Upper bound analysis of oblique cutting: improved method of calculating the friction area

This paper describes further development of the upper bound analysis of oblique cutting with nose radius tools described previously by Adibi-Sedeh et al. [[1]] by incorporation of an improved method for calculating the friction area at the chip-tool interface. Previously, the friction area was obtained from the shear surface area assuming that the ratio of these areas is the same as in orthogonal machining. Our results showed that this led to overestimation of the effect of friction on the chip flow angle, thereby resulting in smaller changes in the chip flow angle with inclination angle as compared to experimental data. In the new approach, the chip-tool contact length is obtained from the length of the shear surface assuming that the ratio of the lengths is the same as in orthogonal machining and the friction area is calculated using this length. The chip flow angle predicted using the new approach shows much better agreement with experimental data. In particular, the dependence of the chip flow angle on the inclination angle is accurately reproduced. Upper bound analysis of oblique cutting using this new model for the friction area provides an elegant explanation for the relative influence of the normal and equivalent rake angles on the cutting force.     

Modeling of velocity-dependent chip flow angle and experimental analysis when machining 304L austenitic stainless steel with groove coated-carbide tools

This paper presents an analysis of experimental cutting forces and the study of the chip flow angle when machining 304L austenitic steel with a groove coated tool under dry condition. Experiments were conducted on a wide range of cutting conditions with a particular attention to ensure a great confidence in the obtained results. A detailed analysis of experimental cutting forces and the identification of empirical cutting force equations similar to that usually used for flat tools are proposed. The main focus of this work is on the study of chip flow angle deduced here from experimental cutting forces, considering that the chip flow direction is collinear to the friction force. From a comparison between experiments and two often used approaches, it appears that the experimental chip flow angle estimation, based on neglecting the complex tool geometry and adopting a zero rake angle, is bounded by the two considered modelings that renders useful for the proposed study. From experiments it is also observed an increase of the chip flow angle as the cutting velocity is increased. A velocity-dependent modeling with two distinct strategies of identification is then proposed in order to capture the cutting velocity effect on the chip flow angle.     

Machining Ti–6Al–4V alloy with cryogenic compressed air cooling

A new cooling approach with cryogenic compressed air has been developed in order to cool the cutting tool edge during turning of Ti–6Al–4V alloy. The cutting forces, chip morphology and chip temperature were measured and compared with those measured during machining with compressed air cooling and dry cutting conditions. The chip temperature is lower with cryogenic compressed air cooling than those with compressed air cooling and dry machining. The combined effects of reduced friction and chip bending away from the cutting zone as a result of the high-speed air produce a thinner chip with cryogenic compressed air cooling and a thicker chip with compressed air cooling compared to dry machining alone. The marginally higher cutting force associated with the application of cryogenic compressed air compared with dry machining is the result of lower chip temperatures and a higher shear plane angle. The tendency to form a segmented chip is higher when machining with cryogenic compressed air than that with compressed air and dry machining only within the ranges of cutting speed and feed when chip transitions from continuous to the segmented. The effect of cryogenic compressed air on the cutting force and chip formation diminishes with increase in cutting speed and feed rate. The application of both compressed air and cryogenic compressed air reduced flank wear and the tendency to form the chip built-up edge. This resulted in a smaller increase in cutting forces (more significantly in the feed force) after cutting long distance compared with that observed in dry machining.     

A model for cutting forces generated during machining with self-propelled rotary tools

In this paper, a force model for self-propelled rotary tool is presented. Conventional oblique cutting force predictions were reviewed and extended to predict the cutting forces generated during machining with the self-propelled rotary tools. The model presented is based on Oxley's analysis and was verified by cutting tests using a typical self-propelled tool. Good agreement was obtained between the predicted and the experimentally measured forces under a wide range of cutting conditions. The effect of different cutting conditions on the friction coefficient along the chip/tool interface and tool rake face normal force were also presented and discussed.     

Effect of micro-textured tools on machining of Ti–6Al–4V alloy: An experimental and numerical approach

In this work, an attempt is made to reduce the detrimental effects that occurred during machining of Ti–6Al–4V by employing surface textures on the rake faces of the cutting tools. Numerical simulation of machining of Ti–6Al–4V alloy with surface textured tools was employed, taking the work piece as elasto-plastic material and the tool as rigid body. Deform 3D software with updated Lagrangian formulation was used for numerical simulation of machining process. Coupled thermo-mechanical analysis was carried out using Johnson-cook material model to predict the temperature distribution, machining forces, tool wear and chip morphology during machining. Turning experiments on Ti–6Al–4V alloy were carried out using surface textured tungsten carbide tools with micro-scaled grooves in preferred orientation such as, parallel, perpendicular and cross pattern to that of chip flow. A mixture of molybdenum disulfide with SAE 40 oil (80:20) was used as semi-solid lubricant during machining process. Temperature distribution at tool–chip interface was measured using an infrared thermal imager camera. Feed, thrust and cutting forces were measured by a three component-dynamometer. Tool wear and chip morphology were captured and analyzed using optical microscopic images. Experimental results such as cutting temperature, machining forces and chip morphology were used for validating numerical simulation results. Cutting tools with surface textures produced in a direction perpendicular to that of chip flow exhibit a larger reduction in cutting force, temperature generation and reduced tool wear.     

