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6 DOWNHOLE TOOLS. BITS

The bit at the bottom of the drill string is the indispensable
tool. Without it all other items of the conventional rotary drilling
rig are useless. Other rig items merely support bit functions. Bits
of many designs have been used in rotary drilling but all require
downward thrust plus rotary motion in order to make hole. The
rotary rig is similar to an out-size drill press.
The forerunner of all rotary drilling bits is the fishtail, so
called because the cutting blade resembles the tail of a fish. These
bits plow through the earth and are not suitable for drilling hard
formations. The outlet for drilling mud is near the bit shank, with
considerable length of blade below.
After a period of use in drilling, the blade became dull and
had to be dressed or sharpened. This usually was done at the drill
site, using forge, anvil and manpower at hand. The long blade of a
new bit provided enough steel for many sharpening jobs. Later the
mud outlet was moved down on the blade and improved results
were noted. Sharpening these bits was done by building up and
hardfacing cutting edges.
Fishtail bits were first referred to as drag bits. Included were
diamond point bits used for sidetracking junk and chisel bottom
hits used for spudding through thin hard shells. Four-bladed drag
bits remained popular for many years especially after jet nozzles
were used. Four-way drag bits are dressed by applying hardfacing
metal to the cutting structure.
Diamond bits (Fig. 6.1) currently in use are in fact drag bits,
but are designed expressly for the hardest formations likely to be
encountered. A modified diamond bit combining the basis design
of a 4-way drag bit with diamond studded wear surfaces has
proved useful in drilling medium-to-soft formations. Because
fewer carats of diamonds are used, it is much cheaper than the
conventional diamond bit.

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