Abstract:
Method for machining a metal component which has a three-dimensional shape produced by removing and/or shaping material, wherein one or more superior component sections are electrochemically finish-machined by means of a nozzle-like cathode, via which an electrolyte is delivered into the working region, and wherein the cathode or the metal component is moved freely in space by means of a manipulator element.

Description:
[0001]    This application claims priority to German Patent Application No. 1020010032701.8 filed Jul. 29, 2010, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for machining a metal component which has a three-dimensional shape produced by removing and/or shaping material. 
         [0003]    Metal components are machined or shaped using various methods in order to obtain a desired three-dimensional geometry. Material-removing methods such as, for example, drilling, turning, milling, EDM (electrical-discharge machining) and ECM (electrochemical machining) or material-shaping methods such as, for example, stamping, pressing or forging are known. These methods normally serve for rough contouring, i.e. the three-dimensional shape is substantially fashioned by these working processes. Superior component sections, however, still have to be subjected to finish machining in order for any burrs, projections, edges, corners and the like to be removed, etc. This finish machining is effected in most cases by milling. Whereas this is often straightforwardly possible in the case of large metal components on account of the accessibility of the superior component sections which are to be finish-machined, problems often arise in particular with smaller metal components or with metal components in which the superior component sections are either very narrow or are difficult to reach on account of the component geometry, since the tool can be guided into the desired region only some of the way, if at all. Further problems are often caused by the material of the metal component. Special alloys, such as, for example, titanium alloys or nickel alloys, are often used in particular for special applications. Whereas components made of titanium alloys still have sufficient machinability and can therefore, for example, be satisfactorily milled, components made of nickel alloys have relatively poor machinability. In conjunction with complex geometrical relationships, this gives rise to even greater problems in the course of the finish machining. 
       SUMMARY OF THE INVENTION 
       [0004]    The problem addressed by the invention is therefore to specify a method which is intended for machining a metal component and makes possible finish machining of superior component sections even in the case of complex component geometry and difficult machinability of the component material. 
         [0005]    To solve this problem, provision is made in a method of the type described at the beginning for one or more superior component sections to be electrochemically finish-machined by means of a nozzle-like cathode, via which an electrolyte is delivered into the working region, wherein the cathode or the metal component is moved freely in space by means of a manipulator element. 
         [0006]    In the method according to the invention, the finish machining of the superior component region or regions, such as in the region of edges, surface transitions, burrs and the like, is effected by electrochemical machining, normally called ECM. For this purpose, use is made of a nozzle-like cathode, via which the electrolyte is delivered directly into the working region. That is to say that the electrolyte is fed to the cathode and, at the tip of the nozzle-like cathode, flows directly onto the metal component (workpiece) to be finish-machined. The application of a working voltage between metal component and cathode produces a process current which causes material to be removed. The metal component itself here is anodically polarized. Via the nozzle-like cathode, material can thus be removed from the workpiece in a precisely defined region, specifically only in the contact region of the impinging electrolyte stream. According to a first alternative of the invention, the cathode itself is arranged on a manipulator element, preferably a multi-axis robot, preferably one having at least five motion axes, via which robot, with associated control device, the cathode can be moved freely (three-dimensionally) in space. That is to say that the cathode can be moved in any desired manner and can therefore also traverse any desired geometries. Since the cathode itself is a very thin, narrow component having a diameter of only a few millimeters, it can consequently be moved even into extremely narrow, constricted component regions, and displaced there, via the robot. This in turn makes it possible even for components having a very complex geometry to be machined free of machining forces in superior, otherwise scarcely accessible regions. According to a second alternative of the invention, the metal component can be moved freely (three-dimensionally) in space by means of the manipulator element, again preferably a multi-axis robot, preferably one having at least five motion axes; that is to say that, here, the metal component is moved in any desired manner relative to the thin cathode. As a result of the free mobility, even complex geometries can be machined by means of this motion variant. The invention is therefore based on the idea that there is spatially free relative mobility between cathode and metal component, and this relative mobility is realized by means of the manipulator element. 
