METHOD FOR PRODUCING A TOOL ELECTRODE AND TOOL ELECTRODE

Description

[0001] The invention lies in the field of mechanical engineering and is particularly advantageously applicable in the field of mold and tool making. Spark erosion machining, for example spark erosion countersinking or cutting as well as grinding, are machining methods that are frequently used in mold and tool making, but also in other areas of application, for example as an alternative or in addition to milling or turning. In contrast to the aforementioned machining processes, the spark erosion machining method is essentially independent of the mechanical material properties of the workpiece, such as strength, hardness, wear resistance or toughness. Spark erosion is therefore used in particular for machining difficult-to-machine or non-machinable materials, such as highly tempered tool steels, nickel-based alloys or similarly hard materials.In principle, however, spark erosion can be used on a wide range of electrically conductive materials.

[0002] In principle, during spark erosion in a dielectric, the material to be machined is removed thermally and without mechanical impact using very low process forces. The removal behavior is determined or co-determined by, for example, the melting temperature, the evaporation temperature, the thermal conductivity, and the heat capacity of the material to be machined, as well as by the density and atomic binding energy of the material.

[0003] During electrical discharge machining, particularly during countersinking or drilling, the tool electrode represents the negative form of the recess to be created, with the shape of the tool electrode being transferred into the workpiece. The tool electrode moves with its front surface toward or into the workpiece, with a frontal working gap within a target value range, for example, between 3 micrometers and 50 micrometers, between the front surface of the tool electrode and the base surface of the recess in the workpiece being controlled based, among other things, on the ignition delay time. The feed rate of the tool electrode depends on the progress of the removal process.

[0004] Electrical discharge machining can be performed in several coarse and fine machining steps (roughing steps, finishing steps) to achieve the desired roughness values ​​and a high dimensional and shape accuracy of the recess to be created (e.g., less than 10 micrometers). During machining, local temperatures in the range of 4000°C to 20000°C can be reached.

[0005] The material removal particles created during machining in the area of ​​the front surface of the tool electrode and the base surface of the recess in the workpiece are transported away by the flowing dielectric fluid surrounding the tool electrode and the workpiece via the working gap that forms between the front surface of the tool electrode and the base surface of the recess, as well as the side gaps between the outer surfaces of the tool electrode and the side surfaces of the recess in the workpiece. Due to the electrical conductivity of the material removal particles, electrical bridges can form between the tool electrode and the workpiece, leading to short circuits in undesirable locations.In addition, the physical effect of induction in the gap regions with removal particles leads to local changes in the electric field, so that the dielectric strength of the dielectric can be exceeded not only in the working gap at the front of the tool electrode but also at the side gaps. The resulting discharges lead to process instabilities, increased tool wear, and the increased formation of a white surface layer on the workpiece. This layer is caused by the thermal removal process and is subject to residual stress and roughness, and may contain foreign particles from the tool electrode and reaction products of the dielectric.

[0006] As the machining depth, i.e., the feed depth of the tool electrode, increases, the frequency of these secondary discharges in the area of ​​the side gap increases. This also leads, for example, to a limitation of the aspect ratio of the recesses to be created in the workpiece (ratio of the recess depth to its width).

[0007] Against the background of the prior art, the object of the present invention is to provide a tool electrode which allows for precise machining during spark erosion, whereby discharges and the formation of electrical bridges in the region of the side gap between the outer surface of the electrode and the side surfaces of a recess to be created in the workpiece are to be minimized.

[0008] The object is achieved with the features of the invention according to claim 1 by a method for producing a tool electrode. Claims 2 to 6 present embodiments of the manufacturing method. Furthermore, the invention relates to a tool electrode according to claim 7.

[0009] Accordingly, the invention relates to a method for producing a tool electrode for the spark erosion countersinking of a workpiece with the aim of producing a recess / countersink in the workpiece which has a base surface and side surfaces, wherein the tool electrode has a front surface which faces the surface to be eroded or the base surface of the workpiece and one or more lateral surfaces which, during the countersinking process, face the side surfaces of the recess / countersink eroded in the workpiece.

[0010] It is also provided that one or more lateral surfaces of the tool electrode are at least partially brought into contact with a substance which reacts with the material of the tool electrode to form an electrically insulating reaction layer, in particular an oxide, a nitride or phosphate.

