DRILLING CROWN (EN: drilling bit; DE: Bohrkrone, Bohrkopf; FR: taillant; ES: broca, trepano, barrena; RU: буровая коронка) is the variety of the drilling rock-destructing tool, which is used for the drilling of the geological-exploratory boreholes with the retrieval of the core, and of the blasting boreholes and blastholes by the continuous bottomhole. The drilling crown for the geological-exploratory drilling represents by itself the steel ring, the lower working end of which is reinforced with the hard alloys, and with the diamonds (see the "Diamond drilling" article), while at the other end, there is created the screw thread for the connection of the crown with the core-retrieving pipe, or with the funnel. Within the USSR, they use for the drilling within the rocks of the low and medium hardness the drilling crowns, which are reinforced with the plates of the VK-6, VK-8 (the rotary drilling), and VK-15 (the hydraulic-striking drilling) hard alloys.
During the drilling of the weak rocks of the I-IV categories (according to the ENV 1971 classification of the Ministry for Geology of the USSR), they use the ribbed crowns, which are made of the hard alloys (of the M type, the diameter is 93-151 millimetres, see the Figure # 1, a). The drilling of the rocks of the V-VII categories is performed by the crowns with the sharpened cutters (of the SM type, the diameter is 46-151 millimetres, see the Figure # 1, b), while the drilling of the rocks of the VII-IX categories is performed by the self-sharpening crowns (of the SA type, the diameter is 46-151 millimetres), with the combined cutters, which are made of the thin hard-alloy and steel plates (see the Figure # 1, v). For the hydraulic-striking drilling, they use the annular drilling crowns with the diameter of 59.76 and 93 millimetres, which are reinforced with 4 or 6 plates, which are made of the hard alloy (see the Figure # 2).
During the drilling of the very strong rocks of the VIII-XII categories, there yield the greatest effect the diamond drilling crowns with the diameter of 76, 59, and 46 millimetres. For the drilling of the blastholes, and of the blasting boreholes, they use the removable drilling crowns, which are reinforced with the plates or pins, which are made of the VK-15 hard alloy. They manufacture the casings for the drilling crowns of the high-strength alloy steels, and connect these casings with the rods with the help of the cone with the angle of 3 degrees 30 minutes, or using the special screw thread, which is calculated for the transmission of the energy of the strike onto the blade through the casing of the crown. The angle of the sharpening for the blade of the drilling crown is 80-120 degrees; the harder is the rock, the greater is the angle of the sharpening.
For the drilling of the shallow blastholes, with the diameter of 35-50 millimetres, they use mostly the single-bit crowns (see the Figure # 3, a). They use the drilling crowns of the cross shape (see the Figure # 3, b) for the drilling of the stronger and cracked rocks. They drill the deep blastholes with the diameter of more than 50 millimetres by the drilling crowns of the stepped shape with the leading blade (see the Figure # 3, v).
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