| 摘要 |
<p>A drill rig narrow-gauge holes to be drilled to great depths consists of a drill bit, made of cemented carbide or steel and connected by a press fit with the end of a tube. In-side the tube, a drill collar can be raised and dropped in quick succession by a thin rope to hit the rounded rear of the drill bit. The drill collar has a one-sided chamfer or recess at the upper end, and a spherical face, offset in the direction opposite to the chamfer, at the lower end. This is a low-cost rig which automatically corrects any tendency of hole deviation by the combination of a recess or chamfer with an offset hemispherical surface.</p> |
