Method for manufacturing noble metal electric discharge chips for spark plugs

Abstract

In a wire drawing process for forming a wire rod containing iridium as a main component and which further contains nickel and at least one of platinum, rhodium and ruthenium, and having a cross-sectional area of not smaller than 0.05 mm 2 and not larger than 1.2 mm 2 . The worked material is continuously heated red hot and/or white hot in a heating region 103 extending up to 60 mm from the work inserting surface 101 a of a die 101 in a direction opposite that in which the work 102 moves, the work being heated to a temperature of 1000 to 1150° C. at a temperature measuring position 105 which is removed from the work inserting surface 101 a by 20 mm. Furthermore, the temperature in the region 106 extending from the temperature measuring position 105 to the work inserting surface 101 a is set to not lower than 1000° C., and the wire drawing rate is set to 300 to 1600 mm/min.

1. 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.  
 
     
     
       2. The method as claimed in  claim 1 , wherein the metal component other than iridium comprises nickel. 
     
     
       3. The method as claimed in  claim 1 , which further comprises welding said noble metal electric discharge chip to at least one of each of  the free end portion of the central electrode and the opposing portion of the earth electrode. 
     
     
       4. The method as claimed in  claim 1 , which comprises fixing said noble metal electric discharge chip to both each of  the free end portion of the central electrode and the opposing portion of the earth electrode. 
     
     
       5. The method as claimed in  claim 1 , wherein said heating region extends from a work inserting surface of the die to within a distance of 60 mm opposite the wire drawing direction. 
     
     
       6. The method as claimed in  claim 1 , wherein said wire rod of an iridium alloy is not heated outside of said heating region extending from a work inserting surface of the die to a position of predetermined distance opposite the wire drawing direction. 
     
     
       7. The method as claimed in  claim 1 , which comprises drawing said wire rod a plurality of times to form said drawn wire rod having a cross- sectional area of not smaller than  0 . 05  mm   2    and not larger than  1 . 2  mm   2   .