See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/323383984 Recent Advances in Micro/Nano-cutting: Effect of Tool Edge and Material Properties Article in Nanomanufacturing and Metrology · February 2018 DOI: 10.1007/s41871-018-0005-z CITATIONS READS 49 651 2 authors: Fengzhou Fang Feifei Xu University College Dublin Tianjin University 347 PUBLICATIONS 4,073 CITATIONS 14 PUBLICATIONS 174 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: the motion control for voice coil actuator based on Moore Lathe View project light field technology View project All content following this page was uploaded by Feifei Xu on 27 February 2018. The user has requested enhancement of the downloaded file. SEE PROFILE Nanomanufacturing and Metrology https://doi.org/10.1007/s41871-018-0005-z (0123456789().,-volV)(0123456789(). ,- volV) REVIEW PAPERS Recent Advances in Micro/Nano-cutting: Effect of Tool Edge and Material Properties Fengzhou Fang1,3 • Feifei Xu1,2 Received: 7 October 2017 / Revised: 9 January 2018 / Accepted: 13 January 2018 International Society for Nanomanufacturing and Tianjin University and Springer Nature 2018 Abstract Micro- and nano-machining technology has been applied in industry to generate high-precision parts with micro/nanometric accuracy or feature size in the recent decades. Cutting is one of the most powerful manufacturing processes, and the material removal mechanism is urgently demanded by the industry to understand and improve the micro/nano-machining process efficiently at a low cost. This paper presents the recent advances in cutting mechanism and its applicability for predicting the surface generation and chip formation, especially when material is removed in micro- and nanoscale. In addition to the industry-concerned performance parameters, fundamental physical parameters such as stresses, strains, temperatures, phase transformation, minimum uncut chip thickness and size effects are discussed in this paper for the indepth understanding of the micro/nano-cutting process. Keywords Micro/nano-cutting Material removal mechanism Tool edge Size effect Surface generation 1 Introduction 1.1 Definition of Micro/Nano-cutting In micro-machining, micro indicates the range from 1 to 999 lm [1]. It means the feature size or machining accuracy of the products is ‘very small’ and not easy to fabricate. In definition of micro-cutting, it has to be distinguished from the traditional macro-cutting where the materials are seen as continuous and the individual grains in materials are averaged and ignored. In micro-cutting, the uncut chip thickness (UCT) is in microscale and always & Fengzhou Fang fzfang@gmail.com Feifei Xu yuanfei1116@163.com 1 State Key Laboratory of Precision Measuring Technology & Instruments, Centre of MicroNano Manufacturing Technology, Tianjin University, Tianjin 300072, China 2 Institute of Mechanical Manufacturing Technology, China Academy of Engineering Physics, Mianyang 621900, Sichuan, China 3 School of Mechanical and Materials Engineering, MNMTDublin, University College Dublin, Dublin, Ireland smaller than the average size of the material grains. The micro-cutting process has to take grain size and its distribution in materials into consideration. Simoneau et al. [2] defined the micro-cutting as the point where the UCT is less than the average grain size of the smallest grain type. Similar to the grain size, the tool edge is usually ignored in macro-cutting process. But it should be considered in micro-cutting process. Therefore, the micro-cutting could be defined in two aspects. One is the definition based on the accuracy or feature size to be attained, such as cutting parts with micro-metric accuracy or making products with micro-metric feature size. Another is based on the differences from the macro-cutting; for example the UCT is in microscale and less than the average grain size of the smallest grain type, or the effect of the tool edge which is in microscale could not be ignored. Nano-cutting could be defined in the same way as the micro-cutting. Therefore, the nano-cutting is generating parts with nano-metric accuracy or making products with nano-metric feature size. Another definition is that the UCT of the nano-cutting process is in nanoscale and less than the average grain size of the smallest grain type. The effect of the tool edge which is in nanoscale too could not be ignored. 123 Nanomanufacturing and Metrology 1.2 Significance of Micro/Nano-cutting With the development of manufacturing technology in the recent decades, the cutting process including turning, milling still takes a large portion of machining processes. Due to the continuous progress in machine tools and cutting tools, the machining accuracy and feature size have approached micro- and nano-metric scale. In this scale, problems same as those in conventional cutting process, like the stresses, strains, strain rates, temperatures, chip formations, should be re-considered to optimize the cutting parameters (feed, cutting speed, depth of cut, etc.), cutting tool geometry parameters (tool materials, rake angle, clearance angle, nose radius, edge shape, etc.) and coolant or lubricant type (cryogenic cooling, high-pressure coolant jet along the flank face or the rake face, etc.). Therefore, the cutting mechanism in micro- and nano-metric scale should be deeply understood and models for predicting the performance and surface generation in micro/nano-cutting processing should be established. However, in micro/nano-cutting, the tool edge shape, the size effect of materials which is almost ignored in conventional cutting process plays an important role in cutting mechanism. In this paper, recent advances about the influence of tool edge shape and material properties on micro/nano-cutting process are presented. Mallock identified the shearing mechanism [10], which might be the first work in which a shear theory was suggested. In the 1930s, Piispanen, one of the great pioneers exploring the physics of cutting, introduced the so-called card model [11]. His work stated that the material removal process of metal is similar to the cutting of a deck of stacked cards and the cards are inclined at an angle that matches the shear plane angle. However, it was the work of Ernst et al. [12] and Merchant [13, 14] in the 1940s that made the shear plane model more recognized. Their work has been seen as a milestone of the research of cutting mechanism and is used as a basis for analyzing various machining processes. The well-known Merchant’s cutting model is shown in Fig. 1. The relationship between the cutting force and other parameters can be derived based on the model. The relationship of the shear angle u with the tool rake angle a and the friction angle b deduced by the principle of minimum required cutting energy is written as a b u ¼ 45 þ 2 2 The models mentioned above are always analyzed based on some assumptions, which is systematically presented by Shaw [11]: (1) 2 Investigation Approaches (2) The ultra-precision machine tool which is a powerful tool in realizing the machining of freeform surfaces [3] and micro/nano-structural elements [4] is also an effective tool in investigating the micro/nano-cutting process by applying particular methods, such as the tapper cutting [5], orthogonal cutting [6] and so on. Besides that, a self-developed cutting device which could be integrated to scanning electron microscope (SEM) could be used to realize the online observation of cutting process [7]. The AFM is an instrument which could be used to imitate the cutting process and identify the machinability of the workpiece materials before cutting process [8, 9]. To fundamentally understand the cutting process, analytical method and numerical method are often employed and discussed in this section. (3) (4) (5) (6) The tool is perfectly sharp and straight, cuts perpendicular to the direction of motion and has a width greater than that of the workpiece; The shear surface is a plane extending upward from the cutting edge; The cutting edge generates a plane surface, constant depth of cut; The workpiece moves relative to the tool with uniform velocity; The chip does not flow to either side; A continuous chip is produced without built-up edge; 2.1 Analytical Method Cutting mechanism has attracted much attention of many researchers over decades. Various models have been proposed to improve the description of the cutting mechanism. In principle, material is removed in shearing. In 1881, Fig. 1 Schematic illustration of conventional cutting model [12] 123 ð1Þ Nanomanufacturing and Metrology (7) There is no contact between the workpiece and the clearance surface of the tool. Slip-line field method is an effective analytical method to study the cutting process and was used by Lee and Shaffer [15] in 1951 to develop a cutting model. After that, many slip-line models have been proposed. In 1977, the parallel-sided shear zone theory proposed by Oxley et al. [16] is an important progress in modeling cutting process, as shown in Fig. 2. Oxley’s study considered the dependence of material flow stress upon strain, strain rate and temperature and obtained the shear angle and other quantities of interest, which is the unique feature of Oxley’s machining theory. In 2001, Fang et al. [17] developed a universal slip-line model and it is further extended by taking into consideration the effects of strain rates, temperatures [18] and the tool edge radius [19, 20]. 