Fig Extreme Variations Of Form Allowed By A Size Tolerance

2.7.1.1 Variations of Size. The actual local size of an individual feature at each cross section shall be within the specified tolerance of size.

2.7.1.2 Variations of Form (Envelope Principle). The form of an individual feature 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 feature 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 feature is produced at its MMC limit of size except as specified in para. 6.4.1.1.2.

(b) Where the actual local size of a feature has departed from MMC toward LMC, a variation in form is allowed equal to the amount of such departure.

(c) There is no requirement for a boundary of perfect form at LMC. Thus, a feature 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.

2.7.1.3 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 in dustry 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.

(b) parts subject to free state variation in the unrestrained condition. See para. 6.8.

2.7.2 Perfect Form at MMC Not Required.

Where it is desired to permit a surface or surfaces of a feature to exceed the boundary of perfect foim at MMC, a note such as PERFECT FORM AT MMC NOT REQD is specified, exempting the pertinent size dimension from the provision of para. 2.7.1.2(a).

2.7.3 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 controlled for location or orientation to avoid incomplete drawing requirements. These controls may be specified by one of the methods given in Sections 5 and 6. If it is necessary to establish a boundary of perfect form at MMC to control the relationship between features, the following methods are used.

(a) Specify a zero tolerance of orientation at MMC, including a datum reference (at MMC if ap plicable), to control angularity, perpendicularity, or parallelism of the feature. See para. 6.6.1.2.

(b) Specify a zero positional tolerance at MMC, including a datum reference (at MMC if applicable) to control coaxial or symmetrical features. See paras. 5.11.1.3 and 5.13.2.

(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.

(d) Relate dimensions to a datum reference frame by a local or general note indicating datum precedence. See para. 4.4. The noted dimensions define only the maximum material condition envelope related to the datum reference frame defined by the datums. For LMC, see para. 2.7.1.2(c).

2.8 APPLICABILITY OF RFS, MMC, AND LMC

Applicability of RFS, MMC, and LMC is limited to features subject to variations in size. They may be datum features or other features whose axes or center planes are controlled by geometric tolerances. In the case of straightness covered in paras. 6.4.1.1.2 and 6.4.1.1.3, it is the derived median line and the derived median plane, rather than the axis and center plane that are controlled. In all cases, the following practices apply for indicating RFS, MMC, and LMC:

(a) All Applicable Geometric Tolerances (Rule #2). RFS applies, with respect to the individual tolerance, datum reference, or both, where no modifying symbol is specified. MMC or LMC must be specified on the drawing where it is required.

NOTE: Circular runout, total runout, concentricity, and symmetry arc applicable only on an RFS basis and cannot be modified to MMC or LMC.

(b) Alternative Practice. For a tolerance of position (Rule #2a), RFS may be specified on the drawing with respect to the individual tolerance, datum reference, or both, as applicable. See Appendix D (Fig. D-l).

2.8.1 Effect of RFS. Where a geometric tolerance is applied on an RFS basis, the specified tolerance is independent of the actual size of the considered feature. The tolerance is limited to the specified value regardless of the actual size of the feature. Likewise, referencing a datum feature on an RFS basis means that a centering about its axis or center plane is necessary, regardless of the actual size of the feature.

2.8.2 Effect of MMC. Where a geometric toler ance is applied on an MMC basis, the allowed tolerance is dependent on the actual mating size of the considered feature. The tolerance is limited to the specified value if the feature is produced at its MMC limit of size. Where the actual mating size of the feature has departed from MMC, an increase in the tolerance is allowed equal to the amount of such departure. The total permissible variation in the specific geometric characteristic is maximum when the feature is at LMC. Likewise, referencing a datum feature on an MMC basis means the datum is the axis or center plane of the feature at the MMC limit. Where the actual mating size of the datum feature has departed from MMC, a deviation is allowed between its axis or center plane and the axis or center plane of the datum.

2.8.3 Effect of Zero Tolerance at MMC. Where a tolerance of position or orientation is applied on a zero tolerance at MMC basis, the tolerance is totally dependent on the actual mating size of the considered feature. No tolerance of position or orientation is allowed if the feature is produced at its MMC limit of size; and in this case, it must be located at true position or be perfect in orientation, as applicable. Where the actual mating size of the considered feature has departed from MMC, a tolerance is allowed equal to the amount of such departure. The total permissible variation in position or orientation is maximum when the feature is at LMC, unless a maximum is specified. See Figs. 6-41 and 6-42.

2.8.4 Effect of LMC. Where a positional tolerance is applied on an LMC basis, the allowed tolerance is dependent on the actual mating size of the considered feature. The tolerance is limited to the specified value if the feature is produced at its LMC limit of size. Where the actual mating size of the feature has departed from LMC, an increase in the tolerance is allowed equal to the amount of such departure. The total permissible variation in position is maximum when the feature is at MMC. Likewise, referencing a datum feature on an LMC basis means the datum is the axis or center plane of the feature at the LMC limit. Where the actual mating size of the datum feature has departed from LMC, a deviation is allowed between its axis or center plane and the axis or center plane of the datum.

2.8.5 Effect of Zero Tolerance at LMC. Where a tolerance of position or orientation is applied on a zero tolerance at LMC basis, the tolerance is totally dependent on the size of the considered feature. No tolerance of position or orientation is allowed if the feature is produced at its LMC limit of size; and in this case, it must be located at true position or be perfect in orientation, as applicable. Where the actual mating size of the considered feature has departed from LMC, a tolerance is allowed equal to the amount of such departure. The total permissible variation in position or orientation is maximum when the feature is at MMC unless a maximum is specified. See Figs. 5-13, 5-14, and 6-42.

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