Patent Publication Number: US-2007101793-A1

Title: Metal bending with an anti-galling bend fixture

Description:
1. FIELD OF THE INVENTION  
      The present invention relates to the bending of metal sheet and more particularly to a method and apparatus for bending metal sheet without detrimentally affecting surface quality in a bending zone.  
     2. BACKGROUND OF THE INVENTION  
      Techniques are known in the prior art for imparting to metal sheet a substantial angular bend, for example a 90° bend. Prior art methods for bending metal sheet at a 90° angle usually require positioning the metal sheet between a punch or punch tool and a die. However, interaction between the punch and the die galls the metal in a bend zone adjacent a bend line where the metal sheet is shaped so that visible streaks appear on the metal surface. Interaction between the punch and the die also risks metal excess or metal deficiency in the bending zone. Accordingly there still remains an unmet need for a bend fixture and method capable of bending metal sheet without detrimentally affecting metal surface quality in the bending zone.  
      A principal objective of the present invention is to provide a method and a bend fixture for bending metal sheet without galling surfaces of the metal sheet.  
      A related objective of the invention is to provide a method and a bend fixture for bending metal sheet without positioning the sheet between a punch and a die.  
      An advantage of the invention is that the claimed method and apparatus are capable of being automated for efficient large volume production of bent metal sheet. Additional objectives and advantages of my invention will become readily apparent to persons skilled in the art from the following detailed description of a particularly preferred embodiment.  
     3. SUMMARY OF THE INVENTION  
      In accordance with the present invention there is provided a bend fixture including a first platen, a second platen, at least one hinge connected with at least one of the platens, and a clamp or clamping means for holding a metal sheet adjacent exterior surface portions of the first and second platens. Preferably the first platen is stationary and the second platen is connected with at least one hinge. The hinge preferably includes hinge pins attached to opposite lateral sides of the hinged platen. The stationary platen and the hinged platen are initially separated by a gap.  
      The stationary platen and the hinged platen each include an exterior surface portion, an interior surface portion spaced from the exterior surface portion, and a slanted surface portion extending angularly between an inner edge of the exterior surface portion adjacent to the gap and an inner edge of the interior surface portion spaced from the gap. The inner edge of the interior surface portion is spaced longitudinally outwardly of the inner edge of the exterior surface portion. The exterior surface portion preferably includes a generally planar principal portion spaced longitudinally from the gap and a convexly curved end portion between the principal portion and the gap.  
      The metal sheet to be bent in accordance with my invention may be made of a zirconium alloy, steel, aluminum, or copper. In a particularly preferred embodiment the metal bending apparatus and method are utilized for applying a 90° bend to a Zircalloy perimeter grid strap making up a part of a lattice for supporting nuclear reactor fuel elements. The perimeter grid straps interlock with ends of inner grid straps to form a lattice, as described in greater detail in Richards U.S. Pat. No. 5,859,887, the disclosure of which is incorporated herein by reference to the extent consistent with the present invention. Zirconium alloy sheet is particularly preferred because of its low neutron absorption cross-section. As used herein the term “sheet” refers to metal having a thickness of about 0.006 inch to 0.060 inch.  
      In the metal bending method of the invention a metal sheet is supported adjacent the planar principal portions of the first and second platens, spanning the gap between the platens. A first clamp holds the metal sheet to the first platen and a second clamp holds the metal sheet to the second platen. The first platen is preferably stationary while the second platen is preferably rotated on its hinge, preferably to an angle of greater than 90°. The hinged platen is preferably rotated by an angle of about 2.5° greater than the desired bend angle in order to overcome metal stress. Rotating the hinged platen bends the metal sheet, preferably to an angle of about 90°. When the hinged plate is rotated to the desired angle, the slanted portions of the stationary and hinged platens approach each other and portions of the metal sheet adjacent the gap approach the curved end portions of the platens in a bend zone extending transversely across the sheet. A bend line extending across the entire width of the metal sheet in the bend zone is remarkably free of streaks and other visible defects resulting from galling.  
      In a preferred embodiment of the invention wherein the metal sheet includes laterally outwardly extending vanes or tabs, the stationary platen and the hinged platen each include at least one exteriorly extending pilot pin positioned adjacent the vanes. The pilot pins stabilize the metal sheet as it is bent. 
    
    
     4. BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a first side elevational view of a bend fixture of the invention.  
       FIG. 2  is a second side elevational view of a bend fixture of the invention.  
       FIG. 3  is a top plan view of a bend fixture of  FIG. 1 .  
       FIG. 4  is an end view of the bend fixture of  FIG. 1 .  
       FIG. 5  is an enlarged fragmentary cross-sectional view of the hinge pin area of  FIGS. 1-4 .  
       FIG. 6  is a first schematic view of a bend fixture of the invention, showing a metal sheet before bending.  
       FIG. 7  is a second schematic view of a bend fixture of the invention, showing a bent metal sheet. 
    
