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This paper summarizes the design and implementation of a model-based torque control strategy for drilling. During drilling, the torque often increases due to difficulties with chip evacuation from the drill flutes. Excessive torque can accelerate tool wear or cause torsional failure of the drill. To avoid problems associated with excessive torque, closed loop torque control by manipulation of feedrate was pursued. This strategy simultaneously avoids tool breakage and decreases the cycle time compared to conventional practice. There can be significant cost benefits of torque control due to eliminating tool breakage. For example, reductions in scrap, rework, and machine maintenance costs may be realized. Dynamic models were developed for the drive system, sensing system, and drilling process. These models were subsequently used to design a model-based torque control strategy. Experimental results are presented for conventional twist drilling and form tool drilling applications.

The semantic differential (SD) method was used to characterize the size and shape of burrs created under various cutting conditions, drill size, tool geometry and coatings. Human subjects visually rated the burr using a SD evaluation form. Significant differences were found in tool type and feed rate. A high performance drill with titanium-nitride coating and a high depth/diameter ratio, yielded minimum burr. A lower feed rate resulted in less burr formation in the majority of the cases. Three primary factors emerged, and accounted for 83% of the variances. Factor scores were mapped into the SD space to show the effect of treatments.