Patent Publication Number: US-2018029151-A1

Title: Electrochemical machining device and electrochemical machining method

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to an electrochemical machining device and electrochemical machining method. 
     Description of the Related Art 
     Electrolyte jet machining (refer to non-patent publication 1 and patent publications 1 and 2 below) is known as one type of electrochemical machining. Electrolyte jet machining is a method for selectively machining only directly below jet flow of an electrolyte, by discharging electrolyte from a nozzle and applying a voltage to a gap between the nozzle and a workpiece. In this case, the nozzle acts as a tool electrode. At the time of electrochemical machining, a voltage is applied between electrodes so that a workpiece becomes an anode, and electric current flows via an electrolytic solution. This machining method has the advantage that since the machining principle is electrolytic action, which is a chemical reaction, it is possible to machine a workpiece regardless of hardness as long as it is a conductive material, and affected layers or residual stress, burrs and cracks etc. do not arise. It is also possible to perform maskless machining of an arbitrary shape by scanning a nozzle. 
     The present inventors have heretofore disclosed obtaining a mirror surface of small surface roughness under high current density, and obtaining complex porous properties under low current density, by controlling current density, using SUS304 as a workpiece material (non-patent publication 2 below). With the machining method disclosed in these publications, surface roughness of a workpiece is deteriorated due to passage of a low current density region as a result of scanning. 
     The present inventors have therefore disclosed being able to perform mirror finishing in an arbitrary shape by scanning a nozzle (tool electrode) at high speed and through multiple reciprocations (non-patent publication 3 below). 
     However, generally, in the case of machining complex shapes, in order to scan an electrode a plurality of times at high speed considerable device cost and running cost become necessary. Also, in the case of carrying out high speed scanning, a problem arises in that the electrolytic solution flies out due to movement of the electrode. 
     It should be noted that non-patent publication 4 below shows attempts to achieve machining precision through the use of electrochemical machining using ultrashort pulse current that utilizes formation of an electrical double layer on an electrode surface. 
     NON-PATENT DOCUMENTS 
     Non-Patent Document 1
     Kunieda M, Yoshida M, Yoshida H, Akamatsu Y (1993) Influence of Micro Indents Formed by Electro-chemical Jet Machining on Rolling Bearing Fatigue Life. ASME PED 64:693-699.   

     Non-Patent Document 2
     Kawanaka T, Kunieda M (2014) Selective Surface Texturing Using Electrolyte Jet Machining. Procedia of 2nd CIRP Conference on Surface Integrity (CSI) (13): 345-349.   

     Non-Patent Document 3
     Natsu W, Ikeda T, Kunieda M (2007) Generating Complicated Surface with Electrolyte Jet Machining. Precision Engineering 31: 33-39.   

     Non-Patent Document 4
     Schuster R, Kircher V, Allonfue P, Etrl F (2000) Electrochemical Micromachining. Science 289(5476): 98-101.   

     PATENT PUBLICATIONS 
     
         
         Patent Publication 1: Japanese patent laid-open 2006-55933 
         Patent Publication 2: Japanese patent laid-open 2011-110641 
       
