Patent Publication Number: US-9421594-B2

Title: Method for manufacturing curved hollow pipe

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing a bent hollow pipe, and more particularly to a method for manufacturing a suspension arm. 
     BACKGROUND ART 
     An example of method for manufacturing a bent hollow pipe such as a suspension arm for a vehicle includes the manufacturing method described in Patent Literature 1. This method is a method for pressing a flat plate, which is a material, into ultimately a two-dimensionally bent hollow pipe through press-forming in a stepwise manner using multiple forming dies, and thus the bent hollow pipe can be formed without using a core. In this method, first, flange portions extending in directions away from each other are formed in a flat plate, subsequently, both of the flange portions are bent in a press direction so as to be in substantially parallel with each other, and thereafter, both of the flange portions are brought into contact with each other along the inner wall of a forming die, so that a pipe is formed. 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: Japanese Patent No. 3114918 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, when a three-dimensionally bent pipe is formed according to the above method, there is a problem in that, when both of the flange portions are bent in a substantially parallel with each other, the flange portion is too long in some portions, and the flange portion is drawn and become too short in other portions, and unstable shrinking occurs, which makes it difficult to form a preferable pipe. 
     The present invention is made to solve the problem associated with the above conventional techniques, and it is an object of the present invention to provide a method for manufacturing a bent hollow pipe, wherein even when a three-dimensionally bent pipe is formed, a gap, an uneven thickness, and the like would not occur at contact portions at the ends of both of the flange portions when press work is finished, and thus a pipe having the high quality contact portions can be formed. 
     Means for Solving Problem 
     The above object is achieved by the inventions described in (1) to (4) below. 
     (1) A method for manufacturing a bent hollow pipe, in which a material, which is to be processed and which is a flat plate extending in a first plane formed by a first direction and a second direction perpendicular to the first direction, is pressed in a stepwise manner using a plurality of forming dies from a third direction perpendicular to the first plane, so that two sides of a second plane formed by the second direction and the third direction of the material are brought into contact with each other, and the bent hollow pipe is made to be bent and extend in a three dimensional manner that is bent in the first plane and in the second plane, and the method includes a step in which a push out portion is formed that is pushed out onto the material in the third direction and that is bent and extends within a third plane formed by the first direction and the third direction in a press work, and extension portions are formed to extend away from each other at both sides which sandwich the push out portion of the material, and flange portions are formed that are bent, in a direction opposite to a push out direction of the push out portion, at ends of the extension portions, a step in which while bending is maintained between the extension portions and the flange portions, a bent portions between the push out portion and the extension portions are bent in the opposite directions, so that the both of the flange portions are formed in a direction in which the both of the flange portions face each other, a step in which the both of the flange portions of a three-dimensionally bent portion that is bent in the first plane and in the second plane are bent and brought closer to each other as compared with both of the flange portions in a portion other than the three-dimensionally bent portion, and a step of bringing ends of the flange portions into contact with each other. 
     (2) The method for manufacturing the bent hollow pipe described in (1) explained above, wherein in the step of bringing the flange portions into contact with each other, the flange portions are slid along an inner wall of the forming die, whereby the flange portions are brought into contact with each other. 
     (3) The method for manufacturing the bent hollow pipe described in (1) explained above, wherein in the step of bringing the flange portions into contact with each other, the flange portions are slid along an inner wall of a forming die, whereby the flange portions are brought into contact with each other, and at the same time, the flange portions are brought into contact with each other by the forming die partially using a core. 
     (4) The method for manufacturing the bent hollow pipe described in (3) explained above, wherein the core is partially applied to a position corresponding a portion where a length of the bent hollow pipe in the first direction in a cross section is longer than a length thereof in the third direction. 
