Spindle vibration suppression for advanced milling process by using self-tuning feedback control

The goal of this work is to concurrently counterbalance the dynamic cutting force and regulate the spindle position deviation under various milling conditions by integrating active magnetic bearing (AMB) technique, fuzzy logic algorithm, and an adaptive self-tuning feedback loop. The experimental data, either for idle or cutting, are utilized to establish the database of milling dynamics so that the system parameters can be on-line estimated by employing the proposed fuzzy logic algorithm as the cutting mission is engaged. Based on the estimated milling system model and preset operation conditions, i.e., spindle speed, cut depth, and feed rate, the current cutting force can be numerically estimated. Once the current cutting force can be real time estimated, the corresponding compensation force can be exerted by the equipped AMB to counterbalance the cutting force, in addition to the spindle position regulation by feedback of spindle position. At the end, the experimental simulations on realistic milling are presented to verify the efficacy of the fuzzy controller for spindle position regulation and the capability of the dynamic cutting force counterbalance.     

Anti-windup embedded magnetic actuator applied for near-high-speed milling

A fuzzy anti-windup (AW) compensator is proposed and applied to the embedded cylindrical-array magnetic actuator (ECAMA) to ensure the superior performance of spindle position regulation for milling machines under actuator saturation. Since ECAMA is a type of active magnetic bearing (AMB), the supplied coil current and the induced magnetic force are both limited by the maximum current and power output of the AMB and the associated amplifier. Once the magnetic actuator is saturated, the required control input cannot be realized by ECAMA and may lead to drastic tremble of spindle position. In this work, an AW compensator, based on fuzzy logic algorithm, is therefore proposed to rectify the control input to ECAMA. By employing commercial software MATLAB/Simulink and signal processing interface, Module DS1104 by dSPACE, the efficacy of the fuzzy AW compensation is practically verified by intensive experiments.     

Fuzzy Logic Based In-Process Tool-Wear Monitoring System in Face Milling Operations

A fuzzy logic based in-process tool-wear monitoring (FL-ITWM) system is developed and presented in this paper. The fuzzy membership function and rule bank were based on observations during cutting experiments using artificial tool-wear inserts in face milling operations. Maximum resultant force and cutting parameters (feedrate and depth of cut) were used as independent variables, and the system was verified using 25 test experimental runs. A t-test using an alpha value of 0.05 demonstrated that the system has greater than 90% accuracy when predicting tool-wear magnitude on-line.     

In-Process Tool Fracture monitoring in Face Milling Using Spindle Motor Current and Tool Fracture Index

A new algorithm for tool fracture detection using spindle motor current is suggested for face milling. A tool fracture index (TFI) is suggested to represent the magnitude of tool fracture. Dynamic cutting force variation in the face milling process is measured indirectly using spindle motor current. Even though the dynamic sensitivity of the spindle motor current is low, the cutting force can be correctly represented by the spindle r.m.s current in rough face milling. In rough milling, tool fracture is distinguished well from cutter run-out and transient cutting. The magnitude of tool fracture can be predicted by the proposed tool fracture index using the spindle motor current.     

Cutting deflection control of the blade based on real-time feedrate scheduling in open modular architecture CNC system

Machining accuracy of thin-walled parts which have low-rigidity is greatly influenced by cutting deflection in flank milling. In this paper, cutting deflection of aero-engine blade during processing is controlled within a required dimensional accuracy based on the strategy of real-time feedrate scheduling which is integrated into an open modular architecture CNC system (OMACS) of five-axis milling machine. The maximum deflection position of blade is determined through combining analytical cutting force model in flank milling and finite element analysis (FEA)-based transient dynamic analysis. Then, the numerical model of blade deflection is established to obtain the numerical relationship among feedrate, cutting force, and blade deflection, which is usually used to get optimized cutting force and feedrate by setting allowable value of blade deflection. To implement blade deflection control during machining, a real-time control strategy of feedrate scheduling based on nonlinear root-finding algorithm of Brent-Dekker and principle of feedrate smooth transition is developed and integrated into OMACS which has functions of real-time cutting force signal processing and real-time feedrate adjustment. Experimental results show that blade deflection is effectively controlled by proposed strategies, machining accuracy, and efficiency are improved.     

