A bolt loaded on and off usually breaks in the thread, where the stress is concentrated. Rolling cold-works the root and leaves it in compression, much like shot peening, and the grain of the bar follows the thread instead of being cut across. In one published test on M12 quenched-and-tempered alloy bolts, rolled threads reached about two and a half times the fatigue strength of cut threads.
When the thread is rolled matters. Heat treatment after rolling recrystallises the root and removes the compressive stress, so the full benefit comes from rolling after the bar is hardened and tempered. ASTM A962, behind A193, A194 and A320, requires threads to be formed after heat treatment unless the parts are heat treated in controlled-atmosphere furnaces.
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model dataTwo cylindrical dies turning the same way with the blank on a work-rest blade between them, one die fed in, and dwell turns that round the thread: published thread-rolling practice (US Patent 350,906; university and maker technical papers). Through-feed: die axes skewed in opposite directions, by the thread's lead angle when the dies carry plain rings; the work feeding about one lead per turn; threads longer than the dies; a lead-in taper and an exit taper on the dies; feed-in rollers and a roller trough out: US Patents 3,800,580, 3,651,678, 4,408,418 and 4,463,588, and maker and trade technical literature. Blank at about the pitch diameter: ASTM F1554 Table 4 sets the minimum body diameter under a rolled 1-1/4 in. 8 UN Class 2A thread at its minimum pitch diameter, 1.1597 in. Class 2A and the 8 UN series from 1-1/8 in.: ASTM A962. Root compression and fatigue: NASA RP-1228. The stud is drawn as a 1-1/4-8 UN-2A thread, 6 in. long, and the through-feed bar as the same thread, 12 ft long in this example; the lead angle, about 2°, is worked out at the pitch diameter. The machine, dies, blade, supports and gauges are generic. The infeed dies are drawn as a multi-start thread of the opposite hand, which meshes with a right-hand thread without moving it along its axis; the through-feed dies as plain rings with a small lead correction so they sit in the bar's roots across the whole face. With ring dies the tilt follows the thread's hand, so the bar is sent from front to back by running both dies the other way round. The rolling is slowed down, the through-feed run and the gauging sped up (at that speed the die holes are drawn blurred); no speeds, forces or die sizes are given, and the enlarged sections are schematic.
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