(57) A rotary drill bit comprises a bit body having a shank for connection to a drill
string, a plurality of cutting elements mounted at the surface of the bit body, and
a passage in the bit body for supplying drilling fluid to the surface of the bit body,
each cutting element comprising a thin hard facing layer, for example polycrystalline
diamond, bonded to a less hard backing layer, for example tungsten carbide. The maximum
instantaneous load to which each cutting element may be subject in use is reduced
by reducing the thickness of the cutting element. The thickness of cutting element
necessary to achieve the required object is defined, alternatively, in terms of the
absolute dimensions of the cutting element or backing layer, in terms of the relative
thickness of the backing layer and hard facing layer, or in terms of the thickness
of the cutting element or backing layer in relation to the overall dimensions of the
cutting face.
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