Method for making a joint between a main pipe and a branch pipe

Abstract: In a method for making a joined pipe, a main pipe (10) is cut so that two V-shaped edges of a right angle are formed in the diametrical positions on the periphery of the main pipe, and a branch pipe (20) is cut so that two peaked edges of a right angle are formed in the diametrical positions on the periphery of the branch pipe (20). Then, the facets of the V-shaped edges and the peaked edges are fused by heating, and butt-welded under pressure. A heating apparatus includes a heater with a V-shape recessed attachment and a peaked attachment. A cutting apparatus includes a circular rail and a slit inside the circular rail on a base, a clamp for clamping a pipe, and a cutter which moves in the slit. A butt-welding apparatus includes two cradles which can move near to and far from each other, a heater which can position between the two cradles, and a clamp which can rotate on each base.

Description: FIELD OF THE INVENTION This invention relates to a method for making a joint between a main pipe and a branch pipe having the same diameter. BACKGROUND OF THE INVENTION An injection molding method has been generally used is the method for making a joint between a main pipe and a branch pipe made of synthetic resin, such as polyethylene. However, the injection molding method has a disadvantage due to manufacturing cost, as different molds are required for pipes having different sizes and shapes, especially when the manufacturing amount is small and requires a specific size. Therefore, in order to reduce the facility cost, other methods have been adopted in which pipes are worked afterwords, which are: the drawing method, the blow molding method and the welding method. In the drawing method, first, a hole is made in the wall of a main pipe and then the periphery of the hole is softened by heating. Next, a mold having a flat top is inserted into the main pipe from one end thereof, and then the mold is pulled out of the main pipe through the hole by pulling a member which is hooked to the mold. The periphery of the hole rises and a flat top is formed at the periphery of the hole by pulling the mold out of the main pipe. Then, a branch pipe is fixed to the flat top of the periphery of the hole of the main pipe by fitting or cementing. This method has been described in Japanese Patent Provisional Publication No. 62868/73. In the blow molding method, first, a predetermined part of the wall of a main pipe is softened by heating, and then a hill having a flat top is formed on the softened part, and then a hole having a diameter slightly larger than the outer diameter of a branch pipe is formed. Then, the branch pipe is inserted through the hole and fixed with the main pipe by fitting or cementing. This method has been described in Japanese Utility Model Publication No. 2955/83. In the welding methed, first, such as known from JP-A-1 309 783 a hole is made in the wall of a main pipe, while one end of a branch pipe is cut to a have the corresponding profile of the hole of the main pipe, and then the cut end of the branch pipe and the periphery of the hole are welded. In many cases, the welded peripheries may be coated with fiber-reinforced plastic (FRP) for reinforcing the joint. However, there are disadvantages to these methods. In the drawing method, it is required to make a periphery of a flat top having a diameter equal to or larger than the outer diameter of the branch pipe, so this method can not be applied for making a joint between the main and branch pipes having the same diameter, as it is impossible to raise the periphery of the hole of the main pipe. In the blow molding method, the center of the softened part in the wall of the main pipe is elongated first and then the outer part is elongated gradually to form a flat top of the periphery in blow molding. As a result, the root of the elongated periphery becomes thin, though most of load is concentrated thereto and therefore requires strength. Consequently, the endurance limit of the joint is not satisfied resulting in poor reliability. In the welding method, considerable skill is required for welding, so that the strength in fixing the branch pipe to the main pipe can be stably obtained only by a skilled operator. Further, the welding operation is time-consuming. These disadvantages may be resolved by adopting the butt-welding method by fusion, in which the two pipe ends of the main and branch pipes are fused by heating and then butted to make a joint under pressure. However, there are disadvantages in the butt-welding method by fusion. One of the disadvantages in the butt-welding method by fusion, is that it requires the process for removing a bead. A bead is generated in the inner wall of the joint between the main and branch pipes after butt-welding, so it is required that the bead be removed. If a cutting tool much as a rotary cutter or a pressure cutter is used for removing a bead, it is required that the cutting tool be inserted into the main pipe to remove the bead generated in the inner wall thereof and into the branch pipe to remove the bead generated in the inner wall thereof. However, the bead in the branch pipe should be removed by hand instead of the cutting tool, because the inner wall of the branch pipe may be damaged by the cutting tool when inserted deeply into the branch pipe to remove all the bead. Accordingly, the operation becomes complicated, such that the operation consumes time and a stable result can not be obtained. Further, if the pipes are made of crystalline polymer material such as polyethylene resin, the pipes become deformed to some extent due to stress and relaxation of the material, so that it is required to obtain accuracy in butting the periphery of the hole of the main pipe and the peaked edge of the branch pipe by some means. If butt-welding is carried out with cores inserted into the pipes from three directions to be located at the inside of the joint as described in Japanese Patent Publication No. 32671/76 and UK Patent No. 1549169, holding members are required for holding the main and branch pipes in three points and supporting members used in inserting and withdrawing the pipes. Additionally, the cores are not always fitted tightly and there may be generated spaces therebetween due to deformation of the main and branch pipes and manufacturing error of the cores and so on, so that there is the possibility of the generation of a bead in the case where fused resin flows into the spaces between the cores. Second, one of the disadvantages in the butt-welding method by fusion is that appearance of the joint is poor and the strength of the butt-welding is not sufficient. When the main and branch pipes are joined by butt-welding, in which the hole of the main pipe and the peaked edge of the branch pipe are fitted, the fused resin in the periphery of the hole of the main pipe and the peaked edge of the end of the branch pipe is extruded towards the inside and outside of the pipes by applying pressure. As a result, the hole becomes deeper as the fused resin is extruded, even though the hole was made to have the two V-shaped edges located diametrically on the periphery of the main pipe. On the other hand, the peaked edge of the opening of the branch pipe does not change in shape, though the branch pipe becomes short. If the hole of the main pipe becomes deeper, the lowest point of the V-shaped edge of the hole moves lower and becomes narrow, so that more fused resin is extruded out in the vicinity of the lowest point. Consequently, the peaked edge of the opening of the branch pipe is butt-welded deep into the hole of the main pipe and an irregular bead is generated in the butt-welded joint, so that the butt-welded joint has a poor appearance and the fixing strength is not stable due to the difference between the width of the V-shaped edge of the hole of the main pipe and the peaked edge of the opening of the branch pipe. Third, one of the disadvantages in the butt-welding method by fusion is that a burr is generated in the butt-welding joint. If the branch pipe is fixed to the main pipe such that the axis of the branch pipe is at an angle of 60° with the axis of the main pipe, one of two facets of the hole of the main pipe is required to be formed at an angle of 30° with the axis of the main pipe, so that the inner periphery of the first facet becomes acute. On the other hand, if one of the two facets of the peaked edge of the opening of the branch pipe is required to be formed at an angle of 30° with the axis of the branch pipe, the inner periphery of the peaked edge becomes acute. As a