Vibration suppression and coupled interaction study in milling of thin wall casings in the presence of tuned mass dampers

Damping of machining vibrations in thin-wall structures is an important area of research due to the ever-increasing use of lightweight structures such as jet engine casings. Published literature has focussed on passive/active damping solutions for open geometry structure (e.g. cantilever thin wall), whereas more challenging situations such as closed geometry structures (e.g. thin wall ring-type casings) were not taken into consideration. In this study, a passive damping solution in the form of tuned viscoelastic dampers is studied to minimise the vibration of thin wall casings while focussing on the change in coupled interaction between tool and workpiece due to added tuned dampers. Finite element simulation was carried out to evaluate the effectiveness of tuned dampers in single impact excitation, and this was further validated experimentally through modal impact testing. A reduction in root mean square value, with tuned dampers, of about 2.5 and 4 times is noted at higher and lower depths of cut, respectively, indicating a moderate dependency on depth of cut. A change in coupled interaction of workpiece with tool’s torsional mode (in undamped state) to that of tool’s bending mode (with tuned dampers) was also noted. Variation in machined wall thickness of the order of 6 mm is noted due to the change in coupled interaction from torsional mode to bending mode of tool

Slender workpiece cutting process stability prediction and monitoring based on internal signals

Chatter is commonly found when machining slender components on a lathe due to the low stiffness and damping. Aspects such as accuracy and roughness are usually compromised owing to large variation in stiffness along the length of the workpiece, which is a problem for the machining industry. The main contributions of this thesis are the proposal of (I) models for determine the chatter-free cutting conditions in slender workpiece machining and (II) a process monitoring system to detect chatter using available internal signals from the regulator. The clamping of a workpiece in the spindle of a lathe is not completely stiff, so the classical boundary conditions such as clamped–free, are not enough. The natural frequencies of the machine-workpiece system are usually lower than the theoretical ones obtained with classical boundary conditions. As the system is different for each machine and clamping method, it is necessary to characterize them. In order to be industrially feasible, a experimental method based on models with elastic boundary conditions is proposed to identify the system parameters. A exciter tool prototype based on a piezo actuator is developed to excite the system and natural frequencies have been measured by an accelerometer. The system parameters for a specific clamping and machine tool have been identified by means of model updating method. Once the system parameters have been identified, the effective stiffness of the system at each tool position of the workpiece is obtained. In slender workpieces, two nearby local modes appear due to the clamping, so workpiece chatter may occur by mode coupling in addition to regenerative. The stability of the process for a facing operation has been modelled in the state space taking into account both phenomena. An eigenvalue analysis is carried out by freezing the physical parameters of the time varying system at each time instant. The stability of the process is determined for certain cutting conditions. The theoretical results are validated by experimental tests. As the system is variant periodically in time due to the workpiece rotation, a Linear Time Periodic (LTP) stability model for slender workpiece machining has been proposed in order to define the process stability for each cutting condition. The stability is analysed by Floquet theory and the Stability Lobe Diagram (SLD) are obtained. Finally, the development and validation of a process monitoring system using internal signals available in the Computer Numerical Control (CNC) is presented. The feasibility of different internal signals for chatter monitoring are evaluated. An internal signal based monitoring system is implemented on a fast prototyping generic hardware and the monitoring system is experimentally evaluated in a CNC lathe.Oso ohikoa da chatterra aurkitzea tornu batean pieza lirainak mekanizatzen direnean, sistemaren zurruntasun eta amortiguazio txikiagatik. Mekanizatzean zehaztasuna oso txikia izan ohi da piezaren zurruntasunaren aldakortasun handiagatik luzeran zehar, eta hori arazo bat da mekanizatu industrian. Tesi honen ekarpen nagusiak (I) pieza lirainak mekanizatzeko chatterrik gabeko ebaketa baldintzak zehazteko modeloak eta (II) erregulatzailean eskuragarri dauden barne seinaleak erabiliz mekanizazio prozesua monitorizatzeko sistema dira. Tornu baten pieza amarratze sistema ez da erabat zurruna, beraz, gainazaleko baldintza klasikoak, ez dira nahikoak. Maiztasun