Abstract:
An electrode assembly for electro-hydraulic forming tool in which the insulation sleeve is assembled over an electrode rod. The insulation sleeve has a deflection surface that is beveled or oriented at an angle relative to the axis of the electrode rod. The electrode assembly may include a replaceable conical receptacle that facilitates placement of an electrode tip an insulation sleeve tip. An electrode tip profile that is shaped to minimize erosion caused by discharge of the electro-hydraulic forming machine.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to the structure and operation of electrode assemblies for electro-hydraulic forming processes. 
         [0003]    2. Background 
         [0004]    Electro-hydraulic forming (EHF) is a process in which a high voltage, stored charge is discharged across spaced electrodes that are disposed in a fluid filled chamber. An EHF system generally includes a chamber that is filled with fluid, for example water with a rust preventative. The electrodes are immersed in the fluid within the chamber. A sheet metal blank is placed on the chamber. A one-sided die is then placed on the blank. Air is then evacuated from both sides of the blank. The capacitor bank stores a charge and may provide between 5 to 50 kV through the electrodes. 
         [0005]    The voltage applied to the electrodes creates a high temperature plasma channel. Current from the capacitors expands the plasma channel and fills the region surrounding the plasma channel with gas in the form of super heated steam which then transitions to a steam/water interface. Pressure builds within the chamber and results in a high intensity high velocity shock wave being formed in the liquid that is transmitted to the sheet metal blank. The sheet metal blank is driven into the die by the high velocity shock wave. 
         [0006]    Traditional advantages over conventional sheet metal forming processes that are known include reduction in capital cost because only a one-sided die is required to form a panel. Recently, EHF processes have been proposed for forming sheet metal materials that are more difficult to form than low carbon steel. For example, the EHF process could be used to manufacture automotive and truck components from high strength steel, stainless steel or aluminum alloys. EHF processes are generally targeted at low volume stamping processes that may take advantage of the savings resulting from the use of a one-sided die. 
         [0007]    The electrode assembly is subject to erosion caused by the shock wave and high temperature plasma. In addition, the fluid in the chamber tends to be corrosive. These conditions lead to erosion of both electrodes and insulation that fills the gap between the chamber and the electrodes. The shock wave resulting from discharge of the capacitors through the electrodes exerts a force on the electrode assembly tending to expel the electrode and insulation from the chamber. The electrode system must withstand the high pressure and high temperature plasma, as well as the corrosive water in the chamber. The electrodes must conduct energy into the chamber, maintain electrical isolation through the insulation, maintain the chamber in a water tight condition and constrain electrode motion during pressure pulses tending to eject the electrode from the chamber. 
         [0008]    These and other problems are addressed in applicants&#39; development as summarized below. 
       SUMMARY 
       [0009]    Applicants&#39; development, as summarized below, is directed to providing a robust electrode assembly for the EHF forming that resists erosion of the electrode, erosion of the insulation material, expulsion of the electrode or its insulation from the chamber. The electrode assembly must also be adjustable to assure maintenance of the desired inter-electrode gap. In addition, the disclosure also provides for a renewable replaceable electrode tip and ways to reduce erosion and wear of the insulation. 
         [0010]    According to one aspect of the present development, an electrode assembly for an electro-hydraulic forming tool that has a fluid filled chamber is provided. The electrode assembly includes an electrode rod upon which an insulation sleeve is assembled. The insulation sleeve has a chamber end and a connector end. The insulation sleeve has a deflection surface at the chamber end that forms a frustum of cone between the electrode rod and the chamber. The deflection surface is narrowest near the electrode to deflect the force of a discharge of the electro-hydraulic forming tool. 
         [0011]    According to other aspects of the invention, the deflection surface may be formed as a tapered part of the insulation sleeve on the inner end of the insulation sleeve. Alternatively, the deflection surface may be formed on a metal cap that is assembled over the electrode rod and against the inner end of the insulation sleeve. 
         [0012]    According to other aspects of the invention relating to providing an adaptor, an adaptor may be disposed about the insulator sleeve that secures electrode and the insulation sleeve in an opening defined by the chamber. A holder may be provided that receives the electrode and insulation sleeve that is in turn received within the adaptor. 
