The aim of the thesis is to propose active damping as a potential control strategy for chatter instability in machine tools. The regenerative process theory explains chatter as a closed loop interaction between the structural dynamics and the cutting process. This is considered to be the most dominant reason behind machine tool chatter although other instability causing mechanisms exist. The stability lobe diagram provides a quantitative idea of the limits of stable machining in terms of two physical parameters: the width of contact between tool and the workpiece, called the width of cut and the speed of rotation of the spindle. It is found that the minimum value of the stability limit is proportional to the structural damping ratio for turning operations. This important finding provides the motivation of influencing the structural dynamics by active damping to enhance stability limits of a machining operation. A direct implementation of active damping in an industrial environment may be difficult. So an intermediate step of testing the strategy in a laboratory setup, without conducting real cutting is proposed. Two mechatronic "Hardware in the Loop" simulators for chatter in turning and milling are presented, which simulate regenerative chatter experimentally without conducting real cutting tests. A simple cantilever beam, representing the MDOF dynamics of the machine tool structure constitutes the basic hardware part and the cutting process is simulated in real time on a DSP board. The values of the cutting parameters such as spindle speed and the axial width of cut can be changed on the DSP board and the closed loop interaction between the structure and the cutting process can be led to instability. The demonstrators are then used as test beds to investigate the efficiency of active damping, as a potential chatter stabilization strategy. Active damping is easy to implement, robust and does not require a very detailed model of the structure for proper functioning, provided a collocated sensor and actuator configuration is followed. The idea of active damping is currently being implemented in the industry in various metal cutting machines as part of the European Union funded SMARTOOL project (www.smartool.org), intended to propose smart chatter control technologies in machining operations.

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

Chatter é uma vibração auto-excitada que ocorre durante usinagens e limita a produtividade do processo. Esta instabilidade causa qualidade superficial inaceitável, diminuição da vida da ferramenta e ruído. Estratégias para definição de modelos e controle desta vibração são importantes, devendo ser avaliadas e implementadas. Neste trabalho foram realizados experimentos e características como frequências naturais, respostas em frequência e respostas temporais foram obtidas. Analisando tais resultados é possível a visualização do acoplamento existente nas duas direções de vibração. Uma estratégia de redução de chatter foi implementada, através do uso de shunts passivos conectados ao sistema mecânico por meio de material piezoelétrico, e sua viabilidade foi verificada. A estratégia foi adaptada para ser utilizada nas duas direções de vibração e o resultado da redução da vibração se provou mais eficiente após esta adaptação. Diagramas de fase, respostas temporais e espectros foram obtidos durante a usinagem e um comportamento não-linear se mostrou presente. Após a validação do uso de material piezoelétrico para o controle de chatter, existe a necessidade de modelos numéricos para a descrição do fenômeno, para que controles ativos e mais efetivos possam ser desenvolvidos. Devido ao acoplamento entre as duas direções de vibração e ao comportamento não linear do fenômeno, modelos que contenham tais características foram estudados, modificados e adaptados. Os resultados numéricos obtidos pelos modelos estudados foram então comparados aos resultados experimentais e conclusões sobre similaridades foram apresentadas. Considerando os resultados obtidos, acredita-se que o modelo que melhor representa o sistema real pode ser utilizado para o desenvolvimento de controles ativos, que garantam uma redução mais efetiva do chatter. / Chatter is a self-excited vibration that leads to instability during ongoing machining, which affects productivity. Chatter instability causes poor surface quality, diminishes the tool\'s life and may cause clatter. Therefore, strategies to control chatter and chatter models are highly necessary, and must be evaluated and implemented. In an experimental campaign done during this work, characteristics such as natural frequencies, frequency responses and temporal responses were obtained. Trough these analysis, it was observed that the system presents a coupling in its two normal directions of vibration. One strategy for chatter reduction was then implemented, in which a passive shunt using piezoelectric material was used. The feasibility of this chatter reduction strategy for one direction could be verified. In addition, the strategy was adapted in order to be utilized in both main vibration directions and the results confirmed that this approach grants better results for the reduction of chatter. Phase-planes, temporal responses and spectras could also be derived from the turning experiments and a nonlinear behavior could be seen present. Being verified the possibility of using a piezoelectric material in chatter control, numerical models that describe the phenomena should be pursued, so that more effective active control could be developed. Because the experiments show the mode coupling between two directions and a nonlinear behavior, models that represent such characteristics were studied, modified and adapted. The numerical results from this models were then compared to the experiments and conclusions were drawn. Considering the obtained results, it is believed that the most similar model should be used in the development of active control that could guarantee a better chatter reduction.

