GB2312390A - Techniques and apparatus for cutting a workpiece - Google Patents

Abstract

Using a die 15 having a cutting groove 24 therein, which groove 24 has a relatively sharp first edge 24A and a relatively blunt second edge 24B, a method of cutting a web of material comprises positioning the web over the cutting groove 24 and forcing a portion 53 of the web into the groove 24, bending the web about the first and second edges 24A, 24B, to the extent that the web is caused to divide along the first edge 24A. In the preferred embodiments described, the web is forced into the groove 24 under the action of a pressurised fluid, which fluid comprises an elastomeric material in the form of a membrane 39 pressurised by and impervious to an hydraulic fluid 52.

Description

TECHNIQUES AND APPARATUS FOR CUTTING A WORKPIECE This invention relates to techniques and apparatus for cutting a workpiece, preferably using a high-pressure fluid to exert pressure upon the workpiece in a cutting operation.

Traditional cutting techniques such as sawing, snipping and stamping have been used for many years in the fabrication of articles, which process usually involves the cutting and/or forming of at least one workpiece. More recently, new techniques such as laser cutting have come into large-scale commercial use. Selection of an appropriate cutting technique depends upon various factors such as the shape of the workpiece, the material from which the workpiece is made, the accuracy and finish required, and the production volume of the articles concerned.

The Applicants' business in the fabrication of metal articles requires cutting of numerous workpieces from sheet or tube stock. Such workpieces often have a complex shape requiring precise cuts in two or three axes. For example, a pipe joint such as a manifold may be fabricated from two or more tubular workpieces that have sections cut out of their walls, the cut edges being brought together to define a junction and then being welded together along the junction. It will be appreciated that the cuts must be accurate so as to ensure that the components will fit together and define a continuous junction for welding, yet accurate cuts are particularly difficult to achieve on a tubular workpiece in view of its curved surface.

When contemplating the range of cutting techniques available to them, the Applicants concluded that no existing cutting technique was ideal for their purposes. Existing cutting techniques are variously slow - requiring multiple operations to achieve a desired cut inaccurate, or expensive in terms of capital cost; many of these existing techniques also lack uniformity or repeatability. Furthermore, the components produced by existing cutting techniques are often far from finished: they will usually require deburring and may require further processing steps including forming, polishing and rounding of cut edges. The Applicants therefore resolved to develop their own cutting technique, better suited to their requirements than existing techniques.

As the Applicants' production volumes justify some investment in specialised tooling, they particularly considered variations on stamping which involves the application of pressure to a workpiece or blank held in a die. The Applicants have also looked to highpressure fluids to apply pressure to a blank and are aware of at least one forming technique that employs both aspects, i.e. a high-pressure fluid used to shape a blank within a die.

In that known forming technique, which is disclosed in EP-A-0195157 to Standard Tube Canada Inc., a tubular metal blank is formed into a box-section frame member, for example for a car body. The blank is enclosed within a shaped cavity in a die and is then expanded circumferentially by the application of internal fluid pressure until the external surfaces of the blank conform to the internal surfaces of the die cavity. For this purpose, both ends of the blank are sealed except for an opening that can admit hydraulic fluid into the interior of the blank, the fluid being supplied at such high pressure that the blank expands, undergoing plastic deformation to conform, permanently, to the shape of the cavity.

The forming technique of EP-A-0195157 is an interesting one that is believed to have met with considerable commercial success in its own field, but it is quite unsuitable for the Applicants' purposes because there is no provision for cutting the blank, only for shaping it. The Applicants have noted that the forming technique of EP-A-0195157 will be rendered useless if the blank is intentionally cut through or otherwise fails under the pressure of the fluid within: the fluid would then immediately flow through the perforated wall of the blank to flood the die, thus equalising pressure on the internal and external walls of the blank so that the fluid would cease to deform the blank. Indeed, the essential teaching of EP-A-0195157 is to avoid cracking or yielding of the blank through over-expansion.

