Method for forming cylindrical member

Abstract

To provide a method for forming a cylindrical member which has a high degree of freedom for a working condition and a moldable shape and enables the integral formation of a cup-shaped large diameter part at one end of the cylindrical member.SOLUTION: The method includes the steps of: molding an intermediate material through the formation of a large inner diameter part by fixing a tip side end of a cylindrical material inserted into a first die hole, pushing a first mandrel from a base end side end to maintain an outer diameter of the base end side end of the cylindrical material, and extending to the base end side through backward extruding treatment while expanding an inner diameter; and molding a cylindrical member through the formation of a small diameter part by pressing the base end side end of the intermediate material inserted into a second die hole with a sleeve, driving a second mandrel while being interlinked with the sleeve to push the tip side end of the intermediate material into the second die hole through forward extruding treatment, and extending to the tip side while reducing an outer diameter and an inner diameter of an unprocessed part left on the tip side of the intermediate material.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a method for forming a cylindrical member, and more particularly to a method for forming a cylindrical member having a cup-shaped large diameter portion at an end thereof. [Background technology]

[0002] In this technical field, a method combining backward extrusion and forward extrusion is frequently used to obtain a long, bottomed hollow member from a solid material. For example, Patent Document 1 (JP 2003-103340 A) discloses a method in which, as shown in FIG. 18, a punch 1 is forced into the center of a solid material 5 inserted into a space formed in a die 2 and supported at its lower end by an opposing punch 3 to form a bottomed intermediate member 6 by backward extrusion. The opposing punch 3 is then removed, and the intermediate member 6 is forced deep into a reduced-diameter portion 20 of the die 2 to form an extended portion 10A by forward extrusion. This method is said to produce a bottomed hollow member having a lug 6a at one end and a bottom at the other end. Patent Document 2 (JP Patent No. 6240244 A) also discloses a method in which a punch pin 1 and a sleeve 2 are maintained in a predetermined positional relationship during the same forward extrusion process to prevent damage due to buckling of the punch pin 1.

[0003] However, all of the above-mentioned conventional methods are methods for forming a hollow member with a bottom, and a step of cutting off the bottom is required to obtain a tubular member. In addition, since the forming is performed from a solid member and the bottom remains until the forming is completed, the processing conditions and the shape that can be formed are limited. Therefore, it is difficult to form a tubular member having an arbitrary shape and / or an arbitrary thickness using the above-mentioned conventional methods.

[0004] In response to the above, Patent Document 3 (Japanese Patent No. 5597764) proposes a method in which the bottom portion is removed after backward extrusion, followed by forward extrusion. This method expands the degree of freedom in terms of processing conditions and formable shapes to a certain extent. However, since the bottom portion still exists in backward extrusion, the degree of freedom in terms of processing conditions and formable shapes is limited.

[0005] Furthermore, Patent Document 4 (Japanese Patent No. 4722324) proposes a method in which a cylindrical member, rather than a solid member, is used as the raw material B1 from the beginning, a mandrel 7 is inserted into the raw material B1, the lower end of the raw material B1 is supported by an auxiliary lower die 8, and a backward extrusion process is performed in which the outer periphery of the raw material B1 is pressed by a cylindrical upper die 6 to bulge the inner periphery of the raw material B1 backward, and then the auxiliary lower die 8 is retracted downward to perform a forward extrusion process in which the raw material B1 is extruded into the gear portion forming die 3. It is said that this method makes it possible to form a gear portion on the outer periphery of the portion extended by the forward extrusion process.

[0006] Meanwhile, recently, there has been an increasing demand for a cup-shaped large-diameter portion to be integrally formed at one end of a cylindrical member such as a shaft, and for another cylindrical member such as a shaft to be fitted into the enlarged-diameter portion, or for serration teeth to be carved into the inner surface of the enlarged-diameter portion to be engaged with another cylindrical member such as a shaft or a columnar member. However, none of Patent Documents 1 to 4 above discloses a method for forming a cup-shaped large-diameter portion at one end of a cylindrical member. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-103340 [Patent Document 2] Patent No. 6240244 [Patent Document 3] Patent No. 5597764 [Patent Document 4] Patent No. 4722324 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, there is a need in the art for a technique that allows a high degree of freedom in terms of processing conditions and moldable shapes and that allows a cup-shaped large-diameter portion to be integrally formed at one end of a tubular member. One object of the present invention is to provide a method for forming a tubular member that allows a high degree of freedom in terms of processing conditions and moldable shapes and that allows a cup-shaped large-diameter portion to be integrally formed at one end of a tubular member. [Means for solving the problem]

[0009] As a result of extensive research, the inventors have found that the above-mentioned problems can be solved by forming an intermediate blank by fixing the distal end of a tubular blank inserted into a first die hole, and pushing a first mandrel into the proximal end of the tubular blank from the proximal end, thereby maintaining the outer diameter of the proximal end while expanding the inner diameter, thereby extending the tubular blank toward the proximal end by backward extrusion to form a large inner diameter section, and by pressing the proximal end of the intermediate blank inserted into a second die hole with a sleeve, while driving the second mandrel in conjunction with the sleeve, thereby forcing the distal end of the intermediate blank into the second die hole by forward extrusion, thereby reducing only the outer diameter or both the outer and inner diameters of the unmachined section left at the distal end of the intermediate blank and extending it toward the distal end, thereby forming a small diameter section, thereby forming a tubular member.

[0010] Specifically, the method for forming a tubular member according to the present invention (hereinafter sometimes referred to as the "method of the present invention") is a method for forming a tubular member from a tubular material, the tubular member having a large diameter portion integrally formed at one end thereof. The method of the present invention includes a backward extrusion step and a forward extrusion step.

[0011] In the backward extrusion step, a first extrusion molding device is used to fix the other end of the tubular material, which is the end opposite to the one end inserted into the first die hole, and a first mandrel is pushed in from the one end, and a large inner diameter portion with an enlarged inner diameter is formed at one end of the tubular material by backward extrusion, thereby forming an intermediate material having the large inner diameter portion integrally formed at one end. The first extrusion molding device includes a first mandrel which is a columnar core metal having a predetermined shape, a first die which is formed with a first die hole which is a hole having a predetermined shape, and a first drive mechanism which drives the first mandrel and the first die to approach each other in the axial direction.

[0012] In the forward extrusion step, a second extrusion molding device is used to press one end of an intermediate material inserted into a second die hole with a sleeve, while driving a second mandrel in conjunction with the sleeve to force the other end of the intermediate material into the second die hole, thereby reducing the diameter of the other end of the intermediate material by forward extrusion to form a small-diameter portion, and leaving the large inner-diameter portion formed at one end as a large-diameter portion, thereby forming a tubular member having one end integrally formed with a large-diameter portion and the other end integrally formed with a small-diameter portion. The second extrusion molding device includes a second mandrel which is a columnar core metal having a predetermined shape, a sleeve which is a cylindrical member arranged coaxially with the second mandrel, a second die which is formed with a second die hole which is a hole having a predetermined shape, and a second drive mechanism which drives the second mandrel, the sleeve, and the second die to approach each other in the axial direction.

[0013] The tubular material is a cylindrical member having a first outer diameter which is a predetermined outer diameter, a first inner diameter which is a predetermined inner diameter, a first wall thickness which is a predetermined wall thickness, and a first length which is a predetermined length in the axial direction.

[0014] The intermediate blank includes a large inner diameter portion, an unprocessed portion, and an intermediate reduced inner diameter portion. The large inner diameter portion is a cylindrical portion formed at an end portion on the first base end side, which is upstream in a first pressing direction, in which the first mandrel approaches the first die, and has a second outer diameter equal to the first outer diameter, a second inner diameter larger than the first inner diameter, and a second wall thickness smaller than the first wall thickness. The unprocessed portion is a portion remaining as a tubular blank at an end portion on the first tip end side, which is downstream in the first pressing direction. The intermediate reduced inner diameter portion is formed between the large inner diameter portion and the unprocessed portion, and has an inner diameter that decreases from the second inner diameter to the first inner diameter as it approaches the unprocessed portion side.

[0015] The tubular member includes a large diameter portion, a small diameter portion, and a tapered portion. The large diameter portion is a large inner diameter portion of the intermediate material remaining at an end portion on the second base end side, which is upstream in a second pressing direction, which is a direction in which the second mandrel and sleeve approach the second die. The small diameter portion is a cylindrical portion formed on the second tip side, which is downstream in the second pressing direction, of the large diameter portion, and has a third outer diameter smaller than the first outer diameter, a third inner diameter equal to or smaller than the first inner diameter, and a predetermined third thickness. The tapered portion is formed between the large diameter portion and the small diameter portion, and is a portion whose outer diameter decreases from the second outer diameter to the third outer diameter and whose inner diameter decreases from the second inner diameter to the third inner diameter as it approaches the small diameter portion.

[0016] The first mandrel includes a first small outer diameter portion, a first large outer diameter portion, and a first increasing outer diameter portion. The first small outer diameter portion is a cylindrical portion formed at an end portion on the first distal side and having a fourth outer diameter corresponding to the first inner diameter. The first large outer diameter portion is a cylindrical portion formed at the first proximal side and having a fifth outer diameter corresponding to the second inner diameter. The first increasing outer diameter portion is formed between the first small outer diameter portion and the first large outer diameter portion and has an outer diameter that increases from the fourth outer diameter to the fifth outer diameter as it approaches the first small outer diameter portion. Furthermore, a second length, which is the size of the first small outer diameter portion in the first pressing direction, is equal to or greater than the first length.

