Patent Publication Number: US-RE36612-E

Title: Mash seam welding process and mash seam welding apparatus

Description:
FIELD OF THE ART 
     This invention relates to a mash seam welding process and a mash seam welding apparatus for continuously welding together two plate-like workpieces by overlapping the workpieces at their edges and applying a welding current between a pair of roller electrodes while the overlapping edge portions of the two plate-like workpieces are squeezed to be mashed, by and between the pair of roller electrodes. 
     BACKGROUND TECHNOLOGY 
     High-speed welding is possible by mash seam welding in which two plate-like workpieces are continuously welded together at their overlapping edge portions while the edge portions are squeezed to be mashed by and between a pair of roller electrodes, with a welding current being applied between the pair of roller electrodes. Since the mash seam welding is suitable for mass production, it is widely employed in production lines for steel plates, automobiles, etc. 
     In the mash seam welding described above, the two plate-like workpieces are positioned relative to each other by a clamping or guiding device that predetermined portions of the workpieces overlap each other, and the workpieces are fed so that the overlapping portions are squeezed by the pair of roller electrodes. Then, the two plate-like workpieces are continuously welded with a welding current applied between the roller electrodes while the overlapping portions of the workpieces are firmly squeezed and mashed by the roller electrodes. 
     In such mash seam welding, a so-called &#34;weld nugget&#34; N is formed based on a welding current applied to the pair of roller electrodes, such that the weld nugget N is located in a central part of the thickness of a portion of two plate-like workpieces 12 H , 12 M  squeezed by the pair of roller electrodes, as shown in FIG. 10. The formed weld nugget N extends across an interface G of the two plate-like workpieces 12 H , 12 M , so that the weld may be given a predetermined strength (tensile strength). In FIG. 10, W G  represents a width dimension of the interface G, while W N  represents a width dimension of the weld nugget N in the direction of the interface. The weld strength obtained by the mash seam welding depends upon a ratio W N  /W G , namely, a nugget ratio R N , which is the ratio of the width dimension W N  of the weld nugget N to the width dimension W G  of the interface G. The weld strength of the mash seal welding increases as this nugget ratio R N  increases. 
     Where two plate-like workpieces having different thickness values are welded according to a conventional mash seam welding process, the interface G of the two plate-like workpieces 12 H , 12 M  mashed by the pair of roller electrodes is formed at a position which is offset from the center of the thickness of the mashed portion in the direction toward the thin plate-like workpiece 12 M , as indicated in FIG. 11. However, the weld nugget N is formed at a position intermediate between the pair of roller electrodes, that is, at a central part of the thickness of the mashed portion of the two plate-like workpieces 12 H , 12 M  squeezed between the pair of roller electrodes. Accordingly, it is difficult to form the weld nugget N in alignment with the interface G of the two plate-like workpieces 12 H , 12 M , and is therefore difficult to obtain the desired weld strength in some cases. 
     While it is of course possible to increase the nugget ratio R N  indicated above by increasing the amount of the welding current to thereby form a large weld nugget N, an increase in the nugget ratio R N  leads to easy occurrence of cracking on the surfaces of the plate-like workpieces due to enlargement of the weld nugget N which undergoes rapid volumetric expansion. This cracking may develop into rupture in a subsequent press-forming process, for example. In view of this, the application of the mash seam welding is conventionally limited to the workpieces whose thickness ratio is not larger than a predetermined upper limit, for example, about 1.5, and the mash seam welding is not conventionally practiced where the thickness ratio of the workpieces exceeds the upper limit. 
     On the other hand, there has been proposed a seam welding process in which a shim is superposed on a thinner one of two plate-like workpieces so that the interface G of the two plate-like workpieces is located intermediate between a pair of roller electrodes, as disclosed in JP-A-63-63575. This seam welding process requires the shim as well as the two plate-like workpieces to be clamped by a clamping device adapted to position the two plate-like workpieces relative to each other. Therefore, this process has a drawback that the operation efficiency is inevitably lowered. Further, where the two plate-like workpieces are positioned relative to each other by using guide rollers while the workpieces are fed toward the roller electrodes, there is a drawback that the above seam welding process using the shim cannot be practiced. 
     The present invention was made in the light of the above background situation. It is therefore an object of the present invention to provide a mash seam welding process and a mash seam welding apparatus, which permit a sufficient weld strength even where two plate-like workpieces have different thicknesses. 
     DISCLOSURE OF THE INVENTION 
     The above object may be achieved according to the principle of the present invention, which provides a mash seam welding process continuously welding a first plate-like workpiece and a second plate-like workpiece which has a smaller thickness than the first plate-like workpiece and which overlaps the first plate-like workpiece, by applying a welding current between a pair of roller electrodes disposed rotatably about respective axes thereof, to thereby form a weld nugget at an interface of the two plate-like workpieces while the overlapping portions are squeezed by the pair of roller electrodes, wherein an amount of mashing of the first plate-like workpiece by one of the pair of roller electrodes is made larger than that of the second plate-like workpiece by the other of the pair of roller electrodes. 
     In the present process in which the amount of mashing of the first thick plate-like workpieces by the first plate-like workpieces is larger than that of the second thin plate-like workpiece, the position of the interface of the two plate-like workpieces is shifted to a position intermediate between the pair of roller electrodes, so that the weld nugget formed at the position intermediate between the pair of roller electrodes extends across the above-indicated interface, permitting a sufficient weld strength. 
     Preferably, a surface area of contact of the above-indicated one roller electrode with the first plate-like workpiece is made smaller than a surface area of contact of the other roller electrode with the second plate-like workpiece. This arrangement is advantageous in that the amount of mashing of the first thick plate-like workpiece is made larger than that of the second thin plate-like workpiece, when the pair of plate-like workpieces are squeezed and mashed by the pair of roller electrodes. 
     It is also preferable that wherein a length of contact of the above-indicated one roller electrode with the first plate-like workpiece in an axial direction of the pair of roller electrodes is made smaller than that of the other roller electrode with the second plate-like workpiece in the axial direction. This arrangement is also advantageous in that the amount of mashing of the first thick plate-like workpiece is made larger than that of the second thin plate-like workpiece, when the pair of plate-like workpieces are squeezed and mashed by the pair of roller electrodes. 
     It is also preferable that the above-indicated one roller electrode located on the side of the first plate-like workpiece has a smaller diameter than the other roller electrode located on the side of the second plate-like workpiece. This arrangement is advantageous in that the amount of mashing of the first thick plate-like workpiece is made larger than that of the second thin plate-like workpiece, even though the lengths of contact of the pair of roller electrodes with the first and second plate-like workpieces are the same. 