     Advantageous Effects 
     According to the invention described in (1) explained above, in a case where a three dimensionally bent pipe is formed by forming bending between an extension portion and a flange portion in advance and thereafter bending bent portions between a push out portion and extension portions in a direction opposite to each other while maintaining this bending, a gap, an uneven thickness, and the like would not occur at contact portions at the ends of both of the flange portions when press work is finished, and thus a pipe having the high quality contact portions can be formed. Further, the method has the step for bending the both of the flange portions of the three-dimensionally bent portion and bringing the both of the flange portions of the three-dimensionally bent portion closer to each other as compared with the both of the flange portions other than the three-dimensionally bent portion, and therefore, in the step for forming the bent hollow pipe, the ends of the both of the flange portions of the three-dimensionally bent portion are first brought into contact with each other, and thereafter the peripheral portions are brought into contact with each other. Therefore, a gap, an uneven thickness, and the like would not occur at contact portions at the ends of both of the flange portions when press work is finished, and thus the three dimensionally bent hollow pipe having the high quality contact portions can be formed. 
     According to the invention described in (2) explained above, the flange portions are slid along the inner wall of the forming die, whereby the flange portions are brought into contact with each other, and therefore the bent hollow pipe can be formed without using any core. 
     According to the invention described in (3) explained above, the flange portions are slid along the inner wall of the forming die, whereby the flange portions are brought into contact with each other, and further the core is partially used, and therefore, this method can form even a bent hollow pipe that could not be formed without any core. 
     According to the invention described in (4) explained above, a core is partially applied in order to form a bent hollow pipe of which length in the first direction in the cross section is longer than the length thereof in the third direction, and therefore, the shape explained above, which could not be formed according to the conventional method, can be formed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1  (A) to  1  (E) are figures illustrating a bent hollow pipe, in which  FIG. 1  (A) is a perspective view,  FIG. 1  B) is a front view,  FIG. 1  (C) is a top view taken along line C-C of  FIG. 1  (B),  FIG. 1  (D) is a cross sectional view taken along line D-D of  FIG. 1  (B), and  FIG. 1  (E) is a cross sectional view taken along line E-E of  FIG. 1  (B). 
         FIG. 2  is a perspective view illustrating a non-pressed flat plate of a bent hollow pipe according to the present embodiment. 
         FIG. 3  is a cross sectional view illustrating a rough forming step of the method for manufacturing the bent hollow pipe according to the present embodiment. 
         FIG. 4  is a cross sectional view taken along line  4 - 4  of  FIG. 3 . 
         FIG. 5  is a perspective view illustrating a material, which is to be processed, when the rough forming step is finished. 
         FIG. 6  is a cross sectional view illustrating a trimming step of the method for manufacturing the bent hollow pipe according to the present embodiment. 
         FIG. 7  is a cross sectional view taken along line  7 - 7  of  FIG. 6 . 
         FIG. 8  is a perspective view illustrating a material, which is to be processed, when the trimming step is finished. 
         FIG. 9  is a cross sectional view illustrating a bending step of the method for manufacturing the bent hollow pipe according to the present embodiment. 
         FIG. 10  is a cross sectional view taken along line  10 - 10  of  FIG. 9 . 
         FIG. 11  is a perspective view illustrating a material, which is to be processed, when the bending step is finished. 
         FIG. 12  is a top view illustrating the material, which is to be processed, when the bending step is finished. 
         FIG. 13  is a cross sectional view illustrating an inner bending step of the method for manufacturing the bent hollow pipe according to the present embodiment. 
         FIG. 14  is a cross sectional view taken along line  14 - 14  of  FIG. 13 . 
         FIG. 15  is a cross sectional view taken along line  15 - 15  of  FIG. 13 . 
         FIG. 16  is a cross sectional view taken along line  16 - 16  of  FIG. 13 . 
         FIG. 17  is a perspective view illustrating the material, which is to be processed, when the inner bending step is finished. 
         FIG. 18  is a top view illustrating the material, which is to be processed, when the inner bending step is finished. 
         FIG. 19  is a cross sectional view illustrating an O press step of the method for manufacturing the bent hollow pipe according to the present embodiment. 
         FIG. 20  is a cross sectional view taken along line  20 - 20  of  FIG. 19 . 
         FIG. 21  is a cross sectional view taken along line  21 - 21  of  FIG. 19 . 
         FIG. 22  is a cross sectional view taken along line  22 - 22  of  FIG. 19 . 
         FIG. 23  (A) is a top view illustrating the material, which is to be processed, during the O press step, and  FIG. 23  (B) is a top view illustrating the material, which is to be processed, when the O press step is finished. 
