Plasma arc cutting process and apparatus using an oxygen-rich gas shield

• US 5,695,662 A
• Filed: 02/02/1996
• Issued: 12/09/1997
• Est. Priority Date: 06/07/1988
• Status: Expired due to Fees

• Current Assignee: Hypertherm Incorporated
• Original Assignee: Hypertherm Incorporated
• Inventors: Luo, Lifeng, Couch, Richard W. Jr., Sobr, John, Sanders, Nicholas A.
• Primary Examiner(s): Paschall, Mark H.
• Application Number:
  US08/595,797
• Time in Patent Office: 676 Days
• Field of Search: 219/121.48, 219/121.5, 219/121.51, 219/121.52, 219/121.39, 219/74, 219/75
• US Class Current: 219/121.39
• CPC Class Codes: B23K 10/006 Control circuits therefor circuits for plasma torches H05H1/36
• CPC Class Codes: H05H 1/341 Arrangements for providing coaxial protecting fluids
• CPC Class Codes: H05H 1/3421 Transferred arc or pilot arc mode
• CPC Class Codes: H05H 1/3442 Cathodes with inserted tip
• CPC Class Codes: H05H 1/3457 Nozzle protection devices
• CPC Class Codes: H05H 1/3468 Vortex generators
• CPC Class Codes: H05H 1/3478 Geometrical details

Abstract

A plasma arc torch has a secondary gas flow that is extremely large during piercing of a workpiece to keep splattered molten metal away from the torch and thereby prevent "double arcing". The secondary flow exits the torch immediately adjacent the transferred plasma arc and is an extremely uniform, swirling flow. A swirl ring is located in the secondary gas flow path at the exit point. A prechamber feeds gas to the swirl ring, which is in turn fed through a flow restricting orifice. For certain applications the secondary gas is a mixture of an oxidizing gas, preferably oxygen, and a non-oxidizing gas, preferably nitrogen, in a flow ratio of oxygen to nitrogen in the range of 2:3 to 9:1. Preferably the flow ratio is about 2:1. A network of conduits and solenoid valves operated under the control of a central microprocessor regulates the flows of plasma gas and secondary gas and mixes the secondary gas. The network includes valved parallel branches that provide a quick charge capability and a set of venting valves, also electrically actuated by the microprocessor, to provide a quick discharge. In a preferred high-definition embodiment, a nozzle with a cut back outer surface and a large, conical head allows a metal seal and enhanced cooling. A two-piece cap protects the nozzle during cutting.

Claims

1. A plasma arc cutting system comprising:

a torch body including a plasma gas flow path for directing a plasma gas from a plasma gas inlet to a plasma chamber in which a plasma arc is formed and a secondary gas flow path directing a secondary gas flow from a secondary gas inlet to a secondary gas flow exit orifice;

a nozzle mounted at a first end of the torch body and being formed of an electrically conductive material, the nozzle comprising (a) a hollow body portion formed having a generally conical, thin-walled configuration sloping toward an exit port for the plasma jet, and (b) a head portion formed integrally with the body portion (i) being generally solid except for a central passage aligned with the exit port, and (ii) having at least a partially conical outer surface sloping toward the exit port;

an electrode mounted at the first end of the torch body in a mutually spaced relationship with the nozzle to define the plasma chamber;

a water-cooled cap mounted on the torch body and substantially enclosing the outer surface of the nozzle;

a secondary gas cap mounted on the torch body in a spaced relationship with the water-cooled cap to define a portion of the secondary gas flow path which includes the secondary gas flow exit orifice; and

a swirl ring formed of an insulating material and positioned in the portion of the secondary gas flow path, the swirl ring configured to direct the secondary gas flow therethrough in a highly uniform flow.

2. The cutting system of claim 1 wherein the swirl ring comprises a set of canted holes which introduce a swirling movement to the secondary gas flow, thereby facilitating the interaction of the secondary gas flow with the plasma arc.

3. The cutting system of claim 1 wherein the nozzle has a diameter adjacent the body portion that exceeds the diameter of the body portion to define a cut back recess.

4. The cutting system of claim 1 wherein the secondary gas cap comprises a replaceable metallic shield having a central circular opening concentric with the exit port of the nozzle and sized to closely surround the plasma jet, the shield intercepting molten metal sprayed from the workpiece toward the torch during piercing and cutting.

5. The cutting system of claim 4 wherein the shield further comprises a set of ports that surround the central opening, and a seal located at the outer edge of the shield for sealing the shield to the body portion of the secondary gas cap.

