Abstract:
A method and apparatus for holding a part on a fixture in a precisely known predetermined position includes pressing a surface of the part against a vacuum head and sealing an annular elastomeric sealing ring against the part surface. A valve button of a valve is engaged by the part surface to open the valve and establish communication between a vacuum source. The part is pulled by the vacuum against a hard stop in the vacuum head having a fixed precisely known relationship to said vacuum head reference surface. The hard-stop can be mounted in the base of the vacuum head for pivoting about at least one axis to self-align the hard-stop face with the part face.

Description:
This application claims the benefit of Provisional Application No. 60/157,141, filed Sep. 30, 1999. 
    
    
     TECHNICAL FIELD 
     This invention relates to a method and apparatus for securing parts, such as webs for wing spars and ribs, to a holding apparatus for machining and/or assembly into a larger part, such as wing spars and ribs, and more particularly to a vacuum head and method for holding a part in a fixed and precisely known position for machining and/or drilling to close tolerances. 
     BACKGROUND OF THE INVENTION 
     Conventional manufacturing techniques for machining and assembling large mechanical assemblies such as airplane wing spars and ribs to a specified contour have, in the past, relied on fixtured “hardpoint” tooling techniques utilizing floor assembly jigs and templates to locate and temporarily fasten detailed structural parts together to position the parts correctly relative to one another. This traditional tooling concept usually requires primary assembly tools for each subassembly produced, and large assembly tools in which the subassemblies are assembled into the assembled structure. 
     Assembly tooling is intended to accurately reflect the original engineering design of the product, but using the conventional tooling concept in which the tooling sets the configuration of the final assembly, there are many steps between the original design of the product and the final manufacture of the tool. It is not unusual that the tool as finally manufactured produces missized spars or wing components that would be outside of the dimensional tolerances of the original spar or spar component design without extensive, time consuming and costly hand work to correct the tooling-induced errors. More seriously, a tool that was originally built within tolerance can become out of tolerance from the hard use it typically receives in the factory. Moreover, dimensional variations caused by temperature changes in the factory can produce a variation in the final part dimensions as produced on the tool, particularly when a large difference in the coefficient of thermal expansion exists between the tooling and the spar, as in the usual case where the tooling is made of steel and the spar components are made of aluminum. Since dimensions in airplane construction are often controlled to within 0.005″, temperature induced dimensional variations can be significant. 
     Hand drilling of the part on the tool can produce holes that are not perfectly round or normal to the part surface when the drill is presented to the part at an angle that is slightly nonperpendicular to the part, and also when the drill is plunged into the part with a motion that is not perfectly linear. Parts can shift out of their intended position when they are fastened in non-round holes, and the nonuniform hole-to-fastener interference in a non-round hole lacks the strength and fatigue durability of round holes. The tolerance buildup on the spar subassemblies can result in significant growth from the original design dimensions, particularly when the part is located on the tool at one end of the part, forcing all of the part variation in one direction instead of centering it over the true intended position. 
     One effective solution to the hard tooling problem is shown in U.S. patent application Ser. No. 09/155,236 entitled “Determinant Spar Assembly” by Clayton Munk, Paul Nelson and David Strand. The process and apparatus of that invention eliminates hard tool and uses instead a fixture that holds the part in a position that can be probed to determine its actual location in space without reference to the fixture itself. However, it is still desirable to mount the part on the fixture in a position and orientation that is as close to a known and flat shape as possible, and to do so quickly and inexpensively. The use of a vacuum head on the settable pogos of the holding apparatus solves the requirement. 
     SUMMARY OF THE INVENTION 
     Accordingly, this invention makes it possible to hold parts in precisely known positions for machining and assembly operations. It provides vacuum to a vacuum head only if a part or other tool is presented to the vacuum head to avoid compromising the vacuum source by open vacuum heads, and eliminates the need to manually turn off or seal vacuum heads that are not used to hold a particular part on the holding apparatus. 
