Patent Publication Number: US-7901268-B2

Title: Method for grinding journal section of workpiece

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims priority under 35 U.S.C. 119 with respect to Japanese patent application No. 2007-044469 filed on Feb. 23, 2007, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for grinding a journal section of a workpiece with a steady rest device being in contact at its rest shoe with a part of the workpiece to decrease the flection of the workpiece. 
     2. Discussion of the Related Art 
     In a grinding field of crankshafts for automotive engines, it has been a practice that such a crankshaft is ground at its crankpins while being supported by a work spindle of a grinding machine. In grinding each of the crankpins, a rest shoe of a steady rest device is brought into contact with a cylindrical surface of a journal section which is at a position shifted in the axial direction from a crankpin to be ground, as described in U.S. Pat. No. 6,409,573 (equivalent of Japanese unexamined, published patent application No. 2000-296444) for example. The steady rest device decreases the flection of the crankshaft which flection is caused by grinding resistance applied from a grinding wheel, so that the machining accuracy of the crankpin can be enhanced. 
     A journal section of the crankshaft is ground in the manner illustrated in  FIG. 5 . That is, in grinding a journal section J 1  on one end side of the crankshaft W with a grinding wheel  17 , the grinding of the journal section J 1  is performed with a rest shoe  34  of a steady rest device  30  being in contact with a center journal section J 3  of the crankshaft W. It is often the case that the grinding of the journal section J 1  is performed with a sizing device measuring the diameter of the journal section J 1  and the width between shoulder surfaces at the opposite ends of the journal section J 1 . Where the grinding of the journal section J 1  is performed using the sizing device, it becomes difficult to bring the rest shoe  34  into contact with the journal section J 1  being ground, and thus, the grinding is practically performed with the rest shoe  34  being in contact with another journal section J 3  which is axially spaced from the journal section J 1  being ground. 
     First of all, description will be made regarding a cylindrical grinding machine which is employed for an exemplified grinding method (hereafter referred to as “grinding method in a compared example”) for grinding the journal section J 1  with the rest shoe  34  of the steady rest device  30  being in contact with the cylindrical surface of the center journal section J 3  of the crankshaft W. As shown in  FIG. 5 , at the rear part of an upper surface of a bed  10  of the cylindrical grinding machine, a pair of guide rails  11  are secured in a horizontal left-right direction (Z-direction), and a feed table  12  is supported and guided on the guide rails  11 . Another pair of guide rails  13  are secured on the feed table  12  in a horizontal front-rear direction (X-direction), and a wheel head  14  is supported and guided on the guide rails  13 . A wheel spindle (not shown) extending in the Z-direction is rotatably carried in the wheel head  14  and is drivingly rotated by a grinding wheel motor (not shown), built in the wheel head  14 , together with a grinding wheel  17  secured to one thereof. As shown in an enlarged fragmentary view of a grinding area encircled in  FIG. 5 , the grinding wheel  17  is of the configuration that an annular grinding wheel layer in which CBN abrasive grains have been bonded with a vitrified bonding agent is securely provided on the circumferential surface of a grinding wheel core made of, e.g., a metal disc and that the thickness in the axial direction of the grinding wheel layer is somewhat greater than that of the grinding wheel core. 
     A Z-axis servomotor  15  attached to the bed  10  is drivingly controllable by a numerical controller  18  and feeds the feed table  12  and the wheel head  14  and the grinding wheel  17 , which are supported on the feed table  12 , through a feed screw  15 a in the Z-direction. An X-axis servomotor  16  attached to the feed table  12  is drivingly controllable by the numerical controller  18  and feeds the wheel head  14  and the grinding wheel  17 , supported on the wheel head  14 , through a feed screw  16   a  in the X-direction. The respective servomotors  15 ,  16  are provided with encoders, which respectively detect the positions of the feed table  12  and the wheel head  14  to feed the detected positions back to the numerical controller  18 . 