         FIG. 24  is a perspective view illustrating the material, which is to be processed, when the O press step is finished. 
         FIG. 25  is a cross sectional view illustrating a contact step of the method for manufacturing the bent hollow pipe according to the present embodiment. 
         FIG. 26  is a cross sectional view taken along line  26 - 26  of  FIG. 25 . 
         FIG. 27  is a cross sectional view taken along line  27 - 27  of  FIG. 25 . 
         FIG. 28  is a cross sectional view illustrating a cutting step of the method for manufacturing the bent hollow pipe according to the present embodiment. 
         FIG. 29  is a cross sectional view taken along line  29 - 29  of  FIG. 28 . 
         FIG. 30  is a cross sectional view for explaining a conventional method for manufacturing a bent hollow pipe. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be explained with reference to drawings. In the present embodiment, a plane in which a flat plate is arranged is defined as an XY plane (first plane), a direction in which the flat plate extends which is perpendicular to the X direction (first direction) is defined as a Y direction (second direction), and a direction perpendicular to the XY plane is defined as a Z direction (third direction). The bent hollow pipe according to the present embodiment extends while bending in a three-dimensional manner in the XY plane and the YZ plane (second plane), and the cross section in the XZ plane (third plane) is in a circular shape and a rectangular shape. 
       FIGS. 1  (A) to  1  (E) are figures illustrating a bent hollow pipe, in which  FIG. 1  (A) is a perspective view,  FIG. 1  (B) is a front view,  FIG. 1  (C) is a top view taken along line C-C of  FIG. 1  (B),  FIG. 1  (D) is a cross sectional view taken along line D-D of  FIG. 1  (B), and  FIG. 1  (E) is a cross sectional view taken along line E-E of  FIG. 1  (B). 
     For example, the bent hollow pipe  1  is a suspension arm used as a vehicle member, and in the present embodiment, the bent hollow pipe  1  is manufactured by bringing two sides of a flat plate in the YZ plane into contact with each other by pressing the flat plate. 
     The bent hollow pipe  1  extends while bending in a three dimensional manner as shown in  FIG. 1 , and has a circular cross section at one of the end portions and has a rectangular cross section at other of the end portions. The bent hollow pipe  1  is made from the flat plate, and therefore, a welding line  2  is formed over the Y direction. A rectangular portion  3  formed by the rectangular cross section is a portion where a collar of a suspension arm is attached, and is formed with a notched portion  4  for attaching a circular collar by welding and the like. 
     The cross sectional shape of the rectangular portion  3  is such that a length L 1  of two sides, which are a side where the welding line  2  is provided and a side opposite thereto, is configured to be longer than a length L 2  of the other two sides. 
     Subsequently, the method for manufacturing the bent hollow pipe according to the present embodiment will be explained. 
       FIG. 2  is a perspective view illustrating a non-pressed flat plate of a bent hollow pipe according to the present embodiment. In  FIG. 2 , after the forming, the lower right portion becomes the rectangular portion, and the upper left portion becomes the circular portion. Each of L 3 , L 4  is set so as to be same as the lengths of the external peripheries of the rectangular portion and the circular portion after the pipe is formed. 
       FIG. 3  is a cross sectional view illustrating a rough forming step of a rough forming step of the method for manufacturing the bent hollow pipe according to the present embodiment.  FIG. 4  is a cross sectional view taken along line  4 - 4  of  FIG. 3 .  FIG. 5  is a perspective view illustrating a material, which is to be processed W, when the rough forming step is finished. 
     First, as shown in  FIGS. 3, 4 , the material W which is a metal flat plate is roughly formed by a first forming die  6  (rough forming step). The first forming die  6  includes a first upper die  7 , a first lower die  8  provided to face the first upper die  7 , and a blank holder  9 . The first lower die  8  is formed with a protruding portion  10  that protrudes toward the first upper die  7  and extends on the surface facing the first upper die  7 . The first upper die  7  is provided with a groove portion  11  that is depressed so as to correspond to the protruding portion  10  of the first lower die  8 . Blank holders  9  are provided at the external peripheries of the first lower die  8 , and the first upper die  7  is formed with holder surfaces  12  so as to face the blank holders  9 . The holder surfaces  12  are formed at the external peripheries of the groove portion  11 , and the holder surfaces  12  extend in the external peripheral direction. At the external peripheries thereof, the holder surface  12  is inclined in a direction toward the blank holder  9 . The blank holder  9  is formed in a shape corresponding to the shape of the holder surface  12  of the first upper die  7 . 
