can result in axial misalignment between the electrode and tip. Also, by establishing the pilot arc between the electrode and the tip at the bottom of the electrode, damage is caused to the tip adjacent the central exit orifice of the tip. Axial misalignment of the electrode and tip, as well as any damage to the tip, can result in decreased torch performance and/or cut quality. Consequently, frequent replacement of the tip is required. For conventional contact start torches in which the tip is movable for establishing electrical contact with the electrode, the tip is in different longitudinal positions in the on and off modes of the torch, making it cumbersome for an operator to control the relative position of the tip with respect to a workpiece being cut. It is also difficult to conduct drag cutting of a workpiece, where the tip is set down onto the workpiece during cutting, because the tip would be undesirably moved into contact with the electrode upon being set down onto the workpiece. 
     SUMMARY OF THE INVENTION 
     Among the several objects and features of the present invention is the provision of a contact start plasma torch and method of operating such a torch which reduces the frequency of torch tip replacement; the provision of such a torch and method which reduces the risk of axial misalignment between the electrode and the tip; the provision of such a torch which reduces the risk of tip damage adjacent the central exit orifice of the tip; and the provision of such a torch and method which eliminates the need for axial movement of the electrode and/or the tip to generate a pilot arc. 
     In general, a contact start plasma torch of the present invention comprises a cathode body adapted for electrical communication with the negative side of a power supply and an anode body adapted for electrical communication with the positive side of the power supply. A primary gas flow path directs working gas from a source of working gas through the torch. A conductive element of the torch is constructed of an electrically conductive material and is free from fixed connection with the cathode body and the anode body. The torch is operable between an idle mode in which the conductive element provides an electrically conductive path between the cathode body and the anode body and a pilot mode in which a pilot arc formed between the conductive element and at least one of said cathode body and said anode body is adapted for initiating operation of the torch by exhausting working gas in the primary gas flow path from the torch in the form of an ionized plasma. 
     Another embodiment of the present invention is directed to a contact start plasma torch of the type having a primary gas flow path for directing a working gas through the torch whereby the working gas is exhausted from the torch in the form of an ionized plasma. The torch of this embodiment generally comprises an electrode having a longitudinally extending side surface and a bottom surface. A tip surrounds the electrode in spaced relationship therewith to at least partially define the primary gas flow path of the torch for directing a working gas through the torch in a downstream direction. The tip has a central exit orifice in fluid communication with the primary gas flow path for exhausting working gas from the torch. The bottom surface of the electrode is in longitudinally opposed relationship with the central exit orifice of the tip. Opposed contact surfaces are disposed in the torch, with at least one of the contact surfaces being movable relative to the other one of the contact surfaces. The torch is operable between an idle mode in which the contact surfaces are positioned relative to each other to provide an electrically conductive path therebetween and a pilot mode in which the contact surfaces are in spaced relationship with each other whereby a pilot arc is formed between the contact surfaces. The contact surfaces are disposed in the torch upstream from the bottom surface of the electrode whereby the pilot arc is formed generally within the primary gas flow path upstream from the bottom surface of the electrode and is blown by working gas in the primary gas flow path toward the central exit orifice of the tip for exhausting working gas from the tip in the form of an ionized plasma. 
     A conductive element of the present invention is adapted for use in a contact start plasma torch of the type having an electrode in electrical communication with the negative side of a power supply and a tip surrounding the electrode in spaced relationship therewith to at least partially define a primary gas flow path of the torch, the tip being in electrical communication with the positive side of the power supply and having a central exit orifice in fluid communication with the primary gas flow path for exhausting working gas from the tip in the form of an ionized plasma. The conductive element generally comprises a generally cup-shaped body constructed of an electrically conductive material. The conductive element is adapted for movement relative to the electrode and the tip between a first position is corresponding to an idle mode of the torch in which the conductive element provides an electrically conductive path between the positive side of the power supply and the negative side of the power supply and a second position spaced from the first position of the conductive element. The second position of the conductive element corresponds to a pilot mode of the torch whereby movement of the conductive element toward its second position forms a pilot arc generally within the primary gas flow path capable of initiating operation of the torch for exhausting working gas from the torch in the form of an ionized plasma. 
     An electrode of the present invention is adapted for use in a contact start plasma torch of the type having a primary gas flow path for directing a working gas in a downstream direction through the torch, a tip surrounding the electrode in spaced relationship therewith to at least partially define the primary gas flow path of the torch, a contact surface in the torch for forming a pilot arc in primary gas flow path of the torch and a central exit orifice in the tip communicating with the primary gas flow path for exhausting working gas from the tip in the form of an ionized plasma. The electrode generally comprises a generally cylindrical body having a longitudinally extending side surface. A bottom surface of the electrode is oriented generally radially relative to the longitudinally extending side surface for longitudinally opposed positioning relative to the central exit orifice of the tip. A contact surface is disposed above the bottom surface of the electrode and is engageable with the contact surface said tip being generally cup-shaped and having a central exit opening adapted for fluid communication with the primary gas flow path for exhausting working gas from the tip in the form of an ionized plasma, the tip further having a top surface and an annular projection extending up from the top surface for use in radially positioning the tip in the torch. 
     A tip of the present invention is adapted for use in a contact start plasma torch of the type having a primary gas flow path for directing a working gas through the torch whereby the working gas is exhausted from the torch in the form of an ionized plasma. The tip is generally cup-shaped and has a central exit opening adapted for fluid communication with the primary gas flow path for exhausting working gas from the tip in the form of an ionized plasma. The tip further has a top surface and an annular projection extending up from the top surface for use in radially positioning the tip in the torch. 
     In another embodiment, a tip of the present invention is adapted for use in a plasma torch of the type having a primary gas flow path for directing a working gas through the torch whereby the working gas is exhausted from the torch in the form of an ionized plasma and a secondary gas flow path for directing gas through the torch whereby the gas is exhausted from the torch other than in the form of an ionized plasma. The tip is generally cup-shaped and has a central exit opening adapted for fluid communication with the primary gas flow path for exhausting working gas from the tip in the form of an ionized plasma. The tip further has at least one metering orifice adapted for fluid communication with the secondary gas flow path for metering the flow of gas through the secondary gas flow path. 
     A contact assembly of the present invention is adapted for use in a contact start plasma torch of the type having a primary gas flow path for directing a working gas through the torch, an electrode in electrical communication the negative side of a power supply and a tip surrounding the electrode in spaced relationship therewith to at least partially define the primary gas flow path of the torch. The contact assembly generally comprises a conductive element constructed of an electrically conductive material and an enclosure surrounding the conductive element in fluid communication with a source of pressurized gas for receiving gas into the enclosure. The conductive element is disposed at least partially within the enclosure and is moveable relative to the enclosure, the electrode and the tip in response to pressurized gas received in the enclosure whereby movement of the conductive element forms a pilot arc in the torch. 
     An electrode assembly of the present invention is adapted for use in a contact start plasma torch of the type having a cathode body adapted for electrical communication with the negative side of a power supply and an anode body adapted for electrical communication with the positive side of the power supply. The electrode assembly generally comprises an electrode extending longitudinally within the torch and defining at least in part the cathode body of the torch. An insulating sleeve surrounds at least a portion of the electrode and is constructed of an electrically non-conductive material to insulate the at least a portion of the electrode against electrical communication with the anode body of the torch. 
