Patent Publication Number: US-7901262-B2

Title: Spark plug manufacturing method ensuring accurate and effective adjustment of spark gap

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority from Japanese Patent Application No. 2007-182930, filed on Jul. 12, 2007, the content of which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to methods of manufacturing spark plugs for use in internal combustion engines of, for example, motor vehicles and cogeneration systems. More particularly, the invention relates to a method of manufacturing a spark plug for an internal combustion engine, which ensures an accurate and effective adjustment of a spark gap in the spark plug. 
     2. Description of the Related Art 
     In manufacturing a spark plug for an internal combustion engine, it is necessary to adjust the spark gap between a pair of center and ground electrodes of the spark plug. Further, to adjust the spark gap, the ground electrode is generally pressed and bent toward the center electrode. Since the ground electrode generally springs back after being pressed, it is necessary to perform the press while taking into account the amount of springback of the ground electrode. However, the amount of springback of the ground electrode is unique to each spark plug. In other words, the amounts of springback of ground electrodes vary among individual spark plugs. Therefore, if the amount of springback was not suitably taken into account, it would be difficult to define a desired spark gap for each individual spark plug. 
     Japanese Patent First Publication No. 2000-164322 discloses a method of manufacturing a spark plug, according to which: the amount of springback of the ground electrode to be made by a regular press is first estimated based on the amount of springback of the ground electrode made by a test press; then the regular press is performed to bend the ground electrode by an amount that is determined based on the estimated amount of spring back of the ground electrode. However, with this method, since there is no fixed relationship between the amounts of springback of the ground electrode made by the test and regular presses, the actual amount of springback of the ground electrode made by the regular press does not always agree with the estimated amount. 
     Japanese Patent First Publication No. H11-121144 discloses a method of manufacturing a spark plug, according to which: preliminary strikes are first made against the ground electrode; the reduction in the spark gap by the preliminary strikes are measured; the number and/or force of finishing strikes required to bring the size of the spark gap into agreement with a target valve are determined based on the reduction in the spark gap by the preliminary strikes; then the finishing strikes are made in accordance with the determined number and/or force of finishing strikes. However, with this method, since there is no fixed relationship between the reductions in the spark gap by a preliminary and a finishing strike, the size of the spark gap cannot always be brought into agreement with the target value. 
     Japanese Patent First Publication No. H3-64882 discloses a device for forming a spark gap in a spark plug, which repeatedly presses the ground electrode while measuring the spark gap. With this device, however, the ground electrode is pressed, each press stroke by a fixed amount that is determined by taking into account the amount of springback of the ground electrode, until the size of the spark gap is decreased below a target value. Therefore, when the amount of springback of the ground electrode varies among individual spark plugs, the spark gap may be formed too small in some spark plugs. 
     Japanese Patent First Publication No. H8-153566, an English equivalent of which is U.S. Pat. No. 5,741,963, discloses an adjustor for adjusting a spark gap in a spark plug. The adjustor bends the ground electrode by means of a hammering device so as to minimize the amount of springback of the ground electrode. However, with this adjustor, the number of hammerings is determined based only on the size of the spark gap measured prior to the adjustment. Consequently, when the amount of springback of the ground electrode varies among individual spark plugs, the spark gap also varies among the individual spark plugs. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problems. 
     It is, therefore, a primary object of the present invention to provide a method of manufacturing a spark plug for an internal combustion engine, which ensures an accurate and effective adjustment of a spark gap in the spark plug. 
     According to the present invention, there is provided a first method of manufacturing a spark plug for an internal combustion engine. The first method includes the steps of: (a) preparing a tubular metal shell, an insulator, a center electrode, and a ground electrode; (b) assembling the metal shell, the insulator, and the center and ground electrodes together, so that the metal shell holds therein the insulator, the center electrode is secured in the insulator, and the ground electrode is fixed to the metal shell to form a spark gap between the center and ground electrodes; and (c) adjusting the spark gap to bring the size of the spark gap into agreement with a target value. Further, in the step (c), the ground electrode is repeatedly pressed, by a hammer, toward the center electrode from a state where the size of the spark gap is greater than a predetermined value that is greater than the target value. The hammer operates in a first mode when the size of the spark gap falls in a rough-process range which is above the predetermined value, and in a second mode when the size of the spark gap falls in a finish-process range which is between the target value and the predetermined value. The amount of pressing the ground electrode in any press stroke of the hammer in the second mode is less than the amount of pressing the ground electrode in any press stroke of the hammer in the first mode. Furthermore, the amount of pressing the ground electrode in every press stroke of the hammer in the second mode is equal to a fixed value. 
