Image Number 3 for United States Patent #6637987.
A drill bit capable of axial cutting as well as side cutting, having a generally cylindrical elongate body with a bit diameter, includes a frontmost drilling end for axial cutting, a cutting section extending from the drilling end for side cutting, and a shank extending from the cutting section. The cutting section is helically cut at a helix angle of substantially 15.degree..+-.2.degree. to form one pair of diametrically opposite helical flutes whose outermost edges are sharpened to form respective cutting edges, arranged around a solid central core whose diameter is substantially directly proportional to the bit diameter. The length of the cutting edges in the axial direction of the drill bit body is calculated according to the formula: Cutting Edge=Bit Diameter.times..pi..times.C.sub.c /Tan (Helix Angle), C.sub.c being a constant in a range from 0.85 to 0.90.