Re-evaluation of the basic mechanics of orthogonal metal cutting: velocity diagram, virtual work equation and upper-bound theorem

This paper re-evaluates the known velocity relationships expressed in the form of a velocity diagram in orthogonal metal cutting, arguing that the metal cutting process be considered as cyclic and consisting of three distinctive stages. The velocity diagrams for the second and third stages of a chip-formation cycle are discussed. The fundamentals of the mechanics of orthogonal cutting, which are the upper-bound theorem applied to orthogonal cutting and the real virtual work equation, are re-evaluated using the proposed velocity diagram and corrected relationships are proposed. To prove the theoretical results, the equation for displacements in the deformation zone is derived using the proposed velocity relationships. To prove that the displacements in the deformation zone follow the derived equation and that this zone consists of two unequal parts, a metallographical study of chip structures has been carried out. To estimate the variation of stress and strain in the deformation zone quantitatively, a microhardness scanning test was conducted.Because it is proved that the chip formation process is cyclic, its frequency is studied. It is shown that when the noise due to various inaccuracies in the machining system is eliminated from the system response and thus from the measuring signal, and when this signal is then properly processed, the amplitude of the peak at the frequency of chip formation is the largest in the corresponding autospectra.     

Residual stresses and strains in orthogonal metal cutting

The finite element method is used to simulate and analyze the orthogonal metal cutting process under plane strain conditions, with focus on the residual stress and strain fields in the finished workpiece. Various modeling options have been employed. The frictional interaction along the tool-chip interface is modeled with a modified Coulomb friction law. Chip separation is modeled by the nodal release technique based on a critical stress criterion. Temperature-dependent material properties and a range of tool rake angle and friction coefficient values are considered. It is found that while thermal cooling increases the residual stress level, the effects of the rake angle and the friction coefficient are nonlinear and depend on the range of these parameters. The predicted residual stress results compare well with experimental observations available in the literature.     

An investigation into the mechanics of cutting using data from orthogonally cutting Nylon 66

Cutting force data for Nylon 66 has been examined in terms of various different models of cutting. Theory that includes significant work of separation at the tool tip was found to give the best correlation with experimental data over a wide range of rake angles for derived primary shear plane angle. A fracture toughness parameter was used as the measure of the specific work of separation. Variation in toughness with rake angle determined from cutting is postulated to be caused by mixed mode separation at the tool tip. A rule of mixtures using independently determined values of toughness in tension (mode I) and shear (mode II) is found to describe well the variation with rake angle. The ratio of modes varies with rake angle and, in turn, with the primary shear plane angle. Previous suggestions that cutting is a means of experimentally determining fracture toughness are now seen to be extended to identify the mode of fracture toughness as well.     

Revisiting the fundamentals of metal cutting by means of finite elements and ductile fracture mechanics

This paper investigates Atkins’ idea that the modelling of metal cutting must include the significant work involved in the formation of new surfaces as well as the traditional components of plastic flow and friction. New finite element and algebraic calculations are presented together with specially designed orthogonal metal cutting experiments performed on lead specimens under laboratory-controlled conditions. Independent determinations of the mechanical properties of lead were made and comparisons are given between theoretical predictions and experimental results. Calculations cover a wide range of topics such as material flow, chip-compression factor, primary shear plane angle, cutting force and specific cutting pressure. It is shown that the choice of lead as workpiece material reveals important facts that would be obscured were the usual sort of workpiece metals to be cut.The paper demonstrates quantitatively that while material flow, chip formation and the distribution of the major field variables can be modelled successfully by traditional ‘plasticity and friction only’ analyses, the contribution of ductile fracture mechanics is essential for obtaining good estimates of cutting forces and of the specific cutting pressure.     

