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A method of producing a metallic shell for a spark plug, including steps of preparing a first intermediate article by using a first die including a first large diameter cavity larger in diameter than a metal blank, a first small diameter cavity smaller in diameter than the metal blank, a curved tape

A method of producing a metallic shell for a spark plug, including steps of preparing a first intermediate article by using a first die including a first large diameter cavity larger in diameter than a metal blank, a first small diameter cavity smaller in diameter than the metal blank, a curved tapered cavity between the first large diameter cavity and the first small diameter cavity, and an oil drain cavity between the first large diameter cavity and the tapered cavity and substantially same in diameter as the metal blank, wherein the metal blank is inserted in the first large diameter cavity and extruded to allow a configuration of the metal blank to conform to the first large diameter cavity, the oil drain cavity, the tapered cavity and the first small diameter cavity while being pressed by a punch, and preparing a second intermediate article by using a second die.

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1. A method of producing a metallic shell for a spark plug from a cylindrical metal blank cut to a predetermined length, the metallic shell including a screw shaft portion to be screwed into a spark plug mounting tap hole of an engine, a stop portion that is disposed on one axial end side of the scr

1. A method of producing a metallic shell for a spark plug from a cylindrical metal blank cut to a predetermined length, the metallic shell including a screw shaft portion to be screwed into a spark plug mounting tap hole of an engine, a stop portion that is disposed on one axial end side of the screw shaft portion and larger in diameter than the spark plug mounting tap hole, and a through hole extending through the metallic shell in an axial direction of the metallic shell, the method comprising the steps of: preparing a first intermediate article including a large diameter portion and a tapered portion which are to be later formed into the stop portion of the metallic shell and a small diameter portion which is to be later formed into the screw shaft portion of the metallic shell, by using a first die including a first large diameter cavity which is larger in diameter than the metal blank and configured to form a portion of the metal blank which is to be later formed into the stop portion of the metallic shell, a first small diameter cavity which is smaller in diameter than the metal blank and configured to form a portion of the metal blank which is to be later formed into the screw shaft portion of the metallic shell, a curved tapered cavity between the first large diameter cavity and the first small diameter cavity which has one axial end connected with an axial end of the first small diameter cavity in an axially opposed relation thereto, and an oil drain cavity between the first large diameter cavity and the curved tapered cavity which is substantially same in diameter as the metal blank, wherein the metal blank is inserted in the first large diameter cavity and extruded to allow a configuration of the metal blank to conform to the first large diameter cavity, the oil drain cavity, the curved tapered cavity and the first small diameter cavity while being pressed by a punch; andpreparing a second intermediate article by using a second die including a second large diameter cavity larger in diameter than the large diameter portion of the first intermediate article and a second small diameter cavity into which the small diameter portion of the first intermediate article is insertable, the second large diameter cavity and the second small diameter cavity being continuously connected with each other to form a stepped cavity, wherein the large diameter portion and the tapered portion of the first intermediate article are inserted in the second large diameter cavity of the second die and the small diameter portion of the first intermediate article is inserted in the second small diameter cavity of the second die, and then the first intermediate article is pressed by a punch such that a hole later serving as the through hole of the metallic shell is formed, and at the same time, the large diameter portion and the tapered portion of the first intermediate article are expanded in a radial direction thereof and allowed to conform to the second large diameter cavity,said method further comprising after the second intermediate article preparing step, a step of preparing a third intermediate article by subjecting the second intermediate article to extrusion. 2. The method as claimed in claim 1, further comprising after the third intermediate article preparing step, a step of preparing a fourth intermediate article by subjecting the third intermediate article to punching. 3. The method as claimed in claim 2, further comprising after the fourth intermediate article preparing step, a step of preparing a fifth intermediate article by subjecting the fourth intermediate article to extrusion. 4. The method as claimed in claim 3, further comprising after the fifth intermediate article preparing step, a step of subjecting a small diameter portion of the fifth intermediate article to rolling to form a male-thread portion which corresponds to the screw shaft portion of the metallic shell. 5. The method as claimed in claim 3, further comprising after the fifth intermediate article preparing step, a step of subjecting a large diameter portion of the fifth intermediate article to machining to form a grooved portion corresponding to a grooved portion between the stop portion and a tool engagement portion of the metallic shell which is engageable with a tool. 6. The method as claimed in claim 1, wherein the metal blank has a locally recessed portion on an outer circumferential surface thereof. 7. The method as claimed in claim 1, wherein the metal blank is made of a low carbon steel.