Method Of Forming Pipe End Structure

Abstract

A method of forming a pipe end structure includes providing a pipe assembly including a metal pipe blank, a ridge, and a tubular fitting, and pressing an inner surface of the pipe blank outwardly. The ridge protrudes outward from an outer surface of the pipe blank. The fitting includes a threaded portion having a thread on an outer surface thereof. The fitting has a first edge and a second edge and is fitted over the pipe blank with the first edge of the fitting being in contact with the ridge protruding from the pipe blank. An inner surface of the pipe blank is pressed outwardly such that a part of the pipe blank protrudes from an outer surface of the pipe blank outwardly at a position adjacent to the second edge of the fitting, thereby forming a locking portion that is in contact with the second edge of the fitting.

Description

[0001] This application claims priority from Japanese Patent Application No. 2013-120822 filed on Jun. 7, 2013. The entire contents of the priority application are incorporated herein by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to a method of forming a pipe end structure.

BACKGROUND

[0003] As described in JP-A-2004-122151, a metal pipe is used as a hydraulic passage in a hydraulic drive system for driving a fork of a forklift, because the metal pipe has a sufficient rigidity to withstand high-pressure fluid (oil pressure). To connect the metal pipe to another pipe, a metal connector (joint) having a tubular shape is provided at an end of the metal pipe As described in JP-A-2004-122151, a screw having a helical thread (an external thread) is provided on an outer surface of the connector. The connector and the metal pipe are connected at their end portions by known welding technologies such as an arc welding, a gas welding, and a brazing.

[0004] U.S. Pat. No. 5,727,303 describes a metal pipe (tube) for an orifice. A tubular connector (a first fitting) having threads on its outer surface is fitted over an end portion of the metal pipe. A front end of the metal pipe, which is on the front side of the connector, is expanded outwardly so as to be in close contact with a front opening edge of the connector. As described in U.S. Pat. No. 5,727,303, a conical extension on an anvil is inserted into an internal bore of the metal pipe to expand the front end portion of the metal pipe.

[0005] The metal pipe is also expanded at a position adjacent to a rear end of the connector so as to be in close contact with a rear opening edge of the connector. The opening edge at the rear end has a recess extending around the metal pipe. A portion of the metal pipe is expanded into the recess. The metal pipe over which the connector is fitted at its end portion is vertically compressed (in a longitudinal direction of the metal pipe) to expand its end portion into the recess.

SUMMARY

[0006] A method of forming a pipe end structure includes providing a pipe assembly including a metal pipe blank, a ridge, and a tubular fitting, and pressing an inner surface of the pipe blank outwardly. The pipe blank has a through hole extending from one end to another end along an axial direction thereof. The ridge protrudes outward from an outer surface of the pipe blank. The fitting includes a threaded portion having a thread on an outer surface thereof. The fitting has a first edge and a second edge and is fitted over the pipe blank with the first edge of the fitting being in contact with the ridge protruding from the pipe blank. An inner surface of the pipe blank is pressed outwardly such that a part of the pipe blank protrudes from an outer surface of the pipe blank outwardly at a position adjacent to the second edge of the fitting, thereby forming a locking portion being in contact with the second edge of the fitting.

BRIEF DESCRIPTION OF DRAWINGS

[0007] FIG. 1 is a side view of a tube end structure according to an embodiment.

[0008] FIG. 2 is a front view of the tube end structure.

[0009] FIG. 3 is a cross-sectional view of the tube end structure taken along a line A-A in FIG. 2.

[0010] FIG. 4 is a perspective view of a fitting.

[0011] FIG. 5 is a side view of a pipe blank in which a front part thereof is illustrated in a cross section.

[0012] FIG. 6 is a schematic view of a fixture, the pipe blank held by the fixture, and a pressing machine in a standby state.

[0013] FIG. 7 is a schematic view illustrating a state in which a front end surface of a holder is in contact with a front end surface of the fixture.

[0014] FIG. 8 is a schematic view illustrating a state in which a front end portion of the pipe blank is pressed and plastically deformed by the punch.

[0015] FIG. 9 is a schematic view illustrating a state in which the front end portion of the pipe blank is further pressed and plastically deformed by the punch.

[0016] FIG. 10 is a view illustrating a general configuration of a locking portion formation apparatus.

[0017] FIG. 11 is a cross-sectional view of a part including the holder and a diameter expander of the locking portion formation apparatus.

[0018] FIG. 12 is a top plan view of the diameter expander.

[0019] FIG. 13 is a cross-sectional view illustrating a state in which the pipe blank, over which the fitting is fitted, is held by the holder.

[0020] FIG. 14 is a cross-sectional view illustrating a locking portion formation step of a method of forming the pipe end structure according to an embodiment.

[0021] FIG. 15 is a side view of a press-fit bar.

[0022] FIG. 16 is a side view of a press-fit bar used in a method of forming a pipe end structure in an embodiment.

[0023] FIG. 17 is a front view of the press fit bar illustrated in FIG. 16.

DETAILED DESCRIPTION

[0024] The end portion of the metal pipe and the connector having threads may be welded to be connected with each other. In such a case, droplets of a welding material are likely to be attached to the threads during the welding. The droplets may lower the quality of a product that includes the metal pipe and the connector. This may lead a big problem. Specifically, this may deteriorate the quality of the threads, which may result in oil leakage. In addition, the welding between the end portion of the pipe and the connector is difficult for not only for an inexperienced worker, but also for a skilled worker. The welding lowers work efficiency. A technology for connecting the end portion of the pipe and the connector without welding is expected.

[0025] The metal pipe over which the connector having a tubular shape is fitted may be compressed in a longitudinal direction of the metal pipe to expand the metal pipe as described in U.S. Pat. No. 5,727,303. However, it is difficult to evenly expand the metal pipe. If the metal pipe is not evenly expanded, the connector, which may be referred to as a fitting, is easily separated from the metal pipe. In addition, the metal pipe may be buckled at a position where the connector is not attached when the metal pipe is compressed in the longitudinal direction to expand the end portion thereof. Such problems occur due to the compression of the metal pipe.

[0026] An object of the technology described herein is to provide a method of forming a pipe end structure in which a fitting having a tubular shape is properly fixed to a predetermined position of a metal pipe blank without welding.

