Patent Publication Number: US-6337460-B2

Title: Plasma arc torch and method for cutting a workpiece

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from provisional U.S. application Ser. No. 60/181,111 filed Feb. 8, 2000, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to plasma arc torches and, in particular, to dual gas plasma arc torches that utilize both a primary working gas and a secondary gas. 
     Plasma torches, also known as electric arc torches, are commonly used for cutting and welding metal workpieces by directing a plasma consisting of ionized gas particles toward the workpiece. In a typical plasma torch, a gas to be ionized is supplied to a lower end of the torch and flows past an electrode before exiting through an orifice in the torch tip. The electrode, which is a consumable part, has a relatively negative potential and operates as a cathode. The torch tip (nozzle) surrounds the electrode at the lower end of the torch in spaced relationship with the electrode and constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the electrode, an arc is caused to jump 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 an arc that extends externally off the tip. As the head or lower end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece because the impedance of the workpiece to ground is made lower than the impedance of the torch tip to ground. During this “transferred arc” operation, the workpiece itself serves as the anode. A shield cap is typically secured on the torch body over the torch tip and electrode to complete assembly of the torch. 
     One type of conventional plasma torch is a dual gas torch in which a secondary gas flows through the torch concurrently with the primary working gas for purposes of cooling various parts of the torch or for affecting the plasma arc or the quality of the cut made in the workpiece. For example, it is common to direct the secondary gas flow onto the plasma arc as the arc exits the central orifice of the tip. However, this can lead to plasma arc instabilities, especially at low amperages, such as less than or equal to about 15 amps. These instabilities can adversely effect both the bevel angle of the cut and the surface quality of the cut. 
     SUMMARY OF THE INVENTION 
     The quality, such as surface finish, bevel angle and dross, of the cut made by the dual gas torch has been found to be a strong function of the composition of the secondary gas. A novel dual gas plasma arc torch is provided having a tip with bleed holes in fluid communication with both the primary and secondary gas flow paths in the torch so that a substantial portion of the primary working gas (e.g., oxygen) is bled off into the flow path of the secondary gas to form an oxygen rich secondary gas mixture in the torch. The size and number of the bleed holes regulates the amount of primary working gas bled into the secondary gas flow path. It is known that the optimal secondary gas mixture composition is a function of the current level at which the torch operates. Thus, the secondary gas mixture may be optimized for a particular torch by simply interchanging the tip with another tip having the desired number and size of bleed holes. 
     The torch of the present invention also incorporates a novel tip and shield cap design in which the shield cap sealingly engages the torch, and more particularly the tip, to prevent secondary gas mixture formed in the torch from impinging or otherwise being directed onto the plasma arc as the plasma exits the central orifice of the tip. Instead, the secondary gas mixture is exhausted from the torch through openings in the shield cap spaced radially from the central orifice to flood the kerf region of the cut with the oxygen (or other primary gas) enriched secondary gas mixture. 
     In general, a plasma arc torch of the present invention comprises a primary gas flow path in the torch for receiving a primary working gas and directing it through the torch to a central exit opening of the torch for exhaustion from the torch onto a workpiece in the form of an ionized plasma. A secondary gas flow path in the torch receives a secondary gas separate from the primary working gas and directs it through the torch. The primary gas flow path is in fluid communication with the secondary gas flow path substantially upstream of the central exit opening of the torch to bleed primary working gas in the primary gas flow path into the secondary gas flow path for admixture therewith to form a secondary gas mixture to be exhausted from the torch. 
     A tip of the present invention for use in a plasma arc torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch generally comprises an inner surface at least partially defining the primary gas flow path and an outer surface. At least one bleed hole extends from the inner surface to the outer surface for bleeding gas in the primary gas flow path into the secondary gas flow path for admixture with the secondary gas to form a secondary gas mixture. The at least one bleed hole is located in the tip such that admixture of the primary and secondary gases occurs generally within the torch. 
