Method for manufacturing cylindrical pipe having flared portion at pipe end

Abstract

To reduce defects such as cracks in a flare part and damage to a die in a method for manufacturing a cylindrical tube having the flare part at a tube end.SOLUTION: A first bottomless cylindrical tube is formed by performing punching in a transition part connecting a straight tube part and a bottom part of a bottomless cylindrical tube, a second bottomless cylindrical tube is formed by performing push-in drawing with the transition part remaining in the first bottomless cylindrical tube as a head, and flare working is performed in an end part on the transition part side of the second bottomless cylindrical tube. Preferably, the shape of each of the straight pipe portion, the transition portion and the bottom portion provided in the bottomed cylindrical pipe is a rotating body having a pipe axis, which is an axis of the straight pipe portion, as a rotation axis, and the first punch in the first step is a rotating body having a pipe axis as a rotation axis and the direction in which the first punch is driven is parallel to the pipe axis. Further, the inner peripheral surface of the second bottomless cylindrical tube may be subjected to ironing before the flaring.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a cylindrical pipe having a flared portion at the pipe end. [Background technology]

[0002] For example, a cylindrical member formed from a sheet material by drawing has a bottom. A technique is known in which the bottom of such a cylindrical pipe material is removed and then the diameter is expanded using a punch to form a flared portion at the end of the cylindrical pipe.

[0003] For example, Patent Document 1 (JP Patent Publication No. 11-309520) discloses a technology in which a hole is formed by punching out the flat portion of the bottom of a bottomed tubular portion formed by drawing a flat plate with a punch, and a burring punch is inserted into this hole from the inside to expand the periphery of the hole and make it bulge to the same diameter as the bottomed tubular portion, forming a straight tubular portion, and then an expanding punch is inserted from the outside to bend the thin-walled portion of the tubular portion outward, thereby forming an expanded portion (sometimes called a "flared portion") at the tip of the tubular portion.

[0004] Furthermore, Patent Document 2 (JP 2016-187826 A) discloses a technology in which a metal tube is formed by removing the bottom of a bottomed tubular section formed by drawing a metal plate and the section connecting the bottom and a straight tube section (sometimes referred to as an "R section"), and then a punch having an outer diameter larger than the inner diameter of the metal tube is inserted into the metal tube to plastically deform the metal tube and expand it outward in the radial direction (diameter expansion). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-309520 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-187826 Summary of the Invention [Problem to be solved by the invention]

[0006] As mentioned above, various techniques are known in the art for forming a flared portion at the end of a cylindrical tube or for expanding the diameter of a cylindrical tube by removing the bottom of a cylindrical tube material with a bottom and then expanding the diameter of the cylindrical tube with a punch.

[0007] However, in the process of forming the flared portion in the technology disclosed in Patent Document 1, the rough cut surface punched by the punch is the largest in the process of forming the flared portion, increasing the risk of defects such as cracks originating from cracks on the cut surface. Furthermore, a problem common to the technologies disclosed in Patent Documents 1 and 2 is that the inner peripheral surface (cut surface) of the hole punched by the punch and the end surface (cut surface) of the opening end formed by removing the bottom and R portion are perpendicular to the pipe wall. Therefore, in the next process of forming the flared portion (flaring) or expanding the diameter of the metal pipe (diameter expansion), the sharp edges of the cut surface come into contact with a mold or the like, potentially damaging the mold or the like.

[0008] One possible solution to alleviate the above problems is to perform a facing process on the cut surface, such as by pressing and / or cutting, before the next process. However, adding such a facing process increases the number of steps and costs involved in the manufacturing method of a cylindrical tube with a flared end.

[0009] As described above, there is a need in the technical field for a technique that can reduce defects such as cracks in the flared portion and damage to the mold in a manufacturing method of a cylindrical tube having a flared portion at the tube end. [Means for solving the problem]

[0010] Therefore, as a result of extensive research, the inventors have found that the above-mentioned problems can be solved by forming a first bottomless cylindrical tube by performing a hole-punching process on the transition section connecting the straight section and the bottom section of a bottomed cylindrical tube, performing a press-drawing process starting from the remaining transition section of the first bottomless cylindrical tube to form a second bottomless cylindrical tube, and performing a flaring process on the end section side of the second bottomless cylindrical tube.

[0011] Specifically, the method for manufacturing a cylindrical pipe having a flared portion at its end according to the present invention (hereinafter sometimes referred to as the "method of the present invention") is a method for manufacturing a cylindrical pipe having a flared portion at its end, in which a cylindrical pipe having a flared portion at its end is manufactured from a bottomed cylindrical pipe. The bottomed cylindrical pipe has a straight pipe portion, a bottom portion, and a transition portion interposed between the straight pipe portion and the bottom portion, where the outer diameter decreases as the pipe approaches the bottom portion. The method of the present invention includes the first to third steps listed below.

[0012] The first step is a step of obtaining a first bottomless cylindrical tube by inserting a first punch, which is a hole-punching punch, into a bottomed cylindrical tube placed in a first die, which is a hole-punching die, and performing a hole-punching process in which the first punch punches out and removes the first transition portion, which is part of the transition portion, and the bottom portion.

