Patent Publication Number: US-7581998-B2

Title: Method for regulating aground electrode position in spark plug

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus for manufacturing an internal combustion spark plug. 
   2. Description of the Related Art 
   Various methods and apparatus for manufacturing spark plugs for internal combustion engines have heretofore been proposed (see, for example, Japanese Patent. Laid-Open No. 121143/1999 (page 10, FIG. 1) and EP 0540159 A1 (FIG. 4)). 
   3. Problems to be Solved by the Invention 
   In recent years, there has been a demand for spark plugs for internal combustion engines capable of igniting a fuel gas at a proper timing under any of various conditions. To this end, recent spark plugs face severe requirements as to accuracy of an X-directional position of a front end surface (or rather a fee end surface) of a ground electrode that is bent and formed over a front end of a center electrode for the spark gap. This is because the X-directional position or the covering length of the ground electrode extending over the front end of the center electrode is critical to ignition timing of fuel gas injected into present day high performance engine cylinders. 
   Notably, the covering length of the ground electrode herein means a distance from an X-directional position of the front end surface (i.e., free end surface) of the ground electrode to an X-directional position of the front end surface of the center electrode, as measured in an X direction that is transverse to an axis direction (i.e., Y direction) of a metal shell of a spark plug and normal to a width direction of a fixation portion of the ground electrode formed on the ground electrode. 
   If the X-directional position of the ground electrode is not fixed at an appropriate position with respect to that of the front end of the center electrode, for instance, if the front end portion of the ground electrode is positioned too far away or too close to the front end of the center electrode, the ignition tinting of the fuel gas by spark discharge of the spark plug in the engine cylinder is delayed or hastened, resulting in reduced total combustion efficiency of the engine. This is because the covering length of the ground electrode extending over the front end of the center electrode affects an approach of the injected fuel gas to the spark gap or aggravates propagation of the ignited gas through the engine cylinder, even if the spark occurs properly at the spark gap formed between the ground and center electrodes of the spark plug. Therefore, since constant-ignition timing is required to a high degree in present day high performance internal combustion engines, the covering length or X directional position of the ground electrode must be fixed or controlled with very small deviation as possible deviating from a predetermined length thereof in manufacturing high performance spark plugs. 
   In conventional methods and apparatus for manufacturing spark plugs, it has been difficult to fix the X-directional position of the front end surface of the ground electrode at a constant predetermined position with respect to the front end of the center electrode. For example, in the manufacturing method and apparatus disclosed in Japanese Patent Laid-Open No. 121143/1999 (page 10, FIG. 1), the ground electrode is pressed and bent for a spark gap without regulating the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode. 
   On the other hand, in the manufacturing method and apparatus disclosed in EP 0540159 A1 (FIG. 4), the ground electrode formed on the metal shell is pressed and bent while the X-directional position of the front end surface of the ground electrode is regulated. However, because the insulator, center electrode inserted therein and the metal shell holding the insulator and having the pre-bent ground electrode are assembled afterwards, it is difficult to keep or re-fix a constant X-directional positional relationship between the front end of the center electrode and the front end of the ground electrode in the assembled spark plug. This is because a large dimensional deviation is generated during the assembly of these components into the spark plug. 
   SUMMARY OF THE INVENTION 
   It is therefore a first object of the present invention to provide a method and apparatus for manufacturing a spark plug, in which the X-directional position of a front end surface of a ground electrode is fixed at a constant predetermined position so as to result in only a small deviation with respect to the X-directional position of a front end of the center electrode during manufacture of the spark plug. 
   Another object of the present invention is to provide a method and apparatus for manufacturing a spark plug in which the Z-directional position of a front end surface of the ground electrode is fixed so as to result in only a small deviation with respect to the Z-directional position of a front end surface of the center electrode, the Z-direction being perpendicular (or rather normal) to the central axis of the metal shell on which the ground electrode is formed. 
   The above objects of the invention have been achieved by providing a method for manufacturing a spark plug, the spark plug including: a cylindrical insulator having an axial hole or bore penetrating said cylindrical insulator in an axial direction thereof; a center electrode inserted into said axial hole of said insulator and having a front end projecting beyond a front end surface of said insulator; a metal shell surrounding said insulator; and a ground electrode fixedly attached to said metal shell by a fixation portion and bent to form a spark discharge gap between a front end portion of said ground electrode and said frond end of said center electrode; said method comprising: measuring an X-directional position of said front end of said center electrode; positioning an X-directional position of a regulation member based on said X-directional position of said front end of said center electrode measured in said center electrode position measuring step, said regulation member regulating an X-directional position of a front end surface of said ground electrode when said ground electrode is pressed and bent in a Y direction; and forming said spark discharge gap using said regulation member to regulate said X-directional position of said front end surface of said ground electrode while pressing and bending said ground electrode in said Y direction so as to form said spark discharge gap; wherein: said X direction designates, of directions perpendicular (or rather transverse) to a central axis of said metal shell and perpendicular(or rather normal) to a width direction of said fixation portion of said ground electrode, a direction from said fixation portion of said ground electrode toward said central axis of said metal shell; and said Y direction designates a direction from a front end side of said internal combustion spark plug to a base side thereof along said central axis of said metal shell. 
   More specifically, according to the method of the invention, the X-directional position of the front end of the center electrode is firstly determined. Next, the X-directional position of the regulation member for regulating the X-directional position of the front end surface of the ground electrode is predetermined based on the X-directional position of the front end of the center electrode. Then, the X-directional position of the front end surface of the ground electrode is regulated or controlled by the regulation member that is positioned at the predetermined X-directional position, while the ground electrode is pressed and bent in the Y direction so as to form a spark discharge gap between the center and ground electrodes. 
   An advantage of this method according to the invention is that even if the position of the front end of the center electrode differs from one spark plug to another, the X-directional position of the front end surface of the ground electrode with respect to that of the front end of the center electrode can be made constant, and resultantly, a spark plug having a small X-directional positional deviation of the ground electrode deviating from its predetermined position with respect to a X-directional position of the front end surface of the center electrode can be made by mass production. Thereby, a high-quality spark plug providing a constant ignition timing for the fuel gas under various engine conditions is attained. 
   As used herein, when the center electrode includes a noble metal tip fixedly attached to the front end of a center electrode body, a distal end of the noble metal tip corresponds to the front end of the center electrode. 
   Another advantage of the method according to the invention is that even if the ground electrode includes a noble metal tip formed on a side surface of the ground electrode, an X-directional position of the noble metal tip of the ground electrode with respect to that of the front end of the center electrode can be accurately positioned by positioning the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode by means of the regulation member. The regulation member regulates or controls an X directional movement of the position of the front end surface of the ground electrode when the ground electrode is bent to form an accurate spark gap between the center electrode and the metal tip formed on the ground electrode. Accordingly, even if the position of the front end of the center electrode differs from one spark plug to another, deviating in a radial direction from a central axis of the spark plug due to a deviation in assembly of the spark plug, the X-directional position of the noble metal tip of the ground electrode with respect to that of the front end of the center electrode can be made constant. 
   Further, in a preferred embodiment, the X-directional position of the regulation member is positioned such that the front end surface of the ground electrode is regulated or fixed within a predetermined range in the X direction with respect to the front end of the center electrode once the gap forming step has been completed. Namely, the front end surface of the ground electrode will have been fixed by the time that the gap forming step has been completed. 
   The deviation range is preferably ±0.1 mm. 
   The ground electrode may include a convex noble metal tip projecting toward the center electrode so that a spark discharge gap is formed between the noble metal tip of the ground electrode and the front end of the center electrode. The X-directional position of the regulation member determines the X-directional position of the front end of the ground electrode. As such, the noble metal tip formed on the ground electrode can be positioned within a predetermined range or deviation in the X direction with respect to the front end of the center electrode once the gap forming step has been completed. Accordingly, the X-directional position of the noble metal tip formed on the ground electrode with respect to the front end of the center electrode can be made constant within a predetermined range. 
   In another preferred embodiment, the regulation member has a regulation abutment surface which abuts against the front end surface of the ground electrode in the gap forming step. Preferably, the front end surface of the ground electrode abuts against the regulation abutment surface of the regulation member in forming the spark gap, and the X-directional position of the front end surface of the ground electrode is regulated during formation of the spark plug. It is most preferable to position the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode and to almost simultaneously bend the ground electrode in forming the spark gap by using the regulation member. This is because simultaneous positioning and bending is simple, contributes to a reduction in manufacturing cost, and provides a spark plug having a small deviation in covering length or distance of the ground electrode extending over the front end of the center electrode, deviating from the predetermined length in the X-direction. 
   In another preferred embodiment, the X-directional position of the regulation abutment surface of the regulation member is fixed at least by the time that the spark discharge gap is formed after the ground electrode has been pressed. In other words, when the spark discharge gap is formed in a gap forming step (wherein the ground electrode is completely bent), the X-directional position of the front end surface of the ground electrode with respect to that of the front end of the center electrode is regulated or controlled. As one example, the X-directional position of the regulation abutment surface of the regulating member is fixed before pressing and bending the ground electrode in the Y direction. As another example, when the ground electrode is pressed and bent in the Y direction, the regulation abutment surface is moved in the X direction together with the front end surface of the ground electrode while abutting against the front end surface of the ground electrode to a predetermined X-directional position of the regulation abutment surface of the regulation member, before the spark discharge gap is formed. 
   In yet another preferred embodiment, the ground electrode may be bent while the X-directional position of the front end surface of the ground electrode is partly regulated by the regulation abutment surface of the regulation member. The regulation abutment surface of the regulation member may then be fixed such that that the X-directional position of the front end surface of the ground electrode is finally regulated or fixed. Accordingly, the X-directional position of the front end surface of the ground electrode can be completely regulated by the X-directional position of the regulation abutment surface of the regulation member. 
   In yet another preferred embodiment, the method may further comprise returning the regulation member to its original position after forming the spark gap (in a gap forming step). The returning step includes a separation step wherein the regulation member abutting against the ground electrode moves in a direction including the X-direction so as to separate the regulation member from the ground electrode without varying the distance of the spark discharge gap formed in the gap-forming step. 
   When the regulation member abutting against the ground electrode is separated from the ground electrode in the returning step, and the regulation member is returned to its original position waiting for the next regulation member positioning step, the spark gap distance should neither be affected nor varied. Therefore, in the separation step, the regulation member abutting against the ground electrode is preferably moved in the X direction so as to separate the regulation member from the ground electrode. 
   Further, in yet another preferred embodiment, the ground electrode is pressed and bent in the Y direction by a pressure member having a pressure abutment surface abutting against the ground electrode. This pressure abutment surface of the pressure member is surface-treated such that a coefficient of friction between the pressure abutment surface of the pressure member and the ground electrode abutment surface of the ground electrode is not higher than 0.2, according to the invention. When the friction coefficient is not higher than 0.2, the ground electrode can easily slide on the pressure abutment surface of the pressure member in the X direction so that the ground electrode is smoothly deformed not only in the Y direction but also in the X direction. As a result, the X-directional position of the front end surface of the ground electrode can be accurately regulated by the regulation member. 
   As for the surface treatment applied to the pressure abutment surface of the pressure member and/or also applied to the regulation abutment surface of the regulation member abutting against the ground electrode, a low friction coefficient material such as DLC (diamond-like carbon) and a lubricating oil is preferably coated on at least the abutment surface of the pressure member. Among them, a DLC coating is most preferred, because it is hard and has a low coefficient of friction. The pressure abutment surface may be polished. In addition, the pressure abutment surface may be cut such that a plurality of convex portions each having a substantially triangular shape in section tapered in the Y direction and extending in the X direction are formed as an array in the pressure abutment surface. By thus treating the convex portions abutting against the abutment surface of the ground electrode, the abutment surface of the ground electrode can move smoothly in the X direction while the ground electrode abutment surface of the ground electrode is pressed by the pressure member. Accordingly, the friction drag action between the ground electrode and the abutment surface of the pressure member is reduced so that the ground electrode can be deformed and slide smoothly in the X direction in forming the spark gap. 
   The pressure member may be integrally formed with the regulation member or may be formed independent and separate therefrom. 
   Further, the aforementioned method may further include: a deviation measuring step of measuring a Z-directional deviation of the front end surface of the ground electrode deviating in a direction normal to the X and Y directions with respect to the front end of the center electrode, the deviation measuring step being carried out after the gap forming step or the returning step; and a deviation adjusting step of adjusting the Z-directional position of the front end surface of the ground electrode based on the deviation measured in the deviation measuring step, so that the front end surface of the ground electrode is located within a predetermined range in the Z direction with respect to the front end of the center electrode. The Z direction designates a direction normal to the axis of the metal shell and also normal to the X direction. Accordingly, not only is it possible to adjust the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode in the spark discharge gap forming step, it is also possible to adjust the Z-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode in the deviation adjusting step. Thus, even if the Z-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode shifts and is located outside a predetermined range of deviation when the spark discharge gap is formed, the Z-directional position of the front end surface of the ground electrode can be adjusted (corrected) to be within the predetermined range. When the ground electrode includes a noble metal tip in its front end portion, the Z-directional position of the noble metal tip of the ground electrode with respect to the front end of the center electrode can be also adjusted by adjusting the Z-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode in the aforementioned manner. 
   When the ground electrode includes a convex noble metal tip formed thereon, the noble metal tip projecting toward the center electrode such that the spark discharge gap is formed between the noble metal tip of the ground electrode and the front end of the center electrode, the method may further include: a deviation measuring step of measuring a Z-directional deviation of the noble metal tip of the ground electrode with respect to the front end of the center electrode, the deviation measuring step being carried out after the gap forming step or the returning step; and a deviation adjusting step of adjusting the Z-directional position of the noble metal tip of the ground electrode based on the deviation measured in the deviation measuring step, so that the noble metal tip of the ground electrode is located within a predetermined range in a Z direction with respect to the front end of the center electrode. The Z direction designates a direction perpendicular to the central axis of the metal shell and perpendicular to the X direction. 
   Accordingly, not only is it possible to adjust the X-directional position of the noble metal tip of the ground electrode (or the front end surface of the ground electrode) with respect to the front end of the center electrode in the spark discharge gap forming step, it is also possible to adjust the Z-directional position of the noble metal tip of the ground electrode with respect to the front end of the center electrode in the deviation adjusting step. Thus, even if the Z-directional position of the noble metal tip of the ground electrode with respect to the front end of the center electrode shifts and is located outside a predetermined deviational range when the spark discharge gap is formed, the Z-directional position of the noble metal tip of the ground electrode can be adjusted (corrected) to be within the predetermined range. 
   Further, the aforementioned method may include: a gap size measuring step of measuring a gap size of the spark discharge gap after the deviation adjusting step; and a gap adjusting step of regulating the X-directional position of the front end surface of the ground electrode while pressing and bending the ground electrode in the Y direction to thereby adjust the gap size within a predetermined range based on the gap size measured in the gap size measuring step. 