[0027] The technology described herein provides a method of forming a pipe end structure that includes providing a pipe assembly including a metal pipe blank, a ridge, and a tubular fitting, and pressing an inner surface of the pipe blank. The metal pipe blank has a through hole extending from one end to another end along an axial direction thereof. The ridge protrudes outward from an outer surface of the pipe blank. The tubular fitting has a thread on an outer surface thereof. The fitting has a first edge and a second edge and is fitted over the pipe blank with the first edge of the fitting being in contact with the ridge protruding from the pipe blank. An inner surface of the pipe blank is pressed outwardly such that a part of the pipe blank protrudes from an outer surface of the pipe blank outwardly at a position adjacent to the second edge of the fitting, thereby forming a locking portion that is in contact with the second edge of the fitting.

[0028] The step of pressing the inner surface may include inserting a diameter expander configured to expand in a radial direction of the pipe blank into the through hole of the pipe blank and expanding the diameter expander inserted in the pipe blank in the radial direction of the pipe blank to press the inner surface of the pipe blank outwardly at the position adjacent to the second edge of the fitting.

[0029] The diameter expander may include a plurality of elastic plates arranged in a tubular shape with a predetermined distance therebetween. Each of the elastic plates may be a fixed end at one end thereof and a free end at another end thereof, whereby the elastic plates are expandable in a radial direction of the pipe blank. The inner surface of the pipe blank may be pressed outwardly by the other ends of the elastic plates.

[0030] The step of pressing the inner surface may include fixing the pipe blank of the pipe assembly to a fixture at a position adjacent to the rear edge of the fitting, and inserting a press fit bar into the though hole of the pipe blank from the one end toward the other end of the pipe blank. The press fit bar may have an elongated shape and include a press fit portion having an outer diameter larger than an inner diameter of the pipe blank, whereby the inner surface of the pipe blank is pressed outwardly such that the pipe blank has the locking portion.

[0031] The press fit portion may have a cylindrical shape.

[0032] The press fit portion may have a polygonal prism shape.

[0033] The step of providing the pipe assembly may include compressing an end portion of the pipe blank in the axial direction of the pipe blank to increase a thickness of the end portion of the pipe blank in a radial direction, whereby the ridge protruding from the outer surface of the pipe blank is formed at the end portion of the pipe blank and fitting the fitting over the pipe blank so as to be in contact with the ridge at the first edge of the fitting.

[0034] The step of providing the pipe assembly may include chamfering an one portion of the pipe blank to have an inclined surface that extends from the inner surface of the pipe blank outwardly at an angle.

[0035] The step of expanding the diameter expander may include inserting an diameter adjuster having a cylindrical shape into a space defined by the elastic plates of the diameter expander and moving the diameter expander in the axial direction of the tube blank toward the one end of the tube blank. The diameter adjuster may have an outer diameter larger than that of the diameter expander.

[0036] Each of the elastic plates may have a projection at the other end thereof. The projection may protrude radially outwardly from the outer surface of each elastic plate and may be configured to press the inner surface of the pipe blank when the diameter expander expands.

[0037] The step of fixing the pipe assembly to the fixture may include positioning the pipe assembly to have a space between the second edge of the fitting and the fixture, whereby the part of the pipe blank protrudes at a position corresponding to the space to form the locking portion.

[0038] The step of inserting the press fit bar into the though hole of the pipe blank may include moving the press fit bar inserted in the through hole of the pipe blank until a tip end of the press fit portion reaches a position corresponding to the second edge of the fitting.

[0039] According to the method of forming the pipe end structure described herein, the fitting having a tubular shape is properly fixed to a predetermined position of the metal pipe blank without welding.

[0040] First embodiment will be described with reference to FIGS. 1 to 13. A side view, a front view, and a cross-sectional view of a pipe end structure 1 according to a first embodiment are illustrated in FIG. 1, FIG. 2, and FIG. 3, respectively. A right side and a left side in FIG. 1 correspond to a front side and a rear side of the pipe end structure 1, respectively.

[0041] Pipe End Structure

[0042] As illustrated in FIG. 1, the pipe end structure 1 includes a pipe 2 having an elongated shape and a fitting 3 having a threaded portion. The fitting 3 fits over a front end portion of the pipe 2. The pipe end structure 1 is a component of a hydraulic passage for a fork of a forklift. The pipe end structure 1 is configured to be connected to another pipe end structure (not illustrated) via the fitting 3.

[0043] Pipe

[0044] As illustrated in FIG. 1, the pipe 2 has a tubular shape elongated in substantially one direction. The pipe 2 includes a circumferential wall 21 having an elongated tubular shape, a front ridge 22 located on a front end of the circumferential wall 21, and a fluid passage 23 extending through the pipe 2 along its longitudinal direction. The circumferential wall 21 has a tubular shape extending in a front-to-rear direction (an axial direction) and is formed of a metal pipe blank, which will be described later. An inner surface 21b of the circumferential wall 21 defines the fluid passage 23. As illustrated in FIG. 3, the front ridge 22 of the pipe 2 on the front side of the circumferential wall 21 protrudes radially outwardly from an outer surface of the circumferential wall 21 and extends around the circumferential wall 21. The front ridge 22 has an outer diameter larger than that of the circumferential wall 21 located on a rear side thereof. The front ridge 22 has a surface that is machined to have a shape like a single thread, for example. As illustrated in FIG. 2, a front surface 22a of the front ridge 22 (i.e., a front surface of the pipe 2) has a circular ring shape. The fluid passage 23 defined by the inner surface of the circumferential wall 21 is a passage for an operating oil.

[0045] The fitting 3 is fitted over the front end portion of the pipe 2 so as to be in contact with a rear edge of the front ridge 22. The pipe 2 includes a locking portion 24 that protrudes radially outwardly from the outer surface of the circumferential wall 21 so as to be in contact with the rear edge of the fitting 3. The pipe 2 is formed by processing the pipe blank by a plastic working and cutting. This step will be described later. An inner surface of the circumferential wall of the pipe blank (the circumferential wall 21 of the pipe 2) is pressed outwardly to plastically deform a part of the pipe blank. The plastically deformed part of the pipe blank becomes the locking portion 24 that has a larger diameter.

[0046] Fitting

[0047] A perspective view of the fitting 3 is illustrated in FIG. 4. The fitting 3 has a tubular shape elongated in substantially one direction. The fitting 3 having a tubular shape has a shorter length (a length along an axis L) than the pipe 2 having a tubular shape. The fitting 3 has an inner diameter that is larger than the outer diameter of the circumferential wall 21 of the pipe 2. The fitting 3 includes a threaded portion 31 at its front portion. The threaded portion 31 includes helical threads 31 a on an outer surface 3a of the fitting 3. The fitting 3 includes a flange 32 at its rear portion. The flange 32 protrudes radially outwardly from the outer surface 3a of the fitting 3. In this embodiment, the flange 32 protrudes more from the outer surface 3a of the fitting 3 than the threaded portion 31 (the thread 31a). In other words, the flange 32 has a larger outer diameter than that of the threaded portion 31. Between the threaded portion 31 and the flange 32 of the fitting 3, the outer surface 3a of the fitting 3 includes a cylindrical surface portion 33 that has an outer diameter smaller than those of the threaded portion 31 and the flange 32.

