Method of manufacturing pipe

Abstract: In order to manufacture a pipe having a plurality of flow channels (a,b), by minimizing die jigs and steps, and thus to permit facilitation of manufacture, reduction of cost, and improvement of accuracy, the method of manufacturing a pipe comprises the steps of holding a sheet material (1) at a center portion (1a) in the width direction thereof from both surfaces thereof by means of core members (2a,2b) composing a split structure, folding both sides (1b,1c) in the width direction of the sheet material (1) in opposite directions and causing the thus folded both sides (1b,1c) of the sheet material (1) to follow the shapes of the outer surfaces of the core members (2a,2b), thereby forming a substantially an S-shaped cross-section. In a preferred embodiment, the method comprises the steps of providing outer dies (3,4), each of which has a concave surface (3b,4b) substantially identical with the outer surface of one of the core members (2a,2b), and folds one of both ends of the sheet material (1) toward the outer surface of the core member (2a,2b) by means of the outer die (3,4) while moving the outer die toward the core member (2a,2b).

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a method of manufacturing a pipe. DESCRIPTION OF THE RELATED ART It is a conventional practice, in an automobile exhaust pipe or the like, to provide a partition plate in the pipe to make a plurality of parallel flow channels. For the manufacture of such a pipe, there have conventionally been available a method of forcing a long plate-shaped partition plate into a cylinder, and a method of dividing a sheet type base plate into three in the width direction, and folding the base plate at both sides in opposite directions into an S-shaped cross-section so that the center portion of the base plate forms a partition plate, thereby providing a plurality of parallel flow channels, as disclosed, for example, in JP-U-63-196425, JP-A-04-6397 and JP-A-04-294835. In the former manufacturing method, it is difficult to force a long partition plate into a long cylinder. In the latter manufacturing method, a side of the base plate is first folded, and then the other side is folded, i.e., in different steps, both sides are folded, respectively. It is therefore necessary to use special die jigs such as exclusive inner dies and exclusive outer dies for folding the respective sides in the different steps, thus resulting in a longer manufacturing time and a higher manufacturing cost. In addition, necessity of replacing jigs and dies during an interval between the two folding steps may lead to occurrence of a working error, thus making it difficult to obtain an accurate sectional shape. This results in an unstable shape of the bent portion (weld joint) of the partition plate with which the leading end of the folded side comes into butt contact, thus leading to a decrease in welding quality. The closest prior art is represented for claim 1 by the document DE 20 31 904 A and for claim 8 by the document FR 1 454 335 A. Both disclose general methods for the manufacturing of a pipe with at least two channels from a flat sheet of metal, the first by fixing a general portion of the sheet and bending the outer portions into opposite directions until they meet again the central portion, the second by fixing one end of the sheet and bending the other portion around the fixed end into a circular shape. The present invention has an object to provide an improved method of manufacturing a pipe. SUMMARY OF THE INVENTION To solve the foregoing problems, a first aspect of the present invention provides a method of manufacturing a pipe, comprising the steps of holding a sheet material at a center portion in a width direction thereof from both surfaces thereof by means of core members composing a split structure, folding both sides in the width direction of the sheet material in opposite directions and causing the thus folded both sides of the sheet material to follow the shapes of the outer surfaces of the core members, thereby forming a substantially an S-shaped cross-section. A second aspect of the present invention provides the method of manufacturing a pipe according to the foregoing first aspect, wherein the method comprises the steps of folding both sides of the sheet material in opposite directions relative to the center portion in the width direction thereof, further folding the sheet material gradually from the sides thereof toward the center portion thereof while holding the sheet material from both surfaces at the center portion thereof by means of the core members, thereby causing both sides of the sheet material to follow the shapes of the outer surfaces of the core members. A third aspect of the present invention provide the method of manufacturing a pipe according to the foregoing second aspect, wherein the method comprises the steps of providing outer dies, each of which has a concave surface substantially identical with the outer surface of one of the core members, and folds one of both ends of the sheet material toward the outer surface of the core member by means of the outer die while moving