EP2337172A1

EP2337172A1 - Method for manufacturing spark plug and device for manufacturing spark plug

Info

Publication number: EP2337172A1
Application number: EP09818919A
Authority: EP
Inventors: Keisuke Kure; Masahiro Enuma
Original assignee: NGK Spark Plug Co Ltd
Current assignee: Niterra Co Ltd
Priority date: 2008-10-06
Filing date: 2009-09-24
Publication date: 2011-06-22
Anticipated expiration: 2029-09-24
Also published as: CN102160247B; JP2010092669A; WO2010041379A1; CN102160247A; EP2337172A4; JP5134486B2; EP2337172B1

Abstract

Even after a metallic shell and an insulator are assembled together, a ground electrode can be bent at a position closer to the proximal end thereof, and a spark discharge gap can be formed accurately. A bending apparatus 51 used in a process of manufacturing a spark plug includes a pre-bending apparatus 61, a main bending apparatus 71, and a gap adjustment apparatus 81. The pre-bending apparatus 61 includes first deformation prevention means which has a curved surface portion corresponding to a bent shape of the ground electrode and prevents deformation of the ground electrode, and a roller which presses the ground electrode against the curved surface portion to thereby perform pre-bending work on the ground electrode. The main bending apparatus 71 includes second deformation prevention means for preventing deformation of the ground electrode, and pressing means for performing main bending work on the ground electrode. The gap adjustment apparatus 81 includes gap adjustment means for relatively moving a distal end portion of the ground electrode in relation to the center electrode along a direction perpendicular to an axis.

Description

TECHNICAL FIELD

The present invention relates to a technique for manufacturing a spark plug used for an internal combustion engine, and more particularly to a method and apparatus for manufacturing a spark plug characterized by bending work performed on the ground electrode of the spark plug.

BACKGROUND ART

A spark plug used for an internal combustion engine such as an automotive engine includes, for example, a center electrode extending in an axial direction, an insulator which surrounds the center electrode, a tubular metallic shell fitted onto the insulator, and a ground electrode having a proximal end portion joined to a front end portion of the metallic shell. The ground electrode is bent and disposed such that a distal end portion of the ground electrode faces a front end portion of the center electrode, whereby a spark discharge gap is formed between the front end portion of the center electrode and the distal end portion of the ground electrode. In general, the spark discharge gap is formed as follows. A metallic shell, to which a ground electrode having a straight-bar-like shape has been joined, and an insulator having a center electrode provided therein are assembled together. Subsequently, the straight-bar-like ground electrode is bent toward the center electrode.
In a known method for bending the ground electrode toward the center electrode, a bending spacer is disposed to face a front end surface of the center electrode. In this state, by use of a bending punch having an inclined surface for butting against the ground electrode, the ground electrode is pressed against the bending spacer, to thereby perform pre-bending. Subsequently, the bending spacer is removed, and the ground electrode is bent, by use of a bending punch having a flat butting surface, to a position where a spark discharge gap of a predetermined size is formed (see, for example, Patent Document 1).

PRIOR ART DOCUMENT

PATENT DOCUMENT

Patent Document 1: Japanese Patent No. 3389121

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

Incidentally, in recent years, in order to improve durability, etc., there have been proposed a so-called lateral-spark-type spark plug in which a distal end portion of a ground electrode is disposed to face a side surface of a center electrode so that spark discharge occurs along a direction approximately perpendicular to the axis, and a so-called inclined-discharge-type spark plug in which a distal end portion of a ground electrode is disposed to face a front end edge portion of a center electrode so that spark discharge occurs obliquely in relation to the axial direction.
In the lateral-discharge-type or inclined-discharge-type spark plug, the ground electrode must be bent at a position closer to the proximal end side (the side toward the metallic shell). However, when the above-described technique is used, a limit arises on moving the bending spacer closer to the proximal end side due to the presence of the center electrode, and therefore, the technique encounters difficulty in bending of the ground electrode at a position closer to the proximal end side. In order to avoid such difficulty, there can be employed a method in which the ground electrode is bent in advance before assembly of the metallic shell and the insulator, and then the metallic shell and the insulator are assembled together. However, in this case, since the ground electrode is not bent upon grasp of the actual position of a front end portion of the center electrode, the size of the spark discharge gap may vary with a variation in the position of the front end portion of the center electrode, which variation may arise depending on the state of assembly of the metallic shell and the insulator.
Notably, the above-described problem occurs not only in lateral-discharge-type and inclined-discharge-type spark plugs, but also in longitudinal-discharge-type spark plugs in which spark discharge occurs along the axial direction.
The present invention has been accomplished in view of the foregoing, and its object is to provide a method and apparatus for manufacturing a spark plug which can bend a ground electrode at a position closer to the proximal end thereof even after a metallic shell and an insulator are assembled together.

MEANS FOR SOLVING THE PROBLEMS

Hereinbelow, configurations suitable for achieving the above-described objects will be described in an itemized fashion. Notably, when necessary, action and effects peculiar to each configuration will be added.
Configuration 1. A spark plug manufacturing method according to the present configuration is a method for manufacturing a spark plug including:

a rod-like center electrode extending in the direction of an axis;
an approximately cylindrical tubular insulator having an axial hole extending in the direction of the axis, the center electrode being provided at a front end portion of the axial hole;
an approximately cylindrical tubular metallic shell surrounding the insulator; and
a ground electrode whose proximal end portion is joined to the metallic shell, and which is bent at an intermediate portion thereof so as to form a spark discharge gap between a distal end portion of the ground electrode and a front end portion of the center electrode,
the method comprising a bending process of bending the ground electrode toward the axis in a state in which the metallic shell with the ground electrode joined thereto and the insulator in which the center electrode is provided are assembled together, wherein
the bending process includes a bending step performed such that
curved surface forming means having a curved surface portion corresponding to a bent shape of the ground electrode is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode,
the ground electrode is supported by deformation prevention means so as to prevent at least one of deformation of the proximal end portion of the ground electrode toward the center electrode and deformation of the proximal end portion toward a side opposite the center electrode,
the ground electrode is pressed against the curved surface portion from the side opposite the center electrode by use of bending means, whereby bending work is performed on the ground electrode.