   When deviation adjustment is performed on the ground electrode in the deviation adjusting step, there is a concern that the gap size of the spark discharge gap may change so as to be outside a predetermined range. This concern is addressed by measuring the gap size after the deviation adjusting step, and the gap size is adjusted to be within the predetermined range based on the measured gap size. Accordingly, even when the gap size is outside the predetermined range due to the deviation adjusting step, the gap size can be adjusted to be within the predetermined range. 
   The gap size is preferably set at a value within the predetermined range or slightly larger than the predetermined range in the spark discharge gap forming step prior to the gap adjusting step, and any slight variation in gap size incurred in the deviation adjustment step is corrected in the gap adjusting step. In such manner, the distance that the ground electrode is pressed in the Y direction in the gap size adjusting step becomes slight. Accordingly, deviation of the ground electrode in the Z direction with respect to the center electrode in the gap adjusting step becomes extremely slight, so that there is no concern that the deviation is outside the predetermined range due to the gap adjustment. 
   Further, the aforementioned method for manufacturing the spark plug may be adapted so that in the gap forming step, the spark discharge gap is formed so that the gap size of the spark discharge gap has a first gap size value; and in the gap adjusting step, the gap size is adjusted to have a second gap size value smaller than the first gap size value. 
   As described above, when deviation adjustment is performed on the ground electrode in the deviation adjusting step, the gap size of the spark discharge gap may change. 
   To cope with the change in the gap size, the spark discharge gap is formed in the gap forming step so that the gap size has a first gap size value, and the gap size is adjusted to a second gap size value smaller than the first gap size value in the gap adjusting step after the deviation adjusting step. In such manner, the gap size can be adjusted to have the second gap size value surely within a predetermined range due to gap adjustment by application of pressure in the gap adjusting step. 
   The above object of the invention has also been achieved by providing an apparatus for manufacturing a spark plug, the spark plug comprising: a cylindrical insulator having an axial hole (or rather bore) penetrating said cylindrical insulator in an axial direction thereof; a center electrode inserted into said axial hole of said insulator and having a front end projecting beyond a front end surface of said insulator; a metal shell surrounding said insulator; and a ground electrode fixedly attached to said metal shell by a fixation portion and bent to form a spark discharge gap between a front end portion of said ground electrode and said front end of said center electrode; said apparatus comprising: a center electrode position measuring unit for measuring an X-directional position of said front end of said center electrode; a pressure member for pressing and bending said ground electrode in a Y direction to thereby form said spark discharge gap between said front end of said center electrode and said front end surface of said ground electrode; a regulation member for regulating an X-directional position of said front end surface of said ground electrode when said ground electrode is pressed and bent by said pressure member; and a positioning unit for positioning an X-directional position of said regulation member based on said X-directional position of said front end of said center electrode measured by said center electrode position measuring unit; wherein: said X-direction designates, of directions perpendicular (or rather transverse) to a central axis of said metal shell and perpendicular (or rather normal) to a width direction of said fixation portion of said ground electrode, a direction from said fixation portion of said ground electrode toward said central axis of said metal shell; and said Y-direction designates a direction from a front end side of said internal combustion spark plug to a base side thereof along said central axis of said metal shell. 
   The apparatus for manufacturing a spark plug according to the invention includes a center electrode position measuring unit for measuring the X-directional position of the front end of the center electrode, a pressure member for pressing and bending the ground electrode to thereby form the spark discharge gap, a regulation member for regulating the X-directional position of the front end surface of the ground electrode, and a positioning unit for positioning the X-directional position of the regulation member based on the X-directional position of the front end of the center electrode as measured by the center electrode position measuring unit. Accordingly, when the inventive apparatus is used, the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode can be made constant while the spark discharge gap is formed. 
   When the center electrode includes a center electrode body portion and a noble metal tip fixedly attached to the front end of the center electrode body portion, the front end of the noble metal tip corresponds to the front end of the center electrode. 
   When the ground electrode includes a noble metal tip in its front end portion, the X-directional position of the noble metal tip of the ground electrode with respect to the front end of the center electrode can be positioned (adjusted) by positioning (adjusting) the X-direction position of the front end surface of the ground electrode with respect to that of the front end of the center electrode by means of the regulation member. Accordingly, in this case, the X-directional position of the noble metal tip of the ground electrode with respect to the front end of the center electrode can be made constant while the spark discharge gap is formed. 
   In a preferred embodiment, the positioning unit positions the X-directional position of the regulation member so that the front end surface of the ground electrode will be located within a predetermined range in the X direction with respect to the front end of the center electrode once formation of the spark discharge gap by the pressure member has been completed. 
   In yet another preferred embodiment, the X-directional position of the regulation member is positioned by the positioning unit so that the front end surface of the ground electrode is located within a predetermined range in the X direction with respect to the front end of the center electrode once formation of the spark discharge gap by the pressure member has been completed. Accordingly, the X-directional position of the front end surface of the ground electrode with respect to that of the front end of the center electrode can be made constant within a predetermined range. 
   Alternatively, the spark plug may be adapted so that the ground electrode includes a convex noble metal tip in the front end portion thereof, the noble metal tip projecting toward the center electrode; the spark discharge gap is formed between the noble metal tip of the ground electrode and the front end of the center electrode; and the positioning unit positions the X-directional position of the regulation member so that the noble metal tip of the ground electrode will be located within a predetermined range in the X direction with respect to the front end of the center electrode once formation of the spark discharge gap by the pressure member has been completed. Accordingly, the X-directional position of the noble metal tip of the ground electrode with respect to the front end of the center electrode can be made constant within the predetermined range. 
   Further, any one of the aforementioned apparatuses may be adapted so that the regulation member has a regulation abutment surface which abuts against the front end surface of the ground electrode. 
   In yet another preferred embodiment, the regulation member has a regulation abutment surface for abutting against the front end surface of the ground electrode. Accordingly, the front end surface of the ground electrode abuts against the regulation abutment surface of the regulation member so that the X-directional position of the front end surface of the ground electrode is regulated. In such manner, the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode can be regulated by the regulation member having a simple structure. Thus, the cost is low. 
   The aforementioned apparatus may further include a fixation unit for fixing an X-directional position of the regulation abutment surface of the regulation member. The X-directional position of the regulation abutment surface of the regulation member is fixed by the fixation unit at least by the time that the spark discharge gap is formed after the ground electrode has been pressed. Thus, the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode can surely be regulated when the spark discharge gap is formed. When the ground electrode includes a noble metal tip in its front end portion, the X-directional position of the noble metal tip of the ground electrode with respect to the front end of the center electrode can be regulated by regulating the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode as described above. 
   For example, the fixation unit according to the invention may fix the X-directional position of the regulation abutment surface of the regulation member before the pressure member presses and bends the ground electrode in the Y direction. Alternatively, when the ground electrode is pressed and bent in the Y direction, the X-directional position of the regulation abutment surface of the regulation member may be not fixed but the regulation abutment surface of the regulation member is moved in the X direction together with the front end surface of the ground electrode while abutting against the front end surface of the ground electrode. In this case, the X-direction position of the regulation abutment surface is fixed before the ground electrode is completely bent (before the spark discharge gap is formed). 
   Further, any one of the aforementioned apparatuses for manufacturing the internal combustion spark plug may further include a separation unit for moving the regulation member abutting against the ground electrode in a direction including an X-directional component so as to separate the regulation member from the ground electrode after the spark discharge gap has been formed by the pressure member. 
   When the regulation member is returned to the position (initial position) where the X-directional position of the front end surface of the ground electrode has not yet been regulated, there is a concern that the regulation abutment surface of the regulation member may engage the front end surface of the ground electrode so as to change the gap size when the regulation member abutting against the ground electrode is moved away from the ground electrode. 
   To address this concern, the manufacturing apparatus may include a separation unit for moving the regulation member abutting against the ground electrode in a direction including an X-directional component so as to separate the regulation member and the ground electrode from each other. Accordingly, due to use of the separation unit, the regulation member abutting against the ground electrode can be separated from the ground electrode in the directional in which the regulation abutment surface of the regulation member leaves the front end surface of the ground electrode. Accordingly, when the regulation member abutting against the ground electrode is separated from the ground electrode, there is no concern of the regulation abutment surface of the regulation member engaging the front end surface of the ground electrode. Thus, the regulation member can be returned to its initial position without changing the gap size. 
   Further, in the aforementioned apparatus, the separation unit preferably moves the regulation member abutting against the ground electrode in the X direction so as to separate the regulation member and the ground electrode from each other. Since the ground electrode abuts against the regulation member in the X direction, the change in gap size due to engagement between the regulation member and the ground electrode can surely be prevented if the regulation member is moved in the X direction by the separation unit so that the regulation member and the ground electrode are separated from each other. 
   Further, any one of the aforementioned apparatus for manufacturing the internal combustion spark plug may be adapted so that the pressure member has a pressure abutment surface for abutting against the ground electrode, and the pressure abutment surface of the pressure member is subjected to surface treatment such that a coefficient of friction between the pressure abutment surface of the pressure member and the ground electrode abutment surface of the ground electrode is not higher than 0.2. Accordingly, the ground electrode can easily slide on the pressure abutment surface of the pressure member in the X direction. Thus, when the ground electrode is deformed by the pressure member, the ground electrode is deformed smoothly not only in the Y direction but also in the X direction so that the X-directional position of the front end surface of the ground electrode can be regulated by the regulation member. 
   As for the surface treatment applied to the pressure abutment surface of the pressure member, for example, diamond-like carbon, lubricating oil, or the like may be formed or applied on the pressure abutment surface. Alternatively, the pressure abutment surface may be polished and smoothed (to lower its coefficient of friction). 
   The pressure member may be formed separately from the regulation member or integrally with the regulation member. It is preferable to use the pressure member that can move independently from the regulation member, because the spark gap may be adjusted more precisely by only moving the pressure member once the regulation member has bee fixed. Notably, the method and apparatus according to the invention enable production of spark plugs each having a small deviation of the front end surface of the ground electrode extending over the center electrode, deviating from its predetermined position with respect to the front end of the center electrode. That is, the spark plug produced according to the method and apparatus of the invention can have a front end surface of said ground electrode located or positioned within a narrow deviation range, deviating in the X direction with respect to the front end of said center electrode. For instance, the front end surface of the ground electrode can be positioned at a distance of 0.5 mm from the front end of the center electrode with a deviation falling in the range of ±0.1 mm. This deviation range is far improved, and less than half that attained by conventional methods and apparatuses. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a spark plug  100  according to Embodiment 1. 
       FIG. 2  is a top view schematically showing a spark plug manufacturing apparatus  200  according to Embodiment 1. 
       FIG. 3  is a side, view schematically showing a gap size measuring unit  230 ,  270  according to Embodiment 1. 
       FIG. 4  is an explanatory view for explaining a gap size measuring step according to Embodiment 1. 
       FIG. 5  is a side view schematically showing a first, second electrode position measuring unit  240 ,  280  according to Embodiment 1. 
       FIG. 6  is an explanatory view showing a photographed image in the first, second electrode position measuring unit  240 ,  280  according to Embodiment 1. 
       FIGS. 7A and 7B  are explanatory views for explaining a center electrode position measuring step, showing enlarged views of a portion A of the photographed image in the first, second electrode position measuring unit  240 ,  280  according to Embodiment 1. 
       FIG. 8  is a side view schematically showing a gap forming unit  300  and a gap adjusting unit  400  according to Embodiment 1. 
       FIGS. 9A ,  9 B,  9 C and  9 D are explanatory views for explaining a regulation member positioning step, a gap forming step and a return step according to Embodiment 1. 
       FIG. 10  is a side view schematically showing a deviation measuring unit  250  according to Embodiment 1. 
       FIGS. 11A and 11B  are explanatory views for explaining a method for measuring a ground electrode center position  17  in a deviation measuring step according to Embodiment 1. 
       FIGS. 12A ,  12 B and  12 C are explanatory views for explaining a method for measuring a center electrode center position J 6  in the deviation measuring step according to Embodiment 1. 
       FIG. 13  is an explanatory view for explaining a method for measuring a Z-directional deviation P in the deviation measuring step according to Embodiment 1. 
       FIG. 14  is a side view schematically showing a deviation adjusting unit  260  according to Embodiment 1. 
       FIG. 15  is a flow chart showing a flow of steps in a method for manufacturing the spark plug  100  according to Embodiment 1. 
       FIG. 16  is a flow chart showing a flow of steps in a gap size measuring process according to Embodiment 1. 
       FIG. 17  is a flow chart showing a flow of steps in a center electrode position measuring process according to Embodiment 1. 
       FIG. 18  is a flow chart showing a flow of steps in a deviation measuring process according to Embodiment 1. 
       FIGS. 19A ,  19 B,  19 C and  19 D are explanatory views for explaining a second regulation member positioning step, a gap adjusting step and a return step according to Embodiment 1. 
       FIGS. 20A and 20B  are side views schematically showing a gap forming unit  700  according to Embodiment 2. 
       FIGS. 21A ,  21 B and  21 C are explanatory views for explaining a regulation member positioning step, a gap forming step and a return step according to Embodiment 2. 
       FIG. 22  is a flow chart showing a flow of steps in a method for manufacturing the spark plug  100  according to Embodiment 2. 
       FIG. 23  is a side view of a spark plug  1100  according to Embodiment 3. 
       FIG. 24  is a top view schematically showing a spark plug manufacturing apparatus  1200  according to Embodiment 3. 
       FIG. 25  is an explanatory view for explaining a gap size measuring step according to Embodiment 3. 
       FIG. 26  is a side view schematically showing a deviation measuring unit  1250  according to Embodiment 3. 
       FIG. 27  is an explanatory view for explaining a method for illumination using first illuminators  1253 , showing a sectional view taken on line A-A in  FIG. 26 . 
       FIGS. 28A ,  28 B and  28 C are explanatory views for explaining a chuck for a ground electrode  1110  in a deviation measuring step according to Embodiment 3. 
       FIGS. 29A ,  29 B and  29 C are explanatory views for explaining a method for measuring a ground electrode center position I 6  in the deviation measuring step according to Embodiment 3. 
       FIGS. 30A ,  30 B and  30 C are explanatory views for explaining a method for measuring a center electrode center position J 6  in the deviation measuring step according to Embodiment 3. 
       FIG. 31  is an explanatory view for explaining a method for measuring a deviation P in the deviation measuring step according to Embodiment 3. 
       FIG. 32  is a flow chart showing a flow of steps in a method for manufacturing the spark plug  1100  according to Embodiment 3. 
       FIG. 33  is a flow chart showing a flow of a deviation measuring process according to Embodiment 3. 
       FIG. 34  is a flow chart showing a flow of an inter-tip distance measuring process according to Embodiment 3. 
       FIGS. 35A and 35B  are explanatory views for explaining a method for measuring a center electrode center position K 6  in the inter-tip distance measuring process according to Embodiment 3. 
