⚡ Key takeaways
- Two families of thread production: cutting (turning or milling) and rolling (flat die or circular die). About 99 % of fasteners are rolled.
- Flat dies (thread rolling plates): two flat plates slide against each other. Best for small sizes M1–M12, high volume, standard tolerance.
- Circular dies (thread rolling wheels): two rotating discs. Best for M6–M60, high accuracy and long thread lengths.
- Rolled threads are 20–30 % stronger than cut threads because the grain flow follows the thread profile instead of being severed.
- Die material tiers: SKH-9 for general work, DC53 for medium load, cemented carbide for long life.
- Thread tolerance is set by die tolerance. 6g comes off a standard die; 4h needs a ground die.
- Die life runs 50,000 to 5,000,000 pieces, driven by die material, workpiece material and lubrication.
1. A thread that would not gauge
2. Flat dies versus circular dies
3. Die materials
4. Thread tolerance and die tolerance
5. Lubrication
6. Decision tree and FAQ
1. A thread that would not gauge
A plant was running an M6 × 1.0 screw. After thread rolling, 50 % of parts failed the GO/NO-GO gauge check. The investigation found a worn flat die that had not been replaced, so the pitch diameter had drifted large.
A new die set restored the pass rate immediately. The lesson is simple: thread rolling dies are consumables, and they must be replaced on a measured interval, not on a hunch.
2. Flat dies versus circular dies
| Dimension | Flat dies (plates) | Circular dies (wheels) |
|---|---|---|
| Size range | M1–M12 | M6–M60 |
| Production rate | 100–300 pieces/min | 20–60 pieces/min |
| Thread tolerance | 6g as standard | 4h to 6g |
| Long threads | Not suited — plate length limits | Well suited |
| Relative cost | 1× | 3× |
| Typical products | Bolts, machine screws | Studs, large bolts, long thread |
The split is mostly economics. Flat dies are faster and cheaper per piece, so anything that fits their size and length window goes there. Circular dies win when the thread is longer than about three times the diameter, or when the part is too large for a plate.
3. Die materials
| Material | Hardness | Life (thousand pieces) | Typical use |
|---|---|---|---|
| SKH-9 (HSS) | 62 HRC | 100–500 | Carbon steel, general work |
| DC53 | 61 HRC | 300–1,000 | Stainless and alloy steel |
| Cemented carbide | 88 HRA | 2,000–5,000 | High volume, stainless |
4. Thread tolerance and die tolerance
- 6g, the standard commercial class: a standard die produces it, and it covers the vast majority of fasteners.
- 4h, precision class: the die must be ground, which costs 2–3× more and shortens die life because the crest is sharper.
- Rolled threads are 20–30 % stronger than cut threads in fatigue, because the grain flows around the root instead of being interrupted by a cut.
- The blank diameter is as important as the die. An undersized blank produces an incomplete crest; an oversized one overloads the die and splits the part.
5. Lubrication
Thread rolling uses sulphurised or animal-fat-based lubricants. Poor lubrication shows up as torn thread flanks and rapid die wear, and it is often mistaken for a die quality problem. On stainless, lubrication is the difference between 50,000 and 500,000 pieces from the same die.
6. Decision tree and FAQ
Step 1: what size?
Below M12 → flat dies. Above M12 → circular dies.
Step 2: what material?
Carbon steel → SKH-9. Stainless → cemented carbide.
Step 3: what tolerance?
6g → standard die. 4h → ground die.
FAQ
Q: Rolled or cut threads — which is better? Rolled: higher strength, better surface finish, no chips, and lower material waste because the blank is smaller than the finished thread diameter. Fasteners use rolling almost exclusively. Cutting is reserved for threads that cannot be rolled, such as on a shoulder or into a blind hole.
Q: How long does a flat die last? Around 300,000 to 500,000 pieces in carbon steel at M6, and 50,000 to 100,000 in stainless.
Q: Why is my GO/NO-GO result inconsistent? Almost always die wear or the wrong blank diameter. Check the blank first — it is cheaper to fix than a die.
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