Core Bit Types
Core bits are classified according to the cutting structure and type of bearings. There are five basic types of bits used by ODP based on their function or structure: drag, scraper, abrasive, roller cone, and hammer. Drag-type bits have a flat chisel-like surface to plane away soft formations (i.e., clay and chalk).
Polycrystalline diamond compact (PDC) bits use multiple tungsten carbide studs with artificial diamond cutting surfaces in a clawlike scraping action to remove soft formations (e.g., clay and chalk) up to hard claystone and limestone.
Diamond bits use either surface-set or impregnated diamonds to abrade (i.e., sanding-like process) hard formations like shale or basalt.
Roller cone bits rotate cone-shaped rollers encrusted with teeth to remove soft to hard formations through a combination of scraping and crushing processes. Hammer bits use percussion to crush the hard rock around the core.
Smaller bits called "shoes" are screwed onto the bottom of the inner core barrel (e.g., APC, XCB, MDCB, and PCS tools). The shoes on the inner core barrel protrude below the primary roller cone bit and trim the formation to core size. In contrast, the primary core bits in the RCB (Fig. 2, Fig. 3) and ADCB systems cut away most of the formation to create the core (i.e., there is no shoe). ODP most commonly uses a four roller cone type bit.