Abstract:
In one embodiment, an apparatus for constraining an object includes a first clamping surface configured to apply a first holding force to a first surface of the object; a second clamping surface configured to apply a second holding force to a second surface of the object; and an actuator configured to selectively move the second clamping surface relative to the first clamping surface.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of co-pending U.S. patent application Ser. No. 13/168,725, filed Jun. 24, 2011, which claims benefit of U.S. provisional patent application Ser. No. 61/358,855, filed Jun. 25, 2010. Both related applications are hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to vibrational analysis and, more specifically, to a multi-purpose vacuum clamp table. 
         [0004]    2. Description of the Related Art 
         [0005]    Vibrational modal analysis can be accomplished by constraining at least one degree of freedom of a structure and introducing a forcing function to the structure. Generally, one or more control points on a structure are constrained to a fixed acceleration. 
         [0006]    In the manufacture of structures where harmonic resonance is an important characteristic, it is necessary to provide fixtures that constrain a work piece in a certain manner to test for resonant frequencies. In a typical manufacturing cycle, the resonant frequencies of a work piece are first measured. Then, precise machining operations make changes to the work piece in response to the measured resonant frequencies. Finally, the vibrational modal analysis is checked again to determine if the structure has the correct harmonic resonance. 
         [0007]    One drawback to this approach is that machining operations require the fixture to provide a rigid base to constrain movement of the work piece during machining. However, accurate vibrational modal analysis requires uniform clamping characteristics on the boundary of the work piece, such as along the perimeter. Traditionally, this would be accomplished by moving the work piece between different fixtures for each of the operations. Changing the fixtures between phases takes time and reduces the manufacturing efficiency for creating the finished products. For example, with each change of the fixtures between phases, the boundary constraints on the work piece for vibrational modal analysis are also changed. 
         [0008]    As the foregoing illustrates, what is needed in the art is a fixture that can be configured to constrain a work piece with one set of constraints and efficiently reconfigured to constrain the work piece with a second set of constraints. 
       SUMMARY OF THE INVENTION 
       [0009]    In one embodiment, an apparatus for constraining an object includes a first clamping surface configured to apply a first holding force to a first surface of the object; a second clamping surface configured to apply a second holding force to a second surface of the object; and an actuator configured to selectively move the second clamping surface relative to the first clamping surface. 
         [0010]    In another embodiment, a method of constraining an object includes applying a first holding force to the object to hold the object against a first clamping surface; performing a first vibrational modal analysis on the object; applying a second holding force to the object to hold the object against a second clamping surface; changing a resonant characteristic of the object; releasing the second clamping surface from the object; and performing a second vibrational modal analysis on the object. 
         [0011]    In another embodiment, a method of constraining an object includes applying a first holding force to the object to hold the object against a first clamping surface; applying a second holding force to the object to hold the object against a second clamping surface; and moving the second clamping surface relative to the first clamping surface. The second clamping surface may be moved to a coplanar position with the first clamping surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    So that the manner in which the above recited features of the present invention, and other features contemplated and claimed herein, are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0013]      FIG. 1  is a perspective view of an embodiment of vacuum clamp table. 
           [0014]      FIGS. 2A-B  are perspective views of an inner machined plate of the vacuum clamp table of  FIG. 1 . 
           [0015]      FIG. 3  is another perspective view of the vacuum clamp table of  FIG. 1 . 
           [0016]      FIG. 4  is an enlarged partial view of an outer machined plate of the vacuum clamp table of  FIG. 1 . 
           [0017]      FIG. 5  shows a bolt and a compensating spring of the vacuum clamp assembly of  FIG. 1 . 
       
    
    
     DESCRIPTION 
       [0018]    In the following description, numerous specific details are set forth to provide a more thorough understanding of the invention. However, it will be apparent to one of skill in the art that the invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the invention. 
         [0019]    The preferred embodiment of the multi-purpose vacuum clamp table according to the present invention is illustrated in the attached  FIGS. 1-5 . The multi-purpose vacuum clamp table  100  may be used with a machine for performing vibrational modal analysis of a musical instrument soundboard. 
