Numerical analysis and parameter study of a mechanical damper for use in long slender endmills

A mechanical damper has been introduced to reduce tool vibration during the high-speed milling process. The mechanical damper is composed of multi-fingered cylindrical inserts placed in a matching cylindrical hole in the center of a standard end-milling cutter. Centrifugal forces during high-speed rotation press the flexible fingers against the inner surface of the tool. Bending of the tool/damper assembly due to cutting forces or chatter vibration causes relative axial sliding between the tool inner surface and the damper fingers, and dissipates energy in the form of friction work. In this paper, a simple numerical method is presented to estimate the amount of friction work during tool bending. Non-linear static finite element analysis is used to estimate normal and frictional contact forces due to centrifugal forces and cutting forces, and calculate the amount of frictional work dissipated by the damper. The numerical results are compared with analytical results, and show a similar trend. Parameter studies are also carried out using the numerical model to identify the best configuration to maximize the amount of friction work.     

ZL401合金的阻尼行为研究

对铸态ZL401合金进行了低温人工时效处理，利用低频内耗测试技术研究了合金的阻尼性能及实模量与应变振幅、频率和温度的关系，并与铸态合金进行了对比。发现合金阻尼与应变振幅及应变频率相关，随温度的提高而增大，且在低温和高温时合金阻尼与频率的关系出现了相反的变化。ZL401合金振动过程中实模量的亏损随频率的降低和温度的升高而增大。认为ZL401合金的阻尼行为是界面阻尼和位错阻尼二者效应叠加的结果，室温阻尼主要归功于位错阻尼，而高温阻尼主要归功于界面阻尼。     

An optical fibre sensor based cutting force measuring device

With a view to providing a way of obtaining cutting force signals which possess good adaptability to workshop conditions, a cutting force measuring device based on a specially treated standard tool shank and an optical fibre sensor is developed. The tool shank is treated in such a way that during a cutting process a displacement proportional to cutting force F_z will occur at its rear part. The displacement is then detected as a measure of the cutting force by the optical fibre sensor which is mounted on the tool post. With this device no undue extra space is required for the installation and the convenience of the tool changing operation is unaffected. Besides, as the measurement is done at the rear of the tool shank, disturbances from chip and coolant may be avoided. A calibration test and real cutting tests of the device are carried out. The results show that the device possesses satisfactory static and dynamic performances and the spectrum feature of its output signal is sensitive to tool condition.     

Modelling machine tool dynamics using a distributed parameter tool–holder joint interface

Increasing productivity in machining process demands high material removal rate in stable cutting conditions and depends strongly on dynamic properties of machine tool structure. Combined analytical–experimental procedures based on receptance coupling substructure analysis (RCSA) are employed to determine the stability of machine operating conditions at different tool configurations. The RCSA employs holder–spindle experimental mobility measurements in conjunction with an analytical model for the tool to predict the dynamics of different sets of tool and holder–spindle combinations without the need for repeated mobility measurements. In this paper an alternative approach using the concept of tool on resilient support is adopted to predict the machine tool dynamics in various tool configurations. In the proposed model the tool, represented by an analytical model, is partly resting on a resilient support provided by the holder–spindle assembly. The support dynamic flexibility is measured by performing vibration tests on the holder–spindle assembly. Tool–holder joint interface characteristics are included in the model by considering a distributed elastic interface layer between the holder–spindle and the tool shank part. The distributed interface layer takes into account the change in normal contact pressure along the joint interface and comparing with the lumped joint model used in RCSA it allows more detailed representation of the joint interface flexibility and damping which have crucial roles in machine dynamics. Experiments are conducted to demonstrate the efficiency of proposed model in prediction of milling operation dynamics and it is shown that the model is capable of accurately predicting the dynamic absorber effect of spindle in a tool tuning practice.     

