Spark plug and manufacturing method thereof

A method for manufacturing the spark plug comprising:

• a center electrode extending in an axial direction;
• an insulator having an axial bore extending in the axial direction and holding the center electrode in the axial bore at a front end side;
• a cylindrical metal shell radially surrounding the insulator so as to hold the insulator and having a front end constituent face at a front end side opening thereof, wherein the front end constituent face is comprised of a visible external surface as viewed from a front end side in the axial direction of the spark plug, and has a plurality of faces;
• and
• a ground electrode having (a) one end joined to at least one of the plurality of faces constituting the front end constituent face and (b) an other end bent toward an inner circumferential of the metal shell so as to form a spark discharge gap with a front end portion of the center electrode, the ground electrode further comprising (i) an extending portion extending in the axial direction from the one end to the other end and (ii) a bending portion between the extending portion and the other end,
• wherein a virtual sphere having a radius of 1.2 mm is neither in contact with the center electrode nor the insulator, and is assumed to be in contact with an inner face of the bending portion of the ground electrode that faces toward the center electrode,
• wherein the plurality of faces constituting the front end constituent face of the metal shell include a first face adjoining at least a part of an inner circumferential face of the metal shell in the axial direction, said first face being an inclined face having a diameter that increases from a rear end side of the spark plug to the front end side of the spark plug in the axial direction of the spark plug, and
• wherein, as viewed on a cross-sectional outline of the metal shell including the axis thereof, a length of the first face is the longest of the plurality of faces constituting the front end constituent face of the metal shell, said method comprising:
• an inclined face formation step for forming the front end constituent face, said inclined face formation step including chamfering in a circumferential direction at least a part of an end face of a front end side opening of a cylindrical metal shell intermediate body that serves as an original form of the metal shell, the first face formed by said chamfering having a diameter that is enlarged toward the front end side from the rear end side in the axial direction of the spark plug, and a second face formed on a remaining external face of the front end portion of the metal shell intermediate body which is not chamfered;
• a joint face formation step for forming a first joint face and a second joint face that are to be joined together with the first face and the second face of the metal shell, respectively, in an end face of the one end of the ground electrode;
• an electrode joining step for joining the one end of the ground electrode to the front end constituent face of the metal shell intermediate body while the extending portion of the ground electrode extends along the axial direction of the cylindrical metal shell intermediate body that serves as an original form of the metal shell;
• and
• a gap formation step for forming a spark discharge gap between the other end of the ground electrode and the front end portion of the center electrode by orientating the other end of the ground electrode toward the front end portion of the center electrode.

A method for manufacturing the spark plug comprising:

• a center electrode extending in an axial direction;
• an insulator having an axial bore extending in the axial direction and holding the center electrode in the axial bore at a front end side;
• a cylindrical metal shell radially surrounding the insulator so as to hold the insulator and having a front end constituent face at a front end side opening thereof, wherein the front end constituent face is comprised of a visible external surface as viewed from a front end side in the axial direction of the spark plug, and has a plurality of faces;
• and
• a ground electrode having (a) one end joined to at least one of the plurality of faces constituting the front end constituent face and (b) an other end bent toward an inner circumferential of the metal shell so as to form a discharge gap with a front end portion of the center electrode, the ground electrode further comprising (i) an extending portion extending in the axial direction from the one end to the other end and (ii) a bending portion between the extending portion and the other end,
• wherein a virtual sphere having a radius of 1.2 mm is neither in contact with the center electrode nor the insulator, and is assumed to be in contact with an inner face of the bending portion of the ground electrode that faces toward the center electrode,
• wherein the plurality of faces constituting the front end constituent face of the metal shell include a first face adjoining at least a part of an inner circumferential face of the metal shell in the axial direction, said first face being an inclined face having a diameter that increases from a rear end side of the spark plug to the front end side of the spark plug in the axial direction of the spark plug, and
• wherein, as viewed on a cross-sectional outline of the metal shell including the axis thereof, a length of the first face is the longest of the plurality of faces constituting the front end constituent face of the metal shell, said method comprising:
• an electrode joining step for joining the one end of the ground electrode to an end face of a front end side opening of a cylindrical metal shell intermediate body that serves as an original form of the metal shell, while the extending portion of the ground electrode extends along the axial direction of the cylindrical metal shell intermediate body;
• an inclined face formation step for forming the first face having a diameter that increases from the rear end side to the front end side in the axial direction, said inclined face formation step including chamfering, in the circumferential direction, at least a part of the end face of the front end side opening of the cylindrical metal shell intermediate body where the ground electrode is to be joined while avoiding a joint portion with the ground electrode;
• and,
• a gap formation step for forming a spark discharge gap between the other end of the ground electrode and the front end portion of the center electrode by orientating the other end of the ground electrode toward the front end portion of the center electrode.