Gauge blocks are precision-machined blocks that are designed for calibrating measuring devices and for use as precision measurement devices. Also called Jo blocks, a reference to their inventor, Swedish engineer and machinist Carl Johansson, they are created in sets containing many different sizes, so that they can be combined various ways to make up almost any desired measurement, up to several inches (1 inch = 2.5 cm). They are usually made from high grade steel or carbides that resist wear and damage. A gauge block is machined to have two sides that are perfectly flat within very small tolerances, and that are parallel to each other.
These devices are manufactured in different grades, according to precision. The lowest grades of gauge blocks have a precision tolerance of around 25 millionths of a meter variance, which is roughly equivalent to the thickness of a fine strand of human hair. The highest grades of gauge blocks can have precision tolerances as low 0.05 millionths of a meter variance, which is a tiny fraction of the thickness of a strand of spider silk. Gauge blocks are manufactured in both metric and standard United States measurement units.
Wringing is a technique for affixing a stack of these blocks together. It involves placing a very thin layer of oil between the blocks, causing them to adhere to each other almost as if they were glued. The blocks can be separated by sliding them apart, which takes a reasonable amount of force. A stack of gauge blocks properly wrung together can easily support its own weight without falling apart, when held in a horizontal position by one end. It is not fully understood how this process works, but it is thought to be a combination of molecular attraction, the surface tension of the wring film, and a vacuum formed between the blocks.
Lapping, a type of grinding and polishing process, is used to create the extremely flat and smooth surfaces of gauge blocks. For sets of the lower grades, the thickness of the wring film is so small as not to be a factor in calculations and measurements. For the higher, more precise grades of gauge blocks, the actual length of a block is purposely reduced by the thickness of one wring film, so that when they are combined, the combined thickness of the layers of wring film do not compromise any measurement or calibration data.
Machinists and others who use precision equipment have found many uses for gauge blocks, such as calibrating equipment and measuring devices. The extremely precise measurements that are possible when using these tools allow for very accurate calibration of micrometers and other measurement tools. They can be used to very accurately measure objects or distances as well. Gauge blocks are used for quality control purposes as well, such as testing machines and machined parts for precision and adherence to tolerance guidelines.