     The benefits of the invention are attained in a method and apparatus for holding a part on a fixture in a precisely known predetermined position. It includes pressing a surface of the part against a vacuum head and sealing an annular elastomeric sealing lip of a vacuum cup against the part surface. A valve button of a valve is engaged by the part surface to open the valve and establish communication between a vacuum source. The part is pulled by the vacuum against a hard stop in the vacuum head having a fixed precisely known relationship to said vacuum head reference surface. The hard-stop can be mounted in the base of the vacuum head for pivoting about at least one axis to self-align the hard-stop face with the part face. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention and its many attendant objects and advantages will become better understood upon reading the following detailed description of the preferred embodiment in conjunction with the following drawings, wherein: 
     FIG. 1 is a perspective view of a vacuum head in accordance with this invention having a universal swiveling hard-stop; 
     FIG. 2 is a sectional elevation of the vacuum head shown in FIG. 1 taken parallel to the central axis the view of a spar built in accordance with the process and on the apparatus of this invention; 
     FIG. 3 is an exploded sectional elevation of the vacuum head shown in FIGS. 1 and 2; 
     FIG. 4 is a side elevation of the vacuum head shown in FIG. 1 showing the base clamp for securing the vacuum head base to a support arm; 
     FIG. 5 is a sectional plan view of the vacuum head base along lines  5 — 5  in FIG. 4; 
     FIG. 6 is a sectional elevation of the valve body shown in FIGS. 2 and 3; 
     FIG. 7 is a top plan view of the hard-stop shown in FIGS. 1-3; 
     FIG. 8 is a sectional elevation of the hard-stop along lines  8 — 8  in FIG. 7; 
     FIG. 9 is a perspective view of an exemplary holding apparatus on which the vacuum head of this invention could be used; 
     FIG. 10 is a sectional elevation of a second embodiment of a vacuum head in accordance with this invention having a fixed hard-stop; 
     FIG. 11 is a sectional elevation of the valve body shown in FIG. 10 along lines  11 — 11  in FIG. 12; 
     FIG. 12 is an elevation of the valve body shown in FIG. 10; 
     FIG. 13 is a sectional elevation of a third embodiment of the inventive vacuum head having a hard-stop that swivels about a lateral axis; 
     FIG. 14 is an exploded elevation of the swiveling hard-stop shown in FIG.  13  and the cylindrical bearing on which it is mounted; and 
     FIG. 15 is a top plan view of the hard-stop shown in FIG.  14 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, wherein like reference characters designate identical or corresponding parts, and more particularly to FIGS. 1 and 2 thereof, a vacuum head  25  is shown for holding a part on a fixture for machining operations and assembly. Such a fixture and process, for example is disclosed in U.S. patent application Ser. No. 09/155,236 now U.S. patent application Ser. No. 6,170,157 entitled “Determinant Spar Assembly” by Clayton Munk, Paul Nelson and David Strand, the disclosure of which is incorporated herein by reference. 
     The vacuum head  25  includes a cylindrical base  30  having a central axis  35  oriented vertically in FIGS. 2 and 3. The base  30  has an upper recess  38  and a lower recess  40  each having a flat floor  39  and  41 , respectively, which define opposite sides of an intermediate web  45 . The web  45  serves as a partition to isolate vacuum conveyed to the recess  40  from the recess  38  until a valve in the web  45  is opened, as explained below. 
     The terms “upper”, “lower” and other such orientation-dependent terms are used herein to assist the reader in relating FIGS. 2 and 3 to the description of this embodiment. Naturally, the invention is not limited to any particular orientation and can be used in any desired orientation. 
     The recess  40  of the base  30  receives the end of a support arm of a holding apparatus and is sealed by an O-ring  43  in a groove  42  in the cylindrical wall of the recess  40 . The vacuum head is secured on the support arm by a clamp structure  46 , best shown in FIGS. 4 and 5, and described in more detail below. Vacuum is typically conveyed to the vacuum head through the support arm. 
