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  • PAGE 1 OF 28EDITION 1 Z Z JANUARY 1995

    EAL-G17 Z CMM CALIBRATION

    Coordinate Measuring Machine

    Calibration

    EAL European cooperation forAccreditation of LaboratoriesPublication ReferenceWELAC Guidance Document WGD 8 EAL-G17

    PURPOSE

    This document has been produced by EAL to provide information on the measurementcapabilities of coordinate measuring machines (CMMs) and the treatment of measureddata, and to provide guidance to national accreditation bodies accrediting laboratoriesto perform task-related calibrations of CMMs, or to use CMMs for the calibration ofcomponents.

  • EAL-G17 Z CMM CALIBRATION

    EDITION 1 Z Z JANUARY 1995PAGE 2 OF 28

    Authorship

    This publication has been written by the EAL Expert Group Dimensional Metrology working groupon CMMs.

    Official language

    The text may be translated into other languages as required. The English language version remainsthe definitive version.

    Copyright

    The copyright of this text is held by EAL. The text may not be copied for resale.

    Further informationFor further information about this publication, contact your National member of EAL, whose telephoneand facsimile numbers are given below:

    Calib National member Testing Nationalmember

    Austria BMwA BMwATel: +431 711 02 233 Tel: +431 711 02 249Fax: +431 714 35 82 Fax: +431 714 35 82

    Belgium BKO/OBE BELTESTTel: +322 233 6111 Tel: +32 2 548 15 11Fax: +322 230 8300 Fax: +32 2 514 47 56

    Denmark DANAK DANAKTel: +45 35 86 86 86 Tel: +45 35 86 86 86Fax: +45 35 86 86 87 Fax: +45 35 86 86 87

    Finland FINAS FINASTel: +358 0 61671 Tel: +358 0 61671Fax: +358 0 6167 467 Fax: +358 0 6167 467

    France COFRAC COFRACTel: +33 1 44 68 82 20 Tel: +33 1 44 68 82 20Fax: +33 1 44 68 82 21 Fax: +33 1 44 68 82 21

    Germany DKD DARTel: +49 531 592 8320 Tel: +49 30 8104 1710Fax: +49 531 592 9292 Fax: +49 30 8104 1717

    Greece Ministry of Commerce ELOTTel: +30 1 36 14 168 Tel: +30 1 201 5025Fax: +30 1 36 42 642 Fax: +30 1 202 0776

    Iceland ISAC ISACTel: +354 1 681 122 Tel: +354 1 681 122Fax: +354 1 685 998 Fax: +354 1 685 998

    Ireland ICLAB ICLABTel: +353 1 370 101 Tel: +353 1 370 101

    Calib National member Testing Nationalmember

    Fax: +353 1 368 738 Fax: +353 1 368 738

    Italy SIT SINALTel: +39 11 348 8933 Tel: +39 6 487 1176Fax: +39 11 34 6384 Fax: +39 6 481 4563

    Netherlands NKO/STERIN/STERLAB NKO/STERIN/STERLABTel: +31 10 413 6011 Tel: +31 10 413 6011Fax: +31 10 413 3557 Fax: +31 10 413 3557

    Norway NA NATel: +47 22 20 0226 Tel: +47 22 20 0226Fax: +47 22 20 7772 Fax: +47 22 20 7772

    Portugal IPQ IPQTel: +351 1 52 3978 Tel: +351 1 52 3978Fax: +351 1 3153676 Fax: +351 1 3153676

    Spain SCI RELETel: +34 1 349 4711 Tel: +34 1 564 96 87Fax: +34 1 457 8066 Fax: +34 1 563 04 54

    Sweden SWEDAC SWEDACTel: +46 33 17 7700 Tel: +46 33 17 7700Fax: +46 33 10 1392 Fax: +46 33 10 1392

    Switzerland SAS SASTel: +41 31 963 3111 Tel: +41 31 963 3412Fax: +41 31 963 3210 Fax: +41 31 963 3210

    United Kingdom NAMAS NAMASTel: +44 181 943 7068 Tel: +44 181 943 6266Fax: +44 181 943 7134 Fax: +44 181 943 7134

  • PAGE 3 OF 28EDITION 1 Z Z JANUARY 1995

    EAL-G17 Z CMM CALIBRATION

    Section Page

    Contents

    1 Summary 4

    2 Introduction 4

    3 Calibration and performance verification of CMMs 5

    4 Accreditation of calibration laboratories 6

    5 Calibration procedures 7

    6 Calibration certificates 10

    7 Future 11

    8 References 11

    Appendix A Terms and definitions 13

    Appendix B Accreditation of laboratories for the calibration of CMMs 16

    Appendix C Accreditation of laboratories for calibration of artefactsusing CMMs 17

    Appendix D Comparator approach 18

    Appendix E Error synthesis approach 20

    Appendix F Recommendations for future developments 27

  • EAL-G17 Z CMM CALIBRATION

    EDITION 1 Z Z JANUARY 1995PAGE 4 OF 28

    1 Summary

    1.1 The aim of this guideline is to demonstrate the ways in which traceability formeasurements using a coordinate measuring machine (CMM) can be achieved.

