(In accordance with Y14.5M-1994 standard)
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There are two kinds of straightness tolerance - (1) Straightness of an axis or center plane and (2) surface straightness. The type of straightness is determined by the placement of the feature control frame.
When the feature control frame is next to the size dimension, it is controlling the axis or center plane. An inspector must derive the median plane or median axis. This can be extremely time consuming and expensive to measure.
When the feature control frame is on a leader line pointing to a surface, the straightness is applied to line elements in the surface.
