Active vibration control in palletised workholding system for milling

This paper presents the development implementation and testing of an active controlled palletised workholding system for milling operations. The traditional approach to controlling vibration in a machining system is to develop control systems for cutting tools or machine spindles as in the case of milling machines. This work is a deviation from the traditional approach and targets a workholding system for the control of unwanted vibration. Palletised workholding systems, due to their compact design, offer an opportunity to design active control systems that are economical and easier to implement in the case of milling machines. The active control system developed here is based on an adaptive filtering algorithm, the filtered X-LMS, and employs piezo-actuators for dynamic control force. The system has been tested experimentally to demonstrate the reduction in dynamic force due to vibration. Extensive testing has been carried out to validate the performance of the system in terms of parameters of practical importance such as improvement in surface finish and increase in tool life.     

影响铣削颤振稳定性的因素与规律分析

应用Altintas切削颤振理论实现了铣削颤振的预测,并对影响铣削稳定性的机床系统因素进行了分析。研究发现,稳定性叶瓣图会受到机床的主轴-刀具系统模态参数影响,尤其是模态刚度、阻尼比和固有频率。另外,通过系统动刚度相同的条件下不同的阻尼比和模态刚度组合对铣削稳定性的影响分析发现,模态刚度对系统稳定性的影响要大于阻尼比的影响程度。分别对影响铣削加工稳定性的刀具参数、工件材料特性以及切削参数等因素及其对铣削稳定性的影响规律进行了分析。结果显示:减小刀具齿数、刀具螺旋角和刀具悬伸量,并增大刀具直径对于改善切削颤振有益;具有较小切向切削力系数和径向切削力系数的材料更容易实现稳定切削;减小铣削宽度,并采用顺铣方式,系统的临界切深更大。     

Active vibration control for peripheral milling processes

In this paper, an active vibration control system is developed for peripheral milling processes. In this system, the workpiece is driven by a specially designed active stage to control the relative vibration between the tool and workpiece during milling processes. The stage is modeled as a control plant, and the cutting vibrations are treated as disturbances to the system. Robust mixed sensitivity method is employed in feedback controller synthesis to achieve robust disturbance rejection and stabilization. Numerical simulations and cutting experiments are carried out to validate the effectiveness of the control system. Results indicate the relative vibration between the workpiece and tool, as well as the rough of surface finish decrease significantly in the case of milling processes with the presented controller.     

Coupled interaction of dynamic responses of tool and workpiece in thin wall milling

Chatter free thin wall machining requires knowledge of the dynamics of a machine-tool system and workpiece either for designing damping solutions or for modelling impact dynamics. Previous studies on thin wall milling mostly focussed on stability studies. However studies on the interaction between the tool and workpiece responses in thin wall machining are scarce in the literature. In this work, the coupled dynamic response of tool and workpiece is presented both for an open (thin wall straight cantilever) and for closed (thin wall ring type casing) geometry structures. Experiments were carried out for different tool overhangs and depths of cut and the machining vibration signal was analysed in time–frequency domain to study the interaction, i.e. coupling, of tool–workpiece dynamic response at various cutting tooth engagement/idle times. The findings from this study highlight the importance of tool's frequency, particularly torsional and first bending modes, in impact dynamics of thin wall milling. Moreover, the differences in dynamic response interaction between a cutting tool and thin wall plate and a cylinder are identified. While the analysis of the open geometry structure showed the presence of tool and workpiece responses for any depth of cut, results on closed geometry structure exhibited a complete dominance of tool mode at higher depths of cut. These findings are of critical importance in understanding the impact dynamics in thin wall milling and also of effectiveness of passive damping solutions.     

基于切削力的铣削加工误差数学模型

针对铣削加工过程中由于工件、设备系统弹性变形以及刀具磨损导致的加工误差进行了分析和研究.提出了以切削分力作为主要参数的由于刀具磨损和工件与机械系统的弹性变形所带来的加工误差的数学模型.     

