The drawing below solves both problems. Datums B and C are now center planes. The dimensioning is implied to be symmetric. The dimensions needed to locate the position tolerances are basic.
Stop Mixing Dimensions (#54)
(In accordance with the ASME Y14.5-2018 standard)
(In accordance with the ASME Y14.5-2009 standard)
(In accordance with Y14.5M-1994 standard)
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A recurring problem occurs when position tolerancing is used but the dimensions locating the features are not all basic. Below is a redraw of a print I recently received. The 20 and 80mm dimensions are not basic. In addition, this appears to be a symmetric part. The part could be set up two different ways (if datum feature A is not identified in some way, there are 4 possible ways to align the part). All dimensions locating position tolerance zones must be basic.
This tip is in accordance with ASME Y14.5M-1994, Y14.5-2009 and Y14.5-2018
This tip was originally released in May 2002.
