Abstract:
A grinding machine includes a spindle head for rotatably driving a crankshaft around a journal center as a rotational axis and, two wheel heads that support respective two grinding wheels and that advance and retract in a direction perpendicular to the rotational axis independently with each other. Two of plural pin portions of the rotating crankshaft are simultaneously ground by the respective two grinding wheels, in which rotational phases of the two pin portions are different from each other. Further, the rotational phases of the two pin portions are stored as a combination in a memory. The two pin portions are simultaneously ground in accordance with the combination by the respective two grinding wheels.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for and a method of grinding a crankshaft, more particularly, to an apparatus for and a method of preventing a machining accuracy from deteriorating by restraining a load fluctuation acting on a main spindle when grinding pin portions of a crankshaft. 
     2. Description of the Related Art 
     Since a pin portion of a crankshaft used in an engine is rotatably connected to a connecting rod, it is required to accurately machine the pin portion in its radial dimension and roundness. 
     As disclosed in Japanese Patent Publication (Kokai) No. S54(1979)-71495, it is known such a grinding machine that grinds a pin portion of one crankshaft eccentrically moving around a journal portion as a rotational center, in which two wheel heads are independently advanced and retracted synchronously with a rotation of a main spindle. 
     In such a conventional grinding machine, the pin portion revolves around the rotational center of the journal portion eccentrically by an eccentric distance between the rotational center of the journal center and a center of the pin portion. Namely, as shown in FIG. 9, a rotational direction of the pin portion relative to a normal component of a grinding resistance changes during a grinding operation either in a case that the pin portion exists at a position represented by (a) in FIG. 9 or in a case that the pin portion exists at a position represented by (b) in FIG.  9 . In another words, at the position (a) the grinding resistance acts on the pin portion in a same direction as the rotational direction of the pin portion and however, at the position (b) it acts thereon in a reverse direction relative to the rotational direction of the pin portion. Therefore, there is such a demerit that a grinding accuracy is deteriorated by a load fluctuation acting on the main spindle. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to solve the above mentioned problems and is to provide a machining method for grinding pin portions of a crankshaft in which a deterioration is prevented in a machining accuracy of the pin portions by restraining a load fluctuation acting on a main spindle rotating the crankshaft. 
     Briefly, according to the present invention, two pin portions of one rotating crankshaft having different rotational phase are respectively ground by respective two grinding wheels which are controllably moved synchronously with a rotation of the crankshaft in accordance with pin portion data. In the pin portion data, the two pin portions to be ground simultaneously are memorized as a combination. The two pin portions are different from each other in rotational phase, so that directions of grinding resistance acting on the respective pin portions are also different from each other, Therefore, a load fluctuation acting on a main spindle can be reduced compared with either case that only one pin portion is ground or case that two pin portions having the same rotational phase are simultaneously ground. 
     Further, a rotational phase difference between the two pin portions in the combination is set as 180°. In a case that the grinding wheels on the wheel heads rotate in the same condition, the grinding resistances act on the two pin portions by the same amount in positive and negative directions. Accordingly, the grinding resistances can be almost canceled in each other, so that loads acting on the main spindle by the grinding resistances can be almost canceled also, whereby load fluctuation acting thereon can be reduced. Therefore, grinding accuracy (i.e., roundness) on the two pin portions can be improved. Even if a rotational phase difference between the two pin portions in the combination is set as 60° or 120°, the grinding resistances can be reduced in each other, so that loads acting on the main spindle by the grinding resistances can be also reduced. 
     The load fluctuation acting on the main spindle can be reduced, so that the grinding accuracy (i.e., roundness) on the two pin portions can be improved compared with either case that only one pin portion is ground or case that two pin portions having the same rotational phase are simultaneously ground. The combination of the two pin portions to be simultaneously ground can be freely changed in a condition that the rotational phase difference is set as 60° or 120°. Even if the adjacent two pin portions cannot be simultaneously ground due to the machine construction, the grinding accuracy (i.e., roundness) on the two pin portions can be improved by changing the combination of the two pin portions. 