基于模态分析的高速铣削主轴转速选择

分析了高速铣削的特点以及切削加工中的振动现象,研究了高速切削和普通切削中的工件振动对加工精度的影响程度,提出了在高速铣削情形下工件振动会影响加工精度的假设。然后采用振动力学中的谐响应方法分析了铣削加工中工件振动的简化模型,研究了工件在刀具作用力下的振动情形;并通过有限元分析软件进行实例仿真,结果表明在高速切削情况下工件的振动会影响要求较高的加工质量。最后,给出了利用模态分析来优化主轴转速的方法,为高速切削情况下减小振动、提高加工质量提供了一种途径。     

Theoretical-experimental modeling of milling machines for the prediction of chatter vibration

Productivity of high speed milling operations can be seriously limited by chatter occurrence. Chatter vibrations can imprint a poor surface finish on the workpiece and can damage the cutting tool and the machine. Chatter occurrence is strongly affected by the dynamic response of the whole system, i.e. the milling machine, the tool holder, the tool, the workpiece and the workpiece clamping fixture. Tool changes must be taken into account in order to properly predict chatter occurrence. In this study, a model of the milling machine-tool is proposed: the machine frame and the spindle were modeled by an experimentally evaluated modal model, while the tool was modeled by a discrete modal approach, based on the continuous beam shape analytical eigenfunctions. A chatter identification technique, based on this analytical-experimental model, was implemented. Tool changes can be easily taken into account without requiring any experimental tests. A 4 axis numerically controlled (NC) milling machine was instrumented in order to identify and validate the proposed model. The milling machine model was excited by regenerative, time-varying cutting forces, leading to a set of Delay Differential Equations (DDEs) with periodic coefficients. The stability lobe charts were evaluated using the semi-discretization method that was extended to n>2 degrees of freedom (dof) models. The stability predictions obtained by the analytical model are compared to the results of several cutting tests accomplished on the instrumented NC milling machine.     

Chatter suppression in micro end milling with process damping

Micro milling utilizes miniature micro end mills to fabricate complexly sculpted shapes at high rotational speeds. One of the challenges in micro machining is regenerative chatter, which is an unstable vibration that can cause severe tool wear and breakage, especially in the micro scale. In order to predict chatter stability, the tool tip dynamics and cutting coefficients are required. However, in micro milling, the elasto-plastic nature of micro machining operations results in large process damping in the machining process, which affects the chatter. We have used the equivalent volume interface between the tool and the workpiece to determine the process damping parameter. Furthermore, the accurate measurement of the tool tip dynamics is not possible through direct impact hammer testing. The dynamics at the tool tip is indirectly obtained by employing the receptance coupling method, and the mechanistic cutting coefficients are obtained from experimental cutting tests. Chatter stability experiments have been performed to examine the proposed chatter stability model in micro milling.     

Modeling and simulation of milling processes including process damping effects

Especially in high speed milling of aluminum alloys in the aviation industry, chamfered milling tools have proven themselves. Due to the chamfer, an extended contact between the tool and the workpiece at the flank face is evoked, which leads to additional process damping forces opposed to tool vibrations. Hence, the cutting process shows improved stability characteristics. This article presents an approach for the identification and modeling of these process damping effects in transient milling simulations. For this purpose, a simulation- and experiment-based procedure for the identification of required simulation parameters depending on the tool chamfer geometry is introduced and evaluated. Finally, the identified parameters are used for transient simulations of milling processes with extended stability due to the tool chamfer. The suitability of the proposed identification method and simulation model for milling with process damping is finally proved by a comparison between simulations and experiments.     

Modelling and simulation of chatter in milling using a predictive force model

A predictive time domain chatter model is presented for the simulation and analysis of chatter in milling processes. The model is developed using a predictive milling force model, which represents the action of milling cutter by the simultaneous operations of a number of single-point cutting tools and predicts the milling forces from the fundamental workpiece material properties, tool geometry and cutting conditions. The instantaneous undeformed chip thickness is modelled to include the dynamic modulations caused by the tool vibrations so that the dynamic regeneration effect is taken into account. Runge–Kutta method is employed to solve the differential equations governing the dynamics of the milling system for accurate solutions. A Windows-based simulation system for chatter in milling is developed using the predictive model, which predicts chatter vibrations represented by the tool-work displacements and cutting force variations against cutter revolution in both numerical and graphic formats, from input of tool and workpiece material properties, cutter parameters, machine tool characteristics and cutting conditions. The system is verified with experimental results and good agreement is shown.     