         [0007]    The electrochemical working process also enables a wide variety of different materials to be machined, that is to say that even materials which are difficult to machine using conventional working processes, because they are very difficult to cut, can be readily machined by ECM. In conjunction with the freedom of movement of the cathode or of the metal component, that is to say the mobility in any desired manner in space, the method according to the invention therefore offers the possibility of being able to finish-machine any desired components or complex geometries virtually irrespective of the material used. 
         [0008]    As described, the manipulator element provided is a robot which should have preferably five motion axes, but it can equally also have six axes, thereby ultimately providing for even more degrees of freedom of movement. This multi-axis configuration enables both translational movements in the three space directions and rotational movements about the space directions. 
         [0009]    According to a development of the invention, the movement is not restricted only to the cathode according to the first alternative of the invention or to the metal component according to the second alternative of the invention. Rather, it is also possible to move the metal component (with free mobility of the cathode) or the cathode (with free mobility of the metal component) in addition, that is to say to permit, for example, a translational movement, for example along one or more space axes, or to rotate said metal component or cathode, possibly in addition, about one or more space axes. That is to say that there are additional degrees of freedom of movement at the respectively other central working element, in addition to the movements which are possible via the manipulator element. 
         [0010]    As stated, the cathode is a thin tube having a diameter of a few millimeters, provided it is round in cross section and delivers a round electrolyte stream. Alternatively, an as it were “squeezed” cathode, which is longer than it is wide, or a hollow cathode having any other desired cross section can be used. The geometry of the working region is defined according to the electrolyte stream geometry, for which reason the corresponding cathode geometry is selected according to the component section to be machined. In order for workpiece sections which are difficult to access to be reached more easily, curved or angled cathode embodiments can be used. 
         [0011]    The essential process parameters are the distance between cathode and workpiece, the working voltage or the process current, the dwell time of the cathode above the location to be machined or the associated feed rate of the cathode relative to the metal component, and the composition and the volumetric flow of the electrolyte. By suitable selection of these parameters, the material removal can be controlled with regard to removal depth and removal rate, care always having to be taken when setting the parameters that as few stray currents as possible occur, which would possibly lead to material also being removed outside the actual working region. For example, it is possible for the working voltage which is applied between cathode and metal component, and typically between 5V and 200V, or the process current flowing therebetween to be constant or pulsed and/or for the volumetric flow of the electrolyte to be between 10 and 1001/h. 
         [0012]    The control of the robot, that is to say the movement of the cathode for traversing the component geometry to be machined, is effected via a suitable control device which has a corresponding control program. The control is based on a model of the component or of the component geometry, said model being stored in a suitable program which serves for the control. During operation, then, the cathode or the metal component (depending on which is moved by the manipulator element), supported by the program, is moved along the component section to be machined. Since, as described, the material removal depends ultimately on the set process parameters and in particular on the distance of the cathode from the component surface, an expedient development of the invention provides for the distance between the cathode and the surface of the metal component to be determined from the ratio of working voltage and process current and/or, as a function thereof, for the process sequence to be controlled by varying any desired abovementioned process parameter, e.g. the feed rate, it being possible for the feed to be constant or intermittent. 
         [0013]    The working voltage and the process current are detected, and the cross-sectional area of the electrolyte stream and thus the working area are also known. These variables, in first approximation, have a clearly defined, formal relationship in accordance with Ohm&#39;s law, to the distance between the cathode and the surface of the metal component. The removal depth and thus the working progress are therefore clearly linked to the aforesaid parameters. Any desired parameter, e.g. the dwell time of the cathode above the metal component surface or the feed rate of the cathode along the component surface, can therefore be set according to the continuously determined cathode spacing in such a way that the desired working result is achieved. 
         [0014]    In a development of the invention, a gas curtain, preferably an air curtain, laterally enclosing the electrolyte stream at least partly, preferably completely, is blown out via the cathode. That is to say that not only is the electrolyte stream delivered via the cathode but so too is a gas stream, which encloses the electrolyte stream preferably completely. The result of this is that the electrolyte stream is delivered onto the component surface in a concentrated manner and is kept away from the vicinity of the working region by being “blown out”. This avoids the situation where adjacent surfaces are undesirably affected and therefore stray currents lead to undesirable removal in adjacent regions. The nozzle itself is therefore of double-walled design, with an inner electrolyte passage and an outer air passage, which are connected to corresponding supply lines. In addition to air, some other gas, e.g. inert gas such as nitrogen or helium, can also be used for forming the gas curtain. 
