Abstract:
The actual axial center position of a helical gear relative to the axial center position of a rotary table is calculated on the basis of input gear dimensions and information from a touch probe. Correction values for the positions and motions of the rotary table and a grindstone are calculated on the basis of the actual axial center position of the helical gear.Operation values for the rotary table, a column, a saddle, and a grindstone head are calculated by adding the correction values to reference values for the positions and motions of the rotary table and the grindstone. Motors are controlled in such a way that operation is carried out at the operation values to thereby carry out form grinding.

Description:
TECHNICAL FIELD 
     The present invention relates to a gear grinding machine and a gear grinding method and is effective particularly when used to grind large-sized gears having helical tooth traces such as large-sized helical gears and crossed helical gears. 
     BACKGROUND ART 
     Gear grinding machines are configured to sequentially grind the tooth surfaces of a gear mounted on a rotary table by rotating the gear and inserting the circumference of a disk-shaped grinding wheel, which is being driven to rotate, into each tooth space of the gear. In this way, the gear&#39;s meshing accuracy can be improved. Such a gear grinding machine cannot properly grind the tooth surfaces of the gear when the axis of the gear is displaced (offset) with respect to the axis of the rotary table. Thus, the axis of the rotary table and the axis of the gear need to be accurately positioned to each other (centering). However, in a case of machining a large-sized gear measuring several meters in diameter, the centering cannot be done easily because such a gear is extremely heavy, weighing several tons. 
     In this respect, for example, Patent Literature 1 listed below and the like disclose the following. Specifically, a rotary table is rotated to measure the runout of a gear to thereby read, from a graph, the relation between the value of the position of the axis of the gear (eccentricity) and the phase position. As position vectors associated with the center of the rotary table, the following are calculated: the eccentricity at the center in the face width; the crossing angle between the axis of the rotary table and the axis of the gear; the eccentricity with respect to any planes perpendicular to the axis of the rotary table; and the phase angle. Then, the above adjustment data thus obtained are displaced perpendicular to the axis of the rotary table so that the axis of the rotary table and the axis of the gear can intersect at the center in the face width, for example. Then, the axis of the rotary table and the axis of the gear are tilted by the crossing angle between these two axes, followed by the moving of a tool in accordance with such speeds, positions, and paths that the guiding axis of the tool and the axis of the gear coincide, to thereby machine the gear. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Publication No. 2003-191131 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The gear machining method described in Patent Literature 1 listed above and the like is capable of machining a spur gear having straight tooth traces along with the correction. However, there is no description regarding accurate machining of a gear having helical tooth traces such as a helical gear or a crossed helical gear. Moreover, in profile grinding of a helical gear, a crossed helical gear, or the like, the machining accuracy is affected by the position of the grinding wheel, errors in operations involving synchronization of the motion of the grinding wheel and the rotation of the gear, and other factors. For this reason, the profile grinding needs to be performed while correcting these factors, as well. 
     In view of the above, it is an object of the present invention to provide a gear grinding machine and a gear grinding method capable of achieving an improved machining accuracy in profile grinding of a gear with helical tooth traces using a grinding wheel for profile grinding. 
