Method for manufacturing a box-shaped door
BACKGROUND OF THE INVENTION 
     This invention relates to manufacture of a box-shaped door, for example, the door of an icebox, and more particularly to a method and device for manufacturing said box-shaped door. 
     The body of the above-mentioned box-shaped door has generally been constructed by pressing a metal blank. However, the pressing work has the drawbacks that it is necessary to provide a die having a complicated shape and a large-scale machine has to be installed. To date, therefore, it has been proposed to carry out bending and shaping separately. In this case, too, the manufacturing equipment is accompanied with the drawbacks that a large number of dies have to be provided; and the work steps and consequently work hours increase in number, resulting in the low efficiency of said manufacturing process as a whole. 
     SUMMARY OF THE INVENTION 
     It is accordingly the object of the invention to provide a method and device for manufacturing a box-shaped door which can assure a reduction in the cost of manufacturing equipment and work expenses by effecting the bending and shaping of workpieces in a single step on a fabricating machine of simple arrangement. Further, the manufacturing device embodying this invention which can automatically locate a workpiece contributes to improvement on product quality and the elevation of manufacturing efficiency. Moreover, according to the invention, a welding process is applied which is accompanied with little residual stress, thereby improving the product quality. 
     The method of this invention for manufacturing a box-shaped door comprises the steps of cutting off the corner sections of a rectangular metal blank in the square form; vertically bending the four sides of the remaining metal blank to provide the rectangular side walls of the box-shaped door by means of a lower female die on the inner wall of which an upper bending section and lower shaping section are formed in the stepped state and a male die whose outer wall in shaped complementary to said bending and shaping sections of the female die; curving the mutually abutting edges of the four vertically bent sides of the metal blank to cause said edges to have a plane contact; and finally welding said mutually abutting edges of the four vertically bent sides of the metal blank. 
     A device embodying this invention for manufacturing a box-shaped door consists of a lower female die on whose inner wall an upper bending section and a lower shaping section are formed in the stepped state and an upper male die whose outer wall is shaped complementary to said bending and shaping sections of the female die. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional view of the box-shaped door of an icebox to which the manufacturing method and device of this invention is applied; 
     FIGS. 2A to 2F illustrate the sequential steps of manufacturing the subject box-shaped door; 
     FIG. 3 is a longitudinal sectional view of a device for vertically bending the four sides of a metal blank remaining after the cutting of its four corners and curvedly shaping the mutually abutting edges of said four vertically bent sides of the metal blank; 
     FIG. 4 is an oblique view of said bending and shaping device; 
     FIG. 5 is an enlarged longitudinal sectional view of one of the lower corners of a male die; 
     FIG. 6 is an oblique view of said lower corner; 
     FIG. 7 is an enlarged oblique view of one of the four corners of a female die; 
     FIG. 8 is enlarged oblique view of the end section of a locating wedge; 
     FIG. 9 is a fractional longitudinal view of the metal blank-locating wedge in operation; 
     FIG. 10 is a fractional longitudinal view of the metal blank-locating wedge after the location operation is brought to an end; 
     FIG. 11 is a plan view of the metal blank including four wedges after the completion of their locating operation; 
     FIG. 12 is a longitudinal sectional view of the four sides of the metal blank which have been vertically bent by the bending device of FIG. 3; 
     FIG. 13 is a longitudinal sectional view of the mutually facing edges of the four vertically bent sides of the metal blank which have been shaped; 
     FIG. 14 is an enlarged cross sectional view of the mutually abutting edges of the four sides of the metal blank in the process of being welded; and 
     FIG. 15 is an oblique view of said mutually abutting edges in the process of being welded (showing a box body turned upside down from that of FIGS. 2A to 2F). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Description is now given with reference to the accompanying drawings of a method and device embodying this invention for manufacturing a box-shaped door of an icebox. Referring to FIG. 1, reference numeral 1 denotes a box-shaped front door of the upper freezing chamber 3 constituting the upper section of a icebox 2. Said front door 1 consists of a box body 6, a heat-insulating material 8 prepared from, for example, foamed polyurethane resin and filled in said box body 6, and a cap 7 fitted to the backside of said front door 1. Reference numeral 4 shows a front door of a cooling chamber 5 constituting the lower section of the icebox 2. This front door 4 similarly consists of a box body 9, heat-insulating material 11 prepared similarly from foamed polyurethane resin and filled in said box body 9, and a cap 10 fitted to the backside of said box body 9. The front doors, 1, 4 are swingably supported by hinges 2. Reference numeral 13 denotes a cooling device for the cooling chamber 5; reference numeral 14 shows a condenser; reference numeral 15 is a compresser; and reference numeral 16 represents an evaporation dish. 
