Abstract:
Adaptable spring force clamping apparatus and methods are disclosed. In one embodiment, an apparatus includes at least one elongated member adapted to be positioned proximate to a surface of a work piece. The elongated member includes first and second end portions adapted to be secured to substantially prevent movement of the elongated member relative to the surface. A plurality of pressure applying devices are operatively attached to the elongated member, each pressure applying device including a contact member moveable relative to the elongated member and adapted to engage the surface of the work piece, and a resilient member operatively coupled to the contact member and to the elongated member.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This patent application is a divisional application of co-pending, commonly-owned U.S. patent application Ser. No. 10/648,535 entitled “Adaptable Spring Force Clamping Apparatus and Methods” filed on Aug. 25, 2003, which application is incorporated herein by reference. 
     
    
     GOVERNMENT LICENSE RIGHTS  
       [0002]     This invention was made with Government support. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     This invention relates generally to assembly tools, and, more specifically, to clamping mechanisms.  
       BACKGROUND OF THE INVENTION  
       [0004]     Equipment and vehicles, including aircraft, often utilize sheets or long sections of metal that need to be clamped together or to a frame during assembly of the equipment. Assembly may include welding or other bonding processes. Typically clamps of various configurations are utilized to hold materials in place during welding, bonding or other assembly. For example, if welding is being utilized to bond segments of skin to each other or the underlying framing, the skin segments typically are held in place during that process.  
         [0005]     Existing clamps often span the surface being clamped, being anchored at their ends. For example, metal bars, shaped to conform to the section or surface being worked on have been utilized as clamps, by being anchored at their respective ends and held tightly against the surface being worked on. It will be appreciated that clamping a bar at its ends across a work surface results in a differential amount of pressure being applied to the work surface across the span of the bar. This occurs because the bar deflects somewhat with the application of clamping force from the ends. Typically the clamped work piece is not held as securely in the center of the span of the clamp bar as it is near the ends where the ends are anchored. In some circumstances, this differential clamping force can result in insufficient clamping force being applied to the work piece, or differential stresses in the work piece after assembly. While larger clamping bars may be utilized to reduce deflections across their span when clamped against a work piece, increased size requires increased effort to manipulate and to use in jigs and assembly modules.  
         [0006]     Therefore, an unmet need exists for adaptable clamps that provide improved clamping of surfaces during manufacturing operations.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed to adaptable spring force clamping apparatus and methods. In one embodiment, an apparatus includes at least one elongated member adapted to be positioned proximate to a surface of a work piece. The elongated member includes at least one portion adapted to be secured to substantially prevent movement of the elongated member relative to the surface. A plurality of pressure applying devices are operatively attached to the elongated member and disposed therealong, each pressure applying device including a contact member moveable relative to the elongated member and adapted to engage the surface of the work piece, and a resilient member operatively coupled to the contact member and to the elongated member. The resilient member biases the contact member away from the elongated member to apply a clamping force to the surface of the work piece. In alternate embodiments, two elongated or spanning members with pressure applying devices may be mounted parallel to each other, or alternately, a single elongated or spanning member with a slot formed therein may provide access for welding or other manufacturing work in the clamped area.  