         [0015]    As already described, the method according to the invention is suitable in particular for machining components made of special (metallic and intermetallic, high-temperature) materials; it is primarily useful for the machining of metal components made of steels, titanium alloys and in particular nickel alloys, which are extremely difficult to cut. 
         [0016]    A metal component which is in particular preferably to be machined with the method according to the invention is a component of a turbomachine, for example a casing component, but in particular a blade component. Blade components which are especially difficult to machine and have a very complex geometry are integral rotors (“blisks”), guide vane clusters or guide vane rings, as are used, for example, in high-pressure compressors of a gas turbine. Such integral rotors, guide vane clusters or guide vane rings are subject to stringent requirements, for which reason they consist either of a titanium alloy or high-temperature steel, but preferably of a nickel alloy. In particular the guide vane clusters and the guide vane rings have a very complex geometry, normally consisting of two shrouds, between which the twisted guide airfoils extend. The distances between the airfoils go from a few millimeters up into the centimeter range. As a result, the accessibility of the regions between the airfoils is greatly restricted. Nonetheless, in particular the edges/corners or the transition surfaces in these regions require the finish machining according to the invention. If such a guide vane cluster or a guide vane ring is produced from solid material by cutting or other removal processes, this inevitably results in the finish machining, in particular in the region between the airfoils, involving considerable outlay. It is precisely in the production of blade components, in particular of the guide vane clusters or guide vane rings, that the method according to the invention can be used in an especially advantageous manner, in particular if there are very small airfoil and shroud spacings. This is because, with the method according to the invention, the edges and surfaces present there in the region between two airfoils and/or the airfoils themselves can be readily machined, since the very thin, narrow cathode can be moved even into these extremely narrow regions, and positioned there with high precision, via the robot moving it in any desired manner in space. 
         [0017]    In addition to the method, the invention also relates to an apparatus for implementing the method, comprising a manipulator element in the form of a multi-axis, preferably five- or six-axis, robot, on which an ECM tool in the form of a nozzle-like cathode or a work holder holding the metal component is arranged, an electrolyte feed device for feeding the electrolyte from an electrolyte reservoir to the cathode, a process energy source connected to the cathode and the metal component, and a control device controlling the operation of the apparatus. The cathode, which of course is interchangeably arranged on a corresponding cathode holder on the robot, has a round, elongated or any other desired cross section; the desired cathode shape is dictated by the machining task. In order to be able to more easily reach workpiece sections where access is difficult, curved or angled cathode embodiments are possible. 
         [0018]    In a development of the invention, the cathode can be designed for delivering gas, in particular air, fed to it via a gas feed device, in the form of an air curtain laterally enclosing the electrolyte stream at least partly, preferably completely, said electrolyte stream discharging from the cathode. The material removal is thereby concentrated on the working region, and the effect on adjacent zones can thereby be reduced. The nozzle itself is therefore, for example, of double-walled design, with an inner electrolyte passage and an outer gas passage enclosing said electrolyte passage, said passages being connected to corresponding supply lines. 
         [0019]    In an advantageous development, the work holder is additionally movable, preferably along or about a plurality of space axes, when the cathode is arranged on the manipulator element, or the cathode is additionally movable, preferably likewise along or about a plurality of space axes, when the work holder is arranged on the manipulator element. That is to say that two movement modalities are provided in the apparatus according to the invention, namely, firstly, the robot for moving the cathode or the work holder together with metal component and, secondly, also the work holder or the cathode, such that an adapted relative motion sequence between cathode (tool) and metal component (workpiece) can be set for the respective application. 