Optimal spindle speed determination for vibration reduction during ball-end milling of flexible details

In the paper a method of optimal spindle speed determination for vibration reduction during ball-end milling of flexible details is proposed. In order to reduce vibration level, an original procedure of the spindle speed optimisation, based on the Liao–Young criterion [1], is suggested. As the result, an optimal, constant spindle speed value is determined. For this purpose, non-stationary computational model of machining process is defined. As a result of modelling, a hybrid system is described. This model consists of following subsystems, i.e. stationary model of one-side-supported flexible workpiece (modal subsystem), non-stationary discrete model of ball-end mill (structural subsystem) and conventional contact point between tool and workpiece (connective subsystem). The method requires identification of some natural frequencies of stationary modal subsystem. To determine them, appropriate modal experiments have to be performed on the machine tool, just before machining. Examples of vibration surveillance during cutting process on two high speed milling machines Mikron VCP 600 and Alcera Gambin 120CR are illustrated.     

Measurement of milling tool vibrations during cutting using laser vibrometry

Spindle and tool vibration measurements are of great importance in both the development and monitoring of high-speed milling. Measurements of cutting forces and vibrations on the stationary spindle head is the most used technique today. But since the milling results depend on the relative movement between the workpiece and the tool, it is desirable to measure on the rotating tool as close to the cutters as possible. In this paper the use of laser vibrometry (LDV) for milling tool vibration measurements during cutting is demonstrated. However, laser vibrometry measurements on rotating surfaces are not in general straight forward. Crosstalk between vibration velocity components and harmonic speckle noise generated from the repeating revolution of the surface topography are problems that must be considered. In order to overcome the mentioned issues, a cylindrical casing with a highly optically smooth surface was manufactured and mounted on the tool to be measured. The spindle vibrations, radial tool misalignment, and out-of-roundness of the measured surface were filtered out from the signal; hence, the vibrations of the cutting tool were resolved. Simultaneous measurements of cutting forces and spindle head vibrations were performed and comparisons between the signals were conducted. The results showed that vibration velocities or displacements of the tool can be obtained with high temporal resolution during cutting load and therefore the approach is proven to be feasible for analysing high-frequency milling tool vibrations.     

Experimental research on cutting force variation during primary chatter vibration occuring in plain milling operation

Plain milling operation is characterized by a transient and intermittent cutting process, in which undeformed chip thickness varies continuously. The undeformed chip thickness variation is opposite in the up milling and down milling processes. First, the property of primary chatter vibration in plain milling operation is investigated. In the up milling process, the transient vibration generated in the initial stage of the cutting operation develops into primary chatter vibration, along with the chip thickness increase. On the other hand, a large amount of vibration energy is supplied during the initial collision of the cutting edge with the workpiece at a large undeformed chip thickness in the down milling process; and immediately after this collision, the primary chatter vibration of almost stable amplitude continues. Secondly, the vibration energy supply during the primary chatter vibration of plain milling operations is investigated on the basis of the experimental results. The exciting mechanism can be explained by considering the interference between the tool flank and the workpiece surface accompanying the arbor vibration. An unusual phenomenon is also discussed, in which the normal cutting force component has two maxima during one period of vibration in up milling. From the above results, the cutting edge shapes (effective relief angle and cutting edge radius), and the torsional rigidity of the milling arbor must be carefully determined, to prevent the primary chatter vibration in plain milling operation.     

