Patent Publication Number: US-9889496-B2

Title: Method for manufacturing metal fitting, method for manufacturing spark plug, and method for manufacturing sensor

Description:
RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2015/003113 filed Jun. 22, 2015, which claims the benefit of Japanese Patent Application No. 2014-132790, filed Jun. 27, 2014. 
     FIELD OF THE INVENTION 
     The present invention relates to a metal fitting. 
     BACKGROUND OF THE INVENTION 
     A spark plug used for ignition in an internal combustion engine, such as gasoline engine, has a metal fitting for mounting the spark plug to a cylinder head of the engine. The metal fitting of the spark plug generally includes: a thread portion formed with an external thread for screwing into a threaded hole of the engine cylinder head; a tool engagement portion formed engageable with a tool e.g. a spark plug wrench; a seal portion formed between the thread portion and the tool engagement portion at a position continuous to the thread portion so as to ensure the gas-tightness of the inside of the engine; and a thin compression deformation portion formed between the seal portion and the tool engagement portion. 
     Conventionally, the metal fitting of the spark plug is completed through the processes of cold forging, cutting and thread forming. By the cold forging process, the metal fitting is provided in semi-finished form close to the finished product (see, for example, Japanese Laid-Open Patent Publication No. H07-016693). 
     As discussed in Japanese Laid-Open Patent Publication No. H07-016693, the cold forging process of the conventional metal fitting manufacturing method contains a plurality of steps. In the step of formation of the tool engagement portion of the finished metal fitting product, it is conceivable to form the tool engagement portion by “drawing” or “bulging”. 
     Herein, “drawing” and “bulging” operations for the formation of the tool engagement portion will be described below with reference to  FIG. 11 .  FIG. 11  is a schematic view showing drawing and bulging operations in the cold forging process.  FIGS. 11(A)  and (B) shows one example of drawing operation; and  FIGS. 11(C)  and (D) shows one example of bulging operation.  FIG. 11  refers to the case where the tool engagement portion is formed in a substantially regular hexagonal cross-sectional shape. When the tool engagement portion is formed by drawing a base material of substantially cylindrical column shape (including hollow cylindrical column shape), a diagonal dimension of the tool engagement portion becomes smaller than or equal to an outer diameter of the base material as shown in  FIGS. 11(A)  and (B). When the tool engagement portion is formed by bulging a base material of substantially cylindrical column shape, a diagonal dimension of the tool engagement portion becomes larger than or equal to an outer diameter of the base material as shown in  FIGS. 11(C)  and (D). When the tool engagement portion is formed in a non-regular polygonal cross-sectional shape by drawing, a maximum diagonal dimension (longest diagonal length) of the tool engagement portion becomes smaller than or equal to an outer diameter of the base material. When the tool engagement portion is formed in a non-regular polygonal cross-sectional shape by bulging, a minimum diagonal dimension (shortest diagonal line length) of the tool engagement portion becomes larger than or equal to an outer diameter of the base material. 
     To manufacture the metal fitting in which an outer diameter of the seal portion is smaller than the maximum diagonal dimension of the tool engagement portion, the tool engagement portion is conventionally formed by either of the following two processes: 1) forming the tool engagement portion by bulging; and 2) enlarging a part of the base material to an outer diameter larger than the maximum diagonal dimension of the tool engagement portion, thereby providing a portion to be formed into the seal portion (hereinafter referred to as “pre-seal portion”) integral with a portion to be formed into the tool engagement portion (hereinafter referred to as “pre-tool engagement portion”), and then, forming the tool engagement portion by drawing (see Japanese Laid-Open Patent Publication No. H07-016693). 
     The process 1) has the problem that the die for the bulging is high in cost and short in lifetime. The process 2) attains a reduction of die cost as compared to the process 1). In the process 2), however, the pre-seal portion and the pre-tool engagement portion are integrally formed by diameter enlargement such that the outer diameter of the pre-seal portion becomes larger than the outer diameter of the seal portion. The process 2) thus causes an increase of cutting amount during the formation of the seal portion in the cutting process, which leads to the problem of increase in chip treatment workload, deterioration in cutting edge lifetime, increase in material input etc. In other words, these conventional processes face the common problem of high manufacturing cost. This problem is common to various metal fittings with tool engagement portions, such as those for not only spark plugs but also sensors e.g. temperature sensors and other devices. Consequently, there has been a demand to develop a technique for reducing the manufacturing cost of metal fittings. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to address the above problem and can be embodied as the following application examples. 
     (1) According to a first aspect of the present invention, there is provided a manufacturing method of a metal fitting, the metal fitting comprising a tool engagement portion engageable with a tool, the manufacturing method comprising a cold forging process, wherein the cold forging process includes: a step (a) of forming a body portion and a butt portion, the body portion having a first maximum length, the butt portion being continuous to the body portion and having a second maximum length larger than the first maximum length; and a step (b) of drawing at least a part of the butt portion in an axis direction of the metal fitting, thereby forming the tool engagement portion. The first maximum length refers to a maximum length of the body portion in a direction perpendicular to the axis direction. The second maximum direction refers to a maximum length of the butt portion in the direction perpendicular to the axis direction. 
     In the above metal fitting manufacturing method, the tool engagement portion is formed by drawing during the cold forging process. It is thus possible to, at the time of manufacturing the metal fitting in which the outer diameter of the seal portion is smaller than the diagonal dimension of the tool engagement portion, attain a reduction of die cost and improvement of die lifetime during the cold forging process as compared to the case of forming the tool engagement portion by bulging. It is also possible to achieve a reduction of cutting amount in the subsequent cutting process as compared to the case of enlarging the outer diameter of the entire body portion and then forming the tool engagement portion by drawing. It is accordingly possible to reduce the manufacturing cost of the metal fitting. 
     (2) In accordance with a second aspect of the present invention, there is provided a manufacturing method of a metal fitting according to the above aspect of the present invention, wherein the manufacturing method further comprises a cutting process of cutting at least a part of the butt portion; and wherein, in the step (b), the tool engagement portion is formed on a region of the butt portion including a first end thereof not adjacent to the body portion. 
