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percussion boring Drilling with a chisel-shaped bit which is repeatedly struck against rock to form a hole. It produces very fine debris, which can be washed away by drilling fluid. If only the drill bit can oscillate up and down to produce the percussive effect, the drilling equipment is called a ‘down-the-hole hammer’. If the whole series of drilling rods and bit are oscillated from outside the drill hole, the equipment is called a ‘drifter’. See DOWNHOLE HAMMER DRILLING.