Field of Invention

[001] The present invention is in the technical field of plasma cutting torches. More particularly, relates to consumables for use in a plasma cutting torch typically known as an electrode.

Background of the Invention

[002] Prior art plasma cutting devices, which include plasma torches, have been well- known for many years and are used in cutting and piercing metal work pieces. Plasma cutting devices use an anode and cathode to generate an electrical arc that ionizes a working fluid, usually compressed air or oxygen. Plasma torches generate a significant amount of heat during operation and require active cooling to prolong the useful life of the components of the plasma torch. Active cooling can be provided by a continuous flow of a working fluid, before ionization, and or a shielding gas flow. Active cooling can be augmented by closed-circuit liquid cooling. [003] Plasma torches typically require an electrical ignition to generate an electrical arc between the cathode and the anode, generally known as a pilot arc. The electrical ignition can be generated by a high frequency alternating current that creates an initiation arc at an initiation point between the shortest gap between the anode and the cathode. In this type of plasma torch the electrode and nozzle are fixed in position after installation. In another type of plasma torch, generally known as a contact start plasma torch, the initiation arc is generated by current flow from the anode or nozzle to the cathode or electrode and is facilitated by physical contact between the nozzle and electrode. After the initiation arc, the electrode and nozzle are separated. This is generally accomplished using the working fluid flow to overcome a biasing means that urges the cathode and anode together during the absence of working fluid flow. The most common type of contact start configuration is a fixed nozzle and an electrode that is urged against the nozzle via a biasing means, typically a spring arrangement, prior to the flow of the working fluid. After the initiation of a plasma arc, a working fluid (typically compressed air) is circulated through the plasma torch and out of the exit orifice of the plasma torch. As the flow rate and pressure increases within the plasma torch, the electrode is moved away from the nozzle. The nozzle and electrode are generally concentric about a central axis. The electrode is moved towards a contact surface on the plasma torch along the central axis by the pneumatic pressure created by the working fluid flow. The pneumatic pressure of the working fluid presses the electrode against the contact surface and is typically the only force that urges the surfaces together. This arrangement has several disadvantages associated with a gap existing between the contact surfaces of the plasma torch and of the electrode, when the plasma torch is not operating. One disadvantage is the possibility of foreign contamination between the contact surfaces. This contamination may reduce the amount of current or result in micro-arcs that can prematurely damage the electrode or the plasma torch body itself. Another disadvantage of this type of plasma torch arrangement is the reduced thermal conductivity between the electrode and plasma torch body because of the contact resistance between the electrode and contact surface of the plasma torch body. In this type of plasma torch, thermal conduction is dependent on the contact force pressing the parts together, i.e. the pneumatic pressure that presses the electrode into the contact surface of the plasma torch. Additionally, contaminants can reduce the surface area available for contact and thereby reduce the amount of thermal or electrical conduction between the electrode and plasma torch body.

[004] Prior art plasma torches such as disclosed in US Patent No. 5,897,795 have used fixed electrodes and moveable nozzles, including embodiments in which the electrodes are fixed in position by threading into the torch body. Threaded connections inherently wear out after repeated use. Moreover, electrodes tend to have a relatively small diameter which makes hand threading of an electrode, to the required torque for a reliable electrical conduction, difficult. For this reason, embodiments of threaded electrodes have a hex or parallel flat section on the tip or body of the electrode to facilitate the use of a tool to tighten and install the electrode. The nozzle is generally the highest wear rate consumable in a plasma torch assembly, with the electrode generally being the second highest wear rate consumable in a plasma torch assembly. The addition of a hex section and threaded section to the electrode increase the cost and manufacturing time of the part. Considerable cost savings can be had by eliminating these features from a high-volume part, such as an electrode. With elimination of these features the end user is not required to use a tool to install and remove the electrode. A threaded connection increases the amount of thermal conduction between the electrode and the torch body due to the reduced thermal contact resistance that occurs from the relatively high contact force generated by a threaded connection, as opposed to an electrode that is pressed against a contact surface by pneumatic pressure alone.

