Bore bit is a modification of drill bit differed by (in most modifications) smaller linear dimensions. Bore bit is designed for bleeding core drilling out (during prospect boring); by means of bore drill full hole drilling is performed (e.g. holes, slim and sometimes mean diameter shot holes). Depending on mechanical-and-physical properties of the rock being drilled hard alloy and diamond bore bits are generally used.
In the Russia hard alloy bore bits are used for medium hard rock drilling (sedimentary rocks mainly); they are reinforced with metal ceramic hard alloy VK-6 and VK-8 chisels. For gels, chalk-stones, poorly consolidated sandstones, gypsums, chalky clays drilling ribbed hard alloy bore bits (fig. 1, а) are used; for siltstones, argillites, clay and sand slates, lime cutting heads (fig. 1, b) are used. Self-sharpening hard alloy bits are designed for drilling in abrasive rock like sandstones, sand slates, basalts, gabbros, etc. Characteristic feature of self-sharpening bits is thin hard alloy chisels and soft steel supporting plates assuring chisels self-sharpening (fig. 1, c). Generally bore bits of these types are of 59, 76, 93 mm diameter; considerably less are used bore bits of 112, 132, 151 mm diameter; bore bits for hard rock hydropercussion drilling are composed of large ductile VK-15 hard alloy plates and have diameters of 59, 76, 93 and 112 mm. For shot hole auger drilling spear-shaped and cutting bore bits are widely used. Chisel, cross and pilot drill bits reinforced with hard alloy VK-15 and VK-11 plates are used for percussion and rotary pneumatic drilling.
Diamond drill bits are used during tough hard rock drilling. Bore
bit consists of metal ceramic diamond bearing business end (matrix) and
steel body. During manufacturing matrix and body are securely joined against
each other. Matrix hardness is selected in the range of 10-50 HRC depending on
drilled rock hardness and abrasiveness. The more abrasive rock is the
harder matrix should be. To reinforce core bits different quality bortz diamonds
are used. Diamond bore bits are manufactured of three main types: single-layer,
multilayered and impregnated. In single-layer bits diamonds with grit of 10-90
pieces per carat are placed according to special layout securing bit nose
strength balance, besides about 60% of diamonds reinforce bit nose, and about
40% are placed as reaming diamonds in bit side walls. Larger in size and
higher quality diamonds are used as reaming diamonds. Depending on diameter
single-layer bits have from 4 to 12 water grooves. Impregnated bore bits
are used for very tough hard abrasive and fissured rocks. Diamonds in
impregnated bits (with grit of 150-500 pieces per carat) are evenly mixed with
matrix powder mix material and diamonds with grit of 30-40 pieces per carat аre
used as reaming diamonds. There are matrixes impregnated with very fine
synthetic diamonds used. Diamond bits perform at their best with 500—1500
rev/min and 6-12 kN weight on the bit (600—1200 kgf).
Fig. 1. Hard alloy bore bits: а - ribbed (1 - body, 2 - rib, reinforced with hard alloy chisels); b - cutting (1 - body, 2 - cutter); c - self- sharpening (1 - body, 2 - self-sharpening chisel).