[0017] The first die hole includes a first large inner diameter portion, a first small inner diameter portion, and a first step portion. The first large inner diameter portion is formed on the first base end side and has a fourth inner diameter that is an inner diameter corresponding to the first outer diameter. The first small inner diameter portion is formed on the first tip end side and has a fifth inner diameter equal to the first inner diameter. The first step portion is formed at the boundary between the first large inner diameter portion and the first small inner diameter portion and has an outer diameter corresponding to the fourth inner diameter and an inner diameter equal to the fifth inner diameter, and has a support surface that is an annular flat surface perpendicular to the first pressing direction.

[0018] The second mandrel includes a second small outer diameter portion, a second large outer diameter portion, and a second increasing outer diameter portion. The second small outer diameter portion is a cylindrical portion formed at an end portion on the second distal side and having a sixth outer diameter corresponding to the third inner diameter. The second large outer diameter portion is a cylindrical portion formed at the second proximal side and having a seventh outer diameter corresponding to the second inner diameter. The second increasing outer diameter portion is formed between the second small outer diameter portion and the second large outer diameter portion and has an outer diameter that increases from the sixth outer diameter to the seventh outer diameter as it approaches the second large outer diameter portion.

[0019] The sleeve includes a pressing portion that is a cylindrical portion formed at an end portion on the second tip side and has an eighth outer diameter equal to the first outer diameter, a sixth inner diameter equal to the second inner diameter, and a fourth thickness equal to the second thickness.

[0020] The second die hole includes a second large inner diameter portion, a second small inner diameter portion, and a second reduced inner diameter portion. The second large inner diameter portion is formed on the second base end side and has a seventh inner diameter corresponding to the second outer diameter. The second small inner diameter portion is formed on the second tip end side and has an eighth inner diameter corresponding to the third outer diameter. The second reduced inner diameter portion is formed between the second large inner diameter portion and the second small inner diameter portion and has an inner diameter that decreases from the seventh inner diameter to the eighth inner diameter as it approaches the second small inner diameter portion.

[0021] The backward extrusion process described above includes the first and second steps listed below. The first step is a step of supporting the tubular material at a predetermined position inside the first die hole by inserting the tubular material into the first die hole and abutting the end of the tubular material on the first tip side against the support surface of the first step portion. The second step is a step of forming a large inner diameter portion at the first base end of the tubular material by inserting a first mandrel from the first base end side of the tubular material and driving the first mandrel toward the first tip end side using a first driving mechanism to expand the inner diameter of the first base end side end of the tubular material.

[0022] The forward extrusion step described above includes the third and fourth steps listed below. The third step is a step of supporting the intermediate material at a predetermined position inside the second die hole by inserting the intermediate material into the second die hole and abutting the first tip end of the intermediate material against the second base end of the second inner diameter reducing portion. The fourth step is a step of inserting a second mandrel from the second base end side of the intermediate material and inserting a sleeve into the space between the outer peripheral surface of the second mandrel and the inner peripheral surface of the second large inner diameter section of the second die hole, and driving the second mandrel and sleeve toward the second tip side by the second drive mechanism so that the inner peripheral surface of the intermediate inner diameter decreasing section of the intermediate material is pressed toward the second tip side by the outer peripheral surface of the second outer diameter increasing section of the second mandrel and the end of the first base end side of the large inner diameter section of the intermediate material is pressed toward the second tip side by the pressing portion of the sleeve, thereby forming a small diameter section and a tapered section.

[0023] Furthermore, in the fourth step, the second mandrel and the sleeve are driven by the second drive mechanism over a period from the first time point to the second time point so that a portion of the second mandrel located closer to the second tip than the second large outer diameter portion faces the second base end of the second small inner diameter portion of the second die hole in the radial direction. The first time point is a predetermined time point before the material constituting the first tip end of the intermediate material starts to flow toward the second tip beyond the second base end of the second inner diameter reduced portion of the second die hole. The second time point is a predetermined time point at which the second mandrel stops moving in the second pressing direction before the second tip end of the second large outer diameter portion of the second mandrel reaches the inner circumferential surface of the second inner diameter reduced portion of the second die hole. [Effects of the Invention]

[0024] As described above, in the method of the present invention, the distal end of the tubular material inserted into the first die hole is fixed, and the first mandrel is forced into the proximal end of the tubular material, maintaining the outer diameter of the proximal end while expanding the inner diameter, thereby forming a large inner diameter portion and forming an intermediate material. Next, the proximal end of the intermediate material inserted into the second die hole is pressed by a sleeve, and the second mandrel is driven in conjunction with the sleeve to form a forward extrusion, forcing the distal end of the intermediate material into the second die hole, thereby forming a small diameter portion and forming a tubular member.

[0025] As described above, in the method of the present invention, the tip end of the intermediate material formed by the prior backward extrusion process is left as an unprocessed tubular material. The forward extrusion process is then performed on this unprocessed portion to form the small diameter portion of the desired tubular member. In this forward extrusion process, the large inner diameter portion of the intermediate material formed by the prior backward extrusion process remains as the large diameter portion of the desired tubular member. In this way, in the method of the present invention, plastic processing is not repeated multiple times on the same portion, thereby reducing work hardening of the workpiece and, as a result, reducing the processing load.

[0026] Furthermore, the second outer diameter, which is the outer diameter of the large inner diameter portion of the intermediate material that will become the large diameter portion of the desired tubular member, is determined by the first outer diameter, which is the outer diameter of the tubular member, the second inner diameter, which is the inner diameter of the large inner diameter portion of the intermediate material, is determined by the fifth outer diameter, which is the outer diameter of the first large outer diameter portion of the first mandrel, and the second thickness, which is the wall thickness of the large inner diameter portion of the intermediate material, is determined by the difference between the fourth inner diameter, which is the inner diameter of the first large inner diameter portion of the first die hole, and the fifth outer diameter, which is the outer diameter of the first large outer diameter portion of the first mandrel. Therefore, by appropriately adjusting these dimensions, the outer diameter, inner diameter, and wall thickness of the large diameter portion of the desired tubular member can be set as desired.

[0027] Furthermore, the third outer diameter, which is the outer diameter of the small diameter portion of the desired tubular member, is determined by the eighth inner diameter, which is the inner diameter of the second small inner diameter portion of the second die hole, the third inner diameter, which is the inner diameter of the small diameter portion of the tubular member, is determined by the sixth outer diameter, which is the outer diameter of the second small outer diameter portion of the second mandrel, and the third wall thickness, which is the wall thickness of the small diameter portion of the tubular member, is determined by the difference between the eighth inner diameter, which is the inner diameter of the second small inner diameter portion of the second die hole, and the sixth outer diameter, which is the outer diameter of the second small outer diameter portion of the second mandrel. Therefore, by appropriately adjusting these dimensions, the outer diameter, inner diameter, and wall thickness of the desired small diameter portion of the tubular member can be set as desired.

[0028] In addition, the axial dimension (thickness) of the tapered portion of the desired tubular member is determined by the distance between the second increasing outer diameter portion of the second mandrel and the second reducing inner diameter portion of the second die hole at a predetermined time (i.e., the second time point) when the advance of the second mandrel in the second pressing direction is stopped before the second tip end of the second larger outer diameter portion of the second mandrel reaches the inner circumferential surface of the second reducing inner diameter portion of the second die hole in the fourth step, which is included in the forward extrusion process. Therefore, the axial dimension (thickness) of the tapered portion of the desired tubular member can be arbitrarily set by changing the position at which the advance of the second mandrel is stopped in the fourth step.

[0029] As is clear from the above, the method of the present invention can provide a method for forming a tubular member that has a high degree of freedom in terms of processing conditions and moldable shapes, and that can integrally form a cup-shaped large-diameter portion at one end of the tubular member.

[0030] Other objects, other features and attendant advantages of the present invention will be readily apparent from the following description of the embodiments of the present invention which will be given with reference to the drawings. [Brief explanation of the drawings]

[0031] [Figure 1] 1 is a flowchart illustrating the flow of each step included in a method (first method) for forming a tubular member according to a first embodiment of the present invention. [Figure 2] 3 is a schematic cross-sectional view showing an example of the configuration of a cylindrical material and an intermediate material used in the first method. FIG. [Figure 3] 3 is a schematic cross-sectional view showing an example of the configuration of a cylindrical member formed by a first method. FIG. [Figure 4] FIG. 3 is a schematic cross-sectional view showing an example of the configuration of a first mandrel and a first die used in a first method. [Figure 5] FIG. 3 is a schematic cross-sectional view showing an example of the configuration of a second mandrel, a sleeve, and a second die used in the first method. [Figure 6] 1 is a flowchart illustrating the flow of each step included in a backward extrusion step and a forward extrusion step included in a first method. [Figure 7] FIG. 1 is a schematic cross-sectional view illustrating the positional relationship between the tubular material, the first mandrel, and the first die at the time when the first step is completed, and the positional relationship between the intermediate material, the first mandrel, and the first die at the time when the second step is completed. [Figure 8] A schematic cross-sectional view illustrating the positional relationship between the intermediate material, the second mandrel, the sleeve, and the second die at the time when the third step is completed, and the positional relationship between the tubular member, the second mandrel, the sleeve, and the second die at the time when the fourth step is completed. [Figure 9] A schematic cross-sectional view showing that the size of the second inner diameter, which is the inner diameter of the large inner diameter portion of the intermediate material formed in the second step included in the backward extrusion step included in the first method, differs depending on the size of the fifth outer diameter, which is the outer diameter of the first large outer diameter portion. [Figure 10] 4 is a photograph illustrating the configuration of a cylindrical member formed by the first method. [Figure 11] FIG. 6 is a schematic cross-sectional view showing an example of the configuration of a tubular member formed by a method (second method) for forming a tubular member according to a second embodiment of the present invention. [Figure 12] FIG. 10 is a schematic cross-sectional view showing an example of the configuration of a second mandrel used in a second method. [Figure 13]A schematic cross-sectional view illustrating the positional relationship between the intermediate material, the second mandrel, the sleeve, and the second die at the time when the third step included in the second method is completed, and the positional relationship between the tubular member, the second mandrel, the sleeve, and the second die at the time when the fourth step is completed. [Figure 14] This is a schematic diagram illustrating the configuration of the outer case of a Rzeppa-type joint, which is often used in drive shafts of front-wheel drive vehicles, and is formed by additional processing on a tubular member having a large diameter portion integrally formed at one end, formed by the tubular member forming method of the present invention (the method of the present invention). [Figure 15] 1A to 1C are schematic diagrams illustrating a process for forming a tubular member from a tubular material via an intermediate material by the method of the present invention. [Figure 16] 16 is a schematic diagram illustrating an intermediate product formed from the cylindrical member illustrated in FIG. 15(c) and a final product which is the outer case of a Rzeppa joint. [Figure 17] FIG. 10 is a schematic diagram illustrating the configuration of an outer case of a tripod joint formed by additionally machining a cylindrical member having a large diameter portion integrally formed at one end thereof, which is formed by the method of the present invention. [Figure 18] 1 is a schematic diagram illustrating a conventional method for obtaining a long, bottomed, hollow member from a solid material by combining backward extrusion and forward extrusion. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0032] First Embodiment A method for forming a cylindrical member according to a first embodiment of the present invention (hereinafter, sometimes referred to as a "first method") will be described below with reference to the drawings.