     In this case, the tool engagement portion, the butt portion and the body portion are arranged continuously in order of mention after the execution of the steps (a) and (b). When the body portion is shaped into a seal portion of the finished metal fitting, the butt portion between the body portion and the tool engagement portion is shaped into a compression deformation portion of the finished metal fitting. The compression deformation portion is thin and is conventionally formed by cutting. In the above metal fitting manufacturing method, the butt portion is formed with a maximum length larger than that of the body portion in order to form the tool engagement portion by drawing in the cold forging process. Then, the tool engagement portion is formed by drawing on the region of the butt portion including the first end not adjacent to the body portion. In the cutting process, the diameter of a portion to be formed into the compression deformation portion (i.e. a part of the butt portion) is reduced by cutting. Since the cutting process is conventionally employed as mentioned above, there is no need to add another cutting process. It is thus possible to prevent an increase in the number of operation processes and reduce the manufacturing cost of the metal fitting. 
     (3) According to a third aspect of the present invention, there is provided a manufacturing method of a metal fitting according to the above aspect of the present invention, wherein the manufacturing method further comprises a cutting process of cutting at least a part of the butt portion; and wherein, in the step (b), the tool engagement portion is formed on a region of the butt portion including a second end thereof adjacent to the body portion such that a maximum cross-sectional diagonal dimension of the tool engagement portion is larger than the first maximum length of the body portion. 
     In this case, the butt portion, the tool engagement portion, and the body portion are arranged continuously in order of mention after the execution of the steps (a) and (b). As mentioned above, the thin compression deformation portion is provided between the tool engagement portion and the seal portion in the finished metal fitting. In the above metal fitting manufacturing method, the compression deformation portion is formed by cutting a part of the body portion in the cutting process. Since the body portion is smaller in maximum length than the butt portion, it is possible to achieve a reduction of cutting amount in the cutting process and reduce the manufacturing cost of the metal fitting. Further, a crimp portion of the metal fitting is formed by cutting the butt portion in the cutting process. Since the crimp portion is conventionally formed by cutting, there is no need to add another cutting process. It is thus possible to prevent an increase in the number of operation processes. 
     (4) In accordance with a fourth aspect of the present invention, there is provided a manufacturing method of a metal fitting according to the above aspect of the present invention, wherein the manufacturing method further comprises a cutting process of cutting at least a part of the butt portion; and wherein, in the step (b), the tool engagement portion is formed such that a maximum cross-sectional diagonal dimension of the tool engagement portion is larger than the first maximum length of the body portion. 
     Even in this case, it is possible to obtain the same effects as above. In the case where the maximum cross-sectional diagonal dimension of the tool engagement portion is larger than the maximum length of the body portion, the tool engagement portion is conventionally formed by bulging or cutting all of the portions including the tool engagement portion. By contrast, the tool engagement portion is formed by drawing in the above metal fitting manufacturing method. It is thus possible to attain a reduction of die cost and cutting cost and obtain a great manufacturing cost reduction effect. 
     (5) According to a fifth aspect of the present invention, there is provided a manufacturing method of a metal fitting according to the above aspect of the present invention, wherein at least parts of the body and butt portions of the metal fitting manufactured by the manufacturing method have the same maximum lengths as those of the body and butt portions formed in the step (a). 
     In this case, some part of the metal fitting is finished by the cold forging process without the cutting process. It is thus possible to achieve a reduction of cutting amount and a decrease in the number of operation processes and improve the manufacturing cost of the metal fitting. 
     It should be noted that the present invention can be embodied in various forms such as a spark plug manufacturing method, a sensor manufacturing method, a metal fitting, a spark plug and a sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view, partially in section, of a metal fitting produced by a metal fitting manufacturing method according to a first embodiment of the present invention. 
         FIG. 2  is a schematic view of a spark plug to which the metal fitting is applied. 
         FIG. 3  is a plan view of the metal fitting as viewed from the rear side. 
         FIG. 4  is a flow chart of the metal fitting manufacturing method according to the first embodiment of the present invention. 
         FIGS. 5(A)-5(F)  are schematic views showing, in half section, semi-finished products obtained in respective steps of cold forging process of the metal fitting manufacturing method according to the first embodiment of the present invention. 
         FIGS. 6(A)-6(E)  are schematic views showing, in half section, semi-finished products obtained in respective steps of cold forging process of a comparative example method 1. 
         FIGS. 7(A)-7(F)  are schematic views showing, in half section, semi-finished products obtained in respective steps of cold forging process of a comparative example method 2. 
         FIGS. 8(A) and 8(B)  are schematic views showing a difference in cutting amount between a cutting process of the metal fitting manufacturing method according to the first embodiment and a cutting process of the comparative example method 2. 
         FIGS. 9(A)-9(F)  are schematic views showing, in half section, semi-finished products obtained in respective steps of cold forging process of a metal fitting manufacturing method according to a second embodiment of the present invention. 
         FIG. 10  is a plan view of a tool engagement portion of a metal fitting according to a modified embodiment of the present invention. 
         FIGS. 11(A)-11(D)  are schematic views showing drawing and bulging operations in a cold forging process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A. First Embodiment 
     A-1. Configuration of Metal Fitting 
       FIG. 1  is a schematic view, partially in section, of a metal fitting produced by a metal fitting manufacturing method according to a first embodiment of the present invention.  FIG. 2  is a schematic view of a spark plug  100  to which the metal fitting is applied. In  FIG. 1 , an axis of the fitting  50  is designated by OL. An appearance of the metal fitting  50  is shown on the left side of axis OL in  FIG. 1 , whereas a cross section of the metal fitting  5 —is shown on the right side of the axis OL in  FIG. 1 . In  FIG. 2 , an axis of the spark plug  100  is designated by OL. (The axis of the spark plug  100  is in agreement with the axis of the metal fitting  50 .) An appearance of the spark plug  100  is shown on the left side of the axis OL in  FIG. 2 , whereas a cross section of the spark plug  100  is shown on the right side of the axis OL in  FIG. 2 . In the following description, a direction parallel to the axis OL is referred to as an “axis direction OD”. Further, the term “front” refers to a bottom side of the figure (i.e. a side on which the after-mentioned ground electrode  30  is located) in the axis direction; and the term “rear” refers to a top side of the figure (i.e. a side on which the after-mentioned metal terminal  40  is located) in the axis direction. 
     As shown in  FIG. 2 , the spark plug  100  includes an insulator  10  as an insulating member, a center electrode  20 , a ground electrode (outer electrode)  30  and a metal terminal  40  in addition to the metal fitting  50 . The insulator  10  has a cylindrical shape, with an axial hole  12  formed therein, so as to accommodate therein the center electrode  20  and the metal terminal  40 . For example, the insulator  10  can be formed by firing a ceramic material such as alumina. The center electrode  20  has a substantially rod-like shape. In the first embodiment, the center electrode  20  includes a cylindrical-shaped cover material  21  and a core material  25  embedded in the cover material  21  and having a higher thermal conductivity than that of the cover material  21 . The center electrode  20  is held in the insulator  10 . The insulator  10  is held in the metal fitting  50 . The ground electrode  30  has a bent, substantially rod-like shape. The ground electrode  30  is joined to a front end portion of the metal fitting  50 . The metal terminal  40  is fitted in a rear end side of the insulator  10 . There is a spark discharge gap defined between a free end of the ground electrode  30  and a front end of the center electrode  20 . 
