On-machine identification of rotary axis location errors under thermal influence by spindle rotation

Position and orientation errors of rotary axis average lines are often among dominant error contributors in the five-axis kinematics. Although many error calibration schemes are available to identify them on -machine, they cannot be performed when a machine spindle is rotating. Rotary axis location errors are often influenced by the machine’s thermal deformation. This paper presents the application of a non-contact laser light barrier system, widely used in the industry for tool geometry measurement, to the identification of rotary axis location errors, when the spindle rotates in the same speed as in actual machining applications. The effectiveness of the proposed scheme is verified by experimental comparison with the R-Test and a machining test. The uncertainty analysis is also presented.This work was supported by JSPS KAKENHI Grant NumberJP15K05721

Traceability of on-machine tool measurement: a review

Nowadays, errors during the manufacturing process of high value components are not acceptable in driving industries such as energy and transportation. Sectors such as aerospace, automotive, shipbuilding, nuclear power, large science facilities or wind power need complex and accurate components that demand close measurements and fast feedback into their manufacturing processes. New measuring technologies are already available in machine tools, including integrated touch probes and fast interface capabilities. They provide the possibility to measure the workpiece in-machine during or after its manufacture, maintaining the original setup of the workpiece and avoiding the manufacturing process from being interrupted to transport the workpiece to a measuring position. However, the traceability of the measurement process on a machine tool is not ensured yet and measurement data is still not fully reliable enough for process control or product validation. The scientific objective is to determine the uncertainty on a machine tool measurement and, therefore, convert it into a machine integrated traceable measuring process. For that purpose, an error budget should consider error sources such as the machine tools, components under measurement and the interactions between both of them. This paper reviews all those uncertainty sources, being mainly focused on those related to the machine tool, either on the process of geometric error assessment of the machine or on the technology employed to probe the measurand

Modeling and Evaluation of the Volumetric Errors for the 5-Axis Machine Tool

Department of Mechanical EngineeringCNC Machine tools are among the most important means of production in metalworking industries and/ have been most widely used. Since an increasing demand for machinery parts with geometric complexity in high efficiency, multi-axialization and multi-functionalization emerged as technology trends in the machine tools field. As a result, 5-axis machine tools have been extensively used in various manufacturing applications requiring higher machining accuracy. In reality, demand in aerospace, medical, electric vehicle, and precision&semiconductor industries are driving. Based on the order composition of machine tools, the proportion of five-axis machine tools has become large remarkably, and also this trend is expected to continue in the future. While this high flexibility to machine complex parts efficiently could have led the 5-axis machine tool to become a great solution in the metalworking industry, at the same time, 5-axis machine tools have encountered challenges that have to overcome hurdles that come from this flexibility and freedom. As is well known, machine tools' accuracy is one of the most important indicators for the performance of machine tools, and 5-axis machine tools accompany more complexities and require more elements and more assembly processes, so encounter more accuracy problems indispensably. Overcoming these challenges and under the motivation to high-accuracy 5-axis machine tools, modeling and evaluating the volumetric errors for a 5-axis machine tool was set as this research objective. Specifically, in order to model the volumetric error, a study on the kinematic structures and identification for systematic error is carried out beforehand, and based on this, the goal aims to establish an error model of the 5-axis machine tool. Then, the error model established for a 5-axis machine tool is applied to a practical machine tool, and the errors propagating sensitively to volumetric error are determined as key errors. Also, the resultant effect of the individual errors and key errors is evaluated in advance through estimation of volumetric error. To estimate the volumetric error in virtual, stochastic estimation with random variables was conducted to obtain tool points??? coordinates within a workspace, and statistical analysis has carried out. Through these analysis processes, whether machine tools' volumetric error is enhanced under the condition when key errors were specified and assembled is confirmed. Lastly, to confirm the utility of modeling and evaluation for volumetric error in advance, the demonstration was conducted on two actual 5-axis machine tools, and each machine tool had been manufactured under the management to be assembled with the tolerance used in virtual evaluations as same as possible. Depending on whether or not the specified key error tolerance range is fulfilled, it has clearly confirmed the superiority and inferiority of 5-axis machine tools' volumetric error. And also by comparing estimation and experimental results, it is found that the approach and methods used in this study were useful and could be applied to the actual 5-axis machine tool manufacturing process.ope

Compensation of Relevant and Compensable Volumetric Errors for Five-Axis Machine Tools Based on Differential Kinematics

