| Summary: | The fundamental problem in rock working is the breakage of fragments out of the face of a solid rock wall rock. Mechanically, this can be done only by forcing a tool into the rock surface, after the manner of an indenter is commonly used in the testing of surface hardness. Since the process breaks rather than cuts the solid rock into small fragments of assorted sizes, it can be regarded as the essential one in crushing. As in the crushing processes generally, energy volume relationships are therefore of interest. The specific energy, defined as the energy requierd to excavate an unit volume of rock is a useful parameter in this context and many also take it as an index of the mechanical efficiency of rock-working processess. In drilling data from a number of sources, its minimum value appears to be very roughly correlated with the crushing strength of the medium drilled in for percussive-rotary. The implications of this are discussed
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