Here is a Tip inspired by one of our regular readers of the Tips. He pointed out that in the examples of composite tolerancing in the Y14.5 standard that explain the meaning of a datum reference in the second segment, the pattern of features seems to always be conveniently oriented squarely to the datum reference frame. His question was, "What if the pattern is at a compound angle to the datum reference framework?" Well, it doesn’t matter what angle the pattern is from the datum reference frame. Any datum reference in the second segment of a composite feature control frame tightens up the orientation of the pattern. Of course, the other purpose of the tolerance in the lower segment is to tighten the location of the features in the pattern to one another. In this example, the tolerance zones of the composite callout are shown in blue and yellow in the isometric views.t, they refine the orientation (perpendicularity, parallelism or angularity) of the pattern relative to the referenced datums.
Datum references in the second segment of a composite feature control frame tighten the orientation of the pattern-no matter what the angle (#186)
(In accordance with Y14.5M-1994 standard)
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The blue zone represents the Ø0.3mm tolerance which would be centered on the basic dimensions. The yellow cylinders represent the Ø0.1mm tolerance. The basic distance between these tolerance zones must be 32mm, and the cylinders are oriented relative to the first datum plane established by datum feature A. Other than that, they can float around; but the axes of the holes must be contained within both tolerance zones.
The figures below show a couple of possible results of the composite tolerance zone callout.
