Concentricity is a GD&T location control that kept the median points of a round feature inside a small cylinder around a datum axis. ASME Y14.5-2018 removed it, so new ASME drawings should use position, runout or profile instead. You will still see it on older drawings made to the 2009 edition, and ISO standards still include a similar coaxiality control.
Concentricity did not look at the axis of the feature directly. It looked at the median points: for every cross-section, the points halfway between opposite surface points. All of those midpoints had to sit inside a cylinder centred on the datum axis. The idea was to balance mass around an axis, for parts that spin at high speed.
Because it works with midpoints of opposed points, an uneven shape could fail concentricity even when its centre was perfectly placed, and the only real way to measure it is to collect a lot of data and calculate the median points. That made it slow, expensive and frequently misapplied.
For the same reasons, most shops have long used circular runout or position instead, and the 2018 edition made that official.
The zone is a cylinder, of the tolerance diameter, coaxial with the datum axis. Every median point of the controlled feature must lie inside it.
The feature can be any shape that has opposed points about an axis, not just a circle, which is part of what made it confusing.
Concentricity always needed a datum axis, usually another diameter on the same part.
| Modifier | With concentricity |
|---|---|
| MMC or LMC | Not allowed. Concentricity always applied regardless of feature size, both to the tolerance and to the datum reference. |
| Diameter symbol | Always shown, because the zone is a cylinder. |
Example: a legacy drawing with a Ø30 mm diameter, concentricity Ø0.05 to datum axis A
Result: Ø0.038 ≤ Ø0.05, so the diameter passes. On a new drawing, a runout of about 0.05 to A would be simpler to check.
ASME Y14.5-2018 removed the concentricity symbol and concept. Use position (RFS or MMC) to locate a coaxial feature, circular or total runout when the surface must spin true, or profile of a surface for non-round shapes. Drawings that call out the 2009 or earlier editions in their title block still use concentricity, and ISO 1101 keeps a coaxiality/concentricity control with its own definition.
Every GD&T symbol page: GD&T symbols chart
It was removed from ASME Y14.5 in the 2018 edition. It is still valid on drawings made to the 2009 or earlier editions, and ISO standards still have a coaxiality/concentricity control.
Position for locating a coaxial feature, circular or total runout when the surface must spin true, and profile of a surface for non-round features.
Concentricity controlled the median points of the feature. Runout controls the actual surface as it spins, so it includes form error and is easier to check.
No. It always applied regardless of feature size.
Strictly with a CMM that evaluates median points. In practice, a runout check within the same value is accepted as proof that the part passes.
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