Investigating dynamics of machine tool spindles under operational conditions

Chatter is one of the major problems in machining and can be avoided by stability diagrams which are generated using frequency response functions (FRF) at the tool tip. During cutting operations, discrepancies between the stability diagrams obtained by using FRFs measured at the idle state and the actual stability of the process are frequently observed. These deviations can be attributed to the changes of machine dynamics under cutting conditions. In this paper, the effects of the cutting process on the spindle dynamics are investigated both experimentally and analytically. The variations in the spindle dynamics are attributed to the changes in the bearing parameters. FRFs under cutting conditions are obtained through the input-output relations of the cutting forces and the vibration response which are measured simultaneously. Experimentally and analytically obtained FRFs are then used in the identification of the bearing parameters under cutting conditions. Thus, bearing properties obtained at idle and cutting conditions are compared and variations in their values are obtained

Alptunc Comak 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., twofluted 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

In-process tool point FRF identification under operational conditions using inverse stability solution

Self-excited vibrations of machine tools during cutting result in process instability, poor surface finish and reduced material removal rate. In order to obtain stability lobe diagrams to avoid chatter vibrations, tool point frequency response function (FRF) must be determined. In classical machine tool studies, tool point FRF is obtained experimentally or analytically for the idle state of the machine. However, during cutting operations, discrepancies are frequently observed between the stability diagrams predicted by using the FRFs measured at the idle state and the actual stability of the process. These deviations can be attributed to the changes in machine tool dynamics under cutting conditions which are difficult to measure. In this study, a new identification method is proposed for the identification of in-process tool point FRFs. In this method, experimentally determined chatter frequency and corresponding axial depth of cut are used in order to identify tool point FRF. The proposed method is applied to a real machining center and by using chatter tests it is demonstrated that the tool point FRF can be accurately identified under operational conditions

Effect of spindle design on spindle dynamics and chatter stability

Spindle-bearing assembly is the most flexible component in machining centers and its dynamics directly affect the performance of the machines. Spindle geometry (outer and inner diameters, segment lengths) and, location of the bearing and their configurations are the crucial parameters that determine the spindle dynamics. Therefore, the selection of the optimum design parameters is the key factor for the spindle design procedures. In this study, a design methodology for the spindle-bearing assembly is offered for the optimized spindle dynamics. In this method, first, the spindle shaft is modeled using the analytical solution of Timoshenko beam and receptance coupling methods. Then, bearing dynamics are included using the structural modification technique. Using the developed analytical model, the effect of each design parameter on spindle dynamics is analyzed. In addition, the effect of operational conditions such as gyroscopic effects and centrifugal forces are included in the structural dynamics model and variations of the spindle dynamics under operational conditions are also considered during the simulations. Simulations show that the proposed method along with the sensitivity analysis can be efficiently used for the selection of the optimum spindle-bearing assembly configuration

Mass loading effect of accelerometers on tool point FRF and stability diagrams

Chatter is one of the major problems in machining resulting in poor surface quality and reduced productivity. Stability diagrams can be used to determine chatter-free process conditions with high productivity. For generation of stability diagrams, frequency re-sponse functions (FRF) at the tool tip is needed to be used in stability model. Impact tests involving accelerometers are commonly used in FRF measurements. Although mass of a typical accelerometer used in these measurements is extremely small com-pared to the cutting tool, it can have a significant effect on the FRF measurement. In this paper, the effect of accelerometer’s on tool point FRFs and stability diagrams will be demonstrated on several cases with different tool-to-accelerometer mass ratios using laser velocity sensor measurement. In addition, a structural modification method which can be used to compensate this effect will also be presented on several cases. The structural modification method can be used to correct the FRFs measured with acceler-ometers, and thus the resulting stability diagram

Analytical modeling of the machine tool spindle dynamics under operational conditions

