Patent Publication Number: US-9431796-B2

Title: Method for manufacturing spark plug

Description:
CROSS REFERENCE OF RELATED APPLICATIONS 
     This application is a U.S. National Phase Application under 35 U.S.C. §371 of International Patent Application No. PCT/JP2012/001713, filed Mar. 13, 2012, and claims the benefit of Japanese Patent Application No. 2011-89754 filed in Japan on Apr. 14, 2011, all of which are incorporated by reference in their entities herein. The International Application was published in Japanese on Oct. 18, 2012 as International Publication No. WO/2012/140833 under PCT Article 21(2). 
     FIELD OF THE INVENTION 
     The present invention relates to a method for manufacturing a spark plug. 
     BACKGROUND OF THE INVENTION 
     A conventionally used spark plug has a noble metal tip provided at a distal end portion of an electrode. Manufacturing such a spark plug usually employs the following processes: a composite tip is formed by joining a noble metal tip to an intermediate tip (e.g., an Ni tip), and the intermediate tip of the composite tip is joined to a distal end portion of an electrode. 
     However, since the noble metal tip and the intermediate tip are such small members as to have a diameter of about 1 mm or so, forming the composite tip by joining the two members together has encountered difficulty in correctly setting up the relative positional relationship therebetween. Also, for example, in manually positioning the noble metal tip and the intermediate tip, the positional adjustment has consumed time. Such problems do not exclusively arise in a process of joining the noble metal tip to the intermediate tip, but have generally arisen in attempting to correctly set up the relative positional relationship between two tips before joining them together. Also, similar problems have arisen in joining a tip, such as a noble metal tip, directly to a center electrode or a ground electrode. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2009-163923 
     Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2002-198157 
     Problem to be Solved by the Invention 
     An object of the present invention is to provide a technique for easily and correctly adjusting the position of a particular tip in relation to a tip-mating member in joining the particular tip to the tip-mating member. 
     SUMMARY OF THE INVENTION 
     Means for Solving the Problem 
     The present invention has been conceived to solve, at least partially, the above problem and can be embodied in the following modes or application examples. 
     Application Example 1 
     A method for manufacturing a spark plug which comprises the steps of: 
     providing a center electrode, 
     providing an insulator disposed externally of an outer circumference of the center electrode, 
     providing a metallic shell disposed externally of an outer circumference of the insulator, 
     providing a ground electrode whose one end portion is joined to the metallic shell and whose other end portion faces the center electrode, 
     providing a first tip which forms a gap in cooperation with the ground electrode or the center electrode, said first tip being disposed at the center electrode and/or the ground electrode and being joined to a tip-mating member, and 
     transferring the first tip to a joining position where the first tip is joined to the tip-mating member, wherein the transfer step comprises a step of performing positional correction for the first tip before the first tip reaches the joining position. 
     According to this configuration, in the transfer step of transferring the first tip to the joining position where the first tip is joined to the tip-mating member, positional correction is performed for the first tip before the first tip reaches the joining position. Therefore, the positional relationship between the first tip and the tip-mating member can be adjusted easily and correctly. Also, since the step of performing positional correction for the first tip and the step of joining the first tip to the tip-mating member can be performed separately at respectively favorable timings, production efficiency can be improved. 
     Application Example 2 
     A method for manufacturing a spark plug according to application example 1, wherein the positional correction for the first tip is performed by gripping the first tip using a position-correcting chuck. 
     According to this method, since the first tip is gripped with the position-correcting chuck and is thereby corrected for position, the positional relationship between the first tip and the tip-mating member can be adjusted easily and correctly. 
     Application Example 3 
     A method for manufacturing a spark plug according to application example 1 or 2, wherein the positional correction is performed at a middle position between a feed position where the first tip is fed, and the joining position. 
     According to this configuration, since positional correction for the first tip is performed at a middle position between the feed position and the joining position, the positional correction can be performed in a sufficiently loose condition in terms of time and position. 
     Application Example 4 
     A method for manufacturing a spark plug according to any one of application examples 1 to 3, wherein 
     the step of transferring comprises 
     (a) a step of moving, by use of a first feed device, the first tip to a middle position between a feed position where the first tip is fed, and the joining position, 
     (b) a step of performing positional correction for the first tip at the middle position by gripping the first tip using a position-correcting chuck which grips the first tip, and 
     (c) a step of, after the positional correction, moving the first tip from the middle position to the joining position by use of a transfer chuck, with the first tip being chucked with the transfer chuck. 
     According to this configuration, after positional correction for the first tip is performed at the middle position by use of the position-correcting chuck, the first tip is gripped with and moved by use of the transfer chuck. Therefore, the first tip can be properly transferred in a positionally corrected condition from the middle position to the joining position. 
     Application Example 5 
     A method for manufacturing a spark plug according to application example 4, wherein 
     the first feed device and the transfer chuck are configured such that transfer is repeated with a horizontal distance therebetween being fixed; 
     there are simultaneously performed 
     a first moving process in which, in the step (a), the first feed device moves one first tip from the feed position to the middle position, and 
     a second moving process in which, in the step (c), the transfer chuck moves another first tip from the middle position to the joining position; and 
     the positional correction in the step (b) is performed in the course of return of the first feed device from the middle position to the feed position and in the course of return of the transfer chuck from the joining position to the middle position. 
     According to this configuration, since the moving processes in the step (a) and the step (c) are performed simultaneously, the overall process can be completed in a short period of time. 
     Application Example 6 
     A method for manufacturing a spark plug according to application example 4 or 5, wherein a gripping member of the transfer chuck has a thickness greater than that of a gripping member of the position-correcting chuck. 