[005] The plasma torch in the ‘795 patent employs a spring to place the movable nozzle in the starting position or pre-energized state. The spring in the ‘795 patent has a spring rate of 48 Ib/in (8.57 kg/cm) and it is disclosed that a force of 3.12 pounds (1.42 kg) is needed to achieve full travel of the nozzle to place the nozzle in the operating position for cutting operations. The pneumatic force generated by the flow of the working fluid during operation is disclosed as 6.08 pounds (2.76 kg) in the ‘795 patent. This is just under double the force needed to compress the spring, only giving the torch assembly a factor of safety of - 2. For example, once the spring wears or is otherwise contaminated it may require a force greater than 6.08 pounds (2.76 kg) such that the torch will no longer be able to properly initiate a plasma arc. The ‘795 patent describes the need to routinely replace the spring in the torch assembly due to thermal stress, cyclic loading and contamination from the compressed air being used as the working fluid. In several of the embodiments of the ‘795 patent the spring is in the direct flow path of the shield gas flow.

BRIEF DESCRIPTION OF THE DRAWINGS

[006] The figures illustrate one or more embodiments of a plasma cutting torch and associated consumables where like reference numerals designate identical or corresponding parts throughout the several views. Embodiments of a plasma cutting torch are described with reference to the following drawings, where:

[007] FIG. 1 A is a cross-section view of a plasma torch in accordance with an embodiment of the invention that is in the starting position.

[008] FIG. IB is an exploded view in cross-section of a plasma torch in accordance with an embodiment of the present invention.

[009] FIG. 2 is a view in cross-section of a torch body of a plasma torch in accordance with an embodiment of the present invention.

[0010] FIG. 3 A is a view in cross-section of an electrode of a plasma torch in accordance with an embodiment of the present invention. [0011] FIG. 3B is a view in cross-section of an electrode, nozzle assembly and swirler in a plasma torch in accordance with an embodiment of the invention that are in the operating position.

[0012] FIG. 3C is a view in partial cross-section of a plasma torch in accordance with an embodiment of the invention with the working fluid flow paths designated by arrows.

[0013] FIG. 4 is a view in cross-section view of a swirler of a plasma torch in accordance with an embodiment of the invention.

[0014] FIG. 5 is a view in cross-section of a retainer assembly for use in a plasma torch in accordance with an embodiment of the invention.

[0015] FIG. 6 is perspective view of an electrode in accordance with an embodiment of the invention.

[0016] FIG. 7 is a view in cross-section of a plasma torch in accordance with an embodiment of the invention that is in the operating position.

[0017] To the extent features are illustrated schematically, details, connections and components of an apparent nature may not be shown or drawn to scale to emphasize certain features of the invention. Suggested dimensions of features are only exemplary.

[0018] The figures illustrate one or more embodiments of a plasma cutting torch and component features thereof.

DETAILED DESCRIPTION OF THE INVENTION

[0019] Referring now to the drawings, the invention provides a plasma cutting torch incorporating a threadless fixed electrode that has a high contact force between the contact surface of the electrode and the contact surface of the plasma torch body.

[0020] Referring to Figures 1A and IB, a plasma cutting torch 1 includes a series of consumables assembled onto a torch body 24. The illustrated consumables are shown installed concentrically along a central axis 10. See Figure IB. The plasma cutting torch has a distal end located at the downstream end of the plasma torch 1 about an exit orifice 37 in the shield body 19; and has an upstream or proximal end that along a handle or mount (not shown). The electrode 4 is mated with the contact post 5 and is fixed in place by swirler 16, which is concentrically installed over the electrode 4. The nozzle assembly 17 is concentrically mated with the swirler 16 and fits over the distal end of the electrode 4. The retainer assembly 22 is concentrically installed over the stack-up formed by the electrode 4, swirler 16, and nozzle assembly 17. In one embodiment the retainer assembly 22 is threaded onto the torch 24 and locks the stack-up in place. In one embodiment a shield 19 can be threaded onto retainer assembly 22. The completed or assembled plasma cutting torch 1 is in a pre-energized or starting position after assembly, as seen in Figure 1A.