         [0020]    Finally, a means for detecting the distance of the cathode from the component surface is provided. This nozzle distance is a measure of the removal capacity and the removal depth and is clearly linked, in first approximation, in accordance with Ohm&#39;s law, to the process parameters working voltage, process current and cross-sectional area of the electrolyte stream, for which reason the detection of the distance is advantageous for the continuous monitoring of the working result. The means in this respect is expediently the control device, which determines the distance. It is possible for any desired parameter, e.g. the dwell time of the cathode above the workpiece surface or the feed rate of the cathode along the component surface, to be set with reference to the detected distance in such a way that the desired working result is achieved. However, the distance can also be determined by means of one or more distance sensors, the control device again controlling the operation, that is to say the relevant process parameters, in accordance with the measuring results from the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Further advantages, features and details of the invention can be gathered from the exemplary embodiment described below and with reference to the drawings, in which: 
           [0022]      FIG. 1  shows a diagrammatic illustration, as a perspective view, of a guide vane cluster to be machined with the method according to the invention and the apparatus according to the invention, 
           [0023]      FIG. 2  shows a diagrammatic illustration of the apparatus according to the invention, 
           [0024]      FIG. 3  shows a diagrammatic illustration of the cathode/metal component working region. 
           [0025]      FIG. 4  shows a diagrammatic illustration of the cathode movement, and 
           [0026]      FIG. 5  shows a diagrammatic illustration of a cathode with a gas curtain laterally defining the electrolyte stream. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]      FIG. 1  shows, in the form of a perspective view, a metal component  1  in the form of a guide vane cluster  2  which for the purpose of forming a complete ring, is assembled with a plurality of such clusters to give a ring shape. Such a guide vane cluster consists of two shrouds  3 ,  4  and a multiplicity of airfoils  5  extending between said shrouds  3 ,  4 . The shrouds  3 ,  4  and the airfoils  5  are fashioned from solid material by cutting and/or other removal processes. The airfoils  5  have a complexly twisted geometry and are very closely spaced apart, i.e. there are only very narrow spaces  6  between the individual airfoils  5 . The complex geometry consequently results in curved edge regions and curved surfaces in the transition between the airfoils  5  and the shrouds  3 ,  4  or at the surfaces of the shrouds  3 ,  4  and at the surfaces of the airfoils  5  themselves, which, after the metal component  1  or its sections (shrouds  3 ,  4 , airfoils  5 ) have been pre-machined using appropriate working processes, have to be finish-machined using the method according to the invention. 
         [0028]    An apparatus as shown in  FIG. 2  as a diagrammatic illustration is used for this purpose. The apparatus comprises a manipulator element  7  in the form of a multi-axis, preferably at least five-axis, robot  8  which carries a cathode  9  which serves for the ECM of the metal component  1 , which is arranged on a corresponding work holder  10 . The cathode  9  is a nozzle-like, narrow tube which can be moved in any desired manner in space via the robot  8 , such that any desired three-dimensional structures can therefore be traversed and machined. In addition to the mobility of the cathode  9 , it is also possible for the work holder  10  to be movable, either translationally along one or more space axes or rotationally about one or more space axes, or both translationally and rotationally, as indicated by the motion arrows. 
         [0029]    A liquid electrolyte, for example an NaCl solution, is delivered via the cathode  9  directly into the working region, for which reason the electrode  9  is embodied, as described, as a nozzle or tube. Provided for this purpose is an electrolyte reservoir  11 , from which the electrolyte  12  is directed to the cathode  9  via a controlled pump  13  and a suitable electrolyte feed line  14 . Provided at the robot  8  is a corresponding connection box  15 , at which the line opens out and at which the cathode  9  is also interchangeably accommodated. The volumetric flow of the electrolyte can be monitored via a flow meter  16 , and the fluid pressure can be monitored via a pressure gage  17 . Furthermore, a temperature measuring device  18 , a heating controller  19  and a pH measuring instrument  20  and a conductivity measuring instrument  21  are provided in the electrolyte reservoir  11  in order to be able to correspondingly set or monitor the electrolyte properties. 
         [0030]    The electrolyte collected after delivery via the cathode  9  is fed back into the electrolyte reservoir  11  by an electrolyte feed line  23 , i.e. a circuit is established. The robot  8  and the work holder  10  are provided in an enclosure  24 , i.e. the apparatus is closed to this extent with regard to the working region. 