     With the above first method, when the size of the spark gap is in the rough-process range, it is possible to deform the ground electrode each press stroke by a large amount, thereby bringing the size of the spark gap into the finish-process range with a small number of reciprocations of the hammer. Moreover, after the size of the spark gap has reached the finish-process range, it is possible to allow the size of the spark gap to gradually approach the target value, thereby preventing the size of the spark gap from being decreased below the target value too much. Further, in the second mode, the hammer repeatedly presses the ground electrode each press stroke by the fixed amount. Therefore, even if the size of the spark gap was decreased below the target value by the last press stroke and the amount of springback of the ground electrode was zero, the final size of the spark gap would deviate from the target value by the fixed amount at the maximum. Accordingly, even when the amount of springback of the ground electrode varies among individual spark plugs, it is still possible to effectively minimize the variation in spark gap size among those spark plugs, thereby accurately and effectively adjusting the spark gap in each individual spark plug. 
     According to the present invention, there is also provided a second method of manufacturing a spark plug for an internal combustion engine. The second method includes the steps of: (a) preparing a tubular metal shell, an insulator, a center electrode, and a ground electrode; (b) assembling the metal shell, the insulator, and the center and ground electrodes together, so that the metal shell holds therein the insulator, the center electrode is secured in the insulator, and the ground electrode is fixed to the metal shell to form a spark gap between the center and ground electrodes; and (c) adjusting the spark gap to bring the size of the spark gap into agreement with a target value. Further, in the step (c), the ground electrode is repeatedly pressed, by a hammer, toward the center electrode from a state where the size of the spark gap is greater than a predetermined value that is greater than the target value. The hammer operates in a first mode when the size of the spark gap falls in a rough-process range which is above the predetermined value, and in a second mode when the size of the spark gap falls in a finish-process range which is between the target value and the predetermined value. The amount of pressing the ground electrode in any press stroke of the hammer in the second mode is less than the amount of pressing the ground electrode in any press stroke of the hammer in the first mode. Furthermore, the difference between the closest positions of the ground electrode to the center electrode in every two consecutive press strokes of the hammer in the second mode is equal to a fixed value. 
     With the above second method, when the size of the spark gap is in the rough-process range, it is possible to deform the ground electrode each press stroke by a large amount, thereby bringing the size of the spark gap into the finish-process range with a small number of reciprocations of the hammer. Moreover, after the size of the spark gap has reached the finish-process range, it is possible to allow the size of the spark gap to gradually approach the target value, thereby preventing the size of the spark gap from being decreased below the target value too much. Further, in the second mode, the hammer repeatedly presses the ground electrode such that the difference between the closest positions of the ground electrode to the center electrode in every two consecutive press strokes is equal to the fixed value. Consequently, even if the size of the spark gap was decreased below the target value by the last press stroke and the amount of springback of the ground electrode was zero, the final size of the spark gap would deviate from the target value only by a limited value. Accordingly, even when the amount of springback of the ground electrode varies among individual spark plugs, it is still possible to effectively minimize the variation in spark gap size among those spark plugs, thereby accurately and effectively adjusting the spark gap in each individual spark plug. 
     According to a further implementation of the invention, in both the first and second modes, the hammer is returned, after each press stroke, to a return position; all the return positions of the hammer are different from one another. 
     Preferably, the return position of the hammer after each press stroke is set to the position of a pressed surface of the ground electrode before the press stroke. 
     In each press stroke of the hammer in the first mode, the amount of pressing the ground electrode may be set in proportion to the difference between the size of the spark gap at the start of the press stroke and the target value. 