     The four corners of, for example, a rectangular steel blank 21 constituting the box body 6 of the front door 1 of the freezing chamber 3 are cut off by a press (not shown) in the form shown in FIG. 2A, thereby forming four side walls 21a, 21b, 21c, 21d of the box body 6 along the edge of the bottom wall 21m. The outer edge portions of said side walls 21a, 21b, 21c, 21d, are formed into four sections 21a&#39;, 21b&#39;, 21c&#39;, 21d&#39; which are later to be bent vertically. Both ends of each of said four sections 21a&#39;, 21b&#39;, 21c&#39;, 21d&#39;, are cut off 45° in the opposite directions. A hole is bored at the required points of the metal blank 21 to allow for the fitting of a gasket, ornamental attachment or handle (not shown). As shown in FIG. 2B, the four sides 21a to 21d of the metal blank 21 are bent vertically. The abutting edge portions 21e, 21f, 21g, 21h of the four sides 21a to 21d are curvedly shaped to be mutually contacted by a plane. The above-mentioned bending and shaping are effected by a device shown in FIGS. 3 and 4. Reference numeral 23 denotes an upper male die body. A male die 25 is fixed to the central part of a rectangular upper base 24. A locating wedge 26 provided with a tapered plane 26a at the outer end is projectively formed at the four corners of the upper base 24. Reference numeral 27 denotes a lower female die body. A rectangular supporting plate 31 slightly smaller than the bottom wall 21m is mounted on the base 28 of the lower female die body 27 by springs 30, 30, thereby fixing the female die 29 whose inner wall is shaped complementary to the outer wall of the male die 25. The bottom wall of the female die 29 is bored with a hole 29a engageable with the supporting plate 31. A guide hole 32 into which the locating wedge 26 is inserted is formed in the four corners of the female die 29. The outer dimensions of the male die 25 and the curvature radii of the peripheral edge 25c of the bottom of said male die 25 and the lower end sections 25a of the four corners of said male die 25 are respectively chosen to be the same as the inner dimensions of the box body 6 and the curvature radius of the four corners of the inner wall thereof. As shown in FIG. 5, an escapement 25d is provided on the edge 25b extending above the lower end sections 25a of the four corners of said male die 25. The edge 25b is chosen to have a larger curvature radius than that of the lower edge of the inner wall of the box body 6. The inner wall 33 of the female die 29 comprises a bending section 33a for vertically bending the four sides 21a to 21d of the metal blank 21 and a shaping section 33b for curving the mutually abutting sections 21e to 21h of said four sides 21a to 21d. Said bending section 33a and shaping section 33b are provided in the stepped form. The inner dimensions of the shaping section 33b and the curvature radii of the inner bottom peripheral edge 33c and the four corners 33d both of said shaping section 33b are respectively chosen to be the same as the outer dimensions, and the curvature radius of the peripheral edge of said box body 6. The inner dimensions of the bending sections 33a is chosen to be larger than those of the shaping section 33b. Now let it be assumed that a metal blank 21 having a thickness of 0.5 mm is bent and shaped to provide a box body 6 whose bottom peripheral edge has a curvature radius of 5 mm and whose four side abutting edges 21e to 21h have a curvature radius of 8 mm. Then the curvature radius of the inner bottom peripheral edge 33c and the curvature radius of the four corners 33d both of said female die 29 are respectively chosen to be 5 mm and 8 mm. The curvature radius of the lower corner edge 25a and that of the peripheral edge 25c both of the male die 25 are set at 4.5 mm. The of the escapement 25d of the edge 25b of the male die 25 is chosen to be about 0.1 mm, and the curvature radius of said edge 25b is set at 7.8 mm. The locating wedge 26 has a square cross section. The outer end portion of said wedge 26 is provided with rectangularly intersecting tapered planes 26a which are used to locate the metal blank 21 by sliding along the cut portions 22 thereof. The locating wedge 26 is provided with chamfered planes 26b in order to widen a space defined between a guide hole 32 drilled in the female die 29 and the inner wall 33 thereof. 
