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Papers by Niccolò Grossi

Research paper thumbnail of Spindle speed ramp-up test: A novel experimental approach for chatter stability detection

International Journal of Machine Tools and Manufacture, 2015

ABSTRACT Chatter is one of the most limiting factors in improving machining performances. Stabili... more ABSTRACT 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.

Research paper thumbnail of Cutting Forces Analysis in Additive Manufactured AISI H13 Alloy

Research paper thumbnail of Investigating Actuation Strategies in Active Fixtures for Chatter Suppression

Research paper thumbnail of A novel experimental-numerical approach to modeling machine tool dynamics for chatter stability prediction

Journal of Advanced Mechanical Design, Systems, and Manufacturing, 2016

Research paper thumbnail of Improved dynamic compensation for accurate cutting force measurements in milling applications

Mechanical Systems and Signal Processing, 2015

ABSTRACT Accurate cutting-force measurements appear to be the key information in most of the mach... more ABSTRACT 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.

Research paper thumbnail of Spindle speed ramp-up test: A novel experimental approach for chatter stability detection

International Journal of Machine Tools and Manufacture, 2015

ABSTRACT Chatter is one of the most limiting factors in improving machining performances. Stabili... more ABSTRACT 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.

Research paper thumbnail of Speed-varying cutting force coefficient identification in milling

Precision Engineering, 2015

ABSTRACT Accurate simulation of the machining process is crucial to improve milling performance, ... more ABSTRACT 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.

Research paper thumbnail of Modeling the orientation-dependent dynamics of machine tools with gimbal heads

ABSTRACT Machine tools with gimbal heads continuously rotate and/or pivot their spindle heads to ... more ABSTRACT Machine tools with gimbal heads continuously rotate and/or pivot their spindle heads to maintain the required relationship between the tool and the surface being cut. This results in orientation-dependent dynamic behavior at the tool-center-point, which further results in orientation-dependent machining stability of the system, potentially limiting productivity during continuous cutting. A dynamic substructuring procedure is proposed to model this behavior by orienting machine tool substructures to their proper configuration prior to synthesis to obtain the synthesized orientation-dependent response. Influence of the changing dynamics on chatter stability is further investigated; and, a strong dependence of machining stability on orientation is observed.

Research paper thumbnail of FEM based cutting velocity selection for thin walled part machining

Monolithic components are commonly used in the aeronautical industry due to their homogeneity and... more Monolithic components are commonly used in the aeronautical industry due to their homogeneity and excellent strength-toweight-ratio: ribs, stringers, spars, and bulkheads can be mentioned as examples. In order to assure enough stiffness to the whole component, monolithic parts are often made of thin walls, and webs, obtained usually starting from a raw block of material and removing up to 95% of the weight of the initial block. Therefore, increasing the removal rate as much as possible is the main condition to reach high productivity. The drawback is that, at high removal rate conditions (high feed, large depth of cut), the low stiffness of the thin walls causes dynamic problems, due to the forced vibrations induced by the milling process: even in case of a stable cutting process, vibrations appear as the result of the combination between the tool and workpiece natural frequencies, excited by the tooth-passing frequency harmonics. This paper presents an overview of a comprehensive milling process plan approach, based on finite element method (FEM), which by considering the effects of fixturing, tooltip dynamics, and material removal, allows to define the optimized cutting velocity in order to reduce deflections and vibrations during machining. The advantages of the proposed model over previous works are: (i) it provides a seamless interface to import the G-code generated by a CAM packages; (ii) it performs a computationally efficient modal superposition frequency response analysis of complex workpieces, considering the tooltip position provided by CAM; (iii) it predicts the workpiece non-linear behaviour during machining due to its changing geometry; (iv) it automatically tunes spindle speed continuously along the toolpath, taking into account both the tooltip dynamics and the local workpiece modal behaviour. The effectiveness of the proposed approach has been experimentally validated.

Research paper thumbnail of Chatter stability prediction in milling using speed-varying cutting force coefficients

Chatter prediction accuracy is significantly affected by reliability of data entry, i.e., cutting... more 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.

Research paper thumbnail of Milled surface generation model for chip thickness detection in peripheral milling

Procedia CIRP, 2013

Prediction of forces between tool and workpiece is essential in order to optimize machining and p... more Prediction of forces between tool and workpiece is essential in order to optimize machining and preserve process stability. In the last decades different predictive approaches have been developed: mainly mechanistic and numerical models. Mechanistic models could be applied to a wide range of cutter geometry and workpiece combination, even if a specific tuning, depending on material and application, is always needed. Numerical models could take in account many operative conditions than analytical ones, and allow predicting other parameters like stress, strain rate, temperature distribution, etc., but the computational time required is often unacceptable. The paper presents an innovative hybrid numerical-analytical approach for uncut chip cross-sectional area calculation in 2.5 axis end milling operations. The proposed model uses a mechanistic cutting force model to couple tool and workpiece finite element (FE) models: FE time domain simulations provide to predict effective paths of tool teeth relative to the workpiece, taking into account the dynamics of the entire system; while an appropriate algorithm, developed in Matlab ® , allows to achieve a more realistic uncut chip area, from which it is possible to calculate the cutting forces. This approach provides an accurate representation of the machined surface. Simulation is compared with experimental results. Finite Element (FE) models were limited to the twodimensional case of orthogonal cutting . Recent advance in software and hardware technologies, and the effort dedicated to develop the Arbitrary Lagrangian

Research paper thumbnail of 3D Finite Element Modeling of Holder-Tool Assembly for Stability Prediction in Milling

Procedia CIRP, 2015

Receptance coupling substructure analysis (RCSA) allows to estimate tool-tip FRF of different spi... more Receptance coupling substructure analysis (RCSA) allows to estimate tool-tip FRF of different spindle-holder-tool configurations with the minimum set of measurements. This technique requires accurate holder-tool connection modelling. In this paper fully predictive modelling strategies of holder-tool connection are presented. Proposed procedures are implemented in FE environment, without the use of any tuning experimental test, using solid elements to model the most common connection types. The main advantage of proposed approaches is to model the entire toolkit without requiring lumped stiffness and iterative procedures. Resulted toolkit FE model can be used in RCSA providing accurate tool-tip FRF for chatter prediction.

Research paper thumbnail of Spindle speed ramp-up test: A novel experimental approach for chatter stability detection

International Journal of Machine Tools and Manufacture, 2015

ABSTRACT Chatter is one of the most limiting factors in improving machining performances. Stabili... more ABSTRACT 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.

Research paper thumbnail of Cutting Forces Analysis in Additive Manufactured AISI H13 Alloy

Research paper thumbnail of Investigating Actuation Strategies in Active Fixtures for Chatter Suppression

Research paper thumbnail of A novel experimental-numerical approach to modeling machine tool dynamics for chatter stability prediction

Journal of Advanced Mechanical Design, Systems, and Manufacturing, 2016

Research paper thumbnail of Improved dynamic compensation for accurate cutting force measurements in milling applications

Mechanical Systems and Signal Processing, 2015

ABSTRACT Accurate cutting-force measurements appear to be the key information in most of the mach... more ABSTRACT 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.

Research paper thumbnail of Spindle speed ramp-up test: A novel experimental approach for chatter stability detection

International Journal of Machine Tools and Manufacture, 2015

ABSTRACT Chatter is one of the most limiting factors in improving machining performances. Stabili... more ABSTRACT 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.

Research paper thumbnail of Speed-varying cutting force coefficient identification in milling

Precision Engineering, 2015

ABSTRACT Accurate simulation of the machining process is crucial to improve milling performance, ... more ABSTRACT 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.

Research paper thumbnail of Modeling the orientation-dependent dynamics of machine tools with gimbal heads

ABSTRACT Machine tools with gimbal heads continuously rotate and/or pivot their spindle heads to ... more ABSTRACT Machine tools with gimbal heads continuously rotate and/or pivot their spindle heads to maintain the required relationship between the tool and the surface being cut. This results in orientation-dependent dynamic behavior at the tool-center-point, which further results in orientation-dependent machining stability of the system, potentially limiting productivity during continuous cutting. A dynamic substructuring procedure is proposed to model this behavior by orienting machine tool substructures to their proper configuration prior to synthesis to obtain the synthesized orientation-dependent response. Influence of the changing dynamics on chatter stability is further investigated; and, a strong dependence of machining stability on orientation is observed.