naturalak orokorrean maiztasun teorikoak baino baxuagoak izan ohi dira. Makina eta amarratze metodo bakoitzerako sistema aldatzen denez, beharrezkoa da horiek identifikatzea. Industrialki bideragarria izan dadin, gainazaleko baldintza elastikoak dituen modeloetan oinarritutako metodo esperimental bat proposatzen da, sistemaren parametroak identifikatzeko. Sistema kitzikatzeko, eragingailu piezoelektriko batean oinarritutako erremienta kitzikagailu prototipo bat garatu da, eta maiztasun naturalak azelerometro baten bidez neurtzen dira. Amarre baterako eta makina-erreminta espezifiko baterako sistemaren parametroak identifikatu dira model updating metodoaren bidez. Sistemaren parametroak ezagutu ondoren, sistemaren zurruntasun eraginkorra lortu da piezaren luzeran zehar. Pieza lirainetan, hurbileko bi modu lokal agertzen dira amarratzearen ondorioz. Beraz, chatterra, moduen akoplamenduaren bidez gerta daiteke, birsorkuntza efektuaz gain. Errefrentatze operazio baterako, prozesuaren egonkortasuna modelatu da egoera-eremuan, bi fenomenoak kontuan hartuta. Balio propioak aztertzeko, sistemaren parametro fisikoak izoztu dira une bakoitzean. Ebaketa baldintza jakin batzuetarako prozesuaren egonkortasuna zehaztu da eta emaitza teorikoak proba esperimentalen bidez baliozkotu dira. Sistema denboran zehar aldakorra da periodikoki piezaren errotazioarengatik. Horregatik, denboran zehar periodikoa den egonkortasun modelo bat garatu da pieza lirainentzako eta haren egonkortasuna Floquet teoriaren bidez aztertu da. Ebaketa baldintza bakoitzerako prozesuaren egonkortasuna zehazteko, egonkortasun lobuluen diagrama lortu da. Azkenik, erregulatzailean eskuragarri dauden barne seinaleak erabiliz prozesua monitorizatzeko sistema bat garatu da. Chatterra monitorizatzeko barne seinale desberdinen bideragarritasuna ebaluatu da. Barne seinaleetan oinarritutako monitorizazio sistema implementatu da hardware generiko batean eta monitorizazio sistema esperimentalki balioztatu da tornu batean.El chatter se encuentra comúnmente cuando se mecanizan componentes esbeltos en un torno debido a su baja rigidez y amortiguación. Aspectos como la precisión y la rugosidad suelen verse comprometidos debido a la gran variación de la rigidez a lo largo de la pieza de trabajo, lo que constituye un problema para la industria del mecanizado. Las principales contribuciones de esta tesis son el desarrollo de (I) modelos para determinar las condiciones de corte sin vibraciones en el mecanizado de piezas esbeltas y (II) un sistema de monitorización del proceso para detectar chatter utilizando las señales internas disponibles del regulador. La sujeción de una pieza de trabajo en el husillo de un torno no es completamente rígida, por lo que las condiciones de contorno clásicas, como por ejemplo, viga empotrada-libre, no son suficientes. Las frecuencias naturales son generalmente más bajas que las teóricas obtenidas con las condiciones de contorno clásicas. Como el sistema varía para cada máquina y método de amarre, es necesario caracterizarlos. Para que sea industrialmente viable, se propone un método experimental basado en modelos con condiciones de contorno elásticas para identificar los parámetros del sistema. Es sistema ha sido excitado mediante un prototipo de herramienta excitadora basado en un actuador piezoeléctrico y las frecuencias naturales han sido medidas por un acelerómetro. Los parámetros del sistema para un amarre y una máquina herramienta específica han sido identificados mediante el método de model updating. Una vez identificados los parámetros del sistema, se ha obtenido la rigidez efectiva del sistema en cada posición de la pieza. En piezas esbeltas, aparecen dos modos locales cercanos debido al amarre, por lo que el chatter de pieza puede ocurrir por acoplamiento de modos además del efecto regenerativo. La estabilidad del proceso para una operación de refrentado ha sido modelada en el espacio de estado teniendo en cuenta ambos fenómenos. El análisis de los valores propios se lleva a cabo congelando los parámetros físicos del sistema en cada instante de tiempo. La estabilidad del proceso ha sido determinada para ciertas condiciones de corte. Los resultados teóricos han sido validados por medio de pruebas experimentales. Como el sistema es variante periódicamente en el tiempo debido a la rotación de la pieza, se ha propuesto un modelo de estabilidad lineal periódica en el tiempo para el mecanizado de piezas esbeltas, con el fin de definir la estabilidad del proceso para cada condición de corte. La estabilidad ha sido analizada con la teoría de Floquet y ha sido obtenido el diagrama de lóbulos de estabilidad. Finalmente, se presenta el desarrollo y validación de un sistema de monitorizado de proceso utilizando señales internas disponibles en el regulador. Se evalúa la viabilidad de diferentes señales internas para la monitorización del chatter. El sistema de monitorizado ha sido implementado en hardware genérico de prototipado rápido y el sistema de monitorización ha sido validada experimentalmente en un torno