         [0020]    In the preferred embodiment, the multi-purpose vacuum clamp table  100  comprises  6  components. The multi-purpose vacuum clamp table  100  is illustrated in  FIG. 1 . The first component is an inner vice assembly  110  that includes an inner machined plate  115  used for applying a clamping force to a work piece. The top surface of the inner vice assembly  110  has an array of holes  210  drilled through the inner machined plate  115  that connect to a series of channels  220  on the back side of the plate  115 . The front and back surfaces of the inner machined plate  115  are illustrated in  FIGS. 2A and 2B , respectively. A back-plate (not shown) is attached to the back surface of the inner machined plate  115  to seal the channels  220  of the inner machined plate  115 . 
         [0021]    The second component is the outer vice assembly  120 . The outer vice assembly  120  includes an outer machined plate  125  configured with a cutout in the center of the outer machined plate  125  in the shape of the inner vice assembly  110 . The outer vice assembly  120  has a vacuum channel  400  cut into the top of the outer machined plate  125 . The vacuum channel  400  may be disposed around the perimeter of the cutout. In one embodiment, the machined vacuum channel  400  is concentric to the cutout in the outer machined plate  125 , as illustrated in  FIG. 4 . Two shoulder bolts  510  and compensating springs  520  connect the outer machined plate  125  of the outer vice assembly  120  to a support plate  135  of the outer vice assembly  120 , as illustrated in  FIG. 5 . The compensating springs  520  force the support plate  135  of the outer vice assembly  120  to rest against the cam lobes  320  of a camshaft  310 . Although the inner vice assembly  110  is shown to be substantially similar in shape to the cutout in the outer vice assembly  120 , it is contemplated that the inner vice assembly  110  may have a different shape from the cutout so long as it provides sufficient support to the work piece during the machining process. 
         [0022]    The third and fourth components are a camshaft  310  assembly and two bearing housings  330 , respectively.  FIG. 3  is an illustration of an isometric view of the multi-purpose vacuum clamp table  100  with a hidden inner vice assembly  110  and a transparent outer vice assembly  120 . The camshaft  310  and the cam lobes  320  are used to move the inner vice assembly  110  relative to the outer vice assembly  120 . The camshaft  310  is constrained by two bearing housings  330  bolted to the bottom surface of the outer machined plate  125  of the outer vice assembly  120 . Two cam lobes  320  are attached to the camshaft  310 . The cam lobes  320  ride against the bottom surface of the support plate  135  of the outer vice assembly  120 . The inner vice assembly  110  is located in the cutout in the outer machined plate  125  of the outer vice assembly  120  and rests on top of the support plate  135  of the outer vice assembly  120 . When the camshaft  310  is rotated to its top-dead-center position, the top surface of the inner vice assembly  110  is coplanar to the top surface of the outer vice assembly  120 . When the camshaft  310  is rotated to its bottom-dead-center position, the top surface of the inner vice assembly  110  is below the top surface of the outer vice assembly  120 . The support plate  135  is configured to limit movement of the inner vice assembly  110  to a predetermined height, such as when the inner vice assembly  110  is coplanar with the outer vice assembly  120 . In one embodiment, the support plate  135  may be in the shape of a rectangle having dimensions larger than the cutout. The support plate  135  may contact the bottom of the outer machined plate  125  to prevent the inner vice assembly  110  rising higher, thereby ensuring the top surface of the inner vice assembly  110  is coplanar with the outer vice assembly  120 . 
         [0023]    In alternative embodiments, alternative mechanisms for actuating the inner vice assembly  110  are contemplated. For example, the inner vice assembly  110  could be actuated by a screw, bellows, pneumatic or hydraulic cylinders, or any other mechanism that could raise and lower the inner vice assembly  110  in relation to the outer vice assembly  120 . 
