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Datum References in the Second Segment Control Orientation
(In accordance with Y14.5M-1994 standard)

When using composite tolerancing, the second segment 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 0.3 mm, the parts may still move 0.8 mm relative to each other. Next months Tip will illustrate how to restrict movement in one direction only.

Back to Tips Tip added Jul 2000