    
     BRIEF SUMMARY 
     The present disclosure has been conceived in view of the above-described situation. The disclosure provides technology that can improve roughness of a machined surface while keeping the relative scanning speed of a tool electrode with respect to a workpiece low. 
     Means for solving the above described problems can be as disclosed in any of the following aspects. 
     (Aspect 1) 
     An electrochemical machining device for machining a surface of a workpiece using electrochemical machining, comprising a power source, a tool electrode, an electrolytic solution supply section, and a charge control means. The power source applies a voltage, for making current for electrochemical machining flow, between the tool electrode and the workpiece. The tool electrode is arranged apart from the workpiece and is capable of being scanned relatively along a surface direction of the workpiece. The electrolytic solution supply section can supply electrolytic solution for electrochemical machining between the tool electrode and the workpiece. The charge control means eliminates an electrical charge that has accumulated between the tool electrode and the workpiece as a result of voltage application from the power source. 
     (Aspect 2) 
     The electrochemical machining device of aspect 1, wherein the power source uses pulse current as the current, and the charge control means eliminates the electrical charge based on a duty factor of the pulse current. 
     (Aspect 3) 
     The electrochemical machining device of aspect 2, wherein an upper limit of an absolute value of a pulse width for the pulse current is set short enough to apply mirror finishing to a surface of the workpiece, and a current density of the current for electrochemical machining is set high enough for application of the mirror finishing. 
     (Aspect 4) 
     The electrochemical machining device of aspect 1, wherein the power source uses alternating current as the current, and the alternating current has a forward direction current component that makes the workpiece an anode and a reverse direction current component that makes the workpiece a cathode, and the charge control means eliminates the electrical charge by applying the reverse direction current component. 
     (Aspect 5) 
     The electrochemical machining device of aspect 4, wherein the charge control means controls the power source so that a value obtained by integrating a forward direction current density of the forward direction current component over and application time, and a value obtained by integrating a reverse current density of the reverse direction current component over an application time, become substantially equal. 
     (Aspect 6) 
     The electrochemical machining device of aspect 5, wherein the charge control means sets a peak value for the reverse direction current component lower than a peak value for the forward direction current component. 
     (Aspect 7) 
     The electrochemical machining device of any one of aspects 4-6, wherein the charge control means eliminates the electrical charge by inserting a current idle period between the forward direction current component and the reverse direction current component of the alternating current. 
     (Aspect 8) 
     The electrochemical machining device of any one of aspects 4-7, wherein the tool electrode is made from monocrystalline silicon, a titanium alloy, a niobium alloy, graphite, or platinum. 
     (Aspect 9) 
     The electrochemical machining device of any one of aspects 1-8, wherein the power source is a constant voltage source or a constant current source. 
     (Aspect 10) 
     The electrochemical machining device of any one of aspects 1-9, wherein at least part of a surface of the workpiece has a curvature that is not 0, and an opposing area of the workpiece and the tool electrode is made small enough, within a range of the opposing area, to be able to regard distribution of distance between the tool electrode and the workpiece as substantially constant regardless of the curvature. 
     (Aspect 11) 
     The electrochemical machining device of any one of aspects 1-10, wherein the charge control means a changes content of control for eliminating the electrical charge in accordance with relative scanning of the tool electrode. 
     (Aspect 12) 
     An electrochemical machining method that uses the electrochemical machining device of any one of aspects 1-11 comprising a step of making a current for electrochemical machining flow between the tool electrode and the workpiece using the power source, a step of scanning the tool electrode relatively along the surface direction of the workpiece, a step of supplying electrolytic solution between the tool electrode and the workpiece using the electrolytic solution supply section, and a step of eliminating the electrical charge that has accumulated between the tool electrode and the workpiece using the charge control means. 
     (Aspect 13) 
     A surface roughness adjustment method that uses the electrochemical machining device of any one of aspects 1-11 comprising a step of making a current for electrochemical machining flow between the tool electrode and the workpiece using the power source, a step of scanning the tool electrode relatively along the surface direction of the workpiece, a step of supplying electrolytic solution between the tool electrode and the workpiece using the electrolytic solution supply section, and a step of adjusting surface roughness of the workpiece surface by controlling the electrical charge that has accumulated between the tool electrode and the workpiece using the charge control means. 
     (Aspect 14) 
     An electrochemical machining device, comprising a tool electrode arranged apart from the workpiece, an electrolytic solution that fills a gap between the workpiece and the tool electrode, and a power source that supplies forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, a peak value of the reverse direction current pulses is made lower than a peak value of the forward direction current pulses, and a pulse width of the reverse direction current pulses is set wider than a pulse width of the forward direction current pulses. 
     (Aspect 15) 
     An electrochemical machining device, comprising a tool electrode arranged apart from the workpiece, an electrolytic solution that fills a gap between the workpiece and the tool electrode, and a power source that supplies forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, a peak value of the reverse direction current pulses is made higher than a peak value of the forward direction current pulses, and a pulse width of the reverse direction current pulses is set narrower than a pulse width of the forward direction current pulses. 
     (Aspect 16) 
     An electrochemical machining device, comprising a tool electrode arranged apart from the workpiece, an electrolytic solution that fills a gap between the workpiece and the tool electrode, and a power source that supplies forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, a pulse width of the reverse direction current pulses is set to less than or equal to a pulse width of the forward direction current pulses, and when switching from the reverse direction current pulses to the forward direction current pulses, an idle period is provided. 
