In a previous article, we saw that the orientation and location of tolerance zones for groups of features can be constrained to each other using various grouping mechanisms.
Now we will see that the tolerance zones for features and groups can be constrained to each other using the concept of Simultaneous Requirements.
Y14.5-2009 defines Simultaneous Requirements in the following way:
• A simultaneous requirement is where two or more geometric tolerances apply as a single pattern or part requirement.
• A simultaneous requirement applies to position and profile tolerances that are located by basic dimensions, related to common datum features referenced in the same order of precedence at the same boundary conditions.
• In a simultaneous requirement there is no translation or rotation between the datum reference frames of the included geometric tolerances, thus creating a single pattern.
Simultaneous Requirements apply by default in the Y14.5-2009 standard. The overall effect is that the tolerance zones for features and groups are constrained relative to each other when common datum features are referenced. In other words, a larger pattern or group is created. This becomes important when the datum features do not fully constrain all six degrees of freedom, and datum shift is possible. The shift must be applied the same way to the entire larger pattern of tolerance zones.
The applicability of Simultaneous Requirements depends on the geometric characteristic:
Position Tolerances and Profile Tolerances
Simultaneous Requirements applies by default to all Position and Profile FCF’s that reference common datum features. This means that groups of Position zones can be created and groups of Profile zones can be created. Most CMM software allows these types of feature groups to be created.
This also means that mixed groups involving both Position zones and Profile zones can be created. This is much more difficult to simulate, and most CMM software does not allow the creation of these mixed groups. Approximations and workarounds are usually necessary to optimize datum shift with mixed patterns of Position and Profile zones.
Circular Runout and Total Runout Tolerances
Simultaneous Requirements is not currently applied to runout tolerance zones in the Y14.5-2009 standard. This is because runout tolerances are generally envisioned to apply to single features with stable datum axes and no datum shift. However, the application can be envisioned in certain situations.
Orientation Tolerances – Perpendicularity, Parallelism, Angularity
Simultaneous Requirements is not currently applied to orientation tolerance zones in the Y14.5-2009 standard. It is possible to envision mutual orientation constraint between features linked with simultaneous requirements, but the Y14.5 standard has not explored this possibility.
Size Tolerances and Form Tolerances
There is no application of Simultaneous Requirements to Size tolerance zones and Form tolerance zones, because these zones apply individually to each feature and do not involve datum features.
||Applicability of Simultaneous Requirements
||Not currently defined, but possible
||Not currently defined, but possible
Simultaneous Requirements creates tolerance zone constraints between features and groups controlled by Position and Profile tolerances that reference common datum features. The overall effect is the features and groups are combined into a single group, that must be optimized together if datum shift and fitting are applied.
CMM software does not usually handle Simultaneous Requirements by default, and care must be taken to define feature groups in the appropriate way in order to obey the default Simultaneous Requirements rule defined in the Y14.5 standard.