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Australian Government: National Measurement InstituteAustralian Government: National Measurement Institute
National Measurement Institute
      

Length and Related Quantities Capabilities

As shown below, NMI is well equipped to perform the calibration of many types of dimensional measuring instruments, gauges and components. Our facilities are also available for appropriate scientific and industrial measurement applications. Click here:

Before consigning an instrument for calibration, always consult us to discuss your requirements. For further information contact calibration.coordinator@measurement.gov.au.

Important Notes

Temperature and other environmental factors can influence the accuracy of measurements and considerable care is taken to reduce these effects to negligible levels wherever possible, or to make appropriate corrections where necessary. Unless otherwise requested the results of measurement are normally referred to a standard temperature of 20 °C.

In many length measurements the method adopted may require the application of a small compressive force. Corrections for this compression are applied where necessary and, unless otherwise stated, all quoted results are referred to zero applied force.

The final reported uncertainty for a calibration can be affected by many factors. In the case of gauges or components these may include the precision of the geometry, surface finish, material, amount of wear and the simplicity of form (i.e. a complex form may make a part more difficult to measure). For instruments, factors that may affect the reported uncertainty may include resolution, repeatability, discrimination (ability to reliably detect a small change in input) and sensitivity to the environment. When necessary, the appropriate uncertainty requirements for a calibration should be discussed with NMI staff.

Laser Interferometers

A performance test can be carried out on laser length measuring interferometers over a distance of up to 10 m. Calibration is via a direct comparison to NMI’s reference interferometer using a common beam path, beam splitter and retro-reflector. All distance measurements are referred to standard environmental conditions (temperature = 20 °C, pressure = 1013.25 hPa, relative humidity = 50%). The calibration of any associated temperature, pressure and humidity sensors can also be arranged, although this work will be carried out by the relevant sections within NMI.

Laser Interferometer

End Standards

End StandardEnd standards often take the form of gauge blocks or length bars. Materials can vary, but most often they are made from steel. Cross-sectional forms are usually rectangular (as specified in ISO3650) or square (as specified in ASME B89.1.9) although some older length bars may have a circular cross section (consistent with BS5317). The end faces of gauge blocks and length bars are finely finished by lapping and are very flat and parallel.

The calibration of end standards of nominal length up to 125 mm is by direct comparison with NMI standard gauge blocks in a vertical orientation. For end standards of nominal length greater than 125 mm, calibration is by comparison to a laser interferometer in a horizontal orientation. In addition, flatness, parallelism and wringing quality are also reported. Typical uncertainties (95%) for nominal length gauge blocks and length bars:

  • 0.5 mm < L ≤ 125 mm are ±(0.043 µm + 0.000 5 L)
  • 125 mm < L ≤ 1 300 mm are ±(0.19 µm + 0.000 75 L)

Another type of end standard is the step gauge. Step gauges are used as reference standards for the calibration of coordinate measuring machines (CMMs) and machine tool scales. Step gauges up to 1200 mm long can be calibrated on NMI's CMM using a substitution method with reference length bars.

Line Standards

NMI examines rigid linear scales up to 1 m in length. The accuracy with which scale intervals can be determined depends on many factors, among which are the material and form of the scale, the nature of the surface on which the lines are ruled, and, importantly, the quality of the lines.
Measurement of the length of an interval is carried out at a temperature close to 20 °C. The coefficient of thermal expansion of the material of the scale must be known with sufficient accuracy to enable the observations to be corrected to standard temperature.

NMI examines stage micrometers used as reference standards in the calibration of eyepiece graticules, microscope stages and micrometer eyepieces.

Although uncertainties as low as ±0.3 µm can be achieved, a more typical uncertainty is of the order of a few micrometres.

Coordinate Measuring Machine (CMM)

NMI has an industrial accuracy CMM with an automatic probe changing rack, indexing head, contact scanning probe and a measuring volume of 1200 mm x 900 mm x 800 mm. By using substitution and reversal methods of measurement, this machine is capable of sub-micrometre accuracies. It is available for the measurement of reference artefacts, components, fixtures and for the provision of training to CMM operators. In addition, NMI also has a non-contact vision based CMM with a measuring volume of 300 mm x 300 mm x 100 mm. This is particularly useful for measuring small or soft parts.

Advice and consultation on CMM performance and applications can be provided. NMI actively collaborates with the MSA CMM Users Group, a technical group for users, potential users and suppliers of CMMs. Upon request, reference artefacts such as step gauges and ballplates can be provided for user verification of their CMMs.