     According to this configuration, the transfer chuck can more reliably grip the first tip in transfer of the first tip, thereby reducing the possibility of a positional shift of the first tip in the midst of transfer. 
     Application Example 7 
     A method for manufacturing a spark plug according to any one of application examples 1 to 6, wherein 
     positional correction for the first tip is performed in a state in which a bottom surface of the first tip is vacuum-chucked by use of a vacuum chuck port provided in a placement table on which the first tip is placed. 
     According to this configuration, since positional correction is performed with the bottom surface of the first tip being vacuum-chucked, when the position-correcting chuck is to grip the first tip, an unintended movement (for example, the position-correcting chuck flicks the first tip) can be restrained. 
     Application Example 8 
     A method for manufacturing a spark plug according to any one of application examples 1 to 7, wherein 
     at least one of the center electrode and the ground electrode has a composite tip; 
     the composite tip is such that a first tip which forms a gap in cooperation with the center electrode or the ground electrode, and a second tip which connects the first tip to the center electrode or the ground electrode, are joined together; and 
     the second tip is the tip-mating member. 
     According to this configuration, in the transfer step of transferring the first tip to the joining position where the first tip is joined to the second tip, positional correction for the first tip is performed by means of gripping the first tip with the position-correcting chuck; therefore, the positional relationship between the two tips which constitute the composite tip can be adjusted easily and correctly. 
     Application Example 9 
     A method for manufacturing a spark plug according to any one of application examples 1 to 7, wherein 
     the center electrode is the tip-mating member. 
     According to this configuration, the positional relationship between the first tip and the center electrode can be adjusted easily and correctly. 
     Application Example 10 
     A method for manufacturing a spark plug according to any one of application examples 1 to 7, wherein 
     the ground electrode is the tip-mating member. 
     According to this configuration, the positional relationship between the first tip and the ground electrode can be adjusted easily and correctly. 
     The present invention can be embodied in various forms. For example, the present invention can be embodied in a spark plug, a metallic shell for the spark plug, a method for manufacturing the spark plug, and a method for manufacturing the metallic shell. 
    
    
     
       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 partially sectional view of a spark plug according to an embodiment of the present invention. 
         FIG. 2  is a perspective view showing a noble metal tip and an intermediate tip before joining them together. 
         FIG. 3  is a perspective view showing a composite tip in which the noble metal tip and the intermediate tip are joined together. 
         FIG. 4  is an enlarged view showing a forward end portion of a center electrode and its periphery. 
         FIG. 5  is an explanatory view showing an example of a joining apparatus in a first embodiment. 
         FIG. 6  is a set of explanatory views showing the shapes of chucks. 
         FIG. 7  is a set of explanatory views showing a joining process for forming the composite tip. 
         FIG. 8  is a set of explanatory views showing the joining process for forming the composite tip. 
         FIG. 9  is a set of explanatory views showing the joining process for forming the composite tip. 
         FIG. 10  is a set of explanatory views showing an example of a joining apparatus in a second embodiment of the present invention. 
         FIG. 11  is a set of explanatory views showing positional correction work for a tip to be conducted at a middle position in a third embodiment of the present invention. 
         FIG. 12  is a flowchart showing a method for manufacturing a spark plug. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Modes for Carrying Out the Invention 
     A. First Embodiment 
       FIG. 1  is a partially sectional view of a spark plug  100  according to an embodiment of the present invention. In the following description, the direction of an axis O of the spark plug  100  in  FIG. 1  is referred to as the vertical direction in the drawings; the lower side is referred to as the forward side of the spark plug  100 ; and the upper side as the rear side. The spark plug  100  includes a ceramic insulator  10 , a metallic shell  50 , a center electrode  20 , a ground electrode  30 , and a metal terminal  40 . 
     The ceramic insulator  10  is formed from, for example, alumina through firing. The ceramic insulator  10  is a tubular insulator and has an axial bore  12  coaxially extending therethrough in the direction of the axis O. The ceramic insulator  10  electrically insulates the center electrode  20  and the metallic shell  50  from each other. The ceramic insulator  10  has a collar portion  19  formed substantially at the center in the direction of the axis O and having the greatest outside diameter, and a rear trunk portion  18  formed rearward (upward in  FIG. 1 ) of the collar portion  19 . The ceramic insulator  10  also has a forward trunk portion  17  formed forward (downward in  FIG. 1 ) of the collar portion  19  and being smaller in outside diameter than the rear trunk portion  18 . The ceramic insulator  10  further has a leg portion  13  formed forward of the forward trunk portion  17  and being smaller in outside diameter than the forward trunk portion  17 . The leg portion  13  reduces in outside diameter toward the forward end thereof. When the spark plug  100  is mounted to an engine head  200  of an internal combustion engine, the leg portion  13  is exposed to a combustion chamber of the internal combustion engine. A stepped portion  15  is formed between the leg portion  13  and the forward trunk portion  17 . 
     The center electrode  20  is a rodlike electrode held in the ceramic insulator  10  along the direction of the axis O. The center electrode  20  has a structure in which a core  25  is embedded in an electrode base metal  21 . The electrode base metal  21  is formed of nickel or a nickel alloy which contains nickel as a main component, such as INCONEL (trade name) 600 or 601. The core  25  is formed of copper or a copper alloy which contains copper as a main component, copper and the copper alloy being superior to the electrode base metal  21  in thermal conductivity. Usually, the center electrode  20  is manufactured as follows: the core  25  is fitted into the electrode base metal  21  formed into a closed-bottomed tubular shape; then, the resultant assembly is subjected to extrusion from the bottom side for prolongation. The core  25  has a substantially fixed outside diameter at its trunk portion and has a tapered forward end portion. 
