Conventionally, a workpiece is processed by a processing apparatus including a base, a workhead table, a grinding wheel table, and a numerical control apparatus, for example. With reference to FIG. 4A, a workpiece Wp is rotated about a workpiece rotational axis (hereinafter referred to as C-axis), and a cylindrical grinding wheel 300 is rotated about a grinding wheel rotational axis which is parallel to the C-axis, thus processing (grinding) the workpiece Wp. When grinding the workpiece Wp, as illustrated in FIG. 4B, the grinding wheel 300 is relatively moved to be close to the workpiece Wp along a direction being perpendicular to the C axis (workpiece rotational axis). A cylindrical surface We of the workpiece Wp is ground by a cylindrical surface 300E of the grinding wheel 300 accordingly. Herein, a workhead motor for rotating the workpiece Wp about the C axis (the workpiece rotational axis), a grinding wheel table driving motor for moving the grinding wheel 300 to be close to and away from the workpiece Wp in a direction being perpendicular to the C axis (hereinafter, the direction is referred to as X-axial direction), a grinding wheel driving motor for rotating the grinding wheel 300 about the grinding wheel rotational axis, and a workhead table driving motor for slidably moving the grinding wheel 300 relative to the workpiece Wp in a direction being parallel to the C axis (hereinafter, the direction is referred to as Z-axial direction), are driven by driving signals outputted from the numerical control apparatus (control means).
Herein, when processing (grinding) the workpiece Wp as illustrated in FIG. 4B by means of the known processing apparatus, regular spaced recesses and/or protrusions may be formed at a processed surface (cylindrical surface We) of the workpiece WP and a riblike mark Bm (so called chatter mark) is formed at the processed surface, as illustrated in FIG. 4C. When grinding the workpiece Wp by moving the grinding wheel 300, which rotates about the grinding wheel rotational axis, so as to contact the workpiece Wp while rotating the workpiece Wp about the workpiece rotational axis (C-axis), a deviation of a position of the grinding wheel 300 in the X-axial direction (i.e., a difference between a target position and an actual position) is periodically generated because of vibration due to an unbalance of the grinding wheel 30. Then, a distance (distance between central points) between a central point of the grinding wheel (grinding wheel rotational axis) and a central point of the workpiece (the workpiece rotational axis) in the X-axial direction is periodically changed. Then, a correlation between the rotational speed of the C-axis and a frequency, which is generated by a change of a distance between central points of the grinding wheel 300 and the workpiece Wp, is in an integral multiple proportion. Such process may cause the formation of the riblike mark Bm on the cylindrical surface We of the workpiece Wp. On occasions, the riblike mark Bm may be so small that only a person skilled in the art can recognize the riblike mark Bm. In a case where the workpiece Wp is assembled to a manufactured product, for example, without noticing that the riblike mark Bm is formed at the completely processed (ground) workpiece Wp, noise and/or vibration may be generated. Therefore, it is preferable that the riblike mark Bm is not formed even if it is so small so as not to be recognized by a naked eye. Further, as illustrated in FIG. 5, when grinding an end surface Wt of the workpiece Wp (i.e., a surface being perpendicular to the workpiece rotational axis) by means of an end surface 300T of the grinding wheel 300 (i.e., a surface being perpendicular to the grinding wheel rotational axis) by relatively moving the grinding wheel 300 in the Z-axial direction, the riblike mark BM may be formed at the end surface Wt of the workpiece Wp.
Therefore, JP1996-174379A (hereinafter referred to as reference 1) discloses a chattering vibration restraining method for repeating a process, where the vibration occurring when the workpiece is ground is monitored and the generation of chattering vibration is detected by means of a vibration detecting device (a vibration sensor) and a frequency analyzing device (an FFT analyzer) assembled to the processing apparatus, and further where driving conditions, such as the rotational speed of the grinding wheel and the rotational speed of the workpiece, are changed, until the chattering vibration is restrained.
According to the processing apparatus disclosed in the reference 1, the chattering vibration is detected at the workpiece Wp, the grinding wheel 300, and various driving conditions such as the rotational speed of the workpiece Wp (rotational speed of C-axis), the rotational speed of the grinding wheel 300, the feeding speed of the grinding wheel 300, and so on, by means of the frequency analyzer. Further, a driving condition where the chattering vibration is not generated should be assured. Thus, a large effort is required. Further, in a case where one of the rotational speed of the workpiece Wp (the rotational speed of the C-axis), the rotational speed of the grinding wheel and the feeding speed of the grinding wheel is changed when the chattering vibration is detected, the speed is normally changed to be slower, thus extending a cycle time and leading to a reduction of a processing efficiency. In addition, when one of the above described speeds is changed, the processing accuracy and/or a quality of the processed surface of the workpiece Wp may be affected. Still further, a three-phase motor, of which rotational speed is constant, may be employed as the driving motor for rotating the grinding wheel 300. In a case where such three-phase motor is used, the rotational speed of the grinding wheel 300 is not changed. In such a condition, in order to change the rotational speed of the grinding wheel, the driving motor is required to be replaced by a motor, such as an inverter motor, of which rotational speed is changeable, so that extra man-hour and cost is required to replace the driving motor.
A need thus exists for an apparatus and method for grinding a workpiece which are not susceptible to the drawback mentioned above.