| 摘要 |
<p>A self-centering twist drill bit (20) is formed with a ''fishtail'' cutting section (32) having major cutting lips (72, 74) on a working end (26) thereof at a point angle of 190°. Tip relief surfaces (76, 78) trail cutting lips (72, 74). A pilot tip (38) on the working end (26) extends axially from the ''fishtail'' cutting section (32) and is formed with a split point having leading cutting edges (40, 42) and minor cutting lips (46, 48). Outer surfaces (58, 60) of pilot tip (38) are each formed with a radial relief and a back taper and joins with cutting section (32) along fillets (92, 100) which are also radially relieved.</p> |