Notably, a noble metal tip formed of a noble metal alloy may be joined to the front end portion of the center electrode and the distal end portion of the ground electrode. In this case, the noble metal tips partially constitute the center electrode and the ground electrode, respectively.
Configuration 2. A spark plug manufacturing method according to the present configuration is characterized in that, in the above-mentioned configuration 1, the bending means is a roller which is configured to be movable in relation to the ground electrode along a direction approximately perpendicular to the axis and which is supported for free rotation, and an outer circumferential surface of the roller is pressed against a side surface of the ground electrode opposite the center electrode so as to perform the bending work on the ground electrode.
Configuration 3. A spark plug manufacturing method according to the present configuration is characterized in that, in the above-mentioned configuration 1 or 2, the deformation prevention means includes inward deformation prevention means which is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode so as to support a side portion of the proximal end portion of the ground electrode located on the side toward the center electrode, to thereby prevent deformation of the ground electrode toward the center electrode.
Configuration 4. A spark plug manufacturing method according to the present configuration is characterized in that, in the above-mentioned configuration 3, the deformation prevention means is configured such that the curved surface forming means and the inward deformation prevention means are united together.
Configuration 5. A spark plug manufacturing method according to the present configuration is characterized in that, in the above-mentioned configuration 3 or 4, the deformation prevention means further includes outward deformation prevention means which is disposed on a side of the ground electrode opposite the center electrode so as to support a side portion of the proximal end portion of the ground electrode opposite the center electrode, to thereby prevent deformation of the ground electrode toward a side opposite the center electrode.
Configuration 6. A spark plug manufacturing method according to the present configuration is characterized in that, in the above-mentioned configuration 5, the inward deformation prevention means and the outward deformation prevention means are disposed to face each other via the proximal end portion of the ground electrode.
Configuration 7. A spark plug manufacturing method according to the present configuration is characterized in that, in the above-mentioned configuration 5 or 6, the spacing between the inward deformation prevention means and the outward deformation prevention means can be changed.
Configuration 8. A spark plug manufacturing method according to the present configuration is characterized in that, in any of the above-mentioned configurations 1 to 7, the curved surface forming means is disposed such that the center of curvature of the curved surface portion is located on the proximal end side of a plane including a front end surface of the center electrode with respect to the direction of the axis.
Configuration 9. A spark plug manufacturing method according to the present configuration is characterized in that, in any of the above-mentioned configurations 1 to 8, the bending process includes a gap adjustment step performed after the bending step so as to adjust a size of the spark discharge gap along a direction perpendicular to the axis by use of gap adjustment means capable of moving the distal end portion of the ground electrode in relation to the center electrode along a direction approximately perpendicular to the axis.
Configuration 10. A spark plug manufacturing method according to the present configuration is characterized in that, in the above-mentioned configuration 9, the gap adjustment means includes gap enlargement means which is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode so as to relatively move the distal end portion of the ground electrode in a direction away from the center electrode, to thereby expand a size of the spark discharge gap along a direction perpendicular to the axis,
wherein the gap enlargement means is disposed on a front end side of a plane including a front end surface of the insulator with respect to the direction of the axis.
Configuration 11. A spark plug manufacturing method according to the present configuration is characterized in that, in the above-mentioned configuration 9 or 10, in the gap adjustment step, restriction means is used to restrict at least relative movement of the distal end portion of the ground electrode in relation to the center electrode toward the front end side with respect to the direction of the axis.
Configuration 12. A spark plug manufacturing method according to the present configuration is characterized in that, in any of the above-mentioned configurations 1 to 11, the spark plug is configured such that a shortest distance between the center electrode and the ground electrode is formed between a front end edge portion or side surface portion of the center electrode and the distal end portion of the ground electrode.
Configuration 13. A spark plug manufacturing apparatus according to the present configuration is a manufacturing apparatus used for manufacture of a spark plug including:

a rod-like center electrode extending in the direction of an axis;
an approximately cylindrical tubular insulator having an axial hole extending in the direction of the axis, the center electrode being provided at a front end portion of the axial hole;
an approximately cylindrical tubular metallic shell surrounding the insulator; and
a ground electrode whose proximal end portion is joined to the metallic shell, and which is bent at an intermediate portion thereof so as to form a spark discharge gap between a distal end portion of the ground electrode and a front end portion of the center electrode,
the apparatus comprising a bending apparatus for bending a straight-bar-shaped ground electrode toward the axis in a state in which the metallic shell with the straight-bar-shaped ground electrode joined thereto and the insulator in which the center electrode is provided are assembled together, wherein
the bending apparatus comprises:
curved surface forming means which has a curved surface portion corresponding to a bent shape of the ground electrode and is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode;
first deformation prevention means for preventing at least one of deformation of the proximal end portion of the ground electrode toward the center electrode and deformation of the proximal end portion toward a side opposite the center electrode; and
bending means for pressing the ground electrode against the curved surface portion of the curved surface forming means from a side opposite the center electrode.

Configuration 14. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in the above-mentioned configuration 13, the bending means is a roller which is supported for free rotation and which can move toward and away from the ground electrode along a direction approximately perpendicular to the axis.
Configuration 15. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in the above-mentioned configuration 13 or 14, the deformation prevention means includes inward deformation prevention means which is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode so as to support a side portion of the proximal end portion of the ground electrode located on the side toward the center electrode, to thereby prevent deformation of the ground electrode toward the center electrode.
Configuration 16. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in the above-mentioned configuration 15, the deformation prevention means is configured such that the curved surface forming means and the inward deformation prevention means are united together.
Configuration 17. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in the above-mentioned configuration 15 or 16, the deformation prevention means further includes outward deformation prevention means which is disposed on a side of the ground electrode opposite the center electrode so as to support a side portion of the proximal end portion of the ground electrode opposite the center electrode, to thereby prevent deformation of the ground electrode toward a side opposite the center electrode.
Configuration 18. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in the above-mentioned configuration 17, the inward deformation prevention means and the outward deformation prevention means are disposed to face each other via the proximal end portion of the ground electrode.
Configuration 19. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in the above-mentioned configuration 17 or 18, the spacing between the inward deformation prevention means and the outward deformation prevention means can be changed.
Configuration 20. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in any of the above-mentioned configurations 13 to 19, the curved surface forming means is disposed such that the center of curvature of the curved surface portion is located on the proximal end side of a plane including a front end surface of the center electrode with respect to the direction of the axis.
Configuration 21. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in any of the above-mentioned configurations 13 to 20, the bending apparatus further comprises gap adjustment means capable of moving the distal end portion of the ground electrode in relation to the center electrode along a direction approximately perpendicular to the axis, wherein the gap adjustment means adjusts a size of the spark discharge gap along a direction perpendicular to the axis.
Configuration 22. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in the above-mentioned configuration 21, the gap adjustment means includes gap enlargement means which is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode so as to relatively move the distal end portion of the ground electrode in a direction away from the center electrode, to thereby expand a size of the spark discharge gap along a direction perpendicular to the axis,
wherein the gap enlargement means is disposed on a front end side of a plane including a front end surface of the insulator with respect to the direction of the axis.
Configuration 23. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in the above-mentioned configuration 21 or 22, the gap adjustment means comprises restriction means for restricting at least relative movement of the distal end portion of the ground electrode in relation to the center electrode toward the front end side with respect to the direction of the axis.
Configuration 24. A spark plug manufacturing apparatus according to the present configuration is characterized in that, in any of the above-mentioned configurations 13 to 23, the spark plug is configured such that a shortest distance between the center electrode and the ground electrode is formed between a front end edge portion or side surface portion of the center electrode and the distal end portion of the ground electrode.

