Formed vs removed. Compressed vs stressed. Fatigue matters.

Three reasons: (1) Compressive residual stress in thread roots — the most stressed location during loading. Compression must be overcome before tensile stress initiates fatigue cracks. (2) Surface finish — rolled threads have Ra 0.4-0.8 µm vs 1.6-3.2 µm for cut. Smoother surface reduces fatigue crack initiation sites. (3) Grain flow — rolled threads have continuous grain structure around thread profile. Cut threads have interrupted grain where material was removed. Combined effect: 2-4× better fatigue life.

No. Rolled threads require: (1) Ductile material — cold-formable without cracking. Steels up to ~40 HRC, aluminum, brass, copper, some nickel alloys. Not: very hard materials (60+ HRC), brittle materials (cast iron, some bronzes), some tool steels after hardening. (2) External threads only (form tapping for internal is limited). (3) Threads ending before part features (thread roll dies have fixed length). For most aerospace bolts, rolled threads work. For highly hardened fasteners, cutting after hardening required.

Rolled threads: fast (1-3 seconds per thread vs 30-60 seconds for cutting). Equipment expensive ($50K+ machine) but amortizes over volume. Cut threads: slower but cheap equipment, flexibility. For production volumes 10,000+/year: rolled always cheaper. For prototype or <1000/year: cutting economical. For specialty threads not standard: cutting (custom roll dies expensive).

Cannot roll threads on hardened steel — material too hard to cold-form. Standard sequence: (1) Machine part near-final dimensions. (2) Roll threads while still soft. (3) Heat treat (harden + temper). Results: threads retain rolled properties through heat treatment. Note: heat treatment can slightly distort threads — precision applications may need post-heat-treat thread grinding. For hardness >45 HRC: usually thread grinding after hardening.

Form tapping (roll tapping) exists for internal threads in ductile materials. Uses specialty form tap instead of cutting tap. Benefits: stronger threads, better finish, no chips produced. Limitations: harder to break tap if hole is undersized (form tap stronger). Works best in: aluminum, brass, mild steel. Not suitable for: hardened steel, cast iron, brittle materials. Increasingly used in high-volume manufacturing for internal threads.

Default drawing thread callout (like "M10x1.5") doesn't specify rolled vs cut. For fatigue-critical applications, add note: "Rolled threads per AMS 2760" or "Threads shall be rolled" on drawing. For aerospace: typically specified. For general: left to manufacturer discretion — we choose based on quantity and material. If rolled threads required, specify explicitly.