Abstract:
On a grinding machine, a workpiece having eccentric cylindrical portions is rotated around the work spindle axis so that the eccentric cylindrical portions effects a planetary motion. A grinding wheel is reciprocated toward and away from the work spindle axis in synchronism with the planetary motion and is also fed toward the work spindle axis to grind the eccentric cylindrical portion into a target cylindrical profile. Upon completion of a part of the grinding operation, the eccentric cylindrical portion under machining is indexed to a predetermined angular position. A diameter measuring device is moved from its rest position to a measuring position to measure the diameter of the eccentric cylindrical portion. Comparing the measured diameter with a target diameter to obtain a residual difference in diameter, the grinding machine further operates to perform a finish grinding according to the residual difference until the diameter reaches the target size, whereby the eccentric cylindrical portion can be finished precisely using a general purpose measuring device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method and an apparatus for grinding eccentric cylindrical portions of a workpiece such as crankpins on a crankshaft, and more particularly to a method and apparatus for grinding an eccentric cylindrical portion while rotating the workpiece around its central axis to effect a planetary motion of the eccentric cylindrical portion.  
           [0003]    2. Description of the Prior Art  
           [0004]    In a conventional process of grinding eccentric cylindrical portions such as crankpins of a crankshaft used in an internal combustion engine or a compressor, a crankshaft workpiece is rotated around the workpiece spindle axis of a grinding machine to effect a planetary motion of the crankpin portion, and at the same time, a grinding wheel is infed against the crankpin portion while it is reciprocated toward and away from the spindle axis in synchronism with the planetary motion. The feature of grinding operation of this type resides in that there is no need of providing various kinds of dedicated chucks for the respective individual shapes of the crankshafts, as the crankshaft workpiece is rotated around its central (principal) axis. This provides versatile machining process using a general-purpose machine tool. More specifically, in the grinding machine, parameters for controlling the grinding machine in accordance with various different types of crankshafts are registered (or stored) beforehand, so that the machine is flexibly adaptable to the manufacturing of various different types of crankshafts.  
           [0005]    An example of such a type of conventional method and apparatus in the art is disclosed in unexamined Japanese patent publication No. 2000-218531. In this technology, a crankpin of a trial workpiece is first ground based on the theoretical grinding parameters (data), and the diameter of the crankpin thus ground is measured by means of a measuring device equipped on the machine. Then, the measured diameter is employed to prepare modified parameters for the rough grinding step, the finish grinding step and so forth by compensating for the errors or differences in diameter involved in the theoretical parameters for the rough grinding step, the finish grinding step and so forth, and thereafter crankpins of workpieces to be produced are ground based on the thus modified parameters. The measuring device used in such a conventional machine is of the type that measures the radius of the crankpin for the diameter thereof. More specifically, a V-block provided at the tip of the measuring device contacts the crankpin with a probe provided projectably at the bottom of the V-cut touching the crankpin surface continually, and then the amount of the probe advance is detected electrically to compute the diameter. Such a measuring device is capable of measuring the diameter during the grinding process, but requires a link mechanism for making the V-block of the measuring head move horizontally and vertically in synchronism with the planetary motion of the crankpin. From the measured crankpin diameter, the measuring device of this type judges the respective timings of switching the grinding operations between two continuous steps among a plurality of grinding steps including a rough grinding step, a fine grinding step, a finish grinding step and so forth and feeds back the judgments to the control unit of the machine in order to properly change the speed of infeeding the grinding wheel against the workpiece.  
           [0006]    However, the measuring device as described above uses a V-block for the measuring head which is designed to measure the radius of the workpiece, and therefore, cannot measure the diameter directly with a high accuracy. In addition, the device has such another drawback that it becomes expensive due to the use of a dedicated unit including a link mechanism which permits the reciprocal movement of the measuring head in synchronism with the planetary motion of the crankpin.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore a primary object of the present invention to provide a novel method and apparatus capable of grinding an eccentric cylindrical portion of a workpiece with a high accuracy in the dimension of diameter by the use of a general-purpose measuring device which is not only inexpensive to make, but also capable of directly measuring the diameter of the eccentric cylindrical portion on the workpiece with a high accuracy.  
