Patent Publication Number: US-2019177872-A1

Title: Electrolytic polishing method and device and method for producing a cathode

Description:
The invention relates to a method and a device for electrolytically polishing an inner surface of a recess in a workpiece made of metal, in particular a recess in a workpiece produced by a generative manufacturing method. The invention further relates to a method of manufacturing a cathode. 
     Methods for the electrolytic treatment of workpieces are known. The workpiece is placed in an electrolyte bath and connected as an anode. A cathode as a tool is positioned as close as possible to the workpiece, wherein the shape of the upper side of the cathode facing the workpiece should correspond as closely as possible to the surface on the workpiece to be produced later. In addition, the so-called working gap between the workpiece and the cathode should be small and uniform in order to make the dissolution of the material on the workpiece swift and uniform. 
     Such workpieces produced by a generative manufacturing method, in particular printed in three dimensions, often have recesses which cannot be subjected to mechanical fine machining because the profiles of these recesses are not linear. Also, the cross-sections of the recesses can be so small that a mechanical polishing method cannot, or at least not economically, be carried out. 
     It is therefore the object of the invention to remove these drawbacks. 
     The invention provides a method for electrolytic anodic polishing of an inner surface of a recess in a workpiece made of metal, in particular in a workpiece produced by a generative manufacturing method, which is characterized by the following steps: 
     (A) immersing the workpiece in an electrolyte containing a salt, water and alcohol having a minimum weight proportion of from 20 to 70%, in particular 40 to 70% per liter of electrolyte; 
     (B) connecting the workpiece to a positive pole so that the workpiece forms an anode; 
     (C) providing at least one cathode in the electrolyte and in the recess; 
     (D) providing a relative movement between the electrolyte and the workpiece; and 
     (E) applying an electric voltage between the anode and the at least one cathode. 
     The method according to the invention and the device according to the invention, which will be presented further below, is not limited to the deburring of a workpiece. Rather, the invention relates to the polishing of surfaces. 
     The idea on which the invention is based is to machine or treat a workpiece that is rough on the inside of a recess by an electrolytic polishing in its entirety, wherein the interaction between process parameters, electrolyte properties and tool design are coordinated in such a way that when a direct current is applied, a large-area, uniform, supersaturated, viscous layer is formed on the anode surface. 
     The following factors provide for this layer, individually and in combination, namely
         selecting an electrolyte basis which distinguishes itself by a viscosity higher than that of water. Viscosity is the decisive factor for reducing the drift velocity of the ions;   keeping the process temperature relatively low, in particular at 7-15° C.;   keeping the water activity/relative moisture (the so-called “aw value”) as low as possible. The aw value is in the range of from 25-70%, ideally between 27 and 50%;   selecting suitable salts which generate an appropriate conductivity of the electrolyte; and/or   having uniform flow conditions in the gap in order to provide for an even layer build-up on the anode surface and for the exchange of the charge carriers.       

     By selecting the suitable direct current parameters, the structure and thickness of the layer can be controlled. Current pulses with the parameters specified on the following pages have turned out to be useful here. 
     Advantages of the invention:
         Large-area parts can be machined using relatively small currents.   Large clearances can be used.   The tool geometry has to follow the workpiece geometry only approximately and not exactly, since the reduced drift velocity greatly minimizes the sensitivity to fluctuations in the current density on the anode surface.       

