Dissertations / Theses on the topic 'Chatter Stability'

Despite a large demand from industry, a realistic chatter modeling for hard turning has not been available due to the complexity of the problem, which is mainly caused by flank wear and nonlinearity in hard turning. This thesis attempts to develop chatter models for predicting chatter stability conditions in hard turning with the considerations of the effects of flank wear and nonlinearity. First, a linear model is developed by introducing non-uniform load distribution on a tool tip to account for the flank wear effect. Second, a nonlinear model is developed by further incorporating nonlinearity in the structure and cutting force. Third, stability analysis based on the root locus method and the describing function approach is conducted to determine a critical stability parameter. Fourth, to validate the models, a series of experiment is carried out to determine the stability limits as well as certain characteristic parameters for facing and straight turning. From these, it is shown that the nonlinear model provides more accurate predictions than the linear model, especially in the high-speed range. Furthermore, the stabilizing effect due to flank wear is confirmed through a series of experiments. Fifth, to fully account for the validity of linear and nonlinear models, an empirical model is proposed to fit in with the experimental stability limits in the full range of cutting speed. The proposed linear and nonlinear chatter models will help to improve the productivity in many manufacturing processes. In addition, chatter experimental data will be useful to develop other chatter models in hard turning.

The prediction of chatter instability in machining steel and thermal-resistant alloys at low ‎cutting speeds has been difficult due to unknown process damping contributed by the ‎contact mechanism between tool flank and wavy surface finish. This thesis presents ‎modeling and measurement of process damping coefficients, and the prediction of chatter ‎stability limits for turning and milling operations at low cutting speeds. ‎ The dynamic cutting forces are separated into regenerative and process damping ‎components. The process damping force is expressed as a product of dynamic cutting ‎force coefficient and the ratio of vibration and cutting velocities. It is demonstrated that ‎the dynamic cutting coefficient itself is strongly affected by flank wear land. In ‎measurement of dynamic cutting forces, the regenerative force is eliminated by keeping ‎the inner and outer waves parallel to each other while the tool is oscillated using a piezo ‎actuator during cutting. ‎ Classical chatter stability laws cannot be used in stability prediction for general turning ‎with tools cutting along non-straight cutting edges; where the direction and magnitude of ‎the dynamic forces become dependent on the depth of cut and feed-rate. A new dynamic ‎cutting force model of regeneration of chip area and process damping, which considers ‎tool nose radius, feed–rate, depth of cut, cutting speed and flank wear is presented. The ‎chatter stability is predicted in the frequency domain using Nyquist stability criterion.‎ The process damping is considered in a new dynamic milling model for tools having ‎rotating but asymmetric dynamics. The flexibility of the workpiece is studied in a fixed ‎coordinate system but the flexibility of the tool is studied in a rotating coordinate system. ‎The periodic directional coefficients are averaged, and the stability of the dynamic ‎milling system is determined in the frequency domain using Nyquist stability criterion. ‎ The experimentally proven, proposed stability models are able to predict the critical ‎depth of cut at both low and high cutting speeds.‎

The objective of this study is to develop and implement practical and accurate methods for prediction of the workpiece dynamics during a complete machining cycle of the workpiece, so that FRFs of the workpiece can be used in chatter stability analysis. For this purpose, a structural modification method is used since it is an efficient tool for updating FRFs due to structural modifications. The removed mass is considered as a structural modification to the finished workpiece in order to determine the FRFs at different stages of the process. The method is implemented in a computer code and demonstrated on representative parts such as turbine blades. The predictions are compared and verified with the data obtained using FEA. The FRFs are used in chatter stability analyses, and the effect of part dynamics on stability is studied.