       FIGS. 36A and 36B  are explanatory views for explaining a method for measuring a ground electrode center position M 6  in the inter-tip distance measuring process according to Embodiment 3. 
       FIG. 37  is a flow chart showing a flow of a tip angle measuring process according to Embodiment 3. 
       FIGS. 38A and 38B  are explanatory views for explaining a method for determining a center line P 10  of a noble metal tip  121  of a center electrode  120  in the tip angle measuring process according to Embodiment 3. 
       FIGS. 39A and 39B  are explanatory views for explaining a method for determining a center line N 10  of a noble metal tip  113  of the ground electrode  1110  in the tip angle measuring process according to Embodiment 3. 
       FIG. 40  is an explanatory view for explaining an angle □ between the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110  in the tip angle measuring process according to Embodiment 3. 
       FIGS. 41A ,  41 B,  41 C and  41 D are explanatory views for explaining a second regulation member positioning step, a gap adjusting step and a return step according to Embodiment 3. 
       FIG. 42  is a flow chart showing a flow of steps in a method for manufacturing the spark plug  1100  according to Embodiment 4. 
       FIG. 43  is a flow chart showing a flow of a center electrode position measuring process according to Embodiment 4. 
       FIG. 44  is an explanatory view showing a photographed image in a first electrode position measuring unit  240  according to Embodiment 4. 
       FIGS. 45A and 45B  are explanatory views for explaining a center electrode position measuring step, showing enlarged views of a portion A of the photographed image in the first electrode position measuring unit  240  according to Embodiment 4. 
       FIG. 46  is a flow chart showing a flow of a ground electrode tip position measuring process according to Embodiment 4. 
       FIGS. 47A ,  47 B and  47 C are explanatory views for explaining the ground electrode tip position measuring step, showing enlarged views of a portion B of the photographed image in the first electrode position measuring unit  240  according to Embodiment 4. 
       FIGS. 48A ,  48 B,  48 C and  48 D are explanatory views for explaining a regulation member positioning process, a gap forming process and a return process according to Embodiment 4. 
   

   DESCRIPTION OF REFERENCE NUMERALS AND SYMBOLS 
   Reference numerals used to identify various structural features in the drawings include the following.
       100 ,  1100  spark plug     110 ,  1110  ground electrode     111  fixation portion of ground electrode     112  front end portion of ground electrode     112   c  front end surface of ground electrode     112   d  ground electrode abutment surface     113  noble metal tip of ground electrode     120  center electrode     120   b  front end of center electrode     130  metal shell     140  insulator     200 ,  600 ,  1200  spark plug manufacturing apparatus     300 ,  700  gap forming unit     320 ,  720  second electric actuator (positioning unit, fixing unit, separation unit)     330  pressure member     331 ,  731   b  pressure abutment surface     340  regulation member     341 ,  732   b  regulation abutment surface     730  gap forming member     731  pressure portion (pressure member)     732  regulation portion (regulation member)   C central axis of metal shell   g spark discharge gap   g 1  first gap size value   g 2  second gap size value   

   DETAILED DESCRIPTION OF THE INVENTION 
   Next, preferred embodiments of the invention will be described in detail with reference to the drawings. However, the present invention should not be construed as being limited thereto. 
   Embodiment 1 
   First, Embodiment 1 of the invention will be described with reference to the drawings. 
     FIG. 1  is a side view of a spark plug  100  manufactured in Embodiment 1. The spark plug  100  has a cylindrical insulator  140 , a center electrode  120  inserted into the cylindrical insulator  140 , a metal shell  130  surrounding the insulator  140 , and a ground electrode  110  fixedly attached to the metal shell  130 . The ground electrode  110  is fixedly attached to a front end surface  130   c  of the metal shell  130  by a fixation portion  111 , and bent. A spark discharge gap g is formed between a front end  120   b  of the center electrode  120  and an opposed surface  112   b  of a front end portion  112  of the ground electrode  110  opposed to the front end  120   b . In the spark plug  100  according to Embodiment 1, the front end  120   b  of the center electrode is constituted by a front end of a noble metal tip  121  welded with a front end of an electrode body portion  122  made of an Ni alloy. 
   Further, in the spark plug  100 , as shown in an enlarged form in  FIG. 1 , the gap size of the spark discharge gap g is g 2  (mm) (corresponding to a second gap size value), and a covering size value of the ground electrode  110  with respect to the center electrode  120  is Q (mm). Here, the covering size means an X-directional distance or length of the ground electrode  110  between a position (right end in  FIG. 1 ) of the front end  120   b  of the center electrode  120  farthest from the fixation portion  111  of the ground electrode  110  and a front end surface  112   c  of the ground electrode  110  when the spark plug  100  is viewed laterally as shown in  FIG. 1 . 
   In Embodiment 1, assume that an X direction designates, of directions perpendicular to a central axis C of the metal shell  130  and perpendicular to a width direction of the fixation portion  111  of the ground electrode  110 , a direction going from the fixation portion  111  of the ground electrode  110  toward the central axis C of the metal shell  130 , and a −X direction designates a direction opposite to the X direction (see  FIG. 1 ). Further, assume that a Y direction designates a direction extending along the central axis C of the metal shell  130  and going from the front end side (where the spark discharge gap g is located) of the spark plug  100  to the base side (opposite to the side where the spark discharge gap g is located) thereof, and a −Y direction designates a direction opposite to the Y direction (see  FIG. 1 ). Furthermore, assume that a Z direction designates a direction perpendicular to the central axis C of the metal shell  130  and perpendicular to the X direction (see  FIG. 3 ). 
     FIG. 2  is a top view schematically showing a spark plug manufacturing apparatus according to Embodiment 1. The spark plug manufacturing apparatus  200  has a linear conveyor  210  serving as a conveyance mechanism for conveying to-be-formed spark plugs (hereinafter also referred to as “works”)  101  intermittently along a conveyance path D (which has a linear shape in Embodiment 1, but may have any shape such as an annular shape). The spark plug manufacturing apparatus  200  has a setter  220 , a first gap size measuring unit  230 , a first electrode position measuring unit  240  and a gap forming unit  300  in order of increasing distance from the upstream side (left side in  FIG. 2 ) along the conveyance path D. The setter  220  sets each work  101 . The first gap size measuring unit  230  measures the gap size of each work  101 . The first electrode position measuring unit  240  measures the position of the center electrode  120  of each work  101 . The gap forming unit  300  forms the spark discharge gap g of each work  101 . 
   Further, the spark plug manufacturing apparatus  200  has a deviation measuring unit  250 , a deviation adjusting unit  260 , a second gap size measuring unit  270  and a first unloading unit  290 . The deviation measuring unit  250  measures a deviation of the ground electrode  110  with respect to the center electrode  120  of each work  101 . The deviation adjusting unit  260  adjusts the deviation of the ground electrode  110  with respect to the center electrode  120  of each work  101  to a size within a predetermined range. The second gap size measuring unit  270  measures the gap size of each work  101 . The first unloading unit  290  unloads each work  101 . Further, the spark plug manufacturing apparatus  200  has a second electrode position measuring unit  280 , a gap adjusting unit  400  and a second unloading unit  390 . The second electrode position measuring unit  280  measures the position of the center electrode  120  of each work  101 . The gap adjusting unit  400  adjusts the spark discharge gap g of each work  101 . The second unloading unit  390  unloads each work  101 . 
   Carriers  211  on which the works  101  are removably mounted are attached to the linear conveyor  210  at predetermined intervals (see  FIG. 2 ). A cylindrical work holder  212  which can open and shut in its upper end is attached integrally to each carrier  211  (see  FIG. 3 ). Each work  101  fixed by the work holder  212  is subjected to processing in each unit. 
     FIG. 3  is a side view schematically showing the first gap size measuring unit  230 . As shown in  FIG. 3 , the first gap size measuring unit  230  is constituted by an illuminator  232 , a photographing camera  231  and a not-shown computer connected to the photographing camera  231 . The illuminator  232  is, for example, an LED illuminator having a surface emitting LED or a large number of LEDs arrayed two-dimensionally. The illuminator  232  is provided to radiate light in the −Z direction so as to illuminate the center electrode  120  and the ground electrode  110  of the work  101 . The photographing camera  231  is, for examples, designed as a CCD camera having a two-dimensional CCD image pickup device as an image detection portion. The photographing camera  231  is disposed in a position opposed to the illuminator  232  in the Z direction, so as to photograph the contours of the center electrode  120  and the ground electrode  110  of the work  101 .  FIG. 4  shows an image picked up by the photographing camera  231 , in which the center electrode  120  and the ground electrode  110  are contoured, and their contours are clarified by the contrast with the bright background. Based on this image, the gap size is determined by the not-shown computer. Incidentally, the second gap size measuring unit  270  is configured in the same manner as the first gap size measuring unit  230 . 
     FIG. 5  is a side view schematically showing the first electrode position measuring unit  240 . As shown in  FIG. 5 , the first electrode position measuring unit  240  is constituted by an illuminator  242 , a photographing camera  241  and a not-shown computer connected to the photographing camera  241 . The illuminator  242  is, for example, an LED illuminator having a surface emitting LED or a large number of LEDs arrayed two-dimensionally. The illuminator  242  is provided to radiate light in the −Z direction so as to illuminate the center electrode  120  and the ground electrode  110  of the work  101 . The photographing camera  241  is, for examples, designed as a CCD camera in the same manner as the photographing camera  231 . The photographing camera  241  is disposed in a position opposed to the illuminator  242  in the Z direction, so as to photograph the contours of the center electrode  120  and the ground electrode  110  of the work  101 .  FIG. 6  shows an image picked up by the photographing camera  241 , in which the center electrode  120  and the ground electrode  110  are contoured, and their contours are clarified by the contrast with the bright background. Based on this image, the position of a front end  120   b  of the center electrode  120  is determined by the not-shown computer. Incidentally, the second electrode position measuring unit  280  is configured in the same manner as the first electrode position measuring unit  240 . 
     FIG. 8  is a side view schematically showing the gap forming unit  300 . As shown in  FIG. 8 , the gap forming unit  300  has a first electric actuator  310  including a servo motor with a brake, a substantially columnar pressure member  330 , a second electric actuator  320  including a servo motor with a brake, a slider  350 , and a substantially rectangular parallelepiped regulation member  340 . The pressure member  330  is attached to the front end of a piston rod  311  of the first electric actuator  310 . The pressure member  330  is moved in the Y or −Y direction, (upward or downward in  FIG. 8 ) by the first electric actuator  310 . The slider  350  is attached to the second electric actuator  320 , and the regulation member  340  and the first electric actuator  310  are attached to the slider  350 . Thus, the slider  350  is moved in the X or −X direction (left or right direction in  FIG. 8 ) by the second electric actuator  320 , while the first electric actuator  310 , the pressure member  330  attached thereto, and the regulation member  340  move together in the X or −X direction. Since the pressure member  330  and the regulation member  340  are formed separately, the pressure member  330  can move in the Y direction independently. 
   In the gap forming unit  300  configured thus, the pressure member  330  is moved in the Y direction by the first electric actuator  310 , so that the ground electrode  110  is pressed and bent in the Y direction by the pressure member  330 . Thus, the spark discharge gap g having a predetermined gap size (first gap size value g 1  in Embodiment 1) can be formed between the opposed surface  112   b  of the front end portion  112  of the ground electrode  110  and the front end  120   b  of the center electrode  120  (see  FIG. 9D ). In this event, in order to prevent the front end surface  112   c  of the ground electrode  110  from moving too far in the X direction, the X-direction position of the front end surface  112   c  of the ground electrode  110  is regulated by the regulation member  340  in the gap forming unit  300  when the ground electrode  110  is pressed in the Y direction by the pressure member  330  (see  FIG. 9B ). 
   Specifically, first, a difference between a gap size value gn (see  FIG. 4 ) measured by the aforementioned gap size measuring unit  230  and the first gap size value g 1  is calculated by the not-shown computer. This size difference is determined as the pressure stroke distance of the first electric actuator  310  after a pressure abutment surface  331  of the pressure member  330  abuts against the ground electrode  110 . Further, based on the position of the front end  120   b  of the center electrode  120  measured by the aforementioned first electrode position measuring unit  240 , the moving distance of the second electric actuator  320  is determined by the not-shown computer so that a regulation abutment surface  341  of the regulation member  340  is located in a predetermined X-direction position. After that, the brake of the servo motor of the second electric actuator  320  is operated to fix the regulation member  340 . 
   The determined moving distance of the second electric actuator  320  is converted into an intended number of revolutions of the servo motor of the second electric actuator  320  correspondingly. On the other hand, the number of revolutions of the servo motor of the second electric actuator  320  is measured by a not-shown pulse counter. When the regulation member  340  is moved in the −X direction (left direction in  FIG. 8 ) by the second electric actuator  320  so that the value measured by the pulse counter reaches the intended number of revolutions, the regulation stroke is regarded as terminated (movement termination), and the motor of the second electric actuator  320  is suspended. After that, the brake of the servo motor of the second electric actuator  320  is operated to fix the regulation member  340  in a predetermined X-direction position. 
   Next, the pressure member  330  is moved in the Y direction (downward in  FIG. 8 ) by the first electric actuator  310 , so that the pressure abutment surface  331  of the pressure member  330  is allowed to abut against the ground electrode  110 . Incidentally, the gap forming unit  300  is designed so that a not-shown load cell detects a change of pressure in the pressure abutment surface  331  of the pressure member  330  caused by abutment as soon as the pressure abutment surface  331  abuts against the ground electrode  110 . After that, the pressure member  330  is moved in the Y direction correspondingly to the determined pressure stroke distance by use of a pulse counter in the same manner as the regulation member  340 . Thus, the ground electrode  110  is pressed and bent in the Y direction so that the spark discharge gap g having the first gap size value g 1  can be formed. In this event, the front end surface  112   c  of the bent ground electrode  110  abuts against the regulation abutment surface  341  of the regulation member  340  so that the X-directional position of the front end surface  112   c  of the ground electrode  110  is regulated. In such a manner, in the gap forming unit  300 , the ground electrode  110  is pressed and bent in the Y direction while the X-direction position of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  is regulated in a predetermined X-direction position. Thus, it is possible to form the spark discharge gap g having the first gap size value g 1  (see  FIGS. 9A-9D ). 
   After that, the pressure member  330  is moved in the −Y direction (upward in  FIG. 8 ) by the first electric actuator  310  so that the pressure member  330  and the ground electrode  110  are separated from each other. Next, the regulation member  340  is moved in the X direction (right direction in  FIG. 8 ) by the second electric actuator  320  so that the ground electrode  110  and the regulation member  340  are separated from each other. Incidentally, in the gap forming unit  300 , the second electric actuator  320  and the not-shown computer connected thereto correspond to a positioning unit, a fixation unit and a separation unit. 
