EP0933982B1 - Plasma generator device - Google Patents

Description

The invention relates to a plasma generator with holder, e.g. for a welding machine, according to the preamble of claim 1.

In the plasma generators known devices, the cathode and the anode is usually connected with terminal connections to connecting cables and mechanically connected to each other via appropriate spacers, which also determine the chamber through which a corresponding gas is supplied to the burning between the anode and the cathode arc.

Due to the inevitable wear of the cathode, this must be reworked after a certain number of operating hours. For this purpose it is necessary to disassemble the plasma generator and to regrind the cathode. Thereafter, the cathode must again be set very accurately with respect to the anode to allow for the intended machining of a workpiece, e.g. the attachment of a weld or the application of a metal layer to perform. These adjustments are associated with a very considerable amount of time and cause significant downtime of the tool and thus production losses.

For this reason, in mass production, laser devices or electron beam devices, e.g. used for welding or cutting, although these are considerably more expensive to manufacture than plasma-working devices, e.g. Welding equipment, and the required preparation of the workpieces to be welded is considerably more expensive, since just laser or electron beams can only generate heat by absorbing, but even no heat radiating, as is the case with the plasma.

Furthermore, US Pat. No. 5,328,516 A, US Pat. No. 5,258,599 A and EP 0 079 019 A1 disclose devices in which plasma generators are provided which operate with consuming cathodes and in which the anodes together with the holder are designed as modules and can be handled together are. This module can be removed from a holder, which also has the holder or guide for the consumable cathode.

In these known solutions, an axial supply of the cathode and also the supply of gas is provided and also the connection of the electrical supply takes place in the axial direction of the chamber.

However, there is the disadvantage that the plasma generator has a very large length. Since, as a rule, the plasma generator is clamped at its end region remote from the nozzle orifice into a receptacle of a welding robot, correspondingly large inaccuracies in the guidance of the machining and the reproducibility, in particular after a change of the module, are decisive due to the unavoidable tolerances the nozzle.

In addition, in plasma generators for currents greater than 1000A, very large diameters of the plasma generators and therefore very large masses thereof are produced. In addition, the necessary for such currents cross sections of the current paths can hardly reach, so that you must work with very high current densities, resulting in a corresponding heating of the live parts.

The inevitable in such axially divisible plasma generators in slim design close distance of the current-carrying parts leads at high ignition voltages, as required in particular in helium operation, to internal flashovers and thus to ignition failure.

The aim of the invention is to avoid these disadvantages and to propose a device with which a largely continuous use with high reproducibility of the settings even after a replacement of a module is possible.

According to the invention this is achieved in a device of the type mentioned by the characterizing features of claim 1.

The proposed measures make it possible to easily exchange the plasma generator designed as a module in a simple manner. It is sufficient to remove the module from the holder and replace it with a new one. This eliminates the previously required disassembly of the leads, since they are connected to the arranged in the holder pins.

In the production of the modules, the cathode can be set optimized for the intended use and fixed in their position. The latter can be done in any way. Thus, for smaller power plasma generators, the cathode may be operatively connected to the holding member in a non-releasable manner, e.g. be soldered.

The radial connections for the voltage and gas supply a very compact design of the module is possible, which can be kept very short compared to the conventional solutions. In addition, electrical connections can be provided with relatively large cross-sections, without resulting in very bulky modules. In this case, correspondingly large distances between the electrically conductive parts can be provided, so that there are no problems with higher ignition voltages.

Due to the orientation of the holder perpendicular to the nozzle axis of the plasma generator a very firm and secure mounting of the module is possible in a simple manner, whereby an exact guidance of the mouth of the same is ensured.

Due to the features of claim 2 results in a very simple manufacturing solution, which is particularly suitable for smaller power, wherein such a plasma generator can be designed as a disposable component, which is fed to a Recyklierung just after use.

Due to the features of claim 3 results in a simple construction of the holder, wherein the contact pins for securing the module in the axial direction.

For devices with more powerful plasma generators according to the preamble of claim 4, it is expedient to provide the characterizing features of claim 4.

The proposed measures also make it possible to exchange the plasma generator in a simple and rapid manner. At the same time also results in a very simple construction of the module, whereby the possibility is given to solve the cathode after disassembly of the module from the collet and regrind. It is also possible to make better use of the cathode whose two end portions conical, the cone angle is adapted to the particular application.