     A forward end portion  22  of the center electrode  20  protrudes from the forward end of the ceramic insulator  10  and reduces in diameter toward the forward end thereof. In order to improve resistance to spark-induced erosion, a substantially circular columnar noble metal tip  90  formed of a noble metal having high melting point is joined to the forward end surface of the forward end portion  22  of the center electrode  20 . The noble metal tip  90  can be formed of, for example, iridium (Ir) or an Ir alloy which contains iridium as a main component and one or more additive elements selected from among platinum (Pt), rhodium (Rh), ruthenium (Ru), palladium (Pd), rhenium (Re), etc. 
     The center electrode  20  and the noble metal tip  90  are joined together by full-circle laser welding with a laser beam radiated to the boundary between the noble metal tip  90  and the forward end portion  22  of the center electrode  20 . In laser welding, since the two materials irradiated with a laser beam are fused and mixed, the noble metal tip  90  and the center electrode  20  are firmly joined together. The center electrode  20  extends rearward within the axial bore  12  and is electrically connected to the rear (upper in  FIG. 1 ) metal terminal  40  via a seal member  4  and a ceramic resistor  3 . A high-voltage cable (not shown) is connected via a plug cap (not shown) to the metal terminal  40  provided at the rear end of the ceramic insulator  10  so as to apply high voltage to the metal terminal  40 . 
     The ground electrode  30  is welded at its proximal portion  32  to a forward end surface  57  of the metallic shell  50  and is disposed such that one side surface of its distal end portion  31  faces the forward end portion  22  of the center electrode  20 . The ground electrode  30  is formed of a metal having high corrosion resistance; for example, a nickel alloy, such as INCONEL (trade name) 600 or 601. The ground electrode  30  has a substantially rectangular cross section across its longitudinal direction. The distal end portion  31  of the ground electrode  30  is bent such that one side surface of the distal end portion  31  faces, on the axis O, the noble metal tip  90  welded to the center electrode  20 . 
     An intermediate tip  60  is joined to the distal end portion  31  of the ground electrode  30  on a plane which faces, on the axis O, the forward end portion  22  of the center electrode  20 . The intermediate tip  60  can be formed of, for example, an Ni alloy which contains chromium (Cr), silicon (Si), manganese (Mn), aluminum (Al), etc. A noble metal tip  70  is joined to the intermediate tip  60  on a side (the upper side in the drawing) toward the forward end portion  22  of the center electrode  20 . The intermediate tip  60  and the noble metal tip  70  are joined together by laser welding. As a result of fusion of the noble metal tip  70  and the intermediate tip  60 , a fusion zone  80  is formed. The noble metal tip  70  can be formed of, for example, a Pt alloy which contains Pt as a main component, and one or more elements selected from among Rh, Ni, etc., as an additive(s). 
     As will be described later, in the course of manufacture of the spark plug, a composite tip is formed by joining the intermediate tip  60  and the noble metal tip  70  together, and the composite tip is joined to the distal end portion  31  of the ground electrode  30 . Notably, the noble metal tip  70  may be called the “first tip,” and the intermediate tip  60  may be called the “second tip.” 
     The metallic shell  50  is a cylindrical metallic member adapted to fix the spark plug  100  to the engine head  200  of the internal combustion engine. The metallic shell  50  holds the ceramic insulator  10  therein. The metallic shell  50  is formed of low-carbon steel and has a tool engagement portion  51 , to which an unillustrated spark plug wrench is fitted, and a mounting threaded portion  52 , which has a thread formed thereon and is threadingly engaged with a mounting threaded hole  201  of the engine head  200  provided at an upper portion of the internal combustion engine. 
     The metallic shell  50  has a collar-like seal portion  54  formed between the tool engagement portion  51  and the mounting threaded portion  52 . An annular gasket  5  formed by folding a sheet is fitted to a screw neck  59  between the mounting threaded portion  52  and the seal portion  54 . When the spark plug  100  is mounted to the engine head  200 , the gasket  5  is crushed and deformed between a seat surface  55  of the seal portion  54  and a peripheral-portion-around-opening  205  of the mounting threaded hole  201 . The deformation of the gasket  5  provides a seal between the spark plug  100  and the engine head  200 , thereby preventing gas leakage from inside the engine through the mounting threaded hole  201 . 
     The metallic shell  50  has a thin-walled crimped portion  53  located rearward of the tool engagement portion  51 . The metallic shell  50  also has a buckled portion  58 , which is thin-walled similar to the crimped portion  53 , between the seal portion  54  and the tool engagement portion  51 . Annular ring members  6  and  7  intervene between the ceramic insulator  10  and an inner circumferential surface of the metallic shell  50  extending from the tool engagement portion  51  to the crimped portion  53 ; furthermore, a space between the two ring members  6  and  7  is filled with a powder of talc  9 . When the precursor of the crimped portion  53  is bent inward and is thereby crimped, the ceramic insulator  10  is pressed forward within the metallic shell  50  via the ring members  6  and  7  and the talc  9 . Accordingly, the stepped portion  15  of the ceramic insulator  10  is supported via the annular sheet packing  8  by a stepped portion  56  formed on the inner circumference of the metallic shell  50  at a position corresponding to the mounting threaded portion  52 , whereby the metallic shell  50  and the insulator  10  are united together. At this time, gastightness between the metallic shell  50  and the ceramic insulator  10  is maintained by means of the annular sheet packing  8 , thereby preventing outflow of combustion gas. The precursor of the buckled portion  58  is designed to be deformed outwardly in association with application of compressive force in a crimping process, thereby contributing toward increasing the length of compression of the talc  9  in the direction of the axis O and thus enhancing gastightness within the metallic shell  50 . A predetermined clearance is provided between the metallic shell  50  and the insulator  10  in a forward end region. 