Determination of micro-scale plastic strain caused by orthogonal cutting

An electron beam lithography technique has been used to produce microgrids in order to measure local plastic strains, induced during an orthogonal cutting process, at the microscopic scale in the shear zone and under the machined surface. Microgrids with a 10 μm pitch and a line width less than 1 μm have been printed on the polished surface of an aluminium alloy AA 5182 to test the applicability of the technique in metal cutting operations. Orthogonal cutting tests were carried out at 40 mm/s. Results show that the distortion of the grids could successfully be used to compute plastic strains due to orthogonal cutting with higher accuracy compared to other techniques reported in the literature. Strain maps of the machined specimens have been produced and show high-strain gradients very close to the machined surface with local values reaching 2.2. High-resolution strain measurements carried out in the primary deformation zone also provide new insight into the material deformation during the chip formation process.     

正交试验法在光纤激光切割厚板中的应用

在光纤激光切割厚板的工艺参数采集过程中,使用正交试验的方法,在最短的时间获得最优的切割工艺参数,有效、合理地减少试验时间与成本。     

An experimental investigation into the orthogonal cutting of unidirectional fibre reinforced plastics

This paper aims to understand the machinability of epoxy composites reinforced by unidirectional carbon fibres when subjected to orthogonal cutting. It was found that the subsurface damage and its mechanisms of a machined component are greatly influenced by fibre orientation. The material’s bouncing back is a characteristic phenomenon associated with the cutting of a fibre-reinforced composite. Three distinct deformation zones appear, i.e., chipping, pressing and bouncing when the fibre orientation is <90°. Otherwise, fibre-bending during cutting will become more significant and subsurface damage caused by fibre-matrix debonding will be severer. As a result, surface roughness, subsurface damage and cutting forces all change dramatically with the fibre orientation. It was also found that the curing conditions of making the composites do not have an obvious effect on the machinability, though the mechanical properties of the materials vary.     

Analysis of the inverse identification of constitutive equations applied in orthogonal cutting process

Flow stress identification of work-piece materials for its use in machining operation simulation models has been long treated. The interest in defining the flow stress in an easy and fast way without using complicated dynamic characterization tests leads to analyse the inverse identification of flow stress employing cutting operations. This paper presents a revision of different aspects concerning the inverse algorithms applied to the primary shear zone (PSZ). It also presents a new approach for studying material's behaviour on the secondary shear zone (SSZ) where experimentally measured temperatures have been included in the inverse algorithm. Two steels, 42CrMo4 and 20NiCrMo5 are employed and finite element method (FEM) simulations are carried out in order to evaluate the usefulness of the calculated flow stress laws and proposed algorithm.     

Experimental and numerical modelling of the residual stresses induced in orthogonal cutting of AISI 316L steel

Residual stresses in the machined surface layers are affected by the cutting tool, work material, cutting regime parameters (cutting speed, feed and depth of cut) and contact conditions at the tool/chip and tool/workpiece interfaces. In this paper, the effects of tool geometry, tool coating and cutting regime parameters on residual stress distribution in the machined surface and subsurface of AISI 316L steel are experimentally and numerically investigated. In the former case, the X-ray diffraction technique is applied, while in the latter an elastic–viscoplastic FEM formulation is implemented. The results show that residual stresses increase with most of the cutting parameters, including cutting speed, uncut chip thickness and tool cutting edge radius. However, from the range of cutting parameters investigated, uncut chip thickness seems to be the parameter that has the strongest influence on residual stresses. The results also show that sequential cuts tend to increase superficial residual stresses.     

Prediction of tool-chip contact length using a new slip-line solution for orthogonal cutting

Tool–chip contact length is an important parameter in machining. Several ways had been proposed in different works to find its value, which gave discordant results for the same set of cutting conditions. In this paper, a new slip-line solution for orthogonal cutting by a tool with unrestricted rake face is suggested. Based on the proposed solution, a new formula for tool–chip contact length has been obtained. Comparative analysis of different methods to predict tool–chip contact length has been done and experimental verification conducted. The suggested formula has shown to correspond well with experimental data and predicts tool–chip contact length better than other known solutions.     

