•Speed-dependency of cutting force coefficients is investigated and highlighted.•A robust approach to identify speed-dependent force coefficients is described.•Improved compensation technique is used ...to extend dynamometer measureable bandwidth.•Proposed instantaneous method including run-out, and average method are tested.•Efficiency and accuracy of the developed instantaneous approach is shown.
Accurate simulation of the machining process is crucial to improve milling performance, especially in High-Speed Milling, where cutting parameters are pushed to the limit.
Various milling critical issues can be analyzed based on accurate prediction of cutting forces, such as chatter stability, dimensional error and surface finish. Cutting force models are based on coefficients that could change with spindle speed. The evaluation of these specific coefficients at higher speed is challenging due to the frequency bandwidth of commercial force sensors. On account of this, coefficients are generally evaluated at low speed and then employed in models for different spindle speeds, possibly reducing accuracy of results.
In this paper a deep investigation of cutting force coefficient at different spindle speeds has been carried out, analyzing a wide range of spindle speeds: to overcome transducer dynamics issues, dynamometer signals have been compensated thanks to an improved technique based on Kalman filter estimator. Two different coefficients identification methods have been implemented: the traditional average force method and a proposed instantaneous method based on genetic algorithm and capable of estimating cutting coefficients and tool run-out at the same time.
Results show that instantaneous method is more accurate and efficient compared to the average one. On the other hand, the average method does not require compensation since it is based on average signals. Furthermore a significant change of coefficients over spindle speed is highlighted, suggesting that speed-varying coefficient should be useful to improve reliability of simulated forces.
Accurate cutting-force measurements appear to be the key information in most of the machining related studies as they are fundamental in understanding the cutting processes, optimizing the cutting ...operations and evaluating the presence of instabilities that could affect the effectiveness of cutting processes. A variety of specifically designed transducers are commercially available nowadays and many different approaches in measuring cutting forces are presented in literature. The available transducers, though, express some limitations since they are conditioned by the vibration of the surrounding system and by the transducer׳s natural frequency. These parameters can drastically affect the measurement accuracy in some cases; hence an effective and accurate tool is required to compensate those dynamically induced errors in cutting force measurements. This work is aimed at developing and testing a compensation technique based on Kalman filter estimator. Two different approaches named “band-fitting” and “parallel elaboration” methods, have been developed to extend applications of this compensation technique, especially for milling purpose. The compensation filter has been designed upon the experimentally identified system׳s dynamic and its accuracy and effectiveness has been evaluated by numerical and experimental tests. Finally its specific application in cutting force measurements compensation is described.
•Cutting force measurements can be drastically affected by machine tool dynamics.•Dynamic compensation can extend the bandwidth of commercial dynamometers.•Two different approaches suitable for milling applications have been developed.•The effectiveness of the proposed approaches has been experimentally evaluated.•Examples in cutting force measurements compensation are shown.
Chatter vibration induces a characteristic pattern on the milled surface, known as chatter marks, causing a poor surface quality. While several works deal with the prediction of the machined surface ...in stable condition, surface under chatter vibration has not been extensively studied: it is not clear how vibrations at high chatter frequency return highly spaced chatter marks on the surface. This paper investigates the chatter marks generation mechanisms focusing on this issue, i.e., on the surface spectral proprieties. The generation of the surface profile is regarded as a problem of sampling at the tooth pass frequency (in the time domain) and reconstruction (in the spatial domain) of the cutting tool displacements. Using this analogy, the paper highlights two main effects (aliasing and pseudo moiré), proposing specific formulations. The method is validated by a numerical investigation, based on a surface generation model coupled with a time-domain simulator of the milling process. Finally, an experimental validation is proposed. The formulations presented in this work provide an insight in the relation between chatter frequency and chatter marks pattern. Therefore, if the chatter frequency pattern over the spindle speed is known (e.g., identified via simulations or experiments), the proposed method could support the selection of cutting parameters which results in an acceptable surface, even in highly unstable cutting conditions.
•An insight of the surface generation mechanism in milling is proposed.•Surface profile is regarded as a problem of sampling at the ftp of the tool vibrations.•Two fundamental spatial frequencies are identified, composing chatter marks surface.•Formulations to predict spectral proprieties of chatter marks are provided.•Numerical and experimental tests prove the accuracy of the proposed approach.
Chatter is one of the most limiting factors in improving machining performances. Stability Lobe Diagram (SLD) is the most used tool to select optimal stable cutting parameters in order to avoid ...chatter occurrence. Its prediction is affected by reliability of input data such as machine tool dynamics or cutting coefficients that are difficult to be evaluated accurately, especially at high speed.
This paper presents a novel approach to experimentally evaluate SLD without requiring specific knowledge of the process; this approach is called here Spindle Speed Ramp-up (SSR) test. During this test spindle speed is ramped up, and chatter occurrence is detected by the Order Analysis technique. As result one single test ensures optimal spindle speed identification at one cutting condition, while if few tests are performed the entire SLD could be obtained. Results of the method applied to slotting operation on aluminum are provided and a comparison between different measurements devices is presented. This quick, easy-to-use and efficient test is suitable for industrial application: no knowledge of the process is required, different sensors can be used such as accelerometer, dynamometer or microphone.