the outer die toward the core member. A fourth aspect of the present invention provides the method of manufacturing a pipe according to the foregoing first aspect, wherein the method comprises the steps of folding both sides of the sheet material in opposite directions with respect to the center portion in the width direction thereof, holding the sheet material at the center portion thereof from both surfaces thereof by means of the core members, and folding both sides of the sheet material gradually from the center portion toward the ends thereof, thereby causing both sides of the sheet material to follow the shapes of the outer surfaces of the core members. A fifth aspect of the present invention provides the method of manufacturing a pipe according to the foregoing fourth aspect, wherein both sides of the sheet material are wound on the core members by turning the core members in a direction around a center line passing through the split surface thereof, and at the same time, both sides are pressed against the outer surfaces of the core members, thereby causing both sides of the sheet material to follow the shapes of the outer surfaces of the core members. A sixth aspect of the present invention provides the method of manufacturing a pipe according to the foregoing first or fourth aspect, wherein the method comprises the steps of rotatably providing the core members, providing on the outer surface sides of the core members pressing rollers, each of which rotates around a rotation axis running in parallel with a rotary shaft of the core members, and pressing both sides of the sheet material against the outer surfaces of the core members by means of the pressing rollers while rotating the core members which hold the sheet material. A seventh aspect of the present invention provides the method of manufacturing a pipe according to the foregoing fourth aspect, wherein the method comprises the steps of rotatably providing the core members, pressing a flat surface of each of flat plates against one of the outer surfaces of both sides of the sheet material held by the core members, and moving the flat plate so that the core members rotate in the pressed state, thereby folding both sides of the sheet material toward the core members. An eighth aspect of the present invention in order to solve the foregoing problems provides the method of manufacturing a pipe, comprising the steps of holding a side in a width direction of a sheet material by means of core members composing a split structure, and pressing another side of the sheet material against the outer surfaces of the core members while rotating the core members, thereby causing another side of the sheet material to follow the shapes of the core members and forming a ⊖-shaped cross-section. A ninth aspect of the present invention provides the method of manufacturing a pipe according to any one of the foregoing first to eighth aspects, wherein holes and/or notches used for attaching another member to the base plate before the folding step of the sheet material or as communicating holes are formed. A tenth aspect of the present invention provides the method of manufacturing a pipe according to any one of the foregoing first to eighth aspects, wherein a projection is formed on an end face in the folding direction of the base plate before the folding step of the sheet material. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 1E illustrate a first embodiment of the manufacturing method of the present invention: Fig. 1A is a sectional side view of a base plate before folding; Fig. 1B is a sectional view of two core members holding a sheet material; Fig. 1C is a sectional side view illustrating a process in the middle of manufacture; Fig. 1D is a sectional side view illustrating completion of folding of the sheet material; and Fig. 1E is a sectional view of a product pipe welded after folding; Fig. 2 is a sectional side view illustrating a second embodiment of the manufacturing method of the present invention; Fig. 3 is a sectional side view illustrating a third embodiment of the manufacturing method of the present invention; Fig. 4 is a sectional side view illustrating a fourth embodiment of the manufacturing method of the present invention; Figs. 5A and 5B illustrate a fifth embodiment of the manufacturing method of the present invention: Fig. 5A is a sectional side view of core members holding a sheet material; and Fig. 5B is a side view illustrating a product pipe; Figs. 6A and 6B illustrate a sixth embodiment of the present invention: Fig. 6A is a perspective view of a product pipe; and Fig. 6B is a plan view of a sheet material comprising a sheet-type base plate having an attachment hole pierced therein; Figs. 7A to 7E illustrate a seventh embodiment of the present invention: Fig. 7A is a plan view of a sheet material comprising a sheet-type base plate having a hole and a notch formed therein; Fig. 7B is a plan view illustrating the sheet material folded into a pipe shape; Fig. 7C is an enlarged sectional view taken along the line VIIC-VIIC in Fig. 7B; Fig. 7D is a plan view illustrating formation of an additional connecting portion; and Fig. 7E is a sectional plan view illustrating the formed pipe in service; and Figs. 8A to 8D illustrate an eighth embodiment of the present invention: Fig. 8A is a plan view of a sheet material comprising a sheet-type base plate having a projection formed therein; Fig. 8B is a plan view of the sheet material folded into a pipe shape; Fig. 8C is an enlarged sectional view taken along the line VIIIC-VIIIC in Fig. 8B; and Fig. 8D is a plan view of a sheet material formed into a door impact beam. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, embodiments according to the present invention will be described below with reference to the drawings. Figs. 1A to 1E illustrate a first embodiment of the present invention. In these drawings, 1' is a base plate for forming a pipe, and a single flat sheet as shown in Fig. 1A having, prior to forming, prescribed length and width. In Fig. 1A, the right/left direction is the width direction of the sheet and the depth direction is the longitudinal one thereof. A pipe as shown in Fig. 1E having a partition plate 1a, a semi-circular first side plate 1b for forming a first flow channel a on one side of the partition plate 1a, and a semi-circular second side plate 1c for forming a second flow channel b on the other side is formed through the following steps. The both sides 1b and 1c of the aforesaid flat sheet-shaped base plate 1' are first folded by about right angles in opposite directions relative to the center portion 1a thereof. The thus folded sheet material is represented in Fig. 1B. This center portion 1a forms the partition plate 1a in the above-mentioned Fig. 1E, and the length in the width direction thereof is set to a value equal to the diameter in Fig. 1E. The first side 1b forms the foregoing first side plate 1b, and the second side 1c forms the foregoing second side plate 1c. The length of these side plates in the width direction thereof is set to a value equal to the length of the portion of the leading end faces 1d' and 1e', after folding into prescribed shapes as shown in Fig. 1E, in contact with folded base portion of the partition plate. Simultaneously with the above-mentioned folded portions, leading end portions 1d and 1e of the both sides 1b and 1c are also folded into an arcuate shape toward the side including the center portion 1a. Then, as shown in Fig. 1B, the center portion 1a is held with two core members 2a and 2b by interposing the center portion 1a of the sheet material 1 between flat surfaces of a core member 2a having a semi-circular cross-section and another core member 2b also having a semi-circular cross-section. These core members 2a and 2b compose a split structure divided into two, moving from right to left or vice versa by means of respective opening/closing means not shown to open or close the gap between the flat surfaces of both core members 2a and 2b which are arranged opposite to each other. The center portion 1a is interposed between the both core members 2a and 2b by opening the same, and is held under pressure by closing the same. While the core members 2a, 2b compose the split structure divided into two in this embodiment, the split structure may be divided into three or more. The core members 2a and 2b holding the sheet material 1 therebetween as described above are kept fixed without rotating the same. In this state, outer dies 3 and 4 arranged on both sides in a horizontal direction of the drawing are moved in the core member direction (in the directions of arrows B and C). These right and left outer dies 3 and 4 have flat surfaces 3a and 4a flush with the extension surface (outer surfaces) of both sides 1b and 1c of the sheet material 1 held in the state shown in Fig. 1B, and die surfaces 3b and 4b comprising concave surfaces having substantially the same diameter (larger by the thickness of the sheet material 1) as the outer surface shape of the core members 2a and 2b, and corresponding to the core members 2a and 2b. The flat surfaces 3a and 4a run continuously to the die surfaces 3b and 4b. These outer dies 3 and 4 are movable forward and back in the directions of the arrows B and C and vice versa by means of driving means not shown. When the outer dies 3 and 4 are moved from the state shown in Fig. 1B in the directions of the arrows B and C, the respective flat surfaces 3a and 4a slide from both ends of the sheet material 1 onto the outer surfaces of both sides 1b and 1c into the state shown in Fig. 1C. Further, when the outer dies 3 and 4 are moved in the directions of the arrows B and C while fixing the core members 2a and 2b, both ends of the sheet material 1 engage with the die surfaces 3b and 4b. Then, following the ends, the sides 1b and 1c are sequentially guided by the die surfaces 3b and 4b, thus folding sequentially both side plates 1b and 1c from the ends toward the center portion 1a. When the outer dies 3 and 4 reach the foremost positions so that both die surfaces 3b and 4b hold both sides of the core members 2a and 2b therebetween as shown in Fig. 1D, the sides 1b and 1c are folded so as to follow the shape of the outer surfaces of the core members 2a and 2b as shown in Fig. 1D by means of