     In the rough forming step, first, the material W is placed in the first forming die  6 , and while the first lower die  8  is kept away from the first upper die  7 , the blank holder  9  and the first upper die  7  are brought close to each other, so that the material W is sandwiched by the first upper die  7  and the blank holder  9 . At this occasion, a portion of the material W which is sandwiched between the first upper die  7  and the blank holder  9  is formed to extend in the external peripheral direction according to the shape of the holder surface  12 , and incline so that the end thereof is in the direction toward the blank holder. 
     Thereafter, the first lower die  8  is brought close to the first upper die  7 , and the material W is formed with a push out portion  13  that protrudes in a direction toward the first upper die  7  in association with the groove portion  11  of the first upper die  7 . This push out portion  13  is pushed in the direction toward the first upper die  7 , and is formed to be bent and extended thereto. 
     In the rough forming step, the material W is sandwiched by the blank holder  9 , and therefore, this reduces unevenness of flow of the material W, so that generation of wrinkles and the like can be prevented. 
     As described above, in the rough forming step, extension portions  15  and flange portions  16  are formed. The extension portions  15  extend to be away from each other, and the extension portions  15  are formed at two sides at the outside of the push out portion  13  in the material W. The flange portions  16  are bent in the direction toward the blank holders  9  at the ends thereof. A bent portion  17  bent in the direction toward the first upper die  7  is formed between the push out portion  13  and the extension portion  15  (see  FIG. 5 ). 
     Subsequently, an unnecessary portion of the external periphery portion of the material W that is roughly formed is trimmed (trimming step). 
       FIG. 6  is a cross sectional view illustrating the trimming step of the method for manufacturing the bent hollow pipe according to the present embodiment.  FIG. 7  is a cross sectional view taken along line  7 - 7  of  FIG. 6 .  FIG. 8  is a perspective view illustrating a material W when the trimming step is finished. 
     As shown in  FIGS. 6, 7 , the roughly-formed material W is trimmed by a second forming die  20 . This second forming die  20  includes a second upper die  21  and a second lower die  22 . The second lower die  22  is formed with a lower die cutting blade  23  on the external peripheral end of the surface facing the second upper die  21 . The second upper die  21  includes a holder unit  24  and an upper die cutting blade  25 . The holder unit  24  is arranged to face the second lower die  22  and arranged with springs on the back surface. The holder unit  24  is urged in the direction toward the second lower die  22 . The upper die cutting blade  25  is arranged on the external periphery of the holder unit  24 , and the upper die cutting blade  25  and the lower die cutting blade  23  make a pair. This holder unit  24  and the second lower die  22  have shapes corresponding to the shape of the material W. 
     In the trimming step, first, the roughly formed material W is placed in the second forming die  20 , and the second upper die  21  and the second lower die  22  are brought close to each other. When the material W is sandwiched between the holder unit  24  of the second upper die  21  and the second lower die  22 , the holder unit  24  is moved backward by the spring. When the holder unit  24  is moved backward, the material W is sandwiched between the upper die cutting blade  25  and the lower die cutting blade  23 , and the external peripheral portion of the material W is cut off (see  FIG. 8 ). Thereafter, when the second upper die  21  and the second lower die  22  are spaced apart from each other, the material W is pushed out from the second upper die  21  by the repulsive force of the springs. 
     Subsequently, the trimmed material W is bent (bending step). 
       FIG. 9  is a cross sectional view illustrating the bending step of the method for manufacturing the bent hollow pipe according to the present embodiment.  FIG. 10  is a cross sectional view taken along line  10 - 10  of  FIG. 9 .  FIG. 11  is a perspective view illustrating a material W when the bending step is finished.  FIG. 12  is a top view illustrating the material W when the bending step is finished. 