[0013] The second step is a step of obtaining a second bottomless cylindrical tube by forcing the first bottomless cylindrical tube, starting from the second transition portion, which is the transition portion remaining in the first bottomless cylindrical tube, into a drawing hole of a second die, which is a drawing die formed with a drawing hole including a portion having an inner diameter smaller than the outer diameter of the straight tube portion, thereby performing a forcing drawing process to reduce the diameter of at least a portion of the straight tube portion and the second transition portion.

[0014] The third step is a step of performing flaring processing in which a second punch, which is an expanding punch with a tapered portion, is pushed into the opening on the second transition section side of the second bottomless cylindrical tube installed in the third die to expand the end on the second transition section side of the second bottomless cylindrical tube and form a flared portion, thereby obtaining a cylindrical tube with a flared portion at the tube end.

[0015] Furthermore, the outer peripheral surface of the transition portion of the bottomed cylindrical tube at the position where the cut surface is formed by punching in the piercing process executed in the first step is inclined with respect to the first direction, which is the punching direction by the first punch.

[0016] Furthermore, a first plane, which faces the bottomed cylindrical tube and is perpendicular to the first direction, is formed at least on the periphery adjacent to the punching hole formed in the first die.

[0017] In addition, as the hole punching process is performed in the first step, the tip of the second transitional portion is pressed against the first plane, and a second plane, which is a plane onto which the first plane is transferred, is formed at the tip of the second transitional portion. [Effects of the Invention]

[0018] As described above, in the method of the present invention, in the first step, a first bottomless cylindrical tube is formed by punching a transition section connecting the straight section and the bottom section of a bottomed cylindrical tube. The outer peripheral surface of the transition section of the bottomed cylindrical tube at the position where the cut surface is formed by punching is inclined with respect to a first direction. In other words, the punching direction (first direction) in the punching process performed in the first step is inclined with respect to the outer peripheral surface at the cut position of the transition section. In other words, the punching process performed in the first step corresponds to so-called "slope punching." As a result, the punched portion of the first bottomless cylindrical tube (i.e., the second transition section, which is the transition section remaining in the first bottomless cylindrical tube) has a tapered shape in which the outer diameter and wall thickness decrease toward the tip.

[0019] As described above, the first die has a first plane, which faces the bottomed cylindrical tube and is perpendicular to the first direction, which is the punching direction of the first punch, formed at least on the peripheral portion adjacent to the punching hole formed in the first die. Therefore, as the hole punching process is performed in the first step, the tip of the second transition portion is pressed against the first plane, and a second plane, which is a plane onto which the first plane is transferred, is formed at the tip of the second transition portion. Because this second plane is a plane onto which the first plane is transferred, it has a smoother surface than a cut surface onto which the first plane is not transferred.

[0020] Next, in the second step, a second bottomless cylindrical tube is formed by performing a forced drawing process starting from the second transition section. In this forced drawing process, as described in detail below, the machining reference surface that comes into contact with the inner peripheral surface of the drawing hole in the second die is the outer peripheral surface of the first bottomless cylindrical tube. The resulting outer diameter of the reduced-diameter portion of the second bottomless cylindrical tube is a constant value corresponding to the inner diameter of the drawing hole. Therefore, the second bottomless cylindrical tube has a shape in which the outer diameter is constant, but the inner diameter increases and the wall thickness decreases toward the tip of the second transition section.

[0021] Next, in the third step, a second punch, which is a tube-expanding punch with a tapered portion, is inserted into the opening of the second bottomless cylindrical tube on the second transition section side to flare the end of the second bottomless cylindrical tube on the second transition section side, thereby obtaining a cylindrical tube with a flared portion at the tube end. During this flaring, the tapered portion of the second punch presses against the inner circumferential surface of the second bottomless cylindrical tube, which is inclined so that the inner diameter increases toward the tip of the second transition section. Therefore, damage to the second punch is less than in the case of the prior art described above, where the sharp edge of the cut surface punched out by piercing is pressed by the tube-expanding punch.

[0022] Furthermore, the tip of the second transition portion, which is the area that is most greatly expanded during flaring, is formed with a second flat surface that is a smooth and favorable flat surface as a result of the transfer of the first flat surface as described above. Therefore, compared to the prior art described above, there is a smaller possibility of defects such as cracks originating from cracks present on the cut surface.

[0023] That is, according to the present invention, defects such as cracks in the flared portion and damage to the mold can be reduced in a method for manufacturing a cylindrical tube having a flared portion at the tube end.

