Abstract:
A machine cutter including a cutting bit positionable in a slot of a machine; a drive shaft rotatably driving the bit; a bracket having a plurality of angular positions for supporting and pivoting the bit wherein bit pivots about a pivot point on the bracket proximate to the bit, and a frame supporting the bracket on a slidable support providing linear movement to the bit.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/551,352, filed on Mar. 10, 2004, and incorporates by reference the entirety of that provisional application.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to the repair of cracks in teeth of an industrial rotary machine and machining tools for making such repairs. In particular, the invention relates to cutting machines which when mounted on a rotor facilitate relief cutting of rotor teeth to remove cracks in the teeth.  
         [0003]     Large industrial power generators, such as those that are driven by steam gas turbines, have rotors with longitudinal slots that receive conductive winding coils. The slots are cut lengthwise into the surface of the cylindrical rotor. Rows of slots are arranged around the poles of the rotor. The teeth of the rotor are the metal fingers between the slot rows. The teeth typically extend the length of the rotor, and are parallel to each other and to the rotor axis. The sides of adjacent teeth form the slots. In cross-section, each slot has a dovetail profile near the rotor surface and a rectangular profile radially inward of the dovetail. Conductive coils are stacked in the rectangular portion of the slots.  
         [0004]     Wedges are inserted above the coils and into the dovetail section of the slots. The sides of the wedges abut against the sides of the pair of adjacent rotor teeth that define the slot in which the wedge is inserted. The wedges are aligned end-to-end in each slot. They secure the underlying coils in the slot. The wedges in some slots are formed of a hard metal, such as steel, which wears against the sides of the rotor teeth. The wedges in other slots are formed of a soft metal, such as aluminum.  
         [0005]     During long term operation of the generator (such as during decades of operation), the dovetail surfaces on rotor teeth may crack due wear of the wedges abutting against the teeth. Cracks tend to form in teeth that have slots that had been capped with hard metal wedges. Cracks are most likely to form on the upper surfaces of the dovetail section of the a rotor tooth. The cracks typically occur in the dovetail surfaces adjacent to an end-to-end joint of wedges.  
         [0006]     If not repaired, small cracks in a rotor tooth may propagate circumferentially through the tooth and to an adjacent slot. Rotors are periodically inspected, such as every five, ten or twenty year, to determine if cracks have formed in their teeth. The inspection of the rotor requires the generator to be taken offline, the rotor removed from the stator and the retaining rings removed from the ends of the rotor. A crack detection probe, e.g., an eddy current probe, is passed over the surface of the rotor. If cracks are detected in a slot, the wedges and windings are removed from the slot to expose the sides of the teeth and the crack.  
         [0007]     Repair of cracks in rotor teeth is done by a machinist that machines metal out of the surface of the rotor tooth to remove the crack and smooth the tooth surface surrounding the crack. Prior techniques for machining rotor teeth to remove cracks have required labor intensive machining operations that may require the rotor to be offline for over a dozen of days. Such long offline periods are extremely expensive in lost power generation production. There is a long felt need for techniques to assist machinist in repairing cracks in rotating machines, and to reduce the number of days needed to repair the cracks in a rotor.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0008]     The invention may be embodied as a machine tool comprising: a cutting bit positionable in a slot; a drive shaft rotatably driving the bit; a bracket having a plurality of angular positions for supporting the drive shaft and bit, wherein the shaft and bit are pivotable attached to the bracket at a pivot point proximate to the bit, and a frame supporting the bracket on a slidable support, said slidable support providing movement to the bit in a direction in a plan common to a pivoting plane of the bit.  
         [0009]     The invention may include a machine cutting tool comprising: a cutting bit positionable in a slot of a machine; a bracket having a plurality of angular positions for supporting and pivoting the bit, wherein bit pivots about a pivot point on the bracket proximate to the bit, and a frame supporting the bracket on a slidable support providing linear movement to the bit in a plane parallel a pivoting plane of the bit.  
         [0010]     The invention may be embodied as machine cutting tool comprising: a cutting bit positionable in a slot; a drive shaft rotatably driving the bit; a cutting assembly slidably supports the bit and drive shaft such that the bit moves reciprocally with respect to the assembly; a bracket supporting the cutting assembly at a plurality of angular positions, wherein cutting assembly pivots with respect to the bracket about a pivot point proximate to the bit, and a frame supporting the bracket wherein the frame is slidably attached to the bracket.  