the die surfaces 3b and 4b, and plastically deform in this state. As a result, the leading end face 1d' of one side comes into contact with one of the folding bases of the center portion (partition plate) 1a, and the leading end face 1e' of another side comes into contact with another folding base of the center portion (partition plate) 1a, thus forming an S shape. By previously folding the leading ends 1d and 1e of both sides 1b and 1c as shown in Figs. 1B and 1C, the sides 1b and 1c are, when the outer dies 3 and 4 move to the outer surfaces of the sides 1b and 1c as shown in Figs. 1B and 1C, guided smoothly by the die surfaces 3b and 4b because of the folded leading ends 1d and 1e, and the outer dies 3 and 4 never come into contact with the leading end faces 1d' and 1e'. By previously folding the leading ends 1d and 1e, furthermore, there is available an improved butt contact between the leading end faces 1d' and 1e' with the folding base of the center portion 1a. After folding as shown in Fig. 1D, the outer dies 3 and 4 are caused to retreat in the directions counter to the respective arrows, and the formed sheet material 1 is pulled out from the core members 2a and 2b by slightly opening the core members 2a and 2b. The leading end faces 1d' and 1e' of the formed product thus pulled out are welded together by welding W with the folded base of the center portion 1a to form a pipe having two parallel flow channels a and b as shown in Fig. 1E. According to the first embodiment, it is possible to fold the sides 1b and 1c at the same time while clamping the sheet material 1 at the center portion thereof with the core members 2a and 2b. This ensures easier manufacture. Since the core members 2a and 2b holding the sheet material 1 directly serve as inner dies, die jigs can be omitted. The final state after causing the sheet material 1 to follow the shape of the core members 2a and 2b directly serves as the desired cross-section. It is therefore possible to achieve a high shape accuracy, eliminate the necessity of shape correction, and ensure a high welding quality. When forming concave die surfaces 3b and 4b on the outer dies 3 and 4 as in this first embodiment, the sheet material 1 is secured by inner dies comprising the core members 2a and 2b and outer dies comprising the die surfaces 3b and 4b. A particularly high shape accuracy is therefore available. Further, by forming the outer surface shape of the core members 2a and 2b and the inner surface shape of the die surfaces 3b and 4b into a desired shape, apart from the true circular cross-section in the embodiment shown, it is possible to form a pipe of any desired shape including an elliptical or rectangular shape. In the present embodiment, therefore, the outer surface shape of the core members 2a and 2b and the inner surface shape of the die surfaces 3b and 4b are not limited to the semi-circular one as shown in the drawings. Fig. 2 illustrates a second embodiment of the present invention. In the second embodiment, the core members 2a and 2b in the foregoing first embodiment are provided on a rotary shaft 2C rotatably around the rotation shaft 2c, which rotates around the center line passing through the split surface, and outer dies 5 and 6 having flat surfaces 5a and 6a similar to those of the outer dies 3 and 4 in the foregoing first embodiment movably in the arrow B and C directions and vice versa. In the second embodiment, both sides 1b and 1c in the width direction of the same base plate 1' as above are first folded in opposite directions relative to the center portion 1a (the leading ends 1d and 1e are folded at the same time) in the same manner as in the foregoing first embodiment to form a sheet material 1. Then, the sheet material 1 is held by the core members 2a and 2b at the center portion 1a of the sheet material as shown in Fig. 2, and the outer dies 5 and 6 are moved forward in the directions of the arrows B and C as in the first embodiment. At the moment when the leading ends of the outer dies 5 and 6 reach the center portion 1a of the sheet material 1, the outer dies 5 and 6 are moved forward in the directions of the arrows B and C while pressing the outer dies 5 and 6 by pressing means not shown in the directions of the arrows D and E toward the core members 2a and 2b. As a result, frictional force produced between the flat surfaces 5a and 6a of the outer dies 5 and 6 and the outer surfaces of the sides 1b and 1c of the sheet material 1 causes the core members 2a and 2b to rotate while following in the arrow A direction. Along with this rotation, the sides 1b and 1c of the sheet material are wound on the outer surfaces of the core members 2a and 2b, and at the same time, are pressed against the outer surfaces of the core members 2a and 2b by the flat surfaces 5a and 6a, following the outer surface of the core members 2a and 2b through plastic deformation, and are thus formed into an S-shaped cross-section as in Fig. 1D. The outer dies 5 and 6 are retreated, and the formed product is taken out from the core members 2a and 2b by slightly opening the core members 2a and 2b, thereby forming a pipe having a plurality of flow channels