     The ion movement velocity during operation is 0.1 to 5 m/sec. This is the opposite of what would actually be provided for an internal treatment of a recess for a surface treatment, because due to the already poorer accessibility of the electrolyte and the cathode in or to the recess, an electrolyte that provides for a high diffusion rate is at first glance more suitable for a large-area treatment in order to compensate for the above-mentioned disadvantages. However, the invention takes the opposite path. The electrolyte allows larger gap widths to be achieved, which in turn results in that the cathodes have to follow the course of the recess less exactly because the gap width is at the same time a distance that influences the geometry tolerance of the cathode. In addition, the slower diffusion rate and thus the slower treatment provides for a kind of compensation effect within the gap with regard to removal peaks. This in turn means that this effect serves to allow working at a significantly lower current density. This compensation effect can be explained by the fact that the charge at the so-called peaks on the workpiece surface is stronger than on a flat surface, which is why ions are exchanged here first. The special electrolyte solution causes a semiconductor-like behavior in the electrolyte, i.e. exposed areas on the component are removed preferentially, whereas the flat surfaces or, more generally, the surfaces without peaks are protected by the electrolyte properties and are attacked only later. The removal is thus effected in a concentrated manner at the peaks. The velocity of the ions themselves can be influenced not only by the composition of the electrolyte and its viscosity, but also by the temperature of the electrolyte or by a relative movement between the electrolyte and the workpiece. The above compensation effect can also be explained in somewhat greater detail by the fact that the peaks are exposed to an intensified electric field and that in the electrolyte the salts release their electrical charge to the anode, which leads to the formation of oxygen and thus in turn to oxidation of the metal surface. The thin metal oxide layer resulting thereby is an at least temporary protective layer against the electrolyte. 
     The preferred embodiment of the invention provides that the method is employed in a generative metal manufacturing method (additive manufacturing). These manufacturing methods include the 3D printing processes including ALM, DMLS and SLM processes. The idea here is to turn disadvantages of the additive manufacturing product into advantages for the polishing process. In order to make additive manufacturing products, i.e. 3D printed products, economical, in particular in the aviation industry, the manufacturing process, i.e. the printing process, would have to proceed as quickly as possible, i.e. so-called large grain sizes would have to be produced (e.g. in laser sintering). These large grain sizes, however, cause inacceptable surface qualities and can be regarded as peaks. The grains are polished extremely well to be smooth on the surface by the method according to the invention. The advantages of the electrolyte employed in the invention are utilized here in that the electrolyte preferentially removes the projecting sections of the grains. 
     According to the invention, the alcohol used is preferably a polyhydric alcohol. 
     Ammonium salt or alkali salt, for example (in this case e.g. alkali sulfamate or sodium sulfamate), may be used for the salt. The salt content, for example, is at least 50 g/l of the electrolyte, in particular more than 150 g/l and less than 400 g/l. 
     Glycol or other higher viscosity media may, for example, be used as alcohol, which are either able to absorb salt directly or can form single-phase liquids miscible with water. 
     The water content of the electrolyte may optionally be a maximum of 20%. 
     The temperature of the electrolyte during the treatment should preferably be in the range between 0° C. and 30° C., in particular in the range of from 5° C. to 20° C., preferably 7° C. to 15° C. 
     The pH value of the electrolyte should be either a maximum of 6.8, in particular be in the range between 5.0 and 6.8. 
     It has been found that the electrolyte should have a so-called conductance of 10 to 40 mS at 20° C. 
     The relative movement between the electrolyte and the workpiece may be effected by moving the workpiece within the electrolyte or moving the electrolyte, i.e. pumping the electrolyte to generate a flow along the surface of the workpiece. Of course, it is also possible to set both the electrolyte and the workpiece in motion. 
     In order to achieve a uniform movement of the electrolyte on the workpiece surface, the workpiece is preferably moved along a guide. 
     The workpiece consists in particular of a metallic material, e.g. a nickel or chromium alloy or a light metal such as aluminum. These metals have turned out to be preferred metals for the use of the method according to the invention and the device according to the invention. 
     The cathode, which must not touch the anode, should be connected to a negative pole of a current source by means of an electric line, in particular a flexible cable, and inserted into the recess in the workpiece in order to treat the inner surface of the recess. The cathode is not a so-called false cathode, which sits near another cathode connected to the current source and is coupled to it only via the electrolyte, but a cathode that is directly connected to the current source. This cathode allows the removal to be realized on a larger area of the cathode, rather than only on a tip of the cathode, for example. 
