WO2010114358A1

WO2010114358A1 - Method for producing an ecm tool and use thereof as a cathode in electrochemical machining of a workpiece

Info

Publication number: WO2010114358A1
Application number: PCT/NL2009/000073
Authority: WO
Inventors: Harm Gerrit Knol
Original assignee: Stork Veco B.V.
Priority date: 2009-03-30
Filing date: 2009-03-30
Publication date: 2010-10-07

Abstract

According to a first aspect, the invention relates to a method for producing a tool (20) which is used as a cathode in the electrochemical machining (ECM) of a workpiece (70) made from metal or a metal alloy to produce a product, which method comprises electroforming a negative (20) of one or more surfaces of the product. A second aspect relates to the use of a tool produced by the method according to the invention as cathode in the electrochemical machining (ECM) of a workpiece (70).

Description

Short title: Method for producing an ECM tool and use thereof as a cathode in electrochemical machining of a workpiece

According to a first aspect, the present invention relates to a method for producing a tool which can be used as a cathode in the electrochemical machining (ECM) of a workpiece made from metal or a metal alloy.

A known method for electrochemically machining (ECM) a workpiece made from metal or a metal alloy comprises the steps of: positioning a work surface of a tool at a distance from the surface of a workpiece to be machined, with an electrolyte being present in the gap between the work surface of the tool and the surface of the workpiece to be machined, - applying a voltage between the tool which is connected as cathode and the workpiece which is connected as anode, as a result of which material is removed from the surface of the workpiece to be machined.

ECM is an electrolysis process, in which metal or a metal alloy is electrochemically dissolved from a workpiece which is connected as anode. The anode forms part of an electrochemical cell, the cathode of which is formed by the tool, which is positioned at a small distance from the workpiece. In this manner, the shape of the tool, that is the shape of that side of the tool facing the workpiece, can, as it were, be imprinted into the opposite side of the workpiece, with the material of the workpiece (the anode) being removed and being taken up in the electrolyte as a result of the applied voltage. The workpiece which has thus been formed is the intended product. The cathodically connected tool, which serves as a die, is not subject to wear. Thus, ECM makes it possible, inter alia, to produce workpieces from metal or metal alloys which are harder than the materials used for the tool. ECM is used, for example, for smoothing surfaces, drilling small holes and forming complex structures.

Since, with ECM, dissolution from the workpiece takes place atom by atom, this technique makes it possible to produce very accurate "imprints" of the tool in the workpiece. The tools themselves are usually produced by a mechanical process, or by means of laser engraving or spark erosion. However, the accuracy of these manufacturing techniques of the tool is inferior to that of the ECM process, in which the respective tool is used. Thus, the accuracy used in manufacturing the tool determines the accuracy of the end product. For example, in practice, the lower limit for the diameter of round small holes or recesses in the workpiece is of the order of magnitude of 30-40 micrometres with a tolerance of ± 2-3 micrometres. Smaller holes, having a diameter of 3-5 micrometres for example, would have the same absolute deviation, which is unacceptable from a practical point of view.

WO-A1 -03/06840 describes a method for the manufacture of an electrode for electromechanically machining a workpiece, the object of which is to produce an electrode with high accuracy and reproducibility at low cost. With this known method, an electrode body is used, which is made from a conductive starting material and has a smooth surface. This electrode body is then covered with a coating made from an electrically insulating material. In one embodiment, this is a plastic, in particular a photoresist, into which a pattern of trenches is made, for example by means of etching, laser-machining or photoresist techniques (in other words using a film, exposure and developer), which trenches usually extend into the electrode body. These trenches are filled with a highly conductive material. Thereafter, the photoresist is removed and replaced by a hard wear-resistant ceramic material. It is said that the combination of the hard wear-resistant ceramic layer and the trenches filled with highly conductive material results in a smooth electrode surface, which is relatively insensitive to mechanical influences. The insulating ceramic material has the dual function of electrically separating the "active" electrode surfaces and simultaneously mechanically protecting the electrode surface.

