FOS Feature of size (FOS)
Feature of size (FOS) · Anotaciones de plano · ASME Y14.5
Example in plan
How to interpret it
A feature of size (hole, shaft, slot with associated dimensions) can carry a geometric tolerance with MMC/LMC modifiers and generate bonus tolerance.
What controls
A geometric feature that has an associated size dimension: a cylinder (shaft or hole), a sphere, or two opposed parallel surfaces (a slot, a tab, a thickness). Its distinguishing trait is that it has a derived axis or median plane, and that it can be at maximum or least material condition. This classification is not academic: it determines which controls can be applied to it, because material modifiers only make sense on features of size and most location controls apply to their axis or median plane, not to their surface.
How to verify
Size is measured with two-point instruments (micrometer, bore gauge, caliper) or with limit gauges, and the derived axis or median plane is obtained on a CMM from points spread over the surface. When there is a material modifier, the actual size of each part must be measured to calculate the bonus: neither the nominal nor the lot average will do. A functional gauge checks size and geometry at the same time, but only on features of size with a maximum material modifier.
Common mistakes when interpreting it
- Applying material modifiers to a single flat face, which is not a feature of size.
- Measuring the position of a hole's surface instead of its derived axis.
- Calculating the bonus with the nominal size instead of the actual size of the specific part.
How to get it in the workshop / if it comes out of tolerance
The practical question is simple: does this feature have a size that can be measured between two opposing points? If the answer is yes, it is a feature of size and accepts modifiers; if it is a single face, a cone or an open profile, it is not. This also tells you where the leader of the feature control frame points: on the dimension line it controls the axis or median plane; on the surface, the surface.
If a feature control frame with a material modifier appears on something that is not a feature of size, the drawing is wrong and must be queried before manufacturing. If size readings vary depending on where they are taken, you have a form problem that should be cleared up before arguing about location. And if a part is rejected for position, check the actual size before anything else: the bonus may save it.
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