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Aramid fiber–reinforced plastic (AFRP) composites have been widely used in automotive, aerospace, and defense industries. The common AFRP drilling process tends to cause damage to the composite structures which subsequently affects their fatigue lives and in-service performance. Understanding the mechanism of cutting force generation is crucial in controlling the cutting process for achieving desired hole quality and machining accuracy. This study proposes a novel mechanistic model considering both the cutting action and the extrusion action of the chisel edge. For the first time, the extrusion force generated by the chisel edge has been considered as a rigid wedge penetrating into an elastic half space based on the Hertz contact theory. The total thrust force in AFRP drilling is divided into three components: (i) thrust force generated by the cutting lips, (ii) thrust force generated by the chisel edge cutting action, and (iii) extrusion force generated by the chisel edge extrusion action. The proposed model was then validated by experiments and data was compared with the case where extrusion was not considered. The results show that our novel mechanistic model can provide a more accurate thrust force prediction. The average error of our model was 2.54% against the experimental data, whereas the error seen in conventional model without accounting extrusion was 8.22%. This suggests that the chisel edge extrusion plays a significant part in the drilling of AFRP and hence confirms the necessity of considering extrusion in establishing the associated mechanistic model. [ABSTRACT FROM AUTHOR]

The chisel-edge axial rake angle has an important effect on the processing performance of helical point micro-drills. In this research, the mathematical modeling for the spiral groove, helical flank, and cross-shaped chisel edge are conducted, and then three types of helical point micro-drills, each having a thinned chisel edge with an unequal axial rake angle, are designed and fabricated. Then, the finite element simulations and processing experiments of micro-drilling are performed separately, and the thrust force, torque, chip morphology, tool wear, and micro-hole machining quality are measured and evaluated. With the rise of the axial rake angle of the chisel edge, the curling degree of the chips and the thrust force were found to decrease owing to the increase of the rake angle along the inner cutting edge. The angle between the tool axis and chip axis was found to gradually decrease due to the reduction of the rake angle gradient along the cutting edge. Additionally, the thickness and width of the uncut chip of the inner cutting edge were found to increase, bringing about the rise in the torque. The wear degree of the micro-drills with a 0° axial rake angle (Type II) was the smallest due to their medium thrust force and torque, and a better micro-hole entrance morphology and minimum roundness were obtained. When drilling to the 30th hole, compared with the micro-drills with a − 10° axial rake angle (Type I) and 10° axial rake angle (Type III), the chisel edge maximum wear width of Type II (δ = 0°) was respectively reduced by 82.43% and 19.69%, and the micro-hole roundness respectively decreased by 26.7% and 11.33%. The thrust force of Type I (δ = − 10°) was the largest, so it caused greater tool wear and poorer micro-hole quality. The larger torque and lower rigidity of the inner cutting edge of Type III (δ = 10°) led to poorer drilling performance. Thus, helical point micro-drills having the 0° chisel edge axial rake angle are suitable for drilling 304 austenitic stainless steel. [ABSTRACT FROM AUTHOR]

