US3450926A

US3450926A - Plasma torch

Info

Publication number: US3450926A
Application number: US585528A
Authority: US
Inventors: Joseph F Kiernan
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1966-10-10
Filing date: 1966-10-10
Publication date: 1969-06-17
Anticipated expiration: 1986-06-17

Description

B8958 REFEREUE ammawa mum J. F. KIERNAN June 17, 1969 PLASMA TORCH Sheet Filed Oct. 10, 1966 FIG.

GAS IN H.F. OSCILLATOR POWER lNl/ENTOR BVJOSEEH F. K/ERNAN ATTORNEP June 17, 1969 J. F. KIERNAN 3,450,926

PLASMA TORCH Filed Oct. 10, 1966 H FIG. 2

. lNl/E/VTOA JOSEPH F. K/ERNAN A T TOR/V5 Y Llfi US. Cl. 313-231 8 Claims ABSTRACT OF THE DISCLOSURE A plasma torch having a combined electrode and gas whirler member of substantially cylindrical form with an external threaded portion surrounded by a hollow concentric heat insulating spacer member having a smooth inner cylindrical wall is disclosed. The insulating spacer member is spaced from the threaded portion of the electrode member to form a clearance space around the threaded portion so that gas supplied by way of the space divides between the space within the threads of the threaded electrode portion and the clearance space surrounding the threads to provide a sheath of relatively non-whirled gas surrounding the whirled portion of the gas.

This invention relates to plasma torches and more par" ticularly to improved structure for forming a more con stricted plasma beam which will follow a path substantially invariable in direction relative to the torch, and for increasing the cutting strength of such a beam.

An object of the invention is to reduce the frictional drag upon a whirling mass of gas employed to constitute and constrict the plasma beam, in order to increase the vortical velocity of the gas and so to increase the degree of constriction of the beam and render the beam more nearly invariable in direction along the center line of the nozzle relative to the torch.

Another object is to increase the rate of heat withdrawal from the arc electrode in order to increase the rate of cooling of the electrode and at the same time to increase the rate of transfer of this heat and direct it more etfectively to pre-heating the gas before the same becomes a part of the arc discharge.

A feature of the invention is a combined arc electrode and gas whirler in which the electrode is externally threaded to provide a plurality of passages for eifecting the whirling of the gas.

A related feature is the use of an increased number of separate thread-like grooves together with a correspondingly decreased pitch of thread whereby there is eifected an increased intimacy of contact between the gas and the electrode over a longer path in order to increase the rate of heat transfer between the electrode and the gas.

Another feature is the provision of a sheath of substantially non-whirling gas surrounding the threaded portion of the electrode and continuing through the nozzle as a sheath of relatively non-whirling gas surrounding the whirling gas and the plasma beam which is enclosed in the whirling gas.

A further feature is the provision of a ceramic spacer for defining the outer wall for the non-whirling sheathing gas as it passes the electrode and particularly the threaded portion of the electrode, and for inserting heat lag between the sheathing gas and the metal mass of the torch assern= bl Another feature is that the aforesaid ceramic spacer is so shaped and dimensioned as to contribute effectively to the attainment of more complete concentricity of the various component parts of the torch and of the conse quent straightness and constancy of direction of the re sultant plasma beam.

at fit Other features, objects and advantages will appear from the following more detailed description of an illustrative embodiment of the invention, which will now be given in conjunction with the accompanying drawings.

In the drawings,

FIG. 1 is a longitudinal sectional view of a portion of a plasma torch embodying the invention, together with an electrical schematic diagram of connections to the torch from a power source;

FIG. 2 is a cross-sectional view of the portion of the torch shown in FIG. 1, taken along the line 2'2' in FIG. 1;

FIG. 3 is a magnified view of that portion of FIG. 1. within the circle 3' in FIG. 1, and

FIG. 4 is a fragmentary sectional view of a modified form of the spacer member and electrode-whirler member shown in FIG. 1.

Relative positions of component parts or portions of such parts, as Well as designations of directions relative to the central axis of the torch, are, in the drawings, particularly in FIG. 1, arbitrarily designated as forward, rearward and the like. Forward in FIG. 1 is toward the bottom of the sheet, while rearward is toward the top. It is to be understood that these designations are not intended to imply any particular orientation of the axis of the torch, which axis is very frequently a vertical one.