2.2 Numerical Method Compared to the analytic method, numerical method could give a result more straight forward. Finite element method (FEM) is a powerful numerical method which has a great capacity in helping people to understand the fundamental cutting mechanism [21]. The reliability of the FEM simulation depends on the mechanical and thermo-physical parameters input to the model. Therefore, the characteristic parameters of the materials at the strain of 100–700%, strain rates up to 106 S1 , temperature up to 1400 C, pressures near 2–3 GPa need to be obtained based on different experiments [21]. Generally, the material properties used in cutting model are homogeneous and isotropic. However, in micro/nano-cutting process, the UCT is smaller than the grain size or the material which is actually multiphase. The anisotropy or the non-homogeneity of the material properties should be taken into consideration. Abouridouane et al. [22, 23] described the thermo-mechanical behavior of the ferrite and pearlite phase of the carbon steel by different constitutive model, and the results are verified experimentally regarding chip formation and Fig. 2 Model of chip formation used in Oxley’s analysis for conventional machining [16] processing forces. For anisotropic materials, such as the single-crystal material, a crystal plasticity theory was developed to help investigating the anisotropic plastic deformation considering the crystal orientation and activated slip systems [24]. The crystal plasticity theory has been implemented in FEM to study the micro-compression [25] and micro-cutting [26, 27] behaviors of single-crystal materials. In FEM simulations, the meshing strategy is an important factor in influencing the simulation results. Niesłony et al. [28] investigated the meshing strategies of the cutting tools considering the tool edge radius and found that the accurate representation of the tool micro-geometry would influence the simulation results. Smoothed particle hydrodynamic (SPH) simulation which is a mesh-free technique has been coupled with the FEM to investigate the micro-machining of FCC materials [29]. Molecular dynamics (MD) simulation is another useful approach in investigating the material removal mechanism in nanoscale. MD simulation was first applied by Lawrence Livermore National Laboratory (LLNL) and Precision Engineering Department at Osaka University to investigate the cutting process [11]. In 1990s, Shimada et al. [30, 31] conducted a series of investigations on the mechanism of nano-cutting of single crystals by MD simulation. The Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [32] is a public-domain computer code used to simulate the cutting process. And the simulation results could be visualized and analyzed by Visual Molecular Dynamics (VMD) [33] and Open Visualization Tool (OVITO) [34]. The microstructural or dislocation evolution of the workpiece could be analyzed based on common neighbor analysis (CNA) [35], dislocation extraction algorithm (DXA) [36] and the displacement vectors module in OVITO. The accuracy of the potential function used in MD simulation determines the reliability of the simulation results [37]. Lennard–Jones (LJ), Morse and embeddedatom method (EAM) potentials were employed to investigate the effect of interaction potential on the MD simulation of nano-cutting by Oluwajobi et al. [38]. The results showed that the EAM potential is most suitable in the three potentials, because it describes the metallic bond better. Morse potential is usually used to depict the interaction between the atoms of cutting tool and workpiece materials. Goel et al. [37] thought it is not robust enough to describe covalent bond interactions between silicon and diamond. Therefore, the analytical bond order potential (ABOP) proposed by Erhart and Albe [39] is used to describe the interaction within and between the diamond tool and silicon. Tersoff potential [40, 41] is usually applied in investigating the brittle materials, such as the silicon and germanium. However, it is failed to simulate the brittle fracture in nano-cutting [42]. It is because the Tersoff 123 Nanomanufacturing and Metrology potential is a short-ranged potential which could not describe the transition events, such as the bond dissociation, accurately [43]. Stillinger–Weber potential is applied by Zhang et al. [44] to investigate the phase transformations and dislocation activated in single-crystal silicon. In nano-cutting of silicon carbide (SiC), the Vashishta potential [45] which could reproduce the brittle fracture and the fracture toughness accurately [46], is applied by Xiao et al. [47] on a self-developed GPU-accelerated MD codes to simulate the fracture in cutting process. However, the MD simulation is usually used in nanoscale system, due to the limitation of the computational power. The FEM model could be used in a larger scale, but microstructure evolution in plastic deformation could not be displayed. Multiscale simulation has been developed to bridge the gap between the MD and FEM simulation. It was applied by Son [48] and Pen [49] et al. to simulate the nano-cutting of single-crystal copper. Based on the development of the numerical method, the micro/nano-cutting mechanism would be fully revealed in the future. 3 Advances in Investigating Cutting Mechanism 3.1 Influence of Tool Edge The cutting tool edge has a significant influence on the cutting force, stress, temperature and other fundamental physical parameters. Especially when it is comparable to the UCT, the effect of the tool edge is non-ignorable and making a larger part of the workpiece materials under the plowing or extrusion of the tool whose effective rake angle is mainly negative. It changes the material flow, the chip formation, and the stagnation point or the dead metal zone in front of the tool edge. Therefore, the generated surface quality would be influenced by the cutting tool edge due to the side flow, recovery and so on. In this section, the influences of the tool edge on the cutting mechanism mentioned above, including the characterization of the edge, are displayed and discussed. The effect of tool edge radius on cutting process was early mentioned by Chien [56], Albrecht [57] and Masuko [58]. After that, many researches have been conducted to investigate the effect of the tool edge on the cutting force, temperature, chip formation, etc. However, the characterization of the tool edge is almost depending on individual researchers, measurement uncertainty and fitting algorithm. These factors would cause discrepancies in describing the tool edge with tool edge radius. Therefore, a common understanding for the influence of tool edge on cutting process is inhibited by the poor detected consistency. Besides that, there is no international standard to characterize the micro-geometry of tool edges, although the macro-geometry of the cutting tool is internationally standardized [59]. It becomes indispensable to draft a standard and characterize the shape of tool edge based on it. Wyen and Wegener [60] have proposed a new algorithm to increase the repeatability in characterizing the cutting edge radius by making the choice of fitting area user independent. As shown in Fig. 3, the first step is to generate straight lines for the rake and flank face by least squares fitting of the initial given data. Then, the fitted straight lines cross at the virtual tool tip pc and the wedge angle b is obtained. The intersection point pint of the wedge angle bisector and tool edge profile is the point where a circle needs to intersect and is tangent with the fitted straight lines. The tangent points determine the upper fitting limit for refitting the rake and flank face lines. The fitting of the lines is repeated until the distance between the tangent point and the foregoing tangent point approximates zero. Therefore, the two tangent points determine the limit of fitting area for fitting the tool edge with a cycle. The tool edge radius rb is calculated by a least squares fitting using all points within the micro-geometry limit. For the cutting edge which is not a symmetrical circle, only tool edge radius cannot describe the shape of cutting edge precisely. Therefore, Wyen et al. [59] develop a 3.1.1 Characterization of Tool Edge Tool edge which was usually neglected in conventional macro machining process, such as the shear plane model proposed by Merchant [50], has become an important parameter in influencing the cutting process [51–54]. The shape of the cutting edge is defined as the transition between the rake face and the flank face of a cutting wedge [55]. Generally, the shape of the tool edge is simply considered as rounded and characterized by an edge radius rb . Fig. 3 Proposed method for characterizing rounded tool edge [59, 60] 123 Nanomanufacturing and Metrology method in characterizing the asymmetry of cutting edges. As shown in Fig. 4, a line is drawn through the intersection point pint perpendicular to the wedge angle bisector and intersects with the fitted rake and flank face lines. The distances from the intersection points to the profile of cutting edge in the direction parallel to the wedge angle bisector are Dc and Da . The ratio of them indicates the asymmetry S(S ¼ Dc Da ). Denkena et al. [61] proposed the form-factor method (also referred to as K-factor method) to characterize the shape of tool edge, especially when the cutting edge is asymmetrical, as shown in Fig. 5. Four parameters Sa, Sc, Dr, and u were introduced. and the average cutting edge rounding S and the form-factor K (Kappa) were deduced to, respectively, specify the dimension and shape of the rounding at the cutting edge. The ratio K ¼ 1 indicates a symmetrically rounded cutting edge. When the ratio is below or above 1, the shape of the cutting edges is in waterfall or trumpet style, respectively. The distance Dr indicates the flatness of tool edge. The smaller the Dr value is, the sharper the cutting edge is. Yussefian et al. [62] proposed a method to identify the cutting edge by the adaptive placement of the knots that minimizes the residual error from fitting the B-spline to the tool profile data. Subsequent to edge identification, the edge is modeled by parametric quadratics. And four parameters are derived to characterize the cutting edge which is symmetrical or asymmetrical. Fig. 4 Asymmetry determination of a rounded cutting edge [59] However, too many parameters may inhibit their applications in practice. In 2012, Denkena et al. [63] proposed a new parameter called as normalized plowing zone A0a ¼ Aa =la . Aa is the area between the cutting edge profile and the workpiece bordered with stagnation point and the contact point in flank face side. la is the corresponding contact length of the cutting edge profile between these two points. The influence of the parameter on the tool wear, burr formation and residual stress has been investigated. In 2017, Xu et al. [64] considered the tool edge radius rb could be fitted only using the cutting edge profile between stagnation point and the contact point mentioned above. Because of the process force, the surface integrity is mainly influenced by the tool edge profile under the stagnation point [63]. The influence of the newly proposed radius rb on the stagnation region, chip formation, subsurface damage, and cutting forces has been investigated and found that it could be used to characterize the cutting performance of a cutting tool, and reduce discrepancies in describing the tool edge, especially when it is asymmetrical [64]. Comparing the method to that proposed by Denkena et al. [63], the normalized plowing zone is in linear relationship with the new proposed tool edge radius, if taking the profile in the flank face side as a circle. In nano-cutting process, diamond cutting tools are usually employed to generate parts with nano-metric precision and finish. The tool edge radius can be down to 10–100 nm which is smaller than those of the tungsten carbide or PCBN inserts, due to the unique material properties of single-crystal diamond and reliable tool edge preparation processes [65, 66]. Therefore, how to accurately measure the shape of diamond tool edge and to analyze the effect of the tool edge shape on micro/nano-cutting process become a significant issue. Optical microscope