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These are large output fluid passages
that are moulded into the outside diameter of a surface-set
diamond bit crown to permit the ejection of large cuttings
from the face of the bit. Typically, the addition of
junk slots to a surface-set bit crown is restricted
to large diameter coring and non-coring bits used in
geotechnical and petroleum drilling applications where
the formations are relatively soft and abrasive, the
bit is set with either large particle size diamonds
or PCD elements and the resulting cuttings tend to be
long and sheared.
The number of junk slots that may be placed
on a bit crown is a function of both its diameter and
waterway configuration and must be divisible by the
number of input waterways in order to establish a uniform
pattern. The preferred number of junk slots is three
(equally spaced) in order to preserve the dynamic balance
of the bit during operation. Consideration should also
be given to the effect of reducing the amount of outside
gauge diameter setting resulting from the incorporation
of junk slots into the bit crown design.
Junk slots may be provided in either the
single channel or double channel designs illustrated
below. Note that while the double channel design is
the most efficient at clearing large cuttings, the single
channel design tends to maximize the amount of gauge
diameter setting on the bit crown outside set diameter.
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