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down on the bit so the bit cutters can bite into the formation and drill it. Most of the
drill string is made up of drill pipe but crew members make up enough drill collars to
put the required weight on the bit.
Drill collars are either 30 or 31 feet (9.1 or 9.4 metres) long and those made to
API specifications range in diameter from 27/s to 12 inches (73.03 to 304.8
millimetres). To give you an idea of a drill collar’s weight, one that is 30 feet (9.1
metres) long and 6 inches (152.4 millimetres) in diameter weighs about 3,000
pounds (1,361 kilograms). Thus, if the drill crew made up ten joints of this particular
drill collar, the assembly would weigh 30,000 pounds (13, 610 kilograms). The
amount of weight a bit requires to drill efficiently varies considerably and depends
on the type of bit and the type of formation it is drilling. Nevertheless, 30,000
pounds is a good example of bit weight required.
A length of drill pipe is about 30 feet (9.1 metres) long, and drilling people call
each length a “joint of pipe”.
Each end of each joint is threaded. One end has inside or female threads; the
other has outside or male threads. The female end is called the “box,” and the male
end is called the “pin.” When crew members make up drill pipe, they insert, or stab,
the pin end in the box and tighten the connection. Crew members call the threaded
ends of drill pipe “tool joints.” Normally, the manufacturer welds the tool joints onto
the ends of the drill pipe and cuts the threads to API specifications.
Manufacturers do not add tool joints to drill collars. The walls of drill collars
are so thick that it is not necessary. Instead, the manufacturer cuts the threads directly
onto and in the drill collars. Like drill pipe, drill collars also have a box and pin end.
Thus, you can easily distinguish drill pipe from drill collars because drill
collars do not have the bulge at either end that characterizes the tool joints of drill
pipe.

Bits
A rig’s primary job is to rotate a bit on the bottom of the hole. The bit is the
business end of a drilling rig, because the bit drills, or makes, the hole. Bit
manufacturers offer several types of bit in many sizes. They design them to drill a

particular size of hole in a particular kind of formation.
Bits fall into two main categories: (1) roller cone and (2) fixed head. Both have
cutters, which remove rock as the bit drills. Bits have several kinds of cutters. Cutters
for roller cone bits are either steel teeth or tungsten carbide inserts. Cutters for fixed-
head bits are natural diamonds, synthetic diamonds, or a combination (a hybrid) of
cutters. Hybrid bits combine natural and synthetic diamonds, and may have tungsten
carbide inserts. Table 3 lists bits and cutters.

Table 3
Roller cone and fixed-head bit cutters
Roller Cone Fixed-Head Bits
Bits
Steel teeth Natural
diamonds
27