     Furthermore, a process table is provided in the memory, in which the combination of the two pin portions and a workpiece No. designating variety of the crankshafts are related, so that a machining process is determined based upon the process table. Therefore, the two pin portions having the different rotational phases can be automatically ground by designating the workpiece No. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in with the accompanying drawings, in which: 
     FIG. 1 is a top plane view of a machine tool according to the present invention; 
     FIG. 2 is block diagram of a numerical control unit according to the present invention; 
     FIG. 3 is an explanatory chart for grinding pin portions of a crankshaft used in a straight four-cylinder engine according to the present invention; 
     FIG. 4 is an explanatory chart showing a phase relationship between each of pin portions of a crankshaft in FIG. 3; 
     FIG. 5 an explanatory chart for grinding pin portions of a crankshaft used in a V-type six-cylinder engine according to the present invention; 
     FIG. 6 is an explanatory chart showing a phase relationship between each of pin portions of a crankshaft in FIG. 5; 
     FIG. 7 shows a table for grinding pin portions of a crankshaft according to the present invention; 
     FIG. 8 is a flowchart showing a machining program according to the present invention; 
     FIG. 9 is an explanatory chart showing a relationship between a rotation of a main spindle and a load acting on a main spindle by a grinding resistance; and 
     FIG. 10 is an explanatory chart showing a machining method in the others of a crankshaft according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment according to the present invention will be described hereinafter with reference to the drawings. FIG. 1 shows a top plane view of a grinding machine according to the present invention, and FIG. 2 shows a block diagram of a numerical control unit according thereto. 
     In FIGS. 1 and 2, Z-axis guide rails  2   a ,  2   b  and  2   c  are secured to a base  7  of a grinding machine  1 . Further, a left-side table motor  3  is fixed on the base  7 , to which a ball screw is rotatably connected. On the other hand, a right-side table motor  4  is fixed on the base  7 , to which a ball screw  4   a  is rotatably connected. An encoder  3   a  is attached to the left-side table motor  3  to detect a rotational position thereof, while an encoder  4   a  is attached to the right-side table motor  4  to detect a rotational position thereof A left-side table  10  and a right-side table  20  are slidably arranged along the axis Z-rails  2   a ,  2   b  and  2   c  in a Z-axis direction (direction indicated by an arrow  5 ). On the left-side table  10 , there are arranged fixed pair of rails  11   a  and  11   b , a left-side wheel head motor  12  and a ball screw  12   b , in which an encoder  12   a  is attached to the left-side wheel head motor  12  to detect a rotational position thereof. Similarly, on the right-side table  20 , there are arranged pair of rails  21   a  and  21   b , a right-side wheel head motor  22  and a ball screw  22   b , in which an encoder  22   a  is attached to the right-side wheel head motor  22  to detect a rotational position thereof. 
     A left-side wheel head  30  is slidably arranged along the rails  11   a  and  11   b  in an X-axis direction (direction indicated by an arrow  6 ), on which a grinding wheel  31  is mounted. The grinding wheel  31  takes the form of a disc and is rotated at a high rotational speed by a wheel motor  32  disposed on the wheel head  30 . Besides,  31   a  denotes a rotational center axis of the grinding wheel  31 . 
     On the other hand, a right-side wheel head  40  is slidably mounted along the rails  21   a  and  21   b  in the X-axis direction, on which a grinding wheel  41  is mounted. The grinding wheel  41  takes the form of a disc and is rotated by a wheel motor  42  at the same high rotational speed as that of grinding wheel  31 . Similarly,  41   a  denotes a rotational center axis of the grinding wheel  41 . 
     A work head  50  and a tailstock  52  are arranged on a worktable  53  fixed on the base  7 . A workpiece such a crank shaft  80  is rotatably held at a journal portion  81  thereof around a center axis of the journal portion  81  by the work head  50  and the tailstock  52 . The crank shaft  80  is rotated as described above by a main spindle motor  51  (refer to FIG. 2) arranged on the work head  50 . On the main spindle motor  51 , there is attached an encoder  51   a  to detect a rotational position of the main spindle motor  51 . 
     A truing device  33  is fixed on the spindle head  50  for truing a grinding surface of the grinding wheel  31 , while a truing device  43  is fixed on the tailstock  52  for truing a grinding surface of the grinding wheel  41 . 
     In a numerical control unit  60  (refer to FIG.  2 ), there are provided an input device  61 , a signal bus line  63 , a RAM  64 , a ROM  65 , a CPU  66  for controlling the left-side table  10 , wheel head  30  and a main spindle of the spindle head  50 , a CPU  67  for controlling the right-side table  20  and wheel head  40 , and interfaces (IFs)  62 ,  68  and  69 . The input device  61  is composed of a key input section  61   a  and a display section  61   b , and is connected to the signal bus line  63  through the interface (IF)  62 . Further, the RAM  64 , ROM  65  and CPUs  66  and  67  are connected with each other through the signal bus line  63 . 
