Continuous workpiece speed variation (CWSV): Model based practical application to avoid chatter in grinding

The continuous variation of rotation speeds provides a means of avoiding chatter instability in different machining processes. This paper presents a time domain dynamic model that simulates chatter vibration during infeed centerless grinding for any continuously variable work rotation speed strategy. As a result of taking machine dynamics and main non-linear effects of the process into account, part roundness error is predicted for the whole grinding cycle. Lastly, experimental results have validated the model and verified that adequate speed variation strategies are capable of avoiding chatter and improving workpiece roundness and roughness, both for infeed and throughfeed centerless grinding.     

An active system of reduction of vibrations in a centerless grinding machine using piezoelectric actuators

Centerless grinding has been extensively used in production engineering to produce accurate cylindrical parts together with high productivities. On the other hand, regenerative chatter vibrations are one of the major problems that limit the ability to produce round workpieces. This constraint can be solved selecting proper machine setup conditions, which still largely relies on a trial and error method, and sometimes this approach is not optimum in a productivity sense. This paper shows a novel method to reduce chatter vibrations in a centerless grinding machine using actively controlled piezoelectric actuators. A simplified model of the machine is used to simulate the behavior of several commercially available piezoelectric actuators in two different locations of the machine. Based on these simulations, a selection of proper actuators and their optimal location is obtained and the control system is implemented experimentally. Experimental results show that the control strategy provides a stabilizing effect on chatter. Thus, the viability of using piezoelectric actuators as active components is demonstrated, providing an important advance in the knowledge of chatter control in centerless grinding machines.     

Analysis of chatter in contour grinding of optical materials

Chatter that limits ground surface finish has been observed in the deterministic microgrinding of brittle optical materials. In this article, the classical single degree of freedom model for chatter, accounting for both work and tool regenerative effects, is adapted for contour grinding optical surfaces. A linearized expression for the cutting stiffness is developed for the contour grinding geometry based on Preston’s law. Techniques developed to measure the machine frequency response function (FRF) and the Preston coefficient, needed as inputs to the simulations, are described. Numerical simulations based on this model are used to predict the grinding system behavior and to investigate process parameters affecting chatter stability. The stability limits observed during grinding experiments on optical glasses are in good agreement with the simulation results.     

基于BP神经网络的外圆磨削颤振在线识别和监测方法

为提高机床磨削加工过程中对颤振现象识别的能力,提出一种基于BP(back?propagation)神经网络模型的颤振识别方法。通过对加工过程中传感器采集到的高频声发射信号以及振动信号相关特征值的提取,获得关于颤振的多特征参数样本库,并用其对BP神经网络模型进行学习和训练,建立BP神经网络在线识别颤振的算法模型,实现对机床加工过程中是否发生颤振的在线监测和识别。试验结果表明:这种基于BP神经网络模型的颤振识别测试结果与磨削加工试验中的磨削颤振现象结果相符合。该方法能够有效地识别磨削加工过程中的颤振,并起到在线监测识别的作用。      

A study on chatter boundaries of cylindrical plunge grinding with process condition-dependent dynamics

This paper presents a systematic study on the chatter characteristics of cylindrical plunge grinding processes. Chatter occurrence under a wide range of operating conditions is experimentally identified. The unique transient chatter behaviors during spark-in and spark-out of plunge grinding are also studied; and the transient chatter occurrence is separated from the steady-state chatter occurrence, which is used to define the chatter boundaries.It is shown that the measured structure dynamics of the wheel/workpiece exhibit strong dependency on grinding conditions. The dynamic/chatter model of cylindrical plunge grinding [H. Li, Y.C. Shin, A time-domain dynamic model for chatter prediction of cylindrical plunge grinding processes, ASME Journal of Manufacturing Science and Engineering 128(2) (2006) 404–415] is improved to handle the grinding force-dependent structure dynamics, and the modified model is validated by comparing the predicted chatter boundaries with experimental chatter conditions.     

Increased productivity in centerless grinding using inertial active dampers

Regenerative chatter during centerless grinding processes is one of the main factors limiting the productivity. This paper presents a novel chatter suppression technique for centerless grinding using an active damping system based on inertial actuators. Dynamic characterisation of the machine with and without active damping is used first to compute theoretical stability maps. The active nature of the system allows stabilising a wide working range of infeed operations. Finally, experimental results validate the predicted increase of stable grinding area and verify that the technology is very effective for avoiding chatter, ensuring workpiece quality and increasing process productivity.     

