Runout Thread
Is thread runout the same as spindle runout?
No. Thread runout usually means the transition zone at the end of a thread, while spindle or tool runout means rotational deviation of the machine tool, holder, or workpiece from its true axis.
When should I specify runout on a threaded feature?
Use it only when the design intent is to control the rotating surface relative to a datum axis; for thread location and coaxiality, position tolerance is often preferred.
What shop feature reduces thread runout issues most reliably?
A properly sized thread relief groove or undercut that gives the thread tool room to exit and gives the mating part full seating clearance.
Thread runout is the transition zone at the end of a threaded section where the full thread profile fades into an unthreaded shank or undercut. In GD&T, runout also means rotational deviation of a surface relative to a datum axis, but for threads it is often a poor control compared with position tolerance.
In CNC turning and thread milling, the thread runout zone is the tapered remnant where the full thread form disappears into a relief groove or unthreaded shank. A machinist programs an undercut so the tool can exit cleanly before a shoulder, and so a nut or mating part can travel far enough to clamp against the seating face. If that relief is missing or too short, the assembly bottoms out on partial threads, leaving a gap and reducing preload. In inspection, runout on a threaded feature is sometimes called out as a GD&T control, but it mixes lead error, form error, and pitch-diameter variation into a single reading. Position tolerance is usually the clearer way to control thread axis location. The practical floor concern is getting the transition length right and keeping spindle and toolholder runout tight enough to produce symmetrical thread flanks.
No relief at a shoulder: The thread ends too abruptly, so the mating nut or part stops on partial threads before seating fully against the shoulder; causing preload loss, stack-up error, or visible gap at assembly.
Using runout as the wrong GD&T control: Specifying circular runout on a thread when the real requirement is thread axis location produces confusing inspection results because runout blends lead error, form error, and pitch-diameter variation into one reading.
Tool-path or spindle eccentricity during thread generation: If the tool, holder, or spindle has runout, one side of the thread cuts heavier than the other, distorting pitch diameter, worsening flank contact, and reducing gageability or fatigue performance.