Method and apparatus for forming a compound curvature metal skin

Abstract

A method of forming a compound curvature metal skin includes providing a blank defining at least a sector of a truncated cone, providing an arcuate or annular punch substantially corresponding in shape to an inner surface of the compound curvature metal skin, gripping a leading edge of a sheet metal blank between mating faces of a first blank holder on a first side of the punch, gripping a trailing edge of the blank between mating faces of a second blank holder on a second side of the punch opposite to the first side, displacing the blank holders with respect to the punch to draw the blank over the punch while permitting the blank to flow between mating surfaces of at least one of the blank holders, including displacing each of the blank holders with respect to the punch by different amounts and at different rates.

1 . A method of forming a compound curvature metal skin from a sheet metal blank comprising the steps of:
 providing a blank defining at least a sector of a truncated cone having a central axis of symmetry, a leading edge and a trailing edge;   providing an arcuate or annular punch substantially corresponding in shape to an inner surface of the compound curvature metal skin and having an outer surface, a leading edge and an inner surface;   gripping the leading edge of the blank between mating faces of a first blank holder located on a first side of the punch adjacent the inner face of the punch;   
       gripping the trailing edge of the blank between mating faces of a second blank holder located on a second side of the punch, opposite to the first side and adjacent the outer face of the punch; and
 displacing the first and second blank holders with respect to the punch in a single direction to draw the blank over the punch while permitting the blank to flow between mating surfaces of at least one of the first and second blank holders, comprising the steps of displacing each of the first and second blank holders with respect to the punch by different amounts and at different rates. 
 
     
     
         2 . The method of  claim 1 , wherein the first and second blank holders are displaced in a direction substantially aligned with the central axis of the blank. 
     
     
         3 . The method of  claim 1 , wherein, in a first step, both the first and second blank holders are displaced with respect to the punch, the second blank holder being moved faster than the first blank holder, drawing material from between the mating faces of the first blank holder and over an outer surface of the punch. 
     
     
         4 . The method of  claim 3 , wherein the surface of the punch is lubricated to minimise friction between the blank and the surface of the punch. 
     
     
         5 . The method of  claim 3 , wherein the second blank holder reaches its limit of travel as the first blank holder becomes aligned with the leading edge of the punch. 
     
     
         6 . The method of  claim 3 , wherein, in a second step, the first blank holder is moved past the leading edge of the punch and over the inner surface of the punch to redraw the blank through the first blank holder and over the surface of the punch. 
     
     
         7 . The method of  claim 1 , wherein the leading edge of the blank flows between the mating surfaces of the first blank holder as the blank is drawn over the punch. 
     
     
         8 . The method of  claim 7 , wherein the blank is drawn over a trailing edge of a portion of the first blank holder as the first blank holder is displaced past a leading edge of the punch. 
     
     
         9 . The method of  claim 7 , wherein the second blank holder grips the trailing edge of the blank substantially without slippage as the blank is drawn over the punch. 
     
     
         10 . wherein the mating surfaces of the first and second blank holders are at least initially aligned with one another to define at least part of a frustoconical surface adapted to match the shape of the blank. 
     
     
         11 . The method of  claim 1 , wherein the mating faces of the first blank holder grip the leading edge of the blank with sufficient force to permit the blank to flow therebetween in a controlled manner without tearing or wrinkling such that the blank is drawn between the mating surfaces of the first blank holder as the blank is drawn over the punch during relative displacement of the first and/or the second blank holder relative to the punch. 
     
     
         12 . The method of  claim 1 , comprising an initial step of forming the blank to define at least a sector of truncated cone. 
     
     
         13 . The method of  claim 12 , wherein the initial forming step comprises a rolling and/or pressing process. 
     
     
         14 . An apparatus for forming a compound curvature metal skin from a sheet metal blank comprising:
 a blank defining at least a sector of a truncated cone having a central axis, a leading edge and a trailing edge;   an arcuate or annular punch substantially corresponding in shape to an inner surface of the compound curvature metal skin having an outer surface, a leading edge and an inner surface;   a first blank holder having a support surface for supporting the leading edge of the blank;   a second blank holder having a support surface for supporting the trailing edge of the blank;   wherein the punch is located between the first and second blank holders;   a first die member for retaining the leading edge of the blank against the support surface of the first blank holder;   a second die member for retaining the trailing edge of the blank against the support surface of the second blank holder;   wherein the first and second blank holders and the punch are displaceable with respect to one another in a single direction to draw the blank over the punch and wherein the first and second blank holders are displaceable with respect to the punch by different amounts and at different rates.   
     
     
         15 . The apparatus of in  claim 14 , wherein the first and second blank holders and the punch are displaceable with respect to one another in a direction substantially aligned with the central axis of symmetry of the blank. 
     
     
         16 . The apparatus of  claim 14 , wherein the first and second blank holders are displaceable with respect to the punch in a direction substantially aligned with the central axis of the blank. 
     
     
         17 . The apparatus of  claim 14 , wherein the support surfaces of the first and second blank holders are at least initially aligned with one another to define at least part of a frustoconical surface adapted to match the shape of the blank. 
     
     
         18 . The apparatus  claim 14 , wherein the first blank holder and first die member are adapted to grip the leading edge of the blank with sufficient force to permit the blank to flow therebetween in a controlled manner without tearing or wrinkling, such that the blank is drawn between the mating surfaces of the first blank holder and the first die member as the first and second blanks holders and the punch are displaced with respect to one another to draw the blank over the outer surface of the punch. 
     
     
         19 . The apparatus of  claim 18 , wherein a trailing or lower edge of the first die member defines a forming surface over which the blank is drawn as the first die member moves past a leading edge of the punch during the forming operation. 
     
     
         20 . The apparatus of  claim 18 , wherein the second blank holder and the second die member are adapted to hold the trailing edge of the blank substantially without slippage as the blank is drawn over the outer surface of the punch. 
     
     
         21 . The apparatus of  claim 14 , wherein the punch comprises an annular body, replicating an entire nacelle lipskin. 
     
     
         22 . The apparatus of  claim 14 , wherein the punch comprises an arcuate sector corresponding to a sector of the lipskin to be formed. 
     
     
         23 . The apparatus of  claim 14 , wherein a tool base is provided, upon which the punch, and first and second blank holders are mounted. 
     
     
         24 . The apparatus of  claim 23 , wherein the first and second blank holders are mounted on the tool base to permit relative displacement between the first and second blank holders and the punch to draw the blank over the punch. 
     
     
         25 . The apparatus of  claim 24 , wherein the first and second blank holders are independently moveable with respect to the punch by respective actuators. 
     
     
         26 . The apparatus of  claim 25 , wherein the actuators comprise hydraulic or pneumatic rams, or cam dies. 
     
     
         27 . The apparatus of  claim 14 , wherein one or both of the mating surfaces of the first blank holder and first die member are provided with a replaceable wear plate against which the leading edge of the blank may slide as it is drawn between the first blank holder and first die member. 
     
     
         28 . The apparatus of  claim 14 , wherein one or both of the mating surfaces of the second blank holder and the second die member are textured or otherwise formed to ensure that the trailing edge of the blank can be gripped therebetween without slippage.