    1.2 Coordinate metrology has become essential for industrial dimensional metrology.In recent years, standards and guidelines have been developed to harmonizethe performance specification of a CMM for a user when purchasing a machineand, once purchased, to provide a well-defined way in which the specifiedperformance can be verified.

    1.3 A further step in the harmonization process is to establish ways of calibratingcomponents with a CMM, i.e. measuring a component, assessing theuncertainties of the respective measurands, and issuing a certificate, theauthenticity of which would be recognised throughout the Western EuropeanCalibration Cooperation (WECC). However, due to the complex errors andversatility of a CMM, it is difficult to specify and verify the uncertainty of allmeasurement tasks that can be executed by a CMM, in any position within itsworking volume, using any measurement strategy.

    1.4 The solution suggested in this guideline is for a laboratory to demonstrate itsability to assess the uncertainty of those measurement tasks for whichaccreditation is desired. This leads to the concept of task-related calibration,where only those error sources are analysed that affect the particularmeasurement task and associated measurement procedure specified in thecertificate. For some tasks, this will be relatively easy to achieve.

    1.5 This guideline describes two basic principles of CMM task-related calibration:the comparator principle and the error synthesis principle. It further summarisesthe views of experts about the related calibration techniques.

    1.6 The functions of this document are to:

    W emphasise the difference between CMM performance verification and CMMcalibration,

    W define CMM task-related calibration and to describe the problems involved,

    W provide guidance to the various national accreditation bodies accreditinglaboratories to perform task-related calibrations of CMMs and/or to useCMMs for the calibration of components.

    2 Introduction

    2.1 In response to requests from firms and organisations, various nationalcalibration organisations are studying the calibration of Coordinate MeasuringMachines (CMMs) and the use of CMMs to calibrate components. Someorganisations have issued accreditations for the calibration of componentsalready. In order to harmonize these efforts, a WECC working group wasestablished, which held its first meeting on 4 April 1990 at the PTB,

  • PAGE 5 OF 28EDITION 1 Z Z JANUARY 1995

    EAL-G17 Z CMM CALIBRATION

    Braunschweig. It was recognized by the working group that existing standardsand guidelines, formulated to verify the performance of a CMM according to astrictly defined set of rules, were inadequate if a CMM was to be used to calibratecomponents. It was agreed that more information on the measurementcapabilities of coordinate measuring machines and the treatment of themeasured data was required. This guideline is the result of the subsequentmeetings and discussions of the group.

    3 Calibration and performance verificationof CMMs

    3.1 CMM calibration

    3.1.1 According to the International Vocabulary of Basic and General Terms inMetrology, a calibration is the assessment of the uncertainties in the final results(measurands) of the measurement task.

    Note: Here 'uncertainty' is used as a synonym for 'error'. The uncertainty maycontain uncorrected known and unknown systematic effects as well as randomeffects.

    3.1.2 In contrast to simple and single-purpose measuring instruments, CMMs areable to measure a very large family of geometrical parameters. For each of theseparameters, there are many possible measurement strategies (eg regarding thenumber and position of the measured points, or the used probe stylus) that arenot standardized. Both the measurement task and the used measurementstrategy determine the way errors are introduced in the measurement systemand the way they propagate in the hardware and software of the CMM. Sincegenerally there is also no predominant source of error, it is currently notpracticable to calibrate a CMM for all the measurement tasks it can handle.The solution is to calibrate a CMM for individual measurement tasks, whereboth the measurement strategy and measurement conditions are well specified.This is called a task-related calibration.

    3.1.3 Based on the concept of task-related calibration, methods and procedures arediscussed in the following chapters to establish traceability of measurementsperformed with a CMM. Furthermore, recommendations are given for theaccreditation of calibration laboratories.

    3.2 CMM performance verification

    3.2.1 Methods to enable a CMM user to specify and verify the performance of a CMM,mainly for the purpose of purchasing a CMM (acceptance testing), have beendiscussed very actively within the CMM community since the mid-1970s. (TheCMM acceptance test, as set out in the appropriate specification standards,provides a mechanism by which a go/no-go decision on the ability of a CMM toperform a series of specific tests can be made. Similar tests are used toperiodically check that this level of performance is maintained.) The discussion

  • EAL-G17 Z CMM CALIBRATION

    EDITION 1 Z Z JANUARY 1995PAGE 6 OF 28

    has resulted in the publication of several national and international standardsand guidelines describing the requirements for the performance verification,periodic reverification and interim checking of CMMs. Furthermore,supplementary documents have been prepared to help the CMM user with theinterpretation of the requirements of particular specification standards.

    3.2.2 Most CMM performance verification standards and guidelines are based onsampling the length-

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