Patent Abstract:
An ECM method involves the use of a thin hollow electrode assembly that carries the electrolyte within and that is advanced relatively to the workpiece. The small profile of the electrode results in a minimal removal of metal in forming the desired rotor or stator shape. The electrode profile allows significant power consumption reduction or increased machining speed for a given rate of power input. The electrode can be a unitary ring shape or can be made of segments that are placed adjacent each other so that a continuous shape is cut. Not all the lobes of the stator or rotor have to be cut in the same pass. Electrode segments can be used to sequentially provide the desired lobe count in separate passes. The lobe shapes in the electrode can be slanted to get the desired rotor or stator pitch or they can be aligned with the workpiece axis.

Full Description:
FIELD OF THE INVENTION 
       [0001]    The field of this invention is electrochemical machining (ECM) and more particularly electrochemical machining of elongated parts. The elongated parts may be parts for machines to transform energy such as but not limited to pumps or motors. These machines may comprise elongated rotors or stators having a complex shape. ECM allows to manufacture such parts meeting the requirements for accuracy of these complex geometries. 
       BACKGROUND OF THE INVENTION 
       [0002]    An ECM apparatus that is described in U.S. Pat. No. 7,479,214 includes a stationary cathode tool having a passage, and a drive mechanism for moving a bar-shaped workpiece through the passage of the cathode tool while simultaneously rotating the workpiece. Electrolyte flows, from a manifold on one end of the cathodic tool to a manifold at the other end, through the passage, between the wall of the passage and the workpiece. An electric current is simultaneously established in the electrolyte, between the wall of the passage and the workpiece. The internal shape of the cathodic tool wall has a gradual transition from a circular entry opening to a lobed exit opening, and lobes formed in the wall of the tool are shaped so that they twist in the direction of workpiece rotation, in order to form helical lobes in the workpiece. 
         [0003]    The issue with this design is the high power consumption that is directly related to the amount of metal that the process has to dissolve to get the finished shape for the workpiece from the initial blank. By making use of the teaching of U.S. Pat. No. 7,479,214, all the metal outside the final shape has to be dissolved. The need to dissolve this much metal not only causes the high power requirements but also affects the speed at which the workpiece can be advanced through the electrode. 
         [0004]    The present invention uses a similar ECM process as taught by U.S. Pat. No. 7,479,214 but reduces the power requirements in a variety of ways. The principle way this result is achieved is to use an electrode that has a much smaller thickness than the maximum thickness of the part that is to be cut away from the workpiece. The electrode can be shaped in various ways including open shapes or closed shapes like rings, for example. The electrode might be hollow or might have at least passages through which the electrolyte is delivered through openings in the electrode that can be at a leading end in the direction of electrode movement with respect to the workpiece to be machined. Alternatively, the outlets for the electrolyte can be on or in or near other surfaces of the electrode or the electrolyte might be directed with seals or other appropriate means to a part of the electrode for dissolving and removing a narrow band of metal to cut into the workpiece. The electrode can be rotated or otherwise moved with respect to the workpiece to create a desired shape of the cut as it is axially advanced, e.g. to create a desired pitch. The electrode can have lobe shapes which might be already inclined for a desired pitch. By using an electrode with lobe shapes, it is a function of controlling the axial speed and rate of rotation to get the desired lobe profile on the workpiece that is being machined. 
         [0005]    Also related to ECM are U.S. Pat. Nos. 6,250,340; 6,413,407; 7,192,260; 5,310,468; 5,244,548; 5,149,405 and 6,309,195. 