Referring to the drawings, there is shown a cylindrical metallic lead 1, of length short relative to its diameter At two points displaced from its axis in opposite directions in the plane of FIG. 1 the head is traversed by re=

spective holes

8 and 9 parallel to the axis, and from the enlarged rear mouths of these holes there extended rearwardly respective metallic tubes 10 and 11 whose end portions are brazed to the head. These tubes also appear in the transverse cross section of FIG. 2, in which there will also be seen a third metallic tube 12 useful in con= nection with feed of gas to the torch, whose end portion may also be brazed to the head (at a point obsecured in FIG. 1 by other elements, and without any associated hole through the head). The three

tubes

10, 11 and 12 may be surrounded by a unitary cylinder 13, of nylon or other mechanically strong electrically insulating material, whose forward end abuts against the head 1. The cylinder 13 be held in this relationship to the head and tubes by suitable means (not shown) at the rear extremities of the cylinder and tubes.

The head is provided with an axial bore whose forward portion 2 may have a diameter somewhat less than the separation of the

holes

8 and 9 from each other and whose rearward portion 3 is of somewhat further reduced diameter. The cylinder 13 is provided with an axial bore whose short forward portion 14 may be somewhat greater diameter than, and whbse next more rearward short portion 15 may be of the same diameter as, that of the headbore portion 3. An annular rearward projection of rim 4 of the head 1 may fit part-way into the cylinder-bore portion 14, and a rubber or other resilient O-ring 5 may be sealingly compressed between that projection and the shoulder formed between bore portions 14 and 15. Rearwardly of the bore portion 15 the bore of the cylinder 13 may have a portion 16 of slightly reduced diameter and of substantial length. The cylinder 13 may be surrounded by a metallic jacket 19 extending forwardly but stopping materially short of the head 1.

Centrally supported within the bore 16 by means not shown, there is provided a tubular central stem 17 within which there is fixed a hollow internally tapered member 18 which matches a tapered portion of a combination whirler-electrode member 31. A fluid-tight seal is provided between the

members

31 and 18 in the form of a rubber or other resilient O-ring 20 in an annular groove in the whirler-electrode member 31. Supported centrally within. the tapered member 18 and the whirler-electrode member 31 by means not shown is a tube 25 for conducting coolant fluid away from the hollow interior of the whirler-electrode member.

The electrode tip which. supports the arc is preferably a separate part 35 of high electron emitting refractory metal such as tungsten, the rearward portion of which may just fit within the forward mouth of the tubular member 31, to which it may be secured as by brazing.

The function of the forward face of the electrode 35 is to serve as one terminal, typical the cathode, of an arc stream having the form of an intense and constricted ionized gas plasma which will issue from the torch and by which, as is known in the art, various functions such for example as cutting may be usefully performed. .A purely illustrative example of the gas may be a mixture of 90% nitrogen and 10% hydrogen.

In order to provide for the constriction of the arc stream the electrode assembly includes a nozzle member 40 extending forwardly of the electrode tip 35. As seen in FIG. 1, this nozzle member may be a generally spoollike metallic member coaxial with the tubular member 31 and tip 35 and having an externally thick near portion 46 fitting within the forward head-bore portion 2, to which it may have a fluid-tight seal provided by a rubber or other resilient O-ring 44 retained within an appropriate perpiheral groove in said rear portion. In its marginal part said rear portion 46 may be provided with a short rear wardly extending annular portion or rim 45 which seats against the shoulder formed in the head 1 between the

bore portions

2 and 3. The nozzle member 40 may extend forwardly, from its rear portion 46, in reduced diameter, and its forward portion may have a forwardly directed annular surface 39 formed for example on the front of an enlarged-diameter flange 47. From the surface 39 the nozzle member may if desired extend a short distance in again-reduced diameter steps to form a nozzle tip 49.

In the rear face 41 of the nozzle member there may be provided the rear orifice 42, and in the nozzle tip 49 there may be provided the front orifice of a much smaller diameter appropriate to the desired constriction of the arc stream. Between the orifices 42 and 0 there may extend a truncatedly conical bore 43 through which the two orifices communicate,

. Of especial significance with respect to the present invention are the whirler-electrode member 31 and an insulating spacer member 60.

The spacer member 60 is preferably made of a suitable ceramic material. It is a cylindrical member, the outer surface of which is of substantially uniform diameter, and which fits snugly into the space defined by the bore 3 in the head 1 and by the bore 15 in the insulating member 13. The spacer 60 is tubular, having a forward bore 62 of somewhat larger diameter than the rear orifice 42 of the nozzle 40, and a rear bore 64 of somewhat larger diameter than the bore 62. The bore 62 surrounds the forward por tion of the whirler-electrode member 31 with a material annular clearance space 56 which provides a non-whirling passage for gas surrounding the member 31. The bore 64 is somewhat greater in inside diameter than the outside diameter of the stem 17, providing between the members 17 and 60 a passage 38 for gas through which the gas may pass forward toward the bore 62. The rearward end of the spacer 60 may be chamfered as shown and may be seated against a shoulder between the bores 15 and 16 of the insulating member 13. The forward end of the spacer 60 is preferably fiat and bears upon the face 41 of the nozzle 40 and against the annular portion 45 of the nozzle.