           [0008]    According to the present invention, the object is accomplished by providing a method of grinding an eccentric cylindrical portion of a workpiece on a grinding machine with a measuring device, which method comprises the steps of: rotating the workpiece around a work spindle axis of the grinding machine, the eccentric cylindrical portion being eccentric away from the work spindle axis to effect a planetary motion when the workpiece is rotated around the work spindle axis; infeeding a grinding wheel toward the work spindle axis while reciprocating the grinding wheel toward and away from the work spindle axis in synchronism with the planetary motion for grinding the eccentric cylindrical portion into a cylindrical profile; indexing the eccentric cylindrical portion to a predetermined angular position upon completion of a part of the grinding operation; moving a diameter measuring device from a rest position to a measuring position to engage the eccentric cylindrical portion held at the predetermined angular position; measuring the diameter of the eccentric cylindrical portion; retracting the diameter measuring device to the rest position; and resuming the feeding of said grinding wheel toward said work spindle axis while reciprocating said grinding wheel toward and away from said work spindle axis in synchronism with said planetary motion, for performing the remaining part of said grinding operation. With this feature of the method according to the present invention, the diameter measurement of the eccentric cylindrical portion is carried out with the eccentric cylindrical portion being held at the predetermined angular position wherein the measuring device is advanced to the standstill object so as to measure the diameter. Therefore, the measurement with a high accuracy can be realized using a simple and thus inexpensive general-purpose measurement device.  
           [0009]    According to the present invention, the object is further accomplished by providing a method of the character set forth above wherein the diameter measured upon completion of a part of the grinding operation is compared with a target diameter to obtain the difference in diameter and wherein the eccentric cylindrical portion is further ground by the difference in diameter. With this feature of the method according to the present invention, the further or finish grinding is conducted based on the diameter difference between the measured value and the target value, whereby the workpiece can be finished with a high accuracy.  
           [0010]    According to the present invention, the object is also accomplished by providing an apparatus for grinding an eccentric cylindrical portion of a workpiece, comprising: a workpiece rotating device including a workpiece spindle having a work spindle axis for rotating the workpiece around the work spindle axis, the eccentric cylindrical portion being eccentric away from the work spindle axis for effecting a planetary motion when the workpiece is rotated around the work spindle axis; a grinding wheel movable toward the work spindle axis while being reciprocated toward and away from the work spindle axis in synchronism with the planetary motion, for grinding the eccentric cylindrical portion into a cylindrical profile; an indexing device associated with the workpiece rotating device for indexing the eccentric cylindrical portion to a predetermined angular position upon completion of a part of the grinding; and a diameter measuring device movable from a rest position to a measuring position and having a pair of feelers which are engageable with the eccentric cylindrical portion at diametrically opposite surfaces thereof for measuring the diameter of the eccentric cylindrical portion held at the predetermined angular position. With this feature of the apparatus according to the present invention, the rotation of the workpiece is stopped to index the eccentric cylindrical portion to the predetermined angular position for measurement of the diameter thereof, and the measuring device is advanced to the standstill object to measure the diameter with the diametrically opposed feelers. Therefor, the grinding apparatus can be configured with a general-purpose measuring device which is simple in configuration, reliable in operation, reduced in cost and high in measuring accuracy.  
           [0011]    According to the present invention, the object is still further accomplished by providing an apparatus of the character set forth above which further comprises a comparing device for comparing the diameter measured upon completion of a part of the grinding operation with a target diameter to obtain the difference in diameter, and a finish grinding control device for causing the grinding wheel to effect a finish grinding on the eccentric cylindrical portion based on the difference in diameter. With this feature of the apparatus according to the present invention, the grinding apparatus can be configured with the comparison device for obtaining the difference in diameter between the state in mid course of grinding and the state on target and can conduct the finish grinding according to the difference so obtained, whereby the workpiece can be finished precisely to the target diameter.  