[0048] The fitting 3 includes a through hole 34 extending through the fitting 3 in the front-to-rear direction. An inner surface 3b of the fitting 3 defines the through hole 34. The fitting 3 has a front edge (first edge) 35 and a rear edge (second edge) 36. The front edge 35 has a front opening edge 35a extending around a front edge of the thorough hole 34. The rear edge 36 has a rear opening edge 36a extending around a rear edge of the thorough hole 34. A front portion of the circumferential wall 21 is inserted in the through hole 34 of the fitting 3, and thus the fitting 3 is fixed to the pipe 2. In this configuration, the fitting 3 fits over the circumferential wall 21 of the pipe 2 (a pipe blank, which will be described later). The inner surface 3b of the fitting 3, which is fixed to the pipe 2, is in close contact with the outer surface 21a of the circumferential wall 21.

[0049] The front ridge 22, which is located in front of the fitting 3, is in close contact with the front opening edge 35a of the fitting 3. The locking portion 24, which is located behind the fitting 3, is in close contact with the rear opening edge 36a of the fitting 3. In other words, the fitting 3 is sandwiched between the front ridge 22 and the locking portion 24 in the front-to-rear direction.

[0050] Method of Forming the Pipe End Structure

[0051] Next, a method of forming the pipe end structure 1 will be described with reference to FIGS. 5 to 13. The above-described pipe end structure 1 is formed by a method described below.

[0052] Step 1: a Step of Forming the Front Ridge

[0053] In this step, a pipe blank 20 that will become the pipe 2 is pressed at its front end portion to have the front ridge 22 at the front portion thereof.

[0054] Specifically, the pipe blank 20, which will become the pipe 2, is provided at first. A partial side cross-section of the pipe blank 20 is illustrated in FIG. 5. FIG. 5 illustrates a front portion of the pipe blank 20 in the cross section. In this embodiment, a metal pipe (Carbon Steel Precision Tubes for Hydraulic Line Service) that has an outer diameter of 9.5 mm, an inner diameter of 6.5 mm, and a length of at least 150 mm is provided as the pipe blank 20. The pipe blank 20 has a tubular shape extending straight. The pipe blank 20 has a front surface 20a that is a flat circular ring surface. The pipe blank 20 has an inclined surface 20b at its inner front end. The inner front end of the pipe blank 20 is chamfered to have the inclined surface 20b that extends from the inner surface of the pipe blank 20 outwardly at an angle. The pipe blank 20 has an outer surface 20c having a cylindrical surface. The pipe blank 20 includes a through hole 230 extending through the pipe blank 20 in the front-to-rear direction. The through hole 230 will be the fluid passage 23 of the pipe 2. The pipe blank 20 has a circumferential wall 221 that will be the circumferential wall 21 of the pipe 2.

[0055] Next, the pipe blank 20 is held by a fixture 4 for a step of pressing the front end portion of the pipe blank 20. The pipe blank 20, which is held by the fixture 4, and a pressing machine 5, which is in a standby state, are schematically illustrated in FIG. 6. The fixture 4 is a device for holding the pipe blank 20 (i.e., a clamping device for pipe) and has a cuboidal shape. The fixture 4 includes a first fixing member 41A and a second fixing member 41B. The first fixing member 41A is positioned above the second fixing member 41B. The pipe blank 20 is sandwiched and held between the first fixing member 41A and the second fixing member 41B. The fixture 4 has a space 43 extending in the front-to-rear direction between the first fixing member 41A and the second fixing member 41B. The pipe blank 20 is fitted in the space 43. The pipe blank 20 is held by the fixture 4 with a front end portion 20d thereof sticking out of a front end surface 41a of the fixture 4. The end surface 41a of the fixture 4 has an opening edge 41b at a position corresponding to an end of the space 43. The front end portion 20d of the pipe blank 20 sticks out of the opening edge 41b. The front end portion 20d of the pipe blank 20 sticks out of the end surface 41a of the fixture 43 by a predetermined length (a predetermined length M1) that is required for the formation of the front ridge 22.

[0056] After the pipe blank 20 is held by the fixture 4, the front end portion 20d of the pipe blank 20 is compressed by the pressing machine 5. As illustrated in FIG. 6, the pressing machine 5 is arranged in front of the pipe blank 20 held by the fixture 4. The pressing machine 5 is reciprocatory movable in the front-to-rear direction along an axial line L by a known mechanism. The pressing machine 5 includes a punch 51 and a holder 55. The punch 51 is configured to enter into the through hole 230 of the pipe blank 20 and press a front end of the pipe blank 20. The holder 55 surrounds the punch 51 and has a surface to be in contact with the end surface 41a of the fixture 4. A front side and a rear side of the pressing machine 5 correspond to the left side and the right side in FIG. 6, respectively. The punch 51 is provided on a front end of a slide shaft, which is not illustrated. As illustrated in FIG. 6, the punch 51 includes an insertion portion 52, a pressing portion 53, and a body 54. The insertion portion 52 has a shaft like shape with a rounded end. The pressing portion 53 extends from a rear edge (a base side) of the insertion portion 52. The body 54 has a cylindrical shape and extends from a rear edge of the pressing portion 53. The insertion portion 52 and the pressing member 53 are located on an extension line of a center line (an axial line) of the body 54.

[0057] The insertion portion 52 has a diameter slightly smaller than the inner diameter of the pipe blank 20. The pressing portion 53, which extends from the rear edge (the base side) of the insertion portion 52, has a concave shape opening toward the front side as a whole. The pressing portion 53 has a larger diameter than the insertion portion 52. The pressing portion 53 is integrally formed with the insertion portion 52. More specifically described, the pressing portion 53 includes a central pressing portion 53a, a front-end pressing portion 53b, and an outer pressing portion 53c. The central pressing portion 53a is configured to press the inner surface of the front end portion 20d of the pipe blank 20 so as to expand the front end portion 20d of the pipe blank 20 outwardly. The central pressing portion 53a extends from a rear edge of the insertion portion 52 toward the rear side and has a shape like a truncated cone (a conical shape) with a sloped surface. The central pressing portion 53a has an outer diameter gradually increasing from the front side toward the rear side. The front-end pressing portion 53b is configured to press the front end surface 20a of the pipe blank 20 from the front side toward the rear side of the pipe blank 20, for example. The front-end pressing portion 53b extends outwardly from an outer circumferential edge of a rear edge of the central pressing portion 53a and has a circular ring surface surrounding the central pressing portion 53a. The outer pressing portion 53c is configured to press the outer surface of the front end portion 20d of the pipe blank 20 inwardly. The outer pressing portion 53c is located outward of the front-end pressing portion 53b and has a slope surface that is a circular ring surface having an inner diameter gradually increasing from the rear side toward the front side.