The forward portion of the whirler-electrode member 31 immediately rearward of the electrode proper 35 is externally threaded as at 48. The pitch of the thread is preferably relatively slight and several distinct threads are preferably provided in intertwining helices. In an embodiment which has been used successfully, the length of the threaded portion 48 of the member 31 is approxi- 4 mately one-half inch, containing a six-fold thread each fold of which comprise approximately one full turn. The threaded portion is preferably substantially coextensive with the length occupied by the electrode proper 35.

I have also used successfully a four-fold thread each fold of which has a pitch of three-fourths inch per thread turn; and a six-fold thread each fold of which has a pitch of one-fourth inch per thread turn.

The spacer 60 is preferably of an insulating material that is refractory to heat and which provides a substantial heat lag between the gas surrounding the electrode and the metallic mass constituting the head 1.

Although it may have broader utility, the electrode assembly of FIG. 1 is especially intended for use in a socalled transferred arc system, in which after the initiation of an arc stream its anode extremity is transferred to the work, e.g. the material to be cut. It will accordingly be understood that the positive terminal of a direct current power source P will typically be connected to the work W, while the negative terminal of that source will be connected to the electrode member 31, for example as shown schematically through the medium of the stem member 17 and the tapered member 18, in FIG. 1. For the initiation of the arc stream the positive terminal of the source P may be made temporarily connectible, as by temporary closure of a suitable switch 5, through a current-limiting resistor R and a high-frequency oscillator H to the nozzle 40, for example via the wall of tube 10 and head 1.

The gas flow having first been established, the switch S may be temporarily closed, whereupon a high-frequency pilot spark will jump between the most closely adjacent portions of the electrode and nozzle members. These portions will be at some point at or closely adjacent to the periphery of the forward face of the electrode tip 35 and a therewith-substantially-aligned point on the nozzle member surface 41. This high-frequency pilot spark will create sufficient ionization so that there will be promptly invoked between those portions a temporary direct current discharge, limited in maximum magnitude by the resistor R. The cathode extremity of the temporary discharge will rapidly extend itself circumferentially around and will then contract somewhat along the face of the electrode tip. The anode extremity will meanwhile shift into a ring on and creeping forwardly along the conical bore 43 toward the front orifice 0.

This temporary discharge will quickly become a limitedcurrent pilot arc stream emerging from, though having as its anode extremity, that orifice. Once it touches the work W (which touching may be facilitated by temporarily bringing the torch and work abnormally close together), the arc stream will preferentially adopt the work as its anode extremity in view of the absence of any current-limiting resistor from the connection of source to work. The temporary connection of the source to the nozzle member 40 may then be broken as by opening of switch S (and the normal spacing of torch and work, if that had been temporarily departed from, restored). The are stream, now of full magnitude, may typically have a configuration such as is approximately shown in dash-dot lines as A in FIG. 1.

The gas passing through the bore 16 and surrounding the central stem 17 is forced under pressure through the passage 38 between the spacer 60 and the stem 17 and divides in substantial proportions between the approximately cylindrical passage 56 and the set of parallel helical passages in the threaded portion 48 of the whirler-electrode member 31. The relative dimensions of these two passages are preferably such as to carry a material portion of the entire gas stream in each passage, typically from 40 to 60 percent in one passage and 60 to 40 percent in the other, illustrated approximately in FIG. 3.

The gas in the threaded portion 48 is in intimate heat transfer contact with the metallic mass of the member 31. As the pitch of the threading is low, and as multiple parallel threads are provided, the mass of gas in each thread is relatively small and the path length within each thread is relatively long. These factors promote efficient heat transfer from the electrode tip 35 through the sur= rounding mass of themember 31 to the gas in the threaded portion 48. The surrounding layer of relatively low veloc ity gas in the straight passage 56 as well as the heat lag effect of the ceramic spacer 60 tend to concentrate and retain in the whirling mass of gas in the passage 48 the greater part of the heat supplied by the electrode, thereby providing efficient pre-heating of the whirling gas, and reducing loss of heatto the heat sink provided by the head 1 and metal parts in contact therewith.