刀具表面织构对刀-屑界面摩擦学特性的影响

刀具快速磨损限制了金属切削的进一步发展,表面织构技术的提出为改善刀-屑界面摩擦提供了新思路,也是目前降低刀具表面磨损的有效方法之一。从刀-屑界面切削液存储与润滑、微/纳织构、织构方向、织构类型及形状、织构位置等五个方面,概述了现有刀具表面织构对刀-屑界面摩擦学特性的影响,总结了刀具表面织构的共性作用机理,并进行了分类归纳。相应分析结果表明,刀具表面织构改变了刀-屑界面内空体集团的数量及微通道分布,增加了界面内切削液的渗入存储,同时捕捉存储了界面内的磨屑颗粒,改善了刀-屑界面润滑摩擦;且在此过程中,润湿性因素对界面内切削液的渗入、存储及润滑油膜的形成等均存在影响。最后对刀具表面织构技术的发展进行了总结与展望,为刀具表面磨损的调控提供了参考。     

Design,fabrication and properties of a self-lubricated tool in dry cutting

Micro-holes were made using micro-EDM on the rake and flank face of the cemented carbide (WC/Co) tools. Molybdenum disulfide (MoS₂) solid lubricants were filled into the micro-holes to form self-lubricated tools (ML-1 and -2). Dry cutting tests on hardened steel were carried out with these self-lubricated tools and conventional tools (ML-3). The cutting forces, the tool wear, and the friction coefficient between the tool–chip interface were measured. It was shown that the cutting forces with ML-1 and -2 self-lubricated tools were greatly reduced compared with that of ML-3 conventional tool, the ML-1 self-lubricated tool with one micro-hole in its rake face possessed the lower friction coefficient at the tool–chip interface; while the ML-2 self-lubricated tool with one micro-hole in its flank face revealed more flank wear resistance. The mechanism responsible was explained as the formation of a self-lubricating film between the sliding couple, and the composition of this lubricating film was found to be MoS₂ solid lubricant, which was released from the micro-hole and smeared on the rake or flank face, and can be acted as lubricating additive during dry cutting processes.     

Predictive force model for ball-end milling and experimental validation with a wavelike form machining test

This paper presents a predictive force model for ball-end milling based on thermomechanical modelling of oblique cutting. The tool geometry is decomposed into a series of axial elementary cutting edges. At any active tooth element, the chip formation is obtained from an oblique cutting process characterised by local undeformed chip section and local cutting angles. This method predicts accurately the cutting force distribution on the helical ball-end mill flutes from the tool geometry, the pre-form surface, the tool path, the cutting conditions, the material behaviour and the friction at the tool-chip interface. The model is applied for a complex surface which is a wavelike form used as a validation machining test. The results are compared with experimental data obtained from ball-end milling tests performed on a 3-axis CNC equipped with a Kistler dynamometer.     

Identification of a friction model for modelling of orthogonal cutting

Numerical approaches to high-speed machining are necessary to increase the productivity and to optimise the tool wear and the residual stresses. In order to apply such approaches, rheological behaviour of the antagonists and friction model of interfaces have to be correctly determined. The existing numerical approaches that are used with the current friction models do not lead to good correlations of the process variables, such as the cutting forces or the tool–chip contact length. This paper proposes a new approach for characterizing the friction behaviour at the tool–chip interface in the zone near the cutting edge. An experimental device is designed to simulate the friction behaviour at the tool–chip interface. During this upsetting-sliding test, an indenter rubs in a specimen with a constant speed, generating a residual friction track. Contact pressure and friction coefficient are determined from the test’s numerical model and are then used to identify the friction data according to the interface temperature and the sliding velocity. These initial findings can be further developed for implementation in FEA machining models in order to increase the productivity.     

Statistical investigation of modal parameters of cutting tools in dry turning

It is very important that optimized cutting parameters be selected in controlling the quality required for surface finishes. Unfortunately, surface roughness does not depend solely on the feed rate, the tool nose radius and cutting speed; the surface can also be deteriorated by excessive tool vibrations, the built-up edge, the friction of the cut surface against the tool point, and the embedding of the particles of the materials being machined. Hence, the forces, which can be considered as the sum of steady, harmonic and random forces, act on the cutting tool and contribute to the modification of the dynamic response of the tool, by affecting its stiffness and damping. These stiffness and damping variations are attributable to parameters that cannot be easily predicted in practice (regenerative process, penetration rate, friction, variation in rake angle, cutting speed, etc.). Furthermore, the effects of cutting parameters, which also contribute to the variation in the tool’s modal parameters, are more useful for controlling tool vibration. This study focuses on the collection and analysis of cutting-force, tool-vibration and tool-modal-parameter data generated by lathe dry turning of mild carbon steel samples at different speeds, feeds, depths of cut, tool nose radii, tool lengths and workpiece lengths. A full factorial experimental design (288 experiments) that takes into consideration the two-level interactions between the independent variables has been performed. This analysis investigated the effect of each cutting parameter on tool stiffness and damping, and yielded an empirical model for predicting the behavior of the tool stiffness variation.     