[0058] The holder 55 is moved along the axial line L together with the punch 51 until the holder 55 strikes the front end surface 41a of the fixture 4. The holder 55 has a tubular shape and surrounds the punch 51. The holder 55 has a through hole extending therethrough in the front-to-rear direction (along the axial line L). The punch 51 is arranged in the through hole of the holder 55. The punch 51 is configured to be moved forward along an inner surface 55b of the holder 55, which defines the through hole, beyond the holder 55 that is in contact with the fixture 4. The holder 55 has a front end surface 55a that is in contact with the front end surface 41 of the fixture 4. The inner surface 55b of the holder 55 and the outer surface of the punch 51 define a substantially tubular space 56 therebetween (see FIG. 7). The pressing machine 5 is coaxially arranged with the pipe blank 20. In other words, the axial line of the punch 51 or the axial line of the holder 55 extends on the same line as the axial line L of the pipe blank 20. As illustrated in FIG. 7, the punch 51 is arranged in the holder 55 with the front end portion of the insertion portion 52 sticking out of the front end surface 55a of the holder 55. The space 56 of the holder 55 has a size that can house the front end portion 20d of the pipe blank 20. In this embodiment, the punch 51 and the holder 55 are metal molded products that has high abrasion resistance.

[0059] An operation of the pressing machine 5 in a standby state as illustrated in FIG. 7 is started when the slide shaft (not illustrated) starts moving. The punch 51 of the pressing machine 5 starts moving toward the pipe blank 20 along with the slide shaft. The holder 55 moves together with the punch 51 at the same speed with the moving speed of the punch 51. The punch 51 continues to move toward the pipe blank 20 and the front end portion of the insertion portion 52 enters the through hole 230 of the pipe blank 20. When the insertion portion 52 enters the through hole 230 by a predetermined distance, the front end surface 55a of the holder 55 strikes the front end surface 41a of the fixture 4. The front end surface 55a of the holder 55 that is in contact with the front end surface 41a of the fixture 4 is schematically illustrated in FIG. 7. As illustrated in FIG. 7, the punch 51 does not press the front end portion 20d of the pipe blank 20 at a moment when the front end surface 55a of the holder 55 strikes the front end surface 41a of the fixture 4.

[0060] Then, the holder 55 remains stationary while the front end surface 55a thereof is in contact with the front end surface 41a of the fixture 4. Contrary to this, the punch 51 housed in the holder 55 moves further inside the pipe blank 20 as the slide shaft moves. The body 54 of the punch 51 moves forward along the inner surface 55b of the holder 55. Then, the pressing part 53 of the punch 51 compresses the front end portion 20d of the pipe blank 20. This plastically deforms the front end portion 20d of the pipe blank 20 to have a larger diameter and a shorter length (dimension along the axial line L). Namely, the wall of the pipe blank 20 at the front end portion 20d becomes thicker. The front end portion 20d of the pipe blank 20 which is plastically deformed by the punch 51 is schematically illustrated in FIG. 8. The insertion portion 52 and the pressing portion 53 of the punch 51 enter the pipe blank 20 by a predetermined distance that is properly determined depending on an intended shape of the pipe end portion. The moving distance of the punch 51 inside the pipe blank 20 can be properly controlled by setting a sliding distance (stroke) of the slide shaft that moves the punch 51. After the punch 51 compresses the front end portion 20d of the pipe blank 20, the pressing machine 5 moves back along the axial line L and returns to the standby state as illustrated in FIG. 6.

[0061] In this embodiment, the step of compressing the front end portion 20d of the pipe blank 20 by the punch 51, which is illustrated in FIG. 8, is repeatedly performed multiple times. The moving (inserting) distance of the punch 51 inside the pipe blank 20 (in the through hole 230) is controlled for each step. Specifically, the moving (inserting) distance of the pipe 51 inside the pipe blank 20 is set to be longer (deeper) with each step. The shapes of the insertion portion 52 and the pressing portion 53 of the punch 51 may be varied for each step. Specifically, the insertion portion 52 and the pressing portion 53 (the central pressing portion 53a) may have a larger diameter with each step.

[0062] The front end portion 20d of the pipe blank 20 which is plastically deformed by the punch 51 is schematically illustrated in FIG. 9. In this embodiment, as illustrated in FIG. 9, the front end portion 20d of the pipe blank 20 is kept pressed until the front end portion 20d has a length M2 that is half the predetermined length M1 (the length along the axial line L). The front end portion 20d of the pipe blank 20 is plastically deformed in a space defined by an outer surface of the punch 51 (the insertion portion 52 and the pressing portion 53), the inner surface 55b of the holder 55, and the front end surface 41a of the fixture 4.

[0063] As described above, the front end portion 20d of the pipe blank 20 is pressed to have a larger thickness by the pressing machine 5, and thus the outer circumferential edge of the pipe blank 20 at the front end portion 20d protrudes outwardly in a ring shape. As a result, the front end portion 20d of the pipe blank 20 will become the front ridge 22 of the pipe 2. The front end portion 20d of the pipe blank 20 may be shaped by grinding as necessary.

[0064] Step 2: a Step of Fitting the Fitting

[0065] In this step, the fitting 3 is fitted over the pipe blank 20 that has the front ridge 22. The fitting 3 is a separate member from the pipe blank 20. The fitting 3 may be obtained by cutting a substantially hexagonal tubular member. The tubular member has a hole extending therethrough which will be the through hole 34 of the fitting 3. In this embodiment, a metal nut having an outer diameter (a maximum diameter) of 21.9 mm, an inner diameter of 9.9 mm, and a length of 20.0 mm is used as the tubular member.