•A new experimental approach to detect chatter stability is developed.•Spindle speed is ramped-up in a single test to reduce experimental time required.•Order Analysis is performed on sensors signals to detect chatter frequency.•Thanks to one single test stable parameters can be obtained at one depth of cut.•Few tests at different depths of cut result in an experimental chatter map.
•A new receptance coupling method to accurately predict tool-tip FRF is developed.•Translation and rotational FRFs of the machine are identified with a single test.•An inverse RCSA approach is ...formulated to extract required FRFs of the machine.•No additional experimental phase is required to compute rotational FRF.•Accuracy of the method is proven by experimental validation on different tools.
Chatter prediction requires Frequency Response Function (FRF) at the tool-tip, which can be reliably identified by modal testing. However this procedure is time-consuming and unsuitable for industrial application since measurements must be performed for each tool attached to the machine. Receptance coupling technique can be adopted to speed up FRF identification. This technique computes tool-tip response combining experimental spindle-holder dynamics with numerical/analytical model of the tool. The main drawback is to identify rotational degree of freedom responses that are generally obtained by performing additional calibrations tests. This work presents a new method overcoming this limitation, obtaining rotational responses by a novel inverse formulation that reduces the number of required experiments. The proposed method is based on receptance coupling theory and requires FRF measurements of a single machine setup, thus increasing the efficiency of receptance coupling approach for tool-tip FRF prediction. The proposed technique is proven by experimental validation and the same accuracy of state of the art methods is obtained.
Chatter prediction is crucial in high-speed milling, since at high speed, a significant increase of productivity can be achieved by selecting optimal set of chatter-free cutting parameters. However, ...chatter predictive models show reduced accuracy at high speed due to machine dynamics, acquired in stationary condition (i.e., without spindle rotating), but changing with spindle speed. This paper proposes a hybrid experimental-analytical approach to identify tool-tip frequency response functions during cutting operations, with the aim of improving chatter prediction at high speed. The method is composed of an efficient test and an analytical identification technique based on the inversion of chatter predictive model. The proposed technique requires few cutting tests and a microphone to calculate speed-dependent chatter stability in a wide range of spindle speed, without the need of stationary frequency response function (FRF) identification. Numerical and experimental validations are presented to show the method implementation and assess its accuracy. As proven in the paper, computed speed-dependent tool-tip FRF in a specific configuration (i.e., slotting) can be used to predict chatter occurrence in any other conditions with the same tool.
Joint attention (JA), whose deficit is an early risk marker for autism spectrum disorder (ASD), has two dimensions: (1) responding to JA and (2) initiating JA. Eye-tracking technology has largely ...been used to investigate responding JA, but rarely to study initiating JA especially in young children with ASD. The aim of this study was to describe the differences in the visual patterns of toddlers with ASD and those with typical development (TD) during both responding JA and initiating JA tasks. Eye-tracking technology was used to monitor the gaze of 17 children with ASD and 15 age-matched children with TD during the presentation of short video sequences involving one responding JA and two initiating JA tasks (initiating JA-1 and initiating JA-2). Gaze accuracy, transitions and fixations were analyzed. No differences were found in the responding JA task between children with ASD and those with TD, whereas, in the initiating JA tasks, different patterns of fixation and transitions were shown between the groups. These results suggest that children with ASD and those with TD show different visual patterns when they are expected to initiate joint attention but not when they respond to joint attention. We hypothesized that differences in transitions and fixations are linked to ASD impairments in visual disengagement from face, in global scanning of the scene and in the ability to anticipate object's action.
In High Pressure Die Casting (HPDC), geometrical distortions usually happen during the cooling phase, due to the reduced cooling time and the high thermal gradient inside the product itself. This ...phenomenon affects most the thin walled products. The usual die design practice considers only the linear shrinking of the product during the cooling as a consequence of the difficult to take in account also the geometrical deformations. In this essay a simple finite element design strategy that allows the designer to improve the die shape is presented. The proposed approach uses an automatic iterative optimization technique based on a heuristic algorithm, which could be easily applied to most of the Finite Element (FE) commercial software: the basic concept of the method is simply to move the nodes defining the die surface in the opposite direction to the error due to the cooling phenomena. An automotive component has been selected as a case study: the aim was to improve the planarity tolerance of a planar surface of the casted product. Results show the efficiency of the proposed method that, despite its simplicity, is able to provide an optimal solution with a small number of iterations.
Chatter prediction accuracy is significantly affected by reliability of data entry, i.e., cutting force coefficients and frequency response, both influenced by spindle speed. The evaluation of ...specific cutting force coefficients in High-Speed Milling (HSM) is challenging due to the frequency bandwidth of commercial force sensors. In this paper specific cutting coefficients have been identified at different spindle speeds: dynamometer signals have been compensated thanks to an improved technique based on Kalman filter estimator. The obtained speed-varying force coefficients have been used to improve the reliability of stability lobe diagrams for HSM, as proven by experimental tests.
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