6. The cutting system of claim 5 wherein the set of ports are sized to divert a significant portion of the secondary gas flow during the cutting, but not so divert a flow of the secondary gas at a higher rate associated with the piercing, the ports venting the high rate secondary gas flow upon termination for a flow for cutting.

7. The cutting system of claim 5 wherein the ports are angled to direct the secondary gas flowing therethrough radially away from the plasma jet.

8. The cutting system of claim 4 wherein the shield includes a shielding portion surrounding said central opening that extends generally parallel to the workpiece with an interior surface spaced from the exterior of the nozzle to define therebetween a main exit orifice for the secondary gas flow.

9. The cutting system of claim 4 wherein the shield includes a step recess formed on an outer edge to locate and lock the shield into the secondary gas cap and an annular groove formed in the outer edge and facing the secondary gas cap that is adapted to receive the seal.

10. The cutting system of claim 1 wherein the secondary gas flow is a mixture of a non-oxidizing gas, selected from the group consisting of nitrogen and argon, and at least 40% of an oxidizing gig, selected from the group consisting of oxygen and air, as measured by flow rate.

11. The cutting system of claim 1 wherein the secondary gas flow is a mixture of a non-oxidizing gas and at least 40% of an oxidizing gas, as measured by flow rate.

12. The cutting system of claim 11 wherein said ratio is about 2:

1, oxidizing gas flow to non-oxidizing gas flow.

13. A plasma arc cutting system comprising:

a torch body including a plasma gas flow path for directing a plasma gas from a plasma gas inlet to a plasma chamber in which a plasma arc is formed and a secondary gas flow path directing a secondary gas flow from a secondary gas inlet to a secondary gas flow exit orifice;

a nozzle mounted at a first end of the torch body and having a central passage aligned with an exit port;

an electrode mounted at a first end of the torch body in a mutually spaced relationship with the nozzle to define the plasma chamber;

a water-cooled cap mounted on the torch body and substantially enclosing the outer surface of the nozzle; and

a secondary gas cap mounted on the torch body in a spaced relationship with the water-cooled cap to define a portion of the secondary gas flow path which includes the secondary gas flow exit orifice; and

a replaceable metallic shield for intercepting molten metal sprayed from the workpiece toward the torch during piercing and cutting, the shield having (i) a central circular opening aligned with the exit port of the nozzle and sized to closely surround the plasma jet, (ii) a set of ports that surround the central opening and are sized to divert a significant portion of the secondary gas flow during the cutting, but not so divert a flow of the secondary gas at a higher rate associated with the piercing, the ports venting the high rate secondary gas flow upon termination for a flow for cutting, and (ii) a seal located at the outer edge of the shield for sealing the shield to a body portion of the secondary gas cap.

14. The cutting system of claim 13 wherein the ports are angled to direct the secondary gas flowing therethrough radially away from the plasma jet.

15. The cutting system of claim 13 wherein the shield includes a shielding portion surrounding said central opening that extends generally parallel to the workpiece with an interior surface spaced from the exterior of the nozzle to define therebetween a main exit orifice for the secondary gas flow.

16. The cutting system of claim 13 wherein the shield includes a step recess formed on an outer edge to locate and lock the shield into the secondary gas cap and an annular groove formed in the outer edge and facing the secondary gas cap that is adapted to receive the seal.

17. The cutting system of claim 13 further comprising a swirl ring disposed in the secondary gas flow path adjacent the secondary gas flow exit orifice and configured to direct the secondary gas flow therethrough in a highly uniform flow.

18. The cutting system of claim 13 wherein the nozzle is formed of an electrically conductive material.

19. The cutting system of claim 13 wherein the nozzle further comprises:

a hollow body portion formed having a generally conical, thin-walled configuration sloping toward an exit port for the plasma jet; and

an enlarged head portion formed integrally with the body portion and (i) being solid except for a central passage aligned with the exit port, and (ii) having at least a partially conical outer surface sloping toward the exit port.

20. The cutting system of claim 13 wherein the nozzle has a diameter adjacent the body portion that exceeds the diameter of the body portion to define a cut back recess.

21. The cutting system of claim 13 wherein the torch is a high definition torch.

22. The cutting system of claim 13 wherein the secondary gas flow is a mixture of a non-oxidizing gas and at least 40% of an oxidizing gas, as measured by flow rate.

