Patent ID: 7131191

Claim:
A method of manufacturing noble metal electric discharge chips for a spark plug, said spark plug comprising: a central electrode, an insulator covering a circumference of the central electrode with a free end portion of the central electrode being exposed, and an earth electrode fixed to the main metal member and having a opposing portion forming a discharge gap g between the earth electrode and the free end portion of the central electrode, the method comprising: wire drawing an ingot of an iridium alloy containing not lower than 0.5 percent by mass and not higher than 35 percent by mass of a metal component other than iridium with a wire drawing die and rolling to form a wire rod having a cross-sectional area of not smaller than 0.05 mm 2 and not larger than 1.2 mm 2 , and cutting the wire rod to a predetermined length to produce a noble metal electric discharge chip for fixing on at least one of the free end portion of the central electrode and the opposing portion of the earth electrode, wherein said wire drawing comprises continuously red heating and/or white heating the iridium alloy at a portion thereof within a heating region extending from a work inserting surface of the die to a position of predetermined distance opposite the wire drawing direction, the iridium alloy being heated to a temperature of not lower than 1000° C. and not higher than 1150° C. at a temperature measuring position which is 20 mm away from the work inserting surface in a direction opposite the wire drawing direction, the temperature in a region extending from the temperature measuring position to the work inserting surface of the die is set to not lower than 1000° C., and the wire drawing rate is not lower than 1300 mm/min and not higher than 1600 mm/min.