STABLE AND EFFICIENT DRILLING PROCESS BY ACTIVE CONTROL OF THE THRUSTFORCE

The objective of thrust force control is to maintain the thrust force at a desired level under large variation of the cutting stiffness that occurs during drilling multi-layered workpieces. An approximate dynam ic model of the drilling process, relating feedrate and thrust force, is formulated as a linear first order system with time-varying gain. T his model is used to formulate a closed-loop controller of the thrust force in drilling a workpiece of composite cutting stiffness. An adapt ive control scheme is applied to cope with the time-varying characteri stics of the plant. The experimental results show that stabilisation o f the thrust force and reduction in machining time can be achieved by the automatic change of the feedrate as a function of workpiece materi al properties.