The role of tool geometry in process damped milling

Spindle speed variation is a well-known technique to suppress regenerative machine tool vibrations, but it is usually considered to be effective only for low spindle speeds. In this paper, the effect of spindle speed variation is analyzed in the high-speed domain for spindle speeds corresponding to the first flip (period doubling) and to the first Hopf lobes. The optimal amplitudes and frequencies of the speed modulations are computed using the semidiscretization method. It is shown that period doubling chatter can effectively be suppressed by spindle speed variation, although, the technique is not effective for the quasiperiodic chatter above the Hopf lobe. The results are verified by cutting tests. Some special cases are also discussed where the practical behavior of the system differs from the predicted one in some ways. For these cases, it is pointed out that the concept of stability is understood on the scale of the principal period of the system—that is, the speed modulation period for variable spindle speed machining and the tooth passing period for constant spindle speed machining.     

Speed-varying cutting force coefficient identification in milling

Accurate simulation of the machining process is crucial to improve milling performance, especially in High-Speed Milling, where cutting parameters are pushed to the limit.Various milling critical issues can be analyzed based on accurate prediction of cutting forces, such as chatter stability, dimensional error and surface finish. Cutting force models are based on coefficients that could change with spindle speed. The evaluation of these specific coefficients at higher speed is challenging due to the frequency bandwidth of commercial force sensors. On account of this, coefficients are generally evaluated at low speed and then employed in models for different spindle speeds, possibly reducing accuracy of results.In this paper a deep investigation of cutting force coefficient at different spindle speeds has been carried out, analyzing a wide range of spindle speeds: to overcome transducer dynamics issues, dynamometer signals have been compensated thanks to an improved technique based on Kalman filter estimator. Two different coefficients identification methods have been implemented: the traditional average force method and a proposed instantaneous method based on genetic algorithm and capable of estimating cutting coefficients and tool run-out at the same time.Results show that instantaneous method is more accurate and efficient compared to the average one. On the other hand, the average method does not require compensation since it is based on average signals. Furthermore a significant change of coefficients over spindle speed is highlighted, suggesting that speed-varying coefficient should be useful to improve reliability of simulated forces.     

高速铣削下PCBN刀具的有限元分析

通过对PCBN铣刀高速铣削淬硬45钢试验所得的数据进行分析,得出在不同的主轴转速下的切削力。再通过Solid Edge建立铣刀模型,导入ANSYS,将不同主轴转速下的切削力施加于所建模型PCBN刀片的切削刃上并进行应力分析。选取不同应力分布图中的最大应力值进行分析、比较,可知其应力变化是随着主轴转速的增大而逐渐减小,与实际情况相符。因此,通过建立有限元模型并进行分析,对铣削加工时选择合理的主轴转速提供依据。     

Extension of process damping to milling with low radial immersion

This paper investigates the stabilizing effect of process damping at low cutting speeds for regenerative machine tool vibrations of milling processes. The process damping is induced by a velocity-dependent cutting force model, which takes into account that the actual cutting velocity is different from the nominal one during machine tool vibrations. The chip thickness and the cutting force are calculated according to the direction of the actual cutting velocity. This results in an additional damping term in the governing delay-differential equation, which is time-periodic for milling and inversely proportional to the cutting speed. In the literature, this term is often assumed to be constant and is considered to improve stability properties at low spindle speeds. In this paper, it is shown that the velocity-dependent cutting force model captures the improvement in the low-speed stability only for turning operations and milling with large radial immersion, while it results in a negative process damping term for low-immersion milling. Consequently, an extended process damping model is needed to explain the low-speed stability improvement for low radial immersion milling.     

Spindle speed variation in turning: technological effectiveness and applicability to real industrial cases

Spindle speed variation is a well-known technique to suppress regenerative machine tool vibrations especially for low spindle speeds. Although a lot of research effort was made over the years the technique is not widespread in real turning applications. In this paper, the reasons that can limit the diffusion of the spindle speed variation were investigated. Therefore, the analysis of spindle speed variation strategy was not only focused on its chatter suppression properties but also on some more general technological aspects: the surface quality of the machined components, the cutting edge-spindle bearings load and the thermal overload the electrical spindle motor is subjected to when the speed modulation is used. A time-domain numerical model of the turning process was developed and exploited to support the analysis. A lot of cutting tests were also performed both to validate the numerical model and to evaluate the effect of variable spindle speed on surface quality. Finally, some real industrial applications were analyzed focusing on thermal overload issue of the spindle motor.     