         [0008]     In accordance with still further aspects of the invention, the apparatus of the present invention may be curved or angled. The apparatus may also be two-dimensional, such as cruciform shaped. In accordance with another aspect of the invention a clamping apparatus in accordance with the present invention is included with an assembly manufacturing jig holding the work piece. In further aspects of the invention, a method is provided for welding skin segments of an aircraft to adjoining sections or to underlying framing utilizing the clamping method of the present invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0010]      FIG. 1A  is an isometric view of an angled clamp bar assembly in accordance with an embodiment of the present invention;  
         [0011]      FIG. 1B  is an end elevational view of a single clamp bar of the angled clamp bar assembly of  FIG. 1A ;  
         [0012]      FIG. 1C  is a side elevational view of the angled clamp bar assembly of  FIG. 1A ;  
         [0013]      FIG. 1D  is a partial top elevational view of the angled clamp bar assembly of  FIG. 1A ;  
         [0014]      FIG. 2  is an end cross-sectional view of the clamp bar assembly of  FIG. 1A  in place on a work piece;  
         [0015]      FIG. 3  is an isometric view of a straight clamp bar assembly in accordance with an alternate embodiment of the present invention;  
         [0016]      FIG. 4  is an isometric view of an assembly jig incorporating a cruciform clamp bar assembly in accordance with yet another embodiment of the present invention;  
         [0017]      FIG. 5  is a flow chart of an exemplary assembly method in accordance with yet another embodiment of the present invention;  
         [0018]      FIG. 6  is an end elevational view of an offset clamp bar assembly in place on a work piece;  
         [0019]      FIG. 7  is an isometric view of the offset clamp bar assembly of  FIG. 6  in place on a work piece.  
         [0020]      FIG. 8  is an isometric view of a cantilevered clamp bar assembly in place on a work piece.  
     
    
     DETAILED DESCRIPTION  
       [0021]     The present invention generally relates to adaptable spring force clamping apparatus and methods. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 1-8  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description.  
         [0022]     By way of background, in one embodiment, an apparatus includes a member arranged to at least partially span a work piece with at least one pressure applying device attached to the member. The at least one pressure applying device may include a plurality of spring-loaded pads arranged to apply pressure to a work piece. Two spanning members with pressure applying devices may be mounted parallel to each other, or a single spanning member with a slot suitably may provide access for welding or other work in the clamped area.  
         [0023]     Turning to  FIG. 1A , an exemplary angled clamp bar assembly  20  in accordance with an embodiment of the present invention is shown. The assembly  20  includes two clamp bars  22  arranged in parallel with a slot  24  between them. The clamp bars  22  in this exemplary embodiment are metal bars with a single obtuse angle bend in the middle. The clamp bars  22  may be any suitable material and cross-section that carries sufficient stress to apply a desired clamping force to a work piece. The work piece (not shown) is spanned by the clamp bars  22  with the work piece underneath or inside the angle of the clamp bars  22 . In a presently preferred embodiment of the present invention, 1018 steel alloy bars 1¾″×1⅛″ are utilized for the clamp bars  22 .  
         [0024]     The clamp bars  22  may be parallel and may be coupled together by two steel angle stock hold-downs  38  fastened to the ends of the clamp bars  22  by fasteners  39 . In this embodiment, the hold-downs  38  are suitably ⅜″ steel 900 angle stock with one limb of the angle stock fastened to the ends of the clamp bars  22 . The hold-downs  38  thus hold the clamp bars  22  in position parallel to each other but separated by the width of the slot  24 . The other limb of the hold-down  38  has an opening  40  through which a bolt or other fastener may be inserted to attach the assembly  20  over a work piece, as described more fully below. In this embodiment, there are two hold-downs  38 , one at each end of the clamp bars  22 . The hold-downs  38  thus hold the clamp bars  22  in position with respect to each other, and also suitably attach the entire assembly  20  in position over the work piece. It will be appreciated that the clamp bars  22  may be curved, straight, segmented, or any other desired shape. It will also be appreciated that the two clamp bars  22  and the hold-downs  38  may suitably be manufactured from one piece, or a different combination of pieces forming the assembly  20 .  
         [0025]     In this example embodiment, the clamp bars  22  have a plurality of spring-loaded pads  32  along their lengths that may apply force against the work piece. The pads  32  extend outwardly from a bottom side of the assembly  20 . Both clamp bars  22  have a plurality of spring-loaded pads  32  along their lengths. The spring-loaded pads  32  are mounted on keepers  30  which project through the clamp bars  22 . The keepers  30  have springs  36  or other pressure-applying devices (see  FIG. 1B ) mounted in counter-bored holes  27  on the bottom of the clamp bars  22 , as described in more detail with reference to  FIG. 1B  and  FIG. 2  below. The slot  24  between the clamp bars  22  allows access to the work piece between the clamp bars  22 . Thus the assembly  20  allows clamping pressure to be applied by the pads  32  to the work piece while allowing access to the work piece between the clamp bars  22  through the slot  24 .  