       SUMMARY OF THE INVENTION 
       [0006]    An ECM method involves the use of an electrode having a relatively small profile with electrolyte delivered through openings in or near the surface of the electrode and that is advanced relative to the workpiece. The profile of the electrode that is relatively small compared to the part of the workpiece that is to be removed results in a minimal dissolution of metal while forming the desired cut. The relatively small electrode profile allows significant power consumption reduction or increased machining speed for a given rate of power input compared to the prior art. The electrode can be a unitary closed shape or can be made of segments. Electrode segments might be placed adjacent to each other so that a continuous shape is cut. However, not all details have to be cut in the same pass. Electrode segments can be used to sequentially create the desired shape details in separate passes. Parts of the electrode may be slanted with respect to the direction of movement to get the desired shape or they can be parallel with the direction of movement. A desired pitch may be produced that way by a combination of translational and rotational movement. The produced profiles can have all kind of shapes. In particular, they do not need to be rotationally symmetric. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective view of the leading end of an electrode and holder to manufacture a rotor; 
           [0008]      FIG. 2  is a perspective view of the trailing end of the electrode and holder of  FIG. 1 ; 
           [0009]      FIG. 3  is a section through the  FIG. 1  electrode and holder to show the electrolyte flow passages; 
           [0010]      FIG. 4  is the electrode of  FIG. 1  without the holder; 
           [0011]      FIG. 5  is a detailed view of a part of the  FIG. 1  electrode with one of the supports; 
           [0012]      FIG. 6  is a perspective view of an assembly for an electrode for making stators; 
           [0013]      FIG. 7  is the view of  FIG. 6  with the guide removed from within the shaft; 
           [0014]      FIG. 8  is a perspective view of the  FIG. 6  electrode showing the electrolyte entrances from the shaft; 
           [0015]      FIG. 9  is a part cutaway showing the stator being cut; 
           [0016]      FIG. 10  is a close-up of the arrangement shown in  FIG. 9  view showing the electrolyte feed to the electrode; 
           [0017]      FIG. 11  is a section view of the workpiece shown in  FIG. 9  showing the resulting stator and the excess material cut away from the workpiece. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    In the examples and illustrations herein, apparatuses and methods are discussed for electrochemical machining of parts with reduced power consumption compared to the prior art of electrochemical machining. The teachings are in particular beneficial for the manufacturing of elongated parts. Elongated parts are defined by having a first dimension significant longer than the other two dimensions perpendicular to the first dimension. The examples and illustrations provided in this patent application are dealing with the manufacturing of a rotor and a stator of a hydraulic motor or a progressing cavity pump which are examples for elongated parts with a complex geometry. The outer or inner geometry of the rotor or stator may comprise symmetrically or asymmetrically twisted lobes that are shapes which are challenging to manufacture with the required accuracy by conventional methods (milling for example). However, these examples are not meant as a limitation. Those skilled in the art will appreciate that the teachings disclosed herein can be used to manufacture other parts with a different use than rotors or stators that are described here to illustrate the invention. 
         [0019]    Referring to  FIG. 1  the electrode holder  10  has a circular inlet guide  12  for the workpiece that is not shown to enter so that the holder  10  can be axially advanced and otherwise moved as the electrolyte represented by arrow  16  ( FIG. 3 ) enters inlets  14  ( FIG. 3 ). Referring to  FIG. 3 , the electrolyte path is from inlets  14  through supports  18  and into the electrode  20  and out the leading end of the electrode  20  as represented by arrow  22 . A seal  24  engages the workpiece that is not shown as the shape of the electrode is cut into the workpiece to eventually cut away one or more excess pieces for making elongated parts like a rotor or a stator. While not shown in the figures, parts of the electrode  20  might be electrically isolated so as to steer the electric field in a way to keep power consumption low and to avoid dissolution at locations where it is not desired. The basic method of ECM is well known but the difference of the proposed method in producing elongated parts like a rotor or a stator involves the use of a relatively thin profile for the electrode  20  that is preferably hollow to handle the electrolyte flow until the leading end exit  28  as best seen in  FIG. 5 . For the scope of this patent application a relatively thin electrode  20  is defined by an electrode  20  with a thickness  26  ( FIG. 1 ) that is smaller than the maximum thickness of the part that is to be cut away from the workpiece. While not shown in the figures, the electrode length in the direction of the relative movement between electrode and workpiece can be in the same range as the thickness  26  of the electrode. This can have significant benefits in terms of steerability of the electrode during the relative movement between electrode and workpiece. The term “hollow” as used in this application includes any type of passage through the electrode that allows to deliver electrolyte next to the electrolyte. A hollow electrode, therefore, includes bores in the electrode, electrodes made of porous material, gaps inbetween two electrode wings, etc. In the preferred embodiment, the cross section of the electrode  20  has a shape similar to the surface contour that will be produced with the electrode  20 . In the case of a rotor or a stator the electrode  20  might have multiple lobes  30  ( FIG. 2 ). The lobes might be understood to comprise an undulating radius along the circumference of the workpiece. The lines connecting the radius maxima substantially along the individual lobe are skewed with respect to the axis of translational movement of the electrode shape  20  such that axial advancement of the electrode  20  coupled with rotation results in the production of continuous lobes that have a desired pitch around the remaining part of the workpiece. While the figures show such a skewed electrode, the electrode can also be designed such that it does not have any inherent direction. For instance, the electrode could be made with a substantially cylindrical cross section or any other cross section in a plane perpendicular to the movement of the workpiece. The gap that is created by the metal that is dissolved and removed is about as wide as the thickness  26  of the electrode  20  so that the electrode  20  can advance relative to the workpiece. 