Chatter Stability of Metal Cutting and Grinding

This paper reviews fundamental modeling of chatter vibrations in metal cutting and grinding processes. The avoidance of chatter vibrations in industry is also presented. The fundamentals of orthogonal chatter stability law and lobes are reviewed for single point machining operations where the process is one dimensional and time invariant. The application of orthogonal stability to turning and boring operations is presented while discussing the process nonlinearities that make the solution difficult in frequency domain. Modeling of drilling vibrations is discussed. The dynamic modeling and chatter stability of milling is presented. Various stability models are compared against experimentally validated time domain simulation model results. The dynamic time domain model of transverse and plunge grinding operations is presented with experimental results. Off-line and real-time chatter suppression techniques are summarized along with their practical applications and limitations in industry. The paper presents a series of research topics, which have yet to be studied for effective use of chatter prediction and suppression techniques in industry.     

An investigation of in-process measurement of ground surfaces in the presence of vibration

Recent work on chatter in grinding has shown that the presence of torsional vibration is potentially significant. Controlling the torsional characteristics of the workpiece drive may eliminate chatter. These findings lead to a re-examination of the fundamental grinding force equation, where the surface speeds are conventionally assumed to be constant. If torsional vibration is present for both the grinding wheel and the workpiece, there will be two extra terms in the grinding force equation. Traditionally, experimental measurements used to try and verify the cutting force model have been undertaken under non-vibrating conditions. Any attempt to verify a cutting force model under vibrating conditions requires the continuous measurement of several parameters as a function of time. One of these is the instantaneous depth of cut, δ. This paper presents experimental results for an investigation into the in-process measurement of δ under chatter conditions on a cylindrical grinding machine. This initial investigation has indicated that such a measurement is very difficult and is prone to errors if chatter is present. It is proposed and anticipated that controlled (vibration) excitation under inherently stable conditions will allow for the required measurements to be made with sufficient accuracy.     

新型陶瓷托辊专用磨床磨削颤振稳定性分析

为了避免陶瓷托辊专用磨床在加工过程中产生颤振而降低磨削质量,根据陶瓷托辊磨削加工机制,建立了陶瓷托辊磨削加工过程数学模型,通过MATLAB计算分析得出磨削深度与主轴转速的稳定性叶瓣图,确定了陶瓷托辊磨床在磨削深度低于4 mm时可无颤振稳定磨削。研究结果为陶瓷托辊专用磨床稳定磨削加工提供了参考。     

Prediction of machining chatter based on FEM simulation of chip formation under dynamic conditions

Machining chatter is an inherently nonlinear phenomenon that is affected by many parameters such as cutting conditions, tool geometry e.g., nose radius and clearance angle and frictional conditions at the tool/workpiece interface. Models for chatter prediction often ignore nonlinearities or introduce them through simple models for friction and geometry. In particular, the effect of chip–tool interaction on the occurrence of chatter is not investigated thoroughly. This paper presents a novel approach for prediction of chatter vibration and for investigation of the effects of various conditions on the onset of chatter. This approach uses finite element simulation to investigate the inter-relationship between the chatter vibration and the chip formation process. Simulation of chip formation is combined with dynamic analysis of machine tool to determine the interaction between the two phenomena. Mesh adaptation technique is used to move the tool inside the workpiece to form the chip, while a flexible tool is used to allow the tool to vibrate under variable loading conditions. By repeating the simulations under various widths of cut, it is shown that the onset of chatter can be detected, and the simulation is able to realistically predict various phenomena observed in actual machining process such as variation of shear angle and the increase of stability at lower speeds known as process damping. The stability map obtained from simulations is compared with experimental data attained through orthogonal cutting tests. Reasonable agreement observed between the two sets of results demonstrates the effectiveness of the simulation approach.     

