Abstract:
A resistance welding process in which the welding electrodes are moved at least via a wording piston-cylinder unit, wherein before the start of the welding process, the welding electrode is in a ready position in which it gently touches the workpiece or there is a gap in the tenth of a millimeter range between the welding electrode and the workpiece, and during the subsequent welding process pneumatic force is applied to the working piston-cylinder unit, where the maximum possible stroke of the working piston-cylinder from the position of readiness corresponds to 1 to 40 times the travel of collapsing bulges in projection welding or 1 to 120 times the penetration distance of the welding electrode in an originally hard material becoming pasty during the welding process in spot welding, or 1 to 60 times the penetration distance of the welding electrode in an originally soft material becoming pasty during the welding process in spot welding. The invention also relates to a device for implementing the process.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of resistance welding in which the welding electrode movement takes place at least by means of one working piston-cylinder unit. 
     Furthermore, the invention relates to a resistance welding plant with a working piston-cylinder unit for carrying out an electrode movement during the welding process. 
     Resistance welding is a generally known method in which, for example, two pieces of sheet metal are connected together, in that they are pressed together by two electrodes, of which at least one is movable, and fusing and subsequent solidification of the material takes place at the joint location by means of adequate electrical resistance heating through the welding current. 
     The quality of the welded connection is decisively influenced by how good the contact is between the electrode and the workpiece, because the contact resistances and thus the distribution of the heat produced are influenced by this contact. 
     The manner in which the electrode is placed onto the workpiece is thus, first of all, of significance, because if the placement movement is too rapid, the electrode rebounds so that the contact pressure force initially oscillates and the decay of this oscillation must be waited for prior to switching on the welding current, which, however, reduces the working speed in a disadvantageous manner. During spot welding rapid placement can lead to an uncontrolled high striking energy, which leads to cold deformation and rapid wear of the electrodes. 
     During projection welding the too rapid placement brings about an uncontrolled cold deformation of the projection prior to switching on of the welding current, and thus to an uncertain welding result. 
     One has hitherto attempted to regulate the speed of placement through the association of damping and restrictor valves. One has achieved a certain improvement of the placement behavior through the use of double stroke and feed stroke units. 
     The force responsible for the contact pressure between the electrode and the workpiece must, however, not only be made available at the start of welding but rather also during welding. 
     Of significance in this connection is the fact that the material to be welded becomes soft during the welding, i.e. pasty, and fuses at the joint position. When the electrode cannot be adequately quickly replenished, the force between the workpiece and the electrodes can, in unfavorable cases, reduce to such an extent that no adequate electrical contact is any longer present between the electrodes and the workpiece. This leads, having regard to the extremely high current density, to disadvantageous spray formation and premature electrode wear. If, during projection welding, the electrode is not adequately rapidly replenished, the contact pressure between the impressed projection of the one piece of sheet metal and the counter-sheet reduces, whereby the softened projection can spray away or pore formation can arise in the welding spot. 
     In the previously known methods, the electrode force is primarily made available during welding by single stroke cylinders, or by the working stroke of twin or feed stroke cylinders, which are, as a rule, pneumatically actuated. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention is to improve the welding of workpieces, and in particular the quality of the welding and/or the welding speed is to be increased. In this respect a rapid, precise follow-up movement of the welding electrode should be ensured during the welding procedure. 
     This object is satisfied in a method of the initially named kind in that the welding electrode is located prior to the start of the welding process in a position ready for use in which it gently contacts the workpiece or in which a distance in the range of tenths of millimeters exists between the welding electrode and the workpiece, and in that, during the subsequent welding procedure, a pneumatic loading of the working piston-cylinder unit takes place, with the maximum possible stroke of the working piston-cylinder unit starting from the position ready for use, corresponding to 
     1 to 40 times the follow-up movement path of the collapsing workpiece projections during projection welding, or 
     1 to 120 times the penetration path of the welding electrode into an originally hard workpiece, which becomes pasty during the welding process when spot welding, or 
     1 to 60 times the penetration path of the welding electrode into an originally soft workpiece which becomes pasty during the welding process when spot welding. 
