Suppression of chip load variations by real-time spindle speed modulation

Abstract

In machining a fixed depth of cut at a constant feedrate to generate a desired curvilinear shape, substantial variations in chip load can occur whenever the smallest concave radius of curvature is comparable to the tool radius. These chip load variations may result in a poor quality of the machined surface or premature tool wear. Conversely, attempting to suppress chip load variations by modulating the feedrate may incur high rates of feed acceleration, that may tax the machine drive systems or induce large contour errors. To address these conflicting influences, the feasibility of minimizing variations in chip load through real-time spindle speed modulation is examined herein. A second-order model is employed to determine the tool angular speeds and accelerations that are required to achieve a specified constant chip load for a given depth of cut along a desired part shape defined by a parametric curve, using a constant feedrate for a tool with a given radius and number of cutting edges. For a spindle driven by a DC motor, the motor voltage variation required to generate the modulated spindle speed is also determined. These analyses facilitate an a priori assessment of the part geometries and process parameters for which spindle speed modulation is a viable approach to the suppression of chip load variations.