This known method has the drawback that the ECM tool is formed indirectly. This is due to the fact that, taking a desired shape of the product to be produced by means of ECM as a basis, requires a conversion, for example digitalization, for controlling a laser or producing the film. The precision of the active electrode surfaces is thus determined by this conversion and by the techniques used to produce the trenches by removing insulating material, for example with lasers, inter alia the step size of the relative movement and the influence of heat, and, in the case of photoresist techniques, by the film resolution. These factors result in a shaped end product which may have deviations regarding the desired shape and accuracy. The electrochemical machining conditions themselves also contribute thereto.

It is an object of the invention in a general sense to improve the reproducibility of the desired shape (also referred to as the original shape) of an end product produced by means of ECM. According to a first aspect of the invention, this object is achieved by means of a method for producing a tool which is used as a cathode in the electrochemical machining (ECM) of a workpiece made from metal or a metal alloy to produce a product, which method comprises electroforming a negative of one or more surfaces of the product as a cathode.

According to a second aspect, the invention provides for the use of a tool which has been produced in this way as cathode in the electrochemical machining (ECM) of a surface of a workpiece made from metal or a metal alloy.

A third aspect of the invention relates to a method for electrochemically machining (ECM) a workpiece made from metal or a metal alloy, comprising the steps of: positioning a work surface of a tool at a distance from a surface of the workpiece to be machined, with the gap between the work surface of the tool and the surface of the workpiece to be machined being filled with electrolyte, - applying a voltage between the tool which is connected as cathode and the workpiece which is connected as anode, as a result of which material is removed from the workpiece, with the tool being produced by means of electroforming.

The preferred embodiments to be described below relate to all aspects of the invention.

With the method according to the first aspect of the invention, an ECM tool is produced by means of electroforming. Electroforming is forming and constructing a self-supporting object by electrolysis - also referred to as galvanic forming - through the deposition of metal or metal alloy onto an electrically conductive die in an electroplating bath, which object is then removed from the die. With this technique, atoms are deposited individually on the conductive surface portions of the die from an electrolyte bath, which comprises ions from one or more metals, by passing a current through. This makes it possible to copy work surfaces of tools with complex and detailed structures directly, without loss of accuracy. With the method according to the invention, one or more surfaces of the original of a product to be manufactured are used as a die. This original surface is or these original surfaces are galvanically copied in order to produce an electroformed tool having a work surface which is an exact negative of the surface of the product to be produced by means of electrochemical machining. The work surface is then used to make reproductions of the original in metal or metal alloy by means of electrochemical machining. The original itself can be produced in any desired way. By electroforming the ECM implement based on an original which is thus built up atom by atom, a more accurate negative of the product is directly produced than can be achieved by using the abovementioned prior-art manufacturing techniques. This results in accurate copies of the original being produced by means of electrochemical machining.

Electroforming can be used for producing a work surface for an ECM tool from virtually any metal or metal alloy. Nickel, cobalt, copper and alloys thereof are particularly preferred due to the fact that the galvanic deposition thereof is simple. If desired, a corrosion-resistant top layer, partially insulating layer or, for example, a chromium-containing layer or a layer of a noble metal, may be provided on the electroformed work surface, for example by electroplating. In electroplating, a relatively thin, non self-supporting layer is deposited on the surface by electrolysis in order to change the surface properties thereof, for example corrosion resistance. Advantageously, with such an embodiment, electroforming starts with the application of a corrosion-resistant top layer, partially insulating layer or, for example, a chromium-containing layer or a layer of a noble metal on the initial shape, following which electroforming is continued with another metal or metal alloy, such as nickel or copper. In this way, no detail of the initial shape is lost.

In electrochemical machining, sodium nitrate is preferably used as electrolyte for machining workpieces, with only hydrogen being produced on the cathode (the tool) as a gas, e.g. Me — > Me ²⁺ + 2e ; 2 H₂O +2e^" — > H₂ + 2OH^'. Therefore, there is no wear on the tool. Usually, the gap between the tool and the workpiece will be kept substantially constant and therefore is is advantageous to move one or both with respect to each other in order to prevent the gap from increasing as a result of the fact that material is dissolved from the workpiece which is being machined. Advantageously, the cathode is made to vibrate, which contributes to the replenishment of electrolyte between anode and cathode. The cathode is then gradually moved in the direction of the anode. The applied voltage is a direct current voltage or a pulsed current voltage. The pulses of the current are preferably in phase with the vibration of the cathode.

