DIE SET AND PROCESSING METHOD USING DIE SET

TECHNICAL FIELD

The present invention relates to a die set for bending a plate-like workpiece into a pipe shape and a processing method using the die set.

BACKGROUND ART

Conventionally, a UO bending processing method has been known as a processing method using this type of dies. The UO bending processing method includes a first process in which a workpiece is processed into a U-shape, and a second process in which the workpiece having been processed into a U-shape is processed into an O-shape. In this processing method, a metal plate material as a workpiece is placed on a die (fixed part) having a groove, and a punch (movable part) is used to press the workpiece from above (see, for example, Patent Literatures 1 and 2).

Patent Literature 1 discloses a processing method in which a plate material is placed on a lower die having an inwardly recessed semi-cylindrical surface, and the plate material is pressed by an upper die having a projecting semi-cylindrical surface to process it into a U-shape.

Patent Literature 2 discloses an apparatus including a punch having a projecting part and a die having a recessed part. When a steel sheet is shaped by the apparatus, under control of a control unit, the steel sheet is pressed against the punch while being applied with pressure.

CITATION LIST
Patent Literature

Patent Literature 1: JP S58-107220 A

Patent Literature 2: JP 2004-195504 A

SUMMARY OF INVENTION
Technical Problem

In recent years, there have been known bent workpieces formed by bending sheet-like metal matrix composites composed of metals such as aluminum and ceramics. Among such metal matrix composites, composites composed of aluminum and ceramics in particular have the property of low ductility of workpiece. Therefore, when bending is performed using the conventional processing method, tensile stress (hereinafter referred to as “tension”) acts on the outer side of the bent part, and fractures and cracks are likely to occur. Further, compressive force is easily applied to the inner side of the bent part, and wrinkles are easily generated.

For a case where such metal-based composite material (workpiece) is processed into a U-shape, the processing method described in Patent Literature 1 would cause workpiece breakage when pressurizing the workpiece with the upper punch, due to tension generated from the starting point of the bend along the outer side surface.

Meanwhile, the processing method described in Patent Literature 2 enables processing while pressing the workpiece from beneath and holding it in place with the upper and lower punches. However, because the end face of the recessed part of the die is not a rounded surface, the tension generated at the starting point of the bend of the workpiece can accumulate in a gap that resides between the punch and the die, so that the workpiece is likely to break.

The present invention has been devised to solve this problem and an object of the present invention is to provide a die set capable of suppressing breakage or wrinkling of the workpiece during bending and a processing method using such die set.

Solution to Problem

In order to solve the problem, a die set according to the present invention is a die set used for bending a plate-like workpiece, which die set comprises a lower die on which the workpiece is placed, and an upper die having a pressing surface formed to press the workpiece against the lower die. The lower die comprises a lower movable part slidable in the same direction as a moving direction of the upper die, a reaction force generating member configured to elastically support the lower movable part from beneath, and receiving members located on both sides of the lower movable part. The pressing surface has an arc-shaped cross-section protruding toward the lower die and extends in a longitudinal direction of the workpiece. The maximum push-in position of the lower end portion of the pressing surface is deeper than a position lowered by a finished inner diameter dimension of the workpiece from an upper surface of the lower die.

According to the present invention, when the upper die is moved downward and the workpiece is pressed by the pressing surface, the workpiece is pressed against the lower die while applying compressive force to the workpiece from upper and lower directions, so that U-shape bending is performed. In this process, since the lower end portion of the pressing surface is pushed in a position deeper than the position lowered by the finished inner diameter dimension of the workpiece from the upper surface of the lower die, the workpiece is fixed so as to be held in place over the entire U-shaped bent range along the circular cross-sectional pressing surface, and sufficient stress is applied to the workpiece while preventing slippage and deviation of the workpiece. This can prevent brakeage of the workpiece and occurrence of wrinkle during the bending process.

It is preferable that the reaction force generating member is a gas spring. The use of a gas spring provides a stronger initial reaction force than other reaction force generating members such as a spring, and the workpiece can be held tightly by the upper die and the lower die. Therefore, the workpiece is held tightly in place along the circular cross-sectional pressing surface, and sufficient stress is applied to the workpiece while preventing slippage and deviation of the workpiece. This can improve formability.