[0066] The rear end of the pipe blank 20 is inserted from an inner edge of a front opening edge 35a of the fitting 3 into the through hole 34 of the fitting 3 to fit the fitting 3 over the pipe blank 20. A position of the fitting 3 on the pipe blank 20 is adjusted so as to be in contact with the front ridge 22 located at the front end of the pipe blank 20 at its front edge 35 (the front opening edge 35a). In this way, the fitting 3 is fitted over the pipe blank 20. In this state in which the fitting 3 is fitted over the pipe blank 20, the fitting 3 is movable on the pipe blank 20. Specifically, the fitting 3 is slidable on the pipe blank 20 in the front-to-rear direction and in a circumferential direction along the outer surface of the pipe blank 20. Such an assembly of the pipe blank 20 and the fitting 3 may be referred to as a pipe assembly 100.

[0067] Step 3: a Step of Forming a Locking Portion Formation

[0068] In this step, an inner surface of the pipe blank 20 of the pipe assembly 100 is pressed outwardly to expand the pipe blank 20 and form the locking portion 24. A schematic configuration of a locking portion formation apparatus 6 is illustrated in FIG. 10. The locking portion formation apparatus 6 is configured to form the locking portion 24 by expanding the pipe blank 20 of the pipe assembly 100. As illustrated in FIG. 10, the locking portion formation apparatus 6 includes a base 61, supports 62, a table 63, a holder 64, a diameter expander 65, an adjuster 66, a drive device 67, a swing member 68, and a link connector 69.

[0069] The base 61 is a base of the locking portion formation apparatus 6, which is placed on a floor, for example. The base 61 is a metal flat plate, for example. The supports 62 are provided on the base 61 and support the table 63 above the base 61. As illustrated in FIG. 10, in this embodiment, two supports 62 are provided on the base 61. One of the supports 62 is provided at one end of the base 61 which is on the left side in FIG. 10. The other one of the supports 62 is provided at another end portion of the base 61 which is on the right side in FIG. 10. The supports 62 each may be a metal flat plate. The supports 62 are provided on the base 61 so as to extend in an up-to-down direction (a vertical direction). A lower end and an upper end of each support 62 are fixed to the base 61 and the table 63, respectively.

[0070] The table 63 is supported by the supports 62 from below. The table 63 is a metal plate and extends between the supports 62. The holder 64 is configured to hold a member to be processed by the locking portion formation apparatus 6 (the pipe blank 20 of the pipe assembly 100). The holder 64 is fixed on the table 63. The holder 64 has a shape like a container that opens upward. At a middle of the holder 64, the diameter expander 65 that is configured to be inserted into the through hole 230 of the pipe blank 20 is arranged.

[0071] In FIG. 11, a portion around the holder 64 and the diameter expander 65 of the locking portion formation apparatus 6 is illustrated in a cross section. As illustrated in FIG. 11, the holder 64 includes a mount 64a, a wall 64b, and a securing portion 64c. On the mount 64a, a member to be processed is mounted. The wall 64b extends upward from an outer end of the mount 64a and surrounds the member to be processed on the mount 64a. The securing portion 64c is provided in the middle of the mount 64c and the diameter expander 65 is secured thereto.

[0072] The mount 64a has a substantially cylindrical shape and is fitted in a concave attachment portion of the table 63 as illustrated in FIG. 11. An upper surface of the mount 64a is a flat surface on which the front end of the pipe blank 20 of the pipe assembly 100 is arranged. The wall 64b is high enough to surround the threaded portion 31 of the fitting 3 of the pipe assembly 100. The securing portion 64c has a surface defining a through hole extending in the mount 64a in an up-and-down direction (the vertical direction). A lower end portion of the diameter expander 65 having the elongated shape is fitted in the through hole defined by the securing portion 64c so that the diameter expander 65 is secured by the securing portion 64c.

[0073] The diameter expander 65 is configured to press the inner surface of the pipe blank 20 of the pipe assembly 100 outwardly to form the locking portion 24 in the pipe blank 20. The diameter expander 65 has an elongated cylindrical shape and is made of metal. The diameter expander 65 includes a base 65a secured by the securing portion 64c and a plurality of plates 65b extending upward from the base 65a. The base 65a has a cylindrical shape and constitutes a lower end portion of the diameter expander 65 which is arranged in the vertical position.

[0074] The diameter expander 65 is illustrated in the top plan view in FIG. 12. The diameter expander 65 of this embodiment includes four plates 65b. Each of the plates 65b is an elastic plate (plate spring). The plates 65b each have an elongated shape and have a circular arc cross section taken along perpendicular to the longitudinal direction thereof. The plates 65b are arranged in a tubular shape (a ring shape) with a predetermined distance (a space S1) therebetween. The plates 65b adjacent to each other provide a slit therebetween. The slit corresponds to the space S1. The slit extends along a long side of the plate 65b from one end thereof toward the other end thereof. The slit extends to a position near the base 65a of the diameter expander 65.

[0075] Each plate 65b has a projection 65b1 at a front end thereof. The projection 65b1 protrudes radially outwardly from the outer surface of the plate 65b. In this embodiment, four projections 65b1 are arranged in a circular shape with a space therebetween. The projections 65b1 are configured to press the inner surface of the circumferential wall 221 of the pipe blank 20 outwardly. Pressing the inner surface of the circumferential wall 221 by the projections 65b1 forms the locking portion 24 of the pipe blank 20.

[0076] The diameter expander 65 includes a through hole 65c extending in the up-and-down direction (the vertical direction, the axial direction of the diameter expander 65). The diameter expander 65 has the outer diameter smaller than the inner diameter of the pipe blank 20. Each of the plates 65b of the diameter expander 65 is arranged in a vertical position as illustrated in FIG. 11, for example. The table 63 includes a through hole 63b extending through the table 63 in the up-and-down direction (the vertical direction) as illustrated in FIG. 11. The through hole 63b of the table 63 communicates with the through hole 65c of the diameter expander 65.

[0077] The adjuster 66 that is configured to adjust the diameter of the diameter expander 65 is placed on the front end portion of the diameter expander 65. The adjuster 66 has a cylindrical shape (a nut like shape) as a whole and an outer diameter larger than that of the diameter expander. The adjuster 66 includes a lower portion 66a that has a conical shape with a diameter decreasing toward the lower side. The lower portion 66a of the adjuster 66 is in contact with the front end portion of the diameter expander 65. Inside the diameter expander 65, a link connector 69 having a bar-like shape is arranged to extend through the through hole 65c. The link connector 69 is formed of an elongated metal bolt in this embodiment. The adjuster 66 is fixed to an upper end portion 69a of the link connector 69.

