Good day, may you suggest a reference method for micrometers calibration?
Thanks in advance
Stefano
Stefano,
Gauge blocks at a certain temperature are the conventional way.
https://www.mitutoyo.com/video/gage-block-introduction-how-to-use-and-calibrate-gauge-blocks/
and, more formally;
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4654600/
To achieve tighter accuracy, interferometric methods exist.
The procedure should cover:
1. Zero error: simply touch measuring surfaces together and read result
2. length error (±2 µm, ±4 µm) - calibration blocks,
3. parallelism error of measuring surfaces - the parallel-flat glass gauge or it can be as well the ball 5-6 mm size measured in different position on the measuring surfaces. The range of readings is the error.
When is going about the lengths, please remember that typical micrometer base on the screw thread (pitch 0,5 mm) in this case the end of the lenght should be not full mm but some parts of mm eg. (0; 6,12; 12,25; 20,75; 25 )mm or something like this
For an in-house reference gauge blocks are the simplest to implement. They are available in various grades of accuracy (getting more expensive as the accuracy increases).
Before investing in gauge blocks its worth taking a step back and looking at the measurement system as a whole. A typical micrometer is accurate up to within 0.001mm, however the measurement system errors don't end there they also include the accuracy of the reference method (in this case the gauge block) and the tolerance of the calibration (essentially allowable error). All of this can be simply calculated via a simple B Type Measurement Uncertainty evaluation taking the root mean sum of the micrometer, gauge block and tolerance.
The first question to ask is what accuracy do you need of your measurement system. This number will guide you as to the level of accuracy you need in the gauge blocks along with an appropriate sized test tolerance in order to meet you measurement system goals.
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