Experimental and analytical issues in drilling.

Abstract

This research investigates several key issues dealing with the metal cutting principles that govern the operation of drills. A model has been developed for predicting the heat partition into the workpiece in dry drilling. An oblique cutting analysis and an advection heat partition model calculate the heat flux loads on a finite element analysis of the workpiece. Experiments using embedded thermocouples have verified that the model predicts the temperature field in the workpiece reasonably well for a range of drilling speeds and feeds. A study of the effects of thermal distortions in dry drilling combines analyses of the flow of heat into the workpiece and drill. The model predicts the effects of thermal distortions of the drill and workpiece on the diameter and cylindricity of dry drilled holes. Experiments using thermocouples embedded in the workpiece and drill have verified the accuracy of the model. The model predicts that for typical industrial drilling processes, thermal distortions of the drill and workpiece lead to oversized holes with a bell shape. A drill-foil thermocouple system has been developed, which measures the temperature distribution along the cutting edges of a drill. The method can be used for high-speed drilling processes and is simple to implement. Experiments have shown that the method is accurate, produces repeatable measurements, and is sensitive to the drilling conditions, including the speed, feed, and drill geometry. The nature of the indentation zone and its contribution to the total drilling thrust force have been revealed. A three-dimensional mathematical model for the geometry of the indentation zone and an expression for the radius of the indentation zone have been developed. Experiments using a ductile workpiece and an enlarged chisel edge have verified the accuracy of the expression for the radius of the indentation zone. Simulations indicate that the thrust force contributed by the indentation zone increases with feed and wedge angle, and for a typical twist drill, the indentation zone can contribute almost four percent of the total drilling thrust force.