Abstract:
A gas turbine component blank is shaped by clamping the gas turbine component blank into a fixture that accurately positions the gas turbine component blank in three dimensions. The positioning is accomplished against stops accurately machined into a base of the fixture, by first supporting the gas turbine component blank on one set of stops that prevents movement in the direction perpendicular to a plane of the base, and then operating a movable clamp to force the gas turbine component blank against other sets of stops that limit the movement of the gas turbine component blank in directions lying in the base plane. The clamp has a compound movement that simultaneously forces the gas turbine component blank against stops that prevent movement in orthogonal directions lying in the base plane. The gas turbine component blank is thereafter shaped, preferably by grinding the sides of the root precursor of the gas turbine component blank.

Description:
This invention relates to fixturing to support a gas turbine component blank, and more specifically, to clamping the gas turbine component blank in the fixture and shaping the root of the gas turbine component blank. 
   BACKGROUND OF THE INVENTION 
   In the most commonly practiced approach, turbine blades for gas turbine engines are cast to approximately the final shape. Then portions of the turbine blade, such as the root and the shroud, if any, are shaped to the final desired form by a technique such as grinding. The turbine blade is thereafter processed by depositing protective coatings or by other procedures. 
   The finished turbine blades are assembled into a turbine disk or wheel, with a “dovetail” form on the root of each turbine blade engaging a respective conformably shaped slot on the turbine disk. The turbine disk is in turn supported on a shaft in the gas turbine engine. The turbine blades must have precisely established positions and angular orientations in the turbine disk. Any mispositioning and misorientation may lead to aerodynamic inefficiency and the introduction of unacceptable vibrations in the turbine disk and the turbine blade as the turbine disk turns during service. 
   Because it is the root of each turbine blade that engages the slot on the turbine disk, the root must be shaped very precisely. Two techniques have been widely used to hold the turbine blade in an exact location and orientation for the shaping of the root. In one, the airfoil of the turbine blade is cast into a matrix of a metal with a low melting point, which is used to hold the turbine blade with its root precursor positioned for grinding or other shaping. This approach, while operable, requires that the low-melting-point metal be cleaned from the surface of the airfoil after the shaping of the root is completed. Even traces of the metal remaining after careful cleaning of the surface of the airfoil may adversely affect the subsequent application of the coatings. Mechanical fixtures or jigs have been developed to hold the turbine blade. These fixtures avoid the use of the low-melting-point metal, but have not been fully satisfactory because they misposition the root precursor or because they do not hold the turbine blades sufficiently repeatably and securely so that each root is shaped the same. 
   There is a need for an improved approach to the shaping of the roots of turbine blades and other gas turbine components. The present invention fulfills this need, and further provides related advantages. 
   SUMMARY OF THE INVENTION 
   The present invention provides a fixture for holding a gas turbine component blank, such as a turbine blade, a compressor blade, or some types of vanes, in a specific fixed position for the shaping of the gas turbine component blank, and a method for performing the shaping. The approach does not use a molten metal whose complete removal is difficult. The fixture holds the gas turbine component blank using features of the gas turbine component blank whose positions are precisely defined. This approach allows each gas turbine component blank to be processed precisely, quickly, securely, reproducibly, without contamination, and with minimal dependence upon operator skill. 
   A fixture is provided for clamping a gas turbine component blank that is to be shaped into a corresponding gas turbine component. The gas turbine component blank comprises an airfoil having a direction of elongation, and a platform extending transversely to the airfoil and having a top side adjacent to the airfoil and a bottom side oppositely disposed from the top side. The gas turbine blank includes a root precursor at the first end of the airfoil and extending away from the airfoil, wherein the root precursor has a pair of oppositely disposed parallel ends, and a pair of sides which are to be shaped into a dovetail form. There is a rotating shroud located at a second end of the airfoil and extending transversely to the airfoil. 
   The fixture is used in conjunction with this gas turbine component blank. The fixture comprises a base lying in a base plane and having a datum locator. The datum locator includes an x-axis datum locator upon which the gas turbine component blank is supported so that the direction of elongation of the airfoil is generally parallel to the base plane. The x-axis datum locator prevents movement of the gas turbine component blank perpendicular to the base plane. A y-axis datum locator comprises a first y-axis stop and a second y-axis stop, wherein the first y-axis stop is contacted by a first one of the ends of the root precursor and the second y-axis stop is contacted by the rotating shroud. A z-axis datum locator is contacted by the gas turbine component blank and prevents movement of the gas turbine component blank parallel to the direction of elongation of the airfoil. The fixture further includes a clamp movable between an unclamped position in which the gas turbine component blank may be inserted onto the x-axis datum locator of the base, and a clamped position wherein the clamp simultaneously forces the first end of the root precursor against the first y-axis stop, the rotating shroud against the second y-axis stop, and the gas turbine component blank against the z-axis datum locator. 
