Does Thermal Expansion Impact Gage Blocks?
Since gage blocks were invented over a century ago, they have been one of the main tools used in length standardization. Some of the properties of gage blocks include: the material it is made of, its thermal expansion characteristics, mechanical deformation, and its geometry. Gage blocks are typically made of steel but can also be made of other material such as ceramic. One of the reasons that steel is the preferred material is that it is hard and can withstand common wear and tear. Another reason has to do with the coefficient of thermal expansion. All objects change size to some extent due to changes in temperature. A material’s coefficient of thermal expansion describes the amount that it will change in size due to a change in temperature. Steel is the material of choice for gage blocks in part because many machines and manufactured parts are made of steel and thus have a similar coefficient of thermal expansion as a steel gage block. This means that when using steel gage blocks to standardize length measurements on a piece of steel equipment, the coefficient of thermal expansion does not have to be taken into account since both the gage block and the piece of equipment are expanding at the same rate. In addition to a gage block’s material makeup and thermal expansion characteristics, it is important to take into account how its surface deforms in response to contact from a measuring probe. Any surface that contacts another surface becomes deformed to some extent. When calibrating gage blocks, we want to make sure that our comparisons are between blocks of the same material because different materials may deform to different extents in response to contact from another surface. Similar to the discussion on the coefficient of thermal expansion, as long as our comparison is of artifacts made from the same material, the effects of surface deformations cancels out. Another property of gage blocks to keep in mind is their geometry, specifically their flatness and parallelism. If gage block surfaces are completely flat and parallel, then the contact points for the measuring instrument are perpendicular to the block. However, gage block surfaces are not perfectly flat and parallel, which can introduce error to our measurement process. This is especially troublesome with very thin gage blocks which have surfaces that can become easily warped.
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