     The entire configuration of the spark plug  100  shown in  FIG. 1  is a mere example. The spark plug can employ various other configurations. 
       FIG. 2  is a perspective view showing the noble metal tip  70  and the intermediate tip  60  before they are joined together. The noble metal tip  70  has a substantially circular columnar shape and has a gap formation face SF (also called the “top face” or “upper bottom face”) perpendicular to the axis. In the spark plug  100 , the gap formation face SF is disposed in such a manner as to face the forward end portion  22  of the center electrode  20 . The gap formation face SF has a substantially circular shape with its edge  71  serving as the circumference of the circle. The intermediate tip  60  has a columnar portion  61  having a substantially circular cross section and a collar portion  62  radially expanding from the columnar portion  61 . The top face of the columnar portion  61  functions as a disposition face DF on which the noble metal tip  70  is disposed. The disposition face DF has a substantially circular shape. The noble metal tip  70  is disposed on the disposition face DF of the intermediate tip  60  in such a manner that the axis of the noble metal tip  70  and the axis of the intermediate tip  60  are aligned with each other. Usually, a diameter D 1  of the noble metal tip  70  is slightly smaller than a diameter D 2  of the displacement face DF of the intermediate tip  60 . 
       FIG. 3  is a perspective view showing a composite tip CP in which the noble metal tip  70  and the intermediate tip  60  are joined together. The intermediate tip  60  and the noble metal tip  70  are joined together by laser welding or the like, yielding the composite tip CP. As a result of the welding, the fusion zone  80  is formed at the boundary between the intermediate tip  60  and the noble metal tip  70 . The collar portion  62  of the composite tip CP is joined to the distal end portion  31  of the ground electrode  30  by resistance welding or the like. 
       FIG. 4  is an enlarged view showing a forward end portion of the center electrode  20  and its periphery. The composite tip CP is disposed where its axis is aligned with the axis of the center electrode  20 . A spark gap G is formed between a bottom face CF of the center electrode  20  (herein, the bottom face of the noble metal tip  90 ) and the top face SF of the composite tip CP. In the example of  FIGS. 1 to 4 , the composite tip CP is provided at the distal end portion  31  of the ground electrode  30 . However, the composite tip may be provided at a forward end portion of the center electrode  20 . That is, preferably, the composite tip is provided at least one of the center electrode  20  and the ground electrode  30 . 
       FIG. 5  is an explanatory view showing an example of a joining apparatus for forming a composite tip through joining in the first embodiment. The joining apparatus includes a transfer device  300  having a first transfer device  310  and a second transfer device  320 ; a tip-pressing device  500 ; a first-tip feed device  410 ; a position-correcting device  420 ; a laser welding machine  600 ; and a tip support device  700 . In the following description, the noble metal tip  70  is called the “first tip  70 ,” and the intermediate tip  60  is called the “second tip  60 .” Since manufacture of spark plugs involves a large number of the first tips  70  and the second tips  60 , when these tips need to be distinguished from one another, affixes indicative of order, such as “n” and “n−1,” are affixed to the reference numerals  70  and  60  of the tips. 
     The first-tip feed device  410  is a part feeder for feeding the first tips  70 . In the first-tip feed device  410 , the position where the first tip  70  is picked up is called the “first-tip feed position P 1 .” 
     The first transfer device  310  picks up the first tip  70  from the first-tip feed position P 1  and transfers the picked-up first tip  70  to a position Pm on the position-correcting device  420 . The first transfer device  310  has a vacuum chuck  314  for vacuum-chucking the first tip  70  at its top face, and a drive mechanism  312  for vertically moving the vacuum chuck  314 . 
     The position-correcting device  420  has a placement table  422 ; a position-correcting chuck  424  provided on the placement table  422 ; and a tip suction device  426 . The first tip  70  transferred by the first transfer device  310  is placed on the placement table  422 . The placement table  422  on which the first tip  70  is placed has a vacuum chuck port  423  at the position Pm. Hereinafter, the position Pm may be called the “middle position Pm.” The position-correcting chuck  424  corrects the position of the first tip  70  at the middle position Pm. The shape of the position-correcting chuck  424  and a method of positional correction by the position-correcting chuck  424  are described later. During the process of positional correction, the tip suction device  426  exerts suction on the bottom face of the first tip  70  through the vacuum chucking port  423  of the placement table  422 , thereby securing the first tip  70  on the placement table  422 . The tip suction device  426  and the vacuum chucking port  423  may be eliminated. 
     The second transfer device  320  transfers the first tip  70  from the position. Pm on the position-correcting device  420  to a position P 2  on the tip support device  700 . The second transfer device  320  has a transfer chuck  324  for gripping the first tip  70  at its side, and a drive mechanism  322  for vertically moving the transfer chuck  324 . 
     The tip support device  700  supports the second tip  60 . Specifically, the tip support device  700  has a plurality of grippers  710 , each having a placement surface  712  and a gripping claw  714 . These grippers  710  are configured such that their gripping claws  714  can shift or pivotally move toward the position P 2  of the center of the tip support device  700 . These grippers  710  grip the collar portion  62  of the second tip  60  from the outside radial direction, thereby supporting the second tip  60  at the position P 2 . In the gripped condition, the bottom face of the collar portion  62  rests on the placement surfaces  712  of the grippers  710 , and an upper edge of the collar portion  62  is pressed by the inner surfaces of the gripping claws  714 . Since a plurality of (e.g., three) grippers  710  are provided around the second tip  60 , by means of the plurality of grippers  710  gripping the second tip  60 , the center of the second tip  60  is correctly positioned at the center position P 2  of the tip support device  700 . In order to enhance the positioning function of the grippers  710 , preferably, as shown in  FIG. 5 , the placement surface  712  and the inner surfaces of the gripping claws  714  form an acute angle. 
