Abstract: A twist drill is made of a carbide alloy and has a pair of helical flutes. The flutes are defined by heel surfaces. Each heel surface bulges, or is formed arcuate, and contacts a margin. The center of the arc is located in the drill. The radius of curvature of each heel surface is from one sixth to one third of the diameter of a drill portion. Stress that acts on heels due to forming holes is dispersed by the arcuate shape. This improves the rigidity of the twist drill. Also, swarf is reliably cleared and is not stuck in the flutes. This improves the smoothness of the surface of the formed hole.

Abstract: A plurality of cutting tips are mounted on the distal end of the cylindrical main body of a drilling tool for performing trepanning. These tips are disposed along the circumferential direction of the main body and equidistantly spaced apart from one another. The force, which is produced by all of the cutting tips and acting at right angles to the axis of the main body, is smaller than the component of the cutting force, which is produced by each tip and acting at right angles to the axis of the main body. The cutting widths of the tips in the radial direction of the main body are substantially equal. One of the tips, which is located at an intermediate position in the radial direction, has a V-shaped leading portion.