     A motor control circuit  71  for controlling the left-side Z-axis table motor  3  is connected to the CPU  66  via the interface (IF)  68 , to which an output from the encoder  3   a  is feedbacked as a detected angle position (rotational position) of the left-side Z-axis table motor  3 . The left-side Z-axis table motor  3  can be controlled by the motor control circuit  71  so as to make zero a difference between a detected value of the encoder  3   a  and a target value in the rotational position of the left-side Z-axis table motor  3 . 
     Further, a motor control circuit  72  for controlling the left-side wheel head motor  12  is connected to the CPU  66  via the interface (IF)  68 , to which an output from the encoder  12   a  is feedbacked as a detected angle position (rotational position) of the left-side wheel head motor  12 . The left-side wheel bead motor  12  can be controlled by the motor control circuit  72  so as to make zero a difference between a detected value of the encoder  12   a  and a target value in the rotational position of the left-side wheel head motor  12 . 
     Furthermore, a motor control circuit  73  for controlling the right-side Z-axis table motor  4  is connected to the CPU  67  via the interface (IF)  69 , to which an output from the encoder  4   a  is feedbacked as a detected angle position (rotational position) of the right-side Z-axis table motor  4 . The right-side Z-axis table motor  4  can be controlled by the motor control circuit  73  so as to make zero a difference between a detected value of the encoder  4   a  and a target value in the rotational position of the right-side Z-axis table motor  4 . 
     Moreover, a motor control circuit  74  for controlling the right-side wheel head motor  22  is connected to the CPU  67  via the interface (IF)  69 , to which an output from the encoder  4   a  is feedbacked as a detected angle position (rotational position) of the right-side wheel head motor  12 . The right-side wheel head motor  12  can be controlled by the motor control circuit  74  so as to make zero a difference between a detected value of the encoder  12   a  and a target value in the rotational position of the right-side wheel head motor  12 . 
     Similarly, a motor control circuit  75  for controlling a main spindle motor S 1  is connected to the CPIJ  66  via the interface (IF)  69 , to which an output from the encoder  51   a  is feedbacked as a detected angle position (rotational position) of the main spindle motor  51 . The main spindle motor  51  can be controlled by the motor control circuit  75  so as to make zero a difference between a detected value of the encoder  51   a  and a target value in the rotational position of the main spindle motor  51 . 
     In the event that a power supply switch of the grinding machine  1  is turned on and that machining data for the crankshaft is input through the key section  61  a of the input device  61 , the machining data therefor is memorized in the RAM  64 . Next, after the grinding wheels  31  and  41  are operated (rotated), the motor control circuits  71 - 75  are respectively controlled in accordance with the machining data memorized in the RAM  64  and programs stored in the ROM  65  by the CPUs  66  and  67 , so that the motors  3 ,  4 ,  12 ,  22  and  51  can be controllably rotated with the motor control circuits  71 - 75 , respectively. 
     The grinding wheel  31  is movable in the Z-axis direction upon rotation of the motor  3 , and is retractably advanced in the X-axis direction upon rotation of the motor  12 . Similarly, the grinding wheel  41  is movable in the Z-axis direction upon rotation of the motor  4 , and is retractably advanced in the X-axis direction upon rotation of the motor  22 . 
     Next, a machining method in a case of using the grinding machine  1  as constructed above will be explained hereinafter. 
     FIG. 3 shows a case grinding pin portions of the crankshaft used for a straight four-cylinder engine, and FIG. 4 shows a phase relationship between the respective pin portions therefor. Besides, a P-axis and Q-axis represent a coordinate axis perpendicular to each other in FIG.  3 . 
     In FIGS. 3 and 4, the crankshaft  80  is to be used for the four-cylinder engine, and there are provided the journal portions  81  as a rotational axis, four pin portions  82   a ,  82   b ,  82   c  and  82   d , and arm portions  83 . The pin portions  82   a - 82   d  are rotatably connected with connecting rods of the engine (not shown), respectively. Further, the pin portions  82   a - 82   d  are fixed to the journal portions  81  through the arm portions  83 , respectively. 
