| 摘要 |
An elongate drill bit (20) with a longitudinal axis (X-X), the drill bit comprising a shank (28), a working end (24), and a fluted section (26) located between the shank and the working end, wherein the shank, the working end and the fluted section are all formed integrally in the drill bit, wherein the working end comprises an outer cutting portion (34) and a central pilot tip (36) extending axially ahead of the outer cutting portion, wherein the pilot tip is formed with at least one pair of minor cutting edges (40, 42; 44, 46) arranged on opposite sides of the axis, and the outer cutting portion is formed with a pair of major cutting edges (70, 72) arranged on opposite sides of the axis, wherein the outer cutting portion has a major diameter (D) defined by the pair of major cutting edges and the pilot tip has a minor diameter (d) defined by the at least one pair of minor cutting edges, wherein the fluted section has a pair of helical lands (32) each with a respective inner surface (92) extending axially rearward from one of the major cutting edges and from one of the minor cutting edges, wherein the helical lands define a web (94) and each helical land defines a respective helical flute (30), wherein the helical lands have a pitch (S) and the web has a web thickness (K) at the pilot tip, characterised in that the ratio of pitch to major diameter (S:D) falls within a range of 380% to 410% and the ratio of minor diameter to web thickness at the at the pilot tip (d:K) falls within a range of 220% to 380%. |
