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Impact of thermal gradients on the geometry correction of a bridge coordinate measuring machine

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Book
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This book focused on effective methods for assesing the accuracy of both coordinate measuring systems and coordinate measurements. IT mainly reports on original research work conducted by Sladek's team at University of Technology's Laboratory of Coordinate Metrology. The book describes the implementation of different methods, including artifical neural networks, the Matrix Method, the Monte Carlo method and virtual CMM, and demonstrates how these methods can be effectively used In practice to gauge the accuracy of coordinate measurements. Morecover, the book includes an introduction to the theory of measurements uncertainty and to key techniques for assesing Measurement accuracy. All methods and tools are presented In detail, using suitable Mathematical formulations and illustrated with numerous examples. The book fills and important gap in the literature, providing readers with an advanced text on a topic that has been rapidly developing in recent years. The book is intended for master and PhD studenta, as well as for metrology engineers working at industrial and reaserch laboratories. It not only provides tchem with a solid background for using existing coordinate metrology methods; IT is also meant to inspire them to develop the state-of-the-art technologies that will play an important role in supporting quality growth and innovation in advanced manufacturing.
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3 Centre Technique des Industries Mecaniques (CETIM) - 52, avenue Felix-Louat - BP 80067 - 60304 Senlis Cedex2 Abstract. Monte Carlo method have been introduced in metrology in the 1990's and integrated in the GUM (Supplement 1) in 2008. This method is more and more used. Typically the users of this method realize a complete simulation in one step, like the GUM, one step for one model. This is unfortunate, the simulation loses its physical sense. The study aim is to present a multi-level Monte Carlo approach which allows being near of the reality of the measurement process. Two applications are developed: evaluation of the uncertainties on CMM and on AACMM. This principle has been developed with CETIM for COFRAC accreditation on CMM for gear measurands. The simulation is divided into two principle stages, namely the first is the comprehensive evaluation of possible changes in the geometry of the CMM and the second step, directly related to the measure of the piece, is the evaluation of the analyzed measurand. For AACMM, same principle is realized but the first level is divided into three sub-levels. The division into several levels has many advantages. Indeed, it makes it easier to understand the key sources of uncertainty and thus optimize processes.
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This paper presents a multivariate regression predictive model of drift on the Coordinate Measuring Machine (CMM) behaviour. Evaluation tests on a CMM with a multi-step gauge were carried out following an extended version of an ISO evaluation procedure with a periodicity of at least once a week and during more than five months. This test procedure consists in measuring the gauge for several range volumes, spatial locations, distances and repetitions. The procedure, environment conditions and even the gauge have been kept invariables, so a massive measurement dataset was collected over time under high repeatability conditions. A multivariate regression analysis has revealed the main parameters that could affect the CMM behaviour, and then detected a trend on the CMM performance drift. A performance model that considers both the size of the measured dimension and the elapsed time since the last CMM calibration has been developed. This model can predict the CMM performance and measurement reliability over time and also can estimate an optimized period between calibrations for a specific measurement length or accuracy level.
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A new methodology to measure thermal distortion in large machine tools is proposed in this paper. The advantage of this method is that a single tracking interferometer can be used to measure thermal distortion of machines with large work volumes while maintaining low enough measurement cadence and uncertainty. A multilateration scheme is conducted using a single laser tracking device positioned on top of the machine table which is automated, and for each target point, all laser stations are reached prior to moving to the next target point; then, the whole measurement cycle is repeated during the test. For measuring angular thermal distortion, precision electronic levels are located in machine ram and column top; also, temperatures are registered in key points of the machine. Experimental measurements on a large column-type milling machine are done, and the effectiveness of the proposed methodology is verified.
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This paper presents a short review of standards pertaining to uncertainty of coordinate measurements, new software developed by the authors as well as theoretical models used within the software. The software has been developed with the following assumptions: in the coordinate measurements the uncertainty is analyzed separately for each characteristic, and the uncertainty of measurement for a particular characteristic is calculated with a formula describing the characteristic as a function of differences of coordinates of characteristic points of the workpiece. Examples of use of the software for measurement uncertainty evaluation of different GPS characteristics are also presented.
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For measuring machines and machine tools, geometrical accuracy is a key performance criterion. While numerical compensation is well established for CMMs, it is increasingly used on machine tools in addition to mechanical accuracy. This paper is an update on the CIRP keynote paper by Sartori and Zhang from 1995 [Sartori S, Zhang GX (1995) Geometric error measurement and compensation of machines, Annals of the CIRP 44(2):599–609]. Since then, numerical error compensation has gained immense importance for precision machining. This paper reviews the fundamentals of numerical error compensation and the available methods for measuring the geometrical errors of a machine. It discusses the uncertainties involved in different mapping methods and their application characteristics. Furthermore, the challenges for the use of numerical compensation for manufacturing machines are specified. Based on technology and market development, this work aims at giving a perspective for the role of numerical compensation in the future.
