WO2008110635A1

WO2008110635A1 - A spot welding gun

Info

Publication number: WO2008110635A1
Application number: PCT/EP2008/053175
Authority: WO
Inventors: Lars Lorentzson; Mikael Ohlsson; Bernard Negre; Fabrice Legeleux
Original assignee: Abb Technology Ab
Priority date: 2007-03-15
Filing date: 2008-03-17
Publication date: 2008-09-18

Abstract

The invention relates to a spot weld gun (1) comprising a base unit (2) arranged to be attached to a tool holder (), a movable gun unit (4) with a first (5) and a second electrode carrying arm (6). The spot weld gun further comprising an equalizing means (25) for balancing the movable gun unit (4) in relation to the fixed base unit (2) during welding operations. The base unit (2) comprises linear guide means (12, 13). The equalizing means (25) is arranged connecting the base unit (2) and the gun unit (4). The first (5) and the second electrode carrying arm (6) are sliding along the linear guide means (12, 13) during balancingof the movable gun unit (4).

Description

A SPOT WELDING GUN

TECHNICAL FIELD

The present invention relates to a tool for spot welding.

BACKGROUND OF THE INVENTION

A tool for spot welding or so-called spot welding gun may be used for spot welding work within different types of industry, and a particular frequent application of such a tool is within the automobile industry for carrying out spot welding work on car bodies.

There are mainly two types of known spot welding guns: So-called X-guns and C-guns. An X-gun comprises a first movable electrode arm and an opposite fixed electrode arm. The first welding electrode is moving in an arc towards the second welding electrode.

A so called C-gun has the approximate shape of a C comprising two arms comprising electrodes arranged in parallel. A first welding electrode is movably arranged in a guide at one arm of the C and moves linearly towards and away from an opposite second welding electrode at the opposite arm.

When joining together at least two sheets by the influence of force and at a definite point, a spot welding gun, comprising a first movable electrode arm and a second fixed electrode arm, is used. The electrode arms are arranged on opposite sides of the at least two sheets that are to be joined together. For the joining to provide a joint that is centred in the thickness direction of the sheets, the free end of the second fixed electrode arm is to lie in the same plane as the sheets. Usually, the position of the second fixed arm deviates from the plane of the underneath surface of the lowest of the sheets to be welded, which results in the welding causing a deformation of the sheets. This may lead to deteriorated surface quality on the finished product. The reason for the electrode arms not being centred are, for example, that the sheet is not accurately positioned in the fixture that holds the sheet in place, or that the tip of the welding electrodes has been worn down during the course of the welding.

A welding gun is usually controlled by compressed air or by a servo motor. For a welding gun controlled by compressed air, the movement of the first electrode towards and away from the second electrode is achieved with a compressed-air cylinder. Patent publication US 6,337,456 teaches a resistance welding machine comprising an arm including one electrode tip, a drive unit including a linear guide for driving another electrode tip. A welding unit including a welding transformer is arranged for passing a current through a gun bracket for supporting the resistance welding machine on a robot. The resistance welding machine can be mounted and removed individually as a unit with a common base or a rail main body including the linear guide as the reference thereof.

It is pointed out that normally one of the arms of tools of this type is fixed with respect to a frame, whereas the other is movable for obtaining said mutual movement of the arms and by that of the electrodes.

When carrying out spot welding work with a tool of this type and the application is suddenly changed, a tool having longer, shorter or otherwise differently designed arms may be required for being able to carry out the welding, for instance for joining members in a position hidden behind other members. It is in such a case necessary to replace the tool by another tool being differently designed.

When carrying out spot welding work on car bodies with a spot weld gun, the positions of the planned spot welds are not easily accessible due to the complicated design of the car body and due to the size and the design of the gun. This results in a frequent and time consuming repositioning and/or reorientation of the tool between the welding moments. The relatively high weight of a spot welding gun is a limiting factor seen in this context since it has an influence on the speed of a robot carrying the spot weld gun.