[0011] It is already known from the prior art to coat tool electrodes for electrical discharge machining in the area of ​​their outer surfaces. This insulates the electrode outer surfaces to prevent discharges in the side gaps. Examples of coatings that are known for this purpose include silicon-based ceramics or pure silicon. The disadvantage of using such coatings is that the applied layers sometimes detach and break off. Furthermore, the coating can burn off during the erosion process, with the burning off sometimes occurring unevenly across the outer surface of the tool electrode, thus calling into question the dimensional accuracy of the erosion process.

[0012] It is also known to coat the outer surfaces of tool electrodes with polymers. The use of composite materials as coating materials is also known, and these (e.g., copper zirconium diborite) can be applied electroplatingly.

[0013] The problem with all known coatings is, on the one hand, the stability of the coating and, on the other hand, the dimensional stability, since the additional layers are first built up and the coatings are partially removed during the erosion process.

[0014] In addition, the erosion process introduces particles or atoms of the coating into the workpiece. This can be particularly undesirable for medical technology workpieces. If such particles / atoms adhere to the surface of the workpiece, they can permanently change the material properties of the workpiece.

[0015] For example, US Patent 4,341,939 discloses a wire electrode with a metallic coating (zinc).

[0016] Patent US 4 977 303 also discloses a zinc-coated wire electrode.

[0017] From the international patent application WO 2012 / 097183 A2 a method for producing a tool electrode and a tool electrode according to the preambles of claims 1 and 7 are known.

[0018] In contrast to the prior art, the present invention does not apply an additional foreign layer to the surface of the tool electrode. Instead, a portion of the tool electrode material is chemically converted in the area of ​​its surface by reacting the tool electrode material with a suitable reactant. This results in virtually no change in volume or shape of the tool electrode, thus achieving excellent dimensional stability. Furthermore, an optimal bond between the tool electrode material and the reaction product on its surface is ensured.

[0019] A further advantage of the manufacturing method according to the invention is that, apart from the reaction partner, for example the oxidant, no other substances are deposited on the tool electrode, thereby minimizing the deposition of unwanted foreign particles on a workpiece during the erosion process.

[0020] An embodiment of the method according to the invention can provide that the reaction is terminated when a layer thickness of less than 15 micrometers, in particular less than 10 micrometers, further in particular less than 5 micrometers, further in particular less than 1 micrometer is formed, wherein in particular undercuts and recesses on the lateral surface(s) are brought into contact with the substance, so that all areas of the lateral surface(s) form a uniform electrically insulating reaction layer.

[0021] The tool electrode can be brought into contact with a substance up to and including the front surface to create a reaction layer. Before the reaction layer is formed, the tool electrode can have a recessed contour in the area of ​​the outer surface(s). This recessed contour is, in particular, formed completely circumferentially around the electrode and can have rounded edges.

[0022] Advantageously, the reaction step for producing the reaction layer can be designed in such a way that a material build-up is produced on the lateral surface(s) which brings the recessed area of ​​the contour of the electrode to the dimensions of the front surface.

[0023] One advantage of the method presented here over coating with a foreign material is that, in known coating processes, individual areas of the outer surface(s) of the tool electrode often remain uncoated or form an insufficient coating thickness, such as undercuts or recesses. In the method according to the invention, the tool electrode can, for example, be flushed with a reactant in the form of a fluid, i.e., a gas or liquid, in order to induce or promote the reaction on the surface of the tool electrode, whereby it can be ensured that all areas of the outer surface of the tool electrode are brought into uniform contact with the reactant.

[0024] A further embodiment of the invention can provide that during the contact of the lateral surface(s) with the substance for forming the reaction layer, a temperature control, in particular a temperature increase, takes place.

[0025] Examples of this are given below in the description of the figures.

[0026] The intention here is that the substance with which the outer surface(s) is / are brought into contact enters into a chemical reaction with the material of the tool electrode, in which an electrically insulating reaction layer is formed with the material of the tool electrode.

[0027] In the method according to the invention, it is provided that a tool electrode made of brass is brought into contact with salt water or a solution of NaCl in vinegar, a solution of sodium bicarbonate in distilled water, a mixture of liver of sulfur and distilled water, a solution of copper carbonate in ammonia water or ammonia vapor.

[0028] In particular, it can also be provided that the contact takes place at least temporarily at a temperature above 100 degrees Celsius, in particular above 120 degrees Celsius.

[0029] As mentioned above, following the process of reaction layer formation described here, in particular oxidation, the tool electrode can be heated again by means of a liquid reactant after the reaction and cooling to remove layer defects.

