Patent Publication Number: US-2005126925-A1

Title: Drive head and ECM method and tool for making same

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of co-pending U.S. Provisional patent Application Ser. No. 60/332,944 filed Nov. 14, 2001. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates generally to a drive head, such as for a bolt, coupling, nut or the like, particularly those made from metals that are less malleable at forging temperatures and thus require special methods in making to prevent cracking. The present invention further relates to an electrochemical machining method, as well as a tool, useful in making such a drive head.  
      Fasteners such as bolts or nuts are typically provided with a drive head that can be gripped by the tool, e.g., wrench, so that sufficient torque can be generated to twist or rotate the fastener. The external surface of the drive head can have a number of different configurations. Some typical configurations include square heads (i.e., having four points or corners) and hexagonal heads (i.e., having six points or corners). See, for example, U.S. Pat. No. 3,492,908 (Thurston), issued Feb. 3, 1970, which shows a bolt having a hexagonal head. Other configurations include those having as many as twelve points or corners (sometimes referred to as a “double hexagonal” configuration) to provide the ability to increase the amount of torque that can be imparted by the wrench. See U.S. Pat. No. 3,352,190 (Carlson), issued Nov. 14, 1967; and U.S. Pat. No. 3,354,757 (Grimm et al), issued Nov. 28, 1967. For fasteners such as bolts, the drive head is typically connected to an externally threaded shank. (For fasteners such as nuts, the drive head typically has an internally threaded interior bore and no shank.)  
      The difficulty in fabricating drive heads for bolts becomes greater as the number of points or corners increases in number. This is particularly the case for bolts where the drive head has a double hexagonal configuration. Relative to a hexagonal configuration, the double hexagonal configuration has a much smaller radius from point to point. This makes it more difficult to provide a double hexagonal configuration in drive head having points or corners that are sufficiently well-defined and sharp so that the wrenching tool can engage the drive head without stripping the points or corners over time.  
      The difficulty in fabricating drive heads for bolts can also be affected by the ductility (malleability) of the metal, especially at the forging temperature used to make the bolt. Forging temperatures are chosen to be high enough to reduce the energy required for deformation and to reduce the propensity of the metal to crack, but low enough to preclude undesirable metallurgical changes in the metal. For metals having relatively high ductility over a large range of forging temperatures (i.e., are more malleable), the drive head can typically be formed by extrusion, forging or cold forming techniques. These fabrication methods basically form the desired configuration for the drive head by either directly deforming the metal, or by heating the metal either directly (or indirectly due to friction) and then deforming the metal. See, for example, U.S. Pat. No. 3,352,190 (Carlson), issued Nov. 14, 1967. See also U.S. Pat. No. 4,417,464 (Tosa), issued Nov. 29, 1983 (nib tool for cold head forming of a bolt having a hexagonal head); and U.S. Pat. No. 4,023,225 (Tochilkin et al), issued May 17, 1977 (cold shaping of bolt having a hexagonal head). However, for bolts made from metals that need to be forged in a very narrow temperature range, such as powder metal alloys (e.g., nickel alloys containing significant levels of nickel (e.g., at least about 40%) and other metals such as cobalt and chromium), conventional deforming techniques typically used to make bolts have not been found to be suitable. Conventional bolt forging in particular has been found to have a propensity to crack bolts made from less malleable metals, especially when a double hexagonal configuration is formed in the drive head of the bolt.  
      Other methods that have been used to form drive heads on bolts are electrical (electrode) heating techniques, such as by electrical discharge machining (EDM). See, for example, U.S. Pat. No. 4,473,738 (Wolfe et al), issued Sep. 25, 1984, which discloses an apparatus for forming a polygonal head on the end of a tie rod using a pair of reciprocating electrodes that define a die cavity having walls forming the desired polygonal contour (e.g., hexagonal). Electrical (electrode) heating techniques either etch the surface by moving the electrode so as to melt off material to form the desired drive head configuration, or by generating enough heat from the electrode to melt and deform the drive head within a die having the desired configuration. However, electrical (electrode) heating techniques such as EDM having been found to be unsuitable for forming drive heads from bolts made from less malleable metals, especially those having a double hexagonal configuration. In particular, EDM has been found to undesirably create a large recast layer on the shaped drive head, and can result in reduced material strength and fatigue life for the bolt.  
