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In a previous article, we saw that the orientation and location of a tolerance zone for a single feature can be constrained to datums. Now we will see that the orientation and location of tolerance zones for groups of features can be constrained to each other, depending on the geometric characteristic.

 

Grouping Methods

The geometric tolerance from a feature control frame can be applied to a group of several features at once, using one of the following methods:


• nX multiplier for number of places (e.g. 4X for a hole pattern)
• n SURFACES (e.g. 3 SURFACES)
• n COAXIAL HOLES (e.g. 2 COAXIAL HOLES)
• All Around Symbol (usually groups multiple surfaces)
• Between Symbol (usually groups multiple surfaces)
• Multiple Leader Lines
• General Note (e.g. a general Profile of a Surface tolerance)

When one of these grouping methods is used, the geometric tolerance applies to all of the features in the group.


Size Tolerances:

When applied to a group of features, a Size tolerance applies independently to each feature in the group. There is no mutual constraint between the features. If the nX multiplier is applied to the diameter tolerance for a hole pattern, for example, the size is evaluated individually for each feature.

Form Tolerances (Flatness, Straightness, Circularity, Cylindricity)

Form tolerances also apply independently to each feature in the group.

Orientation Tolerances – Perpendicularity, Parallelism, Angularity

Orientation tolerances also apply independently to each feature in the group. The tolerance zones will all be oriented to one or more datums. It is possible to envision mutual orientation constraint between grouped features, but the Y14.5 standard has not explored this possibility.

Circular Runout and Total Runout Tolerances

Runout tolerances also apply independently to each feature in the group. The tolerance zones will all be located to the datum axis. It is possible to envision mutual constraint between the grouped features in the axial direction, but this possibility is not explored in the Y14.5 standard.


Position Tolerances

When applied to a group of features, a Position tolerance imposes orientation and location constraints to the tolerance zones for all of the features in the group. If the nX multiplier is applied to the Position tolerance for a hole pattern, for example, the Position tolerance controls the relative orientation and location of the holes. This control is independent of any constraint to datums. So a Position tolerance can be used to control the relative location of a group of features, with or without the use of datum features.

Profile of a Surface and Profile of a Line Tolerances

Profile tolerances also impose mutual orientation and location constraints to the tolerance zones for all of the features in the group. If the all-around symbol is applied to a Profile of a Surface tolerance, for example, the tolerance controls the relative orientation and location of all of the surfaces that are included. This is independent of any constraint to datums.

Summary

Most of the geometric tolerances defined in the Y14.5 standard are intended for application to individual features. When applied to a group of features, these tolerances apply to each feature independently and to not impose any mutual constraints. Position and Profile are the two geometric tolerance types that impose mutual constraints on the features within the group. This allows these tolerances to control the relative orientation and location of the features in the group, with or without the use of datums.

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