What Is Concentricity? GD&T Definition & Status

What Concentricity Means in GD&T

In everyday engineering language, concentric simply means “sharing a center”: a bore centered on an outside diameter, or the steps of a turned shaft lined up on one axis.

As a GD&T control in ASME Y14.5-2009 and earlier, concentricity was much narrower. It required the inspector to derive the median points of the feature — the midpoints between diametrically opposed points on the surface — and check that they all fell within a zone around the datum axis. That definition ignores the feature’s size, roundness and cylindricity. A lobed, out-of-round diameter can pass a concentricity check if its midpoints happen to line up.

Why ASME Y14.5-2018 Removed Concentricity

Concentricity had long been misunderstood and misused. Designers chose it because they wanted features to be coaxial, but deriving median points is slow, difficult to inspect and rarely tied to how the part works. The 2018 edition removed it and left four ways to control coaxial features: position, circular runout, total runout and profile.

Government drawing standards point the same way: NASA Kennedy Space Center’s engineering drawing practices direct designers to use positional tolerances and runout rather than concentricity. Drawings made to the 2009 or earlier editions still carry the symbol and are read to the edition they cite.

Concentricity vs Runout vs Position

How to Control Concentric Features Today

• Start from function. Where a surface spins against a bearing or seal, runout is usually the right control, and it is quicker and cheaper to verify than concentricity ever was.
• Where the concern is that two parts assemble, use position, adding Ⓜ where the fit is a clearance fit.
• If dynamic balance is the real requirement, specify a balance note; a static geometric check does not guarantee balance.
• When you receive a legacy drawing with a concentricity callout, agree with the supplier how it will be measured before production, or convert it to runout or position.

Concentric features are one of the main strengths of turning: CNC turning produces axially symmetric parts with high concentricity, and critical diameters are verified by CMM or manual gauging on CNC machining orders.