RÉSUMÉ Les erreurs géométriques d’une machine-outil ont un impact direct sur la précision des pièces usinées. Cette thèse traite de la compensation d'erreur des machines-outils CNC à cinq-axe. Dans la première phase, une formulation générale de l’erreur volumétrique et un système de compensation hors ligne sont proposés pour améliorer la précision de la pièce. En utilisant la cinématique des corps rigides et les paramètres d'erreur estimés de la machine, les commandes de position de la machine contenues dans un code G standard sont utilisées pour calculer l’erreur de position de l'outil. Le Jacobien, exprimant le différentiel entre l’espace articulaire et l'espace cartésien, est également développé et utilisé pour calculer les modifications de commande articulaire de telle sorte que l'effet des erreurs de la machine peut être annulé par de petits changements directement sur le code G. Lorsque la compensation est implémentée, sa validation est requise. Des machines à mesurer tridimensionnelles (MMT) ou d'autres dispositifs de mesure externes sont couramment utilisés pour mesurer la précision de la pièce usinée à des fins de validation. Dans ce travail, une série de tests de défauts surfaciques issus de l’usinage sont proposés pour comparer la précision d'usinage avant et après la compensation en utilisant des mesures sur machine seulement. Les écarts sur les surfaces produites découlent de l'erreur volumétrique et proviennent d’erreurs géométriques spécifiques de la machine qui sont mesurées en utilisant un palpeur placé sur la machine erronée elle-même. L'effet de la stratégie de compensation est ensuite validé en comparant l’écart entre les surfaces avec usinage compensé et non compensé. Les résultats des mesures sont compatibles avec les valeurs d'erreur volumétrique prévues et montrent une amélioration de la précision (réduction de décalage) d'environ 90% après compensation. Finalement, deux nouvelles notions, la pertinence de l'erreur et l’aptitude à la compenser, sont introduites et quantifiées pour la machine-outil. La compensation des erreurs pertinentes et compensables seulement conduit à une compensation optimisée dans laquelle des modifications de commandes minimales mais efficaces sont faites. Une pièce est conçue spécialement pour le test, contenant des caractéristiques communes est usinée, en utilisant les cinq axes d’usinage simultanément, pour la validation expérimentale. Les résultats de simulation montrent jusqu'à 75% de réduction dans la 1-norme des compensations linéaires et angulaires alors que les erreurs pertinentes demeurent efficacement corrigées.----------ABSTRACT Machine tool geometric errors directly impact on the accuracy of machined parts. This thesis addresses the error compensation in five-axis CNC machine tools. In the first phase, a general volumetric error formulation and an off-line compensation scheme are proposed to improve part accuracy. Using rigid body kinematics and estimated machine error parameters, the machine position commands contained in a standard G-code are used to calculate the tool erroneous location. The Jacobian, expressing the differential joint space to Cartesian space relationship, is also developed and used to calculate minute joint command modifications so that the effect of machine errors can be canceled by making small changes directly to the G-code. When compensation is implemented, its validation is sought. Coordinate measuring machines (CMM) or other external measurement devices are commonly used to measure the accuracy of the machined part for validation purpose. In this work, a series of surface mismatch producing machining tests are proposed to compare the machining accuracy before and after the compensation using only on-machine measurements. The produced surface mismatches that represent the volumetric error and come from specific machine geometric errors are measured using touch probing by the erroneous machine itself. The effect of the compensation strategy is then validated by comparing the surface mismatch value for compensated and uncompensated slots. The measurement results are compatible with the predicted volumetric error values and show an accuracy improvement (mismatch reduction) of about 90 % after compensation for the machine tested. Finally, two new notions, error relevance and error compensability, are introduced and quantified. Compensation of only relevant and compensable errors leads to an optimized compensation in which minimal but effective command modifications are made. A specially designed test part containing common features is machined, using up to five-axis simultaneous machining, for the experimental validation. Simulation results show up to 75% reduction in the 1-norm of the linear and angular compensations while the relevant errors are still effectively corrected

Characterising geometric errors in rotary axes of 5-axis machine tools

It is critical to ensure that a 5-axis machine tool is operating within its geometric tolerance. However, there are various sources of errors influencing its accuracy; testing them with current methods requires expensive equipment and long machine down time. This motivates the development of a simple and fast way to identify and characterise geometric errors of 5-axis machine tools. A method using a Double Ball Bar (DBB) is proposed to characterise rotary axes Position Independent Geometric Errors (PIGEs), which are caused by imperfections during assembly of machine components. An established method is used to test the same PIGEs, and the results are used to validate the developed method. The Homogeneous Transformation Matrices (HTMs) are used to build up a machine tool model and generate DBB error plots due to different PIGEs based on the given testing scheme. The simulated DBB trace patterns can be used to evaluate individual error impacts for known faults and diagnose machine tool conditions. The main contribution is the development of the fast and simple characterisation of the PIGEs of rotary axes. The results show the effectiveness and improved efficiency of the new methods, which can be considered for 5-axis machine tool maintenance and checking

