Metal deforming – By use of closed-die and coacting work-forcer – Cup or shell drawing
Reexamination Certificate
2002-07-03
2004-09-21
Compton, Eric (Department: 3726)
Metal deforming
By use of closed-die and coacting work-forcer
Cup or shell drawing
C029S03400A, C029S890150, C072S347000, C072S356000, C445S007000
Reexamination Certificate
active
06792786
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to an improved fabrication method of a metal shell installed on a spark plug which may be employed in automotive internal combustion engines.
2. Background Art
Typical plug metal shells are installed on spark plugs by staking an annular wrapping end of the metal shell on a porcelain insulator of the spark plug. The wrapping end of the metal shell is usually made by cold forging. The metal shell also has a hollow cylindrical base portion and a hexagonal boss which are also shaped by the cold forging. The hollow cylindrical base portion has threads formed in an exterior surface thereof by rolling.
FIG. 6
illustrates a conventional forging process for fabricating a metal shell of a spark plug, as disclosed in Japanese Patent First Publication No. 7-16693, which forms a wrapping end
11
and a small-diameter base portion
12
of the metal shell in a single process. The formation of the wrapping end
11
is accomplished by striking a large-diameter head portion
13
of a hollow cylindrical workpiece with a cylindrical punch
50
to decrease the diameter of the head portion
13
. An outer wall of the small-diameter base portion
12
is shaped by a die
52
.
The simultaneous formation of the wrapping end
11
and the small-diameter base portion requires a punch holder
51
. It is impossible for the punch holder
51
to have an outer diameter greater than that of the large-diameter head portion of the workpiece. The punch holder
51
must, therefore, be formed to be thin, so that it has a low strength. Forging the workpiece requires exertion of a large pressure on the punch holder
51
, which will lead to a problem that cracks or physical deformation of the punch holder
51
arise within a short period of time.
Further, it is difficult to form a large rounded inner wall in an end of the punch holder
51
because it is thin, which results in a drop in fluidity of material of the workpiece around the end of the punch holder
51
. This causes a boundary between inner walls of the wrapping end
11
and the large-diameter head portion
13
to be subjected to shrinkage, which may result in formation of cracks near the boundary between the wrapping end
11
and the large-diameter head portion
13
.
SUMMARY OF THE INVENTION
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a fabrication method for fabricating a metal shell which is less susceptible to cracks when installed on a spark plug and has an increased service life.
According to one aspect of the invention, there is provided an improved fabrication method of a metal shell to be installed on a park plug which may be employed in automotive engines. The metal shell has a given length and is made up of a small-diameter portion, a large-diameter portion, and a wrapping portion. The wrapping portion is to be wrapped by staking about a porcelain insulator of a spark plug to achieve installation of the metal shell on the spark plug. The method comprises the steps of: (a) preparing a cylindrical workpiece which has a given length with a first and a second end opposed to each other; (b) preparing a punch and a die; (c) placing the workpiece in the die and pressing the workpiece with the punch from the second end of the workpiece to shape the wrapping portion of the metal shell on a side of the first end of the workpiece in a first cold forging process; and (d) processing the workpiece to shape the small-diameter portion of the metal shell on a side of the second end of the workpiece in a second cold forging process.
In the preferred mode of the invention, the method further comprises the step of forming threads on an outer peripheral wall of the small-diameter portion for installation of the spark plug.
The method further comprises the step of processing the workpiece to form a large-diameter portion on the side of the second end and a small-diameter portion on the side of the first end prior to the first cold forging process in which the wrapping portion is formed.
In the first cold forging process, a portion of the workpiece on the side of the first end is pressed within the die stepwise to decrease, in sequence, the portion of the workpiece in outer diameter to shape the wrapping portion of the metal shell.
A hexagonal boss may be formed on the large-diameter portion of the workpiece in the first cold forging process.
The hexagonal boss may alternatively be formed on the large-diameter portion of the workpiece in a third process different from the first and second cold forging process.
REFERENCES:
patent: 3186209 (1965-06-01), Friedman
patent: 4352283 (1982-10-01), Bailey
patent: 4416141 (1983-11-01), Nippert
patent: 4882925 (1989-11-01), Ichie et al.
patent: 4932251 (1990-06-01), Okada et al.
patent: 4939829 (1990-07-01), Maki et al.
patent: 5088311 (1992-02-01), Inoue
patent: 6357274 (2002-03-01), Tanaka et al.
patent: 56-059555 (1981-05-01), None
patent: 57-014440 (1982-01-01), None
patent: 58-065541 (1983-04-01), None
patent: 01-166847 (1989-06-01), None
patent: 04-197544 (1992-07-01), None
patent: 7-16693 (1995-01-01), None
patent: 2001-121240 (2001-05-01), None
Denso Corporation
Nixon & Vanderhye PC
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