A groove at the end of a thread that lets the tool run out and the nut seat fully.
Without a relief groove, a threading tool has nowhere to stop — it either runs into a shoulder and breaks, or the thread fades out before reaching it and the nut cannot seat against the face.
A relief groove also creates a stress concentration, so on fatigue-critical parts a rounded runout is preferred over a square groove. The trade-off is that a rounded runout does not let the nut seat, so it is only usable where the thread does not need to bottom against a face.
Omitting the relief and expecting the nut to seat against the shoulder. The thread cannot be cut all the way to a square corner.
This site uses Google Analytics to count visits and see which pages are useful. It sets cookies only if you agree. See the privacy policy.