[0024] 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]

[0025] [Figure 1] 1 is a schematic cross-sectional view illustrating the configuration of a bottomed cylindrical tube used as a raw material in a manufacturing method (first method) for a cylindrical tube provided with a flared portion at its end according to a first embodiment of the present invention. FIG. [Figure 2] 1 is a flowchart showing the flow of each step included in a first method. [Figure 3] 1A to 1C are schematic cross-sectional views illustrating the change from a cylindrical tube with a bottom to a cylindrical tube having a flared portion at the tube end as each step, including the first step to the third step, performed in the first method progresses. [Figure 4] This is a schematic cross-sectional view illustrating the state of the first die, the first punch, and the first bottomless cylindrical tube at the point when the punching process performed in the first step has completed punching out the first transition section and the bottom from the bottomed cylindrical tube. [Figure 5] FIG. 1 is a schematic cross-sectional view illustrating the manner in which the first transition portion and bottom portion of the bottomed cylindrical tube are punched out from the bottomed cylindrical tube in the first step; FIG. 2 is a schematic cross-sectional view illustrating the configuration of the tip portion of a portion of the transition portion (second transition portion) remaining at the tip of the first bottomless cylindrical tube obtained by performing the first step; and FIG. 3 is a schematic isometric view of the first bottomless cylindrical tube obtained by performing the first step. [Figure 6] A schematic cross-sectional view illustrating the state in which the outer surface of the second transition portion of the first bottomless cylindrical tube is forced into the drawing hole of the second die to reduce its diameter in the second step, a schematic cross-sectional view illustrating the configuration of the second transition portion of the second bottomless cylindrical tube obtained by performing the second step, and a schematic isometric projection view of the second bottomless cylindrical tube obtained by performing the second step. [Figure 7] 10 is a schematic cross-sectional view illustrating the formation of a flared portion by pressing a second punch into the second bottomless cylindrical tube from the second transition portion side in the third step. FIG. [Figure 8] A schematic cross-sectional view illustrating the state in which a second punch is pushed into the opening on the second transition section side of a second bottomless cylindrical tube in the third step included in the first method, a schematic cross-sectional view illustrating the state in which an expansion punch is pushed into the opening formed by cutting off the bottom of a bottomed cylindrical workpiece in flaring processing included in a conventional method, and a schematic isometric projection view of a cylindrical tube having a flared section at the tube end obtained by performing the third step. [Figure 9] 10 is a flowchart showing the flow of each step included in a manufacturing method (third method) for a cylindrical tube provided with a flared portion at the tube end according to a third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] First Embodiment A method for manufacturing a cylindrical pipe having a flared portion at its pipe end according to a first embodiment of the present invention (hereinafter, sometimes referred to as "first method") will be described below with reference to the drawings.

[0027] <composition> The first method is a method for manufacturing a cylindrical pipe having a flared portion at its end, in which a cylindrical pipe having a flared portion at its end is manufactured from a bottomed cylindrical pipe. The bottomed cylindrical pipe has a straight pipe portion, a bottom portion, and a transition portion interposed between the straight pipe portion and the bottom portion, where the outer diameter decreases as the pipe approaches the bottom portion.

[0028] The basic configuration of an apparatus for carrying out the first method is well known to those skilled in the art, and therefore a detailed description thereof will be omitted. However, the components, including the die and punch, are made of materials having properties (e.g., mechanical strength and durability) that can withstand processing conditions, such as the load acting on the components, during the process of carrying out the first method. Furthermore, for example, the drive mechanism that drives the various punches and presses the first bottomless cylindrical tube, which is the intermediate material, into the drawing hole in steps 1 to 3 described below can be appropriately selected from various drive mechanisms well known in the art depending on the properties (e.g., mechanical strength and hardness) of the material that constitutes the material. Typically, a press, such as a hydraulic press, is used as the drive mechanism.

[0029] The bottomed cylindrical tube used in the first method is a bottomed cylindrical member made of a material that can be formed by cold forging. The material that makes up such a raw material is not particularly limited as long as it can be formed into a desired shape by cutting and plastic deformation in the piercing, drawing, and flaring processes described below. Typically, the material that makes up the bottomed cylindrical tube is a metal such as lead, tin, aluminum, copper, zirconium, titanium, molybdenum, vanadium, niobium, and iron, or an alloy containing two or more of these metals.

[0030] Furthermore, the method for manufacturing the bottomed cylindrical tube described above is not particularly limited and can be appropriately selected from various methods known to those skilled in the art. For example, the bottomed cylindrical tube can be manufactured by drawing a flat plate-shaped member made of the above-mentioned material.

[0031] Figure 1 is a schematic cross-sectional view illustrating the configuration of a bottomed cylindrical tube used as a raw material in method 1. The bottomed cylindrical tubes 11 and 12 illustrated in Figure 1(a) and (b), respectively, each include a straight pipe section PS, a bottom section PB, and a transition section PC (hatched in a diagonal grid pattern) located between the straight pipe section PS and the bottom section PB, where the outer diameter decreases as the tube approaches the bottom section PB.

[0032] However, the transition portion PC of the bottomed cylindrical pipe 11 connects the straight pipe portion PS and the bottom portion PB in a curved line in a cross section taken along a plane including the pipe axis AX, which is the axis of the straight pipe portion PS. On the other hand, the transition portion PC of the bottomed cylindrical pipe 12 connects the straight pipe portion PS and the bottom portion PB in a linear line in a cross section taken along a plane including the pipe axis AX, which is the axis of the straight pipe portion PS. As such, the transition portion of the bottomed cylindrical pipe used in the first method is not particularly limited as long as it is a portion interposed between the straight pipe portion and the bottom portion and whose outer diameter decreases as it approaches the bottom portion. In the following description, the bottomed cylindrical pipe 11 illustrated in FIG. 1(a) is used as an example of the bottomed cylindrical pipe used in the first method.

