Page 152 - Drilling Technology in Nontechnical Language

Chapter 6 – DRILL BITS 143

Recently the technology was developed that allows tungsten carbide
teeth to be coated with a layer of diamond. This can significantly increase

the useful life of a TCI bit in abrasive rock.
The outside cutters on a roller cone bit cut at the outside diameter of the
hole. These cutters, called gauge cutters, are especially vulnerable to wear,
and if drilling in an abrasive sandstone, these outer teeth lose material
and cause the hole to be drilled undergauge. (As previously explained,
underguage refers to a less-than-normal bit diameter. For example, if a
12¼" drill bit wears at the outer edge, and as a result, drills a 12" hole, the
hole is considered undergauge because it is less than the unworn (new)
bit diameter.

Fixed Cutter Bits

Fixed cutter bits are divided into diamond bits and polycrystalline

diamond compact (PDC) bits (fig. 6–3). Fixed cutter bits have no moving
parts, so they can drill for a long time as there are no bearings to wear out,
only the cutting surfaces.

Diamond bits drill by wearing out the rock under the bit, producing
very small cuttings called rock flour. Although diamond bits can drill the

hardest rock, they drill slowly and are very expensive. Diamond bits are
generally used in the formations with the highest compressive strength or
in formations that are very abrasive and thus would destroy other bit types
too quickly.

The diamonds used in these bits are normally naturally occurring
industrial-grade diamonds.
PDC bits drill with a disk of diamond mounted on a tungsten carbide
stud. The cutting action is similar to a lathe tool cutting steel. In the right
conditions, they can drill very fast (over 100 ft/hr) for great distances
(thousands of feet). They are quite costly, especially large ones.

PDC bits may be constructed from a machined steel body, where the
tungsten carbide studs are mounted on steel pegs that fit into holes machined

in the body. They may also be constructed from molded tungsten carbide;
these are called matrix body bits. As always, there are trade-offs; steel
bodies are cheaper to produce than matrix bodies, but matrix bodies are
harder wearing and can be produced in complex shapes more easily.

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