     A work table  20  is fixed at the front part on the operator side (i.e., lower side in  FIG. 5 ) of the upper surface of the bed  10  of the grinding machine, and a work head  21  rotatably carrying a work spindle  22  and a foot stock  23  are provided on the work table  20  in axial alignment to face each other in the Z-direction. Centers  22   a ,  23   a  provided on the work spindle  22  and the foot stock  23  support the opposite ends of the crankshaft (i.e., workpiece) W. The work spindle  22  is drivingly rotatable by a work spindle servomotor  24 , which is mounted on the work head  21  to be controllable by the numerical controller  18 . The crankshaft W is rotated together with the work spindle  22  with its left end portion engaged with a driving dog (not shown) secured to the work spindle  22 . The work spindle servomotor  24  is also provided with an encoder, which detects the rotational position of the work spindle  22  to feed the detected position back to the numerical controller  18 . 
     The crankshaft W is a one-body article, which has five journal sections J 1 -J 5  arranged in axial alignment with a space between each journal section and the next thereto, four pairs of crank arms CA radially extending at opposite end portions of the respective journal sections J 1 -J 5  in parallel relation and four crankpins P 1 -P 4  each jointing the radial outmost end portions of an associated pair of crank arms CA. A large-diameter portion K is formed at one end portion of the crankshaft W which portion is outside a first journal section J 1  at the leftmost as viewed in  FIG. 5 , so that the first journal section J 1  has a cylindrical surface S 1  and a pair of shoulder surfaces S 2 , S 3  which extend from the opposite end portions of the cylindrical surface S 1  radially outward. 
     A base  31  of the steady rest device  30  is fixed on an operator side edge portion of the bed  10  which portion is on a side opposite to the wheel head  14  with the crankshaft W therebetween. A rest head  32  is supported and guided on the base  31  to be movable in the Z-direction. A servomotor  33  attached to the base  31  is drivingly controllable by the numerical controller  18  and feeds the rest head  32  through a feed screw  33   a  in the Z-direction. A rest shoe  34  which is supported and guided by the rest head  32  to be movable in the X-direction is moved by a servomotor  35  back and forth between predetermined advanced and retracted positions. The respective servomotors  33 ,  35  are provided with encoders, which respectively detect the positions of the rest head  32  and the rest shoe  34  to feed the detected positions back to the numerical controller  18 . 
     Next, with reference to  FIGS. 5 to 7 , description will be made regarding the grinding method in the compared example which is implemented in the grinding machine as constructed above. In this compared example, the feed table  12  is moved by the Z-axis servomotor  15  and positions the grinding wheel  17  drivingly rotated by the built-in grinding wheel motor (not shown), to a position where the grinding wheel  17  comes to align with and face the first journal section J 1  of the crankshaft W which has been center-supported by the work spindle  22  and the foot stock  23 . Then, the wheel head  14  is moved by the X-axis servomotor  16  to make the grinding wheel  17  approach the crankshaft W, whereby a shoulder grinding is first performed with opposite end surfaces of the grinding wheel  17  to simultaneously grind the left and right shoulder surfaces S 2 , S 3  of the first journal section J 1  and whereby a cylindrical grinding is then performed with the circumferential surface of the grinding wheel  17  to grind the cylindrical surface S 1  of the first journal section J 1 . Before the feed of the wheel head  12  toward the crankshaft W, the servomotor  33  of the steady rest device  30  is operated by the numerical controller  18  to position the rest head  32  to a position where the rest shoe  34  is aligned with a third journal section J 3  (another journal section) which is at a position different axially from the first journal section J 1  on the crankshaft W. As is a practice in the grinding field, for a small quantity of allowance in the grinding operation, a pre-machining groove Sa (shown in  FIG. 7(   a )) has been formed at the position of the first journal section J 1  of the blank of the crankshaft W through a preceding step such as, e.g., turning, milling or the like. The pre-machining groove Sa has an axial width which is somewhat narrower than the axial width of the annular grinding wheel layer of the grinding wheel  17 . 
     In the inoperative state, as shown in  FIG. 5 , the grinding wheel  17  is away from the first journal section J 1  of the crankshaft W, and the rest shoe  34  of the steady rest device  30  is at a retracted position where it is away from the circumferential surface of the third journal section J 3 . Thus, the axial center CL of the crankshaft W extends in parallel to the Z-axis, as shown in  FIG. 7(   a ). In this state, the work spindle servomotor  24  is operated by the numerical controller  18  to rotate the work spindle  22  and the crankshaft W supported thereby, and as indicated by the solid line A in  FIG. 6 , the wheel head  14  is advanced at a rapid feed rate, whereby the grinding wheel  17  advancing together with the wheel head  14  approaches the first journal section J 1  of the crankshaft W. 
