Plasma Arc Torch Having Liquid Laminar Flow Jet For Arc Constriction

Abstract

In the plasma arc torch disclosed herein, the flow of plasma toward a workpiece is constricted and accelerated by a radially inward, laminar jet of liquid which forcefully impinges upon the plasma flow. As the liquid is relatively dense in relation to the gases comprising the plasma, the plasma flow is apparently mechanically pinched to reduce its cross section thereby concentrating the application of heat on the workpiece.

Parent Case Text

This application is a continuation-in-part of U.S. application Ser. No. 814,288, filed Apr. 8, 1969, now abandoned.

Description

BACKGROUND OF THE INVENTION

During the development of plasma arc torches, various proposals have been made for concentrating the application of heat on the workpiece. Most of these proposals have pertained to methods of forming the gas flow which is ionized to form the plasma or have related to ways of cooling the boundary layer of the plasma flow by radiation, e.g., as shown in Gage U.S. Pat. No. 2,806,124. While these proposals have met with some success, considerable room for improvement still existed.

Among the several objects of the present invention may be noted the provision of a novel means for concentrating the application of heat of a plasma arc torch on a workpiece; the provision of a means for constricting and accelerating the plasma flow in such a torch; the provision of a plasma arc torch which will produce a relatively straight sided cut in a workpiece; the provision of a plasma arc torch which will cut rapidly; and the provision of such a torch which is relatively simple and inexpensive. Other objects and features will be in part apparent and in part pointed out hereinafter.

SUMMARY OF THE INVENTION

Briefly, in a plasma arc torch according to the present invention, an arc discharge is created between an electrode and a workpiece. An annular flow of an ionizable gas is provided around the electrode to produce a plasma which flows toward the workpiece. The apparatus further includes annular nozzle means for projecting a radially moving, inwardly substantially laminar jet of liquid. This liquid jet impinges upon the plasma flow and, apparently by virtue of its greater density, constricts and accelerates the plasma flow thereby concentrating the application of heat on the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation, in section, of a plasma arc torch of this invention;

FIG. 2 is a cross-sectional view of the torch taken substantially on the line 2--2 in FIG. 1;

FIG. 3 is a cross-sectional view of the torch taken substantially on the line 3--3 in FIG. 1;

FIG. 4 is a simplified schematic circuit diagram of plasma arc apparatus employing the torch of FIG. 1;

FIG. 5 is a graph illustrating the operation of the FIG. 1 torch;

FIG. 6 is a side elevation, in section of another embodiment of a torch of this invention, employing a ceramic nozzle element; and

FIG. 7 is a bottom view of the torch of FIG. 6.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a plasma arc torch head 11 of this invention is shown substantially in normal operational relationship to a workpiece 13 which is to be cut by the torch. The torch employs a cathode 15 which is preferably constructed of conductive material, e.g., 2 percent thoria-tungsten, which can withstand relatively high temperatures. The cathode 15 is brazed to a heavy copper tube 17. A water flow for cooling the cathode structure is provided through a tube 19 which fits loosely within the central bore of the larger tube 17, the water being allowed to return through the space between the two tubes.

The lower end of the heavy copper member 17 is surrounded by a ceramic sleeve 21 which is constructed of an insulating material, e.g., a boron nitride ceramic, which can withstand high temperatures. The electrode structure, together with the sleeve 21, fits within an insulating body member 25 constructed of a suitable plastic such as delrin, suitable sealing being provided by an O-ring 27.

An annular nozzle is formed by a pair of disclike members 31 and 33. The disclike members 31 and 33 may be constructed of copper and are clamped to the body member 25 by means of a brass cap 35 which is threaded onto the body member. Suitable sealing is provided by O-rings 37, 38 and 39. A spacer 41 is clamped between the nozzle member 33 and the body 25, sealing being provided by a pair of O-rings 43 and 45. As may be seen in FIG. 2, the spacer 41 is bored to provide a series of jet openings 51 which open tangentially into the chamber containing the cathode 15. As may be seen in FIG. 1, the insulating torch body 25 includes a longitudinal passageway 55 through which a gas flow can be provided to the annular chamber surrounding spacer 41. From this annular chamber, the gas can pass through the jet openings 51 to provide a swirling annular gas flow which passes around the cathode 15 and then out through the bore of nozzle members 31 and 33. As is understood by those skilled in the art, the gas used is typically a relatively inert, ionizable gas. For the cutting applications for which the present torch illustrated is designed, nitrogen is usually employed.