      Fabricating drive heads in bolts can be further complicated if it is desired to have an integral flange in the bolt adjacent to the drive head to provide an integral washer or to provide a washer-engaging face. See U.S. Pat. No. 3,492,908 (Thurston), issued Feb. 3, 1970, where bolt 7 has a cylindrical flange 12 adjacent to drive head 11. See also U.S. Pat. No. 3,352,190 (Carlson), issued Nov. 14, 1967, where fastener 10 has a thin integral washer 30 adjacent to drive head 12. This problem of providing such a flange is exacerbated in forming drive heads in bolts made from lower ductility materials, especially if the drive head is to have a double hexagonal configuration where the points or corners need to be well-defined and sharp.  
      Accordingly, it would be desirable to provide a bolt, nut or other driveable article made from a less malleable metal that has well-defined and sharp points or corners, even when the drive head has a double hexagonal configuration, and having a flange adjacent to the drive head. It would also be desirable to provide a method for making such a bolt that does not have propensity to crack the bolt, to create a recast layer, to reduce material strength or fatigue life, or to impart other undesired properties.  
     SUMMARY OF THE INVENTION  
      The present invention relates to a drive head for a bolt, nut, coupling, or other driveable article made from a less malleable metal or metal alloy such as powder metal nickel alloys comprising at least about 40% nickel. The drive head comprises: 
          (a) an upper drive portion having at least six convex corners spaced around the outer periphery thereof, each corner terminating in an edge; and     (b) a lower flange portion adjacent to the drive portion and having an edge extending radially outwardly to at least the edge of each corner;     (c) the drive portion and flange portion being formed by electrochemical machining.        

      The present invention also relates to a method for forming the corners in the drive portion of the drive head, as well as the flange portion. This method comprises the steps of: 
          (a) providing a blank made from a metal or metal alloy and having a generally circular head; and     (b) subjecting the head to electrochemical machining to form the drive portion and the flange portion of the drive head.        

      The present invention further relates to a tool useful in this electrochemical machining method for forming the corners in the drive portion of the drive head, as well as the flange portion. This tool comprises: 
          (1) a shaping end having a cutting face;     (2) a chamber for the passage of electrolyte fluid that opens at the cutting face;     (3) the cutting face having an inner portion adjacent to the chamber, the inner portion of the cutting face having at least six circumferentially spaced concave recesses; and     (4) each recess having a concave relief.        

      The drive portion of the drive head of the present invention can be provided with sharp and well-defined edges at each corner to allow the drive tool (e.g., wrench) to grip the drive portion and easily supply sufficient torque to twist or rotate the drive head. This can be achieved by using the electrochemical machining method and tool of the present invention, even when the drive portion of the drive head has a double hexagonal configuration (i.e., twelve corners), as well as a flange portion adjacent to the drive portion. In particular, the electrochemical machining method and tool of the present invention avoids problems (e.g., cracking, creating recast layers and reducing material strength or fatigue life) of prior forging and electrical discharge machining (EDM) methods in forming the drive head of the present invention from less malleable, harder metals or metal alloys, such as powder metal nickel alloys comprising at least about 40% nickel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side view of an embodiment of a bolt of the present invention.  
       FIG. 2  is a top plan view of the bolt of  FIG. 1 .  
       FIG. 3  is a side view of an embodiment of a blank used in making the bolt of the present invention.  
       FIG. 4  is a top plan view of the blank of  FIG. 3 .  
       FIG. 5  is a side sectional view of an embodiment of the electrochemical machining (ECM) apparatus that is shown forming the drive head for the bolt of the present invention.  
       FIG. 6  is a complete view taken along line  6 - 6  of  FIG. 5  that shows the drive head for the bolt of the present invention being formed from a blank. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      As used herein, the term “driveable article” refers to those articles that have a drive head and are generally twisted or rotated about a longitudinal axis by a tool (e.g., wrench) or other drive component. Driveable articles include fasteners such as bolts, or nuts, couplings such as curvic couplings, splines, gears, etc.  