     Solution to Problem 
     A gear grinding machine according to the present invention for solving the above problems is a gear grinding machine for performing profile grinding of a gear with a disk-shaped grinding wheel, the gear having a helical tooth trace and mounted on a rotary table with an axis thereof oriented in a vertical direction, and is characterized in that the gear grinding machine comprises: rotary-table driving means for rotating the rotary table about an axis extending in the vertical direction; grinding-wheel rotating means for rotating the grinding wheel with a circumference of the grinding wheel facing the gear on the rotary table; displacement measuring means for measuring displacement of the gear on the rotary table in a direction perpendicular to a direction of the axis of the gear; X-axis direction moving means for moving the grinding wheel and the displacement measuring means in a depth direction of a tooth space of the gear on the rotary table; Y-axis direction moving means for moving the grinding wheel and the displacement measuring means in a horizontal direction perpendicular to the depth direction of the tooth space of the gear on the rotary table; Z-axis direction moving means for moving the grinding wheel and the displacement measuring means in the vertical direction; and controlling means for: actuating the rotary-table driving means and the axis direction moving means in such a way that the displacement measuring means measures a position of a circumference of the gear on the rotary table in the direction perpendicular to the direction of the axis of the gear, at a plurality of points in a circumferential direction and an axial direction of the gear, calculating an actual position of the axis of the gear with respect to a position of an axis of the rotary table on the basis of information from the displacement measuring means; calculating correction values for a position and motion of each of the rotary table and the grinding wheel on the basis of the actual position of the axis of the gear, the correction values, in the profile grinding, cancelling out differences, caused by the actual position of the axis of the gear, from reference values of a position and motion of the rotary table in the profile grinding performed in a condition where the axis of the gear coincides with the axis of the rotary table and reference values of a position and motion of the grinding wheel corresponding to a rotational phase of the rotary table in the condition; adding the correction values to the reference values to calculate operation values of the rotary-table driving means and operation values of the X-axis direction moving means, the Y-axis direction moving means, and the Z-axis direction moving means corresponding to the rotational phase of the rotary table; and controlling the rotary-table driving means, the X-axis direction moving means, the Y-axis direction moving means, and the Z-axis direction moving means in such a way as to actuate the rotary-table driving means, the X-axis direction moving means, the Y-axis direction moving means, and the Z-axis direction moving means in accordance with the operation values, to perform the profile grinding of the gear with the grinding wheel. 
     Further, in the gear grinding machine described above, the gear grinding machine according to the present invention is characterized in that the controlling means calculates the actual position of the axis of the gear with respect to the position of the axis of the rotary table by: calculating, on the basis of the information from the displacement measuring means, a value of eccentricity εu of an upper side of the gear with respect to the position of the axis of the rotary table on a horizontal plane in a state where the rotary table is at a reference phase, and a value of eccentricity εd of a lower side of the gear with respect to the position of the axis of the rotary table on the horizontal plane in the state where the rotary table is at the reference phase; and calculating, on the basis of the values of eccentricity εu and εd, a value of eccentricity εm of a center portion, in the axial direction, of the gear with respect to the position of the axis of the rotary table on the horizontal plane in the state where the rotary table is at the reference phase, and angles of inclination Σx and Σy of the axis of the gear with respect to the axis of the rotary table on vertical planes in the state where the rotary table is at the reference phase. 
     Furthermore, in the gear grinding machine described above, the gear grinding machine according to the present invention is characterized in that: on the basis of the value of eccentricity εm, the controlling means calculates correction values for a horizontal position of a profile-grinding start point, the horizontal position being influenced by offset of the axis of the gear, and correction values for a horizontal motion associated with a motion in a Z-axis direction in the profile grinding; and on the basis of the angles of inclination Σx and Σy, the controlling means calculates a correction value for displacement of a position in the vertical direction caused by the inclination of the axis of the gear, correction values for displacement of a motion in the horizontal directions associated with a motion in the vertical direction in the profile grinding operation, the motion in the vertical direction being influenced by the inclination of the axis of the gear, and correction values for a motion in the horizontal directions associated with a motion in the vertical direction, the correction values correcting a tooth profile error caused by displacement of a tooth-profile grinding line of the grinding wheel on a tooth surface of the gear due to the influence of the inclination of the axis of the gear. 
     A gear grinding method according to the present invention for solving the problems described above is a gear grinding method for grinding a gear with a disk-shaped grinding wheel, the gear having a helical tooth trace and mounted on a rotary table with an axis thereof oriented in a vertical direction, and is characterized in that the gear grinding method comprises: calculating an actual position of the axis of the gear with respect to a position of an axis of the rotary table by measuring a position of a circumference of the gear on the rotary table in a direction perpendicular to a direction of the axis of the gear, at a plurality of points in a circumferential direction and an axial direction of the gear; calculating correction values for a position and motion of each of the rotary table and the grinding wheel on the basis of the actual position of the axis of the gear, the correction values, in the profile grinding, cancelling out differences, caused by the actual position of the axis of the gear, from reference values of a position and motion of the rotary table in a condition where the axis of the gear coincides with the axis of the rotary table and reference values of a position and motion of the grinding wheel corresponding to a rotational phase of the rotary table in the condition; adding the correction values to the reference values to calculate operation values of the position and motion of the rotary table and operation values of the position and motion of the grinding wheel corresponding to the rotational phase of the rotary table; and actuating the rotary table and the grinding wheel in accordance with the operation values to perform the profile grinding of the gear with the grinding wheel. 