A Novel Geometric Theory of On-Machine Tool Measurement and Practical, Optimal Approaches to Highly Accurate and Efficient On-Machine Measurement

Modern industry trends to smart machining that improves productivity at a low cost. The kernel technology of intelligent manufacturing is the automatic on-machine measurement (OMM). When applying OMM technology to computer numerical control (CNC) machines, in-situ measurement takes place in the machining environment without the need of unloading the tool and the part. However, adverse measurement environment, limitations on the efficiency of data capturing and processing, and diversified measured objects render efficient and accurate OMM very difficult. Holistic solutions are needed to advance OMM technology and therefore many scientific topics are involved. This work primarily focuses on geometric modeling of the on-machine cutting tool measurement and kinematic modeling for the calibration process of both the probe and the machine. On-machine cutting tool measurement often takes place on a laser tool setter. However, the geometry principles of the gauging mechanisms of laser tool setters are complicated and had not been studied before. This dissertation modeled such a gauging mechanism and presented virtual simulations of the measurement processes on laser tool setters based on geometry principles. The virtual simulations can predict and compensate the measurement errors, allowing for accurate tool setter calibration processes in practical situations. For cutting tool measurement, the tool length characteristic curve for measurement of round-insert mills is discovered. The derivation of the tool length characteristic curve was carried out by modeling the geometries of tool length measurement processes on a laser tool setter. Based on this characteristic curve, an accurate and efficient approach to measuring lengths of mills with round inserts and bottom cutting edge wear is proposed. Current techniques for probe calibration and machine calibration assume the impractical situations where either the machine is accurate or the location of the probe is accurately known. To address these drawbacks, the actual kinematic model of a six-axis belt grinding CNC machine with a customized add-on probe is built in this dissertation. Using this model along with a specially designed artifact can facilitate the simultaneous calibration of the probe position and the machine geometry error

Volumetric error compensation for industrial robots and machine tools

“A more efficient and increasingly popular volumetric error compensation method for machine tools is to compute compensation tables in axis space with tool tip volumetric measurements. However, machine tools have high-order geometric errors and some workspace is not reachable by measurement devices, the compensation method suffers a curve-fitting challenge, overfitting measurements in measured space and losing accuracy around and out of the measured space. Paper I presents a novel method that aims to uniformly interpolate and extrapolate the compensation tables throughout the entire workspace. By using a uniform constraint to bound the tool tip error slopes, an optimal model with consistent compensation capability is constructed. In addition to machine tools, industrial robots, are also becoming popularly used in manufacturing field. However, typical robot volumetric error compensation methods only consider constant errors such as link length and assembly errors while neglecting complicated kinematic errors such as strain wave gearing and out of rotating plane errors. Paper II presents a high-order joint-dependent model which describes both simple and complicated robot kinematic errors. A laser tracker with advantages of rapid data collection and a self-oriented position retroreflector are used for data collection. The experimental results show that nearly 20% of the robot kinematic errors are joint-dependent which are successfully captured by the proposed method. Paper III continues using the high-order joint-dependent robot error model while utilizing a new retroreflector with the ability of measuring robot position and orientation information simultaneously. More than 60% of measurement time is saved. Both position and orientation accuracy are also further improved”--Abstract, page iv

