CROSS REFERENCE TO RELATED APPLICATION
TECHNICAL FIELD
[0002] The present invention relates to a spark plug for use in an internal combustion engine,
an insulating member for the spark plug which is used in the spark plug, and a method
of manufacturing the same.
BACKGROUND
[0003] A spark plug for an internal combustion engine is installed in the internal combustion
engine, and is used to ignite an air-fuel mixture in a combustion chamber. In general,
the spark plug includes an insulating member having an axial hole, a center electrode
inserted into the front end side of the axial hole, a terminal electrode (terminal
shell) inserted into the rear end side of the axial hole, a metal shell installed
on the outer circumference of the insulating member, and a ground electrode installed
on the front end portion of the metal shell and forming a spark discharge gap between
the center electrode and the ground electrode.
[0004] Further, the insulating member for the spark plug is generally manufactured by the
following. That is, raw powder including alumina as a main component thereof is filled
in a cavity of a cylindrical rubber mold for shaping, and a rod-shaped press pin is
inserted into the cavity. The raw powder is pressurized and compressed by applying
hydraulic pressure to the rubber mold for shaping in a diameter direction, thereby
obtaining a molding. After a support pin is inserted into a hole portion serving as
the axial hole, of which a rear end side is opened and a front end side is closed,
from the rear end side, the grinding rotation roller having a predetermined shape
of the insulating member on the outer circumference is rotated around a center shaft
parallel with the axis as a rotation axis, and is brought in contact with the molding.
Accordingly, the molding is ground to obtain an unfired insulating member having the
shape of the insulating member. After that, the unfired insulating member is fired
to obtain the insulating member. In a grinding process of the molding, it is required
to communicate the hole portion to form the axial hole. For this reason, the grinding
is started from the front end portion of the molding, of which a grinding amount is
relatively large in comparison with other portions.
[0005] However, if the grinding is started from the front end portion of the molding, high
stress is applied to the proximal end portion of the support pin. It causes the support
pin to be deformed or broken down. In addition, according to the deformation of the
support pin, the thickness of the unfired insulating member is varied in a circumferential
direction, so that the withstanding voltage performance or mechanical strength of
the insulating member may be deteriorated.
[0006] Consequently, in order to prevent deformation or breakdown of the support pin, a
method of increasing the rigidity of the support pin by relatively enlarging the outer
diameter of the support pin has been proposed (e.g., Patent Document 1).
SUMMARY
[0008] However, there is a demand for a compact spark plug (reduced in diameter), and thus
it is desirable to reduce the diameter of the insulating member. In a case of employing
the above method, since the outer diameter of the support pin is necessarily maintained
to some extent, the thickness of the insulating member has to be thinned in order
to reduce the diameter of the insulating member. As a result, there is a concern that
the mechanical strength or withstanding voltage performance of the insulating member
is not sufficiently ensured.
[0009] In view of the above-described circumstances, a method according to claim 1, an insulating
member according to claim 11, and spark plugs according to claims 12 and 13 are proposed.
Further advantages, features, aspects and details of the invention are evident from
the dependent claims, the description and the drawings.
An advantage of aspects of the invention is to provide a method of manufacturing an
insulating member for a spark plug which can form the insulating member with high
precision by reliably preventing deformation or breakdown of a support pin and can
meet a demand for a compact spark plug. Also, an insulating member for the spark plug
manufactured by the same method and having the related advantages, and a spark plug
for an internal combustion engine including the insulating member for the spark plug
are provided.
[0010] Hereafter, configurations suitable for achieving the above-described advantages will
be described in an itemized fashion. Each configuration may be combined with any other
configuration or aspect unless clearly indicated to the contrary. In particular any
feature indicated as being preferred or advantageous may be combined with any other
feature or features indicated as being preferred or advantageous, both in the configurations
and in the embodiments that are described further below. In addition, when necessary,
operations and effects peculiar to each configuration will be added.
[0011] Configuration 1. According to this Configuration, a method of manufacturing an insulating
member for a spark plug (hereinafter referred to as 'insulating member') including
an axial hole extending in an axial direction, in which a center electrode is held
at a front end side of the axial hole and a (e.g. metal) terminal is held at a rear
end side of the axial hole, is characterized by including a pressurized forming step
of compressing raw powder to form a molding having a hole portion, of which a rear
end side is opened and a front end side is closed; a support pin inserting step of
inserting a rod-shaped support pin into the hole portion from the rear end side of
the molding; and a grinding step of grinding an outer circumferential surface of the
molding inserted with the support pin to form an unfired insulating member having
a shape of the insulating member. According to a variant of configuration 1, after
the pressurized forming step and before the grinding step, a front end removing step
of cutting a front end portion of the molding and/or grinding the front end portion
in an axial direction to penetrate the hole portion is included.
[0012] According to the Configuration 1, in the front end removing step before the grinding
step, the front end portion of the molding is cut and/or ground from the axial direction
(front end side) (toward the rear end side) to penetrate the hole portion. Consequently,
it is not necessary to start the grinding from the front end portion of the molding
in the next grinding step. That is, it is possible to start the grinding from other
portions except for the front end portion of the molding in the grinding step. For
this reason, it is possible to reliably suppress high stress from being applied to
the proximal end portion of the support pin, thereby reliably preventing damage or
breakdown of the support pin. As a result, the insulating member can be formed with
high precision.
[0013] Further, according to the Configuration 1, since the support pin is hardly ever deformed
and it is not necessary to enlarge the diameter of the support pin, the diameter of
the axial hole can be reduced. As a result, in a case in which the outer diameter
of the insulating member is set to be small in order to meet the downsizing of the
spark plug, it is possible to sufficiently ensure the thickness of the insulating
material. That is, according to the Configuration, it is possible to perform the downsizing
of the spark plug, while sufficiently ensuring the mechanical strength or withstanding
voltage required for the insulating member.
