Conventional core drilling is used mainly for mineral exploration, large diameter coring for coal drilling or other bulk sampling, coring underground in areas of conined space, or when drilling for geotechnical or environmental reasons to take core samples at the botom of holes drilled by other methods (e.g. hollow auger, rotary mud, or air drilling).
Drill rods are rotated at relatively high speed and pressure is applied to the bit using either hydraulic rams or a screw feed applied through a gearbox. Bits are hollow and made from diamond or tungsten chips set into matrixes of various hardnesses. These bits are screwed onto the front of a hollow barrel that receives the core left after the drill bit cuts its way into the rock that surrounds it.
The core barrel is pulled to the surface each time the barrel is full of core, or if the core blocks will not move up into the barrel. Heavy wall hollow rods, with course threads to assist coupling and uncoupling, are used. Situated inside the bit, a core lifter grips the core when the barrel is pulled back. This breaks the core of the face of the hole and holds it in the barrel until it is extracted.
A stationary inner tube prevents the spinning barrel from damaging or grinding the core. This tube is atached to the back end of the barrel using thrust bearings. Drilling luid is directed via back-end waterways to outside the inner tube to prevent erosion of the core.
