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ASME Y14.5 -2018

5.8.1 Rule #1: The Envelope Principle (Variations of Form)

Perfect form at MMC

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The form of an individual regular feature of size is controlled by its limits of size to the extent prescribed

Case-1

Case-2

Case-3

Case-4

Case-5

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  1. The actual machined size should not be more than Max. Limit & should not be less than Min. Limit.
  2. This part has to enter into an envelope of maximum material condition.

Case-1

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  1. The actual machined size should not be more than Max. Limit & should not be less than Min. Limit.
  2. This part has to enter into an envelope of maximum material condition.

Case-2

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  1. The actual machined size should not be more than Max. Limit & should not be less than Min. Limit.
  2. This part has to enter into an envelope of maximum material condition.

Case-3

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  1. The actual machined size should not be more than Max. Limit & should not be less than Min. Limit.
  2. This part has to enter into an envelope of maximum material condition.

Case-4

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  1. The actual machined size should not be more than Max. Limit & should not be less than Min. Limit.
  2. This part has to enter into an envelope of maximum material condition.

Case-5

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If it is not perfect form at MMC not meet this rule.

Perfect form at MMC

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  1. The surface or surfaces of a regular feature of size shall not extend beyond an envelope that is a boundary of perfect form at MMC.( Refer Case-1)
  2. Where the actual local size of a regular feature of size has departed from MMC toward LMC, a local variation in form is allowed equal to the amount of such departure. ( Refer Case-2)
  3. There is no default requirement for a boundary of perfect form at LMC, but a requirement may be invoked as explained in (d). Thus, a regular feature of size produced at its LMC limit of size is permitted to vary from true form to the maximum variation allowed by the boundary of perfect form at MMC. ( Refer Case-2)
  4. When a geometric tolerance is specified to apply at LMC, perfect form at LMC is required and there is no requirement for perfect form at MMC.
  5. Where a portion of a regular feature of size has a localized area that do not contain opposed points, the actual value of an individual distance at any cross section between the unrelated AME to a point on the surface may not violate the LMC limit.

5.8.1 Rule #1: The Envelope Principle Requirements

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The control of geometric form prescribed by limits of size does not apply to the following:

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  1. Items identified as stock, such as bars, sheets, tubing, structural shapes, and other items produced to established industry or government standards that prescribe limits for straightness, flatness, and other geometric characteristics. Unless geometric tolerances are specified on the drawing of a part made from these items, standards for these items govern the surfaces that remain in the as-furnished condition on the finished part.
  2. tolerances applied with the free state modifier.
  3. when form tolerances of straightness or flatness are applied to a feature of size.
  4. when the “independency” symbol is used.
  5. when average diameter is specified.

5.8.2 Form Control Does Not Apply (Exceptions to Rule #1)