     Otherwise, the difference between the closest positions of the ground electrode to the center electrode in every two consecutive press strokes of the hammer in the first mode may be set in proportion to the difference between the size of the spark gap at the start of the latter one of the two consecutive press strokes and the target value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description given hereinafter and from the accompanying drawings of preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
       In the accompanying drawings: 
         FIG. 1  is a schematic view illustrating a process of adjusting a spark gap in a spark plug according to the first embodiment of invention; 
         FIG. 2  is a schematic view showing part of a spark gap adjustment system used for adjustment of the spark gap in the spark plug; 
         FIG. 3  is a schematic view showing the overall configuration of the spark gap adjustment system; 
         FIG. 4  is a flow chart illustrating the process of adjusting the spark gap in the spark plug according to the first embodiment; 
         FIG. 5  is a schematic view illustrating the initial height O of a hammer of the spark gap adjustment system in the process of  FIG. 4 ; 
         FIG. 6  is a schematic view illustrating the change in height of a ground electrode of the spark plug in the process of  FIG. 4 ; 
         FIG. 7  is a side view showing part of the spark plug around the spark gap; 
         FIG. 8  is a schematic view illustrating a process of adjusting a spark gap in a spark plug according to the second embodiment of invention; 
         FIG. 9  is a schematic view illustrating a process of adjusting a spark gap in a spark plug according to the third embodiment of invention; 
         FIG. 10  is a schematic view illustrating a conventional process of adjusting a spark gap in a spark plug; and 
         FIG. 11  is a schematic view illustrating the overall configuration of a conventional spark gap adjustment system used for implementation of the conventional process. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described hereinafter with reference to  FIGS. 1-11 . 
     It should be noted that, for the sake of clarity and understanding, identical components having identical functions in different embodiments of the invention have been marked, where possible, with the same reference numerals in each of the figures. 
     First Embodiment 
     Referring first to  FIGS. 1-7 , a method, according to the first embodiment of the invention, of manufacturing a spark plug  1  for an internal combustion engine will be described. 
     The spark plug  1  includes, as shown in  FIG. 7 , a center electrode  11 , an insulator  12  that retains therein the center electrode  11 , a tubular metal shell  13  that holds therein the insulator  12  along with the center electrode  11 , and a ground electrode  14  that is fixed to the metal shell  13  to define a spark gap  10  between itself and the center electrode  11 . 
     In adjusting the size of the spark gap  10 , as shown in  FIGS. 1 and 2 , the ground electrode  14  is repeatedly pressed, by a hammer  2 , in the axial direction of the spark plug  1  (i.e., the axial direction of the center electrode  11 ) toward the center electrode  11  from a state where the size of the spark gap  10  is greater than a predetermined value H. The predetermined value H is greater than a target value (or desired value) G 0  of the spark gap  10 . 
     For example, when the spark plug  1  has a thread diameter of M 14 , the target value G 0  may be in the range of 1.00 to 1.10 mm. Further, when the target value G 0  is equal to, for example, 1.05 mm, the predetermined value H may be in the range of 1.08 to 1.10 mm. 
     In the present embodiment, the hammer  2  operates in a first mode when the size of the spark gap  10  falls in a rough-process range A which is above the predetermined value H, and in a second mode when the size of the spark gap  10  falls in a finish-process range B which is between the target value G 0  and the predetermined value H. Further, the amount of pressing the ground electrode  14  in any press stroke of the hammer  2  in the second mode is less than the amount of pressing the ground electrode  14  in any press stroke of the hammer  2  in the first mode. Furthermore, the amount of pressing the ground electrode  14  in every press stroke of the hammer  2  in the second mode is equal to a fixed value Kb. 
     More specifically, to adjust the spark gap  10 , the spark plug  1  is first installed to a spark gap adjustment system  3 , as shown in  FIG. 3 . The spark gap adjustment system  3  includes a holder  31  for holding and carrying the spark plug  1 , a positioner  32  for positioning the spark plug  1 , the hammer  2  for pressing the ground electrode  14  toward the center electrode  11  of the spark plug  1 , a servomotor  33  for actuating the hammer  2 , a lighting device  34  for lighting the spark gap  10 , a camera  35  for capturing an image of the spark gap  10 , and a control device  36  for processing the image captured by the camera  35  and controlling the servomotor  33  based on the information derived from the processed image. In addition, the lighting device  34  may be implemented by an LED lighting device; the camera  35  may be implemented by a CCD camera. 