[0033] Fig. 2 is a flowchart showing the flow of each step included in the first method. Fig. 3 is a schematic cross-sectional view illustrating the change from a bottomed cylindrical tube to a cylindrical tube having a flared portion at the tube end as each step, including the first step to the third step, performed in the first method progresses. As illustrated in Figs. 2 and 3, the first method includes the first step (step S10), the second step (step S20), and the third step (step S30) listed below.

[0034] In addition, in Fig. 3, step 0, in which a bottomed cylindrical tube used as a raw material in the first method is produced by drawing, is depicted before step 1, but the step of producing a bottomed cylindrical tube is not an essential component of the first method, and as described above, the method of producing a bottomed cylindrical tube is not particularly limited. Also, in Fig. 3, step 4, in which ironing is performed, is depicted between steps 2 and 3, but such step 4 is also not an essential component of the first method. Details of step 4 will be described in detail later in the description of other embodiments of the present invention.

[0035] The first process performed in step S10 is a process of obtaining a first bottomless cylindrical tube 41 by inserting a first punch 31, which is a hole-punching punch, into a bottomed cylindrical tube 11 placed on a first die 21, which is a hole-punching die in which a punching hole has been formed, and performing a hole-punching process in which the first transition portion, which is part of the transition portion, and the bottom portion are punched out and removed with the first punch 31.

[0036] In the above-mentioned Fig. 1, the position punched out by the piercing process performed in the first step is indicated by a thick solid arrow. As illustrated in Fig. 1, the outer peripheral surface of the transition portion PC of the bottomed cylindrical tube (11 or 12) at the position punched out by the piercing process performed in the first step to form a cut surface is inclined with respect to the first direction, which is the punching direction by the first punch 31. In other words, the punching direction (first direction) in the piercing process performed in the first step is inclined with respect to the outer peripheral surface at the cutting position of the transition portion PC. In other words, the punching in the piercing process performed in the first step corresponds to so-called "slope punching."

[0037] 4 is a schematic cross-sectional view illustrating the state of the first die 21, the first punch 31, and the first bottomless cylindrical tube 41 at the time when punching of the first transition portion PC1 and the bottom portion PB from the bottomed cylindrical tube 11 by the piercing process performed in the first step is completed. As illustrated in Fig. 4, the portion of the first bottomless cylindrical tube 41 that has been pierced (i.e., the second transition portion PC2, which is the transition portion that remains in the first bottomless cylindrical tube 41) has a tapered shape in which the outer diameter and wall thickness become smaller toward the tip (the lower end in Fig. 4).

[0038] In the first method, a first plane, which is a plane facing the bottomed cylindrical tube 11 and perpendicular to the first direction, is formed at least on the peripheral portion adjacent to the punching hole formed in the first die 21. Therefore, as the punching process is performed in the first step, the tip of the second transition portion PC2 is pressed against the first plane, and a second plane, which is a plane onto which the first plane is transferred, is formed at the tip of the second transition portion PC2.

[0039] 5(a) to 5(c) are schematic cross-sectional views illustrating how the first transition portion PC1 and the bottom portion PB of the bottomed cylindrical tube 11 are punched out of the bottomed cylindrical tube 11 in the first step. FIG. 5(d) is a schematic cross-sectional view illustrating the configuration of the tip portion of a part of the transition portion PC (second transition portion PC2) remaining at the tip of the first bottomless cylindrical tube 41 obtained by performing the first step. FIG. 5(e) is a schematic isometric view of the first bottomless cylindrical tube 41 obtained by performing the first step.

[0040] 5(a) to 5(c), in the first step, a first punch 31, which is a piercing punch, is inserted into a bottomed cylindrical tube 11 placed in a first die 21, which is a piercing die in which a piercing hole has been formed, and a piercing process is performed in which a first transition portion PC1, which is a part of the transition portion PC of the bottomed cylindrical tube 11, and a bottom portion PB are punched out and removed by the first punch 31. This results in a first bottomless cylindrical tube 41, in which a second transition portion PC2, which is a part of the transition portion PC, remains at the tip (the lower end in FIG. 5), as shown in FIG. 5(e).

[0041] As illustrated in (a) of Figure 5, a first plane FS1, which is a plane facing the bottomed cylindrical tube 11 and perpendicular to the first direction, is formed on the peripheral portion adjacent to the punching hole formed in the first die 21. Therefore, as illustrated in (c) of Figure 5, as the punching process is performed in the first step, the tip of the second transition portion PC2 is pressed against the first plane FS1, and a second plane FS2, which is a plane onto which the first plane FS1 is transferred, is formed at the tip of the second transition portion PC2.