It is preferable that a plurality of reaction force generating members are arranged in a longitudinal direction of the lower die. With this configuration, even if the workpiece has a long shape, a uniform reaction force can be generated in the longitudinal direction, so that the same bending process can be performed at each portion in the longitudinal direction. Further, since a plurality of gas springs are arranged in the longitudinal direction of the lower die, it is possible to provide a die set having a length adapted to the longitudinal dimension of the workpiece.

It is preferable that the reaction force generating member is configured such that a magnitude of a reaction force is adjustable. With this configuration, the reaction force required can be set appropriately according to the size and strength of the workpiece, the pressing force of the upper die, etc., and sufficient reaction force can be generated with respect to the bending stress required for the bending process.

It is preferable that the lower movable part has a pressing force receiving surface opposite to the pressing surface of the upper die and having an inwardly recessed curved shape, and that the difference between a diameter dimension of the pressing force receiving surface and a diameter dimension of the pressing surface of the upper die is smaller than a thickness dimension of the workpiece. This configuration allows the workpiece to be pressed against the lower die while applying compressive force to the workpiece from the upper and lower directions, so that the workpiece is held tightly in place along the circular cross-sectional pressing surface. This can improve formability.

It is preferable that the pressing surface of the upper die is formed by a cylindrical mandrel member extending in a longitudinal direction of the lower die. With this configuration, the pressing surface can be formed easily. If the mandrel member is configured to be attachable to and detachable from the upper die, mandrel members with different outer diameters can be easily installed according to the thickness dimensions of workpieces.

It is preferable that the lower movable part is accommodated inward of the receiving members, and a movable range of the lower movable part is equal to the maximum stroke distance of the reaction force generating member. This configuration makes it possible to further push in the upper die toward the workpiece after the full stroke of the reaction force generating member, so that the formability of the workpiece can be improved.

It is preferable that an inner edge portion of each receiving member has a rounded shape having a radius smaller than a radius of the pressing surface. This configuration can alleviate the concentration of stress during the bending process.

It is preferable that a hard chrome plating layer is formed on a surface of the lower movable part and surfaces of edge portions of the receiving members. This configuration can prevent wear of the die.

Preferably, a processing method comprises: a setting process in which the workpiece is placed on the lower die; a pressing process in which the workpiece is pressed by the upper die; and an advancing process in which the lower movable part and the upper die are moved downward while the reaction force generating member urges the workpiece by a reaction force thereof in a direction opposite to the moving direction of the upper die.

Preferably, a processing method comprises: a setting process in which the workpiece protected by a protective sheet is placed on the lower die; a pressing process in which the workpiece is pressed by the upper die; and an advancing process in which the lower movable part and the upper die are moved downward while the reaction force generating member urges the workpiece by a reaction force thereof in a direction opposite to the moving direction of the upper die.

Further, a processing method preferably comprises a process of further bending a U-shaped member having been processed using the die set into an O-shape to finish the U-shaped member into a pipe member.

Advantageous Effects of Invention

According to the present invention, a die set capable of suppressing breakage or wrinkling of the workpiece during bending and a processing method using such die set can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating an overall structure around a die set, to which a die set and a processing method using such die set according to one embodiment of the present invention are applied.

FIG. 2 is a plan view illustrating a lower die of the die set.

FIG. 3 is a side view of the overall structure, in which the lower die is shown in cross-section taken along lines in FIG. 2.

FIG. 4 is a longitudinal cross-sectional view illustrating a U-bending process.

FIG. 5 is an enlarged longitudinal cross-sectional view illustrating an essential part of the U-bending process.

FIG. 6A is a process drawing showing a process of setting a workpiece on the lower die in the U-bending process.

FIG. 6B is a process drawing showing a process of pressing the workpiece with an upper die in the U-bending process.

FIG. 6C is a process drawing showing the process of pressing the workpiece with the upper die in the U-bending process.

FIG. 6D is a process drawing showing a full stroke state of a reaction force generating member in the U-bending process.

FIG. 6E is a process drawing showing a retracted state of the upper die in the U-bending process.

FIG. 7 is a longitudinal cross-sectional view illustrating an O-bending process.

