Method for manufacturing cylindrical body having different diameters by cold forging

Abstract

A multi-diameter tubular body is cold-forged by forming a large-diameter hole portion in a formed body having a preliminary hole through subjection of the preliminary hole to deep hole forming and by punching out the bottom surface of the large-diameter hole portion to thereby form a small-diameter hole portion. Since a punch having a central protrusion on its forward end surface is used, an internal flaw is generated by dead metal in the inner circumferential surface of a depression, formed by the protrusion, in the bottom surface of the large-diameter hole portion. An outside diameter of a protrusion of a deep hole forming punch is rendered smaller than an inside diameter of the small-diameter hole portion to be formed later by punching out the bottom surface of the large-diameter hole portion. As a result, the internal flaw is removed when the small-diameter hole portion is formed.

Claims

1. A method for manufacturing, by cold forging, a multi-diameter tubular body having a forward end and an axially extending through hole, the multi-diameter tubular body including a small-diameter hole portion having a small inside diameter and a large-diameter hole portion having a relatively large inside diameter, which are coaxially arranged rearward from the forward end of the multi-diameter tubular body, the multi-diameter tubular body further having a rearward-facing annular ledge surface tapering forward and located at a boundary between the small-diameter hole portion and the large-diameter hole portion, the method comprising: providing a columnar starting material having a rear end surface; subjecting the starting material to one or a plurality of forming steps to form a preliminary hole for the large-diameter hole portion in the rear end surface of the columnar starting material; thrusting a deep hole forming punch into the preliminary hole, the deep hole forming punch having a forward end surface and an annular inclined surface which is located in a region of the forward end surface extending along an outer circumference of the forward end surface and which is inclined toward a center, the deep hole forming punch further having a protrusion which is located coaxially at the center of the forward end surface and inward of the annular inclined surface, the protrusion protruding forward and having a predetermined outside diameter, the annular inclined surface of the deep hole forming punch forming an annular surface which is to become the rearward-facing annular ledge surface, and the protrusion of the deep hole forming punch forming a depression inward of the annular surface; and driving a punch for punching into a bottom surface of the large-diameter hole portion so as to punch out the bottom surface of the large-diameter hole portion forming the small-diameter hole portion and leaving the rearward-facing annular ledge surface, the method being characterized in that the outside diameter of the protrusion of the deep hole forming punch is smaller than the inside diameter of the small-diameter hole portion.

2. The method for manufacturing a multi-diameter tubular body by cold forging according to claim 1, wherein, when the punch for punching is driven, an outer circumferential surface of the punch for punching is guided by an inner circumferential surface of the large-diameter hole portion formed through deep hole forming by the deep hole forming punch.

3. The method for manufacturing a multi-diameter tubular body by cold forging according to claim 1, wherein the punch for punching has a communication hole extending therethrough for establishing communication between a forward end surface thereof and a rear region thereof so as to prevent the forward end surface of the punch from closing the depression formed by the protrusion.

4. The method for manufacturing a multi-diameter tubular body by cold forging according to claim 2, wherein the punch for punching has a communication hole extending therethrough for establishing communication between a forward end surface thereof and a rear region thereof so as to prevent the forward end surface of the punch from closing the depression formed by the protrusion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 Sectional views for explaining an embodiment of a manufacturing method of the present invention; specifically, a deep hole forming (large-diameter hole portion forming) step (a fourth step) in which a preliminary hole of a third-step formed body (FIG. 6C), or an intermediate formed body yielded after formation of the preliminary hole, undergoes deep hole forming, including the illustration of a die, etc.

(2) FIG. 2 Enlarged view of the P2 region in FIG. 1 for explaining the position of flaw (crack) generated in a rearward-facing annular ledge surface of a fourth-step formed body yielded after deep hole forming (large-diameter hole portion forming) in FIG. 1.

(3) FIG. 3 Sectional views for explaining a punching (small-diameter hole forming) step (a fifth step) in which a fourth-step formed body yielded after deep hole forming (large-diameter hole portion forming) in FIG. 1 undergoes punching, including the illustration of a die, etc.

(4) FIG. 4 Sectional view for explaining an example of a conventional gas sensor, showing the schematic configuration of the gas sensor.

(5) FIG. 5 Sectional view of a pre-machining workpiece, or a formed body (a multi-diameter tubular body) formed by cold forging, of a metallic shell (a finished component) for use in the gas sensor of FIG. 4.

(6) FIG. 6 Half-sectional views showing an example of steps for forming the formed body by cold forging (the multi-diameter tubular body) of FIG. 5, and adapted to explain formed bodies formed in the respective steps.