The insulating body 25 also includes a second longitudinal passage 57 for providing a flow of liquid to the annular space provided by the cap 35 around the nozzle-forming members 31 and 33. As may be seen in FIG. 3, the under surface of the peripheral flange of member 33 includes a plurality of radial slots 16 which permit the liquid to flow between the members 31 and 33. The member 33 includes a central bore 58 which provides a relative short channel for the gas flowing around the cathode 15. The members 31 and 33 are shaped so that the inner edge of the member 33 cooperates with the lower end of the channel to form an inwardly directed annular nozzle for the liquid flow. While other liquids might be used, water has been found practical for most applications. The water flow also cools those parts of the torch through which it passes. A lead 56 is attached to the cap 35 for providing an electrical connection to the nozzle structure.

In operation, the torch is energized in a circuit essentially as represented in FIG. 4. Typically, the workpiece 13 is grounded and a relatively high negative potential is applied to the cathode 15 by a power supply as indicated at 63. As is understood by those skilled in the art, the power supply 63 will typically include means for providing a brief high voltage pulse to the cathode for initiating an arc discharge. During starting, the nozzle structure is typically connected to ground through a set of contacts K1 and a resistor R1. Initially, the arc may start to run to the nozzle structure but the voltage drop across the resistor R1, causes the arc to transfer to the workpiece when the annular gas flow around the cathode is started. Once the arc is initiated and both the gas and water are running, the contacts K1 can be opened thereby allowing the nozzle structure to float in potential.

As is understood by those skilled in the art, a high-power electric arc discharge between the cathode 15 and the workpiece 13, together with the annular flow of an ionizable gas around the cathode, causes a plasma to be generated which is then projected toward the workpiece. As noted previously, the members 31 and 33 form a circumferentially continuous annular nozzle around the plasma stream. Water provided through passageway 57 flows into the space between the cap 35 and body 25 and through the slots 16 into the annular space between the members 31 and 33. From thence the water is injected radially inwardly relative to the plasma stream so as to impinge thereupon. Since the portions of the members 31 and 33 which guide the injected water terminate in relatively sharply defined edges, as illustrated, the injected water is projected against the plasma flow as a jet of substantially laminar flow.

While the physical phenomenon occuring in the immediate vicinity of a plasma arc are difficult to investigate, it is believed that the kinetic energy of the inwardly flowing water stream causes the plasma stream to be mechanically constricted, since the liquid is relatively dense in relation to the gases making up the plasma stream. Further, as the plasma stream is constricted, it is also accelerated since there is a reduced cross section for the flow. Preferably, the inwardly directed water jet impinges upon the plasma stream substantially at the point of its ejection from the torch head so that intermixing and contamination of the plasma stream is minimized.

It has been found that, by using a swirling gas flow around the cathode 15 such as is produced by the tangential jet openings 51, an unsymmetrical cut or kerf may be obtained, substantially as illustrated at 62. Such a kerf is desirable in that one side thereof is substantially parallel to the axis of the torch. Thus, when the torch is used for cutting a workpiece to size, relatively little material has to be removed to obtain a perfectly squared edge thereby reducing the cost of various manufacturing and fabricating processes. It also appears that the liquid, after impinging upon the plasma stream, sprays against the upper surface of the workpiece and tends to minimize the rounding off of the upper edge of the kerf. This likewise is desirable in that it reduces the amount of material which must be removed to obtain a square edge. In one example, a kerf having a cross section substantially as illustrated was obtained using the torch of the present invention to cut type 304 stainless steel one-inch thick at a speed of 45 inches per minute.

In addition to producing a kerf of desirable cross section, the concentration of heat upon the workpiece by the radially impinging water jet increases the maximum cutting rate of the torch, an increase of more than 50 percent being typical.

The graph of FIG. 5 illustrates the effect of the impinging water jet on the arc in terms of the electrical effects which are concomitant with the construction of the arc described previously. In the graph, the resistance of the arc is plotted against water velocity. As the water velocity is increased and the arc is constricted, it resistance increases since the current must pass through a smaller area. It should be understood that the resistance is not a linear function of the arc diameter and that the improvement in performance which is obtained is in fact greater than the change in resistance might appear to indicate.

In certain types of cutting operations, the torch can easily be damaged by shorting of the lower nozzle element to the workpiece. Such a short can be caused by a cut portion of the workpiece flipping up into the torch or by a glob of molten metal adhering thereto. If the nozzle becomes shorted to the workpiece, the arc will typically transfer to the nozzle, this being a shorter path, and damage to the nozzle and cathode will typically result.

