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In general, a peripheral speed of 8
to 13 feet per second (2.4 to 4 metres per second)
measured on the outside diameter of the impregnated
diamond bit crown will provide an acceptable rate of
penetration. Frequently, a higher rotational speed will
yield a higher rate of penetration. Consideration should
be given to the fact that while a higher rotational speed
will usually reduce the rate of wear on the matrix layer,
it will not allow new layers of synthetic diamonds to become
exposed thus causing the crystals in the original layer
to become flat and polished. At this stage, the rate of
penetration will be reduced to the point that the bit will
no longer cut effectively. Conversely, a low rotational
speed will cause the matrix layer to abrade more readily,
particularly under higher bit loads and will result in the
premature failure of the bit crown.
This type of bit or casing shoe is used
in predominantly medium to soft geological formations.
When compared with impregnated bits whose cutting mechanism
is largely attributed to the creation of local stress
cracks in the rock by the point loading of the individual
diamond crystals, surface-set
bits tend to cut with more of a sawing or ploughing
action. As a result, the rotational speeds used on surface-set
diamond bits are usually lower than those used on similarly
sized impregnated diamond bits. In general, a peripheral
speed of 3 to 8 feet per second
(0.9 to 2.4 metres per second) measured on the
outside diameter of the bit crown will provide an acceptable
rate of penetration. It should be noted that the rotational
speed selected is a function of the size
of the diamonds set on the bit face as well as the
formation conditions. For example, large diamonds (»
3 stones per carat) that are used to drill soft, abrasive
formations would be expected to cut with a pronounced
ploughing action and as such would require a lower rotational
speed approaching the minimum value of the calculated
rotational speed range. Conversely, small diamonds (»
100 stones per carat) that are used to drill harder,
non-abrasive formations would be expected to cut by
means of stress-crack propogation and as such would
require a substantially higher rotational speed approaching
the maximum value of the calculated rotational speed
range.
These types of bits are used in soft to
very soft geological formations. The mechanism by which
they cut is to simply shear the formation and as such,
the rotational speeds required are relatively low when
compared to similarly sized impregnated diamond and
surface-set diamond bits. In order to achieve an acceptable
rate of penetration, the recommended peripheral speeds
measured on the outside diameter of the bit crown for
the various bit types in this category are as follows:
• Triangular-TSP: 3 to 8 feet / sec
(0.9 to 2.4 m /sec)
• Cubic-TSP: 3 to 6 feet / sec (0.9 to 1.8 m /
sec)
• Pax (PCD)-Set: 2 to 5 feet / sec (0.6 to 1.5
m / sec)
• TSP-Sawtooth: 0.7 to 3 feet / sec (0.2 to 0.9 m /
sec)
• Carbide-Chip: 2 to 6 feet / sec (0.6 to 1.8 m
/ sec)
• T/C Sawtooth: 0.7 to 3 feet / sec (0.2 to 0.9
m / sec)
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