Background In gas metal arc-welding, hereinafter referred to as MIG/MAG welding, the equipment consists of a welding torch, a welding power source, a wire feeding mechanism, a wire bobbin for welding electrode, welding cabling and protective gas equipment. In the case of mechanized or automated welding, a manipulator for bringing about relative movement between the object being welded and the welding torch is also included.

Sometimes both a manipulator for moving the welding torch and a manipulator for moving the object being welded are included.

It has been found especially favourable for productivity to weld using two or more welding electrodes fed simultaneously to the same molten pool.

Welding torches for this purpose have been produced, in which the welding electrodes are supplied to the pool in a common laminar protective gas flow.

A very effective form of such multi-wire welding in terms of welding speed involves welding using two electrodes, under common gas protection, which are advanced in the main one after the other in the longitudinal direction of the joint, where each welding electrode is fed from its own wire feeding mechanism and is provided with fusion power from its own welding power source working independently of the other power source. This form is usually referred to as Tandem MIG/MAG welding. By rotating the two electrode positions in relation to the center line of the joint

in the longitudinal direction, improved tolerance towards gap variations in the joint can also be achieved. Maximum welding speed is obtained, however, when the electrodes are located one directly behind the other in the longitudinal direction of the joint.

In such multi-wire welding, the welding electrodes can have different dimensions. It is likewise possible for one welding power source to deliver direct current, while the other delivers alternating current, or for one to deliver direct current with the welding wire positive in relation to the object being welded, while the other delivers direct current of opposite polarity.

Welding parameters such as welding voltage and wire feed speed are set so that the welding power supplied for fusion of the respective welding electrode corresponds to the wire feed speed. The advance speed of the torch along the weld joint is adapted to that for suitable filling up of the joint for the set wire feed speed is achieved.

DD 153 596 describes a welding torch for such Tandem MIG/MAG welding. This torch is provided with two contact tubes usually elastically attached at the rear end of the torch and with a setting device at the front end of the torch for setting the spacing between the front openings of the contact tubes. The setting device consists of an eccentric disk which can be operated from the outside and forces the contact tubes into radially mutually different positions depending on the rotary position of the disk. This welding torch therefore affords the possibility of setting the positions of the welding electrodes in relation to one another. One disadvantage is that the electrodes are adjustable in relation to one another in only the lateral direction. A further disadvantage is that the adjustment range is limited owing to the fact that the elasticity of the contact tubes themselves or of their attachment at the rear end of the torch determines the size of the adjustment range. The dependence on elastic elements is also a disadvantage because the elasticity

changes with time, for example as a result of the high temperature present in a welding torch. Problems of tightness, for the protective gas flowing through the torch, at the attachment of the contact tubes also arise as a consequence of the interaction of the contact tubes with these elastic attachment elements. A further disadvantage is the lack of setting accuracy over a greater setting range which results from the solution using an eccentric disk as a setting device.

Moreover, adjustment is effected in the front part of the torch, where the heat is greatest, which makes adjustment during operation virtually impossible.

WO 97/45227 describes on page 17 as of line 20 and also in the two claims 16 and 17 a welding torch with a spacing between two contact tubes which can be set in the lateral direction. The purpose is, on the one hand, to prevent the electrodes coming so close to one another that either arc flashes over to the other electrode and, on the other hand, to prevent the arcs burning apart for so long that two separate molten pools arise with the accompanying welding spatter.

There is no indication of setting means here either, or even the need for such, for interrelationships between the welding wires other than the spacing in the lateral direction.

The object of the invention The object of the invention is to improve the prerequisites for creating optimum welding conditions as far as the MIG/MAG process is concerned in welding using a number of welding electrodes in a common pool.

It was mentioned above that welding voltage and wire feed speed are parameters which control the welding process. Another parameter which influences the welding result to a great extent is the inclination of the welding torch in relation to the weld joint. Another important parameter for the process is the length of the free electrode end, that is to say the length of the electrode projection from the outlet of the contact

nozzle to the arc. In this part of the welding electrode, resistive preheating of the electrode takes place by means of the current passing through. This in turn influences how much of the energy of the arc is needed to fuse the electrode end. The electrode projection is therefore of vital importance for the welding result. In single-wire welding, it is the height of the torch above the object being welded which determines the electrode projection. Different electrode materials require different lengths of electrode projection.

In multi-wire welding, the situation is more complicated. It has been found to be favorable sometimes to use different materials in the different welding wires which materials, for example, can have different compositions in terms of alloying materials.

It has also proved to be especially effective for the welding speed to weld using one solid electrode and one electrode of the cored wire type. Achieving optimum welding conditions is not then possible merely by individual setting of welding voltage and wire feed speed. Further problems occur when a multi-wire torch is angled in relation to the object being welded, in particular in a plane oriented along the joint and essentially at right angles to the surface of the workpiece. Welding spatter then occurs, and the process becomes unstable. The invention solves the problems in ways indicated in the characterizing parts of the independent claims. By virtue of the fact that means for adjusting the electrode projection (s) is/are integrated in the torch, it is possible to make optimum settings for each electrode type. By virtue of the accessibility of the adjusting means when the torch is equipped and connected for welding, it is possible not only to preset suitable projection relationships but also dynamically to change these during ongoing welding, for example as a consequence of a change in torch angle in relation to the workpiece, because adjustment can be carried out without any form of

demounting of the torch or its accessories.