result, the contact area of the first facets of the hole of the main pipe and the peaked edge of the opening of the branch pipe becomes far too large as compared with that of the second facets, so that the pressure to be applied to the joint in butt-welding decreases, especially if the first facets are very acute. Consequently, a bead is generated in the inner peripheries of the facets, as fused resin between the first facets of the hole of the main pipe and the peaked edge of the opening of the branch pipe stays within the inner peripheries, so that a burr is generated at the inner peripheries due to the resin pressure. In order to allow fluid to flow smoothly through the joint, it is required to remove the burr and smooth the inner surface of the pipes. Next, a problem with the cutting apparatus for cutting pipes used in the method for making a joint between the main and branch pipes will be described. A conventional cutting apparatus for cutting pipes includes a base for fixing a pipe and a cutter, and either the base or the cutter is moved in a straight line. In such a conventional cutting apparatus, there is no difficulty in cutting a pipe in one direction. However, it is required to change the direction of the base versus the pipe to cut the pipe in a different direction after cutting it in one direction when the pipe is to be cut at two different directions in order to obtain a V-shaped edge of a hole or a peaked edge of an opening in the pipe. Therefore, it is required to move the base in an accurate manner to avoid cross cutting of the two cutting facets of different directions or deviation of cutting angle. Next, problems with the butt-welding apparatus for butt-welding pipes by fusion, used in a method for making a joint between main and branch pipes will be described. One known method in the butt-welding methods by fusion, joins three synthetic resin pipes each of which is cut at an edge at an angle of 45 degrees. In this method, an L-shaped pipe is made by butt-welding two pipes by fusion, at their angular cut edges, and then the corner of the L-shaped pipe is cut at an angle of 45 degrees. Finally, the joint is made by butt-welding the cut opening of the remaining pipe to the cut corner of the L-shaped pipe. When a conventional butt-welding apparatus is used in this method, it is difficult to butt-weld the pipes in an angled direction, because the conventional butt-fusing apparatus is able to butt-fuse pipse only in a predetermined direction, that is a right angle. Additionally, different apparatuses are required for making joints having different shapes, so that the cost becomes high. SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a method for making a joint between a main pipe and a branch pipe in which the main and branch pipes having the same diameter are joined with predetermined joint strength which is not inferior to other portions of the pipes. It is another object of this invention to provide a method for making a joint between a main pipe and a branch pipe in which a bead is easily removed. It is further object of this invention to provide a method for making a joint between a main pipe and a branch pipe in which the appearance is excellent and strength of the butt-welding joint is sufficient. It is still further object of this invention to provide a method for making a joint between a main pipe and a branch pipe in which the appearance is excellent and strength of the butt-welding joint is sufficient. It is yet another object of this invention to provide a method for making joint between a main pipe and a branch pipe in which a burr is not generated at the junction of the inner peripheries of a hole of the main pipe and the peaked edge of an opening of the branch pipe. According to the invention, a method for making a joint between a main pipe and a branch pipe includes the steps of cutting a main pipe to have a hole in the pipe wall so that two V-shaped edges of a right angle are formed in the diametrical positions on the periphery of the main pipe, cutting a branch pipe to have facets in the pipe opening so that two peaked edges of a slightly more acute angle than a right angle are formed in the diametrical positions on the branch pipe, fusing the facets of the V-shaped edges of the hole of the main pipe and the peaked edges of the pipe opening of the branch pipe by heating, and butt-welding the facets of the V-shaped edges of the hole of the main pipe and the peaked edges of the opening of the branch pipe to form a joint. In the above method, the largest pressure is applied to the portion between the lowest point of the V-shaped edges of the main pipe and the peak of the peaked edges of the branch pipe when the fused V-shaped edges of the main pipe and the fused peaked edges of the branch pipe are butted under pressure in the direction of the axis of the branch pipe. Therefore, the fixing strength of the portion of the joint where the largest load may be applied and where water leakage easily occurs and becomes large, can be obtained with a high degree of accurary while in the butting position. In order to make a joint between a main pipe and a branch pipe with increased accuracy, the method may include the steps of: a cutting step of a main pipe including the steps of determining two points e 1 and e 2 which are located diametrically on the periphery of the main pipe, determining a point f 1 which is located to have an angle  3 (<90° ) with the points e 1 and e 2 in the direction of the pipe axis and a right angle in the radius direction, determining a point f 2 which is located to have an angle  4 (=90° - 3 ) with the points e 1 and e 2 in the direction of the pipe axis (the same radius direction and opposite axial direction with f 1 ) and a right angle in the radius direction, and cutting the main pipe in a straight line and forming an arc connecting the points e 1 , f 1 and e 2 , an arc connecting the points e 1 , f 2 and e 2 , and V-shaped edges in the facets; a cutting step of a branch pipe including the steps of determining two points g 1 and g 2 which are located diametrically on the periphery of the branch pipe, determining a point h 1 which is located to have the angle  3 with the points g 1 and g 2 in the direction of the pipe axis and at a right angle in the radius direction, determining a point h 2 which is located to have the angle  4 with the points g 1 and g 2 in the direction of the pipe axis (the opposite radius direction and same axial direction with h 1 ) and at a right angle in the radius direction, determining points i 1 and i 2 which are a little distance from the points g 1 and g 2 in the direction of the pipe axis (the opposite axial direction of h 1 and h 2 ), and cutting the branch pipe in a straight line and forming an arc connecting the points i 1 , h 1 and i 2 , an arc connecting the points i 1 , h 2 and i 2 and peaked edges in the facets. According to another embodiment of the invention, a method for making a joint between a main pipe and a branch pipe includes the steps of cutting a main pipe to have a hole in the pipe wall so that two V-shaped edges of a right angle are formed in the diametrical positions on the periphery of the main pipe, cutting a branch pipe to have facets in the opening so that two peaked edges of a right angle or smaller are formed in the diametrical positions on the periphery of the branch pipe, fusing the facets of the V-shaped edges of the hole of the main pipe and the peaked edges of the opening of the branch pipe by heating, butt-welding the facets of the V-shaped edges of the hole of the main pipe and the peaked edges of the opening of the branch pipe in which a core has been inserted and is in contact with all the area of the inner surface of the peaked edges to form a joint, and removing a bead generated at the inner wall of the main pipe by a cutting tool inserted into the main pipe after pulling the core out of the branch pipe. In the above method, the core is inserted into the branch pipe to contact all the inner surfac of the peaked edges in the butt-welding process, so that deformation of the peaked edges of the branch pipe generated by stress and relaxation of the crystalline polymer material of which the pipe consists, is corrected. As a result, butting accuracy between the V-shaped edges of the main pipe and the peaked edges of the branch pipe is improved. Further, all of fused resin extruded from the joint between the facets of the main pipe and the branch pipe by pressing is extruded toward the main pipe due to presence of the core contacting all of the inner surface of the peaked edges of the branch pipe, so that a bead is generated only