[0030] In addition to a method for producing a tool electrode of the type explained above, the invention also relates to a tool electrode for spark erosion countersinking a workpiece with the aim of creating a recess on the workpiece which has a base surface and side surfaces, wherein the tool electrode has a front surface which faces the surface to be eroded or the base surface of the workpiece and one or more lateral surfaces which, during the countersinking process, face the side surfaces of the recess / countersink eroded in the workpiece.

[0031] It is provided that one or more lateral surfaces of the tool electrode have / have a layer on their surface which is formed by reacting the material of the tool electrode with a reaction partner and which has a higher electrical resistance than the material of the tool electrode.

[0032] The invention is shown below with reference to figures in a drawing and then explained.

[0033] This shows Fig. 1 schematically shows the machining of a workpiece using a tool electrode by spark erosion, Fig. 2 the detailed representation of the removal process according to the prior art, Fig. 3 the details of the removal process during spark erosion with a tool electrode according to the invention, Fig. 4 the formation of a coating on a tool electrode according to the prior art, Fig. 5 the formation of an oxide layer / reaction layer on a tool electrode according to the invention, Fig. 6 a method for using a tool electrode in a processing machine with a handling system, Figs. 7 and 8 special designs of the reaction layer.

[0034] The Fig. 1 schematically shows a tool electrode 1 for spark erosion for a method for spark erosion countersinking. Using such a method, a recess can be created in a workpiece 10 by setting a specific discharge voltage and discharge current between the tool electrode 1 and the workpiece 10 using a voltage supply 12 in a dielectric fluid.

[0035] In the Fig. 2 It is shown that the tool electrode 1 according to the prior art is gradually lowered into the recess 11, which is to be introduced into the workpiece 10, in order to allow the material removal to take place on the base surface 4 of the recess 11 by erosion. The corresponding discharges are represented by dashed ovals 13, 14, 15. In the Fig. 2 Discharges 13, 15 are shown in the area of ​​the side surfaces 5 of the recess 11, which are difficult to avoid according to the prior art. The dielectric flowing out in the side gaps between the side surfaces 5 and the tool electrode 1 entrains the ablated particles of the workpiece 10, so that electrical bridges easily form in the area of ​​the side gaps, which can lead to arcing. Even the application of an insulating layer in the area of ​​the outer surface(s) of the electrode only inadequately solves the problems, since on the one hand there are problems with the dimensional accuracy of the tool electrode with such a layer and on the other hand unwanted foreign particles are dissolved out or even parts of the coating flake off.

[0036] In the Fig. 3 shows the situation during spark erosion with a tool electrode 1 which has an insulating layer, in particular an oxide layer, on its outer surface 3, which layer is formed by reaction with a suitable reaction partner on the surface of the tool electrode 1. The spark erosion takes place in the area of ​​the front surface 2 between this and the base surface 4 of the workpiece 10, and the detached particles of the workpiece 10 are transported away through the side gaps between the side surfaces 5 and the outer surface 3 of the tool electrode 1 by the flowing dielectric and are symbolized by the arrows 16, 17. Because the tool electrode 1 is electrically insulated in the area of ​​the outer surface 3 and is dimensionally stable, with a shallow depth and high stability, no regions at risk of arcing are formed there.The recess 11 in the workpiece 10 can thus be produced with high quality, in particular with high dimensional accuracy and surface quality as well as high productivity and low susceptibility to errors.

[0037] In Fig. 4 A tool electrode 1 is shown in a cross-section, with the left half of the illustration showing the Fig. 4 An uncoated tool electrode is shown, and in the right half, a tool electrode covered with an additional layer 18 as a protective layer. The layer 18 is applied to the material of the tool electrode 1 and adheres to it more or less well. When applying the layer 18, it is difficult to evenly coat undercuts on the outer surface of the tool electrode and also to create a good bond to the electrode surface in order to minimize the risk of the layer 18 flaking off.

[0038] The Fig. 5 shows a tool electrode according to the invention in cross section, with the left area of ​​the Fig. 5 an unprocessed electrode is shown, while in the right area the Fig. 5 The situation after the reaction with a suitable reactant is shown, when a layer, in particular an oxide layer 6, has formed on the surface of the tool electrode 1 as a result of a chemical reaction with the reactant. The volume and dimensions of the electrode remain virtually unchanged, since only material in the area of ​​the surface of the electrode is converted by reaction. The bond between the layer and the body of the tool electrode is very stable.

[0039] In the Fig. 6 A machine tool setup is shown with three stations 7, 8, 9 and a handling system 19. In the first station 7, a tool electrode for spark erosion is produced from blanks 1' in a milling station 20 by machining, the shape of which essentially corresponds to a recess to be made in a workpiece 10, taking into account a defined undersize. The dimensions and shape for the desired tool electrode are transferred to station 7 via an input system 21.

