Forging method for a hollow article

A forging method comprises the following four steps: a first step of disposing a columnar material into a first die and extruding one end portion of the material with a first punch in a direction opposite to the advancing direction of the first punch, a second step of deepending a first blind hole in a second stepped die with a cavity having large and small internal diameter portions, and extruding the other end portion of the material into the inside of the small internal diameter portion of the cavity with a second punch which has been previously extended thereinto, a third step of fitting the first and second blind holes to third punches while exposing the small outer diameter portion from a third die and pressurizing, by means of a sleeve, only an end face portion of the small outer diameter portion formed with the second blind hole to expand the small outer diameter portion of the material toward an outer periphery, and a fourth step of punching out a blind portion of the material.

BACKGROUND AND SUMMARY OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a forging or heading method for hollow 
articles, which is suitable for providing plastic processed hollow 
articles having a circumferential groove formed in its outer surface. 
2. Description of the Related Art 
In producing a hollow article having a circumferential groove formed in the 
outer surface and a through hole formed at the center, like the one shaped 
as shown in, for example, in FIG. 6, a conventional manufacturing process 
involves a cutting process. 
In forming a circumferential groove in the outer surface of a columnar 
material by cutting work, however, excessive processing time is needed, 
reducing the productivity. As well, cutting wastes or chips are produced, 
reducing the yield and deteriorating the working environment. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a forging method for a hollow 
article which has a significantly shorter processing time, which yields 
higher productivity, which requires no cutting work, and which prevents 
degradation of the working environment. 
This object can be achieved by a forging method for a hollow article, which 
comprises four steps, namely: 
a first step of disposing a columnar material into a first stepped die with 
a cavity having large and small internal diameter portions and extruding 
one end portion of the material with a first punch in a direction opposite 
to an advancing direction of the first punch to thereby form a first blind 
hole in the center portion of said material; 
a second step of deepening the first blind hole in a second stepped die 
with a cavity having large and small internal diameter portions and a 
stepped portion until the hole reaches the proximity of the stepped 
portion, and extruding the other end portion of the material into the 
inside of the small internal diameter portion of the cavity with a second 
punch previously extended thereinto to thereby simultaneously form a 
stepped portion at the middle section of said material and a small outer 
diameter portion ad a second blind hole at the other end portion thereof 
while leaving a blind portion between the first and second blind holes; 
a third step of fitting the first and second blind hole to third punches 
while exposing the small outer diameter portion of said stepped material 
from a third die and pressurizing, by means of a sleeve, only the end face 
portion of the small outer diameter portion formed with the second blind 
hole to expand the small outer diameter portion of the stepped material 
toward the outer periphery, thereby forming a circumferential groove 
between the expanded portion and the stepped portion of the stepped 
material; and 
a fourth step of punching out the blind portion between said first and 
second blind holes of the stepped material to cause the first and second 
blind holes to communicate with each other. 
According to the forging method of this invention, a hollow article having 
an annular groove formed in the outer surface can be formed through a 
forging method which begins by forming a first tubular portion with a 
first blind hole at the front end portion of a columnar material. A second 
tubular portion having a relatively small diameter and with a second blind 
hole at the rear end portion while leaving a blind portion between the 
first and second blind holes. The second tubular portion is then expanded 
and deformed in an outwardly radial direction to form an annular groove in 
that outer surface portion of the material which corresponds to the blind 
portion. Finally, the blind hole is punched out to form a through hole. 
This significantly shortens the overall processing time of production of 
hollow articles as compared with the time required using the cutting work. 
This considerably improves the productivity of hollow articles. In 
addition, because of the elimination of cutting work, the yield is 
significantly higher and degradation of the working environment due to the 
otherwise possible generation of chips can be prevented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of this invention will be explained below referring to the 
accompanying drawings. 
FIGS. 1A through 6 illustrate one embodiment of this invention, with FIGS. 
1A to 5B illustrating essential portions of a die used for the individual 
steps. 
FIGS. 1A and 1B illustrate the preliminary step, and the forging apparatus 
used to realize the forging method of this invention. This apparatus 
comprises a front punch 11, a sleeve 12, a rear punch 14 and stepped die 
15. Sleeve 12 is disposed concentrically over the outer surface of front 
punch 11. Stepped die 15 is disposed concentrically over the outer surface 
of rear punch 14, has a mold activity defining inner wall 15a defining a 
die cavity 16 between front punch 11, rear punch 14 and a through hole 15b 
in which the rear punch 14 is inserted, and further includes stepped 
portion 15c formed between inner wall 15a and through hole 15b. 