     The spark plug  1  is carried to a predetermined position right below the hammer  2  with the metal shell  13  being held by the holder  31 . Then, the positioner  2  grips an end portion  131  of the metal shell  13 , thereby positioning the spark plug  1  at the predetermined position. The lighting device  34 , which is located on one side of the spark gap  10 , emits light to the center and ground electrodes  11  and  14  in a radial direction of the spark plug  1  (i.e., in a direction perpendicular to the axial direction of the spark plug  1 ). The camera  35 , which is located on the other side of the spark gap  10  opposite to the lighting device  34 , captures an image of the center and ground electrodes  11  and  14  and sends an image signal indicative of the captured image to the control device  36 . 
     The controlling device  36  includes, as shown in  FIG. 2 , a CPU  360 , an image processor  361 , a motor controller  362 , and an Input/Output (I/O) interface  363 . The CPU  360  controls both the image processor  361  and the motor controller  362 . The image processor  361  processes the image signal sent from the camera  35  and determines values of parameters, such as the height of the ground electrode  14  and the size of the spark gap  10 , based on the processed image signal. Further, based on the values of parameters determined by the image processor  316 , the motor controller  362  controls the servomotor  33 , thereby controlling the pressing operation of the hammer  2 . The I/O interface  363  is provided to input/output data between the controlling device  36  and an external control device  30 . In addition, the CPU  360  may be implemented by, for example, a duel-core CPU, so that it can concurrently control the operations of the image processor  361  and the motor controller  362  at high speed. 
       FIG. 4  shows the entire process of adjusting the spark gap  10  in the spark plug  1  according to the present embodiment. 
     This process starts, in step S 1 , upon installation of the spark glug  1  to the spark gap adjustment system  3  and transmission of a start command signal from the external control device  30  to the controlling device  36  of the spark gap adjustment system  3 . 
     In step S 2 , the hammer  2  is moved, by the servomotor  33 , to an initial height O as shown in  FIG. 5 . 
     In step S 3 , the height Qn of an outer side surface  141  of the ground electrode  14  is determined by the controlling device  36 . As shown in  FIG. 7 , the outer side surface  141  faces the opposite side of the ground electrode  14  to the spark gap  10 , and is to be pressed by the hammer  2 . Hereinafter, the outer side surface  141  is to be simply referred to as pressed surface  141 . 
     In step S 4 , the current size Gn of the spark gap  10  is determined by the controlling device  36 . 
     In step S 5 , a preliminary process is performed on the ground electrode  14 . Specifically, the hammer  2  is moved down from the initial height O toward the center electrode  11  by a given amount, thereby pressing the ground electrode  14  toward the center electrode  11 . The given amount is so given as not to bring the size of the spark gap  10  into the finish-process range B through the preliminary process. 
     In step S 6 , the hammer  2  is returned to the height Qn of the pressed surface  141  of the ground electrode  14  before the preliminary process. That is, the hammer  2  is returned only to the height Qn of the pressed surface  141  determined in step S 7 , but not completely to the initial height O thereof. 
     In step S 7 , the height Qn of the pressed surface  141  of the ground electrode  14  is determined by the controlling device  36 . 
     In step S 8 , the current size Gn of the spark gap  10  is determined by the controlling device  36 . 
     In step S 9 , a determination is made, by the controlling device  36 , as to whether the current size Gn of the spark gap  10  falls in the finish-process range B. 
     If the determination in step S 9  produces a “NO” answer, in other words, if the current size Gn of the spark gap  10  falls in the rough-process range A, then the process proceeds to step S 10 . 
     In step S 10 , the ground electrode  14  is pressed, at the pressed surface  141  by the hammer  2 , by an amount Ka toward the center electrode  11 . The amount Ka is determined in proportion to the difference between the current size Gn and the target value Go of the spark gap  10 . That is, Ka=α (Gn−G 0 ), where α is a preset coefficient. Additionally, α may be preset to, for example, 0.8. 
