Datum Feature of Size Representation
James D. Meadows · 2017
When datum reference frames used to locate features on a part are changed from feature-to-feature or pattern-to-pattern, an accumulation of tolerance is experienced. Each feature is indirectly related back to the first origin of measurement and orientation through a series of other features—each of which has been allowed its own error of location and/or orientation. Unless controlled with straightness of a derived median line or plane, if a datum feature of size is referenced as primary and modified at maximum material condition (MMC), it will be represented at its MMC size. Any departure from that virtual condition boundary allows a shift of any pattern of features referenced to it a total amount equal to that departure. The size at which these datum features are represented in the statements are the sizes functional gages are dimensioned at to represent these datum features when inspecting the patterns of features controlled to them.