[0078] The link connector 69 is arranged inside the diameter expander 65 (in the through hole 65c) so as to extend in the up-and-down direction. As illustrated in FIG. 10, the link connector 69 includes a lower end portion 69b that is attached to a swing member 68 configured to swing on a shaft 70 in the up-and-down direction. As illustrated in FIG. 10, the swing member 68 has a shape elongated in the right-to-left direction (a horizontal direction). The swing member 68 is supported by the shaft 70 so as to be able to swing in the up-and-down direction. In the locking portion formation apparatus 6, the swing member 68 is arranged between the base 61 and the table 63. The locking portion formation apparatus 6 further includes a shaft bearing 71 that extends from a lower surface of the table 63 toward the base 61. The shaft bearing 71 supports both ends of the shaft 70.

[0079] As illustrated in FIG. 10, a first end portion 68a of the swing member 68, which is located on the right side in FIG. 10, is connected to the lower end portion 69b of the link connector 69. As illustrated in FIG. 10, a second end portion 68b of the swing member 68, which is located on the left side in FIG. 10, is connected to a drive device 67 that is configured to move the swing member 68 in the up-and-down direction. The drive device 67 may be any known drive device such as a fluid pressure cylinder (for example, an air cylinder or an oil hydraulic cylinder). The drive device 67 is an oil hydraulic cylinder in this embodiment. The drive device 67 is arranged on the base 61.

[0080] In the locking portion formation apparatus 6, when the second end portion (the other end) 68b of the swing member 68 is moved up by the drive of the drive device 67, the first end portion (the one end) 68a of the swing member 68 moves down. This moves the link connector 69, which is attached to the first end portion 68a of the swing member 68, down. In other words, the lower end portion 69b of the link connector 69 is gradually pulled down as the first end portion 68a of the swing member 68 moves down. The link connector 69 connects between the first end portion 68a of the swing member 68 and the adjuster 66. The adjuster 66 moves down into the diameter expander 65 while pushing the plates 65b of the diameter expander 65 outwardly as the link connector 69 is pulled down. In this configuration, when the adjuster 66 moves down in the diameter expander 65, the plates 65b are pushed outwardly and the diameter expander 65 is forced to have a larger diameter at the front end portion thereof.

[0081] Then, in the locking portion formation apparatus 6, when the second end portion (the other end) 68b of the swing member 68 is pulled down by the driving of the drive device 67, the first end portion (the one end) 68a of the swing member 68 moves up. This moves the link connector 69, which is attached to the first end portion 68a of the swing member 68, up. In other words, the lower end portion 69b of the link connector 69 is gradually pulled up as the first end 68a of the swing member 68 moves upward. When the link connector 69 is pulled up, the adjuster 66 that is in the diameter expander 65 moves upward to the original position at the front end portion of the diameter expander 65. The plates 65b of the diameter expander 65 are each formed of a plate spring. The plates 65b that are pushed outwardly return to its original vertical position and the diameter of the diameter expander 65 returns to its original diameter when the adjuster 66 returns to the original position at the front end portion of the diameter expander 65. The driving of the drive device 67 is controlled by a controller, which is not illustrated.

[0082] In the locking portion formation step, the above-described locking portion formation apparatus 6 forms the locking portion 24 in the pipe blank 20 of the pipe assembly 100. The step of forming the locking portion 24 in the pipe blank 20 will be described with reference to FIG. 13. In FIG. 13, the pipe blank 20 of the pipe assembly 100 is held by the holder 64. Before the pipe blank 20 of the pipe assembly 100 is held by the holder 64, the diameter expander 65, to which the adjuster 66 is fitted, is inserted inside (the through hole 230) the pipe blank 20 from the front end thereof. Then, the front end portion of the pipe blank 20 is placed on the mount 64a such that the pipe blank 20 is arranged in a vertical position on the mount 64a. The pipe blank 20 of the pipe assembly 100 is held by the holder 64 in this way. At this stage where the pipe blank 20 of the pipe assembly 100 is held by the holder 64, the projection 65b1 at the front end of the diameter expander 65 is positioned at a position corresponding to the rear edge 36 of the fitting 3, which is fitted over the pipe blank 20, or a position slightly above the rear edge 36. In this embodiment, the projection 65b1 is positioned slightly above the rear edge 36 of the fitting 3. The rear opening edge 36a at the rear edge 36 of the fitting 3 is inclined downward toward the inside of the fitting 3, and thus the rear opening edge 36a and the outer surface of the pipe blank 20 provide a recess.

[0083] After the pipe blank 20 of the pipe assembly 100 is held by the holder 64, the drive device 67 of the locking portion formation apparatus 6 is driven to move down the adjuster 66. The adjuster 66 pushes the inner surfaces of the plates 65b of the diameter expander 65, and thus the plates 65b are pushed outwardly so that the diameter expander 65 expands in a radial direction toward the wall 221 of the pipe blanks 20. When the diameter expander 65 expands in the radial direction, the projections 65b1 at the front end of the diameter expander 65 press the inner surface of the wall 221 of the pipe blank 20 outwardly. As a result, the wall 221 of the pipe blank 20 is plastically deformed to have a larger diameter at the parts pressed by the projections 65b1 of the diameter expander 65, and thus the locking portion 24 in contact with the rear edge 36 (the rear opening edge 36a) of the fitting 3 is formed. The formation of the locking portion 24 in the pipe blank 20 provides the pipe end structure 1 illustrated in FIG. 1 to FIG. 3. As illustrated in FIG. 3, the locking portion 24 protrudes from the outer surface of the pipe 20 (the pipe 2) and is in close contact with the rear edge 36 (particularly, the rear opening edge 36a) of the fitting 3.

[0084] After the formation of the locking portion 24, the adjuster 66 is returned to the original position by the drive device 67 of the locking portion formation apparatus 6. As a result, the diameter expander 65 contracts (the diameter of the diameter expander 65 decreases) to its original size, and thus the diameter expander 65 is allowed to be pulled out of the through hole 230 (the fluid passage 23). Subsequently, the diameter expander 65 is pulled out of the through hole 230 and the processed pipe blank having the locking portion 24 (the pipe end structure 1) is detached from the holder 64. According to the above described method, the locking portion 24 is formed at a predetermined position of the pipe blank 20 over which the fitting 3 is fitted.

[0085] As described above, the pipe end structure 1 of this embodiment is produced. After the locking portion formation step, any step such as shaping may be performed on the pipe end structure 1.