Further disadvantages inherent in the known art, as described previously, are overcome by the subordinate claims, in which advantageous embodiments of the invention are also described.

Brief description of the drawing Fig. la shows a welding torch in an embodiment according to the invention with means for projection adjustment and spacing adjustment, and with two supply lines for welding current connected, a gas cover connected and two contact nozzles connected.

Fig. lb shows in a cross section of Fig. la an embodiment of spacing-adjusting means.

Fig. lc shows an example of the passage of a welding electrode through a contact tube and a contact nozzle.

Fig. 2 shows three different possible positions for a contact nozzle with projection-adjusting means (not shown) in the form of longitudinal position- shifting means of the contact tube and also three different possible positions for a contact nozzle with spacing-adjusting means (not shown) in the form of rotators for the contact tube.

Fig. 3 shows an embodiment with a worm gear as the spacing-adjusting means and a pinion with a rack integrated in the contact tube as the projection- adjusting means.

Fig. 4 shows a welding torch with a revolving device mounted for adjustment of the positions of the arcs relative to the center line of the joint.

Fig. 5 shows a welding torch with two motors integrated for adjusting projection and, respectively, spacing.

Fig. 6 shows a torch according to Fig. 5 with a further motor for revolving adjustment.

Description of preferred embodiments Examples of embodiments of the invention will now

be described.

The welding torch 100 comprises two contact tubes 110,115 which run essentially parallel through the welding torch body 101. At the front opening of the contact tubes, these are each provided with a connection 140,145 in the form of an internal thread, or alternatively some form of bayonet mounting or friction coupling, for a contact nozzle 150,155. At least one of these can be designed so that the contact nozzle forms an angle of the order of 3-15° in relation to the longitudinal symmetry axis of the contact tube.

The contact tubes are electrically insulated from one another and are provided with connection devices 120, 125 for electrical connection to supply lines 165,170, which can be connected to the torch, for welding current normally from two different welding power sources. Arranged between the first contact tube 110 and the welding torch body 101 is a device 190 for longitudinal displacement of the contact tube within the welding torch body. Alternatively, this device can be mounted on the second contact tube 115 instead of on the welding torch body. In this example, the device is designed as a threaded rod 191 with a wrench grip in the center. The direction and/or pitch of the thread is/are different at the two ends, as a result of which rotation of the rod brings about movement of the contact tube in the axial direction. The design at one end can also be in the form of a flange which runs in a groove in the connecting part, or another corresponding solution which brings about a rotatable but axially fixed connection to one connecting part. This can be regarded as a thread without pitch and with a very short extent. Instead of a wrench grip, a transmission element such as a worm wheel, a gear wheel, a rim gear or a pulley can be arranged on the rod, which then makes it possible to connect a motor so as thus to be capable of automating the adjustment procedure. It is advantageous to position the adjusting device described in the rear part of the torch, where the heat is lower

during welding. It is important that the adjusting device is accessible for adjustment without the torch or its accessories having to be dismantled. It is thus possible not only to preset different relative positions between the contact tubes and thus different electrode projection lengths but also to adjust these during operation and thus compensate for changed relationships between the welding torch and the object being welded in terms of angle and/or height. Another embodiment also includes an electric motor in the welding torch as shown in Figs 5 and 6. It can then be advantageous to use a linear transmission element such as a rack 393 for longitudinal displacement of the contact tube 310,410.

Spacing adjustment of the arcs during welding can be brought about by means of a simple angle gear arrangement in which the worm screw consists of an ordinary screw 192. This presupposes that, for this second contact tube 115, connection of the contact nozzle 155 has been carried out so that an angle has been formed between the contact nozzle and the contact tube, as described previously. By rotating the contact tube, the contact nozzle will be made to take up different positions in the lateral direction in relation to the other contact nozzle, as can be seen in Fig. 2. This affords simple adjustment of the spacing between the points of entry of the arcs into the molten pool, without the disadvantages described in the previously known art. There is no need for elastic attachment elements, the risk of leakage of welding gas in the rear part of the torch is reduced etc.

By also connecting a rotating device 253 for the whole torch according to Fig. 4, adjustment of the width of the molten pool can be achieved, which can be an advantage if varying gap widths occur in the joint.

By incorporating an electric motor into one or more of the adjusting devices, the advantage is afforded that it is possible to remote-control the adjustment. This means that adjustments can be effected

at a greater distance from the light-emitting and heat- radiating parts during welding. Furthermore, automation of the adjustment operations becomes possible with a control system and a control device for each adjusting device. The adjusting devices should then also comprise speed and/or position sensors of, for example, the absolute type or incremental optical sensors, resolver sensors, tachometers etc. It is then advantageous if a control system which manages the relative movement of the welding torch in relation to the object being welded also manages the control of one or more adjusting devices present in the welding torch. This can be effected by the positions of these adjusting devices being stored for the same points as are stored for the abovementioned relative movement. A programmed movement both for movements within the welding torch and for its movement along the object being welded is then possible, and these movements can also be synchronized, so that, for example, a smooth transition between different stored positions of the adjusting devices in the torch is obtained. It is then possible, in particular, to adapt the speed so that the starting and stopping of movements in the adjusting devices is simultaneous with associated starting and stopping of movements relating to the movement of the torch along the joint.

In addition to the illustrative embodiments above, embodiments of different types are also possible within the scope of the claims.