| 摘要 |
A raise head having a multiplicity of cutters for enlarging a pilot hole into a larger diameter hole by disintegrating the earth formations that surround the pilot hole is provided that will require lower cutter loads to penetrate the formations being bored by directing the rock fracture planes toward the pilot hole forcing the rock to yield with less input energy. The cutters are positioned on the raise head to provide an earth formation contact profile with a major portion of said earth formation contact profile extending outward and upward from said pilot hole. The included angle between the major portion of the earth formation contact profile and the axis of the pilot hole is less than 90 DEG .
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