      As used herein, the term “less malleable metal” refers to those metals that are difficult or impossible to shape or form by conventional bolt forging techniques. These harder, less malleable metals and metal alloys include powder metal nickel alloys comprising at least about 40% nickel (e.g., from about 40 to about 75%), more typically at least about 45% nickel (e.g., from about 45 to about 60%). These powder metal nickel alloys can also comprise at least about 5% cobalt (e.g., from about 5 to about 21%), more typically at least about 12% cobalt (e.g., from about 12 to about 14%) and at least about 10% chromium (e.g., from about 10 to about 22%), more typically at least about 15% chromium (e.g., from about 15 to about 17%). See U.S. Pat. No. 4,957,567 (Krueger et al), issued Sep. 18, 1990 (herein incorporated by reference) which discloses representative powder nickel alloys suitable for use in the present invention. These powder nickel alloys can also include other metals such as molybdenum (e.g., from about 3.5 to about 4.5%), tungsten (e.g., from about 3.5 to about 4.5%), aluminum (e.g., from about 1.5 to about 2.5%), titanium (e.g., from about 3.2 to about 4.2%), niobium (e.g., from about 0.5 to about 1%), zirconium (e.g., from about 0.01 to about 0.06%), vanadium (e.g., up to about 0.01%), hafnium (e.g., up to about 0.3%) and yttrium (e.g., up to about 0.01%).  
      Referring to the drawings,  FIG. 1  shows an embodiment of the driveable article of the present invention in the form of a bolt generally indicated as  10 . In addition to bolts, driveable articles of the present invention can be in the form of a nut, coupling, spline, gear, etc. However, for purpose of illustrating the present invention, the description hereafter will be with reference to a bolt.  
      As shown in  FIGS. 1 and 2 , bolt  10  comprises an upper drive head indicated generally as  14  and lower generally cylindrical shank generally indicated as  18 . Shank  18  typically has a threaded exterior surface for securing it to a fastener such as a nut. Drive head  14  comprises an upper drive portion indicated generally as  22  and a lower flange portion adjacent to one end of shank  18 . As shown particularly in  FIG. 2 , drive portion  22  has a plurality of generally V-shaped convex corners  30  spaced around the outer periphery thereof. (In the case of a spline configuration, such as a spline gear, corners  30  would be squared-off instead of generally V-shaped.) As shown in  FIG. 2 , drive portion  22  has twelve such corners, i.e., is a double hexagonal configuration. However, the bolt of the present invention can have at least six such corners, for example, six (i.e., hexagonal configuration), eight, ten or twelve corners, and typically has at least twelve corners. Each corner  30  has a first face  34  and second face  38 . The intersection of faces  34  and  38  forms an outer edge  42  for each corner  30 . Edge  42  is typically well-defined and sharp so that it is easily gripped by the drive component (e.g., wrench).  
      As shown in  FIG. 2 , flange portion  26  can comprise an upper generally sloping section indicated as  46  and a lower generally circular section indicated as  50  adjacent to shank  18 . Sections  46  and  50  intersect at generally circular edge  54 . As shown in  FIG. 1  and particularly  FIG. 2 , edge  54  extends radially outwardly to at least edges  42  of each corner  30  and typically extends outwardly beyond edges  42 .  
       FIGS. 3 and 4  show an embodiment of a blank generally indicated as  110  that can be used to form bolt  10 . Blank  110  has a generally circular head  114  from which drive head  14  is formed. Blank  110  also has a cylindrical shaft  118  connected to head  114  from which shank  18  is formed.  
      Blank  110  and bolt  10  that is made from it can be made from a variety of metals or metal alloys including those comprising iron, nickel, cobalt, chromium, molybdenum, tungsten, aluminum, titanium, niobium, zirconium, vanadium, hafnium, and yttrium. Of particular interest to the present invention are blanks  110  and bolts  10  made from harder, less malleable metals (and metal alloys) that are difficult to manipulate with conventional bolt forging techniques. These harder, less malleable include powder nickel alloys as previously defined.  
