GD&T symbols
Name the symbol, or say whether it uses a datum. Get 10 right to finish. Pick ASME or ISO below.
Which geometric characteristic is this symbol?
The symbol chart
| Symbol | ASME Y14.5 | ISO 1101 | Group | Datum |
|---|---|---|---|---|
| Straightness | Straightness | Form | Never | |
| Flatness | Flatness | Form | Never | |
| Circularity | Roundness (circularity) | Form | Never | |
| Cylindricity | Cylindricity | Form | Never | |
| Profile of a line | Line profile | Profile | Optional | |
| Profile of a surface | Surface profile | Profile | Optional | |
| Perpendicularity | Perpendicularity | Orientation | Always | |
| Angularity | Angularity | Orientation | Always | |
| Parallelism | Parallelism | Orientation | Always | |
| Position | Position | Location | Usually (see note) | |
| Removed in 2018 | Concentricity or coaxiality | Location | Always | |
| Removed in 2018 | Symmetry | Location | Always | |
| Circular runout | Circular run-out | Runout | Always | |
| Total runout | Total run-out | Runout | Always |
Groups follow ASME. ISO lists profile under form, orientation or location depending on whether datums are used. Position is used with datums in ASME; ISO allows it without datums for some hole patterns.
Common mistakes
- Mixing up flatness and parallelism. Flatness has no datum; parallelism is always to a datum.
- Reading the two runout arrows as one. Two arrows on a base line is total runout.
- Expecting concentricity on a new ASME drawing. It was replaced by position, runout or profile.
Questions
- How many GD&T symbols are there?
- There are 14 geometric characteristics. ASME Y14.5-2018 removed concentricity and symmetry, so a current ASME drawing uses 12. ISO 1101 still has all 14.
- Which GD&T symbols need a datum?
- Orientation (parallelism, perpendicularity, angularity) and runout always need one. Form (straightness, flatness, circularity, cylindricity) never does. Profile can be used either way. Position is used with datums in ASME; ISO allows it without for some patterns.
- Is circularity the same as roundness?
- Yes. ASME calls it circularity, ISO 1101 calls it roundness.
Explained in our own words and checked against several GD&T references. For the full rules, see ASME Y14.5-2018 and ISO 1101:2017.