     It is also preferable that the pair of plate-like workpieces are fed at a speed of 20 m/min. or higher while the plate-like workpieces are squeezed by the above-indicated pair of roller electrodes. In this arrangement in which the welding speed is increased, the contact resistance of the pair of roller electrodes and the two plate-like workpieces is made higher than the specific resistance of the plate-like workpieces, so that the weld nugget formed within the two plate-like workpieces is formed along the interface, with a result of an increase in the nugget ratio, permitting the weld to have a sufficient strength even where the pair of roller electrodes have different surface areas of contact with the two plate-like workpieces having different thickness values. 
     The above object may also be achieved according to another aspect of the invention, which provides a mash seam welding process of continuously welding a first plate-like workpiece and a second plate-like workpiece which has a smaller thickness than the first plate-like workpiece and which overlaps the first plate-like workpiece, while overlapping portions of the two plate-like workpieces are squeezed a pair of roller electrodes disposed rotatably about respective two parallel axes thereof, the process comprising: a welding step of continuously welding the two plate-like workpieces by applying a welding current between a pair of roller electrodes, to thereby form a weld nugget at an interface of the two plate-like workpieces, while the two plate-like workpieces are squeezed by the pair of roller electrodes; and a preliminary treatment step of reducing the thickness of a welding portion of the first plate-like workpiece, prior to the welding step. In this arrangement, the thickness of the welding portion of the first thick plate-like workpiece is first reduced, so as to reduce the thickness difference with respect to the welding portion of the second thin plate-like workpiece, so that the weld nugget formed at the position intermediate between the pair of roller electrodes extends across the interface of the two plate-like workpieces, permitting the weld to have a sufficient strength. 
     The above process of the present invention may be suitably practiced by a mash seam welding apparatus for continuously welding a first plate-like workpiece and a second plate-like workpiece which has a smaller thickness than the first plate-like workpiece and which overlaps the first plate-like workpiece, by applying a welding current between a pair of roller electrodes disposed rotatably about respective axes thereof, to thereby form a weld nugget at an interface of the two plate-like workpieces while the overlapping portions are squeezed by the pair of roller electrodes, the apparatus comprising: (a) a pair of roller electrodes disposed rotatably about respective two parallel axes thereof and biased toward each other, the pair of roller electrodes having a same diameter; and (b) a guiding device for guiding the two plate-like workpieces such that the two plate-like workpieces overlap each other with a predetermined lap amount, while the two plate-like workpieces are fed toward the pair of roller electrodes, so that the two plate-like workpieces are squeezed by the pair of roller electrodes such that a surface area of contact of one of the pair of roller electrodes with the first plate-like workpiece is smaller than a surface area of contact of the other of the pair of roller electrodes with the second plate-like workpiece. 
     In the present apparatus wherein the surface area of contact of one of the above-indicated pair of roller electrodes with the first plate-like workpiece is made smaller than that of the other roller electrode with the second plate-like workpiece, the amount of mashing of the first thick plate-like workpiece is made larger than that of the second thin plate-like workpiece when the pair of plate-like workpieces are squeezed and mashed by the pair of roller electrodes. Accordingly, the position of the interface of the two plate-like workpieces is shifted to a position intermediate between the pair of roller electrodes, so that the weld nugget formed at the position intermediate between the pair of roller electrodes extends across the above-indicated interface, permitting a sufficient weld strength. 
     Preferably, the guiding device guides the two plate-like workpieces such that a length of contact of the one roller electrode with the first plate-like workpiece in an axial direction of the pair of roller electrodes is smaller than that of the other roller electrode with the second plate-like workpiece in the axial direction. This arrangement is advantageous in that the amount of mashing of the first thick plate-like workpiece is made larger than that of the second thin plate-like workpiece when the pair of plate-like workpieces are squeezed and mashed by the pair of roller electrodes. 
     In one arrangement of the above preferred form of the apparatus, the pair of roller electrodes have respective outer circumferential contact surfaces and are disposed such that widthwise centerlines of the outer circumferential contact surfaces are substantially aligned with each other, and the guiding device guides the pair of plate-like workpieces such that an edge of the first plate-like workpiece is located between a widthwise end of the outer circumferential contact surface of the one roller electrode on the side of the first plate-like workpiece and the widthwise centerline of the outer circumferential contact surface of the one roller electrode, while an edge of the second plate-like workpiece is located at an widthwise end of the outer circumferential contact surface of the other roller electrode on the side of the first plate-like workpiece. 
     In another arrangement of the above preferred form of the apparatus, the pair of roller electrodes have respective outer circumferential contact surfaces and are disposed such that widthwise centerlines of the outer circumferential contact surfaces are offset from each other by a predetermined distance in an axial direction thereof, and wherein the guiding device guides the two plate-like workpieces such that a centerline of the overlapping edge portions of the two plate-like workpieces is substantially aligned with the widthwise centerline of the outer circumferential contact surface of the one of the pair of roller electrodes which is located on the side of the plate-like workpiece having a smaller thickness. 
     In a further arrangement of the above preferred form of the apparatus, the roller electrode which is located on the side of the plate-like workpiece having a larger thickness has an outer circumferential contact surface having a width dimension smaller than an outer circumferential contact surface of the other of the pair of roller electrodes which is located on the side of the plate-like workpiece having a smaller thickness, the pair of roller electrodes being disposed such that widthwise centerlines of the outer circumferential contact surfaces are substantially aligned with each other, and wherein the guiding device guides the pair of plate-like workpieces such that a centerline of the overlapping edge portions of the two plate-like workpieces is substantially aligned with the widthwise centerlines of the outer circumferential contact surfaces of the roller electrode. 
     It is also preferable that the one of the pair of roller electrodes located on the side of the first plate-like workpiece has a smaller diameter than the other roller electrode located on the side of the second plate-like workpiece. This arrangement is advantageous in that the amount of mashing of the first thick plate-like workpiece is made larger than that of the second thin plate-like workpiece, even where the guiding device is adapted to guide the pair of plate-like workpieces such that the lengths of contact of the pair of roller electrodes with the first and second plate-like workpieces in the axial direction are equal to each other. 
     It is also preferable that the apparatus further comprises a feeding device for feeding the pair of plate-like workpieces at a speed of 20 m/min. or higher while the pair of plate-like workpieces are squeezed by the pair of roller electrodes. In this arrangement, the feeding device for feeding the plate-like workpieces permits a high welding speed, and causes a higher resistance of contact of the pair of roller electrodes with the two plate-like workpieces than the specific resistance of the plate-like workpieces, whereby the nugget ratio of the weld nugget formed along the interface of the two plate-like workpieces is increased, advantageously permitting a sufficient weld strength. 