     In particular, the cathode may be formed to be flexible and permit lateral bending in order to allow polishing of non-linear recesses so that the cathode can follow the profile of the recess. 
     At least one electrically insulating spacer coupled to the cathode may project laterally from the cathode. This spacer serves to prevent contact with the workpiece. In particular, the spacer may also be used as a guide. This means that the invention provides that the spacer contacts the workpiece. Sections of the cathode that are not electrically insulated are then used for removal. Especially if the cathode is flexible and, for example, is inserted into a deep opening with a curved profile in a workpiece produced by a generative manufacturing method (e.g. a 3D printed part), one or more spacers can make sure that, despite the bend, the cathode follows the profile of the bend, will bend and still does not come into contact with the inner surface of the recess. 
     In a particularly advantageous variant of the cathode used, provision is made that the cathode along with its spacers is configured as a kind of circular brush. The insulating spacers are projecting filaments. The cathode may be formed from one or more twisted wires which clamp the filaments between them. 
     A further variant of the invention provides that the cathode is an elongated, in particular rotating body having a groove that is helical on its outer surface and transports electrolyte. Such a cathode has a kind of drill shape, for example, with the flute serving to transport the electrolyte. The groove extends to a core formed by the electrolyte. The lateral boundaries of the groove may, for example, be defined entirely by the insulation. Such a design has several advantages. By rotating the cathode, the sections of the cathode located at the base of the groove are always opposite to different sections of the inner wall of the workpiece in order to remove material uniformly therefrom. This allows a particularly uniform polishing to be carried out. Furthermore, a flow can be directed through the groove, or by rotating the cathode, electrolyte is transported as in a screw type pump. 
     In the case of blind holes, it would be conceivable to provide an intake hole or rather a discharge hole, e.g. in the center of this cathode tool, so that electrolyte is supplied via the helical groove and is discharged via the central opening. 
     Especially if spacers temporarily cover the inside of the recess, it is advantageous to move the cathode relative to the workpiece after a first polishing step, in particular axially and/or circumferentially, in order to expose a preceding support portion at which the spacer was previously located opposite the workpiece (with or without contact), so that a subsequent polishing step can then be performed in this area, which is carried out for polishing the section of the inner surface of the recess that previously served as the support portion. 
     A preferred embodiment of the invention provides that the outer surface of the workpiece is polished even simultaneously with the inner surface by applying the electrical voltage. This considerably reduces the treatment time. 
     As already indicated above, to achieve the relative movement between the electrolyte and the workpiece, electrolyte can be pumped and flow past the workpiece and/or the workpiece is moved in the container receiving the electrolyte, in particular along a linear guide already mentioned above. The at least one cathode may have a surface which is opposite the workpiece and is profiled, i.e. is neither flat nor circular cylindrical. This profiling can improve the removal process, in particular if the profiling is similar to the later shape of the surface of the workpiece. 
     In this connection, it may be advantageous if the cathode has a side which faces the workpiece and which, prior to the treatment of the workpiece, has a constant gap to the side of the workpiece which faces it. Here, either the constant gap may be present prior to the treatment of the workpiece, or the reference for determining the gap and the side of the cathode is the then finished workpiece. It should be emphasized, however, that the extremely uniform gap between the surface of the workpiece and the opposite surface of the cathode, which has been customary until now, is not necessary in the invention. The gap may extend non-uniformly, which reduces the effort required to produce the shape of the cathode. Furthermore, the same cathode can also be employed for similar surface shapes. Due to the process properties, any possibly rough surface of the cathode is irrelevant to the surface quality to be achieved for the workpiece. 
     It is particularly advantageous if the cathode and/or a support structure, which forms the anode, is also produced or co-printed at the same time or offset in time along with the process of generative manufacturing, i.e. the 3D printing, of the workpiece. This allows an accurately shaped cathode to be positioned close to the surfaces to be treated, without any particular additional effort. The support structure ensures that no spots appear on the workpiece surface after the electrolytic treatment, which could previously occur when the anode was applied. 
