A Novel Helical Milling End-Effector and Its Application

In the aircraft industry, helical milling is utilized to generate holes in difficult-to-machine materials by means of operating a cutting tool on a helical path into the workpiece. This paper proposes a new helical milling device as an end-effector of industrial robots. The end-effector employs a direct drive rotary motor with dual-eccentric structure to achieve high position accuracy of spindle eccentricity (or radial offset), which indicates the distance between the spindle axis and the hole axis in the radial direction. In order to eliminate the adverse effects of spindle eccentric rotation, time-varying cutting force, and other uncertain disturbances during the machining process, a closed-loop servo control system is designed based on the adaptive robust control (ARC) methodology. Comparative experiments and machining experiments have been conducted to verify the effectiveness of the proposed helical milling end-effector. Compared with the PID and deterministic robust control controllers, the ARC controller guarantees higher accuracy of the spindle eccentricity in the presence of parametric uncertainties and uncertain disturbances. Average errors and maximum errors of the spindle eccentricity stay below 0.5 and 1.6 μm when machining holes with the eccentricity ranging from 0.5 to 4 mm, respectively. Surface roughness Ra and roundness of the machined holes in Ti-alloy and carbon fiber reinforced plastic materials are all below 3.2 and 7 μm, respectively. The results show that the proposed helical milling end-effector is suitable for hole-machining tasks in aircraft assembly applications.