Abstract:
A clamp for clamping a workpiece such as a forging of an aerofoil component comprises a support body  24  to which a clamping member  26  is hinged. The clamping member  26  has a workpiece clamping element  54  having pads  58  for engagement with the workpiece. The workpiece clamping element  54  is pivotable relatively to the clamping member  26  by means of a ball and socket connection including a ball element  40.  This enables the workpiece clamping element  54  to self-align with the profile of the workpiece, for example with a surface of the blade  6  of the aerofoil component.  
     In an alterative embodiment, the workpiece clamping element  54  is formed integrally with the clamping member  26,  but is connected thereto by a necked region of the clamping member  26  which permits flexure of the workpiece clamping element  54  relatively to the clamping member  26.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a clamp for clamping a workpiece, and is particularly, although not exclusively, concerned with a clamp for clamping an aerofoiled component for machining purposes.  
         BACKGROUND OF THE INVENTION AND PRIOR ART  
         [0002]    The secure clamping of aerofoiled components presents difficulties because the aerofoil sections do not have flat, parallel surfaces. Also, aerofoil components formed by forging have dimensional and shape differences from one another which means that a clamping structure shaped to fit perfectly with one component will not fit perfectly with another. Such imperfect fits can result in inadequate clamping and/or marking of the aerofoil surface.  
           [0003]    A forged aerofoil component, for example a compressor rotor blade of a gas turbine engine, must undergo various operations after forging in order to remove forging flash and forging locating pips, to form blade edge and root profiles and to tip the blade to the required length. To perform these operations, the component must be held securely while exposing those parts on which the operations are to be performed. Hand polishing, followed by a vibro-polishing operation, is commonly used to form the desired circular profiles of blade chordal edges, but this often results in the chordal edges having flats or facets which, while within engineering tolerances, do not provide optimum aerodynamic performance, resulting in a loss of efficiency of the compressor module. It is thus desirable for the aerofoil finishing operations to be performed by machine, which requires secure fixing of the component while the machining and polishing operations are performed.  
           [0004]    An object of the present invention is to provide a clamp which is able to support a workpiece having variable profiles and dimensions.  
           [0005]    A further object of the present invention is to provide a clamp with a workpiece clamping element which is pivotable to adapt to the surface profile of a clamped workpiece.  
           [0006]    A yet further object of the present invention is to provide a clamp which provides firm support to parts of a component during the performance of machining and polishing operations.  
         SUMMARY OF THE INVENTION  
         [0007]    According to the present invention there is provided a clamp for clamping a workpiece, the clamp comprising:  
           [0008]    a support body;  
           [0009]    a workpiece supporting element mounted on the support body:  
           [0010]    a clamping member mounted on the support body for displacement towards and away from the support body;  
           [0011]    a workpiece clamping element mounted on the clamping member;  
           [0012]    securing means adapted to retain the clamping member in a clamping position in which, in use, the workpiece clamping element applies a clamping force to a workpiece clamped between the workpiece clamping element and the workpiece supporting element; and  
           [0013]    connecting means which connects the workpiece clamping element to the clamping member for pivoting displacement relatively to the clamping member about axes lying in a plane extending transversely of the line of action of the clamping force, and for linear movement relatively to the clamping member in a direction substantially transverse to the line of action of the clamping force, whereby the workpiece clamping element is self-aligning with the surface of a clamped workpiece.  
           [0014]    Because the workpiece clamping element is self-aligning, it is able to adapt to variations in surface profile and dimensions of the clamped workpiece, for example a forged aerofoil component.  
           [0015]    The pivoting displacement of the workpiece clamping element may be limited, for example by contact between the workpiece clamping element and the clamping member.  
           [0016]    In one embodiment in accordance with the present invention, the pivoting displacement is achieved by means of a ball and socket connection. The ball and socket connection may comprise a part-spherical ball element mounted on the clamping member, which engages a socket formed in the workpiece clamping element. Separate means, for example a transverse pin, may be provided to retain the workpiece clamping element on the clamping member.  
           [0017]    Resilient means may be provided for biasing the workpiece clamping element into engagement with a face of the workpiece, the biasing force extending transversely of the line of action of the clamping force.  
           [0018]    In an alternative embodiment, the pivoting displacement of the workpiece clamping element may be provided by connecting the workpiece clamping element to the clamping member by means of a resilient connecting element. This connecting element may comprise a necked region between the clamping member and the workpiece clamping element, the clamping member, the necked region and the workpiece clamping element then being integral with each other. The connecting element may be situated generally centrally of the workpiece clamping element.  