EFFECTS OF THE INVENTION

According to the above-mentioned configuration 1, in the bending step, the curved surface forming means is disposed between the proximal end portion of the ground electrode and a side portion of the center electrode. That is, the curved surface portion, which serves as a reference for a bending position of the ground electrode, is disposed at a position closer to the proximal end of the ground electrode (position closer to the metallic shell) without being hindered by the center electrode. Therefore, even in a state in which the metallic shell and the insulator have been assembled together, the ground electrode can be bent relatively easily on the side toward the proximal end portion thereof. Furthermore, supporting the ground electrode by the deformation prevention means prevents leaning of the ground electrode toward the center electrode and swelling of the ground electrode toward the direction opposite the center electrode more reliably.
As in the above-described conventional technique, a bending punch or the like having an inclined surface for butting against the ground electrode can be used as the bending means for pressing the ground electrode against the curved surface portion. However, in such a case, since the ground electrode is bent while rubbing against the bending punch, the ground electrode may be scratched. In order to prevent formation of scratches on the ground electrode, there may be employed a method of obliquely pressing the ground electrode from the front end side with respect to the direction of the axis, and gradually changing the pressing direction in accordance with the degree of bending of the ground electrode. However, when this method is employed, the apparatus may become complex.
In contrast, according to the above-mentioned configuration 2, the bending means is constituted by a roller which is movable in relation to the ground electrode along a direction approximately perpendicular to the axis and which is supported for free rotation. Therefore, the ground electrode can be pressed against the curved surface portion more reliably without changing the pressing direction of the ground electrode. Moreover, since the roller is supported for free rotation, there can be suppressed, to a possible extent, occurrence of a state in which the ground electrode bends while rubbing against the roller, whereby formation of scratches on the ground electrode can be prevented more reliably.
Notably, from the viewpoint of preventing formation of scratches on the ground electrode more reliably, it is desired to reduce the frictional coefficients of a bearing portion of the roller and a portion of the outer circumferential surface of the roller which comes into contact with the ground electrode. Accordingly, a film formed of diamond-like carbon may be provided on the outer circumferential surface of the roller and the bearing portion thereof so as to reduce the frictional coefficients thereof.
According to the above-mentioned configuration 3, the inward deformation prevention means prevents deformation of the ground electrode toward the center electrode.
According to the above-mentioned configuration 4, the curved surface forming means and the inward deformation prevention means, which are disposed between the ground electrode and the center electrode, etc. in the bending step are united. Therefore, the apparatus can be prevented from becoming complex.
According to the above-mentioned configuration 5, since the deformation prevention means includes both the inward deformation prevention means and the outward deformation prevention means, deformation of the ground electrode in the bending step can be prevented more reliably.
According to the above-mentioned configuration 6, the ground electrode can be supported more reliably, whereby deformation of the ground electrode in the bending step can be prevented more reliably.
According to the above-mentioned configuration 7, the method of the present invention can cope with ground electrodes having different widths, and can prevent deformation of the ground electrode more reliably.
According to the above-mentioned configuration 8, the curved surface forming means is disposed such that the center of curvature of the curved surface portion is located on the proximal end side of a plane including a front end surface of the center electrode with respect to the direction of the axis. Accordingly, the ground electrode can be bent at a position closer to the proximal end, and, in particular, lateral-discharge-type or inclined-discharge-type spark plugs can be manufactured relatively easily.
According to the above-mentioned configuration 9, the size of the gap adjustment means along a direction perpendicular to the axis can be adjusted by the gap adjustment means. Therefore, the spark discharge gap can be formed more accurately.
In general, the space formed between the ground electrode and the insulator is relatively small. Therefore, in the case where the gap enlargement means for relatively moving the ground electrode in a direction away from the center electrode is disposed in that space, the gap enlargement means restricts relative movement of the ground electrode toward the center electrode, and adjustment of the size of the spark discharge gap may become relatively difficult.
In contrast, according to the above-mentioned configuration 10, the gap enlargement means is disposed frontward of a plane containing the front end surface of the insulator with respect to the direction of the axis. That is, the gap enlargement means is disposed in a relatively large space between the center electrode and the ground electrode. Accordingly, the relative movement of the ground electrode toward the center electrode is not restricted by the gap enlargement means, whereby the size of the spark discharge gap can be adjusted more easily and more accurately.
When the size of the spark discharge gap along the direction perpendicular to the axis is adjusted in the gap adjustment step, the relative height of the distal end portion of the ground electrode in relation to the center electrode along the direction of the axis, the height having been adjusted in the bending step; i.e., the size of the spark discharge gap along the direction of the axis may deviate.
By contrast, according to the above-mentioned configuration 11, the restriction means restricts at least relative movement of the distal end portion of the ground electrode in relation to the center electrode toward the axially front end side. As a result, deviation of the size of the spark discharge gap along the direction of the axis can be prevented more reliably, whereby the size of the spark discharge gap can be adjusted more accurately.
As described in the above-mentioned configuration 12 it is beneficial to employ the above-mentioned configuration 1, etc. in a so-called inclined-discharge-type spark plug in which the shortest distance between the center electrode and the ground electrode is formed between a front end edge portion of the center electrode and the ground electrode, or a so-called lateral-discharge-type spark plug in which the shortest distance between the center electrode and the ground electrode is formed between a side surface portion of the center electrode and the ground electrode.
In other words, through employment of the above-mentioned configuration 1, etc., problems which may arise during manufacture of lateral-discharge-type spark plugs or inclined-discharge-type spark plugs; i.e., problems associated with bending of the ground electrode at a position closer to the proximal end and adjustment of the size of the spark discharge gap along the direction perpendicular to the axis, can be solved at a stroke.
As described in the above-mentioned configuration 13, the technical idea of the above-mentioned configuration 1 may be realized in the form of a spark plug manufacturing apparatus. In this case, an action and an effect basically the same as those of the above-mentioned configuration 1 can be attained.
According to the above-mentioned configurations 14 to 24, actions and effects similar to those of the above-mentioned configurations 2 to 12 can be attained, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] Partially sectioned front view showing the structure of a spark plug according to the present embodiment.
[FIG. 2] Partially sectioned enlarged front view showing the structure of a front end portion of the spark plug.
[FIG. 3] Block diagram showing the structure of a bending apparatus.
[FIG. 4] Partial enlarged view showing a spark plug having a ground electrode before being bent.
[FIG. 5] Enlarged schematic view showing a pre-bending apparatus, etc.
[FIG. 6] Enlarged schematic view showing a main bending apparatus, etc.
[FIG. 7] (a) is a front schematic view showing the structure of a gap adjustment apparatus, etc., and (b) is a side schematic view showing the structure of the gap adjustment apparatus, etc.