     Furthermore, this object is satisfied in an apparatus of the initially named kind in that the maximum possible stroke of the working piston-cylinder unit corresponds to 
     1 to 40 times the follow-up movement path of the collapsing projections during projection welding, or 
     1 to 120 times the penetration path of the welding electrode into an originally hard workpiece, which becomes pasty during the welding process when spot welding, or 
     1 to 60 times the penetration path of the welding electrode into an originally soft workpiece which becomes pasty during the welding process when spot welding. 
     Both in the method of the invention and also in the plant of the invention it is of advantage 
     when the maximum possible stroke of the working piston-cylinder unit corresponds, starting from the position ready for use, to 1 to 20 times, in particular 1 to 10 times, and preferably to 1 to 5 times the follow-up movement of collapsing projections during projection welding, or 
     when the maximum possible stroke of the working piston-cylinder unit corresponds, starting from the position ready for use, to 1 to 60 times, in particular to 1 to 30 times, and preferably to 1 to 20 times the penetration path of the welding electrode into an originally hard material, which becomes pasty during the welding process when spot welding, or 
     when the maximum possible stroke of the working piston-cylinder unit corresponds, when starting from the position ready for use, to 1 to 30 times, and in particular to 1 to 15 times, and preferably to 1 to 10 times the penetration path of the welding electrode into an originally soft material, which becomes pasty during the welding process when spot welding. 
     In the context of the invention it has been recognized that in the previously known arrangements the maximum possible stroke lengths of the single, twin or feed stroke cylinders are of large dimensions, so that, during the welding process, a relatively large air volume flows into the cylinder space above the working piston and is compressed, and mainly so that a larger compressed air volume must escape from the cylinder space beneath the working piston. The long airflow times which arise through this influence the follow-up movement behavior of the electrode during the welding very unfavorably. 
     Since, in accordance with the invention, the maximum possible stroke of the pneumatically actuated piston-cylinder unit out of the position ready for use essentially only corresponds to the required working stroke of the welding electrode for the penetration path of the spot welding electrodes into the pasty material during spot welding and/or for the follow-up movement path of the collapsing projections during projection welding, only a very small working cylinder volume has to be loaded with compressed air in order to move the working piston and thus the working electrode in the direction of the workpiece. At the same time only a very small volume beneath the working piston need be displaced out of the working cylinder. 
     This requires less time than the previously known methods so that the pressure relationships required for the necessary welding electrode pressure force set in very rapidly in the working piston-cylinder unit. Moreover, through the low extent of the pressure loaded volume, only a smaller compression effect has to be brought about than with larger volumina, which leads to improved force action of the electrode. 
     Starting from the fact that the welding electrode gently contacts the workpiece in the position ready for use, or has a spacing from the workpiece in the range of tenths or hundredths of a millimeter, an electrode movement is hardly perceptible after pneumatic loading of the working piston-cylinder unit. If now, e.g., two pieces of sheet metal with a sheet metal thickness of 2 mm each are projection welded, then for this the volume above the piston must be filled with compressed air, whereby the workpieces are pressed together by the electrode at the start of the welding process. As a result of corresponding dimensioning, the distance between the working piston and the cylinder cover associated with it amounts to only ca. 1 mm for the said welding tasks and similar welding tasks (including multiprojection welding). 
     Since the relatively small volume brought about by this small spacing can be filled with compressed air within a very short time, the welding current can be switched on after the expiration of only milliseconds from the actuation of the working piston-cylinder unit. This is additionally assisted by the relatively large air entry cross sections and the thereby caused good flow speeds for the compressed air. 
     Following the switching on of the welding current, the fusing of the projection takes place, which, with the said sheet metal thickness, has a height of ca. 1 mm, and from now on a visible electrode movement starts, which terminates after the return deformation of the projection (ca. 1 mm). 
     The electrode pressure is maintained for a short time for the solidification of the weld spot. Thus, for the piston path of ca. 1 mm for the entire welding process, a volume above the piston with a height of a total of ca. 2 mm must be filled, which takes place within a very short period of time and thus enables rapid follow-up movement of the electrode. 
     In order to be able to fully exploit the rapid pressure build up above the piston with the theoretical electrode force which results from it, the compressed air volume located beneath the piston must also be able to escape rapidly when the working piston-cylinder unit is loaded. 