基于现场总线的恒功率约束切削加工自适应控制

以进给速度为控制对象的自适应约束控制(ACC)技术有利于提高数控加工效率和实现刀具保护,在分析Profibus总线和模糊控制特点的基础上,针对铣削加工过程的非线性、时变性和不确定性,提出了基于现场总线的铣削加工过程自适应模糊控制的解决方案.采用恒功率约束,利用比例因子在线自调整的方法对切削参数变化的进给速度进行在线控制.仿真结果表明,方案克服了传统的模糊控制动态响应较慢的鲁棒性较差的缺点,具有响应速度快、实时性和稳定性好等优点.当切削深度突变时,能在线自适应调整进给速度,使切削功率接近参考值,防止刀具损坏和提高加工效率.     

Indirect Cutting Force Measurement Considering Frictional Behaviour in a Machining Centre Using Feed Motor Current

In machine tools, friction exists between the table and the guideways, and in ballscrews. In this paper, feed motor current is measured by a hall effect current sensor. It is used to calculate the motor torque which, in turn, is the frictional torque at steady state. Some frictional phenomena are studied in feed drive systems of a horizontal machining centre, such as the relationship between feedrate and frictional torque, the relationship between frictional torque and table feed position, and the slideway cover effects on frictional torque. Considering all these frictional phenomena, the relationship between the feed force and the feed motor current is obtained. Feed force can be estimated well from the feed motor current considering frictional behaviour. The relationship between the cutting force and the feed motor current is slightly different during up milling and down milling, because y(vertical) directional cutting force changes the frictional force.     

A Fuzzy-Net-Based Multilevel In-Process Surface Roughness Recognition System in Milling Operations

This paper describes a fuzzy-nets approach for a multilevel in-process surface roughness recognition (FN-M-ISRR) system, the goal of which is to predict surface roughness (Ra ) under multiple cutting conditions determined by tool material, workpiece material, tool size, etc. Surface roughness was measured indirectly by extrapolation from vibration signal and cutting condition data, which were collected in real-time by an accelerometer sensor. These data were analysed and a model was constructed using a neural fuzzy system. Experimental results showed that parameters of spindle speed, feedrate, depth of cut, and vibration variables could predict surface roughness (Ra) under eight different combinations of tool and workpiece characteristics. This neural fuzzy system is shown to predict surface roughness (Ra ) with 90% prediction accuracy during a milling operation.     

基于主轴电流的铣削力间接监测方法

实时准确地监测铣削状态对于提高加工质量与加工效率具有重要意义,切削力作为重要的加工状态监测对象,因其监测设备昂贵且安装不便而受到限制,为此提出一种考虑刀具磨损的基于主轴电流的铣削力监测方法.首先基于切削微元理论建立了考虑后刀面磨损的铣削力模型,并通过铣削实验进行铣削力模型系数标定;然后对主轴电流与铣削力的关系进行理论建...     

Optimization of cutting conditions for minimum residual stress,cutting force and surface roughness in end milling of S50C medium carbon steel

Residual stresses are usually imposed on a machined component due to thermal and mechanical loading. Tensile residual stresses are detrimental as it could shorten the fatigue life of the component; meanwhile, compressive residual stresses are beneficial as it could prolong the fatigue life. Thermal and mechanical loading significantly affect the behavior of residual stress. Therefore, this research focused on the effects of lubricant and milling mode during end milling of S50C medium carbon steel. Numerical factors, namely, spindle speed, feed rate and depth of cut and categorical factors, namely, lubrication and milling mode is optimized using D-optimal experimentation. Mathematical model is developed for the prediction of residual stress, cutting force and surface roughness based on response surface methodology (RSM). Results show that minimum residual stress and cutting force can be achieved during up milling, by adopting the MQL-SiO₂ nanolubrication system. Meanwhile, during down milling minimum residual stress and cutting force can be achieved with flood cutting. Moreover, minimum surface roughness can be attained during flood cutting in both up and down milling. The response surface plots indicate that the effect of spindle speed and feed rate is less significant at low depth of cut but this effect significantly increases the residual stress, cutting force and surface roughness as the depth of cut increases.     

Feedrate scheduling strategy for free-form surface machining through an integrated geometric and mechanistic model

In free-form surface machining, it is essential to optimize the feedrate in order to improve the machining efficiency. Conservative constant feedrate values have been mostly used up to now since there was a lack of physical models and optimization tools for the machining processes. The overall goal of this research is the integration of geometric and mechanistic milling models for force prediction and feedrate scheduling in five-axis CNC free-form surface machining. For each tool move, the geometric model calculates the cut geometry, and a mechanistic model is used along with a maximum allowable cutting force to determine a desired feedrate. The results are written into the part NC program with optimized feedrates. When the integrated modeling approach based feedrate scheduling strategy introduced in this paper was used, it was shown that the machining time can be decreased significantly along the tool path.     