Whilst EP-A-0195157 only teaches a forming technique and provides no clues as to how to cut through a blank, a blank will usually require forming at some stage if it is to assume its desired shape. Forming can take place before, during or after a cutting operation, but the Applicants recognise that simultaneous forming and cutting is particularly advantageous for many workpieces. Accordingly, the Applicants have experimented to see if a pressurised fluid might be used to cut a blank within a die whilst optionally also forming the blank within the die. The present invention has resulted from these experiments, although the use of a pressurised fluid is an optional preferred feature of the invention: it is not essential to the invention in its broadest sense.

From one broad aspect of this invention, the Applicants have devised a method of cutting a web of material, comprising positioning the web of material over a groove in a die, said groove having a relatively sharp first edge and a relatively blunt second edge, and forcing a portion of the web into the groove, deforming the web about the first and second edges, to the extent that the web is caused to divide along the first edge.

In this cutting method, deformation about the relatively sharp first edge concentrates stress along the first edge (the desired line of cut) until the web eventually fails and divides precisely along that line. It will be noted, however, that the relatively blunt second edge does not generate such deformation or such stress in the web, and so the web will not divide there. Instead, the portion of the web that entered the groove remains attached to the part of the web lying around the second edge, making it easy to remove all scrap from the die once the cutting operation is complete.

Immediately after division, therefore, a first part of the web - the cut component - is bounded by the first edge and a second part of the web - scrap - includes the portion of the web that entered the groove.

The cutting method of the invention is particularly suitable for metal webs such as sheet or tubing of mild steel or stainless steel. It has been observed that before such a metal web fails during practice of the invention, plastic deformation of the web in the region of the first edge radiuses the cut edge in an advantageous way and that failure of the web then leaves a clean edge, requiring little or no deburring, that is ready for welding or whatever other processing may be required.

Division of the web may be facilitated by restraining the web on opposed sides of the groove to restrict movement of the web into the groove and thus to cause the portion of the web in the groove to stretch as it is forced into the groove. A suitable restraining force can be achieved by pressing the web against support surfaces of the die that lie on opposed sides of the groove.

Whilst not essential in the broadest sense, it is greatly preferred that the web is forced into the groove under the action of a pressurised fluid. The pressurised fluid may usefully be employed to press the web against the aforementioned support surfaces and thus to achieve the desired restraining force.

Whilst "fluid" suggests a liquid or a gas, a fluid may also be a solid but flexible material that behaves as a fluid when under pressure. It is in this sense that the pressurised fluid, or at least the part of the fluid that bears upon the web, is preferably an elastomeric material such as urethane or polyurethane. The elastomeric material may be a membrane, for example a wall of a bladder, pressurised by and impervious to a gas or liquid such as an hydraulic fluid, the membrane confining the pressurised gas or liquid and thus preventing its escape, and the consequent release of pressure, when the web divides.

The invention also encompasses a die for cutting a web of material, the die having a cutting groove therein which groove has a relatively sharp first edge and a relatively blunt second edge.

The cutting groove may be of non-uniform depth along its length, wherein a relatively deep portion of the cutting groove has a depth selected to ensure division of a web forced into the deep portion, and a relatively shallow portion of the cutting groove has a depth selected to avoid division of a web forced into the shallow portion. The depth of the groove can therefore be used to control whether or not a web is cut; cutting may not be desirable along the full length of the groove if it is desired to avoid numerous pieces of scrap.

First and second cutting grooves may be employed in the die, the first groove being relatively wide and the second groove being relatively narrow to determine the pressure at which the web will fail in each groove and thus to control the chronology of the cutting process as pressure is progressively increased.

As the application of hydraulic pressure is the preferred way of forcing a web into the die, the die preferably comprises a recess or cavity having means for receiving a bladder to deform the web and means for receiving hydraulic sealing parts for sealing the recess or cavity.

