| 摘要 |
A rotary drill bit assembly is disclosed comprising a cylindrical body having a central opening in the lower face thereof which defines a chamber therewithin. An annular main cutting head is carried at the lower end of the body, and a supplemental bit assembly is supported from within the chamber which has an annular cutting head disposed perpendicular to the axis of the body and eccentric thereto. The outer radius of the supplemental cutting head and inner radius of the main cutting head substantially coincide along a line parallel to the body axis, with the outer radius of the supplemental head also extending at least to the axis of the central opening. Means are provided for rotating the supplemental cutting head relative to the main cutting head.
|
