ASME Y14.5-2018 reference
The 14 geometric characteristic symbols
Every characteristic, its symbol, what it controls, whether the tolerance zone is stated as a diameter, whether a material condition modifier may be applied to it, and the datum reference it needs. The two profile symbols are the pair most often drawn wrong.
Characteristics, families and datum requirements
| Symbol | Characteristic | Family | What it controls | Diametral zone | Takes a modifier | Datum requirement |
|---|---|---|---|---|---|---|
| ⏥ | Flatness | Form | Hold a surface between two parallel planes. | No | No | None. A form control compares the surface with a perfect plane fitted to it. |
| ⏤ | Straightness | Form | Hold a line element or an axis between two parallel lines or inside a cylinder. | No | Yes, on a feature of size | None. Line elements or the derived median line are compared with a perfect line. |
| ○ | Circularity | Form | Hold each circular cross section between two concentric circles. | No | No | None. Each cross section is compared with concentric circles fitted to it. |
| ⌭ | Cylindricity | Form | Hold a cylindrical surface between two concentric cylinders. | No | No | None. The surface is compared with concentric cylinders fitted to it. |
| ⌓ | Profile of a surface | Profile | Hold a surface inside a zone disposed about the true profile. | No | No | Optional. With datums it locates and orients; without them it controls shape alone. |
| ⌒ | Profile of a line | Profile | Hold each cross section of a surface inside a two dimensional zone about the true profile. | No | No | Optional, on the same terms as profile of a surface. |
| ⊥ | Perpendicularity | Orientation | Hold a feature at 90 degrees to a datum. | No | Yes, on a feature of size | At least one. The feature is held square to the datum named. |
| ∥ | Parallelism | Orientation | Hold a feature parallel to a datum. | No | Yes, on a feature of size | At least one. The feature is held parallel to the datum named. |
| ∠ | Angularity | Orientation | Hold a feature at a basic angle to a datum. | No | Yes, on a feature of size | At least one, with the angle stated as a basic dimension. |
| ⌖ | Position | Location | Locate a feature axis or centre plane inside a tolerance zone at its basic location. | Yes, when applied to an axis | Yes, on a feature of size | Usually a full frame. The zone sits at a location defined by basic dimensions. |
| ◎ | Concentricity | Location | Hold median points of opposed elements about a datum axis. | No | No | A datum axis. Rarely used, and normally replaced by position or runout. |
| ⌯ | Symmetry | Location | Hold median points of opposed elements about a datum centre plane. | No | No | A datum centre plane. Rarely used, and normally replaced by position. |
| ↗ | Circular runout | Runout | Limit the indicator movement at each single circular element as the part rotates. | No | No | Always a datum axis. There is nothing to rotate about without one. |
| ↗̲ | Total runout | Runout | Limit the indicator movement across the whole surface as the part rotates. | No | No | Always a datum axis, as for circular runout. |
Datum precedence in the planar arrangement
| Datum | Role | Degrees of freedom constrained | Which ones |
|---|---|---|---|
| A | Primary plane | 3 | One translation and two rotations. |
| B | Secondary plane or width | 2 | One translation and one rotation. |
| C | Tertiary stop | 1 | The last translation. |
A primary cylindrical datum feature constrains four degrees of freedom, two translations and two rotations, so the three, two, one split above applies to the planar arrangement only.