ercussion drilling uses steel rock bits of various designs to break up the rock while drilling. Drill hole diameter can vary greatly, but is commonly about 7 centimetres. The bit is rotated and the hammer action of the drill is transmitted through a string of drill rods, which have a relatively small center hole through which compressed air (in the case of dry ground), or water (in the case of wet ground), is passed.Reverse circulation These drills produce rock cuttings at the bit which are blown up the hole by compressed air.
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