Chatter is unwanted since it causes deteriorating effects on the milling process. Stability lobe diagrams are developed in order to determine the stable cutting conditions at which chatter-free machining can be made. The need of cutting away more chips to make milling operations quicker has brought the concept of high-speed milling. This increased the importance of estimating stability lobe diagrams of the milling process more accurately. The state-of-art chatter and spindle-toolholder-tool models predict the stability lobe diagram for milling process quite effectively. However, sometimes chatter might occur even at cutting conditions selected using theoretically obtained stability lobe diagrams. One of the reasons for that may be nonlinearities in the system. This being the motivation, in this work, nonlinearities at the bearings of spindle-toolholder-tool system are investigated. In this thesis, cubic nonlinearity is assumed to represent stiffness of a bearing in a spindle-toolholder-tool system. Effects of nonlinearity on stability lobe diagram of a milling process are studied by using the mathematical model developed for such a system. Frequency response function of spindle-toolholder-tool system without bearings is obtained using Timoshenko beam model. Then, bearings are modeled by using describing function theory and coupled to the dynamics of spindle-toolholder-tool modeled. Solution of the equations of motion of the system in frequency domain is obtained via Newton'
s method with ALC. It is an effective frequency domain method in which turning points on frequency response function are traced. This is important for the system studied, as bearing nonlinearity may introduce turn backs in the response of the system. Case studies are carried out to study the effects of bearing nonlinearity on stability lobe diagram. The effects of the following factors are studied: Magnitude of cutting force, degree of nonlinearity and number of teeth on cutter. Displacement amplitude dependent stiffness of bearings affects the dynamic response due to rigid body modes of the system. It is observed that an increase in cutting force magnitude or in coefficient of bearing nonlinearity results in increase of natural frequencies, thus showing hardening behavior. Shifting of frequencies in the response curve shifts stability lobes related to the affected modes, to the right. For increased number of flutes on cutter, effect of nonlinearity at bearings on stability of the milling process becomes lower. Experimental studies to determine the changes in dynamics of a system during cutting are also carried out in this thesis. Inverse chatter analysis is conducted to obtain modal parameters of a single-degree-of-freedom system using the experiment data. Decrease in natural frequency is observed at high cutting speeds for the particular spindle used. This shift may be due to speed-dependent bearing dynamics and real time adjustment of preload on bearings.

Self-excited chatter vibration in machining is one of the most important limitations on utilizing the increasing productivity of modern machine tools. In order to predict stable depth of cuts at high cutting speeds, the stability lobe diagram for a spindle-tool holder-tool combination must be developed. The frequency response function (FRF) of the system must be known for analytical prediction of the stability lobe diagrams. When the flexibility of the workpiece is important, the workpiece itself should be included in the system model by considering the variation of its dynamics at different stages of the machining process. In this thesis, an exact structural modification method is used to find the frequency response functions of the workpiece to be machined at every stage of the machining process. In order to obtain the system matrices and the modal parameters of the original structure, a commercial finite element program MSC. Marc©
is used. The frequency response functions of workpiece are calculated by using the computer program developed in this thesis, and are compared with the ones found by MSC. Marc©
. The stability lobe diagram of the system is obtained by combining the FRFs of the tool with those of the workpiece. The effects of the dynamic of the workpiece on the stability lobe diagrams are studied extensively by using the results of case studies presented in this thesis. In order to increase productivity, minimum chatter-free machining times are also calculated for different cases. For this purpose the effects of the different radial depth of cuts and different cutting strategies on the stability and the machining time are examined with various case studies.

The diploma work deals with a mathematical description of vibration and its generation when machining. Moreover, some techniques of modal parameters measurement in the theoretical part are included. The practical part is designed and based on the measured natural frequencies of the machine with specific tool and materials. In conclusion, a lobe diagram stability for semiautomatic lathe SPN 12 CNC and selected machining operation is specified by means of apparatus.

Factors such as environmental requirements and fuel efficiency have pushed aerospace industry to develop reduced-weight engine designs and thereby light-weight and thin-walled components. As component wall thickness gets thinner and the mechanical structures weaker, the structure becomes more sensitive for vibrations during milling operations. Demands on cost efficiency increase and new ways of improving milling operations must follow. Historically, there have been two “schools” explaining vibrations in milling. One states that the entry angle in which the cutting insert hits the work piece is of greater importance than the exit angle. The other states that the way the cutter leaves the work piece is of greater importance than the cutter entry. In an effort to shed some light over this issue, a substantial amount of experiments were conducted. Evaluations were carried out using different tools, different tool-to-workpiece offset positions, and varying workpiece wall overhang. The resultant force, the force components, and system vibrations have been analyzed. The first part of this work shows the differences in force behavior for three tool-to-workpiece geometries while varying the wall overhang of the workpiece. The second part studies the force behavior during the exit phase for five different tool-to-workpiece offset positions while the overhang is held constant. The workpiece alloy throughout this work is Inconel 718. As a result of the project a spread sheet milling stability prediction model is developed and presented. It is based on available research in chatter theory and predicts the stability for a given set of variable input parameters.

