Excessive vibrations continue to be a major hurdle in improving machining efficiency and achieving stable high speed cutting. To overcome detrimental vibrations, an enhanced understanding of the underlying nonlinear dynamics is required. Cutting instability is commonly studied through modeling and analysis which incorporates linearization that obscures the true nonlinear characteristics of the system which are prominent at high speeds. Thus to enhance cutting dynamics knowledge, a comprehensive nonlinear turning model that includes tool-workpiece interaction is experimentally validated using a commercial laser vibrometer to capture tool and workpiece vibrations. A procedure is developed to use instantaneous frequency for experimental time-frequency analysis and is shown to thoroughly characterize the underlying dynamics and identify chatter. For the tests performed, chatter is associated with changing spectral components and bifurcations which provides a view of the underlying dynamics not experimentally observed before. Validation of the turning model revealed that the underlying dynamics observed experimentally are accurately captured, and the coupled tool-workpiece chatter vibrations are simulated. The stability diagram shows an increase in the chatter-free limit as the spindle speed increases until 1500rpm where it begins to level out. At high speeds the workpiece dominates the dynamics, and excessive workpiece vibrations create another stability limit to consider. Thus, workpiece dynamics should not be neglected in analyses for the design of machine tools and robust control laws. The chip formation mechanisms and high speeds make micro-milling highly non-linear and capable of producing broadband frequencies that negatively affect the tool. A nonlinear dynamic micro-milling model is developed to study the effect of parameters on tool performance through spectral analysis using instantaneous frequency. A lumped mass-spring-damper system is assumed for modeling the tool, and a slip-line force mechanism is adopted. The effective rake angle, helical angle, and instantaneous chip thickness are accounted for. The model produced the high frequency force components seen experimentally in literature. It is found that increasing the helical angle decreased the forces, and an increase in system stiffness improved the dynamic response. Also, dynamic instability had the largest effect on tool performance with the spindle speed being the most critical parameter.

Vibration problems are still the major constraint in modern machining processes that seek higher material removal rate, shorter process time, longer tool life and better product quality. Depending on the process, the weaker structure element can be the tool/tool holder, workpiece/fixture or both. When the tool/tool holder is the main source of vibration, the stability limit is determined in most cases by the ratio of length-to-diameter. Regenerative chatter is the most significant dynamic phenomenon generated through the interaction between machine tool and machining process. As a rule of thumb, the ratio between the tool’s overhang length and the tool’s diameter shouldn’t exceed 4 to maintain a stable machining process while using a conventional machining tool. While a longer tool overhang is needed for specific machining operations, vibration damping solutions are required to ensure a stable machining process. Vibration damping solutions include both active and passive damping solutions. In the passive damping solutions, damping medium such as viscoelastic material is used to transform the vibration strain energy into heat and thereby reduce vibration amplitude. For a typical cantilever tool, the highest oscillation displacement is near the anti-node regions of a vibration mode and the highest oscillation strain energy is concentrated at the node of a vibration mode. Viscoelastic materials have been successfully applied in these regions to exhibit their damping property. The node region of the 1st bending mode is at the joint interfaces where the cantilever tools are clamped. In this thesis, the general method that can be used to measure and characterize the joint interface stiffness and damping properties is developed and improved, joint interfaces’ importance at optimizing the dynamic stiffness of the joint interface is studied, and a novel advancing material that is designed to possess both high young’s modulus and high damping property is introduced. In the joint interface characterization model, a method that can measure the joint interface’s stiffness and damping over the full frequency range with only the assembled structure is presented. With the influence of a joint interface’s normal pressure on its stiffness and damping, an optimized joint interface normal pressure is selected for delivering a stable machining process against chatter with a boring bar setting at 6.5 times overhang length to diameter ratio in an internal turning process. The novel advancing material utilizes the carbon nano particles mixed in a metal matrix, and it can deliver both high damping property and high elastic stiffness to the mechanical structure. / <p>QC 20130521</p> / PoPJIM, HydroMod, XPRES, NanoComfort

This master thesis deals with researching the structure of embedding resin in the Low-Voltage Transmission Electron Microscope (LV TEM). Further, it focuses on researching the surface morphology by the Atomic Force Microscope (AFM) and by the Scanning Electron Microscope (SEM). The principle of each of the microscopes is explained ? the contrast formation and construction of the LV TEM in particular. In the conclusion the evaluation of the effect of the surface structure of the embedding resin on the image contrast in the LV TEM and the analysis of the minimalization of its effect on the final image is given.

Le centre d'usinage est un moyen de production onéreux. Son impact direct sur la qualité des pièces produites, rend de son audit une opération délicate et complexe.L'objectif des travaux s'intègre dans cette démarche globale.Il consiste à concevoir, pour des cas industriels, des méthodes et des outils de caractérisation statique, et dynamique des moyens d'usinage.Il s'agit de savoir caractériser de manière indépendante le comportement dynamiquede la machine et de l'outil afin de juger son aptitude à accomplir des usinages conformes. Dans la démarche qui sera présentée dans ce mémoire de thèse, les développements ont été orientés sur le comportement dynamique du système usinant Couple : Broche/Outil. / Machining centers are very costly. They have a direct impact on the produced parts quality. This fact makes of its audit a comlexe and important operation.The aim of this work is to develop, for industrial cases, methods for the static and the dynamic qualification of machining centers.It is proposed to apply a methodologies in order to evaluate the ability of different machining systems (Spindle/Tool) to run a machining operation in a stable way. The proposed methodologies are based on an experimental measurement of the dynamic behavior for the system including the spindle and the tool holder. Then, by coupling frequencies response functions, a new system (Spindle/Tool) FRF is predicted at the tool tip. Finally, the critical depth of cut is analytically calculated from the eigenvalues of the characteristic equation of the dynamic machining process.