[0086] According to the method of forming the pipe end structure 1 of this embodiment, the inner surface of the pipe blank 20 that will become the pipe 2 is pressed outwardly, and thus the circumferential wall 221 of the pipe blank 20 is radially expanded to form the locking portion 24. The locking portion 24 is a protrusion evenly extending outwardly in a ring shape on the circumferential wall 21 of the pipe blank 20. The method of forming the pipe end structure 1 of this embodiment provides the pipe end structure 1 in which the fitting 3 is hardly detached from the pipe blank 20 (the pipe 2). The fitting 3 is sandwiched between the locking portion 24 and the front ridge 22 on the pipe blank 20 (the pipe 2). In this configuration, the fitting 3 hardly moves in the longitudinal direction and the circumferential direction of the pipe blank 20. According to the method of forming the pipe end structure 1 of this embodiment, the pipe blank 20 (the pipe 2) is hardly buckled.

[0087] According to the method of forming the pipe end structure 1 of this embodiment, the fitting having a cylindrical shape is properly fixed to a predetermined position of the metal pipe blank without welding. In the pipe end structure 1, the pipe 2 is a seamless pipe, and thus the fluid flowing in the pipe 2 does not leak out of the pipe 2.

Second Embodiment

[0088] A second embodiment will be described with reference to FIG. 14 and FIG. 15. In this embodiment, a method of forming a pipe end structure 1A having a configuration illustrated in FIG. 14 will be described. The method according to this embodiment is the same as the method according to the first embodiment except for the locking portion formation step. Hereinafter, only the locking portion formation step will be described for the second embodiment. In the locking portion formation step of this embodiment, a locking portion 24A is formed in a pipe 2A (a pipe blank 20A) by a press fit bar 8 illustrated in FIG. 14 and FIG. 15.

[0089] The press fit bar 8 is pushed into a through hole 23A of the pipe blank 20A of the pipe assembly 100 from a front end of the pipe blank 20A to form the locking portion 24A. The press fit bar 8 is a component of a press fitting machine which is not illustrated. The press fitting machine further includes a member configured to move the press fit bar 8 in the front-to-rear direction. The press fit bar 8 includes an insertion portion 81, a contact portion 82 extending from a rear edge of the insertion portion 81, and a fixed shaft 83 extending from a rear edge of the contact portion 82. The insertion portion 81 has a shaft like shape and is configured to be inserted into the through hole of the pipe blank 20A. The contact portion 82 is configured to be in contact with a front end portion of the pipe blank 20A when the insertion portion 81 is inserted into the through hole of the pipe blank 20A. The fixed shaft 83 is fixed to another component of the press fitting machine.

[0090] The insertion portion 81 has a cylindrical shape as a whole. The insertion portion 81 includes a press-fit portion 81a at a front end side thereof. The press-fit portion 81a has an outer diameter larger than remaining portions of the insertion portion 81. The press-fit portion 81a includes a front end portion 81a1 that has a diameter decreasing from the rear side toward the front thereof. The press-fit portion 81a has an outer diameter that is larger than an inner diameter of the pipe blank 20A without the locking portion 24A.

[0091] The press-fit portion 81a is configured to press the inner surface of the circumferential wall 21A of the pipe blank 20A outwardly when the insertion portion 81 is inserted into the through hole of the pipe blank 20A. The front end portion 81a1 of the press-fit portion 81a is positioned near the rear edge 36 of the fitting 3 when the insertion portion 81 reaches the farthest point in the through hole of the pipe blank 20A.

[0092] The contact portion 82 has a larger diameter than the insertion portion 81. The contact portion 82 has a front end surface 82a having a circular ring shape. As described above, the front end portion 81a1 of the press-fit portion 81a is positioned near the rear edge 36 of the fitting 3 when the insertion portion 81 is inserted into the through hole of the pipe blank 20A and the front end portion of the pipe blank 20A is in contact with the front end surface 82a of the contact portion 82.

[0093] As illustrated in FIG. 14, the pipe blank 20A of the pipe assembly 100 is held by the fixture 4 (the fixture used in the front ridge formation step) at a position adjacent to the rear edge 36 of the fitting 3. The pipe blank 20A of the pipe assembly 100 is held by the fixture 4 with a small space between the front end surface 41a of the fixture 4 and the rear edge (surface) of the fitting 3, which is fitted over the pipe blank 20A. The small space has a size required for the formation of the locking portion 24A.

[0094] When the press fit bar 8 is inserted in (into the through hole 23A of) the pipe blank 20 of the pipe assembly 100, the press-fit portion 81a of the press fit bar 8 moves toward the rear side while pressing the inner surface of the circumferential wall 21A of the pipe blank 20 outwardly. As a result, an outer surface 21Aa of the circumferential wall 21A is pressed against an inner wall 3b of the fitting 3 so as to be in close contact with each other. The inner surface 21Ab of the circumferential wall 21A is plastically deformed as the press-fit portion 81a moves in the pipe blank 20. This forms the locking portion 24A that is in contact with the rear edge 36 (the rear opening edge 36a) of the fitting 3. The front end portion 81a1 of the press-fit portion 81a moves in the through hole 23A while scraping the inner surface 21Ab of the circumferential wall 21A. As a result, the scraped material of the circumferential wall 21A of the pipe blank 20A is gathered to a position behind the fitting 3. The gathered material of the circumferential wall 21A is in a protruded shape protruding from the outer surface 21Aa of the pipe blank 20A at a position between the front end surface 41a of the fixture 4 and the rear edge (surface) 36 of the fitting 3. The gathered material constitutes the locking portion 24A.

[0095] The insertion of the press fit bar 8 into the pipe blank 20 is performed until the locking portion 24A is formed. The press fit bar 8 may be inserted once or twice or more into the pipe blank 20 to form the locking portion 24A.

[0096] As described above, according to the method of forming the pipe end structure 1A of this embodiment, the inner surface of the pipe blank 20A, which will become the pipe 2A, is pressed outwardly such that the circumferential wall 21A of the pipe blank 20A is radially expanded to form the locking portion 24A. The locking portion 24A is a protrusion evenly extending outwardly in a ring shape on the circumferential wall 21A of the pipe blank 20A. The method of forming the pipe end structure 1A of this embodiment provides the pipe end structure 1A in which the fitting 3 is hardly detached from the pipe blank 20A (the pipe 2A). The fitting 3 is sandwiched between the locking portion 24A and the front ridge 22 on the pipe blank 20A (the pipe 2A). In this configuration, the fitting 3 hardly moves in the longitudinal direction and the circumferential direction of the pipe blank 20A. In addition, according to the method of forming the pipe end structure 1A of this embodiment, the pipe blank 20A (the pipe 2A) is hardly buckled.

[0097] According to the method of forming the pipe end structure 1A of this embodiment, the fitting having a tubular shape is properly fixed to a predetermined position of the metal pipe blank without welding.