     The above object may also be achieved according to a further aspect of the invention, which provides a mash seam welding apparatus for continuously welding a first plate-like workpiece and a second plate-like workpiece which has a smaller thickness than the first plate-like workpiece and which overlaps the first plate-like workpiece, while the overlapping portions of the two plate-like workpieces are squeezed by a pair of roller electrodes disposed rotatably about respective two parallel axes thereof, characterized by comprising: a work feeding device for feeding the two plate-like workpieces in one direction; a welding portion for continuously welding the two plate-like workpieces by applying a welding current between a pair of roller electrodes, to thereby form a weld nugget at an interface of the two plate-like workpieces, while the two plate-like workpieces are fed by the work feeding device and squeezed by the pair of roller electrodes; and a preliminary treatment device disposed upstream of the welding portion, for reducing the thickness of a welding portion of the first plate-like workpiece. In this arrangement wherein the thickness of the welding portion of the first thick plate-like workpiece is reduced prior to the welding by the pair of roller electrodes, the thickness difference of the welding portion of the first thick plate-like workpiece with respect to that of the second thin plate-like workpiece is reduced, so that the weld nugget formed intermediate between the pair of roller electrodes extends across the interface of the two plate-like workpieces, advantageously permitting a sufficient weld strength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view schematically showing a manner of feeding plate-like workpieces to explain a mash seam welding process of the present invention; 
     FIG. 2 is a plan view explaining an arrangement of guide rollers and retainer rollers used for feeding the plate-like workpieces, to explain the mash seam welding process according to the embodiment of FIG. 1; 
     FIG. 3 is a view showing portions of guide members by which the plate-like workpieces are slidably guided, to explain a workpiece guiding step in the embodiment of FIG. 1; 
     FIG. 4 is a view explaining a positional relationship between a pair of roller electrodes and the edge portions of the pair of plate-like workpieces pinched by the roller electrodes in a welding step; 
     FIG. 5 is a view explaining the position of a weld nugget formed in the welding step in the embodiment of FIG. 1; 
     FIG. 6 is view in cross section showing the guide rollers and their vicinity of a seam welding apparatus suitable for practicing the seam welding process of FIG. 1, taken in a plane perpendicular to a feeding direction of the plate-like workpieces; 
     FIG. 7 is a view showing in enlargement a portion of the seam welding apparatus of FIG. 6; 
     FIG. 8 is a view showing the roller electrodes and their vicinity of the seam welding apparatus of FIG. 6, taken in the plane perpendicular to the feeding direction of the plate-like workpieces; 
     FIG. 9 is a partly cut-away view explaining the roller electrodes and their vicinity of the seam welding apparatus of FIG. 6; 
     FIG. 10 is a view for explaining a definition of a nugget ratio; 
     FIG. 11 is a view explaining the position of a weld nugget generated in a conventional mash seam welding process; 
     FIG. 12 is a view corresponding to that of FIG. 4, showing another embodiment of this invention; 
     FIG. 13 is a view corresponding to that of FIG. 4, showing another embodiment of this invention; 
     FIG. 14 is a side elevational view explaining diameters of a pair of roller electrodes in another embodiment of this invention; 
     FIG. 15 is a view corresponding to that of FIG. 4, showing another embodiment of this invention; 
     FIG. 16 is a view explaining the position at which a weld nugget is formed in the embodiment of FIG. 15; 
     FIG. 17 is a view and a table indicating results of experiments conducted to confirm a range of welding speed in which the embodiment of FIG. 15 is effective; 
     FIG. 18 is a view explaining the position at which a weld nugget is formed when the embodiment of FIGS. 1-9 and the embodiment of FIG. 15 are combined; and 
     FIG. 19 is a perspective view explaining a preliminary treatment or a preliminary treatment device in another embodiment of this invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     There will be described embodiments of this invention in detail referring to the drawings. FIG. 1 through FIG. 5 are views explaining a seam welding process to be practiced according to this invention, while FIG. 6 through FIG. 9 are views explaining a seam welding apparatus 10 adapted to practice the seam welding process. 
     In FIGS. 1 and 2, there are shown a pair of left and right plate-like workpieces 12 H , 12 M  which are separate plates such as zinc steel plates having given shapes. The plate-like workpiece 12 H  has a larger thickness than the plate-like workpiece 12 M . These plate-like workpieces 12 H , 12 M  are fed in a predetermined feeding direction B by a plurality of sets of guide rollers 14 (Feeding Step), while the two plate-like workpieces 12 H , 12 M  are guided and positioned relative to each other by a pair of upper and lower plate-like guide members 18 U , 18 S  in sliding contact with guide surfaces 24 U , 24 S  such that edge portions of the plate-like workpieces 12 H , 12 M  overlap each other with a predetermined lap amount (lap width) S (Guiding Step). The thus positioned two plate-like workpieces 12 H , 12 M  are squeezed under pressure at their overlapping edge portions, by a pair of upper and lower roller electrodes 20 U , 20 S . In this condition, an electric current supplied from a power source is applied between the pair of roller electrodes 20 U , 20 S , whereby the two plate-like workpieces 12 H , 12 M  are welded together at their overlapping edge portions, along a welding centerline A which is parallel to the feeding direction B and which passes a center of the lap width S of the overlapping edge portions (Welding Step). A set of retainer rollers 16 are disposed on opposite sides of the roller electrodes 20 U , 20 S , for holding the pair of plate-like workpieces 12 H , 12 M  under welding, with a predetermined squeezing force in the direction of thickness of the workpieces, so as to prevent movement of the workpieces away from each other (Holding Step). 
     Each of the above-indicated sets of guide rollers 14 consists of a pair of upper and lower guide rollers 14 HU , 14 HS  disposed to pinch the plate-like workpiece 12 H  with a predetermined pinching force in the direction of thickness of the workpiece, and another pair of upper and lower guide rollers 14 MU , 14 MS  disposed to pinch the other plate-like workpiece 12 M  with a predetermined pinching force in the direction of thickness of the workpieces. The sets of guide rollers 14 are arranged in the feeding direction B, and include at least one set of guide rollers 14 which are positively driven. Each guide roller 14 HU , 14 HS , 14 MU , 14 MS  of each set is supported rotatably about a horizontal axis C1 which lies in a plane inclined a suitable angle θ with respect to a plane perpendicular to the welding centerline A and which is located above or below the welding centerline A. Each pair of upper and lower guide rollers 14 HU  and 14 HS , or 14 MU  and 14 MS  is given a suitable pinching force by a cylinder device 56 H , 56 M  described below, for example, such that the pinching force acts on the upper and lower guide rollers in a direction toward each other. The pinching force is determined so as to generate a friction force enough to permit the upper and lower rollers to generate a force consisting of a component for feeding the plate-like workpiece 12 H , 12 M  in the feeding direction B and component for moving the workpiece in the direction toward the guide members 18 U , 18 S . 