     As shown in FIG. 3, the metal blank 21 whose corners have been cut off is first set on the supporting plate 31 and female die 29. Under this condition, the upper male die body 23 is brought down. If, in this case, the metal blank 21 is displaced from its normal position as shown in FIG. 9, then the tapered planes 26a of the locating wedge 26 slidably contact the cutoff corners of the four sides of the metal blank 21 while said wedge 26 is brought into the guide hole 32. As a result, the metal blank 21 is pushed in the direction of an arrow A indicated in FIG. 9, to have its position corrected as shown in FIG. 10. Thus, the metal blank 21 is correctly set in position by four locating wedges 26 as illustrated in FIG. 11. When under this condition, the upper male die body 23 is lowered, the bottom plate 21m of the metal blank 21 is clamped between the supporting plate 31 supported by the springs 30 and the bottom wall of the male die 25 to remain flat. Only the four sides 21a  to 21d are vertically bent as shown in FIG. 12 by the bending section 33a of the female die 29. When the upper male die body 23 is brought down, the mutually abutting sections 21e to 21h of the four sides 21a to 21d of the metal blank 21 are curvedly pressed by the four corners 33d of the shaping section 33b of the female die 29. As a result, the abutting edges of the four sides 21a to 21d of the metal blank 21 contact each other by a plane. Since the escapement 25d is provided in the four corner edges 25b of the male die 25, the abutting edges 21e to 21h of said four sides 21a to 21d can be neatly curved without being subjected to excess stresses. The lower edges of the four vertically bent sides 21a to 21d of the metal blank 21 are curvedly shaped by the inner bottom edge 33c of the female die 29 and the bottom peripheral edge 25c of the male die 25. The inside of the bottom wall 21m of the metal blank 21 which is securely held by a supporting plate 31 supported by the springs 30, 30 is little subjected to undesirable bending or deformation. 
     The abutting edges 21e to 21h of the four sides 21a to 21d of the metal blank 21 are welded together as shown in FIG. 2c by a welding device shown in FIG. 14. The welding device comprises one inner jig 34, two outer jigs 35, 35 and welding torch 37. This welding torch 37 is the type which ejects plasma 36 by applying, for example, an argon gas. The plasma 36 is ejected on the side walls 21a to 21d of the metal blank 21 held between the inner jig 34 and outer jigs 35, 35. The welding of the abutting edges 21e to 21h proceeds in the direction of an arrow B indicated in FIG. 15 (showing a box body turned upside down) from the bottom of the box body 6 to the open side thereof. Therefore welding can be effected while releasing excess stresses, thereby reducing strains remaining in the welded section. 
     As illustrated in FIG. 2D, the upper edges 21b&#39;, 21d&#39; of the shorter sides 21b, 21d are bent inward. As shown in FIG. 2E, the upper edges 21a&#39;, 21c&#39;, of the longer sides 21a, 21c are similarly bent inward. The planes cut off 45° at both ends of each of the inward bent upper sides 21a&#39; to 21d&#39; are made to abut with each other. The abutting edges welded by, for example, the spot process. Last as shown in FIG. 2F , hinge holes 38 are provided on one side of the box body 6 of the icebox door to finish the manufacture of said box body 6. 
     A method and device embodying this invention for manufacturing a box-shaped door comprises, as described above. The steps of cutting off the four corners of a rectangular metal blank in the square form to provide the four sides of a box-shaped door and later simultaneously carrying out the vertical bending of the four sides of the box body, and the curved shaping of the abutting edges of said four sides. 
     Therefore, this invention offers the advantages that a working device is far more simplified than that which have been used in the conventional press work; the number of dies and working hours can be more reduced from in the conventional device in which the vertical bending of the four sides of a metal blank and the curved shaping of the abutting edges of said four sides are carried out separately; the metal blank is automatically located when the four sides of metal blank are vertically bended, assuring the elevation of working efficiency and the improvement of a product quality; during the working process, the bottom wall of the box body is securely held by a supporting plate supported by springs and undergoes little bending on defermation; the curved shaping of the abutting edges of the four sides of the metal blank causes said edges to contact each other by a plane, assuring the neat welding of said abutting edges and improving the external appearance of the box body as a whole; and the welding of said abutting edges proceeds from the bottom to the opening of the box body, thereby reducing the residual strains of the welded sections.