     As shown in  FIGS. 9, 10 , the trimmed material W is placed in a third forming die  30  in such a manner that the trimmed material W is flipped upside down, and the trimmed material W is bent. The third forming die  30  includes a third lower die  32  having a groove portion  33  formed therein with which the material W is engaged, and a third upper die  31  having a protruding portion  34  engaging with the groove portion  33 . In addition, the third lower die  32  is provided with a push out portion  35  configured to push out the formed material W from the groove portion  33 . 
     In the bending step, first, the trimmed material W is placed in the third forming die  30  in such a manner that the trimmed material W is flipped upside down, and the third upper die  31  and the third lower die  32  are brought close to each other. When the material W is sandwiched between the protruding portion  34  of the third upper die  31  and the groove portion  33  of the third lower die  32 , the bent portion  17  between the extension portion  15  and the push out portion  13  is bent in the opposite direction (direction toward the third upper die  31 ) along the groove portion  33 , and the extension portions  15  at both sides of the push out portion  13  is formed in the direction toward the third upper die  31 . It should be noted that a space larger than the thickness of the material W is provided between the side surfaces of the protruding portion  34  and the groove portion  33 , and when the third forming die  30  reaches the bottom dead center (or top dead center), a gap  36  is formed between the side surface of the protruding portion  34  and the extension portion  15  of the material W. Therefore, bending is maintained between the extension portion  15  and the flange portion  16 , and is formed in a direction in which both of the flange portions  16  face each other (see  FIGS. 11, 12 ). 
     The material W, which has been formed by the third forming die  30 , is pushed out from the groove portion  33  by the push out portion  35 . 
     Subsequently, the material W, which has been bent in the bending step, is further bent, and both of the flange portions  16  of the three-dimensionally bent portion that is bent in the XY plane and the YZ plane are bent to be brought close to each other than both of the flange portions  16  other than the three-dimensionally bent portion (inner bending step). 
       FIG. 13  is a cross sectional view illustrating the inner bending step of the method for manufacturing the bent hollow pipe according to the present embodiment.  FIG. 14  is a cross sectional view taken along line  14 - 14  of  FIG. 13 .  FIG. 15  is a cross sectional view taken along line  15 - 15  of  FIG. 13 .  FIG. 16  is a cross sectional view taken along line  16 - 16  of  FIG. 13 .  FIG. 17  is a perspective view illustrating the material W when the inner bending step is finished.  FIG. 18  is a top view illustrating the material W when the inner bending step is finished. 
     As shown in  FIGS. 13 to 16 , the material W, which has been bent in the bending step, is placed in a fourth forming die  40 , and is further bent. The fourth forming die  40  includes a fourth upper die  41  and a fourth lower die  42  formed with a groove portion  43  with which the material W is engaged. The fourth upper die  41  is constituted by a fourth upper die  41 A in proximity to the three-dimensionally bent portion P, and by a fourth upper die  41 B in portions other than the portion in proximity to the three-dimensionally bent portion P. The fourth upper die  41 A has a taper portion  47  with which both of the flange portions  16  of the three-dimensionally bent portion P are brought close to each other. The fourth upper die  41 B has a protruding portion  44  that engages with the groove portion  43 . The fourth lower die  42  has a push out portion  45  configured to push out the material W which is formed with the groove portion  43 . 
     In the inner bending step, first, the material W is placed in the fourth forming die  40 , and the fourth upper die  41  and the fourth lower die  42  are brought close to each other. Like the third forming die  30 , a space larger than the thickness of the material W is provided, between the side surfaces of the protruding portion  44  and the groove portion  43 , only in the portions other than the three-dimensionally bent portion P. When the fourth forming die  40  reaches the bottom dead center (or top dead center), a gap  46  is formed between the side surfaces of the protruding portion  44  and the material W. Therefore, over the longitudinal direction of the bent hollow pipe  1 , bending is maintained between the extension portion  15  and the flange portion  16 . It should be noted that the protruding portion  44  of the fourth forming die  40  is formed such that, at one side, the protruding portion  44  is formed to be longer in the pressing direction and formed to be narrower than the protruding portion  34  of the third forming die  30  (see  FIG. 14 ), and at the other side, the protruding portion  44  is formed to be shorter in the pressing direction and formed to be wider than the protruding portion  34  of the third forming die  30  (see  FIG. 15 ). In association with the protruding portion  44 , the groove portion  43  of the fourth forming die  40  is formed such that, at one side, the groove portion  43  is formed to be longer in the pressing direction and formed to be narrower than the groove portion  33  of the third forming die  30  (see  FIG. 14 ), and at the other side, the groove portion  43  is formed to be shorter in the pressing direction and formed to be wider than the groove portion  33  of the third forming die  30  (see  FIG. 15 ). Therefore, the formed material W is formed to be longer in the pressing direction at one side and formed to be wider at the other side. 
