Patent Publication Number: US-7896728-B2

Title: Machining methods using superabrasive tool

Description:
BACKGROUND 
     The disclosure relates to machining. More particularly, the disclosure relates to superabrasive machining of metal alloy articles 
     Superabrasive quills for point and flank superabrasive machining (SAM) of turbomachine components are respectively shown in commonly-owned U.S. Pat. Nos. 7,101,263 and 7,144,307. Commonly-owned U.S. Pat. Publication 2006-0035566 discloses a quill having a tip protuberance. 
     SUMMARY 
     One aspect of the disclosure involves a tool for use in an abrasive machining process. A body extends along a central longitudinal axis from a first end to a tip end. The body has a tip end protuberance. An abrasive material is located on the protuberance. A body lateral surface has, over a radial span of at least 20% of a radius of the protuberance, a continuously concave longitudinal profile diverging tipward. 
     In various implementations, the radial span may be at least 30% of said radius. The abrasive material may be along at least half of the radial span. The body may include a threaded portion for engaging a machine, a flange having a pair of flats for receiving a wrench, and a shaft extending tipward from the flange. The abrasive material may comprise a coating. The abrasive material may be selected from the group consisting of plated cubic boron nitride, vitrified cubic boron nitride, diamond, silicon carbide, and aluminum oxide. The tool may be combined with a machine rotating the tool about the longitudinal axis at a speed in excess of 10,000 revolutions per minute. 
     Another aspect of the invention involves a process for point abrasive machining of a workpiece. A tool is provided having a tip protuberance grinding surface coated with an abrasive. The tool is oriented relative to a surface of the workpiece so that there is contact between the surface and the grinding surface. A part is formed by removing material at the contact by rotating the tool about the central longitudinal axis and translating the tool relative to the workpiece and off-parallel to the longitudinal axis. The tool is cooled by guiding a cooling liquid flow to the tip grinding surface along a surface of the shaft and radially diverging to the grinding surface. 
     In various implementations, the tool may be rotated at a speed in the range of 40,000 to 120,000 revolutions per minute. The longitudinal axis may be reoriented relative to the workpiece while machining the workpiece. The workpiece may comprise an integrally bladed disk. The workpiece may comprise or may consist essentially of a nickel- or cobalt-based superalloy or titanium alloy. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a quill according to principles of the invention. 
         FIG. 2  is an enlarged view of a tip area of the quill of  FIG. 1 . 
         FIG. 3  is a view of the quill of  FIG. 1  machining an integrally bladed rotor. 
         FIG. 4  is a view of the quill of  FIG. 1  machining an undercut. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows an abrasive quill  20  mounted in a multi-axis machine tool spindle  22 . The machine tool rotates the quill about a central longitudinal axis  500  and translates the quill in one or more directions (e.g., a direction of translation  502 ) to machine a workpiece  24 . Exemplary rotation is at a speed in excess of 10,000 rpm (e.g., in the range of 40,000 rpm-140,000 rpm). The traversal of the quill removes material and leaves a cut surface  26  on the workpiece. The machine tool may further reorient the axis  500 . Alternatively or additionally, the machine tool may reposition or reorient the workpiece. The exemplary quill  20  includes a metallic body extending from an aft end  30  to a front (tip) end  32  (e.g., at a flat face). An abrasive coating  34  on the tip end provides cutting effectiveness. 
     Near the aft end  30 , the exemplary quill includes an externally threaded portion  36  for mating by threaded engagement to a correspondingly internally threaded portion of a central aperture  38  of the spindle  22 . Ahead of the threaded portion  36 , an unthreaded cylindrical portion  40  fits with close tolerance to a corresponding unthreaded portion of the aperture  38  to maintain precise commonality of the quill/spindle/rotation axis  500 . A wrenching flange  42  is forward (tipward) of the unthreaded portion  40  and has a radially-extending aft surface  44  abutting a fore surface  46  of the spindle. The exemplary flange  42  has at least a pair of parallel opposite wrench flats  48  for installing and removing the quill via the threaded engagement. Alternatively, features other than the threaded shaft and wrenching flange may be provided for use with tools having different quill interfaces such as are used with automatic tool changers. 
     A shaft  50  extends generally forward from the flange  42  to the tip  32 . In the exemplary embodiment, the shaft  50  includes a proximal portion  52  and a horn-like tip protuberance portion  54 . 
     In the exemplary embodiment, the proximal portion  52  is relatively longer than the protuberance  54 . The tip protuberance  54  is sized to make the required cut features. If a relatively smaller diameter protuberance is required, the shaft may be stepped (e.g., as in US Pat. Publication 2006-0035566, the disclosure of which is incorporated by reference in its entirety herein as if set forth at length). The length of the proximal portion  52  (combined with the length of the protuberance) provides the desired separation of the tip from the tool spindle. Such separation may be required to make the desired cut while avoiding interference between the spindle and any portion of the part that might otherwise interfere with the spindle. 
     In longitudinal section, the surface of the protuberance  54  ( FIG. 2 ) has a concave transition  64  to the adjacent straight portion of the shaft (e.g., the proximal portion  52 ). A convex portion  66  extends forward thereof from a junction/inflection  67  through an outboardmost location  68  and back radially inward to form the end  32 . The exemplary quill has a flat end face  70 . As is discussed further below, the exemplary protuberance has an abrasive coating at least along the convex portion  66 . An exemplary coating, however, extends proximally beyond the junction  67  (e.g., along the entirety of the protuberance) and along the end face  70 . 
     Alternative implementations may, for example, include a central recess in the end so as to leave a longitudinal rim. The presence of the recess eliminates the low speed contact region otherwise present at the center of the tip. This permits a traversal direction  502  at an angle θ close to 90° off the longitudinal/rotational axis  500 . 