Calibration of Machine Tools Using on Machine Probing of an Indigenous Artefact

RÉSUMÉ Les centres d’usinage cinq axes avec deux axes rotatifs facilitent la production des pièces complexes grâce à la capacité de positionnement et d’orientation de l’outil par rapport à la pièce en cours d’usinage. Cependant, le centre d’usinage est vulnérable à de nombreuses sources d’erreurs. L’inspection périodique du centre d’usinage est un élément clé pour obtenir la pièce finie souhaitée dans les limites de tolérances. Les méthodes d’inspection existantes nécessitent un personnel qualifié, un montage spécial et un temps additionnel pour installer les équipements. Par conséquent, l’objectif de cette thèse est de surmonter ces contraintes et développer une nouvelle méthode pour estimer les erreurs paramétrique inter- et intra-axes par palpage d’un artefact indigène directement sur le centre d’usinage. Les palpeurs de déclenchement tactile sont utilisés pour mesurer des facettes de la table de la machine-outil. Un modèle mathématique a été développé pour modéliser les erreurs d’installation de la sonde et des artefacts afin d’enlever leurs effets lors du processus d’étalonnage. Le temps d'étalonnage est de 1 heure et 30 minutes. La validation de cet étalonnage est effectuée en comparant l’artefact du modèle estimé avec les mesures du même artefact obtenu par mesurage sur une Machine à Mesurer Tridimensionnelle (MMT). La capacité de prédiction des erreurs volumétriques du modèle est également validée en prédisant la position de la touche du stylet dans le repère de la pièce usinée pour d’autres facettes sondées pour fin de validation seulement, et les comparer avec les données mesurées par MMT. La distance résiduelle maximale entre l’artefact prédit par le modèle et l’artefact estimé par le MMT est 139.50 μm sans aucun paramètre estimé, et 6.92 μm avec 86 inter- et intra-axes paramètres estimés par le modèle. La technique de calibration proposée est appliquée au centre d’usinage intégré avec des tables de formes prismatique et sphérique (Mitsui Seiki HU40T et Huron KX8-five). Un schéma est proposé pour examiner les performances de la machine au cours d’une journée et entre les jours. La qualité du schéma est validée avec les incertitudes des paramètres calibrés venant de la covariance de l’ensemble des résultats de cycles de mesures effectuées pendant des jours consécutifs.----------ABSTRACT Five-axis machine tools with two rotary axes facilitate the production of intricate parts due to the position and orientation capability of the tool with respect to the workpiece but this flexibility also renders the machine tool vulnerable to numerous sources of error. Periodic inspection is the key to obtain finished part within the prescribed tolerance limits. Existing machine tool inspection methods require trained personnel, special setups and additional time to setup the test equipments. Therefore, the aim of this thesis is to overcome these limitations and develop a new method to calibrate inter- and intra-axis error parameters by on-machine probing of an indigenous artefact. A touch trigger probe is used to measure facets on the existing machine tool table. A mathematical model is developed to model the probe and the artefact setup errors and remove their effects from the estimation process. The calibration time is 1 hour and 30 minutes. The validation of the calibration is done by comparing the model estimated artefact with the Coordinate Measuring Machine (CMM) measured artefact. The volumetric error prediction capability of the model is also validated by predicting the stylus tip positions in the last workpiece branch frame (rigidly connected to the machine table frame) for each facet probing and comparing them with the CMM measurements. The maximum residual distance between the model predicted artefact and CMM artefact is 139.50 μm with no parameters estimated and 6.92 μm with 86 inter- and intra-axis parameters estimated. The proposed calibration technique is applied to the machine tools integrated with prismatic and cylindrical shape tables (Mitsui Seiki HU40T & Huron KX8-five). A scheme is proposed to investigate the machine performance throughout a day and between days supported by the uncertainties of the calibrated parameters estimated from the pooled covariance of the repeated measurement cycles performed for consecutive days. The calibration performance is also evaluated by investigating the repeatability of the uncalibrated indigenous artefact probing against artefact probing strategy, rotary axes indexations, parameters’ uncertainties and artefact dismount and remount cycle