           [0019]    Around the connecting element, the workpiece clamping element may be separated from the clamping member by machined slots, closure of the slots upon deflection of the workpiece clamping element limiting the extent of displacement of the workpiece clamping element relatively to the clamping member.  
           [0020]    The clamp may include at least one locating element, spaced from the workpiece supporting element and the workpiece clamping element, for engagement with a surface profile of the workpiece to locate the workpiece within the clamp. The locating element may comprise a recess for receiving a protrusion or pip on the workpiece.  
           [0021]    The support body may be mounted on a fixture for removable fitting to a machine tool or other workpiece processing equipment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:  
         [0023]    [0023]FIG. 1 shows a forged workpiece in the form of an aerofoil component;  
         [0024]    [0024]FIG. 2 is a partly sectioned side view of a clamp holding the component of FIG. 1;  
         [0025]    [0025]FIG. 3 is an enlarged sectional view of part of the clamp indicated by an arrow III in FIG. 2;  
         [0026]    [0026]FIG. 4 is a view taken in the direction of the arrow IV in FIG. 3;  
         [0027]    [0027]FIG. 5 is a sectioned side view of another clamp holding the aerofoil component of FIG. 1, following processing in the clamp of FIG. 2;  
         [0028]    [0028]FIG. 6 is a view taken generally on line VI-VI in FIG. 5;  
         [0029]    [0029]FIG. 7 is a top view of the clamp shown in FIG. 5, with the clamping member omitted; and  
         [0030]    [0030]FIG. 8 is a partially sectioned view corresponding to FIG. 5. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]    The workpiece  2  shown in FIG. 1 comprises an aerofoil component  4  having a blade  6  and a root  8 . The component  4  is surrounded by a region of forging flash  10 . The forging flash  10  has a tip end  14 , a tip location pip  16  and a root location pip  18 .  
         [0032]    The workpiece  2  must undergo several operations in order to finish the aerofoil component  4 . Thus, the forging flash  10  must be removed, along with the location pips  16  an  18 , the root  8  must be machined to a desired configuration, and the blade  6  must have its chordal edges  20  and tip  24  machined to the required dimensions and profiles.  
         [0033]    To perform these operations, the aerofoil component must be clamped firmly so that it is prevented from moving under the machining forces. This clamping is achieved by means of the clamps shown in FIG. 2 to  4 , and FIGS.  5  to  8 . The clamp shown in FIGS.  2  to  4  is employed to hold the workpiece during finishing of the blade chordal edges  20 , while the clamp shown in FIGS.  5  to  8  is employed during machining of the root  8 .  
         [0034]    Referring first to the clamp shown in FIGS.  2  to  4 , this comprises a support body  124  to which a clamping member  126  is hinged for movement about a hinge axis  128 . The support body  124  is secured to an EROWA base fixing  130  having a spigot  132  for releasable fitment to, for example, a machine tool for performing operations on the workpiece  2 .  
         [0035]    The support body  124  has a workpiece supporting element  134  which is formed integrally with the support body  124 . The clamping member  126  has a workpiece clamping element  138  which is formed integrally with the clamping member  126 . The connection between the clamping member  126  and the workpiece clamping element  138  is shown more clearly in FIG. 5. The clamping member  126  at its end region adjacent the workpiece clamping element  138  has a generally rectangular cross-section. Slots  140  having a relatively narrow outer region  142  (having a width, for example, of 0.25 mm) and a wider inner region  144  (having a width, for example, of 0.5 mm) extend inwardly from all four sides of the clamping member  126 , leaving a central connecting element or pillar  146  connecting the clamping member  126  to the workpiece clamping element  138 . This pillar may, for example, have a cross-sectional area which is not greater than 20 mm 2 , for example it may have a square cross-section with a side of 4 mm.  
         [0036]    The slots  140  may be formed by electrical discharge wire cutting.  
         [0037]    The slots  140  permit resilient pivoting of the workpiece clamping element  138  about the pillar  146 . The pivoting movement is limited by closure of the narrow regions  142  of the slots  140 , so that the maximum pivoting angle of the workpiece clamping element  138  is not greater than  20 , for example it may be 1.5°.  
         [0038]    The workpiece clamping element  138  and the workpiece supporting element  134  each have two pads  158  and  160  respectively, for engagement with the blade  6  of an aerofoil component  2 . Each pad  158  extends the full length of the respective workpiece supporting element  134  and workpiece clamping element  138 , and engages the component  2  close to the respective chordal edge of the blade  6 .  