     A method of the present invention is used for starting a contact start plasma torch of the type having a cathode body in electrical communication with the negative side of a power supply and an anode body in electrical communication with the positive side of the power supply, with the anode body being positioned relative to the cathode body to at least partially define a primary gas flow path of the torch and the torch having a central exit orifice in fluid communication with the primary gas flow path for exhausting working gas from the torch in the form of an ionized plasma. The method generally comprises the act of causing an electrical current to flow along an electrically conductive path comprising the anode body, the cathode body and a conductive element electrically bridging the cathode body and the anode body in a first position of the conductive element corresponding to an idle mode of the torch. Working gas is directed from a source of working gas through the primary gas flow path of the torch. Movement of the conductive element relative to the cathode body and the anode body toward a second position corresponding to a pilot mode of the torch is effected whereby a pilot arc is formed between the conductive element and at least one of said cathode body and said anode body as the conductive element is moved toward its second position. The pilot arc is then blown through the primary gas flow path toward the central exit orifice of the torch such that working gas is exhausted from the primary gas flow path of the torch in the form of an ionized plasma. 
     In another embodiment, a method of the present invention involves starting a contact start plasma torch of the type having an electrode positioned on a longitudinal axis of the torch in electrical communication with the negative side of a power supply and having a longitudinally extending side surface and a bottom surface. The method generally comprises positioning opposed contact surfaces of the torch relative to each other generally within the primary gas flow path upstream from the bottom surface of the electrode to provide an electrically conductive path through the contact surfaces. The contact surfaces are then repositioned relative to each other to form a pilot arc therebetween in the primary gas flow path of the torch upstream from the bottom surface of the electrode. Working gas from a source of working gas is directed to flow through the primary gas flow path of the torch to blow the pilot arc downstream within the primary gas flow path toward the central exit orifice of the anode body. 
     Further, a shield cup of the present invention is adapted for use in a plasma torch of the type having a primary gas flow path for directing a working gas through the torch whereby the working gas is exhausted from the torch in the form of an ionized plasma and a secondary gas flow path for directing gas through the torch whereby the gas is exhausted from the torch other than in the form of an ionized plasma, with the torch having at least one metering orifice in the secondary gas flow path for metering the flow of gas through the secondary gas flow path. The shield cup is generally cup-shaped and is adapted for at least partially defining the secondary gas flow path. The shield cup is further adapted to define a tertiary gas flow path in fluid communication with the secondary gas flow path for further exhausting gas in the secondary gas flow path from the torch. The shield cup has at least one metering orifice in the tertiary gas flow path for metering the flow of gas through the tertiary gas flow path. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary section of a contact start plasma torch of the present invention; 
     FIG. 2 is a portion of a section taken in the plane of line  2 — 2  of FIG. 1 with a conductive element shown in a raised position corresponding to an idle mode of the torch; 
     FIG. 2A is a section taken in the plane of line A—A of FIG. 2; 
     FIG. 2B is a section taken in the plane of line B—B of FIG. 2; 
     FIG. 3 is the section of FIG. 2 showing the conductive element in a lowered position corresponding to an pilot mode of the torch; 
     FIG. 3A is a section taken in the plane of line A—A of FIG. 3; 
     FIG. 3B is an enlarged portion of the contact start plasma torch of FIG. 3; 
     FIG. 4 is a section of a portion of a torch head of a second embodiment of a contact start plasma torch of the present invention with a conductive element shown in a raised position corresponding to the idle mode of the torch; 
     FIG. 5 is the section of FIG. 4 showing the conductive element in a lowered position corresponding to the pilot mode of the torch; 
     FIG. 6 is a section of a portion of a torch head of a third embodiment of a contact start plasma torch of the present invention with a conductive element shown in a lowered position corresponding to the idle mode of the torch; 
     FIG. 7 is the section of FIG. 6 showing the conductive element in a raised position corresponding to the pilot mode of the torch; 
     FIG. 8 is a section of a portion of a torch head of a fourth embodiment of a contact start plasma torch of the present invention with a conductive element shown in a raised position corresponding to the idle mode of the torch; 
     FIG. 9 is the section of FIG. 8 showing the conductive element in a raised position corresponding to the pilot mode of the torch; 
     FIG. 10 is a section of a portion of a torch head of a fifth embodiment of a contact start plasma torch of the present invention with a conductive element shown in a lowered position corresponding to the idle mode of the torch; 
     FIG. 11 is the section of FIG. 10 showing the conductive element in a raised position corresponding to the pilot mode of the torch; and 
     FIG. 12 is a section of a portion of a torch head of a sixth embodiment of a contact start plasma torch of the present invention with a conductive element shown in a raised position corresponding to the idle mode of the torch. 
     Corresponding reference characters are intended to indicate corresponding parts throughout the drawings. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the various drawings, and in particular to FIG. 1, a portion of a plasma arc torch of the present invention is generally indicated at  21 . The torch  21  includes a torch head, generally indicated at  23 , having a cathode, generally indicated at  25 , secured in a body  27  of the torch, and an electrode, generally indicated at  29 , electrically connected to the cathode. Annular insulating members  31  constructed of a suitable electrically insulating material, such as a polyamide or polyimide material, surround upper and lower portions of the cathode  25  to electrically insulate the cathode from a generally tubular anode  33  that surrounds the cathode. The anode  33  is in electrical communication with the positive side of a power supply (not shown), such as by cable  35 . The cathode  25  is electrically connected to the negative side of the power supply. The anode  33  has an intake port  37  for receiving a primary working gas, such as pure oxygen or air, into the torch head  23 . More particularly, the primary gas intake port  37  of the anode  33  is in fluid communication, such as by the cable  35 , with a source (not shown) of working gas for receiving working gas into an annular channel  39  formed by the spacing between the anode and the cathode  25 . A central bore (not shown) extends longitudinally within a lower connecting end  41  of the cathode  25 . Slots  43  extend longitudinally within the lower connecting end  41  of the cathode  25  to provide fluid communication between the cathode bore and the anode channel  39 , thereby permitting working gas in the anode channel to flow down into the torch head  23  via the cathode bore. 
     Still referring to FIG. 1, the electrode  29  has an upper connecting end  45  for connecting the electrode with the connecting end  41  of the cathode  25  in coaxial relationship therewith about a central longitudinal axis X of the torch head  23 . As a result, the electrode  29  is electrically connected to the cathode, and hence in electrical communication with the negative side of the power supply. The electrode  29  and cathode  25  together broadly define a cathode body of the torch  21  in electrical communication with the negative side of the power supply. In the illustrated embodiment, the connecting ends  41 ,  45  of the cathode  25  and the electrode  29  are configured for a coaxial telescoping connection with one another in the manner shown and described in co-owned U.S. Pat. No. 6,163,008, which is incorporated herein by reference. To establish this connection, the cathode connecting end  41  and electrode connecting end  45  are formed with opposing detents generally designated  47  and  49 , respectively. These detents  47 ,  49  are interengageable with one another when the connecting end  45  of the electrode  29  is connected to the cathode  25  to inhibit axial movement of the electrode away from the cathode. It is understood, however, that the electrode  29  may be connected to the cathode  25  in other conventional manners, such as by threaded connection, without departing from the scope of this invention. 
     A central bore (not shown) extends longitudinally within the upper connecting end  45  of the electrode  29  and is in fluid communication with the central bore of the cathode connecting end  41  such that working gas in the cathode central bore is directed down through the central bore of the electrode. The central bore of the electrode  29  extends down from the top of the electrode into registry with gas distributing holes  51  extending radially outward from the central bore for exhausting working gas from the electrode. An annular collar  53  having a jogged, or stepped diameter extends radially outward from the upper connecting end  45  of the electrode  29  above the gas distributing holes  51 . The stepped diameter of the collar  53  defines an annular flange  55  for longitudinally positioning the electrode  29  in the torch head  23  as described later herein. 