         [0013]    The insulator sleeve may have a reaction surface disposed between the chamber end and the connector end. The reaction surface may be partially conical in shape with its narrowest end at the end of the reaction surface that is furthest from the chamber end of the insulation sleeve. The adaptor may be disposed about the insulation sleeve to secure the electrode and the insulation sleeve in an opening defined by the chamber. A holder may be provided that receives the electrode and insulation sleeve and that is received within the adaptor. The holder may have a conical force receiving surface that is engaged by a reaction surface to hold the insulation sleeve within the holder. A retainer may be attached to the holder to secure the insulation sleeve and the electrode rod within the holder. A lock ring may be provided that locks the retainer to the holder. 
         [0014]    According to another aspect of the disclosure, an electrode assembly may be provided for an electro-hydraulic forming tool that has a chamber. The electrode assembly may comprise an electrode rod, an insulation sleeve assembled over the electrode rod with the sleeve having an inner end that is closest to the chamber than the outer end. An adaptor may be disposed about the insulation sleeve to secure the electrode in the insulation sleeve in an opening defined by the chamber. 
         [0015]    A conical receptacle may be assembled to the adaptor that defines an opening. A replaceable insulator tip may be assembled within the opening in the conical receptacle. The chamber end of the insulation sleeve may be received within the insulator tip. The conical receptacle may retain the insulator tip when the electrode assembly is discharged and the conical receptacle is removed from the adaptor to replace the insulator tip. 
         [0016]    According to other aspects of the invention as they relate to an electrode assembly having a replaceable insulator tip, the adaptor may define the cup-shaped end that is disposed within the chamber. The conical receptacle may be at least partially received within the cup-shaped end of the adaptor. 
         [0017]    A holder may be provided that receives the electrode and the insulation sleeve. A retainer may be attached to the holder to secure the insulation sleeve and the electrode rod within the holder. A lock ring may be provided that locks the retainer to the holder. 
         [0018]    According to another aspect of the disclosure, an electrode assembly for an electro-hydraulic forming tool may be provided. The tool has a chamber that receives an electrode rod that extends from outside of the chamber to inside the chamber. The electrode rod may be provided with a main portion and a tip. An insulation sleeve is preferably assembled over the electrode rod with the insulation sleeve having a chamber end and a connector end. A holder attaches the electrode rod and insulation sleeve to the chamber. 
         [0019]    Other aspects of the disclosure relate to providing a tip portion that is formed with different material than the main portion of the electrode rod with the tip portion being welded to the main portion. The tip portion may be selected from a more erosion resistant and durable material such as low carbon steel, tungsten, molybdenum, or a tungsten-copper alloy. The main portion may be selected from the group consisting essentially of low carbon steel. The tip portion may have a tip profile that is a flat tip with rounded edges that conform to the shape of a cylindrical electrode after a substantial number of pulses. 
         [0020]    These and other aspects of the present invention would be better understood with the attached drawings and the following detained description of the illustrated embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a perspective view of an electrode assembly for an electro-hydraulic forming tool made according to one embodiment of the disclosure; 
           [0022]      FIG. 2  is a cross section of an electrode assembly assembled to an electro-hydraulic forming tool shown in  FIG. 1 ; 
           [0023]      FIG. 2A  is a fragmentary cross-section of an alternative embodiment of the electrode tip and insulation sleeve with a tip cap; 
           [0024]      FIG. 3  is an exploded perspective of the electrode assembly shown in  FIG. 1 ; 
           [0025]      FIG. 4  is an alternative embodiment of an electrode assembly for an electro-hydraulic forming tool; 
           [0026]      FIG. 5  is a cross sectional view of the embodiment of the electrode assembly shown in  FIG. 4 ; 
           [0027]      FIG. 6  is an exploded perspective view of the electrode assembly shown in  FIG. 4 ; 
           [0028]      FIG. 7  is a fragmentary cross-sectional view of the embodiment of the electrode assembly shown in  FIG. 4 ; and 
           [0029]      FIG. 8  is a graph of a tip profile for an electrode for use in an electro-hydraulic forming tool. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Referring to  FIGS. 1-3 , an electrode assembly  10  is shown that includes an electrode rod  12 . As shown specifically in  FIG. 2 , the electrode assembly  10  is assembled to an electro-hydraulic forming (EHF) chamber wall  16 . The chamber wall  16  defines an opening  18  in which the electrode assembly  10  is received. The electrode assembly  10  has a chamber end  20  that is disposed within the EHF chamber and a connector end  22  that extends outboard of the EHF chamber wall  16 , and is adapted to be connected to an electrical connector. The electrical connector connects the electrode assembly  10  to a source of stored charge such as a bank of capacitors. 
         [0031]    An insulation sleeve  24  has an insulation tip  26  on the side of the insulation sleeve  24  adjacent the chamber end  20  of the electrode rod  12 . A deflection surface  28  is formed on the insulation tip  26  to deflect the force of an electro-hydraulic discharge from directly impacting the insulation sleeve  24 . The deflection surface  28  may be bevelled or formed as a frustum of a cone. 