           [0012]    In an aspect of the present invention, an apparatus for grinding an eccentric cylindrical portion of a workpiece further comprise: a parameter registration device for registering parameters concerning amounts of eccentricity of the eccentric cylindrical portion for different types of workpieces; a workpiece identification device for identifying the type of a workpiece to be ground; and a parameter retrieval device for searching the parameter registration device to retrieve an eccentricity amount which corresponds to the type of the workpiece identified by the workpiece identification device; wherein the measuring position of the diameter measuring device is determined depending upon the retrieved eccentricity amount. With this configuration, it becomes possible to change the measuring position of the diameter measuring device automatically in dependence upon the types of workpieces to be ground, so that various types of workpieces having eccentric cylindrical portions of different eccentricties can be ground on the apparatus.  
           [0013]    In another aspect of the present invention, an apparatus for grinding eccentric cylindrical portions of a workpiece further comprise: a parameter registration device for registering parameters concerning amounts of eccentricity and phase angles of a plurality of the eccentric cylindrical portions per workpiece with respect to the work spindle axis for different types of workpieces; a workpiece identification device for identifying the type of a workpiece to be ground; and a parameter retrieval device for searching the parameter registration device to retrieve an eccentricity amount and phase angles of the plurality of the eccentric cylindrical portions per workpiece with respect to the work spindle axis which eccentricity amount and phase angles correspond to the type of the workpiece identified by the workpiece identification device; wherein the indexing device indexes the eccentric cylindrical portions to respective angular positions determined by the retrieved phase angles; and wherein the measuring position of the diameter measuring device is determined in dependence upon the retrieved eccentricity amount. With this configuration, automatic processing can be easily performed with respect to a plurality of eccentric cylindrical portions having different phase angles on a workpiece.  
           [0014]    In a further aspect of the present invention, the measuring device may include a measuring head and a head traversing device driven by a servomotor to locate the measuring head at desired measuring positions.  
           [0015]    In a still further aspect of the present invention, the measuring device may include a measuring head and a head traversing device driven by a cylinder device to locate the measuring head at any of plural measuring positions.  
           [0016]    In a still further aspect of the present invention, the indexed angular position for the eccentric cylindrical portion may be determined opposite to the grinding wheel with respect to the workpiece spindle axis in a horizontal plane encompassing the workpiece spindle axis.  
           [0017]    In a specific aspect of the present invention, the workpiece may be a crankshaft, and the eccentric cylindrical portion may be a crankpin. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    For better understanding of the present invention, and to show how the same may be practiced and will work, reference will now be made, by way of example, to the accompanying drawings, in which:  
         [0019]    [0019]FIG. 1 is a block diagram showing the outline of overall configuration of a grinding machine and the schematic connection of associated control circuits for performing the grinding of crankpins on a crankshaft according to the present invention;  
         [0020]    [0020]FIG. 2 is a side elevation view of a diameter measuring device according to the present invention;  
         [0021]    [0021]FIG. 3 is a detailed view of FIG. 2 also showing the internal structure of the diameter measuring device according to the present invention;  
         [0022]    [0022]FIG. 4 is a flow chart showing a program for machining operation according to the present invention;  
         [0023]    [0023]FIG. 5 is a flow chart showing a program for crankpin diameter measurement and feed amount computation at a finish grinding process according to the present invention;  
         [0024]    [0024]FIG. 6 is a chart showing a data table containing parameters for grinding crankshafts;  
         [0025]    [0025]FIG. 7 is an explanatory diagram showing grinding infeed steps of a grinding wheel;  
         [0026]    [0026]FIG. 8 is a side elevation view of the diameter measuring device in which a measuring head is locatable by means of a drive cylinder at two positions toward and away from the workpiece horizontally; and  
         [0027]    [0027]FIG. 9 is a side elevation view of another diameter measuring device in which a measuring head is locatable by means of a ball screw unit driven by a servomotor at desired positions vertically. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    Illustrated in FIG. 1 are the outline of overall configuration of a grinding machine and the schematic connection of associated control circuits for practicing a method of grinding crankpins CP on a crankshaft W according to the present invention. The grinding machine  20  comprises a bed  1 , a pair of guide rails  3 ,  3  extending in the longitudinal direction along the Z axis (as defined in FIG. 1) on the bed  1 , and a table  2  slidably supported on the guide rails  3 ,  3 . At the left side area on the table  2 , a headstock  7  is arranged carrying a work spindle  17 . The work spindle  17  is coupled to an output shaft of a servomotor  9 , which in turn is coupled to a rotary encoder  18  for control in rotation. The work spindle  17  has a chuck or the like device (not shown) to grip a journal portion at one end of the crankshaft W. Opposite to the headstock  7 , a tailstock  8  is arranged at the right side area on the table  2 . The tailstock  8  rotatably supports a journal portion at the other end of the crankshaft W by means of a center  19 . With such an arrangement, the rotational axis of the work spindle  17  and the central axis of the journal portions of the crankshaft W coincide with each other, so that the crankshaft W is rotated around the axis of the work spindle  17  to effect a planetary motion of the crankpins CP.  
