For example, the form of thru holes is typically crucial in order to accept the corresponding hardware during assembly. Rule #1 helps fully define features of a part and ensure the form of them is being controlled. Since Rule #1 is the default condition, it is used always and ignored only when specified. Applying Flatness or Straightness to a Feature of Size.When the “ independency” symbol is applied to the appropriate dimension or notation.Tolerances applied with free state modifier.However certain exceptions will overwrite this rule. The ASME Y-14.5 applies Rule #1 by default to all features of size. At LMC the form can vary by the maximum amount.įigure 3: Form can vary by the maximum amount at LMC The combination of Size and Form cannot exceed the Envelope. However, as the local size departs from MMC, the form can vary up to the envelope. This means that if the feature measures at MMC, the form of the feature must be perfect, which in the real world is impossible to achieve.įigure 2: Envelope = the boundary prescribed by the feature of perfect form at MMC The actual surface of a regular feature cannot extend beyond the envelope prescribed by the feature in perfect form at MMC. It is sometimes also known as the “Taylor Principle.”įigure 1: Symmetric, Unilateral, and Bilateral Size Tolerances See Figure 1 for examples of these size tolerances. A few of them are symmetric, unilateral, and bilateral. Rule #1 of Geometric Dimensioning and Tolerancing states that the form of a regular feature of size is controlled by its “limits of size.” Limits of size, or otherwise known as size tolerances, can be seen in many forms. Symbol: None – Always Implied default condition Definition:
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