Abstract:
A boring machine including a rotatable boring bar carrying a tool holder that is movable radially of the boring bar, wherein a sensor is provided on the tool holder to sense the proximity thereof to a work piece having a variable radius of curvature in a plane perpendicular to the boring bar.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a boring machine for turbine casings having caulked-in seal fins. 
       BACKGROUND OF THE INVENTION 
       [0002]    Axial flow steam turbines generally include fixed stator blades, rotating rotor blades, between-blade seal fins, and a casing surrounding them. The seal fins are commonly retained in circumferential grooves in the casing by caulking wire. 
         [0003]    The fins lose their sealing efficiency through wear and need to be refurbished to maintain turbine performance. This typically involves removing the turbine to a workshop, separating the casing into half-casings, and removing both the stator and rotor blades to access the seal fins and caulking wire. The old fins are then machined down to access the caulking wire which is carefully machined out to avoid damaging the grooves of the half-casings. Replacement fins and caulking wire are then mounted in the grooves, and the replacement fins are machined down to precise clearances from the rotor shaft and blades. 
         [0004]    Precision machining of seal fins and caulking wire is extremely difficult because the half-casings deform when the casing split joint (or horizontal half-joint) is opened due to stress changes created after years of use. This causes the radius of curvature of the grooves, and hence the radial position of the seal fins and caulking wire, to differ at different points around the inner diameter of the half-casings. For example, the split half-casings can be up to 1.5 mm out of round. 
         [0005]    It would be advantageous to refurbish caulked-in seal fins on-site where the turbine is normally operated, and with the stator blades in situ in the half-casings. 
         [0006]    A need therefore exists for a boring machine for on-site refurbishment of seal fins caulked in turbine half-casings having a variable radius of curvature. 
       SUMMARY OF THE INVENTION 
       [0007]    According to the present invention, there is provided a boring machine including a rotatable boring bar carrying a tool holder that is movable radially of the boring bar, wherein a sensor is provided on the tool holder to sense the proximity thereof to a work piece having a variable radius of curvature in a plane perpendicular to the boring bar. 
         [0008]    The boring machine can further include a motor to move the tool holder radially, and a controller to control the motor in response to the sensor. 
         [0009]    The controller can be programmable so that in use a tool held by the tool holder is controllably moved radially by the motor to follow the variable radius of curvature of the work piece at a preselected working depth during rotation of the boring bar. 
         [0010]    The tool holder can be carried by a carriage that is movable axially along the boring bar. 
         [0011]    The boring bar can be rotatable between supports adapted to removably mount in a turbine half-casing. 
         [0012]    The work piece can be a seal fin and/or an adjacent caulking wire therefor in a groove in the turbine half-casing. 
         [0013]    The tool holder can exchangeably hold a rotatable disc having an abrasive and/or cutting periphery. 
         [0014]    The present invention also provides a method of refurbishing seals fins caulked in a half-casing of a turbine using the above described boring machine. 
         [0015]    The half-casing can include stator blades, and the method can be performed with the stator blades in situ in the half-casing. 
         [0016]    The method can be performed on-site where the turbine is normally operated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be further described by way of example only with reference to the accompanying drawings, in which: 
           [0018]      FIGS. 1 and 2  are side and perspective views of a boring machine of one embodiment of the invention positioned in a turbine half-casing; 
           [0019]      FIG. 3  is a perspective view of a carriage of the boring machine; 
           [0020]      FIG. 4  is a perspective view of a tool holder of the boring machine with a rim cutter, and cutting and grinding discs; 
           [0021]      FIGS. 5 and 6  are sections through the peripheries of the cutting and grinding discs; 
           [0022]      FIG. 7  is a perspective view of a radial-feed drive for the tool holder; 
           [0023]      FIG. 8  is a section through a turbine half-casing showing the probe of the boring machine positioned against the inner diameter of the turbine half-casing; 
           [0024]      FIGS. 9 to 11  are sections through the turbine half-casing showing sequential operation of the boring machine to trim old seal fins, cut caulking wires, and trim replacement seal fins; and 
           [0025]      FIG. 12  is a detail of seal fins caulked in the turbine half-casing. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Referring to  FIG. 1 , a boring machine of an embodiment to be described includes a rotatable boring bar  1  carried at each end in bearing supports  2 ,  3  which are removably mounted in the inlet and outlet of a steam turbine half-casing  38  via frames  4 ,  5 . The bearing supports  2 ,  3  are adjustable to locate the boring bar  1  perpendicular to the grooves in the half-casing  38  and, preferably, aligned with the central axis of the half-casing  38 . (Note that the sub-components of the turbine half-casing  38  are omitted from  FIG. 1  for clarity.) 
