Gauge blocks, also known as Jo blocks, are precision machined blocks used to calibrate measuring devices and for precise measurements. They come in sets of various sizes and grades of accuracy, made of high-quality steel or carbides. Squeezing and lapping are techniques used to fasten and create flat surfaces. They are used in calibration, measuring, and quality control in precision equipment and machined parts.
Gauge blocks are precision machined blocks designed to calibrate measuring devices and for use as precision measuring devices. Also called Jo blocks, a reference to their Swedish inventor, engineer and machinist Carl Johansson, they are created in sets containing many different sizes, so that they can be combined in various ways to get almost any desired measurement, up to several inches (1 inch = 2.5cm). They are usually made of high quality steel or carbides that resist wear and damage. A gage block is machined to have two sides perfectly flat within very tight tolerances and parallel to each other.
These devices are manufactured in different grades, based on accuracy. The lowest grades of gauge blocks have an accuracy tolerance of about 25 millionths of a meter variation, which is roughly equivalent to the thickness of a thin strand of human hair. The highest grades of gauge blocks can have precision tolerances down to 0.05 millionths of a meter variation, which is a small fraction of the thickness of a thread of spider silk. Gauge blocks are manufactured in both metric and standard US units of measure.
Squeezing is a technique for fastening a stack of these blocks together. It involves putting a very thin layer of oil between the blocks, making them stick together almost as if they were glued together. The blocks can be separated by sliding them apart, which requires a reasonable amount of force. A stack of gauge blocks properly squeezed together can easily support its own weight without falling apart if held horizontally 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 squeeze film, and the void 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 lower grade sets, the wring film thickness is so small that it is not a factor in the calculations and measurements. For the higher and more precise grades of gauge blocks, the effective length of a block is purposely reduced by the thickness of a squeeze film, so that when they are combined, the combined thickness of the layers of squeeze film does not compromise any measurement data of calibration.
Machinists and others who use precision equipment have found many uses for gauge blocks, as calibration equipment and measuring devices. The extremely precise measurements possible when using these tools allow for very accurate calibration of micrometers and other measuring instruments. They can also be used to measure objects or distances very accurately. Gauge blocks are also used for quality control purposes, such as testing machines and machined parts for accuracy and adherence to tolerance guidelines.
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