in the inner wall of the main pipe. Therefore, only a bead generated in the inner surface of the main pipe is to be removed. According to another embodiment of the invention, a method for making a joint between a main pipe and a branch pipe includes the steps of cutting a main pipe to have a hole in the pipe wall so that two V-shaped edges of a right angle are formed in the positions slightly more shallow than the diametrical positions on the periphery of the main pipe, cutting a branch pipe to have facets in the pipe opening so that two peaked edges at a right angle or at a slightly more acute angle than a right angle are formed in the diametrical positions on the periphery of the branch pipe, fusing the facets of the V-shaped edges of the hole of the main pipe and the peaked edges of the opening of the branch pipe by heating, and butt-welding the facets of the V-shaped edges of the hole of the main pipe and the peaked edges of the pipe opening of the branch pipe to form a joint. In the above method, the lowest point of the V-shaped edges of the hole of the main pipe are located in the position slightly more shallow than the diametrical positions on the periphery of the main pipe, however, the hole of the main pipe becomes deeper as the fused resin of the periphery of the hole is extruded to the inside and outside by butt-welding the main and branch pipes under pressure so that the V-shaped edges of the main pipe are fitted to the peaked edges of the branch pipe. As a result, the width of the V-shaped edges of the main pipe becomes large and has almost the same width as the peaked edges of the branch pipe. Therefore, fused resin of the periphery of the hole and the peaked edges is extruded uniformly, so that a bead having a uniform thickness is generated in the butt-welding joint. Further, the peaked edges of the branch pipe are butt-welded and fixed to V-shaped edges of the hole of the main pipe adequately without sinking into the hole. After fixing, the width of the V-shaped edges of the main pipe and the peaked edges of the branch pipe becomes almost equal. Consequently, the butt-welding joint has excellent appearance and stable fixing strength. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be explained in more detail in conjunction with appended drawings, wherein: Figs. 1(a) and 1(b) are respectively a side view and a top view showing a joined pipe made by a method for making a joint between a main pipe and a branch pipe in a first preferred embodiment according to the invention. Figs. 2(a) and 2(b) are explanatory views illustrating the cutting step of a main pipe in the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment according to the invention. Figs. 3(a) and 3(b) are explanatory views illustrating the cutting step of a branch pipe in the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment according to the invention. Fig. 4 is an explanatory view illustrating the heating step in the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment using a heating apparatus. Fig. 5 is an explanatory view illustrating the butt-welding step in the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment according to the invention. Fig. 6 is a side view showing another example of a joined pipe to be made by the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment according to the invention. Fig. 7 is a side view showing another example of a joined pipe to be made by the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment according to the invention. Fig. 8 is a side view showing another example of a joined pipe to be made by the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment according to the invention. Fig. 9 is a side view showing another example of a joined pipe to be made by the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment according to the invention. Fig. 10 is an explanatory view illustrating the cutting step of a main pipe in a method for making a joint between a main pipe and a branch pipe in a second preferred embodiment according to the invention. Fig. 11 is an explanatory view illustrating the cutting step of a branch pipe in the method for making a joint between a main pipe and a branch pipe in the second preferred embodiment according to the invention. Fig. 12 is an explanatory view illustrating the heating step in the method for making a joint between a main pipe and a branch pipe in the second preferred embodiment according to the invention. Fig. 13 is an explanatory view illustrating the butt-welding step in the method for making a joint between a main pipe and a branch pipe in the second preferred embodiment according to the invention. Fig. 14 is an explanatory view illustrating the removing step of a bead in the method for making a joint between a main pipe and a branch pipe in the second preferred embodiment according to the invention. Fig. 15 is a cross-sectional view showing a joined pipe made by the method for making a joint between a main pipe and a branch pipe in the second preferred embodiment according to the invention. Fig. 16 is an explanatory view illustrating the cutting step of a main pipe in a method for making a joint between a main pipe and a branch pipe in a third preferred embodiment according to the invention. Fig. 17 is an explanatory view illustrating the cutting step of a branch pipe in the method for making a joint between a main pine and a branch pipe in the third preferred embodiment according to the invention. Fig. 18 is an explanatory view illustrating the heating step in the method for making a joint between a main pipe and a branch pipe in third preferred embodiment according to the invention. Fig. 19 is an explanatory view illustrating the butt-welding step in the method for making a joint between a main pipe and a branch pipe in the third preferred embodiment according to the invention. Fig. 20 is an explanatory view illustrating the cutting step of a main pipe in a method for making a joint between a main pipe and a branch pipe in a fourth preferred embodiment according to the invention. Fig. 21 is an explanatory view illustrating the cutting step of a branch pipe in the method for making a joint between a main pipe and a branch pipe in the fourth preferred embodiment according to the invention. Fig. 22 is an explanatory view illustrating the heating step in the method for making a joint between a main pipe and a branch pipe in the fourth preferred embodiment according to the invention. Fig. 23 is an explanatory view illustrating the butt-welding step in the method for making a joint between a main pipe and a branch pipe in the fourth preferred embodiment according to the invention. Fig. 24 is a top view showing a cutting apparatus. Fig. 25 is a cross-sectional view taken on line II-II of Fig. 24. Fig. 26 is a cross-sectional view taken on line III-III of Fig. 24. Fig. 27 is a top view showing a cutting apparatus. Fig. 28 is a top view showing a cutting apparatus. Figs. 29(a), 29(b) and 29(c) are explanatory views illustrating the cutting step of a main pipe using the cutting apparatus. Figs. 30(a), 30(b) and 30(c) are explanatory views illustrating the cutting step of a branch pipe using the cutting apparatus. Fig. 31 is a side view showing a joined pipe made by using the cutting apparatus. Fig. 32 is a top view showing a cutting apparatus. Fig. 33 is a cross-sectional view taken on line IX-IX of Fig. 32. Fig. 34 is a top view showing a butt-welding apparatus. Fig. 35 is a side view showing a butt-welding apparatus. Fig. 36 is a cross-sectional taken on line XI-XI of Fig. 35. Figs. 37(a) and 37(b) are side views showing respectively a main pipe and a branch pipe which are to be butt-welded by the butt-welding apparatus. Fig. 38 is an explanatory view illustrating the mutual relation of setting angles of the main and branch pipes to be butt-welded by the butt-welding apparatus. Fig. 39 is a compared explanatory view illustrating the mutual relation of setting angles of the main and branch pipes to be butt-welded by the butt-welding apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENT Next, a method for making a joint between a main pipe and a branch pipe in a first preferred embodiment and a heating apparatus will be explained in conjunction with Figs. 1 to 5. Figs. 1(a) and 1(b) show a joined pipe to be made by the method for making a joint between a main pipe and a branch pipe in the second preferred embodiment. The joined pipe consists of a main pipe 40 and a branch pipe 50 both having the same diameter (R 4 = R 5 ). The branch pipe 50 is fixed to the pipe wall of the main pipe 40 at an angle of 60 degrees. Figs. 1 to 5 show the steps of