FIGS. 2A and 2B illustrate the first step of the forging method according 
to this invention. The forging apparatus used in this step comprises a 
front punch 21 and a sleeve 22 which is disposed concentrically over the 
outer surface of front punch 21. A material pressing member 23 is disposed 
concentrically over the outer surface of front punch 21 at one end of 
sleeve 22. This forging apparatus further comprises a rear punch 24 and a 
stepped die 25, which is disposed concentrically over the outer surface of 
rear punch 24, has an inner wall 25a that defines a die cavity 26 between 
the front punch 21 and rear punch 24. A stepped portion 25c is formed 
between inner wall 25a and through hole 25b. 
FIGS. 3A and 3B illustrate the second step. The forging apparatus used in 
this step comprises a front punch 31 and a sleeve 32 which is disposed 
concentrically over the outer surface of front punch 31. A material 
pressing member 33 is disposed concentrically over the outer surface of 
front punch 31 at one end of sleeve 32. This pressing deformation 
apparatus further comprises a rear punch 34 and a stepped die 35. This die 
35 is disposed concentrically over the outer surface of rear punch 34, has 
a large inner wall 35a, which defines a large diameter die cavity 36a in 
the proximity of an opening portion of die 35, and further includes a 
small inner wall 35b, which defines a cylindrical small diameter die 
cavity 36b in cooperation with rear punch 34. A stepped portion 35c is 
formed between large inner wall 35a and small inner wall 35b. 
FIGS. 4A and 4B illustrate the third step. The forging apparatus used in 
this step comprises a front punch 41 and a sleeve 42, which is disposed 
concentrically over the outer surface of front punch 41, has a small inner 
wall 42a, a conical inclined face 42b both for restricting the movement of 
a material, and a large inner wall 42c that defines a die cavity 46. This 
forging apparatus further comprises a rear punch 44 and a material 
supporting sleeve 47, which is disposed concentrically over the outer 
surface of rear punch 44. A die 45 is disposed concentrically over the 
outer surfaces of rear punch 44 and sleeve 47, is always pushed by a 
compression coil spring 48, and has a large inner wall 45a, which 
restricts the movement of the material. A casing 49 is provided with an 
inner collar portion 49a that guides die 45 and restricts the movement of 
the die 45 caused by coil spring 48. 
FIGS. 5A and 5B illustrate the fourth step. The forging apparatus used in 
this step comprises a front punch 51 and a sleeve 52 which is disposed 
concentrically over the outer surface of front punch 51. A hollow-article 
pressing member 53 is disposed concentrically over the outer surface of 
front punch 51 at one end of sleeve 52. This forging apparatus further 
comprises a rear punch 54 and a material supporting sleeve 57, with a 
discharge hole 54 through which a cutaway piece 50c, cut by front punch 
51, is discharged. A die 55 is disposed concentrically over the outer 
surfaces of rear punch 54 and sleeve 57 and has an inner wall 55a that 
restricts the movement of material. 
The following describes the procedures for manufacturing a hollow article 
50, which has a through hole 50a at the center portion and a 
circumferential groove 50f in the outer surface, as shown in FIG. 6. 
In the preliminary step shown in FIGS. 1A and 1B, columnar material 1, 
provided by cutting a rod material, for example, with a given size as 
shown in FIG. 1A, is disposed in cavity 16 of stepped die 15. Front punch 
11 and sleeve 12 are moved to compress columnar material 1 (see FIG. 1A) 
as shown in FIG. 1B, thereby providing heading material 10 that is 
plastically deformed to have the same shape as the cavity of stepped die 
15. This material 10 is taken out by moving front punch 11 and sleeve 12 
in the reverse direction and moving rear punch 14 forward. 
In the first step shown in FIGS. 2A and 2B, material 10 prepared in the 
preliminary step is disposed in die cavity of another stepped die 25 (see 
FIG. 2A). Front punch 21, sleeve 22 and material pushing member 23 are 
moved to push one end of the material 10 backward, thereby providing blind 
hole material 20 with a first blind hole 20a formed at this end. Then, 
front punch 21 is alone withdrawn from blind hole material 20 and the free 
end of rear punch 24 is moved forward beyond the opening of stepped die 25 
to remove the material 20. 
