9.4 Concentricity & Symmetry Tolerances Under ASME Y14.5-2009

9.4 Concentricity & Symmetry Tolerances Under ASME Y14.5-2009

Quick Answer: Concentricity and symmetry are active, testable geometric characteristics on the Y14.5-2009 exam — they were not removed until Y14.5-2018. Para. 7.6.4: concentricity is the condition where the median points of all diametrically opposed elements of a surface of revolution (or of correspondingly located elements of two or more radially disposed features) are congruent with a datum axis or center point. The tolerance is a cylindrical (or spherical) zone whose axis or center point coincides with the datum axis or point, containing those median points. Para. 7.7: symmetry is the same concept applied to a different part configuration, controlling median points of opposed elements of two or more feature surfaces relative to a datum axis or center plane. Both controls can be applied only RFS, and their datum references only RMB.

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What Makes These Two Controls Different From Everything Else

Every other location control in Section 7 evaluates either a surface or the axis or center plane of an actual mating envelope. Concentricity and symmetry evaluate neither. They evaluate a cloud of median points — the midpoints of diametrically opposed surface elements — and require that cloud to lie within a zone centred on the datum.

           POSITION (coaxial)                    CONCENTRICITY (7.6.4)

    ┌───────────────────────────┐         ┌───────────────────────────┐
    │  ●═══════════════════●    │         │  ○  ○   ○  ○   ○   ○  ○   │
    │  axis of the UNRELATED    │         │  median points of DIAMETRICALLY
    │  ACTUAL MATING ENVELOPE   │         │  OPPOSED elements
    │  must lie in ⌀t zone      │         │  must ALL lie in the ⌀t zone
    └───────────────────────────┘         └───────────────────────────┘
    A gage pin can check this.            No hard gage can check this.
    Modifiers permitted.                  RFS only, datum RMB only.

The Form-Insensitivity Consequence

Because concentricity averages opposed points, a feature can be badly lobed, ovate, or wavy and still be perfectly concentric, provided its errors are symmetric about the datum axis. Conversely, a perfectly round feature that is displaced fails. The control isolates mass distribution about the axis, which is exactly the requirement for dynamic balance in a high-speed rotor — and exactly the wrong requirement for an assembly fit.

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Concentricity (Para. 7.6.4)

Para. 7.6 frames the design choice explicitly. Coaxial relationships may be controlled by positional tolerance, by runout, or by concentricity, and para. 7.6.1 covers the selection among them:

• Position where the requirement is assembly — permits Ⓜ, permits bonus, gages easily.
• Runout where the requirement is rotational behaviour of the surface — full indicator movement, mandatory RMB, cheap to inspect with an indicator.
• Concentricity where the requirement is genuinely about median-point distribution, most often dynamic balance.

The exam's favourite comparison: given a shaft journal that must run true in a bearing, the correct answer is runout, not concentricity — runout catches the surface error that the bearing actually feels, while concentricity is blind to it.

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Symmetry (Para. 7.7)

Symmetrical relationships may be controlled using either positional, profile, or symmetry tolerances, but significant requirement differences are established by these controls. Positional tolerancing for symmetrical relationships establishes a requirement where the center plane of the unrelated actual mating envelope of one or more features is congruent with a datum axis or center plane within specified limits. Symmetry tolerance instead controls median points of opposed elements of two or more feature surfaces relative to a datum axis or center plane. The explanation given in para. 7.6.4 applies, since symmetry and concentricity controls are the same concept except as applied to different part configurations. Symmetry tolerance can only be applied RFS and the datum reference can only be applied RMB.

The Three Ways to Control a Symmetrical Relationship

Design guidance the standard implies: because symmetry tolerance is RFS-only, datum-RMB-only, and not gageable, positional tolerancing at MMC is the default choice for symmetrical relationships that exist for assembly. Reserve symmetry tolerance for the rare case where median-point distribution is genuinely the functional requirement.

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Revision Awareness: A 2009-Exam-Only Topic

This is one of the few places where studying from current shop practice will actively mislead a Y14.5-2009 candidate. Y14.5-2018 eliminated both symbols, reducing the geometric characteristic set from 14 to 12, on the reasoning that median-point controls were rarely applied correctly and were difficult to verify. Modern training material, modern CAD symbol libraries, and colleagues trained on the 2018 revision will all tell you these controls no longer exist.

On the Y14.5-2009 Technologist exam they exist, they are examinable, and the 14-symbol count is correct. Expect at least one item that turns on the RFS/RMB-only restriction, and at least one that asks you to distinguish concentricity from coaxial position or from runout.