Unless otherwise specified, the limits of size of a feature prescribe the extent within which variations of geometric form, as well as size, are allowed. This control applies solely to individual regular features of size as defined in para. 18.104.22.168. The actual local size of an individual feature at each cross section shall be within the specified tolerance of size.
- Variations of form (Rule #1: Envelope principle)
- Form control does not apply (exceptions to Rule #1)
- Perfect form at MMC not required
- Relationship Between Individual Features
- Limits of Size and Continuous Features of Size
Variations of form (Rule #1: Envelope principle)
The form of an individual regular feature of size is controlled by its limits of size to the extent prescribed in the following paragraphs and illustrated in Fig. 2-6.
(a) The surface or surfaces of a regular feature of size shall not extend beyond a boundary (envelope) of perfect form at MMC. This boundary is the true geometric form represented by the drawing. No variation in form is permitted if the regular feature of size is produced at its MMC limit of size unless a straightness or flatness tolerance is associated with the size dimension or the Independency symbol is applied per para. 2.7.3. See Fig. 2-7.
(b) 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.
(c) Where is no default requirement for a boundary of perfect form at LMC. 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.
(d) In cases where a geometric tolerance is specified to apply at LMC, perfect form at LMC is required. See para. 7.3.5.
Form control does not apply (exceptions to Rule #1)
The control of geometric form prescribed by limits of size does not apply to the following:
(a) 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 asfurnished condition on the finished part.
(b) parts subject to free-state variation in the unrestrained condition. See para. 5.5.
Perfect form at MMC not required
Where perfect form at MMC is not required, the Independency symbol may be placed next to the appropriate dimension or notation. See Fig. 3-11 and para. 3.3.24.
form is entirely uncontrolled. See Fig. 2-7.
CAUTION: Without a supplementary form control, the feature form is entirely uncontrolled. See Fig. 2-7.;
Relationship Between Individual Features
The limits of size do not control the orientation or location relationship between individual features. Features shown perpendicular, coaxial, or symmetrical to each other must be toleranced for location or orientation to avoid incomplete drawing requirements. These tolerances may be specified by one of the methods given in Sections 6 through 9. If it is necessary to establish a boundary of perfect form at MMC to control the relationship between features, one of the following methods may be used:
(a) Specify a zero tolerance of orientation at MMC, including a datum reference (at MMB if applicable), to control angularity, perpendicularity, or parallelism of the feature. See para. 6.4.4.
(b) Specify a zero positional tolerance at MMC, including any specified datum reference (at MMB if applicable) to control coaxial or symmetrical features. See paras. 22.214.171.124 and 126.96.36.199.
(c) Indicate this control for the features involved by a note such as “PERFECT ORIENTATION (or COAXIALITY or LOCATION OF SYMMETRICAL FEATURES) AT MMC REQUIRED FOR RELATED FEATURES.”;
Limits of Size and Continuous Features of Size
The note “CONTINUOUS FEATURE” or continuous feature symbol is used to identify a group of two or more features of size where there is a requirement that they be treated geometrically as a single feature of size. When using the continuous feature symbol, extension lines between the features may be shown or omitted; however, extension lines by themselves do not indicate a continuous feature. See Figs. 2-8 through 2-10.