[0042] Fig. 5(d) is an enlarged view of the second transition portion PC2 surrounded by a thick dashed line in Fig. 5(c). Note that the tube axis AX shown in Fig. 5(d) does not accurately indicate the position of the tube axis AX, but rather indicates on which side of the second transition portion PC2 remaining at the tip of the first bottomless cylindrical tube 41 the tube axis AX is located. In Fig. 5(d), the second plane FS2 formed at the tip of the second transition portion PC2 is indicated by a thick solid double-headed arrow, and the outer peripheral surface OPS of the second transition portion PC2 is indicated by a thick dashed double-headed arrow. In addition, a corner PE, which is the boundary between the second plane FS2 and the outer peripheral surface OPS, is surrounded by a thick dashed line.

[0043] The second plane FS2 is a plane onto which the first plane FS1 has been transferred, and therefore has a smoother surface than a cut surface onto which the first plane FS1 has not been transferred. The outer peripheral surface OPS of the second transition portion PC2 has a tapered shape with an outer diameter that decreases toward the tip. Furthermore, the corner portion PE is a boundary line between the outer peripheral surface OPS and the second plane FS2, which is a plane onto which the first plane FS1 has been transferred and has a smooth surface, as described above, and does not include a cut surface resulting from the punching process.

[0044] 5, the first flat surface FS1 is formed over the entire range from the periphery of the punch hole formed in the first die 21 to the inner circumferential surface of the first die 21 facing the outer circumferential surface of the straight pipe portion PS of the bottomed cylindrical tube 11. However, as long as the second flat surface FS2 can be formed at the tip of the second transition portion PC2, which is the portion that will be most greatly expanded in the flaring process performed in the third step described below, it is sufficient that the first flat surface FS1 is formed at least on the circumferential portion adjacent to the punch hole formed in the first die 21. Therefore, for example, the first flat surface FS1 may be formed only in a predetermined range on the circumferential portion adjacent to the punch hole formed in the first die 21, and an inner circumferential surface having a shape that follows the outer circumferential surface of the second transition portion PC2 remaining at the tip of the first bottomless cylindrical tube 41 may be formed in an area radially outward of the first flat surface FS1.

[0045] The second process performed in step S20 is a process of obtaining a second bottomless cylindrical tube 42 by forcing the first bottomless cylindrical tube 41, starting from the side of the second transition portion PC2, which is the transition portion remaining in the first bottomless cylindrical tube 41, into a drawing hole of the second die 22, which is a drawing die formed with a drawing hole including a portion having an inner diameter smaller than the outer diameter of the straight pipe portion PS, as illustrated in (c) of Figure 3, thereby performing a forcing drawing process to reduce the diameter of at least a portion of the straight pipe portion PS and the second transition portion PC2.

[0046] 6(a) to 6(c) are schematic cross-sectional views illustrating the second transition portion of the first bottomless cylindrical tube 41 being forced into the hole of the second die 22 and reduced in diameter in the second step. FIG. 6(d) is a schematic cross-sectional view illustrating the configuration of the second transition portion PC2 of the second bottomless cylindrical tube 42 obtained by performing the second step. For comparison, the second transition portion PC2 of the first bottomless cylindrical tube 41 is also depicted by a thin dashed line. Note that, like the tube axis AX shown in FIG. 5(d), the tube axis AX shown in FIG. 6(d) does not precisely indicate the position of the tube axis AX, but rather indicates on which side of the second transition portion PC2 remaining at the tip of the second bottomless cylindrical tube 42 the tube axis AX is located. FIG. 6(e) is a schematic isometric projection of the second bottomless cylindrical tube 42 obtained by performing the second step.

[0047] In the forced drawing performed in the second step, the processing reference surface that comes into contact with the inner peripheral surface of the drawing hole of the second die 22 is the outer peripheral surface of the first bottomless cylindrical tube 41. As illustrated in Fig. 6(a), the tapered outer peripheral surface of the second transition portion PC2 of the first bottomless cylindrical tube 41 first comes into contact with the inner peripheral surface of the drawing hole of the second die 22. At this time, as illustrated in Fig. 6(b), while the second transition portion PC2 is being reduced in diameter, the tip of the second transition portion PC2 moves radially outward (i.e., toward the inner peripheral surface of the drawing hole) due to a shear force generated by the force pressing the first bottomless cylindrical tube 41 into the drawing hole and the frictional force between the outer peripheral surface of the second transition portion PC2 and the inner peripheral surface of the drawing hole. As the indentation drawing process progresses further, as illustrated in Figure 6(c), the outer diameter of the resulting reduced diameter portion of the second bottomless cylindrical tube 42 becomes a constant value corresponding to the inner diameter of the drawing hole (see the area surrounded by the thick dashed line).

[0048] 6(d), the cut surface of the second transition portion PC2 of the first bottomless cylindrical tube 41 resulting from the piercing process performed in the first step was generally parallel to the first direction (the punching direction of the first punch in the piercing process) (see the thick dashed line with circles at both ends). However, the cut surface of the second transition portion PC2 of the second bottomless cylindrical tube 42 is inclined with respect to the first direction so as to move away from the tube axis AX as it approaches the tip (see the thick solid line with circles at both ends).

