Abstract:
An electrical discharge machining (EDM) apparatus and method is disclosed that utilizes a rotating electrode for performing the EDM operation to form a desired profile in the work piece surface. A dressing tool located at a side of the rotating electrode opposite from the work piece is operable to dress the surface of the rotating electrode. Operation of the dressing tool is controlled in part by a monitor that monitors changes in the character and condition of the surface of the rotating electrode from certain starting parameters and initiates a dressing operation as needed to restore the starting parameters.

Description:
Technical Field  
         [0001]    The present invention relates to apparatus and method for electrical discharge machining using a rotating electrode for machining a desired profile into the surface of a conductive work piece.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electrical discharge machining (EDM) is a well-known technique for the precision machining of hardened metal alloys and conductive ceramic composite materials (such as a cobalt-tungsten carbide among others) that are difficult or impossible to machine by more conventional methods. In EDM, the cutting tool is an electrode attached to a power supply. Material is gently removed from the work piece by striking a microscopically small spark between the forming tool and the electrically conductive work piece. To do this, the electrode is brought close to the surface of the work piece so an electric discharge or spark is generated across the gap between the electrode and the work piece. As the discharge enters the work piece, several molecules of the material of the work piece are ejected from the work piece surface. The material removed by the spark normally is flushed away by a dielectric liquid. In this fashion, the work piece can be machined to a desired shape.  
           [0003]    One drawback of EDM is that the electrode erodes over time. This erosion of the electrode will change the dimensions of the eventual machined shape. Changing the electrode often or building the expected erosion into the starting electrode are ways to compensate for the erosion.  
           [0004]    A common form of EDM and the process featuring the highest rate of material removal employs a wire electrode about 0.008 to 0.015 inches in diameter. The wire is connected to a spark generator and is moved in a direction normal to its longitudinal axis to cut into the work piece. In addition, the wire electrode is continuously indexed in the direction of its axis so as to present the same wire diameter to the work piece.  
           [0005]    An EDM process using a wire electrode is capable of a dimensional precision of 0.001 to 0.00005 inches under ideal set up conditions. Flushing debris from about the electrode is accomplished by pumping the dielectric liquid directly into the cut. Reliance is on the axial movement of the wire to carry the flushing liquid into the cut for sweeping away the material ejected from the work piece.  
           [0006]    Rotating the electrode also is known. For example, U.S. Pat. No. 4,001,538 discloses rotating the electrode about is longitudinal axis and discharging sparks an end face of the electrode to smooth the surface of the work piece. Preferably the electrode is arranged with its rotational axis disposed at a slight angle from the vertical of between about 85° to 89.40° as measured from the plane surface of the work piece. The shape of the working end of the rotating and inclined solid cylindrical electrode is described as “blunt conical”. The work piece is disposed in a bath of a dielectric liquid and the shape of the blunt conical end aids in flushing away debris and material removed from the work piece.  
           [0007]    Another device using a rotating electrode for making threaded and non threaded bores is disclosed in U.S. Pat. No. 4,628,171 and a device for supplying an electric current to a rotating electrode is disclosed in U.S. Pat. No. 5,561,336  
           [0008]    In EDM, controlling the frequency, voltage, current and wave shape of the spark discharge is known to control the surface finish imparted to the work piece. In this respect, there is a given amount of energy available per unit time depending upon the current applied to the electrode. For a greater number of sparks per second (and a reduction of the current and voltage of that spark) there will be less energy per spark so the amount of material removed per spark will be less. Thus, if a smooth surface is desired, a higher spark frequency (and reduced energy content of each spark) and longer work time is used as opposed to a rapid machining operation which uses a lower frequency to remove larger amounts of the work piece with each discharge.  
           [0009]    EDM requires that the work piece be conductive. Accordingly, working certain ceramic materials including silicon carbide is particularly difficult in part because of the poor conductivity of these materials. Ceramics and silicon carbide have been used for surgical instruments such as scalpels and other cutting instruments and the sharpening of these materials to provide a cutting edge has been done by conventional means (such as Diamond Grinding) in part because of the conductivity issues. Conductivity problems may be overcome by known doping techniques to incorporate a conductive metal into the crystalline structure of the ceramic. While silicon carbide (SiC) can be made conductive, a drawback to machining this material by EDM is that SiC readily combines with free oxygen to form SiO 2  on the surface of the work piece. Since SiO 2  is an insulator, a layer of SiO 2  only Angstroms thick, on the surface of the work piece will adversely effect the machining process. Any build up of SiO 2  on the surface of the electrode also will interfere with the EDM process.  