       FIG. 5  shows an embodiment of an electrochemical machining (ECM) apparatus indicated generally as  200  that is shown forming the drive head  14  of bolt  10  from head  114  of blank  110 . As shown in  FIG. 5 , apparatus  200  includes a block or holder  204  having cylindrical bore  208  for receiving and securing shaft  118  of blank  110 , with head  114  of blank  110  extending above holder  204 . Apparatus  200  also includes a shaping tool indicated generally as  210  that has a shaping end  214  and a generally cylindrical bore or chamber indicated as  218  (for the passage of electrolyte fluid) that extends longitudinally through  210  tool and opens at a cutting face indicated as  222  of shaping end  214 . Cutting face  222  has an outer annular relatively planar portion indicated as  223  and an inner sloped or tapered portion indicated as  225  that is adjacent to chamber  218  where it opens at cutting face  222 . As shown in  FIG. 5 , chamber  218  is provided with insulation  226  that extends from just behind or above cutting face  222  along the length of chamber  218 .  
      In carrying out the ECM method of the present invention using apparatus  200 , a direct electrical current (DC) is applied between blank  110  (as the work piece) which is positively charged (i.e., is the anode) and tool  210  which is negatively charged (i.e., is the cathode). An electrolyte fluid is used to conduct the current across the gap between tool  210  (the cathode) and blank  110  (the anode). Suitable electrolyte fluids include aqueous electrolyte fluids where an electrolyte salt, such as sodium chloride, sodium bromide, sodium iodide, sodium chlorate, sodium perchlorate, sodium sulfate, sodium nitrate, and mixtures thereof, is dissolved in water, typically in a concentration of from about 0.5 to about 3 lb./gallon (from about 60 to about 360 g./l). For example, a suitable electrolyte fluid can be prepared by dissolving about 1.1 lb. of sodium chloride per gallon of water (about 132 g./l.).  
      As shown in  FIG. 5 , tool  210  advances in the direction indicated by arrow  228  (the cutting direction) towards head  114  of blank  110 . As the cutting face  222  of tool  210  is brought closer to head  114  of blank  110 , the electrolyte is pumped from a source (not shown) at a controlled rate (for example, in the range of from about 100 to about 200 ft./sec. or from about 30.5 to about 61 m./sec.) into chamber  218  and then passes or flows out through the gap between cutting face  222  and head  114 . An electrical potential (e.g., from about 12 to about 18 volts) is applied across the electrolyte with the current flowing from the head  114  of blank  110  (the anode) to cutting face  222  of tool  210  (the cathode). As a result of the current being applied across the electrolyte, the metal molecules of head  114  and water molecules from the electrolyte breakdown and form a metal hydroxide and hydrogen gas.  
      The rate of breakdown or dissolution is proportional to the rate of current flow (i.e., amperage), as shown by Ohm&#39;s law (I=VR), where I is the current, V is the voltage and R is the resistance. Accordingly, higher voltages (i.e., by increasing the current flow), while keeping the resistance constant, will increase the rate of breakdown or dissolution. (A similar effect can be achieved by lowering the resistance.) The smaller the gap maintained between cutting face  222  of tool  210  and head  114  of blank  110 , the lower will be the resistance, thus leading to a higher rate of current flow; the higher the rate of current flow, the greater will be the rate of dissolution and removal of metal from head  114 . Typically, the gap between planar portion  223  of the cutting face  222  and the top of head  114  (or the sloping section  46  of flange  26  as drive bead  14  is formed) is in the range of from about 0.005 to about 0.015 in. (from about 0.1 to about 0.4 mm.) and is commonly referred to in the art as the “frontal” gap. Typically, the gap between inner portion  225  of cutting face  222  and faces  34 / 38  of each corner  30  of drive head  14  is in the range of from about 0.02 to about 0.05 in. (from about 0.5 to about 1.3 mm.) and is commonly referred to in the art as the “side” gap.  
