The band, and which end is which
The size tolerance is the distance between the two limits, always positive. For a hole the smaller limit is the maximum material condition; for a shaft it is the larger. That single decision governs every material condition calculation on the drawing.
The size tolerance is also the ceiling on any bonus a feature can earn, so reading it off the drawing before starting is a useful habit. Departure from MMC and departure from LMC always sum to it.
What the envelope requirement adds
Rule #1 says that a regular feature of size must not violate a boundary of perfect form at its maximum material condition. A pin at MMC must be perfectly straight and round; a pin that has given back some material may use that departure as form error, provided it still fits inside the MMC envelope.
The practical effect is that form is controlled by the size tolerance for free, and an explicit form callout is only worth adding when it needs to be tighter than that.
Two point size is not the whole story
A micrometer measures local two point sizes and is blind to bow. That is exactly the loophole the envelope requirement closes, and a ring gauge at MMC is the physical realisation of the closure.
The related definition is the actual mating size: the largest perfect pin that enters a hole, or the smallest perfect ring that receives a shaft. It is the size that governs assembly, and it is what material condition calculations use.
When the envelope does not apply
Commercial stock such as bar and sheet is supplied to its own standard and is not held to a perfect form envelope by the part drawing. If a drawing needs stock straightened, it has to say so.
The independency symbol removes the envelope requirement from a specific feature, so size and form are then checked independently. That is useful on flexible or non rigid parts, and it comes with an obligation to state the form control explicitly rather than inherit it.
Form allowance from the size tolerance alone
A pin is toleranced 10.00 mm at MMC down to 9.94 mm at LMC, with no form callout, and is produced at 9.96 mm.
- Size tolerance
- 0.060 mm
- Departure from MMC
- 0.040 mm
- Straightness available under the envelope requirement
- 0.040 mm
- Form permitted at MMC exactly
- 0.000 mm
The permitted bow is the departure from MMC and nothing more, which is why a pin made at its largest limit has to be perfect and a pin made small has room to be slightly bent.