     The metal fitting  50  has a substantially cylindrical shape, with a through hole  59  formed therein along the axis direction, so as to accommodate and hold therein a part of the insulator  10  as shown in  FIG. 2 . An external thread is formed on an outer circumference of the metal fitting  50  such that the spark plug is mounted to a cylinder head of an engine by screwing the external thread into a threaded hole of the engine cylinder head. The metal fitting  50  is formed of a metal material e.g. low carbon steel. 
     As shown in  FIG. 1 , the metal fitting  50  generally includes a crimp portion  53 , a tool engagement portion  51 , a compression deformation portion  55 , a seal portion  54  and a thread portion  52  arranged in this order from the rear side.  FIG. 3  is a plan view of the metal fitting  50  as viewed from the rear side. The tool engagement portion  51  is substantially regular hexagonal in shape when viewed in plan. At the time of mounting the spark plug to the engine cylinder head, a tool (such as spark plug wrench) is engaged on the tool engagement portion  51 . In the first embodiment, the relationship of D1&lt;L1 is satisfied where L1 is an opposite side dimension of the tool engagement portion  51 ; L2 is a diagonal dimension of the tool engagement portion  51 ; and D1 is an outer diameter of the seal portion  54 . 
     The thread portion  52  has the above-mentioned external thread formed on a circumferential surface thereof and screwed into the threaded hole of the engine cylinder head for mounting of the spark plug to the engine cylinder head. 
     The seal portion  54  is formed between the thread portion  52  and the tool engagement portion  51  at a position continuous to the read portion  52  so as to, when the spark plug  100  is mounted to the engine cylinder head, prevent the leakage of gas from inside of the engine through the threaded hole of the engine cylinder head. An annular gasket  5 , which is formed by bending a plate material, is fitted between the thread portion  52  and the seal portion  54 . The seal portion  54  seals the threaded hole of the engine cylinder head through the gasket  5  to prevent air-fuel mixture from leaking from the inside of the engine through the threaded hole. 
     The crimp portion  53  is formed at a rear end side of the metal fitting  50  as shown in  FIG. 1 . The crimped portion  53  is made thin. As shown in  FIG. 2 , the crimp portion  53  is bent radially inwardly by crimping after the insulator  10  is inserted in the through hole  59  of the metal fitting  50 . By such crimping, the insulator  10  is held in the metal fitting  50  as one assembly unit. 
     As shown in  FIG. 1 , the compression deformation portion  55  is formed between the tool engagement portion  51  and the seal portion  54 . The compression deformation portion  55  is made thin as in the case of the crimp portion  53 . With the application of compression force during the formation of the crimp portion  53  by crimping, the compression deformation portion  55  is deflected and deformed outwardly to increase the gas-tightness of the inside of the metal fitting  50  as shown in  FIG. 2 . More specifically, annular ring members  6  and  7  are disposed in a space between an inner circumferential surface of a part of the metal fitting  50  from the tool engagement portion  51  to the crimp portion  53  and an outer circumferential surface of the insulator  10 . Further, a talc powder (talc)  9  is filled between the ring members  6  and  7 . When the crimp portion  53  is inwardly bent by crimping, the insulator  10  is pressed toward the front within the metal fitting  50  via the ring members  6  and  7  and the talc  9  so that the metal fitting  50  and the insulator  10  are held together as one unit with a reduced diameter portion  15  of the insulator  10  retained on a step portion  56  of the inner circumferential surface of the metal fitting  50 . An annular plate packing  8  is disposed between the reduced diameter portion  15  of the insulator  10  and the step portion  56  of the metal fitting  50  so as to maintain gas-tightness between the metal fitting  50  and the insulator  10  and prevent the leakage of combustion gas. For example, the plate packing  8  can be formed of a high thermal conductive material such as copper or aluminum. When the plate packing  8  is high in thermal conductivity, heat of the insulator  10  is efficiently conducted to the step portion  56  of the metal fitting  50  via the plate packing. It is thus possible to increase the heat radiation performance of the spark plug  100  for improvement of heat resistance. The compression deformation portion  55  is outwardly deflected and deformed with the application of compression force during the crimping, thereby enhancing the compression stroke of the talc  9  and increasing the gas-tightness of the inside of the metal fitting  50 . There is a clearance CL of predetermined dimension left between the insulator  10  and a part of the metal fitting  50  located front of the step portion  56 . 
     A-2. Manufacturing Method of Metal Fitting 
     A manufacturing method of the metal fitting  50  according to the first embodiment will be described below with reference to  FIG. 4  and  FIGS. 5(A)-5(F) .  FIG. 4  is a flow chart of the manufacturing method of the metal fitting  50  according to the first embodiment.  FIGS. 5(A)-5(F)  are schematic views showing, in half section, semi-finished products obtained in respective steps of cold forging process of the manufacturing method of the metal fitting  50  according to the first embodiment. In  FIGS. 5(A)-5(F) , appearance and cross section of each semi-finished forging product are shown on the left and right side of the axis, respectively. The axis of each semi-finished forging product is in agreement with the axis of the metal fitting  50 . 
     As shown in  FIG. 4 , the manufacturing method of the metal fitting  50  according to the first embodiment includes the processes of: preparing a starting material (step S 110 ); forming a semi-finished product  500 F ( FIG. 5(F) ) of the metal fitting  50  by cold-forging the starting material (step S 120 ); cutting the semi-finished product  500 F (step S 130 ); welding the ground electrode  30  to the semi-finished product after the cutting (hereinafter also referred to as “semi-finished cutting product”); forming the external thread (step S 140 ); and performing plating treatment (step S 142 ). The metal fitting  50  ( FIG. 1 ) is completed through these processes. 
     In step S 110 , a substantially cylindrical column-shaped metal material is used as the starting material (not shown). The starting material is prepared by e.g. shear-cutting a metal wire. 