     Advantageous Effects of Invention 
     In the gear grinding machine and the gear grinding method according to the present invention, when the axis of the gear is eccentric (offset) with respect to the axis of the rotary table, the coordinates of the position of the profile-grinding start point changes from one tooth space of the gear to another tooth space indexed. Accordingly, the grinding start point moves in a fluctuating manner. Thus, for each tooth space of the gear indexed, the horizontal position of the grinding wheel is corrected so as to follow the fluctuations. Moreover, the axis of the gear moves from one tooth space of the gear to another tooth space indexed, in synchronization with the phase of the rotary table. Accordingly, the profile grinding point moves in such a way as to swing (helical turn) on one and the other sides in the face width direction, i.e. on upper and lower sides. Thus, along with the motion of the grinding wheel in the vertical direction, the horizontal motion of the grinding wheel is corrected in such a way as to follow the swinging grinding point. In addition, when the axis of the gear is inclined with respect to the axis of the rotary table, the tooth-profile grinding line of the grinding wheel on the tooth surface of the gear is displaced from a reference tooth-profile grinding line. Accordingly, the contact between the grinding wheel and the tooth surface fails to be in a prescribed state. This results in an error in the tooth surface of the gear after its profile grinding. Thus, based on the angles of inclination of the axis of the gear with respect to the axis of the rotary table, the motion of the grinding wheel in the horizontal directions and the rotational motion of the rotary table are further corrected in association with the motion of the grinding wheel in the vertical direction in such a way as to cancel out the error in the tooth surface. Accordingly, it is possible to perform accurate profile grinding of a large-sized gear having helical tooth traces with a disk-shaped grinding wheel, even when the axis of the gear is displaced with respect to the axis of a rotary table. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic side view of a main embodiment of a gear grinding machine according to the present invention. 
         FIG. 2  is a front view seen from the position of a II-II line in  FIG. 1 . 
         FIG. 3  is a block diagram of a control system of the gear grinding machine in  FIG. 1 . 
         FIG. 4  is a flowchart of the control system of the gear grinding machine in  FIG. 1 . 
         FIG. 5  is a view illustrating a procedure for measuring the amount of displacement of a gear in the gear grinding machine in  FIG. 1 . 
         FIG. 6  is a view illustrating various types of values to be calculated by the gear grinding machine in  FIG. 1 . 
         FIG. 7  is a view illustrating the path of the outer circumference of the gear turning with the axis of the gear being offset with respect to the axis of a rotary table. 
         FIG. 8  is a view illustrating how profile grinding is done on the tooth surface of the gear in a state where the axis of the gear is inclined with respect to the axis of the rotary table. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinbelow, an embodiment of a gear grinding machine and a gear grinding method according to the present invention will be described based on the drawings. However, the present invention is not limited only to the following embodiment to be described based on the drawings. 
     &lt;Main Embodiment&gt; 
     A main embodiment of a gear grinding machine and a gear grinding method according to the present invention will be described based on  FIGS. 1 to 8 . 
     As shown in  FIG. 1 , a rotary table  112  being rotatable about the Z axis, i.e. an axis extending in the vertical direction is provided on one side, in the X-axis direction, of the top of abed  111 . On the rotary table  112 , a large-sized (a diameter of several meters and a weight of several tons) helical gear  10  having bosses  10   a  and  10   b  is mounted with its axis being oriented in the Z-axis direction, i.e. the vertical direction. 
     As shown in  FIGS. 1 and 2 , a column  113  being movable in the X-axis direction, i.e. the depth direction of a tooth space of the helical gear  10  on the rotary table  112  is provided on the other side, in the X-axis direction, of the top of the bed  111  with rails  111   a  therebetween. A saddle  114  being movable in the Z-axis direction, i.e. the vertical direction is provided on the front face (the face on the rotary table  112  side) of the column  113  with rails  113   a  therebetween. A turn head  115  being turnable about the X axis is provided on the front face of the saddle  114 . A grinding-wheel head  116  being movable in the Y-axis direction, i.e. a horizontal direction perpendicular to the depth direction of a tooth space of the helical gear  10  on the rotary table  112  is provided on the front face of the turn head  115  with a rail  115   a  therebetween. 