FIG. 8A is a process drawing showing a process of setting a U-shaped member on an O-shape lower die in the O-bending process.

FIG. 8B is a process drawing showing a process of advancing an O-shape upper die downward toward the U-shaped member in the O-bending process.

FIG. 8C is a process drawing showing the process of advancing the O-shape upper die downward toward the U-shaped member in the O-bending process.

FIG. 8D is a process drawing showing a state in which the U-shaped member is processed into an O-shaped pipe member in the O-bending process.

FIG. 8E is a process drawing showing a retracted state of the O-shape upper die in the O-bending process.

DESCRIPTION OF EMBODIMENTS

One embodiment of the present invention is described below with reference to the drawings. FIG. 1 is a perspective view illustrating a die set 10 used for a bending machine 1 of this embodiment. In the following description, dies (die set) 10 used for processing a workpiece 100 into a U-shaped member 110 and a processing method using the dies 10 are described, and then dies (die set) 10A used for processing the U-shaped member 110 into a pipe member 120 and a processing method using the dies 10A are described.

The dies 10 are used to bend a plate-like workpiece 100 into a U-shape. The dies 10 include a lower die 20 on which a workpiece 100 is placed, and an upper die 30 by which the workpiece 100 is pressed.

As shown in FIG. 5, the workpiece 100 to be processed is mainly composed of a core part 101, and a skin parts 102, 102 provided on front and back surfaces of the core part 101. The core part 101 is composed of aluminum powder and tungsten powder, gadolinium oxide powder, or a mixed material containing boron (B4C), and has a shielding performance against radiation ray or neutron beam. The core part 101 (workpiece 100) has a flat plate shape before it is processed (see FIG. 6A). This sort of composite material is less ductile as compared with aluminum alloy.

The skin parts 102 are provided so as to respectively cover the front and back surfaces of the core part 101 substantially entirely. The skin part 102 is composed of an aluminum alloy that excels in ductility. Each skin part 102 is formed such that the dimension thereof in the thickness direction is smaller than that of the core portion 101.

The upper die 30 is configured to move upward and downward with the aid of a drive mechanism (not shown in the drawings). The upper die 30 includes a base part 38 having a cross-sectionally recess-shaped lower end portion 30a (see FIG. 5), and a mandrel member 31 attached to the lower end portion 30a of the base part 38. The mandrel member 31 has a pressing surface 32 that is opposite to the lower die 20. The mandrel member 31 is a cylindrical member extending in the longitudinal direction of the upper die 30 (extension direction of the workpiece 100). As shown in FIG. 4, the mandrel member 31 is fixed to the lower end portion 30a of the base part 38 by a plurality of mounting bolts 33 (only one is shown in the figure) arranged spaced apart in the longitudinal direction of the upper die 30. The mandrel member 31 is detachably attached by screwing the mounting bolts 33. For example, the finished inner diameter (outer diameter) of a pipe member to be formed from a workpiece 100 can be changed by using a mandrel member 31 with a different outer diameter.

The pressing surface 32 has an arc-shaped cross-section protruding toward the lower die 20 and extends in the longitudinal direction of the workpiece 100. As shown in FIG. 5, the maximum push-in position P1 of the lower end portion 32a of the pressing surface 32 is deeper than the position P2 lowered by the finished inner diameter dimension L1 of the workpiece 100 from the upper surface 20a of the lower die 20. In other words, the push-in dimension L2 of the pressing surface 32 from the upper surface 20a of the lower die 20 is set larger than the finished inner diameter dimension L1 of the workpiece 100.

When the upper die 30 is in an upwardly retracted standby position, the upper die 30 is located above upper surface portions 54a of receiving members 54 (see FIG. 5) that constitute the lower die 20, so that a predetermined space for placement of the workpiece 100 is ensured between the upper die 30 and the receiving members 54, 54. Meanwhile, when the upper die 30 is in a downwardly advancing pressing position, the pressing surface 32 approaches inner sides of the receiving members 54, 54 of the lower die 20, and then the upper die 30 is inserted between the receiving members 54, 54.