     In the first embodiment, the cold forging process (step S 120 ) contains six cold forging operations (steps). The cold forging process will be explained in detail below with reference to  FIGS. 5(A)-5(F) . In the following explanation, bottom and top sides of the figure are respectively referred to as front and rear sides of the semi-finished product in correspondence with those of the metal fitting  50 . 
     First, the starting material is subjected to extrusion (the first cold forging step) so as to narrow a front end region of the starting material and thereby form a semi-finished product  500 A ( FIG. 5(A) ). The semi-finished product  500 A is provided with a pre-body portion  501  and a leg portion  504 . The pre-body portion  501  has a substantially cylindrical column shape with an outer diameter substantially equal to that of the starting material. The leg portion  504 , which is to be formed into the thread portion  52  in the later process step, has a substantially cylindrical column shape with an outer diameter smaller than that of the pre-body portion  501 . First and second holes  506  and  508  are made in rear and front ends of the semi-finished product  500 A, respectively. 
     It is herein noted that, in the present specification, the expression “substantially cylindrical column shape” includes the concept of “hollow cylindrical column shape” and includes the concepts of “shape having a cross section slightly deviated from a perfect circle” and “shape having an elliptical cross section” without being limited to a perfect circular cross section. 
     Next, the semi-finished product  500 A is processed into a semi-finished product  500 B ( FIG. 5(B) ) in the second cold forging step. In the second cold forging step, the first and second holes  506  and  508  are elongated; and a middle region of the pre-body portion  501  of the semi-finished product  500 A in the axis direction OD is bulged to form a pre-crimp portion  512 , a butt portion  510  and a body portion  502 . Namely, the semi-finished product  500 B is provided with the pre-crimp portion  512 , the butt portion  510 , the body portion  502  and the leg portion  504 . 
     The pre-crimp portion  512 , which is to be formed into the crimp portion  53  in the later process step, has a substantially cylindrical column shape with an outer diameter substantially equal to the outer diameter D1 of the body portion  502 . The butt portion  510 , which is to be formed into the tool engagement portion  510  and the compression deformation portion  55  in the later process steps, has a substantially cylindrical column shape with an outer diameter D2 larger than the outer diameter D1 of the body portion  502  and larger than the diagonal dimension L2 of the tool engagement portion  51  of the metal fitting  50 . The body portion  502 , which is to be formed into the seal portion  54  in the later process step, has a substantially cylindrical column shape with an outer diameter substantially equal to the outer diameter D1 of the seal portion  54 . In the first embodiment, the outer diameter of the body portion corresponds to the claimed first maximum length; and the outer diameter of the butt portion corresponds to the claimed second maximum length. 
     The semi-finished product  500 B is processed into a semi-finished product  500 C ( FIG. 5(C) ) in the third cold forging step. In the third cold forging step, the first and second holes  506  and  508  are further elongated; and the body portion  502  and the leg portion  504  are elongated. 
     The semi-finished product  500 C is further processed into a semi-finished product  500 D ( FIG. 5(D) ) in the fourth cold forging step. In the fourth cold forging step, the first hole  506  is further elongated; and a rear end region of the butt portion  510  of the semi-finished product  500 C (i.e. a region of the butt portion  510  including a first end  510   u  not adjacent to the body portion  502 ) is subjected to drawing to form a tool engagement portion  514 . Namely, the semi-finished product  500 D is provided with the pre-crimp portion  512 , the tool engagement portion  514  the butt portion  510 , the body portion  502  and the leg portion  504 . The tool engagement portion  514  corresponds to the tool engagement portion  51  of the finished metal fitting  50  and has a substantially regular hexagonal outer shape in plan view. 
     The semi-finished product  500 D is processed into a semi-finished product  500 E ( FIG. 5(E) ) in the fifth cold forging step. In the fifth cold forging step, the first and second holes  506  and  508  are further elongated; and the leg portion  504  is further elongated. 
     The semi-finished product  500 E is then processed into a semi-finished product  500 F ( FIG. 5(F) ) by the sixth cold forging operation. In the sixth cold forging operation, the first and second holes  506  and  508  are connected to and communicated with each other to define a through hole  516 . The semi-finished product  500 F, which is the final product of the cold forging process (step S 120  in  FIG. 4 ), has a substantially cylindrical shape (substantially hollow cylindrical column shape) where the through hole  516  is identical to the through hole  59  of the finished metal fitting  50 . 
     In step S 130 , cutting is performed on outer circumferential surfaces of the pre-crimp portion  512 , the tool engagement portion  514 , the butt portion  510  and the body portion  512  of the semi-finished product  500 F obtained in the cold forging process (S 120 ), so as to correspond to the crimp portion  53 , the tool engagement portion  51 , the compression deformation portion  55  and the seal portion  54  of the metal fitting  50 . Thus provided is a semi-finished cutting product. 
     In step S 132 , the ground electrode  30  is welded to the leg portion  504  of the semi-finished cutting product. In step S 140 , thread forming (rolling) is performed on an outer circumferential surface of the leg portion  504  so as to form the thread portion  52  of the metal fitting  50 . In step  142 , nickel plating is applied to the surface of the metal fitting for corrosion protection. In this way, the metal fitting  50  is completed. 
     The spark plug  100  ( FIG. 2 ) is manufactured by assembling the above-obtained metal fitting  50  with the other plug components. 
     A-3. Effects of First Embodiment 
     The effects of the metal fitting manufacturing method according to the first embodiment will be explained below in comparison with comparative metal fitting manufacturing methods 1 and 2. 
     The comparative metal fitting manufacturing methods 1 and 2 are similar to the metal fitting manufacturing method according to the first embodiment, except for the cold forging process (step S 120  in  FIG. 4 ). Hence, an explanation will be given only of the cold forging processes of the comparative metal fitting manufacturing methods; and explanations of the other processes of the comparative metal fitting manufacturing methods will be omitted herefrom. It is herein assumed that, in each of the comparative metal fitting manufacturing methods 1 and 2, a metal fitting of the same shape as the metal fitting  50  of  FIG. 1  is manufactured. 
     The effects of the metal fitting manufacturing method according to the first embodiment against the comparative metal fitting manufacturing method 1 will be now explained below.  FIG. 6  is a schematic view showing, in half section, semi-finished products obtained in respective step of cold forging process of the comparative metal fitting manufacturing method 1. In  FIG. 6 , appearance and cross section of each semi-finished forging product are shown on the left and right side of the axis, respectively, as in the case of  FIG. 5 . 
     In the comparative metal fitting manufacturing method 1, the cold forging process contains five cold forging operations (steps). 