The invention also encompasses a bladder comprising a membrane for use in appropriate methods of the invention. The bladder is suitably hydraulically sealed to an hydraulic sealing part mentioned above.

Whilst the broad aspect of the invention is principally concerned with cutting techniques, it will be apparent that this aspect encompasses combined cutting/forming techniques because the web can be deformed in the manner of EP-A-0195157 to conform to the surface of the die.

Conversely, another aspect of the invention may be applied to forming techniques that do not necessarily involve cutting. This aspect involves the provision and use of a press for holding together parts of a die, the press having a hydraulic ram acting upon the die, wherein hydraulic pressure is supplied to the hydraulic ram and to the interior of the die from a common source and wherein the effective piston area of the ram is greater than the effective piston area within the die.

A non-return valve may be provided between the common source and the ram, which valve can be released manually or automatically at a predetermined stage in the operation performed in the die.

In order that this invention can be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a perspective view of a tubular metal component apt to be made by the method of the invention, which component is used as an example in the method to be described herein; Figure 2 is a plan view of a half die for use in the method of the invention, the half die being adapted to make the component shown in Figure 1; Figure 3 is a partial enlarged cross-section along line III-III of Figure 2; Figure 4 is a partial enlarged cross-section along line IV-IV of Figure 2; Figure 5 is a partial enlarged schematic cross-section along line V-V of Figure 2; Figure 6 is a part-sectioned plan view corresponding to Figure 2, showing the die holding a tubular blank that will become the component of Figure 1; Figures 7(a) to 7(d) are enlarged partial schematic cross-sections showing how the tubular blank is progressively deformed and eventually cut under the application of pressure; Figure 8 is a perspective view showing a die formation capable of cutting a hole in a blank; and Figure 9 is a schematic perspective view of a press suitable for supporting the half dies described herein.

Referring firstly to Figure 1, a tubular metal component 10 for use in, for example, a manifold consists of a straight-walled circular-section body 11 whose boundaries are defined by three planar lines of cut. A first transverse boundary 12 lies in a plane perpendicular to the longitudinal axis of the body 11 and a second transverse boundary 13 lies parallel to the first boundary 12, the boundaries 12 and 13 defining the overall length of the body 11. The second boundary 13 is interrupted by an angled cut-out 14.

The straight-walled component 10 of Figure 1 is a greatly simplified example adopted merely to illustrate the invention. As has been mentioned and as will become clear, the invention gives scope for forming a blank as well as cutting it: it would, for example, be entirely possible for the body 11 to have longitudinally-curved walls even if produced from originally straight-walled tube stock. The inventive concept may also be applied to generally flat or otherwise non-tubular workpieces. The ultimate shape of the component is limited only by the designer's requirements, within the constraints of the elongation and yield properties of the material.

Figures 2, 3, 4 and 5 depict a die half 15 of hardened tool steel, being one half of a die suitable for forming the component 10. The die half 15 is generally rectangular and contains an elongate generally half-cylindrical recess 16; a corresponding mirror-image die half (not shown) completes the die by being laid over the die half 15 with the recesses combining to form an elongate generally cylindrical die cavity. The die halves are held together in use under high internal fluid pressure and so, for sealing purposes, they each have a flat mating face 17.

Recess 16 has a closed distal end portion 18 embedded within the die half 15 having a part-spherical end 19 adjoining a part-cylindrical wall 20. At the opposite end of recess 16, an open proximal end portion 21 penetrates an end wall 22 of the die half 15. A cutting region 23 lies between the respective end portions 18 and 21 and its partcylindrical surface 29 is interrupted by first, second and third transverse cutting grooves 24, 25 and 26.

The first and second cutting grooves 24 and 25 lie parallel to and spaced from one another in planes perpendicular to the longitudinal axis of the recess 16 at positions corresponding to the boundaries 12 and 13 of component 10. The third cutting groove 26 lies in a plane at an acute angle to, and intersecting, second cutting groove 25, the position and angle of the third cutting groove 26 corresponding to the edge of the angled cut-out 14 of component 10. It will be noted that the first and second cutting grooves 24 and 25 are of substantially the same width as one another, but that the third cutting groove 26 is somewhat narrower than the first and second cutting grooves 24 and 25.

