Forming contoured shapes

Abstract

 Contoured shapes, especially elongated saddle-section shapes such as helicopter rotor blade erosion shields, are formed from a sheet metal workpiece by providing the workpiece with lateral flange portions that are engaged with respective lateral tools movable in synchronism adjacent to the flanks of an elongate male tool so as to draw the workpiece, by its flange portions, over the male tool and into contiguity therewith. The lateral tools may cooperate with gripper means to grip the flange portions and facilitate stretch-forming. Apparatus for performing this method is also disclosed.

Description

[0001] This invention concerns the forming of contoured shapes in sheet metal, and especially the forming of elongated saddle-section or channel-section shapes in relatively thin sheet metal.

[0002] There are many available techniques for forming contoured shapes from sheet metal, including pressing and stretch-forming. Where, however, these techniques are applied to the forming of elongated saddle-or channel-section shapes, problems arise owing to distortion of the tools, under load, and/or as a consequence of differential thermal effects in the case of hot forming.

[0003] Thus when elongated mating male and female tools are used for pressing thin sheet metal, the tools (particularly the female tool) tend to distort, especially under thermal stress in the case of hot pressing, when clear of one another, with the consequence that when the tools close upon an interposed sheet metal workpiece the latter is subjected to unpredictable non-uniform stresses and galling. This problem is compounded by wear of the tools and by variations in workpiece thickness and section uniformity. Similar problems arise in the case of stretch-forming with the use of male and female tools.

[0004] The practical consequence of these problems is a high reject rate and short tool life. An objective of the present invention is to overcome these problems and to provide a forming technique that is adaptable to the accurate forming of extended saddle- or channel-section shapes having section variations along the length thereof and/or contouring in planes parallel to the long axis of the shape.

[0005] In one aspect the invention provides a method of forming a contoured shape comprising bending a sheet metal workpiece into contiguity with an elongate male tool to form a shape having a saddle- or channel-section, characterised by providing said workpiece with lateral flange portions and engaging such portions with respective lateral tools movable in synchronism adjacent to the flanks of the male tool to draw the workpiece, by its said flange portions, over the male tool and into contiguity therewith.

[0006] The lateral tools may co-operate with means for gripping the flange portions of the workpiece to enable stretch-forming of the workpiece to be satisfactorily accomplished by the said movement of the lateral tools.

[0007] Preferably, the lateral tools move in paths spaced from the flanks of the male tool by a sufficient distance both to accommodate thickness variations in a workpiece and any lateral distortions of the tools along the lengths thereof.

[0008] The lateral tools may move in parallel paths throughout their respective synchronized movements or they may move in paths that are at least in part convergent towards the male tool axis, this latter feature being especially applicable to the forming of contoured shapes of varying section curvature, for instance shapes having aerofoil section.

[0009] The invention also extends to apparatus for performing the method thereof and in another aspect provides shape-forming apparatus comprising an elongate male tool and a pair of lateral tools movable in synchronism adjacent to the flanks of the male tool for engaging flange portions of a workpiece to draw the latter over the male tool and into contiguity therewith.

[0010] In preferred embodiments of the invention the said lateral tools are carried by a common carrier movable relatively to the male tool, for instance by being fixed to one platen of a press the other platen of which carries the male tool.

[0011] In principle the lateral tools may be fixed to such a common carrier so as to move in parallel paths as a consequence of movement of the carrier. Preferably, however, the lateral tools are articulated to such common carrier so that their individual paths of motion may be controlled both by motion of the carrier and by lateral motions applied to the tools.

[0012] Thus in preferred embodiments of the invention the lateral tools are articulated to a common carrier and have camming elements that co-operate with external cam means to displace the tools laterally as the carrier moves relatively to the male tool. The external cam means may conveniently be associated with the male tool so that the paths of the lateral tools are controlled in relation to the position of the male tool. The cam means and the camming elements on the lateral tools may together constitute gripper means for gripping flange portions of a workpiece during the forming thereof over the male tool by the motion of the lateral tools relatively thereto.

[0013] The invention will be further explained with reference to the single Figure of the accompanying drawing that shows, in semi-diagrammatic transverse section, the principal components of apparatus embodying the invention and in the course of forming a workpiece into an elongate saddle-section shape.