     The above-indicated set of retainer rollers 16 consists of a pair of upper and lower retainer rollers 16 HU , 16 HS  which are disposed on one side of the roller electrodes 20 U , 20 S  to hold the plate-like workpiece 12 H  with a predetermined squeezing force in the direction of thickness of the workpiece, and another pair of upper and lower retainer rollers 16 MU , 16 MS  disposed on the other side of the roller electrodes 20 U , 20 S  to hold the other plate-like workpieces 12 M  with a predetermined squeezing force in the direction of thickness of the workpiece. Thus, the two pairs of retainer rollers 16 are disposed such that the roller electrodes 20 U , 20 S  are interposed between the retainer rollers 16. Each pair of upper and lower retainer rollers 16 HU  and 16 HS , or 16 MU  and 16 MS  is given a comparatively large squeezing force by a cylinder device similar to the cylinder device 56 H , 56 M  described below, for example, such that the squeezing force acts on the upper and lower retainer rollers in a direction toward each other. The squeezing force is determined so as to prevent the edge portions of the pair of plate-like workpieces 12H, 12M under welding from moving away from each other. Each retainer roller 16 HU , 16 HS , 16 MU , 16 MS  is supported rotatably about a horizontal axis C2 which lies in a plane perpendicular to the welding centerline A and which is located above or below the welding centerline A. The plate-like workpieces 12 H , 12 M  under feeding, namely, under welding are retained by the above-indicated squeezing force to prevent the movements of their edge portions away from each other (Holding Step). 
     Each of the above-indicated pair of roller electrodes 20 U , 20 S  is supported rotatably about a horizontal axis C3 which lies in a plane perpendicular to the welding centerline A and which is located above or below the welding centerline A. The pair of roller electrodes 20 U , 20 S  is given a comparatively large squeezing force by a cylinder device by a cylinder device 80 described below, for example, such that the squeezing force acts on the roller electrodes in a direction toward each other. The squeezing force is determined so as to permit the overlapping edge portions of the plate-like workpieces 12 H , 12 M  to be mashed by an amount enough to reduce the overall lap thickness to the approximate thickness of one of the workpieces 12 H , 12 M . The squeezing portion of the roller electrodes 20 U , 20 S  is indicated at P in FIG. 2. 
     As shown in detail in FIG. 3, the above-indicated plate-like guide members 18U, 18S are disposed with a predetermined amount of gap left between mutually opposed end faces thereof, so as to extend along the welding centerline A and between the pair of upper and lower guide rollers 14 HU , 14 HS  for holding the one plate-like workpiece 12 H , and the pair of upper lower guide rollers 14MU, 14MS for holding the other plate-like workpiece 12M, such that the guide members 18U, 18S are spaced apart by a predetermined small distance from the pair of roller electrodes 20 U , 20 S . The upper guide member 18 U  has a projecting rail 26 U  formed at a portion of its end face on the side of the plate-like workpiece 12 M . The projecting rail 26 U  has a guide surface 24 U  for sliding contact with the end face of the plate-like workpiece 12 H . The lower guide member 18 S  has a projecting rail 26 S  formed at a portion of its end face on the side of the plate-like workpiece 12 H . The projecting rail 26 S  has a guide surface 24 S  for sliding contact with the end face of the plate-like workpiece 12 M . 
     As shown in FIG. 2, the rotation axis C1 of at least one set (all sets in this embodiment) of the guide rollers 14 disposed on the left and right sides of the guide members 18 U , 18 S  is inclined a predetermined small angle θ (1°-3° in this embodiment) with respect to the line perpendicular to the welding centerline A, so that the plate-like workpieces 12 H , 12 M  are driven or guided by the guide roller 14 in a direction (in the direction of the driving force F R  described below) which is inclined inwardly by the predetermined angle θ with respect to the feeding direction B. 
     When the plate-like workpiece 12 M  is in abutting contact with the guide surface 24 S , for example, the plate-like workpiece 12 M  is fed while it is pressed toward the guide surface 24 S . Similarly, the plate-like workpiece 12 H  is fed while it is pressed toward the guide surface 24 U . Thus, the pair of plate-like workpieces 12 H , 12 M  are positioned relative to each other with their edge portions overlapping over the predetermined lap width S, before the workpieces are squeezed by the upper and lower roller electrodes 20 U , 20 S . In this arrangement, the pair of plate-like workpieces 12 H , 12 M  are automatically positioned relative to each other during feeding thereof through a feeding path, even if the workpieces do not have the predetermined relative position when they are introduced into the feeding path. The above-indicated guide members 18 U , 18 S  are adapted to guide the plate-like workpieces 12 H , 12 M  such that the edge portions of the plate-like workpieces 12 H , 12 M  squeezed between the roller electrodes 20 U , 20 S  are positioned as indicated in FIG. 4. Namely, the guide members 18 U , 18 S  are disposed at the appropriate positions for positioning the workpieces as indicated in FIG. 4. In the guiding step in the present embodiment, the pair of plate-like workpieces 12 H , 12 M  are guided and positioned relative to each other by the above-indicated guide members 18 U , 18 S  and the guide rollers 14 disposed on the left and right sides of the guide members 18 U , 18 S , so that the edge portions of these plate-like workpieces 12 H , 12 M  may be squeezed by the roller electrodes 20 U , 20 S , as indicated in FIG. 4. In the present embodiment, the above guide members 18 U , 18 S  and the guide rollers 14 disposed on the left and right sides of the guide members 18 U , 18 S  function as a guiding device. 
     FIG. 4 shows the overlapping edge portions of the thick plate-like workpiece 12 H  and the plate-like workpiece 12 M  having a thickness smaller than the plate-like workpiece 12 H , which overlapping edge portions are positioned between the roller electrodes 20 U , 20 S  which have respective outer circumferential contact surfaces 28 U , 28 S  and which are disposed such that widthwise centerlines CL U , CL S  of the outer circumferential contact surfaces 28 U , 28 S  are substantially aligned with each other. The edge of the thick plate-like workpiece 12 H  is located between the widthwise end of the outer circumferential contact surface 28U of the one roller electrode 20 U  on the side of the plate-like workpiece 12 H  and the centerline CL U , while the edge of the thin plate-like workpiece 12 M  is located at the widthwise end of the outer circumferential contact surface 28 S  of the other roller electrode 20 S  on the side of the plate-like workpiece 12 H . That is, a length W H  of contact of the one roller electrode 20 U  with the plate-like workpiece 12 H  is smaller than a length W M  of contact of the other roller electrode 20 S  with the plate-like workpiece 12 M . When the edge portions of the plate-like workpiece 12 H  and the plate-like workpiece 12 S  whose thickness is smaller than the plate-like workpiece 12 H  are squeezed between the roller electrodes 20 U , 20 HS  while these edge portions overlap each other as shown in FIG. 4, the amount of mashing of the thick plate-like workpiece 12 H  is larger than that of the thin plate-like workpiece 12 M , since the contact surface area of the roller electrode 20 U  with respect to the plate-like workpiece 12 H  is smaller than the contact surface area of the roller electrode 20 S  with respect to the plate-like workpiece 12 S . Consequently, the interface G of the two plate-like workpieces 12 H , 12 M  is formed at a position intermediate between the pair of roller electrodes 20 U , 20 S , whereby the weld nugget N formed is more likely to extend across the above-indicated interface G. Accordingly, the present arrangement results in an increase in the nugget ratio R N  which is the ratio W N  /W G  (ratio of the width W N  of the weld nugget N in the direction of the interface G to the width dimension W G  of the interface G), so that the weld can be given a sufficient strength. 