As shown in the cross-sectional view of Figure 3, the first cutting groove 24 is of substantially uniform depth along its length. Reference to the corresponding view of Figure 4, on the other hand, shows that the second cutting groove 25 is of non-uniform depth along its length; second cutting groove 25 has a relatively deep part 27 corresponding to the boundary 13 of component 10 and a relatively shallow part 28 where no cutting is necessary because the line of cut follows the third cutting groove 26.

The deep part 27 of the second cutting groove 25 is of similar depth to the first cutting groove 24.

The first, second and third cutting grooves 24, 25 and 26 have a common feature that is illustrated schematically in the exaggerated cross-sectional view of Figure 5 with reference to the first cutting groove 24. The first cutting groove 24 has a relatively sharp edge 24A, being practically right-angled, and a relatively blunt rounded edge 24B. As a perfectly right-angled edge is not practically achievable and in any event is not essential to the invention, 'relatively sharp' and 'relatively blunt' encompasses arrangements in which one edge has a relatively small radius of curvature and an opposing edge has a relatively large radius of curvature. The 'relatively sharp' and 'relatively blunt' terminology may, however, also embrace edges that are not radiused in the conventional sense but are chamfered or bevelled.

The relatively sharp and relatively blunt edges of the first, second and third cutting grooves 24, 25 and 26 have been annotated with the suffixes A and B respectively in Figure 2.

Completing the reference to Figure 2, the proximal end portion 21 has a part-cylindrical surface 29 bounded at one end by the end wall 22 of the die half 15 and at the other end by a locating groove 30. A frusto-conical locating surface 31 tapers from the locating groove 30 to the cutting region 23, the boundary between the locating surface 31 and the cutting region 23 being marked by a shoulder 32.

Referring now to the cross-sectional view of Figure 6, the die half 15 is shown ready to cut a tubular blank 33 when a second die half is added to form a complete die. The blank 33, typically of mild steel or stainless steel, is generally tubular and of circular cross-section, having a parallel-sided central portion 34 and gently-tapered first and second ends 35 and 36. The central portion 34 of the blank 33 is received within the cutting region 23 of the recess 16 extending along the full length of the cutting region 23 and is of slightly smaller diameter than the cutting region 23, being a loose fit within.

Apart from the blank 33, three further parts are added to the die half 15 in Figure 6, namely a steel cap 37 that occupies the distal end portion 18 of the recess 16, a steel plug 38 that occupies the proximal end portion 21 of the recess 16, and an elongate polyurethane bladder 39 that is carried by the plug 38 and extends along the central longitudinal axis of the recess 16 through the cutting region 23 within the blank 33.

The outer surface of the cap 37 is shaped to fit closely within the distal end portion 18 of the recess 16 and so has a corresponding part-spherical end 40 and a cylindrical skirt 41. The first tapered end 35 of the blank 33 fits snugly into a correspondingly tapered recess 42 within the skirt 41.

Similarly, the outer surface of the plug 38 is shaped to fit closely within the proximal end portion 21 of the recess 16 and so has a corresponding frusto-conical skirt 43 and cylindrical base 44 divided by a circumferential flange 45 that fits into the locating groove 30. A tapered recess 46 within the skirt 43 snugly receives the second tapered end 36 of the blank 33 and a threaded hole 47 within the base 44 provides for the connection of a correspondingly male-threaded hydraulic coupling (not shown) in fluid communication with the interior of the bladder 39.

The tapered ends 35, 36 of the blank 33 may be pre-formed before being inserted into the cap 37 and plug 38 respectively or may preferably be formed upon insertion by being forced into the tapered recesses 42 and 46 of the cap 37 and plug 38. A snug fit between the tapered ends 35, 36 and the cap 37 and plug 38 respectively is desirable so as to confine the bladder 39 when it is pressurised.