     On one side, in the Y-axis direction, of the front face of the grinding-wheel head  116 , there is provided a grinding-wheel spindle part  117  having a grinding-wheel shaft  117   a  with its axis extending along a vertical plane which is parallel to the Y-axis direction. A disk-shaped grinding wheel  118  for profile grinding is attached to the grinding-wheel shaft  117   a  of the grinding-wheel spindle part  117  coaxially, i.e. with its circumference facing the rotary table  112  side. To the other side, in the Y-axis direction, of the front face of the grinding-wheel head  116 , attached is the base end of an arm  119  projecting in such a way that its tip is located closer to the rotary table  112  side than the grinding wheel  118  is. A touch probe  120  being displacement measuring means for detecting displacement of the helical gear  10  in the X-axis direction which is a direction perpendicular to the direction of the axis of the helical gear  10  is attached to the tip of the arm  119 . 
     As shown in  FIG. 3 , the touch probe  120  is electrically connected to an input part of a control device  121  being controlling means. An output part of an input unit  122  being inputting means for performing operations including input of various conditions such as the specifications of the helical gear  10  is electrically connected to the input part of the control device  121 . 
     An output part of the control device  121  is electrically connected to: a motor  112 M being rotary-table driving means for rotating the rotary table  112  about the Z axis; a motor  113 M configured to move the column  113  horizontally in the X-axis direction; a motor  114 M configured to move the saddle  114  up and down in the Z-axis direction; a motor  115 M configured to turn the turn head  115  about the X axis, i.e. to turn the axis of the grinding wheel  118  along a vertical plane; a motor  116 M configured to slidingly move the grinding-wheel head  116  in the Y-axis direction; and a motor  117 M configured to rotate the grinding-wheel shaft  117   a  of the grinding-wheel spindle part  117  about the axis of the spindle rotation. The control device  121  is capable of controlling the actuation of the motors  112 M to  117 M on the basis of information inputted from the touch probe  120  and the input unit  112 , and the like (details will be described later). 
     Note that in this embodiment, the bed  111 , the column  113 , the motor  113 M, and the like constitute X-axis direction moving means; the saddle  114 , the motor  114 M, and the like constitute Z-axis direction moving means; the turn head  115 , the motor  115 M, and the like constitute grinding-wheel turning means; the grinding-wheel head  116 , the motor  116 M, and the like constitute Y-axis direction moving means; and the grinding-wheel spindle part  117 , the motor  117 M, and the like constitute grinding-wheel rotating means. 
     Next, a gear grinding method using the above gear grinding machine  100  according to this embodiment will be described. 
     First, the helical gear  10  is mounted onto the rotary table  112  as coaxially as possible. In addition to this, various conditions such as the specifications of the helical gear  10  (diameter, width, face width, the number of teeth, helix angle, and the like) are inputted to the input unit  112  (S 1  in  FIG. 4 ). In response, the control device  121  actuates the motor  116 M to move the touch probe  120  from the other side toward the one side in the Y-axis direction along the rail  115   a  by means of the grinding-wheel head  116  and the arm  119  so that the touch probe  120  can be located at a center portion, in the Y-axis direction, of the column  113 . Then, the control device  121  actuates the motors  113 M and  114 M to move the column  113  and the saddle  114  along their respective rails  111   a  and  113   a  so that the tip of the touch probe  120  can contact the outer circumferential surface of the boss  10   a  on the upper side of the helical gear  10  in a direction perpendicular to the rotational axis of the rotary table  112  (axis C) (see Part A of  FIG. 5 ). 
     When the tip of the touch probe  120  contacts the outer circumferential surface of the boss  10   a  of the helical gear  10 , the control device  121  figures out the position of the helical gear  10  in the X-axis direction at the current phase position on the basis of information from the touch probe  120 . Then, the control device  121  actuates the motor  113 M to slightly move the column  113  backward along the rails  111   a  so that the tip of the touch probe  120  can momentarily be disengaged from the boss  10   a  of the helical gear  10 . Thereafter, the control device  121  actuates the motor  112 M to rotate the rotary table  112  through a predetermined angle (e.g. 45°). 