     A combination tip and shield cap of the present invention for use in a plasma arc torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch generally comprises the tip having a central exit orifice through which primary gas from the primary gas flow path exits in the form of an ionized plasma. The shield cap substantially surrounds the tip and has a central opening in generally coaxial relationship with the central exit orifice of the tip. The shield cap further has at least one secondary opening in spaced relationship with the central opening of the shield cap and in fluid communication with the secondary gas flow path for exhausting secondary gas in the secondary gas flow path from the torch. At least one of the tip and shield cap are configured for sealing the secondary gas flow path against fluid communication with the primary gas flow path intermediate the secondary opening and the central opening of the shield cap to prevent secondary gas in the secondary gas flow path from impinging on the primary gas as the primary gas exits the torch. 
     In another embodiment, a gas mixture system of the present invention for a plasma torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch generally comprises a plurality of tips each adapted for use in the plasma torch. Each tip comprises an inner surface at least partially defining the primary gas flow path and an outer surface. At least one bleed hole extends from the inner surface to the outer surface of each tip for bleeding gas in the primary gas flow path into the secondary gas flow path for admixture with the secondary gas to form a secondary gas mixture. The at least one bleed hole is located in the tip such that admixture of the primary and secondary gases occurs generally within the torch. The at least one bleed hole of each tip is sized such that the amount of primary gas bled from the primary gas flow path through the at least one bleed hole of each tip is different for each tip and corresponds to a current level. 
     A shield cap of the present invention for use with a plasma torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch comprises a hollow body having a central longitudinal axis. An upper end of the shield cap is adapted for connection to the torch, and a lower end has a central opening on said central longitudinal axis. At least one secondary opening is spaced radially outward from the central opening and is in fluid communication with the secondary gas flow path for exhausting gas in the secondary gas flow path from the torch. The shield cap has an annular sealing surface for sealing engagement with the torch to seal the secondary gas flow path against fluid communication with the primary gas flow path downstream of the fluid communication of the secondary opening with the secondary gas flow path to prevent gas in the secondary gas flow path from impinging on the primary gas as the primary gas exits the torch. 
     Finally, a method of present invention of operating a torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch to cut a workpiece comprises directing primary gas to flow through the primary gas flow path to a central exit opening of the torch for exhaustion from the torch onto the workpiece in the form of an ionized plasma. Secondary gas is directed to flow through the secondary gas flow path of the torch. Primary working gas in the primary gas flow path is bled into the secondary gas flow path substantially upstream of the central exit opening of the torch for admixture with the secondary gas to form a secondary gas mixture to be exhausted from the torch generally toward the workpiece. 
     Among the several objects and features of the present invention is the provision of a plasma torch and method which increases the stability of the plasma arc; the provision of such a torch and method which improves the surface quality, dross and bevel angle of the cut made by the torch; the provision of such a torch and method which floods the kerf region of the cut with an oxygen enriched secondary gas mixture; the provision of such a torch and method which prevents secondary gas in the torch from impinging on the plasma arc as plasma exits the torch; and the provision of such a torch and method in which the secondary gas mixture is optimized for the current level at which the torch operates. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial vertical section of a torch head of a plasma torch of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a torch head of a plasma torch of the present invention is generally indicated at  31 . The plasma torch is of the dual gas type in which both a primary working gas and a secondary gas or fluid are utilized. The torch head  31  includes a cathode  33  having an upper end (not shown) secured in a torch body (not shown) of the torch, and an electrode  35  having an upper end  37  electrically connected to a lower end  39  of the cathode. The cathode  33  and electrode  35  are arranged in coaxial relationship with each other about a longitudinal axis X of the torch. The electrode  33  of the illustrated embodiment is constructed of copper, with an insert  51  of emissive material (e.g., hafnium) secured in a recess  53  in the bottom of the electrode to define a bottom face  55  of the insert. A central insulator  47  (a lower portion of which is shown in the drawing) constructed of a suitable electrically insulating material surrounds a substantial portion of the cathode  33  to electrically isolate the cathode from a generally tubular anode  49  that surrounds the insulator. A cooling tube  41  extends longitudinally within a central bore  43  of the cathode  33  down into a central bore  45  of the electrode  35 . The cooling tube  41  is in fluid communication with a source (not shown) of cooling water to receive cooling water into the tube and direct the water down into the electrode bore  45 . The cooling water flows out from the cooling tube  41  generally at the bottom of the tube to cool the electrode  35 , particularly in the area of the emissive insert  51 . The water then flows upward within the electrode bore  45  and cathode bore  43  and outward therefrom for use in cooling other components of the torch prior to being exhausted from the torch head  31 . 