The bladder 39 is a hollow flexible moulded membrane that terminates within the first tapered end 35 of the blank 33 at a closed distal end 48. An open proximal end 49 of the bladder 39 is rooted within the plug 38 in sealing engagement effected between an outwardly-facing flange 50 moulded in the bladder 39 at its proximal end 49 and an inwardly-facing shoulder 51 provided in the plug 38 within the base of the skirt 43. A notable advantage of the bladder 39 is that the blank 33 itself does not have to be sealed to retain hydraulic pressure, thus speeding production by easing assembly and disassembly of the parts shown in Figure 6.

To assemble the parts shown in Figure 6, the cap 37 is seated loosely over the first tapered end 35 of the blank 33, the bladder 39 is inserted into the blank 33 until the second tapered end 36 seats loosely within the skirt 43 of the plug 38, the parts are placed into the recess 16 of the die half 15, and the cap 37 and the plug 38 are settled into their respective locations. The second die half is then closed over the first die half 15 and the two die halves are held together in a suitable jig or press ready for the application of internal hydraulic pressure by hydraulic fluid introduced through the hole 47 in the plug 38.

The hydraulic fluid may be of any suitable composition. Good results have been obtained with an emulsion of 95% water and 5% oil, the oil being present to lubricate the hydraulic pump that pressurises the fluid.

The effect of a progressively-increasing application of hydraulic pressure upon the blank 33 will now be described.

The bladder 39 quickly expands, filling all available free space within the die, until it bears against the internal surface of the blank 33. Increasing hydraulic pressure causes the blank 33 to expand circumferentially, first elastically and then also plastically, until the blank 33 encounters the internal walls of the die. Further plastic deformation under pressure exerted through the flexible bladder 39 causes the blank 33 to adopt the shape of the cavity.

Whilst the example shows only a straight-sided die producing a straight-sided component 10, it will be clear to those skilled in the art that the shape of the die can be adapted to produce almost any shape within the constraints of the elongation and yield properties of the material, thus forming the blank 33 into a different shape in addition to merely expanding it.

So far, the present invention follows the basic principles of the known forming technique disclosed in EP-A-0195157, with the exception of the bladder 39. However, the present invention now steps beyond EP-A-0195157 in that continued deformation at the first, second and third cutting grooves 24, 25 and 26 is used to cut the blank 33. Figures 7(a) to 7(d) show the deformation and eventual failure of the blank 33 at the first cutting groove 24, which is also representative of the cutting action at the second and third cutting grooves 25 and 26.

Figure 7(a) shows the process at the point where the blank 33 has expanded circumferentially under the action of hydraulic fluid 52 exerted through the bladder 39 and has just encountered the internal wall of the die on respective sides of the groove, bridging the groove with a web of metal that has a central portion 53 suspended over the groove by first and second marginal portions 54 and 55 in contact with the internal wall of the die. At this stage, the web is forced with a pressure of, say, 2,500 psi against the internal wall of the die and so is subject to great frictional forces that resist slippage of the marginal portions 54, 55 of the web with respect to the die.

Increasing hydraulic pressure initiates the situation shown in Figure 7(b) and continuing in Figure 7(c) where the web bulges into the groove under the influence of the bladder 39. As the marginal portions 54, 55 of the web are effectively clamped against the die and so are substantially unable to move into the groove, the web thins as it bulges into the groove and becomes apt to fail under the tensile stress borne as it stretches.

As the bulging web is bent around the sharp edge 24A and the blunt edge 24B, deformation about the sharp edge 24A concentrates stress along that edge and causes the web to neck as it approaches failure as shown in Figure 7(c). The web fails along the sharp edge 24A soon afterwards, dividing precisely along the edge 24A.

