Automatic chatter detection in grinding

Two methods for automatic chatter detection in outer diameter plunge feed grinding are proposed. The methods employ entropy and coarse-grained information rate (CIR) as indicators of chatter. Entropy is calculated from a power spectrum, while CIR is calculated directly from fluctuations of a recorded signal. The methods are verified using signals of the normal grinding force and RMS acoustic emission. The results show that entropy and CIR perform equally well as chatter indicators. Based on the normal grinding force, they detect chatter in its early stage, while only cases of strong chatter are detected based on RMS acoustic emission.     

In-process monitoring and detection of chip formation and chatter for CNC turning

In order to realize the intelligent machine tool, an in-process monitoring and detection of cutting states is developed for CNC turning machine to check and improve the stability of the processes. The method developed utilizes the power spectrum density, or PSD of dynamic cutting force measured during cutting. Experimental results suggested that there are basically three types of patterns of PSD when the cutting states are the continuous chip formation, the broken chip formation, and the chatter. The broken chip formation is desired to realize safe and reliable machining.     

Spindle speed ramp-up test: A novel experimental approach for chatter stability detection

Chatter is one of the most limiting factors in improving machining performances. Stability Lobe Diagram (SLD) is the most used tool to select optimal stable cutting parameters in order to avoid chatter occurrence. Its prediction is affected by reliability of input data such as machine tool dynamics or cutting coefficients that are difficult to be evaluated accurately, especially at high speed.This paper presents a novel approach to experimentally evaluate SLD without requiring specific knowledge of the process; this approach is called here Spindle Speed Ramp-up (SSR) test. During this test spindle speed is ramped up, and chatter occurrence is detected by the Order Analysis technique. As result one single test ensures optimal spindle speed identification at one cutting condition, while if few tests are performed the entire SLD could be obtained. Results of the method applied to slotting operation on aluminum are provided and a comparison between different measurements devices is presented. This quick, easy-to-use and efficient test is suitable for industrial application: no knowledge of the process is required, different sensors can be used such as accelerometer, dynamometer or microphone.     

Recognition of chatter with neural networks

Chatter deteriorates surface finish, reduces tool life, and damages machine tools. A chatter development prediction procedure is proposed for the cylindrical turning of long slender bars. The procedure uses two synthetically trained neural networks to recognize the harmonic acceleration signals and their frequency, and based on these observations, the future vibration characteristics of the system are estimated. The developed neural networks are capable of identifying 98% of the harmonic signals with over 90% certainty and estimate their frequencies with less than ±5% error from very short data sequences (only 11 sampled points). The accuracy of the neural networks is equivalent to time domain time series method based approaches; however, the proposed procedure can be implemented very quickly by using commercially available neural network hardware and software, and can use the new neural network chips to make the estimations very quickly by using parallel processors. The validity of the chatter prediction procedure is also demonstrated on the experimental data.     

On-line chatter detection and identification based on wavelet and support vector machine

Chatter is very harmful to precision machining process. To avoid cutting chatter effectively, a method based on wavelet and support vector machine is presented for chatter identification before it has fully developed. Wavelet transform, which can image the information in both the time and frequency domain, is applied as an amplification for the chatter premonition. Each wavelet packet's energy regularly changes during the development of the chatter, which has a time advantage for the identification. Therefore, a two-dimensional feature vector is constructed for chatter detection based on the standard deviation of wavelet transform and the wavelet packet energy ratio in the chatter-emerging frequency band. A support vector machine (SVM) is designed for pattern classification based on the feature vector. The intelligent recognition system, composed of the feature extraction and the SVM, has an accuracy rate of 95% for the identification of stable, transition and chatter state after being trained by the experiment data. The method can be applied in different machining processes.     

Model-based chatter stability prediction for high-speed spindles

The prediction of stable cutting regions is a critical requirement for high-speed milling operations. These predictions are generally made using frequency response measurements of the tool/holder/spindle set, obtained from a non-rotating spindle. However, significant changes in system dynamics occur during high-speed rotation. In this paper, a dynamic model of a high-speed spindle-bearing system is elaborated on the basis of rotor dynamics predictions, readjusted with respect to experimental modal identification. Variations in dynamic behaviour according to speed range are then investigated and determined with accuracy. Dedicated experiments are carried out in order to confirm model results. By integrating the proposed speed-dependant transfer function into the chatter vibration stability approach of Budak–Altintas [S. Tobias, W. Fishwick, Theory of regenerative machine tool chatter, The Engineer February (1958)] a dynamic stability lobes diagram is predicted. The proposed method enables a new stability lobes diagram to be established that takes into account the effect of spindle speed on dynamic behaviour. Significant variations are observed and allow the accurate prediction of cutting conditions. Finally, experiments are performed in order to validate chatter boundary predictions in practice. The proposed modelling approach can also be used to qualify a spindle design in a given machining process and can easily be extended to other types of spindle.     