It is known to use a balancing system, also referred to as an equalizer arranged between the spot weld gun and the turning plate (tool holder) of the robot. The equalizer ensures that the fixed arm is brought to a level that lies in the same plane as the underneath surface of the lowest of the sheets to be welded. The equalizer is in principle a clutch adapted to be disengaged, which during the movement of the tool is in a fixed position and with the fixed arm at a defined distance from the sheet. At the end phase of the closing movement of the arms and during the joining process, the clutch is disengaged such that the tool is able to move relative to the turning plate of the robot. This ensures a high welding pressure and consequently an increasing usability of a spot weld gun.

In prior art spot welding methods, first both electrode arms are clamping the sheets to be spot welded and then both arms are released by the equalizer. In this situation, the sheets are already pressured in any direction from the clamping moment and then the equalizer compensates by making the both electrode arms "weightless".

It is known to use equalizing systems that are controlled by compressed air or are electrically controlled. During compressed-air equalizing, compressed air is supplied to the tool. As far as electrically controlled equalizing is concerned, cabling is provided to the tool.

The design of prior art spot welding results in relatively big guns of high weight. The size and large moving mass during operation makes the gun and robot more difficult to control. Further, new materials to be welded and/or increased thickness of the sheets to be welded result in the need of a spot weld gun with increased welding pressure.

The corresponding problems arise also for other types of joining methods, such as, for example, during riveting and clinching, where the tool comprises at least two arms which are to join said sheets together, at least partly by the influence of force.

There is need of a spot welding gun of an uncomplicated design. Further, there is a need for a spot welding gun which is reduced in size and weight, and where the result of the joining process is a joint which is centred in the sheets and which hence imparts a good surface quality to the finished product. Further, there is a need of a spot welding gun with increased accessibility into small passages of e.g. a car body to be spot welded.

The known spot welding guns do not satisfy these needs.

SUMMARY OF THE INVENTION

The object of the invention is to provide a spot weld gun for joining at least two sheets which has a flexible and compact design. Another object of the invention is to provide an improved precision, which involves a lower investment cost, which entails lower operating costs than the prior art spot welding guns, and which results in a joint with a good surface quality.

The above problem is according to the first aspect of the invention solved in that a device of the kind in question has the specific features that it comprises a base unit arranged to be attached to a industrial robot tool holder, a movable gun unit with a first and a second electrode carrying arm. Further, the spot weld gun further comprises an equalizing means for balancing the movable gun unit in relation to the fixed base unit during welding operations. Further, the base unit comprises linear guide means and the movable gun unit comprises means for sliding along the linear guide means. Further, the equalizing means is arranged connecting the base unit and the gun unit, such that the first and the second electrode carrying arm are sliding along the linear guide means during balancing of the movable gun unit, (krav 1)

A base unit is defined as a part of a spot weld gun, which is adapted for attachment to e.g. the tool holder/turn table of an industrial robot.

A linear guide means is defined as means for supporting and guiding each of the electrode arms for relative linear movement along at least one common linear guide. The linear guide means comprises at least two parallel guide rails.

An electrode carrying arm is defined to comprise an arm attachment with an electrode arm and an electrode with an electrode tip. The equalizing unit is arranged to compensate deviations in the dimension of a work object to be welded and also to compensate wear of the spot welding electrodes. The equalizing means comprises a pneumatic cylinder, an electro servo motor or a spring means.

The spot weld gun according to the invention is designed to comprise modules e.g. base unit and gun unit. This increased modularity reduces the assembling and production costs. Neither the possible attachments, e.g. top, side or bottom, nor the positions of the spot gun modules have any impact on function of the equalizer. The design of the spot weld gun comprises a reduced number of modules and also less spare parts. It also results in a spot weld gun with a deep "window" since the electrode arms can be relatively long. The gun is also suitable for narrow passages since it is slim and increases the reach/volume within reach. This is both in simulation and reality. Further, the gun is easy to use in robot simulation.

When an industrial robot is carrying a spot weld gun, the fixed base unit of the spot welding gun is attached to a turntable on the robot wrist. The robot is programmed to perform an optional path passing all the positions to be spot welded. The weight of a spot weld gun is a factor to consider and also the centre of gravity of the gun. A spot weld gun designed according to the invention has reduced weight and a centre of gravity close to the robot attachment which has a positive impact on the precision of an industrial robot. In production, a reduction in weight of a spot weld guns gives the possibility to use smaller robots and this in turn gives a cost reduction.