(7) FIG. 7 Sectional views for explaining, of the conventional steps for forming the formed body (FIG. 6E) of FIG. 5, a deep hole forming (large-diameter hole forming) step (a fourth step) in which the preliminary hole of the third-step formed body (FIG. 6C), or an intermediate formed body yielded after formation of the preliminary hole, undergoes deep hole forming (large-diameter hole portion forming), including the illustration of a die, etc.

(8) FIG. 8 Sectional views for explaining, of the conventional steps for forming the formed body (FIG. 6E) of FIG. 5, a punching (small-diameter hole forming) step (a fifth step) in which a fourth-step formed body yielded after deep hole forming (large-diameter hole portion forming) undergoes punching, including the illustration of a die, etc.

(9) FIG. 9 Enlarged view of the P1 region in FIG. 7 for explaining flaw generated in a rearward-facing annular ledge surface of the fourth-step formed body yielded after deep hole forming (large-diameter hole portion forming) in FIG. 7.

MODES FOR CARRYING OUT THE INVENTION

(10) An embodiment of a method for manufacturing a multi-diameter tubular body by cold forging according to the present invention will next be described in detail with reference to FIGS. 1 to 3. The multi-diameter tubular body manufactured in the present embodiment is the same as that shown in FIG. 5 (FIG. 6E). Also, the steps of manufacturing the multi-diameter tubular body are the same as those for forming the respective formed bodies (A to E) shown in FIG. 6. Specifically, by subjecting a columnar starting material to a plurality of forming steps as shown in FIGS. 6A and 6B, there is formed a third-step formed body 10c having a preliminary hole 25c of a large-diameter hole portion 25d formed in its rear end surface as shown in FIG. 6C; subsequently, in a fourth step, the preliminary hole 25c is subjected to deep hole forming by thrusting a deep hole forming punch thereinto, thereby forming the large-diameter hole portion 25d and thus yielding a fourth-step formed body 10d; then, in a fifth step, a bottom surface 27d of the large-diameter hole portion 25d is punched out by driving a punch for punching, thereby forming a small-diameter hole portion 21e and thus yielding a multi-diameter tubular body 10e. Thus, the manufacturing method is basically the same as the conventional manufacturing method, but differs only in that the outside diameter D1 of a protrusion 125d protruding forward from the forward end surface of a deep hole forming punch 120d used to subject the preliminary hole 25c to deep hole forming in the fourth step (see FIG. 7) is rendered substantially smaller by an appropriate amount than the inside diameter D2 of the small-diameter hole portion 21e. The process of yielding a fifth-step formed body (a finished multi-diameter tubular component) from the third-step formed body 10c shown in FIG. 6, together with dies, punches, etc., used in the fourth and fifth step, will be further described in detail.

(11) First, the third-step formed body 10c to be formed in the fourth step will be described (see FIG. 6, etc.). The third-step formed body 10c is formed from a second-step formed body 10b in such a manner as to have a forward circular columnar portion, a rearward cylindrical portion, and a portion which protrudes from an outer circumferential surface located axially between the forward circular columnar portion and the rearward cylindrical portion and which is to become a polygonal portion 15 of a metallic shell 11. The rearward cylindrical portion corresponds to a rearward tubular portion 17e of the multi-diameter tubular body (see FIG. 5) 10e. As shown in FIG. 6C, the third-step formed body 10c has a recess in its rear end surface (center), and the recess serves as the preliminary hole 25c of a large-diameter hole portion 25. The preliminary hole 25c consists of holes having different diameters such that a rearward hole is slightly larger in diameter than a forward hole; the forward hole (the polygonal portion 15) has a diameter approximately equal to the inside diameter of the large-diameter hole portion to be formed by subsequent deep hole forming; and the rearward hole has an inner diameter slightly larger than that of the forward hole so as to be approximately equal to the inside diameter of the rearward tubular portion 17e. In the present embodiment, a bottom surface 27c of the preliminary hole 25c is located at an axially intermediate position of the polygonal portion 15 and slightly tapers forward toward its center.