     This is achieved in accordance with the invention in that the maximum possible stroke is kept as small as possible, and is in particular very small. Taking account of a possible machine bending, the stroke in the described embodiment amounts to ca. 2 mm. The compressed air escapes very rapidly from the relatively small volume through the air outlet cross sections, which are also made large here, and the electrode force quickly becomes fully effective. 
     Through the fact that the welding current is switched on within milliseconds after the loading of the working piston-cylinder unit, an uncontrolled cold deformation of the projection prior to welding is precluded. This advantage also proves to be particularly favorable in conjunction with the subsequent rapid follow-up movement of the electrode during the projection welding of aluminum and similar materials of soft form. 
     Provision is preferably made for the welding electrode to be returned after termination of the resistance welding into the position ready for use by pneumatic actuation of the working piston-cylinder unit. 
     In an alternative manner the return stroke movement of the welding electrode can, however, also take place by spring force. 
     The control of the compressed air for the mutual movement of the working piston-cylinder unit is preferably executed for both types of resetting using corresponding changeover valves. 
     A prestroke arrangement is preferably connected after the working piston-cylinder unit, by means of which the welding electrode can be moved from a starting position removed from the workpiece up to the workpiece into the position ready for use. 
     The prestroke arrangement can selectively be actuated pneumatically, hydraulically, by an electric motor, by muscle power, by hand, or by some other type of kinematics. 
     With respect to the position and direction of movement of the welding electrode the coupling of the prestroke arrangement and the working piston-cylinder unit to one another can take place in a different association. 
     Furthermore, the possibility exists of separating the prestroke arrangement from the working piston-cylinder unit and arranging it, in relation to the workpiece, on the opposite side of the working piston-cylinder unit. In this way the prestroke movement from the starting position into the position ready for use is executed by the counter-electrode. Subsequently, the resistance welding takes place by the loading of the working piston-cylinder unit of the invention. 
     The interplay between the prestroke arrangement and the working piston-cylinder unit is variably designed through corresponding program preselection of the machine sequence control. For the welding tasks which are primarily to be carried out, the workpiece is removed from the machine after the termination of the welding. As a result of this, the prestroke arrangement is also moved back, with the retraction of the working piston-cylinder unit, into the starting position for each working cycle. 
     When using a prestroke arrangement, the working piston-cylinder unit can be arranged between the prestroke arrangement and the welding electrode. In just the same way it is possible to arrange the prestroke arrangement between the working piston-cylinder unit and the welding electrode. 
     Furthermore, in certain applications, for example with hand-guided welding tongues, it can be of advantage when the electrode is moved by the working piston-cylinder unit and the counter-electrode is loaded by the prestroke arrangement. 
     It should, however, once again be mentioned that the prestroke arrangement can also be omitted completely, with the workpiece being pushed from the side beneath the upper welding electrode after the distance between the welding electrode has previously been set. The resistance welding then takes place through the loading of the working piston-cylinder unit of the invention. 
     A further advantage in comparison to the previously used methods lies in the operating mode “series spot welding”. In this generally known type of resistance welding, two pieces of sheet metal are, for example, connected together by weld points, which are to be arranged closely one after the other. After each weld has been completed, the electrode is lifted from the workpiece in the tenth of a millimeter range, the workpiece is moved on further by the desired spot spacing, and the next weld is carried out. These procedures take place continuously until the execution of the last weld point. 
     Through the use of the method of the invention, after the insertion of the workpiece during series spot welding, the welding electrode is moved by means of the prestroke arrangement out of the starting position into the position ready for use. In the welding processes which now take place, only the working piston-cylinder unit is cycled until the last welding spot, and the prestroke arrangement moves the welding electrode back into the starting position only after termination of the welding process. 
     Through the volumina of the working piston-cylinder unit which are made small, the continuous pressurization and venting in each case takes place in a very short time, whereby a high welding speed and spot welds which take place rapidly one after the other are achieved. 
     In accordance with the invention the spacing of the welding electrode from the workpiece in the position ready for use is variable and is set accordingly prior to the start of welding. A gentle contact of the workpiece by the electrode, or a spacing between the electrode and the workpiece in the range of tenths or hundredths of a millimeter, such as is, for example, necessary for series spot welding, is preferred here. With this setting, the electrode is essentially placed onto the workpiece free of blows and jolts on being moved out of the starting position into the position ready for use. 