     (Aspect 17) 
     An electrochemical machining device, comprising a tool electrode arranged apart from the workpiece, an electrolytic solution that fills a gap between the workpiece and the tool electrode, and a power source that supplies forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, with respect to a forward direction electrical charge that is supplied between the workpiece and the tool electrode by the forward direction current pulses, a reverse direction electrical charge that is supplied between the workpiece and the tool electrode by the reverse direction current pulses is set so as to become smaller, and when switching from the reverse direction current pulses to the forward direction current pulses, an idle period is provided. 
     (Aspect 18) 
     An electrochemical machining device, comprising a tool electrode arranged apart from the workpiece, an electrolytic solution that fills a gap between the workpiece and the tool electrode, and a power source that supplies forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, a ratio (A/B) of a forward direction electrical charge (A) that is supplied between the workpiece and the tool electrode by the forward direction current pulses, and a reverse direction electrical charge (B) that is supplied between the workpiece and the tool electrode by the reverse direction current pulses, is set so as to become larger, as speed of the scanning becomes faster. 
     (Aspect 19) 
     An electrochemical machining device, comprising a tool electrode arranged apart from the workpiece, an electrolytic solution that fills a gap between the workpiece and the tool electrode, and a power source that supplies forward direction current pulses between the workpiece and the tool electrode, wherein in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, a duty factor for applying the forward direction current pulses is set so as to become larger, as speed of the scanning becomes faster. 
     (Aspect 20) 
     An electrochemical machining device, comprising a tool electrode arranged apart from the workpiece, an electrolytic solution that fills the gap between the workpiece and the tool electrode, and a power source for supplying forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, a pulse width of the reverse direction current pulses is set so as to become smaller with respect to a pulse width of the forward direction current pulses, as speed of the scanning becomes faster. 
     According to the present disclosure, it is possible to improve the roughness of a machined surface while keeping a relative scanning speed of a tool electrode, with respect to a workpiece, low. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is an explanatory drawing for explaining the overall structure of an electrochemical machining device of a first embodiment of the present disclosure. 
         FIG. 2  is an expanded cross-sectional view of parts of  FIG. 1 . 
         FIG. 3  is a schematic explanatory drawing for describing the machining principle of electrochemical machining used in the machining device of  FIG. 1 . 
         FIG. 4  is an explanatory drawing showing a pulse current waveform used in a practical example 1, with the vertical axis being current [A] and the horizontal axis being time [s]. 
         FIG. 5  is a graph showing experimental results of the practical example 1, with the vertical axis being surface roughness Rz[μm] and the horizontal axis being translation speed [mm/s]. 
         FIG. 6  is an explanatory drawing showing a pulse current waveform (AC waveform) used in a practical example 2, with the vertical axis being current [A] and the horizontal axis being time [s]. 
         FIG. 7  is a graph showing experimental results of the practical example 2, with the vertical axis being surface roughness Rz[μm] and the horizontal axis being translation speed [mm/s]. 
         FIG. 8  is an explanatory drawing showing another pulse current waveform (AC waveform) that can be used in the practical example 2, with the vertical axis being current [A] and the horizontal axis being time [s]. 
         FIG. 9  is an explanatory drawing for describing the schematic structure of an electrochemical machining device of a second embodiment of the present disclosure. 
         FIG. 10  is an explanatory drawing for describing the schematic structure of an electrochemical machining device of a third embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, an electrochemical machining device (hereafter sometimes referred to as “machining device”) of a first embodiment of the present disclosure will be described with reference to the attached drawings. The machining device of this embodiment is for machining a workpiece  1  (refer to  FIG. 1 ) from a surface side thereof using so-called electrolyte jet machining, and in particular is a preferred device in finish machining of a workpiece surface. 
     Structure of the First Embodiment 
     The machining device of this embodiment comprises a power source  10 , a tool electrode  20 , an electrolytic solution supply section  30  and charge control means  40  as basic elements (refer to  FIG. 1 ). This machining device is additionally provided with a workpiece support section  50  and a scanning drive section  60 . 
     (Power Source) 
     The power source  10  applies a voltage, for making current for electrochemical machining flow, between the tool electrode  20  and a workpiece  1 . In more detail, one electrode of the power source  10  is electrically connected to the tool electrode  20 , another electrode of the power source  10  is electrically connected to the workpiece  1 , and it is possible to apply a given voltage between the two. 
     The power source  10  of this embodiment uses a pulse waveform voltage (pulse voltage) as a voltage to be applied, and in this way it is possible to make current having a pulse waveform (refer to practical examples 1 and 2 which will be described later) flow between the electrodes. It should be noted that with this embodiment, as the power source  10 , a constant current source that makes a current value that has been designated by the charge control means  40  flow between electrodes is used, but a constant voltage source may also be used as long as the necessary current value can be obtained. It is also possible to use a so-called high-speed bipolar power source, for example, as the power source  10  of this embodiment. 