         [0011]     The invention may also be embodied as a machine tool that is mounted in a rotor slot and is capable of performing four relief cuts on the sides of the slot from a single tool position. In particular, the machine tool includes a frame having in-line gripping feet that latch to the sides of a rotor slot to position a cutting bit adjacent a crack in the teeth that define the slot. The tool allows the cutting bit to move axially, transversely across the slot and pivot to make two radial and two oblique cuts in the surfaces of the teeth on either side of the slot.  
         [0012]     The invention may be also embodied as a method for machining a crack in a slot defined by opposite teeth, said method comprising: mounting a cutter in said slot such that the cutter is axially aligned with the slot; aligning a cutting bit of the cutter with a section of the opposite teeth to be machined; aligning the cutting bit with a radial line of the slot; moving the cutting bit along a linear path transverse to a slot axis to engage a first tooth of the opposite teeth; machining a radial relief cut in said first tooth; moving the cutting bit along said linear path to a second sooth of the opposite teeth; machining a second radial relief cut in said second tooth; pivoting the cutting bit in a plane parallel to the linear path to a first oblique angle; moving the cutting bit along said linear path to the first tooth; machining a first oblique relief cut in said first tooth; pivoting the cutting bit in said plane to a second oblique angle; moving the cutting bit along said linear path to the second tooth, and machining a second oblique relief cut in said second tooth.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is an enlarged cross-sectional view of a rotor showing a portion of a rotor tooth, slot, winding and wedge.  
         [0014]      FIG. 2  is a perspective side view of a dovetail section a rotor tooth.  
         [0015]      FIG. 3  is an enlarged perspective side view of a dovetail section of a rotor tooth with a wedge (shown in cross-section) adjacent the tooth.  
         [0016]      FIG. 4  is schematic side view of a first tooth machining device, showing features of the device some of which would be otherwise hidden from an outside view.  
         [0017]      FIG. 5  is a schematic top view of the tooth machining device.  
         [0018]      FIGS. 6, 7  and  8  are schematic front views of the tooth machining device showing the cutting jig in left tilt, center and right tilt positions, respectively.  
         [0019]      FIG. 9  is a schematic diagram showing a partial cross-section of a rotor having dovetail slots and a rotor tooth cutting frame having two cutting heads mounted thereon, which is an alternative embodiment to a rotor tooth cutting machine to that shown in FIGS.  4  to  8 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]      FIG. 1  is an enlarged cross-sectional view of the dovetail section  10  of a rotor slot  12  that is cut radially inward along the longitudinal length of a rotor  14 . Conductive windings  16  are mounted in the rotor slot  12 . A wedge  18  caps the winding in the slot and mounts in the slot at the dovetail section  10 . The wedge  18  has extended side surfaces that engage the rotor slot surfaces at the dovetail sections  20 . The metal row between adjacent slots are the teeth  19  of rotor. The side surfaces of the teeth define the slots. The dovetail surfaces  20  of the rotor teeth tend to be heavily loaded by centrifugal and vibration forces because wedges  18  abut against the dovetail surfaces  20  of the teeth  19 .  
         [0021]     Over the course of many years of rotational operation of a power generator, the vibration and forces acting between the wedge and the rotor teeth can induce cracks in the dovetail sections  20  of the teeth. In particular, inspection of certain rotors which have been in continuous operation for decades identified approximately a third of rotors as having cracks in the dovetail section of certain teeth. The cracks can be detected by periodic, e.g., every decade, inspection of the rotor using devices such as eddy current probes. To inspect the rotor for cracks, it is necessary to remove the rotor from the generator and remove the retaining ring from the rotor. When a crack is detected, the wedges (and possibly the coil windings) are removed from at least those slots in which a crack has been detected and may be removed from all slots in a rotor. Cracks tend to form on the dovetail tooth upper surface  22  which is an upper surface of the dovetail surface  20  of the tooth. If the propagation of the crack is sufficiently shallow in the tooth surface, the crack can be removed by machining away a small portion of the rotor tooth at the dovetail surface where the crack is located. The machining removes the crack and smoothes the surface of the tooth in the area surrounding the crack.  