         [0026]     While clamping pressure is applied on both sides of the slot  24  using the clamp bar assembly  20 , a variety of manufacturing operations may be performed on the work piece, including, for example, welding, heat bonding, adhesive application or other bonding methods, milling, sanding, routing, riveting, drilling, or virtually any other desired manufacting operation. Welding processes suitably may include traditional welding or friction stir welding. A tool (such as one used for the bonding process) may be inserted through the slot  24  to access the work piece while the work piece is clamped on both sides of the slot  24 . In this embodiment, clamping force is applied in a parallel with the slot  24  between the clamp bars  22 .  
         [0027]      FIG. 1B  is an end elevational view of an single exemplary clamp bar  22  such as those used in the angled clamp bar assembly  20  of  FIG. 1A . The spring-loaded pad  32  is attached to a keeper  30  which projects through a hole  26  through the clamp bar  22 . The pad  32  is on the bottom of the clamp bar  22  (i.e. on the side towards the work piece when the assembly  20  is attached across the work piece). The pad  32  is spring-loaded against the work piece by the spring  36  installed in the counter-bored hole  27  in the bottom of the clamp bar  22 . The keeper  30  retains the pad  32  and the spring  36  in place on the clamp bar  22 , as described in greater detail in connection with  FIG. 2  below. The spring  36  suitably applies a pre-determined clamping pressure against the work piece.  
         [0028]     It will be appreciated that various embodiments of the instant invention may include one or more clamp bars  22 . A clamp bar assembly having a single clamp bar may suitably be used where, for example, clamping is only desired on one side of a joint or connection. Similarly, multiple clamp bars  22  in different configurations suitably provide clamping pressure over a wide range of configurations and surfaces. A pair of clamp bars  22  arranged close to each other with a slot  24  in between advantageously allow clamping pressure to be simultaneously applied to both sides of a joint line positioned under the slot  24 .  
         [0029]      FIG. 1C  is a side elevational view of the exemplary angled clamp-bar assembly  20  shown in  FIG. 1A . As is shown in this side view, each clamp bar  22  has two straight limbs  28  and a single central angle bend  29 . At both ends  25  of the clamp bar  22  are the 900 angle stock forming the hold-downs  38 . The hold-downs  38 , at each end  25  of the clamp bars  22 , are fastened to the clamp bars  22  by fasteners  39 .  
         [0030]     In this example configuration the clamp bar assembly  20  has eighteen spring-loaded pads  32  with eighteen keepers  30 , nine of each on each clamp bar  22 . Furthermore, in this embodiment, four pads  32  with four keepers  30  are on each limb  28  of the clamp bars  22  and one pad  32  and one keeper  30  is located at the bend  29  of the angled clamp bar  22 . The pads  32  apply pressure to the work piece approximately perpendicular to the clamp bar  22  when the clamp bar  22  is held in place by the hold-downs  38  at each end. The pad  32  and keeper  30  at the bend  29  apply pressure against the work piece approximately parallel to a line that bisects the angle of the bend  29  of the clamp bar  22 .  
         [0031]      FIG. 1D  is a partial top elevational view of an end of the angled clamp bar assembly  20  of  FIG. 1A . Again, as viewed from the top, the two clamp bars  22  are separated by the slot  24 . In this view of one end of the clamp bar assembly  20 , two spring loaded pads  32  and their respective keepers  30  are on portions of the limb shown of the clamp bars  22  (the balance is cut off in this abbreviated top view of the end of the assembly  20 ). Attached to the ends  25  of the clamp bars  22  is the hold-down  38 . The hold-down  38  is attached by fasteners  39  to the clamp bars  22 . The two clamp bars  22  are held in position parallel to and separated from each other by the hold-down  38 . The hold-down  38  has an opening  40 , in this example in the form of a notch, that allows the assembly  20  to be bolted or attached to a frame or support (not shown) with the clamp bars  22  spanning a work piece.  
