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It isn’t always possible or practical to select datum features that are mutually perpendicular to one another when establishing a datum reference frame. Notice that datum feature C is not nominally perpendicular to datum feature B. The datum feature simulator for C would be made at 35° to the datum feature simulator for B (shown here in red). The actual datum planes (shown in blue), which comprise the datum reference framework, would however be mutually perpendicular to one another as is illustrated in the last figure. The deviation of the hole from the 55mm BASIC location would be measured from the third datum plane—not from the sharp point on the actual part.
Note: the drawing below is incomplete by intent.
This tip is in accordance with ASME Y14.5M-1994, Y14.5-2009 and Y14.5-2018.
This tip was originally released in January 2001.