[0014] In the apparatus illustrated in the drawing, a male tool 1, that may have a symmetrical cross-section as shown or an asymmetrical (e.g. aerofoil) cross-section and be of substantial length (for instance three metres or more) in the direction perpendicular to the plane of the paper, is supported on a bed plate 2 that may be the platen of a press or a tool support that in turn is mounted on a press platen (not shown). Depending on the construction adopted, the tool 1 may be integral with the bed plate 2, but preferably it is detachably secured to the bed plate 2 to permit exchange of tools 1 and to minimise the overall capital costs of the tool and bed plate combination for forming a particular contoured shape, and also to permit packing or shimming of the tool away from the bed plate at one or more regions along the length of the tool to obtain a required longitudinal contour of the tool 1, for instance to compensate for distortion under forming loads and/or differential thermal effects.

[0015] The apparatus further comprises a pair of lateral tools 3, 4 that are articulated to a carrier 5, for instance by means of hinge pins or end trunnions indicated in outline at 6. The tools 3, 4 have a length corresponding with that of the male tool 1 in the direction perpendicular to the plane of the paper and preferably any hinge pins or trunnions 6 would be arranged only to support the tools 3 loosely in position on the carrier 5, forming thrusts acting between the tools 3, 4 and the carrier 5 during operation of the apparatus being borne by arcuate end surfaces 7 of the tools engaging correspondingly shaped surfaces of the carrier 5.

[0016] The tools 3, 4 have restricted freedom of movement about their respective axes of articulation to the carrier 5. In particular lateral outwards movement of the tools 3, 4 about such axes is restrained by abutments 8, 9 on the carrier 5 and positioned outboard of the tools 3, 4 respectively. The abutments 8, 9 may extend over the whole length of the carrier in the direction perpendicular to the plane of the paper, or the abutments may be localised at intervals along the length of the carrier. They may be welded or otherwise permanently secured to the carrier or they may be detachably fixed thereto, for instance by bolts such as indicated at 10.

[0017] The free ends of the lateral tools 3, 4 are provided with forming nibs 11 shaped so as to be able to co-operate with flange portions 12 of a workpiece and with ramped surfaces 13 on the bed piate 2 adjaeent to the flanks of the tool 1. For clarity only the flange portions 12 and the immediately adjacent portions 14 of the workpiece are illustrated and the apparatus is shown with its components in approximately their respective positions at or near the end of a forming stroke of the tools 1, 3, 4.

[0018] Before describing the operation of the apparatus, it should be noted that in the following description it is assumed that the tool 1 and bed plate 2 are fixed in space and that relative motion between the tool 1 and the tools 3, 4, respectively is a consequence of movement of the carrier 5 towards and away from the bed plate 2 in a direction defined by the nominal axial plane 15 of the tool 1. However, it should be understood that the relative motion could be obtained with a fixed carrier 5 and movable tool 1, or with movement of both the carrier 5 and the tool 1.

[0019] At the commencement of a forming operation with the aid of the illustrated apparatus, the carrier 5 is retracted, upwardly as seen in the drawing, away from the bed plate 2 to an extent allowing the introduction and appropriate positioning of a workpiece, that may be a flat sheet having upturned flange portions 12 but that is preferably a preform that will automatically align itself with the tool 1 when laid thereon.

[0020] To effect a forming operation, the carrier 5 is advanced towards the bed plate 2. The forming nibs 11 of the tools 3, 4 thereby engage the workpiece on each side of the tool 1 and, depending on the initial form of the workpiece, bend this about and into general conformity with the profile of the tool 1. The nibs 11 eventually engage the flange portions 12 of the workpiece so that further movement of the tools 3, 4 draws the workpiece down over the upper part of the tool 1. The abutments 8 and 9 prevent the tools 3, 4 pivoting outwardly to an extent that would allow the forming nibs 11 to disengage from the workpiece and also assure that when the tools 3, 4 approach the bed plate 2 the forming nibs 11 and the outboard regions of the flange portions 12 engaged thereby coact with the ramps 13 of the bed plate so that continued advance of the tools 3, 4 towards the latter causes the forming nibs 11 to move laterally inwards towards the base of the tool 1 while gripping the flange portions 12 between themselves and the ramps 13. Thus the flange portions 12 of the workpiece are sufficiently firmly anchored to the tools 3, 4 as to allow of the generation of sufficient tension in the workpiece inboard of the forming nibs 11 to assure the forming of the workpiece into contiguity with the profile of the tool 1. In general the workpiece will be stretch-formed into exact contiguity with the profile of the tool 1 by the final motion of the lateral tools 3, 4 towards the bed plate 2.