[0021] In order to generate a pilot or initiation arc, the plasma cutting torch 1 has two current paths; one for the anode and one for the cathode. The torch body 24 accommodates the two current paths by isolating the anode and cathode current paths. In one embodiment, the anode current path begins in the torch body threads 25, continues through the inner retainer 14, and ends in the nozzle assembly 17, See Figure 1 A. The cathode current path begins at the contact post 5 of the torch body 24 and continues through the electrode 4. The pilot arc is generated at a point of physical contact between the distal end of the electrode 4 and the inner nozzle 12 of the nozzle assembly 17. See Figure 1 A.

[0022] Referring to Figures 1 A, IB and 3A, the electrode 4 includes a threadless electrode body which may be symmetrical about a central axis 10 and is generally manufactured from a copper alloy. The electrode 4 has a contact flange 3 on the proximal end and an emissive insert 11 on the distal end. The contact flange 3 can be sized to have a larger outer diameter than the cooling fins 27. In one embodiment the contact flange 3 has the largest outer diameter of the electrode 4 with a ratio of 0.609 or at least 0.609 between the minimum diameter of the electrode 4 and the outer diameter of the contact flange 3. In another embodiment the contact flange 3 has the largest outer diameter of the electrode 4 with a ratio of 0.660 or at least 0.660 between the minimum diameter of the electrode 4 and the outer diameter of the contact flange 3. In one embodiment the contact flange 3 has an outer dimeter of 0.440 inches. The contact flange 3 has a conductive surface 7 that is designed to mate with a contact surface 6 of the contact post 5 of the torch body 24, See Figure 2. The electrode 4 is concentrically installed inside of the swirler 16, prior to assembly with the plasma torch body 24, see Figure 3B. The electrode 4 is inserted into the proximal end of the swirler 16 until the upstream side of the contact flange 3 comes into physical contact with the step 9 of the swirler 16. The inner diameter 41 of the swirler is 16 is sized to produce a clearance in a range of .0127 mm to .1143 mmm between the outer diameter of the cooling fins 27. This clearance allows for the electrode 4 to be inserted easily into the swirler 16 and forces the working fluid flow through the cooling fins 27. In this embodiment, the cooling fins 27 are manufactured as a helical spiral with a single flow path for the working fluid. As the working fluid flows through the cooling fins 27 a reaction force in the proximal direction is generated in the electrode 4 along the central axis 10. In another embodiment, the cooling fins 27 are manufactured as a helical spiral having multiple flow paths for the working fluid. This design has been shown to provide an advantage with respect to electrode cut life. [0023] Referring to Figures 1 A, IB, 2, 3A, 3B, 3C and 4, the torch body 24 has a contact post 5 which has a contact surface 6 that mates with a conduction surface 7 of the electrode 4. When plasma torch 1 is assembled, as seen in Figure 1 A, the electrode 4 and contact post 5 are mated at interface 8. When installed in the plasma torch 1 the contact flange 3 of electrode 4 is urged in the proximal direction, along the central axis 10, towards the contact post 5 of the torch body 24 by physical contact with the step 9 of the swirler 16. The cylindric section 51 of swirler 16 is sized to concentrically fit over contact post 5. In some embodiments the cylindrical section 51 of swirler 16 can include an O-ring groove 54. When the swirler 16 is assembled onto the contact post 5, an O-ring 38 can be installed in O-ring groove 54 to create a pneumatic seal, see Figure 1A.

[0024] The nozzle assembly 17 has an outer nozzle 13 that is mated with a recess 43 in the swirler 16 when installed in the plasma torch 1, see Figures 1 A and 4. The outer nozzle 13 is also mated with the retainer assembly 22 via physical contact with the threaded retainer insulator 15 and inner retainer 14, see Figure 1A and 5. The flange 23 of inner retainer 14 is in physical contact with step 18 of outer nozzle 13. The retainer assembly 22 is formed by attaching the threaded retainer insulator 15 to the inner retainer 14 via any know method of mechanical attachment, including but not limited to press fitting, threaded attachment, adhesive, welding, and brazing. In this embodiment the inner retainer 14 can be manufacture from a material with high electrical conductivity, such as Copper or Brass alloys.