and scanning electron microscope (SEM) can be used to monitor the diamond tool edge [67, 68]. But they both failed in measuring the shape of the diamond cutting tool edge precisely. The SEM image is basically a 2D projection of a 3D object. Therefore, additional efforts should be done, such as the combination of electron-beam-induced deposition (EBID) with SEM by Shi et al. [69]. With this method, quantitative characterization of diamond tool edge radius and wear land length for new and worn diamond tools can be derived from analysis of the EBID-SEM images. But the detailed information of the diamond tool edge shape cannot be measured with this method. Atomic force microscope (AFM) is an effective method in measuring the shape of diamond tool edge for its nano-metric vertical and lateral resolution. The copied profile of a diamond cutting tool edge which is formed by indenting the tool cutting edge into the surface of copper has been measured by AFM [70]. And the elastic spring-back effect has been compensated Fig. 5 Form-factor method for cutting edge characterization [55, 61] 123 Nanomanufacturing and Metrology by Li et al. [71]. When the diamond tool edge is directly measured by AFM, it is difficult to align AFM tip with tool edge due to the low depth-of-field and poor resolution of the optical microscope in conventional AFM, especially when the diamond tool nose radius is in micro-metric scale. Gao et al. [72, 73] combined an AFM with an optical sensor for alignment of the AFM probe tip with the diamond tool edge in submicrometer range more easily. A 3D edge profile measured by the instrument is shown in Fig. 6a, and from the sectional profiles of the tool edges, the tool edge could be evaluated, as shown in Fig. 6b. Actually, the evaluation of the diamond tool edge radius has the same problems mentioned by Bassett et al. [74] which is the measurement inaccuracy caused by different operators and algorithms. In 2010, Shimizu et al. detailedly introduced the method of characterizing the diamond tool edge with AFM, taking into consideration the influence of the shape of AFM probe. And the methods how to handle the data of the cutting edge profile are proposed. Diamond tools edge with nose radius of 0.2 mm and 1.5 lm were measured with good repeatability [75]. The characterization method proposed in [64] may be a simple and effective method to describe the tool edge with feature size of several tens of nanometers, but more investigations based on experimental and theoretical analyses should be done to verify the hypothesis. An international measurement standard of diamond tool needs to be established, and a relationship of the diamond tool edge shape with the cutting quality should be investigated and understood by the researchers and engineers to choose and fabricate a perfect tool for different applications. Therefore, the influences of the tool edge on the cutting mechanism are discussed in the following sections. 3.1.2 Influence on Cutting Force More and more difficult-to-machine materials which have unique material properties are emerging and widely used in the current industry. The shape of the tool edge becomes one of the major factors in influencing the cutting process Fig. 6 a 3D edge profile measurement result of diamond cutting tool, b sectional profiles of the tool edges [72] 123 [52, 54, 76, 77]. In nano-cutting process, it is important to study machining this type of materials, such as hard and brittle materials by using a diamond tool with special chamfer in the tool edge [78]. A large number of researches have to be considered to investigate the cutting force, stress and temperature distribution with the change of cutting tool edge, in order to optimize the shape of cutting edge for micro/nano-cutting of different materials. According to the experiment results, the cutting force as well as the feed force generally increases with the increment of cutting edge radius [79, 80]. The feed forces are more sensitive to a change in cutting edge radius than cutting forces [60]. Same results were obtained by the finite element simulations, not only in the micro-cutting process but also in the nano-cutting process [81, 82]. The plowing force which directly acts on the cutting edge is also determined and separated from the total force in order to better understand the cutting process, as shown in Fig. 7 [57, 60]. Same as the cutting force, the plowing force in the cutting direction is less sensitive to the change of cutting edge radius than the force in the feed direction. Both of them increase linearly with the increment of the edge radius. When the cutting tool edge is characterized by the form-factor method mentioned in the last section, the process forces are mostly affected by the edge segment on the flank face, Sa , whereas the impact of the segment on the rake face, Sc , is negligible [55]. In nano-cutting process, the edge radius of the tool causes a nonlinear variation in the cutting forces when the ratio of UCT to the tool edge radius t ½rb \1 (½rb indicates the tool edge radius remains constant, and in this reference ½rb ¼10–60 nm) [83]. When the ratio t ½rb 1, the cutting force exhibits an approximately linear relationship with the UCT. This phenomenon is the size effect caused by the tool edge radius in cutting process and is described as the specific cutting energy increases rapidly and nonlinearly as the ratio t rb decreases. It is also found in Ref. [70, 84–87]. The specific cutting energy is the ratio of the cutting force to the product of UCT and cut Nanomanufacturing and Metrology in following sections. Lucca et al. [90] performed experiments to study the effect of tool edge geometry (rake angle, tool edge radius) on the cutting forces and specific energy with UCT changing from 20 to 10 nm. They found that at small UCT, the effective rake angle rather than the nominal rake angle determines the direction of the resultant force. 3.1.3 Influence on Stress Fig. 7 Separation of active force Fa into plowing force FPl and chip forming force FCh and into components in feed and cutting direction [60] width in orthogonal cutting [88]. Similar results were obtained by Ranganath et al. [89] while cutting gray cast iron in micro-metric scale, with the tool edge radius change from 15 to 72 lm in orthogonal cutting experiments. As shown in Fig. 8, the cutting force and feed force coefficients which are similar to the specific cutting energy increase nonlinearly with the decrease in the ratio t=rb (begins at the ratio t rb 1 for cutting force coefficient and 2 for feed force coefficient). Further discussion of the size effect caused by the materials’ size effect is displayed Fig. 8 a Cutting force coefficient versus t=rb ratio for different edge radii. b Feed force coefficient versus t=rb ratio for different edge radii [89] The force action on the cutting tool relates the stress distribution at the tool edge which would affect the performance of the cutting tools [91]. The unit forces, such as unit tangential force and unit normal force acting on the contour of the cutting edge, are investigated with tool edge radius range from 4 to 40 lm [92]. The minimum UCT and the unstable region are identified by the distribution of the unit forces in the tool edge. Özel [52] investigates the stress distribution in four different tool edge shape, including uniform chamfer, uniform waterfall hone, uniform hone and variable hone inserts. And results show that the most favorable stress distribution is obtained with variable hone micro-geometry insert. Besides that, the tool edge also influence the stress distribution in the workpiece materials under/ahead of tool edges. When the tool edge radius is negligible, the primary shear zone could be simply seen as a shear plane, which has been assumed by Ernst et al. [12] and Merchant [13, 14] in the 1940s to build the shearing cutting model. But when the ratio of UCT to tool edge radius t rb decreases to a certain value, the shear zone and the von Mises stress distribution become more extensive and extend to the area under the cutting tool edge [84, 93]. As shown in Fig. 9, the plastic deformation behavior for t=rb ¼ 1 and 3 is similar to that of conventional shearing model. When the ratio t=rb reduced below 1, the materials undergo severe plastic deformation and the size and thickness of it increase due to the merger of the primary and secondary deformation zone [93]. Similar results were obtained by Yan et al. while cutting silicon in ductile-zone with the tool edge radius change from 50 to 500 nm using FEM method [82]. With the increase in cutting tool edge, the primary shear region becomes further deeper and broader forming a large triangular high-stress region beneath/ahead of the tool. With the increase in edge radius, the maximum stress decreases slightly, but is still maintained at a high level. In 1998, Fang et al. [94] propose that the hydrostatic pressure under the tool edge is beneficial to cutting brittle materials in ductile region. It is proved that the area of the hydrostatic pressure zone extends with the increase in cutting tool edge radius by finite element modeling of the silicon [82]. Similar results were obtained by molecular 123 Nanomanufacturing and Metrology Fig. 9 von Mises effective stress distributions at the different ratios t=rb [93]. a t=rb ¼ 1; b t=rb ¼ 3; c t=rb ¼ 0:6; and d t=rb ¼ 0:2625 dynamic simulation of copper with the ratio t=rb change from ! to 0.23, as shown in Fig. 10. The hydrostatic pressure tends to cause an increase in critical UCT of brittle materials which is identified by experiments [95]. It seems that a larger tool edge radius causes a bigger critical UCT under which the brittle material is removed in ductile mode. But there is an upper bound of tool edge radius when cutting brittle materials like silicon whose upper bound value is between 700 and 800 nm [96]. The upper bound of tool edge radius is explained by the increase in tensile stress which borders upon the interface of plastic and elastic deformation zones, when the tool cutting edge radius increases. Based on the tensile stress distribution, the critical conditions for the crack initiation have been determined by Li et al. [97]. The stress distribution in the material is also influenced by itself. The magnitude of concentrated stresses under the tool edge is much higher for silicon compared with copper [98]. The distribution of stress rxx which is in the cutting plane at the cutting direction is investigated by Xiao et al. [47] using molecular dynamic simulation, as shown in Fig. 10 Hydrostatic stress distributions at a t=rb ¼ 1; b t=rb ¼ 1; c t=rb ¼ 0:46; and d t=rb ¼ 0:23 [99] 123 Fig. 11. The results show that compressive stress exists in a region near the cutting edge and with the tensile tress around the compressive zone. The maximum tensile stress tends to increase with the increment of UCT which would lead to fractures under the larger UCT. The formation and propagation of the cracks would be discussed detailedly in the section of surface generation. 