Illustrations

Tricone bit:

    The three inter-locked bits rotate.  Cutters are commonly steel although special alloys and even diamonds have been used to drill through hard rock.

Mud Circulation:

There's more to drilling than simply rotating the bit.  Fluid is circulated while the drilling proceeds.  Powerful pumps move the fluid down the pipe, through the bit and back to the surface, carrying the cuttings and other debris with it.  Thus, on a rotary rig (unlike the cable tool), drilling can be continuous as stopping to bail the cuttings is no longer required.  The drilling mud also stabilizes the walls of the hole.

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