An hydraulic pressure of between 10,000 and 20,000 psi has been used to cut mild steel and stainless steel, the precise pressure depending upon factors such as the nature of the blank (especially its gauge, diameter and yield strength) and the width of the cutting grooves. The cutting grooves are preferably at least 4 times wider than the thickness of the blank wall (and more preferably 4 to 8 times wider) and, to ensure cutting, preferably have a depth in excess of half their width. The depth required to ensure cutting will, however, depend to a great extent upon the material from which the blank is made: a shallower groove may suffice for a relatively brittle material whereas a deeper groove may be required for a relatively ductile material.

Figure 7(d) shows the web after it has failed. It will be noted that the web has not divided along the relatively blunt edge 24B in view of the lower deformation and stress created there. Instead, the central portion 53 of the web has been forced by the bladder 39 into the bottom of the groove 24 but rcmains attached to the second marginal portion 55 of the web. The central portion 53 of the web constitutes scrap and its removal is aided by remaining attached to the second marginal portion 55 which is also scrap.

The first marginal portion 54 of the web forms an edge portion of the finished component 10. Figure 7(d) shows that the aforementioned necking of the web in the region of the sharp edge 24A before failure has the effect of radiusing the cut edge 56.

This is advantageous because it leaves the component 10 with a clean edge requiring little or no deburring, ready for welding or whatever other processing may be required.

It will be noted that the bladder 39 prevents the hydraulic fluid 52 contacting the blank 33 and that the bladder also retains the hydraulic fluid 52 when the web fails.

When the cutting operation has been completed, hydraulic pressure is released, the halves of the die are separated and the component 10 is removed from the die. The scrap portions previously forming the respective ends of the blank 33 can be removed readily from the cap 37 and the plug 38 by virtue of their tapered contacting surfaces.

Where cutting is required, the depth of each cutting groove is selected as being sufficient to ensure that the web will fail before it bears against and is supported by the base of the groove. Cutting will not always be required, however: a groove may be used to anchor a portion of a blank and so to prevent flow of material into an adjacent cutting groove, or a portion of a cutting groove may be adapted to ensure that a blank is not cut at that location.

An instance where cutting will not be desirable is where multiple pieces of scrap might otherwise be generated. To make it easier to dispose of scrap from the process, it may be desired to leave a scrap portion of the blank attached to another scrap portion even if the two portions are separated by a cutting groove. This is why the second cutting groove 25 has a relatively deep part 27 and a relatively shallow part 28. The deep part 27 is deep enough to ensure failure of the blank 33 along the deep part 27 of the second cutting groove 25 whereas the shallow part 28 is shallow enough to ensure that the blank 33 will merely deform, and not fail, along the shallow part 28 of the second cutting groove 25. In this way, the small scrap portion left after cutting along the third cutting groove 26 remains attached to the larger scrap portion left after cutting along the second cutting groove 25, and can be removed and subsequently handled in one operation.

The width of the cutting grooves 24, 25 and 26 can also be tailored to determine the pressure at which a web lying across them will fail. A narrow groove will provide more support for the web than a wide groove and so, as hydraulic pressure is progressively increased, a similar web will fail sooner at a wider groove than at a narrower one. This raises the possibility of using a combination of differently-sized grooves to control the progress of the cutting operation. It is for this reason that the third cutting groove 26 is narrower than the second cutting groove 25; the web will fail first at the second cutting groove 25 and then at the third cutting groove 26 as pressure is increased. The aim of this is to ensure that the web will not slip during the intensive cutting operations at this part of the blank 33; the web will grip the wall of the die after failure along the second cutting groove 25 and will thus be supported to avoid slippage during the subsequent failure along the third cutting groove 26. This aim is helped by engagement of the web with the pointed formation 57 defined by the inlersection of the second and third cutting grooves 25 and 26 as shown in Figure 2, but relies mainly upon friction between the web and the die.