         [0029]    Between the pair of guide rails  3 ,  3 , a ball screw  4  is arranged for moving the table  2  in the direction of Z axis. The ball screw  4  is coupled, at its left end, to an output shaft of a servomotor  5  mounted at the left end of the bed  1 . The servomotor  5  is coupled to a rotary encoder  6  to detect the rotational angle of the ball screw  4 . This arrangement enables sliding movement of the table  2  as controlled in the Z axis direction, so that each of the crankpins CP on the crankshaft W may be aligned with a grinding wheel  15 .  
         [0030]    An X axis defines a direction perpendicular to the Z axis on the horizontal plane of the machine. A pair of guide rails  11 ,  11  are provided on the bed  1  extending along the X axis direction to slidably support a wheel head  10  which carries the grinding wheel  15 . Between the pair of guide rails  11 ,  11 , a ball screw  12  is arranged for moving the wheel head  10  in the direction of X axis. The ball screw  12  is coupled, at its rear end, to an output shaft of a servomotor  13  mounted at the rear end (distal end) of the bed  1 . The servomotor  13  is coupled to a rotary encoder  14  to detect the rotational angle of the ball screw  12 . This arrangement permits a back-and-forth movement of the wheel head  10  as controlled in the X direction which is perpendicular to the central axis of the crankshaft W, so that the grinding wheel can be moved back and forth in synchronism with the planetary motion of the crankpin CP. Further, the grinding wheel  15  mounted on the wheel head  10  is to be rotated as driven by a motor (not shown).  
         [0031]    Reference numeral  16  denotes a measuring device featuring the present invention. The measuring device  16  is provided at the front end of the bed  1  of the crankpin grinding machine  20  opposite to the grinding wheel  15 . As shown in more detail in FIG. 2, the measuring device  16  is mounted on the bed  1  by means of a connecting member  52 . A measuring head  41  is of an approximately rectangle shape having a certain thickness and mounts an upper feeler  44  and a lower feeler  45  on one side for measuring the crankpin CP which is indexed to a position opposed to the grinding wheel  15 . The measuring head  41  is supported on a head support  43  via a hinge pin  42 , as will be described hereinafter. The head support  43  is mounted on a head slide  46  which is slidably supported on the guide member  49 . Fixed at the bottom of the head slide  46  is a driving nut  59 , which is engaged with a ball screw  50 . The ball screw  50  is connected to an output shaft of a servomotor  47 , so that the rotation of the ball screw  50  moves the head slide  46  in the X axis direction. A rotary encoder  48  is mounted on the servomotor  47  to control the moving position of the head slide  46 .  
