What Is a Concentricity?

Concentricity means that two or more round features, such as the diameters of a stepped shaft, share the same center axis. In GD&T it was a tolerance on the midpoints of a feature relative to a datum axis, but ASME Y14.5-2018 removed the symbol; coaxial features are now controlled with position, runout or profile.

In short: everyone still wants round features to be concentric, but the concentricity callout itself is no longer part of the current US GD&T standard.

What Does Concentricity Mean 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 Was Concentricity Removed From ASME Y14.5-2018?

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

	Concentricity (Y14.5-2009 and earlier)	Circular / total runout	Position (coaxiality)
What it controls	Median points of the feature, relative to the datum axis	The surface, as the part rotates 360° about the datum axis	The feature's axis, within a cylindrical zone around the datum axis
Catches out-of-round form?	No	Yes	No (form is left to the size tolerance)
How it is checked	Many CMM points and derived midpoints	Indicator on the surface while the part rotates	CMM, or a fixed gauge when MMC applies
Status in Y14.5-2018	Removed	Current	Current
Typical use	Legacy drawings	Rotating surfaces: bearing journals, seal diameters, shafts	Assembly fits between mating parts

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 at NICE Rapid.

Frequently Asked Questions

What is concentricity in GD&T?

It was a tolerance requiring the median points of a round feature to lie within a zone around a datum axis. It ignored the feature's size and form, which made it hard to inspect and often misapplied, and it was removed in ASME Y14.5-2018.

Is concentricity still used in GD&T?

Not in the current ASME standard. Y14.5-2018 removed the concentricity symbol, so coaxial features are now controlled with position, circular runout, total runout or profile. Drawings made to the 2009 or earlier editions may still show it.

What is the difference between concentricity and runout?

Concentricity checked only the midpoints of a feature against the datum axis. Runout checks the actual surface as the part rotates about the datum axis, so it also picks up out-of-roundness. Runout is simpler to measure and is the usual replacement.

Related Services at NICE Rapid

• CNC Machining — CNC turning for shafts, bushings and pins, with tolerances to ±0.01 mm and CMM or manual gauging of critical dimensions.
• Rapid Prototyping — turned and milled prototypes with an engineering review included in every quote.
• Request a Quote — send your drawing, and flag any legacy concentricity callouts so the inspection method can be agreed up front.

Sources

1. Tec-Ease, GD&T Symbol: Concentricity (2018) — concentricity was misunderstood and misused; derived median points are difficult to inspect; "Concentricity has been removed from the Standard"; replacements are position, circular runout, total runout and profile — tec-ease.com
2. Tec-Ease, GD&T Symbol: Concentricity (ASME Y14.5M-1994 §5.12, Y14.5-2009 §7.6.4) — concentricity requires deriving median points, ignores size, roundness and cylindricity; a static check is not a substitute for dynamic balancing — tec-ease.com
3. Tec-Ease, GD&T Tip: Coaxiality (Part 1), per ASME Y14.5-2018 — "In the 2018 standard, concentricity was removed," leaving four ways to control coaxiality; position with MMC suits assembly — tec-ease.com
4. NASA Kennedy Space Center, KSC-GP-435 Vol. I Rev. H Chg. 1, Engineering Drawing Practices — "Positional tolerances and runout should be used to avoid circularity, concentricity, and cylindricity tolerancing" — standards.nasa.gov
5. Tec-Ease, GD&T Symbol: Circular Runout (Y14.5-2018 §12.2) — runout zones are coaxial with the datum axis and apply to the surface — tec-ease.com