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Composite Tolerancing
Another Composite Tolerancing Tip  

When datum references are repeated in the second segment of a composite or single segment control, the meaning is different.

The tolerance in the upper segment of a composite tolerance is located by all applicable basic dimensions. On the drawing above, the red tolerance is located by the red dimensions.

The lower segment of a composite tolerance does not use the basic dimensions which originate at the datums. Only the basic dimensions within the pattern are applicable. If a datum is repeated, it indicates that the orientation of the pattern must be held to the tighter tolerance. In this case, the perpendicularity to datum A must be within 0.2 and the pattern of two holes may not tilt more than 0.2 relative to datum B.

You will now notice that there are two position symbols being used. This callout is not composite. It is called two single segments. The upper segment has the same meaning as the upper segment of the composite callout shown earlier. The lower segment, however, improves the location as well as the orientationof the feature(s) relative to the datums referenced in the second sement. Notice that the 19mm dimension from datum B is shown in red. The pattern must be positioned at the 19mm dimension from B within 0.2 total even though the pattern may be out of position as much as 0.6 total relative to datum C.

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