GlossaryKind of term: Standard and conventionEnvelope requirement
What is the envelope requirement?
The envelope requirement, Ⓔ after a size, requires the feature to stay within perfect form at maximum material size, so size and form are limited together.
The envelope requirement appears as Ⓔ after a size, such as ⌀20 h7 Ⓔ. The feature may not cross the perfect form at maximum material size: a shaft has to fit inside a perfect cylinder of its largest size, a hole has to accept a perfect cylinder of its smallest size. At the same time every local size has to stay within the other limit. Deviations in roundness, straightness or parallelism of opposite faces therefore fall within the size tolerance.
What does the standard say about the envelope requirement?
The indication is defined in ISO 14405-1. Under the ISO 8015 independency principle, size and form apply separately; the envelope then applies only where Ⓔ is shown. Older drawings sometimes carry ISO 2768-mK-E in the title block, which applies the envelope to every single feature of size. In ASME Y14.5 the envelope is the default for features of size, known as Rule #1, with exceptions such as stock material and features marked as independent. Older German drawings without a reference to ISO 8015 often followed DIN 7167, under which the envelope also applied by default.
How does the envelope requirement work in practice?
The envelope mainly changes inspection. A micrometer touches two points and so misses a bent shaft or an odd-lobed section of constant width; ovality shows only if you measure at several orientations. With Ⓔ, form has to be checked at the maximum material side as well. In the shop that is classically done with a limit gauge following Taylor's principle: the go side with full form at maximum material size, the no-go side as a two-point check at the other limit. A slender shaft that bows can fail while every two-point size reads good. That can call for steadier workholding, a steady rest or a finishing operation; new gauges have to be made or bought, and periodically calibrated.
What is the envelope requirement often confused with?
The envelope requirement is not maximum material condition: Ⓔ follows a size and limits form by that size, Ⓜ sits in a feature control frame and lets a geometric tolerance grow with size. Nor is it a circularity requirement: circularity has its own value, the envelope uses the size tolerance as its limit.
Frequently asked questions
What does the E after a size mean?
That the feature has to stay within perfect form at its maximum material size. A shaft has to pass through a perfect ring at its largest size, and no local size may drop below the smallest.
Does the envelope apply without an E?
Under ISO 8015, no: size and form apply separately unless Ⓔ or a general indication such as ISO 2768-mK-E imposes the envelope. Under ASME Y14.5, yes, as the default rule for features of size, with exceptions. The standard the drawing invokes decides.
How do you inspect a size with the envelope?
With a limit gauge: the go side has the full form at maximum material size, the no-go side checks two points at the other limit. A coordinate measuring machine works too, with enough points along the whole length and the right evaluation.