[0033] <composition> The first method is a method for forming a tubular member by forming a tubular member having a large-diameter portion integrally formed at one end from a tubular blank. The material from which the tubular blank is formed is not particularly limited as long as it can be formed into a desired shape by plastic deformation in an extrusion process. Typical materials for forming the tubular blank include metals such as lead, tin, aluminum, copper, zirconium, titanium, molybdenum, vanadium, niobium, and iron.

[0034] Fig. 1 is a flow chart illustrating the flow of each step included in the first method. As illustrated in Fig. 1, the first method of the present invention includes a backward extrusion step and a forward extrusion step.

[0035] In the backward extrusion process performed in step S10, a first extrusion molding device is used to fix the other end of the tubular material, which is the end opposite to the one end inserted into the first die hole, and a first mandrel is pushed into the one end to form a large inner diameter portion with an enlarged inner diameter at one end of the tubular material by backward extrusion, thereby forming an intermediate material having the large inner diameter portion integrally formed at one end. The first extrusion molding device includes a first mandrel which is a columnar core metal having a predetermined shape, a first die which is formed with a first die hole which is a hole having a predetermined shape, and a first drive mechanism which drives the first mandrel and the first die to approach each other in the axial direction.

[0036] As described above, the first drive mechanism may be configured to drive the first mandrel and the first die so that they approach each other in the axial direction. That is, the first drive mechanism may be configured to move the first die closer to the fixed first mandrel, or may be configured to move the first mandrel closer to the fixed first die, or may be configured to move both the first mandrel and the first die closer to each other. Therefore, for the purpose of easily understanding the present invention, the following description will be given assuming that the first drive mechanism is configured to move the first mandrel closer to the fixed first die, but the configuration of the first drive mechanism is not limited to the following description. The same applies to the second drive mechanism described below.

[0037] In the forward extrusion process performed in step S20, a second extrusion molding device is used to press one end of an intermediate material inserted into a second die hole with a sleeve, and to drive a second mandrel in conjunction with the sleeve to force the other end of the intermediate material into the second die hole, thereby reducing the diameter of the other end of the intermediate material by forward extrusion to form a small-diameter portion, and leaving the large inner-diameter portion formed at one end as a large-diameter portion, thereby forming a tubular member having an integral large-diameter portion at one end and an integral small-diameter portion at the other end. The second extrusion molding device includes a second mandrel which is a columnar core metal having a predetermined shape, a sleeve which is a cylindrical member arranged coaxially with the second mandrel, a second die which is formed with a second die hole which is a hole having a predetermined shape, and a second drive mechanism which drives the second mandrel, the sleeve, and the second die to approach each other in the axial direction.

[0038] Since the basic configuration of the extrusion molding apparatus as described above is well-known to those skilled in the art, a detailed description thereof will be omitted. However, the components such as the mandrel, sleeve, and die are made of a material having properties (e.g., mechanical strength and durability, etc.) that can withstand processing conditions such as the load acting on the components in the above-described backward extrusion and forward extrusion. Also, the drive mechanism provided in each extrusion molding apparatus can be appropriately selected from various drive mechanisms well-known in the art according to the properties (e.g., mechanical strength and hardness, etc.) of the material constituting the workpieces (cylindrical material and intermediate material) to be subjected to the extrusion process. Typically, for example, a press such as a hydraulic press is adopted as the drive mechanism.

[0039] FIG. 2(a) is a schematic cross-sectional view showing an example of the configuration of the cylindrical material used in the first method. As illustrated in FIG. 2(a), the cylindrical material W1 is a cylindrical member having a first outer diameter DO1 which is a predetermined outer diameter, a first inner diameter DI1 which is a predetermined inner diameter, a first wall thickness T1 which is a predetermined wall thickness, and a first length L1 which is a predetermined length in the axial direction. Note that AX shown in the figure is the central axis of the cylindrical material W1, and this axis AX becomes the common central axis of the intermediate material, cylindrical member, first mandrel, first die hole, second mandrel, sleeve, and second die hole during the execution of each process described later.

[0040] FIG. 2(b) is a schematic cross-sectional view showing an example of the configuration of the intermediate material used in the first method. As illustrated in FIG. 2(b), the intermediate material IW1 includes a large inner diameter portion PLDI, an unprocessed portion Puf, and an intermediate inner diameter reduction portion PdDI. The large inner diameter portion PLDI is formed at the end on the first proximal side which is the upstream side in the first pressing direction which is the direction in which a first mandrel (not shown) approaches a first die (not shown), and has a second outer diameter DO2 (DO2 = DO1) equal to the first outer diameter DO1, a second inner diameter DI2 (DI2>DI1) larger than the first inner diameter DI1, and a second wall thickness T2 (T2<T1) smaller than the first wall thickness T1. Note that the "end on the first proximal side" referred to here corresponds to the "one end" described above.

[0041] The unprocessed portion Puf is the portion left as the tubular material W1 at the end on the first tip side, which is the downstream side in the first pressing direction. The intermediate inner diameter reduction portion PdDI is a portion formed between the large inner diameter portion PLDI and the unprocessed portion Puf, and the inner diameter decreases from the second inner diameter DI2 to the first inner diameter DI1 as it approaches from the side of the large inner diameter portion PLDI to the side of the unprocessed portion Puf. Here, the "end on the first tip side" corresponds to the "other end" described above.

[0042] FIG. 3 is a schematic cross-sectional view showing an example of the configuration of a tubular member formed by the first method. As illustrated in FIG. 3, the tubular member P1 includes a large diameter portion PLD, a small diameter portion PSD, and a taper portion PT. The large diameter portion PLD is the large inner diameter portion PLDI of the intermediate material IW1 left at the end on the second base end side, which is the upstream side in the second pressing direction, which is the direction in which a second mandrel (not shown) and a sleeve (not shown) approach a second die (not shown). The small diameter portion PSD is formed on the second tip side, which is the downstream side in the second pressing direction from the large diameter portion PLD, and has a third outer diameter DO3 (DO3 < DO1) smaller than the first outer diameter DO1, a third inner diameter DI3 (DI3 ≦ DI1) not exceeding the first inner diameter DI1, and a third wall thickness T3 with a predetermined wall thickness. The taper portion PT is a portion formed between the large diameter portion PLD and the small diameter portion PSD, and the outer diameter decreases from the second outer diameter DO2 to the third outer diameter DO3 and the inner diameter decreases from the second inner diameter DI2 to the third inner diameter DI3 as it approaches from the side of the large diameter portion PLD to the side of the small diameter portion PSD. Here, the "end on the second base end side" corresponds to the "one end" described above, and the "end on the second tip side" corresponds to the "other end" described above.

[0043] 4A is a schematic cross-sectional view showing an example of the configuration of a first mandrel used in the first method. As shown in FIG. 4A, the first mandrel M1 includes a first small outer diameter portion PSDO1, a first large outer diameter portion PLDO1, and a first increasing outer diameter portion PiDO1. The first small outer diameter portion PSDO1 is a cylindrical portion formed at the end of the first distal end and having a fourth outer diameter DO4 corresponding to the first inner diameter DI1. The first large outer diameter portion PLDO1 is a cylindrical portion formed at the first proximal end and having a fifth outer diameter DO5 corresponding to the second inner diameter DI2. The first outer diameter increasing portion PiDO1 is a portion formed between the first small outer diameter portion PSDO1 and the first large outer diameter portion PLDO1, and the outer diameter of the first small outer diameter portion PSDO1 increases from the fourth outer diameter DO4 to the fifth outer diameter DO5 as it approaches the first large outer diameter portion PLDO1 from the side of the first small outer diameter portion PSDO1. Furthermore, a second length L2, which is the size of the first small outer diameter portion PSDO1 in the pressing first direction, is equal to or greater than the first length L1.

