Out-of-balance and eccentricity of the grinding wheel are the main causes of forced vibrations [Inasaki and Yonetsu 1969, Gawlak 1984]. The wheel, as a source of vibration, can be relatively easily identified through frequency measurement. The main concern with wheel-induced vibration is how to eliminate out-of-balance and wheel runout. There are a number of other sources of forced vibration such as vibration from hydraulic devices integrated into a grinding machine and from floor vibration, which are sometimes more difficult to locate and suppress successfully.
8.2.2 Regenerative Vibration
A great deal of effort has been made to understand the mechanisms of self-excited chatter vibration in grinding. There are various conceivable reasons for process instability, for example, gyroscop — ically induced vibration of the grinding wheels [Hahn 1963]. The regenerative effect is considered to be a major cause of self-excited vibration in grinding. Regenerative vibration is similar to the self-excited vibration experienced in cutting processes [Inasaki, Yonetsu, and Shimizu 1974]. Due to the rotational motion of the workpiece during the material removal process, the waves generated on the workpiece surface, caused by the relative vibration between the grinding wheel and the workpiece, results in a change of depth of cut after one revolution of the workpiece. The phase shift between the surface waves (outer modulation) and the current relative vibration (inner modulation) makes the process unstable when a certain condition is satisfied. A characteristic feature of grinding chatter is, however, that such regenerative effect possibly exists on both the workpiece and the grinding wheel surfaces [Gurney 1965, Inasaki 1975]. This fact makes self-excited grinding chatter quite a complicated phenomenon. We can make a distinction between the two types of regenerative vibration as follows:
• Work-regenerative chatter. The waves generated on the workpiece surface through the regenerative effect grow quite rapidly; therefore, this type of chatter vibration is regarded as one of the constraints when the set-up parameters are determined.
• Wheel-regenerative chatter. On the other hand, waves generated on the grinding wheel surface grow rather slowly due to higher wear resistance of the grinding wheels; therefore, this type of chatter is a determinant for wheel life. When the vibration amplitude builds up to a certain critical limit, it is considered that the grinding wheel has reached the end of its re-dress life and those waves should be removed through truing and dressing.