[0049] As a result of the above-described force-drawing process, a second bottomless cylindrical tube 42 is obtained, which has a second transition section PC2 at its tip, the second transition section PC2 having a constant outer diameter equal to the outer diameter of the reduced diameter portion of the straight tube section PS and an inner diameter that increases toward the tip, as shown in (e) of Figure 6. If it is difficult to obtain the second bottomless cylindrical tube 42 having such a configuration from the first bottomless cylindrical tube 41 in a single force-drawing process, the second step may involve performing the force-drawing process multiple times.

[0050] The third step is a step of performing flaring processing in which a second punch, which is an expanding punch with a tapered portion, is pushed into the opening on the second transition section side of the second bottomless cylindrical tube installed in the third die to expand the end on the second transition section side of the second bottomless cylindrical tube and form a flared portion, thereby obtaining a cylindrical tube with a flared portion at the tube end.

[0051] FIG. 7 is a schematic cross-sectional view illustrating the third step in which a second punch 32 is inserted into the second bottomless cylindrical pipe 42 from the second transition portion PC2 side to form a flared portion. In FIG. 7, the stroke of the second punch 32 is indicated by a thick dashed line. FIG. 8(a) is a schematic cross-sectional view illustrating the state in which a second punch is inserted into the second bottomless cylindrical pipe from the opening on the second transition portion side in the third step included in the first method. FIG. 8(b) is a schematic cross-sectional view illustrating the state in which a tube-expansion punch is inserted into the opening formed by cutting off the bottom of a bottomed cylindrical workpiece in a flaring process included in a conventional technique. FIG. 8(c) is a schematic isometric projection view of a cylindrical pipe having a flared portion at its end obtained by performing the third step.

[0052] When a workpiece having a cut surface of the opening perpendicular to the pipe wall is used as in the prior art described above, as illustrated in Fig. 8(b), the sharp edge of the cut surface of the tapered portion of the second punch 32 comes into contact with a die or the like (see the area surrounded by the thick dashed line), which may damage the second punch 32. In contrast, in the flaring process performed in the third step, as illustrated in Figs. 7 and 8(a), the inner circumferential surface of the second transition portion PC2 of the second bottomless cylindrical pipe 42, which is inclined so that the inner diameter increases toward the tip, is pressed and expanded by the tapered portion of the second punch 32. That is, in the flaring process performed in the third step included in the first method, as illustrated in Fig. 8(a), the flaring process is performed in a state in which the inner circumferential surface of the second transition portion PC2 and the second punch 32 are in substantial surface contact with each other.

[0053] Therefore, in the flaring process performed in the third step, the second punch 32 suffers less damage than in the case where the sharp edge of the cut surface punched out by the hole-punching process is pressed by the expansion punch as in the conventional technology described above.

[0054] Furthermore, the tip of the second transition portion PC2, which is the portion that is most greatly expanded during flaring, is formed with the second flat surface FS2, which is a flat surface with good surface smoothness as a result of the transfer of the first flat surface FS1 as described above. Therefore, compared to the prior art described above, there is a smaller possibility that defects such as cracks originating from cracks present on the cut surface will occur.

[0055] <effect> As is clear from the above, according to the first method, defects such as cracks in the flared portion and damage to the mold can be reduced in a method for manufacturing a cylindrical tube having a flared portion at the tube end.

[0056] Second Embodiment A method for manufacturing a cylindrical pipe provided with a flared portion at the pipe end according to a second embodiment of the present invention (hereinafter, sometimes referred to as "second method") will be described below with reference to the drawings.

[0057] As described in the explanation of the first method, the punching in the piercing process carried out in the first step corresponds to so-called "oblique punching." Generally, in oblique punching, a radial load is generated in addition to a thrust load in the axial direction, and therefore, compared to punching perpendicular to the pipe wall, problems such as damage to the punch and / or deterioration in punching accuracy are more likely to occur.

[0058] In order to reduce the above-mentioned problems in the first step included in the method for manufacturing a cylindrical tube having a flared portion at the tube end (the method of the present invention) according to the present invention, it is desirable that the radial load generated when the first transition portion and the bottom portion, which are part of the transition portion, are punched out by the first punch, is canceled out as a whole.

[0059] Furthermore, from the viewpoint of reducing problems such as unintended deformation bias in the first and second bottomless cylindrical tubes during the squeezing process performed in the second step and the flaring process performed in the third step, it is desirable that the shapes of the first and second bottomless cylindrical tubes be symmetrical around the tube axis.

[0060] In the following description of the second method, the reference numerals used in FIGS. 1 to 8 referred to in the description of the first method will be used, so please refer to FIGS.