the method. The method includes the steps of cutting the main pipe, cutting the branch pipe, fusing the pipes by heating and butt-welding the pipes. Either step of cutting the main pipe or cutting the branch pipe may be carried out first, or the two steps may be carried out simultaneously. (the step of cutting the main pipe) In this step, the main pipe 40 is cut to form a hole for branching in the pipe wall. The facet of the hole has two V-shaped edges 41 of a right angle in which the two lowest points are located diametrically on the periphery of the main pipe 40. In more detail, as shown in Figs. 2(a) and 2(b), positions of four points e 1 , e 2 , f 1 and f 2 are determined on the periphery of the main pipe 40. The points e 1 and e 2 are located diametrically on the periphery of the main pipe 40. The points f 1 and f 2 are located on the common line parallel to the axis C-C of the main pipe 40, and located in the direction from the points e 1 and e 2 to make respectively an angle  3 (=60° ) and an angle  4 (=30° ) with the pipe axis C-C and at a right angle with the radius direction (in the opposite direction from each other). Then, an arc e 1 f 1 e 2 consisting of the three points e 1 , e 2 , and f 1 on the common plane is cut from the point f 1 toward the points e 1 and e 2 . In the same way, an arc e 1 f 2 e 2 consisting of the three points e 1 , e 2 and f 2 on the common plane is cut from the point f 2 toward the points e 1 and e 2 . Thus, the two V-shaped edges 41, 41 are made. (the step of cutting the branch pipe) In this step, the branch pipe 50 is cut to form a facet having two peaked edges 51 of an angle slightly more acute than a right angle in which two peaks are located diametrically on the periphery of the branch pipe 50. In more detail, as shown in Figs. 3(a) and 3(b), positions of six points g 1 , g 2 , h 1 , h 2 , i 1 and i 2 are determined on the periphery of the branch pipe 50. The points g 1 and g 2 are located diametrically on the periphery of the branch pipe 50. The points h 1 and h 2 are located diametrically on the periphery of the branch pipe 50, and located in the direction from the points g 1 and g 2 to make respectively the angle  3 and the angle  4 with the pipe axis D-D and a right angle in the radius direction (in opposite direction each other). The point i 1 is located on the common line parallel to the pipe axis with the point g 1 with a slight distance (preferably equal to or shorter than 5 mm), while the point i 2 is located on the common line parallel to the pipe axis with the point g 2 with a slight distance. Then, an arc i 1 h 1 i 2 consisting of the three points h 1 , i 1 , and i 2 on the common plane is cut from the point h 1 toward the points i 1 and i 2 . In the same way, an arc i 1 h 2 i 2 consisting of the three points h 2 , i 1 and i 2 on the common plane is cut from the point h 2 toward the points i 1 and i 2 . Thus, the two peaked edges 51, 51 are made. (the step of fusing the pipes by heating) In this step, the facet including the V-shaped edges 41 of the main pipe 40 and the facet including the peaked edges 51 of the branch pipe 50 are heated to be fused by a heating apparatus 60 in the second preferred embodiment. The heating apparatus 60 includes a heater 61, a recessed attachment 62 having a V-shaped recess 64 of an angle slightly acute than a right angle attached to the heater 61, and a peaked attachment 63 having a peak 65 of a right angle, as shown in Fig. 4. The facets of the V-shaped edges 41 of the main pipe 40 and the peaked edges 51 of the branch pipe 50 are heated to be fused by fitting the V-shaped edges 41 and the peaked edges 51, respectively to the peak 65 of the peaked attachment 63 and the V-shaped recess 64 of the recessed attachment 62 under pressure for a predetermined period. (the step of butt-welding the pipes) In this step, the V-shaped edges 41 of the main pipe 40 and the peaked edges 51 of the branch pipe 50 are fitted by fusion and butt-welded to each other under equal application of pressure in the direction of the center of the V-shaped edges 41 of the main pipe 40 at an angle of 45° from either side of each V-shaped edge 41 (in the direction of line E-E), as shown in Fig. 5. As described above, in the method for making a joint between a main pipe and a branch pipe in the first preferred embodiment according to the invention, neither blow molding nor drawing method is carried out, so that the main and branch pipes have no thinner part than any other parts. Further, the operation does not require a high degree of skill such as the conventional welding method, so that it is possible to fix the branch pipe 50 to the main pipe 40 with a stable fixing strength even by an operator who is not skilled. If the heating apparatus 60 in the second preferred embodiment is used in the step of heating the pipes, all area of the facets of the V-shaped edges 41 and the peaked edges 51 to be fused are heated simultaneously, so that a uniform fusion can be obtained. Consequently, a joined pipe in which a branch pipe is fixed to a main pipe with a stable fixing strength is realized with excellent quality. In the method in the second preferred embodiment, the facets of the V-shaped edges 41 of a right angle and the peaked edges 51 of an angle slightly more acute than a right angle are butted under pressure to be fixed by fusion in the butt-welding step, so that the largest pressure is applied to the part between the lowest points of the V-shaped edges 41 and the peaks of the peaked edges 51. Consequently, it is possible to obtain the greatest fixing strength in this part. The method for making a joint between a main pipe and a branch pipe in the first preferred embodiments has been described above. However, concrete configurations of the invention are not limited to the embodiments, and modification in design may be included in the invention within the scope of the appended claims. For example, the angle at which the branch pipe is fixed to the main pipe may have other values though the branch pipe is fixed to the main pipe at a right angle or an angle of 60° in the embodiments described above. In the method in the first preferred embodiment, if the branch pipe is fixed to the main pipe at an angle , then  4 =/2 and  3 =90° - 4 . Further, the main and branch pipes may be pipes having sockets or bent pipes for making a joined pipe, as shown in Figs. 6 to 9. Fig. 6 shows a joined pipe using a main pipe 70 and a branch pipe 80 having sockets 71 and 81, made by integral molding with the main and branch pipes 70 and 80 in one opening, respectively. Fig. 7 shows a joined pipe using a main pipe 70 having a socket 71 made by integral molding with the main pipe 70 in one opening thereof and a branch pipe 90 having a socket 100 of an opening 101 having a diameter different from that of the socket 71 of the main pipe 70 butt-welded to the branch pipe 90. Fig. 8 shows a joined pipe using a main pipe 70 having a socket 71 made by integral molding with the main pipe 70 in one opening thereof and a branch pipe 110 of a bent pipe having a socket 111 made by integral molding with the branch pipe 110. Fig. 9 shows a joined pipe using a main pipe 120 of a bent pipe having sockets 121 made by integral molding with the main pipe 120 in both openings thereof. Next, a method for making a joint between a main pipe and a branch pipe in a second preferred embodiment will be explained in conjunction with Figs. 10 to 15. The method includes the steps of cutting the main pipe, cutting the branch pipe, fusing the pipes by heating, butt-welding the pipes and removing a bead. Either step of cutting the main pipe or cutting the branch pipe may be carried out first, or the two steps may be carried out simultaneously. (the step of cutting the main pipe) In this step, the main pipe 10 is cut to form a hole for branching in the pipe wall, as shown in Fig. 10. The facet of the hole has two V-shaped edges 11 at a right angle in which the two lowest points are located diametrically on the periphery of the main pipe 10. (the step of cutting the branch pipe) In this step, the branch pipe 20 is cut to form a facet having two peaked edged 21 at a right angle in which the two peaks are located diametrically on the periphery of the branch pipe 20, as shown in Fig. 11. (the step of fusing the pipes by heating) In this step, as shown in Fig. 12, the facet including the V-shaped edges 11 of the main pipe 10 and the facet including the peaked edges 21 of the branch pipe 20 are heated to be fused by a heating apparatus 30. The heating apparatus 30 includes a heater 31, a recessed attachment 32 having a V-shaped recess 34 of a right