Concentricity

Controls derived median points of a cylindrical feature relative to a datum axis.

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0.05

Concentricity controls the derived median points of a round feature to a datum axis, not simply the visible surface or a best-fit axis.

It was removed from ASME Y14.5-2018, but it still appears on drawings using earlier standards or ISO terminology such as coaxiality.

Use it only when balanced mass distribution around an axis is truly required.

For most rotating parts, runout, total runout, or position communicates the function more clearly and is easier to inspect.

Coaxial Shaft Sections Checked By Median Points

Concentricity evaluates calculated center points, which makes it much harder to inspect than runout or position.

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0.05

A cylindrical zone around the datum axis containing all derived median points of the controlled feature.

A CMM samples diametrically opposed points at multiple cross-sections, calculates their midpoints, and compares those points to the datum axis.

Because the controlled points are derived, simple indicator readings do not directly prove concentricity.

A shaft section has concentricity 0.05 mm to datum axis A. Measured derived median point offsets are 0.012, 0.018, and 0.021 mm.

Zone radius

0.05 / 2 = 0.025 mm

The cylindrical zone diameter is the stated tolerance.

Worst point

0.021 mm

Every derived median point must remain within the cylinder.

Compare

0.021 <= 0.025

The sampled median points are inside the zone.

The feature passes concentricity, though a runout requirement would usually be simpler to verify on the shop floor.

Control	Measured Element	Datum	Practicality
Concentricity	Derived median points	Axis	Difficult
Runout	Actual rotating surface	Axis	Practical
Position	Feature axis	Datum frame	Common

Concentricity is the circular counterpart to symmetry.

Runout indirectly controls concentricity and circularity together by measuring the actual surface during rotation.