         [0020]    While the electrode  20  can be axially advanced and turned with respect to the workpiece, it should be recognized that the workpiece can also be advanced and turned relative to a stationary electrode  20  to make the required cut. It is also possible that either one is turned and the other one is advanced and finally both can be advanced and turned as long as a relative movement and rotation between the two parts is created. Either one of these motions can be active or passive. As an example, by using a trailing guide section behind the electrode  20 , the rotation will be caused by the axial advancement. Alternatively, such a trailing guide section can be used in a way that the translational movement will be caused by the rotational movement. Those skilled in the art will appreciate that there are many other possibilities to create the axial and rotational movement with or without the use of a trailing guide section. The supports  18  ( FIG. 3 ) are hollow and flow represented by arrow  16  ( FIG. 3 ) enters inflow opening  34  in the support  18  to get inside the electrode  20 . Once entering through inflow opening  34  ( FIG. 4 ) the flow then exits through outflow opening  36  ( FIG. 5 ) and the body of the electrode  20 . While  FIG. 4  shows an inflow opening  34  in each support  18  ( FIG. 3 ) it is obvious for those skilled in the art that not all of the supports  18  need to have an inflow opening  34  ( FIG. 4).The  electrolyte is collected and recirculated as will be more fully explained below. As in an conventional ECM systems there may be cooling systems and insulating materials to separate the anode and cathode as the metal is removed when the current is applied. Such electrodes can be manufactured by various methods including but not limited to 3D printing such as additive manufacturing, selective laser melting, or direct laser metal sintering. 
         [0021]    While the closed shape for the electrode  20  is preferred in particular for the manufacturing of a rotor or a stator, it might be advantageous to use an electrode comprising one or more discrete electrode segments (not shown). An electrode segment covers only a part of the circumference of the workpiece. At least one of the electrode segments is hollow or otherwise allows electrolyte flow in a similar manner than the unitary electrode. Electrode segments might be adjacent to each other. However, it might be useful to use only one electrode segment or to use more than one electrode segments that are not adjacent to create a desired cut. 
         [0022]    Whichever configuration of the electrode is used, the entire length of the workpiece is not necessarily formed in a single pass. For example, the machining can be interrupted to allow other processes, for example to physically remove the excess material that may have been cut loose from the workpiece, to exchange electrodes, to modify position of electrode or electrode segments (such as rotating), to otherwise machine the workpiece, or to execute further processing steps. The excess material can be removed and the machining process restarted, if necessary. The electrochemical machining process can be combined with other machining processes such as a milling process to come to the desired shape of the workpiece. 
         [0023]    Whether closed shape electrode or one or more electrode segments are used and whether the electrochemical machining is done in one or more than one passes, the final cut that is created can cover the complete circumference of the workpiece or can cover only a part of the workpiece circumference. For example, by adjacent electrode segments, a structure similar to a closed shape structure can be created in one or more passes. Such a structure of adjacent segments can be used to machine the complete circumference of a workpiece. Alternatively, by using more than one non-adjacent electrode segments, it is possible to cover the full circumference of the workpiece even in a single pass if the electrode segments have some distance to each other along the axis of relative movement between workpiece and electrode. Those skilled in the art will appreciate that all other combinations of closed electrodes versus electrode segments, adjacent versus non-adjacent electrodes, and single pass versus multiple passes are possible to create either a cut that covers either the complete circumference of the workpiece or only a part of the circumference of the workpiece. 