The adjustment of the cathode can be carried out after a revision of the cathode with a tailored to the particular application, or use of the module setting gauge. Since such a reworking of the cathode can be done after a replacement of the entire module, such work does not cause significant interruptions of production, e.g. in a production line.

Due to the features of claim 5, there is the advantage of a very precise guidance of the cathode, while also ensuring that an arc can only burn between the mouth of the anode and the next end face of the centering and thus only in a very narrow range , This ensures a very controlled operation of the plasma generator.

In order to increase the service life of the anode, the features of claim 6 may be provided, wherein the insert is suitably soldered into the mouth of the anode.

The features of claim 7 allow the anode to be easily exchanged, e.g. if this is worn out accordingly, or if for a particular use of the device, or specific work a different geometry of the anode is appropriate.

Due to the features of claim 8 results in a very simple construction, since only one leading to a connection port channel is required for each coolant chamber.

Due to the features of claim 9, a complete flow through the coolant chamber in the region of the anode is ensured, dead spaces are avoided.

For a simple construction of the device, in particular of the holder, it is advantageous to provide the features of claim 10. As a result, the contact pins for the production of the electrical connection can be used simultaneously for the supply of the coolant.

In order to allow easy mounting, in particular larger modules, it is expedient to provide the features of claim 11.

Due to the features of claim 12 results in a very uniform flow of the cathode with gas in the chamber.

Due to the features of claim 13, a change in the axial position of the cathode with respect to the anode during clamping of the collet is reliably avoided. In a rework of the cathode can be easily determined their change in length compared to the previous setting and the stop screwed by the appropriate level in the clamping nut and secured in this position.

• Fig. 1 schematisch einen Schnitt durch einen Halter mit Plasmaerzeuger eines erfindungsgemäßen Gerätes,
• Fig. 2 einen Schnitt im vergrößerten Maßstab durch den Plasmaerzeuger nach der Fig. 1,
• Fig. 3 einen Schnitt durch einen weitere Ausführungsform eines Halters mit einem Plasmaerzeuger nach einer weiteren Ausführungsform eines erfindungsgemäßen Gerätes,
• Fig. 4 eine Draufsicht auf den Halter samt Plasmaerzeuger nach der Fig. 3,
• Fig. 5 einen Schnitt durch den Plasmaerzeuger nach der Fig. 3 und 4 in vergrößertem Maßstab,
• Fig. 6 einen Schnitt durch die Kühlmittelkammer des Anoden-Kontaktteiles und
• Fig. 7 einen Schnitt durch die Zentrierhülse.

The invention will now be explained in more detail with reference to the drawing. Showing:

• 1 shows schematically a section through a holder with plasma generator of a device according to the invention,
• 2 shows a section on an enlarged scale through the plasma generator of FIG. 1,
• 3 shows a section through a further embodiment of a holder with a plasma generator according to a further embodiment of a device according to the invention,
• 4 is a plan view of the holder including the plasma generator of FIG. 3,
• 5 shows a section through the plasma generator according to FIGS. 3 and 4 on an enlarged scale,
• 6 shows a section through the coolant chamber of the anode contact part and
• Fig. 7 is a section through the centering sleeve.

In the embodiment of Figures 1 and 2, one of an electrically insulating material, e.g. Ceramics produced substantially hollow cylindrical holder 1 is provided, in one end region of which also made of an insulating material insert 2 is pressed.

This insert 2 is penetrated by a central, a gas supply line 3 forming tube which terminates at the end face of the projecting beyond the front side of the holder 1 insert 2. Furthermore, the insert 2 still has two bores 4 located in a diametral plane, in which press-fit parts 7 serving as abutments are held, which in turn are interspersed with clearance by the cores 5 of connection lines 6.

These connecting lines 6 are connected to a voltage supply, not shown, which is suitable to deliver in addition to the required for the work to be performed operating power also required for the ignition of the plasma ignition pulses.

At this press-fit parts 7 are based on compression springs 8, the contact pins 9, which are soldered to the souls 5, push outward. In this case, the contact pins 9 are provided at their free end with a frontal projection 10 which cooperates with a contact surface of a plasma generator 11 which is held in a arranged on the end face of the holder 1 fastening means 23, which is manufactured as a made of an electrically insulating material bracket is formed, in which the plasma generator 11 is inserted from above.