         [0032]      FIG. 2  is a cross sectional view of the clamp bar assembly  20  of  FIGS. 1A through 1D  in place on a work piece  4 . The cross section in  FIG. 2  is through the two clamp bars  22  at a location in each clamp bar  22  having a pad  32  and a keeper  30 . Thus, the pad  32 , keeper  30 , and the spring  36  spring-loading the pad  32  are also shown in cross section.  
         [0033]     As further shown in  FIG. 2 , the work piece  4  in this example includes the top of an I-beam frame  6  and two segments of skin (or panels)  8  to be installed in place and joined on the frame  6 . The skin segments  8  preferably come together on top of the frame  6  at a joint  7  centered below the slot  24  of the clamp bar assembly  20 . As described above, the slot  24  in the clamp bar assembly  20  allows a bonding tool  10  to be utilized to form a bond of the skin segments  8  to each other and, depending upon the application, to the underlying frame  6  at the joint  7 .  
         [0034]     In  FIG. 2 , the tool  10  may be a shouldered friction stir welding tool. Friction stir welding is a solid state joining process that is a combination of extruding and forging. A cylindrical shouldered tool with a profiled pin is rotated and slowly plunged into a joint line between pieces of material which are clamped together. Frictional heat is generated between the welding tool  10  and the work piece  4 . The heat causes the work piece  4  to soften without reaching the melting point and allows the tool to traverse the joint  7  line. As it does, plasticized material is transferred from the leading edge of the tool  10  to the trailing edge of the tool  10  shoulder and pin. This leaves a solid phase bond along the work piece. While the clamp bar assembly  20  may be utilized to provide access to the joint  7  through the slot  24  for friction stir welding, it suitably may be used for any other welding process, bonding process, or manufacturing process. In operation, with the clamp bar assembly  20  centered with the slot  24  over the joint  7 , the spring-loaded pads  32  hold the skin segments  8  hard against the underlying frame  6  on either side of the joint  7 . The joint  7  can then be worked, welded, or bonded with the tool  10 , using access through the slot  24  between the clamp bars  22 . Access through the slot  24  spans the work piece, with parallel rows of clamping pads  32  on either side of the joint  7 .  
         [0035]     It will be appreciated that the pads  32  used to clamp the skin segments  8  may be attached with any suitable pressure applying device to the clamp bars  22  to provide a desired clamping pressure. In the representative clamp bar assembly  20  shown in  FIG. 2 , the pads  32  suitably include a pad  33  backed by a backing disc  34 . The backing disc  34  is attached to a machine screw  35  which forms the keeper  30 . The machine screw  35  projects from the bottom surface  18  of the clamp bar  22  to above the top surface  16  of the clamp bar  22  through a hole  26 . The hole  26  is counter-bored from the bottom  18  with a counter-bore  27  providing space for a spring  36 . The spring  36  applies clamping force via the pad  32  to the work piece  4 . The counter-bore  27  may be counter-bored into the bottom  18  of each clamp bar  22  a suitable distance so that when the clamp bars  22  are held in place near the work piece  4 , the springs  36  apply the desired amount of pressure to the work piece  4  via the pad  32 .  
