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except, instead of using four plain discs, two discs were used, each
with teeth around the outer circumference. The Grant differential
bit employs a single cutter wheel assembly mounted 15° from
vertical. The small cutter discs on the wheel rim are free to turn as
the whole assembly rotates. It is used primarily as a hole opener.
The Zublin bit, sometimes referred to as the Simplex bit, and
now marketed by, Grant Oil Tools, deserves special mention. The
design is unique in that the single cutter head is angle-mounted and
is retained on its spindle with balls in a bearing race, very much
like cone-type bits. It might be referred to as a 1-cone bit.
Four different types of cutter heads are available for
specialized use such as rolling out collapsed casing, drilling off
whipstocks, drilling hard sand, drilling soft formations and
reaming out ratholes. Zublin bits were first used extensively in
California and foreign fields. They were used to some extent in
East Texas in the early 1930's and have been improved
continuously since.
The rolling cone rock bit has become the most commonly
used type, world-wide. In many respects the basic construction
employed by several manufacturers is similar.
The surfaces in the pressure area – where journal bearing pin
meets cone interior – are carburized mated surfaces.
They are extremely hard and won't shatter under heavy
impact. Thick noncarburized steel core underneath supports
carburized case and acts as a shock absorber to soak up heavy
drilling shocks.
Mated bearing surfaces in Hughes bits' are inlaid with
special metals that add extra wear resistance and protection against
galling.
This marriage of different metals, plus structural clearances
and Hughes' sealed lubrication system, minimizes friction, heat
and turning torque.

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