Using electroforming, tools of various shapes can be produced as negatives for a product to be produced by means of electrochemical machining. Electroforming is particularly suitable for producing panel-shaped tools with detailed surface designs, such as regular or irregular patterns comprising elevations protruding from the plane of a panel, recesses, through-holes, etc. and more particularly for producing complex three-dimensional structures. Using such electroformed tools, it is possible to produce objects having structures which are complementary to the work surface of the tool by means of electrochemical machining. The end products which are produced by means of an electroformed tool via electrochemical machining according to the invention, have a high degree of accuracy of details. Examples thereof comprise perforated screen material, shaving foils, surface structures on rollers which are optionally solid, for example for the purpose of conveying paper, (slotted) grids, coding discs, etc.

In the context of the present description, the term "tool" denotes an implement which is used as an electrode (cathode) in electrochemical machining; "product" denotes an object which is formed from a workpiece made from metal or metal alloy by means of electrochemical machining; "initial shape" is an object which has an exterior or surface portion which corresponds to the original surface of the product to be copied; the rest of the form of the initial shape may deviate from the product; for the sake of clarity, the term "original surface" refers to a surface of the product, whereas the term "exterior" refers to the "initial shape".

Preferably, the method according to the invention comprises the steps of providing an initial shape having an exterior or exterior portion corresponding to the original surface of the product to be produced by electrochemical machining, connecting the initial shape as a cathode in an electroforming bath and eiectroforming the negative of the product as a cathode. The initial shape is placed in an electroplating bath of an electroformable metal or metal alloy and connected as cathode. Metal from the electroplating bath is thus deposited on the conductive surface parts of the initial shape. This deposition is continued until the desired thickness is reached, upon which the growth front covers the entire surface of the initial shape to be copied. The deposition is then detached from the initial shape and is an exact negative of the corresponding surface of the initial shape.

When the original or the initial shape is made from a non-conductive material, a thin electrically conductive coating may be deposited thereon by means of known techniques, such as PVD, CVD. Advantageously, the initial shape or the electrically conductive coating provided thereon is passivated in order to facilitate detachment of the implement which has been electroformed thereon. Examples of suitable passivating agents comprise potassium dichromate, potassium permanganate, chromium or gold.

In a preferred method according to the invention, the original has a panel shape provided with a surface having - preferably separate - recesses. The tool which has been electroformed by means of the method according to the invention, and which is a negative of said panel shape, then has a work surface which is provided with - if applicable separate - elevations. During electrochemical machining, the (separate) elevations again form corresponding recesses or holes which have a shape which is complementary to that of the elevations in the workpiece. In a particular preferred method, the original has through- openings and the method comprises a step of closing one side (outlet/inlet) of the opening. An electroforming die on which the original or the initial shape has been made is a particularly suitable closure means, as is explained below in more detail with reference to the figures. Such a closure is advantageous in order to be able to detach the electroformed tool from the original. In a further preferred method, the recesses or through-openings have, viewed in cross section, a convex surface, and thus the elevations have, viewed in cross section, at least one concave surface, in particular a part of a circle having a constant radius. This bell-mouth shape of the elevations of the ECM implement can easily be detached from the original and from the workpiece. In the context of this description, "in cross section" refers to a cross section in the thickness direction of the panel, which corresponds to the height direction of the elevations and recesses, respectively.

According to a particular preferred embodiment of the invention, the work surface of the tool, in particular the elevations thereof, is provided with a passage duct for electrolyte. The dimensions of the passage duct are so small that they do not leave an "imprint" in the workpiece during electrochemical machining. It is known per se that replenishing of the electrolyte is one of the factors which can positively affect the accuracy and speed of electrochemical machining. With large surfaces, the narrow gap (typically < 1 mm, for example 0.4 mm) between tool and workpiece can render this replenishment more difficult. In this preferred embodiment of the invention, fresh electrolyte is supplied via the passage ducts in the elevations to those regions where material from the workpiece dissolves in the electrolyte. If desired, the tool and/or the workpiece can also be made to vibrate in order to positively affect the replenishment. With a view to correct build-up of pressure of the electrolyte in the passage duct, the latter is also bell mouth-shaped.

With the invention, at least the forming work surface of the ECM tool is formed by means of electroforming. As has been discussed above for panel-shaped tools with separate elevations, these can be formed entirely by means of electroforming.

