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Roller-cone bits are generally used to drill a wide variety of
formations, from very soft to very hard. Milled-tooth (or steel-
tooth) bits are typically used for drilling relatively soft formations.
Tungsten carbide inserts bits (TCI or button bits) are used in a
wider range of formations, including the hardest and most abrasive
drilling applications (see Fig. 1). Fixed-cutter bits can drill an
extensive array of formations at various depths, and include:
 Polycrystalline diamond compact (PDC) drill bits
 Impregnated bits
 Diamond bits

Figure 4.3 – Types of drill bit

The rock drill bit is not the screw-shaped type, but is a
roller-type bit made up of three cone-shaped, toothed cutters
(Figure 8.3). As this bit rotates, under several tons of downward
pressure, it crushes the rock. The drill string is composed of a

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