     With reference to FIG. 2, the electrode  29  has a cylindric mid-section  57  extending longitudinally below the central bore and gas distributing holes  51  and having a substantially enlarged outer diameter. The outer diameter of the electrode  29  gradually decreases as the electrode extends down from the bottom of the mid-section  57  toward a lower end  59  of the electrode to define a tapered contact surface  61  on the electrode. The lower end  59  of the electrode  29  includes a bottom surface  63  oriented generally radially with respect to the central longitudinal axis X of the torch  21  and a side surface  65  extending generally longitudinally up from the bottom surface to the tapered contact surface  61  of the electrode. The electrode  29  of the illustrated embodiment is constructed of copper and has an insert  66  of emissive material (e.g., hafnium) secured in a recess  67  in the bottom surface  63  of the electrode. 
     A generally cup-shaped metal tip  71 , also commonly referred to as a nozzle, is disposed in the torch head  23  surrounding the lower end  59  of the electrode  29  in radially and longitudinally spaced relationship therewith to form a primary gas passage  73  (otherwise referred to as an arc chamber or plasma chamber) between the tip and the electrode. A central exit orifice  75  of the tip  71  communicates with the primary gas passage  73  for exhausting working gas from the torch  21  and directing the gas down against a workpiece. The outer diameter of the tip  71  increases as the tip extends up toward an upper end  77  of the tip to define a tapered lower contact surface  79  engageable by a shield cup  81 , as discussed later herein, for securing the tip in the torch head  23 . An annular projection  83  extends up from the top of the tip  71  and is positioned generally centrally thereon such that the top of the tip defines an upwardly facing annular shoulder  85  disposed radially outward of the annular projection and an upwardly facing contact surface  87  disposed radially inward of the projection. An inner surface  88  (FIG. 3B) of the annular projection  83  slopes upward and radially outward from the upward facing contact surface  87  to the top of the annular projection. 
     With particular reference to FIGS. 2 and 3, a contact assembly of the present invention is generally indicated at  101  and is operable between an idle mode (FIG. 2) and a pilot mode (FIG. 3) of the torch  21 . In the idle mode of the torch, the contact assembly  101 , the tip  71  to and the electrode  29  are relatively positioned such that the contact assembly provides an electrically conductive path between the positive side of the power supply and the negative side of the power supply without working gas being exhausted from the torch in the form of an ionized plasma. In the pilot mode of the torch  21  the contact assembly  101 , the tip  71  and the electrode  29  are relatively positioned so that a pilot arc is formed in the torch head  23  and is adapted for initiating operation of the torch to exhaust working gas from the torch in the form of an ionized plasma. The contact assembly  101  of the illustrated embodiment comprises a tubular casing  103  having a generally cylindrical side wall  105  and an annular bottom wall  107  extending radially inward from the bottom of the side wall. The bottom wall  107  of the casing  103  has a central opening  109  for receiving therethrough the electrode  29  and the annular projection  83  extending up from the tip  71  whereby the bottom wall of the casing seats on the outer annular shoulder  85  formed by the tip and the annular projection to radially and longitudinally position the tip in the torch head  23  relative to the contact assembly and to electrically connect the tip and the casing. 
     The tubular casing  103  of the illustrated embodiment is constructed of an electrically conductive metal, preferably brass, and is sized to extend sufficiently upward in the torch head  23  so that the side wall  105  of the casing contacts the bottom of the anode  33  when the bottom wall  107  of the casing seats on the tip  71  to electrically connect the casing and the anode. As a result, the anode  33 , the tip  71  and the casing  103  are in electrical communication with the positive side of the power supply and together broadly define an anode body of the torch. It is contemplated that the tubular casing  103  of the contact assembly  101  may instead be formed integrally with the tip  71  without departing from the scope of this invention. 
     An interior shoulder  111  is formed in the side wall  105  of the casing  103  slightly below its upper end to seat a cap  113  of the contact assembly within the casing. As shown in the illustrated embodiment, the assembly cap  113  is annular and has a central opening  115  to receive the electrode  29  therethrough. The assembly cap  113  has a jogged, or stepped inner diameter in the opening  115  to define a shoulder  117  sized in accordance with the stepped outer diameter of the annular collar  53  extending radially outward from the electrode  29 . The annular flange  55  defined by the collar  53  is sized for seating on the shoulder  117  in the central opening  115  of the cap  113  to longitudinally position the electrode  29  in the torch head  23  relative to the contact assembly  101  and the tip  71 . The collar also radially positions the electrode in coaxial relationship with the contact assembly and the tip on the central longitudinal axis X of the torch  21 . The tubular contact assembly casing  103  and the assembly cap  113  together broadly constitute an enclosure defined by the contact assembly  101  for containing working gas in the contact assembly. 
     An insulating sleeve  119  constructed of an electrically non-conductive material surrounds the enlarged mid-section  57  of the electrode  29  in close contact therewith to electrically insulate the mid-section of the electrode against electrical communication with a conductive element  121  surrounding the electrode within the contact assembly casing  103 . Diametrically opposed tabs  123  (FIGS. 1,  2 A) extend up from the top of the insulating sleeve  119  and contact the bottom of the annular collar  53  of the electrode  29  to longitudinally position the sleeve on the electrode. Arcuate openings  125  (FIG. 2A) extend circumferentially between the tabs  123  in radial registry with the gas distributing holes  51  of the electrode  29  to permit gas exhausted from the electrode through the gas distributing holes to flow outward through the insulating sleeve to an upper gas chamber  127  (broadly, a high pressure gas chamber) of the enclosure defined by the contact assembly casing  103  and the assembly cap  113  (FIG.  3 ). The insulating sleeve  119  is preferably secured to the electrode  29 , such as by being press-fit onto the electrode, such that the electrode and insulating sleeve together broadly define an electrode assembly that can be installed in or removed from the torch as a unit. 
     The conductive element  121  is generally cup-shaped and is disposed within the tubular casing  103 . The conductive element  121  of the illustrated embodiment has a central passage  129  for receiving the electrode  29  therethrough with the inner surface of the conductive element surrounding the insulating sleeve  119  in closely spaced relationship therewith and the outer surface of the conductive element in closely spaced relationship with the inner surface of the casing  103 . The conductive element  121  is free from fixed connection to the electrode  29  and cathode  25  (i.e., the cathode body) and the anode  33 , contact assembly casing  103  and tip  71  (i.e., the anode body). The term “free from fixed connection” as used herein means that relative movement is possible between the conductive element and the cathode body and anode body in at least one direction, such as axially and/or radially. For example, in the illustrated the conductive element is free to move axially along the central longitudinal axis X of the torch head  23  within the enclosure defined by the casing and the assembly cap  113 . More particularly, the conductive element  121  is axially movable relative to the electrode  29 , insulating sleeve  119 , tubular casing  103  and tip  71  between a first, raised position (FIG. 2) corresponding to the idle mode of the torch  21  and a second, lowered position (FIG. 3) corresponding to the pilot mode of the torch. It is understood, however, that the conductive element  121  may be free to move radially relative to the cathode body and the anode body. It is also understood that the conductive element  121  may instead be stationary within the torch and either the cathode body, the anode body or both may be free to move, axially and/or radially, relative to the conductive element. 