   The clamp preferably comprises a compound mechanical movement that simultaneously forces the gas turbine component blank against the y-axis datum locator and the z-axis datum locator when the clamp is moved from the unclamped position to the clamped position. Most preferably, the clamp comprises a first link pivotably connected to the base and contacting to the root precursor and to the platform when the clamp is in the clamped position, so as to force the first end of the root precursor against the first y-axis stop and to force the platform against the z-axis datum locator, and a second link pivotably connected to the base and contacting to the rotating shroud when the clamp is in the clamped position, so as to force the rotating shroud against the second y-axis stop, the first link having a sliding and pivoting interconnection to the second link. The first link desirably includes a z-positioning spring contacting the bottom side of the platform when the clamp is in the clamped position to force the top side of the platform against the z-axis datum locator. The sliding and pivoting interconnection is preferably a mechanical knuckle. An hydraulic actuator is operable to move the clamp between the unclamped position and the clamped position. 
   Stated alternatively, a fixture is provided for clamping a gas turbine component blank having an airfoil, a root precursor at a first end of the airfoil, and a rotating shroud at a second end of the airfoil. The fixture comprises a base lying in a base plane and having a datum locator. The datum locator includes an x-axis datum locator upon which the gas turbine component blank is supported to prevent movement of the gas turbine component blank perpendicular to the base plane. A y-axis datum locator comprises a first y-axis stop and a second y-axis stop, with the y-axis datum locator preventing movement of the gas turbine component blank in a first direction lying in the base plane. A z-axis datum locator prevents movement of the gas turbine component blank in a second direction orthogonal to the first direction and lying in the base plane. A clamp is movable between an unclamped position in which the gas turbine component blank may be inserted onto the x-axis datum locator of the base, and a clamped position wherein the clamp simultaneously forces the root precursor against the first y-axis stop, the rotating shroud against the second y-axis stop, and the gas turbine component blank against the z-axis stop. Various modifications and preferred forms as discussed above may be used with this embodiment. 
   A method for shaping a gas turbine component blank comprises the steps of providing the gas turbine component blank and fixture as discussed, thereafter placing the gas turbine component blank into the fixture with the clamp in the unclamped position, thereafter operating the clamp to move the clamp to the clamped position, and thereafter shaping the gas turbine component blank. The step of shaping preferably includes the step of shaping the sides of the root precursor into the dovetail form, most preferably by grinding. 
   After the gas turbine component blank is cast and cleaned, the root precursor must be shaped on both its lateral sides, termed the pressure surfaces, and on its end remote from the airfoil, termed the tang. The present fixture may be used to hold the gas turbine component blank for the shaping of the pressure surfaces, while is performed first, and another fixture is used to hold the gas turbine component blank for the shaping of the tang and the final shaping of the end of the root precursor. The present approach provides a convenient fixturing approach which avoids the use of molten metal and also ensures that the gas turbine component blank is properly and securely positioned for shaping of the root precursor, particularly the pressure surfaces of the root precursor. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block flow diagram of an approach for practicing the invention; 
       FIG. 2  is an elevational view of a turbine blade; 
       FIG. 3  is a top perspective view of a fixture in which the turbine blade is held for grinding, but without the turbine blade present; 
       FIG. 4  is a top view of the clamp; 
       FIG. 5  is a top view of the turbine blade mounted in the fixture, with the clamp arms in the unclamped position; 
       FIG. 6  is a top view of the turbine blade mounted in the fixture, as in  FIG. 5 , but with the clamp arms in the clamped position; and 
       FIG. 7  is a detail of  FIG. 6 , showing the z-positioning spring. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts a method for processing a gas turbine component blank. The gas turbine component blank is provided, numeral  20 . Referring to  FIG. 2 , a preferred form of the gas turbine component blank  40  is a turbine blade blank  42  that is processed into a turbine blade. The articles are referred to as “blanks” because they are furnished in a cast and cleaned form that must be given a final shaping, usually by grinding, to achieve the final required shape, and are thereafter final processed. Other gas turbine components such as compressor blades and vanes may, in appropriate cases, be processed according to the approach of  FIG. 1  as well. 
