Engineering · GD&T
Concentricity (removed in Y14.5-2018).
A location tolerance based on median points — the line of centers of opposing surface elements. Traditionally used to control coaxiality of cylindrical features, but the symbol was removed from ASME Y14.5-2018 (still seen on legacy drawings) in favor of position-of-axis or runout. If your drawing has it, here’s what it means and what to use instead.
01 · What concentricity tried to do
Median points lying within a zone.
The historical definition: median points of all opposed surface elements of a feature must lie within a cylindrical tolerance zone coaxial with a datum axis. The tolerance zone is a cylinder whose axis is the datum’s axis; the median-point line of the toleranced feature must fit inside.
The problem: median-point evaluation is computationally awkward and inspection-heavy. The result was that concentricity was rarely measured the way it was specified — most shops verified it via runout or position instead, leaving a gap between drawing intent and inspection reality.
02 · What to use instead
Three better alternatives.
For most coaxiality applications, position with a Ø symbol applied to the feature axis (referenced to the datum axis) gives equivalent functional control with universal CMM measurability and MMC modifier support.
For rotating-shaft applications where dynamic balance matters, total runout is the right tool. It captures coaxiality plus form errors over the full surface as the part rotates.
When only at-section coaxiality matters (e.g., a single bearing journal), circular runout at that location is simpler and easier to inspect than concentricity.
03 · If you still see it on drawings
Pre-2018 drawings, legacy specs.
Many production drawings still use concentricity, especially in defense and aerospace where revision cycles are long and legacy GD&T habits persist. Use the inspection method required by the governing drawing and standard; runout is not a substitute for legacy concentricity without engineering approval.
For drawings under your control, replace concentricity with position-of-axis or runout. Future revisions will be easier to inspect and the intent will be clearer to manufacturing.
Keep exploring
Related tools & references
Full feature-control-frame symbol reference.
The preferred control for coaxial features in current practice.
The practical, inspectable alternative to concentricity.
Size a diametral location tolerance from measured offset.
Why concentricity is rarely the right callout.
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