Stable and efficient drilling process by active control of the thrust force

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 dynamic model of the drilling process, relating feedrate and thrust force, is formulated as a linear first order system with time-varying gain. This model is used to formulate a closed-loop controller of the thrust force in drilling a workpiece of composite cutting stiffness. An adaptive control scheme is applied to cope with the time-varying characteristics of the plant. The experimental results show that stabilisation of the thrust force and reduction in machining time can be achieved by the automatic change of the feedrate as a function of workpiece material properties.