According to a feature of the invention, the equalizing means is arranged comprised in the first electrode carrying arm and is linked to the fixed base unit.

According to a feature of the invention, the equalizing means is arranged comprised in the fixed base unit and is linked to the first electrode carrying arm.

The design with an equalizing means connected directly to the first electrode arm results in a compact spot weld gun comprising relatively smaller external dimensions. Further, the gun provides an increased usability when spot welding complicated work objects comprising narrow passages and a plurality of positions to be spot welded e.g. car bodies. Due to this compact design, a spot weld gun presents an increased accessibility in relation to complicated work objects. Further, the spot weld gun presents an improved and flexible performance within narrow spaces resulting in an increased number of welded spots during a reduced need of time for repositioning of the gun.

The first electrode carrying arm is fixed in relation to the movable gun unit. To arrange the equalizing means between the first electrode carrying arm and the fixed base results in a less complicated design. This in turn results in a gun of less weight and size when compared with prior art guns.

A reduction in weight of a spot weld gun results in a compact gun with improved performance. This in turn makes it easier to control the equalizing movement resulting in a smooth movement. Further, an industrial robot carrying a spot weld gun of relatively low weight can operate in a higher speed. Further a relatively low weight of the gun makes it possible to use a relatively smaller equalizing unit/smaller pneumatic cylinder, which in turn results in a smoother movement.

A weight reduction of the spot weld gun results in less material for the equalizer cylinder to move. This results in increased control over the balancing movement and improved result at the welding moment. It also results in an improved control over the movement of the equalizer and a more stable and safer movement. Further, it is easier to stop in the right moment and in the right position relative the work object/sheet.

According to a feature of the invention, the equalizing means comprises a pneumatic cylinder. This results in smooth movements of the movable arm to the metal sheet to be welded.

According to a feature of the invention, the equalizing means comprises a spring means. This is a cheap alternative to pneumatic cylinders. Feathers can easily be exchanged but it does not provide the same precision and smooth movement as the other solutions mentioned above.

According to a feature of the invention, the linear guide means comprises at least a first and a second rail-like guide means arranged in parallel.

The electrode arms are arranged to slide along the same parallel rail-like guide means during clamping and welding operations and movement of the movable gun unit during balancing is provided for through this sliding of the arms along the same guide rails.

The linear guide means comprising at least two rail-like guide means, which results in a gun of relatively low weight. This increases the advantages already mentioned above.

The first and second arms are arranged for parallel movement and the linear guide means provides the electrode tips to be positioned adjacent each other for optimal spot welding. The second arm is arranged to move in the direction towards the first arm, which is fixed to the movable gun unit. This provides a movement where the first and second electrode tip is getting closer on a straight line.

According to a feature of the invention, the movable gun unit comprises a drive screw shaft connecting the first and second electrode arm for linear relative movement.

The combination of the drive screw shaft, the parallel movement of the electrode arms and the equalizing unit results in spot weld gun with a compact design that can stand high loads and provides an exactly linear movement.

The spot weld gun according to the invention provides an improved result at the welding moment. The movement of the electrode arms is linear and the spot welding moment is achieved to be optimal where the electrode tips are touching the work objects/sheets without exercising any pressure on them. According to a feature of the invention, the fixed electrode carrying arm comprises an arm attachment with means for supporting one end of the drive screw shaft.

According to a feature of the invention, the second/movable electrode carrying arm comprises an arm attachment with a nut arranged on the drive screw shaft.

Rotation of the drive screw shaft results in a linear movement of the electrode arm comprising the nut. This makes it possible to use a simple design of the spot weld gun. The weight of the gun is relatively low, the movement speed of the gun is relatively high and the speed of the robot is relatively high.

According to a feature of the invention, the drive screw shaft is a pulling screw arranged to pull the nut during a welding operation. The screw has one direction of the thread.

This results in a relatively short distance between the nut and the screw supporting means. The pulling gives a decreased bending on the screw shaft at the welding moment, compared to a pushing drive screw shaft. Further, to pull to get needed force at the electrodes needs less power.