23. A plasma arc cutting system comprising:

a torch body including a plasma gas flow path for directing a plasma gas from a plasma gas inlet to a plasma chamber in which a plasma arc is formed and a secondary gas flow path directing a secondary gas flow from a secondary gas inlet to a secondary gas flow exit orifice;

a nozzle mounted at a first end of the torch body and having (a) a central passage aligned with an exit port, (a) a hollow body portion having a generally conical, thin-walled configuration sloping toward the exit port, and (c) a head portion formed integrally with the body portion and being generally solid except for a central passage aligned with the exit port;

an electrode mounted at a first end of the torch body in a mutually spaced relationship with the nozzle to define the plasma chamber;

a water-cooled cap mounted on the torch body and substantially enclosing the outer surface of the nozzle; and

a secondary gas cap mounted on the torch body in a spaced relationship with the water-cooled cap to define a portion of the secondary gas flow path which includes the secondary gas flow exit orifice, the secondary gas cap configured to define a recess within the secondary gas flow path; and

a swirl ring formed of an insulating material and positioned in the recess within the secondary gas flow path, the swirl ring configured to direct the secondary gas flow therethrough in a highly uniform flow.

24. The cutting system of claim 23 wherein the swirl ring comprises a set of canted holes which introduce a swirling movement to the secondary gas flow, thereby facilitating the interaction of the secondary gas flow with the plasma arc.

25. The cutting system of claim 23 wherein the nozzle further comprises:

a hollow body portion formed having a generally conical, thin-walled configuration sloping toward an exit port for the plasma jet; and

an enlarged head portion formed integrally with the body portion and (i) being generally solid except for a central passage aligned with the exit port, and (ii) having at least a partially conical outer surface sloping toward the exit port.

26. The cutting system of claim 23 wherein the secondary gas cap comprises a replaceable metallic shield having a central circular opening aligned with the exit port of the nozzle and sized to closely surround the plasma jet, the shield intercepting molten metal sprayed from the workpiece toward the torch during piercing and cutting.

27. The cutting system of claim 26 wherein the shield further comprises a set of ports that surround the central opening.

28. The cutting system of claim 27 wherein the set of ports are sized to divert a significant portion of the secondary gas flow during the cutting, but not so divert a flow of the secondary gas at a higher rate associated with the piercing, the ports venting the high rate secondary gas flow upon termination for a flow for cutting.

29. The cutting system of claim 27 wherein the ports are angled to direct the secondary gas flowing therethrough radially away from the plasma jet.

30. The cutting system of claim 26 wherein the shield includes a shielding portion surrounding said central opening that extends generally parallel to the workpiece with an interior surface spaced from the exterior of the nozzle to define therebetween a main exit orifice for the secondary gas flow.

31. The cutting system of claim 26 wherein the shield includes a seal located at the outer edge of the shield for sealing the shield to the body portion of the secondary gas cap.

32. The cutting system of claim 31 wherein the shield includes a step recess formed on an outer edge to locate and lock the shield into the secondary gas cap and an annular groove formed in the outer edge and facing the secondary gas cap that is adapted to receive the seal.

33. The cutting system of claim 26 further wherein the water-cooled cap and the secondary gas cap, in combination, produce a flow rate of the secondary gas during the piercing that is very high in comparison with the flow rate during cutting in the transferred arc mode.

34. The cutting system of claim 23 wherein the nozzle has a diameter adjacent the body portion that exceeds the diameter of the body portion to define a cut back recess.

35. A plasma arc cutting system comprising:

a torch body including a plasma gas flow path for directing a plasma gas from a plasma gas inlet to a plasma chamber in which a plasma arc is formed and a secondary gas flow path directing a secondary gas flow from a secondary gas inlet to a secondary gas flow exit orifice;

a nozzle mounted at a first end of the torch body and having a central passage aligned with an exit port;

an electrode mounted at a first end of the torch body in a mutually spaced relationship with the nozzle to define the plasma chamber;

a water-cooled cap mounted on the torch body and substantially enclosing the outer surface of the nozzle; and

a secondary gas cap mounted on the torch body in a spaced relationship with the water-cooled cap to define a portion of the secondary gas flow path which includes the secondary gas flow exit orifice;

a replaceable metallic shield having a central circular opening aligned with the exit port of the nozzle and sized to closely surround the plasma jet;

a seal positioned in a step recess formed on an outer edge of the shield to locate and lock the shield into the secondary gas cap of the shield and an annular groove formed in the outer edge of the shield and facing the secondary gas cap that is adapted to receive the seal; and

a swirl ring formed of an insulating material and positioned within the secondary gas flow path, the swirl ring configured to direct the secondary gas flow therethrough in a highly uniform flow.