Abstract:
A method for cutting a blade root retention slot in a turbine engine disk element includes forming a precursor slot in the element. The precursor slot has first and second sidewalls and a base. A rotating bit is passed through the precursor slot to machine the base. The bit rotates about an axis off-normal to a direction of passing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     The invention relates to machining. More particularly, the invention relates to the machining of blade attachment slots in turbomachine disks.  
         [0003]     (2) Description of the Related Art  
         [0004]     In turbomachines such as gas turbine engines, the blades of fan, compressor, and turbine sections may be secured to separate disks. One attachment means involves providing blade roots having a convoluted section complementary to a convoluted section of slots in the disk periphery. An exemplary configuration involving a convoluted profile that generally increases in transverse dimension from the slot base toward its opening is called a fir tree configuration. A number of methods have been used or proposed for forming the slots. Exemplary methods are discussed in S. L. Soo et al., “Point Grinding of Nickel-Base Superalloys”, Industrial Diamond Review, February 2002, pages 109-116.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, one aspect of the invention involves a method for cutting a blade root retention slot in a turbine engine disk element. A precursor slot is formed in the element. The precursor slot has first and second sidewalls and a base. Subsequent to the forming, a convoluted profile is machined into the first and second sidewalls. Subsequent to the machining, a rotating bit is passed through the precursor slot to machine the base, the bit rotating about an axis off-normal to a direction of the passing.  
         [0006]     In various implementations, the machining may broaden and deepen the base or, alternatively, the machining may leave at least a first portion of the base intact from the forming. The forming may involve machining with a grinding wheel rotating about a wheel axis essentially perpendicular to a direction of passing and essentially circumferential to a central longitudinal axis of the disk element. The machining may involve machining with a profiling bit having a convoluted longitudinal profile complementary to the convoluted profile of the slot sidewalls. The passing may smooth transitions between the first and second sidewalls and the base. During the passing, the bit axis may essentially lie along a radial plane of the disk element at an angle of between 60° and 85° relative to the direction of its passing. During its passing, the bit axis may be inclined relative to the direction of its passing. The forming may involve machining with a grinding wheel having portions of different diameters for forming the precursor slot with the first and second sidewalls as stepped sidewalls. The passing may increase an outward concavity of the base.  
         [0007]     Another aspect of the invention involves a method for cutting a blade retention slot in a turbine disk engine element. A precursor slot is formed in the element having first and second sidewalls and a base. Subsequent to the forming, a rotating bit is passed through the precursor slot to machine the base, the bit rotating about an axis off-normal to a direction of the passing.  
         [0008]     In various implementations, the bit may be an abrasive bit. The bit may have a doubly convex surface portion used to machine the base. The forming may comprise grinding with a grinding wheel. The forming may comprise grinding with a series of grinding wheels of different widths. The axis may be between 5° and 30° off-normal to the direction of passing. The axis may be inclined relative to the direction of passing.  
         [0009]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a partial longitudinal view of a blade attachment disk according to principles of the invention.  
         [0011]      FIG. 2  is a view of a first precursor of the disk of  FIG. 1 .  
         [0012]      FIG. 3  is a radial cutaway view of the precursor of  FIG. 2  during machining.  
         [0013]      FIG. 4  is a view of a secondary precursor of the disk of  FIG. 1 .  
         [0014]      FIG. 5  is a radial cutaway view of the precursor of  FIG. 2  during machining from the precursor of  FIG. 2 .  
         [0015]      FIG. 6  is a view of a tertiary precursor of the disk of  FIG. 1 .  
         [0016]      FIG. 7  is a radial cutaway view of the precursor of  FIG. 6  during machining from the precursor of  FIG. 4 .  
         [0017]      FIG. 8  is a radial cutaway view of the disk of  FIG. 1  during machining from the precursor of  FIG. 6 . 
     
    
       [0018]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0019]      FIG. 1  shows a disk  20  having a central longitudinal axis  500  and a circumferential perimeter  22 . Extending radially inward from the perimeter are a circumferential array of firtree blade attachment slots  24  each defined by on surface  25  forming the sidewalls and base of the slot. In the exemplary embodiment, each slot  24  extends along a symmetry plane  502  which may be a longitudinal radial plane through the axis  500  or may be at an angle to the axis  500 . Each slot  24  has a convoluted profile for receiving a complementary root or button of a blade (not shown) to secure the blade to the disk against centrifugal forces associated with rotation of the disk about its axis  500 . The exemplary slot has two relatively wide portions respectively distally (radially inward) of associated relatively narrow portions. In the exemplary embodiment, an outboard wide portion is defined between sidewall portions  26 A and  26 B of the surface  25  on either side of the plane  502 . Radially outboard thereof the associated relatively narrow portion is formed between sidewall surface portions  27 A and  27 B. Radially inboard thereof is a relatively narrow portion defined by sidewall surface portions  28 A and  28 B. Yet further inboard, the second relatively wide portion (although not as wide as the first) is defined by sidewall surface portions  29 A and  29 B with a base  30  extending therebetween in a smooth continuously curving fashion.  
