Spark plug and method of manufacturing the same

Abstract

A spark plug for internal combustion engines provided with a noble metal tip on the pointed top of a spark plug center electrode. The noble metal tip is welded to the center electrode top end by heating and pressing the portion to be welded. After the welding, the center electrode is shaped to provide the pointed top. The noble metal tip is a platinum alloy consisting of platinum of 85 through 70 in weight percentage and iridium of 15 through 30 in weight percentage.

What is claimed is: 
     
       1. A method of making a spark plug for internal combustion engines comprising the steps of: providing a center electrode made of an electrically conductive base metal;   providing a center electrode made of an electrically conductive base metal having a uniform diameter, and flat top end surface which faces said ground electrode;   resistance-welding a noble metal tip to said flat top end surface of the center electrode by supplying electric current through said center electrode;   circumferentially cutting the center electrode to provide at and adjacent said flat top end surface thereof a pointed top portion which is smaller in diameter than the other portion of the center electrode; and   providing an insulator securely encasing the center electrode and electrically insulating the center electrode from the ground electrode.   
     
     
       2. A method in accordance with claim 1 wherein said welding step includes heating and pressing together the noble metal tip and the top end surface of the center electrode. 
     
     
       3. A method in accordance with claim 2 wherein the step of cutting the center electrode includes the step of forming a tapered portion on the center electrode joining the uniform diameter portion thereof to the smaller in diameter portion thereof. 
     
     
       4. A method in accordance with claim 3 including the step of providing the ground electrode with a further noble metal facing the noble metal tip of the center electrode. 
     
     
       5. A method in accordance with claim 1 wherein the noble metal tip is formed of a platinum alloy including iridium of 15 through 30 in weight percentage. 
     
     
       6. A method in accordance with claim 1 wherein the noble metal tip consists of platinum of 85 through 70 in weight percentage and iridium of 15 through 30 in weight percentage. 
     
     
       7. A method of making a center electrode of a spark plug comprising the steps of: providing an electrically conductive longitudinal base metal having an end surface;   providing a thin noble metal tip having a diameter larger than the thickness thereof;   resistance-welding said noble metal tip onto the end surface of said base metal while pressing the same to the end surface of said base metal; and   circumferentially cutting after said welding step longitudinal side wall of said base metal at a portion adjacent to the end surface so that the diameter of the base metal at the end surface is smaller than that at the other portion of the base metal.   
     
     
       8. A method according to claim 7, wherein: said base metal providing step includes a step of shaping the end surface flat, and wherein said welding step includes a step of pressing said noble metal tip to the flat end surface of said base metal.   
     
     
       9. A method according to claim 7, wherein: said noble metal tip consists of platinum of 70 through 80 in weight percentage and iridium of 30 through 20 in weight percentage.   
     
     
       10. A method according to claim 7, wherein: said noble metal tip providing step includes a step of shaping said noble metal tip into a disk shape having a uniform thickness and diameter. .Iadd.   
     
     
       11.  A method in accordance with claim 1, wherein said noble metal tip has a thickness less than or equal to 0.5 mm. .Iaddend. .Iadd.12. A method in accordance with claim 1, wherein said step of providing a center electrode comprises providing a center electrode having a diameter at the top end surface thereof sufficiently great that said noble metal tip can be completely welded to said flat top end surface without buckling the center electrode. .Iaddend. .Iadd.13. A method in accordance with claim 12, wherein before said welding step, said diameter at said top end surface is 
     
     
        at least about 2.5 mm. .Iaddend. .Iadd.14.  A method in accordance with claim 1, wherein a diameter of the top portion of the center electrode is larger than the diameter of the noble metal tip so that after the welding step, the noble metal tip is embedded in the flat top end surface of the 
     
     
        center electrode. .Iaddend. .Iadd.15.  A method in accordance with claim 1, wherein the noble metal tip has a surface welded to the flat top end surface and has on its opposite face another surface facing the ground electrode so that a spark gap is formed between the another surface and the ground electrode. .Iaddend.