ASME Y14.5-2018 removed concentricity and symmetry from its list of geometric controls — not because the functional requirements they described have disappeared, but because better, more measurable controls exist to fulfill those requirements.
Concentricity (◎) and symmetry (≡) were among the original 14 GD&T symbols in the ASME Y14.5 standard for decades. Both symbols controlled the location of feature axes and center planes using derived median points — an elegant concept that proved extremely difficult and expensive to verify in practice. When ASME Y14.5-2018 was published, both symbols were removed from the standard’s primary controls. ISO 1101:2017 retains them. This article explains what each control means geometrically, why they were removed from ASME, what controls replace them, and when they might still appear on drawings.
Concentricity: Definition and Tolerance Zone
Concentricity (◎) controls the location of the derived median points of a diametrically opposed feature relative to a datum axis. The tolerance zone is a cylinder of the stated diameter centered on the datum axis. Every derived median point must fall within this cylinder.
To understand what “derived median points” means: for any circular cross-section of the controlled feature, take any pair of diametrically opposed surface points (two surface points separated by 180°), find their midpoint. The collection of all such midpoints across all cross-sections and all angular orientations constitutes the derived median line. Concentricity requires this derived median line to lie within the cylindrical tolerance zone about the datum axis.
The critical insight is that concentricity does NOT control the actual surface — it controls the statistical locus of midpoints. A surface could have severe roundness error (out-of-round cross-sections) while the median line is perfectly centered. Conversely, a surface could be perfectly round but displaced from the datum axis — this WOULD violate concentricity.
Measurement challenge: to find the derived median line rigorously, you must:
- Probe the surface at multiple cross-sections and multiple angular positions
- Find diametrically opposed pairs and compute their midpoints
- Do this for many angular orientations at each cross-section
- Verify all midpoints lie within the tolerance zone
This requires extensive CMM probing and is computationally intensive. The result is sensitive to the probing density and the algorithm used to identify “diametrically opposed” points on a non-round surface — different CMM software implementations can give different results on the same physical part.
Symmetry: Definition and Tolerance Zone
Symmetry (≡) controls the location of the derived median points of a width feature (slot, tab, or symmetric part) relative to a datum plane. The tolerance zone is the space between two parallel planes equally disposed about the datum plane. All derived median points must lie within this zone.
Derived median points for a planar feature: take any pair of points on the opposing surfaces of the controlled width feature that share the same surface normal direction (points directly across from each other), find their midpoint. The locus of all such midpoints constitutes the derived median plane, which must lie within the symmetry tolerance zone.
Like concentricity, measurement is non-trivial. The CMM must identify corresponding opposing-surface point pairs, compute midpoints, and verify the resulting median plane against the tolerance zone. Surface roughness and form error create noise in the midpoint calculation.
Why ASME Y14.5-2018 Removed These Controls
The ASME Y14.5 committee removed concentricity and symmetry from the 2018 edition for several interconnected reasons:
- Measurement ambiguity: The derived median line/plane concept is defined clearly in theory but implemented inconsistently by different CMM software packages. Two CMMs measuring the same part can give different results depending on the probing strategy and fitting algorithm. This means concentricity and symmetry tolerances are not reliably verifiable in practice.
- Functional redundancy: The functional requirements that engineers believed they were controlling with concentricity (coaxiality of rotating parts) and symmetry (centering of features) can be more directly controlled with position or runout — and these alternatives are more straightforwardly measurable.
- Inspector and designer confusion: Surveys and industry experience showed that many engineers specified concentricity when they actually meant coaxiality of the surface (better controlled by circular or total runout) or location of the axis (better controlled by position). The conceptual subtlety of derived median points was not well understood in practice.
- Cost: The extensive probing required to measure concentricity or symmetry rigorously makes routine production inspection extremely expensive relative to the functional benefit achieved.
What Replaces Concentricity: Position or Runout?
The replacement depends on the functional requirement:
Use Circular Runout (↗) when: The part rotates and the concern is how the surface performs during rotation — vibration, balance, seal contact. Circular runout captures both roundness and coaxiality error as they would manifest functionally (as TIR during rotation). It is easy to measure with a dial indicator and V-block setup.
Use Total Runout (⇗) when: The entire cylindrical surface (including along the axis) must be controlled during rotation. Replaces concentricity when cylindricity + coaxiality are simultaneously required for rotating parts.
Use Position (⊕) when: The axis location matters independently of rotation — for example, a non-rotating cylindrical boss that must be coaxial with a datum bore for assembly clearance. Position with RFS (no modifier) controls the axis location directly, is measurable on a CMM, and avoids the median-point calculation entirely.
| Old Control | Functional Requirement | Preferred Replacement |
|---|---|---|
| Concentricity ◎ | Coaxiality of rotating surface | Circular Runout ↗ or Total Runout ⇗ |
| Concentricity ◎ | Coaxiality of non-rotating cylinder | True Position ⊕ (RFS) |
| Symmetry ≡ | Centeredness of planar feature | True Position ⊕ (RFS) for center plane |
| Symmetry ≡ | Dynamic balance about center plane | Runout relative to datum plane |
When Old Callouts Still Appear
Despite the ASME removal, engineers will continue to encounter concentricity and symmetry on drawings for years:
- Legacy drawings: Existing drawings from products designed under ASME Y14.5-1994 or Y14.5M-1982 still use these symbols. The drawings remain valid unless formally revised.
- ISO drawings: ISO 1101:2017 retains both symbols. European, Japanese, and many international drawings may use concentricity and symmetry per ISO, where the measurement methodology is defined through ISO’s associated verification standards.
- Customer-mandated formats: Some customers require specific GD&T callouts and may specify concentricity in their design standards without updating to the 2018 ASME revision.
When encountering a concentricity callout on a drawing — whether legacy or ISO — the inspector must use the median-point methodology per the applicable standard. If the drawing is being revised for another reason and the design engineer has the opportunity, the callout should be updated to position or runout with an ECO, with an explanation in the revision description.
Practical Example: Shaft Coaxiality
A two-diameter stepped shaft has a small-diameter end journal (datum A) and a large-diameter section that must be coaxial with datum A. The design intent is that the large diameter runs concentrically on the same axis as the journal in its bearing.
Old callout: ◎ | ∅0.05 | A |
Modern replacements:
- If the part rotates and surface behavior during rotation matters: | ↗ | 0.05 | A | (circular runout — simple TIR measurement with indicator and V-block on datum A)
- If only axis coaxiality location matters (not surface behavior during rotation): | ⊕ | ∅0.05 | A | (position of the large diameter axis relative to datum axis A)
In the rotating shaft case, runout is always preferred — it directly measures what matters functionally (surface deviation during rotation) using simple equipment available in any machine shop, without requiring a CMM or complex median-point calculation.
Conclusion
Concentricity and symmetry were well-intentioned controls that proved too difficult to measure reliably in production. Their removal from ASME Y14.5-2018 is not a loss of functional capability — position and runout controls cover all the functional requirements that concentricity and symmetry were intended to address, with better measurability, lower inspection cost, and less ambiguity. For engineers maintaining or interpreting legacy drawings or working with ISO-drawn parts, understanding what concentricity and symmetry mean geometrically, how they are measured, and what modern alternatives exist ensures correct interpretation and smooth transition to best-practice GD&T.
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