基于高质量和低振动的铣削参数优化方法研究

针对数控铣床在切削过程中产生的振动对工件表面质量的影响,提出以低振动和高表面质量为优化目标,对切削参数进行优化。以VDF-850A铣床为研究对象对45号钢进行铣削正交试验,通过建立振动采集系统,采集振动信号提取振动特征值并测量工件表面粗糙度值,应用最小二乘法拟合数据建立了振动和粗糙度数学模型。利用层次分析法确定两目标函数权重,使用平方和加权法对两目标函数加权拟合成综合目标评价函数,运用粒子群算法优化切削参数。试验结果表明:应用粒子群算法优化后的切削参数进行加工可有效的降低振动和提高表面质量。     

微细铣削时积屑瘤现象的研究

针对微细铣削实验时的积屑瘤现象,在分析其成因的基础上,研究了切削用量、切削液、刀具几何参数、刀具表面粗糙度及工件材料硬度等因素对积屑瘤的影响机制;分析了积屑瘤的产生对加工工件精度、切削力及切削振动的影响;提出了在微细铣削过程中抑制积屑瘤生成的主要方法。     

基于自适应控制技术的铣削参数优化

采用BP神经网络算法应用于铣齿功率建模能较准确地预测铣齿功率大小,进而运用STATISTICA的正交设计优化试验数据对滚齿机进行再制造,通过在主轴箱加设传感器实现了机床振动稳定性的的在线监控,分析各个切削状态下主轴箱振动同铣削功率的关系,进行优化切削参数,实现了数控系统与在线监控技术的自适应闭环监控.完成了4m大型滚齿机向高速铣齿机床SKX-4000的智能化再制造.结果表明,采用的控制策略能适应强力铣削的工况变化,稳定地控制加工过程,达到保护机床、刀具和提高加工效率的目的.     

Identification of machine–tool–workpiece system dynamics

The aim of this paper is to present a method to identify the dynamics of a structure composed of a milling machine, a tool and a workpiece. The excitation is obtained as a result of the interrupted cutting of a narrow workpiece width and single tooth milling operations. This provides a pulse-like cutting force. The three components of the cutting force and the relative motion between the tool and the workpiece are measured simultaneously. A method was developed to determine the nine terms of the structural transfer matrix under a single cutting operation and without any assumption on the excitation direction. The proposed method is experimentally validated.     

On-line control of machine tool vibration in turning

This paper presents a system developed for on-line vibration control on a turning lathe. Relative vibration between the workpiece and the cutting tool is sensed with the help of a bifurcated bunch of optical fibres which is then phase shifted, amplified and fed back to a specially designed piezoelectric vibrator supporting the tool. Whenever a vibration occurs, leading the machine tool-cutting process system to instability, the close-dloop feedback contour, with the help of the vibrator-exciter, generates an equal and opposite force to stabilize the vibration. Results obtained by mathematical modelling and experimental investigations indicate a significant improvement in the dynamic characteristics of the machine tool resulting in considerably higher productivity, accuracy and finish.     

Statistical tool breakage detection schemes based on vibration signals in NC milling

We develop a vibration sensor-based tool breakage detection system for NC milling operations. The system obtains the time-domain vibration signal from the sensor attached on the spindle bracket of our CNC machine and declares tool failures through the on-line monitoring schemes. For the on-line detection, our approach is to use the statistical process control methods where control limits or thresholds are automatically calculated independently of cutting conditions. The main thrust of this paper is to compare the performance of the proposed statistical process monitoring methods including the X-bar control scheme, the exponentially weighted moving average (EWMA) scheme, and the adaptive EWMA scheme. The performance of the control schemes are compared in terms of the type I and II errors calculated from the experiment data.     

Stability analysis of chatter vibration for a thin-wall cylindrical workpiece

The vibration characteristics and the vibration direction angle between the beam mode and shell mode of the three-jaw clamped thin-wall cylindrical workpiece will change according to the relative position of the cutting tool to the chucking jaws. This will induce parametric vibration and cause the chatter characteristics of a thin-wall cylindrical workpiece to become more complicated than those of a solid workpiece. In this study, the three-jaw clamped thin-wall cylindrical workpiece was considered as a parametrically excited vibration system. Initially, the relationship between the parametric vibration and the workpiece system structure was investigated, and then the equivalent stability cutting depth of the thin-wall cylindrical workpiece was analyzed by a numerical calculation method. When the three-jaw clamped thin-wall cylindrical workpiece was considered as a parametric vibration system, the analytical results were consistent with the experimental results.