     In the power source  10  of this embodiment, an upper limit of an absolute value for the pulse width of the pulse current is set small enough that a mirror finishing can be applied to a surface of the workpiece  1 . Also, a lower limit for the current density is set to a value at which machining marks are finished to a mirror surface while the tool electrode is at rest. What level of current density and pulse width should be set to make it possible to accomplish mirror finishing can be determined by experimentation, for example. Here, the current density is obtained by dividing electrical current by an opposing area of the tool electrode and the workpiece. Also, the mirror finishing is machining that makes surface roughness small, for example, machining to make surface roughness Rz 0.3 μm or less. An upper limit for the absolute value of the pulse width is, for example, 150 μs to 100 μs. However, the present disclosure is not limited to these numerical values. 
     (Tool Electrode) 
     The tool electrode  20  is arranged apart from the workpiece  1 , and is capable of being scanned relatively along a surface direction of the workpiece  1 . Specifically, the tool electrode  20  of this example is provided with a base portion  21  and a tip portion  22 . 
     The base portion  21  is formed with a hollow cylindrical shape, and connected to piping  32  (described later) of the electrolytic solution supply section  30  which is constructed so as to feed electrolytic solution to the tip portion  22 . 
     The tip portion  22  extends from a tip side of the base portion  21  in the direction of the workpiece  1  (downward direction in  FIG. 1 ). The tip portion  22  is also formed with a hollow cylindrical shape, so as to be able to discharge a jet of electrolytic solution  3  (described later) to the workpiece  1  (refer to  FIG. 2 ). It should be noted that in  FIG. 2  only portions of the tip portion  22  are shown enlarged. Also, the flow state of the electrolytic solution  3  in  FIG. 2  is only shown schematically and is not an accurate depiction. The tool electrode  20  of this embodiment has the function of a nozzle for so-called electrolyte jet machining. 
     As material for the tool electrode  20 , various materials can be used as long as it is conductive, and has the necessary mechanical strength. In particular, in a case where monocrystalline silicon, a titanium alloy, a niobium alloy, graphite or platinum has been used as the material for the tool electrode  20 , these materials are suitable since they are difficult to electrolyze even if a reverse direction voltage (voltage that makes the workpiece a cathode) is applied. 
     Here, in a case where at least part of the surface of the workpiece  1  has a curvature that is not 0, an opposing area of the workpiece  1  and the tool electrode  20  is preferably sufficiently small that the distance between the tool electrode  20  and the workpiece  1  within this opposing area is effectively uniform regardless of the curvature. If this is done, it is possible to apply appropriate mirror finishing even to a workpiece having a non-flat surface to be machined, by scanning the machined surface. Making the distance between the workpiece  1  and the tool electrode  20  constant, and sufficiently narrow (for example, making a gap of 1 mm or less, more preferably 0.5 mm or less) is preferable for the realization of mirror finishing. Accordingly, using a tool electrode  20  that has a small opposing area like this contributes to satisfying these gap conditions. 
     (Electrolytic Solution Supply Section) 
     The electrolytic solution supply section  30  can supply electrolytic solution for electrochemical machining between the tip portion  22  of the tool electrode  20  and the surface to be machined of the workpiece  1  (upper surface in  FIG. 1 ). In more detail, the electrolytic solution supply section  30  of this embodiment is provided with a tank  31 , piping  32 , a pump  33  and a sink  34 . 
     The tank  31  is a section for accumulating the electrolytic solution  3  for electrochemical machining. Here, it is possible to use various fluids that are normally used for electrochemical machining as the electrolytic solution  3 . 
     The piping  32  connects between the tank  31  and the base portion  21  of the tool electrode  20 . The pump  33  is attached at some point along the piping  32 , and can feed the electrolytic solution  3  to the tool electrode  20  in an appropriate flow amount. With this embodiment, in order to obtain an accurate flow amount, a so-called gear pump has been used as the pump  33 , but this is not restrictive. 
     The sink  34  is a section that temporarily holds electrolytic solution that has been supplied towards the workpiece  1 . A drain section  341  for discharging the electrolytic solution  3  that has been supplied is formed in the sink  34 . The electrolytic solution  3  that has been discharged from the sink  34  is recovered using an appropriate method that is not illustrated. 
     (Charge Control Means) 
     The charge control means  40  is a functional element for eliminating electrical charge that has accumulated between the tool electrode  20  and the workpiece  1  as a result of voltage application from the power source  10 . More specifically, the charge control means  40  of this embodiment is implemented as a controller (for example, a function generator) configured to adjust a current waveform (for example, current pulse width, peak value, pulse frequency etc.) attributable to voltage from the power source  10 . As an actual device structure, the charge control means  40  may be part of the functions within the power source  10 . Basically, the charge control means is not restricted to a mechanical structure as long as it is capable of exhibiting the necessary functions, and can also be implemented, for example, using a combination of a computer and computer programs, and does not need to exist as a single element. 
     The charge control means  40  of this embodiment is configured to eliminate an electrical charge that has accumulated between the electrodes based on a duty factor of pulse current. Detailed operation of the charge control means  40  (specifically a current waveform) will be described later using practical examples 1 and 2. 
     (Workpiece Support Section) 
     The workpiece support section  50  supports the workpiece  1  which constitutes the object of the electrochemical machining, and with this embodiment is constructed using a machining table. 
     (Scanning Drive Section) 
     The scanning drive section  60  comprises an XZ direction drive section  61  and a Y direction drive section  62  with this embodiment. The XZ direction drive section  61  can move the tool electrode  20  at a given speed in an X direction (left to right direction in  FIG. 1 ), and in a Z direction (up and down direction in  FIG. 1 ). The Y direction drive section  62  can move the workpiece support section  50  at a given speed in a Y direction (a direction perpendicular to the sheet of  FIG. 1 ), via the sink  34 . Control of operations such as scanning speed and scanning direction of the scanning drive section  60  can be implemented by a controller, not illustrated. Using the scanning drive section  60 , with this embodiment it is possible to scan the tool electrode  20  relative to the workpiece  1  in an arbitrary direction within an XY plane. 
     Operation of the Machining Device of the First Embodiment 