     The inner surface of the conductive element  121  tapers inward as the conductive element extends down to a lower end  131  of the element to define an upper contact surface  133  of the conductive element. The upper contact surface  133  is tapered at an angle generally corresponding to the tapered contact surface  61  of the electrode  29  and is generally disposed in axially opposed (e.g., face-to-face) relationship therewith. The bottom of the conductive element  121  defines a generally radially oriented lower contact surface  135  disposed in axially opposed (e.g., face-to-face) relationship with the upper contact surface  87  of the tip  71  extending radially inward from the annular projection  83 . As shown in FIG. 3B, a portion  136  of the outer surface of the conductive element slopes generally upward and radially outward from the contact surface  135  and is sized radially to be as close as possible to the inner surface of the annular projection  83  without contacting the annular projection so that the lower contact surface  135  of the conductive element  121  will contact the upper contact surface  87  of the tip  71  when the conductive element is in its lowered position. For example, the conductive element  121  of the illustrated embodiment is spaced about 0.0043 inches from the inner surface of the annular projection  83  in the lowered position of the conductive element. 
     The conductive element  121  also includes an upper end  137  in close, radially spaced relationship with the inner surface of the side wall  105  of the contact assembly casing  103 , beneath the upper gas chamber  127  of the enclosure, to define a relatively narrow (e.g., 0.005 in.) annular passage  139  between the conductive element and the casing. The lower end  131  of the conductive element  121  has an outer diameter substantially less than that of the upper end  137  to define, together with the casing  103 , a lower gas chamber  141  (broadly, a low pressure gas chamber) of the enclosure in fluid communication with the upper gas chamber  127  via the narrow passage  139  formed between the conductive element and the casing side wall  105 . 
     A coil spring  151  (broadly, a biasing member) is disposed in the lower gas chamber  141  of the contact assembly  101  in radially spaced relationship with both the outer surface of the conductive element  121  and the inner surface of the tubular casing side wall  105 . The spring  151  seats on the bottom wall  107  of the contact assembly casing  103  and is sized axially for contacting a bottom surface  153  of the upper end  137  of the conductive element  121 . The coil spring  151  of the illustrated embodiment is constructed of an electrically conductive material such that the spring is electrically connected at one end (its upper end) to the conductive element  121  and at the opposite (lower) end to the contact assembly casing  103 . As a result, the conductive element  121  remains in electrical communication with the contact assembly casing  103  and, therefore, with the positive side of the power supply, as the conductive element moves between its raised and lowered positions. It is understood that the spring  151  may instead be electrically connected to the tip  71 , without departing from the scope of this invention, as long as the conductive element remains in electrical communication with the positive side of the power supply. The spring  151  preferably remains in compression in the raised and lowered positions of the conductive element  121  to maintain electrical communication between the contact assembly casing  103  and the conductive element and to continually bias the conductive element toward its raised position (FIG. 2) corresponding to the idle mode of the torch  21 . 
     When the conductive element  121  is in its raised position, its upper contact surface  133  engages the contact surface  61  of the electrode  29  to provide electrical communication between the conductive element and the electrode, thereby completing an electrically conductive path between the cathode body and the anode body, i.e., between the positive side of the power supply and the negative side of the power supply. The lower contact surface  135  of the conductive element  121  is longitudinally spaced from the upper contact surface  87  of the tip  71  in the raised position of the conductive element  121 . 
     In the lowered position (FIGS. 3 and 3B) of the conductive element  121  corresponding to the pilot mode of the torch, the upper contact surface  133  of the conductive element is positioned down away from the lower contact surface  61  of the electrode  29 . More preferably, the upper contact surface  133  of the conductive element  121  is positioned a distance from the lower contact surface  61  of the electrode  29  approximating the width of the primary gas passage  73 . For example, in the illustrated embodiment the primary gas passage has a width of the about 0.044 inches and the contact surface  133  of the conductive element  121  is positioned a distance of about 0.040-0.045 inches from the lower contact surface  61  of the electrode  29 . 
     As shown in FIG. 3B, the lower contact surface  135  of the conductive element  121  seats on the upper contact surface  87  of the tip  71  in the lowered position of the conductive element such that the conductive element and tip combine to define a portion of the primary gas passage  73 . The portion  136  of the outer surface of the conductive element  121  extending up from the lower contact surface  135  is in closely spaced relationship with the inner surface  88  of the annular projection  83  extending up from the tip to provide sufficient clearance therebetween to permit the lower contact surface  135  of the conductive element to seat on the upper contact surface  87  of the tip. However, the spacing between the conductive element  121  and the inner surface  88  of the annular projection  83  is sufficiently close to restrict the flow of gas therebetween (e.g., the spacing therebetween is about 0.0043 inches, which is one-tenth of the width of the primary gas passage  73 ) to thereby inhibit working gas flowing down through primary gas passage  73  against flowing back into the lower gas chamber  141  between the tip and the conductive element. The inner surface  88  of the annular projection  83  also inhibits the conductive element against radial movement to thereby maintain the conductive element in coaxial relationship with the longitudinal axis X of the torch  21 . It is understood, however, that since the tip  71  is already electrically connected to the contact assembly casing  103 , the lower contact surface  135  of the conductive element  121  need not seat directly on the upper contact surface  87  of the tip to remain within the scope of this invention. It is also understood that the inner surface  88  of the annular projection  83  may extending vertically up from the upper contact surface  87  of the tip  71  without departing from the scope of the this invention. 
     Gas inlet holes  155  (FIG. 3A) extend through the conductive element  121  above its upper contact surface  133  to provide fluid communication between the lower gas chamber  141  of the contact assembly  101  and the primary gas passage  73  formed in part by the conductive element and the electrode  29  and in part by the tip. The gas inlet holes  155  of the illustrated embodiment extend generally tangentially through the conductive element  121  for causing a swirling action of working gas flowing into and down through the primary gas passage  73 . Alternatively, the gas inlet holes  155  may extend radially through the conductive element  121 . 
     Referring back to FIG. 1, the tip  71 , electrode  29  and non-moving elements of the to contact assembly  101  (e.g., the casing  103  and the insulating sleeve  119 ) are secured in axially fixed position relative to each other during operation of the torch  21  by the shield cup  81 . The shield cup  81  is constructed of a non-conductive, heat insulating material, such as fiberglass, and has internal threads for threadable engagement with corresponding external threads on the anode  33 , which is fixed within the torch body  27 . The shield may alternatively include a metal insert  682  (as shown in the alternative embodiments of FIG.  8  and FIG. 12) having internal threads for threadable engagement with the anode  33  without departing from the scope of this invention. A lower end  161  of the shield cup  81  has a central opening  163  sized to permit throughpassage of the tip  71  with the shield cup radially spaced from the tip in the central opening to define an annular secondary exit opening of the torch  21 . The inner diameter of the lower end  161  of the shield cup  81  gradually increases as the shield cup extends up from the central opening  163  to define a contact surface  165  tapered at an angle generally corresponding to the tapered lower contact surface  79  of the tip  71  and in axially opposed (e.g., face-to-face) relationship therewith. 
     When the shield cup  81  is installed on the torch  21 , the contact surface  165  of the shield cup  81  contacts the lower contact surface  79  of the tip  71  to axially secure the tip, and hence the contact assembly  101  and the electrode  29 , within the torch head  23 . The shield cup  81  extends up from the contact surface  165  in radially spaced relationship with the outer surface of the tip  71  to define a secondary gas chamber  166 . Grooves  167  (FIG. 1) are formed in the lower contact surface  79  of the tip  71  to provide fluid communication between the secondary gas chamber  166  and the central opening  163  of the shield cup  81 . Openings  169  (FIGS. 2,  2 B) are disposed in the tubular casing  103  of the contact assembly  101  in fluid communication with the lower gas chamber  141  of the contact assembly to divert a portion of working gas in the lower gas chamber into the secondary gas chamber  166  for exhaustion from the torch  21  via the central opening  163  of the shield cup  81 . 
     The shield cup  81 , tip  71 , contact assembly  101  and electrode  29  are consumable parts of the torch  21  in that the useful working life of these parts is typically substantially less than that of the torch itself and, as such, require periodic replacement. 