by welding W in the same manner as in Fig. 1D. In this second embodiment, the both sides 1b and 1c can simultaneously be folded while clamping the sheet material 1 at the center portion thereof by means of the core members 2a and 2b, thus permitting easy manufacture. The core members 2a and 2b holding the sheet material 1 directly serving as the inner die eliminate the necessity of die jigs. The final state after causing the sheet material 1 to follow the shape of the core members 2a and 2b constitutes the desired cross-sectional shape itself, resulting in a high shape accuracy, making it unnecessary to conduct shape correction, and permitting maintenance of a high welding quality. By forming the outer surface shape of the core members 2a and 2b into a desired shape, it is possible to form a pipe of any desired shape such as an elliptic or rectangular cross-sectional shape, apart from the true circular shape in the embodiment shown in the drawing. Fig. 3 illustrates a third embodiment of the present invention. In the third embodiment, core members 2a and 2b are provided rotatably as in the second embodiment, and pressing rollers 7 and 8 having rotation axes in parallel with the rotary shaft 2c of the core members 2a and 2b are provided on the outer peripheral surfaces of the core members 2a and 2b. The pressing rollers 7 and 8 are provided with rotation means and pressing means not shown, and rotate in the directions of the arrows H and I while being pressed against the core members 2a and 2b in the directions of the arrows F and G. In this embodiment, a sheet material is first prepared by folding the both sides 1b and 1c in the width direction of a base plate relative to the center portion 1a thereof in the opposite directions in the same manner as in the foregoing first embodiment (leading end 1d and 1e are simultaneously folded). Then, the sheet material 1 is held by core members 2a and 2b at the center portion 1a as shown in Fig. 3. By rotating the pressing rollers 7 and 8 from the state shown in Fig. 3 while pressing in the directions of arrows F and G, the core members 2a and 2b rotate in the arrow A direction while following the shape, and simultaneously with the completion of winding of the sides 1b and 1c of the sheet material 1 onto the outer surface of the core members 2a and 2b, the sheet material is pressed by the pressing rollers 7 and 8 against the outer surfaces of the core members 2a and 2b, plastically deforms into a shape following the outer surfaces of the core members 2a and 2b, and is formed into an S-shaped cross-section as in Fig. 1D. Then, the pressing rollers 7 and 8 are caused to retreat in directions counter to the arrow directions, and the formed product is taken out from the core members 2a and 2b by slightly opening the core members 2a and 2b. A pipe having a plurality of flow channels is formed by welding in the same manner as in Fig. 1D. According to this third embodiment, it is possible to fold the sides 1b and 1c at the same time while clamping the sheet material 1 at the center portion thereof with the core members 2a and 2b. This ensures easier manufacture. Since the core members 2a and 2b holding the sheet material 1 directly serve as inner dies, die jigs can be omitted. The final state after causing the sheet material 1 to follow the shape of the core members 2a and 2b directly serves as the desired cross-section. It is therefore possible to achieve a high shape accuracy, eliminate the necessity of shape correction, and ensure a high welding quality. In this embodiment, furthermore, it is recommendable to provide the pressing rollers 7 and 8 movably (vertically in Fig. 3) with an adjustable distance between the rollers (pitch) and to provide the core members 2a and 2b replaceable with one of a different kind. In such a configuration, the pressing rollers 7 and 8 can rotate while following the outer surfaces of the core members 2a and 2b by only replacing the core members 2a and 2b with ones having desired diameter and outer shape, and it is possible to easily cope with a case of forming a pipe of a different diameter or a different outer shape. This embodiment is therefore suitable for the manufacture of many different pipes which usually requires frequent replacement of setup. Fig. 4 illustrates a fourth embodiment of the present invention. The fourth embodiment has the same configuration as that of the third embodiment except that the center portion 1a and the both sides 1b and 1c of the sheet material 1 are not folded, but are held by the core members 2a and 2b in the form of a flat sheet and then folded for forming. The core members 2a and 2b and the pressing rollers 7 and 8 are configured in the same manner as in the third embodiment. In the fourth embodiment, when the pressing rollers 7 and 8 are pressed and rotated in the directions of arrows H and I, the core members 2a and 2b rotate in the arrow direction of A, following the pressing rollers 7 and 8, so that the sides 1b and 1c of the sheet material 1 are wound on the outer surfaces of the core members 2a and 2b, and at the same time, the pressing rollers 7 and 8 press the same against the