Bit Classification

Over the years, bit manufacturers have developed many different types of bits to drill the various formations encountered by the drilling industry. These include I) hard- and soft-rock bits, 2) bits with special bearings, 3) improved lubrication systems, and 4) specialty functions to achieve some specific purpose. The complexity of selecting the proper bit has been further complicated because

Image Drilling Bits
Fig. 7-16 Worn bits (Courtesy Smith Tool Co.)
Types Bit For Drilling Engineering

Fig. 7-17 Bit gauge wear (Courtesy Smith Tool Co.)

manufacturers use different identification names for bits that, for all practical purposes, are similar from one manufacturer to another.

This array of bit names and nomenclature has given rise to the need for a standardized classification system, in 1972, the International Association of Drilling Contractors (IADC) adopted a three-digit classification system for roller hit nomenclature. Most bit manufacturers have adopted the system and generally use the IADC code labeled on the bit transportation box as well as the individual company's brand name. The IADC system was later adopted by the API and now appears as API Recommended Practice 7G.

The system uses a three-digit code for classification, which appears as follows:


A = a number from I to 8, known as the major class B = a number from I to 4, known as the subgroup C = a number from 1 to 9, known as the specialty feature

An overall classification chart is shown in Fig. 7-18.

The major class or group is divided into mill tooth hits (teeth machined as an integral part of the cone) with digits from 1-3 and insert bits (teeth are

Engineering Classification Rock
Fig. 7-18 Bit classification chart (Courtesy Security Rock Bit)
Rock Classification Chart

tungsten carbide segments inserted into the cone) with digits from 4—8. Individual descriptions of each class are shown in Table 7-6,

Each major group is subdivided into four subgroups, denoted by the second digit of the three-digit code. Increasing digits denote a step toward the next higher group. For example, a 1-2 bit is a mill tooth bit designed to drill formations of a slightly greater compressive strength than required for a l-l bit.

The third digit in the three-digit code designates a specialty feature or a combination of specialty features. The designation and description of each is shown in Table 7-7. Note that specialty group 9 is reserved for manufacturers to list bits with designs proprietary to their company.

Table 7-6 Description of Major Group Classification

Group Number Formations

Mill Tooth Bits

1 Soft formations of low compressive strength and high drill-


2 Medium to medium-hard formations with high compressive strengths

3 Hard, semiabrasive or abrasive formations

Insert Bits

4 Very soft formations

5 Soft to medium formations with low compressive strength

6 Medium-hard formations of high compressive strength

7 Hard, semiabrasive and abrasive formations

8 Extremely hard and abrasive formations

Table 7-7 Bit Coding Specialty Groups

Code No. Identification

Table 7-7 Bit Coding Specialty Groups






Gauge insert


Roller seal bearing


Seal bearing and gauge protection


Friction seal bearing


Friction bearing and gauge protection





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