     It is particularly advantageous if a plurality of workpieces are seated on the support structure, which are then electrolytically treated one after the other or simultaneously. This results in a greater stability during the treatment for the workpieces to be treated, and handling is considerably simplified. 
     The support structure is separated from the workpiece after the electrolytic treatment of the recess, preferably after the treatment of the entire workpiece. This separating process can be effected mechanically or by spark erosion. 
     For generative manufacturing, metal grains having diameters of from 20 to 60 μm are used, from which the workpiece is composed. 
     Generally, during the electrolytic treatment, the workpiece, in particular a plurality of workpieces, may be seated on a support plate which forms the anode. This support plate may, for example, be the above-mentioned support structure, which was generatively produced together with the workpiece. 
     A cathode for external treatment of the workpiece, i.e. for treatment of the workpiece outside the recess, is fastened to the support plate, for example by means of an electrically insulating mounting, in particular by means of a plug connection. This cathode or these cathodes may also be produced generatively. 
     For the treatment of the recess, the cathode may be inserted into the recess and electrolyte may be pumped into the gap between the cathode and the inner surface, opposite to it, of the workpiece. To improve the treatment, in particular for an exactly predeterminable treatment of the recess, an electrolyte conduit is provided, for example, which leads to the recess and introduces electrolyte into it. This may also be effected through a passage duct inside the cathode. 
     Movement of the cathode within the recess may be performed by motor or manually. In particular, a reciprocating movement of the cathode can also be realized by the motor drive. 
     As already discussed above, the invention further relates to a device for the electrolytic anodic polishing of an inner surface of a recess in a workpiece made of metal, in particular in a workpiece printed in three dimensions. The device has a container filled with an electrolyte, the electrolyte containing a salt, water, and an alcohol having a minimum weight proportion of from 20 to 70%, preferably 40 to 70%, a holding device for the workpiece, a cathode which is movable relative to the recess and has an electrical insulation in sections on its outside and is connected to a power source by a cable, a drive being provided for generating a relative movement between the electrolyte present in the opening and/or the cathode, on the one hand, and the workpiece, on the other hand. 
     The invention also relates to a method of manufacturing a cathode for the electrolytic treatment of a workpiece, in particular a generatively manufactured workpiece. The cathode has a side facing the workpiece to be treated and is intended, in particular, to carry out the method according to the invention. But the cathode may also be provided for external treatment of the workpiece. The outer geometry of the cathode is ascertained at least in the region of the side facing the workpiece using a simulation program. In the simulation program, the electric and/or magnetic fields that are present between the cathode to be manufactured and the workpiece during the electrolytic treatment are determined. The outer geometry of the cathode on the side previously mentioned is ascertained taking into account a constant current density between the side and the cathode. The cathode is then manufactured by a generative manufacturing method on the basis of the outer geometry determined. This method takes into consideration that in the later electrolytic treatment process there is an increased charge density in the region of the edges, which can be compensated by means of the adapted geometry of the cathode. In these regions, for example, a larger gap to the workpiece could be provided. 
     All of the features and advantages already discussed above in connection with the method according to the invention also apply to the device according to the invention. 
     Also, the device according to the invention is not limited to deburring, but serves to polish an inner surface of a recess. 
     In particular, a flexible cathode is provided, portions of which are provided with an electrical insulation projecting on the outside as a spacer, in particular in the form of projecting bristles or a shell which includes a helical groove and surrounds a core forming the cathode. 
     In addition to the cathode introduced into the recess, at least one external cathode for polishing the outer surface of the workpiece may be provided, which is connected to the same power source as the internal cathode and polishes the outer surface at the same time. 
     The electrolyte used is the electrolyte previously used in connection with the method according to the invention. 
     Numerous tests for the polishing of recesses of products generated by generative manufacturing methods, that is, e.g. 3D printed products, have shown that the method according to the invention and the device according to the invention allowed the surface roughness to be improved from RZ 60 to RZ 3-4. 