         [0022]      FIG. 2  is a side perspective view of the side surfaces of a rotor tooth  19  and, in particular, the dovetail section  10  of the tooth. The upper dovetail surfaces  22  tend to be the surfaces where cracks occur. A series of three relief cuts  24 ,  26  and  28  are made into the tooth surface to remove a crack. Radially relief cut  24  extends radially from the top surface  30  of the rotor, along the radial upper side surface of the tooth and to the top edge  32  of the dovetail section  20  of the tooth. The relief cut is semi-cylindrical in shape and has a depth corresponding to or greater than the depth of the crack being removed. The width and depth of the relief cut  24  may be selected by the machinist removing the crack.  
         [0023]     An oblique relief cut  26  is machined in the upper dovetail surface  22  from the top edge  32  of the dovetail to the outer corner  34  of the dovetail. The oblique relief cut is radially aligned with the radial relief cut  24  and extends along the width of the upper dovetail surface  22  of the tooth, where cracks most likely form. The oblique relief cut may be at an angle of 45 degrees with respect to a radial line through the rotor axis. The width and depth of the oblique relief cut  26  is often substantially the same as the width and depth of the radial relief cut  24 . However, the width and depth of the three relief cuts  24 ,  26  and  28  may vary somewhat depending on the type of crack to be removed and the machine cut settings selected by the operator. The cross-sectional shape of the relief cuts  24 ,  26  tend to be a shallow semi-circular cut that provides smooth surfaces on the slot wall. Moreover, the outer edges  29  of the radial cuts may be feathered to avoid sharp transitions between the uncut surfaces of the slot and the relief cut.  
         [0024]     The lateral relief cut  28  is oblique to the radial end and oblique relief cuts  24  and  26 . The lateral relief cut  28  is at the outer dovetail corner  34 . The lateral relief cut extends laterally from either side of the oblique relief cut and is parallel to an axis of the rotor. The length of the lateral relief cut  28  is selected by the machinist and depends on the depth and extent of the crack. The lateral relief cut may have its greatest depth at its center  36  and become gradually shallower towards the opposite ends  38  of the lateral relief cut. The center  36  of the lateral relief cut is radially aligned with the radial relief cut and oblique relief cut. The width of the lateral relief cut may have a semi-cylindrical groove shape similar to the shapes of the radial and oblique relief cuts.  
         [0025]      FIG. 3  is a perspective view of the sides of a rotor tooth and particularly the dovetail section  10  of the tooth. In addition, a cross-section of a wedge  18  is shown inserted into the slot defined by the side surfaces of the tooth. The radial relief cut  24 , oblique relief cut  26  and lateral relief cut  28  are shown in conjunction with the wedge  18  to show the interface between the relief cuts and the wedge. The series of three relief cuts  24 ,  26  and  28  may be made on both of opposite sides of the adjacent teeth that define the slot. The opposite sets of three relief cuts are made in both sides of the same slot and at the same lateral location along the slot.  
         [0026]     The relief cuts  24 ,  26  and  28  provide stress relief on the surfaces of the dovetail section  20  of the tooth  19  where a crack had previously formed. Because of the relief cuts, the wedge  18  applies substantially less vibrational and centrifugal forces to the surface  22  of the dovetail where the crack had previously formed.  
         [0027]      FIG. 4  is a side view of a rotor tooth machine cutter  40 .  FIG. 5  is a top view of the cutter  40 . The tooth cutter  40  comprises a rotating cutting bit  42  that includes helical cutting surfaces for machining the rotor tooth. The cutting bit is replaceable and the type of bit is selected by the machinist. The length (l) of the cutting bit  42  is sufficient to extend the length of the desired radially and oblique relief cut. The tool cutter  40  is adapted to perform the radial and oblique relief cuts. The tool cutter  40  does not perform the a lateral relief cut. The cutting bit  42  is detachably mounted to a shaft  44  that extends radially upwards to a drive motor  46 . The drive motor  46  may be driven pneumatically and actuated manually by a machinist grasps the handle of the drive motor. The drive motor fits into a collar  48  on a bracket  50  of the cutter. The bracket  50  supports the motor  46  and drive shaft  44 . The bracket  50  includes a first arm  54  that extends radially outward and includes the collar  48  and includes a base  56  that is rectangular in top view. The bracket  50  supports a drive shaft housing  58  that is a column extending around the drive shaft  44  and down to the cutting bit  42 . The drive shaft column  58  includes an aperture coaxial with and to receive the drive shaft  44 . The drive shaft  44  moves reciprocally along the path shown in arrow  60  to provide reciprocal movement of the drive shaft and bit  42  along the axis of the drive shaft  44  and as indicated by arrow  60 .  
