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Break Rule #1 with Independency
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

Rule #1 requires that a feature not violate an envelope of perfect form at MMC. On this part, MMC of the hole is Ø5.95. If an actual hole measures Ø5.95 at every cross section, the only way it could meet this requirement is if it is perfectly straight, circular and cylindrical. Of course, this "perfect" hole cannot exist. But, if a hole is made close to Ø5.95 and had very good form, it would be acceptable. If two-point measurements are close to 6.05, the form error could be nearly 0.1 - the total size tolerance. This default rule helps assure that parts will assemble.

However, this rule often becomes unreasonable as the length to diameter or width becomes large.

On this shaft, the Independency symbol has been applied to the Ø10±0.1. This removes the requirement for the feature to fit an envelope at 10.1. The size of this feature may be verified using micrometers. However, this overrides Rule #1. So, the form of the feature is not controlled by the limits of size. The straightness and circularity of the feature are controlled by the circular runout tolerance. Care must be taken whenever the Independency symbol is applied to assure that the form of the feature is controlled by something other than the limits of size.

Rule of Thumb: Where the length to diameter/width ratio is greater than 5:1 consider applying the Independency symbol to the size dimension and controlling the form of the feature with a geometric tolerance.

Back to Tips Tip added Jun 2009