| 摘要 |
<p>The instrument consists of a shaft (10), equipped at one end with a cutting head holder (12) which has a bayonet joint and lock. A hemispherical or conical rotary cutting head (1) has inner radial rods (4) to engage with the bayonet joint. The cutting head contains an axial guide tube (5) between the inner ends of the radial rods and a central aperture (3) to receive a twist drill (9) which makes a hole in the middle of the recess formed by the cutting head. The lower end of the guide tube has one or more notches to receive corresponding studs on the twist drill so they rotate together. The holder (12) has a projection to block the drill in the guide tube. Additionally, the end of the guide tube, which adjoins the centre of the cutting head, has at least two projecting cutting teeth. Where a central hole in the recess is not required a shorter twist drill is employed.</p> |