     The mash seam welding apparatus 10 for practicing the mash seam welding process explained above is constructed as shown in FIGS. 6-9. FIG. 6 is a cross sectional view perpendicular to the feeding direction, showing the guide rollers 14 and their vicinity of the mash seam welding apparatus 10. FIG. 7 is a view showing in enlargement the guide rollers 14 and their vicinity. FIG. 8 is a view explaining an arrangement of the roller electrodes 20 U , 20 S  and their vicinity, while FIG. 9 is a partly cut-away view explaining the support structure for the roller electrode 20 U . 
     Since the guide rollers 14 and the retainer rollers 16 are basically identical in construction to each other, the construction of the guide rollers 14 and the support structure thereof will be described by way of example. 
     Referring to FIGS. 6 and 7, a base frame 50 extends in the welding direction B, and a pair of posts 52 H , 52 M  are disposed upright on the base frame 50. On the upper end faces of the posts 52 H , 52 M , there are provided the guide rollers 14 HS , 14 MS  via base blocks 53 H , 53 M . A top frame 55 is fixed to the base frame 50 through an outer frame 54. To the top frame 55, there are fixed a pair of left and right cylinder devices 56 H , 56 M  for giving the pinching forces to the pair of upper and lower guide rollers 14 HU , 14 HS  and the pair of upper and lower guide rollers 14 MH , 14 MS , respectively. Movable rods 58 H , 58 M  extend downward from the cylinder devices 56 H , 56 M , such that the movable rods 58 H , 58 S  are movable. To the lower ends of the movable rods 58 H , 58 M , there are fixed movable plates 60 H , 60 M  on which the guide rollers 14 HU , 14 MU . 
     The set of guide rollers 14 HS , 14 MS , 14 HU , 14 MU  are provided with respective integrally formed shafts 66 which are supported by respective shaft cases 62 HS , 62 MS , 62 HU , 64 MU  through bearings 64. The above-indicated shaft cases 62 HS , 62 MS  are fixed to the upper end faces of the posts 52 H , 52 M , while the above-indicated shaft cases 62 HU , 62 MU  are fixed to the movable plates 60 H , 60 M . The guide rollers 14 HS , 14 MS  are rotatable about the axis C1 located below the welding centerline A while the guide rollers 14 HU , 14 MU  are rotatable about the axis C, located above the welding centerline A. As shown in FIG. 3, the guide members 18 U , 18 S  are fixed to the cylinder device 56 H  and the post 52 M , respectively, such that the guide members 18 U , 18 S  are disposed with the predetermined amount of gap left between their mutually opposed end faces, and extend between the pair of upper and lower guide rollers 14 HU , 14 HS  for holding the one plate-like workpiece 12 H , and the pair of upper lower guide rollers 14 MU , 14 MS  for holding the other plate-like workpiece 12 M , such that the welding centerline A is aligned with the widthwise centers of the opposed end faces of the guide members 18U, 18S. 
     Each of the shafts 66 of the lower guide rollers 14 HS , 14 MS  is connected to a drive motor 76 through a pair of joints 68, 70, an intermediate shaft 72, and a chain 74. The intermediate shaft 72 is provided for each of the left and right guide rollers 14 HS , 14 MS  of each set of guide rollers 14, and these intermediate shafts 72 are connected to each other by chains and to the drive motor 76, for simultaneous rotation at the same speed. In this arrangement, the lower rollers 14 HS , 14 MS  of the guide rollers 16 are rotated by the drive motor 76 to feed the pair of plate-like workpieces 14HS, 14MS. In the present embodiment, the drive motors 76 function as a driving device for rotating the guide rollers 14. Similar drive motors are used as a driving device for rotating the retainer rollers 16. 
     The pairs of upper and lower guide rollers 14 HU , 14 HS , 14 MU , 14 MS  are steel rollers whose outer circumferential surfaces are covered by an elastic material such as urethane rubber. The pair of guide rollers 14 HU , 14 HS  and the pair of guide rollers 14 MU , 14 MS  are squeezed toward each other by the respective cylinder devices 56 H , 56 M  which function as a squeezing force applying device, so that the squeezing forces acting on the guide rollers permit the generation of friction forces enough to generate the component for feeding the pair of plate-like workpieces 12 H , 12 M  and the component for forcing the workpieces toward the guide members 18 U , 18 S . 
     The pair of upper and lower retainer rollers 16 HU , 16 HS  and the pair of upper and lower retainer rollers 16 MU , 16 MS  which constitute one set of retainer rollers 16 are constructed similarly to the guide rollers 14 HU , 14 HS , 16 MU , 14 MS , as shown in FIG. 8. The retainer rollers 16 are rotatable about the respective rotation axes C2 described above, and are rotated by the drive motors 76 at the same peripheral speed as the guide rollers 14 HS , 14 MS . The pair of upper and lower retainer rollers 16 HU , 16 HS  and the pair of upper and lower retainer rollers 16 MU , 17 MS  are steel rollers whose outer circumferential surfaces are knurled to increase the generated friction forces in their axial direction, with respect to the plate-like workpieces 12 H , 12 M . The retainer rollers 16 HS , 16 HU  and the retainer rollers 16 MS , 16 MU  are squeezed toward each other by cylinder devices similar to the cylinder devices 56 H , 56 M , which also function as a squeezing force applying device. The squeezing forces given to the retainer rollers 16 are comparatively large permitting the generation of friction forces on their outer circumferential surfaces, so as to prevent the movements of the edge portions of the pair of plate-like workpieces 12 H , 12 M  under welding in the directions away from each other. 
