Electric lamp or space discharge component or device manufacturi – Process – Spark plug or spark gap making
Reexamination Certificate
2002-04-24
2004-02-10
Sember, Thomas M. (Department: 2875)
Electric lamp or space discharge component or device manufacturi
Process
Spark plug or spark gap making
C313S141000
Reexamination Certificate
active
06688932
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a spark plug and a caulking metallic mold for use with the method.
BACKGROUND OF THE INVENTION
The metal shell of the spark plug is typically composed of iron material such as carbon steel. For the corrosion prevention, a metal plating layer of zinc or nickel is coated on the surface of the metal shell, or a chromate film may be further applied on the surface formed with the metal plating layer. Of these surface treatments, the chromate film containing chromium (VI) (hereinafter referred to as a chromium (VI) film) as chromium constituent has a particularly excellent corrosion resistance, and is suitably employed for the sparkplug. However, the chromium (VI) film contains chromium (VI) as its chromium constituent, and tends to be gradually shunned in these days when there is a rising tide of environmental protection. Therefore, the chromium (VI) film is examined to be done away with in the future.
Thus, the chromate film containing little chromium (VI), namely, the chromate film containing chromium (III) as most of chromium constituent (hereinafter referred to as a chromium (III) film) has been developed from relatively early on. This chromate film can be formed in a treatment bath having a relatively small content of chromium (VI), or may be formed in a treatment bath not containing chromium (VI) at all.
The chromium (III) film as described above was difficult to form in large thickness, and to attain a more excellent anticorrosion than the chromate (VI) film. However, with the development of treatment bath, the thickness of chromate film could be increased, and the excellent anticorrosion obtained. Accordingly, the chromate (III) film tends to be suitably used, with the chromate (VI) film, to prevent corrosion in the metal shell of the spark plug.
Generally, a method for attaching the metal shell of the spark plug to the outside of an insulator inserted inside and having a central electrode disposed at a top end of the metal shell involves caulking and fixing a rear end periphery (portion to be caulked) of the cylindrical metal shell that is curved toward the outer circumferential face of the insulator.
However, if the metal shell having the chromate (III) film formed on its surface is employed, various dimensions of the metal shell often deviate from the tolerance after caulking. The deviation of various dimensions from the tolerance (hereinafter referred to as a dimensional deviation) may be confirmed even when other surface treatment including applying the chromate (VI) film is made on the metal shell, but was especially conspicuous when the chromate (III) film was formed. This dimensional deviation impedes the sufficient effect of caulking. Particularly, if the dimensional deviation of the opposite side of the tool-engaging portion or the caulking height is excessive, the bulk density of talc packed between the inner circumferential face of the metal shell and the insulator or the air-tightness of the spark plug itself is unfavorably decreased. Thus, to suppress this dimensional deviation, the caulking metallic mold useful in caulking and fixing the metal shell to the insulator has a deep compression scroll of the portion to be caulked. By deepening the compression scroll of the portion to be caulked, the opposite side size of the tool-engaging portion is kept from expanding more easily.
SUMMARY OF THE INVENTION
However, the above caulking metallic mold is effective at the early time of use, but less effective as the caulking of the metal shell is repeated, resulting in remarkable dimensional deviation of the metal shell after caulking. This dimensional deviation was especially conspicuous in forming a zinc plating layer as the substrate metal plating layer on the metal shell, and forming the chromate (III) film thereon, but tended to occur when other surface treatments were made.
It is an object of the present invention to provide a method for manufacturing a spark plug and a caulking metallic mold for use with the method in which the deviation of various dimensions of the metal shell after caulking is suppressed within a tolerance even though the metal shell is caulked and fixed repeatedly to the insulator.
To achieve the above object, according to the present invention, there is provided a method for manufacturing a spark plug in which a portion to be caulked of a cylindrical metal shell having a tool-engaging portion to be attached on an engine is caulked and fixed around an outer circumferential face of an insulator extending axially and inserted into the metal shell, characterized in that a caulking metallic mold for caulking and fixing the portion to be caulked is formed with a hard carbon film mainly composed of amorphous carbon phase on a surface contact and sliding with the portion to be caulked of the metal shell.
Further, there is provided a caulking metallic mold for use with the method for manufacturing the spark plug, in which the caulking metallic mold is employed to caulk and fix a portion to be caulked of a cylindrical metal shell having a tool-engaging portion to be attached on an engine around an outer circumferential face of an insulator extending axially and inserted into the metal shell, characterized in that the caulking metallic mold has a hard carbon film mainly composed of amorphous carbon phase formed on a surface contact and/or sliding with the portion to be caulked of the metal shell. The surface contains a first layer containing chromium or titanium and a second layer containing silicon or germanium, and the hard carbon film is formed on the second layer. By forming the hard carbon film on an intermediate layer (the first layer and the second layer) that is a double layer structure, the adhering strength of the hard carbon film to a main body can be heightened, and the hard carbon film can be prevented from peeling in a caulking process for a long period of time. The caulking metallic mold contains the surfaces for contacting and/or sliding on both upper and lower surfaces, and each of the surfaces for contacting and/or sliding can be used to the caulking process if the upper and lower surfaces are turned around. Since the hard carbon film (and the intermediate layer) can be formed on the upper and lower surfaces, a cost is not especially taken for composing such a structure. Accordingly, if the caulking metallic mold is made reversible, a cost-up is not taken for producing the metallic mold, but one metal mold can be used twice by turning it around, and the cost for the metal mold may be saved cheaply.
The deviation of various dimensions of the metal shell after caulking arises because undesired stress is applied on the metal shell at the time of caulking to induce an undesired deformation of the metal shell. To reduce the undesired stress, it is effective to increase the sliding property between a surface of the caulking metallic mold contact and sliding with the metal shell and the metal shell. Thus, the present inventors, as a result of minute examination, have found that if the caulking metallic mold having a hard carbon film mainly composed of amorphous carbon phase made on the surface of the caulking metallic mold contact and sliding with the portion to be caulked of the metal shell is employed, the sliding at the time of caulking is excellently conducted, and the deviation of various dimensions of the metal shell after caulking can be effectively suppressed, and have completed this invention.
As used in this specification, the “hard carbon film mainly composed of amorphous carbon phase” means that the skeleton structure of carbon mainly constituting the film is amorphous, and its Vickers hardness is 1500 kg/mm
2
or greater. The preferable range of the thickness of the hard carbon film is 0.6 to 1.2 &mgr;m. If being less than 0.6 &mgr;m, an effect by forming the hard carbon film is less, while being more than 1.2 &mgr;m, an adhering strength of the hard single film itself decreases, and the film is easy to peel. The hardness of film is measured by, f
Ito Hirohito
Matsutani Wataru
Nasu Hiroaki
Sugimoto Makoto
Uemura Yoshito
Dong Dalei
Morgan & Lewis & Bockius, LLP
NGK Spark Plug Co. Ltd.
Sember Thomas M.
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