AN INTELLIGENT FIXTURING SYSTEM WITH OPTIMAL DYNAMIC CLAMPING FOR PRECISION MACHINING

Abstract

Both proper fixture design and optimum fixturing execution are crucial to the quality of the finished workpiece in manufacturing. One of the essential problems in fixture design is the generation of clamping configuration in terms of clamp placement, clamping sequence, and clamping intensities. An ill-conceived clamping layout would cause the final accuracy of the workpiece to be out of the specified tolerances. At the workholding execution stage, under- or over-clamping of the workpiece and inconsistent clamping actuation are a common problem on the shop floor, which are harmful to the workpiece quality assurance and even bring about unnecessary rejects. This research attempts to solve these problems through the development and application of an intelligent fixturing system that provides a comprehensive approach to the workpiece fixturing task. The system consists of an automated clamping generation algorithm for viable fixture configurations and a "live" fixture with sensory feedback and on-line fixturing control strategy to perform an optimal fixturing operation. The highlights of the work include the following: A geometric reasoning methodology for automatic clamping scheme generation in terms of optimal clamping points and positive clamping sequence is developed. The algorithm is drawing on the metric of force closure that guarantees the total constraint of a workpiece. As the basis of the algorithm, a computational geometry approach to force closure verification is derived and a supplementary condition of clamping equilibrium is proposed for generating robust clamping configurations. Both direct computation and non-linear programming approaches to the computation of dynamic minimal clamping intensities are formulated. The direct generation method is applicable to planar fixturing problems while the latter is suitable for general workpiece with surface contacts. Frictional effect is taken into account through a complete representation of contact forces at the interfaces between the part and fixturing elements. Cutting force dynamics is considered in optimising the minimal clamping forces. Experimental investigation of workpiece-fixture interaction in terms of real-time reaction forces on locators throughout the machining process is carried out, providing insight into the dynamic nature of the workpiece-fixture system. It is shown that friction does play an important role in keeping the workpiece stable during machining. An FEM model of the workpiece-fixture system is developed to examine the workpiece deformation under the cutting and clamping forces and the resultant effect on machining errors. It is proven by the validated model that approach to enhance workpiece accuracy through the manipulation of fixturing parameters in terms of clamping dynamics is feasible. An evaluation method to determine the workpiece displacement within a fixture after clamping actuation is derived. A tool path compensation procedure is developed to fit the actual position of the workpiece before the machining starts. Based on a novel dynamic clamping actuator capable of delivering accurate and changeable clamping intensity, the proposed intelligent fixture is instrumented through system software and hardware integration. Through a series of experiments, the developed intelligent fixturing system has proven its value for workpiece quality improvement and productivity enhancement.