         [0036]     A slippery or cushioning pad  33  may be used to cushion or prevent the scratching of the work piece  4  by the spring-loaded pad  32 . In a preferred embodiment, the pad  33  may be TEFLON®. The pad  33  may be mounted to its backing disc  34  utilizing flush-mounted fasteners (not shown), including, for example, counter-sunk machine screws threaded into the metal backing disc  34 . The pads  32  are suitably round, but may be any desired shape. The backing disc  34  for the pad  33  may have a hole at its center tapered to accept the head of the flathead machine screw  35 . The machine screw  35  fits through the backing disc  34  forming a surface that is flush or flat, permitting the pad  33  to be installed over the backing disc  34  and the machine screw  35  forming a flat clamping surface for the spring-loaded pad  32 . In a presently preferred embodiment, the machine screw  35  suitably may include a ¼-20×2½″ flathead machine screw. The backing disc  34  suitably includes a 1/16″ steel disc, and the pad  33  suitably includes a 1/16″ thick TEFLON®disc. The machine screw  35  forming the keeper  30  holding the pad  32  and spring  36  in place projects through the hole  26  in the clamp bar  22 . In this exemplary embodiment, the hole  26  is suitably a ¼″ hole.  
         [0037]     By way of example but not limitation, in one representative embodiment, the two clamp bars  22  suitably are separated from each other by the slot  24  with a width of ¾″, and the counter-bore  27  providing space for the springs may be 0.875″ in diameter and 0.75″ deep. Furthermore, in an exemplary embodiment, the clamp bars  22  themselves are suitably 1.75″ tall and 1.125″ wide. It will be appreciated that any suitable dimensions for the slot  24 , pads  32 , keeper  30 , and the clamp bars  22  that provide access to the work piece and make provision for the keepers  30  to be moveably linked to the clamp bars  32  and apply clamping pressure to the work piece  4  may be utilized.  
         [0038]     In yet another aspect of the embodiment shown in  FIGS. 1A through 2 , the machine screw  35  forming the keeper  30  is held from falling out of the clamp bars  32  with two jammed nuts  31 , threaded on the machine screw  35  where the screw  35  projects through the top surface  16  of the clamp bar  22 . The jam nuts  31  are suitably installed at the very top of the machine screw  35 , above the top surface  16  of the clamp bar  22 . The jam nuts  31  have a larger outside diameter than the hole  26  through the clamp bar  22  and thus hold the machine screw  35  in place. This retains the spring  36  and the pad  32  in place on the clamp bar  22  when the clamp bar is removed from the work piece  4 . The spring  36  within the counter-bore  27  applys force from the clamp bar  22  to the pad  32  against the work piece  4 . The machine screw  35  included in the keeper  30  is suitably long enough to project through the clamp bar  22  and be held by the jam nuts  31  while giving sufficient clearance for the pad  32  to be spring-loaded by the spring  36  away from bottom  18  of the clamp bar  22 .  
         [0039]     The spring (or pressure applying device)  36  suitably provides clamping pressure to the work piece  4 . In a preferred embodiment, the springs  36  may be Danley Die Springs # 9-1204-36. With the clamp bar assembly  20  installed in place spanning the work piece  4 , the die springs  36  can suitably apply a force of approximately 228 lbs each against the work piece  4 . It will be appreciated that the springs  36  may be shimmed, or the counter-bore  27  depths varied, to provide varying degrees of clamping force where, for example, additional or varying clamping force is desired, or to provide for equal clamping pressure across the entire span of the clamp bars  22 . This suitably may compensate for deflection when the clamp bar assembly  20  is held against a work piece  4  with the spring-loaded pads  32  engaging and clamping the work piece  4 .  
         [0040]     The pressure applying devices  36  applying clamping pressure to the work piece  4  via the pads  32  suitably may include any type of pressure applying device other than springs  36 , such as resilient materials, hydraulic pistons, pneumatic bladders, or any other suitable pressure applying device. A resilient material may include, for example, a soft plastic or rubber material. It will be appreciated that springs  36 , even without further shimming, provide relatively even pressure when several spring-loaded pads  32  project from the clamp bars  22  spanning a work piece  4 . As the clamp bar assembly  20  is held against the work piece (or vice versa)  4  with the bars  22  spanning the work piece  4 , the springs  36  are placed in compression applying clamping force against the work piece  4  on both sides of the joint  7  being worked on. It will be appreciated the jam nuts  31  keep the keeper  30  in place when the assembly  20  is removed from the work piece  4 .  