[0021] The carrier 5 is thereafter retracted to withdraw the tools 3, 4 clear of the formed workpiece and to allow the latter to be removed from the tool 1.

[0022] In the typical case, the required workpiece section corresponds with the profile of the tool 1 above a trim line indicated at 16, and the formed workpiece, after removal from the tool 1, would be trimmed to remove its unwanted portions corresponding to those shown below the trim line 16.

[0023] The forming operation may be carried out at ambient temperature or at elevated temperature, as desired and as required by the composition of the metal of the workpiece. For instance in the forming of titanium sheet erosion shields for helicopter rotor blades, the forming operation may be conducted at a temperature of the order of 675°C.

[0024] The method and apparatus of the invention may of course also be applied to forming sheet metal with superplastie properties and under conditions to utilise those properties to produce a formed shape exhibiting no spring-back from the profile of the male tool.

[0025] It will be understood that because the forming nibs 11 of the lateral tools 3, 4 move with substantial clearance relatively to the tool 1 until the last part of the stroke of the tools 3, 4 relatively to the tool 1, any movement of the forming nibs 11 over the surface of the workpiece prior to engagement of the flange portions 12 will not produce galling of the workpiece, even if the tool 1, and/or either or both of the tools 3, 4, exhibits some distortion, along its length, to one side or other of its nominal axial plane; i.e. the plane 15 in the case of the tool 1.

[0026] Moreover the method and apparatus of this invention may be utilised in conjunction with the method and apparatus disclosed in GB-A-2 139 934. For instance, after a basic forming operation performed upon a workpiece by apparatus such as illustrated in the drawing and as described above, the carrier 5 with lateral tools 3, 4 may be withdrawn and replaced by a semi-rigid diaphragm having the desired configuration for, the outer surface of the final workpiece and that is then caused to apply isostatic forming pressure to the workpiece, on the tool 1, to impart a desired final external contour to the workpiece.

[0027] Indeed for certain applications of such combined forming techniques, the lateral tools 3, 4 of apparatus such as illustrated in the drawing might be associated with, or be adapted to act as, a backing member for an inflatable bag member having as its internal wall a semi-rigid diaphragm of the required shape to accomplish a final pressing of the workpiece on the tool 1 after initial forming by movement of the carrier 5 towards the bed plate 2 as described.

[0028] Such a combination of the methods of this invention and of GB-A-2 139 934 would be especially appropriate for the production of shapes having complex and, especially, re-entrant contours.

[0029] The features disclosed in the foregoing description, in the following claims and/or in the accompanying drawing may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims

1. A method of forming a contoured shape comprising bending a sheet metal workpiece into contiguity with an elongate male tool to form a shape having a saddle-or channel-section, characterised by providing said workpiece with lateral flange portions and engaging such portions with respective lateral tools movable in synchronism adjacent to the flanks of the male tool to draw the workpiece, by its said flange portions, over the male tool and into contiguity therewith.

2. A method according to claim 1, wherein said lateral tools co-operate with means for gripping the flange portions of the workpiece to enable stretch-forming of the workpiece to be satisfactorily accomplished by the said movement of the lateral tools.

3. A method according to claim 1 or 2, wherein the lateral tools move in paths spaced from the flanks of the male tool by a sufficient distance both to accommodate thickness variations in a workpiece and any lateral distortions of the tools along the lengths thereof.

4. A method according to any one of claims 1 to 3, wherein the lateral tools move in paths that are at least in part convergent towards the male tool axis.

5. Shape-forming apparatus comprising an elongate male tool and a pair of lateral tools movable in synchronism adjacent to the flanks of the male tool for engaging flange portions of a workpiece to draw the latter over the male tool and into contiguity therewith.

6. Apparatus according to claim 5, wherein the said lateral tools are carried by a common carrier movable relatively to the male tool.

7. Apparatus according to claim 6, wherein the lateral tools are articulated to said common carrier for motion relatively thereto laterally of said male tool.

8. Apparatus according to claim 7, wherein the lateral tools have camming elements that co-operate with external cam means to displace the tools laterally as the carrier moves relatively to the male tool.

9. Apparatus according to claim 8, wherein said external cam means are associated with the male tool so that the paths of the lateral tools are controlled in relation to the position of the male tool.

10. Apparatus according to claim 9, wherein said cam means and said camming elements together constitute gripper means for gripping flange portions of a workpiece.

Drawing