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screwing together several joints of pipe, they put the bit on the bottom
of the hole. As the hole deepens, they add joints of pipe.

Rotating Systems
With the bit on bottom, the rig can rotate it in one of three
ways. Many rigs use a machine called a "rotary table," a sort of heavy-
duty turntable. Others rotate the bit with a top drive, a device with a
powerful built-in electric motor mat turns the pipe and bit. And, in
special cases, a slim downhole motor, usually powered by drilling
fluid but in some cases by electricity, rotates the bit. A long metal
housing with a diameter a little less than the hole's holds the motor.
The bit screws onto the end of it.
Generally, the latest rotary rigs use a top drive to rotate the
pipe and bit. However, rigs using rotary tables have been around a
long time and many drilling companies still own and use them.
Moreover, rotary tables are simple, rugged, and easy to maintain.
Rotary rig owners often use downhole motors where they have to
rotate the bit without rotating the entire string of pipe. Such situations
occur when the rig is drilling a slant, or directional hole, a hole that is
intentionally diverted from vertical to better exploit a reservoir. (A
later chapter in this book covers directional drilling in more detail.)
Regardless of the system used to rotate the bit, the driller, the
person operating the rig, allows some of the weight of the pipe to
press down on the bit. The weight causes the bit's cutters to bite into
the formation rock. Then, as the bit rotates, the cutters roll over the
rock and scrape or gouge it out.

Fluid Circulation
By itself, rotating a bit on pipe does not get the job done. The
cuttings the bit makes must be moved out of the way. Otherwise, they
collect under the bit cutters and impede drilling. Recall that the crew
on a cable-tool rig had to stop drilling and bail the cuttings. A rotary
rig crew does not have to bail cuttings, because the rig circulates fluid

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