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

ASME GDTP Technologist Y14.5-2009

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1 Chapter 1: Scope, Certification Logistics, Fundamental Rules & General Dimensioning

1.1 ASME GDTP Technologist Exam Scope, Prometric Logistics, & Certification Rules 1.2 Fundamental Rules of ASME Y14.5-2009 (Rules a through p) 1.3 Engineering Drawing Standards, Drawing Units, & Basic vs. Toleranced Dimensions 1.4 Dimensioning Features: Diameters, Radii, Chamfers, Countersinks, Counterbores, Spotfaces, Keyseats & Slots 1.5 Application of Dimensions & Location of Features: Lines, Leaders, Reference Dimensions, Rectangular vs. Polar Coordinates

2 Chapter 2: Features, Limits of Size, & Material Condition Principles

2.1 Features, Features of Size, & Limits of Size 2.2 Rule #1 (Taylor Principle / Envelope Rule), Its Exceptions & Overrides 2.3 Material Condition Concepts: MMC, LMC, & RFS Calculations

3 Chapter 3: General Tolerancing & Symbology

3.1 Direct Tolerancing, Limit Dimensioning, & Angular Tolerancing 3.2 The 14 Geometric Characteristic Symbols, Feature Control Frames, & Special Modifiers 3.3 Screw Threads, Gears & Splines, Angular Surfaces, Tapers & Statistical Tolerancing 3.4 Feature Control Frame Placement, Combined Frames, Tolerance Zone Definition & Tabulated Tolerances

4 Chapter 4: Datum Reference Frames & Degrees of Freedom

4.1 Theoretical Datums, Datum Feature Simulators, & Physical Datum Features 4.2 Degrees of Freedom Constraint & The 3-2-1 Datum Precedence System 4.3 Planar, Cylindrical, & Width Datum Feature Establishment

5 Chapter 5: Advanced Datum Applications: Targets, Boundary Modifiers, Customized Frames & Restrained Condition

5.1 Datum Target Points, Lines, Areas, & Movable Datum Simulators 5.2 Material Boundary Modifiers (RMB, MMB, LMB) & Datum Shift Calculations 5.3 Functional Datum Feature Selection, Multiple Datum Reference Frames & Simultaneous vs. Separate Requirements 5.4 Rotational Constraint About a Datum Axis or Point & Customized Datum Reference Frames 5.5 Irregular & Complex Datum Features, Mathematically Defined Surfaces, Restrained Condition & Datum Feature Controls

6 Chapter 6: Form Tolerances

6.1 Flatness & Straightness Controls for Surfaces & Features of Size 6.2 Circularity & Cylindricity Controls, Differences, & Inspection

7 Chapter 7: Orientation Tolerances

7.1 Parallelism Controls for Planar Surfaces & Cylindrical Features of Size 7.2 Perpendicularity & Angularity Tolerances with Basic Angles & Tangent Plane

8 Chapter 8: Location: Position Fundamentals & Calculations

8.1 Position Tolerancing Fundamentals, True Position, & Coordinate Conversions 8.2 Bonus Tolerance & Virtual Condition Calculations for Internal & External Features 8.3 Projected Tolerance Zones (Ⓟ Modifier) & Fixed vs. Floating Fastener Formulas 8.4 Zero Positional and Orientation Tolerancing at MMC and at LMC 8.5 Surface vs. Axis Interpretation of Position & the Boundary Concept for Slots, Tabs & Noncircular Features

9 Chapter 9: Location: Patterns, Composite Controls, Coaxial & Symmetrical Relationships, and Functional Gaging

9.1 Hole & Slot Patterns, Multi-Segment Frames, & Composite Position Tolerancing 9.2 Functional Gaging Principles & Inspection Verification of Position 9.3 Coaxial Holes, Counterbored Holes, Non-Parallel Axes, Spherical Features & Bidirectional Position 9.4 Concentricity & Symmetry Tolerances Under ASME Y14.5-2009 9.5 LMC Applied to Position: Wall Thickness, Breakout Protection & Boundary Calculations

10 Chapter 10: Profile Tolerances

10.1 Profile of a Line & Profile of a Surface Fundamentals & Basic Geometry 10.2 Bilateral, Unilateral, & Unequally Disposed Profile Boundaries (Ⓤ Modifier) 10.3 Composite Profile Tolerances vs. Multiple Single-Segment Profile Controls 10.4 Profile Verification, Coplanarity Controls, & Non-Rigid Parts 10.5 Profile Applications: Conicity, Extruded and Irregular Shapes, and Profile-Controlled Datum Features 10.6 Modifier Rules for Profile & Combined Controls (Profile with Position, Orientation and Runout)

11 Chapter 11: Runout Tolerances & Comparative Control Analysis

11.1 Circular Runout vs. Total Runout Interpretation & Dial Indicator Verification 11.2 Comparative Analysis: Runout vs. Form, Orientation, & Concentricity Controls

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

Key Takeaways Explain This Topic AI

ASME Y14.5-2009 para. 7.6.4 defines concentricity as the condition where the median points of all diametrically opposed elements of a surface of revolution are congruent with a datum axis or center point.

A concentricity tolerance is a cylindrical or spherical zone whose axis or center point coincides with the datum axis or point, and it can be applied only RFS with the datum reference only at RMB.

Para. 7.7 states that symmetry and concentricity are the same concept applied to different part configurations, and that symmetry tolerance can only be applied RFS with the datum reference only at RMB.

Symmetrical relationships may alternatively be controlled by positional tolerancing at MMC, by zero positional tolerancing at MMC which establishes symmetrical boundaries of perfect form, or by profile, all of which permit modifiers and are gageable.

Concentricity and symmetry remain valid geometric characteristics on the Y14.5-2009 exam and are counted among the 14 symbols, even though ASME Y14.5-2018 removed both and reduced the set to 12.

Last updated: September 2026

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.

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.

Concentricity (Para. 7.6.4)

Attribute Concentricity Under Y14.5-2009
Symbol Two concentric circles
Controls Median points of all diametrically opposed elements of a surface of revolution

Tolerance zone A cylindrical zone (or spherical , for a spherical feature) whose axis or center point coincides with the datum axis or point

Datum requirement Mandatory — a datum axis (or center point)
Material condition RFS only. Ⓜ and Ⓛ are prohibited on the tolerance
Datum material boundary RMB only. Ⓜ and Ⓛ are prohibited on the datum reference
Bonus tolerance None — RFS admits no bonus

Verification Requires full surface-point acquisition and median-point computation, effectively a CMM or a scanning instrument

Status in Y14.5-2018 Removed from the standard

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.

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

Method Para. What Is Controlled Modifiers Gageable?

Positional tolerance at MMC 7.7.1 Center plane of the unrelated actual mating envelope ; datum may be MMB, LMB, or RMB per design Yes — Ⓜ, Ⓛ, RFS Yes

Zero positional tolerance at MMC 7.7.1.1 Establishes symmetrical boundaries of perfect form ; variation permitted only as features depart from MMC toward LMC Yes Yes

Positional tolerance RFS 7.7.1.2 Center plane relationship regardless of actual sizes RFS Harder
Symmetry tolerance 7.7 Median points of opposed elements RFS only, datum RMB only No

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.

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.

Y14.5-2009 Y14.5-2018
Geometric characteristic symbols 14 12
Concentricity Present (7.6.4) Removed
Symmetry Present (7.7) Removed
Replacement guidance — Use position, profile, or runout