This invention relates to methods and apparatus for electrochemical machining.

Electrochemical machining (ECM) is a technique used primarily in the production of articles which are of such complicated shape that they are difficult to produce by other methods. It usually involves the erosion of a metal workpiece connected as anode in an electrolyte by approaching a shaped cathode.

In International Patent Application No. PCT/GB90/01622 and also in British Patent Application No. 9022996.4 is described the use of ECM in novel ways to produce etched articles by the selective disposition of a resist on the workpiece. The etching may produce surface features, holes, outlines and fold lines, the latter enabling three dimensional articles to be produced by bending an electrochemically machined plate or foil. In these earlier applications, the technique is described particularly in connection with the rapid production of urgently-needed, custom-designed individual articles such for example as osteosynthetic plates.

The present invention provides methods and apparatus by means of which the technique of ECM can be extended to further engineering applications.

The invention comprises a method for electro¬ chemical machining in which a prepared workpiece connected as anode moves past a cathode with an intervening electrolyte.

The electrolyte may move with the workpiece - whether the electrolyte moves with the workpiece or not affects the way in which material is removed from the workpiece. When there is relative movement between workpiece and electrolyte there will be a distortion in the way metal is removed relative to the way the workpiece has been prepared.. The electrolyte may be made viscous, as by being made as a gel or a paste, the better to be carried along with the workpiece. In conventional electrochemical machining, of course, the cathode and the workpiece do not move relatively, except for any approaching of the one o the other as more of the workpiece is eroded, while the 'electrolyte moves, often at high speed between them.

The electrolyte may be held under pressure,say from two or three to ten atmospheres being advantageous; in the ECM process hydrogen gas bubbles off at the anode and can interfere with the electrolysis process pressurising the electrolyte holds the gas in solution and suppresses the bubbles as well as elevating the boiling point of the electrolyte _r further suppressing

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bubbles due to heating of the electrolyte as a result of the high energy involved in the electrolysis.

The electrolyte may be moved past the cathode - the cathode could, of course, alternatively move through the electrolyte with the anode workpiece. The cathode may however extend along an elongate path along which the workpiece moves and be apertured therealong and the electrolyte removed and refreshed via the apertures. Electrolyte flow may be controlled and constrained by flow guides and baffles.

The workpiece anode may be of flat material and grooved and/or perforated by the electrochemical machining suitably for bending into a three dimensional article.

The workpiece may be prepared for electrochemical machining by selective disposition of a resist. An overall resist may be selectively removed. The resist may be a photoresist and be selectively removed by the usual methods therefor, or a plastic, rubber or wax resist may be removed by vaporisation e.g. by a laser. A self- standing resist, possibly in the form of a plastic film or sheet may be selectively apertured as by a laser and then applied to the workpiece.

A succession of workpieces may be moved past the cathode in a continuous production process, or an elongate strip workpiece with individual workpiece areas or even with an elongate continuous or repeating pattern prepared thereon may be moved continuously past the cathode.

A flat workpiece anode may be electrochemically machined on both faces by passing between two cathodes - thus fold lines and surface relief may be formed together with apertures.

The process can be used to manufacture accurate, complex articles in a wide range of metals and alloys, and can as easily handle thin delicate foils of say 5 μm in thickness to sheet metal of say 2 mm thickness.

The invention also comprises apparatus for electrochemical machining comprising a cathode in an electrolyte vessel and workpiece moving means adapted to move a workpiece connected as anode past the cathode.

The cathode may extend along an elongate path along which the workpiece moves and be apertured therealong and electrolyte circulation means provided to remove and refresh electrolyte via the aperturing. The cathode may comprise a plurality of segments spaced

apart along said path, the spacing between segment forming said aperturing.

The apparatus may comprise cathode coolin means, which may comprise coolant ducting in .the cathod and coolant circulation means.

The electrolyte vessel may comprise a pressure vessel and may comprise sealed workpiece inlet and outlet means.

Said electrolyte circulation means may comprise filter means.

The invention also comprises articles made by a method or apparatus as herein described.

Embodiments of apparatus and methods for electrochemical machining will now be described with reference to the accompanying drawings, in which :-

Figure 1 is a diagrammatic representation of a first method and a first embodiment of apparatus;

Figure 2 is a diagrammatic representation of a second method and a second embodiment of apparatus;

Figure 3 is a diagrammatic section showing the ECM process in operation;

and Figure 4 is a section through a typical electrolyte vessel showing associated equipment.

The drawings illustrate methods and apparatus for electrochemical machining in which a prepared workpiece 11 connected as anode moves past a cathode 12 with an intervening electrolyte 13.

