A method of forming metal

The present invention relates to forming sheet metal into complex or compound shapes and particularly to the use of High Energy Rate Forming Techniques (HERF) in such a method.

The high energy forming techniques of the type under discussion use high explosives to form metal. These techniques normally use water or some other suitable fluid as a transfer medium for the mechanical energy produced by the explosives. It has been found that liquids transmit the mechanical energy generated more efficiently than air. Normally the process happens in an open tank. The charge of high explosive detonates in the water a short distance from the sheet of metal to be formed. The explosion causes pressure waves to transmit momentum to the metal and force it against the surface of a hollow die by plastic deformation.

The detonation wave that passes through the exploding charge interacts with the water in two ways. First, it creates in a liquid a shock wave that strikes the metal. The detonation wave also forms a bubble of compressed gas in the water. The bubble expands and contracts repeatedly as it reflects off the surface of the workpiece and sides of the tank, before venting into the air. Though the peak pressure produced by the oscillating bubble is perhaps only 10 to 20 % of the peak shock wave, the bubble's contribution to forming the metal is also significant. The gas pressure lasts longer than the initial shock wave.

Many different materials are used in the dies for explosive forming. Inexpensive dies of zinc alloys, epoxy resin or even hard wood are tough enough to make small numbers of products with limited accuracy. Plaster is used for dies where single use of die is sufficient. Reinforced concrete dies, usually resin coated, are an efficient way to make large parts in small numbers. If a manufacturer wishes to make a lot of parts then the dies must be made of ductile iron or special steels which can be reused many times.

The advantage of these techniques are that large complex or compound curved shapes can be formed without the need for heavy presses and the very expensive conventional metal dies.

These known techniques (see, for example, DE-A-1218986) form the base for the pre-characterising part of claim 1 and generally require a vacuum to be applied between the mould (die) surface and the sheet metal prior to discharge of the explosive, to remove the air from the space that the metal will ultimately be asked to take up. If this is not done the speed with which the plastic deformation of the sheet metal takes place is so fast as to cause a compressed air bubble to form resulting in the distortion of the finished sheet metal and prevention of it flowing into the desired shape of the female die. The application of such a vacuum is simple when moulding small shapes. However, when large complex shapes are to be produced in a relatively rough mould it is difficult to produce the appropriate vacuum required because of the need to obtain a seal between the workpiece and the die surface. This process also adds costs to the process.

The present invention seeks to overcome this problem and provide a method of using the known high energy rate forming techniques without the requirement of applying a vacuum between the mould and the sheet metal to be formed.

According to a first aspect of the present invention, there is provided a method of forming sheet metal, comprising the steps of: <ul id="ul0001" list-style="none"><li>(i) forming a female mould of the desired shape;</li><li>(ii) placing the mould in supporting means extending therearound;</li><li>(iii) positioning adjacent to the mould the sheet metal to be formed into the desired shape; and</li><li>(iv) detonating one or more explosive charges at predetermined location(s) within a liquid medium to cause deformation of the sheet metal and taking up by the sheet metal of the desired shape defined by the female mould;    characterized in that: the mould is of a cage-like construction having spaces therein and defining an envelope of the desired shape; step (iii) comprises lining the mould with the sheet metal; between steps (iii) and (iv), the method further comprises the step of filling the lined mould with the liquid medium; and, during step (iv), the sheet metal takes up the shape defined by the envelope of the mould, whilst air trapped between the sheet metal and the envelope of the mould escapes through the spaces of the cage-like construction without the need for vacuum assistance.</li></ul>

For preference, when forming large items, the sheet metal may comprise several part formed pieces joined to form a single sheet (see, for example, US-A-3757411). Preferably the sheets are joined by welding. If the sheet metal is not liquid impervious, the die is preferably lined inside the sheet metal with a liquid impervious material liner before filling with the liquid medium. Preferably, the mould comprises a plurality of longitudinally extending, closely spaced ribs.

In a preferred method, the inner surface of the mould is coated with a frangible material to provide a smooth surface to the mould by filling the space between the ribs. The frangible material is shattered during the deformation process and expelled with trapped air through the spaces between the ribs.

According to a second aspect of the present invention, there is provided a mould for use in the method in accordance with the first aspect of the present invention, comprising a plurality of longitudinally extending, closely spaced ribs mounted on a cradle formed from a plurality of webs extending transversely of the ribs, wherein the webs are shaped and arranged to support the ribs so as to form the desired internal shape of the mould, and the spacing between the ribs is sufficient to allow the expulsion of air trapped between sheet metal and the mould during deformation of the sheet metal.

The invention will now be described in relation to its application to the production of moulds for round bilge boats. However, it will be apparent to those skilled in the art that the invention is equally applicable to any application requiring formation of complex or compound curves in sheet metal and the invention is not limited to the particular application described.