This plasma generator 11 has a connecting part 13 made of an electrically insulating material, e.g. Ceramic, which is conically tapered in its lower portion and has an opening 14 at its lower end face.

This opening 14 is penetrated by an annular anode 15, which is made in the usual way of an electrically conductive and thermally highly resilient material and has a nozzle opening 16 in its mouth region.

The anode 15 has an upwardly conically widening region, which rests on the inside of the connecting part 13 and which merges into a cylindrical region.

At the upper end side of the anode 15 there is an intermediate part 17 which is annular and made of an electrically insulating material, e.g. Ceramic, is made.

At the upper end side of the intermediate part 17 is made of a highly electrically conductive material, such as copper, produced holding part 18, in which a cathode 19 is pressed, consisting of an electrically conductive and thermally highly resilient material, such as a tungsten ceria -Alloy is made and in its nozzle opening 16 of the anode 15 near end region is conical.

The anode 15, as well as the holding member 18 are adapted to define the mutual position of the cathode 10 and the nozzle opening 16 of the anode expediently in the connecting part 13.

The anode 15, the intermediate part 17 and the holding part 18 with the pressed-in cathode 19 together with the connecting part 13 form a module of the device, which can be easily installed in the holder and removed therefrom.

On the upper end side of the holding member 18 is made of an insulating material pressure member 20 which has a cathode 19 with clearance receiving bore 21 and projects beyond the end face of the connecting part 13.

This pressure member 20 cooperates with a cover 22 which is screwed onto an in the upper end face of the connecting part 13 near the area arranged external thread 23.

The connecting part 13 is provided with three along a surface line arranged radial bores 24, 25, of which the holes 24 allow the passage of the lugs 10 of the contact pins 9 and in the region of the holding part 18, and the anode 15 are. The bore 25 is disposed in the region of the intermediate part 17 and aligned with a radially extending inlet 26 of the intermediate part which leads to a limited by the inner wall of the intermediate part 17 chamber 27, which is penetrated by the cathode 19.

In this case, the bore 25 is aligned with the plasma generator used in the holder 1, which is constructed as a module, also with the gas supply line 3 provided in the holder 1.

To install the plasma generator 11 constructed as a module, it suffices the connection lines 6, the insulating coats 28 are performed with play in the holes 4 of the insert 2 of the holder 1, withdraw and insert the plasma generator 11 from above into the bracket 12. Thereafter, the leads 6 can be released and the pins 9 engage in the holes 24 of the connecting part 13 and secure the position of the plasma generator 11 in the holder 1. At the same time they are with their faces by means of the springs 8 to the holding part 8, or the anode 15 is pressed and thus made a good electrical contact.

In the operation of the plasma generator 11, a gas via the gas supply line 3, for example, helium, CO _2, inter alia, introduced into the chamber 27, flows around the cathode 19 and cools it simultaneously in operation. This gas flows out via the nozzle opening 16.

If now an arc between the anode 15 and the cathode 19 is ignited by means of a high voltage pulse, a plasma is formed, which emerges from the nozzle opening 16 and is e.g. can be used to make a weld or to cut materials.

If the cathode 19, or its conical end region worn so far that proper operation of the plasma generator is no longer guaranteed, so constructed as a module plasma generator 11 is easily replaced and replaced by a new one. The exchanged plasma generator 11 can then be supplied to a recycle process.

In the embodiment of FIG. 3, a holder 1 'is provided, the holes 4' for receiving the contact pins 9 ', wherein the contact pins 9' are pierced in the axial direction. In this case, the contact pins 9 'are provided in an area outside the holder 1' with an external thread 29, are screwed onto the terminal nuts 30, between which terminal lugs 31 of leads 8 (Fig. 4) are clamped.

The rear end of the contact pins 9 'is designed for the connection of hoses through which cooling water can be supplied.

Furthermore, a gas supply line 3 'is held in the holder 1', which, as can be seen from Fig. 4, via a radial channel 32 which is closed with a grub screw 33 to the outside, and an opening into this axial bore 34, in which a hose nozzle 35 is screwed in, is connected to a gas hose 36, via which a gas required for generating the plasma can be supplied.

In this case, the gas supply line 3 'in the region of the radial channel 32 slots 37 through which the gas into the interior of the Gas supply line 3 'can flow. In this case, the gas supply line 3 'is secured by means of a screw 39 engaging in this position.

