Symmetry is a GD&T location control that kept the median points of a slot, tab or other width feature between two parallel planes centred on a datum plane. ASME Y14.5-2018 removed it, and new ASME drawings use position or profile instead. It is still valid on older drawings and in ISO standards.
Symmetry was the flat-feature twin of concentricity. For each pair of opposed points across a slot or tab, take the midpoint. Every midpoint had to lie between two planes centred on the datum centre plane.
Like concentricity, it needs a lot of data and a midpoint calculation, and it could fail a part whose walls were uneven even when the slot was in the right place. Position of the centre plane does almost the same job and can use MMC and functional gauges.
The symbol is three horizontal lines with the middle one longer. Do not confuse it with the profile or position symbols that replace it.
The zone is two parallel planes, the tolerance apart, centred on the datum centre plane (or axis). All median points of the controlled feature must be inside.
It has no diameter symbol, because the zone is a pair of planes rather than a cylinder.
Symmetry always referenced a datum, usually the centre plane of the part width or another slot.
| Modifier | With symmetry |
|---|---|
| MMC or LMC | Not allowed; symmetry was always RFS. Use position at MMC for bonus tolerance. |
Example: a legacy drawing with a 12 mm keyway, symmetry 0.1 to datum A (the shaft centre plane)
Result: 0.06 ≤ 0.1, so the keyway passes. A new drawing would show position 0.1 to A for the same intent.
ASME Y14.5-2018 removed the symmetry symbol and concept, along with concentricity. Use position of the centre plane (RFS or MMC) or profile of a surface instead. Drawings made to the 2009 edition or earlier still use symmetry, and ISO 1101 keeps a symmetry tolerance with its own definition.
Every GD&T symbol page: GD&T symbols chart
Yes, from ASME Y14.5 in the 2018 edition. Older ASME drawings and ISO drawings may still use it.
Position of the centre plane, at RFS or MMC, or profile of a surface on the walls with datums.
Three horizontal lines stacked, the middle one longer than the other two.
Symmetry controlled median points of opposed surface points. Position controls the centre plane of the feature as a whole, can use MMC and is easier to gauge.
COPLAIN AI PLATFORM
Moving drawings to ASME Y14.5-2018? Coplain updates your inspection instructions when symmetry becomes position or profile.
Generate yours automatically with Coplain →Related Tools & Templates