         [0024]    The fifth components are gaskets used to seal the vacuum clamp surfaces of the inner and outer vice assemblies  110 ,  120 .  FIG. 4  is an enlarged partial view of the outer machined plate  125  of the outer vice assembly  120 . As shown, a gasket  410 ,  411  is placed in concentric channels on both sides of the vacuum channel  400  of the outer vice assembly  120 . In the preferred embodiment, the gaskets  410 ,  411  are made of a conventional vacuum gasket material having a rectangular cross-section. In alternative embodiments, other suitable sealing materials for vacuum sealing and different cross-sections may be employed, such as gaskets with a circular cross-section. The vacuum channel  400  is connected to a vacuum pressure through a fitting in the back of the outer machined plate  125  of the outer vice assembly  120 . The gaskets  410 ,  411  provide a seal between the top surface of the outer vice assembly  120  and a work piece placed on top of the outer vice assembly  120 . A third gasket  140 , of similar construction, is shown in  FIG. 1  and  FIG. 2A  around the outer edge of the inner vice assembly  110 . The third gasket  140  provides a seal between the top surface of the inner vice assembly  110  and a work piece placed on top of the inner vice assembly  110 . 
         [0025]    The sixth component is the frame structure that connects to the bottom of the outer vice assembly  120 . In the preferred embodiment of  FIG. 1 , the frame  160  includes of four vertical legs  161  and two base cross-members  162 . A front leg  161  is connected to one of the back legs  161  by either end of one of the base cross-members  162 . The four legs  161  are connected with bolts to the bottom surface of the outer machined plate  125  of the outer vice assembly  120 . Other suitable frame structures suitable for supporting the inner and outer vice assemblies  110 ,  120  are also contemplated. 
         [0026]    In the preferred embodiment, the multi-purpose vacuum clamp table  100  is used as a fixture in a manufacturing process that includes a first testing stage, a machining stage, and a second testing stage. In the first testing stage, a vibration modal analysis is performed on a work piece clamped to the multi-purpose vacuum clamp table  100 . The camshaft  310  is rotated to the bottom-dead-center position, causing the top surface of the inner vice assembly  110  to drop below the top surface of the outer vice assembly  120 . A vacuum pressure is applied to the vacuum channel  400  in the outer vice assembly  120  to clamp the work piece to the outer vice assembly  120  around the perimeter edge of the work piece. The work piece does not touch the inner vice assembly  110  when the inner vice assembly  110  is in the lowered position. Therefore, the clamp table  100  is optimally configured for performing vibrational modal analysis by placing uniform constraints along the perimeter of the work piece while leaving the lower surface of the work piece unconstrained. 
         [0027]    Next, vibrational modal analysis is performed to determine if machining operations are required to change the resonant characteristics of the work-piece. If the results of the vibrational modal analysis determine a change to the resonant characteristics of the work piece is necessary, then the machining stage is performed. Initially, a machinist rotates the camshaft  310  to its top-dead-center position. The support plate  135  of the outer vice assembly  120  and the inner vice assembly  110  are moved up such that the top surface of the inner vice assembly  110  is coplanar with the top surface of the outer vice assembly  120 . A vacuum pressure connected to the inner vice assembly  110  is switched on so that the work piece is clamped to the top surface of the inner vice assembly  110 . The work piece is now clamped to both the inner and the outer vice assemblies  110 ,  120 . The top surface of the inner vice assembly  110  and the top surface of the outer vice assembly  120  provide a rigid, coplanar support for the work piece during machining operations. 
         [0028]    After the machining operations are complete, the vacuum is disconnected from the inner vice assembly  110 , thereby releasing the clamping force between the work piece and the inner vice assembly  110 . The machinist rotates the camshaft  310  to its bottom-dead-center position, thereby lowering the top surface of the inner vice assembly  110  below the top surface of the outer vice assembly  120 . Subsequently, a second vibrational modal analysis is performed to check the new resonant characteristics of the work piece. 
         [0029]    By not disconnecting the vacuum pressure from the outer vice assembly  120  during the test-machine-test cycle, the boundary constraints on the work piece are maintained throughout the cycle. Therefore, the second vibrational modal analysis is performed using the same boundary conditions as the first vibrational modal analysis. This allows for accurate comparison of the results of the second vibrational analysis to the results of the first vibrational analysis. However, it is contemplated that the clamp table may be used for one or more of the two testing phases and the machine phase. 
         [0030]    While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.