Critical cutting speed for onset of serrated chip flow in high speed machining

The transition of continuously smooth chip flow to periodically serrated chip flow as the cutting speed increasing is one of the most fundamental and challenging problems in high speed machining. Here, an explicit expression of the critical cutting speed for the onset of serrated chip flow, which is given in terms of material properties, uncut chip thickness and tool rake angle, is achieved based on dimensional analysis and numerical simulations. It could give reasonable predictions of the critical cutting speeds at which chips change from continuous to serrated for various metallic materials over wide ranges of uncut chip thickness and tool rake angle. More interestingly, it is found that, as the turbulent flow is controlled by the Reynolds number, the transition of the serrated chip flow mode is dominated by a Reynolds thermal number. Furthermore, the influences of material properties on the emergence of serrated chip flow are systematically investigated, the trends of which show good agreement with Recht’s classical model.     

An analytical finite element model for predicting three-dimensional tool forces and chip flow

A model of three-dimensional cutting is developed for predicting tool forces and the chip flow angle. The approach consists of coupling an orthogonal finite element cutting model with an analytical model of three-dimensional cutting. The finite element model is based on an Eulerian approach, which gives excellent agreement with measured tool forces and chip geometries. The analytical model was developed by Usui et al. [ASME J. Engng Indust. 100(1978) 222; 229], in which a minimum energy approach was used to determine the chip flow direction. The model developed by Usui required orthogonal cutting test data to determine the tool forces and chip flow angle. In this paper, a finite element model is used to supply the orthogonal cutting data for Usui's model. With this approach, a predictive model of three-dimensional cutting can be developed that does not require measured data as input. Cutting experiments are described in which good agreement was found between measured and predicted tool forces and chip flow angles for machining of AISI 1020 steel.     

Thermomechanical modelling of oblique cutting and experimental validation

An analytical approach is used to model oblique cutting process. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening are considered. The chip formation is supposed to occur mainly by shearing within a thin band called primary shear zone. The analysis is limited to stationary flow and the material flow within the primary shear zone is modelled by using a one-dimensional approach. Thermomechanical coupling and inertia effects are accounted for. The chip flow angle is determined by the assumption that the friction force on the tool face is collinear to the chip flow direction. At the chip–tool interface, the friction condition can be affected by the important heating induced by the large values of pressure and sliding velocity. In spite of the complexity of phenomena governing the friction law in machining, a reasonable assumption is to consider that the mean friction coefficient is primarily function of the average temperature at the tool–chip interface. Comparisons between model predictions and experimental results are performed for different values of cutting speed, undeformed chip thickness, normal cutting angle and inclination angle. A critical study is presented in order to show the influences of the input parameters of the model including the normal shear angle, the thickness of the primary shear zone and the pressure distribution at the tool–chip interface. The model permits to predict the cutting forces, the chip flow direction, the contact length between the chip and the tool and the temperature distribution at the tool–chip interface which has an important effect on tool wear.     

Tool wear and chip formation during hard turning with self-propelled rotary tools

This paper presents a performance assessment of rotary tool during machining hardened steel. The investigation includes an analysis of chip morphology and modes of tool wear. The effect of tool geometry and type of cutting tool material on the tool self-propelled motion are also investigated. Several tool materials were tested for wear resistance including carbide, coated carbide, and ceramics. The self-propelled coated carbide tools showed superior wear resistance. This was demonstrated by evenly distributed flank wear with no evidence of crater wear. The characteristics of temperature generated during machining with the rotary tool are studied. It was shown that reduced tool temperature eliminates the diffusion wear and dominates the abrasion wear. Also, increasing the tool rotational speed shifted the maximum temperature at the chip–tool interface towards the cutting edge.     

Analysis on high-speed face-milling of 7075-T6 aluminum using carbide and diamond cutters

This research is concerned with the analytical and experimental study on the high-speed face milling of 7075-T6 aluminum alloys with a single insert fly-cutter. The results are analyzed in terms of cutting forces, chip morphology, and surface integrity of the workpiece machined with carbide and diamond inserts. It is shown that a high cutting speed leads to a high chip flow angle, very low thrust forces and a high shear angle, while producing a thinner chip. Chip morphology studies indicate that shear localization can occur at higher feeds even for 7075-T6, which is known to produce continuous chips. The resultant compressive residual stresses are shown for the variation of cutting parameters and cutting tool material. The analysis of the high-speed cutting process mechanics is presented, based on the calculation results using extended oblique machining theory and finite element simulation.     

