Dynamics and chatter stability of multi delay machining systems

Machining is an industrial process in which parts are shaped by removal of unwanted material in the forms of chips. Manufacturing industry today demands shorter production times and high quality parts at competitive cost. Increased MRR (material removal rate) in milling and turning may provide high productivity but elevated forces and vibrations are still major obstacles to fulfill these requirements. Chatter vibrations may limit the full potential of machining productivity. In this thesis, chatter stability of multi delay systems is investigated. As examples of multi delay systems, variable tooth spacing tools such as variable pitch and helix milling cutters and parallel milling operations are investigated. Although there are few studies about the chatter stability of variable tooth spacing tools, no work has been reported on optimum design for a given cutting condition. Optimization studies are carried out to determine the optimum variable tool geometry and a new design methodology is presented. Moreover, for parallel milling operations, an analytical solution methodology which is based on frequency domain analysis is proposed to solve the chatter stability for the first time in the literature. Optimum cutting conditions are identified and effects of process parameters and workpiece dynamics on the chatter stability of parallel milling are shown. Since the operation contains single time delay, optimization studies are carried out to determine the optimum cutter dynamic properties in parallel turning. Simulated conditions are verified by time domain and experimental tests

Improvement of boring operations by means of mode coupling effect

Boring bars are inherently slender tools which are prone to show chatter problems due to their low dynamic stiffness and damping, being this problem their main limitation in productivity. The onset of chatter is mainly related to the dynamic stiffness of the bending mode of the boring bar when the length L to dia-meter D ratio is higher than 4. Tuned mass dampers (TMD) are effective technical solutions to increase the dynamic stiffness of large ratio boring bars. However, there are many applications where 4-6 L/D ratio tools are required, and the avoidance of chatter without the application of TMDs is interesting due to the high cost of damped tools. This work proposes the use of mode coupling effect to increase the damping and stabilise the machining process avoiding the use of any special device. This effect occurs when the fre-quency of one of the machine's modes is similar to the frequency of the dominant mode of the boring bar. As a result, the shape of the critical mode of the boring bar is mixed with the mode originated in the machine, and the damping and stability will be higher than the one that is not subjected to any dynamic coupling. The main contribution of this work is the application of this concept to increase stability in boring operations. This objective has been achieved by optimising the tool length and material by means of a dynamic model based on Receptance Coupling Substructure Analysis (RCSA). The model combines an analytical model of the elastic body of the boring bar with the experimental characterisation of the effect of the rest of the machine. This way, the shape and materials of the boring bar can be optimised to create an increase of damping. The optimisation procedure has been experimentally validated resulting in an increase of cutting stability and demonstrating that not always a shorter bar supposes a higher stability.The authors thank the collaboration of Markel Sanz from IDEKO. This work has been supported by EUROSTARS FORTH E!12998 pro-ject and the EU Horizon 2020 InterQ project (958357/H2020-EU.2.1.5.1)

Mechanics, dynamics and stability of turn-milling operations

Recent turn-milling machine tools are capable of carrying out turning, drilling, boring, milling and grinding operations simultaneously, hence they are widely used in industry to produce complex parts in a single set-up. Turn-milling machines have translational axes with a high speed spindle to hold the cutting tool and a low speed spindle to carry the workpiece. The resulting five-axis turn-milling machines can machine parts with complex curved tool paths. This thesis presents the mechanics and dynamics of turn-milling operations to predict cutting forces, torque, power, vibrations, chatter stability and dimensional surface errors in the virtual environment. First, the kinematics of five-axis turn milling operation is modeled using homogenous transformations. The engagement of rotating-moving tool with the rotating workpiece is identified using a commercial graphics system, and used in predicting the chip thickness distribution. The relative vibrations between the tool and workpiece are modeled, and superposed on the chip thickness in the engagement zone. Unlike in regular turning and milling operations with a single spindle which leads to a single and constant delay, turn milling has two time delays contributed by two rotating spindles and three translational feed drives. The regenerative chip thickness with dual delay is used to predict the cutting forces at tool-workpiece engagement zone, which are transformed to three Cartesian directions of the machine. The resulting coupled differential equations with two delays and time periodic coefficients are solved in the semi-discrete time domain to predict chatter stability, cutting forces, vibrations, torque, power and dimensional surface errors simultaneously. The thesis presents the first comprehensive digital model of turn milling operations in the literature, and can be used to predict the most productive cutting conditions ahead of costly physical trials currently practiced in the industry.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat

Chatter stability of parallel milling operations

Parallel milling process is the process where more than one milling tool cut the same workpiece simultaneously offering increased productivity. However, the dynamic interaction between the tools may introduce additional stability problems, and thus needs to be analyzed. In this study, a frequency domain model is developed for the chatter stability of parallel milling processes. The stability diagrams are obtained for different parallel machining conditions demonstrating the effects of dynamic interaction between the tools through a flexible workpiece. The predictions are verified by experiments carried out on a multi-tasking machining center

