QMT Features: April 2016
Finishing the job
In the final article in our series on practical metrology by NPL’s Keith Bevan, he reminds us of the need to finish the job properly


Once that you have performed the measurement, and collected and communicated the data, you may feel that the task is complete.  However, in order to ensure that the next measurement task is not delayed, you need to ensure that you leave the measuring instruments so that they are fit for purpose for their next operation.

There are three key points to consider: cleaning, storage and record keeping.

Cleaning

I previously mentioned the need to clean instruments when preparing to take a measurement, but apparatus should also be cleaned at the end of a measurement task. Some tools are more susceptible to contamination than others, so keeping them clean is critical. For example, it is essential to keep the jaws and toothed rack of dial callipers clean as any contamination can cause the pinion gear to jump and lead to the pointer not returning to zero.  At the very least, use a lint free cloth to wipe instruments free of dirt and fingerprints.

Storage
Data from a Coordinate Metrology Society (CMS) Measurement study showed that more than a quarter of delegates questioned believed it was not important to check a measurement device for damage and place it in its storage location. Such misconceptions can be a serious detriment to other measurement professionals who need to use the instrument.

Depending on the instrument used, there can be quite specific requirements. For example, we recommend observing the following precautions when storing callipers. 

  • Choose a space where the callipers will not be subject to dust, high humidity or extreme temperature fluctuations. The storage area should not be damp and it is worth taking the extra precaution of placing a bag of silica gel in the tool drawer to extract any unwanted moisture.
  • Place the instrument in a way such that the main scale beam will not bend and to provide adequate protection from damage to the Vernier scale.
  • Leave the measuring faces so that they are not in contact. We recommend a gap of about 2 mm.
  • Do not clamp the slider.
  • Store the calliper in a case or plastic bag.
  • With large size callipers, which are not frequently used, apply a rust preventative to the sliding and measuring faces and separate the two jaws. Avoid rust preventatives that leave a coating on the material being protected as this can affect the calibration of dial type callipers.
  • If the callipers are rarely used, ensure you check the storage condition and movement of callipers on a monthly basis.
  • Prevent vapours from chemicals such as acids from permeating the storage area.
  • Keep a record of callipers that are stored. Maintain detailed information on all callipers in use on the shopfloor.

Records
Keeping thorough records helps to give extra confidence in the quality of the measurements, and data from these records is necessary for estimating the uncertainty of measurement. Whether you are part of the quality, inspection or calibration process, it is important to record details of calibration, use, and any issues surrounding the instrument. This ensures that other measurement professionals have information on past uses should any problems with either a product or the measuring tool arise, giving a means of tracing its usage back to try to solve the issue.
www.npl.co.uk

  
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