As can be seen from FIG. 3, the contact pins 9 'project beyond the end face 38 of the holder 1' in their spring-loaded rest position and engage in the lateral surface of a plasma generator 11 'constructed as a module. The same applies to the gas supply line 3 ', which engages with mounted plasma generator 11' in this.

The constructed as a module plasma generator 11 'is held by means of a pipe clamp 40, which is held on the end face 38 of the holder 1' fixed part with pins 42. In this case, the clamp 40 has a hinge 43, whose axis is perpendicular to the axis of the holder 1 '.

In the plasma generator 11 ', the holding part 18' of the cathode 19 'is formed by a collet, which is made of a material with good electrical conductivity. This collet is held in a conventional manner in a receptacle 44 which is screwed into a contact part 45.

This contact part 45 is provided with a coolant chamber 46, which is connected via a radial channel 47 with a connection opening 48. In this case, this connection opening 48 is aligned in the holder 1 'mounted plasma generator 11' with the contact pins 9 '.

For clamping and releasing the collet 18 ', a clamping nut 49 is provided, which is supported by two seals 50 on the upper end face of the receptacle 44, whereby a leakage of coolant is avoided, the receptacle 44 for sealing the coolant chamber 46 also via a seal 51 is supported on the contact part 45.

For further sealing of the coolant chamber of the contact part 45, an O-ring 52 is provided, which is inserted in a groove of a bore 53 which is penetrated by the receptacle 44.

To secure the axial adjustment of the cathode 19 'during clamping of the collet 18', the clamping nut 49 is provided with a continuous threaded hole 90, is screwed into a stop 91 which engages in the collet 18 '. This Stop 91 has a smooth head 94 in which a circumferential groove for receiving an O-ring 95 is incorporated, which serves to seal the interior of the collet 18 '.

To secure the position of the stop 91, which is adjustable by means of a screwdriver inserted into the frontal slot 93, a Kontramutter 92 is provided, the same time for a rotationally fixed connection between the stop 91, against which the cathode 19 ', and the clamping nut 49th provides.

By the stop 91 it is ensured that when clamping the collet, the cathode 19 'of the collet 18' can no longer be moved relative to the anode 15 'axially, since the clamping nut 49 bears against the end face of the contact member 45 and the anode 15' against this is fixed.

The contact part 45, which serves to contact the cathode 19 ', lies with the interposition of a seal 54 on an intermediate part 55, which consists of an electrically insulating material, such as. Ceramic, is made. This intermediate part 55 determines the chamber 27 'which is connected via a radial passage 56 with a connection opening 57.

The radial channels 47 and 56 are provided with circumferential grooves 58, in which O-rings 59 are arranged. These serve to seal the contact pins 9 'or the gas supply line 3' engaging in these channels.

In the chamber 27 ', a distribution ring 59' is arranged, which is provided with distributed over the circumference arranged holes 60 whose diameter increases in both directions with increasing angle to the radial channel 56. In this case, the axial bore of the distribution ring 59 'of the cathode 19' passes. In this case, an annular space 61 remains between the inner wall of the intermediate part 55 and the distribution ring 59 '.

The intermediate part 55 is supported via a seal 62 on an anode contact part 63. In this anode contact part 63, a clamping sleeve 64 is screwed into an internal thread 65, wherein a seal 66 between the anode contact part 63 and the end face of the clamping sleeve 64 is interposed.

The clamping sleeve 64 has in the region of its one end a conical contact surface 67, against which a diametrically opposite conical surface 68 of a head 69 of an anode 15 'is applied, which, like the clamping sleeve 64 and the anode contact portion 63 made of a highly electrically conductive material is.

The anode 15 rests with its end facing away from the head 69 with a further head 70, which rests with the interposition of a seal 71 on a shoulder of the anode contact part 63. In this case, the anode 15 'passes through a coolant chamber 46 of the anode contact part 63.

The anode 15 'is pierced in the axial direction, in which bore 72 a sleeve 73 is inserted, which is made of an electrically insulating material, e.g. Ceramic, is made, and is penetrated by the cathode 19 '.

Furthermore, in the bore 72 in the mouth of the anode 15 'near area a centering sleeve 74 is used, which is shown in more detail in FIG. 7 and whose provided on guide ribs 89 guide surfaces 75 abut the lateral surface of the cathode 19'.