         [0032]    [0032]FIG. 3 shows the measuring head  41  in more detail, in which the upper feeler  44  is fixedly attached to the upper side of the measuring head  41  and has a contact pad  57  attached to the inside tip thereof. The contact pad  57  defines a reference point of measurement in measuring the diameter of the crankpin CP. The lower feeler  45  is movably mounted on the lower side of the measuring head  41 , and carries a contact pad  58  attached to the inside tip thereof. The lower feeler  45  is deflectable according to the diameter of the crankpin CP, while the distance between the upper feeler  44  and the lower feeler  45  at its undeflected state is made a little bit narrower than the diameter of the crankpin CP to be measured. Approximately at the middle point along the length of the movable feeler  45  is attached a cross-shape spring  54  to permit the movable feeler  45  to expand outward with the cross-shaped spring  54  serving as a fulcrum of rotation during the diameter measurement. The measuring head  41  contains therein a differential transformer  55  having a movable probe  56  which is secured to the other portion (inside the head box  41 ) of the movable feeler  45 . The differential transformer  55  detects the amount of the movement of the probe  56  electrically, and outputs an electric signal representing such an amount to a numerical control unit  30  which will be described hereinafter. The movable feeler  45  is urged by a tension spring  52  to rotate in the direction of the contact pad  58  moving toward the contact pad  57 . When the feeler  45  is in its rest position not performing the measurement as indicated by the solid line in FIG. 3, the other portion of the feeler  45  abuts a stop member  53  which is provided opposite to the probe  56  so that a further rotation of the feeler  45  is limited. The measuring head  41  is rotatably coupled to the head support  43  by means of the hinge pin  42  and is elastically held at a neutral position where the repulsive force of the leaf spring  51  balances with the horizontally pressing force of the measuring head  41  due to its weight. As the measuring head  41  rotates in the direction away from the leaf spring  51  (the leaf spring remains pressing the measuring head with a decreased force) during the measurement, the feeler  44  presses the upper peripheral surface of the crankpin CP with an adequate contact pressure due to the gravity exerted on the measuring head  41  (minus the decreased repulsive force of the leaf spring), while the movable feeler  45  contacts the lower peripheral surface of the crankpin CP due to the tension spring  52 .  
         [0033]    A numerical control unit  30  (FIG. 1) contains programs for the machining operation  31 , for the crankpin diameter measurement and the feed amount computation at the finish grinding step  71 , and so forth, and further contains data processing subunits for parameter registration  32 , for workpiece identification  33 , for parameter retrieval  34 , and so forth. The parameter registration subunit  32  is to register an eccentricity amount of the crankpins CP and phase angle of the respective crankpins CP about the central axis of the crankshaft with respect to each of plural types of crankshafts W for the operations of the crankpin grinding machine  20 , the workpiece identfication subunit  33  is to identify the type (kind) of a crankshaft W to be ground, and the parameter retrieval subunit  34  is to retrieve parameter data representing the eccentricity amount and the phase angle of the crankpins CP with respect to each type of crankshaft W identified by the workpiece identfication subunit  33  from the above-explained parameter registration subunit  32 .  
         [0034]    [0034]FIG. 6 shows a data table included in the parameter registration subunit  32 . The table contains parameters for grinding crankshafts and more particularly, contains eccentricity amounts S 1 , S 2 , . . . , Sn of the crankpins of the respective types # 1 , # 2 , . . . , #n of crankshafts W which eccentricity amounts S 1 , S 2 , . . . , Sn are to be used for determining the advancing position of the measuring head. The table further contains phase angles P 1 , P 2 , . . . , Pm of the existing crankpins on the respective crankshafts, wherein “0 degree” means the direction of nine o&#39;clock, “120 degrees” the direction of five o&#39;clock, “180 degrees” the direction of three o&#39;clock, and “240 degrees” the direction of one o&#39;clock.  
         [0035]    The numerical control unit  30  outputs machining instructions to a spindle servomotor control circuit  22 , a wheel head servomotor control circuit  23  and a table servomotor control circuit  21  by a CPU  35  via an interface  37  based on the parameters registered in the parameter registration subunit  32 . The rotary encoders  6 ,  14  and  18  respectively attached to the servomotors  5 ,  13  and  9  respectively detect movements of the table  2 , the wheel head  10  and the work spindle  17 , and feed back the respective states in controls thereof to the numerical control unit  30  via the interface  37 .  