[0044] 4(b) is a schematic cross-sectional view showing an example of the configuration of a first die used in the first method. As shown in FIG. 4(b), the first die hole DH1 formed in the first die D1 includes a first large inner diameter portion PLDI1, a first small inner diameter portion PSDI1, and a first step portion PS1. The first large inner diameter portion PLDI1 is formed on the first base end side and has a fourth inner diameter DI4 that corresponds to the first outer diameter DO1. The first small inner diameter portion PSDI1 is formed on the first tip end side and has a fifth inner diameter DI5 (DI5=DI1) equal to the first inner diameter DI1. The first step portion PS1 is formed at the boundary between the first large inner diameter portion PLDI1 and the first small inner diameter portion PSDI1. The first step portion PS1 has an outer diameter corresponding to the fourth inner diameter DI4 and an inner diameter equal to the fifth inner diameter DI5, and has a support surface SS that is an annular plane perpendicular to the first pressing direction.

[0045] Although not shown in FIG. 4, it goes without saying that the first mandrel M1 and the first die D1 are equipped with the components necessary to be connected to and driven by a first drive mechanism provided in the first extrusion molding device (not shown).

[0046] 5A is a schematic cross-sectional view showing an example of the configuration of the second mandrel and sleeve used in the first method. As shown in FIG. 5A, the second mandrel M2 includes a second small outer diameter portion PSDO2, a second large outer diameter portion PLDO2, and a second increasing outer diameter portion PiDO2. The second small outer diameter portion PSDO2 is a cylindrical portion formed at the end of the second distal end and having a sixth outer diameter DO6 corresponding to the third inner diameter DI3. The second large outer diameter portion PLDO2 is a cylindrical portion formed at the second proximal end and having a seventh outer diameter DO7 corresponding to the second inner diameter DI2. The second outer diameter increasing portion PiDO2 is formed between the second small outer diameter portion PSDO2 and the second large outer diameter portion PLDO2, and the outer diameter increases from the sixth outer diameter DO6 to the seventh outer diameter DO7 as it approaches from the second small outer diameter portion PSDO2 side to the second large outer diameter portion PLDO2 side.

[0047] The sleeve SL includes a pressing portion PP which is a cylindrical portion formed at the end on the second tip side and has an eighth outer diameter DO8 (DO8=DO1) equal to the first outer diameter DO1, a sixth inner diameter DI6 (DI6=DI2) equal to the second inner diameter DI2, and a fourth thickness T4 (T4=T2) equal to the second thickness T2.

[0048] 5(b) is a schematic cross-sectional view showing an example of the configuration of a second die used in the first method. As shown in FIG. 5(b), the second die hole DH2 formed in the second die D2 includes a second large inner diameter portion PLDI2, a second small inner diameter portion PSDI2, and a second reduced inner diameter portion PdDI2. The second large inner diameter portion PLDI2 is formed on the second base end side and has a seventh inner diameter DI7 corresponding to the second outer diameter DO2. The second small inner diameter portion PSDI2 is formed on the second tip end side and has an eighth inner diameter DI8 corresponding to the third outer diameter DO3. The second inner diameter decreasing portion PdDI2 is formed between the second large inner diameter portion PLDI2 and the second small inner diameter portion PSDI2, and is a portion whose inner diameter decreases from the eighth inner diameter DI8 to the eighth inner diameter DI8 as it approaches from the second large inner diameter portion PLDI2 side to the second small inner diameter portion PSDI2 side.

[0049] Although not shown in Figure 5, it goes without saying that the second mandrel M2, sleeve SL, and second die D2 are equipped with the components necessary to be connected to and driven by a second drive mechanism provided in the second extrusion molding device, not shown.

[0050] Fig. 6 is a flowchart illustrating the flow of each step included in the backward extrusion step and the forward extrusion step included in the first method. As illustrated in step S10 of Fig. 6, the backward extrusion step described above includes the first and second steps listed below. Fig. 7 is a schematic cross-sectional view illustrating the positional relationship between the tubular material W1, the first mandrel M1, and the first die D1 at the time when the first step is completed, and the positional relationship between the intermediate material IW1, the first mandrel M1, and the first die D1 at the time when the second step is completed.

[0051] In Fig. 7, for the purpose of simplifying the drawing, reference numerals are omitted for the details of the tubular blank W1, the intermediate blank IW1, the first mandrel M1, and the first die D1. However, in the following description, for the sake of accuracy, the reference numerals used in Fig. 2 and Fig. 4 will be used for the details, so please refer to Fig. 2 and Fig. 4 as well as necessary.

[0052] The first process performed in step S11 is a process of supporting the tubular material W1 at a predetermined position inside the first die hole DH1 by inserting the tubular material W1 into the first die hole DH1 and abutting the first tip end of the tubular material W1 against the support surface SS of the first step portion PS1, as shown to the left of the axis AX in Figure 7.

[0053] As described above, the second length L2, which is the size of the first small outer diameter portion PSDO1 of the first mandrel M1 in the first pressing direction, is equal to or greater than the first length L1, which is the predetermined length in the axial direction of the tubular blank W1. Therefore, as shown to the left of the axis AX in Fig. 7, when the first step is completed and the tubular blank W1 is set at a predetermined position inside the first die hole DH1, the first tip end of the first small outer diameter portion PSDO1 of the first mandrel M1 reaches, in the first pressing direction, the same position as the first base end of the first small inner diameter portion PSDI1 of the first die hole DH1 (i.e., the first step portion PS1) or a position further toward the first tip than the first base end of the first small inner diameter portion PSDI1 of the first die hole DH1 (i.e., the first step portion PS1).

[0054] As a result, the first distal end of the tubular blank W1 is accommodated in a closed space surrounded by the outer circumferential surface of the first small outer diameter portion PSDO1 of the first mandrel M1, the inner circumferential surface of the first large inner diameter portion PLDI1 of the first die hole DH1, and the support surface SS of the first step portion PS1. This prevents the material constituting the first distal end of the tubular blank W1 from flowing toward the first small inner diameter portion PSDI1 of the first die hole DH1 in the second step S12. As a result, in the second step, the material constituting the first proximal end of the tubular blank W1 is extruded rearward (toward the first proximal end) through the gap between the outer circumferential surface of the first large outer diameter portion PLDO1 of the first mandrel M1 and the inner circumferential surface of the first large inner diameter portion PLDI1 of the first die hole DH1 by backward extrusion, thereby forming the large inner diameter portion PLDI and producing the intermediate blank IW1.

[0055] The second process executed in step S12 is a process of inserting a first mandrel M1 from the first base end side of the tubular material W1, as depicted to the right of the axis AX in Figure 7, and driving the first mandrel M1 toward the first tip side using a first drive mechanism (not shown) to expand the inner diameter of the end portion on the first base end side of the tubular material W1, thereby forming a large inner diameter portion PLDI at the end portion on the first base end side of the tubular material W1.

[0056] As illustrated in step S20 of Fig. 6, the forward extrusion process described above includes the third and fourth steps listed below. Fig. 8 is a schematic cross-sectional view illustrating the positional relationship between the intermediate material IW1, the second mandrel M2, the sleeve SL, and the second die D2 at the time when the third step is completed, and the positional relationship between the cylindrical member P1, the second mandrel M2, the sleeve SL, and the second die D2 at the time when the fourth step is completed.

[0057] In Fig. 8, similarly to Fig. 7, for the purpose of simplifying the drawing, reference numerals are omitted for the details of the intermediate material IW1, the tubular member P1, the second mandrel M2, the sleeve SL, and the second die D2. However, in the following description, for the sake of accuracy, the reference numerals used in Figs. 2, 3, and 5 will be used to describe the details, so please also refer to Figs. 2, 3, and 5 as necessary.

[0058] The third process performed in step S21 is a process of supporting the intermediate material IW1 at a predetermined position inside the second die hole DH2 by inserting the intermediate material IW1 into the second die hole DH2 and abutting the first tip end of the intermediate material IW1 against the second base end end of the second inner diameter reducing portion PdDI2, as shown to the left of the axis AX in Figure 8.

[0059] The fourth process executed in step S22 is a process of forming the small diameter portion PSD and the tapered portion PT by inserting a second mandrel M2 from the second base end side of the intermediate material IW1, as shown to the right of the axis AX in Figure 8, and inserting a sleeve SL into the space between the outer surface of the second mandrel M2 and the inner surface of the second large inner diameter portion PLDI2 of the second die hole DH2, and driving the second mandrel M2 and the sleeve SL toward the second tip side by a second drive mechanism (not shown). This presses the inner surface of the intermediate inner diameter decreasing portion PdDI of the intermediate material IW1 toward the second tip side with the outer surface of the second outer diameter increasing portion PiDO2 of the second mandrel M2, and presses the end of the first base end side of the large inner diameter portion PLDI of the intermediate material IW1 toward the second tip side with the pressing portion PP of the sleeve SL.

[0060] Furthermore, in the fourth step, the second mandrel M2 and the sleeve SL are driven by the second drive mechanism from the first time point tp1 to the second time point tp2 so that a portion of the second mandrel M2 located closer to the second tip than the second large outer diameter portion PLDO2 faces the second base end of the second small inner diameter portion PSDI2 of the second die hole DH2 in the radial direction. Note that the first time point tp1 is a predetermined time point before the material constituting the first tip end of the intermediate material IW1 begins to flow toward the second tip beyond the second base end of the second inner diameter reduced portion PdDI2 of the second die hole DH2. The second time point tp2 is a predetermined time point at which the second mandrel M2 stops moving in the second pressing direction before the second tip end of the second large outer diameter portion PLDO2 of the second mandrel M2 reaches the inner circumferential surface of the second inner diameter reduced portion PdDI2 of the second die hole DH2.