3.1.4 Influence on Cutting Temperature Cutting temperature has been a research topic for a long period of time [100]. The temperature measurement methods in material removal processes have been reviewed by Davies et al. [101]. Generally, the cutting temperature is the energy dissipated by the materials deformation and friction at the interface between cutting tool and workpiece [102]. The shape of the tool edge has a significant influence on the cutting force and stress distribution in the tool and workpiece, especially in the micro/nano-cutting process when the ratio t=rb reduced to a certain value. Meanwhile, the tool edge shape affects the temperature distribution in Nanomanufacturing and Metrology Fig. 11 Distribution of stress rxx in the cutting zone [47]. a 40 nm, b 35 nm, c 30 nm, d 25 nm, e 20 nm, f 10 nm the tool/workpiece contact zone. In the traditional orthogonal cutting process where the tool edge radius is ignored, the heat is generated in the primary, secondary and tertiary deformation zone [103], due to the plastic work done at the shear plane and the friction work done on the tool/chip and tool/workpiece interface zone. The heat generated in the secondary zone is the main factor to rise the temperature of cutting tool. The primary zone also influences the temperature distribution in the cutting tool by transferring the heat to the chip and then through the interface zone to the tool rake face. Therefore, the temperature distribution on the tool rake face is influenced by the generated heat in the primary and secondary zones [104]. Akbar et al. [105] investigated the temperature distribution in the cutting tool influenced by the heat fraction transferred into the tool. The results show that lower part of the heat transferred to the tool decreasing the temperature in it hence prolongs the tool life. Therefore, conditions should be chosen to remove the larger part of heat by the chip, such as using a coated tool [106] or a high cutting speed. The temperature distribution on the rake face of sharp cutting tool (tool edge radius 2 lm) has been detected by IR-CCD camera, as shown in Fig. 12a, when cutting the steel SS2541 with UCT of 0.1 mm. The measured maximum temperature point is on the rake face with distance 0.15 mm from the cutting edge [80]. It is compared with a round edge tool (tool edge radius 25 lm) in the cutting process, as shown in Fig. 12b, and found only a small increase in maximum temperature (* 10–15 C) on the rake face. The increase in the temperature could be accounted for the increased the secondary shear zone thickness in the chip as well as the tool–chip contact length due to the tool edge radius. Thus, it could be deduced that less effect acts on the temperature distributions of tool rake faces, when the ratio ½t=rb (½t indicates the UCT remains constant) decreases from 50 to 4. Actually, it is still difficult for the IR-CCD camera to detect the temperature distribution in the tool edge radius ranging from several tens of nanometers to several tens of micrometer. Therefore, the simulation methods are used to predict the temperature in the tool edge. Karpat et al. [107] analyze the temperature distribution in different tool edges using the finite-element simulations. Results show that the rake face temperature distributions are more evenly in waterfall hone tools than honed tools. And the lowest tool tip temperature is obtained in waterfall hone tool with Sa : Sc ¼ 30 : 60 lm for 175 m/min cutting speed and 150 lm UCT, and Sa : Sc ¼ 20 : 40 lm for 125 m/min cutting speed and 100 lm UCT [107]. Özel further considered the smaller UCT in the tool tip and designs a tool with variable hone edge to reduce the ratio t=rb in the tool tip [52]. The smallest hot zone is found to be on the variable honed insert which also has a smallest maximum temperature of about 626 C. Denkena et al. develop a setup for temperature measurement in cutting tool with a graded-index-multimode fiber optic to collect and guide the electromagnetic IR radiation to the IR detectors of the pyrometer [53]. From the temperature gradient determined in the wedge, the segment in the rake face Sc has no significant influence on the isotherms for the tool with the same Sa ¼ 100 lm. And waterfall hones with bigger Sa lead to higher thermal load of the wedge [74]. With increase in Sa , the maximum 123 Nanomanufacturing and Metrology Fig. 12 Effect of tool edge micro-geometry on tool isotherms a sharp tool, b round tool [80] temperature shifts from the rake face to the flank face [55]. Similar results were obtained by Yan et al. in nano-cutting of silicon with the tool edge radius increase from 50 to 1000 nm [82]. The ratio ½t=rb ([t] stays constant at 100 nm) decreased from 2 to 0.2, the high-temperature region in front of the too rake face shrinks while that under the tool flank face grows gradually. At the ratio ½t=rb ¼ 0:5, the temperature rise at the flank face side has become higher than that at the rake face side. When the ratio ½t=rb ¼ 0:2, temperature rise only takes place at the flank face side. The temperature increase at flank face causes a severe wear between the tool and the elastic recovered workpiece materials and finally affects the generated surface quality [82]. It is identical to the molecular dynamic simulation of the diamond cutting of silicon; the number of atoms at the tool flank face showing an increase in temperature is larger than that at the rake face, especially at the place where the silicon atoms try to recover elastically [37]. Cai et al. [108] investigate the temperature in three different zones and find that the temperature near the arc part of the cutting tool edge is larger than that under the flank face of the tool and the zone far from the machined surface. The temperature in the tool edge or the flank face tends to accelerate the formation rate of silicon carbide and hence increase the wear of diamond tools. The wear mechanism has been identified by the experiments [109]. The distribution of temperature on the sharp diamond tool has been investigated by Yan et al. [110] and found a hightemperature region forms at the center of the chip. This phenomenon would further soften the workpiece material causing the adhesion on the rake face of the cutting tool, which could be optimized by two-step cutting process. The diamond turning of micro/nano-structures with multitip diamond tool shows a powerful capacity for improving the production efficiency, suppressing the tool wear and increasing the machining area with one diamond tool. The temperature in the tool tips has been investigated by Tong et al. [111], as shown in Fig. 13. The results show that the inner sides of the tool tips are higher than other 123 sides, which would cause a severer wear in the inner side of the multitip diamond tool. 3.1.5 Influence on Chip Formation and Minimum UCT Generally, the material is removed by shearing which is first identified by Mallock [10] and the model is built by Merchant et al. [12–14]. In the shearing model, the tool edge is simplified as sharp. With the development of manufacturing technology, the machining accuracy and feature size have approached micro/nano-metric scale. In this scale, the UCT becomes comparable to the cutting tool edge radius. Therefore, the chip formation and the minimum UCT would be influenced by the tool edge. Similar to the stress distribution in the cutting zone with a round cutting edge, the shear plane would extend to a shearing zone in front of the cutting edge [11, 112]. Woon et al. [112] systematically investigate the effect of tool edge radius on the chip formation behavior of micro-machining. And the results show that the primary deformation zone expands with the shrinkage of secondary deformation zone following the reduction in total tool–chip contact length, when the ratio ½t=rb decreases from 2 to 1. Further reducing the ratio ½t=rb to 0.4, the secondary deformation zone akin to merge with the larger and thicker primary deformation zone. When the ratio ½t=rb is 0.2, the secondary deformation zone disappeared or absolutely merged with the primary deformation zone [112]. In 1989, Moriwaki and Okuda [66] performed diamond cutting experiments on copper with UCT changing from 3 to 2.5 nm and found that the material removal mechanism experiences a transition from cutting to plowing. In nanocutting process, Fang et al. [5, 94, 113] investigated the influence of the tool edge radius on nano-cutting mechanism by both molecular dynamics analysis and experimental study. The results showed that the atoms are extruded in front of the cutting tool, when the UCT is 1 nm and edge radius is 5 nm. A new nano-metric cutting model that material removal at the nanoscale is on extrusion Nanomanufacturing and Metrology Fig. 13 Temperature distribution of tool tips at a cutting distance of 17 nm; a single tip tool (first pass and second pass); b multitip tool [111] mechanism was proposed by Fang et al. [5, 88, 94, 113], as shown in Fig. 14. Woon et al. [114] found that in micro-cutting process the chip formation mechanism also transforms from shearing to an extrusion-like behavior, at a critical combination of UCT and tool edge [114]. Simoneau et al. [2] investigated the influence of tool edge radius on the chip formation by cutting medium carbon steels with TiNcoated tungsten carbide tools whose cutting edge radius is measured to be 8 to 10 lm. Result shows that the continuous chips are generated when the UCT is larger than or equal to the tool edge radius, as shown in Fig. 15a–c. While the UCT decreases, a transition from shearing to quasi-shear-extrusion chip occurred as shown in Fig. 15d– f. Fig. 15 Chip cross sections in different t/rb [2]. a t=rb ¼ 10, b t=rb ¼ 5, c t=rb ¼ 1, d t=rb ¼ 0:4, e t=rb ¼ 0:3, f t=rb ¼ 0:2 Fig. 14 Schematic illustration of nanoscale cutting model [5] When the tool edge cannot be ignored, there is a threshold below which the chip formation cannot form stably or just no chip formation. Kim et al. [115] observed plowing under a certain UCT indicating existence of a threshold. The threshold is defined by Ikawa et al. [116] as the minimum uncut thickness that can be removed stably from workpiece surface with a cutting edge under perfect 123 Nanomanufacturing and Metrology performance. And the minimum UCT is thought to determine the extreme accuracy attainable under specific cutting conditions, tool and workpiece, etc. The minimum UCT is also defined as the threshold whether a chip is formed or not [115, 117, 118]. Liu et al. [119] take orthogonal cutting, turning and milling process into consideration and define the minimum UCT. The minimum UCT strongly relates to the material separation mechanisms in front of