     Somewhat before the circumferential surface of the grinding wheel  17  reaches the shoulder surfaces S 2 , S 3  of the first journal section J 1 , the numerical controller  18  operates the servomotor  35  of the steady rest device  30  to advance the rest shoe  34  to an advanced position where the rest shoe  34  comes into contact with the external surface of the third journal section J 3  of the crankshaft W. Thus, the axial center area of the crankshaft W is somewhat flexed toward the wheel head  14  side, and this causes the first journal section J 1  to tilt counterclockwise, as shown in  FIG. 7(   b ). Although  FIG. 7  depicts the tilt in an exaggerated scale for ease to see, the flection that the push by the rest shoe  34  brings about in the neighborhood of the third journal section J 3  of the crankshaft W is in a range of several ten-micron meters, and the displacement which the flection gives to the maximum diameter portion of each shoulder surface S 2 , S 3  extending perpendicular to the axial center CL at the first journal section J 1  of the crankshaft W is as extremely small as several micron meters or so. 
     With the advance of the wheel head  14 , the circumferential surface of the grinding wheel  17  advancing with the wheel head  14  reaches the shoulder surfaces S 2 , S 3  of the first journal section J 1 , and the numerical controller  18  then operates the X-axis servomotor  16  to switch the feed rate of the wheel head  14  from the rapid feed rate to a shoulder grinding feed rate slower than the rapid feed rate, whereby the grinding of the left and right shoulder surfaces S 2 , S 3  begins. During the shoulder grinding, small grinding resistance is generated, and this causes the first journal section J 1  to tilt slightly clockwise from the state shown in  FIG. 7(   b ). In this state, the shoulder grinding indicated by the solid line B in  FIG. 6  is performed to grind the left and right shoulder surfaces S 2 , S 3 , as shown in  FIG. 7(   c ). 
     The further advance of the wheel head  14  makes the shoulder grinding progress. When the circumferential surface of the grinding wheel  17  reaches a bottom surface of the pre-machining groove Sa of the first journal section J 1  as shown in  FIG. 7(   c ), the feed rate of the wheel head  14  is switched to a cylindrical grinding feed rate slower than the shoulder grinding feed rate in the same manner as described above, whereby the grinding indicated by the solid line C in  FIG. 6  is initiated on the cylindrical surface S 1  of the first journal section J 1 . During this cylindrical grinding, the grinding resistance increases to be considerably greater than that during the shoulder grinding. The first journal section J 1  is tilted clockwise from the state shown in  FIG. 7(   c ), and the cylindrical grinding indicated by the solid line C in  FIG. 6  is performed in this state. In this cylindrical grinding, due to the clockwise tilt, the shoulder surface S 3  on the left side is ground with the left end surface of the grinding wheel  17  to a larger depth as shown in  FIG. 7(d)  than it was ground in the state of FIG.  7 ( c ), and a clearance is made between the shoulder surface S 2  on the right side and the right end surface of the grinding wheel  17 . The cylindrical grinding is subdivided into a first rough grinding (solid line C 1 ), a second rough grinding (solid line C 2 ), a fine grinding (solid line C 3 ) and a minute grinding (solid line C 4 ) wherein the feed rate of the wheel head  14  are in turn reduced stepwise and also into a spark-out grinding (indicated by the first dotted circle at the end of the solid line C 4  in  FIG. 6 ) which is performed, with the infeed of the wheel head  14  being stopped, in succession to the minute grinding. The whole operation in the cylindrical grinding is as described above. 