         [0032]    Alternatively, and as shown in  FIG. 2A , a metal cap  29  may be provided that is assembled to the chamber end of the electrode rod chamber end  20  and insulation sleeve  24 . In this arrangement, the deflection surface  28  would be provided on the cap  29  to deflect the force of an electro-hydraulic discharge from directly impacting the insulation sleeve  24 . 
         [0033]    A reaction surface  30  distributes any force applied to the insulation sleeve  24  by the electro-hydraulic discharge to the other parts of the electrode assembly  10 , as will be more fully described below. A locking surface  32  is also provided on the insulation sleeve  24  that is provided to lock the insulation sleeve  24  in place within the electrode assembly  10 . 
         [0034]    An adaptor  40  may be provided between the chamber end  20  of the electrode rod  12  and the insulation sleeve  24  and the chamber wall  16 . The adaptor preferably includes a bevelled or a frustoconical surface that extends to a shoulder  42 . The shoulder  42  is placed into direct contact with the EHF chamber wall  16 . The shoulder  42  is assembled against the EHF chamber wall  16 . 
         [0035]    A holder  44 , as best shown in  FIG. 2 , is provided within the adaptor  40  to hold the insulation sleeve  24  in place within the adaptor  40 . A holder receptacle  46  is defined within the adaptor  40  and comprises an opening in the adaptor  40  in which the holder  44  is assembled. An adaptor nut  52  is provided to secure the adaptor  40  to the chamber wall  16 . A holder nut  50  is provided to secure the holder  44  within the adaptor  40 . A plurality of o-rings  58  may be provided to provide a seal between the insulation sleeve  24  and the adaptor  40 . 
         [0036]    A retainer  54  holds the insulation sleeve  24  within the holder  44 . The retainer  54  and locking ring  56  combine to hold the insulation sleeve  24  against the holder  44 . The holder  44 , as shown in  FIG. 2 , has a plurality of wrench flats  53  that are intended to be turned by a wrench to advance and retract the holder  44  within the holder receptacle  46 . The holder  44  is advanced and retracted to move the electrode rod  12  and insulation sleeve  24  relative to the EHF chamber  16 . The holder  44  is threaded to facilitate advancement of the electrode rod  12  through the chamber wall  16 . The electrode rod  12  is threaded to facilitate electrical connection to the connector end  22  and to mechanically interlock the insulation sleeve  24  and the electrode rod  12 . Clearance space is provided between the locking ring  56  and the end of the holder receptacle  46  to allow for adjustment of the electrode rod  12  to its desired position within the EHF chamber  16 . 
         [0037]    Referring to  FIGS. 4-7 , an alternative embodiment of the electrode assembly  60  is illustrated. An electrode rod  62  extends through the electrode assembly  60 . The electrode assembly  60  is assembled to an EHF chamber wall  64  as best shown in  FIG. 5 . An opening  66  is defined by the EHF chamber wall  64  in which the electrode assembly  60  is assembled. A tip portion  68  of the electrode rod extends inside the EHF chamber wall  64 . A main portion  70  of the electrode rod  62  is retained within an insulation sleeve  72 . The tip portion  68  is preferably welded to the main portion  70  to form the electrode rod  62 . 
         [0038]    The insulation sleeve  72  includes an insulation tip cap  74 . The insulation tip cap is a bevelled or frustoconical surface. A replaceable insulation tip  76  provides the insulation tip cap  74  and is assembled to the chamber end of the insulation sleeve  72 . The replaceable insulation tip  76  may be replaced with the electrode tip or independently to provide renewed insulation between the electrode rod  62  and the EHF chamber. A deflection surface  78  is provided on the insulation tip cap. A conical insert  80  is assembled to a cup-shaped receptacle  82 . Set screws  84  may be provided to secure the conical insert  80  within the cup-shaped receptacle  82 . A holder receptacle  86  is defined within the adaptor  88  that provides the cup-shaped receptacle  82 . A holder  92  is received within the holder receptacle  86  and may be adjusted by turning the holder with a wrench that engages the wrench flats  93 . 
         [0039]    A plurality of vent holes  94  may be provided within the conical insert  80  to allow entrapped gas to be vented from the electrode assembly  60 . Any fluid that passes between the conical insert  80  and the insulation insert  76  may be collected within pockets  96  that are provided within the conical insert  80 . Any fluid or gas pressure developed within the pockets  96  is vented through the vent holes  94 . 
         [0040]    As previously described with reference to  FIGS. 1 and 2 , a retainer  100  is assembled within the holder  92  to hold the insulation sleeve  72  within the holder  92 . A lock ring  102  is then assembled over the retainer  100  to lock the retainer  100  in place. An adaptor nut  104  is attached to the adaptor  88  to secure the adaptor to the EHF chamber wall  64 . A holder nut  106  is secured to the holder  92  to maintain the position of the holder  92  within the adaptor  88 . A plurality of o-rings  108  may be provided to provide a seal between the insulation sleeve  72  and the adaptor  88 . 