   The turbine blade blank  42  is described in relation to an orthogonal reference system including an x-axis  44 , a y-axis  46 , and a z-axis  48 . The turbine blade blank  42  includes an elongated airfoil  50  extending generally parallel to the z-axis  48  and having an airfoil face  52 . A platform  54  extends transversely to the z-axis  48  at a first end  56  of the airfoil  50 . The platform  54  has a top side  58  adjacent to the airfoil  50  and a bottom side  60  oppositely disposed from the top side  56 . A root precursor  62  is at the first end  56  of the airfoil  50  and extends away from the airfoil  50  along the z-axis  48 . The root precursor  62  has a pair of oppositely disposed parallel ends  64  lying perpendicular to the y-axis  46 , a pair of sides  66  (only one of which is visible in  FIG. 2 ) which are to be shaped into a dovetail form, and a tang  68  at an end of the turbine blade blank  42  remote from the airfoil  50 . A rotating shroud  70  is at a second end  72  of the airfoil  50 , remote from the first end  56 . The rotating shroud  70  generally extends transversely to the z-axis  48  and along the y-axis  46 . The rotating shroud  70  is fixed in relation to the airfoil  50 , and is cast integrally with the airfoil  50 . The shroud is termed a “rotating shroud” not because it rotates relative to the airfoil  50 , but because it rotates with the remainder of the turbine blade about the shaft of the gas turbine engine. The rotating shroud is contrasted to a stationary shroud that is found in the gas turbine engine but is not a part of the turbine blade. 
   A fixture  80  is provided to clamp and hold the gas turbine component blank  40  during subsequent processing, numeral  22 . A preferred form of the fixture  80  is illustrated in  FIGS. 3-7 , with and without the gas turbine component blank  40  present. The fixture  80  includes a base  82  that lies generally in a base plane containing the y-axis  46  and the z-axis  48 . The remainder of the fixture  80  is affixed to and supported on the base  82 . The base  82  has an x-axis datum locator  84 , a y-axis datum locator  86  including a first y-axis stop  88  and a second y-axis stop  90 , and a z-axis datum locator  92 . As used herein, a “datum locator” is a feature of the base  82  that serves as a positioning and fixed locating stop and against which the gas turbine component blank  40  is pushed and held fixed by the clamping structure to be discussed subsequently. The datum locators  84 ,  86 , and  92  are preferably precisely located supports, surfaces, or shoulders machined into the base  82 . Hard inserts may be affixed to the surfaces of the datum locators that are contacted by the gas turbine component blank  40  to avoid excessive wear of the datum locators. The x-axis datum locator  84  includes several, and typically at least three, x-axis stops  94  that are positioned to receive the gas turbine component blank  40  thereon with one side of the airfoil face  52  in a general facing relationship to the base  82   
   The fixture  80  further includes a clamp  96  ( FIG. 4 ) movable between an unclamped position ( FIG. 5 ) in which the gas turbine component blank  40  may be inserted onto the x-axis datum locator  84  of the base  82 , and a clamped position ( FIG. 6 ) wherein the clamp simultaneously forces a first end  98  of the root precursor  62  against the first y-axis stop  88 , a first end  99  of the rotating shroud  70  against the second y-axis stop  90 , and some part of the gas turbine component blank  40 , preferably the top side  58  of the platform  54 , against the z-axis datum locator  92 . 
   To accomplish this simultaneous clamping of the gas turbine component blank  40  against the y-axis datum locator  86  and the z-axis datum locator  92 , the clamp  96  preferably includes a compound mechanical movement  100 , most easily seen in  FIG. 4 , that simultaneously forces the gas turbine component blank against the y-axis datum locator  86  and the z-axis datum locator  92  when the clamp  96  is moved from the unclamped position of  FIG. 5  to the clamped position of  FIG. 6. A  preferred form of the compound mechanical movement  100  comprises an asymmetric Y-shaped yoke  102  that is connected to the base  82 . 
   A first link  104  is pivotably connected to one arm of the yoke  102  at a first pivot point  106 , which is pivotably connected to the base  82 . A contact region  108  of the first link  104  contacts to a second end  110  of the root precursor  62  when the clamp is in the clamped position, so as to force the first end  98  of the root precursor  62  against the first y-axis stop  88  with a hard metal-to-metal contact. The first link  104  further includes a z-positioning spring  112 , preferably in the form of a leaf spring integral with the first link  104 , contacting the bottom side  60  of the platform  54  when the clamp  96  is in the clamped position to force the top side  58  of the platform  54  against the z-axis datum locator  92 . This contacting is shown in detail in FIG.  7 . 
   A second link  114  is pivotably connected to the other arm of the yoke  102  at a second pivot point  116 , which is pivotably connected to the base  82 . A contact region  118  of the second link  114  contacts to a second end  119  of the rotating shroud  70 , which is oppositely disposed to the first end  99 , when the clamp  96  is in the clamped position of  FIG. 5 , so as to force the first end  99  of the rotating shroud  70  against the second y-axis stop  90 . The contact region  118  preferably comprises a spring in the form of a leaf spring. The first link  104  has a sliding and pivoting interconnection  120  to the second link  114 . The sliding and pivoting interconnection  120  preferably comprises a mechanical knuckle  122 . 
   An hydraulic actuator  124  is operable to move the clamp  96  between the unclamped position of FIG.  5  and the clamped position of FIG.  6 . The hydraulic actuator  124 , which preferably is a liquid-driven actuator to generate a large clamping force but may be a gas-driven pneumatic actuator, desirably controllably generates a force between the base  82  and in this case one side of the yoke  102 , on the one hand, and the first link  104  to cause the first link  104  to pivot about the first pivot point  106 , on the other hand. The second link  114 , through the movement of the interconnection  120 , responsively pivots about the second pivot point  116 . Thus, movement of the single hydraulic actuator  124  generates all of the clamping movements and forces required to clamp the gas turbine component blank  40  in the fixture  80 . 
   To use the fixture  80 , the hydraulic actuator  124  is retracted so that the first link  104  pivots (counterclockwise in  FIG. 4 ) and the second link  114  pivots (clockwise in  FIG. 4 ) to the unclamped, open position of FIG.  5 . The gas turbine component blank  40  is placed into the fixture  80  so that the gas turbine component blank  40  rests upon and is supported by the x-axis datum locator  84 , numeral  24 . The hydraulic actuator  124  is then operated and extended so that the first link  104  pivots (clockwise in  FIG. 4 ) and the second link  114  pivots (counterclockwise in  FIG. 4 ) to the clamped position of FIG.  6 . As the links  104  and  114  pivot to the clamped position, three clamping actions occur. First, the z-positioning spring  112  contacts the bottom side  60  of the platform  54  and forces the gas turbine component blank  40  upwardly so that the top side  58  of the platform  54  is resiliently clamped against the z-axis datum locator  92 . Second, the contact region  118  of the second link  114  contacts the second end  119  of the rotating shroud  70  and forces the rotating shroud  70  along the y-axis  46  (in the negative y direction) so that the first end  99  of the rotating shroud  70  is resiliently clamped against the second y-axis stop  90 . A resilient clamping of the gas turbine component blank  40  against the z-axis datum locator  92  and the second y-axis stop  90  is sufficient because relatively small forces are applied through these locations during subsequent shaping of the gas turbine component blank  40 . Third, the contact region  108  contacts the second end  110  of the root precursor  62  and forces the first end  98  of the root precursor  62  hard (non resiliently) against the first y-axis stop  88 . The root precursor  62  is thus clamped held tightly and securely, with hard metal-to-metal contacts, between the first y-axis stop  88  and the contact region  108  of the first link  104 . This hard, metal-to-metal clamping of the root precursor  62  along the y-axis  46  resists the shaping (grinding) forces produced during the subsequent shaping operation. 
   Once the gas turbine component blank  40  is clamped into the fixture  80 , the gas turbine component blank  40  is shaped, numeral  28  of FIG.  1 . The shaping using the fixture  80  is preferably the shaping of the sides  66  of the root precursor  62  to define the dovetail form required to affix the final root, and thence the completed turbine blade, to the turbine disk in the gas turbine engine. The shaping  28  is accomplished by any operable approach, but preferably grinding using a creep feed grinder and grinding technique is used. The grinding direction is generally parallel to the y-axis  46 , which is the reason that the secure metal-to-metal clamping of the root between the first y-axis stop  88  and the contact region  108  is required. The creep feed grinder takes relatively large bites of material with each pass, typically on the order of 0.20 inches per pass, and the grinding tool moves rapidly with respect to the root precursor  62 , typically on the order of 45 inches per minute. The forces transmitted to the root precursor  62  and thence to the gas turbine component blank  40 , and the vibrations potentially introduced into the gas turbine component blank  40 , by the creep feed grinder are therefore relatively large. The root precursor  62  must therefore be clamped very securely by the fixture  80  against movement of the root precursor  62  in the y-direction  46 , and the present fixture  80  provides that secure support of the gas turbine component blank  40 . During grinding, a liquid coolant/lubricant is forced around the area of the root precursor  62  being ground. The fixture  80  serves to channel and direct the flow of the coolant/lubricant to the exact location where the grinding tool is contacting the metal of the root precursor  62 , improving the cooling and lubrication of the root precursor  62 . 
   After the shaping  28  is complete, the hydraulic actuator  24  is retracted to move the clamp  96  to the unclamped position, and the gas turbine component blank  40  is removed from the fixture  80 , numeral  30 . The gas turbine component blank  40  is thereafter further processed, numeral  32 , for any of several reasons and by any of several approaches. There may be further shaping of the root precursor  62 , as for example to shape the tang  68 . There may be coating of the airfoil  50  with protective coatings. Other further processing may be used as desired. 
   The present approach of  FIG. 1  has been practiced using prototype fixturing  80  and found to be fully operable. The clamping of the root precursor  62  in the fixture  80  was found to be more secure and rigid than when the metal encapsulation approach is used, allowing faster grinding rates. 
   Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.