Crown Bit

a type of drilling bit, distinguished (in most of its modifications) by smaller linear dimensions. Crown bits are designed for drilling out cores (during exploratory drilling); they are also used for continuous-face drilling (for example, shot holes and blast holes of small and sometimes medium diameter). Carbide and diamond crown bits are generally used, depending on the physical and mechanical properties of the rock being drilled.

In the USSR, carbide crown bits are used for drilling in medium-toughness (mainly sedimentary) rock; they are equipped with cutters made of VK-6 and VK-8 cemented carbides. Ribbed carbide crown bits are used for drilling in clays, chalks, weakly cemented sandstones, gypsums, and marls; cutter crown bits are used in siltstones, argillites, argillaceous and sandy slates, and limestones. Self-sharpening carbide crown bits are designed for drilling in abrasive rock such as sandstone, sandy slate, basalt, and gabbro. The characteristic feature of these crown bits is the presence of fine carbide-tipped cutters and supporting plates made of soft steel, which provides self-sharpening of the cutters. Crowns of these types generally have diameters of 59, 76, or 93 mm (much less frequently, 112, 132, and 151 mm); crown bits for hydropercussion drilling in tough rock are equipped with large plates made of ductile VK-15 carbide alloy and have diameters of 59, 76, 93, or 112 mm. Spear-shaped and cutting crown bits are widely used for auger (screw) drilling of blast holes. Chisel, cross, and stepped crown bits equipped with VK-15 and VK-11 carbide plates are widely used for rotary pneumatic drilling and pneumatic percussion drilling of blast holes.

Diamond crown bits are used for drilling in resistant, hard rock. The crown consists of a ceramet diamond-bearing working part (the matrix) and a steel body. The matrix and body are firmly connected during the manufacturing process. The matrix hardness is selected according to the abrasiveness and hardness of the rocks to be drilled, with Rockwell C hardness of 10–50. The more abrasive the rock, the harder must be the matrix. Various bort diamonds are used for reinforcing crown bits. Diamond crown bits are manufactured in three main types: single-layer, multilayer, and impregnated. Diamonds ranging in grain size from 10 to 90 pieces per carat are distributed in a single-layer crown bit according to a specific pattern, which provides equal strength of the crown cutting face; approximately 60 percent of the diamonds reinforce the crown cutting face (crystalline diamonds), and approximately 40 percent are placed in the side walls of the crown bit as undercutting diamonds. The crystalline diamonds are often crushed. Larger high-quality diamonds are used as undercutting diamonds. Single-layer crown bits have four to 12 flushing grooves, depending on the diameter. In multilayer crown bits the volume diamonds are usually set in three layers. Impregnated crown bits are used for drilling tougher, abrasive, and fissured rock. In impregnated crown bits, the crystalline diamonds (with a grain size of 150–500 pieces per carat) are uniformly intermixed with the powder mixture of the matrix, and diamonds with a grain size of 30–40 pieces per carat are used as the undercutting diamonds. Crown bits having a matrix impregnated with very fine synthetic diamonds are also used. Diamond crown bits operate most efficiently at 500–1,500 rpm and with an axial load on the bit of 6–12 kilonewtons (600–1,200 kilograms-force).