     Subsequently, the control device  121  actuates the motor  113 M to move the column  113  forward along the rails  111   a  so that the tip of the touch probe  120  again can contact the outer circumferential surface of the boss  10   a  of the helical gear  10 , and again figures out the position of the helical gear  10  in the X-axis direction at the current phase position. Then, the control device  121  actuates the motor  113 M to again move the column  113  backward along the rails  111   a  so that the tip of the touch probe  120  again can be disengaged from the boss  10   a  of the helical gear  10 . Thereafter, the control device  121  actuates the motor  112 M to again rotate the rotary table  112  through the predetermined angle (e.g. 45°). 
     Subsequently, the actuations described above are repeated given times corresponding to the predetermined angle (e.g. eight times) (see Part C of  FIG. 5 ). As a result, the control device  121  measures the amount of displacement, in the X-axis direction, of the boss  10   a  on the upper side of the helical gear  10  entirely in the circumferential direction of the helical gear  10  (S 2  in  FIG. 4 ). Based on this measurement, the control device  121  calculates the position of the axis of the upper side of the helical gear  10  on the X-Y plane (horizontal plane) in the rotation of the rotary table  112 . Specifically, the control device  121  calculates: a distance eu of a line connecting the axis of the rotary table  112  (axis C) and the axis of the upper side of the helical gear  10  in the rotation of the rotary table  112 ; and an angle Φu of the line connecting the axis of the rotary table  112  (axis C) and the axis of the upper side of the helical gear  10  in the rotation of the rotary table  112 , within an X-Y plane (horizontal plane) coordinate system having the axis of the rotary table  112  (axis C) at the center. In other words, the control device  121  calculates a value of eccentricity (value of offset) cu of the upper side of the helical gear  10  with respect to the position of the axis of the rotary table  112  (axis C) on the X-Y plane (horizontal plane) in a state where the rotary table  112  is at a reference phase (S 3  in  FIG. 4 ; see Part A of  FIG. 6 ). 
     Then, as in the case of the boss  10   a  on the upper side, the control device  121  actuates the motors  113 M and  114 M to move the column  113  and the saddle  114  along their respective rails  111   a  and  113   a  so that the tip of the touch probe  120  can contact the outer circumferential surface of the boss  10   b  on the lower side of the helical gear  10  (see Part B of  FIG. 5 ). Thereafter, the same actuations as described above are performed. Thereby, the control device  121  measures the amount of displacement, in the X-axis direction, of the lower side of the helical gear  10  entirely in the circumferential direction of the helical gear  10  (S 4  in  FIG. 4 ). Based on this measurement, the control device  121  calculates the position of the axis of the lower side of the helical gear  10  on the X-Y plane (horizontal plane) in the rotation of the rotary table  112 . Specifically, the control device  121  calculates: a distance ed of a line connecting the axis of the rotary table  112  (axis C) and the axis of the lower side of the helical gear  10  in the rotation of the rotary table  112 ; and an angle Φd of the line connecting the axis of the rotary table  112  (axis C) and the axis of the lower side of the helical gear  10  in the rotation of the rotary table  112 , within the X-Y plane (horizontal plane) coordinate system having the axis of the rotary table  112  (axis C) at the center. In other words, the control device  121  calculates a value of eccentricity (value of offset) εd of the lower side of the helical gear  10  with respect to the position of the axis of the rotary table  112  (axis C) on the X-Y plane (horizontal plane) in the state where the rotary table  112  is at the reference phase (S 5  in  FIG. 4 ; see Part B of  FIG. 6 ). 
