A number of items are commonly formed by high pressure impact moulding.

Tablets are a particular such item but other examples include confectionery and soap bars.

Taking tablet manufacture as a particular example, a single press is normally used to form many tablets simultaneously. This is achieved by having a number of individual impact moulding punches each of which impacts upon a corresponding area of the base plate of the press. Typically these impact areas of the base plate are cavities.

Each base plate cavity is filled well with a powder mixed from the various ingredients of the tablet and is formed into a tablet shape by the pressure of impact. The tablets shape corresponds to the shape of the base plate cavity and the shape of the impact end of the moulding punch.

Each impact moulding punch is generally of the form of an elongate rod of circular cross-section, one end of which may be adapted in some manner to be retained by the press mechanism. The end of the punch not retained by the press is used to impact with the tablet. The impact end is shaped as the negative of one face of the finished tablet and may incorporate intricate features which allow a distinct makers name or other identifying mark to be formed either embossed upon or sunken into the face of the tablet.

The large forces experienced by impact moulding punches on a regular basis cause damage to both the body of the punch and in particular the impact end on a relatively short time scale. Individual moulding punches therefore need to be replaced

regularly. The standard process for manufacturing impact moulding punches is wasteful however and might be improved.

The difficulty of manufacture is in providing an impact end of an accurate shape and a high quality finish. Conventional spark erosion does not give the required accuracy of finish without extensive polishing therefore a different method is used to manufacture impact mould punches.

At present the first stage in manufacturing an impact moulding punch is to create a punch with a flat or blank impact end from mild steel. The blank punch end is then impacted upon a tablet template (hob) made from hardened steel. The force of the impact is sufficient to force the impact end of the punch to adopt a shape corresponding to that of the hob. The force of the impact may cause damage to the punch which is often irreparable. These production losses add greatly to the expense of the process.

The resulting impact moulding punches which are repairable are re-machined to make them useable. The first stage is to straighten the punch if it is not straight. Once this has been achieved the element is checked and if necessary, re-machined to ensure that the centre of the shaped area of the impact end is on the centre line of the punch. A moulding punch that is straight and balanced will survive more impacts than one which is not. This checking and re-machining is intricate and time consuming. It adds greatly to the expense of the process.

As a final stage of the manufacturing process once all necessary re-machining is finished the steel of the punch is hardened, to extend its working lifetime. This adds further time and expense to the manufacturing process.

Overall this manufacturing process takes around twenty four work hours per impact moulding punch. This production time is expensive and also limits a producer's ability to rapidly switch production from one type or style of tablet to another. Further expense is generated by the heavy production losses and the cost of replacing hobs which become damaged after repeated use.

It is therefore an object of the present invention to provide a faster, more efficient and less expensive process for manufacturing impact moulding punches.

According to the present invention there is provided a method of forming a workpiece in a desired form including the steps of placing the workpiece in a position to expose a surface thereof for forming and subjecting the exposed surface of the workpiece to electrochemical machining to produce the desired form.

Preferably, the method is used to make an impact moulding punch of the kind having a supporting body with an exposed shaped impact end for impact with a surface to be moulded wherein the said impact end is shaped by electrochemical machining.

This process prevents the moulding punch being exposed to very large forces during manufacture and as a consequence production losses are reduced. Additionally the repair and re-machining of damaged moulding punches can then be avoided making the process quicker and cheaper.

Preferably the body of the moulding punch is formed of a rod with an exposed shaped impact end. Most preferably the non-impact end of the rod may be adapted to be retained securely by a press mechanism.

Preferably, to extend its working lifespan the impact moulding punch is formed from hardened metal. Most preferably the metal is hardened after it has been worked into

a rod shape but before the impact end has been shaped by electrochemical machining. A preferred choice of metal to form the moulding punch is steel.

Alternatively a number of other materials may be used to form moulding punches, in particular, titanium, titanium alloys, tungsten carbide and stainless steel.

Preferably the impact end is shaped such that its centre is aligned with the centre line of the rod forming the body of the moulding punch.

Preferably the electrochemical machining is carried out by the technique described in our co-pending application WO 01/30526 wherein there is provided an electrochemical machining technique wherein a cathode is advanced towards an anodic work piece in the presence of an electrolyte and a current is passed between the cathode and the work piece through the electrolyte so as to cause material to be removed electronically from the surface of the material characterised in that vibratory movement is imposed on the cathode so as to cause the gap between the cathode and the work piece to vary, and the current is also varied.

