Runout Tolerances and Controls
Rotational variation
Runout controls the variation of a surface or feature as the part rotates about a datum axis. Circular runout controls variation in each cross-section; total runout controls the entire surface simultaneously. Both require a datum axis, typically established by a cylindrical datum feature.
Concepts to command
Know the tolerance zones: circular runout uses two concentric circles in each cross-section; total runout uses two coaxial cylinders (or parallel planes for a face). Understand that runout controls form, orientation, and location relative to the datum axis, but not size. Know that runout is always RFS (no MMC/LMC modifier permitted). Know how to establish the datum axis from a feature of size using the actual mating envelope.
Traps that appear in questions
- Applying an MMC or LMC modifier to runout (not permitted).
- Confusing circular runout (each cross-section independently) with total runout (entire surface at once; total runout also controls straightness and taper of the surface).
- Using runout without a datum axis, or with a datum that is not a feature of size.
- Assuming runout controls size (it does not).
Rule of thumb
Runout needs a datum axis and is always RFS. If a question mentions MMC with runout, the callout is invalid.
Worked example
A shaft with a cylindrical surface and total runout 0.05 to datum A (the shaft axis): the entire surface must lie within two coaxial cylinders 0.05 apart, centered on the datum axis. Circular runout 0.05 would allow each cross-section to pass independently, permitting taper.