angle attached to the heater 31, and a peaked attachment 33 having a peak 35 of a right angle. The facets of the V-shaped edges 11 of the main pipe 10 and the peaked edges 21 of the branch pipe 20 in which a core 130 is inserted to contact with the inner surface thereof, are heated to be fused by fitting the V-shaped edges 11 and the peaked edges 21 respectively, to the peak 35 of the peaked attachment 33 and the V-shaped recess 34 of the recessed attachment 32, under pressure for a predetermined period. The core 130 has a peaked edge at a right angle, like the peaked edges 21 of the branch pipe 20, and is located slightly back from the end of the peaked edge 21 in the branch pipe 20. It is preferable to make a clearance of about 0.25 mm on the average, between the outer surface of the core 130 and the inner surface of the branch pipe 20, thus the operation of inserting and pulling the core 130 in and out of the branch pipe 20 becomes easy. The core 130 may consist of rigid material such as metals or elastic materials such as natural rubber or synthetic rubber. However, it is preferable to plate the core 130 with chrome in the case of metals and to coat the core 130 with polytetrafluoro ethylene (teflon: trade mark) in case of rubber. The core 130 may be a cylinder with or without a hollow center. (the step of butt-welding the pipes) In this step the core 130 is moved toward the main pipe 10 so that the top of the core 130 passes slightly over the peak of the peaked edges 21 of the branch pipe 20, and the V-shaped edges 11 of the main pipe 10 and the peaked edges 21 of the branch pipe 20 are fitted by fusion and butt-welded to each other by applying pressure in the direction of the axis B-B of the branch pipe 20, as shown in Fig. 13. (the step of removing a bead) In this step, as shown in Fig. 14, the core 130 is removed from the branch pipe 20, and then a cutting tool 140 such as a rotary cutter or a reciprocal cutter (a rotary cutter is shown in Fig. 14) is inserted into the main pipe 10 to remove a bead E generated at the inner surface of the main pipe 10. In the method described above, the inner surface of the peaked edges 21 of the branch pipe 20 is contacted with the core 130 in the steps of heating and butt-welding the pipes, deformation of the peaked edges 21 of the branch pipe 20 due to stress and relaxation of the crystalline polymer material of which the pipes consists, is corrected to obtain the proper shape. As a result, accuracy of butting between the facets of the peaked edges 21 and the V-shaped recess 34 of the recessed attachment 32 becomes improved in the step of heating the pipes, so that a uniform fusion can be obtained. Consequently, accuracy of butting between the facets of the V-shaped edges 11 of the main pipe 10 and the peaked edges 21 of the branch pipe 20 becomes improved in the step of butt-welding, so that a uniform fixing strength can be obtained in the joint. Further, as shown in Fig. 13, fused resin is extruded inside and outside of the pipes by the pressure applied in butting the facets of the V-shaped edged 11 of the main pipe 10 and the peaked edges 21 of the branch pipe 20. However, all the fused resin extruded to the inside of the pipes is forced toward the main pipe 10 as the inner surface of the peaked edges 21 of the branch pipe 20 is contacted with the core 130, and the bead E is generated only at the inner surface of the main pipe 10. Therefore, all the bead E in the pipes can be removed by removing the bead E generated in the inner surface of the main pipe 10 in the step of removing the bead. Additionally, in this method, neither blow molding nor drawing method is carried out, so that the main and branch pipes have no thinner part than any other part. Further, the operation does not require a high degree of skill such as the conventional welding method, so that it is possible to fix the branch pipe 20 to the main pipe 10 with a stable fixing strength even by an operator who is not skilled. The method for making a joint between a main pipe and a branch pipe in the second preferred embodiment have been described above. However, concrete configurations of the invention are not limited to the embodiments, and modification in the design are included by the invention within the scope of the appended claims. For example, the angle at which the branch pipe is fixed to the main pipe may have other values though the branch pipe is fixed to the main pipe at a right angle in the embodiment described above. Further, the main and branch pipes may be pipes having sockets or bent pipes for making a joined pipe. Additionally, if the root of the core is made of elastic material which can be easily folded, it is easy to position the core in the branch pipe by folding the root to touch the edge of the opening of the branch pipe and removing the core from the branch pipe by holding the elastic part and applying force. Next, a method for making a joint between a main pipe and a branch pipe in a third preferred embodiment will be explained in conjunction with Figs. 16 to 19. In this embodiment, a method for making a T-shaped joined pipe in which a branch pipe 20 is fixed to a main pipe 10 at a right angle will be described like that of the second preferred embodiment. The method includes the steps of cutting the main pipe, cutting the branch pipe, fusing the pipes by heating and butt-welding the pipes. Either step of cutting the main pipe or cutting the branch pipe may be carried out first, or the two steps may be carried out simultaneously. (the step of cutting the main pipe) In this step, the main pipe 10 is cut to form a hole for branching in the pipe wall. The facet of the hole has two V-shaped edges 11 at a right angle in which the two lowest points 12 are located at slightly more shallow points then the diametrical points on the periphery of the main pipe 10. Each of the two facets makes an angle of 45° with the pipe axis A-A. (the step of cutting the branch pipe) In this step, the branch pipe 20 is cut to form a facet having two peaked edges 21 at a right angle in which the two peaks 22 are located diametrically on the periphery of the branch pipe 10. Each of the two facets makes an angle of 45° with the pipe axis B-B. (the step of fusing the pipes by heating) In this step, the facet including the V-shaped edges 11 of the main pipe 10 and the facet including the peaked edges 21 of the branch pipe 20 are heated to be fused by a heating apparatus 30, as shown in Fig. 18. The facets of the V-shaped edges 11 of the main pipe 10 and the peaked edges 21 of the branch pipe 20 are heated to be fused by fitting the V-shaped edges 11 and the peaked edges 21 respectively to the peak 35 of the peaked attachment 33 and the V-shaped recess 34 of the recessed attachment 32 under pressure for a predetermined period. (the step of butt-welding the pipes) In this step, the V-shaped edges 11 of the main pipe 10 and the peaked edges 21 of the branch pipe 20 are fitted by fusion and butt-welded to each other by applying pressure in the direction of the axis B-B of the branch pipe 20, as shown in Fig. 19. The lowest points 12 of the V-shaped edges 11 are located at slightly more shallow points than the diametrical points. However, the V-shaped edges 11 become deeper as fused resin within the V-shaped edges 11 is extruded to the inside and outside of the pipes by the pressure applied in the direction of the pipe axis B-B of the branch pipe 20. As a result, the width of the lowest points 12 of the V-shaped edges 11 becomes wider to be nearly that of the peaks 22 of the peaked edges 21, so that the peaked edges 21 of the branch pipe 20 and the V-shaped edges 11 of the main pipe 10 are butt-welded adequately without sinking of the peaked edges 21 into the V-shaped edged 11. Further, the pressure applied in the direction of the pipe axis B-B of the branch pipe 20 is dispersed uniformly between the V-shaped edges 11 and the peaked edges 21 in the radius direction, so that a bead F having uniform width is generated. After butt-welding, the lowest points 12 of the V-shaped 11 and that of the peaks 22 of the peaked edges 21 have almost the same width. As described above, a bead F having uniform width is generated and the leaked edges 21 of the branch pipe 20 and the V-shaped edges 11 of the main pipe 10 are butt-welded adequately without sinking of the peaked edges 21 into the V-shaped edges 11, so that the joint has an excellent appearance. Further, the lowest points 12 of the V-shaped 11 and that of the peaks 22 of the peaked edges 21 have the almost same width, so that a joined pipe having a stable fixing strength can be made. Next, a