           [0010]    Accordingly, in view of the current state of the art there is a need for EDM apparatus and process for working harden metals and conductive ceramics to improve surface finish of the work piece. There also is a need for an EDM apparatus and method capable of dealing with the erosion of the electrode. There further is a need for an EDM apparatus and method for dealing with the build up of contaminants and electrical insulating process by-products on the electrode. This is particularly the case where the work piece is a silicon carbide and the contaminant includes SiO 2 .  
         SUMMARY OF THE INVENTION  
         [0011]    In the present invention, the electrode is circular in cross section and may include such structures as a right cylinder, disk, wheel or conical body. The electrode also may be generally cylindrical with a desired profile that is symmetrical about the longitudinal axis of the electrode. The electrode is arranged to spin about its longitudinal axis and is arranged so that successive portions of the electrode periphery are carried into close proximity to the surface of the work piece. At the same time, portions of the electrode periphery on an opposite side are carried away from the surface of the work piece. For example, in the case of a cylindrical electrode, the axis of rotation may be oriented generally parallel to the surface of the work piece or within a few degrees of being parallel to the work piece surface so substantially the entire electrode is used in the EDM process. In the case of a conical electrode, the axis of rotation may be disposed at an angle to the surface of the work piece so the surface of the electrode is substantially parallel to the surface of the work piece.  
           [0012]    The electrode is rotatably supported on high quality bearings and movement of the electrode is controlled such that the surface of the rotating electrode will approach the work piece surface in a predictable and controlled manner without plunging into the work piece. In this manner the surface of the work piece is machined by the adjacent portion of the electrode while a second portion of the electrode is remote from the work piece. In a preferred embodiment, the surface of the electrode is provided with a shape or contour that is machined into the surface of the work piece.  
           [0013]    As the electrode spins about its longitudinal axis, the desired contour is formed in the work piece surface. A spinning electrode also allows for the use of an in situ dressing or sharpening of the electrode to compensate for the erosion of the electrode or to remove impurities that may build up on the electrode surface thereby extending the life of the electrode.  
           [0014]    As the electrode spins in close proximity to the work piece surface, any suitable position monitoring system is used to monitor the position of the electrode for precise machining. Such a system may include for example a laser, optical system or video camera among others. The feed back from the position monitor and feedback from the electrical monitoring of the rate, duration, voltage and current of the pulse are all used as input parameters for computer assisted programming required for precision control of the electrode.  
           [0015]    The spinning electrode may be used as the vehicle to carry a dielectric liquid between the electrode and the work piece to flush away the by-products of the machining operation by accelerating the flow of the dielectric liquid into the cutting area. Grooves can be provided in the surface of the electrode to enhance the pumping action forcing liquid into the cutting area. A pathway also may be provided through the electrode to facilitate the conduct of the dielectric liquid into the cutting area. In the case of a silicon carbide work piece, the dielectric liquid preferably is oil to retard the development of SiO 2  on the surface of the work piece.  
           [0016]    As the electrode spins one portion of the electrode surface is carried towards and into close proximity to the surface of the work piece while an opposite portion of the electrode surface moves away from the work piece. A suitable monitor can be arranged to monitor the profile of the electrode surface. Should the profile of the electrode change over time due to erosion or the build up of contaminants, the monitor sees the change. At such time as the change in the electrode surface become unacceptable, the electrode surface can be dressed to restore the desired profile. This is accomplished by operating a dressing tool to contact that portion of the spinning electrode surface that is spaced or remote from the work piece where the EDM is occurring.  
           [0017]    The EDM process using a spinning electrode in accordance with the present invention preferably is used to provide a sharp edge, free of burrs or damage common to diamond or other abrasive grinding techniques. A preferred use is to sharpen ceramic or metal or harden metal blanks as may be used for surgical cutting instruments and the like. For example, blades can be formed of an electrically conductive ceramic, metal hardened metal, metal matrix composite, electrically conductive polymer or similar electrically conductive material that are near to a desired shape or profile of a finished blade. The spinning electrode, provided with a profile that is a positive (or reverse image) of the desired machined shape, then is used to perform the machining operation to provide the final shape.  