To prevent such shorting, the embodiment illustrated in FIG. 6 employs a lower nozzle member 71 which is constructed of an insulating ceramic material, e.g., alumina. In this embodiment, separation between the upper nozzle member 33 and the ceramic lower nozzle member 71, which determines the width of the annular jet slit, is established by four pads 73 of an epoxy resin. During manufacture, these resin pads are allowed to harden while the correct spacing is maintained. In this way, the need to grind all of the surfaces of the ceramic member 71 to exact tolerances is avoided. The jet passage formed between the two nozzle members 33 and 71 terminates at a sharply defined point of separation at which the liquid flow separates from the passage defining members. Thus, the inwardly directed annular jet comprises a substantially laminar flow which impinges smoothly upon the plasma flow without causing unwanted turbulence, intermixing and contamination. As will be understood by those skilled in the art, the presence of a rounded surface at the mouth of the annular nozzle passage would cause the liquid flow to tend to follow the curved surface. There would thus be an unstable or poorly defined point of separation between the liquid flow and the nozzle-defining members, and the resulting jet would be unstable or fluctuate and would comprise a turbulent as opposed to laminar flow. If such a turbulent liquid flow were to then impinge upon the plasma flow, it would tend to disrupt and contaminate the plasma flow whereas the smoother laminar flow contemplated by the present invention apparently allows a smooth, continuous interface between the liquid and plasma to be preserved as the momentum of the liquid flow is transferred to the plasma. As the lip of the ceramic member is preferably quite thin adjacent the central bore, e.g., 0.030 inches thick, a plurality of rim segments 75 are provided for preventing the lip itself from being driven into contact with the workpiece.

As will be understood by those skilled in the art, the ceramic nozzle member 71 would itself be melted and scoured away by the plasma arc if it were not protected. In the torch of the present invention, however, the annular water jet is interposed between the plasma and the ceramic measure, the plasma flow being constricted by the water jet just as it passes by the ceramic member. Thus this lower nozzle member is protected from the plasma itself. Further, as the water jet recoils from the plasma flow and the workpiece, it tends to cool the ceramic piece and a relatively long life is assured.

Since substantially the entire lower face of the torch is the ceramic material, there is no exposed metal close to the workpiece which can establish a destructive short as described previously. Further, the water spray recoiling from the plasma tends to prevent pieces of molten metal from sticking to the lower nozzle member. Since the annular liquid flow protects and cools this insulating lower nozzle member, it can be seen that other insulating or insulation-coated materials can be used, consistent with the overall environment.

In view of the foregoing, it may be seen that several objects of the present invention are achieved and other advantageous results have been attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it should be understood 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.

Claims

What is claimed is:

1. A plasma arc torch comprising;

an electrode;

means for creating an arc discharge between said electrode and a workpiece;

means for providing an annular flow of ionizable gas around said electrode thereby to produce a plasma flow toward said workpiece;

annular nozzle means providing a sharply defined point of separation for liquid flowing therethrough for providing a radially inwardly moving, substantially laminar jet of liquid which is projected against and forcefully impinges upon said plasma flow and thereby constricts and accelerates said plasma flow so as to concentrate the application of heat on said workpiece.

2. A torch as set forth in claim 1 wherein said means for providing an annular flow of gas around said electrode includes means for causing said gas to swirl around said electrode.

3. A torch as set forth in claim 1 wherein said nozzle is inclined in the direction of said plasma flow.

4. A plasma arc torch comprising:

an electrode;

annular means for providing a flow of an ionizable gas around said electrode;

a first annular member defining a channel extending from said electrode for plasma formed in said gas by an arc extending from said electrode; and

a second annular member cooperating with said first member and forming an annular nozzle for directing a jet of liquid radially inwardly against a plasma flow issuing from said channel, said nozzle providing a sharply defined point of separation for liquid projected from said nozzle whereby said jet comprises a substantially laminar flow.

5. A torch as set forth in claim 4 wherein said second annular member is insulating for preventing shorting of said torch to the workpiece.

6. A torch as set forth in claim 4 wherein said second annular member is constructed of a ceramic material and is protected from said plasma flow by said liquid jet.

7. A plasma arc torch for use with a metallic workpiece, said torch comprising:

an electrode;

means for creating an arc discharge between said electrode and said workpiece;

annular means for providing a flow of ionizable gas around said electrode thereby to produce a plasma flow toward said workpiece, said annular means being configured to admit gas into the space around said electrode substantially tangentially thereby to swirl said gas around said electrode;

a first annular member defining a channel for said plasma flow extending from said electrode towards said workpiece;

a second annular member constructed of an insulating ceramic material and cooperating with said first member and forming therewith an annular nozzle concentric with said plasma flow for directing a liquid in a circumferentially continuous annular jet against said plasma flow substantially at the point at which said plasma flow emerges from the torch, said jet being directed radially inwardly and being inclined in the direction of plasma flow; and

means for providing water under pressure to said nozzle, said nozzle providing a sharply defined point of separation whereby said jet comprises substantially laminar flow thereby to cause said jet to constrict said plasma flow substantially without disruption or contamination of said plasma flow and to thereby produce a relatively narrow cut in said workpiece.