method for making a joint between a main pipe and a branch pipe in a fourth preferred embodiment will be explained in conjunction with Figs. 20 to 23. In this embodiment, a method for making a Y-shaped joined pipe in which a branch pipe 50 is fixed to a main pipe 40 at an angle of 60° will be described as in the first preferred embodiment. The method includes the steps of cutting the main pipe, cutting the branch pipe, fusing the pipes by heating and butt-welding the pipes. Either step of cutting the main pipe or cutting the branch pipe may be carried out first, or the two steps may be carried out simultaneously. (the step of cutting the main pipe) In this step, the main pipe 40 is cut to form a hole for branching in the pipe wall. The facet of the hole has two V-shaped edged 41 at a right angle in which the two lowest points 42 are located at slightly more shallow points than the diametrical points on the periphery of the main pipe 40. The two facets make respectively an angle of 30° and an angle of 60° with the pipe axis C-C. (the step of cutting the branch pipe) In this step, the branch pipe 50 is cut to form a facet having two peaked edges 51 of an angle slightly more acute than a right angle in which two peaks 52 are located diametrically on the periphery of the branch pipe 50. The two facets make respectively an angle of 30° and an angle of 60° with the pipe axis D-D. (the step of fusing the pipes by heating) In this step, the facet including the V-shaped edges 11 of the main pipe 40 and the facet including the peaked edges 51 of the branch pipe 50 are heated to be fused by a heating apparatus 60, as shown in Fig. 22. The facets of the V-shaped edges 41 of the main pipe 40 and the peaked edges 51 of the branch pipe 50 are heated to be fused by fitting the V-shaped edges 41 and the peaked edges 51 respectively to the peak 65 of the peaked attachment 63 and the V-shaped recess 64 of the recessed attachment 62 under pressure for a predetermined period. (the step of butt-welding the pipes) In this step, the V-shaped edges 41 of the main pipe 40 and the peaked edges 51 of the branch pipe 50 are fitted by fusion and butt-welded to each other under equal application of pressure in the direction of the center of the V-shaped edges 41 of the main pipe 40 at an angle of 45° from either side of each V-shaped edge 41 (in the direction of line E-E), as shown in Fig. 23. The lowest points 42 of the V-shaped edges 41 are located at slightly more shallow points than the diametrical points. However, the V-shaped edges 41 become deeper as the fused resin within the V-shaped edges 41 is extruded to the inside and outside of the pipes by the pressure applied in the direction of the pipe axis D-D of the branch pipe 50. As a result, the width of the lowest points 42 of the V-shaped edges 41 becomes wider to be nearly that of the peaks 52 of the peaked edges 51, so that the peaked edges 51 of the branch pipe 50 and the V-shaped edges 41 of the main pipe 40 are butt-welded adequately without sinking of the peaked edges 51 into the V-shaped edges 41. After butt-welding, the lowest points 42 of the V-shaped 41 and that of the peaks 52 of the peaked edges 51 have almost the same width. In the method described above, the lowest points 42 of the V-shaped edges 41 of a right angle and the peaks 52 of the peaked edges 51 of an angle slightly more acute than a right angle are butt-welded by pressure, so that the largest pressure is applied between the lowest points 42 of the V-shaped edges 41 and the peaks 52 of the peaked edges 51. As a result, it is possible to obtain the largest strength in fixing by fusion in the part where the latest load may be applied and water leakage may easily occur, though the bead F becomes slightly wide as it goes toward the lowest points 42 and the peaks 52. The method for making a joint between a main pipe and a branch pipe in the third and fourth preferred embodiments have been described above. However, concrete configurations of the invention are not limited to the embodiments, and modification in design is included by the invention within the scope of the appended claims. For example, the angle at which the branch pipe is fixed to the main pipe may have other values though the branch pipe is fixed to the main pipe at a right angle or an angle of 60° in the embodiments described above, or the joined pipe to be made by the method may have more than two branch pipes. Further, the peaked edges may have a right angle instead of an angle slightly more acute than a right angle as in the third embodiment. Next, a cutting apparatus will be described. First, the structure of the cutting apparatus will be described in conjunction with Figs. 24 to 28. Figs. 24, 27 and 28 are top views of the cutting apparatus. Fig. 25 is a cross-sectional view taken along line II-II, and Fig. 26 is a cross-sectional view taken along line III-III. The cutting apparatus includes a base 201. The base 201 is provided with a circular roll 211 and a slit 212 having a length slightly shorter than the diameter of the circular rail 211 on a line passing the circle center. Two clamps 202, 202 are provided on the circular rail 211. Each clamp 202 includes a bed 221 allowed to slide along the circular rail 211 and a cradle 222 fixed on the top surface of the bed 221. The bed 221 includes four rollers 223 for smooth sliding of the bed 221 on the circular rail 211 in the four corners, and four stoppers for stopping the clamp. Each stopper 224 includes a handle 225 and a cam (not shown in Figs.) which is provided at the tip of the handle 225 in which the cam rotates between the bed 221 and the circular rail 211 by rotating the handle 225. Each cradle 222 has a V-shaped recess 225 for holding a pipe P in the radius direction of the circular rail 211 on the top surface. As shown in Figs. 27 and 28, the pipe P clamped by the clamps 202, 202 is allowed to change its direction by sliding the clamps along the circular rail 211. If the pipe P is a straight pipe, the clamps 202, 202 are located diametrically on the circular rail 211. A cutter 203 fixed to a moving mechanism (not shown in Figs.) is provided within the slit 212. The cutter 203 is allowed to cut the pipe P clamped by the clamps 202 as it moves along the slit 212. The cutter 203 may be a notched straight cutter allowed to cut the pipe P by reciprocal motion in a vertical direction or a rotary notched cutter allowed to cut the pipe P by rotational motion. The moving mechanism is selected in response to the type of the cutter 203. It is preferable to select a moving mechanism by which the cutter 203 is able to move reciprocally within the slit 212. Next, the method for cutting a pipe by the cutting apparatus will be described. Fig. 29 shows a process for cutting the pipe P to make a main pipe (a) of a joined pipe having a V-shaped hole, Fig. 30 shows a process for cutting the pipe P to make a branch pipe (b) of a joined pipe having a peaked edge, and Fig. 31 shows a joined pipe consisting of the main and branch pipes (a) and (b) cut by the cutting apparatus. When the pipe P is cut from two directions, as shown in Figs. 29 and 30, the pipe P is clamped at two positions by the two clamps 202, 202, and then the clamps 202, 202 are fixed to the circular rail 211 so that the part of the pipe P to be cut is located crossing just above the slit 212. The position of the two clamps 202, 202 are determined in response to the direction of the cutting angle of the pipe P. Then, the pipe wall of the pipe P is cut to reach the predetermined position included in the center plane (x) including the pipe axis, as the cutter 203 moves along the slit 212. The direction of the pipe P is changed by moving the clamps 202 along the circular rail 211, and the pipe wall of the pipe P is cut to reach the predetermined position in the center plane (x) including the pipe axis as the cutter 203 moves along the slit 212. Thus, the V-shaped hole is generated in the pipe P. As shown above, it is possible to change the direction of the pipe P by moving the clamps 202 along the circular rail 211 without changing the directions of the cutter 203 and the base 201, so that the error of the cutting angle becomes small and the crossing of cutting facets due to over cutting can be avoided when the pipe P is cut from more than two directions. With much a high cutting accuracy, uniform pressure for fusion can be obtained in butt-welding the facets of the main pipe (a) and the branch pipe (b) to make a joined pipe, so that a joined pipe with excellent quality is realized, as shown in Fig. 29. Further, the cutting direction of the pipe is set in a desired direction in