         [0036]    The journal portion of the crankshaft W gripped by the chuck of the work spindle  17  has a keyway or the like mark defining the reference angular position of the crankshaft W. The plane encompassing the central axis of the journal portion and the center line of the keyway is defined as an angular reference plane, and the plane angle between this reference plane and another plane encompassing the central axis of the journal portion and the central axis of each crankpin CP is defined as a phase angle of each such crankpin CP with respect to the spindle axis. The chuck of the work spindle  17  is provided with a key not shown which is engageable with the keyway formed on the journal portion, so that the crankshaft W is clamped to the work spindle  17  properly in the rotational direction by holding the journal portion by the chuck with the key being engaged with the keyway. Thus, as the work spindle  17  is rotated and indexed to the phase angle of a particular crankpin CP by means of the servomotor  9 , the crankpin CP is indexed and positioned at a proper angle for the diameter measurement by the measuring device  16 .  
         [0037]    The above described embodiment will work as follows. First, the numerical control unit  30  and the crankpin grinding machine  20  are started to operate. The operator then inserts a crankshaft W to be ground into the chuck of the work spindle  17  with the keyway formed at one end of the journal portion of the crankshaft W being engaged with the key formed on the chuck. The operator then inputs a command to make the journal portion to be clamped by the chuck thereby permitting the phase angles of the crankpins to be identifiable, and the shaft center axis of the other journal portion to be supported by the center  19  of the tailstock  8 . The operator next inputs the type # of the crankshaft W to be ground from an input/output device  36  such as a keyboard, and starts the program for the machining operation  31 , the flow chart of which is shown in FIG. 4. At a step  62  of FIG. 4, the type of the crankshaft W so input is identified by the workpiece identification subunit  33  in the numerical control unit  30 , and accordingly the eccentricity amount and the phase angles of the crankpins CP of the crankshaft W of the type as identified by the workpiece identification subunit  33  are retrieved by the parameter retrieval subunit  34  from among the parameter data registered in the parameter registration subunit  32 .  
         [0038]    A step  63  is to index the table position as a preparation for the grinding. The CPU  35  in the numerical control unit  30  outputs move instructions to the work spindle servomotor control circuit  22 , the wheel head servomotor control circuit  23  and the table servomotor control circuit  21 . Thus, the servomotor  5  rotatably drives the ball screw  4  to move the table  2  at a position to index the first crankpin CP to the position opposed to the grinding wheel  15 . The servomotor  9  rotatably drives the work spindle  17  to rotate the crankshaft W gripped by the chuck of the work spindle  17  around the spindle axis, which brings about a planetary motion of the crankpin CP. The servomotor  13  rotatably drives the ball screw  12  to move the wheel head  10  back and forth in synchronism with the planetary motion of the crankpin CP, so that the work spindle  17  and the wheel head  10  perform cooperative movements (i.e. a generating movement) for the grinding wheel  15  to grind the crankpin CP in a cylindrical profile.  
         [0039]    A step  64  is to feed the grinding wheel  15  toward the crankshaft W. In addition to, or in superposition to, the synchronized reciprocating movement of the wheel head  10  for an eccentric cylindrical profile, the wheel head  10  is advanced in the X axis direction in turn at a rapid advance feed rate, a rough grinding feed rate and a fine grinding feed rate as determined based on the retrieved machining parameters for the identified crankshaft W in order to perform a rough grinding process and then a fine grinding process on the crankpin CP with the grinding wheel  15 . When the fine grinding of the crankpin CP is over, the wheel head  10  carrying the grinding wheel  15  is retracted to a predetermined position (standby position), and the work spindle is stopped at the reference angular position (pose) for the diameter measurement of the crankpin as well as for the computation of a feed amount in a finish grinding.  
         [0040]    A step  65  is to measure the diameter of the crankpin in mid course of the grinding process. In the step  65 , a subroutine processing program  71  for measuring the diameter of the crankpin and for computing the feed amount in finish grinding is executed according to a flow chart shown in FIG. 5. Upon starting of this program  71 , the type of the crankshaft W is identified at a step  73  by the workpiece identification subunit  33  based on the type # (e.g. # 1 ) input from the keyboard  36  or the like. The parameter retrieval subunit  34  retrieves from the parameter registration subunit  32  the phase angles of the crankpins CP on the subject crankshaft W (e.g. # 1 ) of the type so identified. At a step  74 , the work spindle  17  is indexed and rotated by the servomotor  9  based on the retrieved phase angle and is positioned to a predetermined phase angle for the first crankpin of the subject crankshaft W, i.e. to the angular direction of nine o&#39;clock in this embodiment. Further, the parameter retrieval subunit  34  retrieves an eccentricity amount S 1  as registered for the # 1  crankshaft W in the parameter registration subunit  32  and determines the measuring position of the crankpin CP of the # 1  crankshaft at a step  75 .  