the tool edge. Two major approaches have been proposed to analyze the material separation mechanism [55, 107]. Both will be discussed in the following. The first approach is based on the existence of a stagnation point on the tool round edge, below which the material flows under the tool to form the machined surface and above which the material flows up along the tool face to form chip [120]. Fang [19, 20] comprehensively defines the tool edge roundness by four variables. Stagnation point is one of the four variables, and the others are tool edge radius, tool–chip frictional shear stress above or below the stagnation point on the tool edge. As shown in Fig. 16a is the material flow around the tool edge with a radius of rb and a UCT of h. A stagnation point S is assumed on the tool edge locating at a stagnation angle hs . At this point, the effective negative rake angle ce ¼ p=2 hs determines the separation height hs ¼ rb ð1 cosðhs ÞÞ. With the decrease in the UCT, the separation height tends to equal the UCT which is the minimum UCT, hm (hm ¼ rb ð1 cosðhc ÞÞ, hc is the corresponding critical stagnation angle, and the corresponding critical effective rake angle cec ¼ p=2 hs ¼ arcsin 1 hm =rb ). Lai et al. [120] applied 3D molecular dynamics to investigate the effective rake angle of the stagnation region which is almost a point. The relationship between the minimum UCT and tool edge radius has been found. One of the simulation snapshots is shown in Fig. 17. The effective rake angle of stagnation point is approximately constant with the same depth of cut and different tool edge radii. The separation height is almost in proportion to UCT and increases with it when cutting using the same tool edge radius. The critical effective rake angle deduced at the point where separation height equals the UCT is between Fig. 16 Cutting with an edge radius tool, a stagnation point, b stagnation zone 123 Fig. 17 Stagnation region on the 3D simulation snapshot [120] - 65 to - 70. Different molecular dynamics simulation results obtained by Hosseini et al. [99] showed that the stagnation angle was approximately constant with the variation of the ratio t=½rb (the tool edge radius ½rb remains constant at 4.9 nm) from 0.22 to 0.59, except the ratio t=½rb 0:15. This indicates that UCT has no obvious effect on stagnation angle. Therefore, the change of the separation height hs with the UCT is always neglected. The separation height is approximately equal to the minimum UCT hm , which has been used by Yuan et al. [117] to investigate the effect of diamond tool sharpness on minimum UCT. The deduced minimum UCT is 0 1 B C Fy þ lFx ffiC hm ¼ r b B @1 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A Fx2 þ Fy2 ð1 þ l2 Þ ð2Þ where Fx and Fy are the horizontal and vertical forces acting on the stagnation point S. According to the equation, if the tool edge radius rb is known, the minimum UCT is determined by the ratio Fy =Fx and the friction coefficient l. The minimum UCT is estimated to be 20–40% of the cutting edge radius [117]. Son et al. [121] took the friction coefficient between the tool and the workpiece into consideration and deduced the equation of minimum UCT: hm ¼ rb ð1 cosðp=4 b=2ÞÞ ð3Þ where b is the friction coefficient angle (b ¼ tanðlÞ). According to the equation, an increase in the friction Nanomanufacturing and Metrology coefficient could reduce the minimum UCT [122]. Malekian et al. [118] investigated the minimum UCT using the minimum energy approach and infinite shear strain method. Results showed that the stagnation point locates at the place where the critical angle (hc ) approximates the friction coefficient angle b. Therefore, the critical effective rake angle cec ¼ b p=2 could be seen as the key factor in determining the minimum UCT, following the equation: hm ¼ rb ð1 cosðbÞÞ ð4Þ According to the equation, the minimum UCT increases with the increment of the friction angle or the friction coefficient. Lai et al. [123] simulated orthogonal cutting of copper and found that the minimum UCT is 0:25rb when the cutting edge radius is 2 lm and the rake angle is 10. Woon et al. [124] investigated the effects of tool edge radius on flow stagnation phenomenon using Lagrangian– Eulerian FE modeling approach. They found that the material separations could be attributed to the counterbalance of shear contact components and it is insensitive to UCT. The stagnation angle in their model is 58:5 0:5 with the UCT of 2 to 20 lm and cutting speed of 100 m/ min. Jin et al. [125] proposed a slip-line model for microcutting considering the effect of tool edge radius. The model based on the existence of stagnation point on the tool edge and the stagnation angle is calculated by the finite element model. The minimum UCT has a significant influence on the generated surface roughness. Weule et al. [51] proposed the relationship of the minimum UCT with surface roughness. It is responsible for the generated sawtooth-like surface profile. Wang et al. [126] applied this relationship and deduced an equation about surface roughness and minimum UCT. The minimum UCT obtained by experiments based on the proposed equation is hm ¼ ð0:28 0:33Þrb . The material flow in front of the asymmetrical tool edge which is characterized by the formfactor method has been investigated by quick-stop experiments and the FEM [55, 127]. For rounded cutting edges with K [ 1, extensive workpiece material deformation occurs. This effect decreases by shifting the form factor to K \ 1. Based on simulation results, the point of material separation locates at effective rake angle cec ¼ 46 2 . Therefore, the minimum UCT is determined by the tool edge shape and the effective rake angle. The second approach is based on the existence of a stagnation region in front of the tool edge like a stable build-up edge (BUE) or a dead material zone (DMZ) which changes the flow of workpiece material. As shown in Fig. 16b, the stagnation-zone tip S is where the workpiece material starts to split into two parts. The formation of stagnation zone or dead metal zone is determined by the tool edge shape, cutting speed, material and frictional properties between the tool and workpiece materials [129]. One of the extreme conditions is to cut materials with a large negative rake angle which could be used for simulating grinding process or helping analyzing the chip formation under low ratio t=rb [128]. As shown in Fig. 18b, when cutting with - 60 rake angle, a stagnation region like a triangle dead metal appeared at the tool tip. It could be seen as a stable built-up edge which influences the shearing process making the formation of serrated chips. Figure 18a is the finite element modeling (FEM) of the chip formation which is similar to the experimentally obtained serrated chips [128]. Asymmetrically designed chamfers in cutting tool edge also change the material flow by causing a stagnation zone. The size and the shape of the stagnation zone are determined by the designed chamfers [55, 127]. Similar results were obtained by Chen et al. [130] in cutting AISI H13 hot work die steel with PCBN tool. He found a small part of the workpiece material adjacent to the cutting edge with no relative velocity between the tool and workpiece. This phenomenon indicates the existence of stagnation region [131]. The position of the generated stagnation region on tool edge obtained by FEM is same as the experimental results obtain by Ng [132]. When cutting with a sharp tool, the stagnation region at the tool tip is small and unstable [99]. Similar results were obtained by finite element modeling. For sharp tool, the size of stagnation region is small and the shape changes during cutting [133]. When cutting with rounded tool edges, the stagnation zone forms at the beginning of cutting and its shape becomes stable when steady-state cutting is reached [99]. The finite element modeling results show that the shape of stagnation region is almost triangular and the size of it increases linearly with tool edge radius. As shown in Fig. 19, the distance from the stagnation-zone tip S to Fig. 18 Chip formation with stagnation region in rake angle of - 60 [128] 123 Nanomanufacturing and Metrology Fig. 19 Total velocity distribution during steady-state cutting [133]. a rb ¼ 50 lm, b rb ¼ 100 lm the machined surface which could be seen as the separation height hs increased with edge radius. And its distance to rake face ts always stays constant [133]. Kountanya et al. [129, 134] experimentally investigated the formation of dead metal cap ahead of the rounded tool edge. A real-time visual picture of the dead metal cap was obtained in cutting cartridge brass with the ratio t=rb ¼ 0:73 (the edge radius rb is 220 lm) and the cutting speed of 7.8 mm/s. However, dead metal cap was not formed when cutting zinc. Kim et al. [135] investigated the formation of build-up edge with the variation of the ratio t=½rb (½rb = 6 nm) from 0.1 to 0.9. When the ratio t=rb is 0.1, no chip was formed. And when the ratio t=rb is 0.2, the material tended to pile up ahead of the tool. As the ratio increased above 0.4, the piled-up material developed into chips, and the transition from plowing regime to cutting regime happens. Build-up edges were always formed in a triangular shape ahead of the round tool edge in the cutting regime according to the molecular dynamic simulations. Liu et al. [136] proposed a slip-line field model which takes into consideration dead metal cap formed in front of the cutting edge to solve minimum UCT and cutting temperature iteratively. With an initial value of the normalized minimum UCT (kn ) which is defined as the ratio of the minimum UCT to edge radius, the cutting temperatures are calculated by the slip-line model and used to compute the yield strength (r) of the work hardened material and the shear strength (sa ) of the adhesive junction. The normalized minimum UCT is recalculated by Kragelskii–Drujuanov equation as sa kn ¼ 0:5 ð5Þ r The calculation is continued until the computed and initial value is smaller than the preset tolerance. The model is experimentally validated with micro-end milling tests on 1040 steel. In the tests, the minimum UCT is obtained by measuring the sudden surface height change on the machined sidewall. Results showed that the slip-line model predictions match the experimental results well. The 123 minimum UCT is ð0:2 0:45Þrb which increases with cutting edge radius and the cutting speed when machining 1040 steel. When cutting using an asymmetrically rounded tool edge which is characterized by the form-factor method, the influence of Sc on the stagnation region is negligible, whereas the influence of Sa is significant. Therefore, for tool edge of large K, the material separation point is located nearly at the end of the flank face which makes the effective UCT approximately the same as the UCT. However, while machining with