Patent Publication Number: US-9893498-B2

Title: Method for manufacturing tubular metal shell including ground electrode bar for spark plug, and method for manufacturing spark plug

Description:
This application claims the benefit of Japanese Patent Application No. 2016-099530, filed May 18, 2016, which is incorporated herein in its entirety by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for manufacturing a tubular metal shell including a ground electrode bar for a spark plug by bringing a front end surface of a tubular metal shell for a spark plug and a back end surface of a bar, which is a ground electrode member, into contact with each other, and passing an electric current between resistance welding electrodes that are electrically connected to the tubular metal shell and the bar, thereby welding the tubular metal shell and the bar together by resistance butt welding. The present invention also relates to a method for manufacturing a spark plug. 
     BACKGROUND OF THE INVENTION 
       FIG. 8  is a longitudinal half-sectional view of a spark plug  1 . The spark plug  1  includes a tubular metal shell (hereinafter also referred to simply as a metal shell)  10  including portions of different diameters; an insulator  30  having a hollow cylindrical shape that extends through the metal shell  10 ; and a center electrode  50  that is disposed in a central axial hole in the insulator  30  and that has a tip exposed at a front end (top end in  FIG. 8 ) of the insulator  30 . The metal shell  10  includes a ground electrode  20   b , which is a bar that is fixed to a front end surface  13  of the metal shell  10  and that is bent inward (toward a central axis O). A spark discharge gap G is formed between the tip of the ground electrode  20   b  and the tip of the center electrode  50 . As illustrated in  FIG. 9 , the metal shell  10  is manufactured by arranging a bar that serves as a ground electrode member (hereinafter also referred to simply as a bar)  20  so that one end (back end surface)  25  of the bar  20  perpendicularly faces the front end surface  13  the metal shell  10  that serves as a part (single item), and resistance-welding the bar  20  to the metal shell  10 . Then, as illustrated in  FIG. 10A , the metal shell  10  is subjected to, for example, a process of forming a thread  16  on an outer peripheral surface  15  thereof, and is transferred to a spark plug assembly process. In the spark plug assembly process, the above-described insulator  30  and other components are attached to the metal shell  10  including the bar (see  FIGS. 10A  and B), and a back end  17  of the metal shell  10  is bent inward and compressed (crimped) toward the front (see  FIGS. 10B and 10C ). After the assembly, as illustrated in  FIG. 10C , the bar  20  is bent inward and serves as the ground electrode  20   b  that defines a predetermined spark discharge gap. 
     The bar  20  has conventionally been welded to the front end surface  13  of the metal shell  10  by resistance butt welding as follows. First, as illustrated in  FIG. 9 , the metal shell (work in process)  10  is secured by being clamped, for example, by one resistance welding electrode (electrode made of copper or a copper alloy, which is not illustrated) of a resistance welding machine (not illustrated). The bar  20  is held by the other resistance welding electrode  210 ,  220 . Then, the back end surface  25  (bottom face in  FIG. 9 ) of the bar  20  is arranged so as to perpendicularly face the front end surface  13  of the metal shell  10  at a predetermined position, and is brought into contact with the front end surface  13 . In the contact state, a large electric current is passed between the resistance welding electrodes (the electrode that holds the metal shell  10  and the electrode that holds the bar  20 ), so that the abutting surfaces of the two members (the front end surface  13  of the metal shell  10  and the back end surface  25  of the bar  20 ) that are in contact with each other are melted by resistance heating, and are pressed and welded together (see, for example, FIG. 6 of Japanese Unexamined Patent Application Publication No. 2014-135213). In the welding process, the bar  20 , which is a ground electrode member, has conventionally been held by clamp-type holding means illustrated in  FIG. 9 . The holding means includes a pair of electrode lugs  210  and  220  that camp the bar  20  at opposite side surfaces thereof so as to be electrically connected to the bar  20 . 
