Abstract: A spark plug is provided, for a provisional bending process for manufacturing the spark plug, as a work in a condition where an earth electrode is straight and substantially in parallel with an axial line of a center electrode. In the provisional bending process, two searchers individually facing the tip of the center electrode with the tip located therebetween are arranged, positions of the searchers in a first direction perpendicular to the axial line being adjusted for every spark plug. Then a bending punch is driven to press a second end-surface of the other end of the earth electrode down to the searchers so that the earth electrode is provisionally bent at a substantially perpendicular angle to the axial line, the second end-surface being opposite to the first end-surface. Preferably, before the provisional bending process, positioning the work and correcting the position and tilt of the work are performed.

Abstract: A spark plug is provided after a provisional bending process, during which the earth electrode is provisionally bent at almost 90 degrees. The earth electrode, one of both ends is joined to a housing member, has first and second end-side surfaces located mutually back to back and a tip surface connecting both end-side surfaces. A noble metal chip is mounted on the first end-side surface. A bending punch is set, which has a first pressing surface to be fitted to the second end-side surface located and a second pressing surface to be fitted to a tip surface. The bending punch is first moved so that the second pressing surface comes in contact with the tip surface of the earth electrode, and then moved so that the first pressing surface presses the second end-side surface in the axial direction of the center electrode. Accordingly, a main bending process is performed.

Abstract: A spark plug is provided, for a provisional bending process for manufacturing the spark plug, as a work in a condition where an earth electrode is straight and substantially in parallel with an axial line of a center electrode. In the provisional bending process, two searchers individually facing the tip of the center electrode with the tip located therebetween are arranged, positions of the searchers in a first direction perpendicular to the axial line being adjusted for every spark plug. Then a bending punch is driven to press a second end-surface of the other end of the earth electrode down to the searchers so that the earth electrode is provisionally bent at a substantially perpendicular angle to the axial line, the second end-surface being opposite to the first end-surface. Preferably, before the provisional bending process, positioning the work and correcting the position and tilt of the work are performed.

Abstract: A spark plug is provided after a provisional bending process, during which the earth electrode is provisionally bent at almost 90 degrees. The earth electrode, one of both ends is joined to a housing member, has first and second end-side surfaces located mutually back to back and a tip surface connecting both end-side surfaces. A noble metal chip is mounted on the first end-side surface. A bending punch is set, which has a first pressing surface to be fitted to the second end-side surface located and a second pressing surface to be fitted to a tip surface. The bending punch is first moved so that the second pressing surface comes in contact with the tip surface of the earth electrode, and then moved so that the first pressing surface presses the second end-side surface in the axial direction of the center electrode. Accordingly, a main bending process is performed.