         [0028]     The motor bracket  52  and drive shaft housing  58  are attached to fan shaped support brackets  62 . The fan support brackets  62  support the cutting assembly  52  of cutting bit  42 , drive shaft  44 , drive motor  46 , motor bracket and drive shaft housing  58 . The fan support brackets  62  enables the cutting assembly  52  to pivot about pivot point  64  which is perpendicular and aligned with the cutting bit  42 . The pivot point  64  allows the cutting bit to be pivoted about its center point so as to allow the cutting angle to be changed without substantially translating the position of the cutting bit. The pivot point  64  may be slightly offset from the center point of the cutting bit depending on the reciprocal position of the cutting bit along its axis. The pair of support brackets  62  are arranged on opposite sides of the cutting assembly  52 .  
         [0029]     The pivot position of the cutting bit  42  is set by thumb screws  66  which engage recesses  68  (see FIGS.  6  to  8 ) on the fan support bracket  62 . The recesses  68  are arranged at angular positions on the bracket  62  corresponding to the cutting angle for the radial and oblique relief cuts to be performed on the two surfaces. For example, a recess  68  may be at a 90° angle which is in radial alignment with the rotor axis. In addition, there may be recesses  68  arranged at oblique angles of 45° so as to tilt the bit  42  to make the oblique relief cut. The number and angular position of the recesses  68  may be determined based on the radial and oblique relief cuts intended to be made in the tooth surface.  
         [0030]     The fan support bracket  62  is fixed to a jig base  70 . The jig base positions the cutting assembly  52  and fan brackets  62  and particularly the cutting bit  42  within a slot of the rotor such that cutting bit  42  are axially aligned with the slot. The jig base  70  includes a frame  71  that supports a pair of opposite fan support brackets  62  on a sliding rail  72 . The rails  72  allows the cutting assembly  52 , and including the cutting bit  42 , to slide transversally from one side of a rotor slot to the opposite side. This transverse movement of the cutting bit  42  allows the bit to engage and cut opposite tooth. The cutter tool  40  is able to make two radial relief cuts and two oblique cuts while the base  70  is at one position in the slot.  
         [0031]     The side to side transverse movement of the cutting bit  42  across a rotor slot and the reciprocal movement of the cutting bit  42  along its axis  60  may be manually adjusted by the machinist and automated by pneumatic valves  73 . The pneumatic valves  73  are coupled to a source of compressed air  74 . The pneumatic valves when activated apply a uniform force to the transverse movement of the cutting bit  42  and/or the axial movement  60  of the cutting bit. The transverse and/or axial movement of the cutting bit is selected by the machinist performing the machining operation. The pneumatic valves allow the actual cutting of the rotor teeth to be performed automatically moving the cutting bit as it engages the rotor teeth. Mechanical hard stops  76  that limit the transverse movement of the cutting bit  42  and/or the axial movement of the cutting bit. These stops are adjustable and set by the machinist in order to control the depth at which the cutting bit  42  cuts into the tooth. By adjusting the stops  76 , the machinist can set the depth of the relief cuts to be made to the rotor tooth.  
         [0032]     The jig base  70  includes feet  78  that fit into the slot of a rotor. The feet are laterally spaced from the cutting bit  42  by several inches. The feet spread apart and walk into the slot of the rotor. The outer surfaces of the feet  78  may have a dovetail surface which fits into the dovetail surfaces of the opposite teeth of the slot. These feet may spread apart by manual operation of a spreading device  80 . Once the feet are spread apart, the jig foot  70  is locked into the rotor slot such that the jig is aligned with the longitudinal axis of the slot. The feet securely hold the jig base and hence the cutting assembly  52  in the slot during machining operations.  
         [0033]     In operation, the machinist manually inserts the tooth cutter  40  into the slot such that the bit  42  is adjacent the crack to be repaired. The feet  78  are spread apart to secure the jig foot  70  to the slot. With the cutting bit  42  aligned with the crack in the tooth to be removed, the cutting bit is placed in a radial position with respect to the fan plate  62  so as to perform the radial relief cut. The cutting bit  42  is rotated by the motor  46  and then brought into cutting contact to the tooth by activating the pneumatic valves  73 . The cutting bit  42  may be moved by transverse movement across the slot to the rotor tooth and/or by axial movement along the axis  60  of the drive shaft.  