     The spark plug manufactured by the above-described resistance butt welding process (hereinafter also referred to simply as a welding process) is required to have high dimensional precision and welding (joining) strength to ensure sufficient performance thereof. The bar  20 , which is a ground electrode member, may be a thin round bar (columnar member), but is typically a small strip-shaped bar (rectangular bar) that is about 2 to 3 mm wide and about 1 to 1.5 mm thick in cross section and that is about 20 mm long. To precisely and strongly weld such a small bar, it is necessary to precisely control not only the welding conditions, such as the current and welding time, but also the vertical orientation in which the bar  20  is held and the orientation in which the bar  20  is positioned relative to the front end surface  13  of the metal shell  10 . In consideration of the above-described requirements and the use of a large amount of current in the welding process, the electrode lugs  210  and  220 , which serve as a resistance welding electrode used to hold the bar  20 , include clamping portions, that is, contact portions, that are capable of clamping the bar (rectangular bar)  20  at the side surfaces thereof over substantially the entire length in the front-back direction. The contact portions of the electrode lugs, which oppose each other and come into contact with the bar to clamp the bar, have the same front-back length so that the bar can be clamped at the side surfaces thereof in a balanced manner. 
     Unfortunately, the welding process in which the bar is clamped by the above-described electrode lugs has the following problems. That is, heat is dissipated into the electrode lugs  210  and  220  during resistance heating, and heat concentration and heating efficiency at the abutting surfaces are reduced due to the heat dissipation. This will be described in more detail below. Since the bar  20  is small and short in the front-back direction as described above, electrode lugs that are capable of clamping the bar  20  over substantially the entire length thereof in the front-back direction are used as the electrode lugs  210  and  220  of the resistance welding electrode. Therefore, the electrode lugs are in contact with the bar over a large contact area (contact area between the contact portions and the bar). The electrode (electrode lugs) that holds the bar is typically made of copper (or a copper alloy), which is highly conductive. In addition, since the bar is small, the volume of the electrode lugs is comparatively large. When the bar is subjected to resistance heating by passing a resistance welding current while the bar is clamped by the electrode lugs that are in contact with the bar over a large area, a large portion of the generated heat is transmitted or dissipated into the electrode lugs that clamp the bar, that is, into an electrode of the resistance welding machine. Thus, in the above-described welding process according to the related art, a large amount of heat dissipation occurs during resistance heating. Therefore, the temperature increase at the abutting surfaces to be welded together (the back end surface of the bar  20  and the front end surface  13  of the metal shell  10 ) is suppressed, and the heating efficiency (heat concentration) is reduced. As a result, in the above-described welding process, the abutting surfaces cannot be efficiently melted and welded together. This is said to lead to a low and insufficient welding strength. 
     The metal shell of the spark plug is made of a ferrous material, such as low-carbon steel. The bar, which is the ground electrode member, is made of a Ni alloy. Thus, members made of different metals are welded together. Therefore, the welding conditions (current, welding time, and pressing force) need to be set within narrow allowable ranges, and it is difficult to obtain sufficient joining strength. In particular, in the case where the bar has a composite structure including a core made of copper and a peripheral wall made of a Ni alloy in cross section, the welding surface contains the Ni alloy. In such a case, the weldability is low, and the joining strength is easily reduced when the joining surface (cross section) that serves as the welding surface (abutting end surface) is small. Therefore, when the bar having a composite structure is welded, resistance heating needs to be performed efficiently and effectively with small heat dissipation from the viewpoint of preventing breakage of the bar in the subsequent bending step. To reduce heat dissipation to the pair of electrode lugs (electrode), the contact area between the contact portions and the bar may be reduced by reducing the size of the contact portions. However, in such a case, there is a possibility that the bar will be held in an unstable orientation and tilted due to, for example, wear of the electrode lugs. As a result, it becomes difficult to maintain high precision. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in light of the above-described problems, and an object of the present invention is to increase welding efficiency and achieve sufficient welding strength, etc., at a welding position in resistance butt welding for welding a bar, which is a ground electrode member, to a front end surface of a tubular metal shell. This is achieved by reducing dissipation of heat, which is generated by resistance heating at the abutting surfaces when a welding current is applied, to electrode lugs that serve as a resistance welding electrode, thereby increasing heat concentration and heating efficiency at the abutting surfaces, without reducing the precision of, for example, the orientation of the bar. 