         [0041]    At a step  76 , the servomotor  47  of the measuring device  16  is rotatably driven with a feedback of the detected signal from the rotary encoder  48 , and advances the head slide  46  to the measuring position by means of the ball screw  50 . In case the crankshaft W is of type # 1 , the head slide  46  is advanced so that the feelers  44  and  45  are located at the position corresponding to the eccentricity amount S 1  of the crankpin CP. Similarly, in case the crankpin shaft W is of type # 3 , the head slide  46  is advanced so that the feelers  44  and  45  are moved to the position corresponding to the eccentricity amount S 3 . In such a manner, even in the situation where the types of crankshaft W to be ground are variously and frequently changed, the advancing position of the measuring head  41  is automatically determined and set in accordance with the eccentricity amount registered in the parameter table of the parameter registration subunit for each crankshaft W to be ground. This eliminates a manual adjustment of the advancing position of the measuring head as would be necessary in the case of using a conventional measuring device. As the head slide  46  advances toward the crankpin CP, the contact pad  57  of the feeler  44  and the contact pad  58  of the movable feeler  45  come in contact with the upper and lower peripheral surfaces of the crankpin CP, at which time the measuring head  41  rotates upward about the hinge pin  42  so that the feeler  44  and the movable feeler  45  slides on the upper and lower surfaces of the crankpin CP. According to the control by means of the rotary encoder  48 , the measuring head  41  is stopped at the position where the centers of the contact pads  57  and  58  touch the uppermost and lowermost points of the crankpin CP for the diameter measurement.  
         [0042]    At this time, the movable feeler  45  in the lower side is pushed outward with the cross-shape spring  54  as the fulcrum of rotation, which in turn moves the probe  56  of the differential transformer  55  into the differential transformer  55 . Then at a step  77 , the position of the probe  56  relative to the differential transformer  55  is converted into an electric value and is transmitted to the numerical control unit  30  as an electric signal representing the diameter of the crankpin CP. The numerical control unit  30  then computes the feed amount of the wheel head  10  for the fine grinding based on the diameter of the crankpin CP thus measured. The feed amount for the fine grinding is a half of the difference between the measured diameter of the crankpin CP and a target diameter registered beforehand. After the measurement is completed, the measuring head  41  is returned to its rest position at a step  78 , whereby the program for measuring the crankpin diameter and computing the feed amount for the finish grinding comes to its end to return to the program  31  for the machining operation resuming at a step  66  of FIG. 4.  
         [0043]    Now back to the program  31  for the machining operation, the step  66  is carried out to infeed the wheel head  10  against the crankpin CP by the amount which has been computed for the finish grinding of the crankpin CP. Then, the wheel head  10  is kept at the final finish position for a predetermined short period of time to continue a zero infeed grinding, i.e. a sparkout. Upon completion of the sparkout, the wheel head  10  is retracted to the grinding start position at a step  67 . Next, at a step  68 , the program checks whether or not, the crankpin CP thus finished is the last crankpin on the crankshaft W under machining. If there still remains another crankpin unfinished on the crankshaft W, the program returns to the step  63  to repeat the aforementioned processing up to the step  67 , whereby the table  2  is indexed to a position to bring another crankpin CP to be ground next before the grinding wheel  15  and whereby such another crankpin CP is finished in the same manner as described above. If the judgment at the step  68  is affirmative (Yes), the processing moves forward to a step  69  to return the table  2  to its rest position before ending the machining operation program  31 . The operator then inputs a command to disengage the crankshaft W from the chuck of the work spindle  17 , and sets another crankshaft W to be ground.  