MODE FOR CARRYING OUT THE INVENTION

An embodiment will now be described with reference to the drawings. FIG. 1 is a partially sectioned front view showing a spark plug 1. Notably, in FIG. 1, the spark plug 1 is depicted in such a manner that the direction of an axis CL1 of the spark plug 1 coincides with the vertical direction in FIG. 1. Further, in the following description, the lower side of FIG. 1 will be referred to as the front end side of the spark plug 1, and the upper side of FIG. 1 will be referred to as the rear end side of the spark plug 1.
The spark plug 1 is composed of a tubular ceramic insulator 2 serving as an insulator, a tubular metallic shell 3 which holds the ceramic insulator 2, etc.
As well known, the ceramic insulator 2 is formed from alumina or the like through firing. The ceramic insulator 2 includes a rear-end-side trunk portion 10 formed on the rear end side; a larger diameter portion 11 projecting radially outward on the front end side of the rear-end-side trunk portion 10; and an intermediate trunk portion 12 formed on the front end side of the larger diameter portion 11 and having a diameter smaller than that of the larger diameter portion 11. The ceramic insulator 2 further includes a leg portion 13 formed on the front end side of the intermediate trunk portion 12 and tapered off toward the front end side with respect to the direction of the axis CL1. Of the ceramic insulator 2, the larger diameter portion 11, the intermediate trunk portion 12, and the greater part of the leg portion 13 are accommodated within the metallic shell 3. A tapered step portion 14 is formed at a connection portion between the leg portion 13 and the intermediate trunk portion 12. The ceramic insulator 2 is engaged with the metallic shell 3 at the step portion 14.
Furthermore, the ceramic insulator 2 has an axial hole 4 which penetrates the ceramic insulator 2 along the axis CL1. A center electrode 5 is inserted into and fixed to a front end portion of the axial hole 4. The center electrode 5 assumes a rod-like shape (cylindrical columnar shape) as a whole. A flat front end portion of the center electrode 5 projects from the front end of the ceramic insulator 2. The center electrode 5 is composed of an inner layer 5A formed of copper or a copper alloy, and an outer layer 5B formed of a nickel alloy whose predominant component is nickel (Ni). Furthermore, a cylindrical columnar noble metal portion 31 formed of a noble metal alloy (e.g., an iridium alloy) is provided on the front end portion of the center electrode 5. More specifically, the noble metal portion 31 is provided through formation of a fused portion 41 by means of laser welding or the like performed along the periphery of the interface between the outer layer 5B and the noble metal portion 31 (see FIG. 2, etc.).
A terminal electrode 6 is fixedly inserted into a rear end portion of the axial hole 4 such that the terminal electrode 6 projects from the rear end of the ceramic insulator 2.
Furthermore, a cylindrical columnar resistor 7 is disposed in the axial hole 4 between the center electrode 5 and the terminal electrode 6. Opposite ends of the resistor 7 are electrically connected to the center electrode 5 and the terminal electrode 6, respectively, via electrically conductive glass seal layers 8 and 9.
In addition, the metallic shell 3 is formed of metal such as low carbon steel and has a tubular shape. A thread portion (external thread portion) 15 for mounting the spark plug 1 onto an engine head is formed on the outer circumferential surface thereof. Further, a seat portion 16 is formed on the outer circumferential surface located on the rear end side of the thread portion 15, and a ring-shaped gasket 18 is fitted into a thread neck portion 17 at the rear end of the thread portion 15. Moreover, a tool engagement portion 19 and a crimped portion 20 are provided at the rear end of the metallic shell 3. The tool engagement portion 19 has a hexagonal cross section, and a tool, such as a wrench, is engaged with the tool engagement portion 19 when the metallic shell 3 is mounted to the engine head. The crimped portion 20 holds the ceramic insulator 2 at the rear end portion.
Furthermore, a tapered step portion 21 with which the ceramic insulator 2 is engaged is provided on the inner circumferential surface of the metallic shell 3. The ceramic insulator 2 is inserted into the metallic shell 3 from its rear end side toward the front end side. In a state in which the step portion 14 of the ceramic insulator 2 is engaged with the step portion 21 of the metallic shell 3, a rear-end-side opening portion of the metallic shell 3 is crimped radially inward; i.e., the above-mentioned crimped portion 20 is formed, whereby the ceramic insulator 2 is fixed. Notably, an annular plate packing 22 is interposed between the step portion 14 of the ceramic insulator 2 and the step portion 21 of the metallic shell 3. Thus, the airtightness of a combustion chamber is secured, whereby an air-fuel mixture which enters the clearance between the inner circumferential surface of the metallic shell 3 and the leg portion 13 of the ceramic insulator 2 exposed to the interior of the combustion chamber is prevented from leaking to the outside.
Moreover, in order to render the sealing by the crimping more perfect, on the rear end side of the metallic shell 3, annular ring members 23 and 24 are interposed between the metallic shell 3 and the ceramic insulator 2, and powder of talc 25 is charged into the space between the ring members 23 and 24. That is, the metallic shell 3 holds the ceramic insulator 2 via the plate packing 22, the ring members 23 and 24, and the talc 25.
A ground electrode 27 which is bent at an intermediate portion thereof toward the center electrode 5 is joined to a front end face of a front end portion 26 of the metallic shell 3. The ground electrode 27 has a noble metal tip 32 provided on an inner surface of a distal end portion thereof, and a front end edge portion of the noble metal tip 32 faces a front end edge portion of the noble metal portion 31. That is, the shortest distance between the center electrode 5 (the noble metal portion 31) and the ground electrode 27 (the noble metal tip 32) is formed between the front end edge portion of the noble metal portion 31 and the front end edge portion of the noble metal tip 32, whereby a spark discharge gap 33, in which spark discharge occurs obliquely in relation to the axis CL1, is formed between the front end portion of the center electrode 5 and the distal end portion of the ground electrode 27.
Notably, in the case of a spark plug of a type in which the front end surface of the center electrode (the noble metal portion) faces a side surface of the ground electrode (for example, see Japanese Patent Application Laid-Open (kokai) No. 2007-234435 ), discharge occurs generally along the axial direction. That is, the size of the spark discharge gap means a distance between the two electrodes as measured along the axial direction. In contrast, in the case of the spark plug 1 according to the present embodiment, since spark discharge occurs obliquely as described above, as shown in FIG. 2, the size of the spark discharge gap 33 is determined by a distance Gx between the two electrodes 5 and 27 as measured along a direction perpendicular to the axis CL1, and a distance Gy between the two electrodes 5 and 27 as measured along the axis CL1. Therefore, in order to set the spark discharge gap 33 to a predetermined size, not only the distance Gy, but also the distance Gx must be adjusted accurately when the ground electrode 27 is bent toward the center electrode 5. Next, there will be described in detail a bending apparatus 51, which is the feature of the present invention and can adjust both the distances Gy and Gx. The bending apparatus 51 is used in a process of manufacturing the spark plug 1 in order to bend the ground electrode 27 toward the center electrode 5, and set the size of the spark discharge gap 33 to a predetermined size.
As shown in FIG. 3, the bending apparatus 51 includes a pre-bending apparatus 61, a main bending apparatus 71, and a gap adjustment apparatus 81. Notably, in the present embodiment, the spark plug 1 is held by holding means (not shown) such that its front end portion is directed upward and the axis CL1 extends vertically, and is conveyed, by transport means such as a conveyer (not shown), to the pre-bending apparatus 61, the main bending apparatus 71, and the gap adjustment apparatus 81 in this sequence (indicated by arrows in FIG. 3). Furthermore, each of the apparatuses 61, 71, and 81 has a positioning apparatus (not shown) for positioning the ground electrode 27 in a predetermined circumferential position. The spark plug 1 is disposed on each of the apparatuses 61, 71, and 81 in a state in which the ground electrode 27 is positioned in the predetermined circumferential position by the positioning apparatus.
The pre-bending apparatus 61 pre-bends the ground electrode 27 of the spark plug 1 in an unbent state (straight bar like shape) (see FIG. 4) at a position corresponding to a bend portion (intermediate portion) of the ground electrode 27. That is, the pre-bending apparatus 61 performs pre-bending work on the ground electrode 27. As shown in FIG. 5, the pre-bending apparatus 61 includes first deformation prevention means 62 for preventing leaning of the ground electrode 27 toward the center electrode 5 and swelling of the ground electrode 27 toward the side opposite the center electrode 5; and a roller 63 serving as first bending means for bending the ground electrode 27.
The first deformation prevention means 62 includes first inward deformation prevention means 62A and first outward deformation prevention means 62B, each assuming the form of a bar and extending along the width direction of the ground electrode 27. The first inward deformation prevention means 62A and the first outward deformation prevention means 62B are configured to be movable toward and away from the spark plug 1. When the pre-bending work is performed on the ground electrode 27, the first inward deformation prevention means 62A is disposed between the proximal end portion of the ground electrode 27 and a side portion of the center electrode 5, and the first outward deformation prevention means 62B is disposed such that it comes into contact with the outer surface of the proximal end portion of the ground electrode 27. Notably, the deformation prevention means 62A and 62B have flat planar portions 64A and 64B, respectively. When the pre-bending work is performed, the deformation prevention means 62A and 62B are disposed such that the planar portions 64A and 64B face each other and come into surface contact with the proximal end portion of the ground electrode 27. In addition, the deformation prevention means 62A and 62B are supported at their proximal end portions by unillustrated coupling portions in a state in which the spacing between the two means can be changed.
