Near the beginning of the 20th century, Carl Johansson of Sweden, developed steel blocks to an accuracy believed impossible by many others at that time. His objective was to establish a measurement standard that not only would duplicate national standards, but also could be used in any shop. He was able to build gage blocks to an accuracy within a few millionths of an inch. When ﬁrst introduced, gage blocks or “Jo” blocks as they are popularly known in shops, were a great novelty. Seldom used for measurements, they were kept locked up and were only brought out to impress visitors. Today gage blocks are used in almost every shop manufacturing a product requiring mechanical inspection.
They are used to set a length dimension for a transfer measurement, and for calibration of a number of other tools. We generally distinguish three basic gage block forms - rectangular, square and round. The rectangular and square varieties are in much wider use. Generally, gage blocks are made from high carbon or chromium alloyed steel. Tungsten carbide, chromium carbide, and fused quartz are also used. All gage blocks are manufactured with tight tolerances on flatness, parallelism, and surface smoothness. Gage blocks should always be handled on the non-polished sides. Blocks should be cleaned prior to stacking with filtered kerosene, benzene or carbon tetrachloride. A soft clean cloth or chamois should be used. A light residual oil film must remain on blocks for wringing purposes. Block stacks are assembled by a wringing process which attaches the blocks by a combination of molecular attraction and the adhesive effect of a very thin oil film. Air between the block boundaries is squeezed out.
Light pressure is used throughout the process.
The old designation of AAA has a new designation of 0.5 and an accuracy of + or – 0.000001.
The old designation of AA has a new designation of 1 and an accuracy of + or – 0.000002.
The old designation of A+ has a new designation of 2 and an accuracy of + 0.000004 and -0.000002.
The old designation of A and B has a new designation of 3 and an accuracy of + 0.000008 and – .000004.
Gage Blocks Sets
Individual gage blocks may be purchased up to 20" in size. Naturally, the length tolerance of the gage blocks increases as the size increases. Typical gage block sets vary from 8 to 81 pieces based upon the needed application. The contents of a typical 81 piece set are:
For the purpose of stack protection, some gage manufacturers provide wear blocks that are either 0.050" or 0.1 00" in thickness.
Calipers are used to measure length. The length can be an inside dimension, outside dimension, height, or depth. Some calipers are used for only one of these lengths, while other calipers can be used to measure all four types of lengths.
Calipers are generally one of four types:
Spring calipers are transfer tools that perform a rough measurement of wide, awkward or difficult to reach part locations. These tools usually provide a measurement accuracy of approximately 1/16 of an inch. Although these calipers are referred to as spring calipers, there are different varieties (spring joint, firm joint, lock joint, etc.) which describe the type of mechanical joint that connects the two sides of the-unit.
A spring caliper measurement is typically transferred to a steel rule by holding the rule vertically on a flat surface. The caliper ends are placed against the rule for the final readings.
The Vernier scales are used on a variety of measuring instruments such as:
Inside or outside vernier calipers
Gear tooth vernier
Except for the digital varieties, readings are made between vernier plate and beam scale. By design, some of these scales are vertical and some are horizontal.
Dial calipers function in the same way as vernier calipers, however the measurement is indicated by a combination of a scale reading to the nearest 0.1 of an inch and a dial indicating the resolution to 0.001 of an inch. The dial hand typically makes one revolution for each 0.1 of an inch of travel of the caliper jaws. Errors in reading the dial calipers often include misreading the scale by 0.1 of an inch or using the calipers in applications which require an accuracy of 0.001 of an inch, which is not realistic for these type of calipers.
Digital calipers use a-digital display instead of the dial and scale found in dial calipers. Most digital calipers have the ability to be read in either inches or millimeters, and the zero point can be set at any point along the travel. Display resolutions of 0.0005 of an inch are common. Errors in reading the digital display are greatly minimized, however like the dial calipers, digital calipers are often used in applications-which require a different device to attain the required accuracy. For example, some digital calipers have data interface capabilities to send measurement data directly into a computer program.
Digital caliper improvements have made them more reliable for use in machine shop conditions including locations where cutting oil and metal chips come in contact with the calipers.