10.4 Coaxial Feature Controls, Concentricity, and Symmetry
Key Takeaways
- Y14.5-2009 offers four ways to control coaxiality: position, runout, profile, and concentricity, and the Senior exam tests which one the function demands.
- Concentricity controls the median points of diametrically opposed elements and symmetry controls the median points of opposed surfaces; both are RFS only and neither accepts a material condition modifier.
- Position applied to a coaxial feature of size controls the derived median line or plane and does accept MMC, which is why it is almost always the cheaper and more functional choice.
- Runout controls the coaxial surface itself as it rotates about a datum axis, so it is the right answer for sealing, bearing, and balance-of-rotation problems.
- Y14.5-2018 deleted concentricity and symmetry entirely, so a 2018 candidate must reach for position, runout, or profile instead.
Coaxial Feature Controls, Concentricity, and Symmetry
Quick Answer: Four controls can hold coaxiality under Y14.5-2009. Position controls the derived median line and accepts MMC. Runout controls the surface as it rotates. Profile controls the surface relative to a DRF. Concentricity controls median points, is RFS-only, and is almost always the wrong answer. Y14.5-2018 deleted concentricity and symmetry.
Coaxial feature controls and tolerancing for symmetrical relationships are explicitly named on the Y14.5-2009 Senior body of knowledge under Tolerances of Location. They are also the purest test of what the Senior level actually measures: not whether you can read a symbol, but whether you can select the control the function demands and defend it against three plausible alternatives.
The four ways to hold coaxiality
| Control | What it controls | Modifier allowed | Verification |
|---|---|---|---|
| Position | The derived median line of the coaxial feature of size, relative to a datum axis | MMC, LMC, RFS | Functional gage at virtual condition, or CMM |
| Runout (circular or total) | The surface itself as the part rotates about the datum axis | RFS only | Indicator sweep while rotating |
| Profile of a surface | The surface relative to a datum reference frame, without rotation | Applies to the surface | CMM against the true profile |
| Concentricity | The median points of diametrically opposed elements | RFS only | Point-cloud median analysis; expensive |
The four are not interchangeable, and choosing among them is the classic Senior question.
Concentricity: what it actually means
Concentricity requires that all median points of diametrically opposed elements of the surface of revolution lie within a cylindrical tolerance zone whose axis is the datum axis.
Read that carefully. It is not about the surface. It is not about the derived median line of an actual mating envelope. It is about the midpoints of opposing element pairs, taken all over the feature. A part can be perfectly round and still fail concentricity if it is lobed such that its median points wander; a part can be badly out of round and still pass if the lobes are symmetric.
Consequences that make concentricity expensive:
- It is RFS only. No MMC modifier, so no bonus tolerance, ever.
- It cannot be verified with a functional gage, because a gage checks a boundary and concentricity checks a statistical property of median points.
- Verification requires enough measured points, in enough cross-sections, to compute median points reliably — which in practice means a CMM routine written for the purpose.
When concentricity is genuinely right: when the function depends on mass distribution rather than on a surface. Dynamic balance of a high-speed rotor is the canonical case — a balanced shaft needs its mass centred, and median points are a proxy for mass distribution in a way that an outer surface is not.
When it is wrong (which is most of the time): whenever the function depends on assembly clearance (use position at MMC), on how the surface runs (use runout), or on where the surface sits (use profile).
Symmetry: the planar analogue
Symmetry applies the same idea to opposed surfaces rather than opposed elements of a revolution. It requires the median points of all opposed surface points to lie within two parallel planes, symmetrically disposed about the datum centre plane.
Symmetry shares every drawback: RFS only, no bonus, no functional gage, expensive verification. The functional alternative is position applied to the width as a feature of size, which controls the derived median plane, accepts MMC, and can be gaged.
The Senior decision procedure
Ask three questions in order:
- Does the part rotate in service, and does the function depend on that rotation? Sealing lips, bearing journals, running clearances, wobble. → Runout. Circular runout for individual circular elements, total runout for the whole surface.
- Does the function depend on assembly — does something have to fit? → Position, almost certainly at MMC, so that departure from MMC earns bonus and a functional gage can verify it.
- Does the function depend on the mass distribution or on true median-point symmetry, independent of the surface? → Concentricity or symmetry, accepting the cost.
If the answer to question 3 is anything less than an unambiguous yes, do not specify concentricity. That is the judgment the exam is testing.
Coaxial holes and multiple features
A separate coaxiality case is a pattern of coaxial holes — a bore that passes through two or three walls of a housing. Options:
- Position on each hole to a common DRF. Each hole is located individually; the alignment between them is a consequence, not a requirement.
- Position applied to the coaxial features as a single requirement. One feature control frame governs the group, so the holes are held coaxial to one another as well as located.
- A composite frame. The upper segment locates the group to the DRF; the lower segment holds coaxiality among the members more tightly. This is usually the answer when the shaft passing through matters more than the group's absolute location.
Y14.5-2018: both symbols deleted
Y14.5-2018 removed concentricity and symmetry from the standard. There is no replacement symbol; the working group's position was that the controls were consistently misapplied where position, runout, or profile were the correct choice, and that the rare mass-distribution case is better handled with an explicit balance requirement.
For the exam this means:
- 2009 Senior candidates must know both controls, their definitions, their RFS-only restriction, and when each is correct.
- 2018 Senior candidates must know that both are gone, and that a question naming concentricity is testing exactly that.
Traps
- Confusing concentricity with runout. Runout controls the surface during rotation; concentricity controls median points and is indifferent to surface form.
- Applying MMC to concentricity or symmetry. Neither accepts a material condition modifier under Y14.5-2009.
- Choosing concentricity for an assembly-clearance problem. Position at MMC is the functional and economical answer.
- Assuming a round part is automatically concentric. Roundness and concentricity are independent properties.
- Carrying 2009 habits into a 2018 drawing. Both symbols are deleted in 2018.
A shaft journal must run true against a lip seal while the shaft rotates at speed. Which control best matches that function?
Under Y14.5-2009, which statement about concentricity is correct?
A designer working to Y14.5-2018 needs the centre plane of a keyway held relative to a datum axis. What should the drawing specify?
A bore passes through three walls of a housing, and a shaft must slide through all three. Which specification best expresses that intent?