     The anode has a pair of intake ports  57 ,  59  for separately receiving a primary working gas and a secondary gas. More particularly, the primary gas intake port  57  is in fluid communication with a source (not shown) of working gas for receiving the primary working gas, and the secondary gas intake port is in fluid communication with a source (not shown) of secondary gas for receiving secondary gas. In the preferred embodiment, the primary gas is pure oxygen and the secondary gas is compressed air, free of oil impurities. However, it is understood that the primary gas may be other than oxygen, such as air, nitrogen, argon or an argon/hydrogen mixture, and that the secondary gas may be other than air, such as oxygen, nitrogen, argon, carbon dioxide or reducing gases, without departing from the scope of this invention. Primary and secondary passages, indicated as  61  and  63 , respectively, extend down through the anode  49  from the corresponding intake ports  57 ,  59  to direct the primary and secondary gases down through the anode. The first passage  61  leads to an annular inner plenum  65  formed between the anode  49  and the outer surfaces of the central insulator  47  and a gas distributor  67 . The second passage  63  leads to an annular outer plenum  69  which is separate from the inner plenum  65  and defined by the anode  49  and the inner surface of a shield cap body  90  of a shield cap assembly  71  of the torch. A lower end  73  of the anode  49  includes longitudinally extending bores  75  in fluid communication with the outer plenum  69  to direct the secondary gas out from the lower end of the anode. 
     A metal tip  77 , also commonly referred to as a nozzle, is disposed in the torch head  31  surrounding a lower portion of the electrode  35  in radially and longitudinally spaced relationship therewith to form a gas passage  79  (otherwise referred to as an arc chamber or plasma chamber) between the tip and the electrode. An inlet passage  80  is defined by the electrode and a lower portion of the generally tubular gas distributor  67  extending longitudinally between the tip  77  and the central insulator  47  in radially spaced relationship with the electrode. The inlet passage  80  is in fluid communication with the gas passage  79  for directing primary gas into the gas passage. In the preferred embodiment, the gas passage  79  has a width w of approximately 0.041 inches. However, the width w may vary without departing from the scope of this invention. An upper end  83  of the tip  77  extends up between the anode  49  and the gas distributor  67  in close contact relationship with the gas distributor. Axially extending grooves (not shown) in the outer surface of the gas distributor are in fluid communication with the inner plenum  65  of the anode  49  for directing primary gas down along the outer surface of the gas distributor between the gas distributor and the upper end  83  of the tip  77 . Openings (not shown) in the gas distributor  67  are in fluid communication with the grooves in the outer surface of the gas distributor and the inlet passage  80  between the gas distributor and electrode  35  to direct primary gas in the inner plenum  65  of the anode  49  to flow into the inlet passage and then down through the gas passage  79 . The openings in the gas distributor are preferably formed generally tangentially thereto for causing a swirling action of the primary gas flowing into and down through the gas passage. A portion of the gas passage  79  generally along the bottom face  55  of the insert  51  defines an arc region in which a plasma arc is attached to the electrode. A central exit orifice  89  of the tip  77  is in fluid communication with the gas passage  79  such that primary gas exits the torch in the form of a plasma arc and is directed down against the workpiece. An upper end  88  of the tip orifice  89  is preferably widened to approximately the width of the insert  51  to inhibit gouging of the tip as the arc flows through the tip orifice. 
     The shield cap assembly  71  secures the tip  77 , electrode  35  and gas distributor  67  in axially fixed position during operation of the torch. In the illustrated embodiment, the shield cap assembly  71  comprises a shield cap body  90  of heat insulating material, an insert  93  of similar heat insulating material secured to the shield cap body and a shield cap  91 . The shield cap body  90  surrounds the anode  49  and has internal threads  94  for threadable engagement with corresponding external threads  96  on the anode. The shield cap  91  has internal threads  98  for threadable engagement with corresponding external threads  100  on the shield cap insert  93 . A central opening  95  in the shield cap  91  is coaxially aligned with the central exit orifice  89  of the tip  77  to define a central exit opening of the torch through which plasma exiting the. tip is directed onto the workpiece. Longitudinally extending bores  97  in the shield  5  cap insert  93  are in fluid communication with the bores  75  in the lower end  73  of the anode  49  so that secondary gas flowing through the anode is further directed down through the bores in the shield cap insert into a secondary gas chamber  99  formed between the shield cap  91 , the shield cap insert and the tip  77 . 
