GD&T Concentricity: Definition, Symbol, and How to Measure It

A feature of revolution has an axis, and assemblies care intensely where it sits. Concentricity was the old-standard answer: a location tolerance demanding that the median points of the feature — the midpoints of diametrically opposed surface points — all fall inside a cylindrical zone centered on a datum axis, with the zone’s diameter equal to the tolerance value.

Mathematically, that’s the cleanest definition of “shares an axis” available. Practically, it’s a measurement nightmare, because no shop instrument reads median points — you derive them, probe pair by probe pair, usually on a CMM, and pay for the privilege. That gap between definition and measurement is the entire story of this symbol.

In the wider framework — our GD&T basics guide is the series entry point — concentricity sat with the location controls, always tied to a datum axis established by other features. On a stepped shaft that datum is typically the bearing journals, and every controlled diameter gets judged against it; the datum guide explains how those axes are built and simulated.

Concentricity is not in the current edition. ASME Y14.5-2018 removed it — along with symmetry — for three documented reasons: it is hard and expensive to verify, because it controls derived median points rather than the surface; it was among the most misapplied controls on drawings; and, in the committee’s own words, other characteristics provide more direct control.

The third reason is the practical one. Everything a designer used to ask concentricity for — a surface that runs true, an axis that sits in a zone — is available from controls a shop can verify with a dial indicator or a standard CMM routine. When a tolerance costs hours to inspect and its job can be done in seconds, drawings stop carrying it, and eventually the standard caught up with the drawings.

The removal is a boundary, not a ban. Drawings govern by their stated revision, so concentricity on a Y14.5-2009 drawing is still valid and must be inspected as written — with the median-point CMM routine the callout implies. What changed is that new designs under 2018 no longer get to choose it.

On a new drawing the question is no longer whether to use concentricity — it is which successor fits:

Runout, when the function is rotation. Bearing seats, seal surfaces, anything that spins and must not wobble. Circular runout checks each cross-section; total runout sweeps the whole surface. One indicator setup either way.

Position, when the function is location. The axis must sit in a zone relative to datums. Position does that with standard verification, composes cleanly with the rest of the location framework, and reports numbers that match the callout.

Profile of a surface, when median-point control was genuinely intended. The rare case — high-speed balance work. The honest modern route is profile of a surface, and usually a conversation about specifying balance directly rather than proxying it through geometry.

The features that wore this callout are the ones where coaxiality is function: stepped shafts with bearing seats at both ends, pump spindles where an impeller seat must run true to the journal, housings where the bearing bore aligns to a face-mounted pilot, bushings pressed into bores that must share an axis with the outside diameter.

On turned parts the process is the first line of control: every diameter cut in one setup shares the spindle axis by construction, which is why the shaft machining guide pushes single-setup strategies. The callout then verifies what the routing already achieved — it doesn’t rescue a part that got re-chucked between operations. For buyers, the actionable version: express these needs as runout (surface function) or position (axis location) on new drawings, and inspection becomes routine. Our tolerances page states what we quote against drawings — standard ±0.13 mm (±0.005 in), precision features to ±0.025 mm (±0.001 in).