Trajectories of forces and displacements in stable and unstable milling (original) (raw)
Related papers
Continuous model for analytical prediction of chatter in milling
International Journal of Machine Tools and Manufacture, 2009
A novel analytical approach for prediction of chatter in milling process is presented. Existing approaches use lumped-parameter models to define the dynamics of tools/workpieces. In this paper a continuous beam model is employed for prediction of milling operations dynamics. The tool boundary conditions are elastic support at the tool/holder/spindle interface and free support at the other end. Employing the continuous model eliminates the need for tool tip frequency response function (FRF) measurements in tool-tuning practice, especially in micro-milling, where FRF measurement is practically very difficult. Tool/holder/spindle interface parameters, once identified, can be used for other tool lengths. The impact hammer test is used to identify stiffness and damping parameters of the tool/holder/spindle interface. Using the new analytical approach and picking single-frequency solution (SFS), stability lobes are obtained for a slotting operation. The resulting lobes are compared to those obtained by the well-proven lumped-parameter model. In addition to a good general agreement between the two approaches, the continuous model prediction is more conservative for critical depth of cut, which is attributed to its ability to consider all participating modes in the response and so represents a more accurate representation of the system.
On process damping induced by vibration-dependency of cutting direction in milling
Procedia CIRP, 2018
The process damping effect is analyzed for regenerative machine tool chatter in milling via a velocity-dependent cutting force model. This model takes into account that the effective cutting direction depends on the vibrations of the machine tool-workpiece system, which modifies the effective rake angle, the chip thickness, and the cutting force. The model was originally introduced for turning operations where it results in a process damping term that improves the stability of metal cutting at low cutting speeds. Now this model is extended to milling. It is shown that the vibration-dependency of the cutting direction induces a time-periodic process damping term that is negative when the radial immersion is low. This decreases the stability at low cutting speeds, thus the low-speed stability improvement phenomenon in low radial immersion milling can be explained by extended process damping models only.
Linear analysis of chatter vibration and stability for orthogonal cutting in turning
International Journal of Refractory Metals & Hard Materials, 2011
The productivity of high speed milling operations is limited by the onset of self-excited vibrations known as chatter. Unless avoided, chatter vibrations may cause large dynamic loads damaging the machine spindle, cutting tool, or workpiece and leave behind a poor surface finish. The cutting force magnitude is proportional to the thickness of the chip removed from the workpiece. Many researchers focused on the development of analytical and numerical methods for the prediction of chatter. However, the applicability of these methods in industrial conditions is limited, since they require accurate modelling of machining system dynamics and of cutting forces. In this study, chatter prediction was investigated for orthogonal cutting in turning operations. Therefore, the linear analysis of the single degree of freedom (SDOF) model was performed by applying oriented transfer function (OTF) and \tau decomposition form to Nyquist criteria. Machine chatter frequency predictions obtained from both forms were compared with modal analysis and cutting tests.
Prediction of Workpiece Dynamics and its Effects on Chatter Stability in Milling
The workpiece dynamics affect stability in machining of flexible parts. However, it is not a straightforward task to include it in the analysis since the workpiece dynamics continuously change due to mass removal and variation of the cutter contact. In this paper, a methodology for prediction of in-process workpiece dynamics is presented, which is based on a structural dynamic modification using the FE model of the workpiece. The cutter location (CL) file is used to determine the removed elements at each tool location along a cycle. The proposed approach is demonstrated on example cases, and simulations are verified through experiments.
Dynamic Characterization of Milling Based on Interrupted Feed Motion
MM Science Journal, 2021
This study presents an experimental method for detecting and avoiding chatter vibrations that occur during general milling processes. The main idea is to capture the so-called dominant spectral properties from the transient vibrations of the milling process, which gives a good approximation for its dynamical behavior and provides a quantitative measure of stability. To induce transient vibration, the machining process is momentarily interrupted. Therefore, the method offers the possibility that the stability limit can be forecasted by extrapolation from stable and accurate measurement points without reaching harmful vibration on the machine tool. We present laboratory tests with momentary interrupted straight tool path to demonstrate the applicability of the proposed method.
Chatter Stability Improvement of a Slender Milling Tool via a Dynamics Variation Mechanism
Research Square (Research Square), 2023
In this study, a milling tool with variable mass and stiffness is developed for chatter reduction. The proposed milling tool is hollow with a solid core threaded inside it. As the core is screwed in or out of the tool body, the equal mass and stiffness of the tool are changed. Therefore, the tool's frequency response function (FRF) is changed to affect the stability lobe diagram (SLD) position. Moving the SLDs can stabilize an unstable cutting process. With respect to the FRFs obtained from modal test, the idea is proven using an experimental and analytical approach. The optimum core position for every spindle speed is also presented. The developed tool stability is then investigated in a realistic cutting condition. The cutting process sound analysis and surface finish visual inspection results reveal the performance of the proposed system in chatter reduction of a slender milling tool.
Machine Tool Chatter and Surface Location Error in Milling Processes
Journal of Manufacturing Science and Engineering, 2006
A two degree of freedom model of the milling process is investigated. The governing equation of motion is decomposed into two parts: an ordinary differential equation describing the periodic chatter-free motion of the tool and a delay-differential equation describing chatter. The stability chart is derived by using the semi-discretization method for the delay-differential equation corresponding to the chatter motion. The periodic chatter-free motion of the tool and the associated surface location error (SLE) are obtained by a conventional solution technique of ordinary differential equations. It is shown that the SLE is large at the spindle speeds where the ratio of the dominant frequency of the tool and the tooth passing frequency is an integer. This phenomenon is explained by the large amplitude of the periodic chatter-free motion of the tool at these resonant spindle speeds. It is shown that large stable depths of cut with a small SLE can still be attained close to the resonant s...
Journal of Sound and Vibration, 2012
In this paper, internal resonance and nonlinear dynamics of regenerative chatter in milling process is investigated. An extended dynamic model of the peripheral milling process including both structural and cutting force nonlinearities is presented. Closed form expressions for the nonlinear cutting forces are derived through their Fourier series components. In the presence of the large vibration amplitudes, the loss of contact effect is included in this model. Using the multiple-scales approach, analytical approximate response of the delayed nonlinear system is obtained. Considering the internal resonance dynamics (i.e. mode coupling), the energy transfer between the coupled x-y modes is studied. The results show that during regenerative chatter under specific cutting conditions, one mode can decay. Furthermore, it is possible to adjust the rate at which the x-mode (or y-mode) decays by implementation of the internal resonance. Therefore, under both internal resonance and regenerative chatter conditions, it is possible to suppress the undesirable vibration of one mode (direction) in which accurate surface finish is required. Under the steady-state motion, jump phenomenon is investigated for the process with regenerative chatter under various cutting conditions. Moreover, the effects of structural and cutting force nonlinearities on the stability lobes diagram of the process are investigated.
Chatter stability prediction in milling using speed-varying cutting force coefficients
2014
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.