The importance of being calibrated

Citation

(1998), "The importance of being calibrated", Aircraft Engineering and Aerospace Technology, Vol. 70 No. 1. https://doi.org/10.1108/aeat.1998.12770aaf.006

The importance of being calibrated

The importance of being calibrated

Traceability is an increasingly important fact of industrial life. By being able to look back through the manufacturing history of a component it should be possible to identify just how it came to have certain characteristics. The aviation and nuclear industries, which court severe consequences for unexpected in-service failure, were pioneers in this respect. However, the need to raise quality standards in the cause of commercial competitiveness has focused the attention of manufacturing industry generally on recording the life cycle of the humblest component, enabling selection of methods and processes which ultimately improve the breed.

Precise measurement at each stage of manufacture, of constituents, dimensions or properties, against a defined specification is the method. In metal goods manufacture this can provide a wealth of information. In the extreme it allows a steel nut and bolt to be traced back to the iron ore from which it was originally manufactured. Along the route from mine to factory we are relying on measurements made using a variety of instruments. This begs the question of how we can be certain that the measurements are accurate. In short, have they been calibrated and to what standard?

Calibration and verification of measuring systems is an industrial necessity and has progressed from obscurity to a position near the top of most companies' priorities. For companies seeking formal BS 5750 approval of their quality systems, regular calibration of measuring instruments as part of a quality assurance programme is essential. For these companies the dilemma is whether to invest in or maintain an in-house calibration facility, or seek assistance from a specialist testing laboratory.

At the heart of any calibration exercise are traceability to a universally recognised National Standard combined with an accurately controlled and monitored environment, using a documented repeatable procedure for performing the calibration itself, the relevant equipment/instrumentation capable of measurements to the required accuracy, with a skilled person carrying out the measurement. In the UK NAMAS, the National Measurement Accreditation Service, is the recognised body for approval of calibration and testing laboratories. The NAMAS schedule, issued to all approved agencies, gives details of what instruments they are qualified to calibrate, to what standard of uncertainty that calibration can be carried out and who are the personnel qualified to sign the calibration certificate.

The cost of building and equipping a facility to do this is high, as is the cost of maintaining the requisite level of calibration for the test equipment. For a NAMAS laboratory like Mitutoyo UK Limited's Warwick facility to state checking accuracy of three millionths of an inch/0.08µm for a customer's calibration its own primary standards and measuring equipment must be calibrated by a measurement system with a resolution ten times higher. The price of this is something over ten times the commercial calibration fee.

This level of expenditure is one argument in favour of using an outside testing laboratory rather than maintaining one in-house, unless the in-house facility also takes in subcontract work. To calibrate equipment properly requires highly skilled personnel working in a well equipped environmentally regulated facility using fully traceable primary measuring standards. Whereas an independent laboratory has to keep abreast of the latest techniques ­ calibration is, indeed, its business ­ its in-house counterpart can be starved of investment and/or personnel, allowing the quality of its work, and ultimately the manufacturing operation as a whole, to decline.

There is a down side to using an outside laboratory in that the skills developed in the standards room and familiarity with the equipment, which can sometimes be brought to bear on other aspects of a comnpany's activities, will be lost. Additionally, there are the risks of loss or transit damage and the extended period of equipment non-availability while it is being calibrated. The capability of the test house is also worth examination: a NAMAS-approved establishment will have a schedule which states what it can and cannot do and its records and capabilities are subject to regular scrutiny but an independent establishment may not.

However, even larger companies, which have traditionally maintained an in-house facility, are tending towards use of outside contractors as measuring equipment becomes more complex. The technology employed in a modern co-ordinate measuring machine and in some in-cycle gauging systems demands a level of technical expertise that it is not cost effective to maintain for the sake of a small number of installations.

In the particular case of CMMs differential rates of expansion of materials used in their construction together with that of workpieces, and widespread variations in operating environment makes them difficult to calibrate with a high degree of confidence. To date only two NAMAS laboratories, including Mitutoyo, have gained approval for verification by error mapping of CMM performance.

At one end of the spectrum a NAMAS approval may apply to a mobile on-site operator who calibrates machines and instruments in situ on the shop floor. At the other extreme are facilities like Mitutoyo's fully environmentally controlled calibration laboratory with controlled bulk air temperature, humidity and atmospheric dust levels that make an operating theatre look unclean.

What approved operators have in common is that their equipment is calibrated and traceable directly to a National Standard, though their approved capabilities will differ markedly. For length, the standard is held by the National Physical Laboratory using laser interferometry and traceability to it qualifies them to calibrate customers' equipment to a NAMAS ratified level of accuracy. As a result the set of slip gauges used in an engineering shop in Aberdeen can be calibrated to the same standard as the set used by another company in Plymouth and is traceable to the NPL standard.

Details from Mitutoyo (UK) Limited, Joule Road, West Point Business Park, Andover, Hampshire, SP10 3UT. Tel: 01264 353123; Fax: 01264 354883.