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Dimensional FAQs

Dimensional metrologists at NPL provide a wide range of services, facilities and expertise to support UK manufacturing, science and the metrology community.

General

• Are inches still legal units? (FAQ - Length)

In some situations.
• Do you know of any good dimensional metrology books? (FAQ - Length)

There are many good books and in addition NPL produces a range of Good Practice Guides relating to dimensional metrology.
• Do you still use the 'metre bar' as the length standard? (FAQ - Length)

No. Since 1983 the metre has been re-defined in terms of the speed of light and is realised using laser wavelengths.
• Does NPL calibrate test sieves? (FAQ - Length)

No, but there are at least two companies that do.
• Does NPL still calibrate sextants? (FAQ - Length)

Not for a long time, and we are unsure of any companies that do?
• Does NPL still perform calibration of items in imperial units, e.g. inch gauge blocks? (FAQ - Length)

It depends on the exact item. Generally we try to offer calibrations in imperial units, where we have the facilities available.
• How do I calculate the compression of a cylinder between flat parallel anvils? (FAQ - Length)

Formulae are given in Elastic Compression of Spheres and Cylinders at Point and Line Contact by M J Puttock and E G Thwaite.
• How often should my gauges/instruments be calibrated? (FAQ - Length)

There is no fixed period for the validity of a calibration certificate - it is not like your cars MOT certificate. However, measurement results stated on certificates are usually 'on the day' values and no allowance is made for subsequent drift.
• Is there a difference between 'accuracy' and 'uncertainty'? (FAQ - Length)

The words accuracy and uncertainty are sometimes interchanged but the difference between them is significant and, in many applications it is vital.
• On what basis is one inch exactly equal to 25.4 mm? Has the imperial inch been adjusted to give this exact fit and if so when? (FAQ - Length)

The (international) inch has been exactly 25.4 mm since July 1959. At this point in time the (international) yard was redefined as 0.9144 metre.
• What is the conversion between inches and metres? (FAQ - Length)

1 inch = 25.4 mm.
• Where can I buy engineering standards such as gauge blocks, angle gauges etc? (FAQ - Length)

An internet search should lead you to companies which sell such items.
• Where can I get my theodolite calibrated? (FAQ - Length)

The only UK laboratory that is UKAS accredited for the calibration of theodolites is Falcon Scientific Instruments.

Accreditation

• What is the difference between an NPL certificate and a UKAS certificate? (FAQ - Length)

Some services at NPL have been accredited by UKAS and for calibrations made using these accredited services, a UKAS certificate is issued.
• What is UKAS? (FAQ - Length)

UKAS is the United Kingdom Accreditation Service.

International

• My customer is American and requires traceability to NIST. Can NPL offer this? (FAQ - Length)

The services of NPL are equivalent to those offered by NIST (and other National Labs), as covered by the Mutual Recognition Arrangement.
• What is the Mutual Recognition Agreement/Arrangement? (FAQ - Length)

This is an arrangement amongst the majority of the world's National Metrology Institutes whereby each institute agrees to recognise measurement standards and calibration and measurement certificates issued by the other.
• What is traceability? (FAQ - Length)

Traceability is the property of a measurement or the value of a standard whereby it can be related to stated references.

Standards

• How do I obtain MOY/SCMI specifications? (FAQ - Length)

MOY/SCMI specifications are available on the NPL Length website.
• What does MOY/SCMI stand for? (FAQ - Length)

Metrology/Specification Certification Measuring Instruments.

Temperature

• How do I correct lengths to 20ºC? (FAQ - Length)

To correct a length you need to calculate the difference in temperature multiplied by the product of the length and the expansion coefficient.
• I have just taken my step gauge from the boot of a car. How long before I can measure it? (FAQ - Length)

The length of time that it will take for the step gauge to reach ambient temperature in the laboratory will depend on both its mass and the temperature differential between the car boot and the laboratory.
• What is the reference temperature for dimensional metrology? (FAQ - Length)

The internationally agreed temperature is 20°C.

Tolerancing

• Is it acceptable to apply rounding to tolerances? (FAQ - Length)

In short the answer is no. What you do need to take into account is the uncertainty of measurement.
• What is the 10% rule? (FAQ - Length)

The 10% rule says that if a component tolerance is given as x then the instrument used to demonstrate compliance should have an uncertainty of better than x/10.

CMMs

I need to calibrate my CMM, or do I mean verify it or error map it or qualify it? I am confused. (FAQ - Length)

A manufacturer will calibrate a CMM and produce an error map that corrects for the various geometric errors in the machine. When delivered an acceptance test will be carried out to verify that it meets its specification. This verification should be repeated periodically, in addition interim checks should be carried out.

End Standards (gauge blocks, length bars)

• How do I select gauge blocks to pick a specific size combination? (FAQ - Length)

The micrometre (0.001 mm) gauge should be taken first, followed by the hundredth, tenth and millimetre gauges.
• How do I take care of my end standards? (FAQ - Length)

Following these steps ...
• How long should a gauge block be in service before re-calibration and is this period based on the last calibration of the block or the last time it was used to calibrate others? (FAQ - Length)

There are several factors which contribute to the discussion on re-calibration intervals for gauge blocks.

• Can you provide any guidance or information as to the apparent difference in the results between the direct method and check plug method of screw gauge calibration? (FAQ - Length)

Check plugs and parametric measurements are not mutually exclusive. It is NPL's opinion that for checking master gauges (i.e. not products) both check plugs and parametric measurements have their place.
• What is rake correction and how do I calculate it? (FAQ - Length)

The complete inspection by direct measurement of parallel screw threads necessitates the measurement of the effective diameter which, in the case of external threads, is obtained by measurement across cylinders placed in the threads on either side of the screw.
• Where can I obtain a copy of NPL Notes on Applied Science No. 1 Gauging and Measuring Screw Threads? (FAQ - Length)

This publication has been updated by NPL and an electronic version has been put on the website.

Surface Texture

• How many calibration artefacts do I need to verify my surface texture measuring instrument's performance? (FAQ - Length)

It depends on the job at hand. However, you should have at least ...
• Is there a comprehensive list of surface texture parameters that also explains where their use is relevant? (FAQ - Length)

Yes, the Measurement of Surface Texture using Stylus Instruments, Good Practice Guide No 37.
• Where can I find surface texture instrument calibration artefacts? (FAQ - Length)

A list of manufacturers and distributors.
• Why do my stylus and optical surface measuring instruments often give different results? (FAQ - Length)

Stylus (mechanical contact) and optical (non-contact) instruments operate on different physical principles and will always tend to differ in the results they give for a surface.

Optical

• Do surface coatings affect the measurements we perform? (FAQ - Length)

To some degree yes. Ideally, the reflectivity of both the reference and test surface should be equal to give maximum contrast fringes.
• How does NPL calibrate its own Reference Flats? (FAQ - Length)

NPL gets the traceability of its 300 mm diameter Master Flat from the surface of a pool of silicone oil, which conforms to the local radius of curvature of the earth and from which the departure from flatness can easily be calculated.
• How often should I get my reference flats, spherical surfaces and test lenses recalibrated? (FAQ - Length)

We would recommend that reference flats in interferometers be recalibrated every three to five years depending upon the precision of measurement required.

Registration

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