Security Engineering Division of Dresser Industries has utilized the basic bit penetration mechanism of a roller-cone rotary bit in the development of a drillability index for "hard" rock mining bits. A 1/8 inch radius, hemispherical drill bit element is pressed into a flat surface of a hand sample with a hydraulic pump and ram until a distinct crater is formed. The crater depth divided by the ram load constitutes a penetration or drillability index, p'/E. From this index, bit type, drilling weight, average penetration rate and approximate bit life may be determined. Field results have been well within the degree of accuracy consistent with sampling error.
For several years there has been a need in mining and construction for a reliable method of determining drillability of hard rock by a simple test of hand samples. Early in 1966, Security Engineering Division of Dresser Industries began development of a method to measure rock drillability relative to a roller-cone bit used in drilling large (5 to 15 inches) blast holes. A rock strength value or drillability index was considered the essential value to be determined. From that value would be determined bit type, bit weight required, average penetration rate at a given rotary penetration rate at a given rotary speed, and bit life. Standard rock properties had not yet been reliably related to drillability, therefore, the basic bit penetration mechanism was adapted to a test device.