     Operation of the machining device of the previously-described first embodiment will be described in the following. 
     (Description of Machining Principle) 
     As a prerequisite for operation description, the machining principle of the electrochemical machining of this embodiment will be described with further reference to  FIG. 3 . 
       FIG. 3  shows potential distribution within a jet flow and current density distribution on the workpiece surface, for the electrolytic solution  3  that has been jetted out from a cylindrical nozzle acting as a tool electrode. With electrochemical machining, the current density at opposing portions of the tool electrode and the workpiece is high, and the current density is lowered moving away from that area. Reference numeral  3   a  in  FIG. 3  schematically represents a current density distribution of the electrolytic solution  3  that exists between the workpiece  1  and the tool electrode  20  (omitted from within  FIG. 3 ). Also, reference numerals  3   b  in  FIG. 3  schematically represent equipotential surfaces, and here reference numeral V 0  is power source voltage applied between the electrodes (in a case where the workpiece side constitutes an anode). 
     With machining that scans a tool electrode (scanning machining), when directly beneath a nozzle of high current density that passes a point on the workpiece, if that current density is sufficiently high that a mirror surface can be obtained when the nozzle is at rest, the workpiece surface directly beneath the nozzle is machined to a mirror surface. However, after that, a peripheral section thereof with low current density passes through that machining portion. In a case where the scanning speed is slow, the workpiece surface is roughened by electrochemical machining due to the peripheral section with low current density, and surface roughness becomes large (that is, from the perspective aimed at mirror finishing, the mirror surface is degraded). 
     However, a machining amount per unit time in the case of low current density is significantly inferior compared to the case of high current density. If scanning speed is raised, the sojourn time of the electrolytic solution jet for a single scan (namely the duration for which electrochemical machining using the opposed electrode is carried out) becomes short, and the effect of the low current region becomes small. As a result, it can be considered that surface roughness improves with an increase in scanning speed, as disclosed in previously-described nonpatent publication 3. 
     On the other hand, in recent years, for the purpose of improving machining precision, electrochemical machining has been carried out using an ultrashort pulse current that utilizes formation of an electrical double layer on an electrode surface (previously-described non-patent publication 4). If a voltage is applied between the workpiece  1  and the tool electrode  20 , an electrical double layer having a form that confronts positive and negative electrical charge is formed on the workpiece surface, as shown in  FIG. 3 . The phenomenon forming this electrical double layer can be realized by charging an electrical charge into a capacitor C DL  and because of differences in current density a difference in charge time of the electrical double layer arises. It should be noted that reference numeral R in the drawing represents a resistive component within the electrolytic solution jet. 
     According to the findings of the present inventors, an electrolytic reaction does not occur if an electrical double layer is not sufficiently formed. Before an electrical double layer is formed at a low current density portion, elution of the workpiece is confined to a high current density portion directly below the tool electrode by turning off pulse current (namely, turning off machining voltage). Electrical charge of the electrical double layer is then discharged in an idle period after a pulse has been turned off, and the next pulse is applied. At this time, if the idle time is not sufficient, the electrical charge of the electrical double layer will not be completely discharged, and an electrolytic reaction will occur even in a low current density region. Accordingly, by making machining current short pulses and providing a sufficient idle time, it is possible to confine elution of the workpiece to only a high current density region, and even in cases of scanning at low speed it should be possible to obtain a machining surface having a good surface roughness. To give further information, if current density so as not to be able to obtain a mirror surface at the time the tool electrode is at rest is provisionally known, it is preferable to set a time for which a workpiece is exposed to that type of low current density region to short pulse width machining current when an electrode has been scanned, so as to be shorter than a time in which machining at that low current density will occur. 
     The above is the machining principle that is the prerequisite for this embodiment, based on the findings of the present inventors. 
     (Electrochemical Machining Operation) 
     In the case of carrying out electrochemical machining using the machining device of this embodiment, first of all the workpiece  1  is arranged on an upper surface of the workpiece support section  50  (refer to  FIG. 1 ). Then electrolytic solution  3  is supplied between the tool electrode  20  and the workpiece  1  by the electrolytic solution supply section  30 . At this time, the electrolytic solution  3  passes through the tip portion  22  of the tool electrode  20  and is supplied to machining locations (refer to  FIG. 2 ). On the other hand a voltage is applied at a given waveform between the tool electrode  20  and the workpiece  1 , by the power source  10 . In this way it is possible to make current for specified electrochemical machining flow between the two. Here, a waveform of electrical current that flows between the tool electrode  20  and the workpiece  1  is set by the charge control means  40  (refer to practical examples 1 and 2 that will be described later). 
     Further, with this embodiment the tool electrode  20  is scanned relatively within an XY plane with respect to the workpiece  1  using the scanning drive section  60 . In this way the surface of the workpiece  1  is machined in the scanning direction, and it is possible to improve the surface roughness. It should be noted that, with this embodiment, a Z direction position of the tool electrode  20  is fixed during machining, and is adjusted as required. 
     In the following, the electrochemical machining operation of this example will be described in more detail with reference to a specific practical example. 
     Practical Example 1 
     Based on the previously-described machining principle, electrochemical machining is carried out for conditions (Pulse) in table 1 below, using the device structure of the first embodiment. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Table 1 Machine conditions 
                 Pulse 
                 AC 
               