     According to a first aspect of the present invention, a method for manufacturing a tubular metal shell of a spark plug including bar used for a ground electrode member of the spark plug, the method comprising the steps of bringing a front end surface of the tubular metal shell and a back end surface of the bar into contact with each other; and passing an electric current between resistance welding electrodes that are electrically connected to the tubular metal shell and the bar. One of the resistance welding electrodes is a pair of electrode lugs that are electrically connected to the bar and have a clamping structure in which the bar is clamped at side surfaces of the bar to be electrically connected. One of the electrode lugs includes a first contact portion that comes into contact with the bar and the other of the electrode lugs includes a second contact portion that comes into contact with the bar, the first contact portion having a front-back length shorter than a front-back length of the second contact portion, and coming into contact with the bar over a contact area smaller than a contact area over which the second contact portion comes into contact with the bar. When the bar is clamped between the electrode lugs, the second contact portion is in surface-to-surface contact with the corresponding side surface of the bar. 
     According to a second aspect of the present invention, in the method according to the first aspect of the present invention, the bar is clamped between the first contact portion and a portion of the second contact portion at or near a back end of the second contact portion. 
     According to a third aspect of the present invention, in the method according to the first aspect of the present invention, the first contact portion includes a flat surface. 
     According to a fourth aspect of the present invention, in the method according to any one of the first to third aspects, at least one of the first contact portion or the second contact portion has a recessed groove capable of receiving the bar. 
     According to a fifth aspect of the present invention, a method for manufacturing a spark plug includes the step of manufacturing the tubular metal shell by the method according to any one of the first to fourth aspects. 
     According to the first aspect of the present invention, one of the electrode lugs includes the first contact portion that comes into contact with the bar and the other of the electrode lugs includes the second contact portion that comes into contact with the bar. The first contact portion has a front-back length shorter than a front-back length of the second contact portion, and comes into contact with the bar over a contact area smaller than a contact area over which the second contact portion comes into contact with the bar. When the bar is clamped between the electrode lugs, the second contact portion is in surface-to-surface contact with the corresponding side surface of the bar. According to the related art, the bar is clamped by a pair of electrode lugs including contact portions that come into contact with the bar, and the contact portions of both electrode lugs are long in the front-back length. In contrast, according to the present invention, the front-back length of the first contact portion, which is the contact portion of one electrode lug, is shorter than that of the second contact portion, which is the contact portion of the other electrode lug, so that the contact area between the resistance welding electrode and the bar is reduced. Accordingly, during resistance welding in which heat is generated between the abutting surfaces of the metal shell and the bar due to resistance heating, dissipation of the generated heat toward the electrode lugs (resistance welding electrode) that clamp the bar can be reduced. As a result, heat concentration and heating efficiency at the abutting surfaces that are welded together can be increased from those in the related art, and therefore the welding efficiency, welding performance such as welding strength, and reliability of the welded portion are increased. From the viewpoint of preventing heat conduction (heat dissipation) to the electrode lugs, the contact area between the first contact portion and the bar is preferably made as small as possible to reduce the contact area (heat conduction area). 
     In addition, according to the present invention, although the contact area between the first contact portion and the bar is smaller than that between the second contact portion and the bar, the bar is prevented from being clamped in an unstable manner owing to the above-described structure. The contact area between the first contact portion and the bar can be reduced by reducing the front-back length of the first contact portion. When the contact portions of both electrode lugs are short, the clamped bar is easily tilted in the clamping direction (direction perpendicular to the clamping surfaces) and is easily held in an unstable orientation due to, for example, wear of the contact portions. However, according to the present invention, since only one of the contact portions is reduced in length, the bar can be prevented from being clamped in an unstable orientation. As a result, heat dissipation can be reduced without reducing the stability of the orientation of the bar. 
