Patent ID: RE41944

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 a wire rod 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 drawn 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 drawn wire rod to a predetermined length to produce a noble metal electric discharge chip for fixing on at least one of the a free end portion of the a central electrode of the spark plug and the an opposing portion of the an earth electrode of the spark plug opposing the free end portion of the central electrode , wherein said wire drawing comprises continuously red heating and/or white heating the wire rod of an 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 wire rod of an 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.