Mechanics of machining of face-milling operation performed using a self-propelled round insert milling cutter

There has been a renewed interest in the technology of rotary tools because of their ability to perform more productive machining and the concurrent evolution of a number of new ‘difficult-to-machine’ materials. This paper presents an investigation into the application of rotary tools in a face-milling operation. The work involved analysis of cutting forces and chip characteristics, and the development of analytical as well as conceptual models to predict the cutting forces. It was evident that the proposed model predicts cutting force magnitude with a fair accuracy.     

Brittle damage and interlaminar decohesion in orthogonal micromachining of pyrolytic carbon

Engineered features on pyrolytic carbon (PyC) have been reported to improve the functional performance of the cardiovascular implants. PyC also finds application in thermonuclear components due to its unique directional thermal properties. Note that PyC comprises of stacked layers of brittle graphite-like material and its machining characteristics differ from plastically deformable isotropic materials due to brittle damage and interlaminar decohesion. Consequently, this study is aimed at understanding the mechanics of material removal in the plane of transverse isotropy (horizontally stacked laminae) of PyC via a finite element model. A damaged plasticity material model has been used to capture the effect of material degradation of a brittle material under machining. Uniaxial tension/compression tests have been carried out to calibrate the damaged plasticity model. A surface based cohesive bonding has been used between the layers to simulate the interlaminar decohesion which results in peeling, slipping and delamination during machining. The model predicts the cutting force and thrust forces under different process conditions. The cutting force predictions from the finite element model have been validated against the experimental data for different cutting conditions. In addition, the model also predicts the chip morphology for different machining conditions. The prediction error in the model lies between 2% and 23%. Parametric studies have also been performed to understand the effect of the machining parameters, such as rake angle, uncut chip thickness on the process response. It is found that use of the positive rake angle decreases the cutting forces up to 72%.     

不同冷却喷雾场下硬质涂层刀具切削蠕墨铸铁性能研究

研究在复合喷雾油膜附水滴冷却时,3种不同结构喷嘴的喷雾场及在硬质涂层刀具切削蠕墨铸铁过程中的切屑流向,并分析刀具和喷射位置对切削力的影响。研究表明:与普通圆柱型和扁平型喷嘴相比,尖嘴型喷嘴的雾化效果更佳,无大液滴现象,喷雾集中,连续雾化稳定。在硬质涂层刀具切削加工蠕墨铸铁过程中,当喷雾场只作用于刀具前刀面时,普通圆柱型喷嘴和尖嘴型喷嘴有良好的断屑作用;而尖嘴型喷嘴和扁平型喷嘴对切屑流具有更好的导向性。相对干切削和冷风辅助切削,尖嘴型喷嘴与外冷复合喷雾条件下切削蠕墨铸铁时,切削力可减小30～60 N,其中喷雾场喷射在刀具前刀面时的切削力最低。不同硬质涂层刀具需要与外冷复合喷雾的喷射位置相互匹配以达到最小的切削力。      

Prediction of cutting force distribution and its influence on dimensional accuracy in peripheral milling

Cutting force has a significant influence on the dimensional accuracy due to tool and workpiece deflection in peripheral milling. In this paper, the authors present an improved theoretical dynamic cutting force model for peripheral milling, which includes the size effect of undeformed chip thickness, the influence of the effective rake angle and the chip flow angle. The cutting force coefficients in the model were calibrated with the cutting forces measured by Yucesan [18] in tests on a titanium alloy, and the model was proved to be more accurate than the previous models. Based on the model, a few case studies are presented to investigate the cutting force distribution in cutting tests of the titanium alloy. The simulation results indicate that the cutting force distribution in the cut-in process has a significant influence on the dimensional accuracy of the finished part. Suggestions about how to select the cutter and the cutting parameters were given to get an ideal cutting force distribution, so as to reduce the machining error, meanwhile keeping a high productivity.     

基于ABAQUS的锯齿形切屑形成机理分析与实验研究

锯齿形切屑的形成会导致机床振动,使刀具的切削性能下降,降低工件的加工质量,因此需要对其形成机理进行分析,合理优化切削参数,减少锯齿形切屑的形成机率。本文利用ABAQUS软件对钛合金的加工过程进行仿真分析,模拟锯齿形切屑的形成机理,并在不同切削条件下进行仿真和实验研究,讨论切削参数对切屑锯齿化程度的影响。结果表明,随着切削速度和进给量的增加,切屑的锯齿化程度逐渐增大,随着刀具前角的增大,切屑的锯齿化程度逐渐减小。研究结果对提高工件加工质量以及设计工艺参数有一定指导作用。     

A three-dimensional model of clip flow, chip curl and chip breaking for oblique cutting

