## 5 The Coal-Rock Cutting Mechanism  
### Coal-Rock Cutting Process by the Pick  
During the coal-rock cutting process of the shearer pick, the pick impacts the coal rock with the speed \(v\) which causes high contact stress at a small contact area [129; 130; 131]. When the contact stress reaches the limit value, the coal rock shows the partial breakage and thus the induced compact core of coal powder. As the cutting process continues, the compact core of coal powder is discharged along the cone surface of pick body at a high speed, causing the expansions of coal breakage and compact core areas. The coal powder in the compact core accumulates the extrusion-induced energy, and the pressure continues to act on the compact core. As the compact core expands to the contact point \(D\), the coal rock shows small pieces spalling. The coal powder in region II is spewed out at a high speed due to strong compression, and presents strong friction with the pick cone surface. The accumulation region I at the pick tip moves with the pick and shows the wedging effect on the coal rock. As the pick move to point \(B\), the compact core, under enough compressive stress, induces shear cracks in the coal-rock. When the cracks expand to the coal surface, the region \(BCD\) of the coal rock spalls. The coal rock, as a material with high brittleness, mainly presents three cracks, i.e., the open crack, sliding crack and tearing crack. However, the open crack plays a dominate role during coal rock breakage by the pick. It is important to evaluate the cutting performance of pick to determine a machine's capacity and operational parameters. The cutting performance of a pick depends on various parameters, such as the geometry (i.e., cutter type, cutter dimensions, cutter tip angle, and shape), cutting conditions, such as cut spacing, penetration depth, cutting angles, cutting pattern, cutting speed, and the mechanical properties of the rocks. Many studies have been conducted to establish the relationship between the cutting performance of various machines and rock properties [132; 133; 134; 135; 136; 137; 138]. [PERSON] et al. [122] conducted a series of cutting tests to explore the effect of the skew angle of a point-attack pick on the cutting performance. They found that the positive skew angle was preferred with around +10\({}^{\circ}\) being optimal for the selected pick cutter in terms of cutting efficiency and structural stability. [PERSON] et al. [125] investigated the cutting process of low- to medium-strength rocks. [PERSON] [139] studied the relationship between specific energy and rock properties, and proposed estimation formulas for specific energy based on rock properties. [PERSON] et al. [133; 140] have set forth the standard test setting with the cutting depth of 5 mm. Meanwhile, the laboratory specific energy has been calculated by \(SE=F\zeta Q\)[58; 133]. In the equation, \(SE\) is the laboratory specific energy, MJ/m\({}^{3}\), \(Fc\) is the cutting force, kN, and \(Q\) is the yield per unit length of cut, m\({}^{3}\)/km. When the shearer drum cuts a coal seam with dirt bands, the cutting object has changed from simple coal to a coal rock mixture. Therefore, the coal-rock interface affects the cutting performance of the shearer to a certain extent. When encountering gauge and hard inclusion coal seams, caving mining or blasting mining is usually employed. However, this mining method easily presents a low coal recovery rate and roof fall, which thereby affects the normal production [17; 18]. Attributed to the large difference between the strengths of various rocks, some coalfield roofs are difficult to collapse, while others are very easy to collapse. During mining of thin and extremely thin coal seams, the shearer can easily cut the top and rub the bottom; the strength of the roof and floor strata affects the cutting performance of the shearer [141]. [PERSON] et al. [142; 143] studied the relationship between the specific energy consumption of the pick and coal rock characteristics under the condition of linear cutting.