   The pressure abutment surface  331  of the pressure member  330  is subjected to surface treatment with diamond-like carbon, so that the friction coefficient with a ground electrode abutment surface  112   d  of the ground electrode  110  is made not higher than 0.2. It is therefore easy for the ground electrode  110  to slide in the X direction on the pressure abutment surface  331  of the pressure member  330 . Thus, when the ground electrode  110  is deformed by the pressure member  330 , the ground electrode  110  is deformed smoothly not only in the Y direction but also in the X direction, so that the X-direction position of the front end surface  112   c  of the ground electrode  110  can be regulated by the regulation member  340 . 
   The gap adjusting unit  400  also has a structure similar to that of the aforementioned gap forming unit  300  (see  FIG. 8 ). The ground electrode  110  is pressed in the Y direction while the X-directional position of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  is regulated in a predetermined X-directional position. Thus, the gap size of the spark discharge gap g can be adjusted to have a predetermined gap size value (second gap size value g 2  in Embodiment 1) (see  FIGS. 19A-19D ). 
     FIG. 10  is a side view schematically showing the deviation measuring unit  250 . As shown in  FIG. 10 , the deviation measuring unit  250  is constituted by a first illuminator  252 , a second illuminator  253 , a background unit  254 , a photographing camera  251 , and a not-shown computer connected to the photographing camera  251 . The first illuminator  252  is an LED illuminator having a surface emitting LED or a large number of LEDs arrayed two-dimensionally. The first illuminator  252  has a through hole  252   b  for securing a field of view for the photographing camera  251 . The first illuminator  252  is provided to radiate light in the −X direction (right direction in  FIG. 10 ) so as to illuminate the center electrode  120  and the ground electrode  110  of the work  101 . The second illuminator  253  is an optical fiber illumination, which is disposed in a clearance between the ground electrode  110  and the center electrode  120 . The second illuminator  253  is provided to radiate light in the −X direction (right direction in  FIG. 10 ) so as to illuminate an inner side surface  110   d  of the ground electrode  110 . 
   The photographing camera  251  is, for examples, a CCD camera similar to the photographing camera  241 . The photographing camera  251  is disposed in a position opposed to the work  101  in the X direction through the through hole  252   b  of the first illuminator  252 . When the work  101  is illuminated by the first illuminator  252 , light is reflected uniformly by the flat front end surface  112   c  of the ground electrode  110 . Thus, the front end surface  112   c  of the ground electrode  110  is photographed in bright relief against any other part (see  FIGS. 11A-11B ). Based on this photographed image, the position of the front end surface  112   c  of the ground electrode  110  is determined by the not-shown computer. 
   On the other hand, when the work  101  is irradiated by the second illuminator  253 , the inner side surface  110   d  of the ground electrode  110  looks bright while the center electrode  120  is contoured. Thus, the contours of the center electrode  120  can be photographed clearly due to the contrast with the bright background (see  FIGS. 12A-12C ). Based on this photographed image, the position of the front end  120   b  of the center electrode  120  is determined. Based on the position of the front end  120   b  of the center electrode  120  and the position of the front end surface  112   c  of the ground electrode  110  measured thus, the Z-directional deviation P of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  is calculated (see  FIG. 13 ). 
     FIG. 14  is a side view schematically showing the deviation adjusting unit  260 . The deviation adjusting unit  260  has a bending member  261 , an internally threaded portion  262 , an externally threaded portion  263 , and a drive motor  264 . The bending member  261  presses the front end portion  112  of the ground electrode  110  in the Z direction. The internally threaded portion  262  is provided integrally with the bending member  261 . The externally threaded portion  263 ,is screwed down to the internally threaded portion  262 . The drive motor  264  rotates the externally threaded portion  263  around its axis. In this deviation adjusting unit  260 , the externally threaded portion  263  is rotated around its axis by the drive motor  264  so as to move the bending member  261  in the Z direction. Thus, the front end portion  112  of the ground electrode  110  is pressed and deformed in the Z direction while a groove portion  261   b  formed in the bending member  261  engages with the front end portion  112  of the ground electrode  110 . In such a manner, it is possible to adjust the Z-directional deviation of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120 . Incidentally, in the deviation adjusting unit  260 , the number of revolutions of the drive motor  264  is calculated to set the Z-directional deviation P at 0.0 mm by a not-shown computer based on the Z-directional deviation P calculated by the deviation measuring unit  250 . 
   Next, description will be made about a method for manufacturing the spark plug  100  using the spark plug manufacturing apparatus  200  configured as described above.  FIG. 15  is a flow chart (main routine) showing a flow of steps in the method for manufacturing the spark plug  100 . Description will be made along this flow chart and the general view of the spark plug manufacturing apparatus  200  in  FIG. 2 . 
   First, in Step S 1  (work setting step), the work  101  is set, in a position E in  FIG. 2 , on one of the carriers  211  attached to the linear conveyor  210  at predetermined intervals (see  FIG. 2 ). Specifically, by the setter  220 , the work  101  is mounted on the cylindrical work holder  212  placed on the carrier  211  (see  FIG. 3 ). 
   Next, in Step S 2  (first gap size measuring step), the gap size of the work  101  is measured in a position F in  FIG. 2  (see  FIG. 2 ). Specifically, in a position where the linear conveyor  210  has been advanced by one stroke, the gap size of the work  101  is measured by the gap size measuring unit  230  having the photographing camera  231  (see  FIG. 3 ). Here, the gap size measuring process will be described in detail with reference to  FIG. 16  showing the subroutine of Step S 2 . First, in Step S 21 , the contours of the center electrode  120  and the ground electrode  110  of the work  101  illuminated by the illuminator  232  is photographed by the photographing camera  231  disposed on the −Z direction side with respect to the work  101  (see  FIG. 3 ). Next, in Step S 22 , an image picked up by the photographing camera  231  is imported by the not-shown computer (see  FIG. 4 ). Next, based on this photographed image, an edge F 1  of the ground electrode  110  facing the spark discharge gap g is determined in Step S 23 , and an edge F 2  of the center electrode  120  is determined in Step S 24 . 
   Next, in Step S 25 , an origin O (X,Y)=(0,0) is set at one end (left end in Embodiment 1) of the edge F 2 . In Step S 26 , a scan position F 4  (X,Y) is set as the origin O. Next, in Step S 27 , a reference line F 3  passing through the scan position F 4  and crossing the edge F 2  at right angles is made up. In Step S 28 , the coordinates of an intersection point F 5  with the edge F 1  are obtained. In Step S 29 , the gap size gn of the spark discharge gap g is calculated as the length of a line segment connecting F 4  and F 5 , and stored. Next, in Steps S 2 A and S 2 B, the X coordinate of the scan position F 4  is increased by a predetermined amount ΔX. Thus, a new reference line F 3  is made up. In Step S 2 C, it is determined whether the reference line F 3  crosses the edge F 1  or not. Next, when the reference line F 3  crosses the edge F 1 , the routine of processing returns to Steps S 28  and S 29 , where an intersection point F 5  with the edge F 1  is obtained. Thus, the gap size gn is calculated and stored in the same manner. In such a manner, the processing from S 28  to S 2 B is repeated till the intersection point with the edge F 1  disappears. After that, in Step S 2 D, of the measured gap sizes gn, a smallest one is determined as a gap size gm. Then, the routine of processing returns to the main routine of  FIG. 15 . 
   Next, in Step  3  (center electrode position measuring step), the position of the front end  120   b  of the center electrode  120  of the work  101  is measured in a position G in  FIG. 2  (see  FIG. 2 ). Specifically, the position of the front end  120   b  of the center electrode  120  of the work  101  is measured by the first electrode position measuring unit  240  (see  FIG. 5 ). Here, the center electrode position measuring step will be described in detail with reference to  FIG. 17  showing the subroutine of Step S 3 . First, in Step S 31 , the contours of the center electrode  120  and the ground electrode  110  of the work  101  illuminated by the illuminator  242  are photographed by the photographing camera  241  disposed on the −Z direction side with respect to the work  101  (see  FIG. 5 ). Next, in Step S 32 , an image photographed by the photographing camera  241  is imported (see  FIG. 6 ). 
   Next, in Step S 33 , a determination area G 5  having a predetermined width in the Y direction and extending like a belt in the X direction is set in an area A where the noble metal tip  121  forming the front end  120   b  of the center electrode  120  is expected to exist in the photographed image. Next, in Step S 34 , a front end position G 1  is determined based on a Y-direction density distribution obtained in each pixel position in the X direction (see  FIG. 7A ). Next, in Step S 35 , a search line G 2  is set in a position shifted from the front end position G 1  by a predetermined distance (e.g. 0.1 mm) in the Y direction (downward in  FIGS. 7A and 7B ) (see  FIG. 7B ). Next, in Step S 36 , a pixel density distribution in the X direction is obtained along the search line G 2 , and the coordinates of a left end position G 3  and a right end position G 4  are determined based on the obtained pixel density distribution. In Embodiment 1, the coordinates (Xg, Yg) of the right end position G 4  are stored as the position of the front end  120   b  of the center electrode  120 . After that, the routine of processing returns to the main routine of  FIG. 15 . 
   Next, in Steps S 4  and S 5  (regulation member positioning step), the X-direction position of the regulation member  340  of the gap forming unit  300  is positioned in a position H in  FIG. 2 . Specifically, first, the predetermined covering size Q (see  FIG. 1 ) of the ground electrode  110  with respect to the center electrode  120  and an amount Sb with which the ground electrode  110  is expected to spring back in the −X direction after pressure are added to the X-coordinate Xg of the front end  120   b  of the center electrode  120  measured in Step S 3  (center electrode position measuring step), so that an X-coordinate Xf (see  FIG. 9A ) is calculated. That is, an X-coordinate Xf in which the covering size Q (see  FIG. 1 ) of the ground electrode  110  with respect to the center electrode  120  and the amount Sb with which the ground electrode  110  is expected to spring back in the −X direction after pressure are added to the X-coordinate Xg of the front end  120   b  of the center electrode  120  measured in Step S 3  is calculated (Xf=Xg+Q+Sb). In order to move the regulation member  340  to a position where the X-coordinate of the regulation abutment surface  341  of the regulation member  340  will be Xf, a moving distance of the second electric actuator  320  is calculated. Next, the calculated moving distance is converted into an intended number of revolutions of the servo motor of the second electric actuator  320  corresponding thereto. 
   Next, the regulation member  340  is moved in the −X direction (left direction in  FIG. 8 ) together with the first electric actuator  310  and the pressure member  330  by the second electric actuator  320 . In this event, the number of revolutions of the servo motor of the second electric actuator  320  is measured by the not-shown pulse counter. As soon as the value measured by the pulse counter reaches the intended number of revolutions, the regulation stroke is regarded as terminated, and the servo motor of the second electric actuator  320  is suspended. After that, in Step S 5 , the brake of the servo motor of the second electric actuator  320  is operated to fix the regulation member  340  in that position. In such a manner, the X-direction position of the regulation member  340  of the gap forming unit  300  is positioned as shown in  FIG. 9A . 
   Next, in Step S 6  (gap forming step), the spark discharge gap g of the work  101  is formed in a position H in  FIG. 2 . Specifically, first, a difference between the gap size gm measured in Step S 2  (gap size measuring step) and the first gap size g 1  (see  FIG. 9D ) set in advance is calculated and set as a pressure stroke distance of the first electric actuator  310 . The pressure stroke distance is converted into an intended number of revolutions of the servo motor of the first electric actuator  310  corresponding thereto. Incidentally, in Embodiment 1, the value of the first gap size value g 1  is set to be slightly larger (for example, 0.05 mm larger) than the second gap size value g 2  equal to the gap size of the spark plug  100 . 
   Next, the pressure member  330  is moved in the Y direction (downward in  FIG. 8 ) by the first electric actuator  310 . As described previously, the servo motor of the first electric actuator  310  is suspended as soon as the number of revolutions of the servo motor of the first electric actuator  310  reaches the determined intended number of revolutions by use of a now-shown rod cell and a not-shown pulse counter. In this event, as shown in  FIG. 9B , the ground electrode  110  is pressed and bent in the Y direction by the pressure member  330 , while the front end surface  112   c  of the ground electrode  110  moves in the X direction and abuts against the regulation abutment surface  341  of the regulation member  340 . Since the regulation member  340  is fixed, the X-direction position of the front end surface  112   c  of the ground electrode  110  is regulated. Thus, the X-direction position of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  can be regulated in a predetermined X-direction position. 
   Next, in Steps S 7  and S 8  (return step), in a position H in  FIG. 2 , the pressure member  330  is returned to its position before Step S 6  (gap forming step), and the regulation member  340  is returned to its position before Step S 4  (regulation member positioning step). Specifically, first, in Step S 7 , as shown in  FIG. 9C , the servo motor of the first electric actuator  310  is rotated reversely by the same number of revolutions as that in Step S 6  (gap forming step). Thus, the pressure member  330  is moved in the −Y direction (upward in  FIG. 9C ) so that the ground electrode  110  and the pressure member  330  are separated from each other. Next, in Step S 8  (separation step), as shown in  FIG. 9D , the servo motor of the second electric actuator  320  is rotated reversely by the same number of revolutions as that in Step S 4  (regulation member positioning step). Thus, the regulation member  340  is moved in the X direction (right direction in  FIG. 9D ) together with the first electric actuator  310  and the pressure member  330  so that the regulation member  340  and the ground electrode  110  are separated from each other. In this event, the covering size of the ground electrode  110  with respect to the center electrode  120  is Q, and the spark discharge gap g whose gap size is equal to the first gap size value g 1  is formed. 
   In Step S 8  (separation step) in Embodiment 1, as described above, the regulation member  340  abutting against the ground electrode  110  is moved in the X direction so as to separate the both from each other. Accordingly, when the regulation member  340  abutting against the ground electrode  110  is separated from the ground electrode  110 , the regulation member  340  can be returned to its position before Step S 4  (regulation member positioning step) without changing the gap size (first gap size value g 1 ). 
   Next, in Step S 9  (deviation measuring step), the deviation of the ground electrode  110  with respect to the center electrode  120  in the work  101  is measured in a position I in  FIG. 2 . Specifically, the deviation of the ground electrode  110  with respect to the center electrode  120  in the work  101  is measured by the deviation measuring unit  250  (see  FIG. 10 ). Here, the deviation measuring step will be described in detail with reference to  FIG. 18  showing the subroutine of Step S 9 . First, in Step S 91 , the first illuminator  252  is turned on. Next, in Step S 92 , the front end surface  112   c  of the ground electrode  110  seen in brighter relief than any other part of the work  101  illuminated by the first illuminator  252  is photographed by the photographing camera  251  disposed on the X-direction side with respect to the work  101  (see  FIG. 10 ). Next, in Step S 93 , an image photographed by the photographing camera  251  is imported (see  FIG. 11A ). 