         [0039]    A stud  162  is secured to the support body  124  and extends through an opening  164  in the clamping member  126 . A nut  166  engages the top end of the stud  162  and can be tightened to exert the required clamping force between the workpiece supporting element  134  and the workpiece clamping element  138 .  
         [0040]    The support body has two locating elements  168  and  170 , which are spring-loaded upwardly, as seen in FIG. 4, by means of springs  172  and  174 . The locating elements  168  and  170  have V-shaped recesses  176  and  178  at their upper ends, which, in use, receive the locating pips  18  and  16  of the aerofoil component (FIG. 1).  
         [0041]    A spring-loaded domed platform location pin  180  is provided in the workpiece supporting element  134  for abutment with the annulus surface of the root  8  from which the blade  6  extends. Although not shown, a separate clamping device may be provided for retaining the tip pip  16  in the locating recess  178 .  
         [0042]    For use, as shown in FIG. 4, the aerofoil component  2  is positioned on the workpiece supporting element  134  and located by engagement of the locating pips  16  and  18  in the recesses  176  and  178 . The position of the workpiece  4  in the lengthwise direction of the blade  6  is established by the location pin  180 . The clamping member  126  is then lowered to bring the workpiece clamping element  138  into engagement with the upper side of the blade  6 , and the nut  166  is tightened to clamp the blade  6  between the workpiece supporting element  134  and the workpiece clamping element  138 . Variations in the profile and dimensions of the blade  6  are accommodated by flexure of the pillar  146 , allowing the workpiece clamping element  138  to align itself with the profile of the upper surface of the blade  6 . When the workpiece  2  is clamped, machining, profiling and polishing of the blade chordal edges can be performed. While in the clamp of FIGS.  2  to  4 , the blade  6  may be tipped to length, and the locating pip  16  removed. However, it is alternatively possible for this operation to be performed separately.  
         [0043]    The workpiece  2  is then transferred to the clamp shown in FIGS.  5  to  8 , which comprises a support body  24  to which a clamping member  26  is hinged for movement about a hinge axis  28 . The support body  24  and the clamping member  26  are formed from aluminium. The support body  24  is secured to an EROWA base fixing  30  having a spigot  32  for releasable fitment to, for example, a machine tool for performing operations on the workpiece  2 .  
         [0044]    The support body  24  has a steel workpiece supporting element  34  secured to it by means of machine screws  36 . The clamping member  26  has a steel workpiece clamping element  38  which is located relatively to the clamping member  26  by means of a ball and socket connection comprising a part-spherical ball element  40  cooperating with a hardened hemi-spherical recess  42  (FIG. 6) formed in the workpiece clamping element  38 . The ball element  40  is secured to the clamping member  26  by a machine screw  44 , in such a way that there is limited freedom of movement of the ball element  40  in the general direction towards and away from the root  8  of the clamped aerofoil component  2 .  
         [0045]    It will be appreciated that the ball element  40  does not retain the workpiece clamping element  38  on the clamping member  26 . This function is performed by a pin  46  which is secured within the clamping member  26  by a grub screw  48 . The pin  46  extends with a clearance through a bore  50  formed in the clamping element  38 .  
         [0046]    The clearance between the pin  46  and the bore  50 , and the freedom of movement of the ball element  40 , permit limited pivotal displacement of the workpiece clamping element  38  relatively to the clamping member  26  about the axes defined by the ball element  40 , as well as limited linear displacement.  
         [0047]    A spring  52  acts between the clamping member  26  and the workpiece clamping element  38  to bias the workpiece clamping element  38  in a direction which is generally transverse of the line of action of the clamping force exerted by the workpiece clamping element  38  in the direction of the workpiece supporting element  34 . Thus, with reference to FIG. 2, the workpiece clamping element  38  is biased towards the annular surface of the root  8  of the aerofoil component  4  shown clamped between the workpiece supporting element  34  and the workpiece clamping element  38 . This brings a hardened support pad  54 , provided on the workpiece clamping element  38 , into contact with the annulus surface of the root  8  from which the blade  6  extends. A similar hardened pad  56  is provided on the workpiece supporting element  34 .  