In recent years, many publications have appeared dealing with chip breaking in orthogonal cutting of metals. However, in industry, oblique cutting and not orthogonal cutting is encountered in almost all actual machining operations. This paper deals with a model of chip flow, chip curl and chip breaking for oblique cutting. To simplify the analysis, a set of equivalent parameters are introduced. The relationship between the machining parameters and their corresponding equivalent parameters is developed theoretically and experimentally. To assess the level of chip breaking, a criterion of chiplbreaking is suggested under the concept of these equivalent parameters. The agreement of the experimental results with the predictive data of the model verifies that the definition of these equivalent parameters is reasonable. The influences of various machining parameters are discussed, in relation to their corresponding parameters. One significant finding is that the effect of each of the machining parameters on chip breaking is not totally inpdependent of one another. This implies that careful attention must be paid to the relationship between various machining parameters in three-dimensional parameters.     

An experimental technique for the measurement of temperature fields for the orthogonal cutting in high speed machining

Cutting temperature and heat generated at the tool-chip interface during high speed machining operations have been recognized as major factors that influence tool performance and workpiece geometry or properties. This paper presents an experimental setup able to determine the temperature field in the cutting zone, during an orthogonal machining operation with 42 CrMo 4 steel. The machining was performed with a gas gun, using standard carbide tools TiCN coated and for cutting speeds up to 50 ms^-1. The technique of temperature measurement was developed on the principle of pyrometry in the visible spectral range by using an intensified CCD camera with very short exposure time and interference filter at 0.8 μm. Temperature gradients were obtained in an area close to the cutting edge of the tool, along the secondary shear zone. Effects of the cutting speed and the chip thickness on the temperature profile in the chip were determined. Maximum chip temperature of about 825 °C was found, for cutting speed close to 20 ms^-1, located at a distance of 300 μm of the tool tip. It was established that this experimental arrangement is quite efficient and can provide fundamental data on the temperature field in materials during orthogonal high speed machining.     

Effect of tool edge geometry and cutting conditions on experimental and simulated chip morphology in orthogonal hard turning of 100Cr6 steel

Understanding chip formation mechanisms in hard turning is an important area of research. In this study, experiments with varying cutting conditions and tool edge geometry were performed concurrently with finite element simulations. The aim was to investigate how the two mechanisms reported in literature namely—surface shear-cracking (SCH) and catastrophic thermoplastic instability (CTI) contribute to overall chip geometry and machining forces. By varying tool edge geometry and cutting conditions predominance of one over another is discussed. The calculation prescribed by Recht [Recht, R., 1964. Catastrophic thermoplastic shear. J. Appl. Mech. 31, 189–193] for representative cutting conditions resulted in a small critical cutting speed of 0.034 m/min indicating CTI was operative in the range of cutting conditions tested. FEM simulations were conducted on a subset of experimental conditions. Chip geometry and forces were compared between experiments and simulations. The experimental results indicated that SCH predominated in a majority of conditions. However, formation of saw-tooth chips in the FEM simulations established the occurrence of CTI also. Specifically, the edge radius did not alter chip geometry parameters. However, machining forces decreased with cutting speed and chip formation frequency increased linearly with cutting speed. A more negative rake angle also increased the chip pitch. The findings also indicate that only an intrinsic length scale governs saw-tooth chip formation in hard turning.     

The prediction of dimensional error for sculptured surface productions using the ball-end milling process. Part 1: Chip geometry analysis and cutting force prediction

The study of machining errors caused by tool deflection in the balkend milling process involves four issues, namely the chip geometry, the cutting force, the tool deflection and the deflection sensitivity of the surface geometry. In this paper, chip geometry and cutting force are investigated. The study on chip geometry includes the undeformed radial chip thickness, the chip engagement surface and the relationship between feed boundary and feed angle. For cutting force prediction, a rigid force model and a flexible force model are developed. Instantaneous cutting forces of a machining experiment for two 2D sculptured surfaces produced by the ball-end milling process are simulated using these force models and are verified by force measurements. This information is used in Part 2 of this paper, together with a tool deflection model and the deflection sensitivity of the surface geometry, to predict the machining errors of the machined sculptured surfaces.     

Modelling of the combined microstructural and cutting edge effects in ultraprecision machining

The mechanics of ultraprecision machining (UPM) is known to be affected by materials microstructures and cutting tool geometries when cutting magnitudes are reduced to micron-scale. To model the combined effects, a flow stress model that correlates the grain size and chip thickness to the relative tool sharpness is first proposed. Subsequently a novel behavioural chip formation model is developed to distinguish the transitions in chip formation regimes due to the microstructural and cutting edge effects. This led to the discovery of a unique finishing regime where surface roughness is improved by 61.7%, 63.9% and 86.4% for Al-alloy, Mg-alloy and Cu-alloy respectively.