Other Embodiments

[0098] The technology described herein is not limited to the above embodiments explained in the above description and the drawings. The technology described herein may include the following embodiments.

[0099] (1) In the first and second embodiments, the pipe 2, 2A included in the pipe end structure 1, 1A extends straight. However, the technology is not limited to this configuration. In some embodiments, the pipe 2 may be bent in various curved shapes depending on the intended use of the pipe end structure (pipe routing).

[0100] (2) In the first and second embodiments, the front ridge 22 has the single thread shape. However, the technology is not limited to this configuration. In some embodiments, the surface of the ridge may be a cylindrical surface. The shape of the front ridge 22 may be changed depending on an end structure of another pipe to which the pipe is connected.

[0101] (3) In the first and second embodiments, the rear opening edge 36a at the rear edge 36 of the fitting 3 is inclined downward toward the inside of the fitting 3, and thus a recess is provided between the rear opening edge 36a and the outer surface of the pipe 20, 20A. However, the technology is not limited to this configuration. In some embodiments, the rear opening edge 36a of the fitting 3 may not have such an inclined part that provides a recess and may be flat.

[0102] (4) In the first and second embodiments, the pipe end structure 1, 1A is a component of the hydraulic passage for driving the fork of the fork lift. However, the technology is not limited to this configuration. The pipe end structure 1, 1A may be a component for any other products in some embodiments.

[0103] (5) In the first and second embodiments, the front ridge 22 has a ring-like front shape. However, the technology is not limited thereto. The front ridge 22 may have a square tube shape, a triangular tube shape, a pentagonal tube shape, a hexagonal tube shape, or other tube shape (a ring shape) in some embodiments.

[0104] (6) In the locking portion formation step according to the second embodiment, instead of the press fit bar 8, another press fit bar 8A as illustrated in FIG. 16 and FIG. 17 may be used. The press fit bar 8A has an insertion portion 81A that has a different shape from the insertion portion 81 of the press fit bar 8 of the second embodiment. The press fit bar 8 of the second embodiment includes the press-fit portion 81a that has a circular shape in a front view viewed along an axial direction of the press fit bar 8. The press fit bar 8A illustrated in FIG. 16 and FIG. 17 includes a press-fit bar 81Aa that has a triangular shape (an example of polygonal shape) in a front view viewed along the axial direction. Like the second embodiment, the front end portion 81Aa1 of the press-fit portion 81Aa is positioned near the rear edge 36 of the fitting 3 when the insertion portion 81 is inserted in the pipe blank 20A. The press fit bar 8A having such a configuration has a smaller area that is in contact with the circumferential wall 21A of the pipe blank 20A than the press fit bar 8 of the second embodiment. With this configuration, the press fit bar 8A can be easily inserted into the pipe blank 20A of the press fit bar 8. The locking portion formation step of forming the locking portion in the pipe blank 20A may be performed by the insertion bar 8A having the insertion portion 81Aa that is in a polygonal shape such as a triangular shape.

Claims

1. A method of forming a pipe end structure, the method comprising; providing a pipe assembly, the pipe assembly including: a metal pipe blank having a through hole extending from one end to another end along an axial direction thereof; a ridge protruding outward from an outer surface of the pipe blank; and a tubular fitting includes a threaded portion having a thread on an outer surface thereof, the fitting having a first edge and a second edge and being fitted over the pipe blank with the first edge of the fitting being in contact with the ridge protruding from the pipe blank; and pressing an inner surface of the pipe blank outwardly such that a part of the pipe blank protrudes from an outer surface of the pipe blank outwardly at a position adjacent to the second edge of the fitting, thereby forming a locking portion being in contact with the second edge of the fitting.

2. The method according to claim 1, wherein the step of pressing of the inner surface includes: inserting a diameter expander configured to expand in a radial direction of the pipe blank into the through hole of the pipe blank; and expanding the diameter expander inserted in the pipe blank in the radial direction of the pipe blank to press the inner surface of the pipe blank outwardly at the position adjacent to the second edge of the fitting.

3. The method according to claim 2, wherein the diameter expander includes a plurality of elastic plates arranged in a tubular shape with a predetermined distance therebetween, each of the elastic plates has a fixed end at one end thereof and a free end at another end thereof, whereby the elastic plates are expandable in a radial direction of the pipe blank, and the inner surface of the pipe blank is pressed outwardly by the other ends of the elastic plates.

4. The method according to claim 1, wherein the step of pressing the inner surface includes: fixing the pipe blank of the pipe assembly to a fixture at a position adjacent to the second edge of the fitting; and inserting a press fit bar into the though hole of the pipe blank from the one end toward the other end of the pipe blank, the press fit bar having an elongated shape and including a press fit portion having an outer diameter larger than an inner diameter of the pipe blank, whereby the inner surface of the pipe blank is pressed outwardly such that the pipe blank has the locking portion.

5. The method according to claim 4, wherein the press fit portion has a cylindrical shape.

6. The method according to claim 4, wherein the press fit portion has a polygonal prism shape.

7. The method according to claim 1, wherein the step of providing the pipe assembly includes: compressing an end portion of the pipe blank in the axial direction of the pipe blank to increase a thickness of the end portion of the pipe blank in a radial direction, whereby the ridge protruding from the outer surface of the pipe blank is formed at the end portion of the pipe blank; and fitting the fitting over the pipe blank so as to be in contact with the ridge at the first edge of the fitting.

8. The method according to claim 1, wherein the step of providing the pipe assembly includes: chamfering an one portion of the pipe blank to have an inclined surface that extends from the inner surface of the pipe blank outwardly at an angle.

9. The method according to claim 3, wherein the step of expanding the diameter expander includes: inserting an diameter adjuster having a cylindrical shape into a space defined by the elastic plates of the diameter expander, the diameter adjuster having an outer diameter larger than that of the diameter expander; and moving the diameter expander in the axial direction of the tube blank toward the one end of the tube blank.

10. The method according to claim 3, wherein each of the elastic plates has a projection at the other end thereof, the projection protrudes radially outwardly from the outer surface of each elastic plate and is configured to press the inner surface of the pipe blank when the diameter expander expands.

11. The method according to claim 4, wherein the step of fixing the pipe assembly to the fixture includes positioning the pipe assembly to have a space between the second edge of the fitting and the fixture, whereby the part of the pipe blank protrudes at a position corresponding to the space to form the locking portion.

12. The method according to claim 4, wherein the step of inserting the press fit bar into the though hole of the pipe blank includes moving the press fit bar inserted in the through hole of the pipe blank until a tip end of the press fit portion reaches a position corresponding to the second edge of the fitting.