Almost all modern cutting bits use a cemented tungsten carbide tip to provide adequate bit life against wear. Laboratory tests by the U.S. Bureau of Mines have indicated that the number of strikes for ignition increased by a factor of 3 with a new nonrotating bit impacting a sandstone block when the clearance angle was increased by 10 deg, and that carbide-tipped bits allowed more strikes before ignition than steel-tipped bits by a factor of 7 to 10. Recent field tests in an operating coal mine indicate a dramatic reduction in the frequency of frictional ignitions and improvement in bit life by using a mushroom-shaped bit cap and a larger initial clearance angle. The latter was achieved by increasing the bit attack angle. Improved bit performance through selection of the grade of tungsten carbide is a compromise between fracture resistance and wear resistance. A large clearance angle should be maintained by various techniques, such as a variable-attack-angle holder or a reusable bit that can be reground.
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