outer surfaces of the core members 2a and 2b. The sheet material 1 thus plastically deforms, following the shape of the outer surfaces of the core members 2a and 2b, and is formed into an S-shaped cross-section in the same manner as in Fig. 1D. In this fourth embodiment as well, the effects similar to those available in the third embodiment are displayed, permitting elimination of the step of once folding the sides 1b and 1c as in the foregoing embodiments. Also in this embodiment, the core members 2a and 2b should have any desired diameter and outer shape. Figs. 5A and 5B illustrate a fifth embodiment of the present invention. The fifth embodiment represents a method of forming a pipe having a plurality of flow channels by folding a sheet material 1 into a ⊖ shape. In Figs. 5A and 5B, the core members 2a and 2b and the pressing roller 7 have the same configuration as in the third embodiment. A side 1f in the width direction of a base plate is previously folded, and the side 1f of the sheet material 1 is held by the core members 2a and 2b so that an end 1g of this side 1f is flush with the outer peripheral surfaces of the core members 2a and 2b as shown in Fig. 5A. Then, the pressing roller 7 is rotated in the arrow H direction while pressing the pressing roller 7 against the core members 2a and 2b as indicated by the arrow F. As a result, the core members 2a and 2b rotate in the arrow A direction by following, and the other side 1h of the sheet material 1 is continuously wound onto the entire outer peripheral surfaces of the core members 2a and 2b. At the same time, the same is pressed by the pressing roller 7 against the outer surfaces of the core members 2a and 2b, plastically deforming to follow the outer surface shapes of the core members 2a and 2b, and thus, being formed into an ⊖-shaped cross-section as shown in Fig. 5B. Then, an end face 1d' is welded W together with the folding base portion of the side 1f to form a pipe having a plurality of flow channels a and b as shown in Fig. 5B. In the fifth embodiment also, the same effects as in the foregoing second embodiment are available. The core members 2a and 2b should have desired diameter and outer shape as in the preceding embodiment. Figs. 6A and 6B illustrate a sixth embodiment of the present invention. In the sixth embodiment, when manufacturing an automobile exhaust pipe having attachment holes for an O 2 sensor and the like pierced in a part thereof, the pipe is formed by the application of any one of the manufacturing methods of the foregoing embodiments. First, before folding a base plate 1', a hole 1k consisting of a semi-circular hole 1i and a stepped hole 1j as shown in Fig. 6B is stamped in the base plate 1' at a folded portion C between the center portion 1a and the side 1b thereof, and a notch 1m comprising a semi-circular hole as shown in Fig. 6B is stamped in the end face of another side 1c. Then, this sheet material 1 is folded into an S-shaped cross-section as in the preceding embodiment, and a pipe is formed by aligning the holes 1i and 1m. As a result, a hole 10 having two flow channels a and b , opening to these flow channels a and b , and having a notch in a partition plate 1a is formed as shown in Fig. 6A. According to this embodiment, therefore, a hole of a complicated shape such as an attachment hole can be easily formed in the state of a flat sheet. More specifically, as compared with the conventional practice of providing such a hole 10 separately in a pipe and in a partition plate by end milling after forming the pipe, it becomes very easy to form an attachment hole. When forming such a hole 10 into the one in the embodiment shown in Figs. 5A and 5B, it suffices to form the holes 1k and 1j as described above at a folded portion between a side 1f and the other side 1h in Fig. 5A, and form the aforesaid notch 1m on the end 1d' side. Figs. 7A to 7E illustrate a seventh embodiment of the present invention. This embodiment covers a case where an exhaust pipe having flow channels a and b on the both sides of a partition wall 1a as described above has a communicating hole communicating the two flow channels a and b , provided in the partition wall 1a, and such an exhaust pipe is formed by the application of any one of the methods of the foregoing embodiments. First, as shown in Fig. 7A, a hole 1n is formed in a center portion which would serve as a partition wall during forming, in a base plate 1' before folding. In case of necessity, a notch 1p is formed in the center portion on the side which would serve as an end of the pipe after forming. Then, the sheet material 1 is folded by the manufacturing method of any of the first to fourth embodiments into an S-shaped cross-section as shown in Figs. 7B and 7C. Thus, an exhaust pipe having flow channels a and b on both sides of the partition wall 1a and a communicating hole 1n communicating both flow channels a and b , and formed in the partition wall 1a, is formed. Further, in case of necessity, the diameter at the end on the notch 1p side is enlarged to form a connecting portion 1q as shown in Fig. 7D. The thus formed connecting portion 1q of the exhaust pipe manufactured