Glossary & Reference
Gauge blocks are the physical definition of length on the shop floor — precision-ground reference blocks that inspectors and toolmakers stack and "wring" together to build an exact, traceable dimension for calibrating everyday measuring tools.
Gauge blocks, also called Jo blocks after their inventor, Swedish machinist Carl Edvard Johansson, are rectangular blocks of hardened steel, chrome carbide, or tungsten carbide. Each block is ground and lapped so its two opposing faces are flat and parallel to within millionths of an inch. A full set contains dozens of blocks across a range of thicknesses, so combining the right blocks lets you build up almost any length you need, in very fine steps.
The trick that makes gauge blocks useful is wringing. Slide two lapped faces together with a slight rotating pressure and they don't just touch — they adhere, resisting a light pull as though welded. The faces are flat and smooth enough that pressing them together squeezes out the air between them, leaving only an extremely thin film of oil or moisture that bonds the surfaces through molecular attraction and atmospheric pressure. A properly wrung stack behaves as a single continuous piece of steel, holding its combined length to within the tolerance of the individual blocks.
Gauge block accuracy is governed by ASME B89.1.9 in the US and ISO 3650 internationally. ASME grades run K, 00, 0, AS-1, and AS-2, from tightest tolerance down to general shop use; ISO 3650 uses the parallel grades K, 0, 1, and 2. Top-grade blocks (K, 00/0) serve as master references used to calibrate working sets, while lower-grade blocks (AS-1, AS-2) are the everyday shop blocks used to check calipers, micrometers, and height gauges. Because steel changes size slightly with temperature, blocks are certified at the standard reference temperature of 68°F (20°C), and precision work should be done near that temperature.
Beyond checking micrometers and calipers, gauge blocks are used to set up height gauges, verify plug and ring gauges, and serve as reference artifacts when verifying a coordinate measuring machine (CMM). Toolmakers also wring blocks together to build an exact stack length for setting sine bars, precision fixtures, and go/no-go inspection setups — letting a shop create an exact dimension from a handful of blocks instead of needing a dedicated gauge for every size.
Every micrometer, caliper, and height gauge in the shop is only as trustworthy as the standard it was last checked against — and that standard is usually a gauge block. Knowing the grade you're working with, handling blocks correctly, and wringing them properly is what keeps your everyday measuring tools honest.