         [0041]    Referring to  FIG. 8 , a preferred shape or profile of an electrode tip may be used in either embodiment of the EHF forming tool described above or other pre-existing EHF forming tools. The preferred electrode tip profile is defined in  FIG. 8 . The design of an electrode tip is impacted by chamber volume, chamber geometry and discharge energy. Pressures at the electrodes have been estimated to be between 1-2 GPa. The electrode system must sustain erosive pressure and high temperature plasma while being submerged in a corrosive fluid such as water that may, if desired, include a rust inhibitor. 
         [0042]    The electrode must also perform the functions of conducting energy into and out of the chamber, maintaining the electrode isolation from the chamber or a source of ground. The electrode assembly must also maintain the chamber&#39;s water tight seal while allowing for measurement and adjustment of the inter electrode gap. The electrode assembly also must constrain motion of the electrode during the pressure pulses that may tend to cause the electrode to be ejected from the EHF chamber. 
         [0043]    The profile of the electrode or tip shape can influence the efficiency of the discharge and the voltage differential occurring during a discharge. Finally, the electrode profile can also affect the rate of erosion of the electrode. In some instances, severe tip erosion may be experienced by the electrode tip. Electrodes must be changed periodically during a production operation. The erosion rate substantially impacts the frequency of electrode replacement. 
         [0044]    The tip profile shown in  FIG. 8  is a relatively stable tip profile that provides a more consistent discharge than other electrode profiles. This stable tip profile minimizes erosion and reduces the increase of the resulting gap. If the gap between opposed electrodes is doubled, the voltage at the moment of discharge may decrease by 5% and the resulting current drop may be nearly 20%. The combined voltage and current loss results in nearly a quarter of the energy input being lost for forming purposes. Tips having a hemispherical profile typically have a gap increase of 5 mm over a  120  discharges. Studies of initially flat electrode tips provide a gap increase of 2 mm over 120 discharges. As a result, it may be necessary to advance the electrode after each 100 discharges. 
         [0045]    Electrode tips having a pointed or conical cross section tend to focus the discharge energy and erode very quickly to rapidly lose their original design and shape. Flat electrode tips erode more slowly, however, their profile tends to change over time as discharges occur. Electrode tip profiles gradually change during the erosion process with a variable rate of erosion and constantly changing discharge conditions. It is proposed that a stable tip profile be provided initially that has a generally flat tip and rounded edges which asymptotically approach the desired stable electrode tip profile. Such a tip profile tends to exhibit the slowest rate of erosion and is more stable over time. 
         [0046]    As shown in  FIG. 8 , an electrode tip is shown in half-section. The solid optimum line shows the tip in the optimum shape. The x and y axes are normalized against the overall electrode half-diameter/width. The optimum profile shown in the solid line represents the nominal shape of the proposed electrode. The maximum and minimum profiles shown in  FIG. 8  describe the operating envelope of stable tip shapes. The optimum, maximum and minimum profiles may be described by the following exponential decay equation: 
         [0000]        y ( x )= y   o   −A   1   e   (1-x)/t1   −A   2   e   (1-x)/t2   −A   3   e   (1-x)t3    
         [0000]    where e is the base of the natural logarithms (2.718 . . . ), and the constants to be used in the equation describing each profile are found in Table 1 below: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 y o   
                 A 1   
                 t 1   
                 A 2   
                 t 2   
                 A 3   
                 t 3   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Maximum 
                 0.00048 
                 0.40939 
                 0.14818 
                 0.72452 
                 0.01097 
                 0.38186 
                 0.00026 
               
               
                 Optimum 
                 0.0038865 
                 0.59789 
                 0.19858 
                 0.71285 
                 0.01263 
                 0.19676 
                 0.00065 
               
               
                 Minimum 
                 0.01164 
                 0.80809 
                 0.23584 
                 0.83625 
                 0.01788 
                 1.61654 
                 0.00031 
               
               
                   
               
             
          
         
       
     
         [0047]    The benefit of an improved tip shape may also be obtained in large measure by providing a tip with a profile that is between lines A and B in  FIG. 8 . The tip is larger than line A that is defined by: 
         [0000]        Y=− 1.6 x   2    
         [0000]    And that is smaller than a polyline identified by B and defined by the horizontal line (y=0) and the vertical line (x=1) with a radius of ⅛R 0 . 
       Where: 
       [0048]    y is the electrode tip length, as measured from the end of the tip; 
         [0049]    x is the distance from electrode centerline; and 
         [0050]    R 0  is the electrode radius equal to half of the diameter of the electrode diameter or half of the width of the electrode. 
         [0051]    Although embodiments of the invention have been disclosed, it will be apparent to persons skilled in the art that modifications may be made without departing from the scope of the invention. All such modifications and equivalents thereof are intended to be defined by the following claims.