     A starting material is first subjected to extrusion (the first cold forging step) so as to narrow a front end region of the starting material and thereby form a semi-finished product  550 PA ( FIG. 6(A) ). The semi-finished product  550 PA is provided with a body portion  552 P and a leg portion  554 P. The body portion  552 P has a substantially cylindrical column shape with an outer diameter substantially equal to that of the starting material and smaller than the diagonal dimension of the tool engagement portion  51 . The leg portion  554 P, which is to be formed into the thread portion  52  in the later process step, has a substantially cylindrical column shape with an outer diameter smaller than that of the body portion  552 P. First and second holes  556 P and  558 P are made in rear and front ends of the semi-finished product  550 PA, respectively. 
     The semi-finished product  550 PA is processed into a semi-finished product  550 PB ( FIG. 6(B) ) in the second cold forging step. In the second cold forging step, the first and second holes  556 P and  558 P are elongated; and a rear end region of the body part  552 P of the semi-finished product  550 PA in the axis direction OD is bulged to form a tool engagement portion  564 P. Namely, the semi-finished product  550 PB is provided with the tool engagement portion  564 P, the body portion  552 P and the leg portion  554 P. The tool engagement portion  564 P corresponds to the tool engagement portion  51  of the finished metal fitting  50  and has a substantially regular hexagonal outer shape in plan view. In the comparative metal fitting manufacturing method 1, the crimp portion  53  is formed by cutting a rear end region of the tool engagement portion  564 P in the later process step. The compression deformation portion  55  and the seal portion  54  are formed by cutting the body portion  552 P in the later process step. 
     The semi-finished product  550 PB is processed into a semi-finished product  550 PC ( FIG. 6(C) ) in the third cold forging step. In the third cold forging step, the first and second holes  556 P and  558 P are further elongated; and the body portion  552 P and the leg portion  554 P are elongated. 
     The semi-finished product  550 PC is processed into a semi-finished product  550 PD ( FIG. 6(D) ) in the fourth cold forging step. In the fourth cold forging step, the first and second holes  556 P and  558 P are further elongated; and the leg portion  554 P is further elongated. 
     The semi-finished product  550 PD is then processed into a semi-finished product  550 PE ( FIG. 6(E) ) in the fifth cold forging step. In the fifth cold forging step, the first and second holes  556 P and  558 P are connected to and communicated with each other to define a through hole  566 P. The semi-finished product  550 PE is the final product of the cold forging process in the comparative metal fitting manufacturing method 1. 
     In the comparative metal fitting manufacturing method 1, the tool engagement portion  564 P is formed by bulging during the second step of the cold forging process. The die for the bulging is expensive. Due to the fact that the hollow part of the bulging die used in the comparative metal fitting manufacturing method 1 has a regular hexagonal shape in plan view according to the regular hexagonal plan shape of the tool engagement portion  51  of the metal fitting  50 , the bulging die is likely to be broken due to the concentration of stress on corners of the hexagonal hollow die part and is short in lifetime. 
     In the metal fitting manufacturing method according to the first embodiment, on the other hand, the body portion  502  is formed in a substantially circular column shape with an outer diameter D1 substantially equal to the outer diameter of the seal portion  54  of the metal shell  50  (i.e. smaller than the diagonal dimension L2 of the tool engagement portion  51  of the metal fitting  50 ) during the second step of the cold forging process. Further, the butt portion  510  is formed, at a position continuous to the body portion  502 , with an outer diameter D2 larger than the outer diameter D1 of the body portion  510  during second step of the cold forging process. The outer diameter D2 of the butt portion  510  is larger than the diagonal dimension L2 of the tool engagement portion  51  of the metal fitting  50 . Thus, the tool engagement portion  51  of the metal fitting  50  is formed by drawing a part of the butt portion in the metal fitting manufacturing method according to the first embodiment. The die for the drawing is lower in cost and longer in lifetime than the die for the bulging. It is therefore possible to attain a reduction of die cost and reduce the manufacturing cost of the metal fitting. 
     Next, the effects of the metal fitting manufacturing method according to the first embodiment against the comparative metal fitting manufacturing method 2 will be explained below.  FIG. 7  is a schematic view showing, in half section, semi-finished products obtained in respective steps of cold forging process of the comparative metal fitting manufacturing method 2. 
     In the comparative metal fitting manufacturing method 2, the cold forging process contains six cold forging operations (steps) as in the metal fitting manufacturing method according to the first embodiment. 
     A starting material is first subjected to extrusion (the first cold forging step) so as to narrow a front end region of the starting material and thereby form a semi-finished product  500 PA ( FIG. 7(A) ). The semi-finished product  500 PA is provided with a pre-body portion  501 P and a leg portion  504 P. The pre-body portion  501 P has a substantially cylindrical column shape with an outer diameter substantially equal to that of the starting material. The leg portion  504 P, which is to be formed into the thread portion  52  in the later process step, has a substantially cylindrical column shape with an outer diameter smaller than that of the pre-body portion  501 P. First and second holes  506 P and  508 P are made in rear and front ends of the semi-finished product  500 PA, respectively. 
     The semi-finished product  500 PA is processed into a semi-finished product  500 PB ( FIG. 7(B) ) in the second cold forging step. In the second cold forging step, the first and second holes  506 P and  508 P are elongated; and a front end region of the pre-body portion  501 P of the semi-finished product  500 PA in the axis direction OD is bulged to form a pre-crimp portion  512 P and a body portion  502 P. Namely, the semi-finished product  500 PB is provided with the pre-crimp portion  512 P, the body portion  502 P and the leg portion  504 P. 
     The pre-crimp portion  512 P, which is to be formed into the crimp portion  53  in the later process step, has a substantially cylindrical column shape with an outer diameter substantially equal to that of the pre-body portion  501 P of the semi-finished product  500 PA. The body portion  502 , which is to be formed into the tool engagement portion  51 , the compression deformation portion  55  and the seal portion  54  in the later process steps, has a substantially cylindrical column shape with an outer diameter D3 larger than the diagonal dimension L2 of the tool engagement portion  51  of the metal fitting  50 . In this comparative example, the outer diameter D3 of the body portion  502  is substantially equal to the outer diameter D2 of the butt portion  510  of the first embodiment and larger than the outer diameter D1 of the seal portion  54  of the metal fitting  50 . 
     The semi-finished product  500 PB is processed into a semi-finished product  500 PC ( FIG. 7(C) ) in the third cold forging step. In the third cold forging step, the first and second holes  506 P and  508 P are further elongated; and the leg portion  554 P is elongated. 