   Next, in Step S 94 , as shown in  FIG. 11A , a belt-like determination area having a predetermined width in the Z direction and extending in the Y direction is set in the photographed image, and upper end lower end positions I 1  and I 2  of the front end surface  112   c  of the ground electrode  110  are determined based on a Y-direction density distribution. Further, in Step S 95 , a belt-like determination area having a Y-direction width equal to the distance between the upper end position I 1  and the lower end position I 2  and extending in the Z direction is set, and left and right end positions I 3  and I 4  of the front end surface  112   c  of the ground electrode  110  are determined based on a Z-direction density distribution. Next, in Step S 96 , a central 1/3 area  15  in which upper and lower 1/3 areas are excluded from an area surrounded by the positions I 1 , I 2 , I 3  and I 4  is obtained, and a barycentric position  16  of the front end surface  112   c  of the ground electrode  110  is determined based on the obtained area  15 . Next, in Step S 97 , as shown in  FIG. 11B , a ground electrode center line I 8  passing through the barycentric position I 6  and extending in parallel to the central axis C of the metal shell  130  is set. In Step S 98 , a Z-coordinate corresponding to the lower end position (regarded as Yi) of the ground electrode  110  which position has been obtained is obtained on the ground electrode center line  18  to be Zi, and a point (Zi, Yi) is regarded as a ground electrode center position I 7 . 
   Next, in Step S 99 , the first illuminator  252  is turned off, and the second illuminator  253  is turned on. In Step S 9 A, an image in which the center electrode  120  is contoured and the contours thereof are clarified by the contrast with the bright background (see  FIG. 12A ) is picked up by the photographing camera  251  disposed on the X-direction side with respect to the work  101  (see  FIG. 10 ). Next, in Step S 9 B, the image photographed by the photographing camera  251  is imported (see  FIG. 12A ). Next, in Step S 9 C, as shown in  FIG. 12A , a belt-like determination area J 1  having a predetermined width in the Y direction and extending in the Z direction is set in an area where the noble metal tip  121  forming the front end  120   b  of the center electrode  120  is expected to exist, and a front end position J 2  is determined based on a Y-direction density distribution obtained in each pixel position in the Z direction. Next, in Step S 9 D, as shown in  FIG. 12B , a search line J 3  is set in a position shifted from the front end position J 2  by a predetermined distance (e.g. 0.1 mm) in the Y direction (downward in  FIG. 12B ). A Z-direction pixel density distribution is obtained along the search line J 3 , and the coordinates of a left end position J 4  and a right end position J 5  are determined based on the obtained pixel density distribution. 
   Next, in Step S 9 E, the average value of the Z-coordinates of the left and right ends of the center electrode is regarded as Zj, and the Y-coordinate of the front end position J 2  is regarded as Yj. Thus, the coordinates of the center position J 6  of the center electrode are determined as (Zj, Yj) (see  FIG. 12C ). Next, as shown in  FIG. 13 , with reference to the ground electrode center line I 8  which has been set, it is determined whether the Z-coordinate of the center electrode center position J 6  is on the left side or on the right side with respect to the ground electrode center line I 8 . Thus, the sign of the deviation is determined. This sign serves to define the direction of bending of the ground electrode  110  in Step SA (deviation adjusting step, see  FIG. 15 ) which will be performed later. In Step S 9 F, the distance between the ground electrode center line I 8  and the center electrode center position J 6  is calculated as a Z-directional deviation P of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120 . Then, the routine of processing returns to the main routine shown in  FIG. 15 . 
   Next, in Step SA (deviation adjusting step), in a position J in  FIG. 2 , the Z-directional deviation P of the ground electrode  110  with respect to the center electrode  120  in the work  101  is adjusted to be within a predetermined range. Specifically, the front end portion  112  of the ground electrode  110  is bent in the direction (which is a direction determined in Step S 9  (deviation measuring step) and a right direction in  FIG. 14  in this embodiment) with which the Z-directional deviation P will be reduced by the deviation adjusting unit  260  shown in  FIG. 14 . The bending amount (moving distance of the bending member  261 ) is set to be a value in which a spring-back distance of the ground electrode  110  due to release from an urging force of the bending member  261  is added to the Z-directional deviation P. In such a manner, the Z-directional deviation of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  is adjusted or controlled to be 0.0 mm. 
   Next, in Step SB (second gap size measuring step), the gap size of the work  101  is measured in a position K in  FIG. 2  (see  FIG. 2 ). Specifically, a gap size measuring step (S 21  to S 2 D) similar to Step S 2  is carried out to determine the gap size gm (see  FIGS. 3 and 4 ). In Step SC, it is determined whether the gap size gm is or not within a predetermined tolerance range (e.g. g 2 ±0.1(mm)) from the second gap size value g 2  (mm) which is an intended value of a final gap size. When the gap size gm is within the predetermined tolerance range, the routine, of processing proceeds to Step SJ (work unloading step), where in a position L in  FIG. 2 , the work  101  is unloaded from the carrier  211  attached to the linear conveyor  210  at the predetermined interval. Specifically, the work  101  is detached from the cylindrical work holder  212  by the first unloading unit  290  so that the work  101  is unloaded from the carrier  211  of the linear conveyor  210 . Thus, the spark plug  100  as shown in  FIG. 1  is completed. 
   On the contrary, when the gap size gm is out of the predetermined tolerance range (e.g. g 2 ±0.1(mm)) from the second gap size value g 2  (mm), the routine of processing proceeds to Steps SD-SI, where the following steps are carried out. The following steps are substantially similar to the aforementioned Step S 3  (center electrode position measuring step), Step S 4  (regulation member positioning step), Step S 6  (gap forming step), and Steps S 7 -S 8  (return step). Thus, description about similar parts will be omitted or simplified. 
   First, in Step SD (second center electrode position measuring step), the position of the front end  120   b  of the center electrode  120  of the work  101  is measured in a position M in  FIG. 2  (see  FIG. 2 ). Specifically, a center electrode position measuring step (Steps S 31 -S 36 ) similar to Step S 3  is carried out so that the coordinates (Xg, Yg) of the right end position G 4  are stored as the position of the front end  120   b  of the center electrode  120  (see  FIGS. 5 ,  6  and  7 A- 7 B). 
   Next, in Step SE (second regulation member positioning step), the X-directional position of the regulation member  340  of the gap forming unit  300  is positioned in a position N in  FIG. 2  (see  FIG. 19A ). Specifically, a regulation member moving step similar to Step S 4  is carried out to move the regulation member  340  in the −X direction so that the X-coordinate of the regulation abutment surface  341  of the regulation member  340  is Xf. In Step SF, a regulation member fixing step similar to Step S 5  is carried out to fix the regulation member  340  of the gap forming unit  300 . Even when the X-directional position of the front end surface  112   c  of the ground electrode  110  varies in the previous Step SA (deviation adjusting step) so that the covering size is larger than Q, the X-directional position of the front end surface  112   c  of the ground electrode  110  can be returned to its predetermined position (whose X-coordinate is Xf) in this Step SE (second regulation member positioning step). 
   Next, in Step SG (gap adjusting step), in a position N in  FIG. 2 , the gap size of the spark discharge gap g of the work  101  is adjusted by the gap adjusting unit  400  (see  FIG. 19B ). Specifically, the difference between the gap size gm measured in Step SD (second gap size measuring step) and the second gap size value g 2  (see  FIG. 19D ) set in advance is calculated and set as the pressure stroke distance of the first electric actuator  310 . Based on the pressure stroke distance, the pressure member  330  is moved in the Y direction (downward in  FIGS. 19A-19D ) by the first electric actuator  310 . In this event, as shown in  FIG. 19B , the ground electrode  110  is pressed in the Y direction by the pressure member  330  while the front end surface  112   c  of the ground electrode  110  abuts against the regulation abutment surface  341  of the regulation member  340 . Therefore, the X-directional position of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  can be regulated and controlled to a predetermined X-directional position. 
   In the previous Step S 6  (gap forming step), the gap size is set as the first gap size value slightly (e.g. 0.05 mm) larger than the second gap size value g 2 . Therefore, the Y-directional pressure distance in Step SG (gap adjusting step) becomes slight. As a result, the Z-directional deviation of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  in Step SG becomes so slight that there is no concern that the Z-directional deviation adjusted in Step SA (deviation adjusting step) is out of a predetermined range. 
   Next, in Steps SH and SI (second return step), in a position N in  FIG. 2 , the pressure member  330  is returned to its position before Step SG (gap adjusting step), and the regulation member  340  is returned to its position before Step SE (second regulation member positioning step). Specifically, a separation step similar to Steps S 7 -S 8  is carried out to separate the pressure member  330  from the ground electrode  110  (see  FIG. 19C ). Next, the regulation member  340  is separated from the ground electrode  110  (see FIG.  19 D). In this event, the covering size of the ground electrode  110  with respect to the center electrode  120  is Q (mm). Thus, the spark discharge gap g whose gap size is equal to the second gap size value g 2  is formed. 
   In Steps SH-SI (second return step), in the same manner as in Steps S 7 -S 8  (first return step), the regulation member  340  abutting against the ground electrode  110  is moved in the X direction so as to separate the both from each other. Accordingly, the regulation member  340  can be returned to its position before Step SE (second regulation member positioning step) without changing the gap size (second gap size value g 2 ). 
   Finally, in Step SJ (work unloading step), in a position R in  FIG. 2 , the work  101  is unloaded from the carrier  211  of the linear conveyor  210 . Specifically, the work  101  is detached from the work holder  212  of the carrier  211  by the second unloading unit  390  so that the work  101  is moved onto a palette. 
   In such a manner, the spark plug  100  as shown in  FIG. 1  is completed. 
   Embodiment 2 
   Next, Embodiment 2 of the invention will be described with reference to  FIGS. 20A-20B ,  21 A- 21 C and  22 . A spark plug manufacturing apparatus  600  according to Embodiment 2 is almost the same as the spark plug manufacturing apparatus  200  according to Embodiment 1 with the exception of a gap forming unit. A method for manufacturing a spark plug  100  according to Embodiment 2 is almost the same as that according to Embodiment 1 with the exception of Steps S 4 -S 5  (regulation member positioning step), Step S 6  (gap forming step) and S 7 -S 8  (return step). Description will focus on parts different from those in Embodiment 1, but description about parts similar to those in Embodiment 1 will be omitted or simplified. 
     FIGS. 20A and 20B  are views schematically showing a gap forming unit  700  according to Embodiment 2.  FIG. 20A  is a top view thereof, and  FIG. 20B  is a side view thereof. As shown in  FIGS. 20A and 20B , the gap forming unit  700  has a second electric actuator  720  including a servo motor with a brake, a guide  760 , a gap forming member  730 , a spring  750 , a linear bearing  780  and a not-shown first electric actuator. Although the pressure member  330  and the regulation member  340  are formed separately in the gap forming unit  300  according to Embodiment 1, the gap forming member  730  includes a pressure portion  731  and a regulation portion  732  in the gap forming unit  700  according to Embodiment 2. That is, the pressure portion  731  and the regulation portion  732  are formed integrally. 
   The guide  760  is linked with the second electric actuator  720 . The gap forming member  730  is linked with the guide  760  by a pin  735  inserted into a long hole  761  of the guide  760 . Although the first electric actuator  310 , the pressure member  330  and the regulation member  340  is moved together in the X direction and the −X direction by the second electric actuator  320  in the gap forming unit  300  according to Embodiment 1, the gap forming member  730  is moved in the X direction and the −X direction (left and right directions in  FIGS. 20A-20B ) in concert with the motion of the guide  760  by the electric actuator  720  in the gap forming unit  700  according to Embodiment 2. 
   Further, the gap forming member  730  is attached to the linear bearing  780  extending in the X direction. Accordingly, when the gap forming member  730  moves in the X direction or the −X direction, the gap forming member  730  goes straight in the X direction or the −X direction while the Z-directional position thereof is regulated. Moreover, the gap forming member  730  is linked with the spring  750 . Thus, as shown in  FIG. 20B , before the pressure portion  731  of the gap forming member  730  applies pressure to the ground electrode  110 , the gap forming member  730  is designed to be pulled in the −X direction (left direction in  FIG. 20B ) by the spring  750  so that the pin  735  is located in the left end of the long hole  761 . 
   Further, a gap forming unit  701  including the second electric actuator  720 , the guide  760 , the gap forming member  730 , the spring  750  and the linear bearing  780  is linked with the not-shown first electric actuator integrally. Although the pressure member  330  is moved in the Y direction independently by the first electric actuator  310  in the gap forming unit  300  according to Embodiment 1, the gap forming unit  701  including the gap forming member  730  is moved in the Y direction integrally by the not-shown first electric actuator in the gap forming unit  700  according to Embodiment 2 (see  FIG. 20B ). 
   In the gap forming unit  700  configured thus, the gap forming unit  701  including the gap forming member  730  is moved in the Y direction by the not-shown first electric actuator so that the ground electrode  110  is pressed and bent in the Y direction by the pressure portion  731  of the gap forming member  730 . Thus, a spark discharge gap g having a predetermined gap size (first gap size value g 1  in Embodiment 2) can be formed between the opposed surface  112   b  of the front end portion  112  of the ground electrode  110  and the front end  120   b  of the center electrode  120  (see  FIG. 21C ). The pressure stroke distance of the not-shown first electric actuator is determined in the same manner as that of the first electric actuator  310  in Embodiment 1. Further, the gap forming member  730  is moved in the Y direction by the determined pressure stroke distance by use of a not-shown load cell and a not-shown pulse counter. 
   When the gap forming unit  701  is being moved in the Y direction, the guide  760  is fixed with respect to the electric actuator  720 . Accordingly, when the ground electrode  110  is pressed and bent by the pressure portion  731  of the gap forming member  730 , the front end surface  112   c  of the ground electrode  110  abuts against a regulation abutment surface  732   b  of the regulation portion  732 . The regulation abutment surface  732   b  of the regulation portion  732  moves in the X direction (right direction in the drawings) together with the front end surface  112   c  of the ground electrode  110  while the regulation abutment surface  732   b  of the regulation portion  732  abuts against the front end surface  112   c  of the ground electrode  110  together with the pin  735  moving in the X direction (right direction in the drawings) in the long hole  761 . After that, as soon as the pin  735  reaches the right end of the long hole  761 , the X-directional position of the gap forming member  730  is regulated so that the X-directional position of the regulation abutment surface  732   b  of the regulation portion  732  is fixed. Thus, the X-directional position of the front end surface  112   c  of the ground electrode  110  can be regulated by the regulation abutment surface  732   b  (see  FIG. 21B ). 
   In Embodiment 2, the ground electrode  110  is pressed by the gap forming member  730 . As soon as the pin  735  reaches the right end of the long hole  761  of the guide  760 , the X-directional position of the long hole  761  of the guide  760  is positioned so that the regulation abutment surface  732   b  of the regulation portion  732  is located in a predetermined X-directional position. In such a manner, the X-directional position of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  can be regulated at a predetermined X-directional position. 