         [0020]     In an exemplary method of manufacture, a precursor of the final disk is initially formed with a substantially uninterrupted perimeter. Initial precursor slots  24 ′ ( FIG. 2 ) may be ground having an outboard portion between relatively widely spaced parallel sidewall surface portions  31 A and  31 B and an inboard portion with more closely spaced surface portions  32 A and  32 B. An essentially circumferential sidewall shoulder surface portion  34 A,  34 B separates these surface portions. A flat base surface  36  joins the surface portions  32 A and  32 B.  FIG. 3  shows the initial slot precursor being ground by a grinding wheel  40  having a large diameter central portion  42  having a perimeter  44  for forming the base surface  36  and first and second sides for forming the surface portions  32 A and  32 B. The grinding wheel has a pair of smaller diameter portions  50  each having a perimeter  52  for forming an associated one of the surface portions  34 A,  34 B and an outboard side surface for forming the associated surface portion  30 A,  30 B. The wheel is driven for rotation about its central axis  510  in a direction  511  while being traversed in a longitudinal direction  512  to pass the wheel between first and second sides  60  and  62  of the disk.  
         [0021]     A convoluted secondary slot precursor  24 ″ ( FIG. 4 ) is then formed from each initial precursor  24 ′. In the exemplary embodiment, this is done using a superabrasive quill  70  ( FIG. 5 ) driven for rotation about its central longitudinal axis  514  while being traversed in the direction  512 . The exemplary quill has a shaft  72  for mounting in an associated milling machine (not shown) and a distal superabrasive coated tip  74 . The tip is dimensioned with proximal and distal relatively wide portions  76  and  78  for forming associated portions of a surface  25 ″ of the secondary slot precursor  24 ″. The surface  25 ″ is slightly toward the plane  502  relative to the ultimate surface  25 . In the exemplary embodiment, the tip  74  includes a flat distal end  80  for forming a flat base portion  30 ″ on surface  25 ″ similarly slightly outboard of the ultimate base.  
         [0022]     Each slot precursor  24 ″ is, in turn, further enlarged to form a third precursor  24 ′″ ( FIG. 6 ). This is done using a second quill  90  having a shaft  92  and tip  94  and rotated about its axis  516  while being traversed in the direction  512  as was the quill  70 . Relative to the tip  74  of the quill  70 , the tip  94  has corresponding portions of slightly greater diameter and is slightly longer. The exemplary quill dimensions bring the surface  25 ′″ into alignment with the ultimate surface  25  except at a flat base  30 ′″ slightly outboard of the ultimate concave base  30 . The base  30 ′″ has abrupt intersections with the smooth surface portions  29 A and  29 B (being the same for the surface  25 ′″ as for the surface  25 ). At least one additional machining stage is provided to form the surface portion  30  with its outwardly concave shape and smooth continuously curving transition to the adjacent portions  29 A and  29 B. This is achieved by means of a quill  120  having a shaft  122  and a superabrasive tip  124  and being rotated about its axis  520  while being traversed in the direction  512 . Because the rotation produces zero movement at the center  126  of the distal end of the tip  124 , the axis  520  is off-radial.  FIG. 8  shows the axis at a non-right angle θ to the longitudinal direction or feed direction  512  and thus off-radial by the component of θ. With reference to the illustration of  FIG. 8 , the quill is defined as inclined relative to the feed direction for values of θ greater than zero and less than 90°. A decline is defined as θ values greater than 90° and less than 180°. In the exemplary embodiments, the axis  520  is off-radial by between 5° and 50° (40°≦θ≦85°), more narrowly, between 10° and 30° (60°≦θ≦80°).  
         [0023]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, in alternate embodiments, the intermediate radial quill machining may not necessarily disturb the base of the initial slot precursor. This base may end up being effected only by the angled quill. Furthermore, the principles of the invention may be replied to a number of existing basic disk configurations and manufacturing techniques. In such implementations, features of the implementation would be influenced by features of the disks and techniques. Accordingly, other embodiments are within the scope of the following claims.