     The semi-finished product  500 PC is processed into a semi-finished product  500 PD ( FIG. 7(D) ) in the fourth cold forging step. In the fourth cold forging step, the first hole  506 P is further elongated; a rear end region of the body portion  502 P of the semi-finished product  500 PC is subjected to drawing to form a tool engagement portion  514 P; and the leg portion  504 P is further elongated. Namely, the semi-finished product  500 PD is provided with the pre-crimp portion  512 P, the tool engagement portion  514 P, the body portion  502 P and the leg portion  504 P. The tool engagement portion  514 P corresponds to the tool engagement portion  51  of the finished metal fitting  50  and has a substantially regular hexagonal outer shape in plan view. 
     The semi-finished product  500 PD is processed into a semi-finished product  500 PE ( FIG. 7(E) ) in the fifth cold forging step. In the fifth cold forging step, the first and second holes  506 P and  508 P are further elongated; and the leg portion  504  is further elongated. 
     The semi-finished product  500 PE is processed into a semi-finished product  500 PF ( FIG. 7(F) ) in the sixth cold forging step. In the sixth cold forging step, the first and second holes  506 P and  508 P are connected to and communicated with each other to define a through hole  516 P. The semi-finished product  500 PF is the final product of the cold forging step in the comparative metal fitting manufacturing method 2. 
     In the comparative metal fitting manufacturing method 2, the body portion  502 P is formed in a substantially cylindrical column shape with an outer diameter D3 larger than the diagonal dimension L2 of the tool engagement portion  51  of the metal fitting  50  during the second step of the cold forging process. The tool engagement portion  51  of the metal fitting  50  is thus formed by drawing a part of the body portion in the comparative metal fitting manufacturing method 2 in the same manner as in the metal fitting manufacturing method according to the first embodiment. 
     In the comparative metal fitting manufacturing method 2, however, the body portion  502 P, which is to be formed into the seal portion  54  in the later process step, is substantially cylindrical column-shaped with an outer diameter D3 larger than the outer diameter D1 of the seal portion  54  in order to form the tool engagement portion by drawing in the cold forging process. Then, the outer circumference of the body portion  502 P is subjected to cutting in the cutting process as will be explained later, thereby forming the seal portion  54 . 
     In the metal fitting manufacturing method according to the first embodiment, on the other hand, the body portion  502 , which is to be formed into the seal portion  54  in the later process step, is substantially cylindrical column-shaped with an outer diameter D1 substantially equal to that of the seal portion  54  in the cold forging process. Further, the butt portion  510  is formed, at a position continuous to the body portion  502 , in a substantially cylindrical shape with an outer diameter D2 larger than the outer diameter D1 of the body portion  502  and larger than the diagonal dimension L2 of the tool engagement portion  51 . The tool engagement portion  514  is then formed by drawing at least a part of the butt portion  510  in the cold forging process. Without performing the cutting process on the outer circumference of the body portion  502 , the seal portion  54  is substantially finished by forging; and the tool engagement portion  515  is formed by drawing. 
     The cutting processes of the first embodiment and the above comparative example will be explained below with reference to  FIG. 8 .  FIG. 8  is a schematic view showing a difference in cutting amount between the cutting process of the metal fitting manufacturing method according to the first embodiment and the cutting process of the comparative metal fitting manufacturing method 2.  FIG. 8(A)  shows the semi-finished product before and after the cutting process in the metal fitting manufacturing method according to the first embodiment; and  FIG. 8(B)  shows the semi-finished product before and after the cutting process in the comparative metal fitting manufacturing method 2. In  FIG. 8 , the final semi-finished product of the cold forging process is indicated by a broken line; and the semi-finished product after the cutting process is indicated by a solid line. In order to clearly indicate the cutting amount during the cutting process, a part of the semi-finished product cut by the cutting process is indicated by cross-sectional hatching different from that of the semi-finished product after the cutting process. 
     As shown in  FIG. 8(A) , cutting is mainly performed on a front end region of the tool engagement portion  514  and the butt portion  510  of the semi-finished product  500 F during the cutting process, thereby forming the tool engagement portion  51  and the compression deformation portion  55 , in the metal fitting manufacturing method according to the first embodiment. During the cutting process, cutting is also performed on the pre-crimp portion  512  and the body portion  502  for deburring, chamfering, fine adjustment e.g. surface state adjustment and the like. The semi-finished cutting product  50 B is provided by cutting the semi-finished product  500 F. The tool engagement portion  51  is formed by cutting the tool engagement portion  514  to a shorter axial length during the cutting process. In the metal fitting manufacturing method according to the first embodiment, the outer circumferential shape (regular hexagonal shape) of the tool engagement portion remains the same even after the cutting process as shown in the figure. Since the body portion  502  is made substantially equal in outer diameter to the seal portion  54  in the first embodiment, the outer circumferential shape (outer diameter) of the body portion  502  remains the same even after the chamfering etc. 
     In the comparative metal fitting manufacturing method 2, cutting is mainly performed on a front end region of the tool engagement portion  514 , a rear end region of the body portion  502 P and an outer circumferential surface of the body portion  502 P of the semi-finished product  500 PF during the cutting process, thereby forming the tool engagement portion  51 , the compression deformation portion  55  and the seal portion  54 , as shown in  FIG. 8(B) . During the cutting process, cutting is also performed on the pre-crimp portion  512 P and the body portion  502 P for deburring, chamfering, fine adjustment e.g. surface state adjustment and the like. The tool engagement portion  51  is formed by cutting the tool engagement portion  514 P to a shorter axial length during the cutting process. In the comparative metal fitting manufacturing method 2, the outer circumferential shape (regular hexagonal shape) of the tool engagement portion remains the same even after the cutting process as shown in the figure. However, the outer diameter D3 of the body portion  502 P is larger than the outer diameter D1 of the seal portion  54  and thus is reduced to D1 by cutting during the cutting process in the comparative metal fitting manufacturing method 2. It is consequently possible to achieve a reduction of cutting amount during the cutting process subsequent to the cold forging step, suppress an increase in chip treatment workload, deterioration in cutting edge lifetime, increase in material input etc. and thereby reduce the manufacturing cost of the metal fitting in the metal fitting manufacturing method according to the first embodiment as compared to the comparative metal fitting manufacturing method 2. 