[0061] <composition> Therefore, the second method is the first method described above, except that the shapes of the straight pipe section, transition section, and bottom section of the bottomed cylindrical pipe are all bodies of revolution with the pipe axis, which is the axis of the straight pipe section, as the axis of rotation. In this specification, the term "body of revolution" means "a solid created by rotating a plane figure around a straight line on the same plane as its axis" (Kojien, Fourth Edition). In other words, in this specification, the term "body of revolution" means "a rotationally symmetric body whose number of symmetries n is positive infinity." The bottomed cylindrical pipes 11 and 12 illustrated in FIG. 1 are bodies of revolution with the pipe axis AX, which is the axis of the straight pipe section PS, as the axis of rotation. In other words, the shapes of the straight pipe section PS, transition section PC, and bottom section PB of the bottomed cylindrical pipes 11 and 12 are all bodies of revolution with the pipe axis AX, which is the axis of the straight pipe section PS, as the axis of rotation. Therefore, the bottomed cylindrical pipes 11 and 12 satisfy the requirements that a bottomed cylindrical pipe used in the second method should satisfy.

[0062] Furthermore, in the second method, in the first step, the first punch is a rotor with the tube axis as its rotation axis, and the first direction, which is the punching direction by the first punch, is parallel to the tube axis. The first direction is also parallel to the tube axis in the first steps illustrated in Figures 3(b) and 4. That is, the first steps illustrated in Figures 3(b) and 4 satisfy the requirements that the first step included in the second method must satisfy.

[0063] With the above configuration, in the first step included in the second method, the radial load generated radially inward (i.e., toward the tube axis) when the first transition portion PC1 and the bottom portion PB, which are parts of the transition portion PC, are punched out by the first punch 31 becomes symmetrical about the tube axis AX. Therefore, the radial load is canceled out as a whole, which can reduce problems caused by oblique punching, such as damage to the punch 31 and / or deterioration in punching accuracy.

[0064] In the second method, the inner circumferential surface (i.e., the cut surface) of the hole formed by punching out and removing the first transition portion PC1 and the bottom portion PB in the first step is parallel to the tube axis. Therefore, the second transition portion PC2, which is the transition portion PC remaining in the first bottomless cylindrical tube 41 after completion of the first step, also becomes a body of rotation about the tube axis AX, and the first bottomless cylindrical tube 41 also becomes a body of rotation about the tube axis AX. As a result, problems such as unintended deformation bias in the press-drawing process performed in the second step are reduced, and the resulting second bottomless cylindrical tube 42 also becomes a body of rotation about the tube axis AX. Therefore, problems such as unintended deformation bias in the flaring process performed in the third step are reduced, and the resulting cylindrical tube 50 having a flared portion at its end also becomes a body of rotation about the tube axis AX.

[0065] As is well known to those skilled in the art, the cut surface produced by punching typically includes sagging, sheared surfaces, fractured surfaces, and burrs. Therefore, the above-mentioned phrase "the inner circumferential surface of the hole formed by punching and removing the first transition portion PC1 and the bottom portion PB in the first step (i.e., the cut surface) is parallel to the tube axis" does not necessarily refer to a state in which the entire cut surface is strictly parallel to the tube axis. Specifically, the phrase "the inner circumferential surface of the hole formed by punching and removing the first transition portion PC1 and the bottom portion PB in the first step (i.e., the cut surface) is parallel to the tube axis" includes a state in which the cut surface as a whole can be recognized as parallel to the tube axis even if there are portions of the sagging, sheared surfaces, fractured surfaces, and / or burrs that are not strictly parallel to the tube axis.

[0066] <effect> As is clear from the above, according to the second method, in a method for manufacturing a cylindrical tube having a flared portion at the tube end, similar to the first method, defects such as cracks in the flared portion and damage to the mold can be reduced, while problems such as damage to the punch 31 and / or deterioration in the processing accuracy of the hole punching caused by bevel punching, and problems such as unintended bias in deformation of the first bottomless cylindrical tube and the second bottomless cylindrical tube during the push-drawing processing performed in the second step and the flaring processing performed in the third step can be reduced.

[0067] Third Embodiment Hereinafter, a method for manufacturing a cylindrical pipe provided with a flared portion at the pipe end according to a third embodiment of the present invention (hereinafter, sometimes referred to as "third method") will be described with reference to the drawings.

[0068] As described in the explanation of the first method, in the method of the present invention, a fourth step of performing ironing may be provided between the second step of performing indentation drawing and the third step of performing flaring.

[0069] <composition> Therefore, the third method is a method for manufacturing a cylindrical tube having a flared portion at the tube end, which is the above-mentioned first or second method, and further includes a fourth step between the second and third steps of pushing a third punch, which is a striking punch, into an opening on the opposite side of the second transition portion of the second bottomless cylindrical tube installed in the fourth die to perform striking on the inner surface of the second bottomless cylindrical tube.

[0070] Fig. 9 is a flowchart showing the flow of each step included in the third method. The flowchart shown in Fig. 9 is similar to the flowchart shown in Fig. 2, except that it further includes a fourth step (step S25) between the second step (step S20) and the third step (step S30). Therefore, in the following explanation, only the fourth step executed in step S25 will be explained.