QC 20121206

Diplomová práce se zabývá analýzou produktivity a efektivity řezného procesu frézování. Pro zjištění kritické hloubky třísky byla analyzována reálná frézka. Model frézky byl vytvořen v programu Autodesk Inventor. Analýza řezného procesu probíhala v programu Ansys Workbench. Výsledky byly použity pro sestavení stabilitních diagramů. Po vyhodnocení výsledků byly navrženy dva přístupy pro zefektivnění procesu frézování. Vliv těchto změn na produktivitu řezného procesu byl ověřen porovnáním výsledků s předchozí analýzou.

Chatter is one of the major problems in today’s milling processes. Theoretical models to calculate stability lobes are used to predict and avoid chatter onset. However, current predictions are not accurate enough and significant deviations between predicted and experimentally observed stability limits have been reported.The causes for these deviations are diverse and can be the result of the sum of multiple effects. According to previous works, main errors in stability prediction are related to lack of knowledge about double period instability (flip lobes) and inappropriate determination of dynamic parameters through standard experimental characterization techniques. This Thesis deals with these two problems that affect accurate chatter prediction, contributing with new knowledge and calculation methods for double period type lobes and developing a new methodology for a more accurate dynamic response identification. Nevertheless, an accurate chatter stability prediction does not necessarily imply an optimum use of the machine to maximize productivity, as it is required in current production environments. For this reason, three novel process stabilization techniques are proposed for those cases in which the designed machining process is subject to chatter vibrations.
El chatter és avui en dia un dels principals problemes en els processos de fresat. Per predir i evitar la seva aparició es disposa de models teòrics per al càlcul dels lòbuls d'estabilitat. No obstant això, les prediccions realitzades amb els models d'estabilitat de fresat no són robustes, presentant casos en què les desviacions entre la predicció i la realitat són importants. Les causes d'aquestes desviacions són variades i poden ser degudes a la suma de múltiples efectes. A la vista dels estudis previs realitzats, els principals errors es troben en l'omissió de lòbuls de doble període (lòbuls flip) i errors en la determinació experimental dels paràmetres dinàmics del sistema mitjançant mètodes tradicionals. Aquesta Tesi aborda aquests dos problemes principals en la predicció, aportant nous coneixements sobre el chatter de doble període i desenvolupant una nova metodologia per a un càlcul més precís de la resposta dinàmica del sistema. No obstant això, una predicció precisa de les condicions que donen lloc a un procés de fresat estable no garanteix l'aprofitament òptim de la màquina per maximitzar la productivitat, tal com s'exigeix en l'entorn productiu actual. Per això, es proposen tres noves tècniques per a l'eliminació de chatter en aquells casos en què, el procés de mecanitzat dissenyat estigui sota el perillós influx del chatter.