     In step S 11 , the hammer  2  is returned to the height Qn of the pressed surface  141  of the ground electrode  14  before the last press stroke. Then, the process returns to the step S 7 . 
     On the other hand, if the determination in step S 9  produces a “YES” answer, in other words, if the current size Gn of the spark gap  10  falls in the finish-process range B, then the process proceeds to step S 12 . 
     In step S 12 , a further determination is made, by the controlling device  36 , as to whether the current size Gn of the spark gap  10  has reached the target value G 0 . 
     If the determination in step S 12  produces a “YES” answer, then the process directly proceeds to step S 18  without performing steps S 13  to S 17 . 
     On the contrary, if the determination in step S 12  produces a “NO” answer, in other words, if the current size Gn of the spark gap  10  is still greater than the target value G 0 , then the process goes on to step S 13 . 
     In step S 13 , the ground electrode  14  is pressed, at the pressed surface  141  by the hammer  2 , by the fixed amount Kb toward the center electrode  11 . The fixed amount Kb, which is less than any of the amounts Ka determined in S 10 , may be in the range of 5 to 20 μm. 
     In step S 14 , the hammer  2  is returned to the height Qn of the pressed surface  141  of the ground electrode  14  before the last press stroke. 
     In step S 15 , the height Qn of the pressed surface  141  of the ground electrode  14  is determined by the controlling device  36 . 
     In step S 16 , the current size Gn of the spark gap  10  is determined by the controlling device  36 . 
     In step S 17 , a determination is made, by the controlling device  36 , as to whether the current size Gn of the spark gap  10  has reached the target value G 0 . 
     If the determination in step S 17  produces a “NO” answer, then the process returns to step S 13 . On the contrary, if the determination in step S 17  produces a “YES” answer, then the process proceeds to step S 18 . 
     In step S 18 , the hammer  2  is returned to the initial height O thereof. Then, the entire process is terminated in step S 19 . 
       FIG. 6  shows the change in height of the ground electrode  14  during the above spark gap adjustment process. 
     In  FIG. 6 , P 0 -P 6  and p 1 -p 6  denote the heights of that point in the ground electrode  14  which is closest to the center electrode  11  in the axial direction of the spark plug  1 . Moreover, in the following explanation, Qn (n=0-6) denote the heights of the pressed surface  141  of the ground electrode  14  which respectively correspond to Pn (n=0-6). 
     First, the ground electrode  14  is pressed, by the preliminary process (step S 5  in  FIG. 4 ), from the height P 0  to the height p 1 . Then, the hammer  2  is returned to the height Q 0  (step S 6  in  FIG. 4 ), releasing the pressing force against the ground electrode  14 . Consequently, the ground electrode  14  springs back to the height P 1 . 
     Next, in the rough-process range A of the spark gap  10 , the ground electrode  14  is pressed, by an amount Ka (step S 10  in  FIG. 4 ), from the height P 1  to the height p 2 . Then, the hammer  2  is returned to the height Q 1  (step S 11  in  FIG. 4 ), so that the ground electrode  14  springs back to the height P 2 . By repeatedly pressing the ground electrode  14  in this manner (steps S 7 -S 11  in  FIG. 4 ), the height of that point in the ground electrode  14  which is closest to the center electrode  11  is changed as p 3 -P 3 -p 4 -P 4 . 
     From the height P 4 , the size of the spark gap  10  comes to fall in the finish-process range B. Thus, the ground electrode  14  is pressed, by the fixed amount Kb (step S 13  in  FIG. 4 ), from the height P 4  to the height p 5 . Then, the hammer  2  is returned to the height Q 4  (step S 14  in  FIG. 4 ), so that the ground electrode  14  springs back to the height P 5 . 
     Since the size of the spark gap  10  has not yet reached the target value G 0  (step S 17  in  FIG. 4 ), the ground electrode  14  is pressed again by the fixed amount Kb, from the height P 5  to the height p 6 . Then, the hammer  2  is returned to the height Q 5 , so that the ground electrode  14  springs back to the height P 6 . 
