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Do Not Mix Basic and Toleranced Dimensions
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

This is a problem that I encounter way too often. Basic dimensions and directly toleranced dimensions are like oil and water - they just don’t mix. There are two examples of where you can combine the two when dealing with a conical feature. There used to be a couple other examples but they have been deleted from the Standard because they could not be clearly defined. The main reason is that directly toleranced dimensions do not relate to datums.

This is a simplified redraw of a part I was asked about. The profile tolerance references datum features A, primary and B, secondary. That means that the origin of measurement is where the axis established by datum feature B intersects the datum plane established by datum feature A. The original drawing had the 36.11 basic dimension coming from a tangent to datum feature B. The basic dimensions must get you back to the origin of measurement. The figure to the right of the drawing shows how the basic dimension should have been placed.

Back to Tips Tip added Aug 2013