     Then, the control device  121  calculates the position of the axis of a center portion, in the axial direction (face width direction), of the helical gear on the X-Y plane (horizontal plane), from the position of the axis of the boss  10   a  on the upper side on the X-Y plane (horizontal plane) and the position of the axis of the boss  10   b  on the lower side on the X-Y plane (horizontal plane). Specifically, the control device  121  calculates a distance em as the mean of the distances eu and ed, an angle Φm as the mean of the angles Φu and Φd, and a value of eccentricity (value of offset) εm in a two dimensional space as the mean of the values of eccentricity εu and εd (S 6  in  FIG. 4 ). In addition, the control device  121  calculates the angles of inclination of the axis of the helical gear  10  with respect to the axis of the rotary table  112  (axis C) on vertical planes (a three dimensional space) in the state where the rotary table  112  is at the reference phase; to be specific, the control device  121  calculates an angle of inclination Σx of the X-axis direction component and an angle of inclination Σy of the Y-axis direction component with respect to the Z axis (axis C) (S 7  in  FIG. 4 ; see Part C of  FIG. 6 ). As a result, the control device  121  calculates the actual position of the axis of the helical gear  10  with respect to the position of the axis of the rotary table  112  (axis C). 
     After calculating the actual position of the axis of the helical gear  10  as described above, that is, after calculating the value of eccentricity (value of offset) εm of the helical gear  10  in the two dimensional space and the angles of inclination Σx and Σy in the three dimensional space, the control device  121  calculates correction values for the position and motion of the rotary table  112  and correction values for the position and motion of the grinding wheel  118  on the basis of the value of eccentricity (value of offset) εm and the angles of inclination Σx and Σy (S 8  in  FIG. 4 ). In profile grinding, the correction values cancel out the differences, caused by the actual position of the axis of the helical gear  10 , from reference values of the position and motion of the rotary table  112  in profile grinding performed in a condition where the axis of the helical gear  10  coincides with the axis of the rotary table  112  (axis C), and reference values of the position and motion of the grinding wheel  118  corresponding to the rotational phase of the rotary table  112  in the above-described condition. 
     Specifically, based on the value of eccentricity (value of offset) εm, the control device  121  calculates (1) correction values for the X- and Y-axis (horizontal) position (two dimensional position) of the profile-grinding start point, the position being influenced by the offset of the axis of the helical gear and (2) correction values for an X- and Y-axis (horizontal) motion (two dimensional motion) associated with a motion in the Z-axis direction unique in the profile grinding (two dimensional eccentricity correction values). Further, based on the angles of inclination Σx and Σy, the control device  121  calculates (3) correction values for displacement of positions in the Z-axis direction (vertical direction) and about the axis C (on the horizontal plane) (three dimensional positions) caused by the inclination of the axis of the helical gear  10 , (4) correction values for displacement of a motion in the X- and Y-axis (horizontal) directions (three dimensional position) associated with a motion in the Z-axis direction (vertical direction) (three dimensional motion) in the profile grinding operation, the motion in the Z-axis direction being influenced by the inclination of the axis of the helical gear  10 , and (5) correction values for motions in the X- and Y-axis directions and about the axis C (on the horizontal plane) (three dimensional motions) associated with a motion in the Z-axis direction (vertical direction) (three dimensional motion), the correction values correcting tooth profile errors caused by displacement of the tooth-profile grinding line of the grinding wheel  118  on the tooth surface of the helical gear  10  due to the influence of the inclination of the axis of the helical gear  10  (three dimensional eccentricity correction values). 
     Meanwhile, along with the calculation of the above correction values of the helical gear  10 , the control device  121  actuates the motor  113 M to momentarily retreat the column  113 , and then causes the touch probe  120  to be located on the other side, in the Y-axis direction, of the column  113 , that is, actuates the motor  116 M to move the touch probe  120  from the one side toward the other side in the Y-axis direction along the rail  115   a  by means of the grinding-wheel head  116  and the arm  119  so that the grinding wheel  118  can be located on the center portion, in the Y-axis direction, of the column  113 . As a result, the touch probe  120  is moved to a retreat position, and the grinding wheel  118  is located at an operation start position. 
     Then, after calculating the correction values, the control device  121  adds the correction values to the reference values to calculate the actual operation values of the position and motion of the rotary table  112  and also to calculate the actual operation values for the position and motion of each of the column  113 , the saddle  114 , and the grinding-wheel head  116  corresponding to the rotational phase of the rotary table  112  (S 9  in  FIG. 4 ). Then, the control device  121  performs the profile grinding of the helical gear  10  with the grinding wheel  118  by controlling the motors  115 M and  117 M to actuate the turn head  115  and the grinding wheel  118  in accordance with the reference values mentioned above, and also by controlling the motors  112 M to  114 M and  116 M to actuate the rotary table  112 , the column  113 , the saddle  114 , the turn head  115 , and the grinding-wheel head  116  in accordance with the operation values mentioned above (S 10  in  FIG. 4 ). 