     The method according to the invention furthermore provides that it is employed for products produced by generative manufacturing methods, that is, e.g. 3D printed products, in which support structures are generated or printed at the same time, for example for internal sections. The method according to the invention and the device according to the invention remove such support structures, which would otherwise be impossible using a mechanical tool because accessibility is not given. 
     The device according to the invention and the method according to the invention more particularly use working ranges for the applied current in the range of from 10 V to 40 V, ideally 15 V to 30 V, and 0.0003 A/mm 2  to 0.01 A/mm 2 , preferably 0.003 A/mm 2  to 0.01 A/mm 2 . 
     The gap sizes are in the range of from 1.0 mm to 100 mm. 
     In particular, a pulse method is applied which serves against local overheating, pitting, irregular metal dissolution, deposit of soot, and flux lines. The optimum pulse-pause ratio is from 4:1 to 1:4, preferably 2:1, and depends in particular on the material of the workpiece and the electrolyte. 
     It has turned out that a halogen content in the electrolyte improves the properties thereof. The total amount of halogen will be in the range of from more than 0 g to 100 g per liter of electrolyte. 
     A change holder may be provided for fixing the cathode, the change holder being equipped with a power and/or electrolyte connection and/or a drive for moving the cathode. 
    
    
     
       Further features and advantages of the invention will be apparent from the description below and from the accompanying drawings, to which reference is made and in which: 
         FIG. 1  shows a cross-section taken through a schematically illustrated device according to the invention for carrying out the method according to the invention; 
         FIGS. 2 to 4  show different embodiments of a cathode employed in the device according to the invention and in the method according to the invention; and 
         FIG. 5  shows a schematic side view of a further embodiment of the device according to the invention. 
     
    
    
       FIG. 1  shows a device for the electrolytic anodic polishing, the device having a container  10  which is filled with an electrolyte  12 . A workpiece  14  is completely immersed in the electrolyte  12  by means of a holding device  16  and can be reciprocated along a linear guide  18  by means of a motor  20 . The workpiece  14  has a recess, here in the form of an arcuate, curved hole. The recess is defined by an inner surface  22 . 
     The workpiece  14  is connected to a power source  26  by a cable  24  and acts as an anode. One or more cathodes  28 ,  30  are also connected to the power source  26  by cable(s)  32 . The cathodes  28  are intended for treating or machining the outer surface of the workpiece and in the present case are plate-shaped or profiled (see vertical cathode  30 ). The shape of the cathodes  28  does not need to correspond to the desired or existing outer shape of the workpiece  14  in the area where the cathodes  28  are positioned opposite the workpiece  14 . 
     The cathode  30  is flexible and, in the non-deflected position, it is preferably linear (see  FIGS. 2 to 4 ). When it is inserted into a non-linearly extending recess, however, it can adapt to it. 
     To prevent contact of the cathode  30  with the inner surface  22  of the recess, a plurality of electrically insulating spacers  34  are attached to the cathode  30 . In the embodiment according to  FIG. 2 , these spacers  34  are filaments which are attached to the wire-type cathode  30  by clamps like in a circular brush. 
     In the embodiment according to  FIG. 3 , for example, block-like or cylindrical spacers  34  are fitted on, injection-molded on, glued on or otherwise attached to the flexible cathode  30 . 
     The cathode  30  according to  FIG. 4  is also laterally pliable and coated with spacers  34  in the form of an insulation, resulting in a kind of drill shape. The insulation has a helical shape so that a helical, electrolyte-carrying groove  36  is obtained between the threads, with the cathode  30  exposed in the base of the groove. 
     The electrolyte contains 50 g to 400 g salt per liter of electrolyte (preferably 150 g to 400 g), in particular an alkali salt, alcohol, in particular polyhydric alcohol such as glycol or glycerol, with a maximum weight proportion of from 20 to 70% per liter of electrolyte, and water as the balance. In addition, halogens may also be contained, in particular with a content of up to 100 g per liter of electrolyte. 
