Development of a new direct test method for estimating cutter life, based on the Sievers’ J miniature drill test

The Sievers’ J-miniature drill test (SJ) has been used extensively at NTNU/SINTEF since the 1960s in connection with drillability testing of rock samples. Nearly 3000 samples have been tested, and the method is at present internationally recognised as a reliable method for determination of surface hardness of rock samples. A new test method for estimation of cutter life has evolved as a result of a recent update of the SJ test. This update was made possible by the development of instrumentation of the test apparatus. The new test method analyses the penetration depth and speed during the testing. The point in time where an interception occurs between the tangent lines for what could be defined as the “effective drilling” part and the “worn out” part of the penetration curve is defined as the Sievers’ J interception point or SJIP. Testing of 212 rock samples, representing different rock types, and correlations of the results to some of the most widely used methods for estimation of cutter life, show that it is possible to obtain a direct cutter life estimate by use of the SJIP value. The demand for representative parameters for rock properties in connection with underground excavations is increasing, as this constitutes the fundamental input for obtaining the most reliable cost estimates. The new test method provides information that was previously unavailable. This will improve the understanding on how different rock types and properties affect the drilling process and the wear on cutter tools. This test method is not meant to act as a substitute for well established and currently used test methods, but rather constitute an additional tool that can contribute to “bridge the gap” between laboratory testing, modelling and the actual excavation process estimates.