Method of Hemming Sheet Metal Panels with Friction Element Applying a Counteraction Force to Prevent Hem Creep

Abstract

A hemming tool for forming a hem securing an outer panel to an inner panel. The hemming tool includes a hemming die that has a first portion of a forming surface with a first coefficient of friction and a second portion that has a second coefficient of friction that is greater than the first portion. A flange on the outer panel is initially engaged by the first surface to begin forming the hem flange toward the inner panel. The flange is engaged by the second hemming surface to continue forming the flange toward the inner panel. A method of hemming a sheet metal assembly including an inner and an outer panel is also disclosed that utilizes the hemming die that has a forming surface with a second coefficient of friction that is greater than the first portion to reduce flange creep and roll-in during a hemming operation.

What is claimed is: 
     
         1 . A hemming tool for forming a hem to secure an outer panel to an inner panel comprising:
 an anvil die that supports the inner panel on the outer panel inside a flange formed on the outer panel that extends away from the anvil die in an initial position; and   a hemming die spaced from the flange in an initial position, the hemming die having a first portion of a forming surface that has a first coefficient of friction and a second portion that has a second coefficient of friction that is greater than the first portion, wherein the first portion engages the flange to begin forming the flange over the inner panel, and wherein the second portion subsequently engages the flange to continue forming the flange towards the inner panel.   
     
     
         2 . The hemming tool of  claim 1  further comprising a clamping pad that engages the inner panel and holds the inner panel against the outer panel on the anvil die. 
     
     
         3 . The hemming tool of  claim 1  wherein the flange is oriented at an oblique angle relative to the outer panel and further comprising a beveled surface outboard of the first portion of the forming surface, and wherein the beveled surface contacts the flange before the first portion of the forming surface to reduce the angle of the flange from an oblique angle to an acute angle. 
     
     
         4 . The hemming tool of  claim 1  further comprising a flange finishing tool that is inserted between the forming surface and the flange after the second portion engages the flange to complete forming the flange. 
     
     
         5 . The hemming tool of  claim 1  wherein the flange has a distal end that slides across the first portion of the forming surface with less friction than when the distal end slides across the second portion. 
     
     
         6 . The hemming tool of  claim 5  wherein the coefficient of friction of the second portion prevents creep on the flange. 
     
     
         7 . The hemming tool of  claim 1  wherein the hemming die defines a recess in the second portion of the forming surface, and further comprising an insert received in the recess and attached to the hemming die, and wherein the insert provides the surface that has second coefficient of friction. 
     
     
         8 . The hemming tool of  claim 7  wherein the insert is formed of polyurethane. 
     
     
         9 . The hemming tool of  claim 1  wherein the anvil die includes an end wall that is engaged by the hem flange as the hem flange is formed over the inner panel, and wherein the hem flange is driven into the end wall by the hemming tool when the second portion of the forming surface is in engagement with the hem flange. 
     
     
         10 . A method of hemming a sheet metal assembly comprising:
 loading an inner panel and an outer panel that includes a flange into a hemming tool that includes a hemming die that has a first hemming surface and a second hemming surface that has a higher coefficient of friction than the first hemming surface;   engaging the flange with the first hemming surface to begin forming the flange towards the inner panel;   engaging the flange with the second hemming surface to continue forming the flange towards the inner panel; and   forming the flange against the inner panel.   
     
     
         11 . The method of  claim 10  further comprising:
 inserting a finish hem tool between the hemming die and the flange after the second hemming surface forms the flange towards the inner panel. 
 
     
     
         12 . The method of  claim 11  further comprising:
 retracting the hemming die away from the inner panel and the flange after the second hemming surface forms the flange towards the inner panel and before the step of inserting the finish hem tool between the hemming die and the flange.