     At the height P 6 , the size of the spark gap  10  is no longer greater than the target value G 0 . In other words, the size of the spark gap  10  has reached the target value G 0  (step S 17  in  FIG. 4 ). Therefore, the hammer  2  is returned to the initial position O thereof (step S 18  in  FIG. 4 ), terminating the entire process of adjusting the spark gap  10 . 
     The method of manufacturing the spark plug  1  according to the present embodiment thus includes the steps of: 1) preparing the center electrode  11 , the insulator  12 , the metal shell  13 , and the ground electrode  14 ; 2) assembling those components  11 - 14  together, so that the center electrode  11  is retained in the insulator  12 , the insulator  12  is held in the metal shell  13  along with the center electrode  11 , and the ground electrode  14  is fixed to the metal shell  13  to form the spark gap  10  between the center and ground electrodes  11  and  14 ; 3) adjusting the size of the spark gap  10  according to the above-described process. 
     The method of manufacturing the spark plug  1  according to the present embodiment has the following advantages. 
     In the present embodiment, to adjust the size of the spark gap  10 , the hammer  2  selectively operates in one of the first and second modes according to the size of the spark gap  10 . More specifically, when the size of the spark gap  10  falls in the rough-process range A, the hammer  2  operates in the first mode in which it repeatedly presses the ground electrode  14  each press stroke by an amount Ka greater than the fixed amount Kb. Further, when the size of the spark gap  10  is decreased to fall in the finish-process range B, the hammer  2  comes to operate in the second mode in which it repeatedly presses the ground electrode  14  each press stroke by the fixed amount Kb. 
     Accordingly, when the size of the spark gap  10  is in the rough-process range A, it is possible to deform the ground electrode  14  each press stroke by a large amount, thereby bringing the size of the spark gap  10  into the finish-process range B with a small number of reciprocations of the hammer  2 . Moreover, after the size of the spark gap  10  has reached the finish-process range B, it is possible to allow the size of the spark gap  10  to gradually approach the target value G 0 , thereby preventing the size of the spark gap  10  from being decreased below the target value G 0  too much. 
     Further, in the second mode, the hammer  2  repeatedly presses the ground electrode  14  each press stroke by the fixed amount Kb. Therefore, even if the size of the spark gap  10  was decreased below the target value G 0  by the last press stroke and the amount of springback of the ground electrode  14  was zero, the final size of the spark gap  10  would deviate from the target value G 0  by Kb at the maximum. 
     Accordingly, even when the amount of springback of the ground electrode  14  varies among individual spark plugs  1 , it is still possible to effectively minimize the variation in spark gap size among those spark plugs  1 , thereby accurately and effectively adjusting the spark gap  10  in each individual spark plug  1 . 
     In the present embodiment, after each press stroke, the hammer  2  is returned only to the height of the pressed surface  141  of the ground electrode  14  before the press stroke. 
     Consequently, it is possible to minimize the return stroke of the hammer  2  without restricting the springback of the ground electrode  14  after the press stroke. As a result, it is possible to improve the productivity without decreasing the accuracy in adjustment of the spark gap  10 . 
     In the present embodiment, in the first mode, the hammer  2  repeatedly presses the ground electrode  14  each press stroke by an amount Ka that is determined in proportion to the difference between the size of the spark gap  10  at the start of the press stroke and the target value G 0 . 
     Consequently, it is possible to minimize the number of reciprocations of the hammer  2  necessary for bringing the size of the spark gap  10  into the finish-process range B, while reliably preventing the size of the spark gap  10  from being decreased below the target value G 0  too much. 
     Second Embodiment 
     This embodiment illustrates a method of manufacturing the spark plug  1 , which is almost the same as the method according to the first embodiment. Accordingly, only the difference between the two methods will be described hereinafter. 
     In the first embodiment, as described previously, the hammer  2  repeatedly presses, in the second mode, the ground electrode  14  each press stroke by the fixed amount Kb. 
     In comparison, in the present embodiment, the hammer  2  repeatedly presses, in the second mode, the ground electrode  14  in such a manner that the difference between the minimum heights of the ground electrode  14  in every two consecutive press strokes is equal to a fixed value. 