The pulling force during welding provides a better control of the welding moment. The design offers less force on the rails and further, less screw diameter to bring the right force. It is possible to design a gun to be used when heavy electrode force is needed.

According to a feature of the invention, the first/fix electrode arm attachment comprises means for supporting the at least two guide rails during operation of the screw shaft in the welding operation. According to a feature of the invention, the supporting means comprises a cantilever and cantilever supporting means.

The supporting means comprises cantilever means arranged to support the guide rails during operation of the drive screw shaft.

According to a feature of the invention, each of the arm attachments comprises a carriage with at least two sliding means, where each sliding means is arranged to slide along one of the rail-like guide means.

It is advantageous that the rails and the carriage sliding along the guide rails fit each other. Deviation in the position of the electrode arm carriages is almost zero. The result is no angle deviation between the arms and they are always arranged moving in parallel. This design saves time when electrode arms are to be attached to the gun.

According to a feature of the invention, the carriages and the rails are standard parts, not specially designed. This makes it easier, quicker and cheaper to exchange spare parts. It is also easier for the costumer to exchange standard spare parts and get the welding robot into work. According to one feature of the invention, the drive screw shaft is a roller screw shaft. A roller screw shaft provides a compact design and a reduced weight. The grooves are self cleaning and results in longer time intervals between lubrication services. Further a roller screw shaft can stand relatively high forces.

The exact parallel movement provides an optimal reshaping of the electrode tips. The electrode tip is cleaned and its top surface is in a plane perpendicular to the direction of the linear movement. The top surface of the electrode tip will be positioned in parallel with the surface of the sheet to be spot welded. The spot weld gum is less sensitive for welding spat. This results in an increased quality of the spot weld. This also provides a better possibility to do a correct reshaping of the electrode tip due to the fact that top surface of the electrode tip is arranged in a plane perpendicular to the longitudinal axis of the electrode. This reduces the angular deviation from the sheet to be welded and the electrode reshaping.

According to a feature of the invention, balls are positioned between guide rail and nut. A ball screw is cheaper, but is heavier due to the double diameter.

According to a feature of the invention, the drive screw shaft comprises grooves and the nut comprises rollers or balls arranged mounted in the nut for rotation during movement in the grooves of the shaft.

According to a feature of the invention, the first/fixed electrode arm attachment is designed to easily be able to create various types of gun i.e. C-gun. The design of the spot welding gun provides a high degree of modularity.

This results in the possibility to use the same gun body for all gun models. Further, the spot welding gun has an exchange system for the drive unit. Further, the spot welding gun is less sensitive for welding spat

The above problem is according to the second aspect of the invention solved in that a method of the kind in question has the specific features that it comprises method for operating a spot welding gun comprising a fix base unit and a movable gun unit with a first and a second electrode carrying arm. The gun further comprises an equalizing means for balancing the movable gun unit in relation to the fix base unit during welding operations, where the equalizing means is arranged to connect the fix base unit and the first electrode carrying arm. The method comprises moving the first and second electrode arm independently during balancing of the movable gun unit.

Due to an air pressure circuit capable of obtaining the weight balance effect of the gun main body with respect to the balancing cylinder, the whole of the movable gun unit can be floated so as to be able to absorb a reaction force generated when a movable-side electrode tip gives pressure to two works, thereby being able to prevent the works against deformation and strain. This results in an elimination of the risk of pressured sheets in any direction from the clamping moment. The movement of the first electrode arm due to balancing pressure changes in the pneumatic equalizing cylinder proceeds simultaneous to the movement of the second arm due to rotation of the drive screw shaft and the both arms are arranged for sliding along the same parallel guide rails during balancing.

According to a feature of the invention, the method comprises moving the first electrode arm due to operation of the equalizing means and simultaneously moving the second arm due to rotation of a drive screw shaft.

According to a feature of the invention, the method comprises moving the second electrode arm due to operation of the equalizing means and simultaneously moving the second arm due to rotation of a drive screw shaft.

