Abstract: A method for manufacturing a spark plug including a reference standard imaging step of imaging a plurality of reference standards to obtain a plurality of reference images, each reference standard having a reference part with a predetermined known dimension; a reference standard measuring step of measuring a measured dimension or a measured number of pixels of the reference part in the plurality of reference images; a regression line deriving step of obtaining a regression line by the least squares method from: the measured dimension or the measured number of pixels of the plurality of reference standards; and the known dimensions; and a determining step of determining whether a target part in a target image is within a predetermined range based on a correction value obtained by correcting a measured dimension of a target part by using a relational expression describing the regression line.

Abstract: A method for manufacturing a spark plug including a reference standard imaging step of imaging a plurality of reference standards to obtain a plurality of reference images, each reference standard having a reference part with a predetermined known dimension; a reference standard measuring step of measuring a measured dimension or a measured number of pixels of the reference part in the plurality of reference images; a regression line deriving step of obtaining a regression line by the least squares method from: the measured dimension or the measured number of pixels of the plurality of reference standards; and the known dimensions; and a determining step of determining whether a target part in a target image is within a predetermined range based on a correction value obtained by correcting a measured dimension of a target part by using a relational expression describing the regression line.

Abstract: Standard image data representing standard image including image of slit of a standard device is produced by photographing the standard device including the slit which includes a standard slit segment having a predetermined width. A width of the standard slit segment in the standard image is determined by analyzing the standard image data. Gap image data representing a gap image which is an image including an image of a discharge gap of a spark plug is produced by photographing the spark plug. A distance of the discharge gap in the gap image is determined by analyzing the gap image data. By using the distance of the discharge gap in the gap image and the width of the standard slit segment in the standard image, it is determined whether an actual distance of the discharge gap is within a predetermined allowable range or not.

Abstract: Standard image data representing standard image including image of slit of a standard device is produced by photographing the standard device including the slit which includes a standard slit segment having a predetermined width. A width of the standard slit segment in the standard image is determined by analyzing the standard image data. Gap image data representing a gap image which is an image including an image of a discharge gap of a spark plug is produced by photographing the spark plug. A distance of the discharge gap in the gap image is determined by analyzing the gap image data. By using the distance of the discharge gap in the gap image and the width of the standard slit segment in the standard image, it is determined whether an actual distance of the discharge gap is within a predetermined allowable range or not.

Abstract: A method for manufacturing the spark plug comprising bending a ground electrode that is joined to a metal shell, along a first die, the first die having an end portion to be pressed against an inner surface of the ground electrode, and a flat surface portion contiguous to the end portion via a curved portion bulging outward, the flat surface portion being formed to be orthogonal to an axial line and includes a first step of pressing the end portion of the first die against the ground electrode.

Abstract: A spark plug includes a center electrode, a ground electrode formed of a metal material containing 95% or more of nickel and a substantially cylindrical metal shell having a front end face to which one end of the ground electrode is welded. In the spark plug, the conditions: 0.15 mm?BD?0.40 mm; and (EW2?EW1)/EW1?0.1 are satisfied where BD is a depth from the front end face of the metal shell to a portion of the ground electrode embedded most deeply in the metal shell; EW1 is a width of a portion of the ground electrode located closest to a portion of the ground electrode deformed by the welding; and EW2 is a width of the portion of the ground electrode deformed by the welding at the front end face of the metal shell.

Abstract: A spark plug includes a portion from a position at a distance of 0.1 mm from a base end to a distal end that is categorizable into a high hardness portion and a low hardness portion using a hardness distribution of a ground electrode, the high hardness portion being a portion from the position at a distance of 0.1 mm from a base end to a position at a distance of 0.1×n (mm) from a base end, the low hardness portion being a portion from a position at a distance of 0.1×(n+1) (mm) from the base end to the distal end, where “n” is a natural number. The low hardness portion includes a portion that has a largest curvature in the ground electrode. A highest hardness of the low hardness portion is lower than a lowest hardness of the high hardness portion.

Abstract: A spark plug includes a portion from a position at a distance of 0.1 mm from a base end to a distal end that is categorizable into a high hardness portion and a low hardness portion using a hardness distribution of a ground electrode, the high hardness portion being a portion from the position at a distance of 0.1 mm from a base end to a position at a distance of 0.1×n (mm) from a base end, the low hardness portion being a portion from a position at a distance of 0.1×(n+1) (mm) from the base end to the distal end, where “n” is a natural number. The low hardness portion includes a portion that has a largest curvature in the ground electrode. A highest hardness of the low hardness portion is lower than a lowest hardness of the high hardness portion.

Abstract: A spark plug (1) includes a metallic shell (3) and a ground electrode (27) resistance-welded to the metallic shell (3). The ground electrode (27) is formed from a metal material in which precipitates PR containing at least one of an oxide, an intermetallic compound, etc., are precipitated in grain boundaries. The diameter of an equivalent circle of a precipitate PR having the greatest area is 50 ?m or less, and the shortest distance between the precipitates PR is 2 ?m or greater. The area occupied by the precipitates PR on the section of the heat-affected zone 27A is equal to 65% or more the area occupied by the precipitates PR (the diameter of an equivalent circle of a precipitate PR having the greatest area) on a section of the normal zone 27B.

Abstract: A spark plug includes a center electrode, a ground electrode formed of a metal material containing 95% or more of nickel and a substantially cylindrical metal shell having a front end face to which one end of the ground electrode is welded. In the spark plug, the conditions: 0.15 mm?BD?0.40 mm; and (EW2?EW1)/EW1?0.1 are satisfied where BD is a depth from the front end face of the metal shell to a portion of the ground electrode embedded most deeply in the metal shell; EW1 is a width of a portion of the ground electrode located closest to a portion of the ground electrode deformed by the welding; and EW2 is a width of the portion of the ground electrode deformed by the welding at the front end face of the metal shell.

Abstract: A spark plug (1) includes a metallic shell (3) and a ground electrode (27) resistance-welded to the metallic shell (3). The ground electrode (27) is formed from a metal material in which precipitates PR containing at least one of an oxide, an intermetallic compound, etc., are precipitated in grain boundaries. The diameter of an equivalent circle of a precipitate PR having the greatest area is 50 ?m or less, and the shortest distance between the precipitates PR is 2 ?m or greater. The area occupied by the precipitates PR on the section of the heat-affected zone 27A is equal to 65% or more the area occupied by the precipitates PR (the diameter of an equivalent circle of a precipitate PR having the greatest area) on a section of the normal zone 27B.