         [0031]    Furthermore, the apparatus comprises a process energy source  26 , via which the working voltage and the process current can be applied. The parameters are correspondingly monitored via an ammeter  27  and a voltmeter  28 . A supply line  29  runs, once again, to the connection box  15 ; it makes contact with the cathode  9 . The supply return line  22  leads from the metal component  1  back to the process energy source  26 . In the process, the circuit is closed by the electrolyte stream. 
         [0032]    Finally, a gas supply, in this case shown embodied as a compressed air supply, for example in the form of a compressor  30 , is provided, from which an air feed line  31  runs likewise to the connection box  15 . This air feed line is connected in turn to the cathode, which is embodied as a double-walled tube. The electrolyte is fed in the central passage; in the outer passage, an air curtain which encloses the electrolyte can be blown out via the fed compressed air. A controlled restrictor valve  32  and a flow meter  33 , via which the air flow can be measured, are provided in the air feed line  31 . 
         [0033]    Three roughly distinguishable regions are therefore provided, namely the “process energy” region A, the “electrolyte supply” region B and the “compressed air supply” region C. 
         [0034]    Finally, a control device  34  is provided. The control device controls the operation of the robot  8 , that is to say the free movement in space of the cathode  9  and the movement of the work holder  10 , if provided. It is of course also possible to control and monitor all the sub-components of the apparatus in  FIG. 2  for the electrolyte and gas supply and the process energy source  26  (that is to say the regions A, B and C) via the control device  34 . 
         [0035]      FIG. 3  shows, as a diagrammatic illustration, the tip of the cathode  9  in a sectional view. The electrolyte  12  flows through the tubular, nozzle-like cathode  9 . The anodically polarized metal component  1 , for example the guide vane cluster  2  known from  FIG. 1 , is at a distance from the cathode  9 . As illustrated by the arrow diagram, the cathode  9  can be moved translationally in the three directions in space, just as it can also be moved rotationally about each of the three directions in space. The robot  8  is accordingly activated for this purpose via the control device  34 . Control is based here on a stored model of the metal component  1 , which defines the surface along which the electrode  9  is to be moved. 
         [0036]    It can be seen that the electrolyte  12  is conveyed through the nozzle-like or tubular cathode  9  and delivered to the metal component  1 . An electric flow field  36  forms in the electrolyte stream  12 . Electrochemical, locally limited metal removal takes place in the region  37 , i.e. a cavity forms in the metal component  1 . The corresponding removal depth is obtained in accordance with the process parameters selected. 
         [0037]      FIG. 4  shows, as a diagrammatic illustration, the movement of the cathode  9 , which, for example in the case of a round cross-sectional geometry, has a diameter of three millimeters and is at a distance of, for example, one millimeter from the original workpiece surface. It can be seen that a linear region  37  can be removed by a horizontal movement of the cathode  9 , as shown by the arrow P. Whereas only a movement along one space coordinate is shown in  FIG. 4 , it is possible, as described, for the cathode  9  to be moved in any desired manner in space, i.e. the round edge regions of the guide vane cluster  2  shown in  FIG. 1  or the three-dimensionally twisted airfoils  5 , etc., can be readily traversed in order to remove material there to the desired extent. 
         [0038]    Finally,  FIG. 5  shows a further embodiment of the cathode  9 , which is embodied as a double-walled tube. The electrolyte  12  is directed in the central passage  38 . The compressed air fed via the gas feed device, shown in  FIG. 2  as air feed device  31 , is discharged in the outer passage  39 . As  FIG. 5  shows, a gas curtain  40  enclosing the electrolyte stream  12  all around is formed. This reduces the effect on adjacent metal component surfaces. 
         [0039]    Even though a cathode  9  of round cross section is shown by way of example in the figures, the cathode can of course also have an elongated cross section or any other desired cross section. It can have, for example, in the electrolyte passage, a length of 10 mm and a width of 3 mm, such that a long, but narrow, zone can be machined, which is expedient in particular for machining relatively large areas. If a gas curtain is present, the corresponding air passage has, of course, a corresponding geometry.