     To follow the spark-out grinding on the cylindrical surface S 1 , the wheel head  14  is retracted by a predetermined or fixed distance, as indicated by the solid line D in  FIG. 6 . If the retraction of the grinding wheel  17  at a rapid feed rate were performed immediately after the grinding of the cylindrical surface S 1  is completed, the flection of the crankshaft W would be released at a moment, resulting in a further infeed of a part of the cylindrical surface S 1  against the grinding wheel  17 , whereby the roundness of the finished cylindrical surface S 1  would be degraded in accuracy. The retraction of the fixed distance indicated by the solid line D in  FIG. 6  is to prevent the cylindrical surface S 1  from being degraded in roundness by retracting the grinding wheel  17  by the predetermined or fixed distance at a slow feed rate, and the distance or amount of the retraction is a small amount. Thus, even though the retraction by the fixed distance decreases the grinding resistance, there remains grinding resistance of the same degree as that in the shoulder grinding. The remaining grinding resistance thus causes the first journal section J 1  to tilt counterclockwise from the state shown in  FIG. 7(   d ). As a consequence, as shown in  FIG. 7(   e ), the clearance which has been made between the shoulder surface S 2  on the right side and the right end surface of the grinding wheel  17  during the cylindrical grinding is reduced to zero (0), whereas a clearance is made between the shoulder surface S 3  on the left side and the left end surface of the grinding wheel  17 . 
     After being stopped momentarily (as indicated by the second dotted circle in  FIG. 6 ) at the retracted end of the fixed-distance retraction indicated by the solid line D in  FIG. 6 , the wheel head  14  is further retracted at a semi-rapid feed rate as indicated by the solid line E. During this retraction state, the grinding resistance caused by the grinding wheel  17  becomes zero (0) to return to the state shown in  FIG. 7(   b ). Thus, the tilt of the first journal section J 1  toward the counterclockwise direction which tilt is brought about by the rest shoe  34  remaining in contact with the third journal section J 3  becomes larger in value than that during the shoulder grinding shown in  FIG. 7(   c ). As a result, during the retraction at the semi-rapid feed rate, the grinding wheel  17  is retracted with the right end surface thereof interfering with the shoulder surface S 2  on the right side of the first journal section J 1 . This causes vortex or spiral-like shallow scrapes to be formed at the shoulder surface S 2  of the first journal section J 1 . This could make grinding burn on the shoulder surface S 2  in the case of the grinding wheel  17  being low in sharpness. Also during the retraction, a clearance is made between the left end surface of the grinding wheel  17  and the shoulder surface S 3  on the left side of the first journal section J 1 . 
     When the semi-rapid feed retraction of the wheel head  14  makes the circumferential surface of the grinding wheel  17  go away radially outside from an area facing the shoulder surfaces S 2 , S 3  of the first journal section J 1 , the numerical controller  18  operates the servomotor  35  of the steady rest device  30  whereby the rest shoe  34  is retracted toward the retracted position where it is away from the external surface of the third journal section J 3 . Thus, the axial center CL of the first journal section J 1  of the crankshaft W is brought into a parallel relation with the Z-direction, as shown in  FIG. 7(   g ). In this state, the wheel head  14  is retracted at the rapid feed rate indicated by the solid line F in  FIG. 6  to return to the initial inoperative state mentioned in the beginning of this operational description. 
     As described above, in the compared example shown in  FIGS. 5-7 , there arises a problem that spiral shallow scratches or scrapes or, on a certain occasion, grinding burn is formed on the shoulder surface S 2  of the first journal section J 1  when the grinding wheel  17  is retracted at the semi-rapid feed rate following the fixed-distance retraction. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary object of the present invention to provide a grinding method which does not involve the aforementioned drawback in grinding a journal section of a workpiece. 
     Briefly, according to the present invention, there is provided a method of grinding one of journal sections axially spaced on the axis of a workpiece with a rotating grinding wheel, with the workpiece being rotatably supported by a work spindle of a cylindrical grinding machine, each of the journal sections having a cylindrical surface and a pair of shoulder surfaces extending radially outward at opposite end portions of the cylindrical surface. The method comprises the steps of feeding the grinding wheel in a radial direction first to grind the shoulder surfaces of one journal section and then to grind the cylindrical surface of the one journal section; advancing a rest shoe, provided on a side opposite to the grinding wheel with the workpiece therebetween, to an advanced position where the rest shoe contacts with another journal section axially spaced from the one journal section to decrease the flection of the workpiece caused by grinding resistance; retracting the grinding wheel by a fixed distance at a slow feed rate upon completion of the cylindrical surface grinding; retracting the rest shoe away from said another journal section upon completion of the fixed-distance retraction of the grinding wheel; and retracting the grinding wheel at a rapid feed rate after the rest shoe goes away from said another journal section. 