[0014] Configuration 2. According to this Configuration, a method of manufacturing an insulating
member for a spark plug including an axial hole extending in an axial direction, in
which a center electrode is held at a front end side of the axial hole and a (e.g.
metal) terminal is held at a rear end side of the axial hole, is characterized by
including a pressurized forming step of compressing raw powder to form a molding having
a hole portion, of which a rear end side is opened and a front end side is closed;
a support pin inserting step of inserting a rod-shaped support pin into the hole portion
from the rear end side of the molding; and a grinding step of grinding an outer circumferential
surface of the molding inserted with the support pin to form an unfired insulating
member having a shape of the insulating member. According to a first variant of configuration
2, in the pressurized forming step, supposing that a length of the molding along the
axis is L, a portion, of which a grinding amount is maximized in the grinding step,
exists in a range of 2L/3 from the rear end of the molding. According to a second
variant of configuration 2, a maximum-grinding-portion of the molding extends within
a range of 2L/3 from the rear end of the molding, wherein the maximum-grinding-portion
is defined by having a larger or equal grinding amount during the grinding step than
any other axis-longitudinal portion of the molding.
[0015] In this instance, the expression 'portion, of which a grinding amount is maximized
in the grinding step' means a portion on a plane perpendicular to the axis, of which
a value (in this instance, the value means the outer diameter of the molding in a
case in which the unfired molding does not exist on the plane) subtracting the outer
diameter of the unfired insulating member obtained by the grinding process from the
outer diameter of the molding is maximized. Herein, a range of some length from the
rear end of the molding refers to the range anywhere between the rear end and the
position at the length from the rear end of the molding, the distance being measured
along the axis.
[0016] According to the Configuration 2, in the pressurized forming process, the molding
is formed in such a manner that the portion, of which the grinding amount is maximized
in the grinding process, exists in the range of 2L/3 from the rear end of the molding.
For this reason, the grinding starts at the portion existing in the range of 2L/3
from the rear end of the molding in the grinding process. Consequently, it is possible
to reliably suppress high stress from being applied to the proximal end portion of
the support pin, thereby reliably preventing damage or breakdown of the support pin.
As a result, the insulator can be manufactured with high precision.
[0017] Further, as described above, since it is not necessary to increase the diameter of
the support pin, it is possible to downsize the spark plug while the thickness of
the insulating member is sufficiently ensured.
[0018] Configuration 3. According to this Configuration, a method of manufacturing an insulating
member for a spark plug including an axial hole extending in an axial direction, in
which a center electrode is held at a front end side of the axial hole and a (e.g.
metal) terminal is held at a rear end side of the axial hole, is characterized by
including a pressurized forming step of compressing raw powder to form a molding having
a hole portion, of which a rear end side is opened and a front end side is closed;
a support pin inserting step of inserting a rod-shaped support pin into the hole portion
from the rear end side of the molding; and a grinding step of grinding an outer circumferential
surface of the molding inserted with the support pin to form an unfired insulating
member having a shape of the insulating member, wherein in the grinding step, supposing
that a length of the molding along the axis is L, grinding is started from a portion
of the molding which is placed in a range of L/3 from the rear end of the molding,
and a portion of the rear end side more than L/3 from the front end of the molding
comes into contact with the grinding member before the grinding amount reaches 10%
of the total grinding amount in the grinding step.
[0019] According to the Configuration 3, in the grinding process of the molding, first,
the grinding is started from the front end side (the portion placed in the range of
L/3 from the front end of the molding) of the molding. The rear end (the portion in
the rear end side than L/3 from the front end of the molding) of the molding comes
into contact with the grinding member (e.g., grinding rotation roller) at the step
in which the grinding amount is 10% or less of the total grinding amount in the grinding
step, or, in other word, in which the integrated grinding amount, integrated along
the axial length, is 10% or less of the total, or final, integrated grinding amount
in the grinding step. That is, in the initial step of the grinding step, the grinding
member comes into contact with the front end portion and the rear end portion of the
molding, thereby preventing high stress from being continuously applied to the support
pin. For this reason, it is possible to reliably prevent the support pin from being
deformed or broken down, and thus it is possible to form the insulating member with
high precision. Further, since it is not necessary to increase the diameter of the
support pin, it is possible to downsize the spark plug while the thickness of the
insulating member is sufficiently ensured.
[0020] Further, in a case in which the grinding is started from the rear end portion of
the molding, it is necessary to relatively thicken the rear end portion, so that the
total grinding amount may be increased in the grinding step. According to the Configuration
3, since the rear end portion can be relatively thickened, it is possible to suppress
the total grinding amount. As a result, it is possible to suppress the increase of
the manufacturing cost.
[0021] Configuration 4. According to this Configuration, a method of manufacturing an insulating
member for a spark plug including an axial hole extending in an axial direction, in
which a center electrode is held at a front end side of the axial hole and a (e.g.
metal) terminal is held at a rear end side of the axial hole, is characterized by
including a pressurized forming step of compressing raw powder to form a molding having
a hole portion, of which a rear end side is opened and a front end side is closed;
a support pin inserting step of inserting a rod-shaped support pin into the hole portion
from the rear end side of the molding; and a grinding step of grinding an outer circumferential
surface of the molding inserted with the support pin to form an unfired insulating
member having a shape of the insulating member, wherein in the grinding step, supposing
that a length of the molding along the axis is L, a portion of the molding which is
placed in a range of 2 L/3 from the rear end of the molding comes firstly into contact
with the grinding member.
[0022] According to the Configuration 4, the grinding is started from the portion of the
molding which is placed in a range of 2L/3 from the rear end of the molding. Consequently,
the same working effect as that of the Configuration 2 is obtained.
[0023] Configuration 5. The method of manufacturing the insulating member according to any
one of the above-described Configurations 1 to 4 is
characterized in that in the pressurized forming step, supposing that a length of the molding along the
axis is L, the molding is formed in such a manner that a width of the portion in an
axial direction, of which the grinding amount is maximized in the grinding step, placed
in the range of 2L/3 from the rear end of the molding is set to 5 mm or more.