As can be seen from FIG. 6, the anode 15 'has radially projecting guide ribs 76, which, as can be seen from FIG. 6, extend from the hexagonal-section anode 15' to the inner wall of the clamping sleeve 64 and perpendicular to the axis of the radial channel 47. In this case, the guide ribs 76 extend away from the head 70 against the head 69 of the anode 15 ', but between the head 69 and the guide ribs 76, a flow path 77 remains.

Thereby, the coolant chamber 46, which is bounded on the one hand by the anode contact part 63 and the clamping sleeve 64, divided by the guide ribs 76.

The two coolant chambers 46 of the contact part 45 and the anode contact part 63 are connected to one another via an overflow channel 78.

This overflow channel 78 consists essentially of axial bores 79 in the contact part 45, and the anode contact part 63 and radial, coaxial with the radial channels 47 holes 80, which open into the axial bores 79.

In this case, the intermediate part 55 is provided with a bore 81 aligned with the axial bores 81.

In this case, 55 seals 82 are provided in the region of the bore 81 of the intermediate part.

In the mouth region of the anode 15 'is provided an insert 83 made of a wear-resistant material, e.g. made of a tungsten-ceria alloy.

The two contact parts 45 and 63 are surrounded by rings 84 of an electrically insulated material, or these sit on collars 85.

As can be seen from Fig. 3, the pipe clamp 40 in the region of the collars 85 of the contact parts 45 and 63 recesses 86, whereby a short circuit between the two contact parts 45 and 63 is avoided.

During operation, gas, for example helium, CO ₂ or the like, is blown into the chamber 27 'and an arc between the cathode 19' and the anode 15 'is ignited by a high voltage pulse. The plasma forming in this way exits through the nozzle opening 16 '.

The cathode 19 'is tapered at both ends.

The two contact parts 45 and 63 and the intermediate part 55 are connected to one another by means of the screws 87 shown in FIG. 4 and constitute the connecting parts, by means of which a modular construction of the plasma generator 11 'is ensured.

As soon as the cathode 19 'is worn out, the plasma generator 11' designed as a module can be removed by loosening the clamping screw 88 and opening the pipe clamp 40, after which the clamping nut 49 can be released and the cathode 19 'removed from the collet. Then the cathode can be turned or its tapered ends can be reground. Subsequently, the cathode can be adjusted with respect to the anode 15 'by means of a gauge. Subsequently, the stop 91 is set with the collet 18 'open and then the cathode 19' by means of the clamping nut 49 in the collet 18 ' fixed again, after which the module 11 'can be reinstalled.

Claims (13)