The method comprises moving the second electrode arm through rotating the drive screw shaft by the drive unit and simultaneously moving the second electrode arm comprised in the movable gun unit due to balancing pressure changes in the pneumatic equalizing cylinder.

The method according to the invention provides a relatively fast spot welding. A great part of the welding process is made during a relatively shorter period of time. The process does not result in any holes in the work object/metal sheet. The el servo motor has the possibility to smoothly take a short cut between the spot positions to be welded.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained in greater detail, by description of embodiments, with reference to the accompanying drawing, wherein:

Figure 1 is a spot welding gun according to the invention,

Figure 2 is a first electrode arm attachment in detail,

Figure 3 is a second electrode arm attachment in detail,

Figure 4 is a part of a spot weld gun in an open position with a nut on a drive screw shaft in detail,

Figure 5 is a spot welding gun according to the invention in a closed position with work pieces to be welded,

Figure 6 is a spot weld gun according to the invention comprising spring means,

Figure 7 is a cantilever means comprised in a first arm attachment according to the invention,

Figure 8 is a part of the spot weld gun in Figure 5,

Figure 9 is a spot welding gun according to the invention in a closed position with work pieces to be welded,

Figure 10 is a spot welding gun according to the invention in a closed position,

Figure 11 is a part of a spot weld gun in a closed position with the free end of a drive screw shaft in detail,

Figure 12 is a spot welding gun according to the invention designed as a C-gun,

BRIEF DESCRIPTION OF THE EMBODIMENTS

A spot weld gun according to the invention will now be further described while making reference to Figure 1. The spot welding gun 1 comprises a body structure 2, denoted base unit in the following, with means 3 for securing the tool to a tool flange (not shown) of an industrial robot or alternatively a stand. These means 3 are constituted by frames 4 provided with holes for securing the tool by for instance bolts. Thus, the tool may be secured by using the means 3 depending upon the desired direction of the tool in the secured state e.g. top, back or side attachment of the gun.

The body structure 2 comprises a transformer 11. Since the body structure alone is rather weak, the transformer 11 is arranged within the body structure 2 as reinforcement. This also improves the compactness of the spot welding gun. The body structure 2 further supports two rail-like guide means 12, 13 arranged in parallel.

The spot weld gun further comprises two electrode arms 5, 6 extending substantially in parallel with each other arranged for linear parallel movements. Each arm 5, 6 is arranged to comprise an electrode 7, 8 at one end of the arm and an arm attachment 14, 15 at the other end. The arms 5, 6 are arranged such that the electrode tips 9, 10 of the electrodes 7, 8 are aligned. In Figure 1, the spot weld gun is in its open position with the electrode arms 5, 6 and respective electrode tips 9, 10 positioned at a distance.

The spot weld gun comprises an equalizing means 25 with a pneumatic cylinder 26. The pneumatic cylinder 26 is arranged attached to the first arm attachment 14 with the end of the piston 40 fastened to the base unit in a joint 46. The equalizing means 25 is arranged to move the movable gun unit 4 in relation to the fixed base unit 2 to balance a minor difference between the planned and the real position of the sheets to be spot welded, as described below. This eliminates the risk of bending the sheets during the spot welding operation.

The movable gun unit 4 comprises the first 5 and second arms 6 as defined above, the drive screw shaft 17, the nut 2, the drive unit 19 and additionally the pneumatic cylinder 26. A balancing movement of the piston 40 results in a linear movement of the complete gun unit 4 together with the pneumatic cylinder along the rail like guide means 12, 13 a small distance to adjust the position of the electrode tips 9, 10 in relation to the real position of the two sheets. This results in a balancing operation of the spot weld gun placing the spot weld in a correct position.

Figure 2 is the first arm attachment 14 arranged to hold the first arm 5 with the first electrode 7 and to comprise a first carriage 16 arranged for simultaneous linear sliding movement along both the guide rails 12, 13. The first carriage 16 is arranged to comprise means 18 for supporting a drive screw shaft 17, where the shaft is arranged extending in parallel with the two rail-like guide means 12, 13. The drive screw shaft 17 is in the first end 17a arranged supported by the supporting means 18 and is arranged for rotational movement along its longitudinal axis. A drive unit 19 is comprised in the first carriage 16 to provide the rotational movement of the drive screw shaft 17 (Figure 1). The first arm attachment also comprises the pneumatic cylinder 26 comprising a piston 40 fastened to the base unit in a joint 46.