[0061] In other words, by the time the intermediate material IW1 is pressed in the second pressing direction by the second mandrel M2 and the sleeve SL in step 4 and the material constituting the first tip end of the intermediate material IW1 begins to flow further toward the second tip than the second base end of the second reduced inner diameter portion PdDI2 of the second die hole DH2, the second tip end of the second mandrel M2 has reached the same position in the second pressing direction as the second base end of the second small inner diameter portion PSDI2 of the second die hole DH2. Then, advancement of the second mandrel M2 in the second pressing direction is stopped at a point before the second tip end of the second large outer diameter portion PLDO2 of the second mandrel M2 reaches the inner circumferential surface of the second reduced inner diameter portion PdDI2 of the second die hole DH2.

[0062] In the fourth step included in the first method, by controlling the driving of the second mandrel M2 and the sleeve SL by the second drive mechanism as described above, the material constituting the first tip side end of the intermediate material IW1 can be extruded forward (toward the second tip side) through the gap between the second small outer diameter portion PSDO2 or the second increasing outer diameter portion PiDO2 of the second mandrel M2 and the second base end side end of the second small inner diameter portion PSDI2 of the second die hole DH2, thereby forming the small diameter portion PSD and the tapered portion PT and forming the tubular member P1.

[0063] <effect> As described above, in the first method, the distal end of the tubular material inserted into the first die hole is fixed, and the first mandrel is forced into the proximal end of the tubular material, maintaining the outer diameter of the proximal end while expanding the inner diameter, thereby forming a large inner diameter portion and forming an intermediate material. Next, the proximal end of the intermediate material inserted into the second die hole is pressed by the sleeve, and the second mandrel is driven in conjunction with the sleeve to form a forward extrusion, forcing the distal end of the intermediate material into the second die hole, thereby forming a small diameter portion and forming an intermediate material, thereby forming a tubular member.

[0064] As described above, in the first method, the tip end of the intermediate material formed by the backward extrusion process performed first is left as an unprocessed tubular material. Then, the forward extrusion process is performed on this unprocessed portion to form the small diameter portion of the desired tubular member. In this forward extrusion process, the large inner diameter portion of the intermediate material formed by the backward extrusion process performed first in the backward extrusion process is left as the large diameter portion of the desired tubular member. In this way, in the first method, plastic processing is not repeated multiple times on the same portion, so work hardening of the workpiece can be reduced, and as a result, the processing load can be reduced.

[0065] Furthermore, the second outer diameter, which is the outer diameter of the large inner diameter portion of the intermediate material that will become the large diameter portion of the desired tubular member, is determined by the first outer diameter, which is the outer diameter of the tubular member, the second inner diameter, which is the inner diameter of the large inner diameter portion of the intermediate material, is determined by the fifth outer diameter, which is the outer diameter of the first large outer diameter portion of the first mandrel, and the second thickness, which is the wall thickness of the large inner diameter portion of the intermediate material, is determined by the difference between the fourth inner diameter, which is the inner diameter of the first large inner diameter portion of the first die hole, and the fifth outer diameter, which is the outer diameter of the first large outer diameter portion of the first mandrel. Therefore, by appropriately adjusting these dimensions, the outer diameter, inner diameter, and wall thickness of the large diameter portion of the desired tubular member can be set as desired.

[0066] For example, Fig. 9 is a schematic cross-sectional view showing that the size of the second inner diameter, which is the inner diameter of the large inner diameter portion of the intermediate material formed in the second step included in the backward extrusion step included in the first method, varies depending on the size of the fifth outer diameter, which is the outer diameter of the first large outer diameter portion. In Fig. 9, for the purpose of simplifying the drawing, reference numerals are omitted for the details of the tubular material W1, the intermediate material IW1, the first mandrel M1, and the first die D1. However, in the following description, for the sake of accuracy, the reference numerals used in Figs. 2 to 4 will be used to describe the details, so please also refer to Figs. 2 to 4 as necessary.

[0067] 9(a) and 9(b) illustrate and compare the results of performing the first and second steps (i.e., the backward extrusion steps) on the same cylindrical blank W1 having the same first outer diameter DO1, first inner diameter DI1, and first wall thickness T1. However, FIG. 9(a) illustrates the case where the fifth outer diameter DO5, which is the outer diameter of the first large outer diameter portion PLDO1 of the first mandrel M1, is set to be relatively large (thick), while FIG. 9(b) illustrates the case where the fifth outer diameter DO5 is set to be relatively small (thin).

[0068] 9(a), when a first mandrel M1 having a relatively thick first large outer diameter portion PLDO1 is used, the second inner diameter DI2 of the large inner diameter portion PLDI of the resulting intermediate material IW1 is relatively large and the second thickness T2 is relatively small. On the other hand, when a first mandrel M1 having a relatively thin first large outer diameter portion PLDO1 is used, as shown in FIG. 9(b), the second inner diameter DI2 of the large inner diameter portion PLDI of the resulting intermediate material IW1 is relatively small and the second thickness T2 is relatively large. Furthermore, the second outer diameter DO2, which is the outer diameter of the large inner diameter portion PLDI of the intermediate material IW1, is determined by the first outer diameter DO1, which is the outer diameter of the tubular material W1.

[0069] The large inner diameter portion PLDI of the intermediate material IW1 obtained as described above remains as the large diameter portion PLD of the final tubular member P1, so by appropriately designing the first outer diameter DO1 of the tubular material W1 and the fifth outer diameter DO5 of the first large outer diameter portion PLDO1 of the first mandrel M1, the outer diameter, inner diameter DI2 and wall thickness T2 of the large diameter portion PLD of the desired tubular member P1 can be set as desired.

[0070] Furthermore, the third outer diameter, which is the outer diameter of the small diameter portion of the desired tubular member, is determined by the eighth inner diameter, which is the inner diameter of the second small inner diameter portion of the second die hole, the third inner diameter, which is the inner diameter of the small diameter portion of the tubular member, is determined by the sixth outer diameter, which is the outer diameter of the second small outer diameter portion of the second mandrel, and the third wall thickness, which is the wall thickness of the small diameter portion of the tubular member, is determined by the difference between the eighth inner diameter, which is the inner diameter of the second small inner diameter portion of the second die hole, and the sixth outer diameter, which is the outer diameter of the second small outer diameter portion of the second mandrel. Therefore, although not shown, by appropriately setting these dimensions, the outer diameter, inner diameter, and wall thickness of the desired small diameter portion of the tubular member can be set as desired.

[0071] Therefore, according to the first method, for example, in a tubular member, it is easy to make the second thickness, which is the thickness of the large diameter portion, thinner than the third thickness, which is the thickness of the small diameter portion, or to make the second thickness, which is the thickness of the large diameter portion, the same as the third thickness, which is the thickness of the small diameter portion, or even to make the second thickness, which is the thickness of the large diameter portion, thicker than the third thickness, which is the thickness of the small diameter portion.

[0072] In addition, the axial dimension (thickness) of the tapered portion of the desired tubular member is determined by the distance between the second increasing outer diameter portion of the second mandrel and the second reducing inner diameter portion of the second die hole at a predetermined time (i.e., the second time point) when the advance of the second mandrel in the second pressing direction is stopped before the second tip end of the second larger outer diameter portion of the second mandrel reaches the inner circumferential surface of the second reducing inner diameter portion of the second die hole in the fourth step, which is included in the forward extrusion process. Therefore, the axial dimension (thickness) of the tapered portion of the desired tubular member can be arbitrarily set by changing the position at which the advance of the second mandrel is stopped in the fourth step.

[0073] Fig. 10 is a photograph illustrating the configuration of a tubular member formed by the first method. In the tubular member illustrated in Fig. 10, a cup-shaped large diameter portion is integrally formed at one end. Furthermore, as illustrated in Fig. 10, in the tubular member formed by the first method, the axial lengths of the large diameter portion and the small diameter portion can be freely designed. For example, the longer the axial length of the tubular material, the longer the axial length of the resulting tubular member can be.

[0074] Furthermore, the smaller the ratio of the area of ​​the gap between the outer peripheral surface of the first large outer diameter portion of the first mandrel and the inner peripheral surface of the first large inner diameter portion of the first die hole to the cross-sectional area of ​​the tube wall of the tubular blank, as viewed in vertical projection onto a plane perpendicular to the first pressing direction, the longer the axial length of the large diameter portion of the tubular member can be.Furthermore, the smaller the ratio of the area of ​​the gap between the second small outer diameter portion or the second increasing outer diameter portion of the second mandrel and (the end portion of) the second small inner diameter portion of the second die hole to the cross-sectional area of ​​the tube wall of the tubular blank, as viewed in vertical projection onto a plane perpendicular to the second pressing direction, the longer the axial length of the small diameter portion of the tubular member can be.

[0075] As is clear from the above, the first method can provide a method for forming a tubular member that has a high degree of freedom in terms of processing conditions and moldable shapes, and that can integrally form a cup-shaped large-diameter portion at one end of the tubular member.

[0076] Second Embodiment A method for forming a cylindrical member according to a second embodiment of the present invention (hereinafter, sometimes referred to as the "second method") will be described below with reference to the drawings.

[0077] In this technical field, a differential wall pipe (also referred to as "batted pipe" and "batted tube", etc.) in which a thick-walled portion is formed in a part in the axial direction of a pipe material for the purpose of achieving a desired mechanical strength in the thick-walled portion and reducing the weight in the thin-walled portion (portion other than the thick-walled portion) is known. For a cylindrical member in which a large-diameter portion is integrally formed at one end formed by the method for forming a cylindrical member according to the present invention (the method of the present invention), there may be a case where a thick-walled portion is required to be formed in a part in the axial direction (for example, the tip of the small-diameter portion).

[0078] <Configuration> Therefore, the second method is the first method described above, and by using a second mandrel provided with a third small outer diameter portion thinner (having a smaller outer diameter) than the second small outer diameter portion on the second tip side further than the second small outer diameter portion to perform the forward extrusion process, a cylindrical member further including a thick-walled portion having a wall thickness larger than that of the small-diameter portion on the second tip side further than the small-diameter portion is formed. It is a method for forming a cylindrical member.