a tool edge of small K, the height of the material separation point approaches the magnitude of Sc [137]. Further investigation shows that a stagnation zone forms in front of tool edges with K [ 1 and in the case of tool edges with K \ 1 a stagnation zone cannot be clearly recognized [55, 74]. However, in nanocutting of single-crystal silicon, the stagnation zone forms in both kinds of the tool edge which is investigated by MD simulations [64]. Besides that, the size and the position of the stagnation region are also determined by the crystallographic orientation of the workpiece material [138]. In nano-cutting of aluminum, the stagnation region forms in front of tool edge, as shown in Fig. 20 [139]. The sharp tool edge with a negative rake face entraps a large number of atoms forming a large stagnation region at its edge, especially at its side. However, when cutting the material by a sharp tool edge with a positive rake angle, almost no atoms are entrapped and no stagnation region forms at the tool edge, as shown in Fig. 20c–d. The shape of the stagnation region could be approximated to an arc with the center locating at the center of tool nose, and its radius is denoted as Rs . The separation height or the stagnation height is calculated according to the equation Rn Rs . The separation height of the tool edge with large edge radius is larger than that of small tool edge radius, as shown in Fig. 20a–b. With the size of the stagnation zone increases, it tends to work as a build-up edge, which sticks to the cutting edge or the rake face forming a new cutting edge. The hardness of the build-up edge is measured by Kummel et al. [140], and the results show that its hardness is two to three times higher than the hardness of the initial workpiece material. The grain size of the build-up edge is refined from the microscale to nanoscale. The size of the build-up edge is changed with the cutting parameters; for example, the area and height of build-up edge decrease with increasing cutting speed from 50 to 100 m/min [140]. The decreasing build-up edge with the increase in speed can be explained by the higher temperature in the cutting zone [141]. The increase in material hardness results in a decrease in the formation of build-up edge [142]. The build-up edge in front of the tool edge tends to reduce the cutting forces due Nanomanufacturing and Metrology Fig. 20 Stagnation region on the cutting tool edges [139]. a rb ¼ 5 nm; c ¼ 0 , {100} \011[, b rb ¼ 2:5 nm; c ¼ 0 , {100} \001[ c rb ¼ 0 nm; c ¼ 15 , {111\} \1–10[ d rb ¼ 0 nm; c ¼ 15 {111} \1–10[ to the generated higher rake angle and sharper tool edge [143]. But during the cutting process, the variation of the build-up edge geometry and its aperiodic separation from the cutting tool would degenerate the machined surface quality [144]. Meanwhile, the separation of the build-up edge from the tool edge could lead to the damage of the tool decreasing the tool life [145]. However, in certain conditions, such as the low cutting speed (50 m/min), the formation of build-up edge also has the potential to protect the tool against the flank wear and crater wear, which has been observed by Kummel et al. [140]. They further stabilized the build-up edge in front of the tool edge by texturing the rake face with dimples [146]. With the stable build-up edge, the wear of the flank face and the corner radius are suppressed. In this section, the influence of the tool edge on the minimum UCT and chip formation has been discussed. The minimum UCT is an important factor in determining the material removal mechanism. Xu et al. [138] presented that the material is removed by shearing mechanism when the UCT is comparably larger than the minimum UCT. When the UCT is similar or smaller than the minimum UCT, the material is removed in extruding mechanism. For further decreasing the UCT, rubbing happens and no material is removed. More discussions about the influence of material deformation mechanisms on the surface generation will be displayed in the next section. 3.1.6 Influence on Surface Generation In addition to the industry-concerned performance parameters, fundamental physical parameters such as stresses, strains, temperatures, phase transformation and minimum UCT have been discussed for the in-depth understanding of surface generation in cutting process. The influence of tool geometries, especially the cutting tool edge, on the generated surface would be discussed in this section. The generated surface affected by the material properties in cutting process will be introduced in the next section. In cutting process, the generated surface is determined by the feed rate and tool nose radius if assuming that the workpiece material is removed ideally and the machined surface texture is the replication of tool nose profile. The theoretical peak-to-valley roughness is formulated as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rth ¼ Rn R2n ðf =2Þ2 f 2 =8Rn , where Rn is tool nose radius and f is feed rate [11]. However, it always deviates from its ideal value due to many factors, such as material properties of workpiece [147, 148] and cutting tool geometry [96, 149–152]. Wu et al. [153] experimentally investigated the influence of the ratio t=½rb (½rb stays constant at 8 lm) on surface roughness and found that the deviation from the theoretical value increases with the decrease in the ratio t=½rb . They thought that the size effect-induced stress nonlinear increase causes the increase in material side flow and then the increase in surface roughness. The minimum surface roughness is achieved near the minimum UCT which is estimated to be ð0:2 0:3Þrb . The deviation from the theoretical value has also been investigated by Childs et al. [154, 155]. They thought the feature of the cutting tool that determined the machined surface roughness changed from the tool nose radius to the tool edge radius, as the feed was reduced in turning process. Woon et al. [114] found that the generated 123 Nanomanufacturing and Metrology surface quality is deteriorated by further decreasing the ratio t=½rb (½rb stays constant at 10 lm) to 0.05, where the discontinuous chips were produced. In the finish hard turning of AISI 52100 steel, Thiele et al. [79] found that cutting edge with small edge radius results in lower average surface roughness values than that with large edge radius. The effect of the cutting edge radius on the surface roughness decreased with an increase in workpiece hardness. In diamond cutting of silicon, Fang et al. [70] found that large tool edge radius increases the machined surface roughness. Similar results were found by Yuan et al. [117] in diamond cutting of aluminum alloys. Ng et al. [83] investigated surface roughness in diamond cutting of aluminum 2075-T6 and found that the generated surface at an UCT smaller than the tool edge radius is rougher than that with larger UCT. The phenomenon that the cutting-induced surface roughness always deviates from the theoretical value could be partly attributed to the effect of the cutting tool edge [156]. The elastic recovery and the side flow of materials are two factors affected by tool edge and further influence the surface roughness. The combined effect of elastic recovery and side flow which causes greater and deeper marks on machined surface was referred as the swelling effect by Cheung et al. [157]. Sata et al. [158] found that there was no swelling for brass when compared with the roughness obtained in cutting of medium carbon steel. Therefore, greater swelling would generate on the machined surface of the softer material. Cheung [157] and Kong et al. [159] thought the amount of recovery which is determined by the forces on the clearance face and Young’ modulus of the workpiece material would cause a wavy machined surface. Schaal et al. experimentally investigate the influence of cutting edge radius on elastic recovery in metal cutting. Results showed that the elastic recovery for the sharp tool is lower than for the blunt tool and the variation of elastic recovery is in coherence with the change of surface roughness [160]. To predict the surface roughness in single point diamond turning, Zong et al. [156] introduced an additional term to the roughness formulation proposed by Grezesik [161]. It is formulated as f2 hm Rn h m H 0 Rth ¼ 1þ þ ð6Þ þ k1 rb k2 E 8Rn 2 2 where H and E are the Vickers hardness and Young’s modulus. k1 relates to the tool nose radius and rake angle, and k2 relates to the maximal UCT and tool edge radius. He et al. [162] considered the contribution of the elastic recovery to the machined surface roughness individually. Based on the prediction method proposed by them, the contribution of material recovery to Al6061 is 0:9162hm ¼ 0:3207rb . 123 The side flow is the plastic deformation of workpiece material at a direction opposite to the feed direction in cutting process [163]. It is a result of the interactions between cutting tool and workpiece material [164]. Kishawy et al. [152] proposed two mechanisms to describe the formation of material side flow. The first one is the squeeze between the machined surface and the tool flank face, especially when the UCT is smaller than the minimum UCT. The second one is that the trailing edge notch induced high temperature and pressure makes the material be pressed aside. They further investigated the formation of side flow by 3D thermo elasto-viscoplastic finite element method and found that more side flow is generated with higher nose radius and lower feed [165]. Cheung and Lee [166] found that the machined surface roughness increased with an increase in tool nose radius at large radius but decreased at small radius. This phenomenon may be partly caused by the side flow. Ge et al. [167] found that the severe worn tool edge caused the severe swell of the material in the machined surface due to the side flow. Lai et al. [168] found that the side flow increases the height of the tool marks left on the machined surface and therefore influences the machined surface roughness of germanium. The side flow of material has been taken into account to predict the machined surface roughness [156, 162, 164, 169]. In 2006, Liu et al. [164] established a relationship between the roughness due to side flow Rp and the rheological factor x, which is formulated as Rp ¼ k1 ln x þ k2 ð7Þ E cot h ry e ð8Þ x¼ ry is the average flow stress and e is defined as the ration of the average flow stress with and without strain gradient strengthening. E is the Young’s modulus. k1 and k2 are two coefficients needed to be calibrated via actual cutting tests. In 2014, Zong et al. [156] introduced a scale coefficient k3 relating to the side flow to the surface roughness prediction formulation. In determining the value of the coefficient k3 , the feed rate and tool nose radius are considered. In 2015, He et al. [169] thought the side flow is