               
                   
               
             
            
               
                 Pulse on time [μs] 
                 100 
                 100 
               
               
                 Duty factor [%] 
                 1, 10 
                 59, 67, 77 
               
            
           
           
               
               
            
               
                 Machining current [A] 
                 +3.0 
               
               
                 Current density [A/cm2] 
                 187 
               
               
                 Gap width [mm] 
                 0.5 
               
               
                 Flow rate [ml/s] 
                 5.2 
               
               
                 Nozzle inner diameter [mm] 
                 1.43 
               
               
                 Electrolyte 
                 NaNO 3 aq 20 wt % 
               
               
                   
               
            
           
         
       
     
     The meaning of the items shown in this table is shown below. It should be noted that for reference one example of a current waveform based on a pulse voltage applied from the power source is shown in  FIG. 4 . Also, in table 1, conditions (AC) for a case where an alternating waveform is applied are also depicted, but the alternating case will be described later. 
     Pulse on time [μs]: Machining current application time (t 1  in  FIG. 4 ); 
     Duty factor [%]: Duty factor (t 1 /T in  FIG. 4 ); 
     Machining current [A]: current flowing between electrodes; 
     Current density [A/cm 2 ]: current density between electrodes; 
     Gap width [mm]: distance between the tool electrode and the workpiece; 
     Flow rate [ml/s]: flow rate of electrolytic solution; 
     Nozzle inner diameter [mm]: inner diameter of nozzle of tip portion of tool electrode; 
     Electrolyte: electrolytic solution. 
     Current density between the electrodes is not uniform, but in the previous description, a current density value was obtained by dividing the machining current value by the internal area of the nozzle vent, for the sake of simplicity. As will be understood, the descriptions in table 1 are merely one example, and other appropriate structures are possible. 
     With practical example 1, pulse current is used. Pulse width t 1  is made a constant value such a 100 μs, and grooving is carried out on the workpiece  1  by scanning the tool electrode  20  while varying pulse idle time t 2  and scanning speed. Reference numeral T in  FIG. 4  shows the pulse period at this time. The workpiece  1  is made an anode and the direction of current flowing from the workpiece  1  to the tool electrode  20  is made the vertical axis direction in  FIG. 4 . Also, for the purpose of comparison, the same machining was also carried out with direct current (DC), namely under conditions of t 2 =0, duty factor=100%. In this way the effect of pulse current duty factor and scanning speed on the surface roughness was investigated. SUS304 was used as the workpiece  1 . With regard to surface roughness of the workpiece  1 , maximum height roughness Rz was measured and averaged at four different locations within a machined surface. 
     Measurement results for surface roughness when the scanning speed was varied are shown in  FIG. 5 . The curved lines indicated as “Duty” in  FIG. 5  show the result data at a given duty factor (%). As shown in  FIG. 5 , in a case where pulse current is used, and in a case where DC is used, it will be understood that the surface roughness is lowered by raising the scanning speed. On the other hand, when the scanning speed is slow, with a pulse width of 100 μs, the surface roughness improves with an increase in length of the idle period (that is, a lowering of the duty factor). In this way, in a case where sufficient voltage idle time has been set for a constant value such as pulse width 100 μs (that is, a duty factor of 1%), it will be understood that even in the case of low speed scanning, a mirror surface of good surface roughness can be obtained. It can be assumed that this is because, as was discussed above, in electrolyte jet machining (namely electrochemical machining where an electrode is scanned in an arbitrary direction) also, a machining region is confined to only a high current density region. 
     Accordingly, according to practical example 1, it is possible to eliminate the electrical charge between the electrodes using the charge control means  40 , and in this way it will be understood that it is possible to improve the surface roughness of the machined surface while keeping the scanning speed low. It is also possible to control the surface roughness of the machined surface by adjusting the duty factor using the charge control means  40 . 
     Practical Example 2 
     After changing the voltage waveform to be applied (namely the current waveform flowing between electrodes) to that shown in  FIG. 6 , electrochemical machining is carried out under the same conditions as for practical example 1. AC in table 1 shows the pulse waveform (alternating current waveform) used in practical example 2. Practical example 2 is configured so that in period t 3  (refer to  FIG. 6 ), a reverse direction voltage is applied and reverse direction current flows. In this way, alternating current flowing between electrodes has a forward direction current component (time t 1 ) that flows, with the workpiece  1  as an anode, from the workpiece  1  to the tool electrode  20 , and a reverse direction current component (time t 3 ) that flows, with the workpiece as a cathode, from the tool electrode  20  to the workpiece  1 . 
     In a case where a unipolar pulse current is used, such as in practical example 1, in order to obtain a mirror surface with low speed scanning it is necessary to lengthen the idle time (refer to  FIG. 5 ). However, if the duty factor is lowered, the removal amount per unit time is low compared to the case of using direct current, which tends to increase the machining time. 
     With practical example 2, by using alternating current as described previously, the polarity of current flowing between the electrodes is reversed and a time t 3  exists where the workpiece becomes a cathode. During this time t 3 , it is possible to forcibly discharge the electrical charge that has been charged into an electrical double layer (refer to capacitor C DL  in  FIG. 3 ). That is, the charge control means  40  of practical example 2 is configured to eliminate the electrical charge that has been charged into the electrical double layer using application of the reverse direction current component. It should be noted that in this specification, “eliminate the electrical charge” means so-called discharging of the electrical charge, and means not only the complete elimination of the electrical charge but also a reduction in the amount of electrical charge that has been accumulated. 
     In this example, it can be considered that mirror finishing will become possible, without setting a long idle time, by using an alternating current waveform. With this practical example 2, therefore, the pulse width t 1  to make the workpiece  1  an anode was made constant at 100 μs, and the time t 3  to make the workpiece  1  a cathode, and the scanning speed, were varied, and the effect of the AC current duty factor and scanning speed on the surface roughness was investigated. 
     Experimental results for practical example 2 are shown in  FIG. 7 . As shown in  FIG. 7 , in a case where AC current is used also, similarly to when direct current and unipolar pulse current is used (refer to  FIG. 5 ), it will be understood that the surface roughness is lowered together with raising of the scanning speed. Also, when the scanning speed is slow, with a pulse width of 100 μs, it is possible to improve the surface roughness with increase in the polarity inversion time t 3  (that is, a lowering of the duty factor). On the other hand, in a case where alternating current is used in low speed scanning machining, then compared to the case of using a unipolar pulse current in practical example 1, it will be understood that a machined surface having good surface roughness can be obtained while the duty factor is high. Specifically, by applying a reverse direction current component using the charge control means  40 , an improvement in the machining speed of mirror finishing can be expected. 
     It should be noted that with practical example 2 a rectangular wave has been used for the alternating current, but it is also possible to use a different waveform, such as a triangular wave or a sine wave ( FIG. 8 ). 
     Also, with this practical example, the charge control means  40  preferably controls output from the power source  10  so that a value obtained by integrating the forward direction current with respect to the application time of that current (t 1  in  FIG. 6 ) (forward direction electrical charge) and a value obtained by integrating the reverse direction current with respect to the application time of that current (t 3  in  FIG. 6 ) (reverse direction electrical charge) become substantially equal. By doing this, then since it is possible to almost completely discharge the electrical charge that has been charged into the electrical double layer (refer to capacitor C DL  in  FIG. 3 ), if the duty factor is the same, it is possible to obtain better surface roughness. 