     In operation according to a method of the present invention for operating a contact start plasma arc torch, the torch  21  is initially in its idle mode (FIG.  2 ), with no current or gas flowing to the torch head. The conductive element  121  is biased by the coil spring  151  toward its raised position corresponding to the idle mode of the torch, with the upper contact surface  133  of the conductive element  121  engaging the downwardly facing contact surface  61  of the electrode  29  to provide an electrically conductive path between the positive and negative sides of the power supply. When operation of the torch  21  is desired, electrical current and working gas are introduced into the torch  21 . More particularly, positive potential is directed from the power supply via the cable  35  to the anode  33  and flows through a circuit including the contact assembly casing  103 , the coil spring  151 , the conductive element  121 , the electrode  29  and the cathode  25  back to the negative side of the power supply. 
     Working gas is directed from the source of working gas into the torch  21  and flows through a primary gas flow path comprising the anode intake port  37 , anode channel  39 , cathode bore, electrode bore, gas distributing holes  51  of the electrode  29 , upper gas chamber  127  of the contact assembly  101 , narrow passage  139  between the conductive element  121  and the inner surface of the casing  103 , lower gas chamber  141  of the contact assembly, gas inlet holes  155  of the conductive element, primary gas passage  73  and central exit orifice  75  of the tip  71 . A portion of working gas in the lower gas chamber  141  is directed to flow through a secondary gas flow path comprising the openings  169  in the contact assembly casing  103 , secondary gas chamber  165  and the grooves  167  in the lower contact surface  79  of the tip  71  for exhaustion from the torch  21  via the central opening  163  of the shield cup  81 . The flow of working gas from the upper gas chamber  127  to the lower gas chamber  141  is restricted by the narrow passage  139  formed between the conductive element  121  and the inner surface of the contact assembly casing  103 . This causes gas pressure in the upper gas chamber  127  to increase and act against the upper end  137  of the conductive element  121 , as in the manner of a piston, to move the conductive element against the bias of the spring  151  toward the lower gas chamber  141 , i.e., toward the lowered position (FIG. 3) of the conductive element corresponding to the pilot mode of the torch  21 . As an example, the pressure differential between the upper (high pressure) gas chamber  151  and the lower (low pressure) gas chamber  141  of the illustrated embodiment is about 1.7 psi. 
     As the conductive element  121  is moved toward its lowered position, the upper contact surface  133  of the conductive element  121  is moved down away from the contact surface  61  of the electrode  29  to substantially increase the spacing therebetween. A pilot arc is formed between the upper contact surface  133  of the conductive element  121  and the electrode contact surface  61 , generally in the portion of the primary gas passage  73  (e.g., the primary gas flow path) formed by the conductive element and the electrode contact surface, and is exposed to a greater flow of working gas through the primary gas passage. The pilot arc is thus adapted for being blown by working gas flowing through the primary gas passage  73  down through the primary gas passage toward the central exit orifice  75  of the tip  71  for initiating operation of the torch by exhausting working gas from the tip in the form of an ionized plasma. 
     In the several embodiments of the contact start torch shown and described herein, including the torch  21  of the first embodiment of FIGS. 1-3, the conductive element  121  is shown and described as engaging the electrode (e.g., the anode body) in the idle mode of the torch to provide an electrically conductive path between the anode body and the cathode body. It is understood, however, that the conductive element  121  need not engage the anode body or the cathode body in the idle mode of the torch, as long as the conductive element is positioned sufficiently close to at least one of the cathode body and the anode body to provide an electrically conductive path between the positive and negative sides of the power supply. In such an instance, an arc may be formed between the conductive element  121  and the anode body or the cathode body in the idle mode of the torch, but such an arc is not considered to be a pilot arc as that term is commonly understood and as used herein because it is not adapted for initiating operation of the torch by exhausting working gas from the torch in the form of an ionized plasma. 
     Rather, any spacing between the conductive element and the anode body or the cathode body in the idle mode of the torch would be relatively small compared to the spacing therebetween in the pilot mode of the torch such that gas flow between the conductive element and the anode body or cathode body is substantially restricted and is therefore incapable of blowing any arc formed therebetween in the idle mode of the torch down toward the exit orifice of the tip to exhaust working gas from the torch in the form of an ionized plasma. Therefore, reference herein to a pilot arc formed in the torch upon movement of the conductive element toward its second position corresponding to the pilot mode of the torch means an arc formed between the conductive element and at least one of the cathode body and the anode body when the conductive element is sufficiently spaced from the cathode body and/or the anode body that the arc formed therebetween can be blown through the primary gas flow path to the exit orifice of the tip for initiating operation of the torch whereby working gas is exhausted from the torch in the form of an ionized plasma. 
     Further operation of the plasma arc torch  21  of the present invention to perform cutting and welding operations on a workpiece is well known and will not be further described in detail herein. 
     As shown in the drawings and described above, the conductive element  121  remains in electrical communication with the positive side of the power supply, via the coil spring  151  and the contact assembly casing  103 , as the torch  21  operates between its idle mode and the pilot mode. However, it is understood that the conductive element  121  may instead remain in electrical communication with the negative side of the power supply as the torch  21  operates between its idle mode and pilot mode without departing from the scope of this invention. For example, the conductive element  121  may be electrically connected to the electrode or cathode (e.g., the cathode body) such that in the first position of the conductive element corresponding to the idle mode of the torch  21  the conductive element is in electrical communication with the tubular casing  103  or the tip  71  to provide an electrically conductive path between the positive and negative sides of the power supply. In the second position of the conductive element  121  corresponding to the pilot mode of the torch  21  the conductive element would remain in electrical communication with the negative side of the power supply and be moved away from the tubular casing  103  or tip  71  to form the pilot arc between the conductive element and the casing or tip in the primary gas flow path of the torch. 
     Additionally, the electrode  29  and the tip  71  are shown and described as being secured in the torch  21  in fixed relationship with each other as the conductive element  121  moves between its raised and lowered positions. However, the electrode  29 , the tip  71  or both may move relative to each other and remain within the scope of this invention, and the conductive element  121  may or may not be secured against movement within the torch, as long as the conductive element is free from fixed connection with the electrode and the tip in at least one direction so that the conductive element can assume different positions relative to the electrode and the tip in the idle mode and the pilot mode of the torch  21 . 
     Also, while the conductive element  121  is moved between its raised and lowered positions pneumatically, such as by a force generated by pressurized gas (e.g., the working gas flowing through the primary gas flow path), it is understood that the conductive element may be mechanically driven between its raised and lowered positions without departing from the scope of this invention. 
     FIGS. 4 and 5 illustrate part of a second embodiment of a contact start plasma torch  221  of the present invention substantially similar to that of the first embodiment (FIGS. 1-3) in that it comprises an electrode  229  in electrical communication with the negative side of the power supply, a tip  271  in electrical communication with the positive side of the power supply, a contact assembly  301  operable between an idle mode and an pilot mode of the torch and a shield cup (not shown, but similar to the shield cup  81  of FIG.  1 ). A conductive element  321  of the contact assembly  301  of this second embodiment is generally cup-shaped and has a central passage  329  for receiving the electrode  229  therethrough. The inner diameter of the conductive element  321  is generally stepped, or jogged, to define an upper contact surface  333  of the conductive element, an intermediate shoulder  343  for seating a gas distributor  267  in the central passage  329  of the conductive element and an upper shoulder  345 . The inner diameter increases along the upper contact surface  333  such that the contact surface is tapered at an angle generally corresponding to a tapered contact surface  261  of the electrode  229 . The gas distributor  267  is generally annular and seats on the intermediate shoulder  343  of the conductive element  321  in closely spaced relationship with at least a portion of the mid-section  257  of the electrode  229 . The gas distributor  267  is constructed of a non-conductive material to electrically insulate the mid-section  257  of the electrode  229  against electrical contact with the conductive element  321 . Thus it will be seen that the gas distributor  267  can be broadly defined as an insulating sleeve similar to the insulating sleeve  119  of the first embodiment. The gas distributor  267  of the illustrated embodiment is connected to the conductive element  321 , such as being press-fit or bonded thereto, so that the gas distributor and the conductive element can be installed in and removed from the torch as a single unit. 
