Control and calibration of gage blocks

This article is an update of Bernard Schatz's text, from which it takes large extracts.

Because of their accuracy, which is significantly superior to that of the various gauges and instruments used for length measurement, and because they are so easy to use, plane-parallel gauge blocks are used not only as reference length standards in research, but also as the most accurate standard available to industry. This is why, alongside lasers and coordinate measuring machines, the gauge block has been and still is the most widely used length standard in the world.

From reference shims, used to calibrate other shims, to the verification of mechanical parts, via the calibration of measuring instruments, industrial needs are diverse and varied, with precision requirements ranging from ± 0.02 μm to a few tenths of a millimeter.

As will be shown in this article, to meet the diversity of industrial needs, the standardizer has recommended 4 classes of gauge blocks (§ 1 ) and the metrologist has invented and developed various calibration methods (§ 3 ). Whatever the industrial requirements, gauge blocks can be subject to instability over time, and are also liable to wear in line with the rate of use. Periodic recalibration is therefore essential.

To give an idea of the importance of gauge shims in French industry, we can estimate the number of shims in use at a few million; around 50,000 shims are marketed each year, and over 90% of these are steel shims. However, only around 100,000 of these standard shims are recalibrated each year, so there's...