as described above is connected to an engine exhaust manifold 11 as shown in Fig. 7E, and two exhaust pipes 11a and 11b of the exhaust manifold 11 are independently communicated with the flow channels a and b of the thus manufactured exhaust pipe. Incidentally, a pipe may be formed by shifting the forming positions of the aforesaid hole 1m and the notch 1p and folding the sheet material 1 by the manufacturing method shown in Figs. 5A and 5B. Figs. 8A to 8D illustrate an eighth embodiment of the present invention. This embodiment covers a case where an automobile door impact beam is formed by the application of the manufacturing method shown in Figs. 5A and 5B. A sheet material 1 having a projection 1s at a side end thereof as shown in Fig. 8A is formed from a base plate 1' before folding. The projection 1s is held by core members 2a and 2b as shown in Figs. 5A and 5B by the application of the manufacturing method shown in Figs. 5A and 5B, and the sheet 1 is folded in the manner as described with reference to Figs. 5A and 5B. As a result, the portion containing the projection (partition wall) 1s is formed into a ⊖-shaped cross-section, and the other portion is formed into a pipe 12 not containing the projection (partition wall) 1s. A door impact beam 14 is formed by providing an attachment piece 13 as shown in Fig. 8D on each of the both ends of the pipe 12. According to this manufacturing method, it is possible to form a door impact beam light in weight which has a sufficient strength, in which only the center portion requiring a satisfactory bending rigidity is reinforced with the projection (partition wall) 1s, with the other portion having no partition wall. The foregoing embodiments cover cases of forming a true circular pipe. It is however possible to manufacture a pipe having any desired outer shape and a plurality of flow channels with the same effects as above by forming the core members 2a and 2b and the die surfaces 3b and 4b into a special shape such as an elliptic or rectangular shape, or changing the diameter. The present invention is not therefore limited to the shapes of the core members 2a and 2b and the die surfaces 3b and 4b as in the foregoing embodiments. Furthermore, it is needless to mention that the present invention is applicable not only to an automobile exhaust pipe and a door impact beam as described above, but also for the manufacture of other pipes. According to the first and second aspects of the present invention, as described above, it is possible to form a pipe having a plurality of flow channels by folding the both sides of the sheet material at a time in the state in which the sheet material is clamped at the center portion by the core members. It is therefore possible, as compared with the conventional manufacturing method, to omit die jigs and steps, simply manufacture, and reduce the cost. The final state after causing the sheet material to follow the shape of the core members directly serves as the desired cross-section. It is therefore possible to achieve a high shape accuracy, eliminate the necessity of shape correction, and ensure a high welding quality. According to the third aspect of the present invention, the both sides of the sheet material are constrained in a pressure-squeezing manner for folding/forming by the inner dies comprising the core members and the outer dies comprising concave surfaces. It is therefore possible to further improve the shape accuracy and form a pipe of a desired shape such as an elliptic or rectangular shaped cross-section in addition to the true circular cross-section, easily and at a high accuracy. The fourth and fifth aspects of the present invention permit display of the same effects as in the foregoing invention as the first aspect. According to the sixth aspect of the present invention, it is possible to easily cope with manufacture of various pipes with different diameters and outer shapes by providing different core members having different diameters and outer shapes and converting the same into desired values, making adjustable the distance between pressing rollers, or causing the pressing rollers to follow the outer shape of the core members. It is therefore useful for manufacture of many different kinds of pipes which requires frequent change in setup. According to the seventh aspect of the present invention, the same effects as in the foregoing first aspect with simple equipment and configuration. According to the eighth aspect of the present invention, a pipe having a plurality of flow channels with a ⊖-shaped cross-section can be manufactured while fully displaying the same effects as in the first aspect. According to the ninth aspect of the present invention, it is possible to easily form a pipe having a plurality of flow channels as described above, in which there are formed attachment holes for other parts and communicating holes for communicating between the plurality of flow channels, with the same effects as above. According to the tenth aspect of the present invention, it is possible to easily form a pipe having a reinforcing wall at a portion requiring reinforcement in the form of a projection.