In-Process Surface Roughness Recognition (ISRR) System in End-Milling Operations

This paper describes a new approach for surface roughness recognition (ISRR) systems to predict surface roughness (Ra) in-process using an accelerometer to measure vibration signals and cutting conditions while end-milling is taking place. The analysis of the data and the model building is carried out using a neural fuzzy system. Experimental results show that the parameters of spindle speed, feedrate, depth of cut, and vibration variables can predict the surface roughness (Ra) effectively. Surface roughness can also be predicted with a 96% accuracy rate by ISRR using the neural fuzzy system.     

基于有限元数值模型和进给速度优化的薄壁件侧铣变形在线控制

为实现在加工过程中对薄壁件侧铣产生的较大切削变形进行在线控制,提出基于有限元数值模型和进给速度优化的在线控制策略。根据薄壁件切削过程的有限元仿真结果,建立数控机床进给速度、切削力、工件切削变形间的数值模型,进而确定用于控制变形的最优目标切削力。在具有开放式模块化的数控系统平台上开发了切削力信号实时采集、滤波功能和基于Brent-Dekker算法的进给速度在线优化策略,并根据滤波后的切削力及相应算法在加工过程中实时调整机床进给速度,保证切削力逐渐接近最优控制目标而实现切削变形的在线控制。试验结果表明,经过进给速度在线优化后的切削过程可将薄壁件侧铣变形控制在规定范围内,同时提高了切削效率。     

A SIMPLIFIED SOLUTION FOR THE DEPTH OF CUT IN MULTI-PATH BALL END MILLING

Abstract

The axial depth of cut is an important factor in the dynamic cutting force analysis of milling. In multi-path ball end milling, it varies with the cutting edge position angle. General equations are derived from which the instant depth of cut in ball end milling can be calculated. Examples are given for four path increment modes. The cutting condition in each mode is discussed with respect to the depth of cut. The conditions needed to disengage the tip of the ball end mill from the cut are determined. The "step-up" increment mode has the most favorable cutting condition for cutter tip relief and high cutting velocity. In order to obtain an instant evaluation of the cutting stability, the equations of maximum depth of cut in ball end milling are derived. The exact solutions are obtained from the general equations for the instant depth of cut. More conservative estimates are obtained from the simplified solutions. The results in this paper can be used as a guide in NC part programming to select an optimal cutting strategy and to ensure a stable cutting process in ball end milling.     

A SIMPLIFIED SOLUTION FOR THE DEPTH OF CUT IN MULTI-PATH BALL END MILLING

The axial depth of cut is an important factor in the dynamic cutting force analysis of milling. In multi-path ball end milling, it varies with the cutting edge position angle. General equations are derived from which the instant depth of cut in ball end milling can be calculated. Examples are given for four path increment modes. The cutting condition in each mode is discussed with respect to the depth of cut. The conditions needed to disengage the tip of the ball end mill from the cut are determined. The "step-up" increment mode has the most favorable cutting condition for cutter tip relief and high cutting velocity. In order to obtain an instant evaluation of the cutting stability, the equations of maximum depth of cut in ball end milling are derived. The exact solutions are obtained from the general equations for the instant depth of cut. More conservative estimates are obtained from the simplified solutions. The results in this paper can be used as a guide in NC part programming to select an optimal cutting strategy and to ensure a stable cutting process in ball end milling.     

3D FEM Simulation of Milling Force in Corner Machining Process

To optimize milling force and machining accuracy quality in corner milling process, the changing law of milling force is revealed by Finite Element Method(FEM). Based on DEFORM software a serial of 3D FEM models for corner milling process are devloped. Tool curved trajectory is achieved by establishing accurate relationship of tool location with milling time. Adaptive remeshing technique and iterative algorithm are adopted to ensure convergence of FEM model. Component force characteristics are revealed by analyzing FEM simulation results. It indicates that the milling force in Y direction becomes negative comparing with forces in X and Z direction. Magnitude of forces in three directions decreases with increase of spindle speed, while it increases with increase of milling feedrate. The simulation results for cutting force are in good agreement with those obtained from experiment. The FEM simulation model is first successfully established for corner milling process in this study, and the results provide a guide for optimizing cutting parameters in cutting process.     

An experimental investigation on micro machining of fine-grained graphite

Industrial applications of the micro milling process require sufficient experimental data from various micro tools. Research has been carried out on micro milling of various engineering materials in the past two decades. However, there is no report in the literature on micro milling of graphite. This paper presents an experimental investigation on micro machinability of micro milling of moulded fine-grained graphite. Full immersion slot milling was conducted using diamond-coated, TiAlN-coated and uncoated tungsten carbide micro end mills with a uniform tool diameter of 0.5 mm. The experiments were carried out on a standard industrial precision machining centre with a high-speed micro machining spindle. Design of experiments (DoE) techniques were applied to design and analysis of the machining process. Surface roughness, surface topography and burrs formation under varying machining conditions were characterized using white light interferometry, SEM and a precision surface profiler. Influence of variation of cutting parameters including cutting speeds, feedrate and axial depth of cut on surface roughness and surface damage was analysed using ANOVA method. The experimental results show that feedrate has the most significant influence on surface roughness for all types of tools, and diamond tools are not sensitive to cutting speed and depth of cut. Surface damage and burrs analysis show that the primary material removal mode is still brittle fracture or partial ductile in the experimental cutting conditions. 3D intricate micro EDM electrodes were fabricated with good dimensional accuracy and surface finishes using optimized machining conditions to demonstrate that micro milling is an ideal process for graphite machining.     

圆角铣削颤振稳定域建模与仿真研究

为避免在圆角铣削加工中产生颤振,建立考虑再生作用的圆角铣削动力学模型,推导其平均方向力系数计算公式。鉴于圆角铣削时主轴转速通常远大于圆角处的进给角速度,两者的平均方向力系数近似相等。因此,经典直线铣削颤振稳定域解析模型适用于圆角铣削,前提是需要用最大径向啮合角代替名义径向啮合角进行仿真。根据铣刀与工件的啮合情况,将圆角铣削分为均匀切宽圆角铣削和非均匀圆角切宽铣削两类,并分别推导出其最大径向啮合角计算公式。在动力学建模基础上开发圆角铣削颤振稳定域仿真模块,仿真结果得到了切削试验的验证,为圆角铣削切削参数的选择提供了一条有效途径。     

高速铣削超高强度钢的切削力及表面粗糙度研究

选用涂层硬质合金刀具对300M超高强度钢进行高速铣削试验,通过单因素试验和多因素正交试验法,得出铣削参数(主轴转速、每齿进给量、铣削深度)对切削力及表面粗糙度的影响规律及主次关系。对正交试验结果做最小二乘法分析,建立切削力及表面粗糙度与铣削参数之间的经验模型;对经验模型的回归方程及系数做显著性检验,并对其进行参数优化,得出铣削参数的最优组合。结果表明:主轴转速和铣削深度对切削力的作用较大,而每齿进给量对其影响相对较弱;每齿进给量对表面粗糙度作用最强,铣削深度次之,主轴转速对其作用最弱。     

Feedrate optimisation/scheduling on sculptured surface machining: a comprehensive review, applications and future directions

Free-form or sculptured surface milling is one of the continually used manufacturing processes for die/mould, aerospace (especially turbine blades), precision machine design, bio-medical devices and automotive industries. Developments of machining technologies for quality enhancement of machining results have become a very important fact in current real industry. Therefore, reducing milling time, tool wear, cutter deflection and improving surface texture quality and machining operations through adaptation and optimisation of tool feedrates based on changing surface geometry in sculptured surface machining is a great step in this direction. Various feedrate optimisation strategies have different feedrate rescheduling control parameters such as chip thickness, material removal rate (MRR), min(mrr,chip,force), max(expo.Acc/dec) and resultant forces. Some commercial CAM softwares come with MRR-based feedrate optimisation algorithms which have a very short calculation time. However, commercial feedrate scheduling systems have some limitations in generating the scheduled feedrates because they use the MRR or the cutting force model which is dependent on milling conditions. However, for the processes in which machining precision/accuracy is very important, it is inevitable that mechanistic force-based feedrate optimisation approaches, for which the calculation time is improved, will be integrated into commercial CAM software packages. Here, developing only the mechanistic cutting force-based algorithm is not enough. In this paper, improvement and optimisation of machining feedrate value, which is one of the cutting parameters which has a tremendous effect on the precise machining of free-form surfaces, was discussed by using the virtual machining framework. For this purpose, the boundary representation solid modelling technique-based free-form milling simulation and feedrate optimisation system integrated with commercial CAD/CAM software is developed for three-axis ball-end milling. This review study includes the information regarding the following topics: The algorithms developed for the feedrate value optimisation, MRR calculation approaches, cutting force computation methods, details of algorithms, the effects on the surface accuracy, the effects on the machining time, the capabilities of the present commercial CAM software packages, the encountered difficulties and overcoming those difficulties, recent developments and future research directions.