Active suppression of machine tool chatter

PhD ThesisThe aim of the work described in this thesis is to design, build, and test an active, chatter suppression system for use on a lathe. Many methods have been developed to minimise the effects of regenerative chatter in machine tools. These include machine redesign and stiffening, the inclusion of additional damping, and the use of passive and active control systems. The method described here is a development of two of these active methods, those of Comstock and Nachtigal, which control the relative position of the cutting tool. An on-line digital computer is used to monitor the cutting force, predict the relative tool-workpiece displacement, and drive the tool to suppress chatter build-up. The work is described in five main sections. After the introductary section, in which the problem is outlined and past work discussed, a theoretical analysis of chattering and its suppression is presented. Digital simulation is used to confirm and expand the theoretical results. The basic on-line identification method used to investigate the machine-workpiece structure is also presented. The third section describes the design and implementation of the experimental rigg especially the computer system and the tool positioner. Its use as a driver for a cheap, bolt-on CNC turning system is also discussed. The fourth section details the experimental work including calibration, cutting tests, suppressor validation and testing. Finally, the theory, simulation and experiments are discussed and related to past work. Suggestions are made for further reearch and development, including other applications of the system. Conclusions are drawn about the various techniques used during the work, with comments on the effectiveness of the suppression method.Science Research Counci

Accuracy Control in the Process of Low-Rigidity Elastic Deformable Shafts Turning

The paper focuses on the problem of control of accuracy of forming elastic-deformable shafts with low rigidity. In particular, results of analysis of basic factors affecting the accuracy of machining of low-rigidity shafts such as the stiffness of the technological system, geometry of cutting tool, lathe temperature, degree of cutting tool wear, cutting tool strength, lubrication-cooling fluid and machining parameters are presented. Moreover, the performed analysis encompassed the effect of stiffness of particular elements of the technological system, the analytical relations determining the changes of defined parameters, values of elasticity of the fixed headstock and tailstock, as well as of the stiffness of the machining system from the zones of rigidity of the parts. To analyse appearing errors in the real industrial conditions, the study was realized on the example of manufacturing precision mechanics tooling in two and a single pass, with the application of specific parameters of machining

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

Modelling of grinding mechanics : a review

Grinding is one of the most widely used material removal methods at the end of many process chains. Grinding force is related to almost all grinding parameters, which has a great influence on material removal rate, dimensional and shape accuracy, surface and subsurface integrity, thermodynamics, dynamics, wheel durability, and machining system deformation. Considering that grinding force is related to almost all grinding parameters, grinding force can be used to detect grinding wheel wear, energy calculation, chatter suppression, force control and grinding process simulation. Accurate prediction of grinding forces is important for optimizing grinding parameters and the structure of grinding machines and fixtures. Although there are substantial research papers on grinding mechanics, a comprehensive review on the modeling of grinding mechanics is still absent from the literature. To fill this gap, this work reviews and introduces theoretical methods and applications of mechanics in grinding from the aspects of modeling principles, limitations and possible future trendencies

Influence of feed drives on the structural dynamics of large-scale machine tools

Milling is one of the most widely used processes in the manufacturing industry and demands machines with high productivity rates. In large machine tool applications, the cutting capability is mainly limited by the appearance of structural chatter vibrations. Chatter arises from the dynamic interaction of the machining system compliance with the cutting process. For the specific case of large-scale machine tools, the low frequency resonances have modal shapes that generate relative displacements in the machine joints. This thesis presents new approaches to minimize the appearance of chatter vibrations by targeting and understanding the machine tool compliance, in particular, from the feed drive of the machine tool. A detailed model of the double pinion and rack feed drive system and the master-slave coupling improves the large machine tools modeling. As the vibrations are measured by the axes feedback sensors, a new strategy for feed drive controller tuning allows increasing the chatter stability using a judicious selection of the servo parameters. Then, in-motion dynamic characterizations demonstrate the important influence of the nonlinear friction on the machine compliance and improve the chatter stability predictions. Finally, an operational method for characterizing both tool and workpiece side dynamics while performing a cutting operation is developed. All the contributions of the thesis have been validated experimentally and tend to consider the influence of the feed drives on the structural dynamics of large-scale machine tools

