PURPOSE: To enhance the effect of breaking rock walls by causing cracks in the side walls of a prepared hole using a boring tool at the same time that a circular prepared hole is bored.
CONSTITUTION: This boring tool 1 comprises a leading bit 3 mounted at the end of an eccentric rod 2 to bore a circular hole and wing bits 4, 5 fitted into the eccentric parts of its middle portion. The eccentric rod 2 is connected via a connection sleeve 55 to a boring rod 57 connected to a rock drill. To bore a circular hole, thrust transmitted from the rock drill is transmitted to the eccentric rod 2 via the boring rod 57, and the hole is bored by the leading bit 3 being pressed against bedrock. The wing bits 4, 5 fitted into the eccentric parts by the rotation of the eccentric rod 2 are made to move back and forth in their radial direction by the action of a cam, but are kept from rotating since the fitting member of a wing holder 65 is fitted into an oblong hole 52 with a clearance between them. Therefore, as a circular prepared hole is bored, cracks can be caused in the side walls of the prepared hole by the wing bits 4, 5.
AKIYOSHI NAOYOSHI
SHIBUYA YOSHINOBU
OKADA KENJI
MIKAMI YOSHIICHI
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