The first inward deformation prevention means 62A has a triangular cross section at at least a portion thereof which comes into contact with the ground electrode 27. A curved surface portion 65 is provided at a corner portion which connects the planar portion 64A and a side surface located at the upper side of a back surface of the planar portion 64A. The curved surface portion 65 is a portion against which the inner surface of the ground electrode 27 is pressed when the ground electrode 27 is pre-bent by the roller 63, and has a shape corresponding to a bent shape of the ground electrode 27. That is, the first inward deformation prevention means 62A is configured such that means for forming a rough bent shape (curved shape) of the ground electrode 27 and means for preventing leaning of the ground electrode 27 toward the center electrode 5 are united together. Notably, in the present embodiment, the curved surface portion 65 is disposed such that, when the pre-bending work is performed on the ground electrode 27, the center of curvature CC of the curved surface portion 65 is located on the proximal end side of a plane PL1 containing the front end surface of the center electrode 5 (the noble metal portion 31), with respect to the direction of the axis CL1.
In addition, the roller 63 has an outer circumferential surface of a predetermined width (e.g., 3 mm), and is supported for free rotation. The roller 63 is configured such that it can be moved horizontally (in directions indicated by arrows in FIG. 5) by unillustrated moving means. Thus, the roller 63 can move toward and press a side surface of the ground electrode 27 opposite the center electrode 5. Furthermore, a film (not shown) formed of diamond-like carbon (DLC) is formed on a portion of the outer circumferential surface of the roller 63, which portion comes into contact with ground electrode 27, and a bearing portion of the roller 63. The frictional coefficient of the surface of the film is 0.2 or less.
The main bending apparatus 71 bends the pre-bent ground electrode 27 at an approximately right angle (performs main bending work), and adjusts the relative height of the distal end portion of the ground electrode 27 (the noble metal tip 32), with respect to the direction of the axis CL1, in relation to the front end portion of the center electrode 5 (the noble metal portion 31). As shown in FIG. 6, the main bending apparatus 71 includes pressing means 72 serving as second bending means, and second deformation prevention means 73 for preventing deformation of the ground electrode 27.
The pressing means 72 is configured to be movable along the vertical direction, and performs the main bending work on the ground electrode 27 by pressing down the distal end portion of the ground electrode 27 toward the center electrode 5. A lower surface portion 72A of the pressing means 72, which comes into contact with the ground electrode 27 at the time of the main bending work, is formed flat, and a film (not shown) formed of DLC is formed on the surface of the lower surface portion 72A. Therefore, the frictional coefficient of the lower surface portion 72A is relatively small.
The second deformation prevention means 73 includes second inward deformation prevention means 73A and second outward deformation prevention means 73B, each assuming the form of a bar and extending along the width direction of the ground electrode 27. The second inward deformation prevention means 73A and the second outward deformation prevention means 73B are configured to be movable toward and away from the spark plug 1. When the main bending work is performed on the ground electrode 27, the second inward deformation prevention means 73A is disposed between the ground electrode 27 and the center electrode 5 or the like such that the second inward deformation prevention means 73A comes into contact with the inner surface of the proximal end portion of the ground electrode 27, and the second outward deformation prevention means 73B is disposed to come into contact with the outer surface of the proximal end portion of the ground electrode 27. The deformation prevention means 73A and 73B have flat planar portions 74A and 74B. When the main bending work is performed, the planar portions 74A and 74B are disposed to come into surface contract with the proximal end portion of the ground electrode 27. As a result, deformation of the ground electrode 27 at the time of performing the main bending work on the ground electrode 27 can be prevented more reliably. Furthermore, the deformation prevention means 73A and 73B are supported at their proximal end portions by unillustrated coupling portions in a state in which the spacing between the two means can be changed.
The main bending apparatus 71 further includes first illumination means (not shown) for projecting a predetermined light beam so as to illuminate a front end portion of the spark plug 1 (at least a front end portion of the center electrode 5 and a distal end portion of the ground electrode 27) with light; and first image capturing means (not shown) for capturing an image of the front end portion of the spark plug 1 irradiated with light. The main bending apparatus 71 also includes control means (not shown) for acquiring the size of the gap between the two electrodes 5 and 27 (the noble metal portion 31 and the noble metal tip 32) from data representing the image captured by the first image capturing means, for calculating, on the basis of the size of the gap, an amount of pressing by the pressing means 72 (corresponding to an amount obtained through addition of an amount of springback to a deviation to be described later), and for controlling the pressing means 72 on the basis of the pressing amount. In the following, operation of the control means will be described in detail.
That is, the control means calculates a deviation of the acquired size of the gap between the electrodes 5 and 27, as measured along the vertical direction, from a designed size of the spark discharge gap 33 along the direction of the axis CL1. Then, the main bending work is performed on the ground electrode 27 by means of causing the pressing means 72 to press downward the distal end portion of the ground electrode 27 by an amount obtained by adding the amount of springback to the deviation. As a result, the spark discharge gap 33 is formed such that the relative height of the distal end portion (the noble metal tip 32) of the ground electrode 27 in relation to the front end portion of the center electrode 5 (the noble metal portion 31) becomes approximately equal to a designed relative height of the distal end portion of the ground electrode 27 in relation to the front end portion of the center electrode 5. That is, the spark discharge gap 33 is formed such that the distance Gy of the spark discharge gap 33 between the electrodes 5 and 27 as measured along the direction of the axis CL1 becomes equal to a designed distance Gy.
The gap adjustment apparatus 81 adjusts the distance Gx of the spark discharge gap 33, as measured along a direction perpendicular to the axis CL1, while maintaining the distance Gy of the spark discharge gap 33 along the axis CL1 adjusted by the main bending apparatus 71. As shown in FIGS. 7 and 8, the gap adjustment apparatus 81 includes second image capturing means 82, gap adjustment means 83, restriction means 84, image processing means 85, and second illumination means (not shown) for projecting a predetermined light beam toward the front end portion of the spark plug 1.
The second image capturing means 82 captures an image of the front end portion of the spark plug 1 irradiated with light emitted from the second illumination means, and outputs data of the captured image to the image processing means 85.
The gap adjustment means 83 is composed of arms 83A and 83B, each assuming the form of a bar, extending along the width direction of the ground electrode 27, and having a rectangular cross section; and a coupling portion 83C which supports the arms 83A and 83B in a state in which they can move in horizontal directions (directions indicated by arrows in FIG. 7) (notably, the arm 83A corresponds to the gap enlargement means). The gap adjustment means 83 is configured to be movable toward and away from the conveyed spark plug 1. When the above-mentioned distance Gx is adjusted, the two arms 83A and 83B approach the spark plug 1 and hold the ground electrode 27 therebetween. By means of moving the two arms 83A and 83B horizontally, the distance Gx can be adjusted.
In addition, the arm 83A is made thinner than the arm 83B, and the arm 83A is disposed on the front end side of a plane PL2 including the front end surface of the ceramic insulator 2, with respect to the direction of the axis CL1. That is, the arm 83A is disposed in a relatively large space between the center electrode 5 and the ground electrode 27.
The restriction means 84 is controlled by the image processing means 85, and has a flat lower surface portion 84A. A film (not shown) of DLC is formed on the lower surface portion 84A. The restriction means 84 is configured to be movable along the vertical direction. When the distance Gx is adjusted, the lower surface portion 84A is immovably disposed at a position at which the lower surface portion 84A comes in contact with the distal end portion of the ground electrode 27. Thus, at the time of adjustment of the distance Gx by the gap adjustment means 83, relative movement (along the direction of the axis CL1) of the distal end portion of the ground electrode 27 in relation to the center electrode 5 can be restricted, whereby the distance Gy formed by the main bending apparatus 71 can be maintained.