[0040] The handling system 19 then transports the tool electrode 1 thus produced to a second station 8, where a reaction layer / oxide layer is created on the surface of the tool electrode 1. This occurs by exposing the tool electrode 1 to a fluid, for example a gas, represented by the arrows 22, 23. At the same time, temperature control by a heating or cooling device 24 can ensure an efficient reaction process by setting a particularly elevated temperature.

[0041] Once the electrode 1 has been surface-treated in the second station 8, the surface-modified electrode is transferred by the handling system 19 to a third station 9, where the electrode 1 is moved relative to a workpiece 10 by means of a holder and a control 25 in order to create a desired recess 11 in the workpiece 10. At the same time, the tool electrode 1 is connected to a voltage supply 26 of an electrical control device, to which the workpiece 10 is also connected. Current and voltage are regulated by electronics, and the axis 25 of the electrode ensures the desired feed and positioning of the electrode 1 relative to the workpiece 10. In the third station 9, a desired recess 11 is created in the workpiece 10 by spark erosion using the appropriately designed electrode 1.

[0042] Examples of tool electrode manufacturing are described below.

[0043] In a first process, copper electrodes are oxidized. First, the surfaces are cleaned with hydrochloric acid and isopropanol. Oxidation then takes place in a furnace at a maximum temperature of 1100°C, creating an atmospheric, oxygen-containing gas environment. The electrode is then quenched, for example, in distilled water or ambient air.

[0044] Annealing can then take place, in which the electrode is heated for a certain period of time to reduce layer defects. The heating temperature remains significantly below the temperature of the previous oxidation. After annealing, a further quenching and rough cleaning take place.

[0045] Comparative measurements of the electrical resistance on the surfaces of the electrode described above show that the electrical resistance before oxidation is 0.5 Ω, and after oxidation, the electrical resistance is in the range of a few kΩ, for example, between 10 and 100 kΩ. The oxidation temperature and time are crucial for the formation of the desired electrical resistance. This can range from 15 minutes to 45 minutes.

[0046] Repeated tests show that oxidation can also be carried out at temperatures between 700 °C and 900 °C, for example.

[0047] As an alternative to the thermal oxidation of a copper electrode mentioned above, chemical oxidation, electrochemical oxidation (by galvanization), sputtering with a magnetron or laser deposition can also be chosen, whereby it is crucial in each case that the ambient conditions and reaction parameters are selected in such a way that not only a layer is deposited on the surface of the electrode, but the particles added to the electrode react with the material of the electrode.

[0048] An example of the invention relates to the oxidation of brass electrodes. The formation of copper(I) oxide is advantageous for the formation of a suitable oxide layer; the formation of copper(II) acetate (verdigris) should be avoided if possible due to its health and environmental impact. Possible manufacturing processes for an oxidized brass electrode may include exposure to liver of sulfur and distilled water, exposure to ammonia water (25%) plus copper carbonate, or exposure to ammonia vapor.

[0049] The following describes the process using salt water, vinegar and salt, distilled water and baking soda while heating.

[0050] For this purpose, the electrode is first cleaned with isopropanol in an ultrasonic bath and then ground. The electrode is then inserted into an open-pore foam or holder, which is placed in a container with the electrode. A solution containing salt water, vinegar, salt, distilled water, and baking soda is saturated and poured into the container. The container is then heated to approximately 130°C until the desired reaction has taken place on the surface of the electrode, forming a functional / reaction layer. The use of distilled water and baking soda quickly causes the formation of a reaction layer, for example, after just a few minutes. The resistance of this layer is in the megaohm range and is therefore sufficient for the electrode to perform favorably during spark erosion.Oxidation under the influence of salt water requires significantly more time (several tens of hours) and only partially leads to the formation of the desired electrical resistance values.

[0051] The use of a salt and vinegar solution is not guaranteed to be successful, so the use of distilled water with baking soda for chemical reaction / oxidation is preferred. The use of baking soda vapor for oxidation thus shows good results with minimal damage to the tool surface during the erosion process. Electrode wear during the erosion process may be slightly reduced, but the erosion rate—that is, the potentially achievable machining speed in the spark erosion process—can be significantly improved by such a coating.