     Auxiliary roller sets 130 HU , 130 HS , 130 MU , 130 MS  are interposed between the roller electrodes 20 U , 20 S  and the pair of retainer rollers 16 HU , 16 HS , and between the roller electrodes 20 U , 20 S  and the pair of retainer rollers 16 MU , 16 MS , in the direction perpendicular to the feeding direction B, that is, in the axial direction of the roller electrodes. These auxiliary roller sets 130 HU , 130 HS , 130 MU , 130 MS  are located so as to press the portions of the workpieces at which the convex deformation F is expected to take place. Described more specifically, the auxiliary roller sets 130 HS  and 130 MS  located below the plate-like workpieces 12 H , 12 M  are rotatably supported by the end portions of brackets 132 HS  and 132 MS  fixed to the base blocks 53 H , 53 M , while the auxiliary roller sets 130 HU  and 130 MU  located above the plate-like workpieces 12 H , 12 M  are rotatably supported by the end portions of brackets 132 HU  and 132 MU  fixed to the movable plates 60 H , 60 M . The above auxiliary roller sets 130 HU , 130 HS , 130 MU , 130 MS  are rotatable about axes C4 which are parallel to the rotation axes C3 of the roller electrodes 20 U , 20 S  and the axes C2 of the pair of retainer rollers 16 HU , 16 HS . The plate-like workpieces 12 H , 12 M  are pressed with predetermined squeezing forces at their portions laterally adjacent to the roller electrodes 20 U , 20 M , by the upper and lower auxiliary roller sets 130 HU , 130 HS  and the upper and lower auxiliary roller sets 130 MU , 130 MS . 
     The pair of roller electrodes 20 U , 20 S  have the same diameter, width and construction, as shown in FIG. 8, and only the upper roller electrode 20 U  will be explained by reference to FIG. 9, which is a partly cut-away view showing the upper roller electrode 20 U  and the support structure of this electrode. As shown in FIG. 9, a cylinder device 80 U  is fixed to the above-described top frame 55, for giving the roller electrodes 20 U , 20 S  a squeezing force. The cylinder device 80 U  is provided with movable rods 82 U  extending downward such that the movable rods 82 U  are movable. A movable plate 84 is fixed to the ends of the movable rods 82U, and a support block 86 for rotatably supporting the roller electrode 20 U  is fixed to the movable plate 84. 
     The roller electrode 20 U  comprised of a rotary shaft 90 U  with a flange portion 88 U  integrally formed at an axially intermediate part thereof, and a circular electrode disk 96 U  fastened by a screw 94 U  to the flange portion 88 U  together with a pressure plate 92 U . The rotary shaft 90 U  is fitted at its opposite end portions through power supply bushings 98 U , in a through-hole 100 U  formed through the above-indicated support block 86 U , whereby the rotary shaft 90 U  is rotatably supported by the support block 86 U . The above electrode disk 96 U  is a highly electrically conductive body formed of a metallic material such as chromium steel, beryllium copper alloy or chromium zirconium copper alloy, which is highly durable with a small amount of wear or damage even in the presence of a comparatively large electric current applied thereto while the metallic material is in contact with the plate-like workpieces 12 H , 12 M . The rotary shaft 90 U , pressure plate 92 U , power supply bushings 98 U  and support blocks 86 U  are highly electrically conductive bodies formed of a metallic material such as copper alloy or aluminum alloy. The electric current is supplied from the above-indicated power source 22 to the electrode disk 96 U  through electric wires not shown, and through the power supply bushings 98, rotary shaft 90 U , etc. 
     A fluid passage 102 U  is formed through the above-indicated rotary shaft 90 U  such that the fluid passage 102 U  is open in the opposite end faces of the rotary shaft 90 U  and has an intermediate portion in the form of a circular recess formed in the surface of the flange portion 88 U  which contacts the electrode disk 96 U . A pair of hose joints 108 U , 110 U  are fixed to the end faces of the rotary shaft 90 U , for connecting the fluid passage 102 U  to coolant fluid hoses 104 U , 106 U  while permitting the rotary shaft 90U to rotate. 
     In the mash seam welding apparatus 10 constructed as described above, the plurality of sets of guide rollers 14 and the one set of retainer rollers 16 are continuously rotated by the drive motors 76, so that the pair of plate-like workpieces 12 H , 12 M  are fed and positioned relative to each other by the guide rollers 14 and the guide members 18 U , 18 S  such that the edge portions of the workpieces overlap each other over the predetermined lap width S. Then, the workpieces are squeezed and mashed by the pair of roller electrodes 20 U , 20 S , and a welding current is simultaneously applied between the roller electrodes 20 U , 20 S , whereby the workpieces are welded at their edge portions as shown in FIG. 5, for example, as described above. 
     In the present embodiment, the pair of plate-like workpieces 12 H , 12 M  are guided in the guiding step by the guiding device (guide rollers 14 and guide members 18 U , 18 S ), in a manner as shown in FIG. 4, so that the workpieces are introduced between the pair of roller electrodes 20 U , 20 S . Namely, the pair of plate-like workpieces 12 H , 12 M  are guided during feeding thereof such that the length W H  of contact of the one roller electrode 20 U  with the plate-like workpiece 12 H  is smaller than the length W M  of contact of the other roller electrode 20 S  with the plate-like workpiece 12 M , in other words, such that the area of contact of the thick plate-like workpiece 12 H  with the roller electrode 20 U  is smaller than the area of contact of the thin plate-like workpiece 12 M  with the roller electrode 20 S . In this arrangement, the amount of mashing of the plate-like workpiece 12 H  by the one roller electrode 20 U  is larger than that of the plate-like workpiece 12 M  by the other roller electrode 20 S . Consequently, the interface G of the two plate-like workpieces 12 H , 12 M  is formed at a position intermediate between the pair of roller electrodes 20 U , 20 S , whereby the weld nugget N formed is more likely to extend across the above-indicated interface G, as shown in FIG. 5. Accordingly, the present arrangement results in an increase in the nugget ratio R N  which is the ratio W N  /W G  (ratio of the width W N  of the weld nugget N in the direction of the interface G to the width dimension W G  of the interface G), so that the weld can be given a sufficient strength. 
     The present inventors conducted an experiment in which a mash seam welding operation was performed in the following condition, for welding the plate-like workpiece 12 H  in the form of a 1.4 mm-thick zinc steel plate and the plate-like workpiece 12 M  in the form of a 0.7 mm-thick zinc steel plate: lap width S of the overlapping edge portions of the workpieces 12 H , 12 M  =1.0 mm; offset distance L 1  of the welding centerline A from the widthwise centerlines CL U , CL S  of the outer circumferential contact surfaces 28 U , 28 S  of the roller electrodes 20 U , 20 S  =2.0 mm; squeezing force acting on the upper and lower roller electrodes 20 U , 20 S  =9800N (=1000 kgf); welding current=27 kA; and welding speed =20 m/min. In the experiment, the nugget ratio R N  was 50%, and the weld obtained had a sufficient strength. 
