Screw & Bolt Cold Heading Dies

What a cold heading die does

A cold heading die is the cavity that a wire blank is forced into, and it does two jobs at once. It holds the outside diameter of the part to a tolerance while the punch drives material into it, and it controls how the material flows when the punch bottoms out. Everything a buyer cares about — concentricity, head height, the absence of laps and folds, tool life — comes out of those two jobs being done properly at every station.

Cold heading dies are not a single part. On a multi-station header a complete set runs from four to eight cavities, and each one has a different job depending on where it sits in the sequence. Buying them as a matched set from one design intent is the difference between tooling that runs and tooling that has to be tuned on the machine for a week.

The die set, station by station

Understanding what each station does explains why the specification of a die set cannot be reduced to a material grade and a hardness figure.

First station: cut-off and first blow

The wire is sheared and the cut face is squared up. The first cavity sets the blank volume and the initial concentricity, and any error here is carried forward and amplified by every later station. The cut-off knife and the first die work as a pair; a knife that leaves a burr or a slanted cut face produces an off-centre part no later station can recover.

Middle stations: reduction and shaping

These carry the heavy reduction. The cavity geometry decides the reduction ratio at each step, and the ratio decides whether the material flows or tears. Too much reduction in one station produces internal laps that only appear as cracks after the part has been in service. Too little and the number of stations no longer fits the machine.

Final station: the geometry that decides release

The last cavity forms the features that make a part hard to eject — undercuts, collars, flanges, stepped shoulders. Where a one-piece cavity cannot release the part, this is where split dies replace the solid die: segments open radially around the profile, form it in closed extrusion, then separate so the finished part lifts out with its geometry intact.

The parts around the die

A cavity alone does not make a working station. The set includes carbide master dies and their stress rings, second-blow punches, punch sleeves, ejector pins or KO pins, trimming dies and cut-off knives. These are matched to each other, not bought from a catalogue at nominal dimensions. A punch sleeve with the wrong bore clearance will crack a carbide die insert long before the insert itself wears out.

Punches follow the same logic. Where a carbide working face is needed without a solid carbide body, the head is brazed onto a steel shank — the brazed carbide cold heading punch page covers that construction, and semi-finished brazed blanks are held in stock for tool rooms that grind their own geometry.

Carbide grade selection

The grade is not a single number and it is not a marketing label. For cold heading, the choice comes down to a trade-off between wear resistance and toughness, and the right point on that curve depends on four things: the material being formed, the reduction ratio at the station, the machine’s stroke rate, and how the customer runs the die.

Typical tungsten carbide grades used in heading dies run from around 10% to 16% cobalt, with grain size selected to suit the impact load. A high-cobalt, coarse-grain grade absorbs shock and resists cracking, which suits high-speed headers and heavy reduction. A lower-cobalt, finer-grain grade holds a polished cavity and resists abrasive wear, which suits long runs of soft material where dimensional drift is the failure mode rather than fracture.

The common grades on the market — VA80, VA90, ST7, ST6, KG5, KG6, KG3 and their equivalents — sit at different points on that curve. We select against the application rather than defaulting to whichever grade is cheapest to buy, and we will tell a customer when the grade they have specified is wrong for the job.

Tolerance, fit and the things that are not on the drawing

Precision grades for cold heading tooling are usually quoted from G4 to G9. The figure matters less than where it is applied. A cavity that is round to G4 but not concentric with its stress ring will produce an eccentric part at every stroke, because the ring, not the drawing, sets the final position of the insert.

Three fits decide whether a die set runs:

• Insert to stress ring. Interference has to be enough to pre-load the carbide in compression and not enough to crack it on assembly. This is a calculation, not a habit.
• Punch to sleeve. Clearance controls whether the punch guides the blank or shaves it. Too tight and the sleeve seizes; too loose and the head runs off-centre.
• Segment to segment, on a split die. The parting line has to close with no step. Segments are wire-cut, then fitted and marked as a matched set. They are not interchangeable between dies, and they ship with their closing ring, because a new segment in a worn ring will not close correctly.

Why cold heading dies fail

Most die failures are attributed to the die and are actually process failures. Knowing the difference is what separates a tooling supplier from a tooling partner.

Carbide cracking at the bore

Almost always a fit or support problem, not a grade problem. Insufficient interference in the stress ring, a bore that is not supported over its full length, or a punch that bottoms with an impact the cavity was not designed for. Changing to a tougher grade hides the symptom and shortens wear life.

