Manufacturing method for a spark plug

In a bond step, one end of ground electrode is bonded to a metallic housing. In a provisional bending step performed after the bonding step, an intermediate portion of the ground electrode is bent with a pressing die so that the other end of the ground electrode is opposed to a distal end surface of a center electrode. In a regular bending step performed after the provisional bending step, the position of the other end of the ground electrode is adjusted so as to adjust the dimension of a spark gap. Receiving dies are prepared for regulating a shift range of a distal end surface of the other end of the ground electrode in a direction perpendicular to an axial line of the center electrode in the process of bending the intermediate portion of the ground electrode in the provisional bending step. In the provisional bending step, the intermediate portion of the ground electrode is bent until the distal end surface of the other end of the ground electrode is brought into contact with the receiving dies.

BACKGROUND OF THE INVENTION

This invention relates to a manufacturing method for a spark plug installed in an internal combustion engine of an automotive vehicle or the like.

A conventional spark plug has a columnar center electrode insulated and held inside a metal housing and a ground electrode having one end being bonded to the housing, wherein the ground electrode is bent at its intermediate portion so that the other end of the ground electrode is opposed to a distal end surface of the center electrode.

A spacer is used for regulating a bending amount in a provisional bending step of the ground electrode, so that a spark gap is suppressed or regulated within a predetermined range at the end of the provisional bending step (refer to Japanese Patent Application Laid-open No. 2000-164320).

However, according to the manufacturing method disclosed in the above-described prior art document, the protruding amount of a distal end surface of the other end of the ground electrode is not constant.

In other words, the unevenness of the protruding amount increases. This is an ignition property deteriorating factor of the spark plug. In this specification, the “protruding amount” is an overhang of the distal end surface of the other end of the ground electrode with respect to the axial line of the center electrode in a direction perpendicular to the axial line of the center electrode.

SUMMARY OF THE INVENTION

In view of the above-described problems, the present invention has a first object to improve dimensional accuracy of the protruding amount. Furthermore, the present invention has a second object to improve dimensional accuracy of both a spark gap and the protruding amount.

In order to accomplish the above and other related objects, the present invention provides a method for manufacturing a spark plug including a columnar center electrode insulated and held inside a metal housing and a ground electrode having one end being bonded to the housing. The ground electrode is bent at its intermediate portion so that the other end of the ground electrode is substantially perpendicular to an axial line of the center electrode. The other end of the ground electrode is opposed to a distal end surface of the center electrode via a predetermined spark gap. The manufacturing method of this invention includes a bonding step, a provisional bending step, and a regular bending step. The bonding step is performed to bond one end of the ground electrode to the housing. After the bonding step, the provisional bending step is performed to bend an intermediate portion of the ground electrode with a pressing die so that the other end of the ground electrode is opposed to a distal end surface of the center electrode. After the provisional bending step, the regular bending step is performed to adjust a position of the other end of the ground electrode so as to adjust the dimension of the spark gap. The receiving dies are prepared for regulating a shift range of a distal end surface of the other end of the ground electrode in a direction perpendicular to the axial line in the process of bending the intermediate portion of the ground electrode in the provisional bending step. And, in the provisional bending step, the intermediate portion of the ground electrode is bent until the distal end surface of the other end of the ground electrode is brought into contact with the receiving dies.

According to this invention, the receiving dies regulate the protruding amount of the other end of the ground electrode in the provisional bending step. Thus, it becomes possible to improve the dimensional accuracy of the protruding amount at the end of the provisional bending step.

Preferably, the position of the pressing die in the axial line in the provisional bending step is determined with reference to the position of the distal end surface of the center electrode.

According to this method, it becomes possible to reduce the unevenness in the size of the spark gap during the provisional bending step. Furthermore, it becomes possible to reduce the dimensional accuracy of both the spark gap and the protruding amount at the end of the provisional bending step.

Preferably, the provisional bending step is performed in a condition that both sides of the ground electrode are clamped by the receiving dies when the ground electrode is seen from the axial line.

According to this method, the receiving dies prevent the ground electrode from shifting in the lateral direction of (i.e., a direction of the side surfaces of) the ground electrode. Thus, it becomes possible to accurately perform the provisional bending step.

Preferably, the position of the distal end surface of the center electrode is measured by image processing.

According to the above-described prior art document, the position of the distal end surface of the center electrode is measured by laser. However, laser is disadvantageous in that the depth of field is shallow. The measurement based on the laser is adversely influenced by surface conditions (flatness, cutout, etc.) of the distal end surface of the center electrode or unevenness in the position of the distal end surface of the center electrode. Thus, the measurement based on the laser is inaccurate.

On the other hand, according to the above method, the position of the distal end surface of the center electrode is measured by image processing. Thus, it becomes possible to perform accurate measurement without being influenced by the surface conditions of the distal end surface of the center electrode or by the unevenness in the position of the distal end surface of the center electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be explained hereinafter with reference to attached drawings.

FIGS. 1 to 8are views showing a preferred embodiment of the present invention.

InFIG. 1, a spark plug1has a housing10made of an electrically conductive steel material and configured into a substantially cylindrical shape. An insulator11, made of an insulating ceramic and configured into a substantially cylindrical shape, is inserted and fixed in the metallic housing10. A center electrode12, made of an electrically conductive metallic material and configured into a substantially columnar shape, is inserted and fixed in an axial bore of the insulator11. A ground electrode13, made of a Ni-based alloy and configured into a plate-like shape, is bonded to the housing10.

The ground electrode13is straight at the time it is bonded to the housing10as shown inFIG. 1, and is processed into a substantially L-shaped configuration through a bending operation (described later in detail) as shown inFIG. 7. More specifically, the ground electrode13has a leg portion13aextending substantially parallel to an axial line X of the center electrode12and an opposed portion13bextending substantially perpendicularly to the axial line X of the center electrode12. One end of the leg portion13ais welded to the housing10. The opposed portion13bis disposed in a confronting relationship with a distal end surface12aof center electrode12. A spark gap G is provided between the opposed portion13band the distal end surface12aof center electrode12. In this invention, the opposed portion13bis also referred to as the other end of the ground electrode13.

As shown inFIG. 1, a camera unit2including a CCD camera21and a lighting device22is provided in the vicinity of the distal end surface12aof center electrode12. Image data of the distal end surface12aof center electrode12, taken by the camera unit2, is sent to an image processing apparatus3.

In the provisional bending step, the image processing apparatus3measures the position of the distal end surface12aof center electrode12based on the image data and determines an operation amount of a provisional bending apparatus4(described later in detail) with reference to the position of the distal end surface12aof center electrode12. Thus, the image processing apparatus3controls the operation of the provisional bending apparatus3.

Furthermore, in the regular bending step, the image processing apparatus3measures the dimension of the spark gap G based on the image data, and determines the operation amount of a regular bending apparatus5(described later in detail) based on the measured amount of the spark gap G, thereby controlling the operation of the regular bending apparatus5.

Next, the provisional bending apparatus4will be explained with reference toFIGS. 2 to 6. The provisional bending apparatus4performing the provisional bending step of bending the ground electrode13has two electrically driven motors41and42whose operations are controlled by the image processing apparatus3. The first motor41drives or actuates a holder43in the direction of axial line X (i.e., an up-and-down direction in the drawings).

The second motor42is installed on the holder43. The second motor42drives or actuates a pressing die44and a cam plate45so as to move together in the back-and-forth direction. When the pressing die44is moved toward the spark plug1(i.e., forward), the pressing die44is brought into contact with the ground electrode13to bend the intermediate portion of the ground electrode13(refer toFIGS. 5 and 6).

A pair of link levers46and47swing in accordance with the shift movement of the cam plate45. The cam plate45has a pair of cam grooves45a. Pins46aand47a, provided at one end of respective link levers46and47, are slidably inserted into the cam grooves45aof the cam plate45. Pivot shafts46band47b, provided at an intermediate portion of respective link levers46and47, are rotatably supported by the holder43. Receiving dies48and49are attached to the other end of respective link levers46and47.

Two receiving dies48and49cooperatively regulate the shape of the ground electrode13during the provisional bending operation. To this end, as shown inFIGS. 2 to 5, the receiving dies48and49have intermediate portion positioning portions48aand49aand distal end surface positioning portions48band49bprovided at their opposed surfaces48cand49c. The intermediate portion positioning portions48aand49aare brought into contact with intermediate portions of the ground electrode13during the provisional bending operation. The distal end surface positioning portions48band49bregulate a forward shift range of a distal end surface13cof the ground electrode13so as to regulate a protruding amount L (refer toFIG. 5) during the provisional bending operation.

In accordance with a shift movement of the pressing die44and the cam plate45from the bending start position (i.e., the position shown inFIGS. 2 and 3) to the bending end position (i.e., the position shown inFIGS. 5 and 6), the link levers46and47rotate about their pivot shafts from the condition shown inFIG. 3to the condition shown inFIG. 6and accordingly two receiving dies48and49approach to each other.

In this case, at the time the pressing die44is brought into contact with the ground electrode13, i.e., at the time the operation of bending the ground electrode13is started, two receiving dies48and49are positioned most closely. Two positioning portions48aand48bof the first receiving die48are brought into contact with two positioning portions49aand49bof the second receiving die49.

Furthermore, in the condition that two receiving dies48and49are positioned most closely, both side surfaces13d(refer toFIG. 3) of ground electrode13are sandwiched between the opposed surface48cof first receiving die48and the opposed surface49cof second receiving die49with tiny clearances, when the ground electrode13is seen from the axial line X.

Next, the regular bending apparatus5will be explained with reference toFIG. 7. The regular bending apparatus5adjusts the position of the opposed portion13bof ground electrode13after accomplishing the provisional bending step so as to adjust the dimension of the spark gap G. The regular bending apparatus5has an electrically driven motor51whose operation is controlled by the image processing apparatus3. The motor51drives a working head52in the up-and-down direction.

Next, the processes for manufacturing the ground electrode13performed by the above-described manufacturing apparatus will be explained.