     Further, with this practical example the charge control means  40  preferably sets a peak value for the reverse direction current component lower than a peak value for the forward direction current component. This is because, generally, if current density is low a proportion of current that is used in removing material on the anode is reduced, and to the extent of that reduction a proportion that is used in oxygen generation and oxidation reactions is increased. In this way it is possible to reduce the amount of electrolysis of the tool electrode  20  when the reverse direction current flows, and to reduce the running cost of the device. 
     In a case where the peak value of the reverse direction current component is set lower than the peak value of the forward direction current component, it is preferable to set the reverse direction current pulse width longer than the forward direction current pulse width in order to sufficiently discharge the electrical charge that has been charged in to the electrical double layer. In this way it is possible to increase machining speed to the maximum limit in a range that does not cause unnecessary electrolysis at the tool electrode  20 . Obviously, as has been described above, discharging the electrical charge of the electrical double layer is an objective purpose, and accumulating the electrical charge of a reverse polarity in the electrical double layer is not the purpose in this embodiment. It is therefore preferable to set the pulse width to an upper limit value without the reverse direction electrical charge exceeding the forward direction electrical charge. 
     With this practical example, the charge control means  40  may insert a current idle period between the forward direction current component and the reverse direction current component of the alternating current. The current idle period is capable of being inserted during period t 1  in  FIG. 6  or in period t 3 , and is a period in which the voltage or current becomes 0. 
     While the reverse direction pulse width (period t 3 ) is preferably set to level such that the reverse direction electrical charge does not exceed the forward direction electrical charge, these phenomena have variations depending on the state of the electrode gap. For example, the potential distribution in  FIG. 3  is as ideal as possible, and in actual fact erosion products exist, and considerable unevenness exists on the surface profile. There is therefore a possibility of the reverse direction electrical charge locally exceeding the forward direction electrical charge. It is therefore possible, for example, to use this current idle period as an adjustment element. 
     Specifically, the reverse direction current is applied after the forward direction current is applied, and after the current idle period has been provided, the forward direction current is applied as the next cycle. Reverse direction pulse width (period t 3 ) at this time is set so that the reverse direction electrical charge becomes about, for example, ⅔ of the forward direction electrical charge, and after that the current idle period is provided. In this way, even if local dispersion is considered, the reverse direction electrical charge does not exceed the forward direction electrical charge, and it is possible to avoid reverse charging into the electrical double layer. Also, compared to practical example 1 where only a simple current idle period was inserted, in a case where means for applying a reverse direction current is adopted, since it is possible to eliminate the electrical charge of the electrical double layer at high-speed, it is possible to carry out electrochemical machining at a high speed while acquiring a mirror surface. 
     While it is desirable to have a low current peak value since the reverse direction current depletes an electrode, it is possible to alleviate or eliminate a problem such as electrode electrolysis as long as the reverse direction pulse width is sufficiently short even with the reverse direction current peak value being set high. Since it is possible to eliminate an electrical charge that has been accumulated in the electrical double layer at an early stage by setting the peak value of the reverse direction current component higher than the peak value of the forward direction current component, it is possible for the reverse direction pulse width (period t 3 ) to be set short. Specifically, since it is possible to increase the duty factor while preventing electrode consumption, it is possible to improve the machining speed. It is obviously also possible to set the current idle period after applying reverse direction current pulses, as described above. 
     In the first embodiment described above, the opposing area of the workpiece  1  and the tool electrode  20  preferably fully covers a surface area of the workpiece  1  to be machined over a fine section span by scanning, and preferably also makes a distance between the tool electrode and the workpiece, within the range of the opposing area, small enough that there is no significant lack of uniformity due to the curvature of the workpiece. 
     Also, the charge control means  40  of the first embodiment can be configured to change control content (i.e., change the content of control instructions) in order to eliminate the electrical charge, in accordance with scanning of the tool electrode  20 . For example, by changing the control content in order to eliminate the electrical charge in accordance with machining conditions such as distance between electrodes, opposed area of the tool electrode and the workpiece, electrolytic solution supply amount, etc., it is possible to carry out more appropriate mirror finishing. Also, by using conditions that are contrary to appropriate conditions for mirror finishing, it is possible to form a partially non-mirrored surface. 
     With this embodiment, the tool electrode  20  is scanned. In a case where reciprocating motion is repeated and machining direction is changed, the relative speed between the tool electrode  20  and the workpiece  1  is lowered at locations where scanning is turned around and the locations where machining direction has been changed. Specifically, since scanning speed on the horizontal axis of  FIG. 5  and  FIG. 7  is changed, in this vicinity it tends to result in nonuniform machining having a rough surface. With the intention of preventing this, it is possible to change the duty factor, the reverse polarity pulse width, and the reverse polarity current value in accordance with the scanning speed. For example, it is possible to obtain a uniform surface roughness by operating the charge control means  40 , at a returning point of the reciprocating motion, so as to make the idle time long compared to the idle time in regions before and after that, or alternatively such that the reverse currents pulse width is widened. 
     Second Embodiment 
     Next, an electrochemical machining device of a second embodiment of the present disclosure will be described with reference to  FIG. 9 . In the description of the second embodiment, for elements that are basically common to the previously-described first embodiment complicated description will be avoided by using the same reference numerals. 
     In the previously-described first embodiment so-called electrolyte jet machining was used. Conversely, with the second embodiment a jet is not used and normal electrochemical machining is assumed. 
     With the machining device of the second embodiment, a rod shaped tool electrode  220  is used instead of the nozzle shaped tool electrode  20  (refer to  FIG. 9 ). One pole of the power source  10  is electrically connected to both ends of the tool electrode  220  by means of two specified fixtures  221 . The other pole of the power source  10  is electrically connected to the workpiece  1 , similarly to the previously-described first embodiment. 
     The tool electrode  220  of this embodiment is arranged so as to be substantially parallel to the surface of the workpiece  1 , and is capable of scanning along the surface of the workpiece  1 . The electrolytic solution (not illustrated) in the second embodiment is previously filled between the tool electrode  220  and the surface of the workpiece  1  using a suitable electrolytic bath (not illustrated). 
     Specifically, the workpiece  1  is machined in a state of being immersed in electrolytic solution. Obviously, since it is assumed that erosion products will arise between the workpiece  1  and the tool electrode  220 , electrolytic solution may also be supplied to the electrode gap using suitable blowing means. Being parallel to the workpiece  1  and the tool electrode  220  is preferable. If the distance between electrodes varies in the vertical direction it becomes difficult to implement uniform electrochemical machining. For this reason the tool electrode  220  is held and electrically connected at upper and lower parts by fixtures  221 , so that it is difficult for vibration and deformation of the tool electrode  220  to happen. However, this is not at all limiting, and the tool electrode  220  may be held by only one of the upper and lower fixtures  221 . 
