In previous articles, we saw that all geometric tolerance specifications control a particular component of the considered feature and define a tolerance zone.
Now we will see that a geometric tolerance specification also applies certain constraints to the tolerance zone. These constraints affect the zone’s magnitude, orientation, and location and thus affect the types of variation that the geometric tolerance permits.
Zone Magnitude Constraint
Certain “hollow” tolerance zone shapes (annular, tubular) have a controlled wall thickness and also have a size or “magnitude” aspect:
• For some characteristics (e.g. Circularity, Cylindricity, Circular Runout, Total Runout), the magnitude of the zone is unconstrained
• This means that the tolerance zone can adjust to the actual size of the feature as long as the separation between the boundaries remains constant
• These characteristics do not control the size of the feature
• For other characteristics (e.g. Line Profile, Surface Profile), the magnitude of the zone is fixed
Most geometric tolerance specifications reference datum features, which in turn define datums. The datums are used to apply “external” constraints to the tolerance zone (orientation and location). Different geometric characteristics constrain the tolerance
zones to the datums in different ways:
• For Position and Profile characteristics, the orientation and location of the tolerance zones are constrained to the datums. This means that the tolerance zones are fixed at their basic orientation and location relative to any datums that are referenced.
• For orientation characteristics, the tolerance zone’s orientation is constrained to the datums but its location is unconstrained. This means that the zone can freely translate away from its basic location to adjust to the actual location of the feature.
• The tolerance zones for form characteristics are never constrained to datums. The zones can freely rotate and translate to adjust to the actual orientation and location of the feature.
• Circular Runout and Total Runout tolerance zones are constrained only to the datum axis. This special constraint condition is not relevant in most runout applications, and only comes into play when a planar datum feature is referenced as primary.
The following table provides examples of the constraints that are placed on the tolerance zone, for several different geometric characteristics that are applied to a cylindrical considered feature:
As can be seen in the above table, different geometric characteristics can define the same tolerance zone shape but apply different constraints to the zone. For example, Cylindricity, Total Runout, and Surface Profile all result in the same tubular tolerance zone when applied to a cylindrical surface. The distinction between each of these geometric controls lies in how the zone’s magnitude and relationship to datums are constrained.
In inspection, the constraints on the tolerance zone govern exactly how the relationship between the controlled component (i.e. the workpiece) and the tolerance zone (i.e. the inspection equipment) may be optimized to achieve the best result. Other types of zone constraints can arise with geometric tolerances that involve sets of controlled components and sets of tolerance zones. Examples include form tolerances that control sets of line elements (Circularity, Straightness), and tolerances applied to sets of features such as hole patterns. These constraints between zones will be discussed in future articles