FINISH-MACHINING STRATEGIES FOR BLADED DISKS

Integrally Bladed Rotors (IBRs) or Bladed Disks (Blisks) are strategic components of compressor or turbine stages of aircraft engines. Development of manufacturing techniques and materials have aided the integration of two components, blades and the disk, which were originally manufactured separately and then assembled. A single component brings great benefits such as weight reduction, which is key the in aerospace sector. IBR components bring new challenges to the manufacturing industry due to the difficult to cut materials used, paired with complex geometries which limit the access of tooling and limits various efficient cutting strategies for the finish milling operations. Instead, a point milling strategy is commonly used to achieve drawing specifications but at a cost of machining time. Therefore, finish milling is by far the most time-consuming machining operation of IBR blades. However, many efforts from industry are directed to optimize machining times through roughing operations, which are faster to implement internally within the manufacturing engineering department, and often are not affected by fixed process approvals that are in place for the last few millimetres of material removal. This includes approval from the materials department on surface integrity modifications of the final surface, and complex approval processes with the final clients. An EngD project is an ideal scenario for the development of finish machining strategies for the reasons explained above. This thesis takes a real IBR case study as a starting point and navigates through a logical path for the development of its blade finish milling operation to provide a novel industrial optimization strategy. The research question evolves as each chapter explores different aspects of this challenging industrial problem. Initially, in chapter 2, surface integrity is explored within the typical working window (range or map of parameters selected for a given experiment), due to the relevancy of the surface integrity in the finished component. This is explored through an experimental approach which concludes surface integrity is not affected in the analysed range. Instead, chatter is identified and research efforts are then directed to improve finish machining of IBR blades through the understanding and mitigation of chatter. Chapter 3 seeks to analyse tool and component dynamics and includes a brief search into literature about process damping to understand how it might play a role in chatter mitigation. A new research line is then investigated to improve finish milling of IBR blades. A very simple concept of modifying finish milling stock is developed, using a scientific method based on Finite Element Analysis (FEA) and parametrizing the blade in order to maximize natural frequencies of interest. Once an optimized blade stock geometry has been obtained, a further literature review is carried out on chatter mitigation techniques. A knowledge gap is found in the current literature regarding time domain model for Sinusoidal Variable Spindle Speed (SVSS) model for ball end mill tools. This is observed as an opportunity to do a theoretical contribution to the predominantly experimental EngD thesis. A current time domain model has been further developed to incorporate SVSS and ball end mill geometry. Finally, implementation of variable speed in industrial environment has been researched. A further knowledge gap is identified in the implementation of variable speed in commercial milling machines, as most research up to date has been realized either theoretically or in laboratory conditions. In response to this need, a new method has been developed to be able to implement variable spindle speed and variable feed straight forwardly in a wide range of commercial milling machines. To end up with, a machine characterisation has been completed in order to identify the working window to apply the Variable Spindle Speed (VSS) method, and experimental trials have been carried out to demonstrate the capability of this approach. This thesis starts presenting a case study of IBRs with the need to improve current finish machining strategies and delivers new solutions from various perspectives, complementing each other and readily available to implement in the industry environment

Development of chatter threshold boundary for milling of metals

This study reports on a novel experimental method for the prediction of chatter based on the chaos theory. The variation of Poincaré sections of the reconstructed phase space attractor is able to identify the transition of the milling system from a stable to an unstable condition, continuously during the milling process. Two mathematical tools are used to measure the variation of Poincaré sections they being; image correlation and a designed regression model. Image correlation uses Poincaré sections as a pattern and the computation of Pearson’s coefficient assists to develop a chatter threshold boundary. Titanium is the main material in this research, as chatter is more applicable during cutting of titanium due to its specific mechanical properties. Moreover, the method is used in detection of chatter during milling of stainless steel and aluminum in order to demonstrate the method can detect chatter during cutting of other metals. The new method can be used to detect chatter on-line, as it is independent of the cutting parameters and dynamics of the milling process, and can be integrated in the cutting machines. The method does not need expensive equipment and complex process, so it can be easily used in normal production workshop environment. A regression model computes the trend of changes in the Poincaré sections and gives a numerical output value to define the boundary between the stable and unstable state of the milling process. These mathematical tools can be used in expert software to monitor the milling process on-line and detect the onset of chatter