Method of cold forming a piece of sheet metal by bending or press moulding

Abstract

A method of cold forming a piece of sheet metal ( 1 ) by bending or press moulding, in which method:—the piece of sheet metal is bent over a bevelled edge ( 12, 13 ) of a tool ( 10 ) so that the piece of sheet metal in the bending area (B 1, B 2 ) is subjected to compressive stress, on the inner side and tensile stress on the outer side of the neutral layer. A gap is left between the sheet metal and an edge surface ( 14, 15 ) of the bevelled edge.—compressive force is then exerted on the piece of sheet metal outside the bending area, which is compressed transversally to the longitudinal direction of the bend, whereupon compressive force is exerted on the outer side of the bending area so that the bending area is subjected to tensile stress on the inner side and compressive stress on the outer side of the neutral layer.

1 . A method of cold forming a piece of sheet metal ( 1 ) by bending or press moulding, in which method
 the piece of sheet metal ( 1 ) is bent over a bevelled edge ( 12 ,  13 ) of a tool ( 10 ) so that the piece of sheet metal in the area (B 1 , B 2 ) of the bend thus formed, being denominated bending area, is subjected to compressive stress on the side of the neutral layer ( 8 ) facing the inner side ( 3 ) of the bending area and tensile stress on the side of the neutral layer ( 8 ) facing the outer side ( 4 ) of the bending area, a gap being left between the piece of sheet metal ( 1 ) and an edge surface ( 14 , 15 ) of the bevelled edge ( 12 ,  13 ) on the inner side ( 3 ) of the bending area; and   compressive force is thereafter exerted on the piece of sheet metal ( 1 ) outside said bending area (B 1 , B 2 ) in such a manner that the bending area is subjected to compression in the extension direction of the piece of sheet metal transversally to the longitudinal direction of the bend, whereupon compressive force is exerted against the outer side ( 4 ) of the bending area so that the bending area (B 1 , B 2 ) is pressed inwards towards said edge surface ( 14 ,  15 ) so that the bending area is subjected to tensile stress on the side of the neutral layer ( 8 ) facing the inner side ( 3 ) of the bending area and compressive stress on the side of the neutral layer ( 8 ) facing the outer side ( 4 ) of the bending area.   
     
     
         2 . A method according to  claim 1 , wherein the method comprises the following consecutive steps:
 A) the piece of sheet metal ( 1 ) is placed with a first part ( 1 A) of the piece of sheet metal received between a first tool ( 10 ) and a second tool ( 20 ), and with second and third parts ( 1 B,  1 C) of the piece of sheet metal extending out over bevelled edges ( 12 ,  13 ) of the first tool ( 10 ) on either side of the first part ( 1 A) of the piece of sheet metal;   B) the first tool ( 10 ) and the third tool ( 30 ) are thereafter subjected to a mutual displacement in the direction towards each other so that the second and third parts ( 1 B,  1 C) of the piece of sheet metal are bent over the bevelled edges ( 12 ,  13 ) of the first tool ( 10 ) under the effect of the third tool ( 30 ) and thereby are bent in relation to the first part ( 1 A) of the piece of sheet metal to form a first bending area (B 1 ) on the piece of sheet metal at the first bevelled edge ( 12 ) and a second bending area (B 2 ) on the piece of sheet metal at the second bevelled edge ( 13 ), while the first and second tools ( 10 ,  20 ) are kept at a distance from each other so that the first part ( 1 A) of the piece of sheet metal is bulged in connection with this bending of the second and third parts ( 1 B,  1 C) of the piece of sheet metal, a gap being left in the respective bending area (B 1 , B 2 ) between the piece of sheet metal ( 1 ) and an edge surface ( 14 ,  15 ) of the bevelled edge ( 12 ,  13 ) on the inner side ( 3 ) of the bending area and between the piece of sheet metal ( 1 ) and the third tool ( 30 ) on the outer side ( 4 ) of the bending area;   C) the first and second tool ( 10 ,  20 ) are thereafter subjected to a mutual displacement in the direction towards each other so that the first part ( 1 A) of the piece of sheet metal is flattened between these tools while the third tool ( 30 ) retains the second and third parts ( 1 B,  1 C) of the piece of sheet metal in the bent state, whereby the bulge formed in step B in the first part ( 1 A) of the piece of sheet metal is flattened so that the piece of sheet metal in the respective bending area (B 1 , B 2 ) is subjected to compression in the extension direction of the piece of sheet metal transversally to the longitudinal direction of the bend;   D) the first and third tools ( 10 ,  30 ) are thereafter subjected to a continued mutual displacement towards each other, while the first part ( 1 A) of the piece of sheet metal is retained in the flattened state between the first and second tools ( 10 ,  20 ), so that the respective bending area (B 1 , B 2 ) of the piece of sheet metal is pressed inwards by the third tool ( 30 ) towards the adjacent edge surface ( 14 ,  15 ) of the first tool so that the respective bending area (B 1 , B 2 ) is subjected to tensile stress on the side of the neutral layer ( 8 ) facing the inner side ( 3 ) of the bending area and compressive stress on the side of the neutral layer ( 8 ) facing the outer side ( 4 ) of the bending area; and   E) the thus formed piece of sheet metal ( 1 ) is thereafter released from said tools ( 10 ,  20 ,  30 ).   
     
