Method for manufacturing engine poppet valve

A method for manufacturing a poppet valve for an engine having a valve head portion and a stem portion made integral by way of a neck portion that increases in diameter as one proceeds toward a tip end, the method for manufacturing the engine poppet valve characterized in that it comprises:

• a forging operation in which an intermediate member made of metal at which the valve head portion and an intermediate stem portion are made integral by way of the neck portion is formed;
• a diameter-decreasing operation in which an intermediate stem portion of the intermediate member is inserted between/among a plurality of diameter-decreasing tools that are formed so as to have stem member compressing surfaces and stem member inlet surfaces formed so as to be progressively distant from the stem member compressing surfaces and toward the tip end and that are arranged at a plurality of circumferentially equipartite locations about a circumference of the intermediate stem portion, application at the intermediate stem portion from a base end portion of compressive forces which are inwardly directed in a radial direction from the compressing surfaces of the respective diameter-decreasing tools which have been made to come in contact with a part of the intermediate stem portion that has been made to rotate while being displaced in relative fashion in a direction along a central axis thereof causing the part of the intermediate stem portion to be decreased in diameter from the base end portion and causing a main body portion of a first stem portion to be formed at the intermediate member and also causing formation by the compressing surfaces of a stepped portion which is continuous with the main body portion, and formation, by an absence of decrease in diameter of a remaining portion at the intermediate stem portion, of a second stem portion which is broader in girth than the main body portion and which is continuous with the neck portion and which is also continuous with the main body portion by way of the stepped portion;
• a joining operation in which a stem end member having a same outside diameter as the main body portion is joined to a base end portion of the main body portion;
• and
• a hole forming operation in which an intermediate hollow portion is formed in a region extending along 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 intermediate member is carried out after the forging operation but before the diameter-decreasing operation;
• at the diameter-decreasing operation, a first hollow portion is formed at an interior of the main body portion simultaneous with the formation of the main body portion of the first stem portion in accompaniment to the decreasing of the diameter of the part of the intermediate stem portion by the stem member compressing surfaces, a reduced-diameter portion that is continuous with the first hollow portion is formed at an interior of the stepped portion simultaneous with the formation of the stepped portion by the stem member inlet surfaces, and a second hollow portion of large inside diameter that is continuous with the first hollow portion by way of the reduced-diameter portion is formed at an interior of the second stem portion which is of broad girth due to the absence of decrease in diameter of the remaining portion at the intermediate stem portion;
• and
• at the joining operation, the stem end member is joined to the base end portion of the main body portion after the first hollow portion and the second hollow portion have been filled with coolant.