Abstract:
An electrode holder for EDM machines using automatic tool changing techniques provides a base plate that may be attached to an adapter of third party chuck systems and a simple compressed slot mechanism for holding the electrode without the need for epoxy or the like.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based on U.S. Provisional application 60/467,048 filed May 1, 2003 hereby incorporated by reference. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates generally to electrical discharge machines (EDM) and in particular to an electrode holder for such machines.  
           [0003]    Electrical Discharge Machines (EDM) make use of specially shaped electrodes to cut complex surfaces into metal molds or the like. The surface cut in the workpiece mirrors the shape of the electrode which is typically machined on a numerically controlled milling machine or the like and then transferred to the EDM machine.  
           [0004]    In order to obtain precise registration between the surface of the electrode cut on the milling machine and the work piece held in the EDM machine, it is known to attach special chucks to each of the these machines in predefined locations with respect to the milling cutter on the milling machine and the workpiece on the EDM machine. Once the chucks are located, the electrode may be transferred freely between the milling machine and the EDM machine while maintaining a precise registration.  
           [0005]    Chucks and electrode holders for this purpose are available from the 3R Company, Midwest Technical Center, Elk Grove Village, Ill. These chucks and electrode holders are particularly designed to allow rapid automatic changing of electrodes on the milling and EDM machine (tool changing).  
           [0006]    The electrode holders that are provide with such systems, which may provide a channel having inwardly directed set screws to grip the surface of the electrode are not well suited for some types of electrodes, in particular, for relatively thin or cylindrical graphite electrodes. Special fixtures may have to be prepared to hold these electrodes and/or the electrodes may need to be glued in place using epoxy or the like. This latter gluing step can make reuse of the relatively expensive electrode holders difficult.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides an improved electrode holder for automatic tool changing systems in which the electrode is held in a slot compressed by a screw spanning the slot gap. This mechanism provides a simple yet robust method of releasably holding both thin, flat and cylindrical graphite electrodes in systems of this type.  
           [0008]    Specifically, the present invention provides an electrode holder that may be used with an automatic tool changing system having at least two chucks that may alternately receive a tool. The chucks hold the tool between times of automatic removal and insertion at a predetermined fixed position with respect to the chuck and the tool has locating surfaces at a first face of the tool engaging locating surfaces at a second face of the chucks. The electrode holder includes a base having a locating surface engaging the locating surfaces of the chucks. A holder shaft extends from the base and has an electrode-receiving slot sized to receive a portion of a standard graphite electrode. A clamp screw draws the sides of the slot together about the electrode to hold the electrode therein.  
           [0009]    Thus, it is one object of the invention to provide an improved method of holding electrodes for automatic tool changers that does not require epoxy or risk breaking of the electrode through point forces of set screws and the like, but rather it holds the electrode in a broad contact area of faces of a slot.  
           [0010]    It is another object of the invention to provide a simple mechanism for holding electrodes of this type.  
           [0011]    The slot may have a bottom conforming to one surface of the standard graphite electrode so that the standard graphite electrode may abut the bottom of the slot for support.  
           [0012]    Thus, it is another object of the invention to provide additional support in the direction of principal force acting on the electrode.  
           [0013]    The clamp screw may be positioned to pass through a hole in the graphite electrode when the standard graphite electrode is received within the slot.  
           [0014]    It is thus another object of the invention to provide a balanced force to the electrode in the slot at a point centrally located in the electrode.  
           [0015]    The holder shaft may include a coolant flow channel exiting from a bottom of the slot receiving the standard graphite electrode.  
           [0016]    Thus, it is another object of the invention to provide an electrode holder particularly suitable for use in EDM machines where coolant must be constantly flushed over the surface of the electrode.  
           [0017]    The slot may provide opposed planer sides or opposed hemi cylindrical sides.  
           [0018]    Thus, it is another object of the invention to provide an electrode holder suitable for both rectangular and cylindrical electrodes.  
           [0019]    The sides of the slot may be separated substantially by the thickness of the standard graphite electrode.  
           [0020]    Thus, it is another object of the invention to provide for relatively little flexure of the slot so as to provide even compression of the electrode.  
           [0021]    The base may include a base plate having threaded holes receiving machine screws securing the base plate to a chuck plate having the locating surfaces engaging locating surfaces of the chucks.  
           [0022]    Thus it is another object of the invention to provide an electrode holder that may be used with standard chucks for automatic tool changers manufactured by third parties by attaching the plate of the electrode holder to an adapter plate manufactured by the third parties.  
           [0023]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0024]    [0024]FIG. 1 is an exploded perspective view of the electrode holder of the present invention with a chuck for use with rectangular electrodes;  
         [0025]    [0025]FIG. 2 is a fragmentary perspective view of an alternative embodiment with a chuck for cylindrical electrodes;  
         [0026]    [0026]FIG. 3 is a cross section taken along lines  3 - 3  of FIG. 1; and  
         [0027]    [0027]FIG. 4 is a fragmentary cross section of the electrode within the chuck of FIG. 1 (unexploded) taken along line  4 - 4  of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    The present invention provides a graphite electrode holder  10  for use with electrical discharge machining (EDM) machines in which electrical discharge is used to shape complex machine surfaces suitable for injection molds and the like.  