(12) Meanwhile, in the fourth step, the fourth-step formed body (FIG. 6D) 10d is formed from the third-step formed body (FIG. 6C) 10c by deep hole forming in such a manner that: a forward end portion of a portion (a cylindrical portion) located forward of the polygonal portion 15 is extruded forward while being reduced in diameter, and the portion is also extruded rearward by deep hole forming; deep hole forming also causes rearward extrusion to thereby form the large-diameter hole portion 25d; and a portion (a tubular portion to be threaded) 14d between the polygonal portion 15 and a forward end portion (a small-diameter portion) 12d is elongated to thereby form a tubular portion greater in diameter than the small-diameter portion 12d of a multi-diameter cylinder. The bottom surface 27d of the large-diameter hole portion 25d to be punched out for forming a small-diameter hole portion is located slightly rearward (upward in the illustration) of the forward end of the relatively large diameter portion to be threaded.

(13) As shown in FIG. 1, a die 100d used to form the fourth-step formed body 10d from the third-step formed body 10c having the preliminary hole by deep hole forming has a cavity which receives a forward portion of the third-step formed body 10c, including the polygonal portion 15, with a very small gap formed therebetween and whose profile includes a shape 105d corresponding to a forward portion of the fourth-step formed body 10d. The cavity has a circular hole 106d and a circular hole 107d so as to allow formation of the tubular portion 11e of the multi-diameter cylinder. The circular hole 106d is located on the forward end side and has a small diameter. The circular hole 107d is located rearward of the circular hole 106d and has a diameter greater than that of the circular hole 106d. The circular hole 107d can receive the outer circumferential surface of a forward end portion of the third-step formed body 10c and is adapted to form the tubular portion 11e to be threaded. The die 100d further has a polygonal hole located rearward of the two circular holes and capable of receiving the polygonal portion 15 of the third-step formed body 10c with a very small gap formed therebetween. These holes (the cavity) of the die 100d assume the form of a concentrically multi-diameter hole which increases in diameter from the forward side to the rearward side. In consideration of rearward extrusion (elongation) of material in the course of deep hole forming, the intermediate circular hole 107d is set shorter in axial length than a tubular portion of the tubular portion 11e which is to be threaded. For supporting the forward end surface of the fourth-step formed body 10d and ejecting a formed body, a knock pin (a circular columnar body) 140d is disposed in the circular hole 106d in a lower region of the die 100d.

(14) The deep hole forming punch 120d used in the fourth step has a shaft portion (a circular columnar portion) 130d formed in such a manner as to be capable of forming the large-diameter hole portion 25d having a predetermined length and a predetermined diameter and to be capable of being inserted into the preliminary hole 25c. The forward end surface of the deep hole forming punch 120d has an annular inclined surface 124d extending along its outer circumference and inclined toward the center for forming a rearward-facing annular ledge surface 24e of the multi-diameter tubular body (see FIGS. 5 and 6E) 10e. The forward end surface of the deep hole forming punch 120d also has a circular protrusion 125d having a predetermined outside diameter Dl and protruding forward slightly (0.2 mm to 0.5 mm) coaxially at its center from the annular inclined surface 124d. The protrusion 125d is formed in such a manner that its outer circumferential edge is located inside (on a center side of) the inner circumferential edge of the rearward-facing annular ledge surface 24e of the multi-diameter tubular body 10e (the inner circumferential surface of the small-diameter hole portion 21e). That is, in contrast to the conventional case in which the outside diameter D1 of the protrusion 125d is approximately equal to the inside diameter D2 of the small-diameter hole portion 21e to be formed later by punching, the outside diameter D1 of the protrusion 125d is set smaller by an appropriate amount than the inside diameter D2 of the small-diameter hole portion 21e. The annular inclined surface 124d has the same taper as that of the rearward-facing annular ledge surface 24e of the multi-diameter tubular body 10e, and, in the present embodiment, the protrusion 125d is also tapered such that the center of its forward end surface slightly protrudes.