   In addition, in the same manner as the pressure abutment surface  331  of the pressure member  330  according to Embodiment 1, the pressure abutment surface  731   b  of the pressure portion  731  according to Embodiment 2 is subjected to surface treatment with diamond-like carbon so that the friction coefficient with the ground electrode abutment surface  112 d of the ground electrode  110  is made not higher than 0.2. It is therefore easy for the ground electrode  110  to slide in the X direction on the pressure abutment surface  731   b  of the pressure portion  731 . Thus, when the ground electrode  110  is deformed by the pressure portion  731 , the ground electrode  110  is deformed smoothly not only in the Y direction but also in the X direction, so that the X-directional position of the front end surface  112   c  of the ground electrode  110  can be regulated by the regulation portion  732 . 
   Next, description will be made about the method for manufacturing the spark plug  100  according to Embodiment 2. 
     FIG. 22  is a flow chart (main routine) showing the flow of steps in the method for manufacturing the spark plug  100 . Description will be made below along this flow chart and the general view of the spark plug manufacturing apparatus  600  in  FIG. 2 . Incidentally, the manufacturing method according to Embodiment 2 is the same as that according to Embodiment 1, except that the processing of Steps S 4 -S 8  in Embodiment 1 is replaced by the processing of Steps T 4 -T 8  as shown in  FIG. 22 . Description about the same parts as those in Embodiment 1 will be omitted or simplified. 
   First, in the same manner as in Embodiment 1, the work  101  is set in Step S 1  (work setting step). Next, in Step S 2  (first gap size measuring step), the gap size of the work  101  is measured. Next, in Step S 3  (center electrode position measuring step), the position of the front end  120   b  of the center electrode  120  in the work  101  is measured. 
   Next, in Steps T 4 -T 5  (guide positioning step), the X-directional position of the guide  760  of the gap forming unit  700  is positioned in a position H in  FIG. 2 . Specifically, first, in Step T 4 , the predetermined covering size Q (see  FIG. 1 ) of the ground electrode  110  with respect to the center electrode  120  and an amount with which the ground electrode  110  is expected to spring back in the −X direction after pressure are added to the X-coordinate Xg of the front end  120   b  Qf the center electrode  120  measured in the previous Step S 3  (center electrode position measuring step). Thus, an X-coordinate Xf is calculated. A moving distance of the second electric actuator  720  is calculated so that the X-coordinate of the regulation abutment surface  732   b  of the regulation portion  732  will be Xf when the pin  735  reaches the right end of the long hole  761  (see  FIG. 21B ). Further, the calculated moving distance is converted into an intended number of revolutions of the servo motor of the second electric actuator  720  corresponding thereto. 
   Next, the gap forming member  730  is moved in the X direction (right direction in  FIGS. 20A-20B ) together with the guide  760  by the second electric actuator  720 . In this event, the number of revolutions of the motor of the second electric actuator  720  is measured by a not-shown pulse counter. As soon as the value measured by the pulse counter reaches the intended number of revolutions, the movement is regarded as terminated, and the motor of the second electric actuator  720  is suspended. After that, in Step T 5 , the brake of the servo motor of the second electric actuator  720  is operated to fix the guide  760  in that position. In this event, the gap forming member  730  is pulled in the −X direction (left direction in  FIGS. 20A-20B ) by the spring  750  so that the pin  735  is located in the left end of the long hole  761  of the guide  760 . In such a manner, the X-directional position of the guide  760  is positioned as shown in  FIG. 21A . Incidentally, Steps T 4 -T 5  (guide positioning step) in Embodiment 2 correspond to the regulation member positioning step. 
   Next, in Step T 6  (gap forming step), the spark discharge gap g of the work  101  is formed. Specifically, first, a difference between the gap size gm measured in Step S 2  (gap size measuring step) and the first gap size value g 1  (see  FIG. 21C ) set in advance is calculated and set as a pressure stroke distance of the not-shown electric actuator. In Embodiment 2, in the same manner as in Embodiment 1, the value of the first gap size value g 1  is set to be slightly larger (for example, 0.05 mm larger) than the second gap size value g 2  equal to the gap size of the spark plug  100 . 
   Next, the gap forming unit  701  (gap forming member  730 ) is moved in the Y direction (downward in  FIG. 21A ) by the not-shown electric actuator. Then, the gap forming member  730  is moved in the Y direction by the determined pressure stroke distance by use of the not-shown load cell and the not-shown pulse counter. In this event, as shown in  FIG. 21B , the ground electrode  110  is pressed and bent in the Y direction by the pressure portion  731  so that the front end surface  112   c  of the ground electrode  110  abuts against the regulation abutment surface  732   b  of the regulation portion  732 . The X-directional position of the regulation abutment surface  732   b  is regulated by the X-directional position of the pin  735  regulated in the right end of the long hole  761  of the guide  760 . Accordingly, the X-directional position of the front end surface  112   c  of the ground electrode  110  with respect to the front end  120   b  of the center electrode  120  can be regulated in a predetermined X-directional position. 
   Next, in Steps T 7  and T 8  (return step), the gap forming member  730  is returned to its position before Step T 4  (guide positioning step). Specifically, first, in Step T 7  (separation step), as shown in  FIG. 21C , the gap forming member  730  is moved in the X direction (right direction in  FIG. 21C ) by the second electric actuator  720  so that the front end surface  112   c  of the ground electrode  110  and the regulation abutment surface  732   b  of the regulation portion  732  are separated from each other. Next, in Step T 8 , the motor of the not-shown first electric actuator is rotated reversely by the same number of revolutions as that in Step T 6  (gap forming step). Thus, the gap forming unit  701  including the gap forming member  730  is moved in the −Y direction (upward in  FIG. 21C ). Next, the gap forming member  730  is moved in the −X direction (left direction in  FIG. 21C ) by the second electric actuator  720 . In such a manner, the gap forming member  730  is returned to its position before Step T 4  (guide positioning step). In this event, the covering size of the ground electrode  110  with respect to the center electrode  120  is Q, and the spark discharge gap g whose gap size is equal to the first gap size value g 1  is formed. 
   In Step T 7  (separation step) in Embodiment 2, in the same manner as in Step S 7  (separation step) in Embodiment 1, the regulation portion  732  of the gap forming member  730  abutting against the front end surface  112   c  of the ground electrode  110  is moved in the X direction so as to separate the both from each other. Accordingly, there is no fear that the gap size (first gap size value g 1 ) is changed when the gap forming member  730  abutting against the ground electrode  110  is separated from the ground electrode  110 . 
   Next, in Step S 9  (deviation measuring step), in the same manner as in Embodiment 1, the deviation of the ground electrode  110  with respect to the center electrode  120  in the work  101  is measured. Further, in Step SA (deviation adjusting step), the Z-directional deviation P of the ground electrode  110  with respect to the center electrode  120  in the work  101  is adjusted to be within a predetermined range. 
   Next, in Step SB (second gap size measuring step), the gap size of the work  101  is measured (see  FIG. 2 ). Specifically, the gap size gm is determined by a gap size measuring process similar to that in Embodiment 1 (see  FIGS. 3 and 4 ). In Step SC, it is determined whether the gap size gm is or not within a predetermined tolerance range (e.g. g 2 ±0.1(mm)) from the second gap size value g 2  (mm) which is an intended value of a final gap size. When the gap size gm is within the predetermined tolerance range, the routine of processing proceeds to Step SJ (work unloading step), where the work  101  is unloaded from the carrier  211  of the linear conveyor  210  (SJ). Thus, the spark plug  100  as shown in  FIG. 1  is completed. 
   On the contrary, when the gap size gm is out of the predetermined tolerance range (e.g. g 2 ±0.1(mm)) from the second gap size value g 2  (mm), Step SD (second center electrode position measuring step), Steps SE-SF (regulation member positioning step), Step SG (gap adjusting step), Steps SH-SI (return step) and Step SJ (work unloading step) are carried out in that order in the same manner as in Embodiment 1. 
   In such a manner, the spark plug  100  as shown in  FIG. 1  is completed. 
   Embodiment 3 
   Next, Embodiment 3 of the invention will be described with reference to the drawings. 
   A spark plug  1100  according to Embodiment 3 is similar to the spark plug  100  according to Embodiment 1, except that a noble metal tip  113  is provided in a ground electrode as shown in  FIG. 23 . Specifically, the noble metal tip  113  is welded with an inner side surface  114  of a ground electrode  1110  so as to be opposed to the front end  120   b  of the center electrode  120 . A spark discharge gap g is formed between a front end  113 B of the noble metal tip  113  and the front end  120   b  of the center electrode  120 . As shown in the enlarged view of  FIG. 23 , the gap size of the spark discharge gap g is g 2  (mm) (corresponding to the second gap size). 
   A spark plug manufacturing apparatus  1200  according to Embodiment 3 is almost the same as the spark plug manufacturing apparatus  200  according to Embodiment 1, except that the deviation measuring unit  250  is replaced by a deviation measuring unit  1250  as shown in  FIG. 24 . 
   In a method for manufacturing the spark plug  1100  according to Embodiment 3, Step S 9  (deviation measuring step) and Step SE (second regulation member positioning step) in the manufacturing method according to Embodiment 1 are replaced by Step U 9  and Step UG (see  FIGS. 15 and 32 ). Further, in the manufacturing method according to Embodiment 3, Step UC (first inter-tip distance measuring step), Step UD (first tip angle measuring step), Step UJ (third gap size measuring step), Step UK (second inter-tip distance measuring step) and Step UL (second tip angle measuring step) are added newly. The other steps are almost the same as those in the manufacturing method according to Embodiment 1. Therefore, description will focus on parts different from those in Embodiment 1, and description about similar parts will be omitted or simplified. 
     FIG. 26  is a side view schematically showing the deviation measuring unit  1250  according to Embodiment 3. As shown in  FIG. 26 , the deviation measuring unit  1250  is constituted by first illuminators  1253 , a ground electrode chuck  1255 , a photographing camera  1251  and a not-shown computer connected to the photographing camera  1251 . 
   The ground electrode chuck  1255  holds a front end portion  112  of a ground electrode  1110  and rotates a to-be-formed spark plug (hereinafter also referred to as “work”)  1101  around a central axis C so as to adjust the direction of the work  1101  with respect to the photographing camera  1251  (see  FIGS. 28A-28C ). 
   The photographing camera  1251  is, for example, a CCD camera. When the direction of the work  1101  is adjusted by the ground electrode chuck  1255  as described above, it is possible to acquire an image in which a center electrode  120  of the work  1101  is located on the near side and a base portion  111  of the ground electrode  1110  is located on the far side so that the center electrode  120  overlaps with the base portion  111  of the ground electrode  1110 . 
   The first illuminators  1253  are optical fiber illuminations, which are fixedly provided in a lower end surface  1255   b  of the ground electrode chuck  1255  so as to illuminate, of the inner side surface  114  of the ground electrode  1110 , a back portion  114   b  located to be farther than the center electrode  120  when the work  1101  is viewed from the photographing camera  1251 . Further, a sectional view taken on line A-A in  FIG. 26  is shown in  FIG. 27  for explaining the first illuminators  1253  in more detail. As shown in  FIG. 27 , two first illuminators  1253  are provided in symmetric positions with respect to the center electrode  120 . Each first illuminator  1253  is disposed to illuminate the back portion  114   b  of the ground electrode  1110  with light in an oblique direction shifted by about 45 degrees from the camera photographing direction (right direction in  FIG. 27 ). When the back portion  114   b  of the ground electrode  1110  is illuminated by the first illuminators  1253 , light reflected by the back portion  114   b  of the ground electrode  1110  is blocked by the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110 . As a result, an image in which the contours of the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110  are clarified by the back portion  114   b  of the ground electrode  1110  serving as the bright background can be acquired by the photographing camera  1251  (see  FIGS. 29A-29C ). 
   Based on the image photographed thus, the position of the noble metal tip  113  of the ground electrode  1110  and the position of the noble metal tip  121  of the center electrode  120  are determined by the not-shown computer as will be described later. Specifically, in Embodiment 3, the center position of the front end  113 B of the noble metal tip  113  of the ground electrode  1110  is determined as a ground electrode center position  16  (see  FIG. 29C ), and the center position of the front end  120   b  of the center electrode  120  (noble metal tip  121 ) is determined as a center electrode center position J 6  (see  FIG. 30C ). Based on the ground electrode center position  16  and the center electrode center position J 6 , a Z-directional deviation P of the noble metal tip  113  of the ground electrode  1110  with respect to the noble metal tip  121  of the center electrode  120  is calculated (see  FIG. 31 ). 
   Next, description will be made about the method for manufacturing the spark plug  1100  according to Embodiment 3. 
     FIG. 32  is a flow chart (main routine) showing a flow of steps in the method for manufacturing the spark plug  1100 . Description will be made below along this flow chart. 
   First, in the same manner as in Embodiment 1, in Step S 1  (work setting step), the work  1101  is set. Next, in Step S 2  (first gap size measuring step), the gap size of the work  1101  is measured in the same manner as in Embodiment 1. Specifically, a minimum gap size gm is determined by a gap size measuring process similar to that in Embodiment 1. In Embodiment 3, based on a photographed image as shown in  FIG. 25 , an edge F 1  on the ground electrode  1110  side and an edge F 2  on the center electrode  120  side are determined in Steps S 23 -S 24  (see  FIG. 16 ). In Embodiment 3, due to the noble metal tip  113  provided in the ground electrode  1110 , the edge F 1  is determined in the position of the front end surface  113 b of the noble metal tip  113 . Next, in Step  3  (first center electrode position measuring step), the position of the front end  120   b  of the center electrode  120  of the work  1101  is measured. Specifically, the coordinates of a right end position G 4  of the noble metal tip  121  are measured and stored as the position of the front end  120   b  of the center electrode  120  (see  FIGS. 7A-7B ). 
   Further, in the same manner as in Embodiment 1, in Steps S 4 -S 5  (regulation member positioning step), the X-directional position of the regulation member  340  of the gap forming unit  300  is positioned. In Step S 6  (gap forming step), a spark discharge gap g of the work  1101  is formed. After that, in Steps S 7 -S 8  (return step), the pressure member  330  and the regulation member  340  are returned to their positions before Steps S 6  and S 4  respectively. Thus, in the same manner as in Embodiment 1, the covering size of the ground electrode  1110  with respect to the center electrode  120  becomes Q, and the spark discharge gap g whose gap size is equal to the first gap size value g 1  is formed (see  FIGS. 9A-9D ). 
   Next, in Step U 9  (deviation measuring step), a deviation of the ground electrode  1110  with respect to the center electrode  120  in the work  1101  is measured in a position I in  FIG. 24 . Specifically, a Z-directional deviation P of the noble metal tip  113  of the ground electrode  1110  with respect to the noble metal tip  121  of the center electrode  120  is measured by the deviation measuring unit  1250  (see  FIG. 31 ). 