Research paper thumbnail of FEM based cutting velocity selection for thin walled part machining

Monolithic components are commonly used in the aeronautical industry due to their homogeneity and... more Monolithic components are commonly used in the aeronautical industry due to their homogeneity and excellent strength-toweight-ratio: ribs, stringers, spars, and bulkheads can be mentioned as examples. In order to assure enough stiffness to the whole component, monolithic parts are often made of thin walls, and webs, obtained usually starting from a raw block of material and removing up to 95% of the weight of the initial block. Therefore, increasing the removal rate as much as possible is the main condition to reach high productivity. The drawback is that, at high removal rate conditions (high feed, large depth of cut), the low stiffness of the thin walls causes dynamic problems, due to the forced vibrations induced by the milling process: even in case of a stable cutting process, vibrations appear as the result of the combination between the tool and workpiece natural frequencies, excited by the tooth-passing frequency harmonics. This paper presents an overview of a comprehensive milling process plan approach, based on finite element method (FEM), which by considering the effects of fixturing, tooltip dynamics, and material removal, allows to define the optimized cutting velocity in order to reduce deflections and vibrations during machining. The advantages of the proposed model over previous works are: (i) it provides a seamless interface to import the G-code generated by a CAM packages; (ii) it performs a computationally efficient modal superposition frequency response analysis of complex workpieces, considering the tooltip position provided by CAM; (iii) it predicts the workpiece non-linear behaviour during machining due to its changing geometry; (iv) it automatically tunes spindle speed continuously along the toolpath, taking into account both the tooltip dynamics and the local workpiece modal behaviour. The effectiveness of the proposed approach has been experimentally validated.

Research paper thumbnail of Chatter stability prediction in milling using speed-varying cutting force coefficients

Chatter prediction accuracy is significantly affected by reliability of data entry, i.e., cutting... more 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.

Research paper thumbnail of Milled surface generation model for chip thickness detection in peripheral milling

Procedia CIRP, 2013

Prediction of forces between tool and workpiece is essential in order to optimize machining and p... more Prediction of forces between tool and workpiece is essential in order to optimize machining and preserve process stability. In the last decades different predictive approaches have been developed: mainly mechanistic and numerical models. Mechanistic models could be applied to a wide range of cutter geometry and workpiece combination, even if a specific tuning, depending on material and application, is always needed. Numerical models could take in account many operative conditions than analytical ones, and allow predicting other parameters like stress, strain rate, temperature distribution, etc., but the computational time required is often unacceptable. The paper presents an innovative hybrid numerical-analytical approach for uncut chip cross-sectional area calculation in 2.5 axis end milling operations. The proposed model uses a mechanistic cutting force model to couple tool and workpiece finite element (FE) models: FE time domain simulations provide to predict effective paths of tool teeth relative to the workpiece, taking into account the dynamics of the entire system; while an appropriate algorithm, developed in Matlab ® , allows to achieve a more realistic uncut chip area, from which it is possible to calculate the cutting forces. This approach provides an accurate representation of the machined surface. Simulation is compared with experimental results. Finite Element (FE) models were limited to the twodimensional case of orthogonal cutting . Recent advance in software and hardware technologies, and the effort dedicated to develop the Arbitrary Lagrangian

Research paper thumbnail of 3D Finite Element Modeling of Holder-Tool Assembly for Stability Prediction in Milling

Procedia CIRP, 2015

Receptance coupling substructure analysis (RCSA) allows to estimate tool-tip FRF of different spi... more Receptance coupling substructure analysis (RCSA) allows to estimate tool-tip FRF of different spindle-holder-tool configurations with the minimum set of measurements. This technique requires accurate holder-tool connection modelling. In this paper fully predictive modelling strategies of holder-tool connection are presented. Proposed procedures are implemented in FE environment, without the use of any tuning experimental test, using solid elements to model the most common connection types. The main advantage of proposed approaches is to model the entire toolkit without requiring lumped stiffness and iterative procedures. Resulted toolkit FE model can be used in RCSA providing accurate tool-tip FRF for chatter prediction.