     The mid-section  257  of the electrode  229  has a stepped outer diameter so that a portion of the outer surface of the mid-section is spaced radially inward of the gas distributor  267  to define a gas inlet  347  upstream of the contact surface  261  of the electrode. The gas distributor  267  has inlet holes  269  extending therethrough and located generally axially above the upper shoulder  345  of the conductive element  321  to provide fluid communication between the upper gas chamber  327  of the contact assembly  301  and the gas inlet  347  for directing gas in the upper gas chamber into the gas inlet. The inlet holes  269  of the illustrated embodiment extend generally tangentially through the gas distributor  267  for causing a swirling action of working gas flowing into the gas inlet and down through the primary gas passage  273 . However, it is understood that the inlet holes  269  may extend radially through the gas distributor  267  without departing from the scope of this invention. 
     As in the first embodiment, the conductive element  321  of this second embodiment is capable of axial movement on the central longitudinal axis X of the torch  221  relative to the electrode  229 , contact assembly casing  303  and tip  271  between a first, raised position corresponding to an idle mode of the torch and a second, lowered position corresponding to a pilot mode of the torch. The gas distributor  267 , supported in the torch  221  by the conductive element  321 , moves conjointly with the conductive element. A biasing member of this second embodiment is defined by an annular, canted coil spring  351  seated on the radially inward extending bottom wall  307  of the contact assembly casing  303  in contact with the side wall  305  of the casing. The spring  351  also contacts a tapered outer surface  349  of the conductive element  321  to bias the conductive element toward its raised position corresponding to the idle mode of the torch and to provide electrical communication between the conductive element and the contact assembly casing  303 , i.e., the positive side of the power supply. 
     In the raised position (FIG. 4) of the conductive element  321 , the upper contact surface  333  of the conductive element engages the downwardly facing contact surface  261  of the electrode  229  to provide electrical communication between the conductive element and the electrode, thereby completing an electrically conductive path between the contact assembly casing  303  and the electrode, i.e., between the positive side of the power supply and the negative side of the power supply. It is understood, however, that in its raised position the conductive element  321  need not engage the contact surface  261  of the electrode  229 , as long as it is positioned sufficiently close to the electrode contact surface to provide an electrically conductive path between the positive and negative sides of the power supply. The lower contact surface  335  of the conductive element  321  is longitudinally spaced from the upper contact surface  287  of the tip  271  in the raised position of the conductive element. The inlet holes  269  of the gas distributor  267  are out of radial registry with the gas inlet  347  defined by the gas distributor and the spaced portion of the mid-section  257  of the electrode  229  to inhibit the flow of working gas in the upper gas chamber  327  of the contact assembly  301  into the gas inlet. 
     In the lowered position (FIG. 5) of the conductive element  321 , the upper contact surface  333  of the conductive element  321  is positioned down away from the contact surface  261  of the electrode  229  (e.g., a distance greater than that between the upper contact surface of the conductive element and the electrode contact surface in the raised position of the conductive element). The gas inlet  347  is in fluid communication with the gas passage  273  formed between the electrode  229  and the tip  271 , with the gas inlet further defining the primary gas flow path of the torch  221  when the conductive element is in its lowered position. The inlet holes  269  of the gas distributor  267  are in radial registry with the gas inlet  347  to direct working gas in the upper gas chamber  327  of the contact assembly  301  into the gas inlet and down through the gas passage  273  to the central exit orifice  275  of the tip  271 . 
     Electrical operation of the contact start plasma torch  221  of this second embodiment is substantially similar to that of the first embodiment and will not be further described herein. To initiate operation of the torch, working gas is introduced into the torch and directed to flow into the upper gas chamber  327  of the contact assembly  301 . With the inlet holes  269  of the gas distributor  267  out of registry with the gas inlet  347 , the narrow passage  339  between the upper gas chamber  327  and the lower gas chamber  341  restricts the flow of working gas to the lower gas chamber. The gas pressure in the upper gas chamber  327  increases and acts down against the gas distributor  267  and the conductive element  321  to urge the conductive element to move down against the bias of the spring  351  toward the lowered position (FIG. 5) of the conductive element. As the upper contact surface  333  of the conductive element  321  is moved away from the contact surface  261  of the electrode  229 , a pilot arc is formed therebetween. Further, the inlet holes  269  of the gas distributor  267  are moved down into radial registry with the gas inlet  347  as the conductive element is moved toward its lowered position. As a result, working gas in the upper gas chamber  327  of the contact assembly  301  is directed through the inlet holes  269  in the gas distributor  267  into the gas inlet  347 . The working gas is then further directed down through the gas passage  273 , blowing the pilot arc formed between the conductive element  321  and the electrode  229  down through the gas passage toward the central exit orifice  275  of the tip  271  to initiate operation of the torch whereby working gas is exhausted from the torch  221  in the form of an ionized plasma. The flow of working gas through a secondary gas flow path of the torch  221  of this second embodiment is the same as for the first embodiment and will not be further described herein. 
     FIGS. 6 and 7 illustrate a contact assembly  501  of a contact start plasma torch  421  of a third embodiment of the present invention in which the conductive element  521  of the contact assembly is electrically neutral. That is, the conductive element  521  does not remain electrically connected to any potential carrying structure, such as the cathode, the electrode  429 , the tip  471  or the contact assembly casing  503 . 
     In this third embodiment, the annular cap  513  of the contact assembly  501  is integrally formed with the tubular casing  503  and is in close, radially spaced relationship with the electrode  429  generally below the gas distributing holes  451  of the electrode. The contact assembly casing  503  seats on a radially outward extending upper surface  489  of the tip  471 . The mid-section  457  of the electrode  429  is substantially narrowed within the casing  503  whereby the narrowed mid-section and the lower end  459  of the electrode form a shoulder defining a radially oriented contact surface  461  of the electrode. The electrode  429  and tip  471  are secured in generally fixed relationship with each other in the torch  421  with the contact surface  461  of the electrode in radially coplanar alignment with the upper surface  489  of the tip. The contact assembly casing  503  has an inlet hole  557  disposed in its side wall  505  adjacent the lower end of the side wall and an outlet hole  559 , also disposed in the side wall, generally adjacent the upper end of the side wall. 
     An annular support plate  571  constructed of an electrically non-conductive material is disposed within the contact assembly casing  503  and has a central opening  573  through which the narrowed mid-section  457  of the electrode  429  extends. The conductive element  521  is also annular and is constructed of an electrically conductive material, such as brass. The conductive element  521  is secured to the underside of the support plate  571 , such as being bonded thereto, and depends therefrom for conjoint movement of the conductive element with the support plate. The conductive element  521  of this third embodiment is axially movable on the central longitudinal axis X of the torch  421  relative to the electrode  429 , the tip  471  and the contact assembly casing  503  between a first, lowered position (FIG. 6) corresponding to the idle mode of the torch and a second, raised position (FIG. 7) corresponding to the pilot mode of the torch. The annular width of the conductive element  521  is substantially greater than the width of the gas passage  473  formed between the tip  471  and the electrode  429  such that in the lowered position (FIG. 6) of the conductive element, the conductive element is in electrical communication with both the electrode and the tip to provide an electrically conductive path between the electrode and the tip, i.e., between the positive and negative sides of the power supply. It is understood that in its lowered position the conductive element  521  need not engage the contact surface  461  of the electrode  429  and the upper surface  489  of the tip  471 , as long as it is positioned sufficiently close to the electrode and tip to provide an electrically conductive path between the positive and negative sides of the power supply. 