Figure 3 is a second arm attachment 15 arranged to hold the second arm 6 with the second electrode 8 (Figure 1). The second arm attachment 15 is arranged to comprise a second carriage 20 arranged for simultaneous linear sliding movement along both the guide rails 12, 13. The drive screw shaft 17 is linking the first 16 and the second carriage 20. Figure 4 is the second carriage 20 comprising a nut 21 arranged on the drive screw shaft 17. The nut is arranged to move linearly along the screw shaft 17 upon rotation of the shaft 17 by the drive unit 19. Figure 4 is a part of the spot welding gun in an opened position whereby the free end 17b of the drive screw shaft 17 is within the nut means 21. The drive screw shaft 17 comprises grooves 22 and the nut 21 comprises rollers or balls (not shown) arranged mounted in the nut for rotation during movement in the grooves 22 of the shaft 17. In this embodiment, there is one nut and one direction of the thread. The first 16 and second carriages 20 need four ball bearing slides each to handle the torque generated by the welding force. Accordingly, the second carriage 20 comprises four ball bearing slides 33 a-d. As can be seen in Figure 4, a first 33a and a second ball bearing slide 33b are arranged to slide along the front side of the first guide rail 12. The third 33c and the fourth ball bearing slide 33d are arranged to slide along the front side of the second guide rail 13. The second carriage 20 comprises only two ball bearing slides due to limit of space and this is described below, see Figure 7.

Figure 5 is a part of a spot weld gun with the electrode arms in a closed position clamping a first 28 and a second sheet 29 to be welded. The spot weld gun comprises an equalizing means 25 with a pneumatic cylinder 26. The pneumatic cylinder 26 is arranged attached to the base unit 2 (only a part of base unit 2 is shown in Figure 5) with the end of the piston 40 fastened to the first arm attachment 14 in a joint 39. The equalizing means 25 is arranged to move the movable gun unit 4 in relation to the fixed base unit 2 to balance a minor difference between the planned and the real position of the sheets to be spot welded, as described below. This eliminates the risk of bending the sheets during the spot welding operation.

The movable gun unit 4 comprises the first 5 and second arms 6 as defined above, the drive screw shaft 17, the nut 21 and the drive unit 19. A balancing movement of the piston 40 results in a linear movement of the complete gun unit 4 along the rail like guide means 12, 13 a small distance to adjust the position of the electrode tips 9, 10 in relation to the real position of the two sheets. This results in a balancing operation of the spot weld gun placing the spot weld in a correct position.

As can be seen when comparing the position of the first arm attachment 14 in relation to the end 13a of the second guide rail 13 (Figures 1 and 5), the gun unit 4 has been moved along the two parallel guide rails 12, 13 due to the balancing movement.

Figure 6 is a spot welding gun comprising an equalizing means with spring means 27. The spring means comprises helical springs attached to the base unit in a first end 27a and attached to the first arm attachment 14 in a second end 27b.

Figure 7 is a detailed part of the first arm attachment 14 of a gun in an open position. In a gun with compact design, there is not room for four ball bearings in the first carriage 16, as mentioned above. Therefore, the first carriage is arranged to comprise two ball bearing slides 33e, 33f and a cantilever 34, which in combination handle the torque generated by the welding force. The cantilever 34 and cantilever supporting means 35a, 35b are arranged to support the two parallel guide rails 12, 13 during the welding moment when the electrode force is very high. At this moment, there is a risk of bending the guide rails and the cantilever design eliminates this risk. The first arm attachment 14 comprises a first pulley (not shown), a second pulley 31, connected to the drive screw shaft 17 and a belt 32. The drive unit 19 is arranged to rotate a first pulley and moving the belt 3, which is arranged to transmit rotation of the first pulley to rotational movement of the second pulley 31. Rotation of the second pulley 31 causes the drive screw shaft 17 to rotate and this in turn causes the nut means comprised in the second arm attachment 15 to move linearly along the drive screw shaft 17.

