Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE: A METHOD OF FORMING METAL
TECHNICAL FIELD
The present invention relates to forming sheet
metal into comple~ or compound shapes and particularly
to the use of High Energy Rate Forming Techniques (HERF)
in such a method.
~9E~UND OF THE INYENTION
The high eneryy orming techniques of the type
under discussion use high explosives to form metalO
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
th~t 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 o a hollow die by plastic deformation.
The detonation wave that passes through the
e~ploding 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 tne 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
e~plosive 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 o die is
sufficient. Reinforced concrete dies, usually resin
coated, are an eficient 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
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the need for heavy presses and the very e~pensive
conventional metal dies.
These known techniques 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 ta~es place is so fast as to cause a compressed
air bubble to form resulting in the distor-tion 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 comple~ 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.
DISCLOSURE OF THE INVEN~Q~
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 the present invention
provides a method of forming sheet metal comprising the
following steps:
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forming a female die/mould of the desired shape,
said mould being of cage-like construction,
placing said die~mould in supporting means
extending therearound,
lining said die with the sheet metal to be formed
into the desired shape,
filling said lined die with a liquid medium,
detonating one or more explosive charges at
pr~determined locations within said medium so as to
cause deformation~of said sheet metal and taking up by
said sheet metal of the shape defined by the female die
mould.
For preferénce, in large formings the sheet metal
to be formed may be constructed of several part formed
pieces joined to form a single sheet. Preferably the
sheets are joined by welding. Further if the sheet to
be formed is not liquid impervious the die is preferably
lined inside said sheet metal with a liquid impervious
material liner before Eilling with said liguid medium.
Preferably the mould is constructed of a plurality of
longitudinally e~tending, closely spaced, steel 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 spacings
between said ribs, said frangible material being
shattered during the deformation process and expelled
with trapped air through said spacings between the ribs.
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Ths invention will now be described in relation to
its application to the production of moulds ~or round
bilge boats, however~ it will be apparent to those
skilled in the art that the inve~tion is equally
applicable to any application requiring formation oE
comple~ 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
comple~ or compound curves associated with the plating,
ie. less attractive Uhard-chine'' construction.
Alternatively if the vessels are large ~greater than 15
metres) and are 'one-off', rather than production line
models, th~y are produced rom individually shaped
plates welded over a preformed set o boat frames, each
panel being independently worked to impart the smoo~h
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
r~ompeting fibreglass manufacturers who can lay-up their
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materials in a female mould and produce uniform smooth
hulls repetitively and less e~pensively. 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 dificult than steel due to the greater
,distortion that takes place on welding, requiring a
higher level of skilled tradesman.
BRIEF DESCRIPTION OF DRAWINGS
A preferred embodiment of the invention, by way of
e~ample only, will now be described in relation to this
particular application and with reference to the
accompanying drawings, in which:
Figure 1 shows a pictorial representation of the
prior art method of forming sheet metal into comple~
shapes using high energy rate forming techniques:
Figure 2 shows a pictorial perspective
representation o the die mould according to the present
invention;
Figure 3 shows a cross-sectional view taken on
lines 3-3 of Figure 2 illustrating a portion of the
sheet panels and rangible material applied to the inner
surface of the mould;
Figure 4 shows a plan view of the die mould with
the preformed, curved panels welded in place; and
Figure S shows an end elevation of one preformed
panel prior to fitting.
PREFERRED EMBODIMENT OF THE~ IENTION
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 e~tending from the surface of
the die to a vacuum pump external of the tank. The tank
is filled with water 2 and the e~plosive 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 e~plosive, the plate 3 is
forced into contact with the die surface and takes up
the shape of the dieO 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 e~tending 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 the ribs
13. The ribs are supported in the correct shape by a
plurality of upstanding webs 16 extending transversely
of the mould and shaped to cradIe the die. The webs 16
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are mounted on a heavy base 17 to provide a rigid robust
construction. The ribs would be typically of 20mm
20mm cross sectored bright steel strip with
approximately 2mm space ~etween 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 e~ample of the application of the method
according to the invention to application of the boat
hull would be as follows:
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.
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 e~ample of a typical die
surface is depicted in Figure 3.
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3. Clamping the panels along the centre and across the
die;
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; Thes0 side searns 19 are in contact with the
die/mould surEace during welding and suffer minimum
stress in later forming;
5. Lining the sheet metal lay-up with a full size~
polyethylene liner and Eilling with water ~This
step is only necessary if the prewelded shell to be
formed into the die shape is not waterproof);
6. Lowering a frame into the water onto which are
mounted strategically placed and sized charges of
high explosive (typically PETN .(~entae.ry~itol Tetranitrate)
detonation CO~E~ (a registered ~ademark owned by Imperial
Chemical Industries L~ted) connected in parallel to detonate
instantaneously.
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,
8. Frame-up the shell by fitting in metal stringer
bulkheads, frames, floors by welding or other
suitable fi~ing means while still supported in the
die and then welding on decking, as would a typical
fibreglass producer of mass market, round bilge,
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pleasure boats~ Decking may also be advantaqeously
formed using the inventive rnethodq
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 ew
skilled trademen to produce a uniform product of high
dimensional accuracy and precision. Further the ~se of
production line techniquzs enables the application of
other advanced manufacturing techniques such as robotics
for welding or spray painting.
It will be apparent to those skilled in the art
that the inv~ntion is not limited to the speciic
e~amples described and further embodiments and
e~emplifications of the invention are possible without
departing from the spirit or scope of the invention
described.