[0025] One method of assembly of the plasma torch 1 is to insert the nozzle assembly 17 into proximal end of the retainer assembly 22 and let gravity place the step 18 of nozzle assembly 17 in contact with flange 23. The operator can then insert the swirler 16 and electrode 4 in the partially assembled retainer assembly 22. The nozzle assembly 17, swirler 16, electrode 4 and retainer assembly 22 can then be assembled onto the torch body 24. The retainer threads 26 of the inner retainer 14 of the retainer assembly 22 are threaded onto the torch body threads 25 to a tightness or torque specification generally known as hand tight, see Figures 1 A and 5. As the retainer assembly 22 is threaded onto the torch body 24, the flange 23 of the inner retainer 14 exerts a compressive force on the step 18 of the nozzle assembly 17, which then exerts a compressive force on the recess 43 of the swirler 16. Then step 9 of the swirler 16 exerts a compressive force on the contact flange 3 of electrode 4 which forces it against the contact post 5 of the torch body 24. This embodiment has been designed to produce a contact force between the conduction surface 7 and the contact surface 6 in a range of 10-1 llbf, when the retainer assembly 22 is installed hand tight by the operator.

[0026] This embodiment has been designed to have a minimum contact force between the electrode 4 and contact post 5 of 10-11 Ibf, at a starting or pre-energized state, without the added cost and disadvantages of a threaded electrode. It is advantageous to move the source of the compressive force to the threads of the retainer assembly 22 for several reasons. The outer diameter of the retainer assembly 22 is sized to be at least 1 inch in diameter, which allows for easy hand tightening by the operator, i.e. no tools are required. The retainer threads 26 and torch body threads 25 are two to three times the size of a thread that can be accommodated by the outer diameter of the electrode 4. A larger thread reduces the possibility of cross threading during assembly which prolongs part life. Additionally, retainer threads 26 and torch body threads 25 are located in areas with lower temperatures and thermal stresses than are experienced by an electrode 4. Finally, the retainer assembly 22 is infrequently replaced which allows for the lower lifetime costs associate with a plasma torch 1 in accordance with an embodiment of the present invention.

[0027] During operation, the working fluid, in this embodiment compressed air, flows into the swirler cavity 29 via the swirler passages 28, See Figures 1 A and 3C. The working fluid then flows downstream toward the discharge orifice 37 of the shield body 19 in a primary flow and upstream toward the contact flange 3 of the electrode 4 in a secondary flow. The secondary flow vents a portion of the working fluid flow to the atmosphere. The contact flange 3 of the electrode 4 is designed to allow the secondary flow to enter the contact post vents 31, see Figure 6. In this embodiment the electrode 4 has a plurality of secondary flow passages 58 which allow the secondary flow to enter a flow cavity 57 that allows the secondary flow to enter the contact post vents 31 regardless of the orientation of the electrode 4 about the central axis 10, see Figures 3 A and 6. The secondary flow passages in the contact flange 3 may be in the form of milled slots, drilled holes or any other suitable passageway formed by a person of ordinary skill in the art. In this embodiment the flow cavity 57 creates an annular cavity when the electrode 4 is assembled in the plasma torch 1 which is designed to correspond with contact post vents 31, see Figure 7.

[0028] When the electrode 4 is installed in the swirler 16, a cooling passage 30 is created in the gaps between the inner wall 41 of the swirler 16 and the cooling fins 27, see Figures 1 A, 3B and 4. The secondary flow performs two functions as it flows from the swirler cavity 29, through the cooling passage(s) 30, through the secondary flow passages 53 of the contact flange 3, into the flow cavity 57 and out the contact post vents 31. The first function is the active cooling of the electrode 4 by convective heat transfer between the cooling fins 27 and working fluid flow. The second function performed by the secondary flow is to provide a supplementary contact force that is dependent on the flow rate and pressure of the working fluid. In this embodiment the supplementary contact force has been designed to be approximately 3.79 Ibf when the working fluid flow entering the swirler cavity 29 is at approximately 62.5 psi. This brings the total contact force between the conduction surface 7 and the contact surface 6 to within a range of 13.788 Ibf and 14.788 Ibf during operation. The contact force generated by threading the retainer assembly 22 onto the torch body 24 is greater than the contact force that can be generated solely by a working gas flow against a movable electrode.

