Abstract:
A fixture for holding a workpiece includes clamps cofigured to engage tie rods secured to the workpiece, locating pins configured to engage a corresponding pin secured to the workpiece, pads, and actuators configured to move clamps for alternately drawing the workpiece into contact with the pads and disengaging the tie rod from the clamp such that the fixture assembly can release the workpiece.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/431,417, filed Jan. 10, 2011, the full disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    The present invention relates generally to fixturing for holding a workpiece while performing manufacturing operations on the workpiece. 
         [0003]    Holding a workpiece and positioning it accurately while machining the workpiece performing multiple operations on it is challenging. This is especially true when tools used to process a complex-shaped workpiece must access the workpiece from multiple sides. For example, a cylinder head for an internal combustion engine may need to have operations performed that required access to several sides for multiple different operations to be performed on the head. 
         [0004]    Attempts have been made to machine and perform manufacturing processing on these types of workpieces by attaching adapter plates to the workpiece and then supporting the adapter plate. However, these processes can be costly and may require extensive part features to be designed into the workpiece, such as a cylinder head. 
       SUMMARY OF INVENTION 
       [0005]    A fixture for holding a workpiece includes clamps able to engage tie rods secured to the workpiece, locating pins able to engage a corresponding pin secured to the workpiece, pads, and actuators able to move clamps for alternately drawing the workpiece into contact with the pads and disengaging the tie rod from the clamp such that the fixture assembly can release the workpiece. 
         [0006]    A method applicable to the fixture for holding the workpiece includes securing a tie rod to the workpiece, providing a fixture that includes a pad and a clamp able to engage the tie rod, moving the tie rod into the clamp, forcing the clamp into engagement with the tie rod, and drawing the workpiece into contact with the pad. 
         [0007]    An advantage of an embodiment is that the fixture incorporates a non-deck face manufacturing location, yet renders a machined part that is essentially machined from the desired deck face. The fixturing allows for access to five faces of a cylinder head for manufacturing operations to be performed on the head. The number of clamping positions can be minimized, resulting in fewer loads and unloads of the component, and thus a fewer number of manufacturing operations. The access to five faces of a cylinder head may also allow for reduced tool length for tools used to process the head, and may improve tolerances in the finished parts. 
         [0008]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIGS. 1 and 2  are a plan view and a side view, respectively, of a cylinder head for an internal combustion engine showing tie rods and locating pins engaged with the cylinder head; 
           [0010]      FIG. 3  is a side view of a fixture assembly supporting the cylinder head and including clamping cartridges engaging the tie rods; 
           [0011]      FIG. 4  is perspective view of the fixture; and 
           [0012]      FIG. 5  is a side view of a clamping cartridge engaged with one of the tie rods. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIGS. 1 and 2 , a cylinder head  10  for an internal combustion engine (not shown) includes locating pins  12 ,  13 , formed with concave surfaces, and threaded bores for receiving tie rods  14 ,  15 ,  16 ,  17 . A threaded shank  18  of each tie rod  14 - 17  is engaged with the threaded bores formed in the cylinder head  10 . The head  20  of each tie rod extends outward from the exhaust face  22  of the cylinder head  10  and is sized and shaped to be received and selectively retained in a fixture assembly. Each tie rod  14 - 17  includes a cylindrical shank, located between the threaded shank  18  and the head  10 , the cylindrical shank extending mutually parallel to the cylindrical shanks of the other tie rods and perpendicular to the exhaust face  22 . 
         [0014]      FIGS. 3 and 4  show a fixture assembly  24  comprising an upper, quick connect (QC) fixture  26 , which includes four clamping cartridges  28 , each cartridge engaged with the head  20  of each tie rod  14 - 17 , a locating pin  30  having a convex surface facing the cylinder head for engaging the concave locating pin  12  on the exhaust face  22 , and a locating pin  31  having a convex surface facing the cylinder head for engaging the concave locating pin  13  on the exhaust face  22 . 
         [0015]    As  FIG. 5  shows, each clamping cartridge  28  includes a rest pad  32  and a clamping device  34 , located in a cylinder  36  and secured to a piston  38 , which is also located in the cylinder. In  FIG. 5  the clamping device  34  is located at the upper end of its travel where a recess  40 , formed in a rod  42  connected to piston  38 , contacts the lower surface of a stop  44 . When the piston  38  is actuated downward to draw the cylinder head  10  into contact with the rest pad  32 , the axially opposite surface of recess  40  contacts the upper surface of stop  44 . Pressure supplied to each clamping cartridge  28  actuates piston  38  upward and downward through a plate  46 . 
         [0016]    Each clamping device  34  also includes a tie rod holder  48  or clamp bar, which engages the head  20  of one of the tie rods  14 - 17 . The tie rod holders  48  and the rest pads  32  extend upward from the upper fixture  26 . The rest pads  32  may also include a part landing detector  50 , which detects when the cylinder head  10  is positioned properly relative to the upper fixture  26 . An air check for cartridge unclamping may be included with the clamping cartridge  50 . Each tie rod holder  48  includes a recess  52  for receiving the head  20  of a tie rod  14 - 17 , and a retention lip  54 , which engages the head  20  of the tie rod that is located in the recess  52 . The tie rod holder  48  may include an internal passage that may be employed to blow out debris from the recess  52 . 
         [0017]    The upper fixture  26  also includes locating rings  56 ,  58  for positioning and securing the upper fixture to the lower fixture  28 . The fixtures  26 ,  28  may also include quick couplers that connect between the two fixtures in order to allow for the flow of oil, coolant and air between the fixtures. The lower fixture  28  may include Pascal clamping devices, located near each of its corners, and pre-locators for ensuring proper location and securement. The lower fixture  28  may be mounted to a linear motor five-axis machine that allows for five axis machining of the cylinder head  10 . 
         [0018]    An RFID tag may also be mounted on the cylinder head  10  in order to allow for identification of each cylinder head as it is loaded into the fixture assembly. 
         [0019]    To mount the cylinder block  10  on the fixture assembly  24 , the tie rods  14 - 17  are oriented to extend from the cylinder block toward the upper fixture  26 , as shown in  FIG. 3 . The tie rods  14 - 17  are located adjacent the openings to the recesses  52  in the tie rod holders  48 . Each of the tie rods  14 - 17  is received in one of the recesses  52 . The clamping device  34  of each clamping cartridge  28  is actuated to cause the retention lip  54  to engage the engagement head  40  of the respective tie rod  14 - 17  and to pull the tie rod toward the upper fixture  24  until the part landing detectors  50  on the rest pads  32  detect that each tie rod  14 - 17  is positioned as desired. The cylinder head  10  is then held firmly in the desired position for various operations, such as machining to be performed on the cylinder head  10 . When finished, the clamping devices  34  are actuated to release the tie rods  14 - 17  from the fixture  24 . The tie rods  14 - 17  may then be removed from the cylinder head  10 . 
         [0020]    The mounting of the cylinder head  10  on the fixture assembly  24  allows for access to five faces of the head  10 . A semi-finish deck face and head-to-block dowels may be created in the same fixturing as rough end features. Finish end machining may be performed using the same concept. The deck face and dowels are completed at the same time as finish machining of all critical features of the head  10  is performed. 
         [0021]    While particularly well suited for use with a cylinder head  10 , this type of fixturing assembly may also be applied to other types of prismatic surfaces where access to multiple surfaces for machining is desired. 
         [0022]    While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.