Coordinate Measuring Machine

Electronic Distance Measuring (EDM) Instruments

For shorter range EDM instruments, calibration is by comparison back-to-back with a laser interferometer in a stable environment. NMI’s standard services include either a 70 m range or a 130 m range (where the EDM measurement signal is folded through 180° to extend the available range). Measurements are performed under ambient laboratory conditions and referred to the instrument environmental settings that correspond to a 0.0 ppm atmospheric correction. For longer range instruments, the EDM’s frequency oscillator is calibrated against the national standard of frequency followed by calibration on NMI’s 7 pillar 650 m baseline, which is traceable to the national standard of length. The EDM baseline measurements are carried out according to ISO17123 part 4. Typical uncertainties (95%) for:

  • comparison to laser interferometer up to 130 m are ±0.22 µm
  • 7 pillar baseline up to 650 m are ±0.50 µm

Electronic Distance Measuring Instrument

Angle Standards

NMI maintains a pair of precision, indexing tables with an uncertainty of less than ±0.1 arc seconds. The tables are periodically calibrated against each other using a least squares method, the property of angle closure (by definition there are exactly 360° in a circle) and a precision autocollimator. The indexing tables are used to calibrate angle gauges, polygons, angle encoders and other angle measuring equipment. A sine table with a base length of 825 mm is used to calibrate autocollimators and precision levels. Typical uncertainties (95%) for some types of angle equipment are:

  • indexing table ±0.1 arc seconds
  • autocollimator ±0.2 arc seconds
  • polygon ±0.2 arc seconds
  • angle gauge ±0.3 arc seconds

Measuring Tools

NMI can calibrate a variety of dimensional measuring equipment, including micrometres, optical flats (of up to 200 mm diameter), optical parallels, toolmakers’ flats, digital scales, dial gauges, surface plates, etc. Typical uncertainties (95%) for some types of equipment are:

  • optical flats ±0.04 µm
  • digital scales  ±0.30 µm (dependent on instrument resolution)
  • dial gauges ±1.0 µm (dependent on instrument resolution)
  • surface plates ±1.2 µm

Diameter Standards

The diameters of external (cylindrical or spherical) and internal (ring) standards are measured using one of several 2-point methods by comparison to either gauge blocks or laser interferometer. For the most accurate requirements, directional diameters, aligned to some inscribed feature on the standard, are reported. Average diameters can be reported based on either the mean of several 2-point diameters or as the result of matching the orientation of a single directional diameter with a roundness measurement. Typical uncertainties are given by ±(0.12 µm + 0.001 D) where D is the nominal diameter in millimetres.

Roundness

Roundness is measured using a rotating spindle stylus instrument in which the residual out-of-roundness of the rotating spindle is accurately known. Roundness standards such as hemispheres, magnification standards and inclined standards can be calibrated, as well as the roundness of diameter standards and many different types of components. In special cases, the sphericity of spherical standards can also be measured. The uncertainty of measurement is dependent on the quality of the item being measured and may range from less than ±0.01 µm for a very high quality hemisphere standard to ±0.5 µm for some components.

rotating spindle stylus instrument

Surface Roughness

Surface roughness is measured using a stylus instrument in accordance with the principles set out in relevant international (ISO) and Australian (AS) standards. A wide range of surface-roughness parameters can be determined and these include arithmetic-mean deviation (Ra), root-mean-square deviation, maximum peak-to-valley roughness height, and ten-point height, along with many others. In particular NMI calibrates surface-roughness standards, grooves and steps used to determine the magnification of static-displacement-measuring instruments. Advice on problems related to surface roughness and its measurement can also be provided.

Surface roughness stylus instrument.

Machine Tools

Facilities for the examination of most types of machine tools are available at a number of NATA-accredited laboratories. NMI advises on problems relating to the examination of machine tools, on measurement techniques, and on appropriate measuring equipment. In special circumstances the examination of a machine tool may be undertaken.

Components, Jigs and Fixtures

A wide variety of manufactured components, jigs and fixtures can be measured by NMI. These mostly require coordinate measurement techniques using either contact or non-contact CMM technology. The provision of appropriate detail drawings with the required geometric dimensioning and tolerancing specifications and/or a CAD model is often required. If these are not available, detailed discussion is often required to ensure that the correct properties are measured.