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When using composite tolerancing, the second segment (feature relating tolerance zone framework or FRTZF) tightens the location between features in the pattern. If datum references are repeated in the second (lower) segment, the orientation of the pattern is tightened to the value specified in that segment. In the assembly shown below, if you want the mounting holes to line up but are not concerned about the rotation of the parts relative to one another (as illustrated by the arrow), the datum references would not be repeated in the lower segment. This is illustrated on the drawing shown.
To limit this rotation, the datum references should be repeated in the lower segment as illustrated on the drawing below. It is important to note that even though rotation between the parts has been limited to 0.3mm, the parts may still move 0.8mm relative to each other. (These two holes meet both requirements. Tolerance zones are enlarged to show the concept.)
These two holes meet both requirements. Tolerance zones are enlarged to show the concept.
This tip is in accordance with ASME Y14.5M-1994, Y14.5-2009 and Y14.5-2018.
This tip was originally released in July 2000.