     The pH of the electrolyte is either a maximum of 6.8 or in the range between 9 and 12. 
     The electrolyte is typically brought to a temperature of 30° C. maximum, in particular 15° C., for treatment. The process temperature is in particular in the range of from 7 to 15° C. 
     The workpiece is preferably a metal workpiece printed in three dimensions, with metal grains that are joined together by laser sintering, for example, and have diameters of from 20 to 60 μm. This means that on its outer surfaces as well as on the inner surface  22 , the workpiece, expressed in a somewhat exaggerated way, is configured like an uncut polystyrene body consisting of numerous beads. 
     To polish the outer surfaces and the inner surface  22 , the workpiece is pulled back and forth through the electrolyte  12  by the motor drive  20 . The cathodes  28  are moved along with it because they are connected to the holding device  16  by means of coupling elements illustrated in dashed lines. 
     The cathode  30  is placed in the recess in advance. The spacer  34  allows electrolyte  12  to flow past it to the end of the recess on the workpiece side. For this purpose, for the spacers  34  according to  FIG. 3 , for example, axial grooves are provided on the outer shell surfaces of the spacers  34 . 
     A current, for example a pulsating current, is applied at a voltage of 10 V to 40 V and a current intensity between 0.0003 A/mm 2  and 0.01 A/mm 2 . Depending on the material of the workpiece and its initial roughness, the application time of the current is 30 s to 1200 sec. 
     Since in the region of the recess, the electrolyte  12  does not have an optimum effect between the recess and the spacers  34 , especially as far as  FIG. 3  is concerned, the cathode  30  is first brought into a certain position and, after a certain application or exposure time, is slightly shifted axially. After the polishing has previously taken place during a first polishing step between the spacers  34 , these spacers  34 , after the adjustment, are then shifted to the previously polished areas, so that the previous support portions are polished. 
     For uniform polishing, it may be useful to make the distances between neighboring spacers  34  so as to be the same as the length of the spacers, so that the cathode is shifted by the length of a spacer  34  between the polishing steps. 
     Movement of the cathode  30  may be effected manually, which, however, is primarily the case if the workpiece  14  is not moved in the electrolyte  12  by motor, but is stationary. Alternatively to this, a movement of the cathode  30  by motor may be useful. 
     To this end,  FIG. 1  schematically shows a change holder  38  provided for fixing the cathode  30 , which is provided with a current and/or electrolyte connection. A quick-change system in the change holder  38  allows new or other cathodes  30  to be inserted quickly. Furthermore, the change holder  38  may include a drive  39  for moving the cathode  30 . For example, the change holder  38  is mounted to the holding device  16  and moves the cathode  30  in steps or slowly continuously in the recess. 
     The cathode  30  shown in  FIG. 4  is caused to rotate so that the complete inner surface can be treated here as well and the spacer  34  only temporarily shields support portions on the inner surface  22 . 
     For optimum treatment, it may be useful, in addition to or as an alternative to moving the workpiece  14 , to set the electrolyte itself in motion, for example by means of a circulation pump  40  which has an inlet on one side of the container  10  and an outlet on the opposite side, so that a uniform flow is achieved in the container  10 . 
     There should be a flow of electrolyte in the recess as well. This may be effected, for example, by reciprocating the cathode  30  or causing it to rotate (see  FIG. 4 ), on the one hand, or by having a separate pump for electrolyte that is seated at the inlet of the recess. Supplying or carrying of electrolyte in the recess may also be done through a hollow cathode  30 . 
     In the case of 3D products, roughnesses of about RZ 60 are reached for the grain diameters mentioned above, the roughnesses being brought to surface qualities of RZ 3 to 4 by the polishing process. 