Preferably a number of different electrolytes are available for use, the particular electrolyte chosen being dependant on the material to be machined. Most preferably electrolytes containing abrasive particles may be used for machining if desired.

Preferably the cathode, or at least the end portion of the cathode is formed as a master tablet template or hob. Advantageously as there is no direct physical contact between the hob and the impact end of the moulding punch whilst the impact end is being formed the hob does not get damaged in the manufacturing process and thereby does not need to be replaced regularly.

After a punch has been used for some time the surface of the impact end is often pitted and blemished in other ways. Electrochemical moulding can be used to controllably and accurately remove thin layers of material and hence it can be used to re- machine the work surface of the impact end of a punch by removing a thin layer of material to leave a smooth and sharply defined surface. Most preferably moulding punches may be re-machined in this manner on a regular basis thereby removing the need to manufacture replacement punches.

Advantageously a manufacturer of tablets may keep in stock a large number of moulding punches which have not yet had their impact ends shaped and thus shape their impact ends by electrochemical machining on an immediate need basis. As it takes only five to ten minutes to shape an impact end using electrochemical machining production can be rapidly switched from one type or shape of tablet to another.

In addition to making moulding punches the method of the present invention may be easily adapted to making other articles as discussed below.

The method may be used to produce deep ribs in plastic injection moulds, which are usually produced at present by spark erosion. Additionally the method may be used for the production, of what are known as sub gates, in the plastic injection moulding industry, these are the holes between runner and cavity through which the molten plastic flows. It is possible with the above method to form the gate in shapes other than the traditional round or'd'shape other forms of mould may also be produced by the above method in particular. Other forms of mould may also be produced by the above method in particular plastic moulds from which confectionery is produced or moulds or dies, for manufacturing teeth as used by drill heads used in oil exploration. This drill heads of this

type are currently made using a carbide die, but because of the speed of the above method it is viable to make them in hard steel but to replace them more often.

The method of the present invention may be used to form cooling holes in turbine blades. These holes are at present difficult to machine repetitively in any other shape than round or rounded, but using the above method can be produced with any desired profile. Other forms that may be produced include the root commonly known as the fir tree root on air foils or blades.

The handle and or the blades and times of cutlery may be produced directly using this method rather than utilising moulds or forging dies. If a suitable cathode, having a reversed form to the item, is used.

The above method may be used to produce any cavity using an electrode that has been produced by sterolithograpy and then electrocoated with a conductive material.

Additionally, medals, coins, jewellery, and watchcases may be produced repetitively without the use of a mould with the above process.

The above method may be applied to the production of rolls of stamps for embossing paper, which are currently etched or engraved or to the forming of extrusion dies, including the lead in. The machining of medical prosthesis, and the machining of orthopedic prostheses of patches of detail commonly pegs or posts that bone can graft, to, to increase the strength of any joint can also be carried out using this method.

The above method may be applied to the forming of special security devices, in which a one off design of peg fits the corresponding hole, thus functioning as a key or to the machining of a nozzle used to extrude material through, in which the nozzle has a complex form that is highly polished on the inner surfaces, in particular a fuel injection

nozzle, or to the machining of impellors for pumping equipment.

The invention will now be described further by way of example only and with reference to the following drawings in which: Figure 1 shows the main features of an impact moulding punch typical of those to be manufactured by the process of this invention; and Figure 2 shows how an impact end is shaped in one end of a moulding punch by electrochemical machining.

Referring to Figure 1 a typical impact moulding punch is formed from a metal rod 1. One end 2 of the rod 1 impacts upon material when in use to force the material to adopt a shape corresponding to the shape of the impact end 2. In Figure 1 the shape of the impact end 2 is a simple rimmed cavity however more complicated shapes may be machined into the impact end if desired. The overall shape of the impact end may be on average convex rather than concave if required.

The other end (not shown) of rod 1 is gripped by the press mechanism when the punch is in use. To achieve this any suitable adaptation of the end of the rod 1 may be made. Typically this might take the form of a groove around the surface of the rod 1.

The process for making an impact moulding punch similar to that shown in Figure 1 is as follows. A rod 1 of circular cross-section is formed on a lathe or by any other suitable technique from mild steel. One end of the rod may then be adapted to be retained by a press by for instance cutting a groove around its surface. The other end of the rod 1 is the impact end 2. At this stage of manufacture it is a flat surface perpendicular to the axis of the rod 1.

