Importance

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Abstract

A rotary drill bit comprising a bit body having at least one depending leg at its lower end and at least one roller cutter rotatably mounted on the leg. The roller cutter comprises a frustoconical roller cutter body and a plurality of cutting elements projecting from the cutter body to tips adapted to bear on the bottom of the well bore, with the tips defining, upon rotation of the bit, a first cutting surface of the bit extending over substantially the entire area of the bottom of the well bore. At least one drag cutter extends down from the bit body and comprises a support and a plurality of drag cutting elements on the support, each having a lower cutting edge. The cutting edges of these elements are so arranged relative to the tips of the hard metal cutting elements as to define, upon rotation of the bit, a second cutting surface of generally the same configuration as the first but spaced above it, whereby upon drilling a relatively brittle formation only the hard metal cutting elements bear on the formation for cutting the formation by fracturing it thereby protecting the drag cutting elements, and upon drilling a relatively plastically deformable material which the hard metal cutting elements penetrate to a relatively deep depth without causing substantial fracturing of the formation, the drag cutting elements also engage the formation for improved drill bit cutting action and increased rates of drilling penetration.

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First Claim