         [0048]    The ball element  40  has a flat guide surface  70  which engages a complementary surface  72  formed on the clamping member  26 . The surface  72 , as shown in FIG. 2, is slightly angularly offset (for example by 3°) from the plane extending perpendicular to the line of action of the clamping force exerted by the clamping member  26 . The result of this is that the clamping force applies a wedging action on the workpiece clamping element  38  so assisting the spring  52  in biasing the workpiece clamping element  38  into contact, via the pad  54 , with the annulus surface of the root  8  of the workpiece  2 .  
         [0049]    The workpiece clamping element  38  and the workpiece supporting element  34  each have four hardened blade-engaging pads, with two adjacent each chordal edge of the blade  6 , as shown in FIG. 6. These pads are plated with cubic boron nitride (CBN) that serves to prevent slip between the pads  158  and  160  and the blade  6 .  
         [0050]    A pair of jaws  182 ,  184  are secured to the workpiece supporting element  34 . The jaw  182  is a fixed jaw and is rigidly fixed to the workpiece supporting element  34  by screws  186 . The jaw  184  is a movable jaw and is retained on the workpiece supporting element  34  by screws  188 . There is a clearance between these screws  188  and the holes in the movable jaw  184  in which they are received, which permits limited movement of the movable jaw  184  towards and away from the fixed jaw  182 . The movable jaw  184  is biased towards the fixed jaw  182  by means of a leaf spring  190  which is fixed at one end to the workpiece supporting element  34  and bears resiliently at its other end on a side face of the movable jaw  184 .  
         [0051]    A retaining bar  192  is resiliently mounted on the support body  24 . The bar  192  extends from a journal  194  which is pivotably mounted on the support body  24  by bearings  196 . An operating lever  198  depends from the journal  194  and is acted upon by a spring  200 .  
         [0052]    To install the workpiece  2  in the clamp shown in FIGS.  5  to  8 , the bar  192  is raised and the blade  6  is laid under it on to the pads  60  on the workpiece supporting element  34 . To assist this, the jaw  184  is displaced away from the jaw  182 , and, when released, is moved by the leaf spring  190  to engage the adjacent chordal edge  20  of the blade  6  to push the blade  6  into contact, at the opposite chordal edge, with the fixed jaw  182 . The bar  192  is released, and is biased by the spring  200 , acting through the lever  198 , to contact the upper edge of the blade  6  so as to provide a preliminary holding force on the workpiece  2 . The workpiece clamping element  38  is then lowered to bring the pads  58  into contact with the upper surface of the blade, these pads straddling the bar  192 .  
         [0053]    A stud  62  is secured to the support body  24  and extends through an opening  64  in the clamping member  26 . A nut  66  engages the top end of the stud  62  and can be tightened to exert the required clamping force between the workpiece supporting element  34  and the workpiece clamping element  38 .  
         [0054]    As shown in FIGS. 5 and 6, the forged workpiece of FIG. 1 is held by the clamp with the root  8  exposed for machining. Thus the pads  58  on the workpiece clamping element  38  and the pads  60  on the workpiece supporting element  34  engage opposite faces of the blade  6  and grip the blade  6  firmly under the action of the nut  66  on the stud  62 . During clamping, the ball and socket connection provided by the ball element  40  and the hemi-spherical socket  42  permit the pads  58  on the workpiece supporting element  38  to align themselves with the contacted surface of the blade  6  so that an adequate clamping pressure can be applied to the blade  6  without marking it. The CBN plating on the pads  58  and  60  avoids slippage of the blade  6  so that the workpiece  2  will remain held securely during machining operations on the root  8 . Additional support is provided by the pads  54  and  56 . When installing the workpiece in the clamp, the annulus surface of the root  8  is abutted against the pad  56  on the fixed workpiece supporting element  34 , the workpiece clamping element  38  then being biased by the spring  52  and the wedging action of the surfaces  70  and  72  to bring the pad  54  into abutment with the annulus surface.  
         [0055]    The workpiece supporting element  34  has a recess  68  to accommodate the tip locating pip  16  if it has not been removed in a previous operation.  
         [0056]    It will be appreciated that various modifications can be made to the clamps shown in FIGS.  2  to  8  to suit particular applications. For example, although four pads are shown on each of the workpiece clamping elements  38 ,  138  and the workpiece supporting elements  34 ,  134 , satisfactory results may be achieved with, for example, three pads on each element.  
         [0057]    Also, a single support body  24 ,  124  could be provided with separate workpiece supporting elements  34 ,  134  and corresponding separate clamping members  26 ,  126  so that a workpiece can be re-positioned on the same fixture to present appropriate parts of the workpiece for machining at different stages of operation.