Determination of workpiece flow stress and friction at the chip–tool contact for high-speed cutting

This paper presents a methodology to determine simultaneously (a) the flow stress at high deformation rates and temperatures that are encountered in the cutting zone, and (b) the friction at the chip–tool interface. This information is necessary to simulate high-speed machining using FEM based programs. A flow stress model based on process dependent parameters such as strain, strain-rate and temperature was used together with a friction model based on shear flow stress of the workpiece at the chip–tool interface. High-speed cutting experiments and process simulations were utilized to determine the unknown parameters in flow stress and friction models. This technique was applied to obtain flow stress for P20 mold steel at hardness of 30 HRC and friction data when using uncoated carbide tooling at high-speed cutting conditions. The average strain, strain-rates and temperatures were computed both in primary (shear plane) and secondary (chip–tool contact) deformation zones. The friction conditions in sticking and sliding regions at the chip–tool interface are estimated using Zorev's stress distribution model. The shear flow stress (k_chip) was also determined using computed average strain, strain-rate, and temperatures in secondary deformation zone, while the friction coefficient (μ) was estimated by minimizing the difference between predicted and measured thrust forces. By matching the measured values of the cutting forces with the predicted results from FEM simulations, an expression for workpiece flow stress and the unknown friction parameters at the chip–tool contact were determined.     

Identification of dynamic cutting force coefficients and chatter stability with process damping

This paper presents a cutting force model which has three dynamic cutting force coefficients related to regenerative chip thickness, velocity and acceleration terms, respectively. The dynamic cutting force coefficients are identified from controlled orthogonal cutting tests with a fast tool servo oscillated at the desired frequency to vary the phase between inner and outer modulations. It is shown that the process damping coefficient increases as the tool is worn, which increases the chatter stability limit in cutting. The chatter stability of the dynamic cutting process is solved using Nyquist law, and compared favourably against experimental results at low cutting speeds.     

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.     

Theoretical and experimental investigation of the frictional behavior of the tool–chip interface in ultrasonic-vibration assisted turning

The frictional behavior of the tool–chip interface has a significant role in the cutting mechanics. The frictional and normal forces, the contact length between the cutting tool and chip, the coefficient of friction and the stress distribution are the influential parameters. The behavior of the tool–chip interface in ultrasonic-vibration assisted cutting is different from conventional cutting and needs to be investigated. The ultrasonic-vibration assisted cutting has several advantages compared with conventional process. In the present study a frictional model has been developed for studying the above mentioned parameters and predicting the tool–chip behavior in ultrasonic-vibration assisted turning at different cutting speeds and vibration amplitudes. The results have been verified by experiments.     

Identification of a friction model at tool/chip/workpiece interfaces in dry machining of AISI4142 treated steels

The characterization of frictional phenomena at the tool–chip–workpiece interface remains an issue. This paper aims to identify a friction model able to describe the friction coefficient at this interface during the dry cutting of a AISI4142 treated steel with TiN coated carbide tools. A new tribometer has been designed in order to reach relevant values of pressures, temperatures and sliding velocities. This set-up is based on a modified pin-on-ring system. Additionally a numerical model simulating the frictional test has been associated in order to identify local phenomena around the spherical pin, from the standard macroscopic data provided by the experimental system. A range of cutting speeds and pressures has been investigated. It has been shown that the friction coefficient is mainly dependant on the sliding velocity, whereas the pressure has a secondary importance. Moreover a new key parameter has been revealed, i.e. the average local sliding velocity at the contact. Finally a new friction model has been identified based on this local sliding velocity.     