(15) In the fourth step, the third-step formed body 10c is placed in the die 100d (left figure (A) of FIG. 1), and, as shown in the right figure (B) of FIG. 1, the deep hole forming punch 120d is thrusted by a predetermined amount into the preliminary hole (recess) 25c of the third-step formed body 10c, thereby yielding the fourth-step formed body 10d. At the initial stage of thrusting, the protrusion 125d of the forward end surface of the deep hole forming punch 120d is pressed against the bottom surface 27c of the preliminary hole 25c to thereby form a depression corresponding to the protrusion 125d in the bottom surface 27c. As thrusting proceeds, a forward end portion of the third-step formed body 10c is extruded forward within the forward circular hole 106d of the die 100d; and, at the same time, deep hole forming proceeds, whereby a rear portion, including the polygonal portion 15, of the third-step formed body 10c is extruded relatively rearward. As a result of completion of thrusting of the deep hole forming punch 120d over a predetermined stroke (by a predetermined amount), forming of the large-diameter hole portion (deep hole forming) ends, thereby yielding the fourth-step formed body 10d shown in the right figure (B) of FIG. 1. In the thus-yielded fourth-step formed body 10d, the profile of the forward end surface of the deep hole forming punch 120d is transferred to the bottom surface 27d of the large diameter hole portion 25d of the fourth-step formed body 10d. Therefore, a depression 28d is formed by the protrusion 125d at the center of the bottom surface 27d; an annular surface 24d which is to become the rearward-facing annular ledge surface 24 is formed in a region of the bottom surface 27d located outward of the depression 28d; and an intersecting region between the annular surface 24d and an inner circumferential surface 29d of the depression 28d becomes a convex corner (see FIG. 2) along the circumferential direction. Such features are similar to the case of the conventional process. As a result, as shown in the enlarged view of FIG. 2, the inner circumferential surface 29d of the depression 28d may have the internal flaw K, such as a crack or wrinkles, caused by dead metal and extending outward from the bottom surface (or its vicinity) of the depression 28d under the annular surface 24d located outward of the inner circumferential surface 29d. Such occurrence of flaw is similar to the case of the conventional process.

(16) However, as mentioned above, the outside diameter D1 of the protrusion 125d of the deep hole forming punch 120d is smaller than the inside diameter D2 of the small-diameter hole portion 21e to be formed in the next step (fifth step) by punching by use of a punch for punching (see FIG. 2). As a result, the internal flaw K, such as a crack, is removed. Specifically, similar to the case of the conventional process, the fourth-step formed body 10d is placed in the die 100e for use in punching in the fifth step as shown in the left figure (A) of FIG. 3, and then, as shown in the right figure (B) of FIG. 3, a punch 120e for punching which has a forward end outside diameter corresponding to the inside diameter D2 of the small-diameter hole portion 21e is driven into the bottom surface 27d so as to form the small-diameter hole portion 21e. As a result, a portion of the annular surface 24d located outward of the depression 28d formed by the protrusion 125d and inward of a surface which is to become the rearward-facing ledge surface 24 (a portion between the inside diameter D2 of the small-diameter hole portion 21e and the outside diameter D1 of the protrusion 125d) is punched out and removed as a punching scrap U. That is, as shown in the enlarged view of FIG. 2, the internal flaw K, such as a crack, etc., present outward of the depression 28d and inside the diameter D2 is removed together with the punching scrap U when the small-diameter hole portion 21e is formed (by punching). Thus, in the multi-diameter tubular body 10e yielded after the punching process, the existence of an internal flaw in the rearward-facing annular ledge surface 24e can be reliably reduced as compared with the conventional manufacturing method in which D1 and D2 are approximately equal to each other.

(17) The die 100e used in the fifth step has substantially the same structure as that of the die used in the fourth step; i.e., the die 100e has a cavity which receives the fourth-step formed body 10d with approximately no gap formed therebetween. However, a forward end support (a knock pin) 150e has such a pipe structure as not to interfere with the punch 120e for punching. In the punch 120e for punching, a rearward shaft portion 125e is rendered greater in outside diameter than a forward shaft portion (a circular columnar portion) having a punching diameter, so as to have such an outside diameter as to be guided by the inner circumferential surface of the large-diameter hole portion 25d in the punching process. The punch 120e has a lubricant discharge hole H which has openings (not shown) in the forward end surface and a rearward side surface and establishes communication between the openings.

(18) In the present embodiment described above, the outside diameter (dimension) D1 of the protrusion 125d of the deep hole forming punch 120d may be determined as mentioned above on the basis of the degree of dependence of the depth of the internal flaw K extending radially outward from the inner circumferential surface of the depression 28d formed by the protrusion 125d; i.e., the degree of dependence of a region of generation of the internal flaw in a surface which is to become the annular ledge surface 24, on dimensions, shape, structure, etc., of the multi-diameter tubular body 10e, which degree of dependence is found by, for example, cutting a formed test sample. The outside diameter (dimension) D1 of the protrusion 125d may be determined such that punching scrap to be removed contains the generated internal flaw K as much as possible in forming the small-diameter hole portion 21e by punching (simultaneous punching).

(19) In the present embodiment, since the rearward shaft portion 125e of the punch 120e for punching has such an outside diameter as to be guided by the internal circumferential surface of the large-diameter hole portion 25d in the punching process, punching can be performed accurately and stably without involvement of any eccentricity. Although the dimensional relation D2>D1 is employed, since the punch 120e for punching has the lubricant discharge hole H establishing communication between the forward end surface thereof and a rearward side surface thereof, the forward end surface of the punch 120e can be prevented from closing the depression 28d formed by the protrusion. Thus, since lubricant remaining in the depression 28d can be discharged rearward through the communication hole H, there is prevented roughening of texture of a formed surface, which could otherwise result from confinement of lubricant.