The image processing means 85 controls operations of the gap adjustment means 83 and the restriction means 84 on the basis of the data of the image captured by the second image capturing means 82. More specifically, the image processing means 85 moves the two arms 83A and 83B horizontally by an amount which is obtained through addition of an amount of springback to a deviation of the distance (along the horizontal direction) between the distal end portion of the ground electrode 27 and the front end portion of the center electrode 5, obtained from the image data, in relation to the designed distance Gx of the spark discharge gap 33 along the direction perpendicular to the axis CL1.
Notably, in the present embodiment, in consideration of springback deformation of the ground electrode 27, etc., after the distance Gx is adjusted by the gap adjustment means 83, the gap adjustment means 83 and the restriction means 84 are withdrawn from the spark plug 1, the image of the spark plug 1 is again captured by the image capturing means 82, and the distances Gx and Gy of the spark discharge gap 33 are checked so as to determine whether or not the distances Gx and Gy are equal to the designed distances. In the case where the formed distances Gx and Gy are approximately equal to the designed distances, the work for bending the ground electrode 27 is completed. Meanwhile, in the case where the distances Gx and Gy differ from the designed distances, the distance Gx is adjusted again by the gap adjustment means 83, and the distance Gy is adjusted again by the restriction means 84. That is, the restriction means 84 in the present embodiment not only can restrict the relative movement of the distal end portion of the ground electrode 27 toward the front end side with respect to the direction of the axis CL1, in relation to the center electrode 5, but also can adjust the distance Gx by pressing the distal end portion of the ground electrode 27 toward the center electrode 5.
Next, a method for bending the ground electrode 27 by use of the above-described bending apparatus 51 will be described.
First, the spark plug 1 whose ground electrode 27 has been positioned (see FIG. 4) is conveyed to the pre-bending apparatus 61 by the above-described transport means. The first deformation prevention means 62 then approaches the spark plug 1, and supports the proximal end portion of the ground electrode 27. Subsequently, the roller 63 moves toward the ground electrode 27 so as to press the ground electrode 27 against the curved surface portion 65, whereby pre-bending work is performed on the ground electrode 27. After that, the first deformation prevention means 62 and the roller 63 retreat from the spark plug 1.
Next, the spark plug 1 having undergone the pre-bending work is conveyed to the main bending apparatus 71 by the transport means. Subsequently, an image of the front end portion of the spark plug 1 irradiated with light from the first illumination means is captured by the first image capturing means, and a pressing amount is calculated by the control means on the basis of data representing the captured image. Next, the second deformation prevention means 73 approaches the spark plug 1 and supports the proximal end portion of the ground electrode 27. The distal end portion of the ground electrode 27 is pressed by the pressing means 72 by the calculated pressing amount, whereby main bending work is performed on the ground electrode 27. As a result, the ground electrode 27 is bent at an approximately right angle, and the distance Gy of the spark discharge gap 33 along the direction of the axis CL1 is formed. After the main bending work, the pressing means 72 and the second deformation prevention means 73 retreat from the spark plug 1.
Next, the spark plug 1 having undergone the main-bending work is conveyed to the gap adjustment apparatus 81 by the transport means. Subsequently, an image of the front end portion of the spark plug 1 irradiated with light from the second illumination means is captured by the second image capturing means 82. Subsequently, on the basis of data representing the image captured by the second image capturing means 82, the gap adjustment means 83 and the restriction means 84 are disposed at predetermined positions by the image processing mean 84. Subsequently, the distance Gx is adjusted by the gap adjustment means 83, while the distance Gy is maintained by the restriction means 84. After that, the gap adjustment means 83 and the restriction means 84 retreat from the spark plug 1, and the second image capturing means 82 again captures the image of the front end portion of the spark plug 1. When necessary, the distances Gx and Gy are adjusted again by the gap adjustment means 83 and the restriction means 84 on the basis of data representing the re-captured image. As a result, the spark discharge gap 33 is formed to have a predetermined size (approximately equal to the designed size), whereby bending of the ground electrode 27 is completed.
As described above, according to the present embodiment, at the time of (pre-) bending work, means for forming a curved surface (the curved surface portion 65) is disposed between the proximal end portion of the ground electrode 27 and a side portion of the center electrode 5. That is, the curved surface portion 65, which serves as a reference for a bending position of the ground electrode 27, can be disposed at a position closer to the proximal end (position closer to the metallic shell 3) without being hindered by the center electrode 5. Therefore, even in a state in which the metallic shell 3 and the ceramic insulator 2 have been assembled together, the ground electrode 27 can be bent relatively easily on the side toward the proximal end portion thereof. Furthermore, supporting the ground electrode 27 by the first deformation prevention means 62 more reliably prevents leaning of the ground electrode 27 toward the center electrode 5 and swelling of the ground electrode 27 toward the direction opposite the center electrode 5.
Moreover, when the (main) bending work is performed, the relative height of the distal end portion of the ground electrode 27 in relation to the front end portion of the enter electrode 5 along the direction of the axis CL1 is adjusted on the basis of the actual position of the front end portion of the enter electrode 5. As a result, the size of the spark discharge gap 33 as measured along the direction of the axis CL1 can be adjusted more accurately. Furthermore, supporting the ground electrode 27 by the second deformation prevention means 73 more reliably prevents leaning and swelling of the ground electrode 27.
In addition, the size of the spark discharge gap 33 as measured along the direction perpendicular to the axis CL1 can be adjusted by the gap adjustment apparatus 81. That is, in the main bending apparatus 71, the size of the spark discharge gap 33 as measured along the direction of the axis CL1 can be set to a predetermined size, and, in the gap adjustment apparatus 81, the size of the spark discharge gap 33 as measured along the direction perpendicular to the axis CL1 can be adjusted. Therefore, the spark discharge gap 33 can be formed accurately.
Furthermore, the first bending means is the roller 63, which can move relative to the ground electrode 27 along a direction approximately perpendicular to the axis CL1 and is supported for free rotation. Therefore, the ground electrode 27 can be pressed against the curved surface portion 65 more reliably, without changing the pressing direction of the ground electrode 27. Moreover, since the roller 63 is supported for free rotation, there can be suppressed, to a possible extent, occurrence of a state in which the ground electrode 27 bends while rubbing against the roller 63, whereby formation of scratches on the ground electrode 27 can be prevented more reliably.
Moreover, in the present embodiment, in order to reduce the frictional coefficients of a bearing portion of the roller 63 and a portion of the outer circumferential surface of the roller 63 which comes into contact with the ground electrode 27, a film formed of DLC is provided on the outer circumferential surface of the roller 63 and the bearing portion thereof. Thus, formation of scratches on the ground electrode 27 can be prevented further more reliably.
In addition, in the pre-bending apparatus 61, the curved surface portion 65 and the first inward deformation prevention means 62A to be disposed between the ground electrode 27 and the center electrode 5, etc. are united together. Therefore, the apparatus can be prevented from becoming complex.
Furthermore, the curved surface portion 65 is disposed such that the center of curvature CC of the curved surface portion 65 is located on the proximal end side of the plane PL1 containing the front end surface of the center electrode 5, with respect to the direction of the axis CL1. Accordingly, the ground electrode 27 can be bent at a position closer to the proximal end, whereby lateral-discharge-type or inclined-discharge-type spark plugs can be manufactured relatively easily.
In addition, the arm 83A is disposed frontward of a plane containing the front end surface of the ceramic insulator 2 with respect to the direction of the axis CL1. That is, the arm 83A is disposed in a relatively large space between the center electrode 5 and the ground electrode 27. Accordingly, the relative movement of the ground electrode 27 toward the center electrode 5 is not restricted by the presence of the arm 83A, whereby the size of the spark discharge gap 33 can be adjusted more easily and more accurately.
Moreover, the restriction means 84 restricts at least the relative movement of the distal end portion of the ground electrode 27 in relation to the center electrode 5 toward the front end side with respect to the direction of the axis CL1. As a result, there can be prevented a deviation of the distance Gy of the spark discharge gap 33 along the direction of the axis CL1, whereby the size of the spark discharge gap 33 can be adjusted more accurately.
Notably, the present invention is not limited to the details of the above-described embodiment, and may be practiced as follows. Needless to say, other application examples and medications which are not exemplified bellow are also possible.