Presently boats are built from sheet metal (mild steel and aluminium alloy) in a production line sense if they are small (less than 6 metres) and do not have complex or compound curves associated with the plating, ie. less attractive "hard-chine" construction. Alternatively if the vessels are large (greater than 15 metres) and are 'one-off', rather than production line models, they are produced from individually shaped plates welded over a preformed set of boat frames, each panel being independently worked to impart the smooth compound curves necessary for the ultimate round bilge hull and then welded in place over the internal framework. These smooth lines often require the application of plastic putty to camouflage the imperfections in shape (e.g. distortion caused by welding plates) thus adding to cost of the final product. The labour cost and time of construction is substantially greater than the equivalent process of competing fibreglass manufacturers who can lay-up their materials in a female mould and produce uniform smooth hulls repetitively and less expensively. This cost difference is such that metal boats are not an economically viable proposition for round bilge production boats in the 6 - 15 metres, mass market, pleasure or work boat range. Aluminium alloy hulls are even more difficult than steel due to the greater distortion that takes place on welding, requiring a higher level of skilled tradesman.

A preferred embodiment of the invention, by way of example only, will now be described in relation to this particular application and with reference to the accompanying drawings, in which: <ul id="ul0002" list-style="none"><li>Figure 1 shows a pictorial representation of the prior art method of forming sheet metal into complex shapes using high energy rate forming techniques;</li><li>Figure 2 shows a pictorial perspective representation of the die mould according to the present invention;</li><li>Figure 3 shows a cross-sectional view taken on lines 3-3 of Figure 2 illustrating a portion of the sheet panels and frangible material applied to the inner surface of the mould;</li><li>Figure 4 shows a plan view of the die mould with the preformed, curved panels welded in place; and</li><li>Figure 5 shows an end elevation of one preformed panel prior to fitting.</li></ul>

Referring to Figure 1 of the drawings, tank 1 has mounted therein a die 7. The die is supported by container 8 resting on a base 9. The metal sheet to be formed 3 is clamped across the opening to the die 7. The space between the die and the plate 3 is evacuated by means by vacuum pipe 10 extending from the surface of the die to a vacuum pump external of the tank. The tank is filled with water 2 and the explosive 5 with associated detonator 6 is lowered to an appropriate stand-off distance 4 from the upper surface of the plate 3. On detonation of the explosive, the plate 3 is forced into contact with the die surface and takes up the shape of the die. The vacuum prevents the formation of air bubbles during the plastic deformation of the sheet metal and avoids distortion thereof.

Referring to Figure 2 of the drawings, a female die 11 of cage construction is shown. This die consists of a plurality of longitudinally extending ribs 12 each spaced sufficiently from one another so as to allow air to pass through without permitting the deformation of the sheet metal 13 into the voids 15 between said ribs. The ribs are supported in the correct shape by a plurality of upstanding webs 16 extending transversely of the mould and shaped to cradle the die. The webs 16 are mounted on a heavy base 17 to provide a rigid robust construction. The ribs would be typically of 20mm x 20mm cross sectored bright steel strip with approximately 2mm space between each metal rib. The die is preferably of fully welded construction and designed structurally to withstand multiple uses. The die would for preference be located in an isolated environment and mounted in a pit of suitable size and uniformly supported with gravel or blue metal (typically 14-20mm round) and sealed in place with a reinforced concrete cap.

A typical example of the application of the method according to the invention to application of the boat hull would be as follows: <ul id="ul0003" list-style="none"><li>1. Coating the inside of the die cage with a smooth plaster of paris 'wash' 14 sufficient to yield a smooth shell of fragile nature. This plaster wash 14 is disposable and replaced between successive uses of the die mould.</li><li>2. Lining this die of suitable hull shape with preformed, planar curved, half or full width metal panels 18 (typically marine grade Aluminium Alloy [5083-H321]). These panels 18 may be typically between 1200-1400mm wide and of 5mm thickness in a 10 metre long boat. An example of a typical die surface is depicted in Figure 3.</li><li>3. Clamping the panels along the centre and across the die;</li><li>4. Internally welding the side seams 19 of the preformed, planar curved panels 18 using current technology for giving a sound joint in Aluminium plate; These side seams 19 are in contact with the die/mould surface during welding and suffer minimum stress in later forming;</li><li>5. Lining the sheet metal lay-up with a full sized polyethylene liner and filling with water (This step is only necessary if the prewelded shell to be formed into the die shape is not waterproof);</li><li>6. Lowering a frame into the water onto which are mounted strategically placed and sized charges of high explosive (typically PETN (Pentaeythanol Tetranitrate) detonation cord-Cortex) connected in parallel to detonate instantaneously;</li><li>7. Detonation of charge, removal of water/plastic to access full formed boat shell. This process may be repeated if imperfections in the skin dictate a second application of the forming energy;</li><li>8. Frame-up the shell by fitting in metal stringer bulkheads, frames, floors by welding or other suitable fixing means while still supported in the die and then welding on decking, as would a typical fibreglass producer of mass market, round bilge, pleasure boats. Decking may also be advantageously formed using the inventive method.</li></ul>

The use of the method according to the invention in the production of aluminium boat hulls enables economic, low volume production lines to be established. As inexpensive mild steel dies can be used the cost of these dies can be economically amortised over relatively low production volumes and further these dies can be readily modified to cope with hull design changes. The process provides the added advantage of requiring few skilled trademen to produce a uniform product of high dimensional accuracy and precision. Further the use of production line techniques enables the application of other advanced manufacturing techniques such as robotics for welding or spray painting.