Improved experimental-analytical approach to compute speed-varying tool-tip FRF

Chatter stability prediction is crucial to improve the performances of modern milling process, and it gets even more important at high speeds, for which very productive cutting parameters can be achieved if the suitable spindle speed is selected. Unfortunately, the available chatter predictive models suffer from reduced accuracy at high speed due to inaccuracies in the input data, especially the machine tool dynamics that is acquired in stationary configurations but could sensibly change with spindle speed. In this paper, an efficient method to identify the speed-varying Frequency Response Functions (FRFs) under operational conditions is presented. The proposed approach is based on the definition of some experimental chatter limits (i.e., chatter frequency and related depth of cut), obtained by a dedicated test, called Spindle Speed Ramp-up. The experimental results are then combined with the analytical stability solution. By minimizing the differences between the experimental and predicted chatter conditions, a dedicated algorithm computes the speed-varying FRFs. Few tests and simple equipment (i.e., microphone) are enough to calculate the FRFs in a wide range of spindle speeds. The proposed technique was validated in real machining applications, the identified tool-tip FRFs are in accordance with expected trend reported in scientific literature. Speed-varying stability lobe diagram reconstructed with the computed FRFs is proven to be accurate in predicting stable cutting parameters.     

Examination of surface location error due to phasing of cutter vibrations

The purpose of this research is to investigate the relative importance of spindle speed, system dynamics, and cutting conditions on the accuracy of surface location in computer numerical-control (CNC) finish machining operations. The relationship between the spindle speed, the most flexible modes of the machine/cutting tool system and the final part dimensions is rather complex. The underlying theory, based on the situation of forced vibrations, is outlined. It is shown that the critical factor is the ratio of the tooth passing frequency to the system most flexible mode and corresponding natural frequency. Simple analytical calculations are carried out to illustrate the overcut/undercut surface error phenomenon. A simple simulation for end milling operations is also described which calculates the force on the cutter, the resulting cutter deflection, and the final error of surface. A comparison between the simulated and experimental results is presented. From experimental data, it shown that a change in surface location (and part dimension) of up to 50 μm is seen for a set of given conditions (i.e., cutter, material, chip load) simply by changing spindle speeds. Furthermore, it is seen that certain spindle speeds produce surfaces with no error introduced by the machining process.     

Accuracy Optimisation for Mould Surface Profile Milling

In the profile milling process for mould surfaces, for any type of curve combination, when implementing the milling process on a CNC machine, the surface can be produced from straight lines or curves in a piecewise or continuous milling process. This work employs the features of a CNC machine: 1. To divide the various sizes of curve at different slopes. 2. To use with different milling spindle speeds. 3. To use cutting feeds for actual milling experimentation. The results of the profile dimension accuracy and profile surface roughness from these experiments are then used in a neural network to establish an experiment result and a model for the milling variables. The neural network is composed of a number of functional nodes. Once the milling parameters (spindle speed, feed speed and milling angle) are given, the milling processing performance (the surface roughness, the surface profile-error) can be accurately predicted by the net-work developed. The optimal milling processing parameters can be searched for by a simulation annealing (SA) optimis-ation algorithm with a performance index to obtain a satisfactory mould surface. The experiment is then used to show that improved conditions for mould profile processing can indeed be obtained.     

Chatter stability prediction in milling using time-varying uncertainties

The chatter stability in milling severely affects productivity and quality of machining. Tool wear causes both the cutting coefficient and the process damping coefficient, but also other parameters to change with cutting time. This variation greatly reduces the accuracy of chatter prediction using conventional methods. To solve this problem, we consider the cutting coefficients of the milling system to be both random and time-varying variables and we use the gamma process to predict cutting coefficients for different cutting times. In this paper, a time-varying reliability analysis is introduced to predict chatter stability and chatter reliability in milling. The relationship between stability and reliability is investigated for given depths and spindle speeds in the milling process. We also study the time-varying chatter stability and time-varying chatter reliability methods theoretically and with experiments. The results of this study show that the proposed method can be used to predict chatter with high accuracy for different cutting times.     

一种新型的变速切削铣床

为了使具有减振效果的变速铣削加工方法能在生产中推广应用,以长春市第二机床厂生产的ＸＤ５０３２型立式升降台铣床为基型,首次研制开发了一台ＸＤ５０３２／ＢＱ型变速切削铣床。主传动采用交流变频调速方式调速,选用ＳＴＤ工控机作为主传动系统的主控单元,研制开发了一套变速切削铣床主轴转速微机控制系统。     

Chatter vibration surveillance by the optimal-linear spindle speed control

The paper concerns self-excited chatter vibration during high speed slender ball-end milling. Non-stationary cutting process, with inclusion of various approaches towards dynamic characteristics of the process, is described. Dynamic analysis of the milling process is performed and dynamics of controlled closed loop system with time-delay is presented. In order to reduce vibration level, instantaneous change in the spindle speed appears as a control command, and thus—the method of vibration surveillance by the spindle speed optimal-linear control is developed. Presented cutting models have been applied for the proposed method and procedure of the chatter vibration surveillance with a use of variable spindle speed has been developed. Computer simulations are performed for selected cases of ball-end milling at constant and variable spindle speed. The results of them are successfully confirmed by experimental investigations on the Alcera Gambin 120CR milling machine equipped with the S2M high speed electrospindle.     

