In our example, the virtual condition boundary would be. This also results in a Virtual Condition Boundary that cannot be violated based the accumulation of size and form variation. The flatness callout then is controlling the overall form of the width (bow, etc). Which means that the only thing the size tolerance is controlling is the “local” size (localized thickness). But, when the flatness at MMC is added to the size tolerance, it removes the Rule #1 requirement. In our example, the size tolerance would require that the entirety of the feature fit within two parallel planes that are. Though often overlooked, a size tolerance also controls form (per Rule #1). Possibly plates that will be stacked or shims which will be flattened when assembled, etc. Simply stated, this callout can be used when the local size (thickness) needs to be controlled more tightly than the overall form. So, if the callout is valid, then we need to address the two questions at the beginning of this post, namely why and how. This same concept can also be applied to a cylindrical feature of size with a straightness callout. The flatness tolerance in this case is controlling the form of the derived median plane. In this case, because the flatness is being applied to a Feature of Size (a width), it is indeed a valid callout based on ASME Y14.5-2009 (para.
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