NOVEL COAL-CUTTING BITS AND THEIR WEAR RESISTANCES.

Based on earlier tests that showed steel to be more likely to cause frictional ignition than tungsten carbide, the Bureau of Mines redesigned coal-cutting bits to reduce the hazard of face ignitions. Two bit designs - a mushroom-shaped bit with a conical shank and a dove-tail bit with a rectangular shank - are described, as well as bit wear tests conducted in the laboratory and in an operating coal mine.