超高速磨削机床主轴系统模态分析

针对液体动静压轴承支撑的超高速磨削主轴系统工作的特殊性,在机床工作过程中,主轴高速旋转,动静压轴承的支撑刚度随转速动态变化.为了解主轴系统工作过程中的动态特性,应用Flunent软件求解液体动静压轴承的动态支撑刚 度,而后在此基础上利用有限元分析软件ANSYS建立超高速磨床主轴系统的三维有限元模型,并对其进行模态分析,得到了各阶固有频率和振型.通过设置不同转速下轴承的支撑刚度,获得主轴系统模态分析结果,并利用图解法求解出主轴系统的临界转速.分析结果表明主轴系统在高速旋转状态下,系统的结构刚度会发生变化,使主轴系统的固有频率改变,并且随着转速提高差异越显著.通过振动试验测试验证仿真分析的可靠性,经分析可知,试验与仿真的误差主要来源于支撑模型的简化.     

Counterbalance of cutting force for advanced milling operations

The goal of this work is to concurrently counterbalance the dynamic cutting force and regulate the spindle position deviation under various milling conditions by integrating active magnetic bearing (AMB) technique, fuzzy logic algorithm and an adaptive self-tuning feedback loop. Since the dynamics of milling system is highly determined by a few operation conditions, such as speed of spindle, cut depth and feedrate, therefore the dynamic model for cutting process is more appropriate to be constructed by experiments, instead of using theoretical approach. The experimental data, either for idle or cutting, are utilized to establish the database of milling dynamics so that the system parameters can be on-line estimated by employing the proposed fuzzy logic algorithm as the cutting mission is engaged. Based on the estimated milling system model and preset operation conditions, i.e., spindle speed, cut depth and feedrate, the current cutting force can be numerically estimated. Once the current cutting force can be real-time estimated, the corresponding compensation force can be exerted by the equipped AMB to counterbalance the cutting force, in addition to the spindle position regulation by feedback of spindle position. On the other hand, for the magnetic force is nonlinear with respect to the applied electric current and air gap, the characteristics of the employed AMB is investigated also by experiments and a nonlinear mathematic model, in terms of air gap between spindle and electromagnetic pole and coil current, is developed. At the end, the experimental simulations on realistic milling are presented to verify the efficacy of the fuzzy controller for spindle position regulation and the capability of the dynamic cutting force counterbalance.     

铣削加工过程的动态稳定性极限分析

在铣削过程中,由于系统模型的非线性和复杂性,加工过程参数很难控制.利用matlab软件,对铣削过程中主轴转速和轴向极限切削深度的关系进行了仿真,分析了系统切削参数对切削稳定性的影响.由仿真结果知,轴向极限切深随着主轴转速的增大呈现周期性变化,系统径向切削参数增大使轴向极限切深变小,增大系统阻尼使轴向极限切深增大.     

单晶DD98微尺度铣削表面质量试验研究

为探究单晶DD98微尺度铣削的表面质量,采用直径为0.6mm的微铣刀对单晶DD98进行三因素五水平的微尺度铣削正交试验。通过极差分析和方差分析发现：主轴转速对DD98表面质量的影响最大,铣削深度的影响次之,进给速度的影响最小;单晶DD98表面质量最好的工艺参数组合为主轴转速36000r/min,铣削深度5μm,进给速度100μm/s。得到了主轴转速、铣削深度和进给速度对表面质量的影响规律,并对其机理进行了分析,从而为单晶DD98材料的微尺度铣削加工提供理论依据。     

An experimental investigation on micro machining of fine-grained graphite

Industrial applications of the micro milling process require sufficient experimental data from various micro tools. Research has been carried out on micro milling of various engineering materials in the past two decades. However, there is no report in the literature on micro milling of graphite. This paper presents an experimental investigation on micro machinability of micro milling of moulded fine-grained graphite. Full immersion slot milling was conducted using diamond-coated, TiAlN-coated and uncoated tungsten carbide micro end mills with a uniform tool diameter of 0.5 mm. The experiments were carried out on a standard industrial precision machining centre with a high-speed micro machining spindle. Design of experiments (DoE) techniques were applied to design and analysis of the machining process. Surface roughness, surface topography and burrs formation under varying machining conditions were characterized using white light interferometry, SEM and a precision surface profiler. Influence of variation of cutting parameters including cutting speeds, feedrate and axial depth of cut on surface roughness and surface damage was analysed using ANOVA method. The experimental results show that feedrate has the most significant influence on surface roughness for all types of tools, and diamond tools are not sensitive to cutting speed and depth of cut. Surface damage and burrs analysis show that the primary material removal mode is still brittle fracture or partial ductile in the experimental cutting conditions. 3D intricate micro EDM electrodes were fabricated with good dimensional accuracy and surface finishes using optimized machining conditions to demonstrate that micro milling is an ideal process for graphite machining.     