         [0044]    While the above embodiment is described about the case where each of the crankpins CP of the crankshaft W is directed to and kept at the nine o&#39;clock position in mid course of the grinding operation for the diameter measurement by the measuring device  16 , the measurements of all the crankpins CP may be performed with the crankshaft W being indexed to a fixed angular position. For example, in the case of a crankshaft for a typical in-line four-cylinder engine, the first and fourth crankpins are indexed to the nine o&#39;clock (or three o&#39;clock) position, while the second and third crankpins lying at a phase angle which is different by 180 degrees from the first and fourth crankpins are indexed to the three o&#39;clock (or nine o&#39;clock) position, so that the measuring head  41  is advanced differently between the first and fourth crankpins and the second and third crankpins. By indexing the crankshaft W to a single angular position for the measurements of all the crankpins CP, the time which would otherwise be necessary for indexing the crankshaft W to respective angular positions for the individual crankpins CP prior to the measurement can be shortened. Moreover, the possibility of erroneous indexing can be minimized, thereby minimizing the possibility of causing damages to the measuring head  41  due to erroneous indexing.  
         [0045]    While in the above described embodiment, the crankshaft W is put manually by the operator on the chuck of the work spindle  17  of the crankpin grinding machine  20 , it may be set automatically by utilizing a robot arm or the like.  
         [0046]    Further, in the above embodiment, the operator inputs the type # of the crankshaft W to be ground from the input/output device  36  such as a keyboard, but alternatively some imprint mark may be affixed to the crankshaft end surface and there may be provided a mark reader for reading such a mark, so that the output signal from the mark reader may be input to the numerical control unit  30  for automatically identifying the type of the crankshaft W.  
         [0047]    Further, although in the above embodiment, the back and forth movement of the measuring head  41  is effected by the ball screw  50  driven by the servomotor  47 , a cylinder type driving mechanism  29  may be employed as shown in FIG. 8 in place of the servomotor  47  in FIG. 2. In the case of FIG. 8, the measuring position of the measuring head  41  is determined by an abutment piece  24  of the cylinder type driving mechanism  29  abutting on a stop member  25 . The stop member  25  may preferably provided with two levels of abutment surfaces on which the abutment piece  24  abuts and the two levels may be selectively designated by shifting the stop member  25  by a cylinder  28 . Thus, the measuring head  41  can be positioned at two different positions in the X axis direction. The cylinders  29  and  28  may be of a hydraulic type or a pneumatic type.  
         [0048]    Although in the above embodiment, the measuring head  41  is movable only in the direction of the X axis but not in the vertical direction, it may be equipped with a servomotor  26  and a ball screw (not shown) to be driven vertically, as shown in FIG. 9, so that the feelers can be placed at an arbitrary height automatically even in the case where the crankpin CP is positioned in the direction of twelve o&#39;clock. In this instance, the height of the measuring head  41  is first adjusted by the servomotor  26 , and thereafter the measuring head  41  is advanced in the X axis direction by a cylinder device  27  to measure the diameter of the crankpin CP. As the mechanism of vertically indexing the measuring head  41 , a cylinder device which selectively positions the measuring head  41  to two different heights may be employed in place of the servomotor  26 . Further, the mechanism for moving the measuring head  41  horizontally in the X axis direction may employ a servomotor in place of the cylinder device  27 , so that the measuring head  41  can be moved to any positions in the X axis direction horizontally.  
         [0049]    Further, although the nine o&#39;clock position is preferred as an angular position to which the crankpin CP is indexed for measuring the diameter of the same, other angular positions such as the ten o&#39;clock position and the like may be selected instead. In such a case, however, it is preferable to advance the measuring head  41  in a direction parallel to the line which passes across the central axis of the crankpin CP and the central axis of the work spindle (i.e. crankshaft).  
         [0050]    While several forms of the invention have been shown and described, other forms will be apparent to those skilled in the art without departing from the spirit of the invention. Therefore, it is to be understood that the embodiments shown in the drawings and described above are merely for illustrative purposes, and are not intended to limit the scope of the invention which is defined by any of the appended claims.