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Manufacturers continue to meet the challenges with ever more powerful units, reports Paul Hull.
The down-the-hole hammer method of drilling is usually used for drill hole diameters of 114mm to 178mm in hard formations, such as in the quarrying, metalliferous and precious metals industries. The down-the-hole hammer and its impact mechanism operate in the hole. The piston strikes directly on the bit and no energy is lost through the joints in a drill string. "With compressed air sent through the center of the drill rods to the impact mechanism (the hammer) in the hole, all the action takes place at the bottom of the hole", explained Jeff Dudley of Svedala Reedrill.
The exhaust air blows the bottom of the hole clean and carries the drill cuttings up the hole. In theory, the penetration rate for a down-the-hole hammer will be independent of the hole depth. With increased air pressure the impact mechanism will produce more energy and an increase in the penetration rate, so down-the-hole hammers are usually designed to operate at pressures from 10 bar to 24 bar. "In the next few years, we shall probably see pressures increase to 28 bar," added Dudley. "Compressor and hammer manufacturers are developing their technologies all the time." For holes with diameters in the 89mm to 203mm range, down-the-hole (DTH) drilling has been popular and successful for rocks such as sandstone, schist, limestone, dolomites, marble, porphyries, granite, gneiss, quartzites, trap rocks, chert, magnetite and taconite. The compressive strength of these rocks varies from 204 bar to more than 5,000 bar. Rotary drilling is another method used for such work.
The relationship between bit pressure and rotation rate determines drilling efficiency when the rotary cutting method is used for drilling, because a high feed force has to keep the bit insert in contact with the bottom of the hole. The energy losses in rods and bits are minimal. The rotary drilling method seems to dominate blast hole drilling in large open pit mines using hole diameters up to 375mm. For DTH drilling, the rock drill is mounted in front of the drill string and operates below the drilling surface with its impact piston striking directly on the bit. The drill pipes transmit the compressed air into...