The attainment of a greatly increased velocity of the whirling gas in the passage 48 is promoted by various features of the design disclosed herein. The increased length of the paths in the passage 48 permit the available pressure driving the gas therethrough to impart to the gas the desired higher velocity. Furthermore, the relatively slow moving layer of gas in the passage 56, by virtue of its viscosity or other fayorable properties, acts as a lubricant between the bore 62 and the whirling gas streams in the passage 48 to reduce the frictional drag of the outer peripheral surroundings of the whirling gas streams, which combined with the frictional drag in the threads tends to greatly increase the vortical forming tendency of the whirling gas.

The advantageous effects of the shielding layer of gas around the whirling gas stream continue as the whirling gas streams from the passage 48 unite into a single stream and this single stream is formed out through the nozzle even though the shielding gas layer may gradually pick up the whirling motion of the central stream and become part thereof as the combined stream leaves the orifice 0.

Among the over-all effects of the disclosed structure and operation is a greatly increased temperature and a greatly increased axial velocity of the plasma stream A, together with a greater constriction of the stream, all of which results in greater linearity of the plasma stream, and increased stability of direction relative to the center line of the torch, resulting in greater cutting power and the production of a narrow cut with substantially straight, parallel sides.

The nozzle 40 and the ceramic spacer 60 are each manufactured under strict tolerances and these parts co operate with each other and with the head 1, insulating member 13, stem 17 and whirler-electrode member 31 to ensure a very highdegree of concentricity and linearity of the plasma beafr'i. In particular, the stem 17 is positioned within the bore 16 in the insulating member 13 with a high degree of concentricity. The whirler-electrode member 31 is centrally positioned within the stem 17 in. mechanical interlock by means of the tapered member 18. The tapers of the

components

18 and 31 are formed with great precision to ,insure this centering of the member 31. The bottom of the stem 17 provides a very slight clearance for the member 31 so that angular deflection of the axis of the member 31 from the central direction is minimized. The bore 15 in the member 13 is precisely centered with respect to the central axis of the member 13 and the spacer 60 is accurately centralized in the bore 15. The bores 62 and 64 or the uniform bore 63 in the spacer 60 are made precisely concentric with the axis of the spacer and so precisely concentric with the central axis of the touch. The spacer 60 fits snugly into a concentric recess in the nozzle 40 defined by the projection 45. Furthermore, the orifices 42 and 0 in the nozzle, as well as the conical bore 43 are all made precisely concentric with the central axis of the nozzle. Also, the bore 3 in the head 1 is made precisely concentric with the central axis of the head, as is also the bore 2, so that the spacer 60 fits snugly into the upper portion of the head in the bore 3 and the nozzle 40 fits snugly into the lower portion of the head in the bore 2.

It is found that by adopting a high degree of precision in the manufacture of these components, the result is a greatly improved alignment of all the parts along the cen tral axis of the torch and a greatly improved performance of the manufactured torches with respect to constancy of direction of the plasma beam and resultant uniformity of cut approaching exact perpendicularity to the plane of the work surface.

Summarizing the structural features for maintaining a high degree of concentricity of the parts, the stem 17 and the insulating member 13 are concentrically mounted, and to one or the other of these members other members are attached with provision for mechanical interlock to preserve the concentric relationship. The insulating member 13 interlocks mechanically directly with the ceramic spacer 60, which in turn interlocks mechanically with the head 1 and with the nozzle 40. In addition, the head 1 interlocks directly with the nozzle 40. The stem 17 interlocks mechanically with the tapered member 18 which in turn interlocks mechanically with the electrode-whirler member 31.

It will be noted that there is a triple mechanical interlock involving the ceramic spacer 60 with the head 1 and the nozzle 40.

The nozzle member 40 may be retained in its described and illustrated position by a metallic cap 50 whose cen= tral portion 52'may be centrally aperture-d to pass the nozzle tip 49. The cap 50 may have a truncatedly conical portion 53 extending from the portion 52 to a posi tion adjacent the perihphery of the head-J1, a cylindrical portion 54 extending rearwardly from the portion 53 in surrounding relationship to the head, and a flared portion 55 extending a short distance diagonally rearwardly from the portion 54 but without contacting theja cket 19 above mentioned. The rear part of the head 1 may be slightly enlarged in external diameter and provided with external screw-threading 7, and the rear part of the cap-member cylindrical portion 54 may be slightly enlarged in internal diameter and provided with internal screw-threading 57, in order to provide for the assembly of the cap. A rubher or other resilient O-ring 6 retained in an appropriate peripheral groove in the head 1 may seal the cap to the head when the cap is assembled to the head. The central cap portion 52 surrounding the nozzle tip 49 may bear rearwardly, for example through a rubber or other resilient O-ring 51, against the annular surface 39 of the noz= zle member.

