This issue of Blue Update introduces the method for choosing optical flats and optical parallels to help maintain the measuring faces of outside micrometer calipers. When starting, we should consider the standard values of flatness and parallelism of outside micrometer calipers according to the JIS B 7502:1994. Outside micrometer calipers require a flatness of 0.6 µm and a parallelism of 2 µm. After considering this, you should compare the correct values of flatness between measuring faces of the outside micrometer calipers with the flatness of the optical flat, which should, when calculated, have an accuracy ratio of 1:3, which is according to the recommendations of ISO 10012-1, item 4.3. The optical flats of Mitutoyo come in 2 versions. The choice of whether to purchase an optical flat with a flatness of 0.1 µm or 0.2 µm must be based on your needs and your budget. Thus, we must also consider the diameter of the optical flat, because, when it is necessary to use the optical flat to inspect the measuring surfaces of the gauge block, optical flats with diameters of 60 mm are the best choice. As for Mitutoyo’s optical parallels, there are also 2 options, which are optical parallels that have a thickness of 12.00 mm – 12.37 mm and ones with a thickness of 25.00 mm – 25.37 mm. In considering which type to use, we should consider carefully whether we will use the optical parallels for measuring a range from 0 mm – 25 mm or a range greater than 25 mm.


However, when seeing the flatness value of the optical parallels, which provides a flatness of 0.1 µm and a parallelism of 0.2 µm, this informs you that you can use an optical parallel instead of an optical flat. An additional benefit is that the optical parallels can be used with both sides, while the optical flat can only be used with the side where the head of the arrow points.


Anusorn Tonmueanwai Chief of Laboratory, Acting Chief – Dimensional Metrology section National Institute of Metrology, Ministry of Science and Technology, Thailand With over 20 years of professional experience in metrology and an invited lecturer at the Faculty of Engineering, King Mongkut’s University of Technology Thonburi