     After the second transfer device  320  transfers the first tip  70  and then places it on the second tip  60 , the tip-pressing device  500  presses the first tip  70  from above. The tip-pressing device  500  has a pressing jig  510  for pressing the first tip  70 , and a drive mechanism  520  for vertically moving the pressing jig  510 . 
     In a state in which the second tip  60  and the first tip  70  are sequentially placed on the tip support device  700 , and the tip pressing unit  500  presses the first tip  70 , the laser welding machine  600  welds the first tip  70  and the second tip  60  at their boundary to join them together, thereby forming the composite tip. This joining work is performed in a state in which the first tip  70  and the second tip  60  are situated at the center position P 2  of the tip support device  700 . Thus, this position P 2  is also called the “joining position.” 
     The first transfer device  310 , the second transfer device  320 , and the tip-pressing device  500  can move horizontally along a horizontally extending rail  330 . The first transfer device  310  and the second transfer device  320  are driven by an unillustrated drive unit and can move simultaneously in the horizontal direction with a distance L 1  therebetween held at a fixed value. Also, the second transfer device  320  and the tip-pressing device  500  are driven by an unillustrated drive unit and can move simultaneously in the horizontal direction with a distance L 2  therebetween held at a fixed value. However, one or two of the three devices  310 ,  320 , and  500  may be moved independently of the other one(s), or the three devices  310 ,  320 , and  500  may be moved independently of one another. 
     Preferably, the distance L 1  between the first-tip feed position P 1  and the middle position Pm is equal to the distance L 2  between the middle position Pm and the joining position P 2 . Through employment of this distance relationship, by means of simultaneous rightward move in  FIG. 5  of the first transfer device  310  and the second transfer device  320  which grip the respective first tips  70 , the two first tips  70  can be simultaneously transferred. 
       FIG. 6(A)  is an explanatory view showing the shape of the chuck. As shown in  FIG. 6(A) , the position-correcting chuck  424  is composed of two chuck members, each having a gripping recess  425 . Each of the gripping recesses  425  is formed of two planes which form an angle θ. When the first tip  70  is gripped at its side between the two gripping recesses  425 , the first tip  70  automatically undergoes positional correction so as to come to the center position between the two gripping recesses  425  (i.e., the center position of the position-correcting chuck  424 ). Notably, the “center position between the two gripping recesses  425 ” is the one in a state (closed state) in which the two gripping recesses  425  grip the first tip  70  at its side therebetween. In this gripping condition, a predetermined gap is present between the two chuck members (i.e., between the two gripping recesses  425 ). The angle θ of the gripping recesses  425  is preferably 10 degrees to 170 degrees, particularly preferably 90 degrees to 160 degrees. This angle is experimentally determined so as to correctly perform positional correction for the first tip  70 . 
     The transfer chuck  324  of the second transfer device  320  can also be configured to be similar to the position-correcting chuck  424  in the shape of a gripping portion. Alternatively, the transfer chuck  324  and the position-correcting chuck  424  may differ in the shape of a gripping portion. However, preferably, the shapes of the gripping portions of the transfer chuck  324  and the position-correcting chuck  424  are determined such that the tip center position of the position-correcting chuck  424  in a gripping condition and the tip center position of the transfer chuck  324  in a gripping condition coincide with each other. 
       FIG. 6(B)  shows the relationship of thickness between the position-correcting chuck  424  and the transfer chuck  324 .  FIG. 6(B)  shows a state in which, after completion of positional correction performed on the placement table  422  by means of the position-correcting chuck  424 , the transfer chuck  324  grips the first tip  70 . In this state, the position-correcting chuck  424  grips the first tip  70  at a lower portion of its side surface, while the transfer chuck  324  grips the first tip  70  at an upper portion of its side surface. Then, the position-correcting chuck  424  opens and thereby releases the first tip  70 , and, while gripping the first tip  70 , the transfer chuck  324  transfers the first tip  70  to the joining position P 2 . In order to prevent deviation of the position of the first tip  70  relative to the transfer chuck  324  from a proper grip position in the midst of the transfer, preferably, the transfer chuck  324  has a sufficiently large thickness T 2 . Specifically, preferably, the thickness T 2  of the transfer chuck  324  is greater than a thickness T 1  of the position-correcting chuck  424 . Also, preferably, the thickness T 2  of the transfer chuck  324  is equal to or greater than half of a thickness Tt of the first tip  70  (0.5 Tt). 
       FIGS. 7(A) and 7(B)  to  FIGS. 9(A) and 9(B)  are explanatory views showing a joining process for forming the composite tip.  FIG. 7(A)  shows a state in which the first tips  70   n  and  70   n− 1 are held. In this state, the first transfer device  310  vacuum-chucks the first tip  70   n  with the vacuum chuck  314  at the feed position P 1  of the first-tip feed device  410 , whereas the second transfer device  320  grips the first tip  70   n− 1 with the transfer chuck  324  at the middle position Pm on the placement table  422 . Subsequently, the first transfer device  310  transfers the first tip  70   n  from the feed position P 1  to the middle position Pm, and, at the same time, the second transfer device  320  transfers the other first tip  70   n− 1 from the middle position Pm to the joining position P 2  ( FIG. 7(B) ). In parallel with the transfer, a second tip  60   n− 1 is fed onto the tip support device  700  by a second-tip feed device (not shown) and is held on the tip support device  700 . 