In the second step shown in FIGS. 3A and 3B, blind hole material 20 
prepared in the first step is disposed in large diameter die cavity 36a in 
third stepped die 35 from the other end i.e., the end where the first 
blind hole 20a is not formed, as shown in FIG. 3A. At the same time, front 
punch 31, sleeve 32 and material pressing member 33 are moved to push 
backward blind hole material 20 from one end surface thereof, thereby 
deepening the blind hole 20a until it reaches the proximity of stepped 
portion 35c of die 35. As a result, a first blind hole 30a is formed and 
annular portion 30b is formed in large die cavity 36a. At the same time, 
or slightly afterward, this material 20 is pushed backward by rear punch 
34, previously extended into die 35. This provides a cylindrical portion 
30e having a second blind hole 30d at the inner wall, and leaves a blind 
portion 30c between first and second blind holes 30a and 30d within small 
diameter die cavity 36b, thereby providing stepped material 30 as shown in 
FIG. 3B. 
Then, after front punch 31 is withdrawn from first blind hole 30a, sleeve 
32 and material pushing member 33 and rear punch 34 are moved forward to 
remove stepped material 30 from die 35. 
In the third step shown in FIGS. 4A and 4B, stepped material 30 prepared in 
the second step is turned upside down in the diagrams and is in this state 
fit through material restricting face 45a of die 45 from the end face of 
annular portion 30b formed with first blind hole 30a. And, first blind 
hole 30a is fitted over rear punch 44 to restrict the inner and outer 
surfaces of annular portion 30b, i.e., the surface of first blind hole 30a 
and the outer surface of stepped material 30. Subsequently, as also shown 
in FIG. 4A, front punch 41 is fitted in second blind hole 30d of 
cylindrical portion 30e, thereby restricting the inner wall of cylindrical 
portion 30d or the outer surface of this portion 30d. Sleeve 42 is then 
moved to pressurize the end face portion of cylindrical portion 30e. 
During the pressurizing action, the free end portion of cylindrical 
portion 30e is restricted between small inner wall 42a formed in sleeve 42 
and front punch 41. Annular die cavity 46 is formed between conical 
inclined face 42b and large inner wall 42c continuous to small inner wall 
42a of sleeve 42 and the outer surface of cylindrical portion 30e. Blind 
portion 30c is left at the base of cylindrical portion 30e in stepped 
material 30. Therefore, the movement of sleeve 42 causes plastic 
deformation along conical inclined face 42b formed in sleeve 42, over the 
region from the restricted free end portion of cylindrical portion 30e to 
the unrestricted portion thereof. As a result, a cylindrical portion 40e 
having the shape as shown in FIG. 4B can be provided. Since it is more 
difficult to plastically deform blind portion 30c than cylindrical portion 
30e, a circumferential groove 40f is formed at that outer surface of 
cylindrical portion 40e which corresponds to blind portion 40c (30c), thus 
providing material 40 with a groove as shown in FIG. 4B. 
According to this embodiment, die 45 is always pressed by compression coil 
spring 48 and the amount of movement of sleeve 42 suitable for plastic 
deformation of cylindrical portion 30e can be properly adjusted. 
After the formation of grooved material 40, front punch 41 is alone 
withdrawn from cylindrical portion 40e. Sleeve 42 is then moved backward 
and rear punch 44 is moved to remove annular portion 40b from die 45, thus 
taking out grooved material 40. 
In the fourth step shown in FIGS. 5A and 5B, grooved material 40 prepared 
in the third step is put through material restricting face 55a of die 55 
and material supporting sleeve 57 is fitted to the opening portion of 
first blind hole 40a of annular portion 40b, as shown in FIG. 5A. Then, as 
shown in FIG. 5B, front punch 51 is moved to cut blind portion 40c, and 
the resultant cutaway piece 50c is discharged through discharge hole 54 to 
thereby form through hole 50a at the center of grooved material 40. 
Through the aforementioned first to fourth steps, hollow article 50 (see 
FIG. 6) having through hole 50a at the center and circumferential groove 
50f at the outer surface thereof can be provided. 
Although the foregoing description illustrates the step-by-step formation 
of hollow article 50 from columnar material 1 through five steps, 
actually, the individual steps are executed in parallel; in the 
preliminary step, front punch 11 reciprocates to prepare columnar material 
1 while in the fourth step front punch 51 reciprocates to complete hollow 
article 50.