     In its raised position (FIG.  7 ), the conductive element  521  is positioned up away from the tip  471  and the electrode  429  (i.e., a distance greater than the distance between the conductive element and the electrode and tip in the lowered position of the conductive element) such that a pilot arc adapted for initiating operation of the torch is formed between the tip and the conductive element and another pilot arc capable of initiating operation of the torch is formed between the electrode and the conductive element. The biasing member of this third embodiment comprises a coil spring  551  that seats on the top of the support plate  571  and extends up into contact with the contact assembly cap  513 . The spring  551  is preferably sized to remain in compression for continuously biasing the conductive element  521  toward its lowered position corresponding to the idle mode of the torch. Since the conductive element  521  of this third embodiment is electrically neutral, the spring  551  may be constructed of an electrically non-conductive material. 
     In the illustrated embodiment, the axial dimension of the conductive element  521  is such that in the lowered position (FIG. 6) of the conductive element, the support plate  571  is axially disposed above the inlet hole  557  in the side wall  505  of the casing  503  to divide the enclosure defined by the casing  503  and assembly cap  513  into a lower, high pressure gas chamber  575  below the plate and an upper, low pressure gas chamber  577  above the plate. The support plate  571  is spaced radially inward of the side wall  505  of the casing  503  to define a narrow passage  539  (e.g., 0.005 in.) between the upper and lower gas chambers  577 ,  575  of the enclosure for providing fluid communication therebetween. In this manner, working gas in the primary gas flow path enters the enclosure via the inlet hole  557  into the lower gas chamber  575 . The narrow passage  539  restricts the flow of gas to the upper gas chamber  577 . 
     As a result, the pressure in the lower gas chamber  575  increases and acts against the conductive element  521  and support plate  571  to urge the support plate and conductive element up against the bias of the spring  551  toward the raised position of the conductive element corresponding to the pilot mode of the torch. The support plate  571  is axially positioned below the outlet hole  559  in the side wall  505  of the casing  503  in both the raised and lowered positions of the conductive element  521 . It is understood that the narrow passage  539  may be omitted, such that the high pressure gas chamber  575  and low pressure gas chamber  577  are not in fluid communication with each other, without departing from the scope of this invention. 
     In operation, working gas flowing through enclosure flows between the conductive element  521  and the tip  471  and electrode  429  down through the primary gas passage  473 , blowing the pilot arcs formed between the conductive element and the tip and between the conductive element and the electrode down through the primary gas passage so that the pilots arc merge into a single arc blown down toward the central exit orifice of the tip for initiating operation of the torch whereby primary working gas is exhausted from the torch in the form of an ionized plasma. 
     FIGS. 8 and 9 illustrate a contact assembly  701  of a fourth embodiment of a contact start plasma torch  621  of the present invention substantially similar to that of the first embodiment in that it comprises an electrode  629  in electrical communication with the negative side of the power supply, a tip  671  in electrical communication with the positive side of the power supply, a contact assembly  701  operable between an idle mode and a pilot mode of the torch, and a shield cup  681  of FIG.  1 . The shield cup  681  of this fourth embodiment has an insert  682  constructed of metal and having internal threads for threadable engagement with the anode to secure the shield cup on the torch body. The side wall  705  and bottom wall  707  of the contact assembly casing  703  of this fourth embodiment are illustrated as being formed integrally with the tip  671 . The biasing member is a coil spring  751  sized for radial, close contact relationship (e.g., frictional engagement) with the outer surface of the conductive element  721  and the annular projection  683  extending up from the tip  671  such that the tip, the spring and the conductive element are held in assembly with each other for removal from and installation within the torch  621  as a single unit. 
     Further construction and operation of the contact start plasma torch  621  of this fourth embodiment is substantially the same as that of the first embodiment and therefore will not be further described herein. 
     FIGS. 10 and 11 illustrate a contact assembly  901  of a contact start plasma torch  821  of a fifth embodiment of the present invention in which the annular cap  913  and the contact assembly casing  903  are formed integrally with the electrode  829  such that the cap and casing broadly define part of the cathode body. The tip  871  is in electrical communication with the positive side of the power supply via an electrically conductive insert (not shown but similar to the insert  1082  shown in FIG. 12) connected to the shield cup (not shown but similar to the shield cup  1081  shown in FIG.  12 ). The contact assembly casing  903  generally seats on a radially outward extending upper surface  889  of the tip  871 , with an annular insulating pad  990  disposed between the casing and the tip to electrically insulate the casing from the tip. The electrode  829  and tip  871  are secured in generally fixed relationship with each other in the torch  821 . The contact assembly casing  903  has an inlet hole  957  disposed in its side wall  905  adjacent the lower end of the side wall and an outlet hole  959 , also disposed in the side wall, generally adjacent the upper end of the side wall. 
     An annular support plate  971  constructed of an electrically conductive material is disposed within the contact assembly casing  903  and has a central opening  973  through which the electrode  829  extends. The conductive element  921  is also annular and is constructed of an electrically conductive material. The conductive element  921  is attached to the underside of the support plate  971 , such as being bonded thereto, and depends therefrom for conjoint movement of the conductive element with the support plate. The conductive element  921  of this fifth embodiment is axially movable on the central longitudinal axis X of the torch  821  relative to the electrode  829 , the tip  871  and the contact assembly casing  903  between a first, lowered position (FIG. 10) corresponding to the idle mode of the torch and a second, raised position (FIG. 11) corresponding to the pilot mode of the torch. In the lowered position of the conductive element  921 , the conductive element is in electrical communication with the upper surface  889  of the tip  871  to provide an electrically conductive path between the electrode and the tip, i.e., between the positive and negative sides of the power supply. It is understood that in its lowered position the conductive element  921  need not engage the upper surface  889  of the tip  871 , as long as it is positioned sufficiently close to the tip to provide an electrically conductive path between the positive and negative sides of the power supply. 
     In its raised position (FIG.  11 ), the conductive element  921  is positioned up away from the tip  871  (i.e., a distance greater than the distance between the conductive element and the tip in the lowered position of the conductive element) such that a pilot formed between the tip and the conductive element is adapted for being blown down toward the central exit orifice of the tip for initiating operation of the torch whereby working gas in the primary gas flow path is exhausted from the torch in the form of an ionized plasma. The biasing member of this fifth embodiment comprises a coil spring  951  that seats on the top of the support plate  971  and extends up into contact with the contact assembly cap  913  (i.e., the cathode body). The spring  951  is constructed of an electrically conductive material to provide electrical communication between the contact assembly cap  913  and the annular plate  971 , and is preferably sized to remain in compression for continuously biasing the conductive element  921  toward its lowered position corresponding to the idle mode of the torch. 
     Further construction and operation of this fifth embodiment is substantially the same as the third embodiment of FIGS. 6 and 7 and therefore will not be further described herein. 