     As in the second aspect of the present invention, the bar is preferably clamped between the first contact portion and a portion of the second contact portion at or near a back end of the second contact portion. In this case, the electrode lug including the first contact portion having a shorter front-back length than the second contact portion can be brought into contact with the bar at a location close to the back end surface of the bar, that is, close to the abutting surfaces to be welded together. Therefore, the abutting surfaces, at which heat generation is to be concentrated, can be effectively and efficiently heated. The material of the bar is not limited to a Ni alloy, and the bar may have a composite structure including a core made of copper and a peripheral wall made of a Ni alloy in cross section. In this case, the bar is difficult to weld because the bar contains copper, and the weldability thereof is low. However, according to the present invention, the weldability of the bar can be increased because heat concentration can be increased. 
     Also in the second aspect, the front-back length of the first contact portion, which is shorter than the front-back length of the second contact portion, is preferably as short as possible to reduce heat dissipation (heat conduction) to the electrode lugs and increase heat concentration. As in the third aspect of the present invention, the first contact portion preferably includes a flat surface. The reason for this is as follows. The front-back length of the first contact portion is preferably reduced to reduce the contact area between the first contact portion and the bar in the clamped state and to reduce heat conduction. As the front-back length decreases, the first contact portion that clamps the bar becomes sharper, and the contact surface of the bar that comes into contact with the first contact portion receives a larger clamping pressure (force per unit area) in a local region. Furthermore, owing to stress concentration, the local region (surface) is easily dented or damaged, and a large stress is easily generated in the local region. The thermal influence of heat concentration at the local region in the welding process may lead to partial embrittlement around the local region and reduction in strength. The reduction in strength leads to, for example, breakage of the bar in the subsequent bending process for forming the spark discharge gap. These risks can be reduced when the first contact portion having a front-back length shorter than that of the second contact portion includes a flat surface as in the third aspect of the present invention. The first contact portion may have a curved surface (or a spherical surface) having a large radius instead of the flat surface. 
     As in the fourth aspect of the present invention, at least one of the first contact portion and the second contact portion may have a recessed groove capable of receiving the bar. In such a case, the clamped bar can be prevented from moving horizontally (in the direction along the clamping surfaces) or being tilted. As a result, the bar can be clamped in a more stable position and orientation. According to the fifth aspect of the present invention, a high-quality spark plug to which a ground electrode is strongly joined can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein like designations denote like elements in the various views, and wherein: 
         FIG. 1  is a schematic front view illustrating the state before a bar clamped by a pair of electrode lugs for resistance welding is brought into contact with a front end surface of a metal shell according to an embodiment of the present invention; 
         FIG. 2  shows an enlarged view of part II in  FIG. 1 , and a further enlarged view of a portion of part II; 
         FIG. 3  is a sectional view taken along line III-III in  FIG. 2 ; 
         FIGS. 4A and 4B  are schematic diagrams illustrating the process in which the bar illustrated in  FIG. 1  is brought into contact with and welded to the front end surface of the metal shell, and  FIG. 4C  is a schematic diagram illustrating the state in which the electrode lugs are removed after the welding process; 
         FIG. 5  is a plan view of  FIG. 4C ; 
         FIGS. 6A and 6B  illustrate other examples of the shapes of a first contact portion and a second contact portion of the pair of electrode lugs; 
