ON THE STABILITY OF VARIABLE HELIX MILLING TOOLS

One of the main aims of the manufacturing industry has been to maximise the material removal rate of machining processes. However, this goal can be restricted by the appearance of regenerative chatter vibrations. In milling, one approach for regenerative chatter suppression is the implementation of variable-helix cutters. However, these tools can lead to isolated unstable regions in the stability diagram. Currently, variable-helix unstable islands have not been extensively researched in the literature. Therefore, the current thesis focuses on studying and experimentally validating these islands. For the validation, an experimental setup that scaled not only the structural dynamics but also the cutting force coefficients was proposed. Therefore, it was possible to attain larger axial depths of cut while assuming linear dynamics. The variable-helix process stability was modelled using the semi-discretization method and the multi-frequency approach. It was found that the variable helix tools can further stabilise a larger width of cut due to the distributed time delays that are a product of the tool geometry. Subsequently, a numerical study about the impact of structural damping on the variable-helix stability diagram revealed a strong relationship between the damping level and instability islands. The findings were validated by performing trials on the experimental setup, modified with constrained layer damping to recreate the simulated conditions. Additionally, a convergence analysis using the semi-discretization method (SDM) and the multi-frequency approach (MFA) revealed that these islands are sensitive to model convergence aspects. The analysis shows that the MFA provided converged solutions with a steep convergence rate, while the SDM struggled to converge. In this work, it is demonstrated that variable-helix instability islands only emerge at relatively high levels of structural damping and that they are particularly susceptible to model convergence effects. Meanwhile, the model predictions are compared to and validated against detailed experimental data that uses a specially designed configuration to minimise experimental error. To the authors' knowledge, this provides the first experimentally validated study of unstable islands in variable helix milling, while also demonstrating the importance of accurate damping estimates and convergence studies within the stability predictions

Nonlinear Time-Frequency Control Theory with Applications

Nonlinear control is an important subject drawing much attention. When a nonlinear system undergoes route-to-chaos, its response is naturally bounded in the time-domain while in the meantime becoming unstably broadband in the frequency-domain. Control scheme facilitated either in the time- or frequency-domain alone is insufficient in controlling route-to-chaos, where the corresponding response deteriorates in the time and frequency domains simultaneously. It is necessary to facilitate nonlinear control in both the time and frequency domains without obscuring or misinterpreting the true dynamics. The objective of the dissertation is to formulate a novel nonlinear control theory that addresses the fundamental characteristics inherent of all nonlinear systems undergoing route-to-chaos, one that requires no linearization or closed-form solution so that the genuine underlying features of the system being considered are preserved. The theory developed herein is able to identify the dynamic state of the system in real-time and restrain time-varying spectrum from becoming broadband. Applications of the theory are demonstrated using several engineering examples including the control of a non-stationary Duffing oscillator, a 1-DOF time-delayed milling model, a 2-DOF micro-milling system, unsynchronized chaotic circuits, and a friction-excited vibrating disk. Not subject to all the mathematical constraint conditions and assumptions upon which common nonlinear control theories are based and derived, the novel theory has its philosophical basis established in the simultaneous time-frequency control, on-line system identification, and feedforward adaptive control. It adopts multi-rate control, hence enabling control over nonstationary, nonlinear response with increasing bandwidth ? a physical condition oftentimes fails the contemporary control theories. The applicability of the theory to complex multi-input-multi-output (MIMO) systems without resorting to mathematical manipulation and extensive computation is demonstrated through the multi-variable control of a micro-milling system. The research is of a broad impact on the control of a wide range of nonlinear and chaotic systems. The implications of the nonlinear time-frequency control theory in cutting, micro-machining, communication security, and the mitigation of friction-induced vibrations are both significant and immediate

Digital Apprentice for Chatter Detection: An On-line Learning Approach to Regenerative Chatter Detection in Machining via Human-Machine Interaction

Regenerative chatter in machining, which is characterized by self-excited vibration, is a common process anomaly that limits productivity and part quality in machining operations. This thesis proposes an on-line approach for chatter detection via effective human-machine interaction, facilitating knowledge transfer from experienced machinists to the “digital apprentice” through the “learnable skill primitive” (LSP) method that establishes a chatter detection threshold. The research focus is to develop the methodology for chatter-specific knowledge acquisition and a human-machine interface inspired by computing techniques and frameworks such as learning from demonstration, reinforcement learning, and interactive agent shaping. In this work, the milling operation is selected as a case study for the proposed LSP method. Digital audio data is acquired from milling experiments through a studio-style condenser microphone mounted inside a milling machine. The data is pre-processed through various digital filters before Fast Fourier Transform (FFT) is performed to identify the chatter frequency contents. During the training phase, data for the human operator’s natural reaction to chatter is collected via a specially designed human-machine interface. The learned chatter detection thresholds are obtained through the “learnable skill primitive” method by temporally mapping the reaction data to the cutting signal. In addition, a variance mitigation strategy is developed to reduce the negative impact of the high variance in the operator’s reaction time to chatter. During the testing phase, experiments are conducted to evaluate the detection accuracy, detection speed, and robustness of the learned chatter detection thresholds. Experimental data support the claim that the learned thresholds can detect chatter with good detection accuracy and detection speed. Finally, the learned threshold is demonstrated to be robust to milling of different workpiece materials under different cutting conditions such as feeds, speeds, axial and radial immersions (depths of cut), and directions of cutting.M.S

