F4:Feature of Size Linkage
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Summary
Some tolerances are conditionally linked to a feature of size (FOS). When F4 detects a linked size dimension, it can enforce additional rules such as comparing the FCF tolerance against the size tolerance (unless an exception applies).
Standard meaning
In ASME Y14.5, some form/orientation controls interact with size and Rule #1-style constraints. The standard defines the intent; software still has to decide how to infer “linked size” from the available model data.
F4 behavior
How F4 detects a linked size dimension
- If the feature control frame has linked dimensions, F4 treats the tolerance as “FOS-linked”.
- If no linked dimensions exist, F4 treats it as “not linked” (and some tolerance types remain valid in that state).
How F4 computes size tolerance
When a size dimension is linked, F4 computes the size tolerance from the first linked dimension.
When F4 compares tolerance vs size tolerance
For conditional tolerance types, F4 may compare the FCF tolerance value to the linked size tolerance.
- If the FCF tolerance exceeds the size tolerance, F4 flags non-compliance.
- If F4 can’t determine either value (missing or non-parsable), the comparison is skipped.
Exceptions (skip comparison)
F4 skips the tolerance-vs-size comparison for specific cases:
- Flatness applied to a planar feature of size.
- Straightness applied to a cylindrical feature of size.