Method of manufacturing sparkplugs

A method of manufacturing a spark plug which comprises:

• an insulator having an axial hole extending therethrough in an axial direction;
• a center electrode provided at a front end portion of the axial hole;
• a substantially tubular metallic shell which holds the insulator;
• a ground electrode whose one end portion is attached to a front end portion of the metallic shell and whose other end portion faces a front end portion of the center electrode;
• and
• a noble metal tip provided on a surface of the ground electrode which faces the front end portion of the center electrode, and having a discharge surface which forms a spark discharge gap in cooperation with the center electrode;
• the method comprising:
• a fusion-zone formation step of forming a fusion zone through radiation of a high-energy beam to a boundary between the ground electrode and the noble metal tip, said fusion-zone formation step including continuously radiating the high-energy beam to said boundary from a substantially vertical direction while moving the high-energy beam relative to the boundary, or vice versa, in a width direction of the ground electrode, wherein the width direction is maintained within a plane substantially parallel to a plane containing the boundary,
• wherein the fusion-zone formation step forms the fusion zone such that when the fusion zone is projected in a direction perpendicular to the discharge surface, 80% or more of an area of an overlap between the ground electrode and the noble metal tip overlaps the projected fusion zone and
• such that a shape of the fusion zone as viewed from a direction perpendicular to the discharge surface is substantially symmetrical with respect to a centerline perpendicular to a width direction of the ground electrode and passing through a center of the noble metal tip.