(20) Meanwhile, the multi-diameter tubular body 10e of the present embodiment has a small-diameter tubular portion (a small-diameter portion) 12e having a relatively small outside diameter in a forward end part of the forward tubular portion 11e thereof. In the course of forming the large-diameter hole portion 25d, a small diameter portion which is to become the small-diameter tubular portion (the small-diameter portion) 12e is thrusted into the forward small circular hole 106d of the die 100d and undergoes extrusion for forming. Thus, in order to perform forming without involvement of eccentricity, etc., in thrusting the deep hole forming punch 120d (the fourth step), it is preferred that the outside diameter Dl of the protrusion 125d be determined such that the following change proceeds in the thrusting process. At the initial stage of the thrusting process, the protrusion 125d of the forward end surface of the deep hole forming punch 120d is pressed against the bottom surface 27c of the preliminary hole 25c of the third-step formed body 10c and presses the bottom surface 27c forward with a relatively small load (pressing load) so as to establish a state in which the forward-facing surface of the polygonal portion 15 of the third-step formed body is supported by a rearward-facing annular polygonal surface 115d of the die 100d; subsequently, the protrusion 125d further presses the bottom surface 27c to thereby form depression in the bottom surface 27c of the preliminary hole 25c; then, a small-diameter portion which is to form the small-diameter tubular portion (small-diameter portion) 12e is extruded forward into the circular hole 106d; subsequently, substantial deep hole forming is performed to thereby form the large-diameter hole portion 25d through rearward extrusion.

(21) Here, L1 is taken as load to be imposed until the protrusion 125d is thrusted into the bottom surface 27c of the preliminary hole 25c, and then, the forward end surface of the deep hole forming punch 120d is pressed against the entire bottom surface 27c of the preliminary hole 25c. Subsequently, as the thrusting process proceeds, load increases, and L2 is taken as load to be imposed until the small-diameter portion which is to form the forward small-diameter tubular portion (small-diameter portion) 12e is extruded forward to thereby form the small-diameter tubular portion 12e. L3 is taken as load to be imposed next until completion of forming of the large-diameter hole portion 25d (deep hole forming) by rearward extrusion (elongation) by progress of deep hole forming as a result of the punch 120d being further thrusted. In this case, in addition to employment of the dimensional relation such that the outside diameter D1 of the protrusion 125d is smaller than the inside diameter D2 of the small-diameter hole portion 21e, it is preferred that these loads L1, L2, and L3 be in the following relation: L1<L2, L1<L3, and L2L3.

(22) The above embodiment is described while referring to the case where the multi-diameter tubular body 10e is formed from a starting material through five steps; specifically, in the third step, the third-step formed body 10c having the preliminary hole 25c is formed; in the fourth step, the third-step formed body 10c is subjected to deep hole forming; and, in the fifth step, punching is performed. However, in the present invention, the number of steps until formation of a multi-diameter tubular body may be determined as appropriate according to a specific dimensional aspect (height, diameter, thickness, etc.) ratio of the multi-diameter tubular body and the degree of difficulty of forming (or deformability of a metal material). The shape and structure of the multi-diameter tubular body (pre-machining workpiece of the metallic shell for use in a sensor or a spark plug) are not limited to those of the above embodiment. The multi-diameter tubular body may have a shape and a structure in which a multi-diameter profile is modified as appropriate according to positions of machining, machining allowances, etc.

DESCRIPTION OF REFERENCE NUMERALS

(23) 10e: multi-diameter tubular body

(24) 21e: small-diameter hole portion

(25) 24e: rearward-facing annular ledge surface

(26) 24d: annular surface which is to become rearward-facing annular ledge surface

(27) 25e: large-diameter hole portion

(28) 25c: preliminary hole for large-diameter hole portion

(29) 25d: large-diameter hole portion

(30) 27d: bottom surface of large-diameter hole portion

(31) 28d: depression formed by protrusion

(32) 120d: deep hole forming punch

(33) 124d: annular inclined surface located toward outer circumference of forward end surface of deep hole forming punch

(34) 125d: protrusion

(35) 120e: punch for punching

(36) D1: outside diameter of protrusion

(37) D2: inside diameter of small-diameter hole portion