         [0027]    Referring to  FIGS. 2 and 9  to  12 , the sub-components of the half-casing  38  include radially inwardly extending stator blades  39  and seal fins  40 . The seal fins  40  are retained between the stator blades  39  in circumferential grooves in the half-casing  38  by caulking wire  41 . (Note that the seal fins  40  and caulking wire  41  are omitted from  FIG. 2  for clarity.) 
         [0028]    The boring bar  1  is driven by a rotary drive which includes a variable speed electric motor  6  connected to a power source  7 . The variable speed motor  6  is connected to the boring bar  1  by a gear box  8 , torque reaction arms  9 , and a bar end coupling  10 . Other equivalent rotary drives may also be used. In use, the boring bar  1  is rotated at a milling speed, for example, 150 to 500 mm per minute around the inner diameter of the half-casing  38  being refurbished. 
         [0029]    A carriage  11  is manually movable axially along the boring bar  1  by a lead screw assembly  31 ,  32  and locked in the desired axial position. As best seen in  FIG. 3 , the lead screw assembly  31 ,  32  is externally mounted to the boring bar  1  by clamps  12 ,  13 , and the carriage  11  is mounted to the boring bar  1  by carriage clamps  14 ,  15 . The external mounting of the lead screw assembly  31 ,  32  allows for easy replacement of the boring bar  1 . Other equivalent but more complicated mounting arrangements may also be used, for example, the lead screw assembly  31 ,  32  may be keyed to, or arranged inside, the boring bar  1 . 
         [0030]    Although not illustrated, a key extends along the length of the boring bar  1  so that the weight of the carriage  11  is supported by the key when the carriage clamps  14 ,  15  are released. A tool holder  20  is rigidly mounted to the carriage  11  by four shafts  19  mounted to a plate  17 . The tool holder  20  is movable radially inwardly and outwardly by runner blocks  16  that slide on the shafts  19 . 
         [0031]    As best seen in  FIG. 4 , the tool holder  20  rotatably carries a shaft  24  parallel to the boring bar  1  close to the inner diameter of the stator blades. The shaft  24  is rotatably driven at one end by a hydraulic drive  25 . Other equivalent drives for the shaft  24  may also be used, for example, an electric drive. The free end of the shaft  24  holds a machining tool, for example, a rim cutter  36 , a narrow-rimmed cutting and/or abrasive disc  26 , and a square-rimmed cutting and/or abrasive disc  27 .  FIGS. 5 and 6  illustrate the respective profiles of the peripheries of discs  26 ,  27  with the solid dark regions representing coatings of an abrasive and/or cutting material, for example, diamond, boron trinitride, etc. The rim cutter  36  and discs  26 ,  27  are exchangeably held on the shaft  24  by a holding plate  28 . 
         [0032]    The tool holder  20 , and hence a tool held thereby, is controllably movable radially inwardly and outwardly relative to the carriage  11  on the boring bar  1 . Referring to  FIG. 7 , this controlled radial movement is provided by a ball screw nut  18  and ball screw rod  21  which is rotatably driven inwardly and outwardly from the plate  17  by a servo or stepper motor  23  via pulleys  22 . The servo motor  23  is mounted on the opposite side of the carriage  11  to the tool holder  20  so as to provide a compact, space-saving arrangement. The radial position of the tool holder  20  relative to the warped or distorted inner diameter of the half-casing  38  is monitored by a sensor  29 , for example, a linearly variable displacement transformer (LVDT) or a digital probe. Referring to  FIG. 8 , the sensor  29  is positioned on the tool holder  20  so as to be as close as practical to seal fins  40  and/or caulking wire  41  being machined. 
