| 摘要 |
PROBLEM TO BE SOLVED: To suppress abrasion and cracking by a first bit stage by preventing a material to be completely pierced by the first bit stage. SOLUTION: The multi-stage drill having a chip discharging groove is for drilling a hole in compliance with a demand using bits having different diameters increasing from stage to stage. The drill is equipped with the first bit stage (1-1) structured as a core bit, and at least one second bit stage (1-2, 1-3) positioned to connect to the first bit stage with a step in between. The hole is drilled by the core bit to leave a hole making core forming a column-like shape which does not get machined. COPYRIGHT: (C)2004,JPO |