Chatter vibration is a common problem for the manufacturing industry that limits the productivity, accuracy and surface quality of machined parts. This study is focused on the out of process methods, such as Stability Lobe Diagrams (SLD), that ensure the selection of the optimal cutting parameters in which the machining process is stable. Previous studies have found that the dynamic properties of the spindle change with the rotational speed. This fact can also affect the accuracy of the SLD predictions, since, the traditional structural dynamic tests such as the Experimental Modal Analysis (EMA) are carried out at static state. An alternative method for the calculation of speed - dependant SLD using a Contactless Excitation Response System (CERS) was proposed. The modal characteristics, such as natural frequencies and damping ratio were determined by EMA tests carried out at idle state whereas CERS measurements were performed at increasing rotational speeds up to 14000 rpm. Subsequently, the SLD at static and dynamic state were computed. Finally, it was concluded that there was not a significant variation of the dynamic properties and SLD prediction with spindle speed at the tested speed range (0 rev/min to 14000 rev/min).
Chatter är ett vanligt problem inom tillverkningsindustrin som begränsar produktiviteten och minskar noggrannheten och kvalitén på bearbetade ytor. Denna studie fokuserar på processkilda metoder, till exempel stabilitetsdiagram (SLD), vilka säkerställer valet av optimala skärparametrar för en stabil skärprocess. Tidigare studier har visat att spindelns dynamiska egenskaper är beroende av rotationshastigheten. Detta påverkar även noggrannheten vid skattningen av SLD eftersom traditionella strukturdynamiska tester, som experimentell modalanalys (EMA), utförs under statiskt tillstånd. En alternativ metod för bestämning av hastighetsberoende SLD med hjälp av ett beröringsfritt excitering- och svarssystem (CERS) föreslås. De modala egenskaperna, som till exempel egenfrekvens och dämpning, bestämdes med hjälp av EMA med stillastående spindel medan mätningar med CERS utfördes med ökad rotationshastighet upp till 14000 varv/min. Efter detta beräknades SLD för de båda fallen. Till sist drogs slutsatsen att testerna inte påvisade någon större skillnad, vare sig dynamiska egenskaper eller SLD skattning, för spindelhastigheter inom det testade intervallet (0 till 14000 varv/min).

La millora de la productivitat i la qualitat són indubtablement dues de les principals exigències del sector productiu modern i factors clau per la competitivitat i la supervivència. Dins aquest sector,la fabricació per arrancada de material juga encara avui en dia un paper protagonista tot i l'aparició de noves tècniques de conformat per addició.Indústries com l'aeronàutica, l'automobilística,la del motlle o l'energètica, depenen en bona part de les prestacions de les màquines-eina. Aquesta Tesi aborda dos aspectes rellevants quan es tracta de millorar de la productivitat i la qualitat del sector productiu: el problema del fimbrament, més conegut per la denominació anglosaxona chatter,i la monitorització de la rugositat superficial en el mecanitzat a alta velocitat.
Productivity and quality improvement are undoubtedly two of the main demands of the
modern manufacturing sector and key factors for competitiveness and survival. Within this sector, material removal processes play, still nowadays, a principal role despite the emergence of additive manufacturing techniques. Industries such as aerospace, automotive, molds and dies or energy largely depend on machine tools performance for improved productivity and quality. This Thesis is focused on two important aspects when it comes to improving productivity and quality of the manufacturing sector: chatter problem, and surface roughness monitoring in high speed milling.

Ce travail de thèse se concentre sur l’étude du comportement dynamique des tubes minces durant leur usinage par tournage et notamment sur la survenue du broutement régénératif. L’étude de l’usinage de ce type de pièce est pertinente car ces pièces font l’objet d’une large diffusion dans divers domaines industriels tels que la construction aéronautique (notamment moteurs), la construction navale, la production de fusées. Ces pièces ayant une faible rigidité, il est fréquent que des vibrations indésirables se produisent en cours d’usinage. Il est donc intéressant d’être à même de les prédire pour les éviter.L’approche proposée vise à permettre un choix rapide et efficace des conditions de coupe, et notamment des profondeurs de passe, pour cette opération d’usinage. Pour cela nous proposons de mettre en place un modèle mécanique analytique du tube (modèle de coque mince utilisant un nombre réduit de degrés de liberté) de manière à réduire les coûts numériques et à faciliter l’analyse du phénomène. L’impact de la taille du modèle sur les résultats est étudié (nombre de formes propres) ainsi que de la prise en compte de l’enlèvement de matière (évolution du comportement dynamique) et du déplacement de l’outil. Afin de valider l’approche une expérience a été mise en place et est également présentée dans ce mémoire
This work focuses on the study of the dynamic behavior of thin tubes during their machining by turning and gives particular emphasis on the occurrence of regenerative chatter. The study of machining of this type of workpiece is relevant because they are widely used in various industrial fields such as aircraft construction (including engines), shipbuilding, rocket production. As these parts have low rigidity, it is common that undesirable vibrations occur during machining. It is therefore of interest to be able to predict them in order to avoid them.The proposed approach is designed to enable a fast and efficient selection of cutting conditions, including cutting depths for this machining operation. We thus propose to elaborated an analytical model for the dynamics of the tube (thin shell based model using a reduced number of degrees of freedom) to reduce the numerical costs and to facilitate the analysis of the phenomenon. The impact of the size of the model on the results is studied (number of shape functions), as well as the impact of material removal (evolution of the dynamic behavior) and of the motion of the tool. An experiment, presented in this thesis, was also set up in order to validate the approach