     Specifically, as shown in  FIG. 7 , when the axis of the helical gear  10  is eccentric (offset) with respect to the axis of the rotary table  112  (axis C), the coordinates of the position of the profile-grinding start point change from one tooth space of the helical gear  10  to another tooth space indexed. Accordingly, the grinding start point moves in a fluctuating manner. Thus, for each tooth space of the helical gear  10  indexed, the X- and Y-axis (horizontal) position of the grinding wheel  118  and the Z-axis (vertical) position thereof are corrected so as to follow the fluctuations. Moreover, the axis of the helical gear  10  moves from one tooth space of the helical gear  10  to another tooth space indexed, in synchronization with the phase of the rotary table  112 . Accordingly, the profile-grinding point moves in such a way as to swing (helical turn) between one and the other sides in the face width direction, i.e. between upper and lower sides. Thus, along with the motion of the grinding wheel  118  in the Z-axis direction (vertical direction), the X- and Y-axis (horizontal) motion of the grinding wheel  118  is corrected in such a way as to follow the swinging grinding point. 
     In addition, when the axis of the gear  10  is inclined with respect to the axis of the rotary table  112  (axis C), a tooth-profile grinding line M 1  of the grinding wheel  118  on a tooth surface  10   c  of the helical gear  10  is displaced from a reference tooth-profile grinding line M 0  as shown in Part A of  FIG. 8 . Accordingly, the contact between the grinding wheel  118  and the tooth surface  10   c  fails to be in a prescribed state. As shown in Part B of  FIG. 8 , this results in an error in the tooth surface  10   c  of the helical gear  10  after its profile grinding. Thus, based on the angles of inclination of the axis of the helical gear  10  with respect to the axis of the rotary table  112  (axis C), the motion of the grinding wheel  118  in the X- and Y-axis (horizontal) directions and the rotational (horizontal) motion of the rotary table  112  about the axis C are further corrected in association with the motion of the grinding wheel  118  in the Z-axis direction (vertical direction) in such a way as to cancel out the error in the tooth surface  10   c.    
     Accordingly, with the present invention, it is possible to perform accurate profile grinding of the helical gear  10  with the grinding wheel  118  for profile grinding, even when the axis of the helical gear  10  is displaced with respect to the axis of the rotary table  112  (axis C). 
     &lt;Other Embodiments&gt; 
     Note that while the foregoing embodiment has described a case of performing profile grinding of a large-sized helical gear  10 , the present invention is not limited to this case and can be applied to a case of performing profile grinding of a small-sized helical gear. However, the operations and effects of the gear grinding machine and the gear grinding method according to the present invention can be demonstrated fully in a case of performing profile grinding of a large-sized gear having helical tooth traces like the foregoing embodiment. 
     Moreover, while the foregoing embodiment has described a case of performing profile grinding of the helical gear  10 , the present invention is not limited to this case and can be applied to a case of performing profile grinding of a gear having helical tooth traces such as a crossed helical gear in a similar manner to the foregoing embodiment. 
     INDUSTRIAL APPLICABILITY 
     The gear grinding machine and the gear grinding method according to the present invention can be utilized remarkably effectively in metal working industries and the like for being capable of performing accurate profile grinding of a large-sized gear having helical tooth traces with a grinding wheel for profile grinding, even when the axis of the gear is displaced with respect to the axis of a rotary table. 
     REFERENCE SIGNS LIST 
     
         
           10  helical gear 
           10   a ,  10   b  boss 
           10   c  tooth surface 
           100  gear grinding machine 
           111  bed 
           111   a  rail 
           112  rotary table 
           112 M motor 
           113  column 
           113   a  rail 
           113 M motor 
           114  saddle 
           114 M motor 
           115  turn head 
           115   a  rail 
           115 M motor 
           116  grinding-wheel head 
           116 M motor 
           117  grinding-wheel spindle part 
           117   a  grinding-wheel shaft 
           117 M motor 
           118  grinding wheel 
           119  arm 
           120  touch probe 
           121  control device 
           122  input unit