         [0033]    In use, the sensor  29  provides input to a controller  30  which controls the radial position of the tool holder  20  in response to the sensor  29  by controlling the servo motor  23  to rotate the ball screw or threaded lead screw  21  so as to maintain a constant distance between the working face or edge of a tool held by the tool holder  20  and inner diameters of the half-casing  38 , the caulked-in seal fins  40 , and their caulking wire  41 . Other equivalent arrangements to control relative radial movement and positioning of the tool holder  20  and tools held thereby may also be used. 
         [0034]    The controlled radial movement of the tool holder  20  provided by the sensor  29  and the controller  30  advantageously automatically adjusts the working depth of tools held by the tool holder  30  to accommodate circumferential warping or distortion in the half-casing  38 . In addition, the sensor  29  and the controller  30  also automatically adjust the working depth of tools held by the tool holder  30  to accommodate axial warping or distortion in the boring bar  1  itself. The boring bar  1  can therefore be made thinner and less rigid, and need not be precisely rotatably supported between complicated bearings. 
         [0035]    Referring to  FIG. 9 , refurbishment of caulked-in seal fins  40  starts by machining down or trimming old caulked-in seal fins  40  using a rim cutter  36  fitted to the shaft  24  of the tool holder  20 . Although not illustrated in  FIG. 4 , cutting blades are received in radial slots on both faces of the disc of the rim cutter  36 . The cutting blades are adjustable radially inwardly and outwardly to allow the rim cutter  36  to accommodate seal fins  40  having different inner diameters. The controlled radial movement of the tool holder  20  allows the rim cutter  36  to follow the variable inner diameter of old seal fins  40  in a warped half-casing  38 . The cross-sectional width of the disc of the rim cutter  36  is less than the spacing of the stator blades  39 . This allows old caulked-in seal fins  40  to be cut down to short stubs with the stator blades  39  in situ in the half-casing  38 . The compact arrangement of the shaft  24  in the tool holder  20  keeps the shaft  24  clear of the inner diameter of the stator blades  39  while old caulked-in seal fins  40  undergo trimming by the rim cutter  36 . 
         [0036]    Referring to  FIG. 10 , after the old seal fins  40  have been trimmed to allow access to the old caulking wires  41 , the rim cutter  36  is replaced with the narrow-rimmed disc  26  to machine out the caulking wires  41 . The narrow-rimmed disc  26  can be an abrasive or cold saw disc. For example, an abrasive disc is preferred if the caulking wire work hardens. The diameter of the disc  26  is preselected so that it can cut the caulking wire  41  without touching the boring bar  1 . For example, the disc  26  may be selected from a range of discs  26  having mutually different working or cutting diameters. In use, the tool holder  20  remains clear of the stator blades  39  with sufficient clearance from the carriage  11  for one size of disc  26  to accommodate a range of stator blade  39  and seal fin  40  inner diameters. 
         [0037]    To cut and machine out the caulking wire  41 , the tool holder  20  is driven above the half-joint face of the half-casing  38  and a correct diameter disc  26  is fitted to the shaft  24 . A high speed hydraulic motor  37  is fitted via mount plate  35  for abrasive cutting, while a low speed hydraulic motor and gear box  25  may also be fitted via mount plate  34  for cold sawing. The disc  26  is lowered close to the half-joint inner edge and the carriage clamps  14 ,  15  are released so that the weight of the machining assembly is supported by the key along the boring bar  1 . Next, the carriage  11 , the tool holder  20  and the disc  26  are moved axially along the boring bar  1  using the manually cranked lead screw  31 ,  32 ,  33  until the cutting edge of the disc  26  is positioned over the caulking wire  41  to be cut. The carriage clamps  14 ,  15  are then re-tightened. 