     As best shown in FIGS. 2 and 3, a stepped axial hole  47  through the web  45  has a lower internally threaded portion  48  to receive an externally threaded stub  49  of a valve body  50 , shown in section in FIG.  6 . An O-ring  52  (not shown in FIGS. 3 and 6) in an annular groove  54  above the threaded portion  49  in the stub  49  engages a smooth bore portion of the stepped axial hole  47  to seal the valve body  50  in the hole  47 . An annular ridge or bourlet  56  on the valve body  50  above the groove  54  limits the depth to which the valve body  50  can be screwed into the axial hole  47 . The upper portion  60  of the valve body is in the form of a cylindrical tube, integral with the lower portion of the valve body  50  from the bourlet  56  on down. The upper portion  60  has a central bore  63  extending down to a step  65  at the level of the bourlet  56 . A spring  70 , shown schematically in FIG.  2  and more representationally in FIG. 3, is seated on the step  65  for biasing the valve to the closed position, as will be described in more detail below. 
     The exterior surface  72  of the upper portion  60  of the valve body  50  is a smooth cylindrical surface dimensioned to have an interference fit with the bore  75  of a conventional spherical bearing  80 . The bearing  80  has an interior element  82  having an exterior convex spherical surface, and an exterior element  84  having a mating interior concave spherical surface engaged with the convex spherical surface of the interior element  82 . The mating spherical surfaces allow the exterior element to swivel universally on the interior element  82  about the center of curvature of the spherical interface between the elements  82  and  84 . The interior element  82  is pressed onto the cylindrical surface  72  of the valve body upper portion  60  until the interior element engages a step  86  formed at the junction of the upper portion  60  and the bourlet  56 . 
     A hard-stop  90 , shown in detail in FIGS. 7 and 8, has a stepped bore  92  and a flat top surface  94 . The hard-stop  90  is pressed onto the exterior element  84  of the spherical bearing  80  so it is able to swivel universally about the center of curvature of the spherical interface of the spherical bearing  80 . The hard-stop  90  is made of a tough, low friction, abrasion resistant material such a Delrin so it does not mar the surface of parts it engages when the vacuum head  25  pulls the part against the hard-stop  90 . A series of shallow radial grooves  96  is cut in the top surface  94  for communication of vacuum from the valve body  50  to the vacuum cup, as described below. 
     A vacuum cup  100  is secured to the top of the base  30 , in a groove  102  provided for that purpose, by a conventional clamp  104  such as a hose clamp or the like. The vacuum cup has a bellows skirt  106  which provides flexibility and an outward resilience to provide a firm resilient engagement with the surface of the part that is pushed against the vacuum head  25 . The vacuum cup may be made of an elastomer that is resilient and provides a good seal with the surface of a part. A suitable material is Buna-N nitril rubber that is economical and widely available. 
     Vacuum is conveyed to the vacuum head  25  only when a part is in contact with the vacuum cup  100 , so the vacuum in the system is not compromised by open vacuum heads. Vacuum is admitted to the volume within the vacuum cup  100  by pushing the part against the vacuum cup and against the head  108  of a nylon screw  110  that is threaded into a tapped hole  112  in the shank  115  of a valve plunger  120 . The head  108  acts as a valve button by which the valve plunger  120  can be pushed to disengage an O-ring  123  in a groove  125  in conical lower portion  127  of the plunger  120  out of contact with a corresponding conical valve seat  128  at the lower end of the central bore  63  through the valve body  50 , thereby establishing vacuum communication between the recess  40  and the interior of the vacuum cup  100 . The valve plunger is biased to its closed position by the compression spring  70  bearing against the step  65  at its lower end, and, at it upper end, against a washer  130  held on the end of the valve plunger  120  by the screw  110 . Thus, when the part is removed from the vacuum head  25 , the spring  70  lifts the valve plunger  120  to engage the O-ring  123  with the valve seat  128  to close the valve. 
     In operation, the vacuum head  25  is mounted on the end of a support arm on which the part is to be supported and is secured thereon by the clamp structure  46 . The clamp structure  46  includes a cut  131  perpendicular to the axis  35  about half way through the lower end of the base  30 , and another radial cut  132  parallel to the axis  35  and intersecting the cut  132 . These cuts  131  and  132  provide a pair of clamp arms that may be flexed inward. A screw  133  threaded into a tapped portion  134  of a hole  136  extending on a secant perpendicular to the cut  132  can be tightened to squeeze the clamp arms against the support arm in the recess  40  to hold the vacuum head  25  on the support arm. 