     As shown in  FIG. 8 , the compression deformation portion  55  of the metal fitting  50  is thin and is formed by cutting even in the comparative metal fitting manufacturing method 2. Without being limited to the comparative metal fitting manufacturing method 2, it has been conventional to form the compression deformation portion  55  by cutting. In the first embodiment, the body portion  502 , which is to be formed into the seal portion  54 , is made substantially equal in outer diameter to the seal portion  54  so that there is no need to perform cutting on the outer circumference of the body portion  502 . Further, the butt portion  510 , a part of which is to be formed into the compression deformation portion  55  in the later cutting process, is formed by diameter enlargement so that the tool engagement portion  514  is formed by drawing a part of the butt portion  510  in the first embodiment. It is thus possible to reduce the manufacturing cost of the metal fitting, without increase of cutting operation, by forming the butt portion  510  so as to correspond to the compression deformation portion  55  in the metal fitting manufacturing method according to the first embodiment. 
     B. Second Embodiment 
     B-1. Manufacturing Method of Metal Fitting 
     A manufacturing method of a metal fitting according to a second embodiment of the present invention will be described below with reference to  FIGS. 9(A)-9(F) . The metal fitting manufactured by the metal fitting manufacturing method according to the second embodiment is the same in shape as the metal fitting  50  ( FIG. 1 ) manufactured by the metal fitting manufacturing method according to the first embodiment. In the second embodiment, like parts and portion of the metal fitting are designated by like reference numerals; and explanations of the configuration of the metal fitting will be omitted herefrom. The metal fitting manufacturing method according to the second embodiment is similar to the metal fitting manufacturing method according to the first embodiment, except for the cold forging process. Hence, an explanation will be given only of the cold forging process of the second embodiment; and explanations of the other processes of the second embodiment will be omitted herefrom. 
       FIGS. 9(A)-9(F)  are schematic views showing, in half section, semi-finished products obtained in respective steps of cold forging processes of the metal fitting manufacturing method according to the second embodiment. In  FIGS. 9(A)-9(F) , appearance and cross section of each semi-finished forging product are shown on the left and right side of the axis, respectively, as in the case of  FIG. 5 . 
     In the metal fitting manufacturing method according to the second embodiment, the cold forging step contains six cold forging operations (steps) as in the metal fitting manufacturing method according to the first embodiment. 
     First, a starting material is subjected to extrusion (the first cold forging step) so as to narrow a front end region of the starting material and thereby form a semi-finished product  550 A ( FIG. 9(A) ). The semi-finished product  550 A is provided with a pre-body portion  551  and a leg portion  554 . The pre-body portion  551  has a substantially cylindrical column shape with an outer diameter substantially equal to that of the starting material. The leg portion  554 , which is to be formed into the thread portion  52  in the later process step, has a substantially cylindrical column shape with an outer diameter smaller than that of the pre-body portion  551 . First and second holes  556  and  558  are made in rear and front ends of the semi-finished product  550 A, respectively. 
     Next, the semi-finished product  550 A is processed into a semi-finished product  550 B ( FIG. 9(B) ) in the second cold forging step. In the second cold forging step, the first and second holes  506  and  508  are elongated; and a rear end region of the pre-body portion  551  of the semi-finished product  550 A in the axis direction OD is bulged to form a butt portion  560  and a body portion  552 . Namely, the semi-finished product  550 B is provided with the butt portion  560 , the body portion  552  and the leg portion  554 . The butt portion  560 , which is to be formed into the crimp portion  53  and the tool engagement portion  51  in the later process steps, has a substantially cylindrical column shape with an outer diameter D2 larger than the diagonal dimension L2 of the tool engagement portion  51  of the metal fitting  50  and larger than the outer diameter D1 of the body portion  552 . The body portion  552 , which is to be formed into the compression deformation portion  55  and the seal portion  54  in the later process steps, has a substantially cylindrical column shape with an outer diameter D1 equal to the outer diameter D1 of the seal portion  54  of the metal fitting  50 . 
     The semi-finished product  550 B is processed into a semi-finished product  550 C ( FIG. 9(C) ) in the third cold forging step. In the third cold forging step, the first and second holes  556  and  558  are further elongated; and the body portion  552  and the leg portion  554  are elongated. 
     The semi-finished product  550 C is processed into a semi-finished product  550 D ( FIG. 9(D) ) in the fourth cold forging step. In the fourth cold forging step, the first holes  556  is further elongated; and a front end region of the butt portion  560  of the semi-finished product  550 C (i.e. a region of the butt portion  560  including a second end  560   d  adjacent to the body portion  552 ) is subjected to drawing to form a tool engagement portion  564 . Namely, the semi-finished product  550 D is provided with the butt portion  560 , the tool engagement portion  564 , the body portion  552  and the leg portion  554 . In the second embodiment, the butt portion  560  is subjected to cutting in the subsequent cutting process to thereby form the crimp portion  53 . The tool engagement portion  564  corresponds to the cool engagement portion  51  of the metal fitting  50  and has a substantially regular hexagonal outer shape in plan view. 
     The semi-finished product  550 D is processed into a semi-finished product  550 E ( FIG. 9(E) ) in the fifth cold forging step. In the fifth cold forging step, the first and second holes  556  and  558  are further elongated; and the leg portion  554  is further elongated. 
     The semi-finished product  550 E is processed into a semi-finished product  550 F ( FIG. 9(F) ) in the sixth cold forging step. In the sixth cold forging step, the first and second holes  556  and  558  are connected to and communicated with each other to define a through hole  566 . The semi-finished product  550 F, which is the final product of the cold forging process in the second embodiment, has a substantially cylindrical shape (substantially hollow cylindrical column shape) where the above-defined through hole is identical to the through hole  59  of the metal fitting  50 . 
     B-2. Effects of Second Embodiment 
     In the metal fitting manufacturing method according to the second embodiment, the body portion  552  is formed in a substantially cylindrical shape with an outer diameter D1 smaller than the diagonal dimension L2 of the tool engagement portion  51  of the metal fitting  50 . Further, the butt portion  560  is formed, at a position continuous to the body portion  552 , in a substantially cylindrical column shape with an outer diameter D2 larger than the diagonal dimension L2 of the tool engagement portion  51  of the metal fitting  50 . The tool engagement portion  564  is then formed by drawing a part of the butt portion. Consequently, it is possible in the metal fitting manufacturing method according to the second embodiment to attain a reduction of die cost, suppress a deterioration in die lifetime and thereby reduce the manufacturing cost of the metal fitting as compared to the comparative metal fitting manufacturing method 1. Moreover, the body portion  552 , which is to be formed into the seal portion  54  in the later process step, is substantially cylindrical column-shaped with an outer diameter D1 substantially equal to that of the seal portion  54  in the metal fitting manufacturing method according to the second embodiment. The seal portion  54  is thus substantially finished by forging without performing the cutting process on the outer circumference of the body portion  552 . It is consequently possible in the metal fitting manufacturing method according to the second embodiment to achieve a reduction of cutting amount during the cutting process subsequent to the cold forging process, suppress an increase in chip treatment workload, deterioration in cutting edge lifetime, increase in material input etc. and thereby reduce the manufacturing cost of the metal fitting as compared to the comparative metal fitting manufacturing method 2. The metal fitting manufacturing method according to the first embodiment is preferable in that the crimp portion  53  is also substantially finished by forging. 