     During multiprojection welding, the advantage results that with the electrode movement from the starting position into the position ready for use the projection is pressed by an intentional amount in the range of tenths of millimeters by the placement of the large area electrode onto the workpiece. Through this the height tolerances, which are unavoidable in the pressing of the projection, are leveled out and the electrical resistances between the individual projections and the counter-sheet are equalized for the subsequent welding process. The welding result is hereby improved. 
     Through the rapid follow-up movement of the electrode during the welding process the method of the invention is advantageous for almost all the types of resistance welding work that are encountered. 
     The improved follow-up movement characteristics in accordance with the invention are also advantageous during the spot and projection welding of sheet steel with metallic coatings and with non-ferrous metals, because a lack of force-transmitting capability would in this case lead to overheating of the workpiece surface. 
     It should be particularly emphasized that in accordance with the invention even aluminum, aluminum alloys and similar materials of soft form can be projection welded, although one is concerned with materials in which the transition from the solid state into the pasty or liquid state takes place within a very short space of time. This physical characteristic presupposes a particularly rapid follow-up movement of the welding electrode during the welding. In this respect it is no longer necessary (as previously) to press the projections with increased strength and special shapes into the sheet aluminum, but rather the round projection customary during steel welding is sufficient. 
     Through the small air volumes of the working piston-cylinder unit on both sides of the piston, the advantage furthermore results that with a pressure change the desired electrode force change sets in very rapidly, i.e. in the millisecond range. 
     The change of pressure can be carried out with the generally customary techniques of pressure program resistance welding. This method advantageously brings additional advantages during many resistance welding tasks, especially during the projection welding and spot welding of aluminum. The pore formation within the weld spot is reduced through a pressure program or also a current pressure program, in which the electrode force is preferably increased towards the end of the welding procedure. 
     The resistance welding plant of the invention can, for example, be a C-shaped stand machine, a portal machine, a multi-cylinder welding plant, hand-guided welding tongues in scissors or C-shape, robot-guided welding tongues in scissor or C-shape, or can also be any other resistance welding plant. The resistance welding plant can be equipped with one of the different current types in accordance with the prior art. 
     In a preferred embodiment the working piston-cylinder unit is formed as a closed, classical cylinder with an associated cylinder cover and cylinder base. In this way the same working stroke is, in particular, available in each position ready for use, which is determined by the position of the prestroke arrangement. 
     The sealing of the piston of the working piston-cylinder unit relative to the air volumes present at both sides (cylinder cover side and cylinder base side) and also the sealing of the piston rod within the cylinder base can, for example, take place via sliding seals or through membranes of rubber or similar materials. 
     Through the very small maximum possible stroke of the invention for the working piston-cylinder unit, the cylinder function is considerably more reliable if the piston and piston rods are sealed by membranes and the wear of the membranes is lower than with the previously, sometimes used roll membrane cylinders. 
     The prestroke arrangement includes, in a preferred embodiment of the invention, a spindle which can be energized by an electric motor and which runs in the prestroke direction, and a threaded element which is moved by the spindle and connected to the working piston-cylinder unit. With this prestroke arrangement the position of the welding electrode ready for use can be set in a simple manner in that the electric motor is actuated until the welding electrode has reached the desired position ready for use. 
     This permits a particularly convenient setting of the electrode spacing from the workpiece in the position ready for use, for example through a numerical control. 
     A plant in accordance with the invention can further include at least one working piston-cylinder unit formed as a twin piston or multiple piston arrangement. In this manner the electrode force is increased. When, in this arrangement, the working piston-cylinder unit is executed as a twin piston or multiple piston arrangement, then the maximum possible stroke of each working piston-cylinder unit is executed in accordance with the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a double stroke cylinder which can be operated in accordance with the resistance welding method of the invention, and 
     FIG. 2 shows an electrode force unit in accordance with the invention for a resistance welding plant in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 an electrode force unit  10 , for the execution of the method of the invention, is schematically shown, close to its starting position in which a welding electrode (not shown) which is to be secured to the connection plate  44  is essentially fully retracted from a workpiece to be welded (not shown). 
     The electrode force unit  10  comprises a housing  14  in which a prestroke cylinder bore  16  and a working cylinder bore  18  are provided on a common axis A—A. 