         FIG. 7  is a sectional view (sectional view corresponding to  FIG. 3 ) illustrating the case in which a recessed groove is formed in one of the pair of electrode lugs (first contact portion); 
         FIG. 8  is a longitudinal half-sectional view of a spark plug according to the related art; 
         FIG. 9  is a schematic diagram illustrating the state in which a bar is held by a resistance welding electrode when the bar is resistance-welded to a front end surface of a metal shell (work in process) in a known method for manufacturing a tubular metal shell including a ground electrode bar for the spark plug illustrated in  FIG. 8  according; and 
         FIGS. 10A to 10C  illustrate a step of assembling the spark plug illustrated in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method for manufacturing a tubular metal shell including a ground electrode bar for a spark plug according to an embodiment of the present invention (example of a welding method) will be described in detail below with reference to  FIGS. 1 to 5 . In the present embodiment, a metal shell  10  that is the same as that illustrated in  FIG. 9  (work in process to which the bar is not yet welded) is prepared, and a ground electrode bar  20  is brought into contact with and resistance-welded to a front end surface  13  (top surface in  FIG. 9 ) of the metal shell  10 . Although the bar  20  may be a round bar (columnar member) or a polygonal bar having a cross section that is not rectangular, it is assumed that the bar  20  is a rectangular bar in this example. As illustrated in  FIG. 1 , a bottom end  11  of the tubular metal shell  10  is placed on a base  103  of a resistance butt welding machine  100  and is clamped, for example, by one resistance welding electrode (not shown) so that the metal shell  10  is positioned and secured. The bar  20  is clamped by a pair of electrode lugs  210  and  220 , which serve as the other resistance welding electrode, at opposite side surfaces  22  and  23  thereof, and is thereby electrically connected to the electrode lugs  210  and  220 . The lugs  210  and  220  are mounted in an electrode lug device  200  including a chuck mechanism that slides the lugs  210  and  220  relative to each other so that opposing end surfaces (clamping surfaces) of the lugs  210  and  220  move toward and away from each other. The lugs  210  and  220  are closed and clamp the bar  20  when driven by lug opening/closing means (not shown). A back end surface  25  of the bar  20  is brought into contact with the front end surface  13  of the metal shell  10 . 
     In this example, the electrode lugs  210  and  220  are substantially horizontally oriented rectangular blocks that oppose each other in the electrode lug device  200 . The left and right end surfaces of the electrode lugs  210  and  220  that oppose each other in the drawings are defined as a first contact portion  213  and a second contact portion  223 , respectively. The bar  20  can be clamped between the first and second contact portions  213  and  223  by sliding one of the lugs. In this example, the pair of electrode lugs (horizontally oriented rectangular blocks)  210  and  220  may seem to have the same size and shape. However, in practice, the first and second contact portions  213  and  223  that clamp the bar  20  have front-back lengths (lengths in the vertical direction in  FIGS. 1 and 2 ) that differ by a large amount, and the ratio between the front-back lengths thereof is, for example, about 3 to 1. In this example, the first contact portion  213  of the right electrode lug  210  in the drawings has a short front-back length L 1 , and the second contact portion  223  of the left electrode lug  220  in the drawings has a long front-back length L 2  (see  FIG. 2 ). The front-back length L 2  of the second contact portion (end surface)  223  of the left electrode lug  220  that contacts the bar  20  corresponds to the length of substantially the entirety of the bar  20  excluding front and back end portions (top and bottom end portions in  FIG. 1 ) thereof, and is substantially equal to the length L 2  of the vertical sides of the end surface of the horizontally oriented rectangular block in the drawings. The second contact portion (end surface)  223  has a width that is greater than the width of the bar  20  (width in cross section), as illustrated in  FIG. 3 , and includes a vertical surface (vertical surface extending in the front-back direction). 