[0024] According to the Configuration 5, it seems that the width of the portion, of which
the grinding amount is maximized in the grinding step, of the rear end of the molding
is set to 5 mm or more, that is, is sufficiently long. Consequently, in the grinding
process, it is possible to disperse the pressure applied to the portion, and thus
it is possible to reliably prevent the molding from being breaking at the portion
as the origin.
[0025] Configuration 6. The method of manufacturing the insulating member according to the
above-described Configuration 5 is
characterized in that in the pressurized forming step, the width in the axial direction is set to 20 mm
or less.
[0026] According to the Configuration 6, since the width of the portion, of which the grinding
amount is maximized in the grinding step, of the rear end portion of the molding is
set to 20 mm or less, it is possible to prevent the grinding amount from remarkably
increasing, thereby suppressing the increase of the manufacturing cost.
[0027] Configuration 7. The method of manufacturing the insulating member according to any
one of the above-described Configurations 1 to 6 is
characterized in that in the grinding step, by bringing the molding into contact with a rotating grinding
rotation roller and simultaneously into contact with a pressing member, the molding
is supported against a frictional force from the grinding rotation roller so that
the outer circumferential surface of the molding is ground to obtain the unfired insulating
member having the shape of the insulating member, and in that, supposing that a length
of the molding along the axis is L, the pressing member is placed at a range of 2L/3
from the rear end of the molding, and is positioned at a position in such a manner
that a distance S along an axis between a center portion of the outer circumferential
surface of the pressing member along the axis line and the portion, of which the grinding
amount is maximized, in the range of 2L/3 from the rear end of the molding is L/5
or less.
[0028] According to the Configuration 7, the grinding of the molding is performed by bringing
the molding into contact with the rotating grinding rotation roller, and the molding
is supported by the pressing member. Consequently, it is possible to perform the grinding
process of the molding with even higher precision, and thus it is possible to form
the insulating member with even higher precision.
[0029] Further, since the pressing member is placed at a range of 2L/3 from the rear end
of the molding, it is possible to suppress high stress from being applied to the support
pin according to the contact of the pressing member against the molding. Consequently,
it is possible to further prevent the deformation or breakdown of the support pin.
[0030] In addition, the distance of the axis between the center portion of the outer circumferential
surface of the pressing member along the axis line and the portion, of which the grinding
amount is maximized, in the range of 2L/3 from the rear end of the molding is L/5
or less. That is, the pressing member is placed so as to approach as much as possible
a portion, which is firstly ground, of the rear end portion of the molding. Consequently,
in the grinding process, it is possible to reduce the load applied to the support
pin from the grinding rotation roller or the pressing member, thereby further effectively
preventing the deformation or the like of the support pin.
[0031] Further, the above-described technical thought may be embodied as the insulating
member set forth in the Configuration 8 or the spark plug set forth in the Configuration
9.
[0032] Configuration 8. The insulating member for the spark plug according to this Configuration
is
characterized in that the insulating member is manufactured by the manufacturing method according to any
one of the above-described Configurations 1 to 7.
[0033] Since the insulating member for the spark plug according to the Configuration 8 is
manufactured by the manufacturing method according to the above-described Configuration
1 or the like, it is possible to prevent a variation in thickness from happening,
thereby obtaining superior mechanical strength and withstanding voltage performance.
[0034] Configuration 9. The spark plug for an internal combustion engine according to this
Configuration is characterized by including the insulating member for the spark plug
according to the above-described Configuration 8.
[0035] Since the spark plug according to the Configuration 9 includes the insulating member
with superior mechanical strength and withstanding voltage performance, it is possible
to improve the durability and prolong the lifespan.
[0036] Configuration 10. The spark plug for the internal combustion engine according to
this Configuration is characterized by including, in the Configuration 9, a center
electrode installed at the front end side of the axial hole, a metal terminal installed
at the rear end side of the axial hole, a metal shell of a substantially cylindrical
shape which is provided on the outer circumference of the insulating member and has
a threaded portion for engaging in a threaded manner with an attaching hole of a head
of the internal combustion engine, and a glass seal layer formed by a glass powder
mixture and sealing at least a space between the metal terminal and the insulating
member in the axial hole, wherein the length of the insulating member for the spark
plug in the direction of the axis is set to 65 mm or more, the maximum outer diameter
of the glass seal layer is set to 3.4 mm or less, and the outer diameter of the threaded
portion is set to M12 or less.
[0037] Recently, there has been demand for a spark plug with a reduced diameter and prolonged
length, and further, it is possible to manufacture the insulating member in a relatively
elongated shape. At the time of manufacturing the insulating member, it is necessary
to relatively elongate the support pin, but if the support pin is elongated, the strength
of the support pin is lowered, and in the grinding process, the stress applied to
the proximal end portion of the support pin from the grinding member is increased.
That is, there is more concern about the deformation or breakdown of the support pin
at the time of manufacturing the elongated insulating member.
[0038] According to the Configuration 10, the length of the insulating member for the spark
plug in the direction of the axis is set to 65 mm or more, the maximum outer diameter
of the glass seal layer is set to 3.4 mm or less, and the outer diameter of the threaded
portion is set to M12 or less. That is, since the spark plug according to this Configuration
includes a relatively elongated insulating member, there is concern about the deformation
or the like of the support pin in the grinding process. However, it is possible to
further reliably suppress the deformation or the like of the support pin by applying
the respective configurations. In other words, the respective configurations are particularly
advantageous when the spark plug with the reduced diameter and the extended length
is manufactured.