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.

The contens of master thesis is theoretical analysis of vibrations during turning and possibillity to eliminate them, measure dynamic compliance and modal paramteters. The goal of experimental part is to determine the effect of material and type of clamping for three shafts at natural frequency, mode shape and dynamic compliance. Using these results we can optimized cutting parameters and to better overcome vibration during machining.

Drilling is widely used in manufacturing of products which need holes, for example for fluid channels, screws or pins. Depending on application, workpiece material, cutting parameters and economic considerations, different types of drills are employed. Indexable insert drills are types of drills which facilitate inserts to make holes. These types of drills can make high pitch noises due to vibrations. The focus of this thesis is to investigate the mechanism behind these vibrations in order to help reducing the generated noise in the future designs. Primary investigations show that the main mechanism which results the mentioned noise is regenerative chatter vibrations due to axial and torsional flexibilities. There is a gap in modeling of chatter vibrations in indexable drills where loadings and geometries are asymmetrical and due to torsional vibrations, delay terms are variable. The first step of simulating regenerative chatter vibrations in the drill is to model static cutting forces in a reliable way. In this thesis, a model is proposed which is capable of predicting static cutting forces through segmentation of cutting edges. Since, using this model, forces can be calculated separately on each insert, it is possible to consider differences of inserts in estimationof the cutting loads. The obtained loads are used in the chatter simulation.A model is proposed to simulate chatter vibrations by considering axialand angular deflections and the coupling between them. The resulted model isa system of delay differential equations with variable delays. Variations in timedelays, tool jump-outs and backward motions of inserts have been included inthe proposed time-domain simulation. A set of experiments is conducted toverify the model.

The application of robotic manipulators in machining systems has gained a great interest in manufacturing because of their lower prices, higher kinematic flexibility and larger workspace compared to conventional CNC machine tools. However, their performance is limited due to the much lower structural rigidity which makes them more susceptible to excessive and unstable vibrations, known as chatter, during the machining process. Highly effective chatter modeling and avoidance methods that have been developed for CNC machining in the past decades are now being used by the industry to design high-performance chatter-free machining operations. The available methods, however, face major difficulties when applied to robotic machining, mainly due to the high flexibility and pose-dependency of the vibration response in robots. High flexibility leads to high-amplitude vibrations which affect the process dynamics and excite structural nonlinearities. The existing approaches to modeling machining vibrations assume linearity of the structural dynamics of the robotic manipulator. This assumption, considering the inherent nonlinearities in the robot’s revolute joints, may cause considerable inaccuracies in predicting the stability of vibrations during the process. This thesis studies the high flexibility and nonlinearity of the robot’s structural dynamics and their effects on chatter vibrations. The research starts with investigating the effects of high flexibility of robot's structure in the process dyamics by considering the modulation of cutting forces by axial vibrations, which is normally ignored in CNC milling due to high rigidity of the machine in this direction. The results of chatter prediction considering this effect are shown and discussed. The rest of the thesis focuses on the structural nonlinearity. Firstly, an experimental study is presented to investigate the extent of nonlinearity in structural dynamics of the robot. The results confirm that structural nonlinearities in robotic machining systems can be effectively excited in the presence of high-amplitude vibrations due to milling forces, such that they cause remarkable differences in chatter prediction. The following step is modeling the structural nonlinearities. For this purpose, the variation of restoring forces with the dynamic response (displacement and velocity) are observed when the robot is subjected to harmonic excitation. Based on the experimental observations, the nonlinear effects are modeled by cubic stiffness and damping characteristics. Parameters of the nonlinear model are then identified using Higher-order Frequency Response Functions (HFRF) extracted from measurements. The identified model can predict the vibration behavior of the robotic machining system when subjected to periodic loads such as milling forces. The developed model of nonlinear structural dynamics is then coupled with the chatter model. Consequently, the system is described by nonlinear Delay Differential Equations (DDE) with periodic coefficients. Bifurcation diagrams for the forced vibrations in the described system are developed using the numerical continuation method. The effects of cutting parameters such as feedrate as well as the nonlinear parameters are studied. The thesis is concluded by proposing the use of in-process FRF in the linear model of chatter stability for quick prediction of stability limits. In this approach, the exact characteristics of the nonlinear mechanisms are not studied; instead, the measured FRF during the milling process are used, which are assumed to represent the nonlinear structural dynamics that are linearized about the applied operational conditions. Two methods of measuring in-process FRF are proposed and employed in the robotic milling system. The measured FRF are then used in the linear chatter model to develop the Stability Lobes Diagram (SLD) which shows the combination of cutting parameters that lead to stable or unstable vibrations. Experimental chatter tests show that better agreement with predictions can be achieved by using in-process FRF instead of FRF measured at the idle state of the system. The results of this thesis contribute to better characterization of vibrations in robotic machining with high-amplitude forces and selecting suitable strategies to enhance productivity of the operation. / Graduate

Yes / This paper presents the development of a new chatter model using measured cutting forces instead of a mathematical model with empirical nature that describes them. The utilisation of measured cutting forces enables the prediction of real-time chatter conditions and stable machining. The chatter model is validated using fast Fourier transform (FFT) analyses for detection of chatter. The key contribution of the developed chatter model is that it can be incorporated in digital twins for process monitoring and control in order to achieve greater material removal rates and improved surface quality in future industrial applications involving machining processes. / Research Development Fund Publication Prize Award winner, Sep 2020.

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