Dynamic Modeling of the Thrust Force and Torque for Drilling

Discrete time dynamic models relating the drilling process outputs of thrust force and torque to feed input are presented in this paper. Drilling to depths of greater than a few drill diameters with conventional twist drills, the torque and thrust force tend to increase due to difficulties with chip evacuation. This could lead to undesirable events such as increased wear rates, fracture of the cutting edges, or catastrophic tool failure. Closed loop control of thrust force or torque is a possible solution to this problem. To do this, dynamic models relating the controllable inputs of feed and speed to outputs of thrust force and torque are required. Transient responses of the thrust force and torque to command changes in spindle speed and feed were recorded. Using this experimental data, batch and recursive least squares estimation techniques were used to estimate the prameters of first order models. Variations in spindle speed had no observable effect on either thrust force or torque. By using actual feed as the input, these models are independent of the machine tool drive system. These models can be utilized for closed loop control of thrust force or torque by manipulating the drilling feed.