Further aspects of the invention are also directed to apparatuses for carrying out
the disclosed methods and including apparatus parts for performing each described
method steps. These method steps may be performed by way of hardware components, a
computer programmed by appropriate software, by any combination of the two or in any
other manner. Furthermore, aspects of the invention are also directed to methods by
which the described apparatus operates. They include method steps for carrying out
every function of the apparatus or manufacturing every part of the apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] Illustrative aspects of the invention will be described in detail with reference
to the following figures wherein:
Fig. 1 is a partially-sectioned, front view showing the configuration of a spark plug;
Fig. 2 is a flowchart showing a method of manufacturing an insulator;
Fig. 3 is a cross-sectional view of a rubber press machine to illustrate a filling
process;
Fig. 4 is a cross-sectional view of a rubber press molding to illustrate insertion
of a press pin;
Fig. 5 is a cross-sectional view of a rubber press molding to illustrate drawing of
a molding after a pressurized forming process;
Fig. 6 is a front view showing the configuration of a molding according to a first
embodiment;
Fig. 7 is a schematic view showing a molding after insertion of a support pin and
before a grinding process;
Fig. 8 is a schematic view showing a position relationship between a molding and a
grinding rotation roller at a start of a grinding process;
Fig. 9 is a schematic view showing unfired insulator formed after a grinding process;
Fig. 10 is a front view showing the configuration of a molding according to a second
embodiment;
Fig. 11 is a schematic view for reference to illustrate a case in which a molding
comes in contact with a grinding rotation roller;
Fig. 12 is a flowchart showing a method of manufacturing an insulator according to
a second embodiment;
Fig. 13 is a front view showing the configuration of a molding after a process of
removing a distal end;
Fig. 14 is a front view showing the configuration of a molding according to a third
embodiment; and
Fig. 15 is a schematic view showing an initial step of a grinding process according
to a third embodiment.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION
[Embodiment 1]
[0040] An embodiment will now be described with reference to the drawings. Fig. 1 is a partially-sectioned,
front view of a spark plug for an internal combustion engine (hereinafter, referred
to as 'spark plug') 1. Notably, in Fig. 1, the spark plug 1 is depicted in such a
manner that the direction of an axis CL1 which passes through the center 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.
[0041] The spark plug 1 is constituted of a cylindrical insulator 2 serving as an insulating
member for a spark plug, and a cylindrical metal shell 3 holding the insulator 2 therein.
[0042] As is well known, the insulator 2 is made of alumina or the like through baking.
The insulator 2 includes in its outer configuration portion a rear end-side barrel
portion 10 formed on the rear end side thereof, a large-diameter portion 11 protruding
radially outward at a position closer to the distal end side than the rear end-side
barrel portion 10, an intermediate barrel portion 12 formed closer to the distal end
side than the large-diameter portion 11 and having a diameter smaller than that of
the large-diameter portion 11, and a long leg portion 13 formed closer to the distal
end side than the intermediate barrel portion 12 and having a diameter smaller than
that of the intermediate barrel portion 12.
Of the insulator 2, the large-diameter portion 11, the intermediate barrel portion
12, and the major part of the long leg portion 13 are accommodated within the metal
shell 3. A stepped portion 14 of a tapered shape is formed at a connection portion
between the leg portion 13 and the intermediate barrel portion 12. The insulator 2
is engaged with the metal shell 3 at the stepped portion 14.
[0043] Further, the insulator 2 has an axial hole 4 which extends through the insulator
2 along the axis CL1. A center electrode 5 is inserted into and fixed to a front end
side of the axial hole 4. The center electrode 5 includes an inner layer 5A made of
copper or a copper alloy, and an outer layer 5B made of a Ni alloy containing nickel
as a main component thereof. In addition the center electrode 5 is formed in a rod-like
shape (cylindrical columnar shape) as a whole, and the distal end surface of the center
electrode 5 is formed flat and protrudes from the distal end portion of the insulator
2. Further, a noble metal tip 31 of a cylindrical columnar shape which is made of
a noble metal alloy is joined to a distal end portion of the center electrode 5.
[0044] Further, a terminal electrode 6 is inserted into and fixed to a rear end side of
the axial hole 4 in such a manner that the terminal electrode 6 projects from the
rear end of the insulator 2.
[0045] Further, a cylindrical columnar resistor 7 is disposed between the center electrode
5 and the terminal electrode 6 of the axial hole 4. Both end portions 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, respectively.
In this instance, the glass seal layer 9 corresponds to a glass seal layer of the
invention.
[0046] The metal shell 3 is made of metal such as low carbon steel and is formed in a cylindrical
shape. A threaded portion (external threaded portion) 15 for mounting the spark plug
1 onto an engine head is formed on the outer circumferential surface of the metal
shell. A seat portion 16 is formed on the outer circumferential surface of the rear
end side of the threaded portion 15. A ring-shaped gasket 18 is fitted into a threaded
neck potion 17 at the rear end of the threaded portion 15. A tool engagement portion
19 having a hexagonal cross-section shape is provided at the rear end side of the
metal shell 3 so that a tool, such as a wrench, engages with the tool engagement portion
19 when the spark plug 1 is mounted to the engine head. Further, a crimping portion
20 is provided at the rear end side of the metal shell to hold the insulator 2 at
the rear end portion.
[0047] Further, a tapered stepped portion 21 with which the insulator 2 is engaged is provided
on the inner circumferential surface of the metal shell 3. The insulator 2 is inserted
into the metal shell 3 from its rear end side toward the front end side. In a state
where the stepped portion 14 of the insulator 2 is engaged with the step portion 21
of the metal shell 3, a rear end-side opening portion of the metal shell 3 is crimped
radially inward. That is, the crimping portion 20 is formed, so that the insulator
2 is fixed. In this instance, an annular plate packing 22 is interposed between the
stepped portion 14 of the insulator 2 and the stepped portion 21 of the metal shell
3. Thus, the airtightness of a combustion chamber is secured, so that a fuel air mixture
which enters the clearance between the inner circumferential surface of the metal
shell 3 and the long leg portion 13 of the insulator 2 exposed to the interior of
the combustion chamber is prevented from leaking to the outside.
[0048] Moreover, in order to render the sealing by the crimping more complete, on the rear
end side of the metal shell 3, annular ring members 23 and 24 are interposed between
the metal shell 3 and the insulator 2, and powder of talc 25 is charged in the space
between the ring members 23 and 24. That is, the metal shell 3 holds the insulator
2 via a plate packing 22, the ring members 23 and 24, and the talc 25.