     In proximity to the three-dimensionally bent portion P, the taper portion  47  is brought close to the fourth lower die  42 , so that both of the flange portions  16  in proximity to the three-dimensionally bent portion P come closer to each other while being bent by the taper portion  47 , and as a result, both of the flange portions  16  in proximity to the three-dimensionally bent portion P are more closer to each other than both of the flange portions  16  in the portions other than the three-dimensionally bent portion P (see  FIGS. 16, 17, 18 ). 
     The material W, which is formed by the fourth forming die  40 , is pushed out from the groove portion  43  by the push out portion  45 . 
     Subsequently, the material W, which has been bent in the inner bending step, is further bent, and while the entire both of the flange portions  16  are brought close to each other, both of the flange portions  16  in proximity to the three-dimensionally bent portion P are brought into contact with each other (O press step). 
       FIG. 19  is a cross sectional view illustrating the O press step of the method for manufacturing the bent hollow pipe according to the present embodiment.  FIG. 20  is a cross sectional view taken along line  20 - 20  of  FIG. 19 .  FIG. 21  is a cross sectional view taken along line  21 - 21  of  FIG. 19 .  FIG. 22  is a cross sectional view taken along line  22 - 22  of  FIG. 19 .  FIG. 23  (A) is a top view illustrating the material, which is to be processed, during the O press step, and  FIG. 23  (B) is a top view illustrating the material, which is to be processed, when the O press step is finished.  FIG. 24  is a perspective view illustrating the material, which is to be processed, when the O press step is finished. 
     As shown in  FIGS. 19 to 22 , the material W, which has been bent in the inner bending step, is placed in a fifth forming die  50  and is further bent. The fifth forming die  50  includes a fifth upper die  51  having an upper die groove portion  53  formed therein with which the material W is engaged, and a fifth lower die  52  facing the fifth upper die  51 . 
     The fifth lower die  52  includes a lower die groove portion  54  and a lower die projection portion  55 . The lower die groove portion  54  is formed to be depressed in association with one end side of the upper die groove portion  53 . The lower die projection portion  55  is formed to protrude in association with the other end side of the upper die groove portion  53 . The lower die groove portion  54  is formed in the portion where the material W is formed to be longer in the push out direction in the inner bending step and in proximity to the three-dimensionally bent portion P, and the lower die projection portion  55  is formed in the portion where the material W is formed to be wider in the inner bending step. 
     In the O press step, first, the material W is placed in the fifth forming die  50  in such an orientation that the side at which the flange portion  16  is arranged is oriented toward the fifth upper die  51 , and a core  56  of which cross section is in a rectangular shape is placed inside of the portion of the material W which corresponds to the lower die projection portion  55 , and thereafter, the fifth upper die  51  and the fifth lower die  52  are brought close to each other. Between the upper die groove portion  53  and the lower die groove portion  54 , the material W is formed to be longer in the push out direction, and therefore, by bringing the fifth upper die  51  and the fifth lower die  52  closer to each other, the ends of the both of the flange portions  16  come closer to each other while moving along the wall surface of the upper die groove portion  53  (see  FIGS. 20, 23, 24 ). 
     Between the upper die groove portion  53  and the lower die projection portion  55 , the material W is formed to be wider, and therefore, when the fifth upper die  51  and the fifth lower die  52  come closer to each other, the ends of the both of the flange portions  16  do not necessarily move along the wall surface of the upper die groove portion  53 , but the core  56  prevents the flange portions  16  from entering into the inner side (see  FIGS. 21, 23, 24 ). 