The techniques of using differently-sized cutting grooves and/or web-engagement formations have been described herein purely for illustration but can be adapted where necessary to suit any particular blank, particularly those with complex three-dimensional curvature where firm frictional contact between the blank and the die is difficult to achieve.

Figure 8 shows that a cutting groove need not be a straight groove and indeed can be endless, e.g. curved back on itself to form a circle, an oval or the like if it is desired to cut a hole in a blank. The groove 58 shown in Figure 8 is circular, having a relatively sharp outer edge 59 and a relatively blunt inner edge 60 defined by a central upstanding pillar 61. After cutting, the edge of the hole will be defined by the relatively sharp outer edge 59 of the groove 58, and a piece of scrap will lie over the pillar 61.

To facilitate removal of the piece of scrap after the hole is cut, the pillar 61 preferably tapers towards the inner edge 60 but may alternately be retractable into the die so as to disengage from the piece of scrap.

It has been found that at the great pressures used in the invention, the bladder 39 effectively becomes a fluid because it fills its container and conforms to its shape. The bladder 39 therefore restrains the hydraulic fluid 52 only in the

A surprising and beneficial effect of the invention may also be due to the surface deformation that has been observed. Specifically, it has been found that the finished component does not spring back appreciably from the walls of the die when pressure is removed, and that the dimensions of the die therefore substantially determine the dimensions of the finished component without a normal allowance for the release of elastic deformation. Why this should be is not clear; possibly, plastic deformation at the surface and indeed throughout the metal layer might have the effect of 'latching' the elastic deformation of the blank and preventing its release. This, however, is speculation on the Applicants' part and is not to be taken as limiting the invention.

Referring finally to Figure 9, a press 62 for holding together two die halves 15 comprises parallel generally vertical uprights 63 connected by parallel generally horizontal beams 64. The beams 64 are located in cut-outs 65 in the uprights 63 and are further located by bolts 66 extending through the uprights 63.

The assembled die halves 15 lie on the lower beam 64 and are pressed together by a plate 67 on the underside of the upper beam 64, the plate 67 being acted upon by hydraulic rams 68 (only one shown) set into the underside of the upper beam 64. Rods 69 attached to the corners of the plate 67 extend through the upper beam 64 in sliding location and are urged upwardly by springs 70 to lift the plate 67 clear of the die halves 15 when hydraulic pressure is released.

The rams 68 are supplied with hydraulic pressure from the same source (not shown) that supplies the bladder 39 within the die and so receive the same pressure as the bladder 39. Thus, by ensuring that the effective piston area of the rams 68 pressing downwardly is always greater than the effective piston area of the bladder 39 pressing upwardly, the force holding the die halves 15 together will always exceed the force that tries to separate the die halves 15.

A non-return valve may be provided in the hydraulic line (not shown) supplying the rams so as to guard against any reductions in the pressure of the hydraulic supply during a cutting operation. The valve can be released by an operator, or automatically, when the cutting operation is over.

Many variations are possible without departing from the inventive concept.

For example, it is entirely possible and desirable for a die to cut more than one component at any one time, possibly cutting more than one component from a single elongate blank using a single bladder.

In another variant, the die halves can be separated slightly while maintaining hydraulic pressure within the die. The resulting gap between the die halves effectively constitutes a deep groove at which the blank will fail under continued hydraulic pressure, thus splitting the blank along the junction between the die halves should that be desired. The die halves could have non-planar mating faces, for example to split the blank along a line other than a straight line.

Hydraulic pressure need not always be increased progressively: it could be created explosively, for example in the form of a shock wave.

In the broadest aspect of the invention, pressure need not be applied hydraulically. For example, the bladder could be replaced by a solid but flexible rod of elastomeric material that is forced into the die cavity by a jack or similar device.

Also, whilst the invention has particular advantages for tubular workpieces, it is not limited to tubular workpieces and may be readily applied to, for example, generally flat workpieces.