Figure 8 is a detailed part of the first arm attachment 14 of a gun in an open position. The first carriage 16 is arranged to comprise a fifth (not shown) and a sixth ball bearing slides 33f and a cantilever means 34, which in combination handle the torque generated by the welding force. The fifth ball bearing slide 33e is arranged sliding along the front side of the first parallel guide rail 12. The sixth ball bearing slide 33f is arranged sliding along the front side of the second guide rail 13. The cantilever means 34 is arranged to slide along the backsides of the first 12 and second guide rail 13. The cantilever means 34 comprises a cantilever supporting means 35.

The cantilever means 34 comprises a first cantilever supporting means 35a (Figure 7) arranged to support the first guide rails 12 during the welding moment when the electrode force is very high. The cantilever 34 comprises a second cantilever supporting means 35b arranged to support the second guide rails 13 during the welding moment when the electrode force is very high. In this embodiment, the two supporting means 35 a, 35b comprises two bronze pads 36 each.

Figure 9 is the spot weld gun in a closed position with a first 41 and a second work object/sheet 42 to be spot welded arranged between the electrode tips 9, 10.

Figure 10 is a spot welding gun in a closed position. A first 43 and a second, secondary connection 44 are arranged to connect the respective electrodes with the transformer 11. The first connection 43 is comprised in the first arm attachment 14. The second connection 44 is comprised in the second arm attachment 15.

Figure 11 is a part of a spot welding gun in a closed position whereby the free end 17b of the drive screw shaft 17 is visible and free in a space/volume 45.

Figure 12 is a spot welding gun designed as a C-gun and in an open position.

A welding operation is described in the following:

-an industrial robot carrying a spot weld gun according to the invention is moving to place the first electrode tip in a desired position, e.g. 3-5 mm (maximum 20 mm) beneath the sheets to be welded.

-air is supplied to the pneumatic cylinder of the equalizing unit, full pressure in the plus compartment and the lower/first/fixed electrode arm is in its outermost position.

-air is supplied into the pneumatic cylinder with regulated/controlled pressure in the minus-compartment. Some air is let out from the plus-compartment, depending on the weight of the lower arm in order to balance it and achieve "weightlessness". The cylinder comprises a valve, which can be programmed to different attachment positions of the tool i.e. horizontal, vertical etc.

-the lower electrode arm is moving slowly in a direction towards the lower sheet thanks to the regulated pressure in the equalizing cylinder and simultaneously the upper arm is moving in a direction towards the upper sheet/the lower electrode and the speed is reduced when it almost reaches the sheet such that there is no mark in the sheet, then it pulls the lower arm towards the lower sheet thanks to the "weightlessness",

-when the two arms have clamped the sheets, the upper arm is driven until a predefined value of the pressure is created.

-current is sent through the electrodes and a melt is achieved creating a weld spot (<lsek).

-after the current has been on, the electrodes are cooled <0,ls.

-the upper arm is driven away from the upper sheet and air is supplied and full pressure is created in the equalizing cylinder such that the lower arm is moving away from the lower sheet.

-the robot moves the spot weld gun to the next weld position.

Starting with a spot welding gun in its open position, the movements of the arms are described as follows. The lower/first arm is moved by the robot to a desired position close to the lower sheet to be welded. Then, the equalizer is starting to balance the movable gun unit in relation to the base unit to make it "weightless". During the balancing, the electrode tip of the first arm is moved and reaches the lower sheet to be welded, without exerting any pressure on the lower sheet.

The upper/second arm is moved independently from the lower arm. The upper arm is pulled towards the upper sheet through rotation of the drive screw shaft. This is a movement of the upper arm along the screw shaft within the movable gun unit. Separately from the rotation of the screw shaft, the equalizer is activated to balance the movable gun unit in relation to the base unit to make the movable gun unit "weightless". During the balancing, the electrode tip of the second arm is moved and reaches the upper sheet to be welded, without exerting any pressure on the upper sheet.