         [0029]    Referring to FIG. 1, a standard rectangular graphite electrode  12  is generally a rectangular parallelepiped having, for example, a width of 0.375 inches measured along a transverse direction between its broad faces and a height and width of arbitrary length. An upper surface of the rectangular graphite electrode is typically machined to provide a suitable tool surface for EDM machining. Rectangular graphite electrodes  12  of this general type are commercially available through a variety of sources.  
         [0030]    When used with the present invention, an axial hole  14  is bored through the rectangular graphite electrode  12  to extend along its vertical height. The axial hole  14  is of relatively small diameter; for example, {fraction (1/16)}th of an inch to allow for coolant flow through the tool as will be described. A transverse hole  16  intersects the axial hole  14  near the lower edge of the rectangular graphite electrode  12 . The transverse hole  16 , for example, may be approximately 0.25 inches in diameter and is intended to provide clearance for coolant flow as will also be described.  
         [0031]    The rectangular graphite electrode  12  may be held in a fixture  18  following a design of the present invention and having a generally square base  20  with an upwardly extending cylindrical post  22  at its center. The post  22  and base  20  may be fabricated of 4140 steel. The upper end of the post  22  is cut with a longitudinal slot  24  having a width corresponding to that of the graphite electrode  12  (e.g., approximately 0.375 inches plus 0.001 and minus 0.000). The longitudinal slot  24  is sized so that the rectangular graphite electrode  12  may fit within the longitudinal slot  24  with the bottom surface of the graphite electrode  12  resting against the bottom surface of the slot  24 .  
         [0032]    Referring to FIG. 3, as so positioned, the axial hole  14  of the rectangular graphite electrode  12  is aligned with an axial hole  28  (shown in FIG. 3) extending vertically through the center of the post  22 . In turn, the axial hole  28  of the post  22  aligns with an axial hole  30  extending through base  20 . The holes  14 ,  28 , and  30  thereby providing a continuous path for coolant flow from beneath the base to the work piece being machined by the rectangular graphite electrode  12 .  
         [0033]    Referring now to FIGS. 1 and 3, a clearance hole  34  is cut in one wall of the slot  24  to align with a threaded hole  36  on the opposite wall of the slot  24 . Once the rectangular graphite electrode  12  is placed within the slot  24 , a machine screw  32  may be inserted transversely through the clearance hole  34  to extend through the transverse hole  16  in the rectangular graphite electrode  12  to pass through the slot  24  to engage the internally tapped threaded hole  36 . When the machine screw  32  is tightened, it pulls the sides of the slot  24  together about the rectangular graphite electrode  12  contacting a broad area of the graphite electrode  12  eliminating the need for epoxy or the like, while providing a secure grip on the electrode.  
         [0034]    Referring to FIG. 4, when the machine screw  32  passes through the transverse hole  16  in the graphite, the size of the transverse hole  16  is larger than the cross-section of the machine screw  32  providing for a circular channel  37  passing around the machine screw  32  allowing clear flow of solvent through axial hole  14 .  
         [0035]    Referring now to FIG. 2, the present invention may be simply modified for use with cylindrical graphite electrode  40  also having an axial hole  14  and a transverse hole  16 . The cylindrical graphite electrode  40  is in this case received within bore  42  cut in the slot  24  at the upper end of the post  22  equal in diameter to the diameter of the cylindrical graphite electrode  40  (e.g. with tolerances of minus 000 and plus 0.001). Again a clearance hole  34  is provided in one side of the slot  24  aligned with a threaded hole  36  (not shown). A machine screw  32  serves to compress the sides of the bore  42  against the graphite electrode  40  to provide a large area contact preventing looseness or movement of the graphite electrode  40 .  
         [0036]    Referring now to FIG. 3, weld fillets  44  may hold the post  22  to the upper surface of the base  20  as has been described. Expandable dowel pins  46  may extend downward from a lower surface of the base  20  to be received by corresponding bores  50  in a chuck plate  52  such as a macro fixture available from the 3R Company, Midwest Technical Center, Elk Grove Village, Ill. For this purpose, each corner of the base  20  may have a hole  54  tapped with M-6 metric threads which may receive machine screws  56  that pass upward through counterbored holes  57  in the chuck plate  52  and are received by threaded holes  54  in the base  20  to hold the base  20  and chuck plate  52  together as aligned by the dowel pins  46 .  
         [0037]    The fixture  18  provides a positive location of a particular graphite electrode  12  with respect to the dowel pins  46  in the bottom surface of the base  20  that may be used to ensure accurate location of the electrode  12  when it and the chuck plate  52  are transferred between chucks (not shown), one located on an electrode shaper (e.g., a milling cutter used to form the upper surface of the electrode  12 ) and, one located on the EDM machine.  
         [0038]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.