Studies on Design of Spindle-tool System and Their Effects on Overall Milling Process Stability

High speed machining using vertical CNC milling centres continues to be a popular approach in a variety of industries including aerospace,automobile,mould and die casting etc.Chatter oscillations have significant influence in restricting the metal removal rates of the machining process.The cutting process instability or chatter is assessed by prediction of frequency response at the tool tip.Present work aims at evaluating the combined effect of a spindle-housing and tool holder on the dynamics of cutting tool by considering the flexibility of spindle unit supported on bearings.The spindle-tool is analysed by using finite element modeling using Timoshenko beam theory.The dynamic characteristics and tool-tip frequency responses are obtained without considering the cutting forces.The results are compared with receptance coupling approach and using 3D modeling in ANSYS.Further experimental modal analysis on the machining spindle of same dimensions has revealed the same dynamic modes.Using the validated FE model of the system,the effects of nonlinear bearing contact forces,spindle-tool holder interface stiffness,bearing span and axial preload, tool overhang and diameter on the frequency response and cutting process stability are studied.Optimal spindle-tool system is designed for achieving maximum dynamic stiffness.The analytically stability lobe diagrams are obtained from the real and imaginary terms of these frequency responses at the tool tip.Dynamic stability issues in helical end-milling using the two and three dimensional cutting force models are considered for the analysis.The stability boundaries are experimentally verified using the cutting tests on both CNC milling spindle and modified drilling tool spindle systems while machining Al-alloy work pieces.Vibration and sound pressure levels are also employed to assure the stability of cutting operations,while surface images are used to identify the chatter marks at various combinations of cutting parameters.Dynamic milling model is employed with the flexible spindle-tool system by considering several effects including variable tool pitch, tool run-out,nonlinear feed forces and process damping. Design and stability studies on the modified drill spindle with a custom-designed work table for milling operations allowed in understanding several interesting facts about spindle-tool systems. Some control strategies including semi-active and active methods are implemented using finite element model of the spindle-tool system to minimize the chatter vibration levels/maximize the stable depth of cut during cutting operations

Nonlinear Time-Frequency Control Theory with Applications

Nonlinear control is an important subject drawing much attention. When a nonlinear system undergoes route-to-chaos, its response is naturally bounded in the time-domain while in the meantime becoming unstably broadband in the frequency-domain. Control scheme facilitated either in the time- or frequency-domain alone is insufficient in controlling route-to-chaos, where the corresponding response deteriorates in the time and frequency domains simultaneously. It is necessary to facilitate nonlinear control in both the time and frequency domains without obscuring or misinterpreting the true dynamics. The objective of the dissertation is to formulate a novel nonlinear control theory that addresses the fundamental characteristics inherent of all nonlinear systems undergoing route-to-chaos, one that requires no linearization or closed-form solution so that the genuine underlying features of the system being considered are preserved. The theory developed herein is able to identify the dynamic state of the system in real-time and restrain time-varying spectrum from becoming broadband. Applications of the theory are demonstrated using several engineering examples including the control of a non-stationary Duffing oscillator, a 1-DOF time-delayed milling model, a 2-DOF micro-milling system, unsynchronized chaotic circuits, and a friction-excited vibrating disk. Not subject to all the mathematical constraint conditions and assumptions upon which common nonlinear control theories are based and derived, the novel theory has its philosophical basis established in the simultaneous time-frequency control, on-line system identification, and feedforward adaptive control. It adopts multi-rate control, hence enabling control over nonstationary, nonlinear response with increasing bandwidth ? a physical condition oftentimes fails the contemporary control theories. The applicability of the theory to complex multi-input-multi-output (MIMO) systems without resorting to mathematical manipulation and extensive computation is demonstrated through the multi-variable control of a micro-milling system. The research is of a broad impact on the control of a wide range of nonlinear and chaotic systems. The implications of the nonlinear time-frequency control theory in cutting, micro-machining, communication security, and the mitigation of friction-induced vibrations are both significant and immediate

Alternative experimental methods for machine tool dynamics identification: A review

An accurate machine dynamic characterization is essential to properly describe the dynamic response of the machine or predict its cutting stability. However, it has been demonstrated that current conventional dynamic characterization methods are often not reliable enough to be used as valuable input data. For this reason, alternative experimental methods to conventional dynamic characterization methods have been developed to increase the quality of the obtained data. These methods consider additional effects which influence the dynamic behavior of the machine and cannot be captured by standard methods. In this work, a review of the different machine tool dynamic identification methods is done, remarking the advantages and drawbacks of each method.The present work has been partially supported by the EU Horizon 2020 InterQ project (958357/H2020-EU.2.1.5.1.) and the CDTI CERVERA programme MIRAGED project (EXP-00,137,312/CER-20191001)