     With this construction, when the rest shoe is retracted upon completion of the fixed-distance retraction of the grinding wheel, the tilt caused by the rest shoe of the journal section being ground is removed, and thus, one of the shoulder sections at the opposite ends of the journal section is ground with one end surface of the grinding wheel before the subsequent rapid feed retraction. Therefore, the rapid feed retraction of the grinding wheel is performed without making each of the end surfaces of the grinding wheel interfere with any shoulder surface of the workpiece, so that the shoulder surfaces of the journal section can be prevented from having any scrape or scratches of spiral-form and any grinding burn thereon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects and many of the attendant advantages of the present invention may readily be appreciated as the same becomes better understood by reference to the preferred embodiments of the present invention when considered in connection with the accompanying drawings, wherein like reference numerals designate the same or corresponding parts throughout several views, and in which: 
         FIG. 1  is a chart depicting operational steps of a grinding cycle in a first embodiment of a workpiece journal section grinding method according to the present invention; 
         FIGS. 2(   a )- 2 ( g ) are enlarged fragmentary views showing respective states in the neighborhood of a first journal section at the respective operational steps of the grinding cycle in the first embodiment; 
         FIG. 3  is a chart depicting operational steps of a grinding cycle in a second embodiment of the workpiece journal section grinding method according to the present invention; 
         FIGS. 4(   a )- 4 ( h ) are enlarged fragmentary views showing respective states in the neighborhood of a first journal section at the respective operational steps of the grinding cycle in the second embodiment shown in  FIG. 3 ; 
         FIG. 5  is a plan view schematically showing the overall construction of a cylindrical grinding machine employed in implementing the respective embodiments according to the present invention and a workpiece journal section grinding method in a compared example; 
         FIG. 6  is a chart depicting operational steps of a grinding cycle in the workpiece journal section grinding method in the compared example; and 
         FIGS. 7(   a )- 7 ( g ) are enlarged fragmentary views showing respective states in the neighborhood of a first journal section at the respective operational steps of the grinding cycle in the compared example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Hereafter, with reference to  FIGS. 1 ,  2  and  5 , description will be made regarding a workpiece journal section grinding method in a first embodiment according to the present invention. A cylindrical grinding machine employed in implementing the grinding method in the first embodiment takes the construction shown in  FIG. 5 . Since the grinding machine has already been described in connection with the foregoing grinding method in the compared example, the detailed description of the grinding machine is omitted for the purpose of avoiding repetition. Further, the material or blank of a crankshaft W to be ground in the grinding method in the first embodiment is the same as that used in the grinding method in the compared example. 
     Also in this first embodiment, the grinding steps are performed in the same way as those described in the foregoing compared example. Specifically, the respective servomotors  15 ,  16 ,  24 ,  33  and  35  are controlled by the numerical controller  18 , and the cylindrical, external surface S 1  of the first journal section J 1  and the pair of shoulder surfaces S 2 , S 3  extending radially outward from the opposite end portions of the first journal section J 1  are ground by positioning the grinding wheel  17  and the rest shoe  34  relative to the crankshaft W which is supported by the respective centers of the work head  22  and the foot stock  23 , moving the wheel head  14  as indicated by the solid lines A-F in  FIG. 1  to effect a rapid feed advance (cf. solid line A), a shoulder grinding (cf. solid line B), a cylindrical grinding (cf. solid line C), a fixed-distance retraction (cf. solid line D), a semi-rapid feed retraction (cf. solid line E) and a rapid feed retraction (cf. solid line F) and at the same time as the grinding feed, advancing the rest shoe  34  as indicated by the solid line at the bottom of the chart depicted in  FIG. 1 . The operational step cycle chart depicted in  FIG. 1  indicates the respective positions of the wheel head  14  in connection with the lapse of time and is the same as that in the compared example shown in  FIG. 6 . However, the operational chart at the bottom in  FIG. 1  depicting the feed operation of the rest shoe  34  differs from that at the bottom in  FIG. 6  for the compared example in that the time at which the rest shoe  34  at an advanced position is switched to be moved toward a retracted position is not the time of completing the semi-rapid feed retraction (as is the case of the compared example) but the time of completing the fixed-distance retraction (as is the case of the first embodiment). 
       FIGS. 2(   a )- 2 ( g ) are enlarged fragmentary views showing the states of the first journal section J 1  at respective time points. As mentioned earlier, the grinding cycle chart depicted in  FIG. 1  for the first embodiment is the same as that depicted in  FIG. 6  for the compared example, whereas the rest feed chart shown in  FIG. 1  for the first embodiment differs from that shown in  FIG. 6  for the compared example only in the time point at which the rest shoe  34  is switched to be moved from the advanced position toward the retracted position. That is,  FIGS. 2(   a )- 2 ( e ) are identical respectively to  FIGS. 7(   a )- 7 ( e ), and  FIGS. 2(   f ) and  2 ( g ) only differ from  FIGS. 7(   f ) and  7 ( g ). Therefore, description will be omitted regarding the states shown in  FIGS. 2(   a )- 2 ( e ) and will be described hereafter regarding the states shown in  FIGS. 2(   f ) and  2 ( g ). 
     Following the fixed-distance retraction indicated by the solid line D in  FIG. 1 , the numerical controller  18  operates the servomotor  35  of the steady rest device  30  to retract the rest shoe  34  toward the retracted position away from the external surface of the third journal section J 3 . Thus, as shown in  FIG. 2(   f ), the axial center line CL of the first journal section J 1  of the crankshaft W goes back to a position where it is laid in parallel with the Z-direction, whereby the shoulder surface S 3  on the left side is given a deeper infeed of the left end surface of the grinding wheel  17  than it is given in the state shown in  FIG. 2(   e ), whereas a clearance is formed between the shoulder surface S 2  on the right side and the right end surface of the grinding wheel  17 . The infeed grinding is completed through a subsequent spark-out grinding indicated by the second dotted circle in  FIG. 1 , and then, the semi-rapid feed retraction is performed as indicated by the solid line E in  FIG. 1 . In this state, because of having the clearance relative to the right end surface of the grinding wheel  17 , the shoulder surface S 2  on the right side does not suffer any scrapes or grinding burn. The shoulder surface S 3  on the left side does not interfere with the left end surface of the grinding wheel  17  though remaining in contact therewith, so that the shoulder surface S 3  does not suffer any scrapes or grinding burn. 
     By the semi-rapid feed retraction of the wheel head  14 , the circumferential surface of the grinding wheel  17  goes away radially outside from the area facing the shoulder surfaces S 2 , S 3  of the first journal section J 1 . Thereafter, a rapid feed retraction indicated by the solid line F in  FIG. 1  is performed, whereby return is made to the initial inoperative state described in the beginning of this operational description. 
     In the foregoing first embodiment, the fixed-distance retraction includes the spark-out grinding (the aforementioned second dotted circle) in which the grinding wheel  17  is stopped momentarily following the retraction through the fixed distance. Although being performed for the purposed of completing the infeed grinding on the shoulder surface S 3 , the spark-out grinding may be omitted as the case may be. In this modified form, an area on the shoulder surface S 3  to be ground with the end surface of the grinding wheel  17  is suppressed to a permissible degree by reducing the feed rate at an early stage of the semi-rapid feed retraction following the fixed-distance retraction, and upon completion of the grinding, the grinding wheel  17  is retracted at the semi-rapid feed rate without making the end surfaces thereof interfere with any of the shoulder surfaces S 2 , S 3 . In another modified form, it is possible to omit the semi-rapid feed retraction indicated by the solid line E in  FIG. 1  and to simultaneously perform the retraction of the rest shoe  34  and the rapid feed retraction (solid line F) of the wheel head  14  in succession to the fixed-distance retraction indicated by the solid line D. Although the simultaneous retractions can be done without performing the spark-out grinding indicated by the aforementioned second dotted circle in  FIG. 1 , they may preferably be done to follow the spark-out grinding. 