Eşzamanlı frezeleme operasyonlarının dinamiği ve kararlılığı

Eşzamanlı frezeleme operasyonu aynı anda birden çok freze takımının kesme işlemi yapmasıdır. Kullanılan tezgaha da bağlı olarak freze takımları tek bir ya da farklı noktalarda bağlanarak aynı ya da farklı parçalar üzerinde işlem yapabilirler. Eşzamanlı frezeleme operasyonları toplam talaş kaldırma miktarını arttırmanın yanı sıra süreç kararlılığı ve tırlama titreşimleri yönlerinden de avantajlar sunabilirler. Tırlama titreşimlerinin giderilmesi sonucunda yüzey kalitesinde de artış sağlanmakta ve üretim verimliliği artmaktadır. Diğer yandan paralel frezeleme operasyonları, geleneksel tek takımlı frezeleme operasyonlarına nazaran dinamik açıdan daha karmaşık bir yapıdadırlar. Geleneksel frezeleme operasyonunda önemsenmeyen ve ihmal edilen çapraz transfer fonksiyonları, paralel frezeleme operasyonunda büyük önem arz etmektedir. Çapraz frekans tepki fonksiyonları kesme esnasında bir noktadaki dinamik kuvvetlerin tezgahın diğer bir noktasında yarattığı titreşimleri hesaplamada kullanılır. Takımların transfer fonksiyonlarının yanı sıra, iş parçasına ait transfer fonksiyonları da kesme işlemi üzerine etkide bulunmaktadır. Kesilecek olan iş parçasının rijitliği, kesme kuvvetlerini ve buna bağlı olarak tırlama titreşimlerini önemli derecede değiştirebilmektedir. Makale kapsamında paralel frezeleme operasyonun dinamiği ve tırlama titreşiminin önlenmesi ele alınmıştır. Gerçekleştirilen simülasyonlar, çok maksatlı NTX2000 tezgahında yapılan testler ile doğrulanmış ve uygun hız çiftlerinin seçilmesi sonucunda toplam kararlılık limitinde artış sağlandığı gözlenmiştir

Stability of Milling Operations With Asymmetric Cutter Dynamics in Rotating Coordinates

High-speed machine tools have parts with both stationary and rotating dynamics. While spindle housing, column, and table have stationary dynamics, rotating parts may have both symmetric (i.e., spindle shaft and tool holder) and asymmetric dynamics (i.e., two-fluted end mill) due to uneven geometry in two principal directions. This paper presents a stability model of dynamic milling operations with combined stationary and rotating dynamics. The stationary modes are superposed to two orthogonal directions in rotating frame by considering the time-and speed-dependent, periodic dynamic milling system. The stability of the system is solved in both frequency and semidiscrete time domain. It is shown that the stability pockets differ significantly when the rotating dynamics of the asymmetric tools are considered. The proposed stability model has been experimentally validated in high-speed milling of an aluminum alloy with a two-fluted, asymmetric helical end mill

Stability and high performance machining conditions in simultaneous milling

Parallel milling offers the advantage of simultaneous machining of a workpiece with two milling tools. Higher material removal rates and machining with fewer fixtures are possible due to the second tool. These advantages make parallel milling an ideal technology for machining of near net shape structures. However, parameter selection is quite challenging due to the dynamic interaction between the tools. In this study, time and frequency domain stability models are developed to aid the process planner. Effects of process parameters are also investigated and high performance machining conditions are identified. The experimental cuts are made to verify the presented methodology

Paralel tornalama işlevini gerçekleştiren CNC torna tezgahının tasarımı, kesici uç ömrü ve kesme kararlılığı incelemeleri (Cutting tool life, cutting stability and process time review on designed and manufactured CNC turning machine which performs the parallel turning function)

Bu çalışmada, tornalama ile üretilen parça esas alınarak, bilgisayar destekli nümerik kontrol (CNC) sistemine sahip, paralel tornalama işlevini gerçekleştiren tezgah tasarlanmış ve imalatı yapılmıştır. İmalatı yapılan CNC paralel tornalama tezgahında paralel tornalama yönteminin, esas işleme zamanı, kesici uç ömrü, parça kalitesi, sistem kararlılığı ve kendinden kaynaklı titreşimler (tırlama) üzerindeki etkileri incelenmiştir. Bu çalışma sonuçlarında elde edilen verilerden yararlanarak; takım ömründe, işleme kalitesinde ve işleme sürelerinde iyileşme sağladığı görülen CNC paralel tornalama tezgahının, seri imalat yapan işletmeler için uygun olduğu değerlendirilmektedir