In machining centers, with the increasing trends in high precision machining, chatter has become an important problem which results in poor surface finish and low material removal rate. Chatter can be avoided with stability diagrams which provide the stable regions in the machining process for the depth of cut and spindle speed combinations. In order to obtain stability diagrams, tool point frequency response function (FRF) of the system should be obtained. Throughout this study, contact parameters which are the most critical part of the analytical modeling of spindle-holder-tool assembly in order to obtain tool point FRF, are examined. For the accurate identification of the contact parameters, a recently suggested closed form approach based on measured FRFs is improved and applied to real structures by solving several application problems. In addition to the identification of contact parameters from experimental results, in order to eliminate the dependency on experiments, artificial neural networks are used to predict contact parameters for cases for which no experiments were carried out. By using trained neural network, contact parameters are predicted for the first seen combination of tool gauge length and diameter with a high accuracy. Such an application will have an important contribution to the machining stability studies since elimination of dependency on experiments will make it possible to predict stability diagrams for different combinations of spindle, holder and tool without performing any experiments. Additionally, since accurate identification of contact parameters, thus tool point FRFs and stability diagrams are highly dependent on accuracy of the performed experiments, possible errors due the mass of the accelerometers are also investigated. In order to compensate the mass effect of the accelerometers, a structural modification with matrix inversion method is applied to the accelerometer based results.

Regenerative chatter is a well-known machining problem that results in unstable cutting process, poor surface quality, reduced material removal rate and damage on the machine tool itself. Stability lobe diagrams supply stable depth of cut &
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spindle speed combinations and they can be used to avoid chatter. The main requirement for generating the stability lobe diagrams is the system dynamics information at the tool tip in the form of point frequency response function (FRF). In this work, an analytical model that uses structural coupling and modification methods for modeling the dynamics of spindle-holder-tool assemblies in order to obtain the tool point FRF is presented. The resulting FRF obtained by the model can be used in the existing analytical and numerical models for constructing the stability lobe diagrams. Timoshenko beam theory is used in the model for improved accuracy and the results are compared with those of Euler-Bernoulli beam theory. The importance of using Timoshenko beam theory in the model is pointed out, and the circumstances, under which the theory being used in the model becomes more important, are explained. The model is verified by comparing the results obtained by the model with those of a reliable finite element software for a case study. The computational superiority in using the model developed against the finite element software is also demonstrated. Then, the model is used for studying the effects of bearing and contact dynamics at the spindle-holder and holder-tool interfaces on the tool point FRF. Based on the results of the effect analysis, a new approach is suggested for the identification of bearing and interface parameters from experimental measurements, which is demonstrated on a spindle-holder-tool assembly. The model is also employed for studying the effects of design and operational parameters on the tool point FRF, from the results of which, suggestions are made regarding the design of spindles and selection of operational parameters. Finally, it is experimentally demonstrated that the stability lobe diagram of an assembly can be predicted pretty accurately by using the model proposed, and furthermore the stability lobe diagram can be modified in a predictable manner for improving chatter stability.

