In the rotary drilling mechanism, the hole is drilled by a top drive rotating bit to which a pull-down pressure is applied. A hydraulic rotary head or kelly-stem drive provides rotation. The bit is fastened to, and rotated by, a drill string in drilling progress. Feed pressure and rotary torque crush and grind the rock while debris and cutting is flushed from the hole by compressed air, mud or foam, which continuously circulated down the inside of the drill string through water-ways or nozzles in the bit, and upward in annular space between the drill pipe and bore hole.
Feed pressure and rotation rate determines penetration and efficiency. And usually, soft rock requires lower feed pressure and faster rotation speed, while harder rock requires high feed pressure and slower rotation speed.
Rotary drilling is generally used for large-diameter blast-holes or for deep hole drilling and is most effective in soft to medium rock. Water and oil wells, exploration drilling, geothermal research and large open-pit mine drilling operations are typical rotary applications.
Rotary Drill Bits：
Optimum bit styles maximize penetration rate and bit life. So the core of all rotary drill strings is the bit. The most concern in selecting a rotary drilling bit should be to ensure optimum penetration rate, service life and cost effectiveness. Besides, bit selection is oriented by the drilling application, type and drillability of rock to be drilled, the type of flush to be used and the ability to apply load to the bit.
Basically there are four types of rotary drill bits: drag bits, roller cone bits, diamond bits and PDC bits (diamond bits and PDC core bits’ information is on other section).