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Some Dimensions Are Implied, but Tolerances Must Be Stated
(In accordance with Y14.5M-1994 standard)

PDF of this Tip

There are many dimensions that you never see on a drawing. That is because these dimensions are implied. If these dimensions were not implied, we would have a lot more notes and dimensions on drawings. Even though some dimensions are implied, this is not true for tolerances. Every feature on a part must be toleranced. The tolerances may be applied directly to dimensions or applied to features using geometric tolerances. The chart shows some of the more common implied dimensions. This drawing shows how many of these implied dimensions are used.

There are many 90° angles. Since geometric tolerancing controls all feature relationships, the implied 90° angles are basic. The two surfaces that make up datum feature B are implied to be in line or coplanar. Datum feature C is made up of the 2 slots. Once again, these features are implied to be in line. In addition, the slots are implied centered on a plane which passes through the datum axis established by datum feature A. Many people are tempted to add a half dimension from the center line to one side of the slots. This would be incorrect. The position tolerance on the slots is controlling the center plane of the slots relative to the datum axis. The goal, or basic relationship, of the center plane of the slots is to pass through the datum axis.

The 6 radial holes are to be equally spaced as is indicated by the EQL SP. There is no need for a 60° angle. Two of the holes are implied to be on the same plane as the slots. It is implied that the axis of the 6 holes should pass through the datum axis. The Ø60 outside diameter is implied to be coaxial with the axis of datum feature A.

All of these are implied dimensions which greatly simplify the drawing. The geometric tolerancing, of course, states how much each of these features may deviate from these goals since production cannot be expected to produce perfect parts.

Back to Tips Tip added Mar 2008