   Here, the deviation measuring process will be described in detail with reference to  FIG. 33  showing the subroutine of Step U 9 . First, in Step U 91 , the ground electrode chuck  1255  is moved down from above the work  1101  (see  FIG. 28A ), and stopped in a predetermined position (see  FIG. 28B ). Next, in Step U 92 , the work  1101  is rotated around the central axis C while the front end portion  112  of the ground electrode  1110  is grasped by the ground electrode chuck  1255 . In this manner, the direction of the work  1101  with respect to the photographing camera  1251  disposed in a predetermined position is adjusted (see  FIG. 28C ). Thus, an image in which the center electrode  120  is located on the near side and a base portion  111  of the ground electrode  1110  is located on the far side so that the center electrode  120  overlaps with the base portion  111  of the ground electrode  1110  can be acquired by the photographing camera  1251 . At the same time, the first illuminators  1253  fixedly provided in the lower end surface  1255   b  of the ground electrode chuck  1255  are disposed in predetermined positions (see  FIGS. 26 and 27 ). 
   Next, in Step U 93 , the first illuminators  1253  are turned on so as to illuminate the back portion  114   b  of the ground electrode  1110  with light (see  FIG. 27 ). Next, in Step U 94 , an image in which the contours of the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110  are clarified by the back portion  114   b  of the ground electrode  1110  serving as the bright background is photographed by the photographing camera  1251  (see  FIG. 26 ). Next, in Step U 95 , the image photographed by the photographing camera  1251  is imported (see  FIG. 29A ). According to Embodiment 3, in such a manner, an image in which both the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110  are clear can be acquired by one-time photographing. 
   Next, in Step U 96 , as shown in  FIG. 29A , a belt-like determination area I 1  having a predetermined width in the Y direction and extending in the Z direction is set in an area where the noble metal tip  113  of the ground electrode  1110  is expected to exist in the acquired image. A front end position I 2  is determined based on a Y-direction density distribution obtained in each pixel position in the Z direction. Next, in Step U 97 , as shown in  FIG. 29B , a search line  13  is set in a position shifted from the front end position I 2  by a predetermined distance (e.g. 0.1 mm) in the −Y direction (upward in  FIG. 29B ). A Z-direction pixel density distribution is obtained along the search line I 3 , and the coordinates of a left end position I 4  and a right end position I 5  are determined based on the obtained pixel density distribution. Next, in Step U 98 , the average value of the Z-coordinates of the left and right end positions I 4  and I 5  is regarded as Zi, and the Y-coordinate of the front end position I 2  is regarded as Yi. Thus, the coordinates of the center position I 6  of the noble metal tip  113  are determined as (Zi, Yi) (see  FIG. 29C ). 
   Next, in Step U 99 , as shown in  FIG. 30A , a belt-like determination area J 1  having a predetermined width in the Y direction and extending in the Z direction is set in an area where the noble metal tip  121  forming the front end  120   b  of the center electrode  120  is expected to exist in the acquired image. A front end position J 2  is determined based on a Y-direction density distribution obtained in each pixel position in the Z direction. Next, in Step U 9 A, as shown in  FIG. 30B , a search line J 3  is set in a position shifted from the front end position J 2  by a predetermined distance (e.g. 0.1 mm) in the Y direction (downward in  FIG. 30B ). A Z-direction pixel density distribution is obtained along the search line J 3 , and the coordinates of a left end position J 4  and a right end position J 5  are determined based on the obtained pixel density distribution. Next, in Step U 9 B, the average value of the Z-coordinates of the left and right end positions J 4  and J 5  is regarded as Zj, and the Y-coordinate of the front end position J 2  is regarded as Yj. Thus, the coordinates of the center position J 6  of the front end  120   b  of the center electrode  120  (noble metal tip  121 ) are determined as (Zj, Yj) (see  FIG. 30C ). 
   Next, in Step U 9 C, as shown in  FIG. 31 , a center line I 8  passing the center position  16  of the noble metal tip  113  and parallel to the Y axis is set. With reference to the center line  18 , it is determined whether the center position J 6  of the front end  120   b  of the center electrode  120  (noble metal tip  121 ) is on the left side or on the right side with respective to the center line I 8 . Thus, the sign of the deviation is determined. This sign serves to define the direction of bending of the ground electrode  1110  in Step SA (deviation adjusting step) which will be performed later. Next, the distance between the center line  18  and the center position J 6  is calculated as a Z-directional deviation P of the noble metal tip  113  of the ground electrode  1110  with respect to the front end  120   b  of the center electrode  120 . Then, the routine of processing returns to the main routine shown in  FIG. 32 . 
   In Embodiment 3, the coordinates (Zi, Yi) of the center position  16  of he noble metal tip  113  of the ground electrode  1110  and the coordinates (Zj, Zj) of the center position J 6  of the front end  120   b  of the center electrode  120  (noble metal tip  121 ) are calculated based on the image where the contours of the noble metal tip  113  and the noble metal tip  121  are clarified. Accordingly, it is possible to acquire the coordinates of the center positions I 6  and J 6  accurately. Further, the deviation P is calculated based on the accurate center positions I 6  and J 6 . Thus, it is possible to acquire the deviation P accurately. 
   Next, in Step SA (deviation adjusting step), in the same manner as in Embodiment 1, the Z-directional deviation P of the noble metal tip  113  of the ground electrode  1110  with respect to the front end  120   b  of the center electrode  120  (noble metal tip  121 ) in the work  1101  is adjusted to be within a predetermined range. 
   Next, in Step SB (second gap size measuring step), the gap size gm of the work  1101  is measured in the same manner as in Step S 2  (first gap size measuring step). 
   Next, in Step UC (first inter-tip distance measuring step), the X-directional distance (X-directional displacement) between the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110  is measured. Here, the inter-tip distance measuring process will be described in detail with reference to  FIG. 34  showing the subroutine of Step UC, and  FIGS. 35A-35B  and  36 A- 36 B. 
   First, in Step UC 1 , the image photographed in Step SB (second gap size measuring step) is imported (see  FIG. 25 ). Next, in Step UC 2 , as shown in  FIG. 35A , a belt-like determination area K 1  having a predetermined width in the Y direction and extending in the X direction is set in an area where the noble metal tip  121  of the center electrode  120  is expected to exist. Next, in Step UC 3 , a front end position K 2  of the noble metal tip  121  of the center electrode  120  is determined based on a Y-direction density distribution obtained in each pixel position in the X direction (see  FIG. 35A ). Next, in Step UC 4 , a search line K 3  is set in a position shifted from the front end position K 2  by a predetermined distance (e.g. 0.1 mm) in the Y direction (downward in  FIG. 35B ) (see  FIG. 35B ). Next, in Step UC 5 , an X-direction pixel density distribution is obtained along the search line K 3 , and the coordinates of a left end position K 4  and a right end position K 5  are determined based on the obtained pixel density distribution. Next, in Step UC 6 , the average value of the X-coordinates of the left and right end positions K 4  and K 5  is regarded as Xk, and the Y-coordinate of the front end position K 2  is regarded as Yk. Thus, the coordinates of the center position K 6  of the noble metal tip  121  are determined as (Xk, Yk) (see  FIG. 35B ). 
   Next, in Step UC 7 , as shown in  FIG. 36A , a belt-like determination area M 1  having a predetermined width in the Y direction and extending in the X direction is set in an area where the noble metal tip  113  of the ground electrode  1110  is expected to exist. In Step UC 8 , a front end position M 2  of the noble metal tip  113  of the ground electrode  1110  is determined based on a Y-direction density distribution obtained in each pixel position in the X direction. Next, in Step UC 9 , as shown in  FIG. 36B , a search line M 3  is set in a position shifted from the front end position M 2  by a predetermined distance (e.g. 0.1 mm) in the −Y direction (upward in  FIG. 36B ). In Step UCA, an X-direction pixel density distribution is obtained along the search line M 3 , and the coordinates of a left end position M 4  and a right end position M 5  are determined based on the obtained pixel density distribution. Next, in Step UCB, the average value of the X-coordinates of the left and right end positions M 4  and M 5  is regarded as Xm, and the Y-coordinate of the front end position M 2  is regarded as Ym. Thus, the coordinates of the center position M 6  of the noble metal tip  113  are determined as (Xm, Ym) (see  FIG. 36B ). 
   Next, in Step UCC, an X-directional distance (X-direction displacement) □X 1  between the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110  is calculated based on the coordinates (Xk, Yk) of the center position K 6  of the noble metal tip  121  and the coordinates (Xm, Ym) of the center position M 6  of the noble metal tip  113 . Specifically, the X-directional displacement ΔX 1  is calculated by ΔX 1 =Xk−Xm. Then, the routine of processing returns to the main routine shown in  FIG. 32 . 
   Next, Step UD (first tip angle measuring step), the angle (angle displacement) between the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110  is measured. Here, the tip angle measuring process will be described in detail with reference to  FIG. 37  showing the subroutine of Step UD, and  FIGS. 38A-38B  and  39 A- 39 B. 
   First, in Step UD 1 , the image photographed in Step SB (second gap size measuring step) is imported (see  FIG. 25 ). Next, in Step UD 2 , as shown in  FIG. 38A , a belt-like determination area P 1  having a predetermined width in the Y direction and extending in the X direction is set in an area where the noble metal tip  121  of the center electrode  120  is expected to exist. Next, in Step UD 3 , a front end position P 2  of the noble metal tip  121  of the center electrode  120  is determined based on a Y-direction density distribution obtained in each pixel position in the X direction (see  FIG. 38A ). 
   Next, in Step UD 4 , as shown in  FIG. 38B , a center line determination area P 3  for determining the center line (axis) of the noble metal tip  121  is set in a position shifted from the front end position P 2  in the Y direction (downward in  FIG. 38B ). Next, in Step UD 5 , in the center line determination area P 3 , for example, three search lines P 4 , P 6  and P 8  parallel to the X axis are set at predetermined intervals (e.g. 0.1 mm) in the Y direction (downward in  FIG. 38B ). X-direction pixel density distributions are obtained along the search lines P 4 , P 6  and P 8 , and center positions P 5 , P 7  and P 9  are determined based on the obtained pixel density distributions. A center line P 10  of the noble metal tip  121  is determined based on the center positions P 5 , P 7  and P 9  (see  FIG. 38B ). 
   Next, in Step UD 6 , as shown in  FIG. 39A , a belt-like determination area N 1  having a predetermined width in the Y direction and extending in the X direction is set in an area where the noble metal tip  113  of the ground electrode  1110  is expected to exist. In Step UD 7 , a front end position N 2  of the noble metal tip  113  of the ground electrode  1110  is determined based on a Y-direction density distribution obtained in each pixel position in the X direction. Next, in Step UD 8 , as shown in  FIG. 39B , a center line determination area N 3  for determining the center line (axis) of the noble metal tip  113  is set in a position shifted from the front end position N 2  in the −Y direction (upward in  FIG. 39B ). Next, in Step UD 9 , in the center line determination area N 3 , for example, three search lines N 4 , N 6  and N 8  parallel to the X axis are set at predetermined intervals (e.g. 0.1 mm) in the Y direction (downward in  FIG. 39B ). X-direction pixel density distributions are obtained along the search lines N 4 , N 6  and N 8 , and center positions N 5 , N 7  and N 9  are determined based on the obtained pixel density distributions. A center line N 10  of the noble metal tip  113  is determined based on the center positions N 5 , N 7  and N 9  (see  FIG. 39B ). 
   Next, in Step UDA, an angle between the noble metal tip  121  of the center electrode  120  and the noble metal tip  113  of the ground electrode  1110  is calculated based on the center line P 10  of the noble metal tip  121  and the center line N 10  of the noble metal tip  113 . Specifically, of angles formed between the center line N 10  and the center line P 10 , an acute one is calculated as a tip angle θ (see  FIG. 40 ). Then, the routine of processing returns to the main routine shown in  FIG. 32 . 
   In Step UE, it is determined whether the gap size gm is or not within a predetermined tolerance range (e.g. g 2 ±0.1(mm)) from the second gap size value g 2  (mm) which is an intended value of a final gap size. In the same manner, it is also determined whether the inter-tip distance ΔX 1  and the tip angle θ are within predetermined ranges respectively or not. When the gap size gm, the inter-tip distance ΔX 1  and the tip angle θ are within the predetermined ranges respectively, the routine of processing proceeds to Step SJ (work unloading step), where the work  1101  is unloaded from the carrier  211  of the linear conveyor  210 . Thus, the spark plug  1100  as shown in  FIG. 23  is completed. 
   On the contrary, when the gap size gm, the inter-tip distance ΔX 1  or the tip angle θ is out of its predetermined tolerance range, the routine of processing proceeds to Step SD (second center electrode position measuring step), where the coordinates (Xg, Yg) of the right end position G 4  of the noble metal tip  121  of the center electrode  120  are measured in the same manner as in Step S 3  (first center electrode position measuring step) (see  FIGS. 7A-7B ). 
   Next, in Steps UG and SF (second regulation member positioning step), the X-directional position of the regulation member  340  of the gap forming unit  300  is positioned. Specifically, first, a difference ΔXg (Xg in Step SD—Xg in Step S 3 ) between the X-coordinate of the position G 4  measured in Step S 3  (first center electrode position measuring step) and the X-coordinate of the position G 4  measured in Step SD (second center electrode position measuring step) is calculated. Next, the aforementioned difference ΔXg is added to the X-coordinate Xf of the regulation abutment surface  341  of the regulation member  340  set in Steps S 6 -S 7  (first regulation member positioning step), and further the inter-tip distance ΔX 1  calculated in Step UC is added thereto. Thus, an X-coordinate Xd (see  FIG. 41A ) is calculated (Xd=Xf+ΔXg+ΔX 1 ). The regulation member  340  is moved and fixed to the position where the X-coordinate of the regulation abutment surface  341  of the regulation member  340  is Xd. 
   In Embodiment 3, the regulation member  340  is positioned thus. Accordingly, in the next step SG (gap adjusting step), the X-directional position of the noble metal tip  113  of the ground electrode  1110  with respect to the front end  120   b  of the center electrode  120  (noble metal tip  121 ) can be adjusted to be located within a predetermined range while the gap size is adjusted. 
   Next, in the same manner as in Embodiment 1, Step SG (gap adjusting step) (see  FIG. 41B ) and Steps SH-SI (return step) (see  FIGS. 41C-41D ) are carried out in that order. Thus, a spark discharge gap g whose gap size is equal to the second gap size value g 2  is formed. In this event, it is also possible to adjust the X-directional position of the noble metal tip  113  of the ground electrode  1110  with respect to the front end  120   b  of the center electrode  120  (noble metal tip  121 ) as described above. 
   Next, Step UJ (third gap size measuring step), Step UK (second inter-tip distance measuring step) and Step UL (second tip angle measuring step) are carried out in that order. Thus, the gap size gm, the inter-tip distance ΔX 1  and the tip angle θ are calculated in the same manner as in the aforementioned Steps SB, UC and UD. 
