The bit is coiled and
types employ a diamond-studded bit to
the hole continuously cleaned by water or compressed air pumped through
the drill rods.
The core-type bit consists of a hollow steel cylinder, the
end of which is studded with diamonds. The bit is fitted to the lower end of a
hollow steel chamber (core barrel) that is rotated rapidly while the bit is
held firmly against the rock so that the diamonds cut an annular channel in
the rock. The rock that lies within the channel and projects into the barrel
constitutes the core.
(b) Plug type. Two varieties of plug bits are available commercially.
One is a concave type, the head of which is depressed toward the center; and
the other, a pilot type, has a protruding element, cylindrical in shape, but of
s mailer diameter than the main bit head. Noncoring diamond bits have a wide