FIELD OF THE INVENTION 
     The present invention relates to an electrode for the electrochemical fine-boring of workpieces, including an electrode tube for an electrolyte to pass through to reach an electrically conductive workpiece which is to be machined, and an electrical conductor which is connected to the electrolyte, and to a process for producing an electrode for the electrochemical fine-boring of workpieces. 
     BACKGROUND INFORMATION 
     A conventional electrode for electrochemical boring includes a glass tube which is secured in a holding device, consists of SiO 2  glass and through which an electrolyte flows. The electrolyte is brought into contact, through an opening in the glass tube which is opposite the holding device, with the area of a workpiece which is to be machined by boring. In the glass tube, there is an electrical conductor, to which a negative electric charge is applied via a DC source. The opposite, positive charge is applied to the workpiece, so that a chemical reaction leads to material being eroded at the area of the workpiece which is to be machined. The electrical conductor generally consists of a metal, such as a platinum wire. 
     A drawback of this arrangement is that when using glass tubes, the electrochemical boring is only able to produce holes or bores with a minimum diameter of 0.2 mm. Moreover, the electrode, which includes a glass tube and wire arranged therein, is of relatively complicated and sensitive construction. 
     It is an object of the present invention to provide an electrode that may be used to produce holes or bores with a very small diameter in combination with a simple electrode construction. Moreover, it is another object of the present invention to provide a process for producing such an electrode. 
     SUMMARY 
     According to the present invention, the electrode tube is formed of a metal with a substantially amorphous structure, is electrically conductive and therefore simultaneously serves as an electrical conductor for applying a charge, and is provided on its outer surface, at least in sections, with a nonconductive insulating layer. 
     One advantage of an electrode made from a tube of this type is that metals of substantially amorphous structure, so-called metallic glass materials, may be processed to form tubes having relatively small external diameters in the range from 0.2 mm to 0.04 mm. Tubes produced from metal with a substantially amorphous structure are resistant to corrosion and wear and have a virtually smooth and flawless surface, along which the flow of electrolyte is optimal. Moreover, it is possible to eliminate the introduction of a metallic conductor, such as a platinum wire, since the negative charge of a DC source is applied to the tube itself. The positive charge of the DC source is applied to the workpiece which is to be machined. 
     The external diameter of the tube may be at most slightly less than 0.2 mm, and may be in the range from 0.2 mm to 0.04 mm, so that it is substantially possible to produce holes or bores with a diameter of 0.2 mm and below. 
     The tube may be of any desired cross-sectional geometry and may be adapted to various applications, so that it is also possible for holes which are not circular in cross section to be formed in the workpiece. 
     The tube may furthermore consist of a metallic solid glass, such as a ZrTiNiCuBe alloy, it also being possible to use other suitable alloys and therefore other metals which are of a substantially amorphous structure. 
     The nonconductive insulating layer may be a layer based on AlON or TiAlON or organic, such as coating, materials, or inorganic oxide ceramic, the layer thickness of this layer being very small. The thickness of the insulating layer may be less than 10 μm and is generally between 4 μm and 10 μm. With an insulating layer based on TiAlON, the layer may be applied to the tube made from a metal of substantially amorphous structure by sputtering or vapor deposition. 
     The entire outer surface of the tube may be provided with a nonconductive insulating layer, in order to prevent electrochemical corrosion or additional undesired erosion during fine-boring at the wall of the bore formed in the workpiece. 
     Those sections of the tube which are provided with the nonconductive insulating layer may be provided with an electrically conductive metal or graphite layer, to which, as a so-called protective current electrode, an opposite charge to that of the tube made from a metal of substantially amorphous structure may be applied. 
     According to the present invention, the process for producing an electrode for electrochemical fine-boring includes the steps of providing an electrically conductive metal with a substantially amorphous structure as material for the electrode tube, drawing the electrode tube from a semi-finished tube of larger external diameter to a smaller external diameter by glass drawing, and applying a nonconductive insulating layer at least to sections of its outer surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an electrochemical boring arrangement with an electrode, according to the prior art. 
     FIG. 2 illustrates a first example embodiment of an electrochemical fine-boring device having an electrode according to the present invention. 
     FIG. 3 illustrates a second example embodiment of an electrochemical fine-boring arrangement having an electrode according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 schematically illustrates an electrochemical boring arrangement which is known from the prior art, with an electrode  1  that includes a glass tube  2  and a platinum wire  3  which is arranged therein and serves as an electrical conductor. An electrolyte  4  flows through the glass tube  2  and, at the end side, emerges in a region of a workpiece  5  which is to be machined by electrochemical boring. A DC source  12  is used to apply opposite charges to the platinum wire  3  and the workpiece  5 , so that the chemical reaction which is required for the electrochemical boring occurs with the exchange of electrons. A negative charge is applied to the platinum wire  3 , and a positive charge is applied to the workpiece  5 . During the boring, the electrode  1  is moved in the direction of arrow V. 
     When using a glass tube, the bores  13  which are to be produced in the workpiece  5  using the electrochemical boring process are limited to a minimum diameter of 0.2 mm. Moreover, the structure of the electrode  1  including the glass tube  2  and the platinum wire  3  arranged therein is complicated and sensitive. 
