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Don’t Rely on Basic Dimension Placement!
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

When locating dimensions were toleranced the placement of the dimensions affected the meaning of the drawing.  This is not true with basic dimensions.  You have to read the feature control frames to determine origins of measurement and relationships of part features.

On this drawing, the dimensions are coming from the edges of the part but the origin of measurement is where the 3 datum planes, established by datum features A, B and C intersect.  The two M10 tapped holes are positioned within 0.1 mm of each other because of the composite position tolerance even though the locating dimensions are originating again from the edges of the part.

The second illustration is of a reduced dimension drawing. When suppliers use CAM packages and inspection equipment that works from the CAD model there might not be any basic dimensions shown. The supplier may make queries from the CAD model. The resulting dimensions are, by default, basic if the ASME Y14.41 standard has been referenced. Since any distance may be queried the dimensions cannot be used to determine feature relationships.

Back to Tips Tip added Dec 2012