     The secondary gas chamber  99  of the illustrated embodiment includes a narrow passage  101  extending generally downward between the shield cap  91  and the tip  77  to secondary exit openings  103  in the shield cap for exhausting gas in the secondary gas chamber from the torch. In the preferred embodiment, the secondary openings  103  are in generally radially spaced relationship with the central opening  95  of the shield cap  91  to direct gas exhausted from the torch through the secondary openings onto the kerf region of the cut made by the plasma arc in the workpiece. As an example, the shield cap  91  of the illustrated embodiment has twelve secondary openings  103  spaced at intervals around the central opening  95 . While the secondary holes  103  shown in the drawing extend axially, it is contemplated that the secondary holes may be angled, such as being directed inward toward or outward away from the central opening  95  of the shield cap  91 . 
     The diameter of the tip  77  substantially decreases at its lower end to form an annular shoulder  105  and a generally cylindrical seat  107  for seating the shield cap  91  of the shield cap assembly  71  on the lower end of the tip. In the preferred embodiment, the diameter of the seat  107  is sized such that the outer wall of the seat is positioned at a location intermediate the secondary openings  103  and the central opening  95  of the shield cap  91 . An O-ring  109 , broadly referred to as a sealing member, seats in an annular groove  111  in the outer wall of the seat  107  of the tip  77  and is sized in cross-section to protrude generally radially outward from the seat  107  of the tip for sealing engagement with a sealing surface  108  of the shield cap  91  when the cap is placed over the tip, thus sealing the torch against gas in the secondary gas chamber  99  from flowing to the plasma arc as plasma exits the tip orifice  89 . Substantially all of the gas in the secondary gas chamber  99  is thus exhausted from the torch through the secondary openings  103  in the shield cap  91 . 
     It is understood that the groove  111  in the seat  107  of tip  77  may be omitted and the O-ring  109  may instead seat in a circular groove (not shown) in the shield cap  91  to define the sealing surface  108  of the shield cap for sealing engagement with the seat of the tip and remain within the scope of this invention. Also, while the seat  107  shown in the drawing at the lower end of the tip  77  extends axially to form a right angle with the annular shoulder  105 , it is understood that the shoulder may be omitted and the seat may be tapered inward (e.g., frusto-conical) or flat, as long as the O-ring  109  is disposed at a location intermediate the central opening  95  and secondary openings  103  of the shield cap  91  to seal gas in the secondary gas chamber  99  against flowing to the plasma arc as plasma exits the tip orifice  89 . Finally, it is understood that the O-ring  109  may be positioned between the tip  77  and part of the torch other than the shield cap  91 , or between the shield cap and part of the torch other than the tip, as long as the secondary gas flow path is sealed against gas in the secondary gas chamber  99  from flowing to the plasma arc as plasma exits the tip orifice  89 . 
     Bleed holes  113  (two are shown in the drawing) are formed in the tip  77  in fluid communication with both the gas passage  79  and the secondary gas chamber  99  to bleed primary working gas in the gas passage into the secondary gas chamber for admixture with the secondary gas in the chamber to form a secondary gas mixture to be exhausted from the torch through the secondary openings  103  in the shield cap  91 . As shown in the drawing, the bleed holes  113  are located in the tip  77  with inner (upper) ends  115  of the bleed holes being in fluid communication with the gas passage  79  and spaced a distance d above the bottom of the electrode  35 . In the preferred embodiment, the distance d of the inner ends  115  of the bleed holes  113  above the bottom of the electrode  35  is sufficient to bleed a portion of the primary working gas from the gas passage before the gas flows down to the arc region extending generally along the bottom face  55  of the insert  51 . As an example, the distance d of the illustrated embodiment is approximately 0.109 inches. However the distance d may vary without departing from the scope of this invention. Also, while the bleed holes  113  shown in the drawing are angled downward away from the gas passage  79 , it is understood that the bleed hole may be at any angle, such as a zero degree angle (e.g., extending radially from the gas passage) or extending upward away from the gas passage, and remain within the scope of this invention. 
