Rock heterogeneity is an important factor controlling fracture initiation, accumulation, and propagation within rock destruction. Traditionally, rock reaction force instability within rock destruction is associated with rock non-homogeneity or considered self-excited, but no clear relations between rock structure options and reaction force oscillations were proposed. This work hypothesizes the idea, that rock structure options are factors of drill bit whirling. The author innovated the Wojtanowicz and Kuru’s force static balance model (1993) to simulate numerically 3-cutter movement in a rock space, assuming a crack propagates by rock grains getting around. Instability of deviation angle between directions of cutting force and crack propagation causes the rock reaction force instability. Novelty of this work is considering mutual influence of rock granulomentric composition options (sizes, shape and distribution of grains) and drill bit design options (cutter diameter, distance between adjacent cutters, etc.) on rock reaction force oscillations. The follow tendencies were observed: rock grain dimension increasing and grain concentration increasing were accompanied with rock reaction force magnitude decreasing; random grain placing and/or grain random dimensions caused increasing of rock reaction force scattering; random grain sizes are more essential for rock reaction forces (and their projections) scattering than random grain distribution.

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