the cutting apparatus, so that it is possible to cut the pipe to make a V-shaped edge or a peaked edge having any desired angle (any angle of 30° , 45° , 60° , 90° , 120° , for example). Next, a cutting apparatus will be described in conjunction with Figs. 32 and 33. The like parts are indicated as like reference numerals and will not be explained again. The cutting apparatus includes a base 301. The base 301 is provided with a circular rail 310 and a slit 320 longer than the diameter of the circular rail 310 on a line passing the circle center. The circular rail 310 is divided to two are rails 311, 311 by the slit 320 with a distance S through which a cutter 203 is able to pass. The cutter 203 is fixed to a moving mechanism (not shown in Figs.) and allowed to cut the pipe P as it moves along the slit 320. A clamp 302 is provided on each arc rail 311. Each clamp 302 includes a cradle 330 having a wide width and an oblique facet facing toward the circle center. In the cutting apparatus, the cradle 330 is wide and the cutter 203 is allowed to move with a large stroke, so that it is possible to clamp and cut a pipe P having a large diameter as compared with the one in the first embodiment. Further, the facet of the cradle 330 facing toward the circle center is made oblique, so that the cradle 330 may not lie over the slit 320 and obstruct the motion of the cutter 203 when the pipe P is cut at an acute angle with the pipe axis. Next, a butt-welding apparatus will be described in conjunction with Figs. 34 to 39. Figs. 34 and 35 are respectively a top view and a side view of the butt-welding apparatus, and Figs. 36 is a cross-sectional view taken along line XI-XI of Fig. 35. The butt-welding apparatus includes a guide rail 401, first and second cradles 402 and 403, first and second clamps 404 end 405, and a heater 406. The guide rail 401 consists of two parallel rods on which the first and second cradles 402 and 403 are slided, to be moved near to and far from each other. The first cradle 402 for supporting the first clamp 404 includes a cylindrical member 421 through which the rods of the guide rail 401 are penetrated, and a lower table 422 fixed on the cylindrical member 421, while the second cradle 403 for supporting the first clamp 405 includes a cylindrical member 431 through which the rods of the guide rail 401 are penetrated, and a lower table 432 fixed on the cylindrical member 431. The first and second cradles 402 and 403 are connected to an oil-hydraulic cylinder 407 and driven thereby. The first clamp 404 for clamping a pipe P1 at a right angle with the sliding direction of the first cradle 402 includes an upper table 441 fixed to the first cradle 402 by two pairs of a bolt 408 and a nut 409, a supporting member 442 of L-shape having a corner 445 fixed on the upper table 441 for supporting the pipe P1, and two holding members 443, 443 each having a curved part corresponding to the wall surface of the pipe P1 fixed at both edges of the top surface of the supporting member 442. The pipe P1 is clamped by the corner 445 of the supporting member 442 and the curved parts of the two holding members 443, 443. The second clamp 405 for clamping a pipe P2 in the sliding direction of the second cradle 403 includes an upper table 451 fixed to the second cradle 403 by two pairs of a bolt 408 and a nut 409, two supporting members 452, 452 allowed to slide on the upper table 451 for supporting the pipe P2, two guides for guiding the supporting members 452, 452, and a threaded bar 455 having a handle 454 for transmitting the rotating power applied to the handle 454 to the supporting members 452, 452 in the sliding direction. The supporting members 452, 452 are moved near to each other by rotating the handle 454 in one direction and move away from each other by rotating the handle 454 in the counter direction. Each of the supporting members 452, 452 has a right angle notch 456 on the surface facing each other, and the pipe P2 is clamped in the right angle notches 456, 456. The upper table 441 of the first clamp 404 has two arc-shaped bolt holes 411 and 412 which are concentric, while the upper table 451 of the second clamp 405 has two arc-shaped bolt holes 413 and 414 which are concentric. The upper tables 441 and 451 are allowed to move in a circle on the lower tables 422 and 432 respectively by changing the fixing positions of the bolts 408 and the nuts 409. The heater 406 is fixed on a base (not shown in Figs.) so as to be allowed to rotate and be located between the first and second cradles 402 and 403. The heater 406 includes a peaked attachment 461 having a peaked edge 464 at a right angle, a recessed attachment 462 having a V-shaped edge 465 at a right angle, and a hand 463. The peaked attachment 461 faces to the first clamp 404 and the recessed attachment 462 faces to the second clamp 405. Next, a method for making a joined pipe by using the butt-welding apparatus. First, the pipe P1 having a hole in the pipe wall is clamped by the first clamp 404, and the pipe P2 having an opening whose shape corresponds to the hole of the pipe P1 is clamped by the second clamp 405. The mutual direction of the pipes P1 and P2 is set in the desired direction, since the first and second clamps 404 and 405 are fixed on the first and second cradles 402 and 403 respectively so as to be allowed to rotate. Then, the heater 406 is positioned between the first and second cradles 402 and 403, and the peaked attachment 461 is fitted to the periphery of the hole of the pipe P1 while the recessed attachment 462 is fitted to the opening of the pipe P2, and these parts are heated to be fused. Then, after removing the heater 406 from the position between the first and second cradles 402 and 403, the first and second clamps 404 and 405 are moved so as to butt-weld the fused parts of the pipes P1 and P2 under predetermined pressure. Consequently, the pipes P1 and P2 are butt-welded in the direction depending on the mutual direction of the first and second clamps 404 and 405. Thus, by using the butt-welding apparatus, a joined pipe having a desired shape is obtained (in which the pipes are fixed each other at a right angle or angles of such as 30° , 45° , 60° , etc.). Next, a method for making a joined pipe in which two pipes having the same diameter are fixed to each other at an angle of 60° using the butt-welding apparatus is described. First, as shown in Fig. 37(a), the wall of the pipe P1 is cut to make a V-shaped hole in which two V-shaped edges P10 at a right angle are formed so that the lowest points thereof are located diametrically on the periphery of the pipe P1. Two facets P11 and P12 forming the V-shaped edges P10 are formed to make an angle of 60° and an angle of 30° respectively with the pipe axis X-X. On the other hand, as shown in Fig. 37(b), the wall of the pipe P2 is cut to have two peaked edges P20 at a right angle in which the two peaks thereof are located diametrically on the periphery of the pipe P2. The two facets P21 and P22 forming the peaked edges P20 make an angle of 60° and an angle of 30° with the pipe axis Y-Y. Then, the pipe P1 having the V-shaped hole P10 is clamped by the first clamp 404 so that the V-shaped hole P10 faces the peaked attachment 461 of the heater 406, and the pipe P2 having the peaked edge P20 is clamped by the second clamp 405 so that the peaked edge P20 faces the recessed attachment 462 thereof. In this step, the first cradle 402 is rotated so that the facets P11 and P12 of the pipe P1 clamped by the first clamp 404 make an angle of 45° with the sliding direction Z-Z of the first and second cradles 402 and 403, and the second cradle 403 is rotated so that the facets P21 and P22 of the pipe P2 clamped by the second clamp 405 make an 45° with the sliding direction Z-Z, as shown in Fig. 38. In more detail, the pipe P1 is rotated in the direction indicated by the arrow shown in Fig. 38 so that the pipe axis X-X makes an angle of 15° with the direction vertical to the sliding axis Z-Z of the first and second cradles 402 and 403, and the pipe P2 is rotated in the direction indicated by the arrow shown in Fig. 38 so that the pipe axis Y-Y makes an angle of 15° with the direction vertical to the sliding axis Z-Z. It is required to clamp the pipe P1 by the first clamp 404 so that the pipe P1 rotates around the lowest point of the V-shaped hole P10 and to clamp the pipe P2 by the second clamp 405 so that the pipe P2 rotates around the peak of the peaked edge P20. Accordingly, the positions of the lowest point of the V-shaped hole P10 of the pipe P1 and the peak of the peaked edge P20 of the pipe P2 do not change after the rotation of the