In this construction the cap 50 together with O-rings 6 and 39 will be seen not only to retain the nozzle member in position but also to form with thehead a roughly annular jacketing space 58 about the nozzle member, that jacketing space being fluid-tight other than for the communication therewith of the holes 8 and. 9. Those holes may be used as ports for the ingress of cooling fluid into and the egress of fluid from the jacketing space 58. By way of example, cool water may be supplied to that space through the tube 10 and hole 8, and the water, by which much of the heat of the nozzle will have been absorbed, may be expelled or withdrawn from the chamber through hole 9 and tube 11. Water from the same source may be supplied as indicated by arrows at the top of FIG. 1 to the interior of the whirler-electrode member 31 via the space between the tube 25 and the member 18 and with drawn through the tube 25 to join water withdrawn from the tube 11.

Gas for forming the plasma stream, the vortex and a shield of relatively non-whirling gas around the vortex may be supplied through the annular space between the stem 17 and insulator 13, thence through the annular space 38 between the stem 17 and the hollow interior of the spacer 60.

For further details of construction and operation of those parts of the torch that cooperate with the whirlerelectrode 31, the spacer 60 and the nozzle 40, reference may be made to the copending application, Ser. No.

7 331,882, filed Dec, 19, 1963, in the name of George R, Spies, Jr., now Patent No. 3,294,953, which application is owned by the assignee herein.

FIG, 4 shows a modification of the electrode-whirler member 31 and the insulating spacer member 60 in which, for various reasons including simplicity in manufacture, the spacer 60 is made with uniform diameter bore 63 in place of the stepped inner form shown in FIG, 1. This construction gives more room for the whirler-electrode member 31 which may then be made of somewhat larger outside diameter at the externally threaded portion than shown in FIG. 1, resulting in a more rugged member While illustrative forms of apparatus and methods in accordance with the invention have been described and shown herein, it will be understood that numerous changes may be made without departing from the general principles and scope of the invention.

What is claimed is:

1, In a plasma torch, in combination, a combined electrode and gas whirler member of substantially cylindrical form, said member having an external threaded portion, a hollow heat insulating spacer member surrounding said threaded portion of said combined electrode and whirler member and having a smooth inner cylindrical wall concentric with and spaced from said threaded portion to form a clearance space around said threaded portion, a nozzle having an inner cavity of substantially frustmconical form; means to align and maintain said electrode whirler member, said spacer member and said nozzle in concentric axial relationship with each other, and means to supply a stream of gas to said nozzle by way of the said space between said electrode-whirler member and said spacer, whereby the said gas stream divides in substantial proportions between the space within the threads of said threaded portion of said electrode-whirler member to be whirled by said threads and said clearance space surrounding said threads to provide a sheath of relatively nonwhirled gas surrounding said whirled portion of gas.

2. Apparatus according to claim 1, in which said spacer member is composed of ceramic material providing a heat lag for concentrating heat from said electrodewhirler member in said gas stream and for reducing heat leak from said gas stream to a surrounding heat sink.

3. Apparatus in accordance with claim 1, in which the said threads in said electrode-whirler member are parallely arranged intertwined multiple threads.

4. Apparatus in accordance with claim 1, in which the maximum pitch of said threads in said electrode-whirler member is one-fourth inch per thread.

5. Apparatus in accordance with claim 3, in which the said threads in said electrode-whirler member are six-fold intertwined helical threads having a pitch of approximately one-fourth inch per thread turn and consisting of approximately one complete turn per thread,

6. Apparatus in accordance with claim 3, in which the said threads in said electrode-whirler member are fourtold intertwined helical threads having a pitch of approximately three-fourths inch per thread turn and consisting of approximately one complete turn per thread.

7, Apparatus in accordance with claim 3, in which the said threads in said electrode-whirler member are six-fold intertwined helical threads having a pitch of approximately one-half inch per thread turn and consisting of approximately one complete turn per thread,

8. Apparatus in accordance with claim 1, in which said clearance space and the space within said threads are mutually so proportioned as to divide the said gas stream 40 to percent in one portion and 60 to 40 percent in the other portion.

References Cited UNITED STATES PATENTS 3,294,953 12/1966 Spies 313-23l X JAMES W. LAWRENCE, Primary Examiner,

R, F, HOSSFELD, Assistant Examiner,

US. Cl. X.R,