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Calibration of the geometry of five-axis machine tools needs to be performed periodically since the machine accuracy has a direct impact on machined parts. Because mechanical adjustment and a software correction may be done using calibration results, the measurement results must be evaluated. In this paper, the scale and master ball artefact (SAMBA) method is evaluated through the estimation of the uncertainty of the identified machine geometric error parameters. This approach has the multi-input multi-output (MIMO) model and an iterative solution that makes it challenging to apply commonly used uncertainty calculation methods The Guide to the Expression of Uncertainty in Measurement Supplement 2 (GUM S2) gives the opportunity to estimate the uncertainty of a MIMO model through the adaptive Monte Carlo method (MCM). In order to include all the uncertainty sources, the input uncertainty is estimated from the repeated calibration tests performed in different thermal conditions (with and without the warm-up cycle). The uncertainty is calculated for each of the identified machine geometric error parameters along with their covariance. The correlation between the output variables and the impact of the machine state, before and during the repeated calibration, are analyzed. The results demonstrate that the machine tool geometry variations occur even without the warm-up cycle performed before the calibration. Moreover, machine performance has an impact the calibration results.
Chapter
Deviations of engineering workpieces from nominal form are often assessed using leastsquares geometric-element fitting software. In a range of circumstances, it is more appropriate to use minimum-zone, circumscribed and inscribed fits, for which good software is less widespread. When such software becomes more widely available it will be important to test it objectively, and for this purpose reference software is currently being developed. This paper discusses the technical basis of reliable algorithms underpinning reference software.
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A method for the evaluation and the correction of thermal deformations of CMMs is presented; the method uses a deformation mathematical model based on continuous temperature and gradient functions: it follows an empiric approach and requires short time and cheap equipment to be applied. Two different CMMs were used to evaluate the method performances, one being a laboratory CMM and the other an industrial-use oriented machine, tested in an industrial environment. After the description of the applied experimental methodology and of the new deformation model, some preliminary results are given, showing the possibility to explain and correct up to 85% of thermal deformations, also under extremely perturbated thermal conditions.
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Abstract A methodology for the design of a thermal distortion compensation system is presented which will assist the manufacturing processes to reach higher levels of accuracy when working with large machines in common shop floor environments. A parametric state-space representation was selected as model architecture, providing multiple inputs and outputs capability and a compact formulation that takes into account previous thermal states of the machine. Inputs for the model are spindle speed and temperatures of main motor gearbox and room air. Outputs are the estimations of the thermal drift of the machine tool centre point along the three axes in different positions within the working volume. Model parameters were numerically identified with initial experimental tests performed in a large gantry-type milling machine, measuring mentioned variables and also thermal distortion values using a reference artifact along with non-contact proximity sensors. Proposed model was finally verified with a new validation measurement in the machine. Obtained results revealed 80% of error reduction in the vertical axis which comprised 70% of total thermal effects and 50% in the longitudinal X axis which comprised 25% of total thermal effects. Also it was concluded that the model benefits from using valuable information about the machine state from previous spindle speed register instead of using only temperature values. Proposed methodology benefits from providing a feasible implementation in real shop floor conditions without the necessity of including additional temperature sensors or probing systems in the machine.
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Advances in modern manufacturing techniques implies more efficient production but also new tasks for coordinate metrology. The main of them is evaluation of accuracy of the measurement, because according to technological requirements, results of measurements are useful only when they are given with their accuracy. Currently used methods for uncertainty assessment are difficult and require knowledge and measuring experience. It is therefore important to implement correct and validated methods that will also be easy to implement and will not require broad metrological knowledge from the personnel. Presented here simulation method, based on Monte Carlo method is one of them. The article presents the conception, implementation and validation of this method.
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This paper presents a review of the latest research activities and gives an overview of the state of the art in understanding changes in machine tool performance due to changes in thermal conditions (thermal errors of machine tools). The topics are focused on metal cutting machine tools, especially on turning and milling machines as well as machining centres. The topics of the paper thermal issues in machine tools include measurement of temperatures and displacements, especially displacements at the tool centre point, computations of thermal errors of machine tools, and reduction of thermal errors. Computing the thermal errors of machine tools include both, temperature distribution and displacements. Shortly addressed is also to avoid thermal errors with temperature control, the influence of fluids and a short link to energy efficiency of machine tools. The paper presents the summary of research work in the past and current. Research challenges in order to achieve a thermal stable machine tool are discussed. The paper apprehend itself as an update and not a substitution of two published keynote papers of Bryan et al. [28] in 1990 and Weck et al. [199] in 1995.