     In the prestroke cylinder bore  16  there is sealingly and slidingly arranged a prestroke piston  22  with a prestroke piston flange  22 ′ at the side remote from the workpiece and a hollow prestroke piston rod  22 ″ closed at the workpiece side. An abutment  26 , which is broadened at the workpiece side, on a setting, spindle or adjustment  24  passes through the prestroke piston flange  22 ′ into the piston rod  22 ″ to restrict the prestroke piston movement in the direction towards the workpiece. The abutment  26  is connected for its positioning to the adjustment spindle  24  via a pressure, tight lead-through  28 . 
     A through-way  30  for a fluid under pressure, such as compressed air, is provided in the housing  14  above the prestroke piston flange  22 ′, in order to connect the interior of the housing  14  via a pressure fluid valve  32 , to a pressure fluid source (not shown), for example to a compressed air source. The pressure of the pressure fluid is so selected that the desired force arises with the preset diameters of the piston. For this purpose a pressure of 1 to 6 bar can be required with customary diameters. 
     At its side facing the workpiece the prestroke piston rod  22 ′ sits in the starting position on a working piston  34 , which is sealingly slidingly arranged in the working cylinder bore  18 , with a working piston rod  34 ′ being secured to the lower side of the working piston. 
     Above the working piston  34 , but still below the lower edge of the prestroke piston flange  22 ′ even with the fully lowered prestroke piston  22 , there is provided a further through-opening  38  for compressed air, via which a second valve  40  can introduce compressed air into the working cylinder  18 . 
     To the side of the housing  14  there is provided a guide bore  42  parallel to the axis A—A, in which a guide rod  42 ′ is slidingly received, which is connected via a connection plate  44  to the piston rod  34 ′ which emerges out of the housing  14  in order to prevent a rotation of the working piston  34 . The electrode force unit  10  is operated in accordance with the invention as follows: 
     First of all, the prestroke piston abutment  26  is adjusted on the adjustment spindle  24  so that the welding electrode is moved closely up to the workpiece to be welded, when the prestroke piston  22  is energized with pressure fluid via the compressed air valve  32  and the compressed air inlet opening  30  above the prestroke piston flange  22 ′, in order to move the working piston  34  and the working piston rod  34 ′ with the welding electrode in the direction of the workpiece. 
     The position ready for use is in this way precisely selected in accordance with the invention by setting of the adjustment spindle  24  and thus of the abutment  26 , so that the working piston  34  can only move via a stroke in the working cylinder  18 , which corresponds essentially to the required working stroke of the welding electrode, but is at the same time preferably at least sufficiently large that the working piston  34  does not strike the limit within the working cylinder  18  on follow-up movement of the welding electrode. 
     In accordance with the setting of the abutment in accordance with the invention the prestroke arrangement is moved by a corresponding opening of the valve  32  from the starting position into the position ready for use. 
     In order to shift the welding electrode from the position ready for use further in the direction towards the workpiece, compressed air is then fed into the working cylinder  18  via the second valve  40  and the passage opening  38 . Since only a very small working cylinder volume is to be filled with compressed air, and since only a small stroke is to be executed, the electrode movement takes place very rapidly. 
     After the electrode presses against the workpiece to be welded, the current can be switched on for the welding time that is provided, or for a predetermined number of mains cycles. The workpiece material which is softened by the welding current yields under the force of the electrode, whereby the electrode moves slightly. This leads to the working piston  34  being shifted further in its working cylinder  18  in the direction towards the workpiece, which brings about a short term reduction of the contact pressure force, with which the welding electrode is pressed onto the workpiece, because the working piston  34  which follows the yielding workpiece must, on the one hand, displace air, and, on the other hand, the volume above the working piston  34  is increased, i.e. the pressure prevailing in the working cylinder is reduced at least until compressed air has flowed in from the compressed air source. Only when this has taken place does the electrode force correspond again to its starting value. 
     This is a problem which exists both in the prior art and also with the invention. However, only a small volume corresponding to a very small stroke is present beneath the working piston  34  of the invention, which is why, in accordance with the invention, these processes, which are necessary for the follow-up movement, take place very much more rapidly than when operating customary electrode force units, whereby the problem that has been addressed is practically overcome by the invention. 
