Design and modeling for chatter control

Boring bars for single-point turning on a lathe are particularly susceptibl e to chatter and have been the subject of numerous studies. Chatter is, in general, caused by instability. Clearly, the cutting process can be limited to regions of known stable operation. However, this severely constrains th e machine-tool operation and causes a decrease in productivity. The more ag gressive approach is to attack the stability problem directly through appli cation of vibration control. Here, we demonstrate a new biaxial vibration c ontrol system (VPI Smart Tool) for boring bars. We present the experimental ly determined modal properties of the VPI Smart Tool and demonstrate how th ese properties may be used to develop models suitable for chatter stability analysis, simulation, and development of feedback compensation. A phenomen ological chatter model that captures much of the rich dynamic character obs erved during experiments is presented. We introduce the notion that the mea n cutting force changes direction as the width of cut increases due to the finite nose radius of the tool. This phenomenon is used to explain the prog ression from chatter that is dominated by motions normal to the machined su rface at small widths of cut to chatter that is dominated by motions tangen tial to the machined surface at large widths of cut. We show experimental e vidence to support our assertion that a biaxial actuation scheme is necessa ry to combat the tendency of the tool to chatter in both directions. We the n present some preliminary theoretical results concerning the persistence o f subcritical instability as we expand consideration to high-speed machinin g.