           [0018]    Accordingly, the present invention may be characterized in one aspect thereof by an electro discharge machining (EDM) apparatus for removing material from a conductive work piece including:  
           [0019]    a) a rotatable electrode having a generally circular cross section, the electrode being arranged for rotation about its longitudinal axis;  
           [0020]    b) drive means for continuously rotating the electrode about its longitudinal axis to bring successive portions of the electrode surface into close proximity to the surface of the work piece such that electrical discharge machining of the work piece surface occurs at one portion of the electrode surface while another portion of the electrode outer surface is brought to a location remote from the work piece surface;  
           [0021]    c) a dressing tool at the remote location for dressing the electrode surface;  
           [0022]    d) a monitor for monitoring and indicating changes in the character and condition of the electrode surface from certain starting parameters; and  
           [0023]    e) the dressing tool being operable in response to a change in the character and condition of the electrode surface for dressing the electrode surface and restoring the starting parameters.  
           [0024]    In another aspect, the present invention may be characterized by an electric discharge machining process for removing material from a surface of a conductive work piece comprising:  
           [0025]    a) providing a rotatable electrode having a generally circular cross section;  
           [0026]    b) arranging the electrode for rotation about its longitudinal axis;  
           [0027]    c) continuously rotating the electrode about its axis to bring successive portions of the electrode surface into close prolixity to the surface of the work piece such that electrical discharge machining of the work piece surface occurs at one portion of the electrode surface while another portion of the electrode surface is brought to a location remote from the work piece surface;  
           [0028]    d) monitoring changes in the character and condition of the electrode surface from certain starting parameters; and  
           [0029]    e) dressing the portion of the electrode surface that is brought to said remote location by said rotating so as to restore the starting parameters. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a schematic drawing showing an EDM system according to the present invention;  
         [0031]    [0031]FIGS. 2 a  and  2   b  are schematic representations showing the front and side elevation views respectively of a spinning electrode according to the present invention; and  
         [0032]    [0032]FIG. 3 is a schematic representation showing a side elevation view of another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    Referring to the drawings, FIG. 1 shows an EDM system according to the present invention generally indicated at  10 . The system includes a worktable  12  for supporting a work piece  14 . Preferably the worktable  12  is controllably movable in an X-Y (horizontal) plane and most preferably it is movable both horizontally and vertically. Supported above the worktable is an EDM electrode  16 . The electrode has a circular cross section and is supported on precision bearings (not shown) to permit rotation of the electrode about an axis  18  that extends generally parallel or near parallel to the work piece. The bearings include conventional slip rings to permit communication of the electrode  16  to a spark generator  20 . The spark generator controls the impedance of the system to maximize the power transfer to the electrode and to the work piece. Preferably the spark generator is set to generate high frequency sparks to provide a slow machining time and a smoother surface finish as opposed to a rapid machining operation which uses a lower frequency to remove larger amounts of the work piece with each discharge.  
         [0034]    A drive  21  is provided for spinning the electrode  16  about it axis  18  at a speed of between about 25 to 2500 rpm. During the course of rotation, a first portion  22  of the electrode surface is brought into close proximity to the work piece while a second portion  26  of the electrode surface is brought to a position remote from the work piece. Adjacent the second portion of the electrode surface is a dressing tool  28 . The dressing tool is arranged for selective operation to dress the peripheral surface  24  of the spinning electrode as set out further hereinbelow.  
         [0035]    Also included in the EDM system is a monitor  30  located to keep track of the position of the spinning electrode as well as the shape and condition of the surface of the electrode. The monitor may be a video camera, laser, optical scanner or any suitable device capable of recognizing a change in the state or condition of the electrode surface. The output of the monitor is connected to a tool controller  32 . The tool controller integrates the spark rate and the input from the monitor  30  and is operatively connected to both the worktable  12  and the electrode. The tool controller acts in response to its inputs to change the position of the worktable and/or the electrode as needed to provide proper dimensional control of the work piece. A further function of the tool controller is to receive input from the monitor to track the condition of the electrode, such as its profile.  
         [0036]    Completing the system is a process control computer  34  that is connected to the tool controller  32 , spark generator  20  and the electrode dressing tool  28 . The process controller  34  receives input from the tool controller  32  to increase or decrease the spark rate to speed or slow the EDM process as desired. The process control computer also may act to increase or decrease the spark rate to correct for some out of standard condition picked up by the monitor  30 . The process controller  34  further operates the tool dressing tool  28  in response to an input from the tool controller indicating that the electrode requires dressing or sharpening to restore a desired profile.  