[0049] A ground electrode 27 is joined to a front end portion 26 of the metal shell 3. A
middle portion of the ground electrode 27 is bent and a lateral surface thereof faces
the center electrode 5. A noble metal chip 32 of a cylindrical columnar shape, which
is made of a noble metal alloy, is joined to a distal end portion of the ground electrode
27. A spark discharge gap 33 is formed between the two tips 31 and 32, in which spark
discharge occurs along a direction approximately perpendicular to the axis CL1.
[0050] In this embodiment, as the outer diameter of the threaded portion 15 is M12 or less
(e.g., M10 or less), the threaded portion is relatively reduced in diameter. For this
reason, the insulator 2 inserted into the metal shell 3 is relatively reduced in outer
diameter. From the viewpoint of sufficiently ensuring the thickness of the insulator
2, the inner diameter of the axial hole 4 is relatively small. Consequently, the maximum
outer diameter of the glass seal layer 9 disposed in the axial hole 4 is 3.4 mm or
less.
Further, the insulator 2 is extended in length, and more specifically, the length
of the insulator 2 is 65 mm or more in the direction of axis CL1.
[0051] Next, a method of manufacturing the spark plug 1 configured as described above will
be described. First, the metal shell 3 is pre-fabricated. That is, cold forging operation
is performed on a cylindrical columnar metal material (e.g., iron material or stainless
steel material such as S17C or S25C) so as to form a through hole therein and impart
a rough shape to the metal material. Subsequently, cutting operation is performed
on the metal material so as to impart a predetermined outer shape to the metal material
to thereby obtain a metal shell intermediate.
[0052] Subsequently, the ground electrode 27 made of a Ni alloy is resistance-welded to
the front end surface of the metal shell intermediate. Since a so-called "sagging"
is produced as a result of the welding, the "sagging" is removed. Subsequently, the
threaded portion 15 is formed in a predetermined region of the metal shell intermediate
by means of rolling. Thus, the metal shell 3 to which the ground electrode 27 has
been welded is obtained. Zinc plating or nickel plating is performed on the metal
shell 3 to which the ground electrode 27 has been welded. Notably, in order to improve
corrosion resistance, chromate treatment may be performed on the surface.
[0053] Next, the insulator 2 is fabricated. A method of fabricating the insulator 2 will
be described in detail with reference to the flowchart of Fig. 2. First, in a raw
powder adjusting process in S10, a powdery matter containing alumina (aluminum oxide)
powder as a main component thereof and a sintering auxiliary agent is mixed with an
acrylic binder, and then is wet-mixed by using water as a solvent to adjust slurry.
The adjusted slurry is spray-dried to obtain raw powder.
[0054] In a filling process of a step S20, the obtained raw powder is filled in the rubber
pressing machine 41 shown in Fig. 3. The rubber pressing machine 41 includes an internal
rubber mold 43 of a cylindrical shape having a cavity 42 extending in the direction
of an axis CL2, an external rubber mold 44 of a cylindrical shape installed on the
outer circumference of the internal rubber mold 43, a machine body 45 installed on
the outer circumference of the external rubber mold 44, and a bottom cover 46 and
a lower holder 47 which close a lower opening portion of the cavity 42. Further, the
machine body 45 is provided with a liquid channel 45a, and hydraulic pressure is applied
to the outer circumferential surface of the external rubber mold 44 in a diameter
direction through the liquid channel 45a, thereby contracting the cavity 42 in the
diameter direction.
[0055] Returning to the description of the manufacturing method, in a filling process, the
cavity 42 of the internal rubber mold 43 is filled with the raw powder PM. Then, as
shown in Fig. 4, a press pin 51 is placed in the cavity 42. The proximal end side
of the press pin 51 is integrally installed with an upper holder 52, and the upper
holder 52 is fitted into the upper opening portion of the cavity 42 to close the cavity
42 in a sealed state.
[0056] Next, in a pressurized forming process of a step S30, since the hydraulic pressure
is applied through the liquid channel 45a, the pressure is applied from the outer
circumference side of the internal rubber mold 43 and the external rubber mold 44,
so that the cavity 42 is contracted. Thus, the raw powder PM is compressed and formed.
After a lapse of a predetermined time, the application of the hydraulic pressure is
released, the internal rubber mold 43 and the external rubber mold 44 are elastically
returned, so that the contracted cavity 42 is returned to original size. As shown
in Fig. 5, if the press pin 51 is lifted up from the rubber pressing machine 41 in
the direction of the axis CL2, a molding CP1 formed by compressing the raw powder
PM is extracted from the cavity 42 together with the press pin 51. After that, if
the press pin 51 is relatively rotated with respect to the molding CP1, the press
pin 51 is extracted from the molding CP1. A hole portion HL of the molding CP1 which
is formed by extracting the press pin 51 constitutes the axial hole 4.
[0057] In the pressurized forming process, as shown in Fig. 6, supposing that the length
of the molding CP1 in the axis CL1 is L, the molding CP1 is formed in such a manner
that a portion (in the same figure, a portion marked by a scattered point pattern),
of which a value subtracting the outer diameter of the unfired insulating member IP
obtained by a grinding process described below from the outer diameter of the molding
CP1 is maximized, that is, a portion for maximum grinding MG, exists in the range
of 2L/3 from the rear end of the molding CP1. In other words, the molding CP1 is formed
in such a manner that the portion for maximum grinding MG, of which a grinding amount
is maximized in a grinding process of a step S50 described below, exists in the range
of 2L/3 from the rear end of the molding CP1. Further, the length of the portion for
maximum grinding MG in the axis CL1 is set to be equal to or more than 5 mm and equal
to or less than 20 mm.