As shown in FIG. 1 and FIG. 4, the lower die 20 includes a placing part 50 fixed to a mount part 40, a lower movable part 60, and gas springs 70, as reaction force generating members, configured to support the lower movable part 60 from beneath. Among them, the placing part 50 has an inwardly recessed groove portion 52 formed in a longitudinal direction A (see FIG. 1). The receiving members 54, 54 are accommodated on both sides of the groove portion 52. The receiving members 54, 54 are removably fixed to the placing part 50 by a plurality of bolts 56 inserted from both side portions of the placing part 50 (see FIG. 4).

The receiving members 54 are located opposite to each other across the lower movable part 60. Each receiving member 54 has an inner edge portion 54b formed in a rounded shape (cylindrical surface shape) having a radius smaller than the radius L1/2 of the mandrel member 31 (see FIG. 5).

The lower movable part 60 is provided between the receiving members 54, 54 and slidable in vertical directions. As shown in FIG. 5, the lower movable part 60 has a pressing force receiving surface 62 that is opposite to the pressing surface 32 of the upper die 30 and having an inwardly recessed curved shape. The radius L3 of the pressing force receiving surface 62 is set to be greater than or equal to the radius L1/2 of the pressing surface 32. According to this embodiment, the difference between the radius L3 of the pressing force receiving surface 62 and the radius L1/2 of the pressing surface 32 of the upper die 30 (i.e., L3−L1/2) is set to be smaller than a thickness dimension L4 of the workpiece 100.

The cross-sectional shape of both end portions 62f, 62f of the pressing force receiving surface 62 is an upwardly protruding arc-shape. The radius r3 of the end portion 62f is set to be smaller than the radius r4 of the inner edge portion 54b of the receiving member 54 (i.e., r3<r4). This makes it possible to reduce a space that is surrounded by the end portion 62f of the lower movable part 60, the inner edge portions 54b of the receiving members 54, and the workpiece 100. Accordingly, tensile pressure generated on the workpiece 100 can be reduced to thereby suppress cracking.

As shown in FIG. 4, the lower movable part 60 is brought into contact with a bottom portion 20e of the lower die 20 when it moves to the lowermost end portion. In other words, the lower movable part 60 contacts the bottom portion 20e of the lower die 20, so that the downward movement of the lower movable part 60 is restricted. The movable range of the lower movable part 60 is equal to the maximum stroke distance of the gas springs 70.

As shown in FIG. 2 or FIG. 3, a plurality of gas springs 70, seven in this embodiment, are arranged immediately underneath the lower movable part 60, along the longitudinal direction A of the lower die 20 (extension direction of the workpiece 100). The gas springs 70 are arranged in a row with a predetermined distance spaced apart from each other.

Each gas spring 70 includes a cylinder 71 and a piston 72. The piston 74 is configured such that the reaction force thereof is adjustable depending on the pressure of the gas filled in the cylinder 71. For example, nitrogen gas is used as a filler gas. It should be noted that the filler gas is not specifically limited thereto and other gases or mixture of these gases may be used.

As shown in FIG. 4, the upper end face 73 of the piston 72 is in contact with the lower surface 61 of the lowermost end portion of the lower movable part 60. The gas springs 70 are configured to elastically support the lower movable part 60 from beneath.

Next, the processing method for the U-shaped member 110 using the dies 10 is explained according to processes shown in FIGS. 6A to 6E.

First, a protective film 103 is attached to a lower surface of the workpiece 100. The protective film 103 is shown in FIG. 6A but omitted in the other figures. FIG. 6A shows a setting process in which the workpiece 100 is placed on the lower die. In this process, as shown in FIG. 6A, the workpiece 100 is placed on top of the lower die 20 such that the attached protective film 103 faces downward. It should be noted that a positioning jig 80 is fixed in advance to the lower die 20 with bolts 81. In this way, the workpiece 100 can be positioned easily. Using the positioning jig 80 makes it easy to position the center portion of the workpiece 100 in the width direction (lateral direction) against the center portion of the lower die 20 in the width direction (lateral direction). This can achieve excellent formability of a pipe member and improve processing accuracy. Since the surface to which the protective film is attached is in contact with the lower die 20, it is possible to prevent the workpiece 100 from being scratched during the bending process.