First of all, the ground electrode13extending straight is bonded to the housing10. Then, the insulator11and the center electrode12are assembled in the housing10to obtain the spark plug1having the arrangement shown inFIG. 1.

After the above-described bonding step, as shown inFIG. 1, the spark plug1is held by a holding means (not shown). The CCD camera21takes an image of the distal end surface12aof center electrode12and its vicinity. The image processing apparatus3measures the position of the distal end surface12aof center electrode12based on the image data.

Next, the image processing apparatus3calculates respective target positions of pressing die44and two receiving dies48and49in the up-and-down direction with reference to the position of the distal end surface12aof center electrode12. These target positions are determined so that the dimension of the spark gap G becomes equal or close to a predetermined value after the provisional bending operation. Then, by controlling the operation of the first motor41, the pressing die44and two receiving dies48and49are shifted to their target position in the up-and-down direction (refer toFIGS. 2 and 3).

Next, the image processing apparatus3controls the operation of second motor42to shift the pressing die44and the cam plate45forward to the predetermined positions. The link levers46and47rotate in accordance with the shift movement of the pressing die44and the cam plate45. Two receiving dies48and49approach to each other in accordance with the rotations of the link levers46and47. At the time the pressing die44is brought into contact with the ground electrode13, two receiving dies48and49are positioned most closely.

After the pressing die44is brought into contact with the ground electrode13, the pressing die44further shifts forward. At this moment, the intermediate portion of ground electrode13are clamped between the intermediate portion positioning portion48aof first receiving die48and the intermediate portion positioning portion49aof second receiving die49. Thus, the ground electrode13is bent at its intermediate portion being guided by the surfaces of the receiving dies48and49. The opposed portion13bof ground electrode13formed through this bending operation extends substantially perpendicularly to the axial line X and is opposed to the distal end surface12aof center electrode12.

In this case, as shown inFIG. 5, the distal end surface13cof ground electrode13is brought into contact with the distal end surface positioning portion48bof first receiving die48and the distal end surface positioning portion49bof second receiving die49. Thus, the forward shift range of the distal end surface13cof ground electrode13is regulated by these positioning portions48band49b. In other words, the protruding amount L is regulated accurately.

Furthermore, the side surfaces13dof the ground electrode13are clamped between the opposed surface48cof first receiving die48and the opposed surface49cof second receiving die49. This prevents the ground electrode13from shifting in the direction of both side surfaces13d. The direction of both side surfaces13dis a right-and-left direction shown inFIG. 3orFIG. 6. In other words, the direction of both side surfaces13dis perpendicular to a longitudinal direction of the opposed portion13bof the ground electrode13on a surface perpendicular to the axle line X.

After the above-described provisional bending step, the dimension of spark gap G is adjusted by using the regular bending apparatus5shown inFIG. 7. Hereinafter, the regular bending step will be explained with reference to the flow of control processing shown inFIG. 8.

First, after accomplishing the provisional bending step, the dimension of spark gap G is measured based on the image data obtained from the CCD camera21(Step S10). Then, the stop position of the electric motor51for lowering the working head52is calculated based on the measured value of the spark gap G (Step S11).

Next, the electric motor51is operated until it reaches the stop position. In accordance with the operation of the motor51, the working head52is lowered to the predetermined position (indicated by an alternate long and two short dashes line inFIG. 7). The working head52presses the opposed portion13bof ground electrode13so as to adjust the dimension of spark gap G (Step S12).

Next, the electric motor51is rotated in the opposite direction to raise the working head52(Step S13). The dimension of spark gap G after accomplishing the regular bending step is measured (Step S14). Then, it is judged as to whether or not the measured value of spark gap G is within an allowable range (Step S15). When the measured value of spark gap G is smaller than the allowable range, this spark plug is excluded as a defective product (Step S16). On the other had, when the measured value of spark gap G is larger than the allowable range, the flow of control processing returns to the step S11to perform readjustment of spark gap G.

According to the above-described embodiment, the distal end surface positioning portion48bof first receiving die48and the distal end surface positioning portion49bof second receiving die49cooperatively regulate the protruding amount L of during the provisional bending operation. Thus, it becomes possible to improve the dimensional accuracy in adjusting the protruding amount L in the provisional bending.

Furthermore, the target positions of pressing die44and two receiving dies48and49are determined so that the dimension of the spark gap G after the provisional bending operation becomes equal or close to the predetermined value. This makes it possible to reduce the unevenness of spark gap dimension caused in the provisional bending. Thus, it becomes possible to improve the dimensional accuracy of spark gap G at the time the provisional bending operation is accomplished.

Furthermore, the opposed surface48cof first receiving die48and the opposed surface49cof second receiving die49clamp the side surfaces13dof ground electrode13, so as to prevent the ground electrode13from shifting in the direction of the side surfaces13d. Thus, it becomes possible to accurately perform the provisional bending operation.

Furthermore, the position of the distal end surface12aof center electrode12is measured by image processing. This makes it possible to perform accurate measurement without being influenced by the surface conditions of the distal end surface12aof center electrode12or the unevenness in the position of the distal end surface12aof center electrode12.