     Furthermore, it is also possible to use a wire electrode that is used in a so-called wire electric discharge machine as the tool electrode  220 . In order to achieve compatibility with a wire electric discharge machine, it is possible for a wire electrode to be used in the both machining actions. For example, after performing wire electric discharge machining on the workpiece  1  and then finishing the surface, it is possible to perform further finishing on the finished machined surface after electric discharge machining, with electrochemical machining using the same wire electrode. By having a structure with which it is possible to switch between wire electric discharge machining and electrochemical machining within the same device, there is the advantage that work involved in positioning the tool electrode  220  and the workpiece  1  parallel to each other becomes easy. 
     With the second embodiment also, similar to the previously-described first embodiment, there is the advantage that it is possible to carry out mirror finishing of the workpiece while reducing the scanning speed, by eliminating the electrical charge of the electrical double layer using the charge control means  40 . Also, by using alternating current it is possible to expect improvements in the machining speed of the electrochemical machining. 
     The remaining structure and advantages of second embodiment are the same as those of the previously-described first embodiment, and so further description will be omitted. 
     Third Embodiment 
     Next, an electrochemical machining device of a second embodiment of the present disclosure will be described with reference to  FIG. 10 . In the description of the third embodiment, for elements that are basically common to the previously-described first embodiment complicated description will be avoided by using the same reference numerals. 
     With the third embodiment, similar to the previously-described second embodiment, a jet is not used and normal electrochemical machining is assumed. 
     With the machining device of the third embodiment, a block-shaped tool electrode  320  is used instead of the nozzle-shaped tool electrode  20  (refer to  FIG. 10 ). One pole of the power source  10  (omitted from  FIG. 10 ) is electrically connected to the tool electrode  320 . The other pole of the power source  10  is electrically connected to the workpiece  1 , similar to the previously-described first embodiment. 
     The tool electrode  320  of this embodiment can be scanned along the surface of the workpiece  1  with the surface of the tool electrode  320  facing the workpiece  1 . The electrolytic solution of the third embodiment is supplied between the tool electrode  320  and the workpiece  1  by a nozzle  36 . 
     With the third embodiment also, similar to the previously-described first embodiment, there is the advantage that it is possible to carry out mirror finishing of the workpiece while reducing the scanning speed, by eliminating the electrical charge of the electrical double layer using the charge control means  40 . Also, by using alternating current it is possible to expect improvements in the machining speed of the electrochemical machining. 
     The remaining structure and advantages of third embodiment are the same as those of the previously-described first embodiment, and so further description will be omitted. 
     It should be noted that the present disclosure is not limited to the previously-described embodiments, and various modifications can be additionally obtained within a scope that does not depart from the gist of the present disclosure. 
     (Additions) 
     The invention(s) described in each of the previously-described embodiments can be considered to be described in the following aspects. 
     (Aspect A) 
     An electrochemical machining device, comprising: 
     a tool electrode arranged apart from a workpiece, 
     an electrolytic solution that is filled between the workpiece and the tool electrode, and 
     a power source for supplying forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein 
     in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, 
     a peak value of the reverse direction current pulses is made lower than a peak value of the forward direction current pulses, and a pulse width of the reverse direction current pulses is set wider than a pulse width of the forward direction current pulses. 
     (Aspect B) 
     An electrochemical machining device, comprising: 
     a tool electrode arranged apart from a workpiece, 
     an electrolytic solution that fills the gap between the workpiece and the tool electrode, and 
     a power source for supplying forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein 
     in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, 
     a peak value of the reverse direction current pulses is made higher than a peak value of the forward direction current pulses, and 
     a pulse width of the reverse direction current pulses is set narrower than a pulse width of the forward direction current pulses. 
     (Aspect C) 
     An electrochemical machining device, comprising: 
     a tool electrode arranged apart from a workpiece, 
     an electrolytic solution that fills the gap between the workpiece and the tool electrode, and 
     a power source for supplying forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein 
     in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, 
     a pulse width of the reverse direction current pulses is set to less than or equal to a pulse width of the forward direction current pulses, and 
     when switching from the reverse direction current pulses to the forward direction current pulses, an idle period is provided. 
     (Aspect D) 
     An electrochemical machining device, comprising: 
     a tool electrode arranged apart from a workpiece, 
     an electrolytic solution that fills the gap between the workpiece and the tool electrode, and 
     a power source for supplying forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein 
     in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, 
     with respect to a forward direction electrical charge that is supplied between the workpiece and the tool electrode by the forward direction current pulses, a reverse direction electrical charge that is supplied between the workpiece and the tool electrode by the reverse direction current pulses is set so as to become small, and 
     when switching from the reverse direction current pulses to the forward direction current pulses, an idle period is provided. 
     (Aspect E) 
     An electrochemical machining device, comprising; 
     a tool electrode arranged apart from a workpiece, 
     an electrolytic solution that fills the gap between the workpiece and the tool electrode, and 
     a power source for supplying forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein 
     in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, 
     a ratio (A/B) of a forward direction electrical charge (A) that is supplied between the workpiece and the tool electrode by the forward direction current pulses, and a reverse direction electrical charge (B) that is supplied between the workpiece and the tool electrode by the reverse direction current pulses, is set so as to become bigger as the speed of the scanning becomes faster. 
     (Aspect F) 
     An electrochemical machining device, comprising: 
     a tool electrode arranged apart from the workpiece, 
     an electrolytic solution that fills the gap between the workpiece and the tool electrode, and 
     a power source for supplying forward direction current pulses between the workpiece and the tool electrode, wherein 
     in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, a duty factor for applying the forward direction current pulses is set so as to become larger as the speed of the scanning becomes faster. 
     (Aspect G) 
     An electrochemical machining device, comprising: 
     a tool electrode arranged apart from a workpiece, 
     an electrolytic solution that fills the gap between the workpiece and the tool electrode, and 
     a power source for supplying forward direction current pulses and reverse direction current pulses between the workpiece and the tool electrode, wherein 
     in the electrochemical machining device that carries out electrochemical machining by relatively scanning the tool electrode and the workpiece, a pulse width of the reverse direction current pulses is set so as to become smaller, with respect to a pulse width of the forward direction current pulses, as the speed of the scanning becomes faster. 
     DESCRIPTION OF THE NUMERALS 
     
         
         
           
               1  workpiece 
               3  electrolytic solution 
               10  power source 
               20 ,  220 ,  320  tool electrode 
               21  base section 
               22  tip portion 
               221  fixture 
               30  electrolytic solution supply section 
               31  tank 
               32  piping 
               33  pump 
               34  sink 
               341  drain section 
               36  nozzle 
               40  charge control means 
               50  workpiece support section 
               60  scanning drive section 
               61  XZ direction drive section 
               62  Y direction drive section 
             t 1  pulse width 
             t 2  pulse idle time 
             t 3  polarity inversion time 
             T pulse cycle 
             R resistive component inside electrolytic solution jet 
             C DL  capacitive component corresponding to electrical double layer 
           
         
       
    
     The various embodiments described above can be combined to provide further embodiments. All of the foreign patents, foreign patent applications, and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.