         [0024]      FIGS. 6-10  show how an inner contour can be produced by cutting away an inner excess piece. The cutting plane can have various even irregular shapes. In the examples of  FIGS. 6-10 , the manufacturing of a stator  50  ( FIGS. 9 ,  11 ) by cutting away an inner excess piece  52  ( FIG. 11 ) along an undulating line  54  ( FIG. 11 ) is illustrated. For the stator manufacturing, the supports  56  ( FIG. 7 ) are on the inside of the electrode  58  and electrolyte flow represented by arrow  60  ( FIG. 10 ) passes through a shaft (see  FIG. 7 ) on the way to the supports  56 . As before, the electrolyte flow  60  ( FIG. 10 ) goes through the hollow passage  63  ( FIG. 10 ) of the electrode  58  ( FIG. 7 ) and out and back around the inside or the outside of the electrode  58  as represented by arrows  64  ( FIG. 10) and 66  ( FIG. 10 ). The electrolyte flow represented by arrow  60  ( FIG. 10 ) goes into openings  78  ( FIG. 8 ) in supports  56  ( FIG. 7 ). While  FIG. 8  shows an opening  78  in each support  56  it is obvious for those skilled in the art that not all of the supports  56  need to have an opening  78 . Supports  56  ( FIG. 7 ) are fixed to shaft (see  FIG. 7 ) that is a hollow shaft for electrolyte delivery. The shaft will be used to support the electrode  58  ( FIG. 7 ) and to convey force or movement to the electrode  58  to create the relative movement of the electrode  58  and the workpiece. The shaft may also house equipment to power the electrode (wires or electronic components for example—not shown). An inner guide  82  ( FIG. 6 ) may be placed inside the tubular workpiece when making an inner contour like a stator. The guide is close in dimension to the inner diameter of the stator workpiece to guide the relative movement of the workpiece and the electrode. In addition or alternatively, the shaft may guide the relative movement of the workpiece and the electrode. The shaft and the inner guide  82  ( FIG. 6 ) may be insulated with a cover  84  ( FIG. 6 ) to electrically isolate metallic parts inside the insulation from the workpiece and the inner excess piece  52  ( FIG. 11 ) that is removed from the workpiece to leave what will be the stator  50  ( FIG. 11 ). As with the production of the rotor, when making the stator, the electrode  58  ( FIG. 7 ) is axially advanced as shown with arrow  86  ( FIG. 9 ) and rotated as shown by arrow  88  ( FIG. 9 ). Beyond that the variations described above with regard to making the stator are applicable to the making of the corresponding rotor. 
         [0025]    The lobes for the rotor and stator do not have to have a specific shape but can have various shapes with all kinds of variation. Also, more than one electrode might be used consecutively. Also, while shown in the figures that the lobes in electrodes for manufacturing rotors or stators are each supported by a single support  18  ( FIG. 3 ) or  56  ( FIG. 7 ), the number of the supports can be lower or higher than the number of the number of the lobes. 
         [0026]    Those skilled in the art will appreciate that significant reductions in power consumption can be achieved because only a narrow sliver of metal is removed when making the rotor or the stator by virtue of the use of the narrow electrode. The electrolyte can meet the workpiece through leading openings in the electrode or with inside or outside openings with seals to redirect the flow to the leading end of the advancing electrode. The electrode can be unitary or in one piece or it can be made of abutting or non-abutting segments. In either case the stator or rotor with all its lobes will be made at one pass. Alternatively, some segments can be used for the electrode in which case less than all the lobes will be made in a single pass. The pass can be interrupted before the finished length is produced and the excess cut away material can be removed. After such removal the cutting can continue to finish the required length. The whole length of a workpiece does not need to be cut but could leave, for example, a section for further installations such as mounting one or more bearings or if additional length of the workpiece is desired for machining of the workpiece. 
         [0027]    When making a stator, the electrode cuts through the workpiece to leave an excess piece or pieces on the interior of the workpiece. On the other hand when making a rotor the electrode cuts through the workpiece leaving an exterior excess piece. The power savings comes into play because the excess piece is created with a thin cutting of the workpiece rather than using ECM to completely dissolve the excess portion from the workpiece and leaving behind only the finished rotor or stator. The power savings results from the elimination of the machining of the entire excess portion and replacing such machining with a thin cut made by the electrode during relative movement with respect to the workpiece. The cut height is just slightly larger than the electrode height to allow the electrode to advance as the shape of the rotor or stator is cut while leaving behind an excess piece to be discarded. 
         [0028]    The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below:

Technology Classification (CPC): 1