Chip geometry can play a crucial role in understanding the rock cutting efficiency in mechanical excavation. In this study, the shape and size distribution of chips produced in laboratory scale rock cutting tests and their relationship with cutting geometry and rock cuttability were analyzed. For this purpose, a series of small-scale linear rock cutting tests (SSLRCT) were carried out with a simple chisel tool in unrelieved mode at cutting depth of 1, 2, 3, and 4 mm to determine the cuttability of four rock types (sandstone, marl, salt, and coal) and two synthetic (dental plaster and concrete) specimens. The size distribution of all cuttings collected from the grooves in different samples was determined by sieve analysis. The results were used to create a comprehensive particle size analysis, including a Rosin–Rammler plot (d′), coarseness index (CI), median (d50) and Krumbein mean particle size (dMPS). Also, the chips remained on the 4-mesh sieve and greater were chosen for chip shape analysis. The results obtained from chip analysis indicated that 85.6% of selected chips were fragmented in very platy and very bladed shape. Also, the analysis results showed that an inverse exponential correlation exists between specific energy and particle size indicators including CI, dMPS, d50, and d′ as anticipated based on rock fragmentation and grinding theories. Furthermore, a strong inverse correlation was found between specific energy and representative chip volume. The findings highlight the relationship of rock cutting efficiency with chip shape and size. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to predict the rock cuttability by considering the peak cutting forces and analyze the shear behavior of chisel picks by numerical simulations on the basis of theoretical and numerical studies. Numerical modeling studies were carried out as two main parts using PFC3D. In the first part, triaxial compression test was modeled on soft and hard rocks at various confining stresses. In part two, a chisel pick was numerically simulated and advanced on artificially generated soft and hard rocks depending on different cutting parameters. Then, the variation of peak cutting forces with the rake angle and cutting depth was monitored by modeling unrelieved rock cutting tests. The simulation results were compared with the results of theoretical cutting mechanisms. In conclusion, it is found that reasonable relationships exist between two predictive methods. In addition, the effects of rake angles on the shear strength were analyzed using Nishimatsu's cutting model. The variation of shear angle during rock cutting was also examined. As a result of numerical studies, it is found that the theoretical formula proposed by Nishimatsu for the estimation of peak cutting forces and shear strength is mainly suitable in negative rake angles of chisel picks and in hard rocks. Furthermore, the shear planes of the microcracks in PFC3D were monitored and the shear angle was found to be ranging between 20° and 35°. These results were also verified by modeling of triaxial tests at different confining pressures. [ABSTRACT FROM AUTHOR]

Chisel edge thinning can effectively improve the drilling performance of austenitic stainless steel. Four kinds of helical point micro-drills, each with a thinned chisel edge of a distinct length, are fabricated based on the mathematics models of the micro-drill by using a six-axis computer numerical control tool grinder. Micro-drilling experiments on 304 stainless steel are conducted. Drilling performance is evaluated according to the measured and observed micro-hole machining quality, chip morphology, thrust force, and tool wear of the micro-drill tip. With a decrease in the chisel edge length, the micro-holes drilled using thinned micro-drills have small entrance burrs, roundness errors, and regular shapes, which are attributed to an increase in self-centering capability. Owing to increased equivalent chip thickness and different directions of chip flow, micro-drills with thinned chisel edge can generate more small-size chips and improve chip breaking. In addition, the inner cutting edge with a positive rake angle replaces the chisel edge with a negative rake angle to cut the workpiece, significantly reducing the chip curl, thrust force, and tool wear of micro-drills. However, helical point micro-drills with critical chisel edge lengths by thinning exhibit less thrust force and tool wear than do helical point micro-drills with no chisel edge because of the increase in the uncut chip width of the micro-drill. Thus, with the selected parameter range, the optimal helical point micro-drill is the micro-drill with a specific chisel edge length by thinning to improve drilling performance. [ABSTRACT FROM AUTHOR]

Cross-shaped chisel edge thinning is an effective method to improve the drilling performance of non-coaxial helical flank micro-drill. In order to study the effect of the cross-shaped chisel edge on drilling performance, the mathematical model of cross-shaped chisel edge is proposed. The rake angle, lip inclination angle, uncut chip thickness, and uncut chip width are calculated by MATLAB software. Finite element models of four kinds of micro-drills with a different thinned chisel edge are established using DEFORM-3D software, and micro-drilling simulations on 304 austenitic stainless steel are carried out. The simulation results show that with a decrease in chisel edge length, the drilling force, tool temperature, tool wear, and curl degree of the chip are clearly reduced. However, for micro-drill without chisel edge, the torque and tool temperature are larger than other micro-drills, and string chips are generated during the drilling process. Furthermore, the non-coaxial helical flank micro-drill with an optimal cross-shaped chisel edge is manufactured by a six-axis CNC grinding machine, and the measurement results indicate that the geometrical parameters meet the designed values. At last, the micro-drill with an optimal cross-shaped chisel edge has a satisfactory drilling performance by the micro-drilling experiment. [ABSTRACT FROM AUTHOR]

