| 摘要 |
A self-centering twist drill bit 20 is formed with a "fishtail" cutting section 32 having major cutting lips 72 and 74 on a working end 26 thereof at a point angle of 190 DEG . Tip relief surfaces 76 and 78 trail cutting lips 72 and 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 and 42 and minor cutting lips 46 and 48. Outer surfaces 58 and 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 and 100 which are also radially relieved. |
