Chatter-free Milling Research Articles

Parameters optimization for chatter-free milling tool path based on spindle speed variation method

Existing methods orienting for optimizing cutting parameters in milling process with spindle speed variation (SSV) only optimally selected the axial depth of cut and only considered the acceleration constraint of spindle. This article proposes a systematic optimization method to obtain the best combination of multiple machining parameters (axial and radial depths of cut) and SSV’s parameters (the ratio of variation amplitude and the ratio of variation frequency) so that the chatter-free milling tool path can be shortened. Both acceleration and jerk constraints of spindle are considered. A C3 continuous variable feedrate scheduling algorithm is used to eliminate the influence of spindle speed variation on feed per tooth, which is thus not required to treat as an optimization variable. The machining parameters of each iterative step are updated by following that the number of axial and radial cutting layers could be decreased. Discrete optimization method is combined with chatter stability analysis to solve the problem. A series of milling experiments verify that the proposed method can achieve the shortest tool path with chatter-free condition.

Chatter-free milling strategy of a slender Blisk blade via stock distribution optimization and continuous spindle speed change

The machining process of Blisk blades poses multiple challenges due to high requirements on surface quality and precision combined with high dynamic compliance of the thin-walled blades. Avoidance of chatter is thus of high priority in Blisk blade machining. However, the geometry of the Blisk blade array where the tool must fit between individual blades significantly limits the possibilities of controlling stability through the relative orientation of the tool and workpiece. Thus, the main parameters that can be used to control the stability of the process are the distribution of stock allowance and the spindle speed. Due to the effect of material removal on the blade’s dynamic properties, spindle speed needs to be adjusted throughout the machining process to keep it within the continuously changing stability gaps. This paper describes in detail an optimization procedure for the design of stock allowance distribution in such a way that a continuous spindle speed modulation is possible that avoids chatter throughout the machining process by maintaining spindle speeds within stability gaps. The presented algorithm uses finite element analysis software to simulate the effects of stock allowance distribution and material removal on workpiece dynamical properties. This information is then coupled with a stability model based on the Jacobian of the cutting force with respect to the regenerative deflection to identify the varying stability gaps throughout the machining process. The proposed method was experimentally verified.

Proposal of novel chatter-free milling strategy utilizing extraordinarily numerous flute endmill and high-speed high-power machine tool

This paper proposes a novel chatter-free milling strategy that utilizes the infinite stable area, i.e., area below the high-speed part of the 0-th lobe, which has not been generally utilized to date. In the conventional methodology, the stable pockets are utilized to realize a high axial depth of cut. However, these pockets tend to shift, e.g., due to long-term deterioration of the spindle, and hence they sometimes cannot be utilized effectively. The proposed strategy combines an extraordinarily numerous flute endmill, e.g., 20 flutes, and a high-performance spindle, e.g., capable of a spindle speed of 33000 min−1, installed on a high-speed high-power machine tool. By increasing the number of flutes, the stability lobes shrink in inverse proportion to that number against the spindle speed and the depth of cut, and the infinite stable area can be utilized in the practical spindle speed zone of recent machine tools. A design method is proposed for the cutting tool and the cutting conditions to utilize this area effectively. Analyses and experiments are conducted to verify the effectiveness of the proposed milling strategy. In the experiments, the material removal rate reaches 2904 cc/min at the spindle speed of 33000 min−1 and the axial depth of cut of 110 mm with no chatter vibration. The infinite stable area is capable of much higher stability and machining ability than the conventionally utilized stable pockets.

An ensemble approach to maximize metal removal rate for chatter free milling

In the present work, a new ensemble approach is adopted to obtain an optimal set of machining parameters resulting in chatter free milling at higher Metal Removal Rate (MRR). In order to achieve this, a modified LMD based on cubic spline interpolation is used to analyze the recorded audio signals during machining. Further, these decomposed chatter signals are explored to ascertain chatter severity in terms of a new dimensionless chatter indicator (CI). Total 27 experiment based on full factorial design are performed considering 3 levels of each machining parameters viz. axial depth of cut (D), feed rate (F) and spindle speed (N). CI and MRR are calculated for these 27 tests. Further, regression models of CI and MRR are developed using RSM (Response Surface Methodology) approach. Finally, multi-objective particle swarm optimization (MOPSO) is invoked to optimize these regression models for obtaining best possible range of input parameters for minimal chatter and maximum MRR. Moreover, to validate the presented methodology, more tests are performed considering the ascertained optimal set of parameters. Verification results proved that the developed optimal solution is feasible and can be used for higher productivity with chatter free milling.

Strategy development for chatter-free milling of Ti-6Al-4V thin-walled surfaces using stability region diagram (SRD)

The uncertainty in the prediction of machining stability increases with the increasing flexibility of the workpiece. The past research showed that the process cannot be stabilized even using the largest pockets in the stability diagrams due to high flexibility of the parts. Therefore, the present research emphasizes extensive experimental analysis where the uncertainty of the stability lobe diagrams was brought into the light, especially in the case of machining of flexible parts. This led to the introduction of a new term “Stability Region Diagram (SRD)” in the present research. The three generalized machining scenarios for chatter-free machining of thin-walled features on low rigidity Ti-6Al-V4 were experimentally analyzed. These include Scenario-I, where workpieces have identical pre-machining stiffness and natural frequency that varies during and after machining. In Scenario-II, workpiece stiffness and natural frequencies vary initially but are identical after machining. In Scenario-III, workpieces have identical pre-machining stiffness and natural frequency but vary during and after machining over central 1/3 part, straddled between bosses. Various machining strategies with an optimum combination of cutting speed-feed-radial depth of cut were developed using stability region diagrams (SRD) to achieve stable machining throughout the length of the cut along the flexible workpiece surface. For Scenario-I and II, stable machining is possible at widths of cut lesser than or equal to the final thickness of workpiece and at a constant spindle speed of 4,000 rpm throughout the length of the workpiece. However, while machining at widths of cut more than the post-machining thickness of the workpiece, the stable machining is possible using a spindle speed ramp-up technique. In the Scenario-III, the surrounding uncut material is found to make machining unstable which can be improved by spindle speed ramp-down technique.

Optimal milling modes identification of a jet-engine blade using time-domain technique

This paper considers a method of determining a chatter-free milling mode using time-domain milling dynamics simulation. The simulation is using a depth-buffer algorithm, an adaptive finite element model of the in-process workpiece, and a phenomenological model of cutting forces. The frequency characteristics of the in-process workpiece are obtained from the experimental modal analysis. The simulation analyzes the in-process workpiece vibration and actual cutting forces for the specified machining trajectory, as well as part surface quality after machining. The chatter-free milling mode is determined using a milling map produced by a multiple simulation for different spindle speed values. The milling map specifies regions where chatter may occur during milling and indicates the variation of the optimal spindle speed along the machining trajectory. The proposed method is used to determine a chatter-free milling mode for the finish milling of a jet-engine compressor blade. The blade is machined under the chatter-free milling mode, and it has good surface finish quality without chatter marks, whereas a blade machined under constant spindle speed contains chatter marks. The method can be used to engineer milling processes for other thin-walled workpieces with complex geometry and low stiffness.

Experimental study of stability prediction for high-speed robotic milling of aluminum

It has been fully demonstrated that the regenerative chatter theory is applicable for predicting chatter-free milling parameters for computer numerical control machine tools, but researchers are still arguing whether it is effective for robotic milling processes. The main reason is that the robot’s modes greatly shift, depending on its varying dynamic parameters and joint configurations. More experimental investigations are required to study and better understand the mechanism of vibration in robotic machining. The present paper is focusing on finding experimental support for chatter-free prediction in robot high-speed milling by the regenerative chatter theory. Modal tests are first conducted on a milling robot and used to predict stability lobes by zeroth order approximation. A number of high-speed slotting tests are then carried out to verify the prediction results. Thus, the regenerative chatter theory is proved to be also applicable to robotic high-speed milling. Furthermore, low-frequency modes of the robot structure are investigated by more modal experiments involving a laser tracker and a displacement sensor. The low-frequency modes are identified as the main part of the prediction error of the zeroth order approximation method, which could also be dominant in low-speed robotic milling processes. In addition, robots are different from computer numerical control machines in terms of stiffness, trajectory following error, forced vibration, and motion coupling. These long-period trend terms have to be carefully taken into account in the regenerative chatter theory for robotic high-speed milling.

Multi-Objective Machining Parameters Optimization for Chatter-Free Milling Process Considering Material Removal Rate and Surface Location Error

In machining parameters optimization of a chatter-free milling process, the inevitable surface location error (SLE) reflecting the machined workpiece dimension accuracy has been barely considered as one objective representing the machining quality, lowering the optimization accuracy. Therefore, this paper provides an approach to establish a multi-objective optimization model, where the material removal rate (MRR) represents the machining efficiency and the SLE predicted in time-domain represents the machining quality. The non-dominated sorting genetic algorithm (NSGA-II) method is used to solve the multi-objective model and provide pareto optimal solutions to first determine some ideal optimal solutions. Then the analytic hierarchy process (AHP) and grey target decision (GTD) methods are combined to select one most satisfactory optimal solution which has a well balance between the MRR and SLE. A multi-objective model was established and taken as a case study to maximize the MRR and minimize the SLE. Comparison study was performed on this multi-objective model and two other mono-objective models for obtaining the optimal MRR and SLE respectively, which was combined with the influences of machining parameters on SLE to show the necessity of conducting a multi-objective optimization. Milling tests were conducted based on the solved optimal machining parameters, and the well consistence between the measured and predicted SLEs shows that the proposed multi-objective optimization method can provide an effective approach to balance the machining efficiency and quality when there are conflicts between different objectives.

A Method of Chatter-Free Milling Parameters Optimization for Processing with High Energy Efficiency

The goal of machining is to fulfill processing for high energy efficiency on the basis that processing quality is guaranteed. Chatter-free cutting is a necessary condition to guarantee processing quality. In order to improve the energy utilization rate of milling, a cutting parameters optimization method for milling is proposed with chatter-free cutting as a constraint and energy utilization rate as an optimization objective. First of all, energy consumption characteristics of machine tool are analyzed and high-speed milling & cutting parameters optimization model is established with high energy efficiency as a goal and chatter-free stable cutting as a constraint. Next, on the basis of energy consumption characteristics of machine tool, an optimization model for high-speed chatter-free milling & cutting parameters is established with high energy efficiency as a goal. Stable domain chart for cutting is used to determine optimization range of rotational speed of main shaft. Optimization range of cutting depth and width and feeding speed is determined in combination with cutting force, cutting power, cutting speed and other constraints. Optimized step length of parameters is selected properly so as to obtain cutting parameters rapidly. In the end, confirmatory experiment is performed on a 5-axis engraving and milling machine. Result of the experiment shows energy utilization rate is improved by 154.5%, total energy consumption reduced by 61.5% and processing time shorten by 74.8%.

Analytical Modeling, Design and Performance Evaluation of Chatter-Free Milling Cutter With Alternating Pitch Variations

Machining efficiency could be increased and chatter vibrations could be suppressed by employing irregular pitch angles onto milling cutters. Applying variable pitch cutters could improve machining stability by disrupting the regenerative chatter mechanism and reducing cutting forces due to decreased nonlinear dynamic chip thicknesses. Previous studies on variable pitch cutters have been mainly investigating vibration suppression and dynamic modeling. There is a lack of efficient and practical variable pitch cutter design methods for practical industrial applications. An analytical method for designing milling cutters with alternating variable pitches has been proposed in this paper. In this method, a new variable, the generic sum of phase difference between the inner and outer waves, has been defined to model the machining dynamics and the stability of the variable pitch cutter. The phase difference value has then been investigated to obtain its generic optimal solution to the stability limit considering different cutting conditions. Based on this, the analytical variable pitch cutter design method is derived. The experimental results have manifested that, compared with uniform pitch cutter, the variable pitch cutter, designed using the proposed method, was able to achieve: 1) a critical stable axial depth of cut improvement of 126% under the desired spindle speed; 2) a cutting force reduction of 53%; 3) a decrease of 75% and 52% in Sq (the root mean square length of the scale limited surface) and Sz (the maximum height of the scale limited surface) of the machined surfaces, respectively. These significant improvements approved the superiority and robustness of the proposed design method for chatter suppression and may promote the application of variable pitch cutter technologies.

Identification of milling process damping using operational modal analysis

Abstract This paper provides a novel approach for identifying the process damping directly from chatter-free milling tests. First, power spectrum density matrix of cutter's deflections is theoretically derived by introducing transfer function and random excitation force, and the spectral decomposition of the power spectrum density matrix is formulated as an explicit function of modal parameters. Then, exponential attenuation method is adopted to extract the damping ratios from the inverse Fourier transformation result of the decomposed form. Finally, tangential and radial ploughing force coefficients, which are utilized to characterize process damping, are simultaneously calculated based on energy balance principle. Besides, experimental setup consisting of displacement sensors is specially designed to measure the cutter's deflections, which are further used to calculate the power spectrum density required in the above identification procedure. It is experimentally proven that the accuracy of chatter stability limits in milling process is improved when the proposed process damping model is considered.

Formation of Stability Lobe Diagram (SLD) for Chatter Free Milling on Aluminium Alloy

Chatter is a vibrational phenomenon which may cause damage of the tooling structure. The border between a stable (i.e., no chatter) and an unstable (i.e., with chatter) can be visualized in terms of the axial depth of cut as a function of spindle speed. The enveloping curve is a stability lobe diagram (SLD). Using SLD, it is possible to select cutting parameters to perform chatter-free cutting operations. In present study, an experimental methodology is followed based on tests in which a tapered workpiece allows gradual increase in axial depth of cut in feed direction, representing the Y-coordinate of the SLD while speed of the spindle is increased for every pass representing X-coordinate. Experiments are carried on Siemens 828D MTAB CompactMill machine. Acoustic sensor SKF CMAC 5084 is used to collect the milling sound. This work will allow operator to choose the combination of spindle speed and depth of cut for chatter free milling.

