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These rods have male threads and they are coupled to

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Unformatted text preview: pes include multiple-insert steels, button steels, double chisel steels and cross edge bit steels, as shown in figure 4.8. 4.8.1 EXTENSION DRILL STEELS Threaded rods can be joined together to form a string, which can be used to drill long holes (longer than the length of one rod). These rods have male threads and they are coupled to other with the use of couplings having internal female threads. There are two types of rods: (i) shank rods – these are the rods with an integral shank and a bit end. (ii) Full section extension rods – they can be round or hexagonal having threads at both ends. In long hole drilling first a shank adapter is inserted into the drill. This transmits the impact energy and rotation from the drill to drilling string. These accessories have been shown in figure 4.8.1,4,7 4.8.2 BITS1,4,7 It is a part of the drilling equipment that performs the crushing work. The part of the bit in contact with the rock is made of cemented carbide in the form of buttons or inserts. The threaded rod is normally screwed into the bit until it bottoms. The impact energy is then transmitted between the end of the rod and the thread bottom of the bit. © 2005 by Taylor & Francis Group, LLC DRILLING 73 The flushing medium is supplied through the flushing hole in the rod and is distributed through flushing holes in the center and/or at the sides of the bit front. Button bits (fig. 4.8) have more wear resistant cemented carbide than insert bits. These bits are available from diameters of 35 mm and upwards. Insert bits are available in wide variety of designs having diameters from 35 mm and upwards. Cross bits and x-bits are the most common insert bits (fig. 4.8). Cross-bits have an angle of 90° between the inserts, where as X bits have 75° and 105°. X bits are used for large dia hole 75 mm. The button bits allow regrinding interval 4–5 times longer than the insert bits. Insert bits are more resistant to heavy gauge wear. Retract bits are used for drilling in a rock where the holes tend to cave behind the bit, making it difficult to withdraw the drilling equipment. DTH bits are made with shanks to fit different drills. The normal size ranges 85–215 mm. Button, core crusher and full face, are the three designs available. The first one is most common and the last one is suitable for drilling in loose rock for filling material. When drilling is made by rotary cutting method use of drag bits is made. Roller bits have been designed for their use during the rotary drilling. This type of bit consists of a bit-body with three movable conical rollers, known as tricone. Buttons are distributed over the three rollers in such a manner that the entire bottom of the hole works when the bit rotates. Different designs are available to suite different rocks. In figure 4.8, the common drilling accessories used during development drifting, raising and tunneling works have been shown. For longhole drilling the common type of drilling accessory has been depicted in figure 4.8. With electric drills different accessories are needed. 4.9 SELECTION OF DRILL4,6 Drill selection: Drill selection for a particular application should be based on the technological and cost factors. It is considered that the lower cost is obtainable in soft rock with rotary drag-bit drilling, in medium and hard rock with rotary roller-bit and rotary-percussion drilling, and in very hard rock with percussion drilling. Use of percussive drills is very common in underground metalliferous mines and tunnels. The rotary drills are common in underground coalmines. In surface mines both types of drills have applications depending upon the rock types. Drilling efficiency: Drilling efficiency can be measured by taking into consideration the following parameters: ● ● ● ...
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