Twist drill

Rock drill comprising an axially elongated drill shank (1) having a first end and a second end, an outside surface, and a cross-sectional area extending transversely of the axial direction thereof, a drill head (2, 12, 22, 32) is formed of hard metal and has a drilling surface and an opposite end surface spaced axially from said drilling surface and secured to the first end of said drill shank, grooves (11) are formed in the outside surface of said shank and extend from the first end thereof towards the second end for conveying drillings from said drill head, wherein the improvement comprises that said drill head (2) has a cross-sectional area extending transversely of the axial direction of said shank smaller than the cross-sectional area of said shank with said first end of said shank forming two circularly segment-like surfaces each on an opposite side of said drill head and the projection of said segment-like surfaces to the drilling surface of said drill head forming a pair of axially extending recesses (3a, 3b), on opposite sides of said drill head, each of said recesses (3a, 3b) registers in the axial direction with one of said grooves at the first end of said shank, said drilling surface of said drill head having a first diagonal located between corners thereof with a main cutter extending along said first diagonal and having a flattened arrowhead-like sloping roof-shaped configuration, said drill head has a generally rectangularly-shaped transverse cross-section with two opposite rounded sides projecting outwardly from said drill shank (1) and two other side surfaces extending between said rounded surfaces and said other side surfaces being planar-shaped, said cross-section of said drill head has four corners with said first diagonal extending between a first pair of said corners and a second diagonal extending between a second pair of said corners, said main cutter having cutting edges extending parallel with said first diagonal with a first section of said main cutting edge located on one side of said first diagonal and a second section of said main cutting edge located on an opposite side of said main diagonal and with a linear apex extending obliquely of said main cutting edge between said first and second sections.