When the elevations are to have a passage duct which ends in the top thereof in order to be able to replenish electrolyte during use as an ECM tool, no continuous layer is formed, but the conditions (time duration, etc.) are chosen such that the passivated base surface of the initial shape is covered completely, but the recesses or through-openings are not filled completely. On said last positions, openings then remain behind in the deposition which can act as passage duct.

As has already been explained above, the metal from which the deposition is made can be any electroformable metal or metal alloy, in particular copper, and preferably nickel. Suitable electroplating baths comprise nickel-containing or copper-containing baths.

In the electroforming method which has been explained above in detail, a self-detaching product is produced. Complex three-dimensional structures cannot be self-detaching copies. In that case, the original (cf. the abovementioned screen layer), of which the imprint, as it were, is made, can be etched away selectively and/or dissolved, so that the electroformed product can be detached from the electroforming die.

The implement is electroformed from an electroformable metal or metal alloy. The separate elevations or separate recesses, respectively, have a substantially uniform height or depth, respectively, with a tolerance in the order of magnitude of one micrometre. The elevations or recesses can be arranged according to a regular pattern, although an irregular (stochastic) pattern can also be used.

The invention will be explained below with reference to the attached drawings, in which

Fig. 1 shows an example of an electroforming die having a regular pattern of insulating regions of photoresist for use in an electroforming method of a screen product;

Fig. 2 shows an insulating photoresist island in detail;

Figs. 3-6 show a number of steps of a method for manufacturing a screen product, followed by the production of an ECM tool starting from this screen product;

Fig. 7 shows a detail of an elevation in the ECM tool illustrated in Fig. 6;

Figs. 8 and 9 show an embodiment of a method for manufacturing an ECM tool having passage ducts in the elevations; and

Figs. 10 and 11 diagrammatically show an embodiment of an electrochemical machining method according to the invention.

Figs. 1-7 show a method for producing a screen product as an original by means of electroforming (Figs. 1-3), followed by an embodiment of the method according to the invention for producing an ECM tool based on this screen product.

Fig. 1 shows a top view of a planar electroforming die 10, on which insulating photoresist islands 12 with a circular cross section are arranged. The electroforming die 10 comprises a flat panel 14 made from electrically conductive material, on which circular regions 12 from insulating material, such as photoresist, are provided. In other words, in this embodiment, the regions 12 form elevations having a low height on the panel 14. The photoresist islands 12 are arranged according to an orthogonal grid. The cross section of the photoresist islands is, for example, in the region of 75 to 100 micrometres, for example 85 micrometres. The height thereof is in the region of a few micrometres to a few tens of micrometres, for example 5 micrometres, while the distance between two neighbouring photoresist islands of a base form of the orthogonal grid is in the order of magnitude of 150 to a few hundred micrometres, for example 200 micrometres.

Fig. 2 shows a detail of such a photoresist island 12 having a diameter d_r and height h_r.

Fig. 3 shows a portion of the electroforming die 10 from Fig. 1 in cross section. The electroforming die 10 is connected as cathode in an electroplating bath (not shown), for example of nickel. When an electric current is passed through, first of all metal is deposited between the insulating regions 12 on the uncovered conductive regions 16 of the electroforming die 10, and subsequently the insulating regions 12 are also partially overgrown.

Figure 4 shows the electroforming die 10 with resist islands 12 and the screen product 18 deposited thereon in this way. The screen product 18 comprises a network of webs 30 which delimit screen openings 32. The webs 30 have partly grown over the insulating regions 12. The webs 30 have a convex surface 31. In this case, this screen product 18 is the original, a surface of which (in top view of webs 30 and openings 32) is copied according to the present invention. The electroforming die 10 closes the openings 32 on the underside.

After the uncovered surface of the screen product 18 has been passivated, metal, such as nickel, is once again deposited on top of the screen product 18, with the electroforming die 10 with screen product 18 thereon being placed in an electroplating bath and connected as cathode. In this way, in the illustrated embodiment, a continuous metal layer 20 is formed on the screen product 18, filling the screen openings 32 and covering the entire surface. This deposition is continued until the desired thickness (strength) has been reached, and generally also a flat growth front has been formed. Cf. Fig. 5.