one of the uncertain components determining the surface roughness which is predicted empirically by a radial basis function (RBF) neural network. In 2016, a new prediction model was proposed by them [162]. The side flow-induced increment of surface roughness is formulated as w ¼ kd kt kf hmin bD , where bD is the effective cutting width, hmin is the minimum UCT, kd is a scale coefficient determined by the material properties of workpiece, and kt and kf are two variable coefficients relating to the tool nose radius and feed rate, respectively. The minimum UCT hmin used in their study is 0:35rb , and the scale coefficient Nanomanufacturing and Metrology determined by the material properties is 0.001 lm-1. However, the tool edge radius or the anisotropy of the material which determines the minimum UCT and the side flow lack direct consideration in predicting the machined surface roughness. The side flow effect on the surface generation or the surface roughness should be investigated deeply. The material deformation mechanism in cutting process is one of the most important factors in influencing the surface generation, especially the side flow and the recovery which is determined by the deformation mechanism. Zhang et al. [170] reviewed the surface roughness formation in ultra-precision machining and thought that the larger tool edge radius would lead to higher plowing phenomenon causing an increase in surface roughness. In orthogonal cutting of aluminum, Xu et al. [138] found that small minimum UCT accompanied with small recovery height and contact length between the flank face and workpiece material caused a better surface quality. The surface quality gets worse when the UCT is similar to the minimum for the cutting direction of {100} \001[ and {100}\011[, due to the extruding removal mechanism taking place at the cutting condition. Xu et al. [139] found the side flow effect on surface generation in nano-cutting of aluminum as shown in Fig. 21. A larger part of the side flow material and residual material are at or under the stagnation region which is characterized by stagnation radius Rs . Then, in the cutting process, they are extruded by the tool edge to form the side flow. The position, shape and size of the stagnation region which are determined by the material properties and tool edge geometry would influence the side flow of nano-cutting process. Small tool edge radius and positive rake angle which could suppress the formation of stagnation region or enlarge the stagnation radius Rs (decrease the separation height Rn Rs ) would decrease the size of side flow [139]. The side flow of the material, on the other hand, would result in Poisson burr which would deteriorate the machined surface quality in cutting process [171]. The increase in tool edge radius which increases the size of side flow would encourage the formation of the Poisson burr and enlarge the size of it [172]. Fang and Liu [173] found Fig. 21 Atoms tend to form the side flow region (yellow) and residual region (blue), a before and b after nano-cutting [139] that the formation of burrs could be suppressed by decreasing the UCT. To minimize the burrs in cutting process, the UCT should be comprehensively matched with the cutting tool edge radius [171]. In this section, the influence of the tool edge on the surface generation has been discussed. However, the surface generation could also be affected by the material properties which would be discussed in the next section. 3.2 Influence of Material Properties The material properties, such as the crystallographic orientation [138, 148, 174], grain size [175], grain boundary [147, 176, 177], or hard particles [178–180] of or in workpiece material, have important effects on the cutting process. Therefore, in this section, the influence of the material properties would be discussed in three aspects: the influence of ductile materials, the influence of brittle materials and the influence of the defects, such as grain size, grain boundary and hard particle, on the micro/nanocutting process. 3.2.1 Influence of Ductile Materials In micro/nano-cutting process, the material is removed under the scale which is usually smaller than the material grain size [181]. Therefore, the size effect of materials could not be neglected [88, 182]. The size effects of the material have been studied by many researchers. Volkert et al. [183] and Lee et al. [184] investigated the mechanical properties of Au columns with a diameter changing from 100 nm to 8 lm. The results that the yield stress and apparent strain hardening rate increased with a decrease in column diameter were found under the uniaxial compression experiments. Similar results were found in niobium single crystals [185] and even amorphous metals [186, 187]. Abad et al. investigated temperature-dependent size effects on the strength of [111]—oriented tantalum and [100]—oriented tungsten pillars by elevated temperature compression tests [188]. Kaira et al. [189] investigated the microscale deformation behavior of bicrystal boundaries in pure tin by the micropillar compression tests and found that Rn-r Rn-r Rn Rs Rn PV A-A Side flow (a) Rs Workpiece (b) Residual region Side flow region C-C Workpiece B-B PV’ 123 Nanomanufacturing and Metrology the random angle grain boundaries in the micropillar act as barriers to the dislocation glide causing the strengthening mechanism. Shan et al. [190] found the annealing of micropillar through mechanical deformation due to the exhaustion of dislocations making the dislocation density fall to zero. The size effects of material in micro/nanoscale would appear in micro/nano-cutting process, partly causing a rapid and nonlinear increase in specific cutting energy, especially at the nano-metric UCT [83, 191]. Lai et al. [192] found the increase in the specific cutting energy with a decrease in UCT in investigating the nano-cutting by three-dimensional MD simulations. Xu et al. [138] found that the size effects of materials make different plastic carriers, such as the twin, stacking faults and dislocations, generate on the plastic deformation zone of different cutting directions, as shown in Fig. 22. And the processing forces would be influenced by the size effect and anisotropy of materials. Therefore, when decreasing the UCT, the effect of cutting direction which is actually the effect of crystallographic orientation or anisotropy of the materials on the cutting process becomes non-ignorable, whether the workpiece is single or polycrystalline material [138, 181, 193]. In 1994, Yuan et al. [181] experimentally investigated the influence of crystallographic orientations on the cutting force and generated surface roughness. They found that the fluctuation in the cutting force is responsible for the variation of the machined surface roughness. Lee et al. [194] found different surface roughness is obtained at different crystal planes. They thought that the anisotropy of materials which would cause the anisotropy of Young’s modulus and different amount of recovery caused the difference on generated surface roughness. The best surface roughness is obtained on aluminum {100} planes by To et al. [195]. The shear angle and shear stress were also determined by the crystallographic orientations, but they are less Chip Frank partial dislocations Cutting direction Chip Diamond tool ISF sensitive to the changes of cutting conditions [196]. The side flow which is another important factor in deteriorating the generated surface roughness would be influenced by the anisotropy of materials. It has been investigated by Xu et al. [139] and found that the side flow could be suppressed by optimizing the cutting directions. Small side flow is formed at the cutting direction of {100}\011[, {110}\001[ and {110}\1–10[, whose stagnation height is smaller than that of others. The stagnation height, to a certain extent, is corresponding with the minimum UCT and the minimum depth of material removal. The effect of the crystal orientation on minimum depth of material removal has been investigated by Zhu et al. [197]. Similar to the effect of crystallographic orientation on side flow in cutting process, the formation of burrs is also affected by the anisotropy of materials. Wu et al. [172] found that the maximum burrs are formed on the {111} planes of copper and the minimum burrs are formed on {100} planes of copper. 3.2.2 Influence of Brittle Materials The subsurface deformation for the brittle materials, such as the silicon and germanium, is also influenced by the crystallographic orientation, and the best cutting directions which lead to the thinnest thickness of deformed layer are obtained [174, 198]. The machining-induced phase transformation and residual stress could be investigated by micro-Raman spectroscopy [199, 200]. Yan experimentally investigated the influence of the cutting direction on surface deformation and the critical UCT of single-crystal silicon [201]. The results showed that the cutting direction of {100}\128–3090[ had the largest critical UCT which meant the best ductile machinability. And the machininginduced amorphization layer at this cutting direction is the thinnest compared to the other cutting directions. For KDP Twin bondary Chip Cutting direction ESF Diamond tool TB Perfect dislocations Twin boundary Other dislocations (a) Shockley partial dislocations Stair-rod dislocations Shockleypartial dislocations Workpiece (b) ISF Flankpartial dislocations Fig. 22 Snapshots of the microstructure evolution at cutting direction of a {100}\011[, b {110}\001[ [138] 123 Workpiece Nanomanufacturing and Metrology crystal, the critical UCT has been investigated under the cutting of {100}, doubler and tripler planes of KDP. The maximum critical UCTs were obtained at different cutting planes [202]. Zong et al. [203] proposed a predictive model for the calculation of critical UCT taking the compression force and micro-friction force into consideration. Cai et al. [204] simulated the crack initiation in the nano-cutting of silicon by MD. They found that cracks may be initialized due to a peak deformation zone in front of cutting tool edge. Xiao et al. [47] investigated the crack initiation and propagation in nano-cutting of silicon carbide with different UCT by MD simulations, as shown in Fig. 23. The crack propagation direction is determined by the UCT. Shallow craters would form on the machined surface when the crack propagates parallel to the cutting direction. Otherwise, steeper crater would be formed if the crack propagates downward the subsurface. Fang et al. [7] realized ductile cutting of silicon in a developed cutting device under the online SEM observation and found that the cutting velocity would influence the brittle-ductile transition. One nm surface roughness (Ra) on single-crystal silicon has been achieved by optimizing the cutting process [5, 70]. Li et al. [205] realized