Vibrações mecânicas oferecem grande limitação para a produtividade, qualidade ou mesmo viabilidade das operações de usinagem, especialmente quando se trata das autoexcitadas (chatter). Neste trabalho, foram desenvolvidas estratégias que tem como objetivo diminuir a intensidade de vibrações em processos de torneamento por meio do acoplamento de material piezelétrico ao suporte de ferramenta em conjunto com uso de shunts resistivo, indutivo e resistivo-indutivo em série ou em paralelo, criando assim estruturas eletromecânicas passivamente amortecidas. Para tanto, foram construídos modelos eletromecânicos de parâmetros distribuídos para mostrar a capacidade que tais estruturas eletromecânicas possuem em oferecer um maior amortecimento quando comparadas com estruturas mecânicas convencionais. Com os modelos construídos, foi possível verificar a influência causada pela espessura da camada de material piezelétrico bem como a influência dos shunts no comportamento da estrutura, sendo constatado que camadas mais espessas aumentam a capacidade de amortecimento da estrutura e que os shunts resistivo-indutivo, tanto em série quanto em paralelo, funcionam como um amortecedor dinâmico de vibrações amortecido e oferecem o melhor desempenho. A seguir, construiu-se o diagrama de lóbulos de estabilidade para comparar as estruturas com e sem shunts e observou-se que as estruturas com shunts resistivo-indutivo possuem um melhor desempenho. Também foram executados testes de impacto (tap tests) para a verificação experimental do comportamento da estrutura quando conectadas aos shunts e os resultados mostraram que há um maior amortecimento. Considerando os resultados obtidos, acredita-se que seja possível melhorar o desempenho de processos de torneamento usando material piezelétrico. / Mechanical vibrations offer great limitation for the productivity, quality or even feasibility of the machining operations when chatter is present. In this work it was developed strategies aiming to diminish the intensity of the vibration in turning processes. By coupling a piezoelectric material with a turning tool and by using different associations of resistive and inductive shunt (series or parallel) it was created electromechanical structures passively damped. Electromechanical models of distributed parameters were developed in order to show the capacity that these structures has to offer a greater dumping when compared with conventional mechanical structures. By using these constructed models it was possible to verify the influence of the thickness of the piezoelectric material as well as the influence of shunts in the behavior of structure. It was observed that thicker layers increase the damping capacity of the structure that resistive-inductive shunt (series or parallel) works as a damped dynamic vibration absorber which offer better performance. Latter was developed a stability lobes diagram in order to compare the structures with and without shunts and it was observed that structures connected to resistiveinductive shunt has a better performance. Tap tests were performed for the purpose of study the experimental behavior of the structure connected to shunt and results showed that there is a better damping in this situation. Considering the results obtained, is fair to believe that is possible to improve turning process by using piezoelectric materials.

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.

Orientador: Paulo Roberto de Aguiar / Resumo: O chatter corresponde a movimentos instáveis e caóticos no sistema de usinagem, resultando em flutuação das forças de corte e na impressão de ondulações na superficie da peça usinada. É um fenômeno indesejável ao processo de usinagem, especialmente ao processo de retificação, pois a sua ocorrência acentuada resulta em um produto acabado com tolerâncias dimensionais e geométricas fora dos padrões, ou até mesmo em danos irreversíveis, como por exemplo, alteração na dureza, alta rugosidade e queima superficial da peça usinada. Na literatura, poucos trabalhos tratam da análise e monitoramento do chatter com técnicas de processamento digital de sinais, especialmente de aceleração. O objetivo desse trabalho é propor uma nova técnica de processamento digital utilizando os sinais de aceleração baseados no cálculo da STFT - Short Time Fourier Transform (Transformada de Fourier de curta duração) e na estatística Relação de Potência (ROP – ratio of power), com a finalidade de detecção do fenômeno de chatter na retificação tangencial plana com rebolo superabrasivo de nitreto cúbico de boro (CBN) e óxido de alumínio. Para tanto, ensaios de retificação foram realizados em corpos de prova de aço ABNT 1045. Um acelerômetro piezelétrico foi acoplado ao suporte das peças e sinais de aceleração foram coletados à uma frequência de amostragem de 2MHz. Dentre as variáveis de saída, obteve-se a dureza Vickers (HV), rugosidade média (Ra) e a análise microestrutural das peças retificadas. Os sinais d... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Chatter corresponds to unstable and chaotic movements in the machining system, resulting in fluctuation of the cutting forces. It is a serious and undesired physical phenomenon that occurs in the grinding process during parts manufacturing. The intense occurrence of this phenomenon during machining can generate a finished part outside the dimensional and geometric tolerances or even cause irreversible damage, such as: changes to the hardness, high surface roughness, and thermal damages to the ground part. Few vibration signal processing techniques have been proposed for monitoring chatter during grinding. Thus, the objective of this study is to propose and validate a new vibration signal processing technique based on the short-time Fourier transform (STFT) and the ratio of power (ROP) statistic for the detection of chatter during the tangential surface grinding of ABNT 1045 steel with different grinding wheels. Experimental grinding tests were conducted, and the vibration signals were recorded at 2 MHz. The Vickers hardness (HV), roughness (Ra) and metallography of the ground workpiece surfaces were performed. Subsequently, a digital processing technique based on the STFT and ROP was applied to the vibration signals to extract the characteristics of the chatter in the grinding process. The results show that this technique can be used to characterize over time the spectral patterns of a frequency band related to chatter. The observed patterns have a strong relationship with th... (Complete abstract click electronic access below) / Doutor

