Abstract: 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.

Abstract: An object of the present invention is to efficiently adjust a positional difference (measured along the direction perpendicular to the center electrode axis) between the center electrode (CE) axis and the center of the earth electrode (EE). The CE tip and EE tip surface are illuminated from the front of the EE tip surface, thereby picking up their images. The image processing unit calculates the positional difference between the CE axis and EE center. Here, the EE center is defined by an area centroid of the EE tip surface area, thereby improving a measurement accuracy. The adjustment unit connected with the image processing unit causes the actual positional difference to decrease by moving EE by the that calculated positional difference, taking a prescribed spring-back into consideration. EE is moved by a jig moved by a screw rotated by a motor.

Abstract: 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.

Abstract: An object of the present invention is to efficiently adjust a positional difference (measured along the direction perpendicular to the center electrode axis) between the center electrode (CE) axis and the center of the earth electrode (EE). The CE tip and EE end tip surface are illuminated from the front of the EE tip tip surface, thereby picking up their images. The image processing unit calculates the positional difference between the CE axis and EE center. Here, the EE center is defined by an area centroid of the EE tip surface area, thereby improving a measurement accuracy. The adjustment unit connected with the image processing unit causes the actual positional difference to decrease by moving EE by the that calculated positional difference, taking a prescribed spring-back into consideration. EE is moved by a jig moved by a screw rotated by a motor.

Abstract: First and second guides of a guide jig are removed from a tip in a direction normal to an axial direction of the tip. This surely prevents the guide jig from touching and moving the tip when the guide jig departs from the tip. Accordingly, the welding operation can be performed properly under a condition that the tip is accurately positioned on a ground electrode. Thus, it becomes possible to reduce a manufacturing error of the discharge gap formed between a center electrode and the ground electrode.

Abstract: First and second guides of a guide jig are removed from a tip in a direction normal to an axial direction of the tip. This surely prevents the guide jig from touching and moving the tip when the guide jig departs from the tip. Accordingly, the welding operation can be performed properly under a condition that the tip is accurately positioned on a ground electrode. Thus, it becomes possible to reduce a manufacturing error of the discharge gap formed between a center electrode and the ground electrode.

Abstract: A composite chip formed by joining a discharging layer and a heat stress relieving layer at a joint interface therebetween beforehand is provided on at least one of a central electrode and a ground electrode in its discharge portion made of an electrode material. The discharging layer is made of a precious metal or a precious metal alloy having superior spark- and wear-resistance, and the heat stress relieving layer is made of a metal or an alloy having a linear expansion coefficient between those of the discharging layer and the electrode material. Formed at the joint interface between both the discharging layer and the heat stress relieving layer through mutual diffusion of those materials developed when the two layers are joined to each other is a diffusion layer, in which concentrations of materials of both the layers are continuously changed. A thickness of the diffusion layer is not less than 3 .mu.m in a state that the composite chip is welded to the discharge portion.