[0079] FIG. 11 is a schematic cross-sectional view showing an example of the configuration of a cylindrical member formed by the second method. As illustrated in FIG. 11, the cylindrical member P1' further includes a thick-walled portion PTW and a wall thickness increasing portion PiT in addition to the large-diameter portion PLD, the small-diameter portion PSD, and the tapered portion PT described above. The thick-walled portion PTW is a cylindrical portion formed on the second tip side further than the small-diameter portion PSD and having a tenth outer diameter DO10 (DO10 = DO3) equal to the third outer diameter DO3, a ninth inner diameter DI9 (DI9 < DI3) smaller than the third inner diameter DI3, and a fifth wall thickness T5 (T5 > T3) larger than the third wall thickness T3. The wall thickness increasing portion PiT is a portion formed between the small-diameter portion PSD and the thick-walled portion PTW, and the inner diameter decreases from the third inner diameter DI3 to the ninth inner diameter DI9 and the wall thickness increases from the third wall thickness T3 to the fifth wall thickness T5 as approaching from the small-diameter portion PSD to the thick-walled portion PTW.

[0080] 12 is a schematic cross-sectional view showing an example of the configuration of a second mandrel used in the second method. As illustrated in FIG. 12, the second mandrel M2′ further includes a third small outer diameter portion PSDO3 and a third outer diameter increasing portion PiDO3 in addition to the second small outer diameter portion PSDO2, second increasing outer diameter portion, and second large outer diameter portion described above. The third small outer diameter portion PSDO3 is a cylindrical portion formed further toward the second tip side than the second small outer diameter portion PSDO2 and having a ninth outer diameter DO9 smaller than the sixth outer diameter DO6. The third outer diameter increasing portion PiDO3 is formed between the second small outer diameter portion PSDO2 and the third small outer diameter portion PSDO3 and has an outer diameter that increases from the ninth outer diameter DO9 to the sixth outer diameter DO6 as it approaches the second small outer diameter portion PSDO2.

[0081] Figure 13 is a schematic cross-sectional view illustrating the positional relationship between the intermediate material IW1, the second mandrel, the sleeve, and the second die at the completion of the third step included in the second method, and the positional relationship between the tubular member P1, the second mandrel M2', the sleeve SL, and the second die at the completion of the fourth step. In Figure 13, as in Figure 8, for the purpose of simplifying the drawing, reference numerals are omitted for the details of the intermediate material IW1, the tubular member P1, the second mandrel M2', the sleeve SL, and the second die D2. However, in the following description, for the sake of accuracy, the reference numerals used in Figures 2, 11, and 12 will be used to describe the details, so please also refer to Figures 2, 11, and 12 as needed.

[0082] The third step included in the second method is also a step of supporting the intermediate material IW1 at a predetermined position inside the second die hole DH2 by inserting the intermediate material IW1 into the second die hole DH2 and abutting the first tip end of the intermediate material IW1 against the second base end end of the second inner diameter reducing portion PdDI2, as shown to the left of the axis AX in Figure 13.

[0083] The fourth step included in the second method is also a step of inserting a second mandrel M2' from the second base end side of the intermediate material IW1, as shown to the right of the axis AX in Figure 13, and inserting a sleeve SL into the space between the outer surface of the second mandrel M2' and the inner surface of the second large inner diameter portion PLDI2 of the second die hole DH2, and driving the second mandrel M2' and the sleeve SL toward the second tip side by a second drive mechanism not shown, so that the inner surface of the intermediate inner diameter decreasing portion PdDI of the intermediate material IW1 is pressed toward the second tip side by the outer surface of the second outer diameter increasing portion PiDO2 of the second mandrel M2', and the end of the first base end side of the large inner diameter portion PLDI of the intermediate material IW1 is pressed toward the second tip side by the pressing portion PP of the sleeve SL.

[0084] However, the second mandrel M2' used in the second method further includes, in addition to the second small outer diameter portion PSDO2, the second increasing outer diameter portion, and the second large outer diameter portion, a third small outer diameter portion PSDO3 and a third increasing outer diameter portion PiDO3, as illustrated in Figure 12.

[0085] Furthermore, in the second method, in the fourth step, over the period from the first point in time tp1 to a third point in time tp3, which is a predetermined point in time before the second point in time tp2, the second mandrel M2' and the sleeve SL are driven by the second driving mechanism so that the third small outer diameter portion PSDO3 or the third increasing outer diameter portion PiDO3 of the second mandrel M2' radially faces the second base end side end of the second small inner diameter portion PSDI2 of the second die hole DH2.

[0086] In other words, by the time the intermediate material IW1 is pressed in the second pressing direction by the second mandrel M2' and the sleeve SL in step 4 and the material constituting the first tip end of the intermediate material IW1 begins to flow further toward the second tip than the second base end of the second reduced inner diameter portion PdDI2 of the second die hole DH2, the second tip end of the third small outer diameter portion PSDO3 of the second mandrel M2' will have reached the same position in the second pressing direction as the second base end of the second small inner diameter portion PSDI2 of the second die hole DH2. Then, at a third time point tp3, the second base end of the third increasing outer diameter portion PiDO3 of the second mandrel M2' (i.e., the second tip end of the second small outer diameter portion PSDO2) will have reached the same position in the second pressing direction as the second base end of the second small inner diameter portion PSDI2 of the second die hole DH2.

[0087] During the above period, the material constituting the first tip side end of the intermediate material IW1 is extruded forward (towards the second tip side) through the gap between the third small outer diameter portion PSDO3 or the third increasing outer diameter portion PiDO3 of the second mandrel M2' and the second base end side end of the second small inner diameter portion PSDI2 of the second die hole DH2, and the thick-walled portion PTW and the increasing thickness portion PiT are formed at the second tip side end of the tubular member P1'.

[0088] Thereafter, at a second time point tp2 before the second tip-side end of the second large outer diameter portion PLDO2 of the second mandrel M2' reaches the inner peripheral surface of the second reduced inner diameter portion PdDI2 of the second die hole DH2, the advancement of the second mandrel M2' in the second pressing direction is stopped. During this time period, the material constituting the intermediate material IW1 is extruded forward (toward the second tip-side) through the gap between the second small outer diameter portion PSDO2 or the second increasing outer diameter portion PiDO2 of the second mandrel M2' and the second base-side end of the second small inner diameter portion PSDI2 of the second die hole DH2, thereby forming the small diameter portion PSD and the tapered portion PT on the second base-side side of the increasing-wall-thickness portion PiT of the tubular member P1'.

[0089] In the fourth step included in the second method, by controlling the driving of the second mandrel M2' and the sleeve SL by the second driving mechanism as described above, it is possible to form a tubular member P1' that further includes a thick-walled portion PTW and an increased-wall-thickness portion PiT at the end of the second tip side, in addition to the large diameter portion PLD, the small diameter portion PSD, and the tapered portion PT, as illustrated in Figure 11.

[0090] <effect> As described above, in the second method, a thick-walled portion having a thickness greater than that of the small diameter portion can be easily and reliably formed at one end of a tubular member having a cup-shaped large diameter portion integrally formed at the other end. [Example]

[0091] A tubular member having a large diameter portion integrally formed at one end thereof, which is formed by the method for forming a tubular member according to the present invention (method of the present invention), including the first and second methods described above, can be used as a component of a fixed constant velocity universal joint and a sliding constant velocity universal joint that constitute a constant velocity universal joint that constitutes a power transmission system in automobiles and various industrial machines, etc., by undergoing additional processing such as cutting and / or drawing.

[0092] Fig. 14 is a schematic diagram illustrating the configuration of an outer case of a Rzeppa-type joint, which is often used in drive shafts for front-wheel drive vehicles and is formed by additionally machining a tubular member having a large-diameter portion integrally formed at one end thereof, formed by the method of the present invention. Fig. 14(a) is a schematic perspective view of the outer case OC1 of the Rzeppa-type joint, and Fig. 14(b) is a top view of the outer case OC1 as viewed from the direction of the black arrow in Fig. 14(a). The portion to the left of the axis AX in Fig. 14(c) is a schematic cross-sectional view of the outer case OC1 taken along a plane including the line segment PP and the axis AX of the outer case OC1, as depicted in Fig. 14(b). The portion to the right of the axis AX in Fig. 14(c) is a schematic cross-sectional view of the outer case OC1 taken along a plane including the line segment QQ and the axis AX of the outer case OC1, as depicted in Fig. 14(b).

[0093] Figures 15 and 16 are schematic diagrams illustrating the process of forming the outer case of the Rzeppa joint shown in Figure 14 from a tubular material. Figure 15 is a schematic diagram illustrating the process of forming a tubular member P2 from a tubular material W2 via an intermediate material IW2 by the method of the present invention. Figure 16 is a schematic diagram illustrating the process of forming the outer case OC1 of the Rzeppa joint shown in Figure 14 from the tubular member P2 shown in Figure 15(c) via an intermediate product IP1.

[0094] 15(a) is a schematic diagram illustrating the shape of the tubular material W2 at the time when the first step included in the method of the present invention is completed, the upper part is a top view of the tubular material W2 observed from the base end side in the direction of the axis AX, and the lower part is a schematic cross-sectional view taken along a plane including the line segment TT drawn in the upper part and the axis AX. FIG. 15(b) is a schematic diagram illustrating the shape of the intermediate material IW2 at the time when the second step included in the method of the present invention is completed, the upper part is a top view of the intermediate material IW2 observed from the base end side in the direction of the axis AX, and the lower part is a schematic cross-sectional view taken along a plane including the line segment UU drawn in the upper part and the axis AX. (c) of Figure 15 is a schematic diagram illustrating the shape of the tubular member P2 at the time when the third and fourth steps included in the method of the present invention are completed, where the upper part is a top view of the tubular member P2 observed from the base end side in the direction of the axis AX, and the lower part is a schematic cross-sectional view along a plane that includes the line segment VV drawn in the upper part and also includes the axis AX.