[0071] FIG. 3(d), which was referred to in the description of the first method, illustrates a fourth step of ironing between the second step (c) of drawing and the third step (e) of flaring. As illustrated in FIG. 3(d), in the fourth step, a third punch 33, which is an ironing punch, is pressed into the fourth die 24 through an opening on the opposite side of the second transition portion PC2 of the second bottomless cylindrical tube 42, thereby ironing the inner circumferential surface of the second bottomless cylindrical tube 42. This improves the dimensional accuracy of the inner diameter and / or wall thickness (plate thickness) of the second bottomless cylindrical tube 42 and further smooths the surface of the inner circumferential surface. As a result, the final cylindrical tube having a flared portion at its end can have improved dimensional accuracy of the inner diameter and / or wall thickness (plate thickness) and further smooths the surface of the inner circumferential surface.

[0072] When the fourth step is performed between the second step and the third step, the second die used in the second step may be used as the fourth die. Although the fourth step is preferably performed between the second step and the third step, depending on, for example, the convenience of the manufacturing process and / or the dimensional accuracy or surface properties of the inner peripheral surface required for the resulting cylindrical tube having a flared portion at its end, the fourth step may be performed between the first step and the second step to perform ironing on the inner peripheral surface of the first bottomless cylindrical tube 41. Alternatively, the fourth step may be performed after the third step to perform ironing on the inner peripheral surface of the cylindrical tube 50 having a flared portion at its end.

[0073] <effect> As is clear from the above, according to the third method, in a manufacturing method for a cylindrical tube having a flared portion at the tube end, similar to the first method, defects such as cracks in the flared portion and damage to the mold can be reduced, while the dimensional accuracy of the inner diameter and / or wall thickness (plate thickness) of the cylindrical tube having a flared portion at the tube end can be improved and the surface properties of the inner surface can be made smoother.

[0074] For the purpose of explaining the present invention, several embodiments 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 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]

[0075] 11,12...Bottomed cylindrical tube PS…Straight pipe section PC…transition part PC1…1st transition part PC2…Second transition part PB…bottom 21...First die FS1…1st plane 22...Second die 23…3rd die 24...4th die 31...First punch 32...Second punch 33...Third punch 41...First bottomless cylindrical tube FS2…Second plane 42…Second bottomless cylindrical tube 50...Cylindrical pipe with a flared portion at the end

Claims

1. A method for manufacturing a cylindrical tube having a flared portion at a tube end, the method comprising: manufacturing a cylindrical tube having a flared portion at a tube end from a bottomed cylindrical tube having a straight tube portion, a bottom portion, and a transition portion interposed between the straight tube portion and the bottom portion, the transition portion being a portion whose outer diameter decreases as the transition approaches the bottom from the straight tube portion, a first step of inserting a first punch into the bottomed cylindrical tube placed in a first die, which is a piercing die with a piercing hole formed therein, and performing a piercing process to punch out and remove a first transition portion that is a part of the transition portion and the bottom portion with the first punch, thereby obtaining a first bottomless cylindrical tube; a second step of performing a press-drawing process to obtain a second bottomless cylindrical tube by forcing the first bottomless cylindrical tube, starting from the second transition portion side, which is the transition portion remaining in the first bottomless cylindrical tube, into a drawing hole of a second die, which is a drawing die having a drawing hole including a portion having an inner diameter smaller than the outer diameter of the straight tube portion, thereby reducing the diameters of at least a portion of the straight tube portion and the second transition portion; a third step of performing flaring by pushing a second punch, which is an expanding punch having a tapered portion, into an opening of the second bottomless cylindrical tube disposed in a third die on the second transition portion side to expand the end of the second bottomless cylindrical tube on the second transition portion side, thereby forming a flared portion, thereby obtaining a cylindrical tube having a flared portion at its end; Including, an outer peripheral surface of the transition portion of the bottomed cylindrical tube at a position where a cut surface is formed by punching in the piercing process executed in the first step is inclined with respect to a first direction which is a punching direction by the first punch, a first plane, which is a plane facing the bottomed cylindrical tube and perpendicular to the first direction, is formed at least on a peripheral portion adjacent to the punching hole formed in the first die, a second plane, which is a plane transferred from the first plane, is formed at the tip of the second transition portion by pressing the tip of the second transition portion against the first plane during the execution of the piercing process in the first step; A method for manufacturing a cylindrical tube with a flared portion at the end.

2. A method for manufacturing a cylindrical pipe having a flared portion at a pipe end according to claim 1, comprising: the straight pipe section, the transition section, and the bottom section of the bottomed cylindrical pipe are all shaped as bodies of revolution whose rotation axis is the pipe axis that is the axis of the straight pipe section, In the first step, the first punch is a rotating body having the tube axis as a rotation axis, and the first direction is parallel to the tube axis. A method for manufacturing a cylindrical tube with a flared portion at the end.

3. A method for manufacturing a cylindrical pipe having a flared portion at a pipe end according to claim 1 or 2, a fourth step between the second step and the third step, in which a third punch, which is a punching punch, is inserted into a fourth die through an opening on the opposite side of the second transition portion of the second bottomless cylindrical tube to perform a punching process on the inner peripheral surface of the second bottomless cylindrical tube; Further comprising: A method for manufacturing a cylindrical tube with a flared portion at the end.

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