     Typically, the part is supported on a holding apparatus having numerous support arms, each provided with a vacuum head  25 . An example is shown in FIG. 9 wherein the support arms are pogos  135  which are adjustable lengthwise and also vertically on a series of stanchions  137  (only one of which is shown in FIG.  9 ), and the stanchions  137  are movable along rails  138 . A holding apparatus of this type allows the vacuum head to be adjusted to any desired position in space within the range of the apparatus to position the vacuum heads exactly where desired so the part may be held for machining and/or assembly operations at a precisely known position and orientation. The universally swiveling hard-stop  90  on the vacuum head  25  allows the hard-stop to swivel to self-align to the plane of the part and lie flat against the part surface. 
     Turning now to FIG. 10, a vacuum head  150  is shown having a hard-stop  90  that is non-swiveling, that is, the hard-stop is fixed with respect to the base  30 . The structure of the vacuum head  150  is identical to that of the vacuum head  25  shown in FIG. 2, except that the valve body  50  in the vacuum head  25  is replaced with a valve body  160  having a fixed mount for the hard-stop  90  instead of the universally swiveling spherical bearing mount used in the embodiment of FIG.  1 . As shown in detail in FIGS. 11 and 12, the upper portion  162  of the valve body  160  is a cylinder having an outside diameter equal to the outside diameter of the spherical bearing, so the same hard-stop  90  can be mounted in a fixed position on the fixed upper portion of the valve body  160  instead of the universally swiveling spherical bearing  80 . The biasing compression spring  70  is shown representationally in FIG. 10 instead of schematically as in FIG.  2 . 
     Turning now to FIG. 13, a vacuum head  170  is shown having a hard-stop  175  mounted to swivel about a lateral axis lying perpendicular to the central axis  35 . This third embodiment of the invention would normally be used when the part that it holds is too narrow to allow more than one vacuum head to engage the part at that location. In that case, the vacuum head  170  would be oriented with the swivel axis of the hard-stop  175  vertical so the part would be restrained from tilting away from the holding apparatus but the hard-stop would be free to swivel about the vertical axis to self-align with the surface of the part. 
     The vacuum head has a base that is-identical to the base  30 , with the addition of two vertical holes  181  drilled on a diameter across the web  182  and spaced equally from the central hole that receives the valve body  185 . The holes  181  receive Allen head machine screws  186  which extend through the holes  181  and are threaded into tapped holes  187  in a truncated cylindrical bearing block  190  to hold the bearing block in place on the floor of the upper recess  38 . The bearing block has a flat surface  194  at about the ⅔ the diameter of the cylinder which is held flat against the floor of the recess  38  by the screws  186 . 
     The hard-stop  175  has a cylindrical recess  197  open at one axial end (the left end in FIG. 13) and closed by a flat end  191  at the other end (the right end in FIG.  10 ). The cylindrical recess  197  receives the bearing block  190  with a snug fit. The cylindrical interface between the bearing block and the hard-stop recess  197  allows the hard-stop to swivel about 10°-20° on the bearing block  190  about the axis  198  of the cylinder and be retained on the bearing block by the portions of the hard-stop  175  which extend around the bearing block below the horizontal plane through the axis  198 . A pin  199  through a hole  193  in the hard-stop  175  spans the cylindrical recess  197  adjacent the end of the bearing block  190  to prevent lateral translation of the hard-stop  175  along the bearing block  190 . Lateral translation of the hard-stop  175  is prevented by engagement of the other end of the bearing block  190  with the end  191  of the cylindrical recess  197 . 
     The bearing block is sealed to the floor of the recess  38  by O-rings  200  lying in shallow counterbores around the holes  181 . The O-rings  200  prevent leakage of vacuum under the bearing block  190  and through the holes  181 . 
     The valve body  185  does not support the hard-stop  175 , so it does not need and does not have an upper portion. It is screwed into the central hole in the web  182  by a spanner (not shown) having pins the fit into diametrically opposite holes in the top surface of the valve body. In all other respects, the structure and function of the valve body and valve plunger are the same as the corresponding parts in the embodiments of FIGS. 1 and 10. 
     Obviously, numerous modifications and variations of the system disclosed herein will occur to those skilled in the art in view of this disclosure.