Make Certain You Have Controlled Your Datum
Features!
In most cases, datums are
established from the extremities or high points of datum
features. Measurements are then made from the true geometric
counterpart of the datum feature. Consequently, datum features
often do not get checked unless there is a tolerance that
affects the datum feature. On the first drawing, the form
(flatness) of datum feature A is controlled by the size
toleranc of ±0.1mm. It would not be necessary to add a
flatness control unless the design requirement for flatness
was tighter than 0.2mm.
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