     In the inner bending step, both of the flange portions  16  in proximity to the three-dimensionally bent portion P are brought close to each other in advance, and therefore, when the fifth upper die  51  and the fifth lower die  52  come closer to each other, both of the flange portions  16  in proximity to the three-dimensionally bent portion P are brought into contact with each other (see  FIGS. 22, 23, 24 ). 
     Subsequently, both of the flange portions  16  of the material W in the portions other than the portion in proximity to the three-dimensionally bent portion P are brought into contact with each other (contact step). 
       FIG. 25  is a cross sectional view illustrating a contact step of the method for manufacturing the bent hollow pipe according to the present embodiment.  FIG. 26  is a cross sectional view taken along line  26 - 26  of  FIG. 25 .  FIG. 27  is a cross sectional view taken along line  27 - 27  of  FIG. 25 . 
     As shown in  FIGS. 25 to 27 , the material Win the portions other than the portion proximity to the three-dimensionally bent portion P where the both of the flange portions  16  are brought close to each other in the O press step is placed in a sixth forming die  60 , and are bent until the ends of the material W are in contact with each other. The sixth forming die  60  includes a sixth upper die  61  having an upper die groove portion  63  formed therein with which the material W is engaged, and a sixth lower die  62  facing the sixth upper die  61 . 
     The sixth lower die  62  includes a lower die groove portion  64  and a lower die projection portion  65 . The lower die groove portion  64  is formed to be depressed in association with one end side of the upper die groove portion  63 . The lower die projection portion  65  is formed to protrude in association with the other end side of the upper die groove portion  63 . The lower die groove portion  64  is formed in the portion where the material W is formed to be longer in the push out direction in the inner bending step, and the lower die projection portion  65  is formed in the portion where the material W is formed to be wider in the inner bending step. 
     In the contact step, first, the material W is placed in the sixth forming die  60  in such an orientation that the side at which the flange portion  16  is arranged is oriented toward the sixth upper die  61 , and while the core  56  placed in the O press step is still held in the inside of the portion of the material W which corresponds to the lower die projection portion  65 , the sixth upper die  61  and the sixth lower die  62  are brought close to each other. Between the upper die groove portion  63  and the lower die groove portion  64 , the material W is formed to be longer in the push out direction and the flange portions  16  are bent to the inside, and therefore, by bringing the sixth upper die  61  and the sixth lower die  62  closer to each other, the ends of the both of the flange portions  16  come closer to each other while moving along the wall surface of the upper die groove portion  63 , and are ultimately brought into contact with each other. In this case, the diameters of the sixth upper die  61  and the sixth lower die  62  are smaller than the diameter of the bent hollow pipe  1 . Therefore, after the ends of the both of the flange portions  16  are brought into contact with each other, compressive force is further exerted on the bent hollow pipe  1 , and therefore, when the sixth upper die  61  is released from the sixth lower die, a phenomenon that the ends of the both of the flange portions  16  move away (spring back) is less likely to occur. 
     Between the upper die groove portion  63  and the lower die projection portion  65 , the sixth upper die  61  and the sixth lower die  62  are brought close to each other, and accordingly, while the material W are sandwiched between the upper die groove portion  63 , the lower die projection portion  65 , and the core  56 , the ends of the material W come closer to each other and are ultimately brought into contact with each other. 
     Subsequently, the notched portion  4  for attaching a circular collar is formed in the rectangular portion  3  of the material W (cutting step). 
       FIG. 28  is a cross sectional view illustrating a cutting step of the method for manufacturing the bent hollow pipe according to the present embodiment.  FIG. 29  is a cross sectional view taken along line  29 - 29  of  FIG. 28 . 
     As shown in  FIGS. 28, 29 , the core  56  is withdrawn from the material W, of which ends of the flange portions  16  are brought into contact with each other in the contact step, and the material W is placed in a seventh forming die  70 . The seventh forming die  70  is formed in association with the shape of the material W, and includes a seventh upper die  71  and a seventh lower die  72  with which the material W can be sandwiched. 
     The seventh upper die  71  is provided with a cutting unit  73  that can move back and forth with respect to the rectangular portion  3  of the material W using a cam mechanism, and the seventh lower die  72  is provided with a cutting hole  74  through which the cutting unit  73  can penetrate. It should be noted that the direction in which the cutting unit  73  moves back and forth is different from the pressing direction of the seventh forming die  70 . 