       FIG. 8(A)  shows a state in which, after the two first tips  70   n  and  70   n− 1 are transferred to positions above the middle position Pm and the joining position P 2 , respectively, the first tips  70   n  and  70   n− 1 are lowering. As a result, the first tip  70   n  transferred by the first transfer device  310  is placed on the placement table  422  of the position-correcting device  420 , whereas the first tip  70   n− 1 transferred by the second transfer device  320  is placed on the second tip  60   n− 1 supported by the tip support device  700  at the joining position P 2 . At this time, the first tip  70   n  placed at the middle position Pm undergoes the aforementioned positional correction ( FIGS. 6(A) and 6(B) ) performed by the position-correcting chuck  424 . This positional correction is performed with the bottom face of the first tip  70   n  being vacuum-chucked through the vacuum chuck port  423  provided in the placement table  422 . Therefore, when the position-correcting chuck  424  is to grip the first tip  70   n , an unintended movement (for example, the position-correcting chuck  424  flicks the first tip  70   n ) can be restrained. Upon completion of the placing operation and the position correcting operation, the first transfer device  310  and the second transfer device  320  release the respective tips and then move back toward the original positions P 1  and Pm, respectively, in an unloaded condition ( FIG. 8(B) ). The position-correcting chuck  424  may perform positional correction in parallel with the returning movement of the first transfer device  310  and the second transfer device  320  toward the original positions P 1  and Pm. In this case, when the first tip  70   n  is placed on the placement table  422 , the first tip  70   n  is vacuum-chucked and held through the vacuum chuck port  423 ; then, after the first transfer device  310  releases the first tip  70   n , the positional correction is performed. 
       FIG. 9(A)  shows a state in which the first transfer device  310  and the second transfer device  320  are back at the positions P 1  and Pm, respectively, and hold the next first tips  70   n+ 1 and  70   n , respectively. At this time, the tip-pressing device  500  is also back at the joining position P 2  and presses downward the top face of the first tip  70   n− 1. Also, the laser welding machine  600  welds the first tip  70   n− 1 and the second tip  60   n− 1 together. As a result, the composite tip composed of the tips  70   n− 1 and  60   n− 1 is formed. Subsequently, the first transfer device  310  transfers the first tip  70   n+ 1 from the feed position P 1  to the middle position Pm, and, at the same time, the second transfer device  320  transfers the first tip  70   n  from the middle position Pm to the joining position P 2  ( FIG. 9(B) ). In parallel with the transfer, unillustrated another transfer device transfers the composite tip formed in  FIG. 9(A)  from the tip support device  700  to another place. Operations of the first transfer device  310  and the second transfer device  320  in  FIGS. 9(A) and 9(B)  are similar to those of the first and second transfer devices  310  and  320  in  FIGS. 7(A) and 7(B) . Subsequently, the operations described above with reference to  FIGS. 7(A) and 7(B)  to  FIGS. 9(A) and 9(B)  are repeated, thereby yielding the composite tips one after another. 
     In the first embodiment, the position-correcting device  420  performs positional correction for the first tip  70  mainly for the following reason. As mentioned above, the first transfer device  310  vacuum-chucks the first tip  70  at its top face and transfers the first tip  70 . Therefore, great variation is likely to arise in the vacuum-chucking position (holding position) on the first tip  70  to be vacuum-chucked (held) by the first transfer device  310 . If the first transfer device  310  transfers the first tip  70  from the feed position P 1  to the joining position P 2 , the first tip  70  may possibly fail to be correctly placed at the joining position P 2 . Thus, in the first embodiment, at the middle position Pm, the position-correcting device  420  corrects the first tip  70  to a correct position; subsequently, the second transfer device  320  which holds the first tip  70  by means other than vacuum chuck transfers the first tip  70  from the middle position Pm to the joining position P 2 . Through employment of such process, the first tip  70  can be placed correctly at the joining position P 2 . 
     As described above, according to the first embodiment, in the course of transfer of the first tip  70  from the feed position P 1  of the first tip  70  to the joining position P 2  where the composite tip is manufactured, positional correction is performed on the first tip  70 ; therefore, the positional relationship between the two tips which constitute the composite tip can be adjusted easily and correctly. Also, according to the first embodiment, particularly, since positional correction is performed in a state in which the first tip  70  is placed at the middle position Pm located at the center between the feed position P 1  and the joining position P 2 , as compared with the case where positional correction is performed on the first tip  70  in the process of transfer, positional correction can be performed easily and correctly. Furthermore, at the middle position Pm, positional correction can be performed in a sufficiently loose condition in terms of time and position. Also, since the middle position Pm where positional correction is performed is located at the center between the feed position P 1  and the joining position P 2 , transfer by the first transfer device  310  from the feed position P 1  to the middle position Pm and transfer by the second transfer device  320  from the middle position Pm to the joining position P 2  can be performed simultaneously. As a result, individual transfer distances become short, thereby shortening working time required to manufacture the composite tip. 
     Also, according to the first embodiment, positional correction for the first tip  70  is performed before the first tip  70  reaches the joining position P 2 . Thus, since the step of positional correction for the first tip  70  and the step of joining the first and second tips together can be performed separately at respectively favorable timings, production efficiency can be improved. 