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).

Cette thèse commence avec un état de l’art des domaines d’études importants pour notre objectif (différentes techniques usuelles de réduction des vibrations en usinage, méthodes de contrôle actif) avant de valider le principe de contrôle actif du fraisage en se plaçant en repère fixe. On a alors développé un modèle d’état d’une poutre d’Euler Bernoulli perturbée en un point et corrigée en un autre via un actionneur piézoélectrique. Ce modèle a permis d’obtenir plusieurs compensateurs, suivant différentes stratégies de commande. Nous avons par la suite procédé, d’un point de vue expérimental, à l’étude sur un dispositif similaire à notre besoin d’un point de vue de l’actionnement et des ordres de grandeurs (amplification mécanique, gamme de fréquences etc.). Les stratégies de commande robustes que nous avons développé pour pouvoir atténuer les déplacements vibratoires de cette poutre ont conduit à des résultats concluants présentés dans le même chapitre, d’abord en simulation (qui nous a permis une étude comparative), avec ou sans la présence du processus d’usinage, puis expérimentalement. La robustesse de ces stratégies de commande a été étudiée (en simulation) en ajoutant des incertitudes au modèle étudié de différentes manières. Ensuite, nous avons identifié le modèle du système étudié, déterminé les correcteurs correspondants et testé ces derniers sur notre banc d’essai pour valider le bon fonctionnement des différentes stratégies de contrôle utilisées tout le long de cette thèse. Enfin, pour préparer un déploiement de ces stratégies en repère tournant (porte-outil de contrôle actif), nous avons modélisé et implémenté les mêmes démarches pour le cas où l’actionnement se situe en repère tournant et concerne deux axes simultanément, situés dans le plan XY du porte-outil. Nous avons d’abord étudié les vibrations transversales d’une poutre en rotation dans le cas général avant de négliger les phénomènes d’inertie et gyroscopique. En effet, on s’intéresse au contrôle actif du fraisage particulièrement dans les applications de finition, là où on utilise des outils longs de faibles diamètres. Les nouvelles expressions des deux fonctions de transfert de notre système usinant ont été déterminées pour obtenir sa représentation d’état, clé du contrôle actif. La projection du processus de coupe sur le repère tournant est indispensable pour effectuer les simulations du fraisage via le porte outil actif. Ce dernier chapitre met en relief les perspectives de cette thèse, à savoir le contrôle actif du fraisage quelque soit le type de l’opération ou du diamètre de l’outil avec un porte outil mécatronique destiné pour ce genre d’opérations. / This thesis deals with the fields of study which are important for our objective (different usual vibration reduction techniques in machining, active control methods) before validating the principle of active control of milling in a fixed reference. We then developed a state space model of an Euler Bernoulli beam excited at one point and corrected in another one by a piezoelectric actuator. This model allowed us to obtain several compensators, according to different control strategies. We then proceeded from an experimental point of view to study a device similar to our need from an actuating point of view and levels of magnitude (mechanical amplification, frequency range, etc.). ). The robust control strategies that we have developed to attenuate the vibratory displacements of this beam have led to conclusive results presented in the same chapter, first in simulation (which allowed us a comparative study), with and without the cutting process and then experimentally. The robustness of these control strategies was studied (in simulation) by adding uncertainties to the model in different ways. Then we have identified the model of the system, calculated the corresponding compensators and tested them on the test bench in order to validate the good functioning of the different control strategies used in this thesis. Finally, in order to use these strategies in rotating reference (active control tool holder), we have modeled and implemented the same steps for the case where the actuation is located in rotating reference and concerns two axes simultaneously, located in the XY plane of the tool holder. We first studied the transverse vibrations of a rotating beam in the general case before neglecting the inertia and gyroscopic phenomena. Actually, we are interested in the active control of milling, particularly in finishing applications, where long tools of small diameters are used. The new expressions of the two transfer functions of the system have been determined to obtain its state space representation, key of the active control. Projection of the cutting process on the rotating reference is essential to perform milling simulations with the active tool holder. This last chapter highlights the prospects of this thesis,that is the active control of the milling for all kinds of milling operations as well as for different tools with a mechatronic tool holder aimed for this kind of operation.