     More specifically, referring to  FIG. 8 , in the last press stroke in the first mode, the hammer  2  presses the ground electrode  14  from the maximum height P 3  to the minimum height p 4 . 
     After entering the second mode, in the first press stroke, the hammer  2  presses the ground electrode  14  from the maximum height P 4  to the minimum height p 5 . The difference between the minimum heights p 4  and p 5  of the ground electrode  14  is equal to a fixed value δ pb. In addition, the fixed value δ pb may be in the range of 0.005 to 0.020 mm. 
     Further, in the second press stroke in the second mode, the hammer  2  presses the ground electrode  14  from the maximum height P 5  to the minimum height p 6 . The difference between the minimum heights p 5  and p 6  of the ground electrode  14  is also equal to the fixed value δ pb. 
     After the second press stroke in the second mode, the ground electrode  14  springs backs from the height p 6  to the height P 6 , making the size of the spark gap  10  reach the target value G 0 . 
     As above, in the present embodiment, when the size of the spark gap  10  is decreased to fall in the finish-process range B, the hammer  2  comes to operate in the second mode in which it repeatedly presses the ground electrode  14  such that the difference between the minimum heights of the ground electrode  14  in every two consecutive press strokes is equal to the fixed value δ pb. 
     Consequently, even if the size of the spark gap  10  was decreased below the target value G 0  by the last press stroke and the amount of springback of the ground electrode  14  was zero, the final size of the spark gap  10  would deviate from the target value G 0  only by a limited value. 
     Accordingly, even when the amount of springback of the ground electrode  14  varies among individual spark plugs  1 , it is still possible to minimize the variation in spark gap size among those spark plugs  1 , thereby accurately adjusting the spark gap  10  in each individual spark plug  1 . 
     Third Embodiment 
     This embodiment illustrates a method of manufacturing the spark plug  1 , which is almost the same as the method according to the first embodiment. Accordingly, only the difference between the two methods will be described hereinafter. 
     In the first embodiment, as described previously, the hammer  2  repeatedly presses, in the first mode, the ground electrode  14  each press stroke by the amount Ka that is determined in proportion to the difference between the size of the spark gap  10  at the start of the press stroke and the target value G 0 . 
     In comparison, in the present embodiment, the hammer  2  repeatedly presses, in the first mode, the ground electrode  14  in such a manner that the difference between the minimum heights of the ground electrode  14  in every two consecutive press strokes is proportional to the difference between the size of the spark gap  10  at the start of the latter one of the two consecutive press strokes and the target value G 0 . 
     More specifically, referring to  FIG. 9 , in the press stroke of the preliminary process, the hammer  2  presses the ground electrode  14  from the maximum height P 0  to the minimum height p 1 . 
     After entering the first mode, in the first press stroke, the hammer  2  presses the ground electrode  14  from the maximum height P 1  to the minimum height p 2 . The difference δ pa1 between the minimum heights p 1  and p 2  of the ground electrode  14  is equal to β (G 1 −G 0 ), where β is a preset coefficient, G 1  is the size of the spark gap  10  at the start of the first press stroke, and G 0  is the target value. 
     In addition, the coefficient β may be in the range of 0.1 to 1.0. Moreover, in  FIG. 9 , the chain line L-L represents the height of the tip of the center electrode  11 . 
     Further, in the second press stroke in the first mode, the hammer  2  presses the ground electrode  14  from the maximum height P 2  to the minimum height p 3 . The difference δ pa 2  between the minimum heights p 2  and p 3  of the ground electrode  14  is equal to β (G 2 −G 0 ), where G 2  is the size of the spark gap  10  at the start of the second press stroke. 
     Furthermore, in the third press stroke in the first mode, the hammer  2  presses the ground electrode  14  from the maximum height P 3  to the minimum height p 4 . The difference δ pa3 between the minimum heights p 3  and p 4  of the ground electrode  14  is equal to β (G 3 −G 0 ), where G 3  is the size of the spark gap  10  at the start of the third press stroke. 