         [0038]    The boring bar  1  is then rotated slowly and the disc  26  is adjusted radially using either manual control or electronic manual feed until the disc  26  is just touching the caulking wire  41 . The hydraulic motor  37  is started and the disc  26  is fed into the desired depth, for example, around 20 to 50% of the depth of the caulking wire  41 , or about 0.2 to 0.7 mm. The feed-in depth of the disc  26  is indicated by a linear scale (not shown) which indicates the radial travel of the tool holder  20 . The linear scale is zeroed and the set depth is then shown as a direct reading on the linear scale. The sensor  29  and the controller  30  are then set to automatically maintain the disc  26  at the desired constant cutting or working depth. Next, the boring bar  1  begins rotation and starts to circumferentially move the disc  26  around the inner diameter of the half-casing  38 . The controller  30  controls the servo motor  23  in response to the sensor  29  so that the caulking wire  41  is cut by the disc  26  to a constant depth regardless of warping or distortion of the half-casing  38 . As mentioned above, the boring bar  1  is positioned perpendicular to the grooves of the half-casing  38  before the machining starts. 
         [0039]    With the caulking wire  41  cut down and weakened, both the wire  41  and the associated seal fin  40  can be pulled out of the groove in the half-casing  38 . The tool holder  20  is then returned by running back on the running blocks  16  through the same cut to the starting point. The cutting sequence can be selectively varied, for example, two small or shallow half-circle cuts can be made in alternate rotations, or a single large or deep half-circle cut can be made. For example, after completing one half-circle through the casing, the disc  26  can be realigned to the next caulking wire  41  and the disc  26  moved through one half-circle in the reverse direction so as to cut a second wire  41  and return to the original angular starting point. Alternatively, the disc  26  can be driven through one full rotation to make one half-circle cut in each caulking wire  41 . 
         [0040]    After removal of old caulking wires  41  and old seal fins  40 , replacement seal fins  40  can be fitted, and replacement wires  41  caulked into the grooves in the half-casing  38 . The inner circumferential edges of the replacement seal fins  40  then need to be ground to create a standardised clearance fit between the replacement seal fins  40  and the rotor shaft and blades (not shown), for example, a H7 fit. This is performed by changing the disc  26  with a disc  27  having a square rim of abrasive material, as illustrated in  FIG. 11 . The disc  27  is aligned over at the top edge of a replacement seal fin  40 . With the disc  27  spinning, it is allowed to touch the seal fin  40  and is fed circumferentially, for example, by about 15 mm to set the position of the disc  27  over the seal fin  40 . The depth of the freshly cut (but undercut) height of the seal fin  40  is measured relative to the inner diameter of the half-casing  38 . The tool holder  20  is fed the required radial distance to create the desired designed-in clearance or fit, with the disc  27  above the seal fin  40  to be ground. The disc  27  is then fed radially down to start the grinding of the seal fin  40 . As soon as the first few millimetres of contact with the seal fin  40  are made, rotation of the boring bar  1  rotation is stopped and the linear scale, sensor  29  and controller  30  are set. The disc  27  is then moved around the half-casing  38  to automatically trim the seal fin  40  to the required set height tolerance regardless of circumferential warping or distortion in the half-casing  38 . The half-casing  38  may return very close to its original circular shape when re-bolted together with the other half-casing (not shown), so small additional tolerances may be provided to ensure that any residual casing warping or distortion does not cause fouling between the seal fins  40  and the rotors, or vice versa. 
         [0041]    Although not illustrated, the boring machine of the invention may also be used to refurbish seal fins caulked in circumferential grooves in the rotor shaft between adjacent rotor blades. In this application, the seal fins and caulking wires of the rotor shaft are machined by the boring machine while the rotor shaft is rotated in a precision lathe, thereby removing the need for automatic axial adjustment of the tool holder of the boring machine. 
         [0042]    Embodiments of the invention therefore enable refurbishment of caulked-in seal fins in turbine half-casings having a variable radius of curvature around their inner diameter due to warping or distortion. Advantageously, the caulked-in seal fins are refurbished on-site where the turbine is normally operated, and with the stator blades in situ in the turbine half-casings. 
         [0043]    It will be appreciated that the invention is suitable for use with any and all conventional turbines having caulked-in seal fins. 
         [0044]    The above embodiment has been described by way of example only and modifications are possible within the scope of the claims which follow.