Which Parts Suit Cold Heading: An Executable Set of Process Criteria

1. Breaking the question into five gates

"Can this part be cold headed" is a question asked far more often than it is answered. The two usual replies are extremes: "cold heading can do anything", or "cut a die and find out". The first is irresponsible; the second is expensive.

The question is in fact quantifiable. Split it into five gates that can be calculated or looked up, and the answer usually becomes obvious:

The order matters: first whether it can be made, then whether it is worth making. If any of the first four gates is clearly exceeded, go straight to turning or another process. Gate five is the classic source of "it can be made but not repeatably" — many cold heading yield problems are rooted in the incoming wire, not in the tooling.

2. Gate one: is the deformation within the material's capability?

Deformation measures how much the material has been changed in one forming step. For upsetting, the standard definition is the difference in cross-section before and after, divided by the cross-section after, equivalently expressed by height or diameter:

Here d is the original wire diameter, D the upset diameter and h the height. Under constant volume the three forms are equivalent; use whichever is convenient.

For example, φ5 mm wire upset into a φ10 mm head gives q = 1 − (5/10)² = 75 %. That is a substantial deformation, and whether it works depends entirely on the material.

The key point: every material has a deformation ceiling, and it falls steeply as carbon and alloy content rise. The data below comes from the classical experimental literature on cold heading.

How to use the table: convert the target shape into q and compare with the allowable for that material. If it is exceeded, there are two routes:

• Stage the deformation. Do not upset to final size in one blow; insert an anneal to restore ductility (recrystallisation softening) and continue. This is precisely why single, double, triple and four-blow cold formers exist.
• Change the incoming condition. Spheroidise anneal the wire and ductility improves markedly. The cost is an extra operation, but it is often the only way to cold form high-carbon steel at all.

Why deformation cannot be unlimited: cold heading is work hardening by definition. The larger the deformation, the further yield point, elastic limit and ultimate strength are pushed up, and the further elongation and reduction of area fall — ductility is being spent. Once it is insufficient to continue deforming, the material cracks. "Just press harder" is not an attitude problem, it is a physical one.

3. Gate two: how many blows

This gate decides machine type — single blow or multi-blow — and the number of die stations. The criterion is the free upset ratio S, the length to be deformed against the wire diameter:

Why it goes wrong without control: at the instant the punch touches the blank, the length to be deformed is projecting freely out of the die with no support. Too long, and it buckles instead of upsetting, with the metal folding over itself into what is called a cold shut — a crease on the outside and a discontinuity inside. It is the most dangerous cold heading defect, because it becomes a crack origin in later heat treatment or under load.

Two further factors push the blow count up beyond what S suggests:

• A large, thin head, high D₁/H₁. L/D may be modest, but a single rough upset tears the head rim, so staged forming is needed.
• Tight surface requirements and complex head forms, which need extra blows so the metal flows more gently.

There is also a reduction (extrusion) criterion that is often overlooked. This is not upsetting but drawing thicker wire down into a shank:

Combining the two is the standard trick: start with thicker wire, extrude the shank and rough form the head in the first blow, finish the head in the second — "extrude the shank, then upset the head". That allows a large head-to-shank ratio without being constrained by the 2–2.5 single-upset limit.

4. Gate three: shank length and slenderness

Whether the shank ejects cleanly is the most overlooked structural rule in cold heading design, and its consequences are immediate.

One related rule about straightness: once the blank departs from the die axis, the eccentricity is amplified three to five times by forming. Cut-off face squareness and wire straightness therefore show up directly in the finished coaxiality — a wire control problem, not a die design problem.

5. Gate four: size limits and machine capability

The same shape becomes impossible once it gets large enough. The underlying reason is forming force: area grows with the square of size, while press tonnage is finite.

One easy misjudgement: machine capability is described by wire diameter and cut-off length, not by "what size head it can make". Producing a φ30 mm head from φ12 mm wire gives q ≈ 84 % and will not work, while the same head from φ20 mm wire gives q ≈ 55 % and is straightforward. So "can you make this head" is a meaningless question; the real one is "from what wire, in how many blows".

6. Gate five: will the wire meet specification?

Passing the first four gates means the part is geometrically feasible. Whether it can be produced repeatably depends on the wire, and the requirements are stricter than most people expect — and testable.

