What Is a Runout?

What Does Runout Mean in GD&T?

A runout tolerance relates a surface to a datum axis, usually established from the diameters the part actually runs on, such as bearing journals. Without a datum axis there is nothing to rotate about, so runout always references one.

Because it checks the real surface rather than a derived center, runout picks up several errors at once: the surface being off-center from the datum axis, tilted relative to it, or out of round. What it never controls is size — that stays with the diameter's own tolerance.

Up to the 2009 edition, ASME Y14.5 defined runout by the full indicator movement (FIM) of a dial indicator on the surface while the part turns 360°. The 2018 edition describes the same tolerance zones geometrically, since Y14.5 is not an inspection standard, but how runout is applied and what it permits did not change.

Shops also use the word for spindle or tool runout — the wobble of a rotating cutter or chuck. That is the same idea applied to the machine rather than a callout on the part.

Circular Runout vs Total Runout

Runout vs Concentricity

Both aim at coaxial features, but concentricity checked only the midpoints of a feature against the datum axis and ignored its form. ASME Y14.5-2018 removed concentricity and lists circular runout, total runout, position and profile as the ways to control coaxial features. Runout is usually the easiest of those to verify on the shop floor.

How to Specify Runout

• Pick datum features the part actually runs on in assembly — two bearing journals, or a face plus a diameter — so the inspection setup matches function.
• Use circular runout where each section just needs to run true, such as a seal diameter; use total runout where the whole surface must be straight and true, such as a long bearing seat.
• Keep the runout tolerance tighter than the size tolerance only where you really need form control; otherwise it adds inspection cost for little benefit.
• For turned parts that will be finish-machined, hold critical diameters and faces in one setup where possible, which keeps them on the same axis.

Turned parts such as shafts, bushings and pins are produced by CNC machining at NICE Rapid, with critical dimensions verified by CMM or manual gauging.

Frequently Asked Questions

What is runout in GD&T? It is a tolerance that limits how much a surface varies as the part rotates 360° about a datum axis. It controls the surface's location, orientation and form relative to that axis, but not its size, and it is traditionally checked with a dial indicator.

What is the difference between circular and total runout? Circular runout checks each circular cross-section separately, so it ignores taper and changes along the length. Total runout checks the whole surface at once, so it also controls straightness, taper and cylindricity — or flatness and perpendicularity on a face.

What is the difference between runout and concentricity? Runout checks the actual surface as the part rotates, so it also catches out-of-roundness. Concentricity checked only the midpoints of the feature and ignored form. Concentricity was removed in ASME Y14.5-2018, and runout is the usual replacement.