     
         3 . A method according to  claim 2 , wherein the first tool ( 10 ) is kept stationary in steps B and D, while the third tool ( 30 ) is displaced towards the first tool. 
     
     
         4 . A method according to  claim 2 , wherein the first tool ( 10 ) is kept stationary in step C, while the second tool ( 20 ) is displaced towards the first tool. 
     
     
         5 . A method to  claim 1 , wherein the piece of sheet metal ( 1 ) by bending is bent along two rectilinear and mutually parallel bending lines. 
     
     
         6 . A method according to  claim 5 , wherein the piece of sheet metal ( 1 ) is bent into U-shape. 
     
     
         7 . A method according to  claim 1 , wherein the piece of sheet metal ( 1 ) by press moulding is bent along one or more curved bending lines. 
     
     
         8 . A method according to  claim 3 , wherein the first tool ( 10 ) is kept stationary in step C, while the second tool ( 20 ) is displaced towards the first tool. 
     
     
         9 . A method according to  claim 8 , wherein the piece of sheet metal ( 1 ) by bending is bent along two rectilinear and mutually parallel bending lines. 
     
     
         10 . A method according to  claim 2 , wherein the piece of sheet metal ( 1 ) by bending is bent along two rectilinear and mutually parallel bending lines. 
     
     
         11 . A method according to  claim 3 , wherein the piece of sheet metal ( 1 ) by bending is bent along two rectilinear and mutually parallel bending lines. 
     
     
         12 . A method according to  claim 4 , wherein the piece of sheet metal ( 1 ) by bending is bent along two rectilinear and mutually parallel bending lines. 
     
     
         13 . A method according to  claim 12 , wherein the piece of sheet metal ( 1 ) is bent into U-shape. 
     
     
         14 . A method according to  claim 11 , wherein the piece of sheet metal ( 1 ) is bent into U-shape. 
     
     
         15 . A method according to  claim 10 , wherein the piece of sheet metal ( 1 ) is bent into U-shape. 
     
     
         16 . A method according to  claim 9 , wherein the piece of sheet metal ( 1 ) is bent into U-shape. 
     
     
         17 . A method according to  claim 2 , wherein the piece of sheet metal ( 1 ) by press moulding is bent along one or more curved bending lines. 
     
     
         18 . A method according to  claim 3 , wherein the piece of sheet metal ( 1 ) by press moulding is bent along one or more curved bending lines. 
     
     
         19 . A method according to  claim 4 , wherein the piece of sheet metal ( 1 ) by press moulding is bent along one or more curved bending lines.