A stratey for geometric error characterization in multi-axis machine tool by use of a laser tracker

This paper aims to present different methods of volumetric verification in long range machine toll with lineal and rotary axes using a commercial laser tracker as measurement system. This method allows characterizing machine tool geometric errors depending on the kinematic of the machine and the work space available during the measurement time. The kinematic of the machine toll is affected by their geometric errors, which are different depending on the number and type of movement axes. The relationship between the various geometrical errors is different from relationship obtained in machine tool whit only lineal axes. Therefore, the identification strategy should be different. In the same way, the kinematic chain of the machine tool determines determines the position of the laser tracker and available space for data capture. This paper presents the kinematic model of several machine tools with different kinematic chains use to improve the machine tool accuracy of each one by volumetric verification. Likewise, the paper thus presents a study of: the adequacy of different nonlinear optimization strategies depending on the type of axis and the usable space available

Traceable onboard metrology for machine tools and large-scale systems

Esta tesis doctoral persigue la mejora de las funcionalidades de las máquinas herramienta para la fabricación de componentes de alto valor añadido. En concreto, la tesis se centra en mejorar la precisión de las máquinas herramienta en todo su volumen de trabajo y en desarrollar el conocimiento para realizar la medición por coordenadas trazable con este medio productivo. En realidad, la tecnología para realizar mediciones en máquina herramienta ya está disponible, como son los palpadores de contacto y los softwares de medición, sin embargo, hay varios factores que limitan la trazabilidad de la medición realizada en condiciones de taller, que no permiten emplear estas medidas para controlar el proceso de fabricación o validar la pieza en la propia máquina-herramienta, asegurando un proceso de fabricación de cero-defectos. Aquí, se propone el empleo del documento técnico ISO 15530-3 para piezas de tamaño medio. Para las piezas de gran tamaño se presenta una nueva metodología basada en la guía VDI 2617-11, que no está limitada por el empleo de una pieza patrón para caracterizar el error sistemático de la medición por coordenadas en la máquina-herramienta. De esta forma, se propone una calibración previa de la máquina-herramienta mediante una solución de multilateración integrada en máquina, que se traduce en la automatización del proceso de verificación y permite reducir el tiempo y la incertidumbre de medida. En paralelo, con el conocimiento generado en la integración de esta solución en la máquina-herramienta, se propone un nuevo procedimiento para la caracterización de la precisión de apunte del telescopio LSST en todo su rango de trabajo. Este nuevo procedimiento presenta una solución automática e integrada con tecnología láser tracker para aplicaciones de gran tamaño donde la precisión del sistema es un requerimiento clave para su buen funcionamiento.<br /

The use of a Laser Tracker and a Self-centring Probe for Rotary Axis Verification

This paper presents a small collection of tests related with the analysis of a rotary axis according to ISO 230-7 but introducing two alternative equipments briefly explaining each method. The disadvantages of the methods in which the movement of a rotary axis engages the translational axes of a Machine Tool are expressed, which leads to the proposed study. The errors of a rotary axis are described as established in standards and the measurement procedures carried out in the tests for verification of a rotary indexing table, based on the use of a self-centring probe and a laser tracker, are explained. Also, the necessary elements setup for measurement are described. Then, the followed calculation process of the measured errors is explained in detail. Finally, the results of the most significant errors obtained from the test measurements are presented

Adequacy of technical and commercial alternatives applied to machine tool verification using laser tracker

Besides presenting a volumetric verification technique that allows characterization of the different geometric errors of a machine tool (MT) depending on its kinematic chain and geometry through a kinematic model, this paper investigates the influence of measurement tools and techniques available on the final accuracy of the MT. Volumetric verification based on a laser tracker (LT) relates the coordinates of the tool with the coordinates of the LT, including it into the kinematic model. Using a non-lineal optimization process, approximation functions that characterize the joint influence of MT geometric errors are obtained. However, measurement data will be affected by previous compensation of the MT, the accuracy of the measurement system, LT measurement technology, the type of retroreflector used, and techniques used to improve data accuracy, among other sources of errors. This paper studies the adequacy of different commercial alternatives such as: retroreflectors, LTs from different manufacturers, etc., that can be applied in MT verification using a long-range MT. As the accuracy is strongly affected by the uncertainty of its angular encoders, the multilateration technique tries to improve data accuracy using only LT radial information. Nonetheless, a new bundle adjustment which uses radial and angular information is presented in current metrology software. This paper studies both techniques and analyzes their adequacy for MT verification too

Analysis of the measurement capacity of a machine tool

Industrial sectors that demand manufacturing of high quality components within specified tolerances are looking for cost reductions without affecting the quality of the product. The verification of workpieces is normally carried out in post-process with coordinate measuring machines which increase the manufacturing cycle time. Machine tools can carry out contact measuring operations with a probe, and since there is a growing need to inspect the workpieces in process, using the machine tool itself for verification while the workpiece remains clamped to the machine can lead to an improvement in manufacturing times, reduction of costs and energy saving

Improving the Accuracy of a Machine Tool with Three Linear Axes using a Laser Tracker as Measurement System

This paper presents a real way to improve machine tool accuracy using interferometry techniques based on laser tracker as measurement system. This is based on an indirect measurement of the geometric error of the MT; characterizing their combined effect though volumetric verification. This technique can be used to verify all types of machine through its kinematic model, relating measured points with the laser tracker and nominal points. Using non-lineal optimization techniques the difference between pair of points are reduced. Finally, it provides the approximation functions of the machine tool geometric errors used to compensate theirs influence on MT accuracy. As it is a mathematical compensation result of an optimization procedure, the improvement on MT accuracy should be validate using the same or external measurement system. This paper shows that volumetric verification provides an real improvement of the MT accuracy

Monte Carlo method to machine tool uncertainty evaluation

Currently machine tools are not only a way to make different parts based on material removal processes. These ones can be used as a measurement system too. In this way, overall inspection time is reduced and equipment productivity is increased. Nevertheless, the use of machine tool probes as measurement tool in manufacturing parts required previous works. Firstly, the machine tool accuracy should be improved, in order to reduce the influence of its geometric errors. This way, volumetric verification based on laser tracker measurement has increased strongly in the last few years, especially in long range machine tools. Secondly, calibration uncertainty should be calculated to provide measurement uncertainty. This way, the paper presents a new tool able to analyze the effect of different influence verification parameters in calibration uncertainty based on Monte Carlo method. Using real tests carried out on a milling machine and its geometric errors, the influence or laser tracker measurement noise in calibration uncertainty is studied using Monte Carlo method

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

Study on Machine Tool Positioning Uncertainty Due to Volumetric Verification

Volumetric verification is based on the machine tool (MT) kinematic model, along with its geometric errors. Although users often ignore the uncertainty of verification, the use of the MT as a traceable measurement system in the manufacturing process has increased the need for professionals to be aware of it. This paper presents an improvement in the MT kinematic model, introducing in it the influence of verification uncertainty sources. These sources have been classified into four groups: the MT, the measurement system itself, the measurement strategy, and the optimization strategy. As the developed model exhibits non-linear behavior, the Monte Carlo method was used to determine the influence of the measurement system on verification uncertainty using synthetic tests. In this manner, an improved estimation of the MT uncertainty can be obtained. Therefore, if the MT is used as a traceable measurement system, its accuracy should not be higher than the laser tracker (LT) verification influence. It hence shows the importance of LT influence