(a) In the above-described embodiment, the first deformation prevention means 62 includes the first inward deformation prevention means 62A and the first outward deformation prevention means 62B. However, the first deformation prevention means 62 may include only one of the first inward deformation prevention means 62A and the first outward deformation prevention means 62B.

(b) In the above-described embodiment, the roller 63 is employed as first bending means for performing pre-bending work on the ground electrode 27. However, the means for performing pre-bending work on the ground electrode 27 is not limited thereto.

(c) In the above-described embodiment, a film formed of DLC is provided on the outer circumferential surface of the roller 63, the lower surface portion 72A of the pressing means 72, etc. However, the film may be omitted.

(d) In the above-described embodiment, the spark plug 1 is of an inclined discharge type in which spark discharge occurs obliquely in relation to the direction of the axis CL1. However, the discharge direction of the spark plug 1 is not limited thereto. Accordingly, the spark plug 1 may be of a lateral discharge type in which spark discharge occurs along a direction approximately perpendicular to the direction of the axis CL1 or a longitudinal discharge type in which spark discharge occurs along the direction of the axis CL1.

(e) In the above-described embodiment, the ground electrode 27 was joined to the front end surface of the front end portion 26 of the metallic shell 3. However, the present invention can be applied to a spark plug in which the ground electrode is formed through cutting a portion of the metallic shell (or a portion of a front end metal piece welded to the metallic shell in advance) (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-236906 ). Furthermore, the ground electrode 27 may be joined to the side surface of the front end portion 26 of the metallic shell 3.

(f) In the above-described embodiment, the tool engagement portion 19 has a hexagonal section. However, the shape of the tool engagement portion 19 is not limited thereto. For example, the tool engagement portion 19 may have a Bi-HEX (modified dodecagonal) shape [IS022977:2005(E)] or the like.

(g) The positions of the first inward deformation prevention means 62A and the first outward deformation prevention means 62B at the time of pre-bending work may be determined on the basis of data representing a captured image of the front end portion of the spark plug 1.

(h) In the above-described embodiment, a pair including the first inward deformation prevention means 62A and the first outward deformation prevention means 62B, a pair including the second inward deformation prevention means 73A and the second outward deformation prevention means 73B, and a pair including the arm 83A and the arm 83B are each formed of separate members. However, each pair may be formed of a single member.

(i) In the above-described embodiment, the gap adjustment means 83 holds the proximal end portion of the ground electrode 27. The gap adjustment means may be configured to press a distal end surface of the ground electrode 27 (in the case where the noble metal tip 32 is provided, a distal end surface of the ground electrode base material excluding the noble metal tip 32).

DESCRIPTION OF REFERENCE NUMERAL

1:: spark plug
2:: ceramic insulator serving as an insulator
3:: metallic shell
4:: axial hole
5:: center electrode
26:: front end portion of the metallic shell
27:: ground electrode
33:: spark discharge gap
51:: bending apparatus
61:: pre-bending apparatus
62:: first deformation prevention means
62A:: first inward deformation prevention means
62B:: first outward deformation prevention means
63:: roller
65:: curved surface portion
71:: main bending apparatus
72:: pressing means
73:: second deformation prevention means
81:: gap adjustment apparatus
83:: gap adjustment means
83A:: arm serving as gap enlargement means
84:: restriction means

