Chatter suppression in fast tool servo-assisted turning by spindle speed variation

This paper focuses on the investigation, based on theoretical analysis and through machining experiments, of using variable spindle speed machining to suppress chatter in a fast tool servo-assisted noncircular turning process. Noncircular turning is accomplished by controlling the position of the radial cutting tool in the direction normal to the surface of a workpiece with noncircular cross-sections. The process stability is thus more involved than in a general cutting process. This is mainly due to strong dynamic feedback between the machining process and the fast tool servo drive. An enhanced closed loop dynamic model of the noncircular turning process is developed, in which the fast tool servo employs an active disturbance rejection control scheme. By using the spindle’s angular position as the independent variable, the dynamics of the variable spindle speed noncircular turning process are described by a differential equation with linear periodic time-varying coefficients and a fixed delay in the angle domain. The Floquet theory is applied to determine the stability limit. Analytically predicted stability boundaries are compared with those of constant spindle speed machining generated by the Nyquist method. By investigating the effects of spindle speed variation amplitude and frequency on the stability limits, it is shown that a modest increase in noncircular turning stability is given by continuous spindle speed variations due to the limited performance of the fast tool servo controller. Experimental results are also presented, which validate the ability and explanation of increasing noncircular turning stability using spindle speed variations.