When the workpiece 100 is placed on the top surface of the lower die 20, both end portions 62f, 62f of the lower movable part 60 and the upper surface portions 54a, 54a of the receiving members 54, 54 are brought into contact with the lower surface of the workpiece 100. It should be noted that the upper die 30 is in the retracted position and is not shown in the figure.

FIG. 6B to FIG. 6D are process drawings showing a pressing process in which the workpiece 100 is pressed by the upper die 30. In this process, as shown in FIG. 6B, the upper die 30 is moved downward so that the pressing surface 32 of the mandrel member 31 presses the workpiece 100. In this case, the pressing surface 32 of the upper die 30 presses the workpiece 100 from the upper side of the workpiece 100, and holds the workpiece 100 against the pressing force receiving surface 62 of the lower movable part 60. At this time, the gas springs 70 start to move downward under the pressing force of the upper die 30.

When the upper die 30 is advanced further downward, as shown in FIG. 6C, both sides of the workpiece 100 are bent to rise between the mandrel member 31 and the receiving member 54, 54, while the center portion of the workpiece 100 is gradually bent along the pressing surface 32 so as to be held in place in the circumferential direction of the mandrel member 31 with advancing of the upper die 30. As a result, the workpiece 100 is gradually bent and deforms to follow curved shapes of the pressing force receiving surface 62 and the pressing surface 32, while suppressing deformation for causing a rapid tension. In other words, as the workpiece 100 is bent and deforms, the workpiece 100 is compressed in the thickness direction around a portion of the workpiece 100 with which the pressing force receiving surface 62 of the lower movable part 60 is in contact. This allows the skin portion 102 at the lower surface of the workpiece 100 to move in the ductile direction together with the material of the adjacent core portion 101 to reduce tension. Therefore, since the tension of the workpiece 100 due to deformation of the bending process is relieved, it is possible to suppress breakage of the workpiece 100 on the lower side thereof. Further, since the compression force of the workpiece 100 at the upper side thereof is reduced, it is possible to suppress wrinkles.

During the downward movement of the upper die 30, the reaction force of the gas springs 70 is always given to the lower movable part 60. As a result, the workpiece 100 is always urged upward, which is the opposite direction of the downward movement of the upper die 30. It should be noted that the initial reaction force of the gas spring 70 is relatively stronger than other reaction force mechanisms such as a spring. As a result, the workpiece 100 is held tightly between the lower die 20 and the upper die 30. In other words, when the workpiece 100 placed on the lower die 20 is pressed by the upper die 30, the workpiece 100 is restrained by the reaction force from the lower die 20. Accordingly, a compressive force is applied to the workpiece 100 in the thickness direction thereof from when the workpiece 100 is pressed by the upper die 30 to when the lower movable part 60 reaches the lowermost point. This compressive force suppresses slippage of the workpiece 100 between the dies 10 and reduces the tension generated in the direction along the area of the workpiece 10 where the pressing force receiving surface 62 is in contact.

Further, since the workpiece 100 is kept restrained from above and beneath during the bending process, the starting point of the bend moves along the lower surface of the workpiece 100, so that an overconcentration of the bending stress generated on the workpiece 100 can be prevented. In other words, the starting point of the bend of the workpiece 100 at the initial stage of the bending process is the point where the pressing surface 32 contacts the top surface of the workpiece 100 and the point where the workpiece 100 contacts the inner edge portions 54b, 54b of the receiving members 54, 54. When the upper die 30 and the lower movable part 60 slide downward with the workpiece 100 being restrained, the portion of the workpiece 100 that is in contact with the inner edge portion 54b moves downward on the inner edge portion 54b. This can prevent overconcentration of the bending stress generated on the workpiece 100.

Further, since each of the receiving members 54, 54 has the inner edge portion 54b formed in a rounded shape (cylindrical surface shape) having a radius smaller than the radius L1/2 of the mandrel member 31, the workpiece 100 smoothly moves downward on the inner edge portions 54b, 54b while ensuring sufficient contact area of the workpiece 100 with respect to the inner edge portions 54b, 54b. Accordingly, the workpiece 100 is gradually held by the mandrel member 31 over the entire U-shaped bent range, and sufficient stress is applied to the workpiece 100 while preventing slippage and deviation of the workpiece 100. In other words, the bending progresses gradually and a rapid bending of the workpiece 100 can be prevented.