     The right electrode lug  210  includes the first contact portion  213  that contacts the bar  20 . The first contact portion  213  is a bottom portion whose length L 1  is about ⅓ of the overall length in the front-back direction (vertical direction in  FIG. 1 ), and includes a vertical surface. A top portion (front portion) that is above the vertical surface in  FIGS. 1 and 2  and whose length is about ⅔ of the overall length in the vertical direction is obliquely cut. Thus, the distance between the right electrode lug  210  and the second contact portion (end surface)  223  of the electrode lug  220  that opposes the electrode lug  210  increases toward the top, as illustrated in  FIGS. 1 and 2 . Thus, in this example, the right electrode lug  210  in  FIG. 1  includes the first contact portion  213  that is short in the front-back direction, and the left electrode lug  220  in  FIG. 1  includes the second contact portion  223  that is long in the front-back direction. The right electrode lug  210  and the left electrode lug  220  oppose each other so that the range of the first contact portion  213  having the front-back length L 1  is within the range of the second contact portion  223  having the front-back length L 2  in the front-back direction. Thus, in this example, the first contact portion  213  opposes the second contact portion  223  of the electrode lug  220  having the front-back length L 2  over the entire length thereof in the front-back direction. However, as long as the bar  20  can be appropriately clamped and welded and a bead can be appropriately formed, a bottom end  215  of the range of the first contact portion  213  may be located below a back end  225  (bottom end in  FIG. 1 ) of the range of the second contact portion  223  of the electrode lug  220  having the front-back length L 2 , and a portion of the first contact portion  213  around the bottom end  215  may protrude downward from the bottom end  225 . 
     The electrode lugs  210  and  220  are arranged so that the left electrode lug  220 , which includes the second contact portion  223  having the long front-back length L 2 , is fixed to the electrode lug device  200 , and the right electrode lug  210 , which includes the first contact portion  213  having the short front-back length L 1 , is slidable so as to clamp or release the bar  20  in the electrode lug device  200 . The bar  20  clamped by the electrode lugs  210  and  220  is positioned in the front-back direction (vertical direction) by bringing a front end  27  (top end in  FIGS. 1 and 2 ) of the bar  20  in the clamped state into contact with a positioning stopper (downwardly facing surface)  240 . 
     In this example, the electrode lug device  200  having the above-described clamping structure is used. The bar  20  is placed between the first and second contact portions  213  and  223  of the right and left electrode lugs  210  and  220  having different front-back lengths, and the electrode lugs  210  and  220  are closed while the bar  20  is positioned in the front-back direction (vertical direction in the drawings). Thus, the bar  20  is clamped at the side surfaces  22  and  23  thereof by the second contact portion (end surface)  223  of the left electrode lug  220 , which has the long front-back length L 2 , and the first contact portion (end surface)  213  of the right electrode lug  210 , which has the short front-back length L 1  and opposes a portion of the second contact portion  223  near the back end  225 . Thus, the bar  20  is clamped by and fixed to the resistance welding electrode lugs  210  and  220  such that the second contact portion  223  of the left electrode lug  220  that is long in the front-back direction is in contact with a large portion of the bar  20  excluding the front and back end portions and having the front-back length L 2 , and such that the first contact portion  213  of the right electrode lug  210  having the short front-back length L 1  is in contact with a portion of the bar  20  near the back end surface  25 . Here, the bar  20  is clamped such that the range of the first contact portion  213  of the right electrode lug  210  in  FIG. 1  having the short front-back length L 1  is within the range of the second contact portion  223  of the left electrode lug  220  in  FIG. 1  having the long front-back length L 2 . Therefore, the bar  20  is stably held in the clamping direction. 
     As illustrated in  FIGS. 4A and 4B , for example, the electrode lug device  200  including the electrode lugs  210  and  220 , which clamp the bar  20  and serve as one resistance welding electrode, is moved to bring the back end surface  25  of the clamped bar  20  against the front end surface  13  of the tubular metal shell  10  at an appropriate contact pressure (pressing force). The tubular metal shell  10  is positioned and secured by being clamped, for example, by the other resistance welding electrode. In this state, an electric current is passed between the resistance welding electrodes so that the back end surface  25  and the front end surface  13  are welded together by resistance butt welding. 