Volumetric Error-Based Condition and Health Monitoring System for Machine-Tools

Résumé Des défaillances ou détériorations imprévues ou non détectées des machines-outils entraînent des pertes de production et de qualité, d'où la nécessité d'une maintenance prescriptive et normative utilisant la surveillance de l'état des machines-outils. Cette recherche présente la méthodologie et les solutions développées pour surveiller l’état de précision des machines-outils à cinq axes en analysant les erreurs volumétriques de la machine-outil. L’erreur volumétrique est définie comme un vecteur d'erreur cartésien représentant l'écart de la position réelle de l'outil par rapport à sa position attendue par rapport au repère de la pièce et projeté dans le repère de base. La méthode SAMBA (Scale and Master Ball Artefact) a été utilisée pour mesurer les erreurs volumétriques de la machine-outil expérimentale à cinq axes. Les erreurs volumétriques acquises contenant les états normaux et défectueux de la machine-outil constituent la base de données pour cette recherche. De plus, des pseudo-fautes et les fautes graduelles et soudaines simulées ont également été utilisées. Les caractéristiques du vecteur d'erreurs volumétriques extraites par des mesures de similarité de vecteur sont utilisées comme entrée pour le graphique de contrôle basé sur les moyennes mobiles pondérées exponentiellement, où le changement anormal du vecteur unique d'erreurs volumétriques peut être détecté. Pour surveiller de manière exhaustive l’état de précision de la machine-outil, une matrice de mesures de similarité vectorielle combinée contenant toutes les caractéristiques d’erreurs volumétriques acquises a été proposée et traitée par le graphique de contrôle de la moyenne mobile pondérée exponentiellement. Pour les mêmes défauts, les deux traitements de données ci-dessus peuvent tous détecter automatiquement le temps exact d’apparition du défaut. Sur la base d'une logique de surveillance complète des erreurs volumétriques, une analyse fractale des coordonnées d'erreur volumétrique a également été explorée. Les résultats des tests révèlent qu’il s’agit d’un outil efficace pour représenter la fonctionnalité des erreurs volumétriques. Pour comprendre le processus de changement de l'état de la machine-outil, les erreurs volumétriques historiques acquises ont été traitées par analyse en composantes principales et par K-moyennes. D'une part, les méthodes proposées séparent les états normaux et défectueux de la machine-outil (près de 100%), d'autre part, les machines-outils désignées fournissent les références pour la reconnaissance de l'état d’autre machines-outils lors du traitement de nouvelles données d'erreurs volumétriques. En résumé, le travail de recherche effectué dans cette thèse a contribué à la mise au point d’une solution efficace de surveillance de l’état de la précision des machines-outils à l’aide des erreurs volumétriques des machines-outils, basées sur des méthodes d’extraction de caractéristiques, de reconnaissance des modifications et de classification des états. Le système développé peut reconnaître les points de changement exacts des défauts réels du codeur d'axe C, des pseudo-défauts EXX et EYX. De plus, il atteint une précision proche de 100% dans la classification de l'état défectueux et normal de la machine-outil. ---------- Abstract Unexpected or undetected machine tool failures or deterioration results in production and quality losses, hence proactive and prescriptive maintenance using machine tool condition monitoring is sought. This research presents the methodology and solutions developed to monitor the accuracy state of five-axis machine tools by analyzing the machine tool volumetric errors which are defined as the Cartesian error vector of the deviation of the actual tool position compared to its expected position relative to the workpiece frame and projected into the foundation frame. The scale and master ball artefact (SAMBA) method has been used for the measurement of volumetric errors of the experimental five-axis machine tool. The acquired volumetric errors containing machine tool normal and faulty states provide the database for this research. In addition, pseudo-faults and the simulated gradual and sudden faults have also been used. Volumetric error vector features extracted by vector similarity measures are used as the input for the exponential weight moving average control chart where the abnormal change of the single volumetric error vector can be detected. To comprehensively monitor the machine tool accuracy state, a combined vector similarity measure array containing all acquired volumetric errors features has been proposed and processed by the exponential weight moving average control chart. Towards the same faults, the above two data processing can all automatically detect the exact fault occurrence time. Based on the logic of comprehensive monitoring of volumetric errors, fractal analysis of volumetric error coordinates has also been explored. The testing results reveal that it is an effective tool for volumetric errors features representing. To understand the change process of the machine tool state, the acquired historical volumetric errors have been processed by principal component analysis and K-means. For one thing, the proposed methods separate the normal and faulty states of the machine tool (Nearly 100%), for another thing, the designated machine tools provide the references for machine tools state recognition when processing new volumetric errors data. In summary, this research contributed to the development of an efficient solution for machine tool accuracy state monitoring using machine tools volumetric errors based on feature extraction, change recognition and state classification methods. The developed system can recognize the exact change points of real C-axis encoder faults, pseudo-faults EXX and EYX. In addition, it achieves close to 100% accuracy in machine tool faulty and normal state classification

Machining centre performance monitoring with calibrated artefact probing

Maintaining high levels of geometric accuracy in five-axis machining centres is of critical importance to many industries and applications. Numerous methods for error identification have been developed in both the academic and industrial fields; one commonly-applied technique is artefact probing, which can reveal inherent system errors at minimal cost and does not require high skill levels to perform. The primary focus of popular commercial solutions is on confirming machine capability to produce accurate workpieces, with the potential for short-term trend analysis and fault diagnosis through interpretation of the results by an experienced user. This paper considers expanding the artefact probing method into a performance monitoring system, benefitting both the onsite Maintenance Engineer and visiting specialist Engineer with accessibility of information and more effective means to form insight. A technique for constructing a data-driven tolerance threshold is introduced, describing the normal operating condition and helping protect against unwarranted settings induced by human error. A multifunctional graphical element is developed to present the data trends with tolerance threshold integration to maintain relevant performance context, and an automated event detector to highlight areas of interest or concern. The methods were developed on a simulated, demonstration dataset; then applied without modification to three case studies on data acquired from currently operating industrial machining centres to verify the methods. The data-driven tolerance threshold and event detector methods were shown to be effective at their respective tasks, and the merits of the multifunctional graphical display are presented and discussed