Chatter is an important problem in machining operations, and can be avoided by utilizing stability diagrams which are generated using frequency response functions (FRF) at the tool tip. In general, tool point FRF is obtained experimentally or analytically for the idle state of the machine. However, during high speed cutting operations, gyroscopic effects and changes of contact stiffness and damping at the interfaces as well as the changes in the bearing properties may lead to variations in the tool point FRF. Thus, stability diagrams obtained using the idle state FRFs may not provide accurate predictions in such cases. Spindle, holder and tool can be modeled analytically; however variations under operational conditions must be included in order to have accurate predictions. In authors previous works Timoshenko beam model was employed and subassembly FRFs were coupled by using receptance coupling method. In this paper, extension of the model to the prediction of operational FRFs is presented. In order to include the rotational effects on the system dynamics, gyroscopic terms are added to the Timoshenko beam model. Variations of the bearing parameters are included by structural modification techniques. Thus, for various spindle speeds, and holder and tool combinations, the tool point FRFs can be predicted and used in stability diagrams

Identification of bearing dynamics under operational conditions for chatter stability prediction in high speed machining operations

Chatter is a major problem causing poor surface finish, low material removal rate, machine tool fail-ure, increased tool wear, excessive noise and thus increased cost for machining applications. Chattervibrations can be avoided using stability diagrams for which tool point frequency response function(FRF) must be determined accurately. During cutting operations, due to gyroscopic moments, centrifugalforces and thermal expansions bearing dynamics change resulting in tool point FRF variations. In addi-tion, gyroscopic moments on spindle–holder–tool assembly cause separation of modes in tool point FRFinto backward and forward modes which will lead to variations in tool point FRF. Therefore, for accuratestability predictions of machining operations, effects of operational conditions on machine tool dynamicsshould be considered in calculations. In this study, spindle bearing dynamics are identified for variousspindle rotational speeds and cutting forces. Then, for a real machining center, tool point FRFs underoperating conditions are determined using the identified speed dependent bearing dynamics and themathematical model proposed. Moreover, effects of gyroscopic moments and bearing dynamics varia-tions on tool point FRF are examined separately. Finally, computationally determined tool point FRFsusing revised bearing parameters are verified through chatter tests

Frezeleme esnasındaki tezgâh dinamiğinin belirlenmesi ve modellenmesi

Tırlama tipi titreşimler talaşlı imalat sırasında üretim verimliliğini etkileyen önemli bir sorundur. Kendinden kaynaklı bu titreşimler işlenen yüzeyin kalitesini etkilemekte, takım aşınmasında artışa ve hatta tezgâhta önemli zararlara sebep olmaktadır. Tırlamanın olmadığı kesme koşulları ise kararlılık diyagramları kullanılarak belirlenebilir ve üretim verimliliğinde önemli artışlar sağlanabilir. Kararlık diyagramlarının elde edilebilmesi için tezgâhın kesici takım ucundaki frekans tepki fonksiyonunun (FTF) belirlenmesi gerekmektedir. Takım uç nokta FTF’si genellikle tezgâhın çalışmadığı duran konumunda ölçülerek elde edilmektedir. Ancak yüksek hızda gerçekleşen kesme işlemlerinde jiroskopik momentten, merkezkaç kuvvetlerinden, ısıl genleşmeden dolayı tezgâh dinamiğinde değişimler olmaktadır. Dolayısı ile tezgâhın çalışmadığı durum için elde edilen takım uç nokta FTF’si hatalı tırlama kararlılığı tahminlerine neden olabilmektedir. Bu çalışmada, kesme koşullarının iş mili dinamiği ve işlem kararlılığına etkileri farklı tutucu-takım kombinasyonları için incelenmiştir. Ayrıca kesme koşulları altında rulman dinamiğinde meydana gelen değişimler belirlenmiştir. Elde edilen hıza bağlı rulman özellikleri geliştirilen analitik modelde kullanılarak farklı tutucu-takım kombinasyonları için takım uç nokta FTF’leri ve kararlılık diyagramları hesaplanmıştır. Elde edilen kararlılık diyagramlarının doğruluğu tırlama testleri ile yapılmış ve deney yapmaya gerek olmadan yüksek hızlarda kesme işlemleri için kararlılık diyagramlarını başarıyla tahmin edilebileceği gösterilmiştir

Identification of spindle dynamics by receptance coupling for non-contact excitation system

International audienceIn today's business environment, the trend towards more product variety and customization is unbroken. Due to this development, the need of agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the similarity between product families by providing design support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach