DatumLab
ASME Y14.5-2018, Symbols

Reading a feature control frame compartment by compartment

A feature control frame is one sentence written in compartments, and it always reads the same way: control this characteristic, to this much, relative to these datum features in this order. Once that sentence is automatic, most of the arguments on a shop floor turn out to be about a single compartment. This guide walks the frame left to right and names the mistakes that live in each part of it.

The characteristic and the tolerance

The leftmost compartment is always the geometric characteristic symbol, and everything to the right modifies that one statement. The second compartment holds the tolerance, and it holds it as a unit: the diameter symbol if the zone is cylindrical, then the value, then the material condition modifier if there is one.

That value is always the total width of the zone, never a plus and minus limit. A profile tolerance of 0.40 mm means 0.20 mm on each side of true profile, and reading it as plus or minus doubles the real allowance.

Why RFS is never written

Regardless of feature size is the default, and it is shown by the absence of a modifier. There is no symbol for it, and the letters RFS never appear in a frame. A rendered compartment reading RFS is not valid drafting, and a tool that produces one is teaching a habit that has to be unlearned.

The same applies on the datum side: regardless of material boundary is the default there, shown by an unmodified datum letter.

Datum compartments and their order

Datum references are read left to right in order of precedence. The compartment nearest the tolerance is the primary datum feature, established first and constraining the most degrees of freedom. Two frames with the same letters in a different order are two different requirements, and on a part with out of square surfaces they will give different verdicts.

A modifier after a datum letter fixes that feature's simulator at a material boundary and permits datum shift. It says nothing about precedence, which comes from position in the frame alone.

What sits outside the frame

Extent indicators such as all around, all over and between answer where the control applies, not how much. They never change the tolerance, the characteristic or the datums.

A projected tolerance zone indicator does change the requirement: it moves the zone out of the part and into the space a fastener will occupy, over a stated height, which is what catches a tapped hole that is on location at the surface and badly angled above it.

Reading one frame aloud

Position, diameter 0.25 at MMC, to A, B at MMB, C. Every compartment carries exactly one piece of information.

Characteristic
Position
Zone shape
Cylindrical, from the diameter symbol
Tolerance value
0.250 mm
Modifier on the tolerance
MMC, so the feature earns bonus
Primary datum feature
A, established first
Modifier on B
MMB, so B permits datum shift

Two modifiers, two meanings, two different features. The one inside the tolerance compartment belongs to the feature being controlled; the one after a datum letter belongs to that datum feature.

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Frequently asked questions

Where does the material condition modifier belong?

Inside the tolerance compartment, directly after the tolerance value. It is not a compartment of its own. A modifier that appears after a datum letter is a material boundary modifier on that datum feature and means something different.

How is RFS shown in a frame?

It is not shown at all. Regardless of feature size is the default implied by the absence of a modifier, so the letters RFS never appear in a feature control frame.

What does the diameter symbol in the tolerance compartment change?

It makes the zone a cylinder rather than a pair of parallel planes, so the axis is controlled equally in every direction. Dropping it converts a two dimensional control into a one dimensional one without any other visible change.

Does the order of the datum letters matter?

Yes. Precedence is read left to right, and the primary datum feature is contacted first and constrains the most degrees of freedom. Swapping the first two letters can tip the part onto different high points and change every measured value.

What does a frame with no datum compartments mean?

That the control is a form control, which compares the feature only with a perfect version of itself. Flatness, straightness, circularity and cylindricity take no datum reference, and a datum on one of those frames is an error.