     It is noted that a guiding device in a conventional mash seam welding apparatus is adapted to guide the plate-like workpieces 12 H , 12 M  such that the surface area of contact of the roller electrode 20 U  with the plate-like workpiece 12 H  is equal to that of the roller electrode 20 S  with the plate-like workpiece 12 M . Although the conventional apparatus permits the weld nugget N to be formed at a position intermediate between the pair of roller electrodes 20 U , 20 S , the position of the interface G of the two plate-like workpieces 12 H , 12 M  is offset from the weld weld nugget N toward the thin plate-like workpiece 12 M , as indicated in FIG. 11, whereby the nugget ratio R N  is as low as 20%, making it impossible to provide a sufficiently large weld strength. 
     There will next be described other embodiments of this invention. In the following embodiments, the reference numerals used in the preceding embodiment will be used to identify the corresponding elements, and no description of these elements will be provided. 
     FIGS. 12 and 13 show the pair of plate-like workpieces 12 H , 12 M  which are guided by guiding devices (guide rollers 14 and guide members 18 U , 18 S ) and squeezed by the roller electrodes 20 U , 20 S , in other embodiments of this invention. 
     In the embodiment of FIG. 12, the pair of roller electrodes 20 U , 20 S  have the respective outer circumferential contact surfaces 28 U , 28 S  which have the same width dimension. However, the roller electrodes 20 U , 20 S  are disposed such that the widthwise centerlines CL U , CL S  of the outer circumferential contact surfaces 28 U , 28 S  are offset from each other by a predetermined distance L 2  in the axial direction. Further, the guiding device (guide rollers 14 and guide members 18 U , 18 S ) is constructed so as to guide the pair of plate-like workpieces 12 H , 12 M  such that the centerline of the lap width S of the overlapping edge portion of the workpieces 12 H , 12 M , that is, the welding centerline A is substantially aligned with the widthwise centerline CL S  of the roller electrode 20 S  located on the side of the thin plate-like workpiece 12 M . 
     In the embodiment of FIG. 13, the the outer circumferential surface 28 U  of the roller electrode 20 U  on the side of the thick plate-like workpiece 12 H  has a smaller width dimension than the outer circumferential contact surface 28 S  of the roller electrode 20 S  on the side of the thin plate-like workpiece 12 M . The pair of roller electrodes 20 U , 20 S  are disposed such that the widthwise centerlines CL U , CL S  of those outer circumferential contact surfaces 28 U , 28 S  are substantially aligned with each other in the axial direction. Further, the guiding device (guide rollers 14 and guide members 18 U , 18 S ) is constructed so as to guide the two plate-like workpieces 12 H , 12 M  such that the centerline of the lap width S of the overlapping edge portion of the workpieces 12 H , 12 M , that is, the welding centerline A is substantially aligned with the widthwise centerlines CL U , CL S  of the roller electrodes 20 S , 20 S . 
     In both of the embodiments of FIGS. 12 and 13, the surface area of contact of the roller electrode 20 U  with the thick plate-like workpiece 12 H  is made smaller than that of the roller electrode 20 S  with the thin plate-like workpiece 12 M . When the pair of plate-like .[.rollers.]. .Iadd.workpieces .Iaddend.12 H , .[.12 S  .]. .Iadd.12 M  .Iaddend.are squeezed by the pair of roller electrodes 20 U , 20 S , the amount of mashing of the thick plate-like workpiece 12 H  by the roller electrode 20 U  is made larger than the amount of mashing of the thin plate-like workpiece 12 M  by the roller electrode 20 S . Therefore, the interface G of the two plate-like workpieces 12 H  .[.,.]. .Iadd.and .Iaddend.12 M  is formed at a position intermediate between the pair of roller electrodes, and the nugget ratio R N  is increased, making it possible to provide a sufficient weld strength. 
     FIG. 14 shows the pair of plate-like workpieces 12 H , 12 S  which are guided by guiding devices (guide rollers 14 and guide members 18 U , 18 S ) and squeezed by the roller electrodes 20 U , 20 S , in another embodiment of this invention. In the embodiment shown in the figure, the outer circumferential surfaces 28 U , 28 S  of the pair of roller electrodes 20U, 20S have the same width dimension. However, the roller electrode 20 U  located on the side of the thick plate-like workpiece 20 H  has a smaller diameter than the roller electrode 20 S  located on the side of the thin plate-like workpiece 20 M . This arrangement is advantageous in that the amount of mashing of the thick plate-like workpiece 12 H  is made larger than that of the thin plate-like workpiece 12 M , even though the lengths W H  and W M  of contact of the pair of roller electrodes 20 U , 20 S  with the plate-like workpieces 12 H , 12 M  in the axial direction of the roller electrodes are equal to each other. 
     FIG. 15 shows the pair of plate-like workpieces 12 H , 12 S  which are guided by guiding devices (guide rollers 14 and guide members 18 U , 18 S ) and squeezed by the roller electrodes 20 U , 20 S , in another embodiment of this invention. In the embodiment shown in the figure, the outer circumferential contact surfaces 28 U , 28 S  of the pair of roller electrodes 20 U , 20 S  have the same width dimension. These roller electrodes 20 U , 20 S  are disposed such that the widthwise centerlines CL U , CL S  of the outer circumferential contact surfaces 28 U , 28 S  are substantially aligned with each other. The above-indicated guiding device (guide rollers 14 and guide members 18 U , 18 S ) is constructed to guide the pair of plate-like workpieces 12 H , 12 M  such that the centerline of the lap width S of the overlapping edge portions of the pair of plate-like workpieces 12 H , 12 M , that is, the welding centerline A is substantially aligned with the widthwise centerlines CL U , CL S  of the pair of roller electrodes 20 U , 20 S . In the present embodiment, the drive motors 76 functioning as a driving device for driving the guide rollers 14 and retainer rollers 16 are arranged to rotate the guide rollers 14 and retainer rollers 16 so that the welding speed, namely, the peripheral speed of the roller electrodes 20 U , 20 S  is 20 m/min. or higher. 
     In the present embodiment, the weld nugget N is formed obliquely along the interface G, as indicated in FIG. 16 by way of example. Accordingly, the nugget ratio R N  is made large permitting a sufficient weld strength, even though the amount of mashing of the thick plate-like workpiece 12 H  is not made larger than that of the thin plate-like workpiece 12 M . This appears to be caused by the specific resistance of the plate-like workpieces 12 H , 12 M  which becomes higher than electrical contact resistance between the pair of roller electrodes 20 U , 20 S  and the plate-like workpieces 12 H , 12 M . 
     FIG. 17 shows a relationship between the thickness ratio of the plate-like workpieces 12 H , 12 M  and the welding speed, which was obtained by experiments conducted by the present inventors in different conditions, each experiment consisting of ten test operations. In the figure, (1) indicates the case where the nugget ratio R N  is lower than a desired value (40%), while (2) indicates the case where the nugget ratio R N  is higher than the desired value (40%). It will be understood from the table of FIG. 17 that the nugget ratio R N  does not reach the desired value even where the thickness ratio is 1.4 (1:1.4), when the welding speed is 10 m/min. or 15 m/min., while the nugget ratio R N  reaches the desired value even where the thickness ratio is 2.0 (1.2.0), when the welding speed is 20 m/min. 
