Modeling and simulation of 5-axis milling processes

5-axis milling is widely used in machining of complex surfaces. Part quality and productivity are extremely important due to the high cost of machine tools and parts involved. Process models can be used for the selection of proper process parameters. Although extensive research has been conducted on milling process modeling, very few are on 5-axis milling. This paper presents models for 5-axis milling process geometry, cutting force and stability. The application of the models in selection of important parameters is also demonstrated. A practical method, developed for the extraction of cutting geometry, is used in simulation of a complete 5-axis cycle.     

Mechanistic modeling and accurate measurement of micro end milling forces

Micro milling operations can fabricate miniaturized components with high relative accuracy. Since micro machining operations are different than conventional macro machining processes, due to the large negative rake angle and elasto-plastic effects, it is important that the modeling of micro end milling forces incorporates the dynamics of the tool, ploughing and elastic recovery. This study examines the mechanistic modeling of shearing and ploughing domain cutting regimes to accurately predict micro milling forces. The tool dynamics are indirectly identified by performing receptance coupling analysis. Furthermore, the Kalman filter compensation method is used to precisely measure the forces to obtain the cutting constants.     

Dynamic Compensation of Spindle-Integrated Force Sensors

This paper presents a dynamically compensated Spindle-Integrated Force Sensor system to measure cutting forces. Piezo-electric force sensors are integrated into the stationary spindle housing to measure cutting forces in three directions. The transfer function of the spindle structure between the cutting forces acting on the tool tip and the measured forces at the spindle housing are identified. Using the cutting force signals measured at the spindle housing, a Kalman filter is designed to filter the influence of structural modes on the force measurements. The frequency bandwidth of the force measurement system is significantly increased with the proposed sensor and the signal processing method. Milling experiments with tooth passing frequencies up to 1000 Hz are presented with effective removal of cutting force distortions caused by three structural modes of the spindle.     

Cylindrical milling tools: Comparative real case study for process stability

Critical comparison is presented related to the stability behaviour of milling processes performed by conventional, variable helix and serrated milling tools. The paper presents a general milling model linked to any non-proportionally damped dynamic system. Extended multi frequency solution and semi-discretization are implemented and used to calculate the stability of stationary milling. Measurements performed in industrial environment validate the general numerical algorithm that is able to predict the stability conditions of milling processes carried out by cylindrical cutters of optional geometry. Both the calculations and the measurements confirm that, for roughing operations, the highest stability gain can be achieved by serrated cutters. It is also demonstrated that variable helix milling tools can achieve better stability behaviour only if their geometry is optimized for the given cutting operation.     

An expert troubleshooting system for the milling process

Common problems experienced in milling processes include forced and chatter vibrations, tolerance violations, chipping and premature wear of the tools. This paper presents an expert system which attempts to troubleshoot the source of milling problems by utilising dynamics data coupled with the opinion of the operator and acoustic Fourier spectrum data taken from the cutting process. The expert system utilises a fuzzy logic based process to interpret the signals and information, and recommends possible alterations to the process to achieve high-performance milling operations.Specific inference engines were developed to assess the chatter stability, variation in cutting force coefficient, tool run-out and forced vibration characteristics of the system. Lastly, a stability lobe plot interpretation engine to automate the lobe selection process and recommend new, chatter free cutting conditions, was also developed. The chatter stability inference engine was tested with real cutting data, through acoustic measurements taken from various cutting conditions on an aluminium milling process. The chatter inference engine successfully determined the stability of the system for each sampled cutting condition. The robustness of the troubleshooting system depends on the accuracy of acoustic and frequency response measurements.     

Feedrate optimization for freeform milling considering constraints from the feed drive system and process mechanics

This paper presents a new and comprehensive strategy for planning minimum cycle time tool trajectories subject to both machining process related constraints, and also limitations of the feed drive control system. The machining process is considered by computing the workpiece-tool engagement along the toolpath and setting local feed limits to maintain a specified resultant cutting force. The drive constraints are considered by limiting the velocity, acceleration, and jerk magnitudes commanded to each actuator. Feed profiling is realized with uninterrupted acceleration transitions, capable of spanning multiple toolpath segments. Effectiveness of the proposed strategy is demonstrated in sculptured surface machining experiments.     

Modeling regenerative workpiece vibrations in five-axis milling

During milling—especially of thin-walled components—the dynamic behavior of the workpiece-tool-machine-system influences the milling process and particularly the quality of the resulting workpiece surface. This article focuses on the presentation of a simulation concept for predicting regenerative workpiece vibrations, which combines a finite element model for analyzing the dynamic behavior of the workpiece with a time domain simulation for the five-axis milling process. Both concepts, their linking, and the experimental setup for verifying the simulation will be described. A comparison of the simulation results with the data measured in experiments with regard to the vibration frequencies as well as the surface quality will be given.     

Gap control for near-dry EDM milling with lead angle

A new gap control strategy for five-axis milling using near-dry electrical discharge machining (EDM) has been experimentally investigated. The conventional EDM control strategy only allows the retraction of the electrode in the direction of machining trajectory, which results in inefficient gap control when the electrode is not perpendicular to the workpiece. The new gap controller is capable of retracting the electrode in the direction of its orientation. This enables more efficient enlargement of the discharge gap leading to faster recovery of average gap voltage. Experimental results show a 30% increase in material removal rate while the tool electrode wear ratio and surface roughness are not affected. Furthermore, EDM efficiency is improved due to the change in the electrode retraction in its axial direction. The gain tuning of the proposed controller is also discussed. This study shows the direction of electrode retraction is important for five-axis near-dry EDM milling.     

Modelling of the electrochemical machining process by the boundary element method

This paper describes the development and application of the boundary element method to model the machining of simple milling and turning features. The 3D model uses linear triangular elements to discretise the workpiece and tool surfaces. Highlights of the program include the use of analytical integration to calculate the element matrices rather than numerical, and the automatic refinement of the mesh as the workpiece is progressively machined. The program has been tested for milling slots using a rectangular tool and for turning a thin-walled tube. It is shown that there is good agreement between the predicted and experimental results.     

基于模糊芯片的加工过程智能控制

以F100模糊芯片为例,设计了基于模糊芯片的控制系统,并在非线性的加工过程进行了仿真实验,同时运用于铣削过程的现场实验。仿真和现场试验表明,用模糊芯片所构造的模糊控制器具有较好的鲁棒性,可适应于加工过程的工况变化。     

Finite element method based machining simulation environment for analyzing part errors induced during milling of thin-walled components

The rigid body motion of the workpieces and their elastic–plastic deformations induced during high speed milling of thin-walled parts are the main root causes of part geometrical and dimensional variabilities; these are governed mainly from the choice of process plan parameters such as fixture layout design, operation sequence, selected tool path strategies and the values of cutting variables. Therefore, it becomes necessary to judge the validity of a given process plan before going into actual machining. This paper presents an overview of a comprehensive finite element method (FEM) based milling process plan verification model and associated tools, which by considering the effects of fixturing, operation sequence, tool path and cutting parameters simulates the milling process in a transient 3D virtual environment and predicts the part thin wall deflections and elastic–plastic deformations during machining. The advantages of the proposed model over previous works are: (i) Performs a computationally efficient transient thermo-mechanical coupled field milling simulation of complex prismatic parts comprising any combination of machining features like steps, slots, pockets, nested features, etc., using a feature based milling simulation approach; (ii) Predicts the workpiece non-linear behavior during machining due to its changing geometry, inelastic material properties and fixture–workpiece flexible contacts; (iii) Allows the modelling of the effects of initial residual stresses (residing inside the raw stock) on part deformations; (iv) Incorporates an integrated analytical machining load (cutting force components and average shear plane temperature) model; and (v) Provides a seamless interface to import an automatic programming tool file (APT file) generated by CAM packages like CATIA V5. The prediction accuracy of the model was validated experimentally and the obtained numerical and experimental results were found in good agreement.     

An overview of approaches to end milling tool monitoring

The increase in awareness regarding the need to optimise manufacturing process efficiency has led to a great deal of research aimed at machine tool condition monitoring. This paper considers the application of condition monitoring techniques to the detection of cutting tool wear and breakage during the milling process. Established approaches to the problem are considered and their application to the next generation of monitoring systems is discussed. Two approaches are identified as being key to the industrial application of operational tool monitoring systems.Multiple sensor systems, which use a wide range of sensors with an increasing level of intelligence, are seen as providing long-term benefits, particularly in the field of tool wear monitoring. Such systems are being developed by a number of researchers in this area. The second approach integrates the control signals used by the machine controller into a process monitoring system which is capable of detecting tool breakage. Initial findings mainly under laboratory conditions, indicate that both these approaches can be of major benefit. It is finally argued that a combination of these approaches will ultimately lead to robust systems which can operate in an industrial environment.     

Delayed-reference control (DRC) applied to machining operations

This paper deals with a non-time based control for machining operations. Such a control can be implemented in most of the CNC centers in a practical way. Basically, in this control, which belongs to the category of non-time based controllers, the desired input reference is a function of the time and of a variable which plays the role of a time delay: x_d(t−T). Indeed, the proposed controller is called delayed-reference control (DRC). It is recalled that in a time-based control, the desired input reference is described as a function of time only, which is referred to as reference time: x_d(t). The calculation of this function is usually implemented off-line by means of the so called path-planning process. According to this traditional approach, during the task execution, at each instant, the control module is required to track an input reference. What is relevant is that such a reference can never be modified by any event or circumstance. The DRC control differs from time-based controllers in the sense that time delay is properly calculated on-line according to the measured force signals in such a way to improve the interaction during the cutting process. In fact, the DRC consists in an outer force feedback loop around an inner position feedback loop. The effectiveness of the controller has been proven by means of its implementation on a three axes CNC center for wood machining. Eventually, experimental results are presented.     

Influence of machining cycle of horizontal milling on the quality of cutting force measurement for the cutting tool wear monitoring

The cutting tools are today used a lot by industry and they are expensive, so it was interesting to optimize their use, by developing a predictive method of their wear, particularly, the flank wear V _b . For this task, the flank tool wear was measured in off-line using a binocular microscope, whereas, the cutting forces are recorded by means of a dynamometer (Kistler 9255B). The acquired signatures are analyzed during the milling operation throughout the tool life. In this paper, we are interested in the extraction of the appropriate indicators which characterize the tool wear by temporal and frequential analyses of the cutting force signals; and highlighting the influence of the clamp holes and the machining cycle to the quality of the measurements.     

Providing stability maps for milling operations

This article presents a new stability prediction tool. The method is based on the dynamic behaviour of both milling tool and workpiece, computed using finite element method. Dynamic behaviour is expressed under the form of transfer functions and used to predict stability lobes at each tool position. The unconditionally stable depth of cut is then stored and displayed on a graphic representation of the machined surface under the form of colour axis, named stability map.An application of the method on a Renault cylinder block is presented as an illustration.     

加工过程物理仿真技术研究

加工过程仿真是虚拟制造的底层关键技术，同时也是数字化制造系统的重要组成部分，而物理仿真是其中的研究重点和难点。建模难，通用性和实用性差是目前物理仿真中存在的主要问题。文章总结了物理仿真的研究内容和方法，分析了研究中存在的问题及加工过程建模的常用方法。     

Fuzzy control of spindle torque for industrial CNC machining

Developing a dedicated control system for each and every machining process or machine is costly and time-consuming. Such a practice has obviously undermined the usefulness of many current systems. This paper presents a fuzzy control system that can be used for different machining processes. This system consists of a basic fuzzy logic controller, a fuzzy rule base, and a tuning mechanism used to enhance the adaptability of the system. Industrial tests have been carried out for both end milling and turning processes. The control signal is spindle torque, readily available on many CNC machines. The test results show that the system performs well on both end milling and turning operations and can easily adapt to tool changes as well as workpiece material changes.     

Thermal modelling of end milling

Determination of the temperatures during machining is one of the most important challenges for accurate milling simulations. Coupled with excessive shearing, plastic deformation and friction in a small region of cutting, the temperatures in milling may have very significant impact on parts and tools such as dimensional errors, residual stresses and tool wear. Temperature exhibits a non-linear complex-modelling problem in milling process. In this article, for the first time, a novel thermal modelling is introduced for fast and accurate prediction of temperatures in end milling processes. A theoretical modelling approach and experimental validations are presented for various cutting conditions.     

A comparison of model-based machining force control approaches

Machining force regulation provides significant benefits in productivity and part quality. Adaptive techniques have typically been utilized due to the tremendous parameter variations that are found in machining processes. While adaptive controllers provide greater stability as compared to fixed-gain controllers, they have found very little headway in industry due to complexity in design, implementation, and maintenance. Recently, model-based techniques, with and without process compensation (i.e., the ability to directly adjust controller gains given known changes in process parameters), have been explored. This paper provides a comparison of four model-based machining force controllers; namely, linearization, log transform, nonlinear, and robust. These controllers are compared to an adaptive machining force controller in terms of transient performance and stability robustness with respect to parameter variations, and in terms of stability robustness with respect to unmodeled dynamics via simulation and experimental studies. The developed stability analyzes for the model-based controllers provide excellent predictions of the stability boundaries in the parameter space. Thus, stability robustness in terms of both model parameter variation and controller parameter adjustments can be systematically explored. Also, the results demonstrate that the stability robustness of the model-based controllers is insensitive to unmodeled servomechanism dynamics. While each force control approach performed satisfactorily in a laboratory environment, it can be generally concluded that their implementation should be dictated by the economics of the production environment.     

Improvement of a delayed velocity reference control (DVRC) for machining operations

This paper deals with a non-time-based controller for machining operations, suitable to control the interaction force between the tool and the workpiece.In the traditional approach usually the tool feed velocity (on which the force depends) is predetermined: it is chosen on the basis of some important parameters, such as tool dimensions, shape and material, the depth of cut, the workpiece material, etc. In this traditional approach, the reference function is calculated off-line, and during the task execution, at each instant, the control module is required to track the input reference; the drawback of this approach is that the system is not able to modify its velocity if unpredictable events occur or if side conditions change; this could produce high forces that could damage the tool or the workpiece.This paper presents a non-time-based controller for machining operations. Basically, in this control, the desired input reference is calculated on-line according to the measured force signal in such a way to improve the interaction. The controller allows to set the optimum interaction force F_max. In addition to this the same control is able to move the tool, during free motion paths, with a constant customizable velocity V_max, thus implementing a velocity control. As a third feature, it is possible to set the maximum allowed acceleration A_max of the end effector during the motion. This control scheme, called delayed velocity reference control (DVRC) belongs to the category of non-time-based controllers, since the reference signal is not directly related to time.The effectiveness of the controller has been proven by means of its implementation on a three-axes CNC center for EPS foam cutting.