Abstract : Performance testing of twist drills on AISI 4140 alloy steel was conducted by measurement of drill geometrics, drilling forces, drill wear, chip formation, and geometrical accuracy, and surface finish, of drilled holes. Major investigations were conducted using 1/2 inch diameter, H.S.S., taper-shank drills, of three manufactures, having six different, standard and special, geometrics. A dynamometer was designed and constructed to measure the drilling thrust, tongue and radial forces; and, effects of cutting conditions on drilling forces, and sources of force variations, were analyzed. Drill wear was studied by photographs of wear progress. Wear on drill lips and chisel edges was visually detectable when using a cutting fluid; however, built-up-edge (BUE) predominated when drilling dry. An extensive study was made of drilled hole accuracies in size, straightness, roundness and concentricity. Criteria for determining and controlling drill life are discussed in terms of the measurements made, differences in drills, and drilling speeds.