Circular Runout vs Roundness: Check the Datum Axis Evidence
Roundness describes the form of a circular feature by itself. Circular runout describes how a surface varies when the part is rotated about a specified datum axis. A surface can be locally round and still be displaced from that axis, so a good roundness result does not answer a runout requirement, and a runout number does not say which error contributed to it. To judge a runout result, find the axis the drawing requires, the stations where the surface was measured, and whether the coverage matches the callout.
Two controls, two questions
KEYENCE, a measurement-equipment maker, describes run-out tolerance as a geometric tolerance that specifies the fluctuation of a feature when the part is rotated on an axis. It states that a datum is always necessary to indicate run-out tolerance, so run-out is a tolerance for features related to datums. That datum is the axis the surface is measured around.
A roundness or circularity requirement works differently. It is a form control that concerns the shape of the circular feature itself, without reference to a datum. Check the drawing's invoked standard for the exact definition, because the governing meaning comes from the drawing language and not from a general description.
This difference is the reason the two results cannot stand in for each other. Roundness can be good when the whole feature is shifted off the datum axis. Runout can be large when the cause is that shift, when the cause is out-of-round form, or when it is a mix of both.
Circular and total runout cover different amounts of surface
KEYENCE separates two types. Circular run-out specifies the run-out of any part of a circumference when the part is rotated, so each measurement plane perpendicular to the axis is judged on its own. Total run-out specifies the run-out of the entire surface, so the whole cylinder surface is judged together. KEYENCE's examples are teaching illustrations with their own tolerance values. They are not this part's tolerance or an inspection procedure.
A report therefore has to say which control was measured. A set of readings taken at two or three planes can support a circular-runout statement for those planes only. It cannot support a whole-surface claim.
What the governing standard decides
ASME Y14.5-2018 (R2024) is described by ASME as the authoritative guideline for the design language of geometric dimensioning and tolerancing. It establishes symbols, rules, definitions, requirements, defaults and recommended practices for stating and interpreting GD&T on drawings and in digital models. Its restructured edition places form, orientation and profile in separate sections, and its changes include datum references and degrees of freedom.
That description is why a drawing's invoked standard and revision matter. How the datum axis is established, including the precedence of datum features, comes from the drawing and its governing standard. If a drawing uses another system, such as ISO GPS, do not translate symbols by appearance.
Build an axis, station and coverage map
Use the map below to check a runout result against its requirement. It is a recommended review aid, not a form required by any standard.
| Field | What the drawing or report must show | What goes wrong if it is missing |
|---|---|---|
| Controlled feature | The surface the tolerance applies to | The result may belong to a different surface |
| Datum feature and precedence | The datum feature or features that define the axis, in order | The axis used may differ from the required one |
| Control type | Circular runout or total runout, and its direction | A circular result is read as whole-surface |
| Axis establishment | How the part was located and rotated about the axis | A convenient axis replaces the required one |
| Stations or coverage | Which planes were measured, or whether the whole surface was | Coverage is unknown |
| Measurement direction | Radial or another direction, per the callout | The reading may not match the control |
| Reported result | The original reading at each station | A single maximum hides where it occurred |
| Roundness evidence | Any separate form result, kept separate | Form and axis errors get blended |
| Uncertainty and open points | Stated uncertainty and any unresolved datum interpretation | Gaps are treated as passes |
A runout reading taken about a different axis, such as one fitted to the feature itself or to a convenient fixture, is a different measurement, even when the reading is small. It should not be reported as meeting a datum-axis requirement unless the drawing allows it.
Request the evidence you need
Ask for the runout result by station, tied to the datum axis the drawing names, and keep any roundness result alongside it as a separate record. Do not try to reconstruct roundness from diameter or runout readings, and do not call every runout deviation eccentricity, since form error can contribute. Diameter checks and roundness are covered in why a two-point diameter check can miss an out-of-round bore.
For a part from KTSU, provide the drawing, the revision and the measurement report you hold when asking what documentation is available. The public excavator undercarriage parts collection identifies the product family. It does not establish a runout capability or a tolerance level for any part.
The request is complete when the drawing's datum axis, the control type, the measured stations and the coverage are written down, the roundness evidence is kept apart, and any unresolved datum interpretation is named.