⚡ Key takeaways
- Ninety percent of standard fastener threads are rolled, not cut. Two threaded dies squeeze the blank and form the thread in about a second, and the result is 20–30 % stronger than a cut thread.
- Why rolling is stronger. Cutting severs the metal fibres; rolling flows them along the thread profile, so the grain structure stays continuous, fatigue strength rises 20–30 % and brittle fracture is less likely.
- Better accuracy and surface. Class 6g is routine, with Ra 0.8–1.6 μm against Ra 3.2 μm for a cut thread.
- Extremely low cost. A die set lasts 100,000–500,000 pieces, so the thread costs a fraction of a percent of a turned thread.
- Material limits apply: elongation of at least 8 % and hardness no more than HRC 32. Hardened or brittle materials cannot be rolled.
- Two processes: flat-die rolling for simple parts, and cylindrical (two- or three-roll) rolling for high accuracy. Flat dies below M6; roll machines above M6 or where precision matters.
- Rolling work-hardens the surface. Thread hardness rises 10–20 HV, improving wear resistance.
Contents
1. How thread rolling works
2. Rolling versus cutting
3. Flat-die versus cylindrical rolling
4. Suitable materials
5. Common defects
6. Why your screw is rolled
2. Rolling versus cutting
3. Flat-die versus cylindrical rolling
4. Suitable materials
5. Common defects
6. Why your screw is rolled
1. How thread rolling works
Rolling is cold bulk forming:
- An annealed wire or blank, at roughly the pitch diameter, is placed between two threaded dies.
- One die reciprocates while the other is fixed.
- The dies press into the surface and the metal flows plastically into the die grooves.
- After one or two seconds the thread is complete.
There is no chip, so material utilisation is effectively 100 %.
2. Rolling versus cutting
| Metric | Rolled | Cut (turning or milling) |
|---|---|---|
| Strength | Higher (+20–30 %) | Baseline |
| Fatigue life | Higher (+50 %) | Baseline |
| Accuracy | 6g consistently | 6g achievable but variable |
| Surface roughness | Ra 0.8–1.6 | Ra 3.2–6.3 |
| Cost per thread | About 1 % of turning | Baseline |
| Material utilisation | 100 % | 60–80 % |
| Economic batch | Above 10,000 | Below 1,000 |
💡 Engineer's instinct: a production screw almost certainly has a rolled thread. Only prototypes, very large sizes and special profiles are cut.
3. Flat-die versus cylindrical rolling
| Process | Machine | Accuracy | Size | Use |
|---|---|---|---|---|
| Flat-die rolling | Flat-die roller | 6g | M1–M12 | Standard bolts and screws |
| Cylindrical rolling (2 or 3 rolls) | Thread roller | 4h–6g | M6–M100 | Precision parts, lead screws, hydraulic fittings |
4. Suitable materials
| Material | Elongation | Hardness | Rollable |
|---|---|---|---|
| Low-carbon steel 1008/1010 | 30–40 % | HRB 60 | ✓ easiest |
| Medium-carbon 1045 | 15–20 % | HRB 80 | ✓ |
| Alloy SCM435 | 15 % | HRB 90 | ✓ annealed |
| Stainless 304 | 40 % | HRB 85 | ✓ but dies wear faster |
| Aluminium 6061 | 12 % | HB 95 | ✓ |
| Hardened steel (HRC 40+) | 5 % | HRC 40 | ✗ too hard |
| Cast iron | < 1 % | HB 200 | ✗ too brittle |
⚠️ Never roll after hardening. Class 8.8 and above are rolled first and then quenched and tempered. Reverse the order and the thread cracks.
5. Common defects
- Torn thread: material too hard or dies worn.
- Oval thread: wrong blank diameter.
- Burr at the ends: incorrect die entry angle.
- Surface cracking: material too brittle, insufficient elongation.
6. Why your screw is rolled
Take any screw apart and look along the flank: you will see a continuous grain flow rather than a helical tool mark. That is the signature of rolling, and it is why standard bolts are both cheap and strong.
FAQ
Q. Can every thread be rolled? Above M3 at volume, yes. Below M1, or for single pieces, cutting is used.
Q. How long do dies last? 50,000–100,000 pieces for standard steel on steel; carbide dies reach one to five million.
Q. Is heat treatment needed afterwards? Rolling happens before heat treatment, otherwise the part is too hard. High-strength parts are rolled and then quenched and tempered.
🛠️ Engineering support: Toprecision has 20 years of rolling experience from M0.6 to M30 at 6g–4h accuracy. Send us the thread specification and volume for a quotation within 24 hours.
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👉 Engineering services ｜ 👉 Request a technical quote