INFLUENCE OF MATERIAL REMOVAL ON THE DYNAMIC BEHAVIOR OF THIN-WALLED STRUCTURES IN PERIPHERAL MILLING

Machining is a material removal process that alters the dynamic properties during machining operations. The peripheral milling of a thin-walled structure generates vibration of the workpiece and this influences the quality of the machined surface. A reduction of tool life and spindle life can also be experienced when machining is subjected to vibration. In this paper, the linearized stability lobes theory allows us to determine critical and optimal cutting conditions for which vibration is not apparent in the milling of thin-walled workpieces. The evolution of the mechanical parameters of the cutting tool, machine tool and workpiece during the milling operation are not taken into account. The critical and optimal cutting conditions depend on dynamic properties of the workpiece. It is illustrated how the stability lobes theory is used to evaluate the variation of the dynamic properties of the thin-walled workpiece. We use both modal measurement and finite element method to establish a 3D representation of stability lobes. The 3D representation allows us to identify spindle speed values at which the variation of spindle speed is initiated to improve the surface finish of the workpiece.     

INFLUENCE OF MATERIAL REMOVAL ON THE DYNAMIC BEHAVIOR OF THIN-WALLED STRUCTURES IN PERIPHERAL MILLING

Machining is a material removal process that alters the dynamic properties during machining operations. The peripheral milling of a thin-walled structure generates vibration of the workpiece and this influences the quality of the machined surface. A reduction of tool life and spindle life can also be experienced when machining is subjected to vibration. In this paper, the linearized stability lobes theory allows us to determine critical and optimal cutting conditions for which vibration is not apparent in the milling of thin-walled workpieces. The evolution of the mechanical parameters of the cutting tool, machine tool and workpiece during the milling operation are not taken into account. The critical and optimal cutting conditions depend on dynamic properties of the workpiece. It is illustrated how the stability lobes theory is used to evaluate the variation of the dynamic properties of the thin-walled workpiece. We use both modal measurement and finite element method to establish a 3D representation of stability lobes. The 3D representation allows us to identify spindle speed values at which the variation of spindle speed is initiated to improve the surface finish of the workpiece.     

Surface texture generation using high-feed milling with spindle speed modulation

This paper presents a new surface texturing technique using ball-end milling with high feed speed and spindle speed modulation. The ratio between feed-rate and cutting tool radius is in the range of 0.2–0.4, which is much larger than the ratio in conventional milling. Sinusoidal modulation signal is added, so the spindle speed becomes time-varying in order to generate different texture profiles. The cutting tool kinematics are modeled considering the tool-tip run-out and deflection due to cutting forces. The effects of amplitude and frequency of the modulation signal on tool-tip trajectories and surface textures are simulated and analyzed. The relationship between the micro features of the surface texture and the process parameters are investigated. Surface texturing experiments are conducted based on the proposed technique, and tribology tests are performed on the textured surface. It is shown that the textured surfaces present frictional anisotropy, which depends on the process conditions and modulation parameters. The proposed technique is able to achieve fast generation of various surface textures without additional instrumentation, and the final texture geometry is controllable based on the presented kinematics model.     

The influence of the crystallographic orientation when micro machining commercially pure titanium: A size effect

Micro machining processes are characterized by the influence of size effects like the influence of the workpiece micro structure on the process and ultimately on the process results. So far, the influence of grain orientation on the process results is state of the art and mostly investigated on single-crystal materials, but the influence of the grain orientation on the process and process results in micro machining of polycrystalline materials with a hexagonal close packed (hcp) crystal structure like commercially pure (cp)-titanium is, to the best knowledge of the authors, not completely understood.Within this research, the correlation between the grain orientation and the surface topography, process forces, and chip formation is researched by orthogonal cutting. This process offers the possibility of clearly linking the cutting behavior with the resulting surface and the influences of the size effect. The results show a clear correlation between the grain orientation and the topography height, the ripping out of material, and the resulting surface. A correlation between the grain orientation and the forces in the X-, Y-, and Z-direction could also be shown. The chip formation and thus the resulting chip thickness differs depending on the grain orientation. In addition, micro milling tests were conducted and influences on burr formation and process forces are shown. At low spindle speeds the burr width or the burr shape are a function of the grain orientation. At high spindle speeds this effect can no longer be observed. With the results of this research, it is possible to compensate the influence of the grain orientation on the process results and to enhance the efficiency and the resulting surface quality of micro machining processes.