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Flatness and Straightness Have Changed
(In accordance with the ASME Y14.5-2009 standard)

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Some revisions to the Standard are made because they just make sense. In the 1982 revision of the Y14.5 standard, "as an extension of principle", straightness could be applied to the size dimension of a width as well as a cylinder. The tolerance was described as two parallel planes separated by the amount of the tolerance. The derived center plane had to lie within this tolerance zone. This sounds like a flatness tolerance zone rather than a straightness zone. Since straightness was already being applied to the size dimension of a cylindrical feature, this seemed like a logical extension. 27 years later the standard was revised to replace straightness with flatness when controlling the derived median plane. Although it might seem a little strange to associate flatness with a size dimension, technically it makes a lot of sense.

Whether straightness or flatness is associated with the size dimension of a width, the tolerance applies regardless of feature size. This can be a difficult tolerance to inspect. In this case, the tolerance may be modified with MMC or LMC. A flatness tolerance applied to a size dimension at MMC may be much easier to inspect and will be illustrated in the next Tip.

Back to Tips Tip added Mar 2013