The development of down-the-hole (DTH) drilling stemmed from the realization that hole accuracy and the efficiency of transmitting power from drill to bit could be improved by placing the hammer as close as possible to the business-end of the drill string. As far as hole accuracy was concerned, by the late 1940s the need for deeper and deeper holes in both surface mines and quarries had led to problems with increasing deviation with depth, especially with small-diameter bits mounted on conventional sectional drill rods. Meanwhile, the nature of the rods themselves, and the way that they were joined using couplings, reduced the energy-transfer efficiency, a problem that only increased as the threads on rods and couplings alike became increasingly worn. The solution, which may appear obvious today but was radical at the time, was to ‘hit the bit’ directly. |
