Electric lamp or space discharge component or device manufacturi – Process – Spark plug or spark gap making
Reexamination Certificate
2000-04-10
2003-03-18
Ramsey, Kenneth J. (Department: 2879)
Electric lamp or space discharge component or device manufacturi
Process
Spark plug or spark gap making
C219S121640, C313S141000
Reexamination Certificate
active
06533628
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of manufacturing a spark plug and the spark plug.
DESCRIPTION OF THE RELATED ART
As spark plugs serving as igniting means for internal combustion engines, spark plugs of a type having a noble-metal igniting portion have been used in recent years to improve resistance against sparks, the spark plugs being formed at the leading end of an electrode by welding a noble metal chip, the main component of which is Pt or Ir. When a noble metal chip is joined to the leading end surface of, for example, a central electrode, a method has been disclosed for example, in JP-A-6-45050 (U.S. Pat. No. 5,320,569, EP-0 583 103 B1) and JP-A-10-112374. The method has the steps of superimposing a disc-shaped metal chip on the leading end of the central electrode and irradiating the outer surface of the superimposed surfaces with a laser beam while the central electrode is being rotated so that a perimeter laser weld portion is formed.
In recent years, the temperature in the combustion chamber has been raised because the performance of the internal combustion engine has been improved. Moreover, engines of a type having a structure that the diameter of the igniting portion of the spark plug is reduced to be 2.0 mm or smaller so that the igniting portion is allowed to project into the combustion chamber in order to improve ignition easiness have been widely employed. To improve resistance of the noble metal chip against separation in the severe atmosphere for use, a method has been employed with which a noble metal chip, the main component of which is Ir or Pt, is welded to an electrode made of a heat resisting alloy composed of Ni groups or Fe groups so as to form a noble-metal igniting portion.
The noble metal chip is usually welded to the spark plug by using pulse laser beams, such as YAG laser beams. The conditions under which the laser beam is applied have been determined such that the pulse generating frequency is a relatively low value of 0.5 pulse/second or lower. The foregoing method, however, requires an excessively long time of about 15 seconds to weld a noble metal chip having a diameter of about 0.7 mm. Therefore, there arises a problem of unsatisfactory productivity for each laser welding apparatus.
Problems to be Solved by the Invention
To solve the problem of deterioration in the productivity, it is effective to raise the pulse generating frequency of the laser beams. As a result of investigations performed by the inventor of the present invention, the following problem has been found. When the pulse generating frequency of the laser beam is simply raised to improve the productivity, the heat resisting alloy composed of Ni groups or Fe groups for use as the material of the electrode and having a low heat conductivity suffers from an unsatisfactory fall in the temperature. When the leading end of the electrode has a small diameter, the fall in the temperature of the electrode is sometimes delayed as compared with application of heat which is performed for each pulse of the laser beam. As a result, the rear portions of a weld portion
10
in the circumferential direction are heated excessively, as shown in
FIGS. 13A and 13B
. Therefore, there arises a problem in that the weld portion is made to be nonuniform such that a rear weld portion
10
s
is deeply welded as compared with a front weld portion
10
p
. As an alternative to this, the width I of the weld portion is enlarged excessively. Since the weld portion
10
is constituted by an alloy of a material of a noble metal chip and a material of the electrode, the resistance against sparks of the weld portion
10
is inferior to that of only the noble metal chip. If the depth or the width of the weld portion
10
is enlarged excessively, the durability of an igniting portion
31
formed by welding the noble metal chip excessively deteriorates. In a region in which the width I of the weld portion
10
has been enlarged, the weld portion
10
is exposed to a discharging surface
31
a
in extreme cases. If the exposure is prevented, exposure of the weld portion
10
occurs in a case where the igniting portion
31
is somewhat consumed. In general, a spark plug is provided with the noble-metal igniting portion for the purpose of elongating the lifetime of the igniting portion (for example, lifetime corresponding to driving for 100,000 km to 160,000 km). If the foregoing problem arises, consumption of the exposed portion proceed. As a result, there sometimes arises a problem of misfire or the like because the spark discharge gap is enlarged in a relatively short time.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of manufacturing a spark plug capable of significantly improving efficiency of a process for welding a noble metal chip to an electrode and obtaining a uniform weld portion and a spark plug having durability of an igniting portion to a degree not heretofore attained.
To solve the problems, according to the present invention, there is provided a method of manufacturing a spark plug comprising a central electrode and a ground electrode disposed such that the side surface of the ground electrode is opposite to the leading end surface of the central electrode and having a structure that a noble-metal igniting portion having a discharge surface is provided for at least either of the central electrode or the ground electrode at a position corresponding to a spark discharge gap by welding a noble metal chip, the method of manufacturing a spark plug comprising the steps of:
constituting at least a chip securing surface forming portion of the central electrode and/or the ground electrode is formed with a heat resisting alloy having Ni or Fe as a main component thereof and superimposing the noble metal chip on a chip securing surface so as to form a superimposed assembly;
providing a perimeter laser weld portion across the noble metal chip and the chip securing surface forming portion for the outer surface of the chip constituting the superimposed assembly so that the noble metal chip is secured to the chip securing surface; and
forming the perimeter laser weld portion having a maximum outer dimension dmax plane-viewed from the chip interposing direction is smaller than 2.0 mm and so formed not to reach the discharge surface in a direction of the thickness of the noble metal chip by using, as a light source for laser welding, a pulse laser beam source having energy per pulse of 1.5 J to 6 J, a pulse length of 1 millisecond to 10 millisecond and a pulse generation frequency is 2 pulse/second to 20 pulse/second.
The chip securing surface forming portion is constituted by the heat resisting alloy, the main component of which is Fe or Ni, and the small-diameter perimeter laser weld portion having a maximum outer dimension dmax which is smaller than 2.0 mm is formed. The inventors of the present invention has energetically performed studies. Thus, the following facts have been detected. That is, it is important that the pulse generating frequency satisfies a specific range to efficiently form a uniform weld portion. Moreover, it is important that conditions of the energy per pulse of a laser beam and the length of each pulse are made to be specific values. The following fact has been detected: in a state where the foregoing conditions are set, the problem of the nonuniform weld portion and the like can be solved if a frequency higher than the frequency employed in the conventional method is employed in only a specific pulse frequency range. Thus, the present invention has been established. That is, the laser beam having energy per pulse of 1.5 J to 6 J, a pulse length of 1 millisecond to 10 milliseconds is employed. Moreover, 2 pulse/second to 20 pulse/second which is a pulse generating frequency which is considerably higher than the pulse generating frequency employed in the conventional method is employed. Thus, a perimeter laser weld portion exhibiting excellent uniformity can significantly efficiently be formed.
In this specification, the maxi
NGK Spark Plug Co. Ltd.
Ramsey Kenneth J.
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