    
     5. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
      A particularly preferred embodiment of a bend fixture  10  made in accordance with the invention is shown in  FIGS. 1-5 . Referring more particularly to  FIGS. 1 and 3 , the bend fixture  10  includes a stationary platen or first platen  12  and a second platen or hinged platen  14 . The fixture  10  also includes a subplate  16  anchored to a work table  18  by several bolts  20 . The stationary platen  12  is fixed to the subplate  16  by bolts  22 . A skirt  24  extends downwardly from the subplate  16 , below the stationary platen  12 . A stop bolt or stop  26  extends through the skirt  24  and outwardly thereof. The stationary platen  12  is separated from the hinged platen  14  by a short gap  27 , best shown in  FIG. 5 .  
      The hinged platen  14  is connected with hinges on opposite sides thereof. Each hinge includes a generally cylindrical hinge pin  30 , two fixed hinge covers  32  attached to the stationary platen  12  (one on each side), and two movable hinge covers  34  attached to the hinged platen  14  (one on each side). The hinged platen includes a handle  36  comprising a socket head cap screw. A manual operator pushes downwardly on the handle  36  in order to rotate the hinged platen  14  between the initial position shown in  FIG. 1  and the rotated position shown in  FIG. 2 .  
      Referring again to  FIG. 1 , the stationary platen  12  includes an exterior surface portion  38  and an interior surface portion  40  spaced from the exterior surface portion. The exterior surface portion  38  is parallel to the interior surface portion  40 . As shown in  FIGS. 1 and 5 , the exterior surface portion  38  includes a generally planar principal portion  38   a  spaced from the gap  27  and a convexly curved end portion  38   b  extending between the principal portion  38   a  and the gap  27 . The stationary platen  12  also includes a slanted surface portion  42  extending angularly between an inner edge of the exterior surface portion  38  adjacent the gap  27  to an inner edge of the interior surface portion  40  spaced from the gap  27 . Similarly the hinged platen  14  includes an exterior surface portion  48  and an interior surface portion  50  spaced from the exterior surface portion. The exterior surface portion  48  is parallel to the interior surface portion  50 . The exterior surface portion  48  includes a generally planar principal portion  48   a  spaced from the gap  27  and a convexly curved end portion  48   b  extending between the principal portion  48   a  and the gap  27 . The hinged platen  14  also includes a slanted surface portion  52  extending angularly between an inner edge of the exterior surface portion  48  adjacent the gap  27  to an inner edge of the interior surface portion  50  spaced from the gap  27 .  
      As shown in  FIGS. 1-3 , the bend fixture  10  includes a first clamp or clamping plate  54  supported above the stationary platen  12  by a support strut  56 . The first clamp  54  includes a generally planar lower surface  57 . The first clamp  54  is moved downwardly toward the stationary platen  12  by a piston activated by air from a pressure cylinder  58 . A threaded screw  60  is rotated to fit into an upwardly extending screw hole  62  in the first clamp  54 . Similarly, a second clamp  64  is supported above the hinged platen  14  by support strut  66 . The second clamp  64  is moved downwardly toward the hinged platen  14  by air from a pressure cylinder  58 . A threaded screw  60  is rotated to fit into an upwardly extending screw hole  62  in the second clamp  64 . The second clamp  64  has a generally planar lower surface  67 . The first clamp  54  and the second clamp  64  each define a shallow recess  68  extending upwardly from the respective lower surfaces  57 ,  67 . In a particularly preferred embodiment wherein the bend fixture  10  is used to bend metal sheet having a thickness of about 0.025 inch, the recess  68  has a depth of about 0.023 inch. This depth is selected to reduce direct contact between clamp lower surfaces  57 ,  67  and their respective platens  12 ,  14 .  
      As shown in  FIGS. 1-3 , the stationary platen  12  and the hinged platen  14  each define several downwardly extending indentations  70 . The indentations are sized and shaped to accommodate structures projecting outwardly from the metal sheet. Such projections include dimples and springs when the metal sheet is a 0.025 inch thick Zircalloy sheet shaped for use as a perimeter grid strap in a lattice supporting nuclear fuel elements. The hinged platen  12  also includes upwardly extending pilot pins  75  for stabilizing the metal sheet by standing aside a vane in the sheet. The vane is preferably concavely shaped to fit the convex outer curvature of the pilot pin  75 . The sheet is bent by rotating the hinged platen  12  from the initial position shown in  FIG. 1  to the final position shown in  FIG. 2 . As shown in  FIG. 2 , such rotation causes the slanted portions  42 ,  52  of the platens  12 ,  14  to approach each other. The stop  26  is adjustable to vary the bend distance by rotating the stop  26  to a preselected position.  
       FIG. 5  is an enlarged, fragmentary view of a portion of the bend fixture in the vicinity of a hinge pin  30 . The bend fixture  10  includes a stationary platen  12  spaced from a hinged platen  14  by a gap  27 . The stationary platen  12  includes an exterior surface portion  38  and an interior surface portion (not shown) spaced from the exterior surface portion. The exterior surface portion  38  includes a generally planar principal portion  38   a  spaced from the gap  27  and a convexly curved end portion  38   b  extending between the principal portion  38   a  and the gap  27 . The stationary platen  12  also includes a slanted surface portion  42  extending angularly between an inner edge of the exterior surface portion  38  adjacent the gap  27  to an inner edge of the interior surface portion (not shown). Similarly the hinged platen  14  includes an exterior surface portion  48  and an interior surface portion (not shown) spaced from the exterior surface portion  48  and extending generally parallel thereto. The exterior surface portion  48  includes a generally planar principal portion  48   a  spaced from the gap  27  and a convexly curved end portion  48   b  extending between the principal portion  48   a  and the gap  27 . The hinged platen  14  also includes a slanted surface portion  52  extending angularly between an inner edge of the exterior surface portion  48  adjacent the gap  27  to the interior surface portion (not shown).  
       FIGS. 6 and 7  schematically illustrate a metal sheet  105  positioned adjacent a bend fixture  110  including a stationary platen  120  and a hinged platen  122 . The platens  120 ,  122  are initially separated by a small gap  124 . The platens each include an exterior surface portion  120   a,    122   a  below the metal sheet  105 , an interior surface portion  120   b,    122   b  on a side opposite the exterior surface portions 120 a,   122   a,  and a slanted surface portion  120   c,    122   c  extending angularly between the exterior surface portions  120   a,    122   a  and the interior surface portions  120   b,    122   b  adjacent the gap  124 . Exterior surface portions  120   a,    122   a  of the platens each include a generally planar principal portion  120   d,    122   d  spaced from the gap  124  and a convexly curved end portion  120   e,    122   e  between the respective principal surface portions  120   d,    122   d  and inner edges  120   f,    122   f  adjacent the gap  124 . The metal sheet  105  to be bent on the fixture  110  is positioned by pilot pins (not shown) extending outwardly of the platens  120 ,  122  and then clamped next to the platens  120 ,  122  by clamping plates (not shown).  
      Initially as shown in  FIG. 6  the metal sheet  105  extends longitudinally across the gap  124 , spaced upwardly of inner edges  120   f,    122   f  of the platens  120 ,  122 . The hinge covers associated with the hinged platen  122  are not shown in  FIGS. 6 and 7  although the location of the center or centerline  130  of the hinge pin is shown. Precise location of the centerline  130  is important in bending a metal sheet  105  in the bend fixture  110 . Fortunately, the centerline  130  can be located precisely by following the following equation:  
       C   =       0.017453   ×     D   ⁡     (     R   +     .333   ×   T       )         2         
 