[0058] Next, in a support pin inserting process of a step S40, as shown in Fig. 7, a support
pin 61. is inserted into the hole portion HL of the obtained molding CP1. In a grinding
process of a step S50, the grinding is performed on the molding CP 1. More specifically,
as shown in Fig. 8, the grinding is performed on the molding CP1 by inserting the
molding CP1 between a grinding rotation roller 62 which rotates around a CL3 parallel
with the axis CL1 as a center axis, and has a shape corresponding to the shape of
the insulator 2 on the outer circumferential portion, and a pressing member 63 has
a circular cross-sectional shape and supports the molding CP1 against a frictional
force received from the grinding rotation roller 62. As described above, the molding
CP1 has the portion for maximum grinding MG in the range of 2L/3 from the rear end,
of which the grinding amount is maximized in the grinding process of the step S50.
For this reason, at the start of the grinding, the portion for maximum grinding MG
of the molding CP1 comes in contact with the grinding rotation roller 62. In this
instance, the pressing member 63 is within the range of 2L/3 from the rear end of
the molding CP1, and is positioned at a position in which the distance S of the axis
CL1 between a center portion 63M of the outer circumference portion of the pressing
member 63 and the portion for maximum grinding MG is 1/5 or less of the length L of
the molding CP1.
[0059] By performing the grinding process, as shown in Fig. 9, the unfired insulating member
IP having the axial hole 4 formed by penetration of the hole portion HL is formed
in the same shape as the insulator 2. After that, the support pin 61 is separated
from the unfired insulating member IP. In a baking process of a step S60, the obtained
unfired insulating member IP is input and baked in the firing furnace to obtain the
insulator 2.
[0060] Further, the center electrode 5 is fabricated, separately from the metal shell 3
and the insulator 2. That is, a forging process is performed on a Ni alloy with a
copper alloy placed at a center portion thereof so as to enhance a heat radiation
performance, thereby fabricating the center electrode 5.
[0061] The insulator 2 and the center electrode 5 which are obtained by the above description,
and the resistor 7 and the terminal electrode 6 are sealed and fixed by the glass
seal layers 8 and 9. In general, the glass seal layers 8 and 9 are formed of a mixture
of borosilicate glass and metal powder. The mixture is charged in the axial hole 4
of the insulator 2 in such a manner that the resistor 7 is disposed between upper
and lower layers of the mixture. While the mixture is pressed from the rear side via
the terminal electrode 6, the mixture is heated within the firing furnace, so that
the mixture is fired. In this instance, a glaze layer may be simultaneously formed
on the surface of the rear end-side barrel portion 10 of the insulator 2 through firing.
Alternatively, the glaze layer may be formed in advance.
[0062] After that, the insulator 2 fabricated as described above and having the center electrode
5 and the terminal electrode 6, and the metal shell 3 fabricated as described above
and having the ground electrode 27 are assembled together. More specifically, the
insulator 2 is fixed by crimping radially inward the rear end-side opening portion
of the metal shell 3 which is relatively thin, i.e., by forming the crimping portion
20.
[0063] Finally, the spark discharge gap 33 between the distal end portion of the center
electrode 5 and the distal end portion of the ground electrode 27 is adjusted by bending
the ground electrode 27, thereby obtaining the spark plug 1.
[0064] As described above in detail, according to this embodiment, in the pressurized forming
process, the molding CP1 is formed in such a manner that the portion for maximum grinding
MG, of which the grinding amount is maximized in the grinding process, exists in the
range of 2L/3 from the rear end of the molding CP1. That is, the grinding starts at
the portion existing in the range of 2L/3 from the rear end of the molding CP1 in
the grinding process. Consequently, it is possible to reliably suppress high stress
from being applied to the proximal end portion of the support pin 61, thereby reliably
preventing damage or breakdown of the support pin 61. As a result, the insulator 2
can be manufactured with high precision.
[0065] Further, since the support pin 61 is hardly ever deformed and it is not necessary
to enlarge the diameter of the support pin 61, the diameter of the axial hole 4 can
be reduced. As a result, as this embodiment, in order to meet the downsizing of the
spark plug 1, it is possible to sufficiently ensure the thickness of the insulator
2 in a case in which the outer diameter of the insulator 2 is set to be small. That
is, according to this embodiment, it is possible to perform the downsizing of the
spark plug 1, while sufficiently ensuring the mechanical strength or withstanding
voltage required for the insulator 2.
[0066] In addition, the unfired insulating member IP is formed by bringing the molding CP1
in contact with the rotating grinding rotation roller 62. In this embodiment, the
molding CP1 is supported by the pressing member 63. Accordingly, it is possible to
perform the grinding process of the molding CP1 with even higher precision, and thus
it is possible to form the insulator 2 with even higher precision.
[0067] Further, since the pressing member 63 is positioned in the range of the 2L/3 from
the rear end of the molding body CP1, it is possible to suppress high stress from
being applied to the support pin 61 due to the fact the pressing member 63 comes into
contact with the molding CP 1, thereby reliably preventing damage or breakdown of
the support pin 61.
[0068] In addition, the distance S between the middle portion 63M of the pressing member
63 and the portion for maximum grinding MG in the axis CL1 is set to L/5 or less.
That is, the pressing member 63 is placed in such a manner that it approaches the
portion (the portion for maximum grinding MG) initially ground in the rear end-side
portion of the molding CP1. Accordingly, at the grinding process, it is possible to
decrease the load applied to the support pin 61 from the grinding rotation roller
62 or the pressing member 63, thereby further effectively preventing the deformation
of the support pin 61.
[0069] Moreover, the width of the portion for maximum grinding MG is set to be sufficiently
long at 5 mm or more. Accordingly, in the grinding process, it is possible to disperse
the pressure applied to the portion for maximum grinding MG, and thus it is possible
to reliably prevent the molding CP1 from breaking down on the portion for maximum
grinding MG as the origin. Further, since the width of the portion for maximum grinding
MG is set to 20 mm or less, it is possible to prevent the grinding amount from remarkably
increasing, thereby suppressing the increase of the manufacturing cost.
