Concentricity — Location Tolerance

Controls the coaxiality of median points of a feature with a datum axis.

Definition

Concentricity controls the median points of a feature — the midpoints of diametrically opposed point pairs taken across its surface — relative to a datum axis. It is a Location control, so it always requires a datum reference. The zone is a cylinder of diameter Øt centered on the datum axis, and every median point must fall inside it. The critical nuance is what is being controlled: not the surface itself, and not the feature's axis as a whole, but the cloud of individual median points. This is the symbol most often miscalled on drawings — many drafters write concentricity when the functional requirement they have in mind is actually coaxiality of the surface, which is what runout or position controls directly.

Why the professional consensus is to avoid it

Concentricity is legal in the standard and occasionally the honest callout, but the working consensus among metrologists and designers is to reach for it last. The reason is the median points themselves: they are derived data, not something a gauge or probe reads directly. Establishing them means measuring many diametrically opposed point pairs around the feature and computing each pair's midpoint — a slow, expensive CMM analysis with no functional gauge possible, because a physical gauge contacts surfaces, not derived midpoints. And because the median points are averages, the control is surprisingly insensitive where it matters: a surface with lobed out-of-roundness can have perfectly centered median points and pass concentricity while it would rattle on a shaft seat.

The alternatives that do the same job

Almost every functional intent behind a concentricity callout is served better by another control. If the requirement is that the surface runs true relative to the datum — the usual case for rotating parts — circular runout controls it directly and is checked with a dial indicator in seconds. If the requirement is assembly clearance or location of a feature of size, position with a diameter zone does that, takes material modifiers, and is gaugeable. If the requirement is genuinely about the derived axis — rare — coaxiality language or a position callout at RFS is the closer match. The honest framing: concentricity survives on drawings mostly by habit, and each surviving instance deserves a challenge.

• Surface must run true: circular runout — fast dial-indicator check, controls the surface directly
• Feature must assemble in the right place: position — gaugeable, MMC bonus available
• Derived axis genuinely matters: position at RFS, which controls the actual axis without the midpoint averaging

Reading a feature control frame

⌔ Ø0.1 A reads as: a concentricity tolerance of 0.1 mm diameter, referenced to datum axis A. Two things to notice in the frame: the diameter symbol is required, because the zone is a cylinder, and there is no material-condition modifier — concentricity never takes Ⓜ or Ⓛ, because it controls derived median points rather than a feature of size. That exclusion is the detail most often gotten wrong; a frame with ⌔ and Ⓜ together is simply not a valid callout.

Measurement

CMM median-point analysis — the only real method: probe many points around the feature in diametrically opposed pairs, compute each pair's midpoint, and fit the tightest cylinder about the datum axis that contains them. Slow, operator-sensitive, and the result depends on how many pairs were taken.

There is no functional gauge — a physical gauge cannot contact derived midpoints. If a drawing shows concentricity with a proposed ring or plug gauge, the callout is being measured as something else — almost always runout — and should be redrawn as that control.

When to use it

Honestly: almost never, and the default answer is to reach for runout or position instead. The narrow remaining cases are balance-critical rotating components — crankshaft and camshaft journals in some specifications, high-speed rotating assemblies — where a standard or customer explicitly requires control of mass centroid distribution about the datum axis, and where the median-point semantics are actually what the balance calculation consumes. Even there, modern practice increasingly substitutes position at RFS or a runout pair. If you write concentricity, be prepared to explain why runout does not already cover the function — and expect the shop to measure it as runout anyway.