     FIG. 12 illustrates a contact assembly  1101  of a sixth embodiment of a contact start plasma torch  1021  of the present invention substantially similar to that of the first embodiment in that it comprises an electrode  1029  in electrical communication with the negative side of the power supply, a tip  1071  in electrical communication with the positive side of the power supply, a contact assembly  1101  operable between an idle mode and a pilot mode of the torch, and a shield cup  1081 . The shield cup  1081  of this sixth embodiment has an insert  1082  connected to its inner surface and constructed of an electrically conductive material. The insert  1082  has internal threads for threadable engagement with the anode (not shown but similar to anode  33  of FIG. 1) to secure the shield cup on the torch body and to provide electrical connection of the insert with the anode (i.e. to provide electrical communication between the insert and the positive side of the power supply). The insert  1082  has an annular shoulder  1091  formed generally at its lower end upon which the upper end  1077  of the tip  1071  is seated. The insert  1082  is otherwise spaced radially outward of the upper end  1077  of the tip  1071  to define the secondary gas chamber  1166 . The insert  1082  also surrounds the contact assembly casing  1103  in radially spaced relationship therewith to define an exhaust channel  1181  in fluid communication with the secondary gas chamber  1166  for directing a portion of the gas in the secondary gas chamber to be exhausted from the torch  1021  other than through the central opening  1163  of the shield cup  1081 . An upper portion  1183  of the inner surface of the shield cup  1081  is spaced radially outward from the insert  1082  to define an exhaust passage  1185  for exhausting gas from the exhaust channel  1183  out of the torch  1021  via the top of the shield cup. Metering orifices  1187  extend radially outward through the insert  1082  to provide fluid communication between the exhaust channel  1183  and the exhaust passage  1185 . 
     The tip  1071  of this sixth embodiment is similar to that of the first embodiment in that an annular projection  1083  extends up from the top of the tip and is positioned generally centrally thereon to define an upwardly facing annular shoulder  1085  disposed radially outward of the annular projection and an upwardly facing contact surface  1087  disposed radially inward of the projection. The bottom wall  905  of the contact assembly casing  903  seats on the annular shoulder  1085  extending radially outward of the projection  1083 . An annular notch  1093  is formed in the peripheral edge of the upper end  1077  of the tip  1071 , radially outward of the annular shoulder  1085 , so that the tip is axially spaced from the bottom wall  1107  of the contact assembly casing  1103 . Three metering orifices  1095  (one of which is shown in FIG. 12) extend axially through the upper end  1077  of the tip  1071  generally at the annular notch  1093  and are in fluid communication with the secondary gas chamber  1166 . The metering orifices  1095  in the tip  1071  are also in fluid communication with the central opening  1163  of the shield cup  1081  for exhausting gas in the secondary gas chamber  1166  from the torch  1021 . 
     The orifices  1095  of the tip  1071  and the metering orifices  1187  of the shield cup insert  1082  are preferably sized relative to each other to meter the flow rate of gas from the secondary gas chamber  1166  in accordance with the current at which the torch is operated. In other words, the metering orifices  1095 ,  1187  are sized relative to each other such that a predetermined portion of gas in the secondary gas chamber  1166  is exhausted from the torch  1021  via the central opening  1163  of the shield cup  1081  and the remaining gas in the secondary gas chamber is exhausted from the top of the shield cup. 
     As an example, for a torch operating at 80 amps, the central exit orifice  1075  of the tip  1071  has a diameter of about 0.052 inches, the tip has three metering orifices  1095  each having a diameter of about 0.052 inches and the shield cup insert  1082  has four metering orifices  1187  each having a diameter of about 0.043 inches. As another example, for a torch operating at 55 amps the central exit orifice  1075  of the tip  1071  has a diameter of about 0.045 inches, the tip has three metering orifices  1095  each having a diameter of about 0.043 inches and the shield cup insert  1082  has four metering orifices  1187  each having a diameter of about 0.043 inches. As a further example, for a torch operating at 40 amps the central exit orifice  1075  of the tip  1071  has a diameter of about 0.031 inches, the tip has three metering orifices  1095  each having a diameter of about 0.040 inches and the shield cup insert  1082  has two metering orifices  1187  each having a diameter of about 0.043 inches. 
     The working gas pressure supplied to the torch is in the range of about 60-70 psi. For example, for a torch operating at about 80 amps, the working gas pressure supplied to the torch is about 70 psi and for torches operating at about 55 amps and 40 amps the working gas pressure supplied to the torch is about 65 psi. The flow rate at which working gas is exhausted from the central exit orifice  1075  of the tip  1071  is preferably in the range of about 50-150 standard cubic feet per hour (scfh), with the flow rate increasing with the current level at which the torch is operated. For example, for torches operating at about 40 amps, 55 amps and 80 amps, the flow rate at which working gas is exhausted from the central exit orifice  1075  of the tip  1071  is about 50 scfh, 80 scfh and 110 scfh, respectively. The flow rate at which working gas is exhausted from the central opening  1163  of the shield cup  1081  is preferably in the range of about 50-300 scfh, with the flow rate increasing with the current level at which the torch is operated. For example, for torches operating at about 40 amps, 55 amps and 80 amps, the flow rate at which working gas is exhausted from the central opening  1163  of the shield cup  1081  is about 125 standard cubic feet per hour (scfh), 200 scfh and 290 scfh, respectively. The flow rate at which working gas is exhausted from the shield cup  1081  via the metering orifices  1187  of the shield cup insert  1082  is preferably in the range of about 50-150 scfh. 
     Thus it will be seen that the cathode body of this sixth embodiment is broadly defined by the cathode (not shown but similar to the cathode  25  of FIG. 1) and the electrode  1029 , and the anode body is broadly defined by the anode, the shield cup insert  1082 , the contact assembly casing  1103  and the tip  1071 . In other words, the tip  1071  provides electrical communication between the insert  1082  and the contact assembly casing  1103 . It is understood that the contact assembly casing  1103  may alternatively be constructed of an electrically non-conductive material without departing from the scope of this invention. For example, the coil spring  1151  may seat on the tip  1071  instead of the contact assembly casing  1103  so that the spring is in electrical communication with the positive power supply via the anode, the shield cup insert  1082  and the tip. It is also contemplated that the contact assembly casing  1103  and the insert  1082  may be integrally formed such that the casing is defined by the insert and is connected to the shield cup  1081  for installation in and removal from the torch  1021  as a single unit without departing from the scope of this invention. 
     Further construction and operation of the contact start plasma torch  1021  of this sixth embodiment is substantially the same as that of the first embodiment and therefore will not be further described herein except with respect to the flow of gas through the secondary gas flow path. Working gas in the lower gas chamber  1141  of the contact assembly  1101  is directed to flow through a secondary gas flow path comprising the openings  1169  in the contact assembly casing  1103 , the secondary gas chamber  1166 , and the metering orifices  1095  in the upper end  1077  of the tip  1071  for exhaustion from the torch  1021  via the central opening  1163  of the shield cup  1081 . Additionally, a portion of gas in the secondary gas chamber  1166  is directed to flow through a tertiary gas flow path comprising the exhaust channel  1183  formed between the insert  1082  and the contact assembly casing  1103 , the metering orifices  1187  in the insert and the exhaust passage  1185  formed between the insert and the shield cup  1081  for exhaustion from the torch via the top of the shield cup. Providing this tertiary flow path allows the gas pressure of working gas received in the torch to be increased for use in moving the conductive element  1121  against the bias of the spring  1151  without negatively effecting the desired gas flow through the central exit opening  1075  of the tip  1071  and the central opening  1163  of the shield cup  1081 . 
     It is understood that the tip  1071  having metering orifices  1095  and the shield cup  1081  having an insert  1082  with metering orifices  1187  may be used in plasma torches other than a contact start plasma torch, such as any plasma torch having a primary gas flow path and a secondary gas flow path, without departing from the scope of this invention. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.