[Embodiment 2]
[0070] The second embodiment will be now described with reference to the drawings, on the
basis of the difference between the first embodiment and the second embodiment.
[0071] In the second embodiment, in particular, the shape of a molding CP2 is different
in the manufacture of the insulator 2. That is, in the first embodiment, the middle
portion of the molding GP1 is formed to have a relatively large diameter so as to
form the portion for maximum grinding MG, of which the grinding amount is maximized
in the grinding process, in the range of 2L/3 from the rear end. On the contrary,
in the second embodiment, since the internal shape of the cavity 42 is changed, the
middle portion of the molding CP2 is formed to have a relatively small diameter, as
shown in Fig. 10. Consequently, as shown in Fig. 11, in a case in which the molding
CP2 comes into contact with the grinding rotation roller 62 while the axis CL1 of
the molding CP2 is parallel with the axis CL3 of the grinding rotation roller 62,
the molding CP2 is formed in such a manner that a portion MG2 (in the figure, a portion
marked by a scattered point pattern) placed in the range of L/3 from the front end
of the molding CP2 comes into contact with the grinding rotation roller 62. That is,
if the grinding process is performed on the molding CP2 in an intact state, the molding
CP2 is configured in such a manner that the grinding is started from the portion MG2,
so that high stress may be applied to the support pin 61.
[0072] Consequently, in the second embodiment, as shown in Fig. 12, a front end removing
process of a step S35 is provided after the pressurized forming process of the step
S30 before the support pin inserting process of the step S40. In the front end removing
process, as shown in Fig. 13, the hole portion HL of the molding CP2 is penetrated
by cutting the portion MG2 of the molding CP2 to form the axial hole 4.
[0073] As described in detail above, according to the second embodiment, the front end portion
of the molding CP2 is cut in the front end removing process before the grinding process,
and the hole portion HL is penetrated. Consequently, it is not necessary to start
the grinding from the front end portion of the molding portion CP2 in the next grinding
process. That is, in the grinding process, it is possible to start the grinding from
other portions except for the front end portion of the molding CP2. For this reason,
it is possible to suppress high stress from being applied to the proximal end portion
of the support pin 61 in the grinding process, thereby reliably preventing the support
pin 61 from being deformed or broken down. As a result, it is possible to form the
insulator 2 with high precision.
[Embodiment 3]
[0074] The third embodiment will be now described with reference to the drawings, on the
basis of the difference between the first embodiment and the second embodiment.
[0075] In the third embodiment, as shown in Fig. 14, in a molding CP3 there is a portion
for maximum grinding MG3 (in the figure, a portion marked by a scattered point pattern),
of which the grinding amount is maximized in the grinding process, in the range of
L/3 from the front end of the molding CP3. In the grinding process, as shown in Fig.
15, the grinding is started from the portion MG3 of the molding CP3, in which the
portion SG placed in the range of 2L/3 from the rear end of the molding CP3 comes
into contact with the grinding rotation roller 62 before the grinding amount reaches
10% of the total grinding amount in the grinding process. That is, in the initial
step, the grinding rotation roller 62 comes into contact with the front end portion
and the rear end portion of the molding CP3.
[0076] According to the third embodiment, in the grinding process of the molding CP3, first,
the front end portion of the molding CP3 comes into contact with the grinding rotation
roller 62, and the rear end of the molding CP3 comes into contact with the grinding
rotation roller 62 at the step in which the grinding amount is 10% or less of the
total grinding amount in the grinding process. That is, in the initial step of the
grinding process, the grinding rotation roller 62 comes into contact with the front
end portion and the rear end portion of the molding CP3, thereby preventing high stress
from being continuously applied to the support pin 61. For this reason, it is possible
to reliably prevent the support pin 61 from being deformed or broken down, and thus
it is possible to form the insulator 2 with high precision.
[0077] Further, in a case in which the grinding is started from the rear end portion of
the molding CP3, it is necessary to relatively thicken the rear end portion, so that
the total grinding amount may be increased in the grinding process. According to the
third embodiment, since the rear end portion can be relatively thickened, it is possible
to suppress the total grinding amount. As a result, it is possible to suppress the
increase of the manufacturing cost.
[0078] It should be noted that the invention is not limited to details of above-described
embodiment, and may be implemented as described below, for example. It goes without
saying that it is also possible to adopt other applications and modifications which
are not illustrated below.
[0079]
- (a) In the embodiment, the outer diameter of the threaded portion 15 is set to M12
or below, but the outer diameter of the threaded portion 15 is not specifically limited.
Further, the maximum outer diameter of the glass seal layer 9 is set to 3.4 mm or
less, but the maximum outer diameter of the glass seal layer 9, that is, the inner
diameter of the axial hole 4, is not specifically limited. In addition, the length
of the insulator 2 in the direction of the axis CL1 is set to 65 mm or more, but it
is not limited thereto. That is, the invention can be applied to the manufacture of
another insulator 2 having the axial, hole 4 of various diameters or total lengths.
[0080] (b) In the embodiment, the molding CP1 is supported by the pressing member 63 in
the grinding process of the molding CP1, but the grinding process may be performed
without forming the pressing member 63.
[0081] (c) In the second embodiment, the front end portion of the molding CP2 is cut and
removed in the front end removing process. However, the front end portion of the molding
CP2 may be removed by grinding the front end portion of the molding CP2 from the front
end side toward the rear end side in the axial direction.
[0082] (d) In the embodiment, the noble metal tips 31 and 32 are provided on the center
electrode 5 and the ground electrode 27, but both or any one of the noble metal tips
31 and 32 may be omitted.
[0083] (e) In the embodiment, the case in which the ground electrode 27 is joined to the
front end portion 26 of the metal shell 3 is exemplified, but the invention is also
applicable to a case in which the ground electrode is formed in such a manner as to
shave off a portion of the metal shell (or a portion of a tip fitting welded in advance
to the metal shell) (e.g., refer to
JP-A-2006-236906).
