Ⓔ Envelope requirement
Envelope requirement · Anotaciones de plano · ISO 1101
Example in plan
How to interpret it
The boxed E (envelope requirement) demands that the real form of a shaft does not exceed the perfect cylinder at maximum size, nor a hole fall inside the perfect cylinder at minimum size.
What controls
A modifier placed next to the size dimension of a feature that couples size and form: the actual surface may not violate the envelope of perfect form at maximum material condition. On a shaft it means the shaft must fit entirely inside a perfect cylinder of its maximum diameter, so if the shaft is at its maximum diameter it has no allowance left to be bent or out of round; on a hole it means the hole must accept a perfect cylinder of its minimum diameter. In ISO it must be specified explicitly, because by default size and form are independent; in ASME it is the default rule (Rule #1) and does not need to be stated.
How to verify
The natural method is an envelope gauge: a plain ring GO gauge at the maximum diameter for a shaft, or a plain plug GO gauge at the minimum diameter for a hole; if it goes, the envelope is met. It cannot be verified with two-point instruments (micrometer, caliper): they measure local size and do not detect bending or multi-lobed form. On a CMM it is evaluated by fitting the minimum circumscribed cylinder for a shaft, or the maximum inscribed cylinder for a hole, and comparing it with the maximum material limit.
Common mistakes when interpreting it
- Assuming the envelope requirement is the default behavior on an ISO drawing: there it has to be specified, and without it size and form are independent.
- Trying to verify it with a micrometer: a two-point measurement does not detect that the part is bent.
- Confusing it with an added form tolerance: it does not add tolerance, it takes it away as the feature approaches its maximum material condition.
How to get it in the workshop / if it comes out of tolerance
It is the assembly guarantee for a fit: it says the part will go into its mating bore without the need to add a separate form tolerance. In practice it shares out the allowance: the closer the feature is to its maximum material condition, the less form error is permitted, and the further it departs from it, the more. That is why a part that meets diameter at every cross-section can still be rejectable if it is bent.
A part that meets every point measurement and still will not go into its mating bore: this is almost always an envelope problem, not a size problem; check straightness and circularity. The practical way out is to make the feature somewhat away from maximum material (the shaft slightly smaller, the hole slightly larger) to gain form allowance. If the process cannot deliver that form, it has to be corrected: a steady rest on long shafts, less aggressive clamping, or a light finishing pass.
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