In reciprocating power drills, the drilling cylinder is mounted on a feed-screw, such that as the hole is drilled and the drilling point recedes from the rock face, the drill-bit continues to move into it, while the anchor point (on the tripod or column) remains in place. The drill bit has to be changed out for a longer one every 12 to 30 inches (30 to 76 cm), depending on the length of the feed screw. [pdf]
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