Comparison of cutting forces on disc cutters in constant vs variable penetration modes, The

Abstract

The application of mechanical excavation using disc cutters is predominantly to cut hard rock. The force required to cut rock is determined from the force penetration curve generated by different models which helps predict the machine performance. The trends of force penetration plot of a disc cutter cutting rock are generated in a laboratory scale by linear cutting machine (LCM) tests. Linear cutting tests measure cutting forces acting on a cutter in full-scale cutting experiments to mimic field conditions where mechanical excavation equipment fragments and mine rock. The trend of varying cutting forces as a function of rock properties, cutter size and shape, and cutting geometry has been studied in the past and is well understood. However, of all the tests in the past on various cutters, and especially disc cutters, have been performed at discrete penetration points and used to develop the best-fit trend line of force variation as a function of increments of penetration. No study was performed to understand the effects of cutting forces on a continuously increasing or decreasing (variable) penetration, even though this mode of operation is very common in partial-face machines. This refers to the crescent shape cutting profile of the contact area when using drum type cutters as opposed to the full-face machines such as TBMs where cutters work in constant penetration mode. In this study a modified testing method is developed and used to perform the LCM tests under variable penetrations and generate a continuously increasing and decreasing force-penetration plot. The main goal was to compare the force-penetration trends over constant or discrete penetration versus variable penetration tests in the same rock. A data analysis routine is developed for this method to ascertain the compatibility of the two approaches for design purposes. Six rock blocks were successfully tested to generate data for comparison of the two methods from which inferences were made followed by possibilities for future work. Three rock types were tested on the LCM using the conventional testing method using increments of penetration (or constant penetration mode) as well as continuously increasing and decreasing (or variable) penetration. When performing the variable penetrations, different slopes have been used. This refers to the change in penetration from zero to a nominal depth of penetration at the end of the LCM stroke, simulating a continuously changing penetration like the conditions in a partial-face machine operation. The test matrix involved cut spacing interval of 62.5 to 100 mm (2.5-4 inch) in different rock types. The results show that while the anticipated force trends were observed relative to the near linear or power function increase with the cutting force with penetration, trends of constant and variable penetration tests were in similar range, except in bedded sandstone, which estimates lower forces when variable penetration mode was used. that the test results indicate that the higher the slope (or higher nominal terminal depth of penetration), the lower the force measured for the same range of penetration. Overall, the tests show that the past practice of using the increments of constant penetration test results to develop force-penetration was valid, although in the limited testing on 5 rock blocks, the measured forces using the variable penetration method was slightly lower than the forces measured in respective constant penetration tests. The sieve analysis of the cuttings from the two methods of tests showed that there is no significant difference in particle size distribution between constant and variable penetration cutting modes.