Fundamental Good Practice in Dimensional Metrology
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Good Practice Guide No. 80 Fundamental Good Practice in Dimensional Metrology David Flack and John Hannaford Measurement Good Practice Guide No. 80 Fundamental Good Practice in Dimensional Metrology David Flack Engineering Measurement Services Team Engineering Measurement Division John Hannaford ABSTRACT This good practice guide is written for those who need to make dimensional measurements but are not necessarily trained metrologists. On reading this guide you should have gained a basic knowledge of fundamental good practice when making dimensional measurements. An introduction to length units and key issues such as traceability and uncertainty is followed by some examples of typical sources of error in length measurement. Checking to specification, accreditation and measurement techniques are also covered along with an introduction to optical measurement techniques. © Queen's Printer and Controller of HMSO First printed July 2005 Reprinted with minor corrections/amendments October 2012 ISSN 1368-6550 National Physical Laboratory Hampton Road, Teddington, Middlesex, TW11 0LW Acknowledgements This document has been produced for the Department for Business, Innovation and Skills; National Measurement System under contract number GBBK/C/08/17. Thanks also to Hexagon Metrology, Romer, Renishaw and Faro UK for providing some of the images and to Dr Richard Leach (NPL), Simon Oldfield (NPL), Dr Anthony Gee (University College London) and Prof Derek Chetwynd (University of Warwick) for suggesting improvements to this guide. i Contents Introduction .............................................................................................................................. 1 Why have you picked up this guide? ................................................................................ 2 Why bother making dimensional measurements at all? ...................................................... 5 Open day at the factory..................................................................................................... 7 Some history of dimensional measurement ...................................................................... 8 Manufacture in the early days – the military ...................................................... 8 The industrial revolution – the beginnings of the modern age and interchangeability ............................................................................................... 8 The 20th century ................................................................................................ 10 So why do we measure things? ...................................................................................... 10 Why make a dimensional measurement? ......................................................... 10 Why do we take so much trouble in making measurements? .......................... 12 How do we ensure consistent measurements throughout the world? ............................. 12 Units of length measurement .......................................................................................... 13 Who defines the metre? .................................................................................... 13 Metres and inches ............................................................................................. 13 A typical transfer standard – the gauge block .................................................. 16 Units of angle ................................................................................................................. 18 Angle gauges .................................................................................................... 24 Resolution, accuracy, tolerance, etc. .............................................................................. 25 Accuracy and precision .................................................................................... 26 Resolution, uncertainty, tolerance and error .................................................... 27 International standards.................................................................................................... 27 Just how well do you have to measure? ............................................................................... 29 Car parts.......................................................................................................................... 30 Introduction to measurement in manufacturing ............................................................. 30 Trend monitoring during production .............................................................................. 31 So your measurement has an uncertainty? ..................................................................... 33 Introduction ...................................................................................................... 33 Expression of uncertainty in measurement ...................................................... 33 The contributions to the uncertainty budget ..................................................... 34 Expanded uncertainty ....................................................................................... 35 The statement of uncertainty ............................................................................ 35 Confidence level ............................................................................................... 36 Type A and Type B contributions .................................................................... 37 Distributions ..................................................................................................... 40 Checking conformance to a specification. How do you know the part meets specification? .................................................................................................................. 42 ii Determining conformance with a specification - ISO 14253 decision rules .... 42 Summary of ISO 14253 .................................................................................... 45 Decision-making rules (production 3:1 versus inspection 10:1) .................................... 45 GO and NOT GO hard gauges (limit gauges) ................................................................ 47 Multiple gauging stations and master parts .................................................................... 48 Future developments ...................................................................................................... 49 Saving money by spending money ........................................................................................ 51 Car parts again ................................................................................................................ 52 Saving money ................................................................................................................. 52 Modern measurement techniques ................................................................................... 55 An overview of co-ordinate metrology ............................................................ 55 Articulated arms ............................................................................................... 56 Laser trackers ................................................................................................... 57 Spending money ............................................................................................................. 58 What happens if things go wrong? ................................................................................. 59 Choosing the right equipment for the job............................................................................ 61 Plane parts ...................................................................................................................... 62 Monday morning .............................................................................................. 62 Later that day .................................................................................................... 63 What bits need measuring .............................................................................................. 66 Choosing the correct tool................................................................................................ 66 Temperature compensation ............................................................................................ 69 Minimising the effects of temperature ............................................................. 69 Correcting for deviations from the reference temperature ............................... 70 Table of errors for typical materials and temperatures .................................... 71 Allowing for temperature effects in uncertainty .............................................. 72 Performance verification - how good is this instrument anyway? .................................... 75 Car parts yet again .......................................................................................................... 76 How do you check out what’s in your toolbox? ............................................................. 76 Visual verification ............................................................................................ 77 Formal verification checks ............................................................................... 77 Formal verification of a CMM ....................................................................................... 78 What happens if the verification fails and the CMM is outside specification? 81 Interim checks .................................................................................................