         [0041]     Turning to  FIG. 3 , it will be appreciated that a clamp bar assembly  120  in accordance with an embodiment of the present invention may be fabricated using a single spanning or elongated member  122 . In the example embodiment shown in  FIG. 3 , the spanning member  122  has a slot  124  formed through it lengthwise, for access to a work piece (not shown). The spanning member  122  suitably may be manufactured from a single bar of steel with the slot  124  cut through the bar. The slot  124  is cut through the bar along its length, but not all the way through to the ends, leaving it as one piece. At the ends (or at one or more locations, see  FIG. 8  below), the spanning member  122  may be suitably bolted to a tower or attachment over a work piece (not shown) with the use of fastening bolts  138 . In this embodiment, the two clamp bars  22 , and the two hold-downs  38  of  FIG. 1A  are combined into a single integral component spanning member  122  of  FIG. 3 , with the cut slot  124  longitudinally cut through the spanning member  122 .  
         [0042]     In the embodiment shown in  FIG. 3 , a straight clamp bar assembly  120  is suitably formed by the straight spanning member  122 . As with other configurations of the present invention, the spanning member  122  suitably may be angled, segmented, or curved to conform to the surface of the work piece (not shown) being clamped. The embodiment shown in  FIG. 3  is straight for use in clamping a flat work piece (not shown).  
         [0043]     As in the angled clamp bar assembly  20  of  FIGS. 1A-1D , the straight clamp bar assembly  120  of  FIG. 3  includes two parallel rows of spring-loaded pads  32  disposed on opposite sides of the slot  124 . The slot  124  suitably provides access to a joint or elongated work area underneath the assembly  120 , when the assembly  120  is in place over a work piece. The assembly  120 , through its pads  32 , may suitably apply pressure on two sides of a joint centered underneath the slot  124  in the spanning member  122  when the spanning member  122  is attached in place or held against a work piece. The pads  32  may be held in place by keepers  30  projecting through the spanning member  22  in the manner described above with reference to  FIG. 2 .  
         [0044]     It will be appreciated that the clamp bar assembly  120  of the present invention may be held in place against a work piece, or the work piece may be pushed against the assembly  120  by a jig, lift, or other assembly. Put differently, clamping force can be applied by holding the assembly  120  against the work piece, or by holding the work piece against the assembly  120 .  
         [0045]      FIG. 4  shows another exemplary embodiment of the present invention including an assembly jig  210  for an aircraft segment. The assembly jig  210  suitably includes a base  214  with a plurality of supports or cradles  12  for holding the frame for a fuselage segment (not shown). Attached to the base  214  are a plurality of towers  250  to which are attached a clamp bar assembly  220  of the present invention. In this example embodiment, the clamp bar assembly  220  is approximately cruciform shaped. The clamp bar assembly  220  includes four limbs  228  intersecting at 900 in a cross shape bridging over a work piece (not shown) when the work piece is held in place on the cradles  12  of the jig  210 . The clamp bar assembly  220  thus has four pairs of rows of spring-loaded pads  232  held in place by keepers  230 . Each limb  228  of the cruciform shaped clamp bar assembly  220  has a slot  224  centered in the limb  228  and extending longitudinally along the limb  228 . Each slot  224  allows access to the work piece when the assembly  220  is in place on the jig  210  for welding, bonding, or other manufacturing work.  
         [0046]     As further shown in  FIG. 4 , in this embodiment, three of the limbs of the cruciform shaped clamp bar assembly  220  are co-planar while a fourth limb is bent at an angle away from the plane of the other limbs. The clamp bar assembly  220  thus may apply clamping force along an intersecting pair of joints in the work piece, including across an angled three-dimensional surface of the work piece.  