The steel forming the rod 1 is now hardened. This allows the rod 1 to withstand both greater instantaneous impact pressures and more impacts than a mild steel rod. As a result the working lifespan of the punch is increased.

Once the hardening process is complete the next step is to use electrochemical machining (ECM) to shape the impact end 2 of rod 1.

Referring to Figure 2 the rod 1 is mounted securely on an ECM apparatus. The impact end 2 which is to be shaped is uppermost. A power source 6 is electrically connected to the rod 1 and the ECM apparatus. The power source 6 holds rod 1 at a positive potential relative to a cathode 4 provided as part of the ECM apparatus.

To shape the impact end 2 the cathode 4 is advanced toward the impact end 2 of the anodic rod 1 in the presence of a flow of an electrolyte 5 and a current is passed between the cathode 4 and the rod 1 through the electrolyte 5 so as to cause material to be removed electrolytically from the impact end 2 of rod 1. The effect of this is to form the impact end 2 into a shape corresponding to the shape of the end portion 7 of the cathode 4. In the case of the punch 1 shown in Figure 1 the impact end 2 has been shaped to contain a simple cavity 3. To improve performance vibratory movement is imposed on the cathode 4 so as to cause the gap between the cathode 4 and the rod 1 to vary. The current passed between the rod 1 and the cathode 4 may also be varied, either in correspondence with the vibration of the cathode 4, or otherwise. When the impact end 2 has been shaped to the desired accuracy the current is switched off and the flow of electrolyte stopped. A particularly suitable ECM apparatus for performing this operation is described in our co-pending application WO 01/30526.

To ensure that the impact end 2 is accurately shaped to impact mould tablets the end portion 7 of cathode 4 is a tablet template or hob. Before the impact end 2 is machined care should be taken to ensure that the centre of the cathode end portion 7 is lined up with the centre line of rod 1. This ensures as far as is possible that the impact end balances the impact forces it experiences during use in the manufacture of a tablet.

The use of ECM to shape the impact end 2 reduces losses of partly formed punches due to bending or cracking during the impact shaping part of the process. It also thereby eliminates the time and expense required to straighten or to re-centre the impact end 2 of the rod 1.

A further advantage of this process is that the cathode end portion 7 (the hob) experiences no direct physical pressure during the shaping of the impact end 2. As a result the wear on the hob is negligible. A single hob may thereby be used to shape a large number of impact ends without deterioration in quality. This means that the expense of manufacturing a number of replacement hobs may be avoided.

It is of course possible to use this ECM process to form moulding punches from materials other than mild steel, for instance titanium, titanium alloys, tungsten carbide or stainless steel. Normally the electrolyte used for the ECM process depends on the choice of Material to be machined. It is even possible to use an electrolyte containing abrasive particles for the ECM process if desired.

The use of ECM to shape the impact ends 2 of pre-hardened rods 1 thereby forming impact moulding punches allows for such punches to be used for tablet manufacture immediately their impact end has been shaped. Punches which are made by the traditional technique require a further twenty four hours machining and hardening

work after the impact end has been shaped before they are ready for use in tablet manufacture.

This means a tablet manufacturer can conveniently install an ECM apparatus in the vicinity of a tablet production line and shape the impact end 2 of impact moulding punches as and when replacements are needed. This saves on the expense of ordering and storing a large number of pre-shaped moulding punches. It also allows a manufacturer the flexibility to machine a new set of punches at very short notice if a new type or design of tablet is to be manufactured.

Another advantage of a process as quick and accurate as ECM is that used punches may have their impact ends 2 re-machined so that they may be reused. The impact end 2 of a used moulding punch often has a multitude of small pits and other minor blemishes in its surface. These pits and blemishes are usually not very deep and as such the controlled removal of a thin layer of material from the surface of the impact end 2 will usually return the moulding punch to mint or near mint condition.

ECM is ideal for this operation as thin layers may be accurately removed from a surface. It is envisaged that regular re-machining of the impact end 2 of a punch can be used as a standard technique to extend the working lifespan of each individual punch and thereby save expenditure on replacement punches.

It is of course to be understood that the invention is not restricted to the details of the above process which is described by way of example only. For instance moulding punches have been described herein relation to tablet manufacture but they may be used in other situations including, but not limited to the manufacture of confectionary or soap bars or for embossing paper or leather products.