the ductile cutting of germanium by fast tool servo (FTS) ultra-precision turning. Near-rotational freeform surface with nano-metric surface roughness has been achieved efficiently. Microlens arrays of single-crystal silicon have also been machined in ductile region by slow tool servo (STS) ultra-precision turning [206]. However, to eliminate the formation of cracks in cutting of brittle materials, the UCT is less than the critical UCT which is a relatively small value and usually in nano-metric scale. Therefore, the machining efficiency is relatively low and the machined surface quality could not attain a stable status due to the other factors in cutting process. To improve the machinability of brittle materials, many assistant methods, such as the ultrasonic elliptical vibration assistant [207], laser beam assistant [208–210] and surface modification methods [211–213], have been proposed. Ultrasonic vibration-assisted cutting which is an effective method in diamond cutting of ferrous metals [214–216] was also a method in improving the machinability of brittle materials [207]. The critical UCT was improved significantly for the soda-lime glass (an amorphous material) by applying the ultrasonic vibration cutting. Based on the advantages of the ultrasonic vibrationassisted cutting process, ultra-precision cutting of glass can be realized. This technology has been used in other hard and brittle materials, such as the tungsten carbide [217]. Laser beam which has high directional characteristics can be focused to a small point creating high power density. It is widely used in processing of materials [218], such as cutting, welding, marking and sintering. Besides that, it is also used in assisting the cutting of brittle materials, such as the silicon [209] and sapphire [210]. In laser beamassisted cutting of silicon, the surface roughness was improved and the radial spokes which are caused by the crystallographic orientation effect can be eliminated [209]. The anisotropy effect of the single-crystal C-plane sapphire on the ductile mode cutting was investigated in laser-assisted cutting technology [210]. The results showed that for different cutting directions, the critical UCT was improved by applying this technology. Surface modification method is used to modify the workpiece surface to be machinable by bringing different physical or chemical characteristics and meanwhile not to change the properties of original materials. In micro/nanocutting process, initial application of this approach is incidental on the nickel. Arnold et al. [219] machined nickel phosphorus alloy by diamond tool and observed that tool wear was decreased with an increase in phosphorus content. After that, Brinksmeier et al. [220] modified the steel molds using plasma nitridation to reduce the wear of diamond tool in cutting process. For brittle materials, such as the machining of single-crystal silicon, Fang et al. [211] proposed a novel method which is the ion implantation surface modification method (NiIM), to enhance the machining efficiency, improve the surface quality and prolong the tool life in nano-cutting. After the ion implantation, the critical UCT of the silicon is increased for 4 times or even higher. By using the modification process, Fig. 23 Molecular dynamics snapshots at various UCT [47]. a 40 nm, b 35 nm, c 30 nm, d 25 nm, e 20 nm, f 10 nm Fig. 24 Mirror surfaces: a flat mirror and b freeform sinusoidal mirror [211] 123 Nanomanufacturing and Metrology flat mirrors and freeform sinusoidal mirrors were machined with nano-metric surface finish, as shown in Fig. 24. The same method of NiIM has been applied in cutting of singlecrystal silicon carbide [212] and germanium [213]. The detailed mechanism about the influence of the surface modification on nano-cutting of silicon has been discussed by Wang et al. [221]. 3.2.3 Influence of Grain Size, Grain Boundary and Hard Particle The grain size of the material accompanied with grain boundary would affect the plastic deformation in cutting process and further influence the generated surface quality. In nano-cutting, both the regular Hall–Petch relation and inverse Hall–Petch relation are found [175]. The regular Hall–Petch relation in nano-cutting is due to the resistance of the grain boundary to the dislocation movement. The inverse Hall–Petch relation is caused by the grain boundary diffusion and movement in nano-cutting [175]. At the grain boundary, tears were found to generate when cutting Al– Mg alloy [177]. The processing force would vary as the tool passed the grain boundaries [182]. Simoneau et al. [176] found that surface dimples occur at the hard to soft grain boundaries but do not occur when cutting through soft to hard grain boundaries. The dimples could be reduced by sizing the grain structure appropriately to the cutting parameters and specially the UCT. Surface treatment processes could be used to change the surface properties of materials and suppress the influence of the grain boundary on the surface generation, such as the friction stir process which has been applied by Tauhiduzzaman et al. [147] on the polycrystalline aluminum surface before cutting process. The results showed that the large grain boundaries were absent and the related defects disappeared on the machined surface. The grain size would also influence the burr formation in cutting process. The materials with small grain size are found to be beneficial to reduce the size of the burrs [172]. Hard particle in workpiece also plays an important role in plastic deformation and surface generation during cutting process. Ding et al. [179] found the formation of voids on the machined surface caused by the hard particles in Al RSA-905 and Al-6061. When the top layer of the machined surface is removed, the hard particle could be found near the voids. In cutting process, the hard particle could be sheared and fractured. The vacation of the broken parts of the hard particle causes the generation of the voids and enlarges the voids as the loose particles are drawn along by the cutting tool [180]. The formation of the voids could be suppressed by making the hard particle be machined in ductile mode, such as the reduction of UCT. Ultrasonic vibration-assisted cutting was also found to be an effective 123 method to achieve a stable-state cutting performance, an improved surface roughness and reduced burr size [222]. MD simulation has been used to investigate the influence of pore and second-phase particles on the subsurface damage and surface integrity in cutting process [178]. In 2017, Xu et al. [223] investigate the influence of the hard particle on the surface generation, plastic deformation and processing forces in nano-cutting of aluminum by MD simulation. The results show that when the hard particle is removed, only a small shallow pit is left on the machined surface. Otherwise, it is pressed down to the subsurface of the workpiece left larger and deeper pit on the generated surface. Besides that, the hard particle in the workpiece would increase the processing force when the cutting tool edge or the plastic carriers interact with the hard particle. In ultra-precision cutting of SiCp =Al composites, in which the SiCp is the hard particle in a soft aluminum material, many defects, such as pits, voids, microcracks, grooves, protuberances, matrix tearing and so on, are generated on the machined surface [224]. Better surface quality could be obtained when the SiCp particles are removed by pressed into or cut through mechanism. Otherwise, when the SiCp particles are pulled out or crushed in cutting process, cracks and pits will form on the machined surface which deteriorates the surface quality. The precipitates (Mg2 Si) which generate at the isothermal heat treatment would introduce scratch marks on the machined surface and deteriorate the surface roughness in cutting or Al6061 [225]. 4 Conclusions and Outlook The recent advances about the influence of the tool edge and material properties on micro/nano-cutting process are reviewed. The material deformation mechanism, such as shearing, extruding, and rubbing mechanisms, has been presented in this paper to explain the plastic deformation and surface generation phenomena in micro/nano-cutting process. For distinguishing these material deformation mechanisms, they are summarized as follows: • • Shearing mechanism: With the UCT that is larger than the minimum UCT, a shearing plane forms in front of the tool edge and expands from the stagnation point or the tip of stagnation region. The material flow direction changes abruptly at the shearing plane. Materials above the stagnation point or stagnation region would be removed as a chip. The material below it would be pressed and flow to the flank face of cutting tool forming as machined surface. Extruding mechanism: With the UCT that is less than or similar as minimum UCT, the stagnation point or Nanomanufacturing and Metrology • • stagnation region does not form stably in front of tool edge and no shearing plane expands, making the material be extruded away from the cutting edge to form as a chip. With a decrease in UCT, the cutting process is combined with extrusion and rubbing mechanisms, making the material be removed less efficiently. Rubbing mechanism: With decreasing the UCT to a value much smaller than minimum UCT, the cutting tool rubs along the workpiece surface and slightly presses the surface down to the bottom of cutting tool edge. The workpiece material experiences elastic– plastic deformation due to the interaction with tool edge and flank face, which, to some extent, affects the generated surface roughness. In rubbing, no chip would form, but the machined surface would be deteriorated. Plowing mechanism: Plowing mechanism is the appearance of the side flow processes. The side flow is the workpiece material flow to the two sides of the tool edge left the material on the machined or unmachined surface. The material flows at the direction opposite to the feed direction deteriorate the machined surface roughness. Based on the fundamental understanding of the material deformation mechanism in micro/nano-cutting process, more efforts should be made on the research field as follows: • • • • The design and fabrication of the cutting tool should be optimized to meet the extreme requirement of the micro/nano-cutting process. The machinability of the workpiece material should be improved by applied surface modification and other assisted cutting technologies. The prediction model of the micro/nano-cutting-induced surface should be further investigated by taking the material properties and tool edge shape into consideration. Atomic and/or close-to-atomic scale manufacturing (ACSM), such as atomic machining, should be taken into account as the main future manufacturing technology [226]. 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