[0029] This embodiment of the invention has been designed to increase the contact force between the conduction surface 7 and contact surface 6 by taking advantage of the differences in thermal expansion of the various parts of the plasma torch 1. In one embodiment, the outer nozzle 13 and inner retainer 14 are made from Brass alloys which have coefficients of thermal expansion of 20.5 m/(m°C) and the swirler 16 is manufactured from a polymer commonly known as Vespel, which has a coefficient of thermal expansion of 50 m/(m°C). During operation, the plasma torch 1 increases in temperature from the extreme heat generated by the emissive insert 11 and the plasma flow that exits the discharge orifice 37. Regardless of the temperature, above room temperature or resting temperature, the swirler 16 will expand at a ratio per linear unit of 100/41 when compared to the inner retainer 14 and outer nozzle 13. The greater expansion of the swirler 16 about the central axis 10 will increase the compressive force transferred by the swirler 16 to the contact flange 3 of the electrode 4. Additionally, the electrode 4 is manufactured from a copper alloy with a coefficient of thermal expansion of 17.8 m/(m°C) and the contact flange 3 will also expand as the temperature increases from conductive heat transfer from the distal end of the electrode 4 during operation, which further increases the contact force during operation. The material properties of the components of the plasma torch 1 have been selected such that plastic deformation will not occur due to the compressive force(s) exceeding the yield strength of the materials of the various components during operation.

[0030] As discussed above the electrode 4 requires active cooling to prevent premature wear of the emissive insert 11 and the distal end of the electrode 4 in general. There are two sources of active cooling in an electrode in accordance with an embodiment of the present invention, conductive and convective heat transfer. As stated above, the secondary flow of the working fluid provides convective heat transfer between the electrode 4 and the working fluid flow. The interface 8 is the location of conductive heat transfer between the conduction surface 7 and contact surface 6. The conductive heat transfer that takes place at the interface 8 is dependent on the contact resistance between the conduction surface 7 and contact surface 6. The contact resistance is dependent on the material properties of the electrode 4 and contact post 5, the temperature of the surfaces, flatness or parallelism of the surfaces, the surface finish of the surfaces and the amount of contact force between the surfaces. In this embodiment the surface finish of the conduction surface 7 is held within a range between 8 and 10 RA, which reduces the amount of microscopic imperfections on the conduction surface 7. As discussed above, the contact pressure between the conduction surface 7 and contact surface 6 that is generated by the hand tightening of the retainer assembly 22 is within a range of 10-11 Ibf when the plasma torch 1 is at room temperature and in the pre-energized state. During operation, the working fluid flow adds about an additional 3.788 Ibf of contact force as it flows through the cooling fins 27. The thermal expansion, about the central axis 10, of the swirler 16 and contact flange 3 relative to the other components of the plasma torch 1, during operation, adds an additional amount of contact force. The plasma torch 1 is designed to increase the contact force, over and above the contact force that exists in the pre-energized state at room temperature, as the flow rate of working fluid and component temperature increase until the plasma torch 1 reaches an operating condition. Since the electrode 4 has a minimum contact for of lOlbf during the pre-energized state, at room temperature, the contact resistance at interface 8 is lower than the contact resistance of any plasma torch that uses a movable electrode design.

[0031] A plasma torch 1 in accordance with an embodiment of the present invention will be able to provide more heater transfer via conduction between the conduction surface 7 and contact surface 5, at the pre-energized state, than a plasma torch that uses a movable electrode can provide during operating conditions. The plasma torch 1 is designed to increase the contact force, between the conduction surface 7 and contact surface 6, during operation and thereby reduce the contact resistance at interface 8, which increases the amount of conductive heat transfer between the electrode 4 and contact post 5 of the plasma torch body 24. Increasing the amount of conductive heat transfer available to the electrode 4 has the benefit of increasing the useful life of the electrode 4 and reducing costs to the end user. Operating costs are reduced because the electrode 4 lasts longer and the operator can reduce maintenance intervals.

