A cut thread and a rolled thread can measure identically and still differ by an order of magnitude in fatigue life. The difference is not geometry. It is what happened to the metal. Cutting removes material. A tool or die severs the grain flow of the drawn bar, leaving fibre ends exposed at the thread root and a machined surface stippled with tool marks. Every one of those marks is a crack initiation site sitting exactly where the stress concentration is highest. Rolling removes nothing. The blank starts near the pitch diameter, smaller than the finished major diameter, and hardened dies press into it, displacing metal plastically. Material squeezed out of the root flows upward to form the crest. Nothing becomes swarf, and the grain flow bends to follow the thread contour rather than being sliced through it. Three things change at once: - Work hardening. Cold plastic flow multiplies dislocation density through the root region, raising local yield strength well above that of the par...