高速切削加工中刀柄的应用

针对高速切削加工刀柄的性能要求,介绍了几种新型结构的手柄的特点及应用。     

Modeling of chip–tool interface friction to predict cutting forces in machining of Al/SiCp composites

In Al/SiCp metal matrix composites, in addition to machine, tool and process-related parameters, a change in composition (size and volume fraction of reinforcement) has a influence on machining force components. In the analytical models in the literature, the effect of abrasive reinforcement particles, which affects the coefficient of friction and consequently the friction angle, has not been considered while predicting cutting forces in machining of MMCs. In this paper, chip–tool interface friction in machining of Al/SiCp composites has been considered to involve two-body abrasion and three-body rolling caused due to presence of reinforcements in composites. The model evaluates resulting coefficient of friction to predict the cutting forces during machining of Al/SiCp composites using theory of oblique cutting. Further, the model considers various frictional forces on the wiper geometry on the cutting edge that has been found to improve the integrity of machined surface on composites. The predicted cutting force values were found to agree well with the corresponding experimental values for finer reinforcements composites with the assumption that 40% of the reinforcement particles contribute to the abrasion at chip–tool interface. However, for the coarser reinforcement composites, assumption that the 60% of the particles contribute to the abrasion yields better results.     

High pressure waterjet cooling/lubrication to improve machining efficiency in milling

The efficiency of the metal cutting operations depends upon the thermal/frictional conditions at the tool-chip interface. The use of high pressure waterjet as a coolant/lubricant to improve the thermal/frictional conditions in milling operations was studied here. The influence of high pressure waterjet delivered into tool-chip interface in two different methods, namely, waterjet injected directly into the tool-chip interface through a hole in the tool rake face, and waterjet injected into tool-chip interface through an external nozzle, was explored in this study. The effectiveness of these developed methods was evaluated in terms of cutting force, surface finish, chip shape and tool wear.     

Cutting under active and inert gas shields: A contribution to the mechanics of chip flow

This paper is focused on the interaction between cutting medium and freshly formed surfaces with the aim at providing a new level of understanding on the mechanics of chip flow in metal cutting.The methodology draws from specially designed orthogonal metal cutting experiments performed in dry conditions under active and inert gas shields for observation of chip flow and measurement of friction, chip-compression factor and forces acting on the cutting tools.The presentation is a step towards clarification of common belief among researchers and practitioners that active gases influence metal cutting by showing that oxygen acting on the freshly cut surfaces of lead may change friction, chip compression factor, chip curling and forces to a level that goes significantly beyond what has been said and written in the context of metal cutting fundamentals.The overall content of the paper is original and discloses new experimental results and explanations about the influence of the surrounding medium on tribological conditions at the tool–chip contact interface and about the correlation between surrounding medium, surface roughness, freshly cut surfaces and chip curling.     

石墨烯涂层刀具车削6061铝合金有限元仿真

针对高效干切削和微量润滑加工中润滑能力不足的难题,通过引入石墨烯涂层自组装及界面优化,降低刀具表面自由能,提高刀具的耐磨性和寿命.采用DEFORM软件模拟不同切削速度下石墨烯涂层和非涂层硬质合金刀具切削加工6061铝合金,提取分析试验的切削力和切削热等数据后表明:在一定的切削条件下,石墨烯涂层能有效提高刀具的润滑性能,...     

The influence of friction models on finite element simulations of machining

In the analysis of orthogonal cutting process using finite element (FE) simulations, predictions are greatly influenced by two major factors; a) flow stress characteristics of work material at cutting regimes and b) friction characteristics mainly at the tool-chip interface. The uncertainty of work material flow stress upon FE simulations may be low when there is a constitutive model for work material that is obtained empirically from high-strain rate and temperature deformation tests. However, the difficulty arises when one needs to implement accurate friction models for cutting simulations using a particular FE formulation. In this study, an updated Lagrangian finite element formulation is used to simulate continuous chip formation process in orthogonal cutting of low carbon free-cutting steel. Experimentally measured stress distributions on the tool rake face are utilized in developing several different friction models. The effects of tool-chip interfacial friction models on the FE simulations are investigated. The comparison results depict that the friction modeling at the tool-chip interface has significant influence on the FE simulations of machining. Specifically, variable friction models that are developed from the experimentally measured normal and frictional stresses at the tool rake face resulted in most favorable predictions. Predictions presented in this work also justify that the FE simulation technique used for orthogonal cutting process can be an accurate and viable analysis as long as flow stress behavior of the work material is valid at the machining regimes and the friction characteristics at the tool-chip interface is modeled properly.