     In the preferred embodiment, where the current level is relatively low, such as about 15 amperes, six bleed holes  113  are provided and are sized so that the portion of primary gas bled off from the gas passage  79  for admixture with the secondary gas is substantially greater than the portion of primary gas flowing to the tip orifice  89 . For example, the primary gas flow rate may be approximately 85 cfh, with 66 cfh being bled from the gas passage  79  into the secondary gas chamber  99 . The remaining primary gas exits through the central orifice  89  of the tip  77 . Thus, in this example, roughly 78% of the primary gas flowing through the gas passage  79  is bled out from the gas passage and into the secondary gas chamber  99 . However, it is understood that the portion of the primary gas bled from the gas passage  79  may vary, such as by changing the number and size of the bleed holes  113  or the pressure of the primary gas, without departing from the scope of this invention. In particular, the optimal secondary gas mixture will vary for different current levels at which the torch operates. 
     In operation, primary working gas, such as pure oxygen, is pumped from the source of working gas into the torch and flows through a primary gas flow path (indicated by single shaft arrows in the drawing) comprising the anode primary intake port  57 , anode passage  61 , inner plenum  65 , the grooves in the outer surface of the gas distributor  67 , gas distributor openings, inlet passage  80 , gas passage  79 , tip orifice  89 , and the central opening  95  of the shield cap  91 . Secondary gas, such as compressed air, is received from the source of secondary gas into the torch and flows through a secondary gas flow path (indicated by double shaft arrows in the drawing) comprising the secondary gas intake port  59 , anode passage  63 , outer plenum  69 , the longitudinally extending bores  75  in the lower end  73  of the anode, the bores  97  in the shield cap insert  93 , the secondary gas chamber  99  and secondary openings  103  in the shield cap  91 . 
     As primary gas flows down through the gas passage  79  to the arc region, a substantial portion (e.g., 78%) of the primary gas bleeds out from the gas passage through the bleed holes  113  in the tip  77  and is directed into the secondary gas chamber  99  for admixture with the secondary gas in the secondary gas chamber to form a secondary gas mixture. As will be seen, in the preferred embodiment where the primary gas is pure oxygen, the amount of oxygen in the secondary gas mixture is substantially increased by bleeding primary gas into the secondary gas chamber for admixture with the secondary gas. Primary gas remaining in the gas passage flows down through the arc region and out through the exit orifice  89  of the tip  77  and the central opening  95  of the shield cap  91  onto the workpiece in the form of an ionized plasma. The secondary gas mixture formed in the secondary gas chamber  99  concurrently flows down between the tip  77  and the shield cap  91  to the secondary openings  103  in the shield cap. Because the secondary gas mixture is sealed by the O-ring  109  against flowing to the plasma arc exiting the tip orifice  89 , substantially all of the secondary gas mixture is exhausted from the torch through the secondary openings  103  in the shield cap  91 , thereby directing a primary gas (e.g., oxygen) enriched gas mixture onto the workpiece, with the enriched gas mixture generally surrounding the plasma arc and being directed at the kerf region of the cut. 
     While the plasma torch of the present invention is shown and described herein as including a shield cap  91  that extends down beyond the lower end of the tip  77  so that the central opening  95  of the shield cap defines the central exit opening of the torch, it is understood that the tip may instead extend down through the central opening of the shield cap such that the tip orifice  89  defines the central exit opening of the torch without departing from the scope of this invention. In such an embodiment, the primary gas flow path of the torch would not include the central opening  95  of the shield cap  91 . 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. Sealing off the flow of the secondary gas mixture against impinging on the plasma arc as plasma exits the tip  77  improves plasma arc stability and also improves the surface finish, dross characteristics and bevel angle of the cut. Bleeding oxygen from the gas passage  77  into the secondary gas chamber  99  to form an oxygen rich secondary gas mixture allows the kerf region of the cut to be flooded with the oxygen rich mixture as the mixture is exhausted from torch through the secondary openings  103  in the shield cap  91 . An oxygen rich secondary gas mixture has been found to positively impact the quality (e.g., surface finish, bevel angle and dross) of the cut made by the torch. A set of tips having various numbers and/or sizes or bleed holes may be provided as a gas mixture system for adjusting the amount of primary gas bled into the secondary gas to form the secondary gas mixture. This allows for optimizing the secondary gas mixture in accordance with the current level at which the torch operates. 
     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.