         [0047]     The clamp bar assembly  220  is held over the top of the cradles  12  by fasteners  248  to towers  250  attached to the base  214 . Two of the limbs of the cruciform shaped clamp bar assembly  220  are held to the towers  250  by angle bar hold-downs  238 , while two other limbs of the assembly  220  are held in place with fasteners  248  through the clamp bars  222 . It will be appreciated that any suitable combination of attachments between the clamp bar assembly  220  and the towers  250  holding the clamp bar assembly  220  to the jig  210  may be utilized. It will also be appreciated that the clamp bar assembly  220  may be fixedly mounted (e.g. to a floor or other suitable support structure), and a work piece on moveable supports or cradles  12  may be moved underneath or alongside the clamp bar assembly  220  and pushed against the clamp bar assembly  220  with a suitable mechanism resulting in a clamping force being applied to the work piece. The assembly jig  210  of  FIG. 4  may be configured with a variety of clamp bar assemblies  220  to quickly and efficiently allow manufacturing operations, including bonding work such as welding or friction stir welding, to be completed on complex shaped equipment and equipment sections. This may include bonding work traversing a variety of contours, as well as multiple segments intersecting at different angles as required for the assembly of the work piece.  
         [0048]     Embodiments of the present invention may also suitably includes the methods of clamping work pieces, and working work pieces, in the manner described in connection with  FIGS. 1-4  above. Embodiments of methods in accordance with the present invention may permit uniform application of clamping force across a work piece, and working the work piece along side or between the clamping mechanism(s) of the present invention.  
         [0049]      FIG. 5  illustrates an exemplary process  300  of performing a manufacturing process (e.g. friction stir welding) on a pair of skin segments of a portion of an aircraft frame in accordance with yet another embodiment of the present invention. The method  300  may begin at a block  310 , in which a portion of a support frame is cradled. Cradling the support frame may include placing the frame within an assembly jig, such as the assembly jig described above in connection with  FIG. 4 . At a block  320 , at least one skin segment is positioned over the frame. It will be appreciated that, as in  FIG. 2 , at the block  320 , two adjoining skin segments may be butted together along the top of the support frame.  
         [0050]     As further shown in  FIG. 5 , at a block  330 , at least part of the portion of the frame and the at least one skin segment are bridged forming a span from which clamping pressure can be applied. The span can be a single span, permitting work to be done on the work piece on either side of that span, or the span can include two parallel spans, permitting working of the work piece in a slot between the spans such as shown and described above in connection with  FIG. 2 . The span may completely bridge the work piece, may be along side the work piece as described in connection with  FIGS. 6 and 7  below, and may be cantilevered over the workpiece as described in connection with  FIG. 8  below. In embodiments where two parallel spans are utilized, at a block  340 , the method  300  may include defining a slot over a joint on the work piece to be worked. In this exemplary embodiment, the joint suitably includes two butted skin segments over a frame member of an aircraft, positioned in the manner as described above in connection with  FIG. 2 .  
         [0051]     With continued reference to  FIG. 5 , in this embodiment, at a block  350  both sides of the joint are clamped. The slot or opening defined by the spans permits the work piece to be worked on between the clamped areas. At a block  360 , a manufacturing operation may be performed on the work piece (e.g. welding). Suitably, in one embodiment of the method  300 , the two abutting skin segments positioned over the frame member of a portion of the frame of the aircraft may be friction stir welded by inserting a tool through the slot in the span, while the skin segments on both sides of the joint may be securely clamped in the manner described above using apparatus in accordance with embodiments of the present invention. Thus, it will be appreciated that the method  300  may advantageously facilitate the secure and rapid clamping and working of a work piece, even when performing a manufacturing operation on the work piece involves the application of substantial force, such as in friction stir welding.  