Optimisation of variable helix end milling tools

High productivity, low cost and high profits are important issues in aerospace, automotive and tool/die metal manufacturing industries. Machining processes are widely used in manufacturing operations for metal manufacturing rather than casting and forming. However, the dynamic deflection of tool and workpiece systems generates unstable cutting forces when machining with high material removal rate. Here, sudden large vibration amplitudes occur when energy input exceeds the energy dissipated from the system, leading to self-excited vibration or chatter. This thesis focuses on the avoidance of milling chatter by using variable helix milling tools. Since milling chatter is strongly influenced by the frequency response function of the dynamic system, a preliminary study is first presented to assess the feasibility of non-contacting electromagnetic modal analysis for milling tools. It is shown that this approach shows some promise for use in real machining problems where traditional modal hammers have some drawbacks. In particular, the amplitude dependency of the frequency response function could be qualitatively illustrated. The main focus of this thesis is the optimisation of variable helix tool geometry for improved chatter performance. A semi-discretisation method was combined with Differential Evolution to optimise variable helix end milling tools. The target was to reduce chatter and maximise performance by modifying the variable helix and variable pitch tool geometry. The performance of the optimisation routine was benchmarked against a more traditional approach, namely Sequential Quadratic Programming. Numerical studies indicated that the Differential Evolution optimisation performed much better than Sequential Quadratic Programming due to the nonlinearity of the optimisation problem. The numerical study predicted total mitigation of chatter using the optimised variable helix milling tool at a low radial immersion. However, in practice, a five-fold increase in chatter stability was obtained, compared to traditional milling tools. In addition to this practical contribution, this study has provided new insight into the experimental nonlinear dynamics of variable helix milling tools, which exhibit period-one bifurcations under certain conditions. There have been very few previous studies that have investigated variable helix milling tools. However, one previous study proposed that the so-called ‘process damping' phenomenon is particularly important for variable helix milling tools. Consequently, the final contribution of this thesis is a study of process damped milling and the influence of different tool geometries. Testing was performed for tools with different rake and relief angle, edge radius and variable helix/pitch. It was found that variable helix/pitch had the greatest influence on the process damping phenomenon.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Active and Passive Control of Machine Tool Vibrations for High Speed and Accuracy

High-performance mechatronic systems are widely used in precision manufacturing equipment such as CNC machine tools, 3D-Printers, photolithography systems, industrial robots, and Coordinate Measuring Machines (CMMs). These equipment are utilized in producing parts and components for aviation, semiconductor, optics, and many other emerging industries, with geometric features and surface properties within micrometer-, or even in some cases nanometer-level accuracy. To keep up with the rapidly increasing productivity and accuracy demands, it is crucial that mechatronic systems of these manufacturing equipment deliver high-speed motion with high precision. In this dissertation, motion control strategies are presented to increase dynamic positioning accuracy and productivity of such mechatronic systems. First, a novel trajectory generation method is presented to avoid exciting low frequency structural vibration modes of machine tools and 3D-Printers, without compromising from productivity. The trajectory generation problem is posed as a convex optimization problem, and a practical windowing method is presented to implement the proposed strategy in real-time for long and realistic manufacturing scenarios. The proposed algorithm is validated on an industrial 3-Axis machine tool, and 4-6 times attenuation of the column vibration mode is achieved with 1[g] acceleration commands, without increasing the cycle time compared to state-of-the-art trajectory generation methods. This is followed by proposition of a data-driven trajectory shaping algorithm designed to eliminate dynamic positioning errors induced by flexible motion transmission components (such as ball-screw drives) and nonlinear friction forces typically caused by mechanical bearings and guiding units. The proposed algorithm is used for optimizing trajectory pre-filters through machine-in-the-loop iterations, in a data-driven fashion, and therefore it can be applied on a wide variety of systems without requiring elaborate dynamic modeling. Effectiveness of the proposed technique is validated on a linear-motor-driven planar motion stage and an industrial 3-Axis machine tool, and it is shown that dynamic errors are reduced by 3-5 times compared to industry-standard approaches. Finally, an active tool position control strategy is proposed to mitigate self-excited (chatter) vibrations for improving stability margins of turning processes. Two motion control algorithms are developed to control the dynamic process defined by the interaction of the tool and the workpiece. An industrial lathe (turning center) is utilized for validating the effectiveness of proposed algorithms. A piezo-actuator driven tool-assembly is utilized to control tool position during the machining process, utilizing tool acceleration feedback, and the experiments show that 4-5 times increase in productivity (widths of cut) is achieved by the proposed strategy