[0032] According to Clause 1, provided is an electrode for use in a plasma cutting torch. The electrode includes the following components: a central axis, a distal and a proximal end, a body, and a contact flange, wherein an outer diameter of the contact flange is larger than any other diameter of the electrode and wherein the electrode does not include threads.

[0033] According to Clause 2, the electrode of Clause 1 is provided, wherein the contact flange is at the proximal end of the electrode.

[0034] According to Clause 3, the electrode of Clause 1 or Clause 2 is provided, wherein the contact flange has a conductive surface and wherein the contact flange is capable of mating with a contact surface of a contact post of a torch body.

[0035] According to Clause 4, the electrode of any of Clauses 1-3 is provided, wherein the electrode body includes a plurality of secondary flow passages for flow of a secondary working fluid.

[0036] According to Clause 5, the electrode of any of Clauses 1-4 is provided, wherein the contact flange includes at least one secondary flow passage for flow of a secondary working fluid.

[0037] According to Clause 6, the electrode of any of Clauses 1-5 is provided, wherein the electrode includes a flow cavity at its proximal end for receiving flow of a secondary working fluid.

[0038] According to Clause 7, the electrode of any of Clauses 1-6 is provided, wherein the electrode further includes an emissive insert at the distal end of the electrode.

[0039] According to Clause 8, the electrode of any of Clauses 1-7 is provided, wherein the ratio between a minimum diameter of the electrode and the outer diameter of the contact flange is at least 0.609. [0040] According to Clause 9, the electrode of any of Clauses 1-8 is provided, wherein the ratio between a minimum diameter of the electrode to the outer diameter of the contact flange is at least 0.660.

[0041] According to Clause 10, the electrode of any of Clauses 1-9 is provided, wherein a flow cavity is created between the contact flange and a contact post of a plasma torch body after installation.

[0042] According to Clause 11, the electrode of any of Clauses 1-10 is provided, wherein the outer diameter of the contact flange is equal to an outer diameter of the contact post.

[0043] According to Clause 12, the electrode of any of Clauses 1-11 is provided, wherein the electrode is symmetrical about the central axis.

[0044] According to Clause 13, the electrode of any of Clauses 1-12 is provided, wherein the electrode is manufactured from a copper alloy.

[0045] According to Clause 14, provided is a method of operating a plasma torch that includes an electrode having a proximal end and a distal end, a swirler, a nozzle assembly, a retainer assembly, a contact post, and a central axis, wherein the method includes the steps of: creating an initial contact force between the electrode and the contact post while the plasma torch is in a pre-energized state; and creating at least one additional contact force between the electrode and the contact post during operation of the plasma torch.

[0046] According to Clause 15, the method of Clause 14 is provided, wherein a secondary working fluid enters the swirler creating pneumatic pressure.

[0047] According to Clause 16, the method of Clauses 15 is provided, wherein pneumatic pressure created by the secondary working fluid entering the swirler creates a secondary contact force between a conduction surface of the electrode and a contact surface of the contact post.

[0048] According to Clause 17, the method of any of Clauses 14-16 is provided, wherein the method further includes the step of creating a third contact force between the electrode and the contact post during operation of the plasma torch.

[0049] According to Clause 18, the method of any of Clauses 14-17 is provided, wherein the third contact force is created by the thermal expansion of the swirler during operation of the plasma torch. [0050] According to Clause 19, the method of any of Clauses 14-18 is provided, wherein the thermal expansion of the swirler increases the compressive force transferred by the swirler to the contact flange of the electrode.

[0051] According to Clause 20, the method of any of Clauses 14-19 is provided, wherein the conductive heat transfer from the distal end of the electrode during operation further increases the contact force between the electrode and the contact post.

[0052] While the electrode and associated components, devices and processes have been described above in connection with various illustrative embodiments, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiments for performing the same function disclosed herein without deviating therefrom. Further, all embodiments disclosed are not necessarily in the alternative, as various embodiments may be combined or subtracted to provide the desired characteristics. Variations can be made by one having ordinary skill in the art without departing from the spirit and scope hereof. Therefore, the present disclosure should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitations of the appended claims.