A comparison of orthogonal cutting data from experiments with three different finite element models

The aim of this study is to compare various simulation models of orthogonal cutting process with each other as well as with the results of various experiments. Commercial implicit finite element codes MSC.Marc, Deform2D and the explicit code Thirdwave AdvantEdge have been used. In simulations, a rigid tool is advanced incrementally into the deformable workpiece which is remeshed whenever needed. In simulations with MSC.Marc and Thirdwave AdvantEdge, there is no separation criterion defined since chip formation is assumed to be due to plastic flow, therefore, the chip is formed by continuously remeshing the workpiece. However, in simulations with Deform2D, the Cockroft–Latham damage criterion is used and elements, which exceed the predefined damage value, are erased via remeshing. Besides this different modeling of separation, the three codes also apply different friction models and material data extrapolation schemes. Estimated cutting and thrust forces, shear angles, chip thicknesses and contact lengths on the rake face by three codes are compared with experiments performed in this study and with experimental results supplied in literature. In addition, effects of friction factor, different remeshing criteria, and threshold tool penetration value on the results are examined. As a result, it has been found that although individual parameters may match with experimental results, all models failed to achieve a satisfactory correlation with all measured process parameters. It is suggested that this is due to the poor modeling of separation.     

Al-9.4Zn-1.9Mg-2.0Cu合金的热变形流变行为与本构方程

本文对Al-9.39Zn-1.92Mg-1.98Cu合金做等温热模拟压缩实验,变形温度为300 ℃~460 ℃,应变速率为0.001 s-1~10 s-1,变形量为60%。结果表明：变形时,合金的流变应力力随着变形温度的降低或应变速率的增大而增大。由于热变形时存在摩擦影响,对流变应力曲线进行修正.结果发现摩擦修正后的应力值低于实验值,摩擦力对流变应力的影响程度随着温度的降低和应变速率的增大而增大。基于经典的Arrhenius方程,考虑应变量对材料常数(α,n,Q和A)的影响,构建该合金在热变形时的本构方程。评价改进的本构方程预测能力发现流变应力值与实测值吻合度较高,其相关度高达93.5%。     

基于正交回归的GH44合金本构关系研究

通过热物理模拟试验，系统地研究了主要热力参数(变形温度、变形速率、变形程度)与流动应力间的数值关系，采用正交设计原理，科学地分析并回归出GH44合金的本构方程，定性地探讨热力参数对GH44合金成形性能的影响规律，为GH44合金的热变形数值模拟和热力参数的合理制定与控制提供了依据。     

A study of the tool-chip interface contact problem under low cutting velocity with an elastic cutting tool

To understand the effects of elastic deformation of the tool and the crater phenomenon generated by the cutting force and high pressure during metal cutting processing on the cutting process, an iterative mathematical model for calculating the tool-chip contact is developed in this paper under the assumption of elastic cutting tools. In this model, the finite-element method is used to simulate the cutting of mild steel by a cutting tool of three different materials. The results obtained in the simulation are found to match experimental data reported by related studies. The simulation results also indicate that tools with a smaller stiffness produce greater elastic deformation. Further, decrease of the rake angle due to elastic deformation of the tool can result in greater difficulty in internal deformation of the material and an increase in cutting force. The micro-crater phenomenon on the tool face generated by high pressure at the tool-chip interface is the preliminary symptom of crater wear on the tool face. Therefore, under some machining conditions, such as in precision machining or in automation processing where tool compensation is required, the phenomenon of elastic deformation of the tool must be considered carefully to ensure product precision.     

FEA modeling and simulation of shear localized chip formation in metal cutting

The finite element analysis (FEA) has been applied to model and simulate the chip formation and the shear localization phenomena in the metal cutting process. The updated Lagrangian formulation of plane strain condition is used in this study. A strain-hardening thermal-softening material model is used to simulate shear localized chip formation. Chip formation, shear banding, cutting forces, effects of tool rake angle on both shear angle and cutting forces, maximum shear stress and plastic strain fields, and distribution of effective stress on tool rake face are predicted by the finite element model. The initiation and extension of shear banding due to material's shear instability are also simulated. FEA was also used to predict and compare materials behaviors and chip formations of different workpiece materials in metal cutting. The predictions of the finite element analysis agreed well with the experimental measurements.     

Basic geometric analysis of 3-D chip forms in metal cutting.: Part 2: implications

The geometric analysis of 3-D chip forms developed in Part 1 is extended and several new implications are identified: (i) the geometric properties at every point on the tool–chip separation line are fully determined once those at any one point are known, (ii) all possible 3-D chip forms are confined to a relatively restricted parameter space defining the chip velocity direction and the orientation of the axis of the helical chip, (iii) 3-D helical chips are only approximately conical, and (iv) the radii of up-curl and side-curl can be determined from a set of simple measurements of the chip-in-hand. Unlike past analyses, the new analysis paves the way to the study of chip forms from empirical data obtained from practical 3-D chips.     

On the evaluation of the global heat transfer coefficient in cutting

The use of numerical simulations for investigating machining processes is remarkably increasing because of the simulation cost is lower than the experiments and the possibility to analyze local variables such as pressures, strains, and temperatures is allowable. Process simulation is very hard from a computational point of view, since it frequently requires remeshing phases and very small time steps. As a consequence, the simulated cutting time is usually of the order of few milliseconds and no steady cutting conditions are generally achieved, at least as far as thermal conditions are concerned. Therefore, nowadays numerical prediction of cutting temperatures cannot be considered fully reliable. In the paper this issue was taken into account: a mixed Lagrangian-Eulerian numerical approach was utilized and the global heat transfer (film) coefficient at the tool-chip interface was derived through an inverse approach. Finally, the dependence of the film coefficient on pressure and temperature on the rake face was investigated.     

On the mechanics and material removal mechanisms of vibration-assisted cutting of unidirectional fibre-reinforced polymer composites

This paper aims to reveal the material removal mechanisms and the mechanics behind the vibration-assisted cutting (VAC) of unidirectional fibre reinforced polymer (FRP) composites. Through a comprehensive analysis by integrating the core factors of the VAC, including fibre orientation and deformation, fibre–matrix interface, tool–fibre contact and tool–workpiece contact, a reliable mechanics model was successfully developed for predicting the cutting forces of the process. Relevant experiments conducted showed that the model has captured the mechanics and the major deformation mechanisms in cutting FRP composites, and that the application of ultrasonic vibration in either the cutting or normal direction can significantly decrease cutting forces, minimise fibre deformation, facilitate favourable fibre fracture at the cutting interface, and largely improve the quality of a machined surface. When the vibrations are applied to both the cutting and normal directions, the elliptic vibration trajectory of the tool tip can bring about an optimal cutting process. There exists a critical depth of cut, beyond which the fibre–matrix debonding depth is no longer influenced by the vibration applied on the tool tip.     

An FEM investigation into the behavior of metal matrix composites: Tool–particle interaction during orthogonal cutting

An analytical or experimental method is often unable to explore the behavior of a metal matrix composite (MMC) during machining due to the complex deformation and interactions among particles, tool and matrix. This paper investigates the matrix deformation and tool–particle interactions during machining using the finite element method. Based on the geometrical orientations, the interaction between tool and particle reinforcements was categorized into three scenarios: particles along, above and below the cutting path. The development of stress and strain fields in the MMC was analyzed and physical phenomena such as tool wear, particle debonding, displacements and inhomogeneous deformation of matrix material were explored. It was found that tool–particle interaction and stress/strain distributions in the particles/matrix are responsible for particle debonding, surface damage and tool wear during machining of MMC.