| 摘要 |
<p>A drill, such as a hammer drill, comprises a shank part adapted to be received in a tool-holder, an axially opposite drill head (13), and a conveying part. (11) interconnecting said shank part and said drill head (13). The drill head has a free end surface defining a diametrically extending main cutting edge ( 16) having a tip (17), and a pair of auxiliary cutting edges (18) extending radially in relation to the tip and defining with the main cutting edge a pair of opposite obtuse angles (180° - o) so as to define a pair of opposite sectors (20) of said free end surface. The conveying part (11) defines only two helical flutes (12) opening into the opposite obtuse angled sectors (20). In order to increase the possible drilling speed of the drill, the total cross sectional area of the flutes (12) amounts to 45 - 55 % of the total circular cross-sectional area of the conveying part (11) of the drill. An axially extending inlet portion (22) of each of the flutes (12) has an axial length (I), which is 25 - 35 % of the axial width (w) of each of the helical flutes. The pitch angle of the helical flutes (12) is 30° - 45°, preferably 35° - 40°.</p> |