[0084] (f) Although the tool engaging portion 19 is provided with a hexagonal cross-sectional
shape, the shape of the tool engaging portion 19 is not limited thereto. For example,
the tool engaging portion 19 may have a Bi-HEX (modified 12-point) shape [IS022977:2005(E)]
or the like.
[Description of Reference Numerals and Signs]
[0085]
1: SPARK PLUG (SPARK PLUG FOR INTERNAL COMBUSTION ENGINE)
2: INSULATOR (INSULATING MEMBER FOR SPARK PLUG)
3: METAL SHELL
4: AXIAL HOLE
5: CENTER ELECTRODE
6: TERMINAL ELECTRODE (TERMINAL SHELL)
8: GLASS SEAL LAYER
15: THREADED PORTION
61: SUPPORT PIN
62: GRINDING ROTATION ROLLER (GRINDING MEMBER)
63: PRESSING MEMBER
CL1:AXIS
CP1, CP2, CP3: MOLDING
HL: HOLE PORTION
IP: UNFIRED INSULATING MEMBER
PM: RAW POWDER
1. A method of manufacturing an insulating member (2) for a spark plug (1), the insulating
member (2) including an axial hole (4) extending in an axial direction, in which a
center electrode (5) is held at a front end side of the axial hole and a metal terminal
is held at a rear end side of the axial hole, the method comprising:
a pressurized forming step (S30) of compressing raw powder (PM) to form a molding
(CP1) having a hole portion (HL), of which a rear end side is opened and a front end
side is closed;
a support pin inserting step (S40) of inserting a rod-shaped support pin (61) into
the hole portion (HL) from the rear end side of the molding; and
a grinding step (S50) of grinding an outer circumferential surface of the molding
(GP1) inserted with the support pin (61) to form an unfired insulating member (IP)
having a shape of the insulating member (2).
2. The method according to claim 1, wherein
after the pressurized forming step (S30) and before the grinding step (S40), a front
end removing step of removing the front end is included, whereby the hole portion
(HL) is caused to penetrate through the insulating member (IP), the front end removing
step including cutting a front end portion of the molding and/or grinding the front
end portion along an axial direction to penetrate the hole portion.
3. The method according to any one of claims 1 to 2, wherein
in the pressurized forming step (S30), supposing that a length of the molding along
the axis is L, a portion (MG), of which a grinding amount is maximized in the grinding
step, exists in a range of 2L/3 from the rear end of the molding.
4. The method according to any one of claims 1 to 3, wherein in the grinding step, supposing
that a length of the molding along the axis is L, grinding is started from a portion
of the molding which is placed in a range of L/3 from the rear end of the molding,
and a portion of the rear end side more than L/3 from the front end of the molding
comes into contact with a grinding member before the grinding amount reaches 10% of
the total grinding amount in the grinding step.
5. The method according to any one of claims 1 to 4, wherein
in the grinding step, supposing that a length of the molding along the axis is L,
a portion of the molding which is placed in a range of 2L/3 from the rear end of the
molding comes firstly into contact with a grinding member.
6. The method of manufacturing the insulating member according to any one of claims 1
to 5,
wherein
in the pressurized forming step, supposing that a length of the molding along the
axis is L, the molding is formed in such a manner that a width, in an axial direction,
of the portion (MG), of which the grinding amount is maximized in the grinding step,
placed in the range of 2L/3 from the rear end of the molding, is set to 5 mm or more.
7. The method of manufacturing the insulating member according to claim 6,
wherein
in the pressurized forming step, the width, in the axial direction, of the portion
(MG) of which the grinding amount is maximized in the grinding step is set to 20 mm
or less.
8. The method of manufacturing the insulating member according to any one of the claims
1 to 7, wherein
in the grinding step, by bringing the molding into contact with a rotating grinding
rotation roller (62) and simultaneously into contact with a pressing member (63),
the molding is supported against a frictional force from the grinding rotation roller
so that the outer circumferential surface of the molding is ground to obtain the unfired
insulating member having the shape of the insulating member, and
wherein
supposing that a length of the molding along the axis is L, the pressing member (63)
is placed at a range of 2L/3 from the rear end of the molding, and is positioned at
a position in such a manner that a distance S along an axis between a center portion
(63M) of the outer circumferential surface of the pressing member (63) along the axis
line and the portion (MG), of which the grinding amount is maximized, placed in the
range of 2L/3 from the rear end of the molding, is set to L/5 or less.
9. A method of manufacturing a spark plug, including:
manufacturing an insulating member according to any one of the previous claims.
10. The method according to claim 9, further comprising at least one of the following:
inserting a center electrode into the front end side of the axial hole;
inserting a terminal electrode into the rear end side of the axial hole,
installing a metal shell on the outer circumference of the insulating member; and
installing a ground electrode on the front end portion of the metal shell, and forming
a spark discharge gap between the center electrode and the ground electrode.
11. An insulating member for the spark plug characterized in that the insulating member is manufactured by the manufacturing method according to any
one of claims 1 to 8.
12. A spark plug for an internal combustion engine characterized by including the insulating member for the spark plug according to the claim 9.
13. The spark plug for the internal combustion engine, preferably the spark plug according
to claim 12, the spark plug comprising:
an insulating member having an axis and an axial hole, the axial hole having a front
end side and a rear end side;
a center electrode installed at the front end side of the axial hole;
a metal terminal installed at the rear end side of the axial hole;
a metal shell of a substantially cylindrical shape which is provided on the outer
circumference of the insulating member and has a threaded portion for engaging in
a threaded manner with an attaching hole of a head of the internal combustion engine;
and
a glass seal layer formed by a glass powder mixture and sealing at least a space between
the metal terminal and the insulating member in the axial hole,
wherein
the length of the insulating member for the spark plug in the direction of the axis
is set to 65 mm or more, the maximum outer diameter of the glass seal layer is set
to 3.4 mm or less, and the outer diameter of the threaded portion is set to M12 or
less.