     After the welding current has flowed through the workpiece for the predetermined time, switching off takes place. The electrode is preferably held for a predetermined further holding time on the workpiece in order to press the workpieces together during the solidification or cooling of the weld position. 
     Thereafter the welding electrode can be moved back into the position ready for use. For this purpose the second valve  40  is first switched. Then the working piston  34  is either pneumatically energized in the direction away from the workpiece via through openings  38 ′, or, for example, is shifted by a compression spring. At the same time the compressed air above the working piston escapes. 
     Insofar as a plurality of weld positions are to be welded one after the other on a workpiece, such as for example in the operating mode “series spot welding”, then the workpiece can be moved beneath the welding electrode which is located in the position ready for use. The electrode can then be started for a new welding process. Since the working piston only needs to move over a small stroke in the working cylinder, and a more rapid pressure, build up can take place, a significantly increased step sequence is possible with this arrangement. 
     Although the principle of an electrode force unit in accordance with the invention can be realized in accordance with FIG. 1, changes are possible in order to improve the operation in accordance with the invention. Thus the prestroke piston rod  22 ′ can, for example, extend close to the lateral prestroke cylinder wall. Since the volume between the prestroke piston rod  22 ″ and the prestroke cylinder wall communicates with the working cylinder  18 , the working cylinder volume, which has to be filled, is further reduced and the follow-up movement behavior is improved. 
     A further arrangement in accordance with the invention is shown in FIG. 2 in which, for similar parts, the same reference numerals are used as in FIG. 1, but with a prefix  1 . 
     In accordance with FIG. 2 an electrode force unit  110  includes a prestroke arrangement  106  with a first housing  114   a  and a working piston-cylinder unit  108  connected after it and having a second housing  114   b  separate from the first. The housings  114   a  and  114   b  each comprise a cylinder cover, a cylinder jacket and a cylinder base. 
     A prestroke cylinder space  116  is provided in the housing  114   a  of the prestroke arrangement, and a prestroke piston  122 , which is sealingly slidable for the movement over large prestroke paths, along the axis of movement B—B is received in the prestroke cylinder space  116 . The prestroke piston  122  is secured to a piston rod  122 ′, which is led out of the housing  114   a  at both sides in the direction of the axis of movement B—B. 
     Pressure fluid through-ways  130 ,  130 ′ are provided in the housing  114   a  above and below the piston  122  and are selectively connected to a pressure fluid source, such as a compressed air source, via a pressure fluid changeover valve  132 . The diameter of the prestroke arrangement and the pressure of the fluid are so matched to one another that the required electrode force is achieved. 
     At the side (at the top of FIG. 2) of the prestroke piston rod  122 ′ remote from the welding electrode (not shown) there is provided an adjustable counter-nut arrangement  124   a  for the coarse restriction of the prestroke piston movement in the prestroke cylinder  116 . 
     The prestroke piston rod  122 ′ is provided with a bore which extends along the axis B—B, through which a fine adjustment spindle  124 , which can be locked by the locking element  124   b , is guided beyond the workpiece end (at the bottom in FIG. 2) of the piston rod  122 ′. The housing  114   b  of the working piston-cylinder unit is secured to the workpiece end of the fine adjustment spindle  124 . 
     In the housing  114   b  there is provided a cylinder space  118  for the reception of a working piston  134 , which can be slidingly and sealingly moved therein over a short stroke along the axis B—B. The working piston  134  is secured to a working piston rod  134 ′, which, for example, emerges through a rolling or sliding bearing in the direction of the workpiece. In the end position of the working piston  134  remote from the workpiece, i.e. in the position ready for use, there is provided a small air distribution gap  118 ′ between the working piston  134  and the working cylinder  118 . A connection plate  144  which connects the working piston  134  to a guide rod  142 ′ guided in a guide bore  142  is preferably mounted outside of the housing  114   b  to the working piston rod  134 ′. The guide rod  142 ′ prevents a rotation of the (not shown) electrode coupled to the connection plate  144 . 
     Compressed air supply and discharge lines  138  and  138 ′ are provided in the housing  114   b , which can optionally energize the working piston  134  from both directions with compressed air from a compressed air source via a compressed air changeover valve  140 . The diameter of the working piston  134  and a compressed air pressure are matched to one another for the required electrode force. The diameter of the working piston  134 , i.e. of the associated working cylinder space  118 , is preferably smaller than the diameter of the corresponding elements of the prestroke piston arrangement  106 . 