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A method has been developed at PTB to carry out calibration, acceptance test and periodic inspection of coordinate measuring machines (CMM) using reference objects. The procedures are done by measuring 2-dimensional patterns in different optimally select positions with a multiple of probe stylii. The measured apparent deformations of the patterns are analysed to yield the corresponding information about the volumetric deviations of position of the inspected instrument. Unlike the conventional method of separately measuring machine errors this concept allows a direct traceablity of a CMM via one single pre-calibrated reference object. A full scope calibration can be performed with considerably less efforts for measurement and evaluation than it was required for so far used techniques but nevertheless with comparable accuracy.
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The use of coordinate measuring machines (CMMs) outside a strictly controlled environment necessitates the management of the temperature-dependent geometry of the machines. This paper presents a thermal error compensation based on physical relations between temperature and deformation, rather than on an empirical model linking inputs and outputs (for example, a regression model or a neural network). Based on an extensive set of laser interferometer measurements, a thermal compensation scheme is composed.A Zerodur hole plate is used to verify the applicability of the model via real measurements. This thermal stable artifact allows monitoring the machine's geometry under various thermal loads. Differet parameters like lengths and angles are measured under various near steady state environment conditions. The developed thermal correction uses only six thermometer inputs. For temperatures different from 20°C, the model reduces the uncertainty of length measurements to the level of the stochastic errors at 20°C. Under spatial temperature gradients, a significant reduction is also obtained.
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For over three years the Physikalisch-Technische Bundesanstalt has been investigating the numerical error correction of cmms, and the relationships between problems in this area and in the calibration of these machines. It has been determined that calibration is possible, for pre-defined conditions, but that the calibration is applicable only to the particular measuring task and conditions. This paper describes the mathematical model used by PTB and gives results of tests on a cmm.
Article
The methods and results are presented for applying software error compensation to a commercial three axis coordinate measuring machine. The technique incorporates compensation for geometric positioning errors and some thermal effects. Geometric error computation is based on a rigid body model of workpiece motion in the machine coordinate frame. Complete kinematic equations of the error motions are presented. The measurement method only requires taking a small set of data for each axis to compute the errors throughout the full workzone. To achieve the desired accuracy, squareness is determined using linear displacement measurements along selected machine diagonals. The dominant thermal effects in the machine are removed by introducing the concept of an “effective” nominal differential expansion coefficient. The entire error compensation computation is incorporated into the machine position reading subroutine to automatically produce compensated readings. The effectiveness of this method is tested by measuring linear displacement along arbitrarily oriented lines through the workzone and by measuring the length of a 500 mm gage block in several orientations. The results show a factor of ten performance improvement (limited by measurement repeatability) over 0.5 C range in temperature.
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Using CMMs under normal workshop conditions necessitates to take into account the influence of environmental temperature on the machine structure. Non-standardised environmental conditions result in temperature dependent measurement errors. The paper presents a parametric approach to describe the relation between transient temperature distributions and resulting deformation of the CMM.The focus lies on broadening the temperature range in which the original accuracy specifications can be guaranteed. Starting from a correction scheme for uniform, invariant temperature situations, an approach for transient environmental loads is developed. Based on a limited number of temperature inputs, the required correction coefficients for the probe position are calculated.
Coordinate Measuring Machines and Systems
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Bosch J A 1995 Coordinate Measuring Machines and Systems (Boca Raton, FL: CRC Press) pp 24 p 496
Geometrical Product Specifications (GPS) -Acceptance and reverification tests for coordinate measuring machines
ISO 10360-1:2000 Geometrical Product Specifications (GPS) -Acceptance and reverification tests for coordinate measuring machines (CMM) -Part 1: Vocabulary
The virtual CMM concept, advanced mathematical tools in metrology
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Trapet E and Wäldele F 1995 The virtual CMM concept, advanced mathematical tools in metrology World Sci. Publ. Comp. 40 238-47
The "Virtual CMM" a software tool for uncertainty evaluation -practical application in an accredited calibration lab
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Trenk M, Franke M and Schwenke H 2004 The "Virtual CMM" a software tool for uncertainty evaluation -practical application in an accredited calibration lab ASPE Proc.: Uncertainty Anal. Meas. Des. (http://doi.org/10.1.1.474.27)
International standard development of virtual CMM (coordinate measuring machine
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Takamasu K et al 2002 International standard development of virtual CMM (coordinate measuring machine) Tokyo, Japan: NEDO International Joint Research Project (FY 1999 -FY 2001) Final Research Report p 159
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Flack D Good Practice Guide No. 130 (Co-ordinate measuring machine, task-specific measurement uncertainties) September 2013 National Physical Laboratory
Multilevel Monte Carlo approach to evaluate the process closer to physics 17th Int. Congress of Metrology hennebel
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Hennebelle F, Coorevits T and Vincent R 2015 Multilevel Monte Carlo approach to evaluate the process closer to physics 17th Int. Congress of Metrology hennebel, EDP sciences 02002 Les Ulis