     After the third press stroke in the first mode, the ground electrode  14  springs back from the height p 4  to the height P 4 , bringing the size of the spark plug  10  into the finish-process range B. 
     As above, in the present embodiment, when the size of the spark gap  10  falls in the rough-process range A, the hammer  2  operates in the first mode in which it repeatedly presses the ground electrode  14  such that the difference between the minimum heights of the ground electrode  14  in every two consecutive press strokes is proportional to the difference between the size of the spark gap  10  at the start of the latter one of the two consecutive press strokes and the target value G 0 . 
     Consequently, as in the first embodiment, it is possible to minimize the number of reciprocations of the hammer  2  necessary for bringing the size of the spark gap  10  into the finish-process range B, while reliably preventing the size of the spark gap  10  from being decreased below the target value G 0  too much. 
     COMPARATIVE EXAMPLE 
       FIG. 10  illustrates a conventional process of adjusting the spark gap  10 . According to this process, the hammer  2  presses the ground electrode  14  toward the center electrode  11  by an amount that is determined based on a prediction of the amount of springback of the ground electrode  14 . 
     More specifically, let Kc represent the amount by which the hammer  2  presses the ground electrode  14  toward the center electrode  11 , x represent the predicted amount of springback of the ground electrode  14 , and G 1  represent the initial size of the spark gap  10 . Then, the amount Kc is determined by the following equation:
 
 Kc=G 11 −G 0 +x.  
 
       FIG. 11  shows the overall configuration of a conventional spark gap adjustment system used for implementation of the conventional process. As shown, the conventional system includes the hammer  2  for pressing the ground electrode  14  toward the center electrode  11 , the servomotor  33  for actuating the hammer  2 , a motor controller  372  for controlling the servomotor  33 , the camera  35  for capturing an image of the spark gap  10 , and an image processor  371  for processing the image captured by the camera  35 . 
     The conventional process starts upon transmission of a start command signal from the external control device  30  to the motor controller  372 . Then, the motor controller  372  sends a command signal to the image processor  371 . Upon receipt of the command signal, the image processor  371  processes the image captured by the camera  35 , determines the initial size G 11  of the spark gap  10  based on the processed image, determines the amount Kc based on G 11 , x, and G 0  as described above, and sends the motor controller  372  a signal representative of the determined amount Kc. Based on the signal, the motor controller  372  controls the servomotor  33  to actuate the hammer  2 , thereby enabling the hammer  2  to press the ground electrode  14  by the amount Kc. 
     With the above conventional process, when the actual amount of springback of the ground electrode  14  agrees with the predicted amount x, the final size of the spark gap  10  also agrees with the target value G 0 . However, when the actual amount of springback of the ground electrode  14  is less than the predicted amount X, the final size of the spark gap  10  is also less than the target value G 0 . 
     Accordingly, when the amount of springback of the ground electrode  14  varies among the individual spark plugs  1 , the spark gap  10  may be formed too small in some spark plugs  1 . Therefore, with the conventional process, it is difficult accurately adjust the spark gap  10  in each individual spark plug  10 . 
     In comparison, with the spark plug manufacturing methods according to the previous embodiments of the invention, even when the amount of springback of the ground electrode  14  varies among the individual spark plugs  1 , it is still possible to accurately and effectively adjust the spark gap  10  in each individual spark plug  10 . 
     In addition, in the conventional spark gap adjustment system, the image processor  371  and the motor controller  372  are separately provided. Accordingly, a relatively long time is necessary for communication between the image processor  371  and the motor controller  372 , lowering the productively. In comparison, in the spark gap adjustment system  3  according to the previous embodiments, both the image processor  361  and the motor controller  362  are integrated into the single control device  36  and controlled by the common high-speed CPU  360 . Accordingly, the time necessary for communication between the image processor  361  and the motor controller  362  is shortened, improving the productivity. 
     While the above particular embodiments of the invention and comparative example have been shown and described, it will be understood by those skilled in the art that various modifications, changes, and improvements may be made without departing from the spirit of the invention. 
     For example, in the previous embodiments, the hammer  2  enters the first mode only after performing the preliminary process. However, it is also possible for the hammer  2  to directly enter the first mode without performing the preliminary process.