Many other variations are possible within the inventive concept. Accordingly, the scope of the invention should be determined with reference to the accompanying claims and other general statements herein rather than the foregoing specific description.

Claims (28)

1. A method of cutting a web of material, comprising positioning the web of material over a groove in a die, said groove having a relatively sharp first edge and a relatively blunt second edge, and forcing a portion of the web into the groove, bending the web about the first and second edges, to the extent that the web is caused to divide along the first edge.

2. A method according to claim 1, wherein the web is restrained on opposed sides of the groove to restrict movement of the web into the groove.

3. A method according to claim 2, wherein the web is pressed against support surfaces of the die that lie on opposed sides of the groove.

4. A method according to any preceding claim, wherein the web is forced into the groove under the action of a pressurised fluid.

5. A method according to claim 4 when appendant to claim 3, wherein the web is pressed against the support surfaces under the action of the pressurised fluid.

6. A method according to claim 4 or claim 5, wherein the pressurised fluid consists of or comprises an elastomeric material.

7. A method according to claim 6, wherein the elastomeric material is a membrane pressurised by and impervious to an hydraulic fluid.

8. A method according to any preceding claim, comprising positioning the web over first and second grooves in a die, each groove having a relatively sharp edge and a relatively blunt edge, forcing the web into the first groove to cause the web to divide along the sharp edge of the first groove, and forcing the web into the second groove to cause the web subsequently to divide along the sharp edge of the second groove.

9. A method according to claim 8, wherein the first groove is relatively wide and the second groove is relatively narrow.

10. A method according to claim 9, wherein the same pressure is applied to the web over the first and second grooves.

11. A method according to any of claims 8 to 10, wherein a portion of the web is engaged by the die for the first time after the web enters the first groove but before the web is divided along the sharp edge of the second groove.

12. A method according to claim 11, wherein the portion of the web is engaged with a pointed formation of the die.

13. A method according to any preceding claim, wherein the surface of the web that contacts the die is deformed to match the surface finish of the part of the die that is contacted by that surface of the web.

14. A method according to any preceding claim, wherein the web is formed into a different shape immediately before or during the cutting operation.

15. A die for cutting a web of material, the die having a cutting groove therein which groove has a relatively sharp first edge and a relatively blunt second edge.

16. A die according to claim 15, wherein the cutting groove is of non-uniform depth along its length.

17. A die according to claim 16, wherein a relatively deep portion of the cutting groove has a depth selected to ensure division of a web forced into the deep portion, and a relatively shallow portion of the cutting groove has a depth selected to avoid division of a web forced into the shallow portion.

18. A die according to any of claims 15 to 17 and having first and second cutting grooves, the first groove being relatively wide and the second groove being relatively narrow.

19. A die according to any of claims 15 to 18, wherein first and second cutting grooves intersect.

20. A die according to any of claims 15 to 19, comprising a recess or cavity having means for receiving a bladder to deform a web and means for receiving hydraulic sealing parts for sealing the recess or cavity.

21. A bladder comprising a membrane for use in the method of the invention as defined in claim 7.

22. A bladder according to claim 21 and being hydraulically sealed to an hydraulic sealing part as recited in claim 20.

23. A press for holding together parts of a die, the press having a hydraulic ram acting upon the die, wherein hydraulic pressure is supplied to the hydraulic ram and to the interior of the die in use from a common source and wherein the effective piston area of the ram is greater than the effective piston area within the die.

24. A press according to claim 23, wherein a non-return valve is interposed in the supply between the common source and the ram.

25. A component produced by the method of the invention or by use of the die, bladder or press of the invention.

26. A method of cutting a web of material, substantially as hereinbefore described with reference to or as illustrated in any of the accompanying drawings.

27. A die for cutting a web of material, substantially as hereinbefore described with reference to or as illustrated in any of Figures 2 to 7 of the accompanying drawings.

28. A press for holding together parts of a die, substantially as hereinbefore described with reference to or as illustrated in Figure 9 of the accompanying drawings.