     FIG. 18 shows the state of a weld when the welding speed is 20 m/min. or higher in the embodiment of FIGS. 1-9. In this case, the interface G is formed at a position intermediate between the pair of roller electrodes 20 U , 20 S , and the nugget N is formed obliquely along the interface G, whereby the nugget ratio R N  is further increased, leading to an advantage of assuring a sufficient weld strength. 
     FIG. 19 shows an embodiment provided with a preliminary treatment device for performing a preliminary treatment step of mashing and thinning a welding portion or one edge portion of the thick workpiece 12 H  prior to a welding operation. In the embodiment of FIG. 19, the pair of plate-like workpieces 12 H , 12 M  are guided relative to each other (Guiding Step) by a plurality of sets of guide rollers 14 and guide members 18 U , 18 S  functioning as a guiding device, in the same manner as in the preceding embodiments, such that the edge portions of the workpieces to be welded together overlap each other with a predetermined lap amount S, while the workpieces are fed in the predetermined feeding direction B by conveyors 140, 142, 144, guide rollers 14 and retainer rollers 16, which function as a work feeding device. Then, the overlapping edge portions of the thus positioned pair of plate-like workpieces 12 H , 12 M  are squeezed at the welding portion by the pair of upper and lower roller electrodes 20 U , 20 S , and are seam-welded (Welding Step) by application of a welding current between the roller electrodes 20 U , 20 S , along the welding centerline A which is parallel to the feeding direction B and which passes the widthwise center of the lap width S of the overlapping edge portions. The pairs of plate-like workpieces 12 H , 12 M  which have been welded together are unloaded by a conveyor 150 and placed in stack on a pallet 152. 
     In a feeding path along which the workpieces are fed as described above, there is provided a presser roller device 148 consisting of a pair of roller dies 146 U , 146 S  for squeezing and thinning the welding portion or edge portion of the thick plate-like workpiece 12 H . The presser roller device 148 is disposed upstream of the roller electrodes 20 U , 20 S , guide rollers 14 and guide members 18 U , 18 S , that is, between the conveyors 140 and 142 provided exclusively for feeding the thick plate-like workpiece 12 H . This presser roller device 148 functions as the preliminary treatment device for thinning the edge portion of the thick plate-like workpiece 12 H  prior to the welding operation. In the mash seam welding apparatus according to the present embodiment, the thickness of the welding portion of the thick plate-like workpiece 12 H  is reduced prior to the seam welding by the pair of roller electrodes 20 U , 20 S , so that the thickness difference of the welding portion of the thick plate-like workpiece 12 H  with respect to the welding portion of the thin plate-like workpiece 12 M  is reduced, whereby the weld nugget N formed at a position intermediate between the pair of roller electrodes 20 U , 20 S  extends across the interface G of the two plate-like workpieces 12 H , 12 M , permitting the weld to have a desired strength. 
     While the embodiments of this invention have been explained referring to the drawings, the invention may be otherwise embodied. 
     In the illustrated embodiments, the plate-like workpiece 12 H  has a larger thickness than the plate-like workpiece 12 M , the workpiece 12 M  may have a thickness larger than the workpiece 12 H . In this case, too, the surface area of contact of the thick plate-like workpiece with the corresponding roller electrode is made larger than that of the thin plate-like workpiece with the corresponding roller electrode. 
     While the illustrated embodiments are adapted to seam-weld the plate-like workpieces 12 H , 12 M  at their edge portions, the plate-like workpieces may be seam-welded at their portions located inwardly from the edge portions. 
     The embodiment of FIG. 18 is a combination of the embodiment of FIGS. 1-9 and the embodiment of FIG. 16. However, the present invention may be embodied as any combination of the embodiments selected from among the embodiment of FIGS. 1-9, embodiment of FIG. 12, embodiment of FIG. 13, embodiment of FIG. 16 and embodiment of FIG. 19. 
     While the illustrated embodiments are adapted to seam weld the pair of plate-like workpieces 12 H , 12 M  while the workpieces are fed in the horizontal direction, the seam welding may be effected while the workpieces are fed in the vertical direction. 
     Although the embodiment of FIG. 19 uses the presser roller device 148 provided with the pair of roller dies 146 U , 146 S  as the preliminary treatment device for thinning the edge portion of the thick plate-like workpiece 12 H , the preliminary treatment device may be a grinding device for thinning the edge portion of the thick plate-like workpiece 12 H  by grinding prior to the welding operation. 
     In the illustrated embodiments, the guide rollers 14 and the retainer rollers 16 include the lower guide rollers 14 HS , 14 MS  and the lower retainer rollers 16 HS , 16 MS  which are located below the plate-like workpieces 12 H , 12 M  and which are driven by the drive motors 76. However, the upper guide rollers 14 HU , 14 MU  and the upper retainer rollers 16 HU , 16 MU  are driven by the drive motors 76, or the upper and lower guide and retainer rollers are both driven by the drive motors 76. 
     While all of the guide rollers 14 HS , 14 MS  are driven by the drive motors 76 in the illustrated embodiments, only the selected ones of these guide rollers may be driven. In the case where the pair of plate-like workpieces 12 H , 12 M  can be sufficiently fed by a plurality of sets of guide rollers 14, the set of retainer rollers 16 in the embodiment of FIGS. 1-9 need not be driven. 
     Further, at least one of the pair of roller electrodes 20 U , 20 S  in the embodiment of FIGS. 1-9 may be driven by the electric motor 140 shown in FIG. 19, for example, at a peripheral speed equal to the feeding speed of the pair of plate-like workpieces 12 H , 12 M . 
     Further, at least one of the pair of roller electrodes 20 U , 20 S  in the embodiment of FIGS. 1-9 may be driven by a drive motor (not shown) at a peripheral speed equal to or higher than the feeding speed of the pair of plate-like workpieces 12 H , 12 M . 
     In the embodiment of FIGS. 1-9, the guide rollers 14 are inclined by a predetermined angle θ for pressing the pair of plate-like workpieces 12 H , 12 M  toward the guide members 18 U , 18 S  while the workpieces are fed. However, slant guiding plates may be provided for contact with the outer edges of the pair of plate-like workpieces 12 H , 12 M , such that the slant guiding plates approach the guide members 18 U , 18 S  as it extends in the feeding direction. 
     It is to be understood that the foregoing embodiments are provided for illustrative purpose only, and that various changes may be made in the illustrated embodiments, without departing from the spirit of the invention.