Feedrate modulation for accurate traversal of trimmed planar offset paths

Abstract

In milling planar shapes with cylindrical tools, CNC machines must employ tool paths offset from the desired part shape by the tool radius. To prevent gouging the part geometry and to ensure continuous paths, the offset path construction invokes trimming and filling operations at tangent discontinuities of the part shape, and smooth concave regions where the tool radius exceeds the part radius of curvature. A constant feedrate of the tool center along the offset path can result in large chip load variations. Conversely, a variable feedrate that (approximately) achieves a constant chip load can cause large acceleration/deceleration rates at the tool center. Moreover, feedrate reduction is required in the vicinity of the trimmed offset path tangent discontinuities, to ensure accurate execution and to minimize vibrations. A feedrate modulation methodology for trimmed offset paths is proposed, incorporating user-specified parameters to balance the competing demands of minimizing path contour error, chip load variations, and the overall traversal time. The feedrate modulation scheme is compatible with simple real-time interpolator algorithms, and its practical performance is demonstrated by real-time data obtained from several experiments performed on a CNC mill with an open-architecture software controller.