Chatter, process damping, and chip segmentation in turning: A signal processing approach

An increasing number of aerospace components are manufactured from titanium and nickel alloys that are difficult to machine due to their thermal and mechanical properties. This limits the metal removal rates that can be achieved from the production process. However, under these machining conditions the phenomenon of process damping can be exploited to help avoid self-excited vibrations known as regenerative chatter. This means that greater widths of cut can be taken so as to increase the metal removal rate, and hence offset the cutting speed restrictions that are imposed by the thermo-mechanical properties of the material. However, there is little or no consensus as to the underlying mechanisms that cause process damping. The present study investigates two process damping mechanisms that have previously been proposed in the machining literature: the tool flank/workpiece interference effect, and the short regenerative effect. A signal processing procedure is employed to identify flank/workpiece interference from experimental data. Meanwhile, the short regenerative model is solved using a new frequency domain approach that yields additional insight into its stabilising effect. However, analysis and signal processing of the experimentally obtained data reveals that neither of these models can fully explain the increases in stability that are observed in practice. Meanwhile, chip segmentation effects were observed in a number of measurements, and it is suggested that segmentation could play an important role in the process-damped chatter stability of these materials

Influence of radial depth of cut on entry conditions and dynamics in face milling application

In this study, acceleration of the workpiece is measured for different radial depths of cut and cutting edge geometries. The influence of the radial depth of cut on the dynamical behavior is evaluated in time and frequency domains. The results for different radial depths of cut and cutting geometries are quantified using the root mean square value of acceleration. The outcome of this research study can be used both for the better cutting data recommendations and improved tool design.У даному дослідженні прискорення оброблюваної деталі вимірювали для різних радіальних глибин різання і геометрії ріжучої кромки фрези. Вплив радіальної глибини різання на динамічну поведінку оцінювали в часових і частотних інтервалах. Отримано кількісні результати для різних радіальних глибин і геометрії різання з використанням середньоквадратичного значення прискорення. Результати цього дослідження можуть бути використані для рекомендацій як покращення процесу різання, так і конструкції інструменту.В данном исследовании ускорение перемещения заготовки измеряли для разных радиальных глубин резания и геометрии режущей кромки фрезы. Влияние радиальной глубины резания на динамическое поведение оценивали во временных и частотных интервалах. Получены количественные результаты для различных радиальных глубин и геометрии резания с использованием среднеквадратичного значения ускорения. Результаты этого исследования могут быть использованы для рекомендаций по улучшению как процесса резания, так конструкции инструмента

Chatter stability of turning and milling with process damping

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.‎Applied Science, Faculty ofMechanical Engineering, Department ofGraduat

In-process identification of modal parameters using dimensionless relationships in milling chatter

Machining parameters needed for stable, high-performance high-speed machining could be found using mathematical models that need accurate measurements of modal parameters of the machining system. In-process modal parameters, however, can slightly differ from those measured offline and this can limit the applicability of simple measurement methods such as impact hammer tests. To study and extract the in-process modal parameters, mathematical models are used to define two key dimensionless parameters and establish their relationships with each other and the modal parameters. Based on these relationships, the modal parameters are extracted using two analytical methods, the two-point method (TPM), and the regression method (RM). As shown with experimental studies, the RM extracts the modal parameters successfully and while being much faster than the existing iteration-based methods, it provides stability lobe predictions that match well the experimental results. Furthermore, it is noted that the natural frequency parameter is estimated with much better relative precision compared to the damping ratio and the modal stiffness parameters. © 2019 Elsevier Lt

Frequency Domain Study of Vibrations above and under Stability Lobes in Machining Systems

Using modified Nyquist contours, the dominant poles of the closed loop delay-differential equation for machining systems such as milling are identified. Contours with constant damping ratio of the dominant poles are constructed using this method. These contours are similar in shape to the stability lobes, but move upwards and to the right as the instability parameter increases. Additionally, it is possible to study the movement of the dominant poles to the right-hand side of the complex plane as the system becomes unstable by increasing the depth of cut at a constant spindle speed. The movement of the dominant pole is shown to be towards the right (unstable) and upward (higher vibration frequency) of the complex plane. In some cases, there would be a jump of vibration frequency due to the change of the lobe number. It is also shown that the damping ratio of the structure strongly affects both the vibration frequency and the damping ratio of the dominant poles in the closed loop system. Finally, in two milling experiments with two different spindle speeds and continuously increasing depth of cuts, vibration frequencies are measured and compared to the theoretical predictions. The measurements agree with the theoretical predictions, particularly in the unstable cutting conditions.InnK

Effect of tool wear on quality in drilling of titanium alloy Ti6Al4V, Part I : Cutting Forces, Burr Formation, Surface Quality and Defects

Titanium's Ti6Al4V, alloy is an important material with a wide range of applications in the aerospace industry.Due to its high strength, machining this material for desired quality at high material removal rate is challenging and may lead to high tool wear rate. As a result,this material may be machined with worn tools and the effects of tool wear on machining quality need to be investigated.In this experimental paper, it is shown how drills of various wear levels affect the cutting forces, surface quality and burr formation. Furthermore, it is shown that high cutting forces and high plastic deformation, along with high temperatures that arise in cutting with worn tools may lead to initiation of microscopic cracks in the workpiece material in proximity of the drilling zone.CC BY-NC-ND 3.0</p

Effect of tool wear on quality in drilling of titanium alloy Ti6Al4V, Part II : Microstructure and Microhardness

Drilling of Ti6Al4V with worn tools can introduce superficial and easily measured features such as increase of cutting forces, entry and exit burrs and surface quality issues and defects. Such issues were presented in the part I of this paper. In part II, subsurface quality alterations,such as changes of the microstructure and microhardness variation is considered by preparing metallographic sections and measurement, mapping of the depth of grain deformation, and microhardness in these sections. Drastic changes in the microstructure and microhardness were found in sections drilled with drills with large wear lands,particularly in the dry cutting tests. These measurements emphasize the importance of detection of tool wear and ensuring coolant flow in drilling of holes in titanium components.CC BY-NC-ND 3.0</p

Minimization of chatter in machining by the use of mobile platform technologies

The risk of vibration in machining can be estimated by identification of the transfer functions of the system. Traditionally the transfer functions may be identified through the use of an impact hammer and a force transducer. This technology needs wiring and therefore cannot be easily used in a production environment. Through the use of the sound that is picked up from the running process, the simplified stability diagram of the system can be deduced. Mobile platforms, like phones and pads can be used to record the sound of the process. The computational capability of these devices are now enough to render the possibility to include the theory and modelling into these devices to make it possible to analyze the process in question as it is running and from that give recommendations to modify the process for minimization of the chatter vibrations. The paper outlines the theoretical considerations and strategy employed to make it possible to develop a useful solution for end-users on the shop floor