     In the resistance welding process, the resistance welding electrode connected to the bar  20  includes the pair of electrode lugs, as described above. One of the electrode lugs includes the first contact portion  213  that contacts the bar  20  and whose front-back length L 1  is shorter than that in the structure of the related art. More specifically, the front-back length L 1  of the first contact portion  213  of the right electrode lug  210  is shorter than that at the left. Therefore, unlike the clamping structure according to the related art in which the contact portions of the electrode lugs are both long in the front-back direction, the front-back length of the first contact portion  213  of the right electrode lug  210  is about ⅓ the front-back length at the left. Accordingly, the contact area between the bar  20  and the resistance welding electrode is greatly reduced. As a result, when an electric current is passed between the electrodes (resistance welding is performed) and heat is generated between the abutting surfaces of the metal shell  10  and the bar  20  due to resistance heating, dissipation of the generated heat to the right electrode lug  210  that clamps the bar  20  can be reduced. The heat concentration and heating efficiency at the abutting surfaces are higher than those in the structure of the related art in which the bar is clamped between the electrode lugs which each include a contact portion having a long front-back length. Accordingly, the welding efficiency, welding performance such as welding strength, and reliability are increased. 
     After the welding process, as illustrated in  FIG. 4C , the electrode lugs are opened, for example, to remove the tubular metal shell  10  including the ground electrode bar from the resistance welding machine  100 . After that, necessary processes, such as a process of forming the thread  16  and a plating process, are performed, and then the tubular metal shell  10  is transferred to a spark plug assembly process. As described above, the insulator and other components are assembled together, and then the bar  20  is bent to form the spark discharge gap. Thus, the spark plug is manufactured. 
     In this example, the bar  20  is clamped by the left and right electrode lugs  210  and  220 , and the front-back length L 1  of the first contact portion  213  of the right electrode lug  210  is about ⅓ the front-back length L 2  of the second contact portion  223  of the left electrode lug  220 . From the viewpoint of suppressing heat conduction (heat dissipation) to the electrode lugs, the front-back length L 1  is preferably set so to be as short as possible to reduce to reduce the contact area (heat conduction area). 
     In this example, in addition to forming the first contact portion  213  of the right electrode lug  210  so that it has the short front-back length L 1 , the first contact portion  213  is arranged so as to oppose the second contact portion  223  of the left electrode lug  220  having the front-back length L 2  over the entire length thereof in the front-back direction. Therefore, the bar  20  is prevented from being clamped in an unstable manner. The contact area between the bar  20  and the electrode lugs is reduced by reducing the length of the contact portion of only one of the electrode lugs instead of reducing the lengths of the contact portions of both of the electrode lugs. Therefore, when the bar  20  is clamped, one side surface  23  of the bar  20  comes into contact with the second contact portion  223  having the long front-back length L 2 , and the opposite side surface  22  of the bar  20  is pressed by the first contact portion  213  having the short front-back length against the second contact portion  223  within the range of the second contact portion  223  having the front-back length L 2 . Therefore, the bar  20  is prevented from tilting in the clamping direction (direction perpendicular to the clamping surfaces) and being held in an unstable orientation. As a result, a tubular metal shell  10  including a ground electrode bar for a spark plug in which heat dissipation can be reduced without reducing the dimensional precision can be provided. 