1. A plasma generator with a holder, with the plasma generator (11, 11') comprising a non-consuming cathode (19, 19') which penetrates a chamber (27, 27') connected to a gas connection and is in connection with an electric connection and is held in a holding part (18, 18') made of an electrically well conducting material, with the holding part (18, 18') and an annular anode (15, 15') enclosing an end region of the cathode (19, 19') with an annular gap being in electrically conductive connection with contact surfaces cooperating with contact pins (9, 9') which are in connection with electric connecting leads (6), and a bore of the anode (15, 15') which is aligned coaxially with the cathode (19, 19') and is in mechanical connection with the cathode (19, 19') in an electrically insulated manner, delimits a nozzle (16, 16') connected with the chamber (27, 27'), and the anode (15, 15') is joined together with an intermediate part (17, 55) delimiting the chamber (27, 27') and at least one connecting part (13, 87) into a module (11, 11') which can be handled as a unit, characterized in that the holding part (18, 18') with the cathode (19, 19') is also a part of the module (11, 11') and can be handled jointly with the same, with the module being clamped into the holder (1, 1') whose axis stands substantially perpendicular to the axis of the cathode (19, 19') and in which the contact pins (9, 9') are held in an axially resiliently displaceable manner, with the holder (1, 1') comprising a detachable fastening device (12, 40) for receiving the module (11, 11') and a gas feed line (3, 3') which is in alignment with a radially extending inlet (26, 56) of the chamber (27, 27') when the module (11, 11') is mounted.
2. A device according to claim 1, characterized in that the anode (15) expands conically at least in one section directly adjacent to its nozzle region and rests with its one face side on an electrically insulating intermediate part (17) which comprises the inlet (26) of the chamber (27), and with the holding part (18) of the cathode (19) resting on its second face side, with a pressure part (20) made of insulating material resting on said holding part, and said parts are enclosed on the outside by a connecting part (13) which is made from an electrically insulating material and onto which a cover (22) can be screwed which presses against the pressure part (20).
3. A device according to claim 1 or 2, characterized in that the holder (1) comprises a bracket (12) for receiving the module (11) and the contact pins (9) are provided in the bracket (12) for fixing the axial position of the module (11), which pins penetrate radial bores (24) of the connecting part (13) when the module is inserted into the bracket (12), with the connecting part (13) further comprising a radial bore (25) which is in alignment with the radially extending inlet (26) of the chamber (27) and which, when the module (11) is mounted in the holder (1), is in alignment with the gas feed line (3) of the holder (1).
4. A device according to claim 1, in which at least one cooling chamber (46) is provided which can be flowed through by a cooling medium, especially water, characterized in that the holding part (18') of the cathode (19') is formed by tongs whose receiver (44) is inserted into a contact part (45) comprising a coolant chamber (46) and on whose one face side rests an intermediate part (55) made of an insulating material, which intermediate part delimits the chamber (27') penetrated by the cathode (19'), and on whose second face side rests an anode contact part (63) which holds the anode (15') at least indirectly and comprises a further coolant chamber (46) and is penetrated by the cathode (19'), with the two contact parts (45, 63) and the intermediate part (55) being clamped together in the axial direction to the module (11') and the anode (15') projects into the coolant chamber (46), and the two coolant chambers (46) and the chamber (27') of the intermediate part (55) are connected to connection openings (48, 57) via radially extending channels (47, 56), which openings are in alignment with feed and discharge lines arranged in the holder (1') when the module (11') is inserted into a holder (1').
5. A device according to claim 4, characterized in that a sleeve (73) made of an electrically insulating material, preferably ceramic material, is inserted into the continuous axial bore (72) of the anode (15'), which sleeve reaches up into the chamber (27') of the intermediate part (55), projects beyond the face side of the anode (15') facing the same, and encloses the cathode (19') with play, and a centering sleeve (74) made of an electrically insulating material, preferably ceramic material, is inserted close to the free face side of the anode (15') whose inwardly facing guide ribs (89) rest on the cathode (19').
6. A device according to claim 4 or 5, characterized in that an insert (83) preferably made of a tungsten alloy such as a tungsten-thorium alloy is inserted in the orifice region of the anode (15').
7. A device according to one of the claims 4 to 6, characterized in that the anode (15') comprises in its region close to its orifice (16') a head (69) with a substantially conical jacket surface (68) which rests on a substantially diametrically opposed clamping sleeve (64) which can be screwed into the anode contact part (63) and delimits its coolant chamber (46) at least in part.
8. A device according to one of the claims 4 to 7, characterized in that the two coolant chambers (46) are connected with each other by way of an overflow channel (78) penetrating the intermediate part (55).
9. A device according to one of the claims 4 to 8, characterized in that the anode (15') is provided with guide ribs (76) which project outwardly in a substantially diagonal manner, extend up to the inner wall of the coolant chamber (46), are arranged between the overflow channel (78) and the channel (47) leading to the connection opening (48), and extend over a portion of the axial extension of the coolant chamber (46).
10. A device according to one of the claims 4 to 9, characterized in that the feed and discharge lines for the coolant are formed by hollow, spring-loaded contact pins (9') which are held in the holder (1') in an axial displaceable manner and engage in extensions of the channels (47) when the module (11') is mounted.
11. A device according to one of the claims 4 to 9, characterized in that the holder (1') comprises pipe bracket (40) for receiving the module (11').
12. A device according to one of the claims 1 to 11, characterized in that a distributor ring (59') is arranged in the chamber (27'), which ring delimits an annular space about the cathode (19') and is enclosed at least over a portion of its outer jacket surface by an annular space (61) remaining between the inside wall of the chamber (27') and is provided with radial bores (60) which are arranged in a distributed manner over the circumference and whose diameter increases with an angular position increasing in the two rotational directions with respect to the channel (56) leading to the chamber (27').
13. A device according to one of the claims 4 to 12, characterized in that the adjusting nut (49) is provided with a continuous threaded bore (90) into which a stop (91) is screwed which projects into the tongs (18') and on which the cathode (19') rests, with the stop being fixable in the adjusting nut (49).

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