     The exemplary transition  64  radially diverges from a junction  80  with the adjacent straight portion of the shaft (e.g., the proximal portion  52 ). At this exemplary junction, the shaft and transition have a radius R S . Along the transition  64 , the radius progressively increases toward the end  32 . The tip has a largest radius R T . The divergence of the transition  64  may provide a structural reinforcement. For example, with R T  larger than R S , and no transition, the protuberance would be formed as a disk at the end of the shaft. The disk would have a tendency to flex/wobble during use. The transition braces against such flex/wobble. 
     The transition  64  may also help direct coolant and/or lubricant to the contact area between the quill and the workpiece (the grinding zone). For example,  FIG. 1  shows a tool-mounted nozzle  180  having a circumferential array of coolant outlets  182  circumscribing the quill. Each of the outlets discharges a stream  184 . The streams impact along the transition  64  and are guided by the transition to form a tipward flow  186  along the transition to the grinding zone. 
     An exemplary transition  64  is concave in longitudinal section. This may provide an advantageous combination of strength, light weight, and guidance of the coolant flow. 
     The exemplary protuberance has a length L T  from the junction  80  to the end  32 . Of this length, the convex or radial rim portion  66  has a length L R . The exemplary concave transition  64  has a length L C . A radius at the junction  67  is R C . Exemplary R C  is at least 80% of R T , more narrowly, 90%, or 95%. An exemplary change in radius over the transition (R C  minus R S ) is at least 20% of R T , more narrowly, at least 30% (e.g., 30-60%). Exemplary L T  and L C  are larger than R S , more narrowly, at least 150% of R S  (e.g., 200-500%). 
       FIG. 3  shows exemplary positioning of the quill  20  during one stage of the machining of an integrally bladed rotor  200  (IBR, also known as a blisk). The unitarily-formed blisk  200  has a hub  202  from which a circumferential array of blades  204  radially extend. Each blade has a leading edge  206 , a trailing edge  208 , a root  210  at the hub, and a free tip  212 . Each blade also has a generally concave pressure side and generally concave suction side extending between the leading and trailing edges. In the exemplary blisk  200 , a fillet  220  is formed between the outer surface  222  (defining an inter-blade floor) of the hub and the blades. The quill  20  is shown grinding a leading portion of a blade suction side and fillet near the interblade floor. The divergence of the protuberance allows access around the curve of the blade span. The same or a different quill may be used to machine surface contours (e.g., pressure side concavity and suction side convexity) of the blades. A traversal at or near normal to the quill axis may permit machining of the floor  222 . 
     Other situations involve machining undercuts. Various examples of undercuts are used for backlocked attachment of one component to another and/or for lightening purposes. In various such undercut situations, during one or more passes of the quill, the grinding zone may extend up along the concave transition  64 . For example,  FIG. 4  shows machining to leave undercuts  250  on each side of a rail  252 . Along the undercuts, a base/root/proximal portion  254  of the rail is recessed relative to a more distal portion  256 . Such recessing on both sides renders the proximal portion narrower than the distal portion (e.g., with a thickness at a minima being at least 10% less (e.g., (20-50%)than a thickness at a maxima). The exemplary grinding zone  258  extends (at least for the pass/traversal being illustrated) partially along the concave transition  64  (e.g., along slightly more than half the longitudinal length of the transition). An exemplary rail  252  serves as a structural reinforcement rib on a gas turbine engine augmentor case segment (e.g., as part of an ISOGRID rib structure (e.g., three groups of intersecting ribs along the inner diameter (ID) or outer diameter (OD) of the case segment). In such a situation, the undercuts may serve to lighten the case with a relatively low reduction in strength. Such undercuts may also provide attachment locations (e.g. for a clamp or other joining member to grasp the rail). In a reengineering situation they may replace baseline non-undercut ribs or may replace baseline undercut ribs formed by chemical milling/etching (thereby reducing chemical waste, contaminations, and/or other hazards). The protuberance permits the undercutting of a geometry that a straight tool (e.g., of similar length and of diameter corresponding either to R S  or R T ) would not have access to cut (e.g., a T-like rail/rib). 
     Another optional feature is elongate recesses (e.g., as in U.S. Pat. Publication 2006-0035566), which may serve to help evacuate grinding debris. 
     In an exemplary manufacturing process, the basic quill body is machined (e.g., via one or more lathe turning steps or grinding steps) from steel stock, including cutting the threads on the portion  36 . There may be heat and/or mechanical surface treatment steps. The abrasive may then be applied as a coating (e.g., via electroplating). Exemplary superabrasive material may be selected from the group of cubic boron nitride (e.g., plated or vitrified), diamond (particularly useful for machining titanium alloys), silicon carbide, and aluminum oxide. The exemplary superabrasive material may have a grit size in the range of 40/45 to 325/400 depending on the depth of the cut and the required surface finish (e.g., 10 μin or finer). A mask may be applied prior to said coating and removed thereafter to protect areas where coating is not desired. For example, the mask may confine the coating to the tip protuberance portion  54 . Particularly for a vitrified coating, the as-applied coating may be dressed to improve machining precision. To remanufacture the quill, additional coating may be applied (e.g., optionally after a removal of some or all remaining used/worn/contaminated coating). 
     An exemplary projecting length L of the quill forward of the spindle is 57 mm, more broadly, in a range of 40-80 mm. An exemplary protuberance radius R T  is 10 mm, more broadly 8-20 mm. An exemplary longitudinal radius of curvature of the convex portion is 1-3 mm, more broadly 0.5-4 mm. 
     One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other embodiments are within the scope of the following claims.