         [0052]     Turning to  FIG. 6 , it will be appreciated that clamping force may be resiliently applied from along side a work piece  4 .  FIG. 6  is an end elevational view of an exemplary clamp assembly  400  with keepers  405  that extend laterally over a work piece, transmitting clamping force from clamp bars  422  along side the work piece  4 . The work piece  4  includes a frame member  6 , and two covering skin sections  8 , to be joined together at a joint  7  over the frame member  6 . A clamp bar  422  is positioned on each side of the work piece  4 . Attached to the clamp bars  422  are keepers  430 , mounted on supports  431  with fulcrums  433  attached to the clamp bars  422 . The keepers  430  form a lever arm projecting across the fulcrums  433  out over the the work piece  4 , in this embodiment from two clamp bars, one on each side of the work piece  4 . The keepers  430 , in this exemplary embodiment, are resiliently biased by springs  436  between their first ends  437  situated over the clamp bars  422 , and the clamp bars  422 . The springs  436  thus bias the keepers&#39; 430 second ends  439 , which project over the work piece  4 , hard against the work piece  4 , applying clamping force to the work piece  4 . In this embodiment, the fulcrums  433  are pivots linking the keepers  430  to the supports  431 , permitting force from the springs  436  applied to the first ends  437  of the keepers  430  to be transmitted to the work piece  4  by the opposite second ends  439  of the keepers  430 .  
         [0053]     The springs  436  are suitably held in place in position between the clamp bars  422  and the first ends  437  of the keepers within counterbores  427  and  428  in the clamp bars  422  and keepers  430 , respectively. The ends of the springs  437  nest within the counterbores  427  and  428 . In this embodiment, the second ends  439  of the keepers  430 , over the work piece  4 , have pads  432 , that apply the clamping force to the work piece  4 .  
         [0054]     The clamp assembly  400  applies clamping force from both sides of the work piece  4 . A distance or slot  424  between opposing second ends  439  of the keepers  430  projecting over the work piece  4  allows the work piece  4  to be worked along the joint  7  without interference from the assembly  400 . The distance or slot  424  between the keepers  430  allows a working tool  10  to be moved along the joint  7 , between the pads  432  applying clamping force.  
         [0055]     As shown in  FIG. 7 , the clamp bars  422  of  FIG. 6  may be of any suitable configuration positionable along side the work piece  4 .  FIG. 7  is an isometric view of the clamp bar assembly  400  of  FIG. 6  in position over a work piece. The assembly  400  iincludes a plurality of keepers  430  that project over the work  4  from both sides. The keepers  430  apply resilient clamping force to the work piece  4  in the manner described with respect to  FIG. 6 . The clamp bars  422  are suitably elongated triangular trusses that form a support for the keepers  430 . Two clamp bars  422  are positioned one on each side of the work piece  4 , each with a plurality of keepers  430  projecting over the work piece  4 . The clamp bars  422  suitably provide sufficient moment resistance when positioned along side the work piece  4  to compress the springs  436  resulting in clamping force being applied to the work piece by the keepers  430 , and their attached pads  432 .  
         [0056]     It will be appreciated that a clamp assembly of the present invention may be positioned with respect to a work piece in a range of configurations. In  FIG. 8 , an exemplary clamp assembly  500  is cantilevered over a work piece  4 . As in  FIG. 3 , an elongated clamp bar  522  has a longitudinal slot  524  cut through the bar  522 . On each side of the slot  524  are a plurality of spring-loaded keepers  530  projecting though the bar  522 . The spring-loaded keepers retain springs (not shown) that apply clamping force biasing pads  532  against the work piece  4 . In this embodiment the clamp bar  522  is attached to a base  502  at one end  523 , while the opposite end  525 , with the slot  524  and the plurality of spring loaded keepers  530  projects over the work piece. The base  502  is attached to a foundation surface  501  that also supports the work piece  4 . It will be appreciated that cantilevering the clamp bar  522  from a base  502  may allow the work piece  4  to be slid or moved in underneath the clamp bar  522 , without detaching the clamp bar  522  from the base  502 .  
         [0057]     While preferred and alternate embodiments of the invention have been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of these preferred and alternate embodiments. Instead, the invention should be determined entirely by reference to the claims that follow.