     C. Modifications 
     The present invention is not limited to the above specific embodiments and can be embodied in various forms without departing from the scope of the present invention. For example, it is possible to appropriately replace or combine any of the technical features mentioned above in “Summary of the Invention” and “Description of the Embodiments” in order to solve a part or all of the above-mentioned problems or achieve a part or all of the above-mentioned effects. Any of these technical features, if not explained as essential in the present specification, may be eliminated as appropriate. For example, the following modifications are possible. 
     C-1. First Modified Example 
     In the above embodiment, the metal fitting  50  satisfies the relationship of D1&lt;L1 where L1 is the opposite side dimension of the tool engagement portion  51 ; L2 is the diagonal dimension of the tool engagement portion  51 ; and D1 is the outer diameter of the seal portion  54 . The metal fitting is not however limited to this dimensional relationship. The metal fitting may alternatively satisfy the relationship of L1≦D1&lt;L2 or D1&lt;L2. Even in such a case, it is possible to reduce the manufacturing cost of the metal fitting by the adoption of the metal fitting manufacturing method according to the present invention. 
     C-2. Second Modified Example 
     Although the tool engagement portion  51  of the metal fitting  50  is substantially regular hexagonal in cross section in the above embodiment, the tool engagement portion  51  is not limited to such a cross-sectional shape. The cross-sectional shape of the tool engagement portion  51  may alternatively be formed in a regular n-sided polygonal shape (where n is a natural number of 3 or greater) other than the regular hexagonal shape, a n-sided polygonal shape other than the regular n-sided polygonal shape, a Bi-HEX shape (modified dodecagonal shape) (according to ISO 22977: 2005(E)) or the like. In the case where the cross-sectional shape of the tool engagement portion is a n-sided polygonal shape other than the regular n-sided polygonal shape, the tool engagement portion is formed such that the maximum opposite side dimension (longest opposite side length) of the tool engagement portion is larger than the outer diameter of the body portion, or the maximum diagonal dimension (longest diagonal length) of the tool engagement portion is larger than the outer diameter of the body portion, in the metal fitting manufacturing method according to the above embodiment.  FIG. 10  is a plan view of a tool engagement portion of a metal fitting according to a modified embodiment of the present invention. In this modified embodiment, the tool engagement portion  51 A of the metal fitting  50 A has a Bi-HEX (modified dodecagonal) cross-sectional shape. As shown in the figure, a distance L1 between opposite sides of the tool engagement portion  51 A is defined as an opposite side dimension; and a distance L2 between diagonally opposite corners of the tool engagement portion  51 A is defined as a diagonal dimension. To manufacture the metal fitting with such a tool engagement portion  51 A, the tool engagement portion  51 A is formed such that the outer dimension of the body portion is smaller than the diagonal dimension of the tool engagement portion. 
     C-3. Third Modified Example 
     It is feasible to form the tool engagement portion by drawing the entire butt portion although the tool engagement portion is formed by drawing a part of the butt portion in the axis direction in the above embodiment. In the case where the tool engagement portion is formed by drawing the entire butt portion  510  in the fourth step of the cold forging process in the metal fitting manufacturing method according to the first embodiment, for example, the compression deformation portion  55  may be formed by cutting a rear end region of the body portion  502  or by cutting a front end region of the tool engagement portion. It is possible even in this case to obtain the same effects as in the above embodiment. 
     C-4. Fourth Modified Example 
     In the above first embodiment, the outer dimension of the body portion  502  of the semi-finished product  500 B is set equal to the outer diameter of the seal portion  54  of the finished metal fitting  50  (that is, in the metal fitting manufacturing method according to the first embodiment, the outer diameter of the seal portion  54  of the finished metal fitting  50  is set equal to the outer dimension of the body portion  502  of the semi-finished product  500 B). The semi-finished product is not however limited to such a configuration. For example, it is feasible to set the outer diameter of the body portion of the semi-finished product larger than the outer diameter of the seal portion  54  of the finished metal fitting  50  and form the seal portion  54  formed by cutting. It is alternatively feasible to set the outer diameter of the butt portion of the semi-finished product equal to the outer diameter of the crimp portion of the finished metal fitting such that the outer diameter of a part of the butt portion remains the same in the finished metal fitting for reduction of cutting workload. 
     C-5. Fifth Modified Example 
     Although the above embodiment refers to the metal fitting for use in the spark plug, the metal fitting is not limited to such use. The present invention is applicable to various metal fittings with tool engagement portions for use in sensors e.g. temperature sensors and any other devices. The present invention is also applicable to a manufacturing method of a sensor using such a sensor metal fitting. 
     C-6. Sixth Modified Example 
     In the above embodiment, the body and butt portions are formed in a substantially cylindrical column shape. The shapes of the body and butt portions are not limited to the substantially cylindrical column shape. The body and butt portions may alternatively be formed into a hexagonal shape, modified dodecagonal shape or other shape. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               5 : Gasket 
               6 : Ring member 
               8 : Plate packing 
               9 : Talc 
               10 : Insulator 
               12 : Axial hole 
               15 : Reduced diameter portion 
               20 : Center electrode 
               21 : Cover material 
               25 : Core material 
               30 : Ground electrode 
               40 : Metal terminal 
               50 ,  50 A: Metal fitting 
               50 B: Semi-finished cutting product 
               51 ,  51 A: Tool engagement portion 
               52 : Thread portion 
               53 : Crimp portion 
               54 : Seal portion 
               55 : Compression deformation portion 
               56 : Step portion 
               59 : Through hole 
               100 : Spark plug 
               500 A to  500 E: Semi-finished product 
               501 : Pre-body portion 
               502 : Body portion 
               504 : Leg portion 
               506 : First hole 
               508 : Second hole 
               510 : Butt portion 
               510   u : First end 
               512 : Pre-crimp portion 
               514 : Tool engagement portion 
               516 : Through hole