Page 151 - Drilling Technology in Nontechnical Language

142 Drilling Technology in Nontechnical Language Second Edition

than modern fixed cutter bits, and they are also a lot less expensive. On a
relatively cheap drilling operation (land rig), it is usually more economical
to use roller cone bits except in the smaller hole sizes (8½" or smaller).

Roller cone bits are available with steel teeth milled from the same
block of metal as the cones. These are called steel tooth bits or mill tooth

bits. The other type of roller cone bit consists of steel cones fitted with

teeth made of tungsten carbide, which are fitted into holes drilled into
the cone surface. Mill tooth bits are very robust and will tolerate severe
drilling conditions, but they wear out relatively quickly. Tungsten carbide
tooth bits will not tolerate shock loadings but can drill for long distances
before wearing out. Of the two types, tungsten carbide teeth bits are more
expensive than the same size of steel tooth bit, all else being the same.

Fig. 6–2. Selection of bits (clockwise from top left): PDC bit, TCI bit, natural
diamond bit, and mill tooth bit.

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