Method for manufacturing engine poppet valve
A method for manufacturing an engine poppet valve comprising a valve head portion manufacturing operation in which hot forging is employed to form a stem/head intermediate product; a stepped stem portion manufacturing operation in which cold drawing is employed to cause a stepped stem portion comprising a first stem portion which has been made to be of decreased diameter, a step portion which has been formed by a stem member inlet surface, and a second stem portion in which there is absence of decrease in diameter and which is continuous by way of the step portion with the first stem portion to be formed from an intermediate stem portion; and a joining operation in which a stem end portion having a same outside diameter as the first stem portion is joined to a base end portion of the first stem portion.
1. A method for manufacturing an engine poppet valve having a valve head portion and a stem portion made integral by way of a neck portion that increases in diameter toward a tip end, the method for manufacturing the engine poppet valve characterized in that it comprises:
a valve head portion manufacturing operation in which hot forging is employed to form a stem/head intermediate product comprising an intermediate stem portion and a valve head portion which is made integral with the intermediate stem portion by way of a neck portion;
a hollow portion manufacturing operation in which a hole is formed at an interior of the valve head portion, the neck portion, and the intermediate stem portion from a base end portion of the intermediate stem portion of the stem/head intermediate product to form an intermediate hollow portion;
a stepped stem portion manufacturing operation in which cold drawing in which a part of the intermediate stem portion is inserted one time in press-fit fashion from a base end portion into a stem member inlet surface and a stem member compressing surface of a hollow circular hole die that has a drawing hole and at which the stem member inlet surface that has a shape that gradually narrows toward a central axis, and the stem member compressing surface that is continuous with a base end portion of the stem member inlet surface and that has a constant inside diameter which is less than an outside diameter of the intermediate stem portion but which is identical to that of the base end portion are provided at least at a portion thereof is employed to cause a stepped stem portion comprising a first stem portion which has been made to be of decreased diameter, a step portion which has been formed by the stem member inlet surface, and a second stem portion at a part in which there is absence of decrease in diameter and which is smoothly continuous by way of the step portion with the first stem portion to be formed from the intermediate stem portion and to cause a stepped hollow portion to be formed from the intermediate hollow portion at an interior of the stepped stem portion in accompaniment to decrease in diameter of the first stem portion; and
a joining operation in which a stem end portion having a same outside diameter as the first stem portion is joined to a base end portion of the first stem portion after the stepped hollow portion has been filled with coolant.
2. The method for manufacturing the engine poppet valve according to claim 1 characterized in that a fractional amount by which an outside diameter of the first stem portion is made to be of decreased diameter at the stepped stem portion manufacturing operation is not greater than 25% of that of the second stem portion.
3. The method for manufacturing the engine poppet valve according to claim 1 characterized in that a fractional amount by which an outside diameter of the first stem portion is made to be of decreased diameter at the stepped stem portion manufacturing operation is not less than 3% but not greater than 12% of that of the second stem portion.