     The device and the method are used in particular for 3D-printed parts for aircraft, such as engine parts, with cooling ducts incorporated during printing. It is also possible by the above-mentioned machining to remove support surfaces or support ribs that were produced during 3D printing or to selectively create openings in normally inaccessible inner ribs so that flow channels are formed here. 
     But the cathode  30  may also be co-printed in 3D printing, so that it is connected to the inner surface  22  e.g. only via thin supporting walls that have to be removed, and is then exposed and only needs to be coupled to the cable  2 . Furthermore, it is conceivable that the supporting walls are made of an insulating material and therefore remain in existence during polishing. 
     The cathode  28  may also be manufactured by a generative method. For example, it is possible to manufacture the cathode  28  jointly with the workpiece  14 . 
     In the case of workpieces having edges, there is an increased charge density in the region of the edges during the electrolytic treatment. The cathode  28  can be customized for any desired outer geometry of the workpiece. For this purpose, a 2D or 3D simulation program is used in which the electrolytic treatment is also taken into account, i.e. electric and/or magnetic fields occurring later during the later treatment of the workpiece are simulated. A constant current density between the facing sides of the cathode and of the workpiece is further specified as a prerequisite in the simulation program. Once the side of the cathode facing the workpiece has been determined, the cathode can be manufactured using a generative manufacturing method based on the outer geometry ascertained in the simulation program. A cathode produced in this way is used in particular for the method described earlier and below. 
     In electrolytic treatment methods, the connection of the anode, more precisely the anode contact, to the workpiece can be problematic. Here, black spots may appear after the treatment. In order to avoid this and also to realize an effective treatment of the workpiece, the anode may be produced together with the workpiece by a generative manufacturing method, for example laser sintering. In this case, the anode is a so-called support structure, for example a support plate, which is then connected to the current source. Black spots on this support structure are not a disadvantage because the support structure is removed after treatment of the workpiece, in particular by spark erosion. 
     Furthermore, a plurality of workpieces can be produced on the same support structure and then electrolytically treated. 
       FIG. 5  shows a support plate  50  which was printed together with the workpiece  14  or, in more general terms, produced generatively. While the support plate  50  and the workpiece  40  are one part, only the support plate  50  is illustrated in hatched form for better distinguishability of the two sections, whereas the workpiece  14  is drawn as a block. In reality, the two sections transition into each other in one piece. 
     However, the support plate  50  does not necessarily have to be in the form of a plate; it may quite generally have any desired shape and constitute a support structure. 
     A singular workpiece  14  on the support plate is shown here as an example only. Preferably, a plurality of workpieces  14  are on the same support plate  15 . 
     The lower side of the support plate  50  fully rests over its entire surface on a tool-side, multi-part holding device. The holding device comprises an electrically insulating base plate  52 , in which electrical, spring-loaded contact pins  54  are guided. The contact pins  54  are connected to the power source  20  via lines. A bearing plate  56  below the base plate  52  forms the holding device together with the base plate  52 .  FIG. 5  shows a plurality of cathodes  28  for external treatment. These cathodes  28  are generatively produced simultaneously or separately from the workpiece  14 , in particular in order to ensure a uniform gap  58  or specified gap  58  between their side  60  facing the workpiece  14  and the workpiece  14 . 
     The cathodes  28  may be mounted via electrically insulating mountings  62  in openings produced in the support plate  50 , so that releasable plug connections are obtained. 
       FIG. 5  also shows the recess to be treated in the workpiece  14  and the cathode  32  inserted into it. The cathode  32  is mounted in an electrically insulating cathode receptacle  64 , which in turn is attached to the holding device  16 . 
     Electrolyte from a second container  66  is pumped directly into the container  10  through a line  70  by means of a pump  68 , on the one hand, and, furthermore, is pumped directly into the recess through a line  72 , a connection  74  and a duct inside the cathode  32 . This results in a separate electrolyte line into the recess. 
     The ratio of water activity to relative moisture (the so-called “aw value”) is set in the range of 25-70%, ideally between 27-50%.