      To keep the “frontal” and “side” gaps relatively small so as to maintain a maximum rate of metal removal, cutting face  222  of tool  210  is moved or advanced in the direction indicated by arrow  228  at a rate equivalent (or substantially equivalent) to the rate that metal is dissolved and removed from head  114  of blank  110 . Typically the cutting face  222  is advanced in the direction indicated by arrow  228  at a rate in the range of from about 0.01 to about 0.4 in./min (from about 0.2 to about 10.2 mm./min.), and more typically in the range of from about 0.04 to about 0.2 in./min (from about 1 to about 5.1 mm./min.). The drive head  14  will begin to take shape since those areas closer to the cathode cutting face  222  dissolve quicker than areas further away from face  222 . Tool  210  is also undercut and insulated in areas where the side of drive head  14  requires walls parallel to the centerline of bolt shank  18 . In particular, insulation  226  attached or adhered to the surface of chamber  218  behind or above cutting face  222  minimizes current flow from the remainder of chamber  218  so that further metal dissolution or removal from those portions of drive head  14  that have been shaped by cutting face  222  is minimized.  
      As cutting face  222  moves concentrically along the exterior length of head  114  in the direction indicated by arrow  228  (i.e., towards shaft  118 ), and as metal is dissolved and removed from head  114 , drive head  14  will take on the desired shape. After the cutting face  222  advances the distance indicated by outline  230  of the shaping end  214 , the drive portion  22  and flange portion  26  will be formed in drive head  14 . The particular distance cutting face  222  advances is determined by the design requirements for drive head  14 . The particular shape of the drive portion  22  of drive head  14  will be determined by the shape or configuration of cutting face  222 , and in particular the inner portion  225 . For example, the shape or configuration for forming a twelve point bolt  10  (i.e., a twelve corner drive portion  22 ) is shown in  FIG. 6 . Referring to  FIG. 6 , cutting face  222  comprises a plurality of generally V-shaped concave recesses indicated generally as  232  that are formed in and circumferentially spaced around the inner portion  225  of cutting face  222  such that recesses  232  are also adjacent to chamber  218 . As shown in  FIG. 6 , in order to form a twelve point bolt, there are twelve such recesses  232  formed in inner portion  225  of cutting face  222 . As also shown in  FIG. 6 , each recess  232  is opposite to and is complementary of a respective corner  30  of drive head  14  that is formed by the inner portion  225  of cutting face  222 . Each recess  232  has a first segment  234  and a second segment  238  that are opposite corresponding first and second faces  34  and  38  of the respective corner  30  that is formed.  
      A concave relief in the form of a generally semicircular groove or notch indicated as  242  is formed in each recess  232  and connects first and second segments  234  and  238 . (Depending on the geometry of corner  30 , relief  242  can be in the form of other concave shapes or configurations besides semicircular, so long as a laminar flow of the electrolyte is provided at corner  30 .) Each relief  240  is opposite and is complementary to an edge  42  of the respective corner  30  of drive head  14 . As shown in  FIG. 6 , there is a “side” gap indicated generally as  246  between the faces  34 / 38  of each complementary corner  30  and segments  234 / 238  of each recess  232 . As also shown in  FIG. 6 , there is also a gap indicated generally as  250  between each complementary edge  42  of the respective corner  30  and the relief  242  of the respective recess  232 .  
      In the absence of the relief  242 , each corner  30  of drive portion  22  would not form a sharp edge  42  but would instead form a much more rounded edge. This is due to current flowing from head  114 /drive head  14  across the electrolyte passing through “side” gap  246  to more than one of segments  234  and  238  of recess  232  of tool  210 . By providing relief  242  in each recess  232 , the current is less likely to flow to more than one of segments  234  and  238  (i.e., because of the increased width of gap  250 ), thus allowing for a relatively sharp edge  42  to be formed at each respective corner point  30 . The size of relief  242  also needs to be relatively small. If relief  242  is too large, an undesired protrusion can form on each of the respective corners  30 .  
      In the absence of relief  242 , current will flow from corner  42  of the drive head  14  to both segments  238  and  234  of each recess  232  of tool  210 , causing more material to be dissolved and removed from the corner  42  than from faces  34  and  38 . This would result in a rounded corner  42 , rather than a sharp, well-defined corner  42 . Without a sharp, well-defined corner  42 , it is usually significantly more difficult to apply proper torque to drive head  14  with a driveable tool (e.g., installation wrenches). The addition of semicircular relief  242  to each of the respective recesses  232  allows a sharp, more well defined corner  42  to be obtained by tool  210  during the ECM method for drive head  14 , while still being able to generate section  46  of flange portion  26 .  
      While specific embodiments of the method of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the present invention as defined in the appended claims.