Why is Circularity used?
A lobed or out-of-round journal can create uneven bearing contact even when its measured diameters meet size limits.
FORM CONTROL / GD&T SYMBOL GUIDE
CIRCULARITY
Circularity, also called roundness, limits the form of each circular cross-section of a cylindrical or spherical surface. Each section fits between two concentric circles whose radial separation is the tolerance value.
INTERACTIVE APPLICATION
Circularity of a shaft section
NO SEPARATE CONTROL
The journal section can be visibly lobed
- No separate roundness limit is stated for each section.
Limitations
- A lobed or out-of-round journal can create uneven bearing contact even when its measured diameters meet size limits.
- Lobing can produce uneven contact and vibration.
CIRCULARITY CONTROL
Each section stays inside a roundness zone
- Each section fits between circles 0.03 mm apart radially.
- At each cross-section, two concentric circles enclose the surface profile; their radial gap is the tolerance.
Advantages
- Rounder sections support steadier contact.
- The drawing states the intended requirement for inspection and assembly.
Where is Circularity used?
Shaft journals, precision bores, valve spools and rotating sealing lands.
How is Circularity used?
Attach the circularity frame to the cylindrical surface. Inspect circular sections individually against the stated roundness zone.
The circularity frame applies to each circular cross-section of the cylindrical surface.
How to read the frame
Read the circularity symbol, then the 0.03 tolerance. The frame has no datum reference.
What the zone controls
At each inspected section, the actual profile must fit between two concentric circles separated radially by 0.03. It does not control the entire length at once.
Commonly defined variable values
The controlling value is a tolerance-zone width, not a universal process grade. For this circularity example, the drawing states the controlled feature and applicable extent.
Scroll the table sideways to see every column.
| Application | Illustrative starting value | Example feature | Notes / caution |
|---|---|---|---|
| General turned journal | 0.01–0.05 mm | Ø30 mm shaft | Check number of lobes sampled. |
| Ground bearing seat | 0.005–0.02 mm | Ø25 mm journal | Use suitable roundness metrology. |
| Machined bore | 0.03–0.10 mm | Ø50 mm bore | Inspect several depths. |
These are planning examples, not universal rules or guaranteed process capabilities. Set the actual tolerance from function, size, length, process capability, inspection method and the standard specified on the drawing.
How is circularity measured in real life?
Inspect without inventing a datum. Measure the indicated feature and compare it with the requirement shown on the drawing.
- Roundness instrument: Rotate the feature against a precision spindle and scan multiple axial sections.
- V-block + indicator: Use as a quick shop-floor screen; the setup can miss some lobe patterns and is not a complete roundness evaluation.
- CMM: Sample dense points around several cross-sections and evaluate each section separately.
ASME Y14.5-2018 (R2024)
ASME Y14.5 circularity is datum-free and section-wise. It does not replace size, straightness or full-surface cylindricity controls.
ASME Y14.5 edition reference. Examples are learning schematics; use the complete requirements on the drawing.
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