     In the cutting step, first, the material W is placed in the seventh forming die  70 , and the cutting unit  73  is moved forward with respect to the rectangular portion  3  of the material W, and the surface adjacent to the cutting hole  74  of the rectangular portion  3  is cut by the cutting unit  73  and the cutting hole  74 , and the notched portion  4  for attaching the circular collar is formed in the rectangular portion  3  of the material W. Thereafter, the material W is retrieved from the seventh forming die  70 , and unnecessary portions of the both end portions  16  are cut off (not shown), and the ends of the both of the flange portions  16  which are brought into contact with each other are welded, and thus the formation is completed. 
     As described above, according to the method for manufacturing the bent hollow pipe according to the present embodiment, the bent flange portions  16  are formed at the ends of the extension portions  15  in the rough forming step, and therefore, the three-dimensionally bent hollow pipe  1  can be formed. More specifically,  FIG. 30  shows a conventional method for manufacturing a bent hollow pipe, and in the conventional method, first, flange portions  101  extended in directions away from each other are formed in a flat plate  100  ( FIG. 30  (A)), and subsequently, the flange portions  101  are bent in a press direction so that both of the flange portions  101  are in substantially parallel with each other ( FIG. 30  (B)), and thereafter, the both of the flange portions  101  are brought into contact with each other along the inner wall of the forming die, and thus a pipe is formed ( FIG. 30  (C)). In this method, a two dimensionally bent hollow pipe can be formed, but when a three dimensionally bent hollow pipe is tried to be formed, and when the both of the flange portions  101  are bent in a press direction so that the both of the flange portions  101  are in substantially parallel with each other (see  FIG. 30  (B)), one of the flange portions  101  may be drawn in to be short or become too long, and therefore, this makes it difficult to do forming. In contrast, in the present embodiment, in the rough forming step, the bent flange portions  16  are formed in advance at the ends of the extension portions  15 , and the bent portions  17  between the extension portions  15  and the push out portion  13  are bent in the opposite directions in the bending step, so that the both of the flange portions  16  can be bent in the opposite directions, and therefore, a gap, an uneven thickness, and the like would not occur at contact portions at the ends of both of the flange portions  16  when press work is finished, and thus the three dimensionally bent hollow pipe  1  having the high quality contact portions can be formed. Further, the method has the step for bending the both of the flange portions  16  of the three-dimensionally bent portion P and bringing the both of the flange portions  16  of the three-dimensionally bent portion P closer to each other as compared with the both of the flange portions  16  other than the three-dimensionally bent portion P, and therefore, in the step for forming the bent hollow pipe  1 , the both of the flange portions  16  of the three-dimensionally bent portion P are first brought into contact with each other, and thereafter the peripheral portions are brought into contact with each other. Therefore, a gap, an uneven thickness, and the like would not occur at contact portions at the ends of both of the flange portions  16  when press work is finished, and thus the three dimensionally bent hollow pipe  1  having the high quality contact portions can be formed. 
     In the conventional method, no core is used, and therefore, a wide hollow member of which length in the pressing direction is short (see  FIG. 1  (D)) cannot be formed, but in the present embodiment, the core  56  is partially used, and therefore, a wide hollow member of which length in the pressing direction is short partially can be formed. 
     It should be noted that the present invention is not limited to the embodiment describe above, and can be modified in various manners within the scope of the claims. For example, the trimming step and the cutting step may be omitted. Depending on the shape of the bent hollow pipe, the embodiment can be carried out without using any core. For example, the embodiment can also be applied to a bent hollow pipe other than a suspension arm. 
     DESCRIPTION OF SYMBOLS 
     
         
         
           
               1 : Bent hollow pipe, 
               2 : Welding line, 
               3 : Rectangular portion, 
               6 ,  20 ,  30 ,  40 ,  50 ,  60 ,  70 : Forming die, 
               13 : Push out portion, 
               15 : Extension portion, 
               16 : Flange portion, 
               17 : Bent portion, 
               56 : Core, 
             L 1 , L 2 : Length of rectangular portion in cross section, 
             P: Three-dimensionally bent portion, 
             W: Material which is to be processed.