A comprehensive dynamic cutting force model for chatter prediction in turning

This paper presents a model of the dynamic cutting force process for the three-dimensional or oblique turning operation. To obtain dynamic force predictions, the mechanistic force model is linked to a tool–workpiece vibration model. Particular attention was paid to the inclusion of the cross-coupling between radial and axial vibrations in the force model. The inclusion of this cross-coupling facilitates prediction of the unstable–stable chatter phenomenon which usually occurs in certain cases of finish turning due to process non-linearity. The dynamic force model developed was incorporated into a computer program to obtain time-saving chatter predictions. Experimental tests were performed on AISI 4140 steel workpieces to justify the chatter predictions of the dynamic cutting process model in both the finishing and roughing regimes. Experimental results corroborate the unstable–stable chatter predictions of the model for different cases of finish machining. In addition, experimental results also confirmed the accuracy of chatter predictions for various cases of rough turning.     

Simulation of an active vibration control system in a centerless grinding machine using a reduced updated FE model

In this paper, a novel and complete process to simulate an active vibration control system in a centerless grinding machine is presented. Based on the updated finite element (FE) model of the machine, the structural modifications performed to incorporate active elements are detailed, as well as the subsequent reduction procedure to obtain a low-order state space model. This reduced structural model was integrated in the cutting process model giving a tool adapted for the purpose of simulating different control laws. Using the developed model, a control algorithm, which previously had been implemented in the centerless grinding machine under study, was checked. The simulation results were in agreement with the experimentally obtained ones, showing that the designed model is able to reproduce machine behaviour with the control activated. This model constitutes a powerful tool to evaluate the effectiveness of different approaches to that of the described one, making it possible to tackle an optimisation process of the control system by means of simulations and, thus, avoiding the costs that would involve the practical implementation of each one.     

Analytical modeling of spindle–tool dynamics on machine tools using Timoshenko beam model and receptance coupling for the prediction of tool point FRF

Regenerative chatter is a well-known machining problem that results in unstable cutting process, poor surface quality and reduced material removal rate. This undesired self-excited vibration problem is one of the main obstacles in utilizing the total capacity of a machine tool in production. In order to obtain a chatter-free process on a machining center, stability diagrams can be used. Numerically or analytically, constructing the stability lobe diagram for a certain spindle–holder–tool combination implies knowing the system dynamics at the tool tip; i.e., the point frequency response function (FRF) that relates the dynamic displacement and force at that point. This study presents an analytical method that uses Timoshenko beam theory for calculating the tool point FRF of a given combination by using the receptance coupling and structural modification methods. The objective of the study is two fold. Firstly, it is aimed to develop a reliable mathematical model to predict tool point FRF in a machining center so that chatter stability analysis can be done, and secondly to make use of this model in studying the effects of individual bearing and contact parameters on tool point FRF so that better approaches can be found in predicting contact parameters from experimental measurements. The model can also be used to study the effects of several spindle, holder and tool parameters on chatter stability. In this paper, the mathematical model, as well as the details of obtaining the system component (spindle, holder and tool) dynamics and coupling them to obtain the tool point FRF are given. The model suggested is verified by comparing the natural frequencies of an example spindle–holder–tool assembly obtained from the model with those obtained from a finite element software.     

Effectiveness of continuous workpiece speed variation (CWSV) for chatter avoidance in throughfeed centerless grinding

The continuous rotation speed variation is demonstrated to be an efficient method to avoid regenerative chatter in different machining processes. This paper presents a time-domain dynamic model for throughfeed centerless grinding process that can predict chatter by means of part roundness error evolution. Continuous workpiece speed variation (CWSV) has been implemented in this model to analyze the influence of this disturbing method on the dynamic instability. Experimental results have validated the model and verified the effectiveness of CWSV for chatter avoidance and surface finish and dimensional tolerances improvement. It has been demonstrated that the selection of the optimal variation parameters is an important factor not only for chatter avoidance, but also for the stability of surface finish and dimensional tolerances since workpiece speed variation has a direct influence on throughfeed rate and grinding forces.     

Identification and modeling of process damping in turning and milling using a new approach

Process damping can be a significant source of increased stability in machining particularly at low cutting speeds. However, it is usually ignored in chatter analysis as there is no model available to estimate process damping coefficients. In this study, a practical identification and modeling method is presented where process damping coefficients are obtained from chatter tests. The method is generalized by determining the indentation force coefficient responsible for the process damping through energy analysis. This coefficient is then used for process damping and the stability limit prediction in different cases, and predictions are verified by time domain simulations and experimental results.     

