<p>This paper aims to propose a three-dimensional peak cutting force (PCF) model of conical picks, grounded in a thorough theoretical understanding of rock-cutting mechanisms by a series of cutting tests. The experimental results, including the fragmentation process, load–displacement curve, and the peak cutting force etc., are obtained with an automatic universal testing machine and observed via an i-SPEED high-speed camera. Accordingly, some controversial rock-cutting issues, such as cracks propagation control factors, PCF occurrence and rock–tool interaction stages were analyzed in detail. It was found that rock fragmentation process with conical pick could be simply categorized into the energy accumulation stage and the energy pure release stage. The crack propagation is controlled by both rock compressive and tensile strength, and then follows a tensile failure pattern. The PCF occurs at the beginning of the pure energy release stage, with higher compressive and tensile strengths in the rock mass leading to higher PCF values and reduced fracturing times. Based on observation of rock-cutting mechanisms and simplifying assumptions, a theoretical three-dimensional PCF model was derived by the moment equilibrium principle. It indicates that rock chips size is primarily governed by the rock brittleness and is independent of the applied cutting force. Meanwhile, the predicted PCF by this theoretical model is significantly much lower than the actual applied force, which can only systematically evaluate the performance of cutter and its design.</p>