Wear and dimensional drift

The cavity grows gradually and the part grows with it until the head diameter or shank OD leaves tolerance. This is the normal end of life. The way to extend it is not a harder grade but a better surface finish in the bore and a lubricant that actually reaches the deformation zone.

Seizing and material build-up

Cold welding of the workpiece to the die wall, producing smearing and eventually a torn surface on the part. It is a lubrication and surface-finish problem, and on stainless and high-alloy wire it is the dominant failure mode. Polished cavity walls, correct coating and a die design that does not trap material matter far more than hardness.

Segment collapse on split tooling

Specific to segmented dies. The petals take the radial forming load on their closing faces, and if the clearance between petals is optimised badly they collapse or chip at the corners under high-speed continuous running. This is where multi-petal geometry earns its development cost.

Matching the die set to the machine

A die set is designed for a specific machine, not for a wire diameter in isolation. Stroke length, station count, die pocket dimensions, knockout arrangement and the machine’s tonnage all constrain the design before the part geometry is even considered.

We build for the machines our customers actually run: the 104S, 105S, 106S and 134L families, the 164L and 204L series, and the nut formers from 19B upward. Where a part is being transferred from an older machine, send the existing die drawing as well — the constraint is often the pocket, not the process.

For long-length products, station count and stroke length interact with the split-die decision. Our optimised split die process for long-length parts was completed in 2020 and is covered by a patent application; the tooling built to it is what we supply today.

How to evaluate a cold heading dies supplier

Price per cavity is the easiest number to compare and the least useful. Four questions separate suppliers who can hold a process from suppliers who can ship a part that fits a drawing.

Do they ask for the machine and the process route, or only the drawing? A supplier who quotes from the print alone is guessing at the reduction sequence. The die cannot be right if the process behind it is unknown.

Can they tell you why a grade was chosen? A specific answer referencing cobalt content, grain size and the failure mode being designed against is a different answer from a grade name repeated back.

Do they supply the whole station set or only the cavity? Mixed sourcing is where interfaces go wrong, and the supplier of the cavity is the one who gets blamed when a punch from elsewhere does not fit.

What happens after the first trial? Tooling that needs correction is normal. A supplier who owns the correction — on their own design intent, without charging it as a new project — is the one to keep.

What we build

BENC MOULD is a cold heading dies supplier working from two plants in Ningbo, one producing cold heading dies and one producing carbide punches and punch pins. The range covers carbide master dies, second-blow dies, split dies, nut forming dies, trimming dies, cut-off knives, punch sleeves, ejector pins and special-shaped punch pins for square, hex, Torx and radiused recesses.

Wire diameters run from M1.0 to M33 equivalent, in UNC, UNF and JIS thread systems, at precision grades from G4 to G9. Die cores are ground to G6 or tighter where the application calls for it. Heat treatment is vacuum hardening followed by cold treatment, which is what stabilises the dimensions before final grinding rather than after. Standard delivery runs 7 to 20 days depending on complexity.

For parts that a solid die cannot release, the tooling route changes: see how we develop cold heading dies for non-standard and special-shaped fasteners. For the design principles behind segmented cavities, read the guide to split die cold forming.

What to send for a quotation

Send the part drawing, the machine model and station layout, the wire material and diameter, the annual volume, and the tolerance that actually governs acceptance. If the part currently runs on existing tooling, send the die drawing or the process sheet — comparing the current route against a proposed one is how we establish whether a change pays back.

If a drawing arrives in Chinese and describes the tooling as 冷镦模具 or 螺丝模具, the engineering conversation is identical: what the part is made of, which machine it runs on, and which feature has to hold tolerance. Request a quotation and we reply within 24 hours.

Related Technical Guides

Three variables decide most cold heading die outcomes. Each has its own guide:

• Cold heading die materials — tungsten carbide vs HSS vs PM steel, cobalt content trade-offs and how to read a failure mode back to a grade.
• Cold heading die precision — what G4 to G9 actually constrain, how bore and concentricity are verified, and why G6 is our default.
• Extrusion ratio limits — safe reduction ranges by material, worked examples, and when a shank needs two blows instead of one.
• Extrusion ratio calculator — enter wire and shank diameter to get the area ratio, true strain and a station split.