Chatter

Chatter is one of the most important limitations in machining processes causing poor surface finish, decreased tool life, and damage to the machine tool. Additional operations are required to clean the marks left on the surface; however this may not be possible in cases of severe chatter. In short, chatter vibrations result in reduced productivity, increased cost, and inconsistent product quality. Chatter is a self-excited vibration type resulting from the dynamic interaction between the cutting tool and the work material (Tobias 1965; Koenigsberger and Tlusty 1967). For forced vibrations arising in mechanical systems, the excitation is independent of the response, i.e., the vibrations do not affect the forces. In self-excited chatter vibrations, on the other hand, the chip thickness becomes modulated due to the vibration marks left on the surface in a previous pass and the present vibrations causing oscillatory cutting forces which vary at the same frequency with the vibrations. This process is called regen-eration of waviness or chip thickness (or simply regeneration) and is responsible for instability and chatter as shown in Fig. 1. Thus, in regenerative chatter, vibrations and cutting forces are coupled through the process where increase in one of them causes the other to increase resulting in instability. For a stable cutting process, on the other hand, vibrations, and thus the dynamic part of the forces, will diminish in a short time although the process may start with the existence of vibrations due to impacts, step forces, and transients. Modulation in the chip thickness depends on the phase between two successive vibration waves shown with e in Fig. 2. The phase depends on the dynamic prop-erties of the cutting system and the cutting speed. The modulation in the chip thickness disappears if the phase angle & is zero in which case the vibra-tions diminish and the system becomes stable.