Drill bit structure
 A drill bit structure has a drill bit body 4 and a plate-like hard tip member 9. The cutting end of the drill bit body 4 has a groove 7 extending substantially at right angles to the central drill bit axis, and the hard tip 9 is fitted and fixed in the groove 7. The web thickness is 25-35% of the drill bit diameter and the flute width ratio is 0.4:1 to 0.8:1. The rake angle of each cutting lip 10 measured at positions spaced at least 2/3 of the drill bit diameter radially outward from the drill bit center is in the range of -5.degree. to positive values. 
Sumitomo Electric Industries, Ltd.
Y B B Y B B Y
Y10T B23B B23B Y10T B23B B23B Y10T
additional additional inventional additional additional additional additional
Yoshikatsu Masaaki Nagatoshi
Mori Jindai Kunimori