After that, as shown in FIG. 6D, the upper die 30 is advanced further downward, and the lower surface 61 of the lowermost end portion of the lower movable part 60 is brought into contact with the bottom portion 20e of the lower die 20. According to this embodiment, as shown in FIG. 5, the maximum push-in position P1 of the lower end portion 32a of the pressing surface 32 is deeper than the position P2 lowered by the finished inner diameter dimension L1 of the workpiece 100 from the upper surface 20a of the lower die 20. Accordingly, the workpiece 100 is fixed so as to be held in place over the entire U-shaped bent range along the circular cross-sectional pressing surface 32, and sufficient stress is applied to the workpiece 100 while preventing slippage and deviation of the workpiece 100. The mandrel member 31 is entirely inserted inside the lower die 20.

After that, the mandrel member 31 is pressed further downward from this contact state by the upper die 30. In other words, an additional load is applied to the workpiece 100 to enhance its formability. Accordingly, a U-shaped member 110 having been bent into a desired U-shape can be obtained.

FIG. 6E shows a process of moving the upper die 30 upward to a retracted position. When the upper die 30 is moved upward, a gap is formed between the lower die 20 and the upper die 30. Since the lower movable part 60 of the lower die 20 is no longer subjected to the pressing force of the upper die 30, the reaction force of the gas springs 70 acts as a force for lifting the workpiece 100 from the mount part 40. Therefore, the workpiece 100 having been bent into a U-shape can be easily removed from the bending machine 1 by retracting the upper die 30.

Next, dies 10A used for processing the U-shaped member 110 into a pipe member 120 and a processing method using the dies 10A are described.

As shown in FIG. 7, the dies 10A are used to bend the U-shaped member 110 into an O-shape. The dies 10A includes an O-shape lower die 20A on which the U-shaped member 110 is placed, and an O-shape upper die 30A by which the U-shaped member 110 is pressed.

The O-shape upper die 30A is configured to move upward and downward with the aid of a drive mechanism (not shown in the drawings). The O-shape upper die 30A includes a base part 37 and a pressing member 35 attached to the base part 37. As shown in FIG. 7, the pressing member 35 has a lower portion having a pressing surface 30g in the form of an inwardly recessed cross-section. The pressing member 35 is fixed to the base part 37 by a plurality of mounting bolts 34 (only one is shown in the figure) arranged spaced apart in the longitudinal direction of the base part. The pressing member 35 is detachably attached by screwing the mounting bolts 34. The pressing surface 30g has an arc-shaped cross-section extending in the longitudinal direction of the U-shaped member 110. The radius of curvature of the pressing surface 30g corresponds to the radius of curvature of the outer diameter of the U-shaped member 110.

Cross-sectionally tapered slanted surfaces (open surfaces) 36, 36 are formed on both sides of the pressing surface 30g so that they are continuous with the pressing surface 30g.

When the O-shape upper die 30A is in an upwardly retracted standby position, a predetermined space for placement of the U-shaped member 110 is formed between the O-shape upper die 30A and a placing part 55 of the O-shape lower die 20A. Meanwhile, when the O-shape upper die 30A is in a downwardly advancing pressing position, both upper end portions 26, 26 (to be described later) of the O-shape lower die 20A are inserted inside the slanted surfaces 36, 36 and face the slanted surfaces 36, 36 in a non-contact manner.

The O-shape lower die 20A includes the placing part 55 fixed to the mount part 40 with bolts 44. The placing part 55 has a generally trapezoidal cross-section. The upper end portion of the placing part 55 includes a pressing force receiving surface 57 having an inwardly recessed curved shape to hold the U-shaped member 110. The radius of curvature of the pressing force receiving surface 57 corresponds to the radius of curvature of the outer diameter of the U-shaped member 110. Both sides of the pressing force receiving surface 57 have upper ends 26, 26 protruding toward the slanted surfaces 36, 36 of the O-shape upper die 30A.

Next, the processing method for the pipe member 120 using the dies 10A is explained according to processes shown in FIG. 8A to FIG. 8E.

