CMM Measurement Strategies and Good Practice

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CMM Measurement Strategies and Good Practice

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This guide covers the selection of the number of measurement points when using Co-ordinate Measuring Machines (CMMs) and gives advice on the compromise between accuracy and speed. It provides guidance on sampling criteria for standard features and advice on measurements that involve projections of features over long distances. It covers cleanliness, part loading/alignment and the effect of temperature, surface finish and geometry on the result. It also contains information on basic measurement principles, common measurement requirements, CMM software functionality in relation to drawing requirements and good metrology practice when using CMMs with CAD data to inspect parts.

Contents
Introduction 1
What this guide is about and what it is not .. 2
Introduction to measurement strategies and this guide . 2
Co-ordinate measuring machines .. 2
General measurement strategy .. 3
Selection of the features to be measured .. 5
Features to be measured 6
Workpiece setup considerations 6
Repositioning methods .. 7
Chapter summary.. 7
Define the workpiece datum to be used within the co-ordinate system 9
Datum features on drawing .. 10
Changing the datum .. 14
Introduction 14
Procedure . 14
An example 14
Measuring small features 18
Chapter summary 18
Selection of the workpiece orientation and holding method . 19
Selection of the workpiece orientation 20
Selection of the workpiece holding method . 20
Measurement force . 20
Heavy items 20
Light items .. 20
Fixtures . 21
Adhesives 21
General . 22
Chapter summary 22
Stylus system qualification . 23
Basic stylus system qualification 24
Additional checks .. 25
The choice of stylus .. 26
Comparison (substitution method) 27
Chapter summary 27
Definition of the probing strategy . 29
Introduction to probing strategies .. 30
Distribution of points 32
The “AD HOC” approach . 33
Lines .. 33
Planes 33
Circles 35
Spheres . 37
Cylinders . 38
Cones . 41
The scientific approach 43
The problem .. 43
A solution 43
Basic principles and limitations .. 45
Using a typical UES package .. 46
Partial features and partial arcs 48
Chapter summary 50
Programming the CMM .. 51
Programming the CMM . 52
The user requirement 52
The functional specification . 52
Testing the program .. 52
Probing considerations in the measurement Strategy .. 53
General points to note .. 53
Recommended details the program should contain .. 54
Program version control . 54
Formatting records. 55
Chapter summary 55
CMM Data Assessment software 57
CMM data assessment software . 58
Data filtering effects . 60
Chapter summary 60
Software functionality .. 61
Non-conventional part alignment .. 63
The CAD interface 63
Graphical feature analysis . 64
Automatic feature measurement . 64
Reverse engineering . 65
Offline programming 65
Programming language 66
Unknown geometry scanning .. 66
Geometric tolerancing . 66
Complex surface measuring . 67
Integrated best fit routines . 67
Integrated statistics 67
Uncertainty evaluating software . 68
Chapter summary 68
Alignment criteria for standard features .. 69
Length measurement 70
Diameter and position of a hole.. 71
Diameter and position of a shaft . 73
An angle between two lines .. 74
Chapter summary 74
Environmental Conditions . 75
Temperature effects .. 76
The effect of vibration . 79
Dust and cleanliness . 79
Chapter summary 80
Summary .. 81
Glossary of terms . 83
Glossary of terms 84
Health and safety.. 87
Mechanical hazards .. 88
Hazards associated with laser illumination .. 88
Chemical hazards 88
Appendices 89
Appendix A Links to other useful sources of information 90
A.1 National and International Organisations. 90
A.1.1 National Physical Laboratory .. 90
A.1.2 National Institute of Standards and Technology (NIST) 91
A.1.3 EURAMET 91
A.1.4 Institute for Geometrical Product Specification . 92
A.2 Networks .. 92
A.2.1 Mathematics and Modelling for Metrology (MMM) .. 92
A.3 National and International Standards . 92
A.3.1 British Standards Institution (BSI) 92
A.3.2 International Organisation for Standardization (ISO) . 92
A.4 Traceability . 93
A.5 Training courses 94
A.5.1 Dimensional Measurement Training: Level 1 – Measurement User 95
A.5.2 Dimensional Measurement Training: Level 2 - Measurement Applier96
A.5.3 Mitutoyo training courses .. 98
A.5.4 NPL E-Learning .. 98
Appendix B Further reading 100
Appendix C User Requirement .. 101
Appendix D Functional Specification . 102
Appendix E Testing 108

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