The classical requirements for annealed cold-heading steel wire. Indentation diameter is the Brinell ball impression, and a larger value means softer material:

How to accept it: beyond the routine tensile test, cold heading has its own — the upsetting test. Take a specimen whose height is twice its diameter and upset it to half that height, q = 50 %, and it must show no cracks or splits. For parts needing higher deformation, raise the compression to match the actual deformation of the part.

There are also hard requirements on surface and composition:

On the phosphate process: a typical sequence is degrease → cold and hot water rinse → phosphate bath above 90 °C for 4–12 minutes → cold and hot water rinse → lime and soap dip → dry and oil. The soap layer carries lubricant and helps the metal flow during heading.

7. Shape boundaries: what cold heading does well

After five gates there is a finer question: does the shape itself look like a cold-formed part? Cold heading fills a die cavity by metal flow, so its boundaries are specific.

Features cold heading does well

Features cold heading handles poorly, or at extra cost

8. Economics: what volume makes it worthwhile

Feasible is not the same as economic. Cold heading has a high front-load and low unit cost structure: tooling and setup are one-off, and unit cost falls quickly with volume.

An illustrative calculation, with your own figures substituted: assume cold heading requires CNY 12,000 of tooling and setup and saves CNY 0.65 per part against turning. Break-even is then 12,000 ÷ 0.65 ≈ 18,500 pieces. Below that annual volume the tooling never pays back; well above it, the saving is pure margin.

Three further items belong in the calculation:

• Material utilisation: cold heading forms rather than removes, so utilisation is typically above 95 %. Turning scrap rates are high — on some solid pins the turned route consumes 8–10 times the material of the cold-headed route.
• Secondary operations: cold-formed parts often still need thread rolling, heat treatment and surface finishing, and a heavy secondary content eats into the advantage.
• Die life: on hard materials, die life is one of the decisive variables in production cost. The closer the deformation is to the material limit, the faster the die wears.

9. A ten-point checklist

Take a drawing and run these ten items; ten minutes tells you whether the project is worth opening:

10. Summary

Cold heading suitability is not really a matter of experience. It is a combination of physical constraints: how much the material can deform, how long it can be upset without support, whether the die can eject it, whether the machine has the capacity, and whether the incoming wire will let you down. Each has an explicit criterion, and running them produces an answer.

The most useful point for buyers and designers is to move these criteria to the design stage. Once the shape is fixed and the die is cut, change costs multiply; on a drawing, turning "large thin head with a 9× shank" into "7× shank formed in two stations" costs nothing at all.

11. Sources and standards

• Metal Cold Heading Technology (B. M. Misuzhnikov, translated by the First Ministry of Machine Building General Machinery Research Institute): definition and three forms of deformation q; allowable deformation by material, including 60–70 % for low-carbon steel at D₀/H = 2.0, only 20–30 % for Y10/Y12 and 65–85 % for steel up to 0.5 % C in three blows; blow count versus carbon content; incoming wire requirements; the upsetting test at q = 50 %; phosphate process; annealing stages at 400–500 °C for reduction and 550–650 °C for recrystallisation; blank diameter and die bore determination.
• Cold Forming Basics: Industrial Fasteners: controlled upset limits of about 2–2.5 diameters in a solid die, 4.5 in two blows and 6.5 with a sliding punch in two; open extrusion up to 30 % area reduction per pass and confined extrusion up to 75 %; die angle up to 30°; multi-station combined extrusion and upsetting.
• Determining Upset Count and Preform Design for Cold-Headed Parts: upset ratio S = L/D bands at ≤ 2.5 for one blow, 2.5–4.5 for two and 4.5–6.5 for three; unsupported shank length L/D < 9.5–10; solid die shank ≤ 8d and the conditions for open dies; eccentricity amplification of about 3–5×; conical preform punches, with best metal flow at 2β = 12–15°.
• ISO 898-1:2013, Mechanical properties of fasteners made of carbon steel and alloy steel, Part 1: property classes and carbon limits — C ≤ 0.55 % for classes 4.6 and 4.8, C ≥ 0.15 % for class 6.8, C ≤ 0.55 % with tempering at not below 425 °C for quenched and tempered class 8.8.