     It is important in this arrangement that with a given diameter of the cylinder space  118  the volume above and below the working piston  134  is kept as small as possible. This is achieved in that the maximum possible stroke of the working piston  134  is kept small. Thus, by way of example, provision can be made for the working piston  134  and the working cylinder  118  to have a stroke of only about 2 mm in the position ready for use, in the direction towards the workpiece, and to have an air distribution gap  118 ′ of approximately 1 mm available away from the workpiece, whereas, in comparison to this, the maximum possible prestroke can, for example, include the range from a few tens of millimeters to a few hundred millimeters. 
     The electrode force unit  110  of the invention in accordance with FIG. 2 is operated as follows: 
     First of all, the desired prestroke is roughly preset at the lock nut  124   a.    
     Then compressed air is applied via the pressure changeover valve  132  via the compressed air opening  130  remote from the workpiece in order to shift the welding electrode through a prestroke movement out of its starting position into the roughly set position ready for use, with the air which is still present beneath the prestroke piston  122  in the prestroke cylinder  116  being displaced through the openings  130 ′. 
     If the welding electrode is located in the roughly set position ready for use, then the desired position ready for use in which the welding electrode is located closely above the workpiece or gently contacts the workpiece can be precisely set at the fine adjustment spindle  124   b.    
     The fine adjustment spindle  124   b  can also be reset to the extent that with multiprojection welding, in particular round projection welding, such as of aluminum or its alloys, the projections can be intentionally cold deformed in the submillimeter range and contact the electrodes through the prestroke movement. 
     Subsequently, through corresponding control of the valve  140  which can be changed over, the working piston  134  can be loaded in the direction of the workpiece to be welded until the welding electrode presses against the workpiece. The filling and venting of the cylinder volume takes place in a very short time. 
     Subsequently, the welding current which is provided can be switched on. When the material to be welded thereby becomes soft, the electrode together with the working piston  134  execute a follow-up movement. 
     The large cross-sections of the working cylinder pressure supply and discharge lines and the low working cylinder volume enable a very precise follow-up guidance. The compressibility of the quantity of gas contained in the working cylinder  118 , which is only very small because of the small volume, also contributes to this. 
     After the welding has been terminated, and the post holding time has elapsed, the pressure valve  140  associated with the working piston-cylinder unit  108  is changed over and the double acting working piston  134  is pneumatically moved away from the workpiece. 
     Insofar as further welds are to be effected on the same workpiece, such as for example in the operating mode “series spot welding” the next weld position can be moved under the welding electrode in the position ready for use and the welding process is repeated. After the conclusion of the desired number of welds, the welding electrode is first retracted by a retraction stroke of the working piston into the position ready for use, and is then moved back into the starting position by a changeover of the prestroke valve  132 . 
     With the apparatus of the invention a rebound of the welding electrode during contact can be largely avoided. Moreover, the follow-up movement is improved to the extent that the round projection welding of aluminum and aluminum alloys is now also possible. Moreover, in the operating mode “series spot welding” a stroke sequence can be achieved which is almost an order of magnitude faster than in the arrangements in accordance with the prior art. 
     Whereas the housing  114   b  of the working piston-cylinder unit  108  is connected via the fine adjustment spindle  124  to the prestroke arrangement  106  in the illustrated electrode force unit  110 , other types of connection in series are also possible. Thus, for example, the working piston  134  can be connected to the prestroke arrangement  106 , with the welding electrode then being secured to the now moved housing  114   b.    
     Furthermore, one further or a plurality of such units can be connected after the illustrated working piston-cylinder unit  108  to increase the electrode force. Of importance in this respect is the fact that the stroke which is possible in a working piston-cylinder unit  108 , and thus also the corresponding volume, is kept as small as possible. 
     Whereas the electrode force unit  110  explained by way of example has a pneumatically actuated prestroke arrangement, it is also possible in place of this to, for example, hydraulically actuate the described working piston-cylinder unit or to mount a prestroke arrangement on a threaded element, which can be shifted on a spindle turned by an electric motor.