     B. Second Embodiment 
       FIGS. 10(A) and 10(B)  are explanatory views showing a joining apparatus in a second embodiment of the present invention and the operation of the joining apparatus and correspond to  FIGS. 7(A) and 7(B)  showing the joining process in the first embodiment. The second embodiment differs from the first embodiment only in that a single transfer device  300   a  replaces collectively all of the first transfer device  310 , the second transfer device  320 , and the position-correcting device  420 . Other configurational features are similar to those of the first embodiment. Specifically, the transfer device  300   a  of the second embodiment has a vacuum chuck  314   a  for vacuum-chucking, from the first-tip feed device  410 , the first tip  70  at its top face; a drive mechanism  312   a  for vertically moving the vacuum chuck  314   a ; a position-correcting chuck  424   a  which grips the first tip  70  at its side and performs positional correction for the first tip  70 ; and a drive mechanism  428   a  for vertically moving the position-correcting chuck  424   a . Notably, each of the position-correcting chuck  424   a  and the drive mechanism  428   a  is divided into right and left halves so as to be able to vertically move on the opposite sides of the vacuum chuck  314   a.    
     As shown in  FIG. 10(A) , when a new tip is to be held, the vacuum chuck  314   a  vacuum-chucks the first tip  70  at the feed position P 1  and then moves upward for picking up the one tip. At this time, the position-correcting chuck  424   a  is in an open state (in a standby state). When the one first tip  70  is picked up in this manner, the transfer device  300   a  moves rightward in  FIG. 10(A)  toward the joining position P 2 .  FIG. 10(B)  shows the state of the movement. During the transfer by the transfer device  300   a , the position-correcting chuck  424   a  moves downward and changes from the open state to a closed state to thereby grip the first tip  70  at its side. The position-correcting chuck  424   a  has a shape similar to that of the first embodiment shown in  FIG. 6(A) . Therefore, by means of the position-correcting chuck  424   a  gripping the first tip  70  at its side, the position of the first tip  70  is corrected to the center position of the position-correcting chuck  424   a . Subsequently, while the first tip  70  is vacuum-chucked at its top face by the vacuum chuck  314   a  and gripped at its side by the position-correcting chuck  424   a , the transfer device  300   a  moves to the joining position P 2 . Then, the drive mechanisms  312   a  and  428   a  operate to lower the vacuum chuck  314   a  and the position-correcting chuck  424   a , respectively, and the first tip  70  is thereby placed on the second tip  60 . 
     In the second embodiment, in transfer after positional correction shown in  FIG. 10(B) , vacuum chucking by the vacuum chuck  314   a  may not be employed. In this case, the position-correcting chuck  424   a  carries out a function similar to that of the transfer chuck  324  in the first embodiment. Also, in the second embodiment, the position-correcting chuck  424   a  may be configured to not vertically move. 
     As mentioned above, in the second embodiment, positional correction for the first tip  70  is performed during transfer of the first tip  70 ; therefore, the configuration of the transfer apparatus becomes simple. Also, since positional correction for the first tip can be performed during transfer (i.e., during movement), there can be shortened time required for the entire process which includes transfer of and positional correction for the first tip  70 . 
     C. Third Embodiment 
       FIGS. 11(A) to 11(C)  are explanatory views showing positional correction work for a tip to be conducted at a middle position in a third embodiment of the present invention and correspond to  FIGS. 6(A) and 6(B)  showing that in the first embodiment. In the third embodiment, in place of the position-correcting chuck  424  shown in  FIG. 6(A) , a servo stage  800  and a camera  820  are used to perform positional correction for the first tip  70 . The servo stage  800  shown in  FIG. 11(A)  is a table which can perform two-dimensional positioning through utilization of a servomechanism. A vacuum chuck block  810  having a vacuum chuck hole  812  is fixed on the servo stage  800 . The vacuum chuck block  810  functions as a placement table on which the first tip  70  is placed. The vacuum chuck hole  812  has a vacuum chuck port which opens at the upper surface of the vacuum chuck block  810 , and is connected to a vacuum pump (not shown). The camera  820  is disposed above the vacuum chuck block  810  and can capture an image of a wide region, including the vacuum chuck hole  812 , of the upper surface of the vacuum chuck block  810 . The servo stage  800  and the camera  820  are electrically connected to a control unit  830 . The control unit  830  includes an image analyzer  832 . 
     As shown in  FIG. 11(A) , when the first tip  70  is placed on the vacuum chuck block  810 , the bottom face of the first tip  70  is vacuum-chucked on the vacuum chuck block  810  by means of vacuum suction through the vacuum chuck hole  812 , and the first tip  70  is thereby held at the position. In this condition, the camera  820  captures an image of the first tip  70 .  FIG. 11(B)  shows an example of a thus-captured image, and X and Y indicate a coordinate system of the camera. In this example, the actual position of the center of the first tip  70  deviates from a target position Pt. The target position Pt is a preset position in the coordinate system of the camera; for example, the target position Pt can be set at the center of the initial position (default position) of the camera  820 . The target position Pt does not need to be marked in an image, but may be set at a position which the image analyzer  832  can recognize. As shown in  FIG. 11(B) , in the case where the actual position of the center of the first tip  70  deviates from the target position Pt, the control unit  830  adjusts the position of the servo stage  800  so as to establish the coincidence between the actual position of the center of the first tip  70  and the target position Pt as shown in  FIG. 11(C) . 
     As mentioned above, according to the third embodiment, the camera  820  captures an image of the first tip  70  whose bottom face is vacuum-chucked on the servo stage  800 , and, through utilization of the captured image, the position of the first tip  70  is corrected; therefore, the third embodiment has an advantage that positioning can be accurately performed by means of a simple configuration. 