pipes. Then, the first and second cradles 402 and 403 are moved near to each other, and the facets P11 and P12 are fitted to the peaked attachment 461 of the heater 406 while the facets P21 and P22 are fitted to the recessed attachment 462, and these parts are heated to be fused. In this step, the facets P11 and P12 of the pipe P1 and the facets P21 an P22 of the pipe P2 are moved near to the heater 406 to make contact while the direction of the facets is kept at an angle of 45° with the sliding direction Z-Z of the first and second cradles 402 and 403, so that all the facets P11, P12, P21 and P22 are fused in a uniform fusion condition. Then, after removing the heater 406 from the position between the first and second cradles 402 and 403, the first and second clamps 404 and 405 move near to each other so as to butt-welding the facets P11 and P12 of the pipe P1 and the facets P21 and P22 of the pipe P2 under predetermined pressure. In this step, the facets P11 and P12 of the pipe P1 and the facets P21 and P22 of the pipe P2 are moved near to each other to make contact while the direction of the facets is kept at an angle of 45° with the sliding direction Z-Z of the first and second cradles 402 and 403, so that the butt-welded facets P11 and P21 make an angle of 60° with the pipe axis X-X and the pipe axis Y-Y and the butt-welded facets P12 and P22 making an angle of 30 with the pipe axis X-X and the pipe axis Y-Y having a uniform fixing strength. As shown in Fig. 39, there is another method in which only the pipe P2 clamped by the second clamp 405 is rotated in the direction indicated by the arrow shown in Fig. 39 so that the pipe axis X-X makes an angle of 30° with the direction vertical to the sliding direction Z-Z of the first and second cradles 402 and 403. In this method, the pipe P1 clamped by the first clamp 404 is slid in the direction vertical to the pipe axis X-X, and the pipe P2 clamped by the second clamp 405 is slid in the direction to make an angle of 30° with the pipe axis Y-Y. However, the facets P11 and P21 which are at an angle of 60° with the pipe axis X-X and Y-Y are fitted to each other in the direction of an angle of 30° with the sliding direction Z-Z of the first and second cradles 402 and 403, while the facets P12 and P22 which are at an angle of 30° with the pipe axes X-X and Y-Y are fitted to each other in the direction of an angle of 60° with the sliding direction Z-Z, thus the butt-welded parts might not have uniform fixing strenght. Therefore, it is recomended to rotate the first and second clamps 404 and 405 so that the facets P11 and P12 of the pipe P1 and the facets P21 and 22 of the pipe P2 make an angle of 45° with the sliding direction Z-Z of the first and second cradles 402 and 403. Thus, it is possible to make a joined pipe with a high fixing strength and excellent quality at low cost by butt-welding the two pipes P1 and P2 with the butt-welding apparatus.

Claims: A method for making a joint between a main pipe (40) and a branch pipe (50), comprising the steps of: cutting a main pipe (40) to have a hole in the pipe wall so that two V-shaped edges of a right angle are formed in the diametrical positions on the periphery of said main pipe; cutting a branch pipe (50) to have facets in the pipe opening so that two peaked edges of an angle slightly more acute than a right angle are formed in the diametrical positions on the periphery of said branch pipe; fusing the facets of said V-shaped edges of said. hole of said main pipe (40) and said peaked edges of said opening of said branch pipe (50) by heating; and butt-welding said facets of said V-shaped edges of said hole of said main pipe and said peaked edges of said opening of said branch pipe to form a joint. A method for making a joint between a main pipe (40) and a branch pipe (50), according to Claim 1, wherein: said cutting step of said main pipe (40) comprises the steps of: determining two points e 1 and e 2 which are located diametrically on the periphery of said main pipe; determining a point f 1 having an angle  3 <90° with said points e 1 and e 2 in the direction of said axis of said main pipe and a right angle in the radius direction; determining a point f 2 having an angle  4 =90°,- 3 with said points e 1 and e 2 in the direction of said axis of said main pipe which is the same radius direction and opposite axial direction with f 1 , and a right angle in the radius direction; and cutting the main pipe in a straight line forming an arc connecting said points e 1 , f 1 and e 2 , an arc connecting said points e 1 , f 2 and e 2 and said V-shaped edges; and cutting step of said branch pipe (50) comprises the steps of: determining two points g 1 and g 2 which are located diametrically on the periphery of said branch pipe; determining a point h 1 having said angle  3 with said points g 1 and g 2 in the direction of said axis of said branch pipe, and a right angle in the radius direction; determining a point h 2 having said angle  4 with said points g 1 and g 2 in the direction of said axis of said branch pipe which is the opposite radius direction and same axial direction with h 1 , and a right angle in the radius direction: determining points i 1 and i 2 which are at a little distance from said points g 1 and g 2 in the direction of said axis of said branch pipe which is the opposite axial direction; and cutting said branch pipe in a straight line forming an arc connecting said points i 1 , h 1 and i 2 , an arc connecting said points i 1 , h 2 and i 2 and said peaked edges. A method for making a joint between a main pipe (40) and a branch pipe (50), according to claim 2, wherein: said points i 1 and i 2 are located in the direction parallel to said axis of said branch pipe with a distance of about 5 mm or less from said points g 1 and g 2 respectively and away from said points h 1 and h 2 . A method for making a joint between a main pipe (10) and a branch pipe (20), comprising the steps of: cutting a main pipe (10) to have a hole in the pipe wall so that two V-shaped edges of a right angle are formed in the diametrical positions on the periphery of said main pipe; cutting a branch pipe (20) to have facets in the pipe opening so that two peaked edges of a right angle or an angle slightly more acute than a right angle are formed in the diametrical positions on the periphery of said branch pipe; fusing the facets of said V-shaped edges of said hole of said main pipe and said peaked edges of said opening of said branch pipe by heating with a core (130) inserted into said branch pipe to contact the whole surface of said peaked edges; butt-welding said facets of said V-shaped edges of said hole of said main pipe and said peaked edges of said opening of said branch pipe to form a joint; and removing a bead generated at the inner surface of said main pipe by inserting a cutting tool (140) after removing said core from said branch pipe. A method for making a joint between a main pipe and a branch pipe, according to Claim 4, wherein: said core (130) is inserted into said branch pipe (20) with a clearance of approximately 0.25 mm on average between the outer surface of said core and the inner surface of said branch pipe. A method for making a joint between a main pipe (10) and a branch pipe (20), comprising the steps of: cutting a main pipe (10) to have a hole in the pipe wall so that two V-shaped edges of a right angle are formed in the points slightly more shallow than the diametrical positions on the periphery of said main pipe; cutting a branch pipe (20) to have facets in the pipe opening so that two peaked edges of a right angle or an angle slightly more acute than a right angle are formed in the diametrical positions on the periphery of said branch pipe; fusing the facets of said V-shaped edges of said hole of said main pipe and said peaked edges of said opening of said branch pipe by heating; and butt-welding said facets of said V-shaped edges of said hole of said main pipe and said peaked edges of said opening of said branch pipe. A method for making a joint between a main pipe and a branch pipe, according to any of claims 1 to 6, wherein: said step of butt-welding is carried out under equal application of pressure in the direction of the center of said V-shaped edges of said hole of said main pipe at an angle of 45° from either side of said V-shaped edges.

Inventors: KITADA MITSUSHIRA, SEKI KAZUO, YAMADA YONEKAZU, YOSHINO YUTAKA, KITADA, MITSUSHIRA, SEKI, KAZUO, YAMADA, YONEKAZU, YOSHINO, YUTAKA
Applicants: SEIKISUI CHEMICAL CO LTD, SEKISUI KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1992
Main classification: F16L47
IPC codes: F16L47, B29C65