The first and second electrode arms have been moved independently and separately and the first electrode tip reaches the lower sheet and the second electrode tip reaches the upper sheet to be welded without pressing the sheets in any direction, i.g. "weightlessly" due to the balancing function of the equalizer. The spot weld gun is in position to start the welding process.

The second electrode arm with the second electrode tip is pulled into contact with the upper sheet and the pulling movement continues until the two electrode tips press the sheets with a desired pressure and then a spot weld is welded.

Claims

1. A spot weld gun (1) comprising a base unit (2) arranged to be attached to a tool holder (), a movable gun unit (4) with a first (5) and a second electrode carrying arm (6), the spot weld gun further comprising an equalizing means (25) for balancing the movable gun unit (4) in relation to the fixed base unit (2) during welding operations, characterized in that that the base unit (2) comprises linear guide means (12, 13), the equalizing means (25) is arranged connecting the base unit (2) and the gun unit (4), and the first (5) and the second electrode carrying arm (6) are sliding along the linear guide means (12, 13) during balancing of the movable gun unit (4).

2. A spot welding gun according to claim 1, wherein the equalizing means (25) is arranged comprised in the first electrode arm (5) and is linked to the fixed base unit (2).

3. A spot welding gun according to claim 1, wherein the equalizing means (25) is arranged comprised in the fixed base unit (2) and is linked to the first electrode arm (5).

4. A spot welding gun according to any preceding claim, wherein the equalizing means (25) comprises a pneumatic cylinder.

5. A spot welding gun according to any of claims 1-3, wherein the equalizing means comprises a spring means.

6. A spot welding gun according to any preceding claim, wherein the linear guide means comprises at least a first (12) and a second rail-like guide means (13) arranged in parallel.

7. A spot welding gun according to any preceding claim, wherein the movable gun unit (4) comprises a drive screw shaft (17) connecting the first (5) and second electrode arm (6) for relative movement.

8. A spot welding gun according to claim 7, wherein the first/fix electrode carrying arm (5) comprises a first arm attachment (14) with means for supporting one end (17a) of the drive screw shaft (17).

9. A spot welding gun according to claim 8, wherein the second/movable electrode carrying arm (6) comprises a second arm attachment (15) with a nut (21) arranged on the drive screw shaft (17).

10. A spot welding gun according to claim 9, wherein the drive screw shaft (17) is a pulling screw arranged to pull the nut (21) during a welding moment.

11. A spot welding gun according to claim 8, wherein the first electrode arm attachment (14) comprises means for supporting the guide rails (12, 13) during operation of the screw shaft in the welding operation.

12. A spot welding gun according to claim 11, wherein the supporting means comprises a cantilever means (34).

13. A spot welding gun according to claim 8, wherein the first arm attachment (14) comprises a first carriage (16) with at least two sliding means (33 e, 331), where each sliding means is arranged to slide along one of the rail-like guide means (12, 13).

14. A spot welding gun according to claim 9, wherein the second arm attachment (15) comprises a second carriage (20) with at least two sliding means (33a, 33b, 33c, 33d), where each sliding means is arranged to slide along one of the rail-like guide means (12, 13).

15. The first and second electrode arms have been moved independently and separately and the first electrode tip reaches the lower sheet and the second electrode tip reaches the upper sheet to be welded without pressing the sheets in any direction, due to the balancing function of the equalizer.

16. A method for operating a spot welding gun (1) comprising a fix base unit (2) and a movable gun unit (4) with a first (5) and a second electrode carrying arm (6), the gun further comprises an equalizing means (25) for balancing the movable gun unit (4) in relation to the fix base unit (2) during welding operations where the equalizing means (25) is arranged to connect the fix base unit (2) and the first electrode carrying arm (5), the method comprises moving the first (5) and second electrode arm (6) independently during balancing of the movable gun unit.

17. A method according to claim 16, comprising moving the first electrode arm (5) due to operation of the equalizing means (25) and simultaneously moving the second arm (6) due to rotation of a drive screw shaft (17).

18. A method according to claim 16 or 17, comprising moving the second electrode arm (6) due to operation of the equalizing means (25) and simultaneously moving the second arm (6) due to rotation of a drive screw shaft (17).