     In a machining operation of such a crankshaft  80  for the straight four-cylinder engine, the pin portions  82   a  and  82   c  are respectively ground as a first grinding process by the left- and right-side grinding wheels  31  and  41 . First, a position of the grinding wheel  31  in the Z-axis direction is coincided with the pin portion  82   a  by moving the left-side Z-axis table  10  with the left-side Z-axis table motor  3 . On the other hand, a position of the grinding wheel  41  in the Z-axis direction is coincided with the pin portion  82   c  by moving the right-side Z-axis table  20  with the right-side Z-axis table motor  4 , at the same time. Subsequently, a movement of the left-side wheel head  30  by the left-side wheel head motor  12  in the X-axis direction is synchronously coincided with a rotation of the main spindle motor  51 . Similarly, a movement of the right-side wheel head  40  by the rightside wheel head motor  22  in the X-axis direction is synchronously coincided with a rotation of the main spindle motor  51 . Therefore, the pin portions  82   a  and  82   c  can be simultaneously ground by the grinding wheels  31  and  41 , respectively. 
     In the above-mentioned situation, a rotational phase difference between the pin portions  82   a  and  82   c  is 180°, i.e., the pin portion  82   c  exists at a position represented by (b) in FIG. 9 when the pin portion  82   a  exists at a position represented by (a) in FIG.  9 . Therefore, a load acting on the main spindle by a grinding resistance of the grinding wheel  31  can be canceled in a rotational direction of the main spindle by that acting thereon due to the grinding resistance of the grinding wheel  41 . According to this result, a load fluctuation in the main spindle is restrained, so that a grinding accuracy on the workpicce can be improved. 
     Next, as a second grinding process similar to the above-described first machining process, the pin portion  82   b  is ground by the left-side grinding wheel  31 , while the pin portion  82   d  is ground by the right-side grinding wheel  41 . In this second grinding process, the rotational phase difference between the pin portions  82   b  and  82   d  is also 180°, so that the load acting on the main spindle by the grinding resistance of the grinding wheel can be canceled. 
     FIG. 5 shows a case grinding pin portions of the crankshaft used for a V-type six-cylinder engine, and FIG. 6 shows a phase relationship between the respective pin portions therefor. Besides, a P-axis and Q-axis in FIG. 6 are the same as that shown in FIG.  4 . 
     In FIGS. 5 and 6, the crankshaft  90  is to be used for the V-type six-cylinder engine, and there are provided a journal portions  91  as a rotational axis, six pin portions  92   a ,  92   b ,  92   c ,  92   d ,  92   e  and  92   f , and arm portions  93 . The pin portions  92   a - 92   f  are rotatably connected with connecting rods of the engine (not shown), respectively. Further, the pin portions  92   a - 92   f  are fixed to the journal portions  91  through the arm portions  93 , respectively. Each of the pin portions  92   a - 92   f  is arranged so that the rotational phase difference between each of the pin portions  92   a - 92   f  is 60° in turn. 
     In the crankshaft  90  for the V-type six-cylinder engine similar to the machining process for the straight four-cylinder engine, two of the pin portions is so selected that its rotational phase difference therebetween is 180°, and are simultaneously ground by the grinding wheels  31  and  41 , respectively. 
     Namely, the pin portions  92   a  and  92   f  are respectively ground by the grinding wheels  31  and  41  in a first grinding process. In a second grinding process, the pin portions  92   b  and  92   d  are ground by the grinding wheels  31  and  41 , respectively. Further, in a third grinding process, the pin portions  92   c  and  92   e  are ground by the grinding wheels  31  and  41 , respectively. In a case that such grinding processes are performed, the load acting on the main spindle by the grinding resistance of the grinding wheel is canceled, so that the machining accuracy on the workpiece can be improved. 
     In the machining operations according to the aforementioned grinding processes, the pin portion  92   b  and the pin portion  92   c  adjacent thereto are simultaneously ground in the second grinding process and thereafter, the pin portion  92   d  and the pin portion  92   e  adjacent thereto are simultaneously ground in the third grinding process. According to a size (a distance in width between adjacent two pin portions) of the crankshaft, it may occur that the adjacent two pin portions cannot be simultaneously ground because of an interference between the left-side wheel head  30  and the right-side wheel head  40 . With this reason, the following grinding processes may be adopted as another embodiment. 
     In a first grinding process, the pin portions  92   a  and  92   f  are respectively ground at the same time by the grinding wheels  31  and  41  and thereafter, the pin portions  92   b  and  92   d  are respectively ground thereby at the same time as a second grinding process. Further, the pin portions  92   c  and  92   e  are respectively ground by the grinding wheels  31  and  41  at the same time. 