The electrochemical machining takes place in an electrolyte vessel 14, more particularly shown in Figures 3 and 4, which is a pressure vessel with sealed inlet and outlet apertures 15,16 for the workpiece. The cathode 12 is comprised of a plurality of segments 12a with intervening spaces constituting ^* apertures through which electrolyte 13, which may be acidic or alkaline, is circulated by a pump 17 with a filter 18 in the circuit. Depending upon the rate of removal of metal from the workpiece, it may be desirable not to recirculate used electrolyte but to allow same to run to waste to be constantly replaced by fresh electrolyte. The electro¬ lyte 13 is circulated (or recirculated) so as to move with the workpiece 11 and may be viscous, e.g. a gel or a paste, the better to move with the workpiece 11 in

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order to avoid distortion in the way material is removed from the workpiece by relative movement between workpiece and electrolyte. Flow guides or baffles 20 with seals 20a guide the electrolyte to and from the workpiece 11, which, in Figure 3, is shown being progressively eaten away and eventually through as the process progresses.

Coolant 19, e.g. water,, is circulated through ducting 21 in the cathode - high currents, e.g. 50-150 amps, are typical in ECM operations. Cooling, to keep temperatures down, and pressurisation to elevate the electrolyte boiling point and increase gas solubility, suppress the formation of gas and vapour bubbles which interferes with the electrolysis.

Air and liquid seals 15,16 are capable of sealing the inlet and outlet of the vessel 14 at the working pressure of up to 10 atmospheres. Last traces of electrolyte are removed by a water rinse/squeegee arrangement 10.

In Figure 4, manifolds 42,43 for electrolyte and coolant correlation respectively are shown together with busbars 44 connecting the cathode segments 12a to the negative pole of the d.c. supply.

Figure 1 illustrates an arrangement in which individual workpieces 11a are carried through the vessel 14 on a conveyor arrangment 22 which is conductive or includes a conductor arrangement to carry current to the workpieces 11a as metal contact rollers 41, Figure 4.

The workpieces 11a are prepared for ECM in any of a variety of ways, of which three are illustrated in Figure 1. A double-sided resist coated workpiece lla(l) has a removal pattern imposed on it as by a laser arrangement 23 controlled by a computer aided design arrangement 24; for a photo-resist, the resist is removed by chemical treatment represented at 25, or a plastic or wax coating can be removed by vaporisation by the laser arrangement 23. Or a loose resist barrier arrangement, shown as separate self-standing film members 26 can be apertured by a laser arrangement 27 again under CAD control and then be fitted to a metal workpiece lla(2).

After the ECM step, the resist is removed, whether by dissolving the same or by peeling off a self-standing film layer and the. processed workpiece washed at stage 28. The processed workpiece may then be electropolished and further operated on as by removal of cut-out pieces, bending along fold lines, locating tabs in slots or otherwise as appropriate.

Figure 2 illustrates a method in which an elongate strip workpiece lib is unrolled from a reel 31 and run through straightening rollers 32 to the electro¬ lyte vessel 14 and connected as an anode of the electro¬ lytic circuit by rollers 41. Plastic film resist 33 is unrolled from reels 34 and passed by laser vaporising devices 35 controlled by the CAD arrangement 24.

After the ECM step, the film is peeled away, and the machined strip electropolished, washed and further operated on as before.

The invention is not limited to the production of flat workpieces, of course, though it will find many applications in that regard. ECM is suitable for machining materials for which other machining methods are unsuitable and such materials may be fashioned in different ways using the invention, for example grooves, apertures and channels of complex cross section can be produced by moving a rod, tube, extrusion or formed plate past a suitably shaped cathode.

The process can be used for the rapid and economical production of single components or of small batches or of large production runs from strip material. It will be of particular benefit in connection with "just-in-time" production methodology and, by storing

designs in the computer memory, can avoid the need to maintain high cost inventories of press tools and finished components.

The most economical way of using a press to produce pressed-out components to a number of different shapes which are to be assembled into a structure - a gas turbine engine say - is to have long production runs on each component before changing the tooling to produce another component. All the components thus produced must be put in store and drawn out in ones and twos whenever needed. With the present invention, on the other hand, the components can be produced in ones and twos as required; to produce a different component involves only a few keystrokes on a computer terminal, and the inventory problem is avoided as also are delays occasioned by the store being out of particular components.

Moreover, rapid response can be made to design changes, say in development of a piece of machinery, and particularly in those metals which because of high tensile strength, high hardness, rapid work hardening, high abrasiveness causing rapid tool wear, or specific crystal structures which can be altered by normal metal proces- sing techniques and so on. Such metals include complex and expensive alloys of nickel, cobalt, tungsten,

vanadium and chromium designed for high temperature applications, titanium alloys, and the nickel-titanium alloys with shape memory properties, duplex structure alloys of titanium and stainless steel, complex alloys produced by powder metallurgy, isostatic pressing and hydrostatic extrusion which are not amenable to conventional machining processes, and the glassy- amorphous metal alloys produced in strip form by super- quenching direct from molten metal.

The process can also be used to produce components from sheet pretreated as by heat treatments such as annealing, surface hardening, through hardening and so on which, using conventional machining methods, would have to be treated afterwards. Into this category fall components such as springs, bimetallic switches and matching keys and lock tumblers.