Possibilities and limitations of rotary drilling in hard rock.

Rock drilling forms an important part of operations in many fields of engineering and may be a. major item in the capital cost of any enterprise. In mining, increased mechanisation, large reorganisation schemes, and the adoption of the horizon system, have resulted in an enormous increase in the amount of drilling taking place. It soon becomes apparent that drilling speeds would have to be speeded up considerably if the desired rates of tunnel drivage and reorganisation were to be attained. This led to the development of interest in the adoption of rotary drilling, already successful in coal and soft rocks, to replace percussive drilling in even harder rock. The rotary technique had long been recognised to be inherently capable of higher drilling rates together with several other important advantages, viz. 1. Electrical power may be used instead of the highly inefficient compressed-air systems. 2. Concentrations of small dust sizes is much lower, thus reducing the health hazard. 3. Less noise is produced. Balanced against this however were two main disadvantages:- 1. The applied thrusts required are considerably higher, necessitating power-feeds and rig-mounted machines. 2. The continuous rotation of the bit in contact with the rock results in rapid wear and consequent rapid fall in drilling rate. Of these, the second disadvantage was the more serious until the introduction of rotary bits tipped with a highly wear resistant alloy metal, tungsten carbide. This has resulted in the successful application of rotary drilling to all but the hardest and most abrasive of rocks. Drill-rigs have been designed capable of' supporting high-powered drilling machines and of exerting thrusts often of several thousands of pounds.