ASME Y14.5
Read a geometric tolerancing callout: each characteristic, its tolerance zone, the datums it needs and the modifiers that change it.
ASME Y14.5 · See ASME catalogue
Original plain-language summaries of each characteristic and modifier. The governing standard remains the controlling document.
| Symbol | Name | Controlled feature | What it controls | Tolerance zone | Datum required | Note |
|---|---|---|---|---|---|---|
| ⊕ | Position | Hole or feature location | Cylindrical zone of diameter t at the true position | 3 – 4 datums | Y14.5-2018 removed the RFS modifier – the zone is always at true position | |
| ∥ | Parallelism | Surface or axis | Two parallel planes t apart, or a cylindrical zone of diameter t | 2 datums | Use the Ⓜ modifier when the tolerance zone may grow | |
| ⊥ | Perpendicularity | Surface or axis | Two parallel planes t apart, 90° to the datum | 1 – 2 datums | Controls orientation only, not location | |
| ⌒ | Flatness | Any surface, no datum | Two parallel planes t apart | None | Applied to a surface that is not tied to a datum | |
| ∠ | Angularity | Surface or axis | A zone t wide between two parallel planes inclined to the datum | 1 – 2 datums | Controls orientation relative to a datum | |
| ○ | Circularity (roundness) | Round surface | An annular zone t wide between two concentric circles | None | Controls form only – it does not control the position of the axis | |
| ⌭ | Cylindricity | Round surface or axis | A zone t wide between two coaxial cylinders | None | Tighter than circularity because it also controls straightness | |
| Profile of a line | 2D profile of a line | Any line feature | A zone t wide between two equidistant lines | 1 datum (usually A) | Written as the profile symbol in a single segment feature control frame | |
| Profile of a surface | 3D profile of a surface | Any surface | A zone t wide between two equidistant surfaces | 1 – 4 datums | Controls form, orientation and sometimes size in one frame | |
| ⌗ | Surface texture | Surface finish | Ra, Rz or other parameter limits | None | Specify the parameter, the value and the production method | |
| ⌵ | Countersink | Hole | Included angle and depth | None | State the nominal diameter and the included angle, for example 90° | |
| ⌴ | Counterbore | Hole | Diameter and depth | None | State the counterbore diameter, the depth and the tolerance | |
| ↧ | Depth | Hole | Depth measured from the surface | None | For blind holes specify the depth and the drill point angle | |
| ⌑ | Square | Square feature | Squareness of the feature | None | Often written as a separate note next to the dimension | |
| ⌯ | Symmetry (deprecated) | Feature | A zone t wide between two parallel planes symmetric about the datum | 1 datum | Replaced by position tolerance in ASME Y14.5-2018 | |
| Concentricity (deprecated) | Axis | Cylindrical zone of diameter t | Cylindrical zone of diameter t | 2 datums | Replaced by position tolerance in ASME Y14.5-2018 |
ASME Y14.5 · 2018 · unit: Modifier
Modifiers change how the tolerance zone grows or shrinks. They are written inside the feature control frame.
| Modifier | Meaning | Applies to | Example | Note | Source |
|---|---|---|---|---|---|
| Ⓜ | Maximum material condition | Size and form features | ⌀10 −0.1/−0.3 Ⓜ | A | B | ⌀0.2 A | B | C | The tolerance may grow as the part departs from maximum material size | ASME Y14.5-2018 |
| Ⓛ | Least material condition | Size features | ⌀10 −0.1/−0.3 Ⓛ | A | B | ⌀0.2 A | B | C | The tolerance may grow as the part departs from least material size | ASME Y14.5-2018 |
| Ⓣ | Tangent plane | Surface | Profile | Ⓣ | A | ⌀0.2 A | The profile must lie between two tangent planes | ASME Y14.5-2018 |
| Ⓢt / Ⓢy | Statistical tolerancing | Statistical process control | Ⓢt on the drawing | Only for parts produced by a statistically controlled process | ASME Y14.5-2018 |
| RFS | Regardless of feature size | Any feature | ⌀0.2 RFS | A | B | C | No longer used in ASME Y14.5-2018 – the tolerance applies at true position | ASME Y14.5-2018 |
| Free state | Part is measured free of clamping forces | Non-rigid parts | Free state note on the drawing | For thin or flexible parts that change shape when clamped | ASME Y14.5-2018 |
| Between | Tolerance lies between two equidistant surfaces | Feature | Profile between two surfaces | Used for profile control of a slot or a tab | ASME Y14.5-2018 |
| All around | The requirement applies to the whole feature | Partial feature | RFS All around on the drawing | Common for a hole in a non-planar surface | ASME Y14.5-2018 |
| All over | The requirement applies over the whole surface | Surface | Continuous feature note | Used for surfaces such as a curved wall | ASME Y14.5-2018 |
| Continuous feature | The requirement follows the feature around | Rounded slot | Continuous feature symbol | Used for slots with a constant tolerance around the ends | ASME Y14.5-2018 |
| Unequally disposed | Unequally disposed profile tolerance | Profile | Ⓤ on the drawing | Provides more tolerance at the middle of the profile than at the ends | ASME Y14.5-2018 |
| Direction of tolerance zone | Displacement of the zone | Position tolerance | Direction symbol on the drawing | Shifts the tolerance zone in one or two directions | ASME Y14.5-2018 |
The explanations are original plain-language summaries, and the controlling document is always the governing standard. ASME Y14.5-2018 removed the RFS modifier and deprecated concentricity and symmetry, so a drawing that still uses them needs a check against the edition in force.
Reference source
ASME Y14.5
Maximum material condition is the condition where the part has the most material: a hole at its smallest size and a shaft at its largest. Least material condition is the opposite. The modifier tells the inspector that the tolerance zone is allowed to grow by the difference between the actual size and the MMC or LMC limit, and it always states the actual size of the feature.
RFS means regardless of feature size, and it used to appear in most position callouts. ASME Y14.5-2018 removed it because the default is now that the tolerance applies at true position with no bonus, so the modifier is no longer allowed. ISO 1101 follows the same direction.
Position, with the tolerance zone described as a cylinder at the true position, referenced to the datums that locate the part. Add a size callout with MMC or LMC if the assembly has to work with parts from different suppliers, and keep the zone small enough that the fastener still fits.
Flatness controls a single surface on its own and needs no datum. Parallelism controls the relationship between a surface and a datum, so it also controls orientation. A part can be flat but tilted relative to its neighbours, so if the mating face must be square to the others, specify parallelism and not flatness.
Use the fewest that control the features you care about. Three datums establish a coordinate system, but a simple flat part often only needs one, and a hole pattern usually needs two. Referencing more datums than necessary adds cost and can make the drawing harder to inspect.
ASME Y14.5-2018 deprecated both, because they were interpreted differently across the industry and could not be measured easily. Replace them with a position tolerance that controls the axis, which is unambiguous and directly measurable on a CMM.
On a threaded hole, where the practical tightening torque pulls the mating part out of position and a zero projected zone would make the assembly impossible. The projected zone is set to the thread pitch so the fastener can seat. Specify it explicitly and state the fastener grade, because the engagement length changes the requirement.
A profile of a line controls a two-dimensional profile, typically the outline of a part in a view, relative to the basic dimensions. A profile of a surface controls a three-dimensional surface with a zone between two equidistant surfaces and can replace many individual form, orientation and location callouts.
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