A single-lip drill including at least one cutting edge on a drill head has a face which is concave along at least 75 percent of the width thereof and substantially along the entire length thereof, all the way to, and inclusive of, the cutting edge.

A drill including: a drill main body that rotates about an axis of the drill main body; a chip discharge groove formed on an outer periphery of a tip portion of the drill main body; and a cutting edge formed on the tip portion of the drill main body, a rake face of the cutting edge being a wall surface of the chip discharge groove facing a rotation direction of the drill is provided. The cutting edge includes: a major cutting edge portion provided on an inner peripheral side of the tip portion of the drill main body; a chamfer portion provided on a posterior outer-peripheral side relative to the major cutting edge portion; and a convex curved portion that is provided between the major cutting edge portion and the chamfer portion, the convex curved portion being directed toward a posterior end side of the drill main body from the inner peripheral side to the outer peripheral side with a convex curved shape viewed from a side facing the rake face.

A ridgeline between the thinning face and the flank face includes a first thinning portion contiguous to the outer cutting edge, and a second thinning portion contiguous to the first thinning portion. When viewed in a direction along the central axis, the first thinning portion is linear and the second thinning portion is curved. When viewed in the direction along the central axis, the first thinning portion, and a boundary between the first thinning portion and the second thinning portion are located rearward in a rotational direction relative to a first straight line, and are located forward in the rotational direction relative to a second straight line, the first straight line being a straight line passing through the central axis and parallel to the first thinning portion, and the second straight line being a straight line perpendicular to the first straight line.

A cutting element may include a substrate having a non-planar upper surface with a peripheral edge, and an ultrahard layer. The upper surface may include at least one depression formed at least proximate the peripheral edge; and a compressive stress hoop extending around the upper surface adjacent the peripheral edge, extending into the at least one depression, and configured to reduce tensile stress in the ultrahard layer. The ultrahard layer may be on the substrate and may have a non-planar top surface and an interface formed between the ultrahard layer and the substrate.

A flat drill (1) with a shank (6) and a cutting part (8) with front-side main cutting edges (16), which each extend in a plane transversely to the drill axis (3) from a cutting edge corner (22) into the drill center. The flat drill (1) has exactly three main cutting edges (16), which converge at the drill axis (3).

A drill has a helical flute surface, a flank face, and a thinning face. A ridgeline between the thinning face and the flank face constitutes a thinning cutting edge. The thinning cutting edge has a curved thinning cutting edge portion and a straight thinning cutting edge portion. The curved thinning cutting edge portion protrudes forward in the rotation direction. The straight thinning cutting edge portion is contiguous to the curved thinning cutting edge portion. The curved thinning cutting edge portion has a first end portion and a second end portion. The first end portion is a boundary between the curved thinning cutting edge portion and the straight thinning cutting edge portion.

A cutting element may include a substrate having a non-planar upper surface with a peripheral edge, and an ultrahard layer. The upper surface may include at least one depression formed at least proximate the peripheral edge; and a compressive stress hoop extending around the upper surface adjacent the peripheral edge, extending into the at least one depression, and configured to reduce tensile stress in the ultrahard layer. The ultrahard layer may be on the substrate and may have a non-planar top surface and an interface formed between the ultrahard layer and the substrate.