Note on a novel method for machining parameters optimization in a chatter-free milling process

Machining parameters optimization in a chatter-free milling process is a common issue for practical engineers at shop floor. The detailed and easily applicable optimization algorithm taking the milling mechanism into account is absent to date. This communication deals with the procedures of optimization of machining parameters in the chatter-free milling process. First, the model of machining parameters optimization is given based on the milling dynamics. The optimization objects are to minimize the tool vibration response and maximize the material removal rate, and the constraint conditions are to keep the milling process chatter-free or stable and let the spindle speed below a predefined one. Emphasis is put on how to select the spindle speed for reducing the tool vibration response and maximization of the material removal rate simultaneously. Augmented Lagrangian function method is employed to solve the nonlinear optimization problem. A numerical example is provided to show the effectiveness of the developed results. Besides, improvements on the calculation scheme of the tool vibration response have been made.

GRADE OF STABILITY IN FUZZY CHATTER STABILITY LOBE MODEL IN MILLING

The machining productivity and part quality are typically limited by the regenerative chatter induced by the dynamic interactions of spindle-holder-tool combination system. The conventional chatter stability model predicts the permissible stable axial depth of cut versus spindle speed by plotting the stability lobe diagram which represents two independent regions as absolutely stable zone and instable zone divided by the critical lobe curve. In fact, it is more reasonable to be a transition stage between the stable and instable zone. This paper introduced the grade of stability (GOS) to improve the conventional chatter stability model and study the transition zone in the stability lobe diagram. The variation of transition zone width with the stability sensibilities for different order lobe curve in milling system was analyzed. Sigmoid function was used as the membership function to develop the fuzzy stability lobe model. Then, the fuzzy stability lobe diagram with an adjustable slope coefficient was implemented to improve the mould steel milling process. The improved fuzzy stability model enhances the reliability of stability lobe diagram and guarantees the chatter-free milling process.

Experimental Introduction to Forced and Self-Excited Vibrations in Milling Processes and Identification of Stability Lobes Diagrams

In milling operations, cutting edge impacts due to the interaction between the cutter and the workpiece excite vibrations. It is possible to distinguish between free, forced and self-excited vibrations. Chatter is a self-excited vibration that can occur in machining processes, and is considered to be a common limitation of productivity and quality. Stability lobes diagrams (SLDs) show the frontier between chatter-free milling operations, i.e. stable dominated by forced vibrations, and operations with chatter, i.e. unstable. These diagrams are usually obtained from impact hammer testing. However, this method requires trained personnel with advanced knowledge and it is not easily applied in engineering studies or operator training. This paper presents an experimental method that allows engineering students and operators-in-training to observe the chatter phenomenon and to distinguish between forced and chatter vibrations and identify process stability diagrams.

Productivity improvement through chatter-free milling in workshops

High-speed machining has improved performance in manufacturing sectors such as the aeronautical, automotive, and mould and die industries. However, one problem of reaching such high speeds is the self-excitation of the spindle-tool system during the machining process. This so-called chatter vibration produces several negative effects: a reduction in surface quality and accuracy, and in tool and spindle life. Nowadays, it is still the most common limitation to increased productivity and quality of metal-removal operations. Although theoretically calculated stability-lobe diagrams can be used to calculate stable cutting conditions, some uncertainties affect the accuracy of this solution. This paper presents a method based on experimental tests where an inclined plane in the workpiece is machined at several spindle speeds. The gradual increase of the axial depth of cut allows the milling to be interrupted when chatter is detected. The final result is that the stability lobes are machined in the workpiece. This method can be used for process optimization and avoiding trial and error tests to determine optimal depths of cut and spindle speeds. The creation of a database with the optimal cutting conditions for a given spindle-tool holder-tool system can help the computer aided manufacturing (CAM) programmer select the cutting conditions that maximize the material removal rate and avoid vibration problems. The method presented is especially suitable for small- and medium-sized workshops where operators are unfamiliar with analytical methods or modelling techniques. It is an interesting way of saving money that does not require complex knowledge, and can easily be applied without additional costs owing to its simplicity.

Analysis of Linear and Nonlinear Chatter in Milling

A general mathematical model that describes all aspects of milling dynamics is presented. The proposed model becomes linear if each tooth of the milling cutter remains in contact with the workpiece along the entire length of the are being machined. For this case a new theoretical approach is used to predict the limit of the axial depth of cut for chatter-free milling. The applicability of this method to the general nonlinear case is also examined. The theoretical predictions are compared to the results of numerical simulations of the milling equations. It is shown that the proposed stability method yields accurate results for many practical milling cases.