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

Although a considerable amount of research exists on geometrical aspects of 5-axis flank milling, the dynamics of this efficient milling operation have not yet been given proper attention. In particular, investigating machining chatter in 5-axis flank milling remains as an open problem in the literature. The axial depth of cut in this operation is typically quite large, which makes it prone to machining chatter. In this thesis, chatter in 5-axis flank milling is studied by developing analytical methods of examining vibration stability, generating numerical simulations of the process, and conducting experimental investigations. The typical application of 5-axis milling includes the machining of thermal resistant steel alloys at low cutting speeds, where the process damping dominates the machining vibration. The results of experimental study in this thesis showed that the effect of process damping is even stronger in flank milling due to the long axial engagement. Accordingly, the first part of the thesis is devoted to studying process damping, and in the second part, the modeling of chatter in flank milling is presented. Linear and nonlinear models have been reported in the literature that account for process damping. Although linear models are easier to implement in predicting stability limits, they could lead to misinterpretation of the actual status of the cut. On the other hand, nonlinear damping models are difficult to implement for stability estimation analytically, yet they allow the prediction of “finite amplitude stability” from time domain simulations. This phenomenon of “finite amplitude stability” has been demonstrated in the literature using numerical simulations. In this thesis, that phenomenon is investigated experimentally. The experimental work focuses on uninterrupted cutting, in particular plunge turning, to avoid unduly complications associated with transient vibration. The experiments confirm that, because of the nonlinearity of the process damping, the transition from fully stable to fully unstable cutting occurs gradually over a range of width of cut. The experimental investigation is followed by developing a new formulation for process damping based on the indentation force model. Then, the presented formulation is used to compute the stability lobes in plunge turning, taking into account the effect of nonlinear process damping. The developed lobes could be established for different amplitudes of vibration. This is a departure from the traditional notion that the stability lobes represent a single boundary between fully stable and fully unstable cutting conditions. Moreover, the process damping model is integrated into the Multi-Frequency Solution and the Semi Discretization Method to establish the stability lobes in milling. The basic formulations are presented along with comparisons between the two approaches, using examples from the literature. A non-shallow cut is employed in the comparisons. Assessing the performance of the two methods is conducted using time domain simulations. It is shown that the Semi Discretization Method provides accurate results over the whole tested range of cutting speed, whereas higher harmonics are required to achieve the same accuracy when applying the Multi Frequency Solution at low speeds. Semi Discretization method is modified further to calculate the stability lobes in flank milling with tools with helical teeth. In addition to the tool helix angle and long axial immersion, the effect of instantaneous chip thickness on the cutting force coefficients is considered in the modified formulation of Semi Discretization as well. Considering the effect of chip thickness variation on the cutting force coefficients is even more important in the modeling of 5-axis flank milling, where the feedrate, and consequently the chip thickness, varies at each cutter location. It also varies along the tool axis due to the additional rotary and tilt axis. In addition to the feedrate, the tool and workpiece engagement geometry varies at each cutter location as well. The actual feedrate at each cutter location is calculated by the dynamic processing of the toolpath. The tool and workpiece engagement geometry is calculated analytically using the parametric formulation of grazing surface at the previous and current passes. After calculating the instantaneous chip thickness and tool/workpiece engagement geometry, they are integrated into the Semi Discretization Method in 5-axis flank milling to examine the stability of vibration at each cutter location. While the presented chatter analysis results in establishing stability lobes in 3-axis flank milling, it results in developing a novel approach in presenting the stability of the cut in 5-axis flank milling. The new approach, namely “stability maps”, determines the unstable cutter locations of the toolpath at each spindle speed. The accuracy of established 3-axis flank milling stability lobes and 5-axis stability maps is verified by conducting a set of cutting experiments and numerical simulations.

