Manufacturing is highly dependent on dimensional measurement, particularly for quality control purposes. Coordinate measurements play a significant role in this activity and are used in automation and the measurement of complex geometries in a wide range of processes ranging from prototyping to mass production and from micro components to large parts. In terms of coordinate measurements, industrial processes should comply with EN ISO 14253-1 (decision rules for proving conformity or non-conformity to specifications, which quantitatively includes the uncertainty of measurement) and the EN ISO 15530 series on methods for evaluating the uncertainty of measurement in coordinate measurements. If you are certified to ISO 9001 your measurements need to be traceable and that includes a statement of measurement uncertainty.
Methods currently in use for determining task specific measurement uncertainties on Coordinate Measuring Machines (CMMs) either involve making time consuming comparison measurements (ISO 15530-3) or using one of the many software applications that enable Monte Carlo simulations to be performed (Calypso OVCMM, Quindos OVCMM, PUNDITCMM and others) as described in ISO 15530-4. These methods are further described in the NPL Good Practice Guide 130. However, the methods currently in use for uncertainty evaluation lack practical application for use in industry and are primarily confined to National Metrology Institutes (NMIs) and a few high-end users.
It was identified within Europe that there is a need to develop simplified and validated methods for predicting the uncertainty of task-specific coordinate measurements in industry as well as a need to developing traceable methods that improve the validity of existing methods, e.g., EN ISO 15530-3 and prEN ISO 15530-4.
To address this need, a number of European partners are working on a project called EUCoM. NPL, the UKs National Metrology Institutes is taking part in this project. Other consortium partners include
- INRIM (coordinator) the Italian NMI,
- IK4 Tekniker (Spain),
- Metrosert, the Estonian NMI,
- NMIJ-AIST the Japanese NMI,
- CMI the Czech Metrology Institute and NMI of the Czech Republic,
- DTI (Danish Technological Institute),
- PTB the German NMI,
- TUBITAK the Turkish NMI,
- ATH University of Biesko-Biala (Poland),
- UNIPD University Degli Studi Di Padova (Italy),
- GUM Główny Urząd Miar (Central Office of Measures, Poland).
The overall objective of the project is to develop viable methods for evaluating the measurement uncertainty in coordinate measurement throughout industry and to support the standardization working group (ISO/TC213/WG10) in further development of related standards in the ISO 15530 series.
This project aims to achieve this objective by delivering simplified methods for evaluating task-specific coordinate measurement uncertainties that are applicable for use in industry, and by providing guidance to practitioners. This will enable companies to make more reliable product inspection decisions, reducing the risk of false acceptance or rejection of parts. The project aims to deliver two methods for evaluating the uncertainty of coordinate measurement.
Disseminating the results to industry will be a key part of the project.
The specific objectives of the project are:
- To develop traceable and standardized methods for evaluating the uncertainty of coordinate measurement a posteriori using type A evaluation.
The EN ISO 15530-3 specification standard provides an a posteriori method based on measurements of a calibrated work-piece. This calibrated work-piece is required to be identical to the actual work-pieces. Effectively, the burden of the uncertainty evaluation is down to the person who calibrates the calibrated work-piece. The project aims to remove this limitation by restricting the experimental investigation to the work-piece under inspection. This will be applicable to a broad range of measurands (sizes, angles and geometrical features – either related to a datum or not related to a datum) and probing conditions (discrete points and scanning).
- To develop a simplified and validated method for predicting the uncertainty of coordinate measurements a priori using type B evaluation (e., expert judgement).
The relevant ISO standards offer no guidance on uncertainty prediction. Currently, experts use their own knowledge while the majority of CMM users are unable to predict the uncertainty on their own. The project will aim to overcome this problem by providing guidance to practitioners so that they can predict the coordinate measurement uncertainty. It is planned that the guidance will include a number of practical cases and example uncertainty components.
- To demonstrate the validity of existing methods, and those from the first two objectives, in industrial conditions and evaluate their consistency and accuracy against the Guide to the Expression of Uncertainty in Measurement (GUM) and its supplements.
The two proposed methods will be validated thoroughly. Several experienced partners from different countries, equipped with different CMMs, will provide robust evidence and ascertain the validity limits. The validation results will be made widely available to the public through country-based EUCoM seminars, an open source data repository, and ISO/CEN standards.
- To contribute to revisions of EN ISO 15530 and EN ISO 14253-2 by providing the necessary data, methods, guidelines and recommendations, in a form that can be incorporated into the standards at the earliest opportunity. In addition, to collaborate with the technical committees CEN/TC290 and ISO/TC213/WG10 and the users of the standards they develop to ensure that the outputs of the project are aligned with their needs and recommendations for incorporation of this information into future standards at the earliest opportunity. To promote early dissemination of the developed methods to industry.
Specification standard ISO/TS 15530-2 (draft) Geometrical Product Specification (GPS). Coordinate measuring machines (CMM): Technique for determining the uncertainty of measurement. Part 2: Use of multiple measurements strategies in calibration artefacts is currently under preliminary study in the working group ISO/TC213/WG10 (CMMs). This approach assumes that a significant part of the uncertainty is repeatability of the measurement of the same workpiece. In order to include in the spread as many as possible factors influencing the uncertainty, the measured workpiece is placed in different places in the measuring volume oriented differently relative to the CMM axes. Use of this approach for uncertainty estimation is limited to workpieces of simple design (e.g. ring gauges or plug gauges) for which the measurement in four different orientations within the CMM measuring volume is simple to perform.
Currently, no activity is foreseen for the new ISO/TS 15530–5 Geometrical Product Specification (GPS). Use of expert judgement. Specification standard EN ISO 14253-2 is being considered for possible revision in ISO/TC213/WG4 (Uncertainty and decision rules). The project will provide input to the ISO/TC213/WG10 for developing ISO 15530-2 and ISO 15530-5. Some input may be given for a possible revision of the EN ISO 15530-3.
The kick-off meeting was held in INRIM (National Metrology Institute of Italy) in Turin on the 10th of July 2018 and work has now started on the project. Subsequent meetings to monitor progress have been held in Prague (April 2019) and in Braunschweig (November 2019). The project is now half way through and is moving from the theoretical stage to the verification stage involving actual measurements.
Updates on project progress will be posted regularly on the project website.
For more information: www.eucom-empir.eu