Methodologies for computer-aided process planning for parallel machining.

Abstract

Flexible, programmable machine tools with parallel machining capabilities are increasingly a part of production systems. These parallel machines perform multiple operations simultaneously using various combinations of workholding and toolholding devices. One class of parallel machine, the mill/turn, has the ability to both turn and mill in the same setup. With multiple workholding devices, mill/turns can also transfer workpieces from one machining location to another, thus exposing the workpieces's backend without the need for a manual setup. These features combine to give mill/turns distinct advantages over traditional machine tools in reducing part cycle time and increasing part accuracy. To realize their full potential, parallel machines like the mill/turn require Computer-Aided Process Planning (CAPP) systems that automate the task of mapping part design specifications to process plans. The primary focus of CAPP research thus far has been the sequential machining domain. This thesis develops CAPP methodologies for the parallel machining domain, using the mill/turn as representative of the capabilities of parallel machine tools. This thesis develops an architectural framework which divides the planning task into three areas: Pre-processing, design interpretation, and process plan building. Within the pre-processing module, methodologies for identifying the machining axis, locating fixturing surfaces and generating the maximum turnable state (MTS) have been developed. The MTS is the state of the workpiece from which no more material can be removed by turning without gouging the final surfaces of the part. This state defines a mill-to-turn ratio for each part which can be used as a global metric of its suitability for mill/turn machining. Within the design interpretation module, a user-interactive machining volume extraction methodology has been developed for extracting turned and milled machining volumes. Genetic Algorithms (GAs) for sequencing operations on mill/turns and for generating tool groups as part of a tool replacement strategy, have been developed as part of the process plan building module. The objective of sequencing is to optimize the utilization of a mill/turn's resources by minimizing part cycle time. The tool grouping algorithm balances the cost of machine downtime with the cost of lost tool life.