     Second Embodiment 
     Next, a workpiece journal section grinding method in a second embodiment according to the present invention will be described with reference to  FIGS. 3 and 4 . Also in the grinding method in this second embodiment, a cylindrical grinding machine and the blank of a workpiece W are the same as those in the foregoing compared example and the foregoing first embodiment. In this second embodiment, as indicated by the solid lines A-F in  FIG. 3 , the wheel head  14  is moved to perform a rapid feed advance (cf. solid line A), a shoulder grinding (cf. solid line B), a cylindrical grinding (cf. solid line C), a fixed-distance retraction (cf. solid line D), a semi-rapid feed retraction (cf. solid line E) and a rapid feed retraction (cf. solid line F). The difference from the first embodiment is in a respect that the duration for which the wheel head  14  is stopped temporarily at the end of the fixed-distance retraction indicated by the solid line D is made to be longer by a period indicated by the solid line D 1  than the corresponding duration in the foregoing first embodiment. Also in this second embodiment, as indicated by the solid line at the bottom in  FIG. 3 , the rest shoe  34  is advanced to, and retracted from, a first advanced position at the same timings as those in the foregoing first embodiment. Then, the rest shoe  34  is re-advanced to a second advanced position prior to the initiation of the semi-rapid feed retraction (the solid line E) and is then retracted from the second advanced position toward the retraction position at the time point when the semi-rapid feed retraction (the solid line E) is terminated. The first advanced position is the same position as the advance position in the foregoing first embodiment, whereas the second advance position is an intermediate position which is along the way to the first advance position and which is another predetermined distance behind the first advanced position. The rest shoe  34  at the intermediate position still remains in contact with the external surface of the third journal section J 3 . That is, the second embodiment differs from the foregoing first embodiment in that at the end of the fixed-distance retraction, the grinding wheel  17  is temporarily stopped by a longer duration D 1  than the corresponding duration in the foregoing first embodiment and that during the duration D 1 , the rest shoe  34  is retracted to a position which is away from the journal section J 3  and is then re-advanced to the second advance position which is the predetermined distance to the first advance position, to contact with the third journal section J 3 . The position to which the rest shoe  34  is retracted away from the journal section J 3  during the duration D 1  may be the retracted position as indicated in  FIG. 3  or another intermediate position between the second advanced position and the retracted position. 
       FIGS. 4(   a )- 4 ( h ) are enlarged fragmentary views showing the states of the first journal section J 1  at respective time points and are similar to  FIGS. 2(   a )- 2 ( g ) for the foregoing first embodiment. The rapid feed advance (cf. solid line A) through the fixed-distance retraction (cf. solid line D) in  FIG. 3  are the same as those in  FIG. 1 , and likewise,  FIGS. 4(   a )- 4 ( f ) are the same as  FIG. 2(   a )- 2 ( f ). Therefore, description regarding  FIGS. 4(   a )- 4 ( f ) will be omitted to avoid repetition, and description will hereafter be made regarding  FIGS. 4(   g )- 4 ( h ) only. 
     In the state shown in  FIG. 4(   f ), the axial center line CL of the first journal section J 1  of the crankshaft W is laid in parallel with the Z-direction, so that the shoulder surface S 3  on the left side is in contact with the left end surface of the grinding wheel  17 , whereas a clearance is formed between the shoulder surface S 2  on the right side and the right end surface of the grinding wheel  17 . At this time, the rest shoe  34  is at the retracted position and is away from the external surface of the third journal section J 3 . In this state, the servomotor  35  of the steady rest device  30  is operated by the numerical controller  18  to bring the rest shoe  34  into contact with the external surface of the third journal section J 3  at the second advanced position which is along the way to the first advanced position. This causes the first journal section J 1  to tilt counterclockwise from the position which is parallel to the Z-direction. However, because the second advanced position is along the way to the first advanced position, that is, behind the first advanced position, the first journal section J 1  is not tilted to the state shown in  FIG. 4(   e ) and is stopped between the positions shown in  FIGS. 4(   e ) and  4 ( f ), as shown in  FIG. 4(   g ). As a consequence, the grinding wheel  17  is brought into the state that the both end surfaces of the grinding wheel  17  are respectively spaced from the shoulder surfaces S 2 , S 3  at the opposite ends of the cylindrical surface S 1  of the first journal section J 1 . In this state, the wheel head  14  is retracted at the semi-rapid feed rate as indicated by the solid line E in  FIG. 3 , so that it can be prevented reliably that any shallow scrape of spiral form or any grinding burn is formed on the shoulder surfaces S 2 , S 3 . 
