| I often get the
question, "How can a feature reference itself in a feature
control frame?" The answer is, "It can't." Datum features
referenced in a feature control frame are establishing a datum
reference framework which serves as the origin of measurement.
The shaft shown has two total runout controls that appear to
be referencing themselves. The total runout tolerance applies
to the surface of the features. The datum is an axis which is
established using the two datum features A and B.
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