FIG. 8A shows a process of setting the U-shaped member 110 on the placing part 55 of the O-shape lower die 20A. In this process, as shown in FIG. 8A, the U-shaped member 110 is placed on the pressing force receiving surface 57 of the O-shape lower die 20A. Then, a cylindrical member 90, which serves as a mandrel, is placed inside the U-shaped member 110. The radius of curvature of the cylindrical member 90 is set equal to or slightly smaller than the radius of curvature of the inner diameter of the U-shaped member 110.

After the cylindrical member 90 is placed inside the U-shaped member 110, positioning of the U-shaped member 110 is carried out using a positioning member 85. The positioning member 85 has a generally L-shape and is detachably attached to the outer surface of the placing part 55. The positioning member 85 includes a reference plate 86 for positioning both end portions 115, 115 of the U-shaped member 110 at the same height. Using the positioning member 85 makes it possible to position the U-shaped member 110 substantially horizontally on the placing part 55. After the positioning is completed, the positioning member 85 is removed from the placing part 55. A mechanism (not shown) may be used to retreat the positioning member 85 to a position where the positioning member 85 does not interfere with the O-shape upper die 30A during the O-bending process.

FIG. 8B to FIG. 8D are process drawings showing a process of pressing the U-shaped member 110 with the O-shape upper die 30A. In this process, as shown in FIG. 8B, the O-shape upper die 30A is advanced downward, so that the pressing surface 30g of the pressing member 35 approaches and is brought into contact with both end portions 115, 115 of the U-shaped member 110.

As shown in FIG. 8C, when the O-shape upper die 30A is advanced further downward, the both end portions 115, 115 of the U-shaped member 110 are bent inward along the inwardly recessed curved pressing surface 30g of the pressing member 35 and curved to hold the outer peripheral surface of the cylindrical member 90. As a result, straight portions of the U-shaped member 110 are gradually bent and deform to follow curved shapes of the pressing surface 30g and the outer peripheral surface of the cylindrical member 90, while suppressing deformation for causing a rapid tension.

After that, as shown in FIG. 8D, the O-shape upper die 30A is advanced further downward. In the final stage of the bending deformation, a pipe member 120 is pressed by the pressing surface 30g, the pressing force receiving surface 57, and the cylindrical member 9. Accordingly, a pipe member 120 having been bent into an O-shape can be obtained.

FIG. 8E shows a process of moving the O-shape upper die 30A upward to the retracted position. The pipe member 120 having been bent into an O-shape, together with the cylindrical member 90, can be easily removed from the bending machine 1 by moving upward and retracting the O-shape upper die 30. Thereafter, the process of processing the pipe member 120 is completed by pulling out the cylindrical member 90 from the pipe member 120. The pipe member 120 is formed as a product in which end portions 121 thereof face each other with a gap formed therebetween.

The above-described dies 10 according to this embodiment are configured such that, when a workpiece 100 is bent into a U-shape, the lower end portion of the pressing surface 32 is pushed in a position P1 deeper than the position P2 lowered by the finished inner diameter dimension L1 of the workpiece 100 from the upper surface (upper surface portions 54a) of the lower die 20. Accordingly, the workpiece 100 is fixed so as to be held in place over the entire U-shaped bent range along the circular cross-sectional pressing surface 32, and sufficient stress is applied to the workpiece 100 while preventing slippage and deviation of the workpiece 100. This can prevent brakeage of the workpiece 100 and occurrence of wrinkle during the bending process.

Since the reaction force generating member is a gas spring 70, the gas spring 70 provides a stronger initial reaction force than other reaction force generating members such as a spring, and the workpiece 100 can be held tightly by the upper die 30 and the lower die 20. Therefore, the workpiece 100 is held tightly in place along the circular cross-sectional pressing surface 32, and sufficient stress is applied to the workpiece 100 while preventing slippage and deviation of the workpiece 100. This can improve formability.

Since a plurality of gas springs 70 are arranged in the longitudinal direction of the lower die 20, even if the workpiece 100 has a long shape, a uniform reaction force can be generated in the longitudinal direction, so that the same bending process can be performed at each portion in the longitudinal direction. Further, since a plurality of gas springs 70 are arranged in the longitudinal direction of the lower die 20, it is possible to provide dies 10 (lower die 20) having a length adapted to the longitudinal dimension of the workpiece 100.