     In this situation, the load acting on the main spindle by the grinding resistance of the grinding wheel cannot be canceled perfectly similarly to a case that simultaneously grinds the two pin portions in which its rotational phase difference therebetween is 180°. However, the two pin portions in which rotational phases are different (120°) are ground simultaneously, so that the load fluctuation acting on the main spindle by the grinding resistance of the grinding wheel can be reduced compared with a case either that only one pin portion is ground or that the two pin portions having the same rotational phase are ground simultaneously. 
     In this embodiment, it is explained about the machining operation for the crankshaft used in the straight four-cylinder or V-type six-cylinder engine and however, a shape of the crankshaft cannot be limited to that in this embodiment. In the other shape of the crankshaft, similar machining operations can be adopted, for example, the combination of the simultaneous machining operation may be adopted as shown in FIG.  10 . 
     FIG. 7 shows a process table for simultaneously grinding by the grinding wheels  31  and  41  two pin portions having the different rotational phases in each variety of workpiece (workpiece No.). If such a process table is memorized in the RAM  64  beforehand, the simultaneous machining operation in the two pin portion having the different rotational phases can be automatically performed by commanding only a workpiece No. 
     In FIG. 7, “workpiece No. 1” and “workpiece No. 2” represent a crankshaft used in the straight four-cylinder engine and a crankshaft used in the V-type six-cylinder engine. 
     Further, “workpiece No. 3” represents another type of a crankshaft used in the V-type sixcylinder engine. 
     In “workpiece No. 1”, a first pin portion (corresponding to the aforementioned pin portion  82   a  ) and a third pin portion (corresponding to the aforementioned pin portion  82   c  ) are simultaneously ground in a first grinding process. Thereafter, a second pin portion (corresponding to the aforementioned pin portion  82   b  ) and a fourth pin portion (corresponding to the aforementioned pin portion  82   d  ) are simultaneously ground in a second grinding process. 
     In “workpiece No. 2”, a first pin portion (corresponding to the aforementioned pin portion  92   a  ) and a sixth pin portion (corresponding to the aforementioned pin portion  92   f  ) are simultaneously ground in a first grinding process Thereafter, a second pin portion (corresponding to the aforementioned pin portion  92   b  ) and a third pin portion (corresponding to the aforementioned pin portion  92   c  ) are simultaneously ground in a second grinding process. Further, a fourth pin portion (corresponding to the aforementioned pin portion  92   d  ) and a fifth pin portion (corresponding to the aforementioned pin portion  92   e  ) are simultaneously ground in a third grinding process. 
     In “workpiece No. 3”, a first pin portion (corresponding to the aforementioned pin portion  92   a  ) and a fourth pin portion (corresponding to the aforementioned pin portion  92   f  ) are simultaneously ground in a first grinding process. Thereafter, a second pin portion (corresponding to the aforementioned pin portion  92   b  ) and a sixth pin portion (corresponding to the aforementioned pin portion  92   d  ) are simultaneously ground in a second grinding process. Further, a third pin portion (corresponding to the aforementioned pin portion  92   c  ) and a fifth pin portion (corresponding to the aforementioned pin portion  92   e  ) are simultaneously ground in a third grinding process. 
     The machining operation using the aforementioned process table will be explained hereinafter with reference to a flowchart shown in FIG.  8 . In step S 10  “workpiece No.” to be machined is input and then, in step S 11  a variable “N” indicative of a grinding process is set to “1”. 
     Next, in step S 12 , a pin portion number to be machined in “N th ” grinding process designated in step S 10  is read from the process table in FIG.  7 . For example, in the first grinding process of workpiece No. 1, the pin portion number “L=1” and “M=3” are read. 
     Thereafter, in step S 13 , the left-side wheel head  30  is moved by the left-side Z-axis table motor  3  so that the grinding wheel  3   1  is indexed at the front of the first pin portion (corresponding to the aforementioned pin portion  82   a  ). Similarly, the right-side wheel head  40  is moved by the right-side Z-axis table motor  4  so that the grinding wheel  41  is indexed at the front of the third pin portion (corresponding to the aforementioned pin portion  82   c  ). 
     In step S 14 , profile data (data indicating a position of the wheel head relative to a rotational angle of the main spindle to synchronize a advance-and-retractive movement of the wheel head to a rotation of the main spindle) is read from the RAM  64  in order to grind each of the pin portions. Thereafter, the two pin portions are simultaneously ground based upon this read profile data. In step S 17 , “N” is counted up (incremented by “1”). The aforementioned steps are repeated until it is judged such a last grinding process in step S 16 . 
     Obviously, numerous 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.