Method for manufacturing hollow engine valve

1. A method for manufacturing a hollow engine valve having a hollow portion formed from a stem part to a widening portion of a valve head part, characterized in that the method comprises:

a first rotary plastic working step of performing rotary plastic working on a semifinished hollow engine valve body which includes a stem part and a valve head part forming part connected to the stem part, to thereby reduce an outer diameter of the stem part while maintaining the size of an inner diameter thereof as is, the semifinished hollow engine valve body having a columnar hole portion formed from the stem part to a widening portion of the valve head part forming part;

a necking step of, after the first rotary plastic working step, performing necking on the semifinished hollow engine valve body to thereby reduce an outer diameter and an inner diameter of the stem part, the necking using as many dies as the number of narrowing operations to thereby narrow the widening portion of the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely, the dies being configured to press the widening portion and the stem part and having die holes with inner diameters decreasing slightly and successively according to a sequence of the operations; and

a sealing step of, after the necking step, sealing a tip of the stem part to thereby obtain the hollow engine valve.

2. The method for manufacturing a hollow engine valve according to claim 1 , characterized in that the rotary plastic working is any one of: rotary swaging including inserting a core inside the hole portion of the semifinished hollow engine valve body, and striking an outer peripheral portion of the stem part with a swaging die while axially rotating the semifinished hollow engine valve body and the core together; and spinning including inserting a core inside the hole portion of the semifinished hollow engine valve body, and pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body and the core together.

3. The method for manufacturing a hollow engine valve according to claim 1 , characterized in that the rotary plastic working is a combination of:

performing any one of rotary swaging including striking an outer peripheral portion of the stem part with a swaging die while axially rotating the semifinished hollow engine valve body, and spinning including pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body; and

performing, after the any one of the rotary swaging and the spinning, any one of rotary swaging including inserting a core inside the hole portion of the semifinished hollow engine valve body, and striking the outer peripheral portion of the shaft part with a swaging die while axially rotating the semifinished hollow engine valve body and the core together, and spinning including inserting a core inside the hole portion of the semifinished hollow engine valve body, and pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body and the core.

4. The method for manufacturing a hollow engine valve according to claim 1 , characterized in that the method further comprises a second rotary plastic working step of performing rotary plastic working on the semifinished hollow engine valve body after performing the necking thereon, to thereby reduce the outer diameter of the stem part while maintaining the size of the inner diameter thereof as is.

5. The method for manufacturing a hollow engine valve according to claim 4 , characterized in that the rotary plastic working in the second rotary plastic working step is any one of: rotary swaging including inserting a core inside the hole portion of the semifinished hollow engine valve body, and striking an outer peripheral portion of the stem part with a swaging die while axially rotating the semifinished hollow engine valve body and the core together; and spinning including inserting a core inside the hole portion of the semifinished hollow engine valve body, and pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body and the core together.

6. The method for manufacturing a hollow engine valve according to claim 4 , characterized in that the rotary plastic working in the second rotary plastic working step is performed only on a portion of the stem part other than an upper portion thereof, so that the outer diameter of the stem part is reduced while the size of the inner diameter thereof is maintained as is except the upper portion, whereas the outer diameter of the upper portion of the stem part is maintained at the size thereof before the rotary plastic working to thereby form a large diameter portion, and

in the sealing step, the large diameter portion is pressed to seal a tip of the stem part and thereby obtain the hollow engine valve.

7. A method for manufacturing a hollow engine valve having a hollow portion formed from a stem part to a widening portion of a valve head part, characterized in that the method comprises:

a first necking step of performing necking on a semifinished hollow engine valve body which includes a stem part and a shaft head part forming part connected to the stem part and has a columnar hole portion formed from the stem part to a widening portion of the valve head part forming part, to thereby reduce an outer diameter and an inner diameter of the stem part, the necking using as many dies as the number of narrowing operations to thereby narrow the widening portion of the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely, the dies being configured to press the widening portion and the stem part and having die holes with inner diameters decreasing slightly and successively according to a sequence of the operations;

a rotary plastic working step of, after the first necking step, performing rotary plastic working on the semifinished hollow engine valve body to thereby reduce an outer diameter of the stem part while maintaining the size of an inner diameter thereof as is; and a sealing step of, after the rotary plastic working step, sealing a tip of the stem part to thereby obtain the hollow engine valve.

8. The method for manufacturing a hollow engine valve according to claim 7 , characterized in that the rotary plastic working is any one of: rotary swaging including inserting a core inside the hole portion of the semifinished hollow engine valve body, and striking an outer peripheral portion of the stem part with a swaging die while axially rotating the semifinished hollow engine valve body and the core together; and spinning including inserting a core inside the hole portion of the semifinished hollow engine valve body, and pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body and the core together.

9. The method for manufacturing a hollow engine valve according to claim 7 , characterized in that the method further comprises a second necking step of, after performing the rotary plastic working on the semifinished hollow engine valve body, performing necking to thereby reduce the outer diameter and the inner diameter of the stem part, the necking using as many dies as the number of narrowing operations to thereby narrow the widening portion of the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely, the dies being configured to press the widening portion and the stem part and having die holes with inner diameters decreasing slightly and successively according to a sequence of the operations.

10. The method for manufacturing a hollow engine valve according to claim 7 , characterized in that

the rotary plastic working is performed only on a portion of the stem part other than an upper portion thereof, so that the outer diameter of the stem part is reduced while the size of the inner diameter thereof is maintained as is except the upper portion, whereas the outer diameter of the upper portion of the stem part is maintained at the size thereof before the rotary plastic working to thereby form a large diameter portion, and

in the sealing step, the large diameter portion is pressed to seal a tip of the shaft part and thereby obtain the hollow engine valve.