19.12.1 Definitions
The following definitions are helpful in making distinctions between different types of vibrations that may be experienced in centerless grinding.
Marginal stability is where waviness neither builds up nor decays as grinding proceeds without direct excitation of the marginally stable frequency. In practice, a linear system excited at a frequency of marginal stability will experience large vibrations only limited by system nonlinearity. Convenient waviness was described earlier in this chapter. Convenient waviness is waviness that can persist and is not corrected by the rounding geometry. Convenient waviness that neither builds up nor decays is termed “marginal geometric stability.”
There has also been discussion of the mechanism of rounding. In the case of rounding, it is implied that the system is stable and roundness errors are reduced as grinding proceeds. In a stable system, waviness decays. However, it should be remembered that a stable system produces waviness when there is a source of vibration excitation.
An unstable situation is where waviness and vibration build up as grinding proceeds without a need for direct excitation. If the waviness builds up primarily on the workpiece, the instability is termed “work-regenerative chatter.” Another type of instability is where waviness primarily builds up on the grinding wheel circumference or on the control wheel circumference. This second type of instability is termed “wheel-regenerative chatter.” Wheel-regenerative chatter often builds up slowly with the grinding of successive workpieces so that the instability may not be apparent until a number of workpieces have been ground. The waviness on each succeeding workpiece increases with the waviness on the wheels.
In machining, an unstable system is said to chatter, since in metal turning and milling, unstable vibrations are characterized by audible vibrations or “chatter.” In grinding, unstable regeneration of waviness and vibrations may occur, but due to a more gentle character the vibrations are not
necessarily audible. The term “chatter” is used to describe regeneration of waviness and vibration due to an unstable system, whether it is audible or inaudible. Chatter should not be confused with forced vibration.