Two diagnostic models for PLC controlled flexible manufacturing systems

The control of flexible manufacturing systems (FMS's) is generally characterised by logical and sequential functions under the auspices of a programmable logic controller (PLC). Operational faults associated with control processes are often confusing to maintenance personnel at workshop level. This has resulted in the development of automatic diagnosis techniques. In this paper two generic diagnostic models based on the logical function chart and sequential control process of the PLC are developed. With the two complementary models, the major operational faults of PLC controlled FMS's can be diagnosed. Application of the models to a typical FMS is presented.     

一种新型振动下料机的PLC控制

在分析了新型振动下料机采用继电器－接触器控制方式存在不足的基础上，指出了对其进行PLC控制的必要性。分析了该振动下料机的基本构成及工作原理，建立了相应的液压系统。针对下料机正常工作对控制系统要求，确定了PLC的输入量及输出量，建立了PLC控制的硬件接线图，编制了相应的梯形图控制软件。大量实验验证了该控制方案的可行性和实用性。     

Magnetorheological fluid-controlled boring bar for chatter suppression

An innovative chatter suppression method based on a magnetorheological (MR) fluid-controlled boring bar for chatter suppression is developed. The MR fluid, which changes stiffness and undergoes a phase transformation when subjected to an external magnetic field, is applied to adjust the stiffness of the boring bar and suppress chatter. The stiffness and energy dissipation properties of the MR fluid-controlled boring bar can be adjusted by varying the strength of the applied magnetic field. A dynamic model of a MR fluid-controlled boring bar is established based on an Euler–Bernoulli beam model. The stability of the MR fluid-controlled boring system is analyzed, and the simulation results show that regenerative chatter can be suppressed effectively by adjusting the natural frequency of the system. Experiments in different spindle speeds utilizing a MR fluid-controlled boring bar are conducted. Under a 1 Hz square wave current, chatter can be suppressed, as evidenced by the elimination of chatter marks on the machined surfaces and the reduction in the vibration acceleration at the tip of the boring bar.     

Tool wear and chatter detection using the coherence function of two crossed accelerations

Tool wear and chatter have been found to be the main causes of rejects in the machining of super alloys. A novel detection technique to identify both tool wear and chatter in turning a nickel-based super alloy is introduced. It uses the coherence function between two crossed accelerations from the bending vibration of the tool shank. The value of the coherence function at the chatter frequency reaches unity at the onset of chatter. Its values at the first natural frequencies of the tool shank approach unity in the severe tool wear stage. The results are interpreted using the analysis of the coherence function for a single input-two output model. The advantage of using this method is that the thresholds for detecting severe tool wear and chatter can be easily set, since values of coherence function are normalized to a range of between zero and unity, and are also not so susceptible to changing cutting conditions, because the value of the coherence function is close to unity at the onset of chatter and severe tool wear.     

On the chatter frequencies of milling processes with runout

The detection of undesirable vibrations in milling operations is an important task for the manufacturing engineer. While monitoring the frequency spectra is usually an efficient approach for chatter detection, since these spectra typically have a clear and systematic structure, we show in this paper that the stability of the cutting process cannot always be determined from solely viewing the frequency spectra. Specifically, the disturbing effect of the tool runout can sometimes prevent the proper determination of stability. In this paper, we show these cases can be classified by alternative analysis of the vibration signal and the corresponding Poincaré section. Floquet theory for periodic systems is used to explore the influence of runout on the structure of milling chatter frequencies. Finally, the results from theoretical analysis are confirmed by a series of experimental cutting tests.     

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.     

Programming spindle speed variation for machine tool chatter suppression

This paper presents a novel method for programming spindle speed variation for machine tool chatter suppression. This method is based on varying the spindle speed for minimum energy input by the cutting process. The work done by the cutting force during sinusoidal spindle speed variation S³V is solved numerically over a wide range of spindle speeds to study the effect of S³V on stable and unstable systems and to generate charts by which the optimum S³V amplitude ratio can be selected. For on-line application, a simple criterion for computing the optimal S³V amplitude ratio is presented. Also, a heuristic criterion for selecting the frequency of the forcing speed signal is developed so that the resulting signal ensures fast stabilization of the machining process. The proposed criteria are suitable for on-line chatter suppression, since they only require knowledge of the chatter frequency and spindle speed. The effectiveness of the developed S³V programming method is verified experimentally.     

A Model-Based Method to Develop PLC Software for Machine Tools

It is difficult to develop PLC software for modern machine tools due to their increasing functionality and the resulting complexity. One approach to managing this is the model-based development and simulation-aided verification of control software. A suitable method for this is described in this paper. It focuses on a method for modeling the control functionality and a simulation system for commissioning the developed programs using a virtual model of the machine.     