The measurement of forces in grinding in the presence of vibration

Most investigations of chatter have made the assumption that torsional vibration is not a significant factor. Some recent work has shown that chatter in grinding is affected by a change in the torsional stiffness of the workpiece drive. Also, a theoretical model of grinding chatter has been developed that confirmed the significance of torsional effects. However, the model for the grinding force was assumed to be a dynamic equivalent of a published steady-state model. This paper describes tests conducted to measure the variation in force caused by an oscillation in workpiece speed. The oscillating test results indicate that the torsional vibration of the workpiece may well be a significant effect on chatter in grinding. Moreover, as the grinding force changes with workpiece speed, it may be possible to use variation of workpiece speed at high frequency to reduce chatter.     

Generalized dynamic model of metal cutting operations

This paper presents a unified mathematical model which allows the prediction of chatter stability for multiple machining operations with defined cutting edges. The normal and friction forces on the rake face are transformed to edge coordinates of the tool. The dynamic forces that contain vibrations between the tool and workpiece are transformed to machine tool coordinates with parameters that are set differently for each cutting operation and tool geometry. It is shown that the chatter stability can be predicted simultaneously for multiple cutting operations. The application of the model to single-point turning and multi-point milling is demonstrated with experimental results.     

The feasibility of eigenstructure assignment for machining chatter control

The eigenstructure assignment algorithm is proposed for controlling machining chatter by changing the response of the machine tool structure to dynamic cutting forces through the change of its modal properties so that the interaction between the tool and workpiece can be altered. The determination of the desired modal shapes is derived from a concept similar to gain scheduling in adaptive control system theory. By using computer simulations, the desired eigenstructure of the machine tool structure for different cutting conditions is determined and used to form the scheduling table. The gain matrix is adjusted according to the scheduling table and cutting conditions. It was found from experimental results that by changing the principal direction of the machine tool structure, the machining system could be stabilized and that the use of the proper eigenstructure to suppress machine tool chatter could significantly increase the material removal rate. Simulations have shown that the responses of the controlled machining system have been altered from unstable to stable, proving the feasibility of the proposed chatter control concept.     

A new perspective on the stability study of centerless grinding process

Regenerative chatter is one of the most complex dynamic processes in machine tools. It is characterized by the presence of self-excited vibrations during machining, limiting the achievable tolerances in the workpieces. In order to predict the set-up conditions that produce these vibrations, it is necessary to model the regenerative mechanism responsible of their appearance accurately, so that the system stability can be studied solving the characteristic equation of the chatter loop. Although the dynamic behavior of machining processes like milling, turning or drilling is governed by a time delayed differential equation with one time delay term, a very particular problem is presented in centerless grinding. In this process, in addition to the dynamic instabilities, geometric instabilities must be analyzed, which are another important factors limiting the workpiece tolerances and lead to three time delay terms in the modeling procedure. This fact complicates its study remarkably, and the resolution of the characteristic roots of the dynamic process of these kinds of machines has not been tackled in the specialized literature as extensively as in other machining processes, being this field a challenging research line. According to this, in this paper an original and efficient method is presented to solve the roots of the characteristic equation of the centerless grinding process, based on the application of the root locus method. The main features of the proposed procedure are its ability to obtain the solutions accurately and that it is capable of determining the origin of the instabilities, so it constitutes a powerful tool to predict machine response for different set-up conditions. These interesting properties are demonstrated through the simulation results presented in this paper.     

基于空化效应的功率超声珩磨再生型颤振研究

功率超声珩磨加工过程中的颤振影响因素很多,不仅包含物理磨削过程和机械振动环节,还包含了高频振动和超声波传动系统,是一个非常复杂的复合振动系统。通过建立珩磨颤振动力学模型,重点研究珩磨中空化泡溃灭产生辐射声压对再生型颤振的影响,结果表明:决定功率超声珩磨再生型颤振的主要原因为磨削厚度;空化泡溃灭产生的辐射声压会加剧系统颤振的频率,但对系统颤振的时域图变化趋势和振幅的大小基本没有影响。