The main objective of this study is to investigate the effect of textural properties of stones on cutting performance of a standard chisel tool. Therewithal, the relationships between textural properties and cutting performance parameters and physical and mechanical properties were statistically analyzed. For this purpose, physical and mechanical property tests and mineralogical and petrographic analyses were carried out on eighteen natural stone samples, which can be grouped into three fundamentally different geological origins, i.e., metamorphic, igneous, and sedimentary. Then, texture coefficient analyses were performed on the samples. To determine the cuttability of the stones; the samples were cut with a portable linear cutting machine using a standard chisel tool at different depths of cut in unrelieved (non-interactive) cutting mode. The average and maximum forces (normal and cutting) and specific energy were measured, and the obtained values were correlated with texture coefficient, packing weighting, and grain size. With reference to the relation between depth of cut and cutting performance of the chisel tool for three types of natural stone groups, specific energy decreases with increasing depth of cut, and cutting forces increase in proportion to the depth of cut. The same is observed for the relationship between packing weighting and both of specific energy and cutter forces. On the other hand, specific energy and the forces decrease while grain size increases. Based on the findings of the present study, texture coefficient has strong correlation with specific energy. Generally, the lower depth of cut values in cutting tests shows higher and more reliable correlations with texture coefficient than the increased depth of cut. The results of cutting tests show also that, at a lower depth of cut (less than 1.5 mm), even stronger correlations can be observed between texture coefficient and cutting performance. Experimental studies indicate that cutting performance of chisel tools can be predicted based on texture coefficients of the natural stones. [ABSTRACT FROM AUTHOR]

This paper presents a cutter partition-based tool orientation optimization algorithm that can identify and eliminate both local and rear gouging in five-axis machining. This algorithm is developed to generate smooth gouge-free tool paths for arbitrary free-form surfaces machined by general Automatically Programmed Tools (APT) cutters. The algorithm involves an iterative process of cutter projection to compute the gouge-free cutter positions. During each iteration, the cutter partition method is used to classify the cutting portion of the cutter, which in turn determines the gouging condition. Based on the classification, the algorithm applies an optimal gouge-avoidance strategy to minimize the size of tool orientation and position changes. The proposed algorithm can handle the combination of local and rear gouging (either separately or simultaneously), guarantee front edge cutting (avoiding damage to the cutter and surface), ensure a safety clearance between the rear edge of the cutter and the machined surface, and achieve minimal interruption to tool orientation and position from gouge avoidance. Simulation and cutting experiments confirm the effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

The effects of the fracture scale (size) and normal stress on the shear behavior of a fracture were investigated in direct shear rig using mortar fracture replicas of different sizes (100-200 mm), which were reproduced from a tensile fracture created in granite. Monotonic shear loading was applied at normal stresses of 0.3 and 1 MPa for shear displacements of 2 and 20 mm. The change in surface topography was measured by using a profile measurement system with a laser profilometer to determine the changes in surface damage and the aperture during shear. Additionally, the effect of gouge material on the non-linearity of the closure curve during shear was investigated. The results show that the standard deviation of the initial aperture during shear increases not only with scale but also with the normal stress, since the matedness decreases due to the fact that asperities are more damaged with normal stress, and accordingly, the non-linearity in the closure curve during shear increases with normal stress. The results also show that the non-linearity of the closure curve during shear increases with the presence of gouge material/debris that is produced in the shearing process. This is mainly because gouge is soft and easily deforms under the application of normal stress. Moreover, gouge material is distributed non-uniformly around the contact areas. Thus, the normal stiffness during shear decreases not only with a decrease in the matedness between the surfaces, but also with the presence of gouge material. [ABSTRACT FROM AUTHOR]

This study presents a dynamic-induced direct-shear model to investigate the dynamic triggering of frictional slip on simulated granular gouges. An incident P-wave is generated as a shear load and a normal stress is constantly applied on the gouge layer. The shear stress accumulates in the incident stage and the frictional slip occurs in the slip stage without the effect of the reflected wave. The experimental results show a non-uniform shear stress distribution along the gouge layer, which may be induced by a shear load induced torque and by normal stress vibration along the layer. The shear stress at the trailing edge strongly affects the frictional slip along the P-wave loading direction, while the rebound stress at the leading edge propagates along the opposite direction. The frictional slip is triggered when the maximum shear stress at the trailing edge reaches a critical value. The normal stress influences the maximum shear stress at the trailing edge, the maximum slip displacement and the slip velocity. The advantages and the limitations of this model are discussed at the end. [ABSTRACT FROM AUTHOR]