[0095] The intermediate material IW2 illustrated in Fig. 15(b) can be formed from the tubular material W2 by performing a backward extrusion process including the first and second steps, as described in the description of the first method (see, for example, Fig. 7). Also, the tubular member P2 illustrated in Fig. 15(c) can be formed from the intermediate material IW2 by performing a forward extrusion process including the third and fourth steps, as described in the description of the first method (see, for example, Fig. 8).

[0096] Further, at the tip side of the small-diameter portion PSD of the cylindrical member P2 illustrated in (c) of FIG. 15, a portion having an outer diameter smaller than that of the base end side is formed. Such a multi-stage small-diameter portion PSD is not an essential component of the cylindrical member formed by the method of the present invention. However, for example, it can be formed by providing a portion having an inner diameter smaller than that of the base end side at the tip side of the second small inner diameter portion PSDI2 of the second die D2 used in the forward extrusion step.

[0097] Next, (a) of FIG. 16 is a schematic diagram illustrating the shape of an intermediate product IP1 at a stage prior to the formation of the outer case OC1 of the Zepar type joint illustrated in FIG. 14. The intermediate product IP1 illustrated in (a) of FIG. 16 can be formed by performing finishing on the large-diameter portion PLD and the tapered portion PT of the cylindrical member P2 illustrated in (c) of FIG. 15.

[0098] The upper part of (a) of FIG. 16 is a top view of the intermediate product IP1 when observing the intermediate product IP1 from the base end side in the direction of the axis AX. The left part of the middle part of (a) of FIG. 16 with respect to the axis AX is a schematic cross-sectional view of the intermediate product IP1 by a plane including the line segment W-W drawn in the upper part and including the axis AX of the intermediate product IP1. The right part of the middle part of (a) of FIG. 16 with respect to the axis AX is a schematic cross-sectional view of the intermediate product IP1 by a plane including the line segment X-X drawn in the upper part and including the axis AX of the intermediate product IP1. The lower part of (a) of FIG. 16 is an enlarged view of the portion surrounded by a thick broken line in the middle part.

[0099] As illustrated in the middle and lower parts of (a) of FIG. 16, the cross-sectional shape of the inner peripheral surface of the large-diameter portion of the intermediate product IP1 by a plane including the axis AX is linear. By performing further finishing on the inner peripheral surface of the large-diameter portion of the intermediate product IP1 having such a structure, the outer case OC1 of the Zepar type joint illustrated in FIG. 14 can be formed.

[0100] Fig. 16(b) is a schematic diagram illustrating the shape of an outer case OC1 of a Rzeppa-type joint formed from the intermediate product IP1 illustrated in Fig. 16(a). The upper part of Fig. 16(b) is a top view of the outer case OC1 when the outer case OC1 is observed from the base end side in the direction of the axis AX. The portion to the left of the axis AX in the middle part of Fig. 16(b) is a schematic cross-sectional view of the outer case OC1 taken along a plane including the line segment YY drawn in the upper part and the axis AX of the outer case OC1, and the portion to the right of the axis AX in the middle part of Fig. 16(b) is a schematic cross-sectional view of the outer case OC1 taken along a plane including the line segment ZZ drawn in the upper part and the axis AX of the outer case OC1. The lower part of Fig. 16(b) is an enlarged view of the portion surrounded by the thick dashed line in the middle part.

[0101] As illustrated in FIG. 16(b), a portion of the inner circumferential surface of the cup-shaped large-diameter portion of the outer case OC1 (the portion shown to the left of the axis AX in the example shown in FIG. 16(b)) is formed as a curved surface (negative angle portion) that bulges gently radially outward (see the black arrow in FIG. 16(b)). As is well known to those skilled in the art, this structure having a negative angle portion on at least a portion of the inner circumferential surface of the cup-shaped large-diameter portion is provided for the purpose of allowing the ball retaining cage to slide in response to axial bending when, for example, the shaft on the outer race side and the shaft on the inner race side tilt (axial bending occurs) as a result of steering, mainly in FF vehicles (front-engine, front-wheel drive vehicles). This structure is suitable for ensuring sufficient tracking ability and reliably transmitting torque between the two shafts. The configuration of the area on the inner circumferential surface of the cup-shaped large-diameter portion where this curved surface is formed is appropriately designed, for example, depending on the configuration of the Rzeppa joint and / or the expected degree of axial bending, etc.

[0102] Note that various methods well known to those skilled in the art can be used for finishing the large diameter portion PLD and tapered portion PT of the cylindrical member P2 shown in Fig. 15(c) to form complex shapes such as the large diameter portions of the outer cases OC1 and OC2 shown in Fig. 16. Specific examples of such methods include a method in which a cup-shaped large diameter portion is formed by a combination of warm forging and cold ironing, or by cold ironing alone if possible, followed by cutting if necessary, and further by heat hardening where necessary.

[0103] As is clear from the above, the method of the present invention makes it possible to form a tubular member having a complex cup-shaped large-diameter portion, such as the outer case of a Rzeppa joint, from a bottomless tubular material, rather than from a bottomed material that requires additional processing to remove the bottom. Furthermore, the method of the present invention reduces the number of times plastic processing is repeated, thereby reducing work hardening of the workpiece and, as a result, the processing load. Furthermore, the method of the present invention offers a high degree of freedom in terms of processing conditions and formable shapes, and allows the cup-shaped large-diameter portion to be integrally formed at one end of the tubular member.

[0104] On the other hand, Fig. 17 is a schematic diagram illustrating the configuration of an outer case of a tripod joint formed by additionally machining a tubular member formed by the method of the present invention, the tubular member having a large-diameter portion integrally formed at one end. Fig. 17(a) is a schematic perspective view of the outer case OC3 of the tripod joint, and Fig. 17(b) is a top view of the outer case OC3 when viewed from the direction of the black arrow in Fig. 17(a). The portion to the left of the axis AX in Fig. 17(c) is a schematic cross-sectional view of the outer case OC3 taken along a plane including the line segment RR and the axis AX of the outer case OC3, as depicted in Fig. 17(b). The portion to the right of the axis AX in Fig. 17(c) is a schematic cross-sectional view of the outer case OC3 taken along a plane including the line segment SS and the axis AX of the outer case OC3, as depicted in Fig. 17(b).

[0105] As shown in FIG. 17, unlike the outer case OC1 shown in FIG. 16(b), the inner circumferential surface of the cup-shaped large-diameter portion of the outer case OC2 does not have a curved surface that bulges outward in the radial direction. Therefore, in this structure, grooves formed on the inner circumferential surface of the outer case are linear and parallel to the axial direction to contact rollers provided on the inner case (inner race) and to retain balls between the grooves formed on the inner race. This structure is suitable for ensuring sufficient compliance and reliably transmitting torque between the shafts on the outer race and the inner race when the degree of axial bending is small and the shafts on the outer race and the inner race move relatively linearly mainly in the axial direction. Specific examples of such cases, as are well known to those skilled in the art, include torque transmission in an inboard joint (differential side) of a vehicle and torque transmission to the rear wheel drive shaft (tire-side shaft) in a front-engine, rear-wheel-drive vehicle (FR vehicle).

[0106] As is clear from Figures 14 to 17, a tubular member formed by the method of the present invention, with a large diameter portion integrally formed at one end, can be used, by additional processing, as the outer case of a Rzeppa-type joint and / or the outer case of a tripod-type joint, which are often used in drive shafts of front-wheel drive vehicles, for example.

[0107] However, the configuration and uses of a tubular member having a large diameter portion integrally formed at one end thereof formed by the method of the present invention are not limited to the configurations and uses exemplified in this specification, and a wide variety of configurations are possible and the member can be used in a wide variety of applications as long as they do not deviate from the scope of the present invention.

[0108] For the purpose of explaining the present invention, several embodiments and examples having specific configurations have been described above, sometimes with reference to the accompanying drawings. However, the scope of the present invention should not be construed as being limited to these exemplary embodiments and examples, and it goes without saying that appropriate modifications can be made within the scope of the claims and the matters described in the specification. [Explanation of symbols]

[0109] AX…Axis (center axis) W1, W2...Cylindrical material DO1…1st outer diameter DI1...First inner diameter T1...First thickness L1...First length IW1, IW2...Intermediate materials PLDI…Large inner diameter part DO2…2nd outer diameter DI2...Second inner diameter T2…Second wall thickness Puf...unprocessed part PdDI…Reduced inner diameter part P1, P1', P2...Cylindrical members, PLD: Large diameter section DO2…2nd outer diameter DI2...Second inner diameter T2…Second wall thickness PSD…Small diameter section DO3…3rd outer diameter DI3...Third inner diameter T3…Third wall thickness PT...Tapered section IP1: Intermediate product OC1...Rzeppa type joint outer case OC2...Outer case of tripod joint M1...First mandrel PSDO1...1st small outer diameter section DO4…4th outer diameter PLDO1...1st large outer diameter section DO5…5th outer diameter PiDO1: First outer diameter increase section D1...1st die DH1...First die hole PLDI1: First large bore section DI4...Fourth inner diameter PSDI1: First small inner diameter section DI5...5th inner diameter PS1: First stage SS…support surface M2, M2'...Second mandrel PSDO2…Second small outer diameter part DO6…6th outer diameter PLDO2…Second large outer diameter part DO7…7th outer diameter PiDO2: Second outer diameter increase section PSDO3…3rd small outer diameter part DO9…9th outer diameter PiDO3: Third outer diameter increase section SL...Sleeve PP...Pressing part DO8…8th outer diameter DI6...6th inner diameter T4…4th wall thickness D2...Second die DH2: Second die hole PLDI2…Second large inner diameter part DI7...7th inner diameter PSDI2…Second small inner diameter part DI8...8th inner diameter PdDI2…Second inner diameter reduced part