In this thesis a new frequency domain approach for the analysis of time delay systems is presented. After linearization of a nonlinear delay differential equation (DDE) with constant distributed delay around a constant or periodic reference solution the so-called Hill-Floquet method can be used for the analysis of the resulting linear DDE. In addition, systems with fast or slowly time-varying delays, systems with variable transport delays originating from a transport with variable velocity, and the corresponding spatially extended systems are presented, which can be also analyzed with the presented method. The newly introduced Hill-Floquet method is based on the Hill’s infinite determinant method and enables the transformation of a system with periodic coefficients to an autonomous system with constant coefficients. This makes the usage of a variety of existing methods for autonomous systems available for the analysis of periodic systems, which implies that the typical calculation of the monodromy matrix for the time evolution of the solution over the principle period is no longer required. In this thesis, the Chebyshev collocation method is used for the analysis of the autonomous systems. Specifically, in this case the periodic part of the solution is expanded in a Fourier series and the exponential behavior of the solution is approximated by the discrete values of the Fourier coefficients at the Chebyshev nodes, whereas in classical spectral or pseudo-spectral methods for the analysis of linear periodic DDEs the complete solution is expanded in terms of basis functions. In the last part of this thesis, new results for three applications with time delay effects are presented, which were analyzed with the presented methods. On the one hand, the occurrence of diffusion-driven instabilities in reaction-diffusion systems with delay is investigated. It is shown that wave instabilities are possible already for single-species reaction diffusion systems with distributed or time-varying delay. On the other hand, the stability of metal cutting vibrations at machine tools is analyzed. In particular, parallel orthogonal turning processes with multiple discrete delays and turning processes with a time-varying delay due to a spindle speed variation are studied. Finally, the stability of the synchronized solution in networks with heterogeneous coupling delays is studied. In particular, the eigenmode expansion for synchronized periodic orbits is derived, which includes an extension of the classical master stability function to networks with heterogeneous coupling delays. Numerical results are shown for a network of Hodgkin-Huxley neurons with two delays in the coupling
In dieser Dissertation wird ein neues Verfahren zur Analyse von Systemen mit Totzeiten im Frequenzraum vorgestellt. Nach Linearisierung einer nichtlinearen retardierten Differentialgleichung (DDE) mit konstanter verteilter Totzeit um eine konstante oder periodische Referenzlösung kann die sogenannte Hill-Floquet Methode für die Analyse der resultierende linearen DDE angewendet werden. Darüber hinaus werden Systeme mit schnell oder langsam variierender Totzeit, Systeme mit einer variablen Totzeit, resultierend aus einem Transport mit variabler Geschwindigkeit, und entsprechende räumlich ausgedehnte Systeme vorgestellt, welche ebenfalls mit der vorgestellten Methode analysiert werden können. Die neu eingeführte Hill-Floquet Methode basiert auf der Hillschen unendlichen Determinante und ermöglicht die Transformation eines Systems mit periodischen Koeffizienten auf ein autonomes System mit konstanten Koeffizienten. Dadurch können zur Analyse periodischer Systeme auch eine Vielzahl existierender Methoden für autonome Systeme genutzt werden und die Berechnung der Monodromie-Matrix für die Lösung des Systems über eine Periode entfällt. In dieser Arbeit wird zur Analyse des autonomen Systems die Tschebyscheff-Kollokationsmethode verwendet. Im Speziellen wird bei diesem Verfahren der periodische Teil der Lösung in einer Fourierreihe entwickelt und das exponentielle Verhalten durch die Werte der Fourierkoeffizienten an den Tschebyscheff Knoten approximiert, wohingegen bei klassischen spektralen Verfahren die komplette Lösung in bestimmten Basisfunktionen entwickelt wird. Im Anwendungsteil der Arbeit werden neue Ergebnisse für drei Beispielsysteme präsentiert, welche mit den vorgestellten Methoden analysiert wurden. Es wird gezeigt, dass Welleninstabilitäten schon bei Einkomponenten-Reaktionsdiffusionsgleichungen mit verteilter oder variabler Totzeit auftreten können. In einem zweiten Beispiel werden Schwingungen an Werkzeugmaschinen betrachtet, wobei speziell simultane Drehbearbeitungsprozesse und Prozesse mit Drehzahlvariationen genauer untersucht werden. Am Ende wird die Synchronisation in Netzwerken mit heterogenen Totzeiten in den Kopplungstermen untersucht, wobei die Zerlegung in Netzwerk-Eigenmoden für synchrone periodische Orbits hergeleitet wird und konkrete numerische Ergebnisse für ein Netzwerk aus Hodgkin-Huxley Neuronen gezeigt werden