     D. Method for Manufacturing Spark Plug 
       FIG. 12  is a flowchart showing a method for manufacturing a spark plug according to an embodiment of the present invention. In step T 10 , the metallic shell  50 , the ceramic insulator  10 , the center electrode  20 , and the ground electrode  30  are prepared. In step T 20 , the composite tip CP is formed by joining the first tip  70  and the second tip  60  together. The step of forming the composite tip CP is performed according to any one of the above-described procedures of the first to third embodiments. In step T 30 , the ground electrode  30  is joined to the metallic shell  50 . In step T 40 , a distal end portion of the ground electrode  30  is bent by use of a bending tool (not shown). In step T 50 , the composite tip CP is joined to the distal end portion  31  of the ground electrode  30  ( FIG. 4 ). This joining work is carried out through utilization of, for example, resistance welding. In step T 60 , an assembling step is performed through insertion of the center electrode  20  and the ceramic insulator  10  into the metallic shell  50 . The assembling step yields an assembly in which the ceramic insulator (insulator)  10  and the center electrode  20  are incorporated into the metallic shell  50 . The assembling step may employ either one of the following methods: a method in which the ceramic insulator  10  incorporated with the center electrode  20  is incorporated into the metallic shell  50 , and a method in which after the ceramic insulator  10  is incorporated into the metallic shell  50 , the center electrode  20  is incorporated into the ceramic insulator  10 . In step T 70 , by use of a crimping tool (not shown), crimping work is performed on the metallic shell  50 . The crimping work fixes the ceramic insulator  10  to the metallic shell  50 . In step T 80 , the gasket  5  is fitted to the mounting threaded portion  52  of the metallic shell  50 , thereby completing the spark plug  100 . 
     The manufacturing method shown in  FIG. 12  is a mere example, and various methods different from the example method are available for manufacturing a spark plug. For example, the order of the steps T 10  to T 80  can be varied to a certain extent. 
     MODIFICATIONS 
     The present invention is not limited to the above-described embodiments or modes, but may be embodied in various other forms without departing from the gist of the invention. For example, the following modifications are also possible. 
     Modification 1 
     In the above-described embodiments, the first transfer device  310  vacuum-chucks and holds the first tip  70 . However, even in the case where the first transfer device  310  employs means other than vacuum chuck for holding the first tip  70 , the present invention can be applied. In this case also, by means of the position-correcting chuck  424  performing positional correction for the first tip  70 , the first tip  70  can be placed correctly at the joining position P 2 . 
     Modification 2 
     The position-correcting chuck  424  can employ various shapes other than that shown in  FIG. 6(A) . Preferably, the position correcting chuck  424  is shaped such that, when the position-correcting chuck  424  grips the first tip  70  at its side, the first tip  70  is automatically moved to the center position of the position-correcting chuck  424 . 
     Modification 3 
     The above embodiments are described while mentioning the case of joining the first and second tips together for forming the composite tip. However, the present invention is not limited thereto, but can be applied to the case of joining a particular first tip to a tip-mating member. For example, the present invention can be applied to the case where a noble metal tip is directly joined or welded to the center electrode or the ground electrode. In this case, the noble metal tip corresponds to the “first tip,” and the center electrode or the ground electrode corresponds to the “tip-mating member.” In the above-described embodiments, it is understandable that the second tip corresponds to the “tip-mating member.” 
     In the case where the tip-mating member is a thin member (a member having a small cross section), such as the second tip  60  or the center electrode, joining the first tip to the tip-mating member requires accurate positioning of the first tip to a greater extent. Even in such a case, the present invention is effective, since positioning of the first tip can be performed simply and accurately. Particularly, in the case where a diametrical difference between the first tip and the tip-mating member is very small (e.g., the diametrical difference is 0.1 mm or less), the present invention is particularly effective. 
     DESCRIPTION OF REFERENCE NUMERALS 
       3 : ceramic resistor 
       4 : seal member 
       5 : gasket 
       6 : ring member 
       7 : ring member 
       8 : seat packing 
       9 : talc 
       10 : ceramic insulator 
       12 : axial bore 
       13 : leg portion 
       15 : stepped portion 
       17 : forward trunk portion 
       18 : rear trunk portion 
       19 : collar portion 
       20 : center electrode 
       21 : electrode base metal 
       22 : forward end portion 
       25 : core 
       30 : ground electrode 
       31 : forward end portion 
       32 : proximal portion 
       40 : metal terminal 
       50 : metallic shell 
       51 : tool engagement portion 
       52 : mounting threaded portion 
       53 : crimped portion 
       54 : seal portion 
       55 : seat surface 
       56 : stepped portion 
       57 : forward end surface 
       58 : buckled portion 
       59 : screw neck 
       60 : intermediate tip (second tip) 
       61 : columnar portion 
       62 : collar portion 
       70 : noble metal tip (first tip) 
       71 : edge 
       80 : fusion zone 
       90 : noble metal tip 
       100 : spark plug 
       200 : engine head 
       201 : mounting threaded hole 
       205 : peripheral-portion-around-opening 
       300 : transfer device 
       310 : first transfer device 
       312 : drive mechanism 
       314 : vacuum chuck 
       320 : second transfer device 
       322 : drive mechanism 
       324 : transfer chuck 
       330 : rail 
       410 : first-tip feed device 
       420 : position-correcting device 
       422 : placement table 
       423 : vacuum chuck port 
       424 : position-correcting chuck 
       425 : gripping recess 
       426 : tip suction device 
       428   a : drive mechanism 
       500 : tip-pressing device 
       510 : pressing jig 
       520 : drive mechanism 
       600 : laser welding machine 
       700 : tip support device 
       710 : gripper 
       712 : placement surface 
       714 : gripping claw 
       800 : servo stage 
       810 : vacuum chuck block 
       812 : vacuum chuck hole 
       820 : camera 
       830 : control unit 
       832 : image analyzer