   Next, in Step UM, it is determined whether the gap size gm, the inter-tip distance ΔX 1  and the tip angle θ are within their predetermined ranges or not. When they are within the predetermined ranges, the routine of processing proceeds to Step SJ, where the work  1101  is unloaded from the carrier  211  of the linear conveyor  210  as an acceptable product. On the contrary, when the gap size gm, the inter-tip distance ΔX 1  or the tip angle θ is out of its predetermined range, the routine of processing proceeds to Step UN where the work  1101  is unloaded from the carrier  211  of the linear conveyor  210  as an inferior product. 
   In such a manner, the spark plug  1100  as shown in  FIG. 23  is completed. 
   Embodiment 4 
   Next, Embodiment 4 of the invention will be described with reference to the drawings. 
   A method for manufacturing a spark plug  1100  according to Embodiment 4 is almost the same as that according to Embodiment 3, except that Step S 3  (first center electrode position measuring step) and Steps S 4  and UG (regulation member positioning step) in the manufacturing method according to Embodiment 3 are replaced by Step V 3  and Steps V 5  and VG respectively, and further Step V 4  (ground electrode tip position measuring step) is added (see  FIGS. 32 and 42 ). Therefore, description will focus on parts different from those in Embodiment 3, and description about similar parts will be omitted or simplified. 
   Description will be made on the method for manufacturing a spark plug  1100  according to Embodiment 4 will be described with reference to the drawings. 
     FIG. 42  is a flow chart (main routine) showing a flow of steps in the method for manufacturing the spark plug  1100 . First, in the same manner as in Embodiment 3, in Step S 1  (work setting step), the work  1101  is set. Next, in Step S 2  (first gap size measuring step), the gap size gm of the work  1101  is measured in the same manner as in Embodiment 3. 
   Next in Step V 3  (first center electrode position measuring step), the position of the front end  120   b  of the center electrode  120  of the work  1101  is measured. Specifically, the center position of the front end  120   b  of the center electrode  120  (noble metal tip  121 ) is measured by a first electrode position measuring unit  240  (see  FIG. 5 ) similar to that in Step S 3  in Embodiment 3. Here, the center electrode position measuring process will be described in detail with reference to  FIG. 43  showing the subroutine of Step V 3 . First, in Step V 31 , an image of the work  1101  is photographed by the photographing camera  241 , and next in Step V 32 , the photographed image is imported (see  FIG. 44 ). 
   Next, in Step V 33 , as shown in  FIG. 45A , a belt-like determination area R 5  having a predetermined width in the Y direction and extending in the X direction is set in an area A where the noble metal tip  121  of the center electrode  210  is expected to exist in the acquired image. Next, in Step V 34 , a front end position RI is determined based on a Y-direction density distribution obtained in each pixel position in the X direction (see  FIG. 45A ). Next, in Step V 35 , a search line R 2  is set in a position shifted from the front end position R 1  by a predetermined distance (e.g. 0.1 mm) in the Y direction (downward in  FIG. 45B ) (see  FIG. 45B ). Next, in Step V 36 , an X-direction pixel density distribution is obtained along the search line R 2 , and the coordinates of a left end position R 3  and a right end position R 4  are determined based on the obtained pixel density distribution. Next, in Step V 37 , the average value of the X-coordinates of the left and right end positions R 3  and R 4  is regarded as Xr, and the Y-coordinate of the front end position R 1  is regarded as Yr. Thus, the coordinates of a center position R 6  of the front end  120   b  of the center electrode  120  (noble metal tip  121 ) are determined as (Xr, Yr) (see  FIG. 45B ). After that, the routine of processing returns to the main routine of  FIG. 32 . 
   Next in Step V 4  (ground electrode tip position measuring step), the position of the noble metal tip  113  of the ground electrode  1110  is measured. Specifically, an X-directional distance between the front end surface  112   c  of the ground electrode  1110  and the center position of the noble metal tip  113  is measured by a first electrode position measuring unit  240  (see  FIG. 5 ) similar to that in Step V 3  (see  FIGS. 47A-47C ). Here, the ground electrode tip position measuring process will be described in detail with reference to  FIG. 46  showing the subroutine of Step V 4 . First, in Step V 41 , an image photographed in the pervious Step V 3  is imported (see  FIG. 44 ). 
   Next, in Step V 42 , an area B where the noble metal tip  113  of the ground electrode  1110  is expected to exist is created in the acquired image (see  FIG. 44 ). This area B is rotated and moved on the XY plane so that the central axis of the noble metal tip  113  is parallel to the Y axis (see  FIG. 47A ). A belt-like determination area Q 1  having a predetermined width in the X direction and extending in the Y direction is set in an area where the front end surface  112   c  of the ground electrode  1110  is expected to exist in the area B. Next, in Step V 43 , a front end position Q 2  is determined based on an X-direction density distribution obtained in each pixel position in the Y direction (see  FIG. 47A ). 
   Next, in Step V 44 , a belt-like determination area Q 3  having a predetermined width in the Y direction and extending in the X direction is set in an area where the front end  113 B of the noble metal tip  113  of the ground electrode  1110  is expected to exist in the area B (see  FIG. 47B ). Next, in Step V 45 , a front end position Q 4  is determined based on a Y-direction density distribution obtained in each pixel position in the X direction. Next, in Step V 46 , a search line Q 5  is set in a position shifted from the front end position Q 4  by a predetermined distance (e.g. 0.1 mm) in the −Y direction (upward in  FIG. 47C ) (see  FIG. 47C ). 
   Next, in Step V 47 , an X-direction pixel density distribution is obtained along the search line Q 5 , and the coordinates of a left end position Q 6  and a right end position Q 7  are determined based on the obtained pixel density distribution. Next, in Step V 48 , the average value of the X-coordinates of the left and right end positions Q 6  and Q 7  is regarded as Xq, and the Y-coordinate of the front end position Q 4  is regarded as Yq. Thus, the coordinates of a center position Q 8  of the noble metal tip  113  of the ground electrode  1110  are determined as (Xq, Yq) (see  FIG. 47C ). Next, in Step V 49 , an X-directional distance Q 9  between the front end position Q 2  of the ground electrode  1110  and the center position Q 8  of the noble metal tip  113  (a distance between the front end surface  112   c  of the ground electrode  1110  and the central axis of the noble metal tip  113 ) is calculated. After that, the routine of processing returns to the main routine of  FIG. 42 . 
   Next, in Steps V 5  and S 5  (regulation member positioning step), the X-directional position of the regulation member  340  of the gap forming unit  300  is positioned. Specifically, first, the X-direction distance Q 9  (distance between the front end surface  112   c  of the ground electrode  1110  and the central axis of the noble metal tip  113 ) measured in Step V 4  and an amount Sb with which the, ground electrode  1110  is expected to spring back in the −X direction after pressure are added to the X-coordinate Xr of the center position R 6  of the front end  120   b  of the center electrode  120  (noble metal tip  121 ) measured in Step V 3  (first center electrode position measuring step), so that an X-coordinate Xf (see  FIG. 48A ) is calculated (Xf=Xr+Q 9 +Sb). The regulation member  340  is moved and fixed to a position where the X-coordinate of the regulation abutment surface  341  of the regulation member  340  satisfies Xf=Xr+Q 9 +Sb. 
   In Embodiment 4, the regulation member  340  is positioned thus. Accordingly, in the next Step S 6  (gap forming step), the spark discharge gap g can be formed while the noble metal tip  113  of the ground electrode  1110  can be disposed in an X-directional position within a predetermined range with respect to the front end  120   b  of the center electrode  120  (noble metal tip  121 ). 
   After that, in the same manner as in Embodiment 3, in Step S 6  (gap forming step) (see  FIG. 48B ), the spark discharge gap g of the work  1101  is formed. After that, in Steps S 7 -S 8  (return step) (see  FIGS. 48C-48D ), the pressure member  330  and the regulation member  340  are returned to their positions before Steps S 6  and V 5  respectively. Thus, the spark discharge gap g whose gap size is equal to the first gap size value g 1  can be formed while the X-directional position of the noble metal tip  113  of the ground electrode  1110  with respect to the front end  120   b  of the center electrode  120  (noble metal tip  121 ) is regulated (see  FIG. 48D ). 
   Next, in the same manner as in Embodiment 3, in Step U 9  (deviation measuring step) and Step SA (deviation adjusting step) in turn, a deviation P of the noble metal tip  113  of the ground electrode  1110  with respect to the noble metal tip  121  of the center electrode  120  is measured, and the deviation P is adjusted to be within a predetermined range. 
   Next, in the same manner as in Embodiment 3, Step SB (second gap size measuring step), Step UC (first inter-tip distance measuring step) and Step UD (first tip angle measuring step) are carried out in that order. Thus, the gap size gm (see  FIG. 25 ), the inter-tip distance ΔX 1  (see  FIGS. 35A-35B  and  36 A- 36 B) and the tip angle θ (see  FIG. 40 ) are calculated. Next, in Step UE, it is determined whether the gap size gm, the inter-tip distance ΔX 1  and the tip angle θ are within their predetermined ranges or not. When they are within the predetermined ranges, the routine of processing proceeds to Step SJ, where the work  1101  is unloaded from the carrier  211  of the linear conveyor  210  as an acceptable product. 
   On the contrary, when the gap size gm, the inter-tip distance ΔX 1  or the tip angle θ is out of its predetermined range, the routine of processing proceeds to Step VF (second center electrode position measuring step), in which the coordinates (Xr, Yr) of the center position R 6  of the front of the center electrode  120  (noble metal tip  121 ) are measured in the same manner as in Step V 3  (see  FIG. 45B ). 
   Next, in Steps VG and SF (second regulation member positioning step), the X-directional position of the regulation member  340  of the gap forming unit  300  is positioned. Specifically, first, a difference ΔXr (Xr in Step VF—Xr in Step V 3 ) between the X-coordinate of the position R 6  measured in Step V 3  (first center electrode position measuring step) and the X-coordinate of the position R 6  measured in Step VF (second center electrode position measuring step) is calculated. Next, the aforementioned difference ΔXr is added to the X-coordinate Xf of the regulation abutment surface  341  of the regulation member  340  set in Steps S 6 -S 7  (first regulation member positioning step), and further the inter-tip distance ΔX 1  calculated in Step UC is added thereto. Thus, an X-coordinate Xd (see  FIG. 41A ) is calculated (Xd=Xf+ΔXr+ΔX 1 ). The regulation member  340  is moved and fixed to the position where the X-coordinate of the regulation abutment surface  341  of the regulation member  340  is Xd. 
   In Embodiment 4, the regulation member  340  is positioned thus. Accordingly, in the next step SG (gap adjusting step), the X-directional position of the noble metal tip  113  of the ground electrode  1110  with respect to the front end  120   b  of the center electrode  120  (noble metal tip  121 ) can be adjusted to be located within a predetermined range while the gap size is adjusted. 
   Next, in the same manner as in Embodiment 3, Step SG (gap adjusting step) (see  FIG. 41B ) and Steps SH-SI (return step) (see  FIGS. 41C-41D ) are carried out in that order. Thus, a spark discharge gap g whose gap size is equal to the second gap size value g 2  is formed. In this event, it is also possible to adjust the X-directional position of the noble metal tip  113  of the ground electrode  1110  with respect to the front end  120   b  of the center electrode  120  (noble metal tip  121 ) as described above. 
   Next, in the same manner as in Embodiment 3, Step UJ (third gap size measuring step), Step UK (second inter-tip distance measuring step) and Step UL (second tip angle measuring step) are carried out in that order. Thus, the gap size gm, the inter-tip distance ΔX 1  and the tip angle θ are calculated. 
   Next, in the same manner as in Embodiment 3, in Step UM, it is determined whether the gap size gm, the inter-tip distance ΔX 1  and the tip angle θ are within their predetermined ranges or not. When they are within the predetermined ranges, the routine of processing proceeds to Step SJ, where the work  1101  is unloaded from the carrier  211  of the linear conveyor  210  as an acceptable product. On the contrary, when the gap size gm, the inter-tip distance ΔX 1  or the tip angle θ is out of its predetermined range, the routine of processing proceeds to Step UN where the work  1101  is unloaded from the carrier  211  of the linear conveyor  210  as an inferior product. 
   In such a manner, the spark plug  1100  as shown in  FIG. 23  is completed. 
   Although the present invention has been described in detail above with respect to Embodiments 1-4, it should further be apparent to those skilled in the art that various changes in form and detail of the invention as shown and described above may be made. It is intended that such changes be included within the spirit and scope of the claims appended hereto. 
   For example, in Embodiment 1 or the like, in Step SG (gap adjusting step), by use of the gap forming unit  300 , the ground electrode  110 ,  1110  is pressed in the Y direction so as to adjust the gap size while the X-directional position of the front end surface  112   c  of the ground electrode  110 ,  1110  is regulated. However, the distance with which the ground electrode  110 ,  1110  is pressed in the Y direction in Step SG (gap adjusting step) is slight because the gap size is set at the first gap size value g 1  slightly (e.g. 0.05 mm) larger than the second gap size value g 2  in the previous Step S 6 , T 6  (gap forming step). Therefore, the change of the X-directional position of the front end surface  112   c  of the ground electrode  110 ,  1110  is extremely slight. Thus, there are some cases in which the X-directional position of the front end surface  112   c  of the ground electrode  110 ,  1110  does not have to be regulated in Step SG (gap adjusting step) where the ground electrode  110 ,  1110  is pressed in the Y direction. 
   In Embodiment 1 or the like, Step SA (deviation adjusting step) is carried out after Step S 9 , U 9  (deviation measuring step). However, Step SA (deviation adjusting step) may be designed not to be carried out when the deviation of the work  101 ,  1101  measured in Step S 9 , U 9  (deviation measuring step) has a value within a predetermined range. 
   In Embodiment 1 or the like, the X-coordinate of the regulation abutment surface  341  of the regulation member  340  calculated in each of the first, and second regulation member positioning steps is used. However, the value calculated in the first regulation member positioning step may be stored so that the same value can be used in the second regulation member positioning step. 
   In Embodiment 1 or the like, a total of two center electrode position measuring steps are carried out before the gap forming step and before the gap adjusting step. However, the coordinates (Xg, Yg) of the right end of the center electrode measured in the first center electrode position measuring step may be stored so that the X-directional position of the regulation member can be positioned in the second regulation member positioning step using the stored coordinates (Xg, Yg) of the right end of the center electrode. In such a manner, the second center electrode position measuring step can be omitted. 
   Further, in Embodiment 1, 2, a deviation measuring step, a center electrode position measuring step, etc. may be carried out after the gap adjusting step in order to confirm whether the deviation value or the covering distance of the work is within a predetermined range or not. The deviation adjusting step etc. may be carried out again when such a value is not within a predetermined range. 
   This application is based on Japanese Patent application JP 2003-24165, filed Jan. 31, 2003, and Japanese Patent application JP 2004-19696, filed Jan. 28, 2004, the entire contents of which are hereby incorporated by reference, the same as if set forth at length.