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Positional tolerances
Positional tolerances are used to control the orientation and location of features. Positional tolerances control feature-to-feature relationships and the relationship between features and a datum reference frame (DRF). If a positional tolerance is applied to multiple features (e.g. with 2X, 4X), the tolerance zones are constrained (locked) together. In Example 1.1 the positional tolerance applies to four holes and is not related to a DRF. The tolerance creates a group of four tolerance zones that are locked together, but the tolerance is not related to a DRF, so the group of zones can move relative to the part. Thus, the positional tolerance controls the location of the holes to one another, but not relative to the part, as the tolerance is not related to a DRF.
There are three levels of constraint in positional tolerancing:
First, the positional tolerance zones are constrained together.
Second, if the tolerance is related to a DRF, the group of tolerance zones is constrained to a DRF.
Third, the datum reference frame with its tolerance zones is constrained to the part.
In example 1.2, the four tolerance zones are locked together, the pattern of zones is constrained to DRF A, and DRF A is constrained to the part. In example 1.3, the four tolerance zones are locked together, the pattern of zones is constrained to DRF A|B, and DRF A|B is constrained to the part. In example 1.4, the four tolerance zones are locked together, the pattern of zones is constrained to DRF A|B|C, and DRF A|B|C is constrained to the part. Table 1 below explains the constraint in these examples.
Constraint | Example 1.1 | Example 1.2 | Example 1.3 | Example 1.4 |
|---|---|---|---|---|
Constraint Between Tolerance Zones | Full constraint | Full constraint | Full constraint | Full constraint |
Degrees of Freedom Constrained Between Tolerance Zones | 6 | 6 | 6 | 6 |
Degrees of Freedom Constrained Between Pattern of Tolerance Zones and DRF | N/A - no DRF | 6 | 6 | 6 |
Constraint Between DRF | N/A - no DRF | Constrained to Datum Feature A | Constrained to Datum Features A & B | Constrained to Datum Features A, B & |
Degrees of Freedom Constrained Between DRF and Part | N/A - no DRF | 3 | 5 | 6 |
Positional Tolerance Constraint Defaults
If a positional tolerance applies to more than one feature, such as by using “2X”, "3X", “4 HOLES”, “6 PLACES”, etc., all six degrees of freedom are constrained between the positional tolerance zones.
If a single-segment positional tolerance is related to a DRF, all six degrees of freedom are constrained between the positional tolerance zones and the DRF. Different rules apply for composite position.
A DRF is constrained to a part as dictated by the datum features referenced in a feature control frame. For example, if only a planar primary datum feature is referenced in a positional tolerance feature control frame, 3 degrees of freedom are constrained between the datum feature and its true geometric counterpart. Thus, only those three degrees of freedom are constrained between the DRF and the part. Positional tolerance zones are fully constrained to the applicable DRF, but depending on the specification, the DRF may or may not be completely constrained to the part.
True position
Many people call a positional tolerance true position. This is a mistake – it’s not a big deal, but true position means something else. In GD&T, the word “true” means perfect. True position is the theoretically exact (perfect) position of features controlled by a positional tolerance. For a pattern of holes, their true position is defined by their perfect orientations and locations, which are usually defined by basic dimensions. Sometimes we define the true position using CAD model geometry instead of explicitly showing basic dimensions. In those cases, we must indicate that the applicable CAD model geometry is equivalent to basic dimensions.
Positional Tolerance and True Position
Positional Tolerance Zone Shape
If a diameter symbol (Ø) precedes a positional tolerance value, the tolerance zone is cylindrical.
If a spherical diameter symbol (SØ) precedes a positional tolerance value, the tolerance zone is spherical.
If no symbol precedes a positional tolerance value, the tolerance zone shape is two parallel planes.
Axis vs. Surface
Axis (RFS – Regardless of Feature Size)
RFS applies if a positional tolerance value is not followed by an MMC or LMC modifier.
A positional tolerance applied RFS to a cylindrical feature of size controls the axis of the feature.
A positional tolerance applied RFS to a width feature of size controls the center plane of the feature.
Surface (MMC and LMC)
A positional tolerance value followed by an MMC or LMC modifier controls the surface of the feature.
Figure 1 - Positional Tolerance Define Different Shape Tolerance Zones
Figure 2 - Positional Tolerances Specified RFS and at MMC
Figure 3 - Part with Positional Tolerances