     After the wheel head  14  is retracted at the semi-rapid feed rate to make the circumferential surface of the grinding wheel  17  go away from the area facing the shoulder surfaces S 2 , S 3  of the first journal section J 1 , the servomotor  35  of the steady rest device  30  is operated by the numerical controller  18  to retract the rest shoe  34  toward the retracted position away from the external surface of the third journal section J 3 . As a result, as shown in  FIG. 4(   h ), the axial center CL of the first journal section J 1  of the crankshaft W becomes parallel to the Z-direction, and in this state, the wheel head  14  is retracted at the rapid feed rate as indicated by the solid line F in  FIG. 3 , to return to the initial inoperative state. 
     In each of the foregoing embodiments, description has been made regarding the example that the present invention is applied to grinding the first journal section J 1  which is on one axial end side of the crankshaft W. The present invention is not limited to grinding the first journal section J 1 . It is possible to apply the present invention to grinding any other journal section of the crankshaft or grinding any journal section of a shaft other than the crankshaft. 
     Various features and many of the attendant advantages in the foregoing embodiments will be summarized as follows: 
     In the grinding method in the foregoing first embodiment typically shown in  FIGS. 1 ,  2  and  5 , when the rest shoe  34  is retracted upon completion of the fixed-distance retraction (D in  FIG. 1 ) of the grinding wheel  17 , the tilt caused by the rest shoe  34  of the journal section J 1  is removed, and thus, one (S 2  or S 3 ) of the shoulder sections is ground with one end surface of the grinding wheel  17  before the subsequent rapid feed retraction including the semi-rapid feed retraction. Therefore, the rapid feed retraction of the grinding wheel  17  is performed without making each of the end surfaces of the grinding wheel  17  interfere with any shoulder surface S 2 , S 3  of the workpiece W, so that the shoulder surfaces S 2 , S 3  of the first journal section J 1  can be prevented from having any scrapes or scratches of spiral-form and any grinding burn thereon. 
     In the grinding method in the foregoing second embodiment typically shown in  FIGS. 3 to 5 , after being retracted from the first advanced position, the rest shoe  34  is re-advanced to the second advanced position which is on the way to the first advanced position and at which the rest shoe  34  comes to contact with the different journal section J 3 . This brings the shoulder surfaces S 2 , S 3  into the state that each of the shoulder surfaces S 2 , S 3  is spaced from the associated end surface of the grinding wheel  17 , and the rapid feed retraction of the grinding wheel  17  is performed in this state. Accordingly, the shoulder surfaces S 2 , S 3  of the first journal section J 1  can be reliably prevented from having any scrape of spiral form and any grinding burn thereon. 
     Also in the grinding method in the foregoing second embodiment typically shown in  FIGS. 3 to 5 , after being retracted by the fixed distance (D in  FIG. 3 ), the grinding wheel  17  is temporarily stopped to perform the spark-out grinding (D 1  in  FIG. 3 ). This ensures completing the grinding of the shoulder surfaces S 2 , S 3  which is performed as a result that the tilt of the first journal section J 1  is removed by the retraction of the rest shoe  34 , and thus, it can be prevented that the end surfaces of the grinding wheel  17  are interfered with the shoulder surfaces S 2 , S 3  during the rapid feed retraction. Accordingly, the shoulder surfaces S 2 , S 3  of the first journal section J 1  can be reliably prevented from having any scrape of spiral form and any grinding burn thereon. 
     Also in the grinding method in the foregoing second embodiment typically shown in  FIGS. 3 to 5 , during the temporal stop (D 1  in  FIG. 3 ) of the grinding wheel  17  following the fixed-distance retraction (D in  FIG. 3 ), the rest shoe  34  is retracted from the first advanced position and after this retraction, is re-advanced to the second advanced position which is on the way to the first advanced position and at which it contacts with the different journal section J 3 . Thus, the subsequent rapid feed retraction of the grinding wheel  17  is performed with the both end surfaces thereof being reliably spaced from the shoulder surfaces of the first journal section J 1 . Accordingly, the shoulder surfaces S 2 , S 3  of the first journal section J 1  can be reliably prevented from having any scrape of spiral form and any grinding burn thereon. 
     Obviously, numerous further modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.