Claims

1. A method for forming a tubular member, the method comprising: forming a tubular member from a tubular material, the tubular member having a large diameter portion integrally formed at one end thereof; a backward extrusion process in which, using a first extrusion molding device including a first mandrel which is a columnar core metal having a predetermined shape, a first die which is a hole having a predetermined shape formed therein, and a first drive mechanism which drives the first mandrel and the first die so as to approach each other in the axial direction, the other end of the tubular material which is an end opposite to the one end inserted into the first die hole is fixed, and the first mandrel is pushed in from the one end to form a large inner diameter portion whose inner diameter is enlarged at the one end of the tubular material by backward extrusion processing, thereby forming an intermediate material having the large inner diameter portion integrally formed at the one end; a forward extrusion process using a second extrusion molding device including a second mandrel which is a columnar core metal having a predetermined shape, a sleeve which is a cylindrical member disposed coaxially with the second mandrel, a second die which has a second die hole formed therein, and a second drive mechanism which drives the second mandrel, the sleeve, and the second die so as to approach each other in the axial direction, to press the one end of the intermediate material inserted into the second die hole with the sleeve and to drive the second mandrel in conjunction with the sleeve to force the other end of the intermediate material into the second die hole, thereby reducing the diameter of the other end of the intermediate material by forward extrusion to form a small diameter portion, and leaving the large inner diameter portion formed at the one end as the large diameter portion, thereby forming the tubular member having the large diameter portion integrally formed at the one end and the small diameter portion integrally formed at the other end; Including, A method for forming a tubular member.

2. 2. A method for forming a cylindrical member according to claim 1, comprising: the tubular material is a cylindrical member having a first outer diameter that is a predetermined outer diameter, a first inner diameter that is a predetermined inner diameter, a first wall thickness that is a predetermined wall thickness, and a first length that is a predetermined length in an axial direction, the intermediate material includes: the large inner diameter portion, which is a cylindrical portion formed at an end portion on a first base end side that is upstream in a first pressing direction, which is a direction in which the first mandrel approaches the first die, and which has a second outer diameter equal to the first outer diameter, a second inner diameter larger than the first inner diameter, and a second thickness smaller than the first thickness; an unmachined portion, which is a portion left as the tubular material at an end portion on a first tip end side that is downstream in the first pressing direction; and an intermediate inner diameter decreasing portion, which is a portion formed between the large inner diameter portion and the unmachined portion, and whose inner diameter decreases from the second inner diameter to the first inner diameter as it approaches the unmachined portion side from the large inner diameter portion side, the cylindrical member includes: the large diameter portion, which is the large inner diameter portion of the intermediate material left at an end portion on a second base end side that is upstream in a second pressing direction, which is a direction in which the second mandrel and the sleeve approach the second die; the small diameter portion, which is a cylindrical portion formed on a second tip end side that is downstream in the second pressing direction from the large diameter portion and has a third outer diameter smaller than the first outer diameter, a third inner diameter equal to or smaller than the first inner diameter, and a third thickness that is a predetermined thickness; and a tapered portion, which is formed between the large diameter portion and the small diameter portion and is a portion whose outer diameter decreases from the second outer diameter to the third outer diameter and whose inner diameter decreases from the second inner diameter to the third inner diameter as it approaches the small diameter portion side from the large diameter portion side, the first mandrel includes: a first small outer diameter portion that is a cylindrical portion formed at an end portion on the first tip side and having a fourth outer diameter that is an outer diameter corresponding to the first inner diameter; a first large outer diameter portion that is a cylindrical portion formed on the first base end side and having a fifth outer diameter that is an outer diameter corresponding to the second inner diameter; and a first increasing outer diameter portion that is formed between the first small outer diameter portion and the first large outer diameter portion and has an outer diameter that increases from the fourth outer diameter to the fifth outer diameter as it approaches the first small outer diameter portion side toward the first large outer diameter portion side, and a second length that is a size of the first small outer diameter portion in the first pressing direction is equal to or greater than the first length, the first die hole includes: a first large inner diameter portion formed on the first base end side and having a fourth inner diameter that is an inner diameter corresponding to the first outer diameter; a first small inner diameter portion formed on the first tip end side and having a fifth inner diameter equal to the first inner diameter; and a first step portion formed at a boundary between the first large inner diameter portion and the first small inner diameter portion, having an outer diameter corresponding to the fourth inner diameter and an inner diameter equal to the fifth inner diameter, and having a support surface that is an annular flat surface perpendicular to the first pressing direction, the second mandrel includes: a second small outer diameter portion that is a cylindrical portion formed at an end portion on the second tip side and having a sixth outer diameter that is an outer diameter corresponding to the third inner diameter; a second large outer diameter portion that is a cylindrical portion formed on the second base end side and having a seventh outer diameter that is an outer diameter corresponding to the second inner diameter; and a second increasing outer diameter portion that is formed between the second small outer diameter portion and the second large outer diameter portion and has an outer diameter that increases from the sixth outer diameter to the seventh outer diameter as it approaches the second large outer diameter portion from the second small outer diameter portion side, the sleeve includes a pressing portion that is a cylindrical portion formed at an end portion on the second tip side and that has an eighth outer diameter equal to the first outer diameter, a sixth inner diameter equal to the second inner diameter, and a fourth wall thickness equal to the second wall thickness, the second die hole includes: a second large inner diameter portion formed on the second base end side and having a seventh inner diameter corresponding to the second outer diameter; a second small inner diameter portion formed on the second tip end side and having an eighth inner diameter corresponding to the third outer diameter; and a second inner diameter decreasing portion formed between the second large inner diameter portion and the second small inner diameter portion and having an inner diameter that decreases from the seventh inner diameter to the eighth inner diameter as the diameter approaches the second small inner diameter portion from the second large inner diameter portion side, The backward extrusion step a first step of supporting the tubular material at a predetermined position inside the first die hole by inserting the tubular material into the first die hole and bringing the end of the tubular material on the first tip side into contact with the support surface of the first step; a second step of inserting the first mandrel from the first base end side of the tubular material and driving the first mandrel toward the first tip side by the first drive mechanism to expand the inner diameter of the end portion of the tubular material on the first base end side, thereby forming the large inner diameter portion at the end portion of the tubular material on the first base end side; Including, The forward extrusion step a third step of inserting the intermediate material into the second die hole and bringing an end portion of the intermediate material on the first tip side into contact with an end portion of the second reduced inner diameter portion on the second base side, thereby supporting the intermediate material at a predetermined position inside the second die hole; a fourth step of inserting the second mandrel from the second base end side of the intermediate material, inserting the sleeve into a space between the outer circumferential surface of the second mandrel and the inner circumferential surface of the second large inner diameter portion of the second die hole, and driving the second mandrel and the sleeve toward the second tip side by the second drive mechanism so that the inner circumferential surface of the intermediate inner diameter reducing portion of the intermediate material is pressed toward the second tip side by the outer circumferential surface of the second outer diameter increasing portion of the second mandrel and the end of the large inner diameter portion of the intermediate material on the first base end side is pressed toward the second tip side by the pressing portion of the sleeve, thereby forming the small diameter portion and the tapered portion; Including, In the fourth step, the second mandrel and the sleeve are driven by the second drive mechanism so that a portion of the second mandrel located on the second tip side of the second large outer diameter portion faces the second base end side end of the second small inner diameter portion of the second die hole in the radial direction over a period from a first time point, which is a predetermined time point before a material constituting the first tip side end of the intermediate material starts to flow toward the second tip side beyond the second base end side end of the second inner diameter reduced portion of the second die hole, to a second time point, which is a predetermined time point at which the second mandrel stops moving in the second pressing direction before the second tip side end of the second large outer diameter portion of the second mandrel reaches the inner circumferential surface of the second inner diameter reduced portion of the second die hole. A method for forming a tubular member.

3. 3. The method for forming a cylindrical member according to claim 2, the cylindrical member further includes: a thick-walled portion that is formed closer to the second tip than the small diameter portion and is a cylindrical portion having a tenth outer diameter equal to the third outer diameter, a ninth inner diameter smaller than the third inner diameter, and a fifth thickness larger than the third thickness; and an increasing-wall-thickness portion that is formed between the small diameter portion and the thick-walled portion and has an inner diameter that decreases from the third inner diameter to the ninth inner diameter and a thickness that increases from the third thickness to the fifth thickness as the small diameter portion approaches the thick-walled portion, the second mandrel further includes: a third small outer diameter portion that is a cylindrical portion formed further toward the second tip side than the second small outer diameter portion and having a ninth outer diameter smaller than the sixth outer diameter; and a third increasing outer diameter portion that is formed between the second small outer diameter portion and the third small outer diameter portion and whose outer diameter increases from the ninth outer diameter to the sixth outer diameter as it approaches the second small outer diameter portion side, in the fourth step, the second mandrel and the sleeve are driven by the second drive mechanism over a period from the first time point to a third time point, which is a predetermined time point before the second time point, so that the third small outer diameter portion or the third increasing outer diameter portion of the second mandrel faces an end portion of the second small inner diameter portion of the second die hole on the second base end side in the radial direction. A method for forming a tubular member.

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