Since each of the gas springs 70 is configured such that the magnitude of the reaction force is adjustable, the reaction force required can be set appropriately according to the size and strength of the workpiece 100, the pressing force of the upper die 30, etc., and sufficient reaction force can be generated with respect to the bending stress required for the bending process.

Since the difference between the diameter dimension L3 of the pressing force receiving surface 62 of the lower movable part 60 and the diameter dimension L1/2 of the pressing surface 32 of the upper die 30 is smaller than the thickness dimension L4 of the workpiece 100. This configuration allows the workpiece 100 to be pressed against the lower die 20 while applying compressive force to the workpiece 100 from the upper and lower directions, so that the workpiece 100 is held tightly in place along the circular cross-sectional pressing surface 32. This can improve formability.

Since the pressing surface 32 of the upper die 30 is formed by a cylindrical mandrel member 31 extending in the longitudinal direction of the lower die 20, the pressing surface 32 can be formed easily. Further, since the mandrel member 31 is configured to be attachable to and detachable from the upper die 30, mandrel members 31 with different outer diameters can be easily installed according to the thickness dimensions L4 of workpieces 100.

Since the movable range of the lower movable part 60 is equal to the maximum stroke distance of the gas springs 70, it is possible to further push in the upper die 30 toward the workpiece 100 after the full stroke of the gas springs 70, so that the formability of the workpiece 100 can be improved.

Since the inner edge portion 54b of each receiving member 54 has a rounded shape having a radius smaller than the radius L1/2 of the mandrel member 31, the concentration of stress during the bending process can be alleviated preferably.

Since a hard chrome plating layer is formed on the surface of the lower movable part 60 and the surfaces of edge portions of the receiving members 54, it is possible to prevent wear of the lower die 20.

Further, since the processing method includes a process of further bending the U-shaped member 110 having been processed using the dies 10 into an O-shape to finish the U-shaped member 110 into the pipe member 120, the pipe member 120, in which breakage or wrinkling of the workpiece 100 (U-shaped member 110) is suppressed, can be obtained.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment and various changes or modifications may be made where necessary. It is to be understood that the above-described embodiment is illustrated for the purpose of easy understanding and explaining the present invention. It is not necessary that the present invention be limited to the specific configuration including all the elements described above.

For example, the gas springs 70 are used as reaction force generating members in the above embodiment. However, the present invention is not limited to this specific embodiment, and other reaction force generating members with other configurations may be used, for example, other mechanisms such as hydraulic cylinders and metal springs, and soft and elastic members including foamed synthetic resin materials such as urethane, and rubber members. In other words, a member for elastically supporting the lower movable part 60 from beneath is suitable. In particular, a member whose reaction force increases as it is compressed cab be suitably used for the reaction force generating member. The shape, number, quantity, and material of the reaction force generating member are not particularly limited.

In the above embodiment, the mandrel member 31 having the pressing surface 32 is used. However, the present invention is not limited to this specific configuration, and another pressing member having other shapes, in which the pressing surface 32 is integrally formed at its lower portion, may be used.

As long as the workpiece 100 is tightly held between the mandrel member 31 and the pressing force receiving surface 62 of the lower movable part 60 during the bending process and the workpiece 100 is guided between the pressing force receiving surface 62 and the inner edge portions 54b, 54b of the receiving members 54, 54 so as to be held in place around the mandrel member 31, the size (width and area) of the pressing force receiving surface 62 of the lower movable part 60 may not be limited, and various sized pressing force receiving surfaces may be used.

REFERENCE SIGNS LIST

- 10 Dies
- 10A Dies
- 20 Lower die
- 20A O-shape lower die (Lower die)
- 30 Upper die
- 30A O-shape upper die (Upper die)
- 31 Mandrel member
- 32 Pressing surface
- 54 Receiving member
- 54
  b Inner edge portion
- 60 Lower movable part
- 62 Pressing force receiving surface
- 70 Gas spring
- 100 Workpiece
- 110 U-shaped member
- 120 Pipe member
- L1 Finished inner diameter dimension
- P1 Maximum push-in position of Lower end portion of Pressing surface
- P2 Position lowered by Finished inner diameter dimension

DIE SET AND PROCESSING METHOD USING DIE SET

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