碩士
國立高雄應用科技大學
模具工程系
106
Chatter vibration is the relative interaction between the cutting tool and the workpiece under specific cutting conditions which can result in waves on the machined surface. Severely chatter vibration can even harm the machine structure. As the cutting speed gets higher, it is even easier to be observed. Therefore, it is important to understand the cutting machine structure configuration and the cutting parameters to avoid the occurrence. In this study, the two flute and four flute high speed steel end-mill were applied to slot milling of the CNS 6061 aluminum alloy. The frequency response function (FRF) of the tool point was first obtained by tap test. Both the Average tooth angle and the Fourier series approach were then used to calculate the stability lobe diagram(SLD) by the FRF results. Experiments were finally carried out to verify the accuracy of cutting stability lobe diagram. Experiment and measurement results show there are three factors affecting the stability lobe diagram, which are natural frequency, elastic constant and damping ratio. The natural frequency affects the density of the stability lobe diagram, while the elastic constant and damping ratio affect the size of the stability lobe diagram. In the case of slotting in this research, the stability lobe diagram calculated by Fourier series approach is more accurate than the Average tooth angle approach. The experimental results are consistent with the calculated SLD results.

碩士
中原大學
機械工程研究所
102
Through this study, the actual milling experiment to capture the vibration acceleration, time domain and frequency domain analysis of vibration changes flutter occurs, the signal changes by transient analysis and spectrum analysis dual threshold formulate chatter judgment rule, while taking advantage of cutting stability diagram auxiliary determine whether the cutting conditions in the region chatter to improve the accuracy of judgment. In terms of inhibition of chatter, chatter suppression in order to allow more efficient, is to establish a stable figure in cutting chatter direct identification method for inhibiting the cutting speed to make a one-time suppress chatter. Research has also established a two-way data transfer controller capture module, to achieve real-time monitoring and control functions via TCP / IP protocol. The method of development, in addition to PLC is compiled, the research to Visual C # to establish the overall monitoring system in Visual C ++ environment, can be CNC controller data transmission, recording real-time vibration signal and cutting unusual information, draw cutting stability graph model and online chatter real-time monitoring and other functions.According to experimental results, show that the system can quickly and accurately find and make immediate chatter suppression.

Chatter, unstable vibration during machining, damages the tool and workpiece. A proper selection of spindle speed and depth of cut are required to prevent chatter during machining. Such proper cutting conditions are usually determined using vibration models of the machining process. Nonetheless, uncertainties in modeling or changes in dynamics during the machining operations can lead to unstable machining vibrations, and chatter may arise even when stable cutting conditions are used in the process planning stage. As a result, online chatter monitoring systems are key to ensuring chatter-free machining operations. Although various chatter monitoring systems are described in the literature, most of the existing methods are suitable for detecting chatter after vibrations become unstable. In order to prevent poor surface finish resulting from chatter marks during the finishing stages of machining, a new monitoring system that is capable of predicting the occurrence of chatter while vibrations are still stable is required. In this thesis, a new approach for predicting the loss of stability during stable turning operations is developed. The new method is based on the identification of the dynamics of self-excited vibrations during turning operations using Operational Modal Analysis (OMA). The numerical simulations and experimental results presented in this thesis confirm the possibility of using Operational Modal Analysis as an online chatter prediction method during stable machining operations.
Graduate

碩士
國立虎尾科技大學
機械與電腦輔助工程系碩士班
105
In recent years, with the rapid progress of industrial technology, high-speed machining has gradually become a trend, but for high-speed machining, the stability of the machine would decrease with spindle speed rising. The traditional experience method has not been able to deal with the variable processing conditions. The chatter prediction technique can be used to avoid the unstable speed area by numerical means, and obtaining the stable cutting conditions. Compared to domestic, this technology is more mature in foreign countries. The reason is that the technical standard is high, complex algorithm, the cutting parameters are not easy to obtain, and the calculation time is longer and so on, so most can not be widely used in the actual process. Therefore, this study intends to develop a simple and fast milling chatter prediction system that provides a basis for selection of machining conditions.This paper is divided into three parts. First, construction a relationship between the spindle speed and cutting depth by regenerative chatter theory and frequency respond function. Second, the modal parameters of the cutting tool system are estimated by curve fitting.Furthermore, we develop a processing stability prediction system with commercial software LabVIEW, The system include multi-modal parameter identification and chatter stability curve drawing function. In this study, uses the zero-order analysis method by professor Altintas and Lowest envelop method (LEM) to obtain the chatter stability curve diagram. Which can calculate by separately considering different dominant modes from the experiment of dynamic stiffness. The second part focuses on the identification of modal parameters, the experiment uses the hammer impact test to obtain the tool frequency response characteristics required for the analysis, and uses the global rational polynomial (GRFP) to perform the curve fitting of the response function to identify the damping ratio. Finally, the predicted chatter boundaries are compared to the experimental results in order to validate the modal and the stability analysis. The experiments show that under the condition of 100% radial feed, three of the eleven test points are not consistent with the predicted results, and the initial prediction accuracy is about 72%. In the phase difference measurement section, when the chatter occurs, the major frequency of the phase difference is greater than 60 degrees and the fluctuation range is also larger；In the case of stability, most of its major frequency located on the frequency of the tool passing frequency and its multiplier and the phase difference of the abnormal frequency is not more than 30 degrees.