Continuous model for analytical prediction of chatter in milling

A novel analytical approach for prediction of chatter in milling process is presented. Existing approaches use lumped-parameter models to define the dynamics of tools/workpieces. In this paper a continuous beam model is employed for prediction of milling operations dynamics. The tool boundary conditions are elastic support at the tool/holder/spindle interface and free support at the other end. Employing the continuous model eliminates the need for tool tip frequency response function (FRF) measurements in tool-tuning practice, especially in micro-milling, where FRF measurement is practically very difficult. Tool/holder/spindle interface parameters, once identified, can be used for other tool lengths. The impact hammer test is used to identify stiffness and damping parameters of the tool/holder/spindle interface. Using the new analytical approach and picking single-frequency solution (SFS), stability lobes are obtained for a slotting operation. The resulting lobes are compared to those obtained by the well-proven lumped-parameter model. In addition to a good general agreement between the two approaches, the continuous model prediction is more conservative for critical depth of cut, which is attributed to its ability to consider all participating modes in the response and so represents a more accurate representation of the system.     

基于PLC的故障诊断及处理

现场层诊断技术在以往智能诊断研究中往往被忽视。现场控制器迅速发展，在完成对设备和生产过程监控任务的同时，完全具备能力完成必要的诊断工作，并具有快速准确、简化诊断过程的特点。文章对于在智能诊断中最底层的智能设备PLC的诊断功能和处理方法加以阐述，指出处理及设计中的基本原则。并对基于PLC的模糊融合智能诊断进行深入探讨。     

A new perspective and analysis for regenerative machine tool chatter

The paper contains a practical perspective on regenerative machine tool chatter. Chatter is a well known phenomenon, occurrence of which is undesired in manufacturing. Aggressive machining conditions, in the sense of removing more metal rapidly, usually cause chatter. In most cases, these conditions can be determined a priori to the operation. A chatter stability study and its reasoning based on root locus plot analysis of time delayed systems is presented as a new and practical perspective in the field. At the junction of root locus and chatter concepts an area of particular interest to the authors arises: a new method for active vibration suppression, the Delayed Resonator. It is an active vibration absorber tuning of which is achieved utilizing a simple time delayed feedback. The cross linking between the Delayed Resonator study and the subject matter, machine tool chatter, is exciting to share. This is the primary motivation in pursuing this study. One of the highlights of the work appears at the phenomenon called Dual Frequency Delayed Resonator. This feature has been conjectured in the literature using the well known “stability lobes”, but never discussed with detail.     

Optimisation of variable helix tool geometry for regenerative chatter mitigation

It is well known that regenerative chatter can result in excessive tool wear, poor surface finish, and hence limited productivity during metal machining. Various mitigation methods can be applied to suppress chatter; however, the current paper focuses on applying optimal variable helix tool geometry. A semi-discretisation method is combined with Differential Evolution to optimise variable helix end milling tools so as to avoid chatter by modifying the variable helix and variable pitch tool geometry. The semi-discretisation method is first validated experimentally. The numerical optimisation procedure is then used to optimise tool geometry for a machining problem involving a flexible workpiece. The analysis predicted total mitigation of chatter using the optimised variable helix milling tool at a low radial immersion. However, in practice a five fold increase in chatter stability was obtained, compared to the traditional milling tool.     

A passive adaptor to enhance chatter stability for end mills

A design procedure is suggested in this paper to enhance the chatter stability of an end mill cutter. Tool chatter is a well recognized self-excited vibration problem where the excitation force is the cutting force required to machine a workpiece. The cutting force magnitude is proportional to the thickness of the chip removed from the workpiece. The thickness of the chip, on the other hand, is affected by two distinct sets of events, namely, the instantaneous oscillations of the tip of the cutter and the undulations left on the surface from the earlier passes of the cutter. Hence, the excitation force, which is the cause of the oscillations, feeds on the oscillations.In this investigation, an end mill is simplified to be a beam cantilevered from a relatively rigid spindle. A local structural modification technique of the authors is then applied to this beam to suppress its proneness to excessive resonance vibrations. An add-on passive adaptor is developed for this purpose. Numerical simulations with random tip excitation and experimental verification with a scaled model are discussed. Suggested designs promise to enhance the chatter stability of a cantilevered cutter. Cutting tests are being planned for the next stage of work.     

Tuneable clamping table for chatter avoidance in thin-walled part milling

Regenerative chatter is one of the main productivity limiting factors for thin-walled part milling due to their inherently low stiffness and damping properties. This paper presents a new concept called tuneable clamping table (TCT) for thin-walled part machining, which permits controlling the table mode through a rotary spring and eddy current modules. This allows introducing damping to one critical mode of the thin-walled part without any direct contact in the machining area. Two novel frequency tuning strategies to increase the dynamic stiffness and milling stability for the TCT are presented. The concept of TCT is experimentally validated through milling tests, demonstrating a notorious stability increase compared to a regular clamping.