         [0037]    In operation the work piece  14  is fixed to the worktable  12  and the worktable is moved by the tool controller  32  to bring the work piece into a proper alignment with the electrode. The drive  21  is activated to spin the electrode about its longitudinal axis  18  and the spark generator  20  is activated to energize the electrode. As the spinning electrode is brought into close proximity with the work piece, material is eroded from the work piece surface by EDM to bring the surface of the work piece to the desired shape. During the EDM process, the tool controller may increase or decrease the spark rate to speed or slow the EDM process as desired. Also during the EDM process, monitor  30  monitors the shape and condition of the electrode. Changes in the electrode are communicated to the tool controller  32 .  
         [0038]    At the time the tool controller  32  recognizes that the electrode surface no longer is within operative parameters, it communicates this state to the process control computer  34 . The process control computer then initiates the sequence for dressing the surface of the electrode to return the electrode to the correct operative parameters. For example, the electrode may itself be eroded by the EDM process or changes may occur in the shape or conformation of the electrode surface so the electrode no longer is configured to impart the desired profile into the work piece surface. When this occurs, the electrode is moved slightly away from the work piece to stop the EDM process. While the electrode continues to spin, the dressing tool is brought into position to dress the electrode surface  24 . The dressing tool may be a grindstone or the like that bears against the electrode. As an alternative, the dressing tool itself may be an electrode that performs an EDM process to dress the surface  24  of the spinning electrode  16 . Once the surface of the spinning electrode restored to proper parameters, the process control computer  34  returns the electrode to a position for continuing the EDM process on the work piece.  
         [0039]    [0039]FIG. 2 a  shows the work piece  14  as being in a bath  36  of dielectric liquid, preferably dielectric oil. As the electrode  16  spins about its axis  18 , the peripheral outer surface  24  of the electrode carries the dielectric liquid into the gap  38  between the work piece and the electrode. Sub micron particles  40  ejected from the work piece by the EDM process are swept clear from the work area by the spinning action of the electrode. As shown in FIG. 2 b , the peripheral surface  24  of the electrode may be provided with grooves or channels  42  to facilitate the movement of the dielectric liquid into the gap  38  between the electrode and the work piece. The potions of the electrode forming the grooves and channels also provide a plurality of locations from which sparks can be discharge to the work piece so there is a high spark frequency. In an alternate embodiment, the electrode may be provided with internal passages (not shown) that communicate with the groove  42  for conducting dielectric liquid from an external supply through the electrode and into the groove.  
         [0040]    [0040]FIG. 2 b  also shows that the outer peripheral surface  24  of the electrode can be provided with a profile or contour that is machined into the surface of the work piece  14  by the EDM process. The profile as shown may be used for machining the work piece  14  to a knife blade. The electrode may range in diameter from 0.5 to 4 or more inches in diameter and have a thickness at its operative end of 0.10 to 6.0 inches. It also is possible for the electrode to feature a contoured profile. For example, the electrode may be conical with a smooth continuously tapered profile or it may have a profile that is shaped or irregular along its length but circular in cross section.  
         [0041]    As shown in the figures, the rotational axis  18  of the electrode  16  is oriented generally parallel to the surface of the work piece to allow substantially the entire periphery of the electrode to sweep into the work piece. This orientation is not essential and the axis may be within a few degrees of being parallel to the work piece surface. For example the electrode as shown in FIG. 2 b  can be more cylindrical wherein the bevel shape to the work piece is accomplished by having the axis of the electrode disposed at angle to the work piece surface. Also, in the case of a conical electrode, the axis of rotation may be oriented at a steeper angle with respect to the work piece surface so as to machine the work piece with a flat profile as shown in FIG. 3. It is important only that a portion of the surface of the electrode pass through a location remote from the work piece so that contact can be made with a dressing tool.  
         [0042]    Thus it should be appreciated that the present invention provides an improved EDM apparatus and process for working harden metals and conductive ceramics. The use of a rotating electrode together with a high spark frequency provides the work piece surface with a smooth finish. The present invention further provides for the monitoring and dressing of the electrode to compensate for the erosion of the electrode. In the case where the work piece is a silicon carbide ceramic, dressing the tool in accordance with the present invention prevents an undesirable buildup of SiO 2  on the work piece.  
         [0043]    Having described the invention in detail, what is claimed as new is: