CAD assisted chatter-free NC tool path generation in milling

A chatter vibration-free NC tool path preparation strategy is presented for CAD/CAM systems. The stability charts, which contain chatter-free axial depth and radial width of cuts at a practical cutting speed range, are identified from time domain simulation of dynamic end milling operations. The time domain simulation system allows multiple modes of the machine tool-end mill structure whose transfer functions are measured. The existing pocketing routines in a CAD/CAM system are corrected by planning chatter-free axial depth and radial width of cuts that are automatically selected from the stability data bank during NC tool path generation. A newly introduced linear tool-path pocketing strategy is shown to be the most productive algorithm for dynamically corrected tool paths. The method is supported by experimental results.     

Linear analysis of chatter vibration and stability for orthogonal cutting in turning

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.     

Generalized dynamic model of metal cutting operations

This paper presents a unified mathematical model which allows the prediction of chatter stability for multiple machining operations with defined cutting edges. The normal and friction forces on the rake face are transformed to edge coordinates of the tool. The dynamic forces that contain vibrations between the tool and workpiece are transformed to machine tool coordinates with parameters that are set differently for each cutting operation and tool geometry. It is shown that the chatter stability can be predicted simultaneously for multiple cutting operations. The application of the model to single-point turning and multi-point milling is demonstrated with experimental results.     

Modelling and simulation of chatter in milling using a predictive force model

A predictive time domain chatter model is presented for the simulation and analysis of chatter in milling processes. The model is developed using a predictive milling force model, which represents the action of milling cutter by the simultaneous operations of a number of single-point cutting tools and predicts the milling forces from the fundamental workpiece material properties, tool geometry and cutting conditions. The instantaneous undeformed chip thickness is modelled to include the dynamic modulations caused by the tool vibrations so that the dynamic regeneration effect is taken into account. Runge–Kutta method is employed to solve the differential equations governing the dynamics of the milling system for accurate solutions. A Windows-based simulation system for chatter in milling is developed using the predictive model, which predicts chatter vibrations represented by the tool-work displacements and cutting force variations against cutter revolution in both numerical and graphic formats, from input of tool and workpiece material properties, cutter parameters, machine tool characteristics and cutting conditions. The system is verified with experimental results and good agreement is shown.     

Virtual process systems for part machining operations

This paper presents an overview of recent developments in simulating machining and grinding processes along the NC tool path in virtual environments. The evaluations of cutter–part-geometry intersection algorithms are reviewed, and are used to predict cutting forces, torque, power, and the possibility of having chatter and other machining process states along the tool path. The trajectory generation of CNC systems is included in predicting the effective feeds. The NC program is automatically optimized by respecting the physical limits of the machine tool and cutting operation. Samples of industrial turning, milling and grinding applications are presented. The paper concludes with the present and future challenges to achieving a more accurate and efficient virtual machining process simulation and optimization system.     

Flank face texture design to suppress chatter vibration in cutting

Process damping is useful in improving chatter stability in a low cutting speed range. This paper presents a texture design on tool flank faces that can effectively generate process damping. A convex structure on the flank face dampens chatter vibration even at general cutting speeds. An orthogonal cutting simulation utilizing a finite element analysis was conducted to estimate process damping force coefficients that are the functions of cutting and vibration conditions and tool geometry. Sufficient damping effect was predicted using the proposed texture via a chatter stability analysis in frequency domain. Face turning experiments verified the significant chatter suppression effect.     

Theoretical-experimental modeling of milling machines for the prediction of chatter vibration

Productivity of high speed milling operations can be seriously limited by chatter occurrence. Chatter vibrations can imprint a poor surface finish on the workpiece and can damage the cutting tool and the machine. Chatter occurrence is strongly affected by the dynamic response of the whole system, i.e. the milling machine, the tool holder, the tool, the workpiece and the workpiece clamping fixture. Tool changes must be taken into account in order to properly predict chatter occurrence. In this study, a model of the milling machine-tool is proposed: the machine frame and the spindle were modeled by an experimentally evaluated modal model, while the tool was modeled by a discrete modal approach, based on the continuous beam shape analytical eigenfunctions. A chatter identification technique, based on this analytical-experimental model, was implemented. Tool changes can be easily taken into account without requiring any experimental tests. A 4 axis numerically controlled (NC) milling machine was instrumented in order to identify and validate the proposed model. The milling machine model was excited by regenerative, time-varying cutting forces, leading to a set of Delay Differential Equations (DDEs) with periodic coefficients. The stability lobe charts were evaluated using the semi-discretization method that was extended to n>2 degrees of freedom (dof) models. The stability predictions obtained by the analytical model are compared to the results of several cutting tests accomplished on the instrumented NC milling machine.     

A novel tool path/posture optimization concept to avoid chatter vibration in machining – Proposed concept and its verification in turning

This research presents novel strategies to optimize tool path/posture to avoid chatter vibration in various machining operations. It is well known that the chatter stability depends on tool geometry and cutting conditions; whereas it is less known that it also depends on tool path/posture relative to the dynamically most compliant direction. In order to realize an intelligent tool path/posture planning with consideration of the chatter stability, a simple index is proposed to represent the machining stability due to the tool path/posture. As an example, the stability in turning is considered, and the use of proposed stability index is verified experimentally.     

The chaotic characteristics of three dimensional cutting

Chatter is an important problem in turning operations. However, there are very few studies on severe chatter because of the difficulties related to data collection and the complexities associated with non-linear cutting dynamics. In this study, cylindrical turning of long slender bars was simulated at three different cutting speeds by using experimentally identified cutting and structural dynamics transfer functions. The simulations were prepared at various depths of cuts, and variations of the cutting forces and displacements were investigated. Visual inspection and the Lyapunov spectrum of the cutting force signals suggested the chaotic nature of the cutting force signals. Although the frequency of the displacement signal remained constant, the amplitude of the signal appeared to have a chaotic nature. The same characteristics were also observed on the experimental data. During the design of a chatter detection or suppression system, the chaotic nature of the force and displacement signals must be considered. The unpredictable nature of the signals may cause prediction errors.     

Implementation of a process and structure model for turning operations

The consideration of the dynamic interaction between the machine tool structure and the cutting process is a prerequisite for the simulative prediction and optimization of machining tasks. However, existing cutting force models are either dedicated to already examined manufacturing operations or require extensive measurements for the determination of cutting coefficients. In this context this paper outlines a modular, analytical cutting force model applicable to common turning processes. It takes into account the dynamic material behavior, nonlinear friction ratios on the rake face as well as heat transfer phenomena in the deformation zones. On the part of the machine tool structure a parametric model based on the Finite Element Method (FEM) is implemented. Both models are coupled for the simulation of process and structure interactions, whereas the influence of the control system is considered as well. The simulation results were verified experimentally on a turning center.     

Chatter Stability of Metal Cutting and Grinding

This paper reviews fundamental modeling of chatter vibrations in metal cutting and grinding processes. The avoidance of chatter vibrations in industry is also presented. The fundamentals of orthogonal chatter stability law and lobes are reviewed for single point machining operations where the process is one dimensional and time invariant. The application of orthogonal stability to turning and boring operations is presented while discussing the process nonlinearities that make the solution difficult in frequency domain. Modeling of drilling vibrations is discussed. The dynamic modeling and chatter stability of milling is presented. Various stability models are compared against experimentally validated time domain simulation model results. The dynamic time domain model of transverse and plunge grinding operations is presented with experimental results. Off-line and real-time chatter suppression techniques are summarized along with their practical applications and limitations in industry. The paper presents a series of research topics, which have yet to be studied for effective use of chatter prediction and suppression techniques in industry.     

Active chatter suppression in turning by band-limited force control

The suppression of chatter vibration is required to enhance the machined surface quality and to increase tool life. In this study, a new, conceptually active approach for chatter suppression in machining is proposed. The hybrid control method developed by applying sensorless force control with a disturbance observer enables the simultaneous and independent control of the position trajectory and band-limited forces. The proposed method is introduced to the carriage of a prototype desktop-sized turning machine, and the ability to suppress chatter is evaluated by end-face cutting tests. The results demonstrate that actively controlling a band-limited force leads to the avoidance of chatter.     

In-process monitoring and detection of chip formation and chatter for CNC turning

In order to realize the intelligent machine tool, an in-process monitoring and detection of cutting states is developed for CNC turning machine to check and improve the stability of the processes. The method developed utilizes the power spectrum density, or PSD of dynamic cutting force measured during cutting. Experimental results suggested that there are basically three types of patterns of PSD when the cutting states are the continuous chip formation, the broken chip formation, and the chatter. The broken chip formation is desired to realize safe and reliable machining.     

A hybrid predictive model and validation for chip flow in contour finish turning operations with coated grooved tools

Unlike straight turning, the effective cutting conditions and tool geometry in contour turning operations are continuously changing with changing workpiece profile. This causes a wide variation in chip flow during the operation. Unfavorable chip flow can cause scratch/damage the machined surface, and lead to tool failure due to chip build up at the cutting edge. This paper presents a new methodology to predict the chip side-flow angle for complex grooved tool inserts in contour turning operations as well as experimental validation. Computer program has been developed to apply the predictive model to any contour workpiece profile and the capability of the predictive program is also presented in this paper.     