 where D is the angle of bend in degrees, R is the inside radius of the bend in inches, T is the thickness of the metal sheet in inches, 0.017543 radians/degree is the factor for converting degrees to radians, and C is the horizontal and vertical distance from the centerline  130  to the center  135  of the inside radius of the bend  140  in the sheet  105 . 
 
      In a preferred example where the bend angle is 90°, the sheet thickness is 0.026 in, and the bend radius is 0.100 in, the centerline distance, C, is calculated as follows:  
             C   =       0.017453   ⁢           ⁢   radians   ⁢     /     ⁢   degree   ×   90   ⁢   °   ⁢           ⁢     (       0.100   ⁢           ⁢   in     +     .333   ×   .026   ⁢           ⁢   in       )       2                 C   =       0.017453   ⁢           ⁢   radians   ⁢     /     ⁢   degree   ×   90   ⁢   °   ⁢           ⁢     (       0.100   ⁢           ⁢   in     +     .00866   ⁢           ⁢   in       )       2                 C   =     0.08534   ⁢           ⁢   in               
 
      In order to bend the metal sheet  105  to a desired angle it is necessary to overbend by about 2.5°, thereby overcoming internal metal stresses. An angular relief  140  of about 2.5° is shown in  FIG. 6 . As shown in  FIG. 7 , after the hinged platen  122  is rotated clockwise by about 92.5° so that its slanted surface portion  122   c  approaches the slanted surface portion  120   c  of the stationary platen  120 , the sheet  105  attains a permanent bend angle of about 90°.  
      The foregoing detailed description of my invention has been provided with reference to a particularly preferred embodiment. Persons skilled in the art understand that numerous modifications can be made in my invention without significantly departing from the spirit and scope of the following claims.