| 摘要 |
A drill bit 10 comprising a main body 22 having an axis about which it is rotated in use; a cutting face 12, connecting means 16 for, in use, attaching the bit to a source of rotary motion; a gauge region 18 intermediate said cutting face 12 and said connecting means 16, said gauge region 18 comprising at least one member 20, 32 movable between a first position in which the gauge region is bounded by an imaginary tubular surface of constant cross-section co-axial to the axis of rotation; and a second position in which a portion of the member is located radially inwards, with respect to the axis of rotation, of its position when said member is in said first position, the gauge region whilst said member is in said second position being bound by an imaginary three dimensional conical sectional surface; and at least one actuator 24, 36.
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