Claims

A spark plug manufacturing method for manufacturing a spark plug including:
a rod-like center electrode extending in the direction of an axis;

an approximately cylindrical tubular insulator having an axial hole extending in the direction of the axis, the center electrode being provided at a front end portion of the axial hole;

an approximately cylindrical tubular metallic shell surrounding the insulator; and

a ground electrode whose proximal end portion is joined to the metallic shell, and which is bent at an intermediate portion thereof so as to form a spark discharge gap between a distal end portion of the ground electrode and a front end portion of the center electrode,

the method comprising a bending process of bending the ground electrode toward the axis in a state in which the metallic shell with the ground electrode joined thereto and the insulator in which the center electrode is provided are assembled together, wherein

the bending process includes a bending step performed such that

curved surface forming means having a curved surface portion corresponding to a bent shape of the ground electrode is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode,

the ground electrode is supported by deformation prevention means so as to prevent at least one of deformation of the proximal end portion of the ground electrode toward the center electrode and deformation of the proximal end portion toward a side opposite the center electrode, and

the ground electrode is pressed against the curved surface portion from the side opposite the center electrode by use of bending means, whereby bending work is performed on the ground electrode.
A spark plug manufacturing method according to claim 1, wherein the bending means is a roller which is configured to be movable in relation to the ground electrode along a direction approximately perpendicular to the axis and which is supported for free rotation, and an outer circumferential surface of the roller is pressed against a side surface of the ground electrode opposite the center electrode so as to perform the bending work on the ground electrode.
A spark plug manufacturing method according to claim 1 or 2, wherein the deformation prevention means includes inward deformation prevention means which is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode so as to support a side portion of the proximal end portion of the ground electrode located on the side toward the center electrode, to thereby prevent deformation of the ground electrode toward the center electrode.
A spark plug manufacturing method according to claim 3, wherein the deformation prevention means is configured such that the curved surface forming means and the inward deformation prevention means are united together.
A spark plug manufacturing method according to claim 3 or 4, wherein the deformation prevention means further includes outward deformation prevention means which is disposed on a side of the ground electrode opposite the center electrode so as to support a side portion of the proximal end portion of the ground electrode opposite the center electrode, to thereby prevent deformation of the ground electrode toward a side opposite the center electrode.
A spark plug manufacturing method according to claim 5, wherein the inward deformation prevention means and the outward deformation prevention means are disposed to face each other via the proximal end portion of the ground electrode.
A spark plug manufacturing method according to claim 5 or 6, wherein the spacing between the inward deformation prevention means and the outward deformation prevention means can be changed.
A spark plug manufacturing method according to any one of claims 1 to 7, wherein the curved surface forming means is disposed such that the center of curvature of the curved surface portion is located on the proximal end side of a plane including a front end surface of the center electrode with respect to the direction of the axis.
A spark plug manufacturing method according to any one of claims 1 to 8, wherein the bending process includes a gap adjustment step performed after the bending step so as to adjust a size of the spark discharge gap along a direction perpendicular to the axis by use of gap adjustment means capable of moving the distal end portion of the ground electrode in relation to the center electrode along a direction approximately perpendicular to the axis.
A spark plug manufacturing method according to claim 9, wherein the gap adjustment means includes gap enlargement means which is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode so as to relatively move the distal end portion of the ground electrode in a direction away from the center electrode, to thereby expand a size of the spark discharge gap along a direction perpendicular to the axis,
wherein the gap enlargement means is disposed on a front end side of a plane including a front end surface of the insulator with respect to the direction of the axis.
A spark plug manufacturing method according to claim 9 or 10, wherein, in the gap adjustment step, restriction means is used to restrict at least relative movement of the distal end portion of the ground electrode in relation to the center electrode toward the front end side with respect to the direction of the axis.
A spark plug manufacturing method according to any one of claims 1 to 11, wherein the spark plug is configured such that a shortest distance between the center electrode and the ground electrode is formed between a front end edge portion or side surface portion of the center electrode and the distal end portion of the ground electrode.
A spark plug manufacturing apparatus used for manufacture of a spark plug including:
a rod-like center electrode extending in the direction of an axis;

an approximately cylindrical tubular insulator having an axial hole extending in the direction of the axis, the center electrode being provided at a front end portion of the axial hole;

an approximately cylindrical tubular metallic shell surrounding the insulator; and

a ground electrode whose proximal end portion is joined to the metallic shell, and which is bent at an intermediate portion thereof so as to form a spark discharge gap between a distal end portion of the ground electrode and a front end portion of the center electrode,

the apparatus comprising a bending apparatus for bending a straight-bar-shaped ground electrode toward the axis in a state in which the metallic shell with the straight-bar-shaped ground electrode joined thereto and the insulator in which the center electrode is provided are assembled together, wherein

the bending apparatus comprises:

curved surface forming means which has a curved surface portion corresponding to a bent shape of the ground electrode and is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode;

first deformation prevention means for preventing at least one of deformation of the proximal end portion of the ground electrode toward the center electrode and deformation of the proximal end portion toward a side opposite the center electrode; and

bending means for pressing the ground electrode against the curved surface portion of the curved surface forming means from a side opposite the center electrode.
A spark plug manufacturing apparatus according to claim 13, wherein the bending means is a roller which is supported for free rotation and which can move toward and away from the ground electrode along a direction approximately perpendicular to the axis.
A spark plug manufacturing apparatus according to claim 13 or 14, wherein the deformation prevention means includes inward deformation prevention means which is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode so as to support a side portion of the proximal end portion of the ground electrode located on the side toward the center electrode, to thereby prevent deformation of the ground electrode toward the center electrode.
A spark plug manufacturing apparatus according to claim 15, wherein the deformation prevention means is configured such that the curved surface forming means and the inward deformation prevention means are united together.
A spark plug manufacturing apparatus according to claim 15 or 16, wherein the deformation prevention means further includes outward deformation prevention means which is disposed on a side of the ground electrode opposite the center electrode so as to support a side portion of the proximal end portion of the ground electrode opposite the center electrode, to thereby prevent deformation of the ground electrode toward a side opposite the center electrode.
A spark plug manufacturing apparatus according to claim 17, wherein the inward deformation prevention means and the outward deformation prevention means are disposed to face each other via the proximal end portion of the ground electrode.
A spark plug manufacturing apparatus according to claim 17 or 18, wherein the spacing between the inward deformation prevention means and the outward deformation prevention means can be changed.
A spark plug manufacturing apparatus according to any one of claims 13 to 19, wherein the curved surface forming means is disposed such that the center of curvature of the curved surface portion is located on the proximal end side of a plane including a front end surface of the center electrode with respect to the direction of the axis.
A spark plug manufacturing apparatus according to any one of claims 13 to 20, wherein the bending apparatus further comprises gap adjustment means capable of moving the distal end portion of the ground electrode in relation to the center electrode along a direction approximately perpendicular to the axis, wherein the gap adjustment means adjusts a size of the spark discharge gap along a direction perpendicular to the axis.
A spark plug manufacturing apparatus according to claim 21, wherein the gap adjustment means includes gap enlargement means which is disposed between a side portion of the center electrode and the proximal end portion of the ground electrode so as to relatively move the distal end portion of the ground electrode in a direction away from the center electrode, to thereby expand a size of the spark discharge gap along a direction perpendicular to the axis,
wherein the gap enlargement means is disposed on a front end side of a plane including a front end surface of the insulator with respect to the direction of the axis.
A spark plug manufacturing apparatus according to claim 21 or 22, wherein the gap adjustment means comprises restriction means for restricting at least relative movement of the distal end portion of the ground electrode in relation to the center electrode toward the front end side with respect to the direction of the axis.
A spark plug manufacturing apparatus according to any one of claims 13 to 23, wherein the spark plug is configured such that a shortest distance between the center electrode and the ground electrode is formed between a front end edge portion or side surface portion of the center electrode and the distal end portion of the ground electrode.