     FIG. 2 schematically illustrates an electrochemical fine-boring arrangement according to one configuration of the electrode according to the present invention, which is denoted overall by  6  and includes an electrode tube  7  made from a metal of substantially amorphous structure, such as, for example, a ZrTiNiCuBe alloy. The tube  7  has an external diameter D which is less than 0.2 mm and, depending on the size of the desired bore  13  in the workpiece  5 , is in the range between 0.2 mm and 0.04 mm. Opposite electric charges are applied to the electric tube  7  and the workpiece  5  by a DC source  12 , a negative charge being applied to the tube  7  and a positive charge being applied to the workpiece  5 . 
     An electrolyte  4  flows through the tube  7  from a holding device  8  to an opening  9  of the tube  7 , where it comes into contact with that area of the workpiece  5  which is to be eroded by electrochemical boring, in order to erode the material of the workpiece  5  by a standard chemical reaction with exchange of electrons. The tube  7  is secured in a conventional manner in the holding device  8 , through which the electrolyte  4  is fed to the tube  7 . 
     In the present configuration, a nonconductive insulating layer  10  is applied to the entire outer surface of the tube  7 , in order to prevent electrochemical corrosion or an additional, undesired erosion during fine-boring at the wall of the bore  13 . The insulating layer  10  ensures that there is no current passing through at this point and therefore there is neither corrosion nor undesirable erosion of material. The chemical reaction occurs exclusively in the region of the opening  9  of the tube  7  and leads to erosion of material and to the bore  13  becoming deeper. The insulating layer  10  is a layer of a mixed oxide, for example based on TiAlON, which has been applied by sputtering or vapor deposition and has a layer thickness which is generally less than 10 μm and therefore only slightly increases the desired small external diameter of the tube  7 . 
     FIG. 3 schematically illustrates an alternative configuration of the electrode according to the present invention, which is denoted overall by  6  and in which an electrode tube  7  consists of an electrically conductive metal which is of substantially amorphous structure. The tube  7  is held in a holding device  8 , from which it is supplied with an electrolyte  4 . The electrolyte  4  flows through the tube  7  and emerges, at an opening  9  of the tube  7 , at that area of a workpiece  5  which is to be machined. A negative charge is applied to the electrically conductive tube  7  via a DC source  12 , and a positive charge is applied to the workpiece  5  in which a bore  13  is to be made by electrochemical fine-boring via the DC source  12 . A nonconductive insulating layer  10  based on an oxide ceramic, such as, for example, Al 2 O 3 , is applied to an outer surface of the tube  7 , so that there is no current passing through this layer. The foregoing arrangement may, for example, prevent electrochemical corrosion or additional, undesired erosion of material in the region of the wall of the bore  13  in the workpiece  5 . 
     The tube  7  has an external diameter D of 0.09 mm, so that a bore  13  of slightly larger than 0.09 mm is produced, taking into account the thickness of the insulating layer  10  of approximately 7 μm. Therefore, the material is eroded exclusively in the region of the opening  9  of the tube  7  as a result of a conventional chemical reaction with exchange of electrons, since the electrolyte is connected firstly to the electrically conductive tube  7  and secondly to the workpiece  5 , which is likewise electrically conductive. During the boring operation, the electrode  6  is moved in the direction of arrow V, in order to achieve the desired depth of the bore  13 . Due to the insulating layer  10 , there is no undesirable erosion of material at the wall of the bore  13 . 
     In the foregoing example embodiment of the present invention, an electrically conductive metal layer  11  is applied to the insulating layer  10  as a so-called protective current electrode, to which a positive charge, i.e., an opposite charge to that applied to the tube  7 , is applied via a DC source  14 , in order in this manner to additionally prevent the partial etching or undesired widening of the wall of the bore  13  in the workpiece  5 . Alternatively, a different electrically conductive layer  11 , such as, for example, a graphite layer, may also be applied as protective current electrode. The opposite, negative charge is applied to the workpiece  5  by the DC source  14 . 
     The electrode  6  for electrochemical fine-boring in workpieces which is illustrated in FIGS. 2 and 3 is produced so that the material used for the electrode tube  7  is an electrically conductive metal of substantially amorphous structure, i.e., a so-called metallic glass material, such as, for example, a ZrTiNiCuBe alloy, and the electrode tube  6  is drawn from a semi-finished tube of larger external diameter to an external diameter D in the range from 0.2 mm to 0.04 mm by glass drawing in an inert gas atmosphere. Then, an insulating layer  10  is applied to the outer surface of the electrode tube  6  produced in this manner, so that during the fine-boring of a bore  13  into a workpiece  5 , there is passage of current only in the region of the opening  9  of the electrode tube  6 , due to the electrolyte  4  emerging at that location. The insulating layer  10  substantially is formed of AlON, TiAlON or an oxide ceramic. In the case of the electrode  6  illustrated in FIG. 3, an electrically conductive metal or graphite layer  11  is then also applied to the insulating layer  10 , for example, by sputtering or vapor deposition, this layer serving as a protective current electrode.
An electrode for the electrochemical fine-boring of workpieces includes an electrode tube for an electrolyte to pass through to reach a workpiece which is to be machined, and an electrical conductor which is connected to the electrolyte, in which electrode the tube, for the purpose of boring bores with the smallest possible diameter, consists of a metal with a substantially amorphous structure, is electrically conductive and is provided on its outer surface, at least in sections, with a nonconductive insulating layer.