Analytical force and dynamic models for material removal processes such as end and face milling do not account for material and process related uncertainties such as tool wear, tool breakage and material inhomogeneity. Optimization of material removal processes thus requires not only optimal process planning using analytical models but also on-line monitoring of the process so that adjustments, if needed, can be initiated to maximize the productivity or to avoid damaging expensive parts. In this thesis, a Polyvinylidene Fluoride (PVDF) sensor based process monitoring method that is independent of the cutting conditions and workpiece material is developed for measuring the cutting forces and/or torque in milling. The research includes the development of methods and hardware for wireless acquisition of time-varying strain signals from PVDF sensor-instrumented milling tools rotating at high speeds and transformation of the strains into the measurand of interest using quantitative physics-based models of the measurement system. Very good agreement between the measurements from the low cost PVDF sensors and the current industry standard, piezoelectric dynamometer, has been achieved. Three PVDF sensor rosettes are proposed for measuring various strain components of interest and are shown to outperform their metal foil strain gauge counterparts with significantly higher sensitivity and signal to noise ratio. In addition, a computationally efficient algorithm for milling chatter recognition that can adapt to different cutting conditions and workpiece geometry variations based on the measured cutting forces/torque signals is proposed and evaluated. A novel complex exponential model based chatter frequency estimation algorithm is also developed and validated. The chatter detection algorithm can detect chatter before chatter marks appear on the workpiece and the chatter frequency estimation algorithm is shown to capture the chatter frequency with the same accuracy as the Fast Fourier Transform (FFT). The computational cost of the chatter detection algorithm increases linearly with data size and the chatter frequency estimation algorithm, with properly chosen parameters, is shown to perform 10 times faster than the FFT. Both the cutting forces/torque measurement methodology and the chatter detection algorithm have great potential for shop floor application. The cutting forces/torque measurement system can be integrated with adaptive feedback controllers for process optimization and can also be extended to the measurement of other physical phenomena.

No description available.

Although a considerable amount of research exists on geometrical aspects of 5-axis flank milling, the dynamics of this efficient milling operation have not yet been given proper attention. In particular, investigating machining chatter in 5-axis flank milling remains as an open problem in the literature. The axial depth of cut in this operation is typically quite large, which makes it prone to machining chatter. In this thesis, chatter in 5-axis flank milling is studied by developing analytical methods of examining vibration stability, generating numerical simulations of the process, and conducting experimental investigations. The typical application of 5-axis milling includes the machining of thermal resistant steel alloys at low cutting speeds, where the process damping dominates the machining vibration. The results of experimental study in this thesis showed that the effect of process damping is even stronger in flank milling due to the long axial engagement. Accordingly, the first part of the thesis is devoted to studying process damping, and in the second part, the modeling of chatter in flank milling is presented. Linear and nonlinear models have been reported in the literature that account for process damping. Although linear models are easier to implement in predicting stability limits, they could lead to misinterpretation of the actual status of the cut. On the other hand, nonlinear damping models are difficult to implement for stability estimation analytically, yet they allow the prediction of “finite amplitude stability” from time domain simulations. This phenomenon of “finite amplitude stability” has been demonstrated in the literature using numerical simulations. In this thesis, that phenomenon is investigated experimentally. The experimental work focuses on uninterrupted cutting, in particular plunge turning, to avoid unduly complications associated with transient vibration. The experiments confirm that, because of the nonlinearity of the process damping, the transition from fully stable to fully unstable cutting occurs gradually over a range of width of cut. The experimental investigation is followed by developing a new formulation for process damping based on the indentation force model. Then, the presented formulation is used to compute the stability lobes in plunge turning, taking into account the effect of nonlinear process damping. The developed lobes could be established for different amplitudes of vibration. This is a departure from the traditional notion that the stability lobes represent a single boundary between fully stable and fully unstable cutting conditions. Moreover, the process damping model is integrated into the Multi-Frequency Solution and the Semi Discretization Method to establish the stability lobes in milling. The basic formulations are presented along with comparisons between the two approaches, using examples from the literature. A non-shallow cut is employed in the comparisons. Assessing the performance of the two methods is conducted using time domain simulations. It is shown that the Semi Discretization Method provides accurate results over the whole tested range of cutting speed, whereas higher harmonics are required to achieve the same accuracy when applying the Multi Frequency Solution at low speeds. Semi Discretization method is modified further to calculate the stability lobes in flank milling with tools with helical teeth. In addition to the tool helix angle and long axial immersion, the effect of instantaneous chip thickness on the cutting force coefficients is considered in the modified formulation of Semi Discretization as well. Considering the effect of chip thickness variation on the cutting force coefficients is even more important in the modeling of 5-axis flank milling, where the feedrate, and consequently the chip thickness, varies at each cutter location. It also varies along the tool axis due to the additional rotary and tilt axis. In addition to the feedrate, the tool and workpiece engagement geometry varies at each cutter location as well. The actual feedrate at each cutter location is calculated by the dynamic processing of the toolpath. The tool and workpiece engagement geometry is calculated analytically using the parametric formulation of grazing surface at the previous and current passes. After calculating the instantaneous chip thickness and tool/workpiece engagement geometry, they are integrated into the Semi Discretization Method in 5-axis flank milling to examine the stability of vibration at each cutter location. While the presented chatter analysis results in establishing stability lobes in 3-axis flank milling, it results in developing a novel approach in presenting the stability of the cut in 5-axis flank milling. The new approach, namely “stability maps”, determines the unstable cutter locations of the toolpath at each spindle speed. The accuracy of established 3-axis flank milling stability lobes and 5-axis stability maps is verified by conducting a set of cutting experiments and numerical simulations.