May Be Applied As A Means Of Preventing An Abrupt Surface Variation Within A Relatively Short Length Of The Feature

-— 1

0.4 diameter tolerance zone

'—016.4 outer boundary

*-2£

— 0.1 diameter tolerance zone in each 25 mm of length

The derived median line of the feature's actual local size must lie within a cylindrical tolerance zone of 0.4 diameter for the total 100 mm of length and within a 0.1 cylindrical tolerance zone for any 25 mm length, regardless of feature size. Each circular element of the surface must be within the specified limits of size.

The derived median line of the feature's actual local size must lie within a cylindrical tolerance zone of 0.4 diameter for the total 100 mm of length and within a 0.1 cylindrical tolerance zone for any 25 mm length, regardless of feature size. Each circular element of the surface must be within the specified limits of size.

FIG. 6-4 SPECIFYING STRAIGHTNESS PER UNIT LENGTH WITH SPECIFIED TOTAL STRAIGHTNESS, BOTH RFS

sis, as in Fig. 6-2, the maximum straightness tolerance is the specified tolerance. When applied on an MMC basis, as in Fig. 6-3. the maximum straightness tolerance is the specified tolerance plus the amount the actual local size of the feature departs from MMC size. The derived median line of an actual feature at MMC must lie within a cylindrical tolerance zone as specified. As each actual local size departs from MMC, an increase in the local diameter of the tolerance zone is allowed that is equal to the amount of such departure. Each circular element of the surface (that is. actual local size) must be within the specified limits of size.

6.4.1.1.3 Application of RFS or MMC to Noncylindrical Features. As an extension of the principles of para. 6.4.1.1.2, straightness may be applied on an RFS or MMC basis to noncylindrical features of size. In this instance, the derived median plane must lie in a tolerance zone between two parallel planes separated by the amount of the tolerance. Feature control frame placement and arrangement as described in para. 6.4.1.1.2 apply, except the diame ter symbol is not used since the tolerance zone is noncylindrical.

6.4.1.1.4 Applied on Unit Basis.

Straightness may be applied on a unit basis as a means of preventing an abrupt surface variation within a relatively short length of the feature. See Fig. 6-4. Caution should be exercised when using unit control without specifying a maximum limit because of the relatively large theoretical variations that may result if left unrestricted. If the unit variation appears as a "bow" in the toleranced feature, and the "bow" is allowed to continue at the same rate for several units, the overall tolerance variation may result in an unsatisfactory part. Figure 6-5 illus-rates the possible condition where straightness per unit length given in Fig. 6-4 is used alone, that is, if straightness for the total length is not specified.

6.4.1.1.5 Straightness of Line Elements.

Figure 6-6 illustrates the use of straightness tolerance on a flat surface. Straightness may be applied to control line elements in a single direction on a flat sur-

Straightness Per Unit Length
FIG. 5-5 POSSIBLE RESULTS OF SPECIFYING STRAIGHTNESS PER UNIT LENGTH RFS, WITH NO SPECIFIED TOTAL

face; it may also be applied in two directions as shown. Where function requires the line elements to be related to a datum feature(s), profile of a line should be specified related to datums. See Fig. 6-18.

6.4.2 Flatness. Flatness is the condition of a surface having all elements in one plane.

6.4.2.1 Flatness Tolerance. A flatness tolerance specifies a tolerance zone defined by two parallel planes within which the surface must lie. When a flatness tolerance is specified, the feature control frame is attached to a leader directed to the surface or to an extension line of the surface, it is placed in a view where the surface elements to be controlled are represented by a line. See Fig. 6-7. Where the considered surface is associated with a size dimension. the flatness tolerance must be less than the size tolerance.

6.4.2.1.1 Applied on Unit Basis. Flatness may be applied on a unit basis as a means of preventing an abrupt surface variation within a relatively small area of the feature. The unit variation is used either in combination with a specified total variation, or alone. Caution should be exercised when using unit control alone for the reasons given in para. 6.4.1.1.4. Since flatness involves surface area, the size of the unit area, for example 25 X 25, is speci-

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  • lorena
    How to use the datum line?
    7 years ago

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