高精度棱体成形车刀的CAD/CAM系统

研究了高精度棱体成形车刀优化设计的数学模型,介绍了用VB语言开发的高精度棱体成形车刀的CAD/CAM子系统的构成及其主要模块的功能.该系统可由工件零件图直接生成加工该工件的高精度圆体成形车刀的零件图及刀具廓形的数控线切割加工程序,同时还能通过仿真检验所设计的刀具廓形及其加工程序代码的正确性.     

机床加工过程中的振动实验研究

分析了数控车床加工过程中切削颤振的产生机理,总结了目前国内外对切削颤振抑制理论和实践的研究现状.基于CJK6140数控车床利用脉冲激振法对刀架系统进行初步实验研究,并且提出了基于计算机仿真技术对加工过程进行动态仿真的方法,研究结果为提高加工效率提供了依据.     

Chatter stability of a slender cutting tool in turning with tool wear effect

An analysis of the chatter behavior for a slender cutting tool in turning in the presence of wear flat on the tool flank is presented in this research. The mechanism of a self-excited vibration development process with tool wear effect is studied. The components contributing to the forcing function in the turning vibration dynamics are analyzed in the context of cutting force and contact force. A comparison of the chatter stability for a fresh cutting tool and a worn cutting tool is provided. Stability plots are presented to relate width of cut to cutting velocity in the determination of chatter stability. Machining experiments at various conditions were conducted to identify the characteristic parameters involved in the vibration system and to identify the analytical stability limits. The theoretical result of chatter stability agrees qualitatively with the experimental result concerning the development of chatter stability model with tool wear effect.     

Virtual High Performance Milling

The goal of future manufacturing is to design, test and manufacture parts in a virtual environment before they are manufactured on the shop floor. This paper presents a generalized process simulation and optimization strategy for 2 1/2 axis milling operations to increase Material Removal Rate (MRR) while avoiding machining errors. The process is optimized at two stages. Optimal spindle speed, radial and axial depth of cut are recommended to process planner by considering the chatter, and spindle's torque/power limits. The cutter-part engagement conditions are extracted from CAD system by geometrically processing the NC program and part geometry. Long tool path segments are broken into smaller segments whenever the geometry varies. The spindle speed and feed fields of the NC program are automatically optimized by constraining maximum torque, power, tool deflection and chip load set by the user. The acceleration and speed limits of the machine tool feed drives are considered to prevent frequent variations of the feed unnecessarily. The optimization is experimentally verified by milling a helicopter gear box cover on a high speed, horizontal machining centre.     

Detection of chatter vibration in end milling applying disturbance observer

Suppression of chatter vibration is required to improve the machined surface quality and enhance tool life. For monitoring the chatter vibrations, additional sensors such as acceleration sensors are generally used, which results in high costs and low reliability of the machine tools. In this study, a novel in-process method to detect chatter vibrations in end milling is developed on the basis of a disturbance observer theory. The developed system does not require any external sensors because it uses only the servo information of the spindle control system. Self-excited and forced chatter vibrations are successfully detected.     

Prediction of machining chatter based on FEM simulation of chip formation under dynamic conditions

Machining chatter is an inherently nonlinear phenomenon that is affected by many parameters such as cutting conditions, tool geometry e.g., nose radius and clearance angle and frictional conditions at the tool/workpiece interface. Models for chatter prediction often ignore nonlinearities or introduce them through simple models for friction and geometry. In particular, the effect of chip–tool interaction on the occurrence of chatter is not investigated thoroughly. This paper presents a novel approach for prediction of chatter vibration and for investigation of the effects of various conditions on the onset of chatter. This approach uses finite element simulation to investigate the inter-relationship between the chatter vibration and the chip formation process. Simulation of chip formation is combined with dynamic analysis of machine tool to determine the interaction between the two phenomena. Mesh adaptation technique is used to move the tool inside the workpiece to form the chip, while a flexible tool is used to allow the tool to vibrate under variable loading conditions. By repeating the simulations under various widths of cut, it is shown that the onset of chatter can be detected, and the simulation is able to realistically predict various phenomena observed in actual machining process such as variation of shear angle and the increase of stability at lower speeds known as process damping. The stability map obtained from simulations is compared with experimental data attained through orthogonal cutting tests. Reasonable agreement observed between the two sets of results demonstrates the effectiveness of the simulation approach.