[0052] The Fig. 7 shows a perspective view of a tool electrode 1 with a square cross-section, which has a contour recess at some distance from the front surface 2. The shoulder, which forms a circumferential edge, is designated S. From the shoulder S onwards, the tool electrode has a reduced thickness, whereby the reduction in dimensions can be precisely compensated for by the thickness of the generated reaction layer so that the outer dimensions are constant throughout and the contour recess is covered. As a result, the lateral surfaces 3 are smooth again, so that the material / fluid flow is not disrupted during operation of the electrode.

[0053] The Fig. 8shows a tool electrode 1 with a round cross-section in a schematic longitudinal section. In the area of ​​the front surface 2, the electrode has the largest diameter of the metallic core material. Positions A, B, C and D are marked along the outer surface, with each letter designating a height along the outer surface 3 in the direction of the longitudinal axis 30 of the electrode, where A designates the height of the front surface itself. In the area between A and B along the outer surface, the electrode has the same diameter / external dimensions as directly at the front surface. In this area, the electrode can be coated with a reaction layer or remain free of the coating. The distance between A and B can be, for example, 1 - 2 mm. From height B onwards, a recess in the contour can be provided, so that a shoulder is formed on the electrode which has the height h. h can be less than 200 µm, but > 10 µm.The thickness of the reaction layer can correspond to the height h of the shoulder, but the thickness of the reaction layer can also be greater than the height h. In the area between height B and height C along the lateral surface, a rounding of the edge / shoulder of the contour recess can be provided. The rounding can be at least partially or completely covered by the reaction layer.

[0054] In most cases, the oxidation / chemical reaction described above is applied to the entire tool electrode. The active surfaces, especially the front surface, are usually either covered / masked before the reaction or exposed afterwards (e.g., by grinding, milling, or similar).

Claims

1. Method for manufacturing a tool electrode (1) for the spark erosion countersinking of a workpiece with the aim of producing a recess / countersink (11) in the workpiece (10), which has a base surface (4) and side surfaces (5), the tool electrode (1) having a front surface (2) which faces the surface for erosion or the base surface (4) of the workpiece, and one or more outward-facing surfaces (3) which face the side surfaces (5) of the recess / countersink (11) eroded in the workpiece (10) during the countersinking process, one or more outward-facing surfaces of the tool electrode (1) at least being partially brought into contact with a substance which reacts with the material of the tool electrode (1) to form an electrically insulating reaction layer (6), characterised in that the tool electrode (1) is made of brass and is brought into contact with salt water or a solution of NaCl in vinegar, a solution of sodium carbonate in distilled water, a mixture of sulphuric acid and distilled water, a solution of copper carbonate in ammonia water, or ammonia vapour.

2. Method according to claim 1, characterised in that the reaction is interrupted by the formation of a layer thickness of less than 15 micrometres, in particular less than 10 micrometres, further in particular less than 5 micrometres, further in particular less than 1 micrometre, wherein in particular undercuts and recesses on the outward-facing surface(s) (3) are brought into contact with the substance, so that all areas of the outward-facing surfaces (3) form a uniform electrically insulating reaction layer (6).

3. Method according to claim 1 or 2, characterised in that while the outward-facing surface(s) (3) is in contact with the substance to form the reaction layer, temperature control, in particular a temperature increase, occurs.

4. Method according to claim 1, 2 or 3, characterised in that the substance with which the outward-facing surface(s) (3) is / are brought into contact undergoes a chemical reaction with the material of the tool electrode (1), in which an electrically insulating reaction layer is formed with the material of the tool electrode.

5. Method according to claim 1, characterised in that the contact temporarily occurs at least at a temperature higher than 100 degrees Celsius, in particular higher than 120 degrees Celsius.

6. Method according to any one of claims 4 or 5, characterised in that the tool electrode (1) post chemical reaction, in particular oxidation, and cooling is reheated to remove layer defects.

7. Tool electrode (1) for the spark erosion countersinking of a workpiece with the aim of producing a recess / countersink in the workpiece, which has a base surface (4) and side surfaces (5), the tool electrode (1) having a front surface (2) which faces the surface for erosion or the base surface (4) of the workpiece, and one or more outward-facing surfaces (3) which face the side surfaces (5) of the recess / countersink (11) eroded in the workpiece during the countersinking process, one or more outward-facing surfaces (3) of the tool electrode having a layer (6) on their surface formed by the reaction of the tool electrode's material with a reaction partner and having a higher electrical resistance than the material of the tool electrode (1), characterised in that the tool electrode is made of brass and the layer (6) is formed by bringing the tool electrode into contact with salt water or a solution of NaCl in vinegar, a solution of sodium bicarbonate in distilled water, a solution of copper carbonate in ammonia water or ammonia vapour.