Claims: A method of manufacturing a pipe, comprising the steps of holding a sheet material (1) at a center portion (1a) in a width direction thereof from both surfaces thereof by means of core members (2a,2b) composing a split structure, folding the both sides (1b,1c) in the width direction of the sheet material in opposite directions and causing the thus folded both sides to follow the shapes of the outer surfaces of the core members, thereby forming a substantially an S-shaped cross-section. The method of manufacturing a pipe according to claim 1, wherein said method comprises the steps of folding the both sides (1b,1c) of said sheet material in opposite directions relative to the center portion (1a) in the width direction thereof, further folding the sheet material gradually from the sides thereof toward the center portion thereof while holding the sheet material (1) from the both surfaces at the center portion (1a) thereof by means of the core members (2a,2b), thereby causing both sides of the sheet material to follow the shapes of the outer surfaces of the core members. The method of manufacturing a pipe according to claim 2, wherein said method comprises the steps of providing outer dies (3,4), each of which has a concave surface (36,46) substantially identical with the outer surface of said core member (2a,2b), and folds both sides (1b,1c) of the sheet material toward the outer surface of the core member by means of said outer die (3,4) while moving the outer die toward the core member. The method of manufacturing a pipe according to claim 1, wherein said method comprises the steps of folding the both sides (1b,1c) of the sheet material (1) toward the center portion (1a) in the width direction thereof in opposite directions, holding the sheet material (1) at the center portion (1a) thereof from both surfaces thereof by means of the core members (2a,2b), and folding both sides of the sheet material gradually from the center portion side toward the ends thereof, thereby causing both sides of the sheet material to follow the shapes of the outer surfaces of the core members. The method of manufacturing a pipe according to claim 4, wherein both sides (1b,1c) of said sheet material are wound on said core members (2a,2b) by turning the core members in a direction around a center line (2c) passing through the split surface thereof, and at the same time, both sides (1b,1c) are pressed against the outer surfaces of the core members, thereby causing both sides of the sheet material to follow the shapes of the outer surfaces of the core members. The method of manufacturing a pipe according to claim 1 or 4, wherein said method comprises the steps of rotatably providing said core members (2a,2b), providing pressing rollers, (7,8) on the outer surface sides of said core members, each of which rotates around a rotation axis running in parallel with the rotation axis of the core members, and pressing both sides (1b,1c) of the sheet material against the outer surfaces of the core members by means of said pressing rollers (7,8). The method of manufacturing a pipe according to claim 4, wherein said method comprises the steps of rotatably providing said core members (2a,2b), pressing a flat surface (5a,6a) of each of flat plates (5,6) against one of the outer surfaces of both sides (1b,1c) of the sheet material (1) held by said core members, and moving the flat surface so that the core members rotates in the pressed state, thereby folding both sides of the sheet material toward the core members. A method of manufacturing a pipe, comprising the steps of holding a side (1f) in a width direction of a sheet material (1) by means of core members (2a,2b) composing a split structure, and pressing another side (1h) of said sheet material against the outer surfaces of the core members while rotating said core members, thereby causing another side (1b) of the sheet material to follow the shapes of the core members and forming a -shaped cross-section. A method of manufacturing a pipe according to any one of claims 1 to 8, wherein holes (1n) and/or notches (1p) used for attaching another member to a base plate before the folding step of the sheet material (1) or as communicating holes are formed. The method of manufacturing a pipe according to any one of claims 1 to 8, wherein a projection (1s) is formed on an end face in the folding direction of a base plate (1) before the folding step of the sheet material.

Inventors: KUNIMINATO YASUSHI, SEKIDO YUTAKA, KUNIMINATO, YASUSHI, SEKIDO, YUTAKA
Applicants: SANGO CO LTD, SANGO CO., LTD.
Application year: 1997
Main classification: B21C37
IPC codes: B21C37, B21D5, F01N13