         [0034]     After the radial relief cut is performed on one tooth, the cutting bit  42  is slid transversely across the slot along rails  72  to cut a radial relief cut in the opposite tooth. The depth of the radial relief cuts in the two opposite teeth are determined by the position of the hard stops  76  as set by the machinist.  
         [0035]     Once the radial relief cuts are completed, the machinist pivots the cutting bit  42  by turning the thumb screw  66  and positioning the cutting assembly  52  at a 45 degree angle and setting the thumb screw in an appropriate recess  68  on the fan support bracket  62 . Once the cutting head is aligned obliquely with the surface of the tooth to be machined, the pneumatic valves  73  are activated and the cutting bit is moved automatically into cutting position against the tooth until the hard stops  76  are reached by the cutting bit at which point the cutting stops. Once the first oblique relief cut is made in the side of one tooth, the machinist repositions the alignment jig  50  such that the cutting bit  42  is repositioned to an opposite 45 degree angle for cutting the tooth on the opposite side of the slot. The jig  50  is repositioned by turning the thumb screw  66  and pivoting the jig  50  about the fan support bracket  62 .  
         [0036]     The tooth cutter  40  provides a cutting mechanism for performing radial relief cuts and oblique relief cuts in both teeth on opposite sides of a rotor slot. The cutter ensures that the cutting bit  42  is positioned properly within the slot and against the teeth surfaces so as to perform the four related radial and oblique cuts on opposite teeth in an accurate and expedited manner. The cutter  40  allows several sets of relief cuts to be performed along the slot of a rotor by repositioning the feet  78  at different positions in the slot corresponding to cracks in the teeth. The tooth cutter  40  is sufficiently mobile that it may be carried manually by a machinist to the rotor and positioned in those slots which require machining.  
         [0037]      FIG. 9  shows an alternative tooth cutting mechanism to the cutter  140 .  FIG. 9  shows in partial cross-section, a rotor on which is mounted a cutting frame  140  having a pair of opposite arms  142  which support respective cutting assemblies  144 . The arms  142  are connected at one end by a pivot joint  146 . The arms are supported by a brace  148  that is attached at its opposite ends to each of the respective arms  142 . The brace  148  fixes the angle between the two arms, for example at 120 degrees. The brace includes a micrometer depth gauge  150  that positions the brace and arms  142  above the rotor surface. The micrometer depth gauge  150  accurately and precisely determines the height above a center location  152  on the rotor surface e.g., rotor pole, for the brace  148  and the arm pivot point  146 . By adjusting the micrometer depth gauge  150 , a machinist can accurately position the pivot point and hence the cutting assemblies  144  above the rotor surface. The brace  148  also includes adjustable positioning blocks  154  to align the brace and arms  142  circumferentially on the rotor. The blocks  154  slide equidistantly away from the center of the template which is aligned with the microdrive. The blocks extend radially inward into a slot  10  beneath the brace. The blocks align the brace and the arms  142  circumferentially with respect to the rotor  14 . The positioning of the blocks on the brace  148  is set such that the blocks  154  abut against the side surfaces of their respective rotor slots.  
         [0038]     Opposite ends  156  of the arms  142  have respective micrometer depth gauges  158 . These micrometer depth gauges  158  position the end  156  of the arm  142  above the surface of the rotor. Normally, the arms  142  will each be positioned an equal distant height above the rotor surface during cutting operation. In the micrometer depth gauge is mounted to the end  156  by an arm by a triangular bracket  160 .  
         [0039]     The cutting tool frame  140  formed by the arms  142 , brace  148  and micrometer depth gauges  150 ,  158  provide a platform which supports the cutting heads  144  as they are aligned with a slot  10  and in particular, a crack in a slot. The cutting assemblies may provide reciprocal, pivoting and transverse movement to the cutting bit  42 , similar to the cutting assembly  52 , fan bracket  62  and frame  71  shown in  FIG. 4 . The frame  140  may be secured to the rotor by straps that fit around the rotor. The frame  140  may be moved axially along the length of the rotor to position one or more of the cutting heads  144  adjacent a crack in a slot.  
         [0040]     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.