โก Key takeaways
- Cold heading is the core fastener process. Steel wire is upset at room temperature by a punch, at 100โ300 parts per minute.
- Tooling is the cost centre in cold heading. A die set runs 50,000 to 5,000,000 pieces, and material and design decide both the cost and the yield.
- Three tooling tiers: HSS SKH-9 for general work, cold-work DC53 for medium loads, cemented carbide for long life.
- About 80 % of tooling failures are fatigue and wear, not fracture.
- Three design rules: reduce stress concentration, keep lubrication adequate, match tool material to the deformation.
- Multi-station heading splits forming, piercing and reducing across stations, each with its own die.
- Tooling is 10โ30 % of fastener cost, amortised over the batch โ painful on small runs, negligible on large ones.
1. A punch that kept shattering
2. How cold heading works
3. The three tooling materials
4. Tooling structure
5. Life and failure modes
6. Design rules that matter
7. Cost, decision tree and FAQ
1. A punch that kept shattering
In 2022 a fastener plant in eastern China was running an M6 ร 20 socket head cap screw in 10B21. Two hours into the shift the punch shattered. It was replaced, ran another hour, and shattered again. Five punches in a row.
The investigation found three things:
- The punch was SKH-9, but the forming reduction was far too aggressive โ a head upset ratio of about 3:1.
- The punch had no relief taper and no radius at the transition, so stress concentrated exactly where it broke.
- Lubrication was insufficient, so the punch ran hot and lost hardness at the tip.
The fixes were a cemented carbide punch (YG20C), an R 0.5 relief, and a higher oil spray frequency. Tool life went from 20,000 pieces to 500,000.
The lesson: cold heading tooling is not a question of "hard enough steel". It is a system of material, geometry and lubrication. This article draws on VDI 3355, cold heading tooling handbooks and twenty years of production experience.
2. How cold heading works
Cold heading feeds coiled wire into a multi-station former at room temperature:
- Feed and cut: wire is fed automatically and sheared to a fixed length.
- Square up: the first station flattens the cut end.
- Form: the second station upsets the head โ hex, cylindrical or countersunk.
- Pierce: the third station punches the hex socket.
- Reduce: the fourth station reduces the shank to the thread diameter.
Everything happens at room temperature, so the steel flows plastically. The larger the deformation, the higher the load on the tooling.
Upset ratio
Upset ratio = head cross-section รท wire cross-section. Rules of thumb:
- Up to 2:1 โ safe with standard tooling.
- 2:1 to 2.5:1 โ moderate; tooling quality starts to matter.
- Above 2.5:1 โ severe; needs cemented carbide or a two-step forming sequence.
3. The three tooling materials
| Material | Hardness | Life (thousand pieces) | Relative cost | Typical use |
|---|---|---|---|---|
| SKH-9 (HSS) | 62โ64 HRC | 50โ200 | 1ร | General work, small batches |
| DC53 (cold-work die steel) | 60โ62 HRC | 100โ300 | 1.5ร | Medium load, complex shapes |
| YG20C (cemented carbide) | 88 HRA | 500โ3,000 | 5โ10ร | High volume, long life |
| ASP23 (powder metallurgy HSS) | 64โ66 HRC | 300โ1,000 | 4ร | Demanding geometry |
How to choose
- Under 100,000 pieces: SKH-9 is enough, and the lowest total cost.
- 100,000 to 1,000,000 pieces: DC53 or ASP23 โ the extra hardness and toughness pay for themselves.
- Over 1,000,000 pieces: cemented carbide. Expensive up front, cheapest per piece.
The crossover is not fixed. It shifts with part geometry, wire hardness and how well the machine is maintained. A shop running three shifts with poor lubrication will not see carbide's theoretical life, and the economics move.
4. Tooling structure
4.1 Main die (female)
Forms the outside of the head. The bore accuracy sets the visible appearance of the part. Typically cemented carbide or DC53 in a shrink-fit steel case.
4.2 Punch (male)
Presses the shape from the front. The punch is the part that breaks most often, because it sees the full alternating load of every stroke.
4.3 Ejector pin
Pushes the formed part out of the die. On small parts the ejector pin is the weak link and breaks under side load.
4.4 Socket punch
Cuts the hex socket. It sees an eccentric load and is prone to breakage if the punch is not guided precisely.
5. Life and failure modes
| Failure mode | Cause | Countermeasure |
|---|---|---|
| Chipping or shattering | Stress concentration plus impact | Add radii, use a tougher grade |
| Wear | Repeated sliding contact | TiN or similar coating, cemented carbide |
| Fatigue cracking | Alternating load | Lower surface roughness, shot peening |
| Pick-up / galling | Poor lubrication | More oil, phosphate the wire |
| Bore growth | Wear plus thermal growth | Carbide insert, better cooling |
6. Design rules that matter
6.1 Reduce stress concentration
- Radius every internal corner at R 0.3 mm or more.
- Give the punch a relief taper of at least 1ยฐ.
- Avoid abrupt changes in cross-section; blend them over a length.
6.2 Lubrication
Cold heading uses phosphate plus soap coating on the wire, or an oil-based lubricant applied at the die. Poor lubrication is the single largest cause of tooling failure, and it is also the cheapest to fix.
6.3 Match material to the workpiece
Stainless work-hardens far more than carbon steel, and the tooling load is roughly 1.5ร higher. Stainless almost always justifies cemented carbide, even at moderate volumes.
7. Cost, decision tree and FAQ
Step 1: what is the batch size?
Under 100,000 โ SKH-9. 100,000 to 1,000,000 โ DC53. Over 1,000,000 โ cemented carbide.
Step 2: what is the workpiece material?
Carbon steel โ standard tooling. Stainless โ carbide. Aluminium โ standard tooling, with attention to pick-up.
Step 3: how complex is the shape?
Multi-station with large reduction โ carbide. Simple shape with modest reduction โ SKH-9.
FAQ
Q: Why does my tooling keep breaking punches? Nine times out of ten it is excessive reduction combined with stress concentration. Reduce the reduction, add radii, and move to carbide if the geometry cannot change.
Q: Is carbide tooling actually worth it? It costs 5โ10ร more and lasts 10โ20ร longer. Above roughly a million pieces the total cost is clearly lower; below that, the amortisation does not work.
Q: Cold heading or hot forging? Cold heading for small carbon steel parts โ it is faster, gives better surface finish and holds tolerance. Hot forging for large sizes and materials that cannot be deformed cold.
Q: Can tooling be repaired? Worn dies can be re-ground and re-coated. A shattered punch is scrap, because the crack path cannot be reliably removed.
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