     In addition, in the above-described example, the first contact portion  213  of the right electrode lug  210  opposes a portion of the second contact portion  223  of the left electrode lug  220  at or near the back end  225 . Therefore, in the resistance welding process, the current flows between the right electrode lug  210  and the bar  20  at a location close to the back end surface (abutting surface)  25 , which is a welding surface, of the bar  20 . Therefore, heat is effectively and efficiently generated at the abutting surfaces. Moreover, in this example, the first contact portion  213  of the right electrode lug  210  having a short front-back length has a flat surface, and is not sharp. Therefore, when the bar  20  is clamped, the contact surface of the bar  20  that is pressed against the first contact portion  213  does not receive an excessive local clamping pressure (force per unit area), and the generated heat can be dissipated along the flat contact surface. Accordingly, the contact surface is prevented from being dented or damaged due to the pressure in the local region thereof, and generation of large stress in the local region can be prevented. In addition, the thermal influence of heat concentration in the welding process on the local region can be reduced. As a result, partial embrittlement around the local region and reduction in strength due to the embrittlement do not occur, and the bar  20  can be smoothly bent to form the spark discharge gap in the subsequent step. To enhance these effects, the front-back length of the first contact portion  213  is preferably as long as possible within a range in which the front-back length is less than that of the second contact portion  223 . However, in such a case, heat dissipation increases. The appropriate front-back length may be determined through welding tests based on which of the above-mentioned factors is important. The contact portion may have a curved surface or a spherical surface having a large radius instead of the flat surface. 
     In the above-described example, the first contact portion  213  having a short front-back length is located so as to oppose a portion of the second contact portion  223  having a long front-back length near the back end surface  225  in the front-back direction. However, in the case where only the reduction in heat dissipation is important, as illustrated in  FIG. 6A , the first contact portion  213  may be arranged so as to oppose a central portion of the second contact portion  223  in the front-back direction. Alternatively, as illustrated in  FIG. 6B , the first contact portion  213  may be arranged so as to oppose a front portion of the second contact portion  223 . However, to achieve heat concentration at the back end surface  25  of the bar  20 , as described above, the first contact portion  213  is preferably arranged so as to oppose a portion of the second contact portion  223  that is as close to the back end surface  25  of the bar  20  as possible within a range in which the welding process is not adversely affected. 
     In addition, in the above-described example, the first and second contact portions  213  and  223  of the electrode lugs  210  and  220  each have a flat surface. However, as illustrated in  FIG. 7 , the first contact portion  213 , for example, may have a recessed groove capable of receiving a side surface of the bar  20 . When such a recessed groove is formed, the clamped bar  20  can be prevented from moving or being displaced horizontally (in a horizontal direction along the clamping surfaces) or being tilted. As a result, the bar  20  can be clamped in a stable position and orientation. It is not necessary for the side walls of the recessed groove to extend continuously in the front-back direction. More specifically, projections that partially restrict the movement of the bar  20  in the horizontal direction may be provided. The recessed groove may be formed in each of the first and second contact portions of the pair of electrode lugs. 
     In the above-described example, the second contact portion  223  of the left electrode lug  220  having the long front-back length L 2  includes a single (continuous) flat surface that extends in the front-back length. However, the second contact portion  223  may include, for example, a recess as shown by the dashed lines H in  FIG. 6B  in a region in which no clamping force is applied when the bar  20  is clamped between the second contact portion  223  and the first contact portion  213  of the right electrode lug  210  and which excludes regions near the front and back ends. In this case, the second contact portion  223  is partially spaced from the side surface  23  of the bar  20 . Also in this case, the bar  20  can be reliably clamped in the clamping direction by the second contact portion  223  having a long front-back length and the first contact portion  213  having a short front-back length. 
     The present invention is not limited to the above-described embodiment, and modifications can be made as appropriate without departing from the scope of the present invention. For example, with regard to the opening-closing structure (chuck mechanism) for the electrode lugs and the shapes of the first and second contact portions, any structures and shapes may be employed as appropriate. In addition, in the above-described example, the bar is clamped in the thickness direction in cross section so that the electrode lug including the first contact portion having a short front-back length comes into contact with the side surface of the bar that faces the central axis of the metal shell, and the electrode lug including the second contact portion having a long front-back length comes into contact with the side surface of the bar that faces away from the central axis (outward). However, it is obvious that the arrangement of the electrode lugs may be reversed, and the bar may instead be clamped in the width direction in cross section. The present invention can, of course, be applied irrespective of the materials of the metal shell and the bar.