博士
國立成功大學
機械工程學系碩博士班
93
In this study, the workpiece was assumed to be elastic and deformable under external force generated from the cutting tool. By considering the workpiece under investigation as an Euler-Bernoulli beam, then a second order partial differential equation was established to simulate the lateral forced vibration. Additionally, a model for the cutting tool vibration is also developed in a second order ordinary differential equation formulation. These developed models permitted the full analysis and discussion of the interaction between the workpiece and the tool. For the calculation of cutting force, the modes of “shear surface was a plane” and of orthogonal cutting were utilized. For the chatter mechanism, considering the regenerative chatter of the self-excited vibration, system characteristic equation could be yielded from the Laplace transform of the dynamic equations. The stability analyses by frequency domain responses and Nyquist charts could imply the governing equations of critical chip width for different materials, sizes and spindle speeds. 　In the machining process, considering the slot cutting, two cases of with or without the tailstock were both discussed. In the cutting force analysis, two cases of with or without the previous cycle deflection were also be discussed. 　In the simulation, the differences of the critical chip widths were studied under different conditions. It was found that, no matter using the tailstock or not; and no matter considering the previous cycle deflection or not, the following conclusions were always obtained. 1.The greater the deflection and the bigger the critical chip width. 2.The smaller the natural frequency and the larger the critical chip width. 3.The critical chip width of bronze is greater than the steel. 　Besides the above results, it was also found that the critical chip width when using the tailstock was always smaller than the case without using the tailstock. Also, the critical chip width with the previous cycle deflection was always greater than the without case.

abstract: In this research, a new cutting edge wear estimator for micro-endmilling is developed and the reliabillity of the estimator is evaluated. The main concept of this estimator is the minimum chip thickness effect. This estimator predicts the cutting edge radius by detecting the drop in the chip production rate as the cutting edge of a micro- endmill slips over the workpiece when the minimum chip thickness becomes larger than the uncut chip thickness, thus transitioning from the shearing to the ploughing dominant regime. The chip production rate is investigated through simulation and experiment. The simulation and the experiment show that the chip production rate decreases when the minimum chip thickness becomes larger than the uncut chip thickness. Also, the reliability of this estimator is evaluated. The probability of correct estimation of the cutting edge radius is more than 80%. This cutting edge wear estimator could be applied to an online tool wear estimation system. Then, a large number of cutting edge wear data could be obtained. From the data, a cutting edge wear model could be developed in terms of the machine control parameters so that the optimum control parameters could be applied to increase the tool life and the machining quality as well by minimizing the cutting edge wear rate. In addition, in order to find the stable condition of the machining, the stabillity lobe of the system is created by measuring the dynamic parameters. This process is needed prior to the cutting edge wear estimation since the chatter would affect the cutting edge wear and the chip production rate. In this research, a new experimental set-up for measuring the dynamic parameters is developed by using a high speed camera with microscope lens and a loadcell. The loadcell is used to measure the stiffness of the tool-holder assembly of the machine and the high speed camera is used to measure the natural frequency and the damping ratio. From the measured data, a stability lobe is created. Even though this new method needs further research, it could be more cost-effective than the conventional methods in the future.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2019