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Basic Dimensions
(In accordance with Y14.5M-1994 standard)

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The Y14.5 standard tells us that the tolerance on basic dimensions is indirect, Section 1.4(a). The tolerance is not on the basic dimension. The tolerance is on the feature whose ideal size, form, orientation and or location is defined with the basic dimensions. That’s why we call the frame containing the geometric tolerance the "feature control frame". Because of this, trailing zeros have no significance, section 2.4. Traditionally, we use trailing zeros for inch dimensions to determine what tolerance in the title block should apply. With basic dimensions that doesn’t work. Section 2.3.2(d) is where there is a problem in the Standard. You might have one-place, two-place, three-place….. tolerances related to the same basic dimension. Metric dimensions and tolerances are not to have trailing zeros (except in non-symmetric bilateral tolerance) for uniformity, Section 2.3.1. What should you do with inch dimensions? It doesn’t really matter from an interpretation or meaning standpoint. The tolerance is the geometric tolerance no matter how many zeros follow the basic dimension. Here is a drawing with three features of size, all located by the .75 dimension. The associated geometric tolerances are one-, two- and three-place tolerances. The drawing meaning will not change if a zero is added to the dimension. Until the Standard is fixed, let’s not worry about trailing zeros on basic dimensions.

Back to Tips Tip added Mar 2005