Fig. 6 shows this continuous metal layer 20 with a flat underside 50 and an upper side 52 with elevations 22 as an exact negative of the screen product 18 after it has been separated therefrom, that is to say, the ECM tool by means of which accurate reproductions can be made of the screen product 18. The elevations 22 are of the form shown in detail in Fig. 7, with a concave surface 26.

Fig. 7 shows an elevation 22 in detail. The elevation 22 has a circular base 24 and, viewed in cross section, a concave outer surface 26, complementary to the convex surface 31 of the webs 30. This concave outer surface 26 describes a part of a circle with a constant radius r_p. The diameter d_t of the flat top 28 of the elevation 22 is, for example, 15 micrometres, at a diameter of the base of 85 micrometres and a height of 35 micrometres. In other words, growth radius r_p is also 35 micrometres. The tolerances achieved using electroforming are in the range of submicrometres.

Figs. 8 and 9 show a diagrammatic cross section similar to Figs. 5 and 6, but with the screen openings 32 not completely filled, so that holes remain which serve as passage ducts 60 for replenishing electrolyte during electrochemical machining.

Figs. 10 and 11 diagrammatically show an electrochemical machining method according to the invention. The tool with work surface 20 according to Fig. 6 comprising elevations 22 is arranged in an electroplating bath so as to be moveble in the vertical direction and at a short distance from a workpiece 70. The tool is connected as cathode and the workpiece as anode. The bath liquid (the electrolyte) can flow - either forced or not forced - through the narrow gap 72 between work surface 20 and workpiece 70. Material from the workpiece dissolves in the electrolyte. After some time, the machined surface 74 acquires a shape which is complementary to that of the work surface 20 of the tool, as Fig. 11 illustrates. The machined surface 74 in this embodiment comprises recesses 76 at the positions of the elevations 22 of the work surface 20. Depending on inter alia the thickness of the workpiece and the time duration of electrochemical machining, the recesses 76 may extend up to the machined surface 74 and thus form through-openings, similar to the screen openings 32.

Claims

C L A I M S

1. Method for producing a tool (20) which is used as a cathode in the electrochemical machining (ECM) of a workpiece (70) made from metal or a metal alloy to produce a product, which method comprises electroforming a negative (20) of one or more surfaces of the product.

2. Method according to claim 1 , comprising the steps of providing an initial shape (18) having an exterior corresponding to the original surface of the product (70) to be produced by electrochemical machining, connecting the initial shape as a cathode in an electroforming bath and electroforming the negative (20) of the product.

3. Method according to claim 1 or 2, further comprising a step of providing the initial shape (18) with an electrically conductive coating.

4. Method according to one of the preceding claims, furthermore comprising the passiva ^■rttiioonn ooff tthhee initial shape (18) or the electrically conductive coating provided thereon.

5. Method according to one of the preceding claims, in which the original surface comprises a relief structure.

6. Method according to one of the preceding claims, in which the original surface has recesses.

7. Method according to one of the preceding claims, in which the original surface has through-openings (32), and the method furthermore comprises a step of closing a side with openings.

8. Method according to one of the preceding claims, in which the original surface comprises recesses or through-openings (32) which, viewed in cross section, have at least one convex surface portion.

9. Method according to claim 8, in which a convex surface portion comprises an arc of a circle with a constant radius.

10. Method according to one of the preceding claims, in which the work surface of the tool has a passage duct (60) for electrolyte.

11. Method according to claim 9, in which a passage duct (60) is provided in an elevation (22), corresponding to a recess or through-opening (32) in the original surface, in the work surface of the tool.

12. Use of a tool produced by the method according to one of the preceding claims as cathode in the electrochemical machining (ECM) of a workpiece (70).

13. Method for electrochemically machining (ECM) a workpiece (70) made from metal or metal alloy, comprising the steps of: positioning a work surface (20) of a tool at a distance from a surface (74) of the workpiece (70) to be machined, with the gap (72) between the work surface (22) of the tool and the surface (74) of the workpiece (70) to be machined being filled with electrolyte, applying a voltage between the tool which is connected as cathode and the workpiece (70) which is connected as anode, as a result of which material is removed from the workpiece (70), with the tool being produced by means of electroforming.