Chatter é uma vibração auto-excitada que ocorre durante usinagens e limita a produtividade do processo. Esta instabilidade causa qualidade superficial inaceitável, diminuição da vida da ferramenta e ruído. Estratégias para definição de modelos e controle desta vibração são importantes, devendo ser avaliadas e implementadas. Neste trabalho foram realizados experimentos e características como frequências naturais, respostas em frequência e respostas temporais foram obtidas. Analisando tais resultados é possível a visualização do acoplamento existente nas duas direções de vibração. Uma estratégia de redução de chatter foi implementada, através do uso de shunts passivos conectados ao sistema mecânico por meio de material piezoelétrico, e sua viabilidade foi verificada. A estratégia foi adaptada para ser utilizada nas duas direções de vibração e o resultado da redução da vibração se provou mais eficiente após esta adaptação. Diagramas de fase, respostas temporais e espectros foram obtidos durante a usinagem e um comportamento não-linear se mostrou presente. Após a validação do uso de material piezoelétrico para o controle de chatter, existe a necessidade de modelos numéricos para a descrição do fenômeno, para que controles ativos e mais efetivos possam ser desenvolvidos. Devido ao acoplamento entre as duas direções de vibração e ao comportamento não linear do fenômeno, modelos que contenham tais características foram estudados, modificados e adaptados. Os resultados numéricos obtidos pelos modelos estudados foram então comparados aos resultados experimentais e conclusões sobre similaridades foram apresentadas. Considerando os resultados obtidos, acredita-se que o modelo que melhor representa o sistema real pode ser utilizado para o desenvolvimento de controles ativos, que garantam uma redução mais efetiva do chatter. / Chatter is a self-excited vibration that leads to instability during ongoing machining, which affects productivity. Chatter instability causes poor surface quality, diminishes the tool\'s life and may cause clatter. Therefore, strategies to control chatter and chatter models are highly necessary, and must be evaluated and implemented. In an experimental campaign done during this work, characteristics such as natural frequencies, frequency responses and temporal responses were obtained. Trough these analysis, it was observed that the system presents a coupling in its two normal directions of vibration. One strategy for chatter reduction was then implemented, in which a passive shunt using piezoelectric material was used. The feasibility of this chatter reduction strategy for one direction could be verified. In addition, the strategy was adapted in order to be utilized in both main vibration directions and the results confirmed that this approach grants better results for the reduction of chatter. Phase-planes, temporal responses and spectras could also be derived from the turning experiments and a nonlinear behavior could be seen present. Being verified the possibility of using a piezoelectric material in chatter control, numerical models that describe the phenomena should be pursued, so that more effective active control could be developed. Because the experiments show the mode coupling between two directions and a nonlinear behavior, models that represent such characteristics were studied, modified and adapted. The numerical results from this models were then compared to the experiments and conclusions were drawn. Considering the obtained results, it is believed that the most similar model should be used in the development of active control that could guarantee a better chatter reduction.

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.

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.

The aim of the thesis is to propose active damping as a potential control strategy for chatter instability in machine tools.<p>The regenerative process theory explains chatter as a closed loop interaction between the structural dynamics and the cutting process. This is considered to be the most dominant reason behind machine tool chatter although other instability causing mechanisms exist.<p>The stability lobe diagram provides a quantitative idea of the limits of stable machining in terms of two physical parameters: the width of contact between tool and the workpiece, called the width of cut and the speed of rotation of the spindle. It is found that the minimum value of the stability limit is proportional to the structural damping ratio for turning operations. This important finding provides the motivation of influencing the structural dynamics by active damping to enhance stability limits of a machining operation.<p>A direct implementation of active damping in an industrial environment may be difficult. So an intermediate step of testing the strategy in a laboratory setup, without conducting real cutting is proposed. Two mechatronic "Hardware in the Loop" simulators for chatter in turning and milling are presented, which simulate regenerative chatter experimentally without conducting real cutting tests. A simple cantilever beam, representing the MDOF dynamics of<p>the machine tool structure constitutes the basic hardware part and the cutting process is simulated in real time on a DSP board. The values of the cutting parameters such as spindle speed and the axial width of cut can be changed on the DSP board and the closed loop interaction between the structure and the cutting process can be led to instability.<p><p>The demonstrators are then used as test beds to investigate the efficiency of active damping, as a potential chatter stabilization strategy. Active damping is easy to implement, robust and does not require a very detailed model of the structure for proper functioning, provided a collocated sensor and actuator configuration is followed. The idea of active damping is currently being implemented in the industry in various metal cutting machines as part of the European Union funded SMARTOOL project (www.smartool.org), intended to propose smart chatter control technologies in machining operations. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished