Error compensation in flexible end milling of tubular geometries

There are many machining situations where slender tools are used to machine thin walled tubular workpieces. Such instances are more common in machining of aircraft structural parts. In these cases, cutting force induced tool as well as workpiece deflections are quite common which result into surface error on machined components. This paper presents a methodology to compensate such tool and workpiece induced surface errors in machining of thin walled geometries by modifying tool paths. The accuracy with which deflections can be predicted strongly depends on correctness of the cutting force model used. Traditionally employed mechanistic cutting force models overestimate tool and workpiece deflections in this case as the change of process geometry due to deflections is not accounted in modeling. Therefore, a cutting force model accounting for change in process geometry due to static deflections of tool and workpiece is adopted in this work. Such a force model is used in predicting tool and workpiece deflection induced surface errors on machined components and then compensating the same by modifying tool path. The paper also studies effectiveness of error compensation scheme for both synclastic and anti-clastic configurations of tubular geometries.     

Tool path accuracy enhancement through geometrical error compensation

Kinematic and geometric errors of CNC machine tools, introduce large deviations in the real path traveled by the cutting tool. Tool path deviation reduces geometrical and dimensional accuracy of the machined features of the component. Tool path modification is an effective strategy to increase accuracy of the machined features. An improved error estimation model based on kinematic transformation concepts has been developed and used to calculate the volumetric overall error. These calculations are applicable for each arbitrary target positions of the machine's work space. Also a NC Program editor software has been developed in order to manage the calculations, modifications and to generate the new compensated NC program. The compensation procedure includes: fragmentation of nominal tool path to small linear elements, translating nominal position of elements to real positions using the Kinematics error model, finding compensated positions using the error compensation algorithm, converting newly generated elements to new tool paths using the packing algorithms and finally editing old NC program using NC code generator algorithm. Experimental tests showed 4-8 times accuracy improvement for linear, and S-pline tool paths deviations.     

Active integration of tool deflection effects in end milling. Part 2. Compensation of tool deflection

This study presents a compensation method in milling machining in order to take into account tool deflection during tool-path generation. Tool deflection that occurs during machining, and especially when flexible tools such as end mills are used, can result in dimensional errors on workpieces. The study presented here is part two of a two-part paper. In part one the cutting force models and the surface prediction method have been presented.Here the focus is on tool deflection effects' integration during the generation of the tool path. A strategy is proposed that modifies the nominal tool trajectory, compensates for the machining errors due to tool deflection, without degrading the production performance and the machined accuracy. The methodology allows optimization of the tool path trajectory in order to achieved a specified tolerance. Some experimental results are presented.     

5-Axis tool path smoothing based on drive constraints

In high speed machining, the real feedrate is often lower than the programmed one. This reduction of the feedrate is mainly due to the physical limits of the drives, and affects machining time as well as the quality of the machined surface. Indeed, if the tool path presents sharp geometrical variations the feedrate has to be decreased to respect the drive constraints in terms of velocity, acceleration and jerk. Thus, the aim of this paper is to smooth 5-axis tool paths in order to maximize the real feedrate and to reduce the machining time.Velocity, acceleration and jerk limits of each drive allow to compute an evaluation of the maximum reachable feedrate which is then used to localize the areas where the tool path has to be smoothed. So starting from a given tool path, the proposed algorithm iteratively smoothes the joint motions in order to raise the real feedrate. This algorithm has been tested in 5-axis end milling of an airfoil and in flank milling of an impeller for which a N-buffer algorithm is used to control the geometrical deviations. An important reduction of the measured machining time is demonstrated in both examples.     

Digital manufacture of large-grade hydro turbine's blades

The blade is one of the vital components and the most difficulty in manufacturing of large hydro turbines. In order to cost-effectively and productively manufacture these kinds of blades, a series of innovative digital techniques have been developed. It includes the digital design of hydro turbine blades based on manufacturers’ requirements, computer-aided location and the machined error evaluation with three-dimensional digitized measure, tool path generation strategy for enhancing machining efficiency and controlling deviation in NC machining, tool path generation and NC machining simulation by means of a virtual NC machining environment for blades, and feasible strategy and the systematic scheme for manufacturing of large blades with 5-axis simultaneous CNC machining technology. The developed innovative digital manufacture techniques have been successfully applied in the manufacturing of both the large grade Kaplan and Francis hydraulic turbine blades. It has been shown that the higher efficiency and the better surfaces finish accuracy can be achieved in practical engineering.     

An advanced FEA based force induced error compensation strategy in milling

The study introduces a multi-level machining error compensation approach focused on force-induced errors in machining of thin-wall structures. The prediction algorithm takes into account the deflection of the part in different points of the tool path. The machining conditions are modified at each step when the cutting force and deflection achieve a local equilibrium. The machining errors are predicted using a theoretical flexible force-deflection model. The error compensation is based on optimising the tool path taking into account the predicted milling error. The error compensation scheme is simulated using NC simulation package and is experimentally verified.     

Estimation of effective error parameters in high-speed micro-endmilling

In micro-endmilling, tool alignment errors and tool manufacturing errors such as flute deviation and cutting edge offset from the tool axis are relatively large compared to the associated chip load and machined features. Two effective error parameters have been defined to represent the effect of these errors on the machining accuracy and process performance: maximum radius of rotation and difference in the radii of rotation. Methods have been developed to estimate these parameters based on feature measurement on a machined artifact and acoustic emission signals during machining. Experiments were conducted to estimate the effective error parameters of three different tools at several different orientations. The estimation results show that there are significant variations in the effective errors among the tools, which indicates that estimation of the effective error parameters in micro-endmilling is important for achieving good processes performance on-line.     

Dimensional errors in longitudinal turning based on the unified generalized mechanics of cutting approach.: Part I: Three-dimensional theory

During the machining of a part, a new surface is generated together with its dimensional deviations. These deviations are due to the presence of several phenomena (workpiece deflection under strong cutting forces, vibration of the machine tool, material spring-back, and so on) that occur during machining. Each elementary phenomenon results in an elementary machining error. Consequently, the accuracy of the manufactured workpiece depends on the precision of the manufacturing process, which it may be controlled or predicted.The first part of this work presents a new model to evaluate machining accuracy and part dimensional errors in bar turning. A model to simulate workpiece dimensional errors in longitudinal turning due to deflection of the tool, workpiece holder and workpiece is shown. The proposed model calculates the real cutting force according to the Unified Generalized Mechanics of Cutting approach proposed by Armarego, which allows one to take into account the three-dimensional nature (3D) of the cutting mechanism. Therefore, the model developed takes advantage of the real workpiece deflection, which does not lie in a plane parallel to the tool reference plane, and of the real 3D cutting force, which varies along the tool path due to change in the real depth of cut. In the first part of the work the general theory of the proposed approach is presented and discussed for 3D features. In the second part the proposed approach is applied to real cases that are mostly used in practice. Moreover, some experimental tests are carried out in order to validate the developed model: good agreement between numerical and experimental results is found.     

Milling error prediction and compensation in machining of low-rigidity parts

The paper reports on a new integrated methodology for modelling and prediction of surface errors caused by deflection during machining of low-rigidity components. The proposed approach is based on identifying and modelling key processing characteristics that influence part deflection, predicting the workpiece deflection through an adaptive flexible theoretical force-FEA deflection model and providing an input for downstream decision making on error compensation. A new analytical flexible force model suitable for static machining error prediction of low-rigidity components is proposed. The model is based on an extended perfect plastic layer model integrated with a FE model for prediction of part deflection. At each computational step, the flexible force is calculated by taking into account the changes of the immersion angles of the engaged teeth. The material removal process at any infinitesimal segment of the milling cutter teeth is considered as oblique cutting, for which the cutting force is calculated using an orthogonal–oblique transformation. This study aims to increase the understanding of the causes of poor geometric accuracy by considering the impact of the machining forces on the deflection of thin-wall structures. The reported work is a part of an ongoing research for developing an adaptive machining planning environment for surface error modelling and prediction and selection of process and tool path parameters for rapid machining of complex low-rigidity high-accuracy parts.     

基于开放式系统的曲面加工误差动态补偿控制

将基于传感器信息的智能型开放加工系统应用于曲面的高精度加工,提出了曲面加工误差的动态补偿方法。为预测加工误差的补偿量,建立了刀具弯曲模型,分析了刀具弯曲对加工误差的影响。实验结果表明,本系统可有效地提高加工精度,使加工误差降低1／3－1／4。     

5-Axis adaptive flank milling of flexible thin-walled parts based on the on-machine measurement

Deformation of the part and cutter caused by cutting forces immediately affects the dimensional accuracy of manufactured parts. This paper presents an integrated machining deviation compensation strategy based on on-machine measurement (OMM) inspection system. Previous research attempts on this topic deal with deformation compensation in machining of geometries in 3-axis machine tools only. This paper is the first time that concerned with 5-axis flank milling of flexible thin-walled parts. To capture the machined surface precision dimensions, OMM with a touch-trigger probe installed on machine׳s spindle is utilized. Probe path is planned to obtain the coordinate of the sampling points on machined surface. The machined surface can then be reconstructed. Meanwhile, the cutter׳s envelope surface is calculated based on nominal cutter location source file (CLSF). Subsequently, the machining error caused by part and cutter deflection is calibrated by comparing the deviation between the machined surface and the envelope surface. An iteration toolpath compensation algorithm is designed to decrease machining errors and avoid unwanted interference by modifying the toolpath. Experiment of machining the impeller blade is carried out to validate the methodology developed in this paper. The results demonstrate the effectiveness of the proposed method in machining error compensation.     

The prediction of dimensional error for sculptured surface productions using the ball-end milling process. Part 1: Chip geometry analysis and cutting force prediction

The study of machining errors caused by tool deflection in the balkend milling process involves four issues, namely the chip geometry, the cutting force, the tool deflection and the deflection sensitivity of the surface geometry. In this paper, chip geometry and cutting force are investigated. The study on chip geometry includes the undeformed radial chip thickness, the chip engagement surface and the relationship between feed boundary and feed angle. For cutting force prediction, a rigid force model and a flexible force model are developed. Instantaneous cutting forces of a machining experiment for two 2D sculptured surfaces produced by the ball-end milling process are simulated using these force models and are verified by force measurements. This information is used in Part 2 of this paper, together with a tool deflection model and the deflection sensitivity of the surface geometry, to predict the machining errors of the machined sculptured surfaces.     

The prediction of dimensional error for sculptured surface productions using the ball-end milling process. Part 2: Surface generation model and experimental verification

This paper presents a surface generation model for sculptured surface productions using the ball-end milling process. In this model, machining errors caused by tool deflections are studied. As shown in Part 1 of this paper, instantaneous horizontal cutting forces can be evaluated from the cutting geometries using mechanistic force models. In this paper, a tool deflection model is developed to calculate the corresponding horizontal tool deflection at the surface generation points on the cutter. The sensitivity of the machining errors to tool deflections, both in magnitude and direction, has been analyzed via the deflection sensitivity of the surface geometry. Machining errors are then determined from the tool deflection and the deflection sensitivity of the designed surface. The ability of this model in predicting dimensional errors for sculptured surfaces produced by the ball-end milling process has been verified by a machining experiment. In addition to providing a means to predict dimensional accuracy prior to actual cutting, this surface generation model can also be used as a tool for quality control and machining planning.     

A new approach for the geometrical calibration of parallel kinematics machines tools based on the machining of a dedicated part

A main limitation of parallel kinematics machine tools (PKM) in high-speed machining tasks is their low level of accuracy compared with serial kinematics machine tools, which is largely due to geometrical transformation errors. These errors can be reduced by identifying the geometrical parameters of the inverse kinematics model integrated in the controller by exteroceptive calibration. The aim of this paper is to propose a new external measurement method in order to perform the geometrical calibration of PKM, taking into account machining requirements. This method is implemented in three steps: machining of a dedicated part, measurement, and identification of the geometrical parameter values. In this paper, the method is described with a particular emphasis on the machined surface profiles of the dedicated part and on the numerical calibration approach. Measurement errors on the machined surface enable the identification of the PKM geometrical parameters. Thus, calibration is performed with respect to machined surface defects without taking into account the entire tool pose defect, as is the case in usual calibration methods. The study is illustrated using the Verne PKM, which is located at IRCCyN (Nantes, France).     

A new tool wear compensation method based on real-time estimation of material removal volume in micro-EDM

Owing to the reduced tool area and poor flushing conditions in deep holes, tool wear in micro-electrical discharge machining (EDM) is more significant than in macro-EDM. In micro-EDM drilling, the z-axis of the tool position is monitored as machining progresses. However, due to significant electrode wear, the machined hole depth is not identical to the programmed depth of the hole, and thus this will result in geometrical inaccuracy. This paper presents a new micro-EDM drilling method, in which the material removal volume is estimated as machining progresses. Compensation length is calculated and adjustment is made repeatedly along the tool path until the targeted material removal volume is reached. A real-time material removal volume estimator is developed based on the theoretical electro-thermal model, number of discharge pulse and pulse discrimination system. Under various energy input and machining depth settings, the experimental and estimated results are found to be in satisfactory agreement with average error lower than 14.3% for stainless steel, titanium, and nickel alloy work materials. The proposed drilling method can compensate the tool wear and produce more accurate micro-holes as compared to other methods. Experimental work also shows that the proposed method is more reliable as compared to the uniform wear method. In drilling micro-holes of 900 μm depth, the depth error can be reduced to 4% using the proposed method.     

Integrated machining error compensation method using OMM data and modified PNN algorithm

This paper presents an integrated machining error compensation method based on polynomial neural network (PNN) approach and inspection database of on-machine-measurement (OMM) system. To improve the accuracy of the OMM system, geometric errors of the CNC machining center and probing errors are compensated. Machining error distributions of a specimen workpiece are measured to obtain error compensation parameters. To efficiently analyze the machining errors, two machining error parameters, W_err and D_err, are defined. Subsequently, these parameters can be modeled using the PNN approach, which is used to determine machining errors for the considered cutting conditions. Consequently, by using an iterative algorithm, tool path can be corrected to effectively reduce machining errors in the end-milling process. Required programs are developed using Ch language, and modified termination method are applied to reduce computation times. Experiments are carried out to validate the approaches proposed in this paper. The proposed integrated machining error compensation method can be effectively implemented in a real machining situation, producing much fewer errors.     

立式加工中心空间误差验证及补偿

为提高某立式加工中心整机加工精度,借助旋量理论建立完备立式加工中心空间误差模型,在此基础上实现机床空间误差有效补偿.以旋量理论为基础推导并建立机床刀具运动链与工件运动链运动学正解,分析机床21项几何误差原理,在考虑21项几何误差的基础上建立该立式加工中心完备空间误差模型;利用九线法完成各项几何误差辨识;基于旋量运动学正解求解机床运动学逆解后得出运动轴实际运动路径,并通过体对角线实验对比补偿前后的效果.结果表明:所提补偿方法补偿效果显著,验证了机床空间误差模型的准确性,实现了提高机床加工精度的目的.     

Modelling geometric and thermal errors in a five-axis cnc machine tool

The total volumetric error within the workspace of a machine tool is induced by the propagation of both scalar and position dependent geometrical errors, as well as time-variant thermal errors. This paper presents a compact volumetric error model which can be used as a basis for a practical compensation scheme. The broad objective is to increase the achievable accuracy of an industrial five-axis CNC machine tool. In place of using Denavit-Hartenberg (D-H) transformations, the method used here directly considers the shape and joint transformations for inaccurate links and joints using small angle approximations and then finds the total volumetric error in the workspace as a function of all the possible errors.The development of the model shows that angular deviations are independent of translational errors. However, the tool point deviations are dependent on both translational and rotational errors. The model has been used for the design and testing of a compensation strategy. The simulation studies indicate that CNC compensation for errors in X, Y and Z axes is possible. However, the capability of the CNC compensation for pitch, roll and yaw errors is dependent on the positioning of the rotary axes on the machine tool. This is shown by an example using the compensation scheme developed.     

Evaluation of the stiffness chain on the deflection of end-mills under cutting forces

This study presents the investigation of the stiffness of the system formed by the machine-tool, shank and toolholder, collet and tool. Cutting forces induce the deflection of the system, and consequently an error appears on the machined surface.Comparing values obtained from cantilever beam models applied to the cutting tool, analytical or FEM, with those experimentally obtained, large differences have been observed, which in some cases are more than 50%. For this reason, we have proceeded to evaluate the stiffness of each of the existing elements between the machine bed and the tool tip. Thus, deflections of the machine-tool, toolholder and toolholder clamping in the spindle, tool clamping in the toolholder, and tool itself, were measured experimentally under the effects of known forces.The final application is the ball-end milling of complex surfaces, an operation commonly performed in the finishing of moulds or forging dies, where errors of more than 70 μm are not unusual. A great part of this error comes from the deflection of the machine-tool assembly, spindle, shank and tool, due to the high cutting forces of the high speed machining of tempered steels. Cutting forces can be estimated using a semi-empirical approach, and from here some values of probable errors may be taken into account to check if the CNC programs are sufficiently adapted. However, a previous study of the deflection chain in the cutting process is needed, as is presented in this work.Results show that stiffness of the slender and flexible tools is 15 times lower than that of the machine and toolholder system. But this correlation is only 5–7 times lower for shorter and thicker tools.     

A study on the machining of high-aspect ratio micro-structures using micro-EDM

Micro-electro-discharge machining (micro-EDM or μ-EDM) has been gaining popularity as a new alternative method to fabricate micro-structures. The main advantages of the micro-EDM method are its low set-up cost, high accuracy and large design freedom. Compared to etching or deposition techniques, micro-EDM has the advantage of being able to fabricate complex three-dimensional shapes with high-aspect ratio. However, there are many operating parameters that affect the micro-EDM process. The fabrication of micro-electrodes on the machine is also an important process to remove the clamping error to maintain high accuracy in the machined micro-structures.

In this paper, the machining of micro-structures is divided into two basic processes. One is the on-machine fabrication of the micro-electrodes with high-aspect ratio, and the other is the EDM of the workpiece in micrometer range. An optical sensor has been developed to measure and control the dimension of the thin electrode during the tool fabrication process. Different methods have been investigated to fabricate a thin electrode into the desired dimension without deflection. The performance of the micro-EDM process is evaluated in terms of the material removal rate (MRR), tool wear ratio (TWR), and the stability of the machining. Influences of the various operating parameters of the micro-EDM process, such as the operating voltage, gap control algorithm, and resistance and capacitance values in the R–C spark control circuit, are discussed.