Home / Processes
From wire to finished part
How a made-to-print fastener is produced
Eight steps take a coil of wire to a packed lot of special bolts. Which of them your part needs, and in which order, decides the route, the tooling and who can quote it.
Capabilities below are typical ranges. The plant that quotes confirms what fits your part.
The production sequence
A cold headed bolt passes through every row. A machined special skips the first three and starts from bar. The last column is what your print or RFQ has to say so the plant can plan that step.
Swipe the table sideways to see all columns.
| Step | What happens | Typical capability | What the print should state |
|---|---|---|---|
| Wire preparation | Coil is drawn to the blank diameter and, for alloy grades, spheroidize annealed so it heads without cracking. | Wire grades such as 10B21, 10B33, SCM435 or 1022, chosen by the property class | Property class or material grade, not a wire grade, unless your customer specifies one |
| Cold heading | The wire is sheared and formed in one to six die stations into head, shank and reduced sections at room temperature. | About M2 to M20, length up to roughly 10 x d. More stations allow more complex heads | Head form and dimensions, under-head radius, recess type, any shank reduction |
| Trimming and pointing | Hex or flange heads are trimmed to size, and the end is pointed or chamfered for assembly. | Often done on the header or on a separate trimmer | Point type (chamfered, dog point, cone) and head across-flats tolerance |
| Thread rolling | Flat or cylindrical dies displace metal into the thread form. No material is removed. | Metric and unified threads, tolerance class 6g or 2A as standard, 4h or 3A on request | Thread size, pitch, class, thread length b and any thread to head requirement |
| Secondary machining | Cross holes, grooves, slots, ground shoulders or tight diameters a header cannot hold. | Drilling, turning or grinding on transfer machines or lathes | Only the features that need it, with their tolerances |
| Heat treatment | Quench and temper for class 8.8 and above, case hardening for tapping screws. | Continuous mesh belt furnaces for volume, batch furnaces for small lots | Property class, or hardness range with the test location |
| Surface finish | Electroplating, zinc flake, phosphate and oil or passivation, applied after heat treatment. | Usually subcontracted or run on a separate line | Finish standard, thickness or corrosion hours and baking if hydrogen risk applies |
| Sorting and inspection | Optical or eddy current sorting for mixed parts and cracks, then lot sampling. | Dimensional, mechanical and coating checks per the agreed plan | Inspection documents and any 100 percent sort requirement |
Cold heading
A header shears a slug from the wire and pushes it through a series of dies at room temperature. A two-blow header makes simple heads. A multi-station former moves the blank through four to six stations and can extrude a reduced shank, upset a flange and punch a recess in one cycle.
The limit is how much metal can be gathered into the head without the slug buckling. Heads up to about two and a half times the wire diameter form in a few blows. Larger heads need more stations, warm forming or hot forging. The return is speed: a former makes parts in fractions of a second with almost no scrap, so the piece price at volume is the lowest of any route.
What it costs is tooling. Dies and punches are made for each part number and take weeks. Cold heading vs machining vs hot forging compares the routes on cost, and tooling cost and blank size covers what drives the die bill.
Design notes for headed parts
- Under-head radiusGive the largest radius the joint allows. A sharp corner is a stress raiser and wears the die.
- Head heightStandard head heights form in fewer blows. A thin, wide head needs extra stations.
- Recess depthA deep recess in a low head leaves too little wall and can crack in heading or tightening.
- TolerancesHeaded diameters typically hold a few hundredths of a millimetre. Tighter fits go to a grinding or turning step.
Thread rolling vs thread cutting
Almost every headed fastener has a rolled thread. Cutting is kept for cases rolling cannot reach. The difference matters most on parts that see vibration or cyclic load.
| Rolled thread | Cut thread | |
|---|---|---|
| How it forms | Metal displaced by dies, grain follows the thread | Metal removed by a single point tool, tap or die, grain is cut |
| Fatigue | Higher, especially when rolled after heat treatment | Lower at the root |
| Blank diameter | About the pitch diameter, so less material per part | Full major diameter |
| Cycle time | Seconds per part | Longer, especially on coarse pitches |
| Where it fits | Most bolts and screws in volume, up to large diameters | Prototypes, very large or short threads, special forms such as ACME |
Metric threads follow ISO 965 tolerance classes and unified threads ASME B1.1. Background on the methods: threading (manufacturing). The cost and strength trade-off in more depth: thread rolling vs cutting.
CNC turning and Swiss-type
When volume is low, the head is too large to form, the material does not cold form well or a diameter must be held tighter than a header can, the part is turned from bar. Most turned fasteners still get a rolled thread, cut on a lathe attachment or a separate roller, so they keep the fatigue benefit.
Swiss-type lathes take small diameters and long slender parts: micro screws, pins and shoulder screws below about M6. Conventional lathes cover larger studs and bolts up to the bar capacity. Titanium, nickel alloys and large diameters are usually turned.
Heat treatment
Class 8.8, 10.9 and 12.9 steel bolts are hardened and tempered to reach the strength and hardness that ISO 898-1 sets for the class. The furnace atmosphere has to be controlled, because a decarburized surface layer is softer than the core and weakens the thread. ISO 898-1 limits the depth of that layer.
Tapping and self drilling screws are case hardened instead, giving a hard skin over a tougher core. Stainless A2 and A4 bolts are not hardened at all. Their class 70 and 80 strength comes from cold work. Quench, temper and decarburization explains the checks to ask for.
Which features change the route
Read the print feature by feature. One feature a header cannot make adds a secondary operation. Several of them can move the whole part to machining.
Swipe the table sideways to see all columns.
| Feature on the print | Can a header form it? | What usually happens |
|---|---|---|
| Hex, flange, button or pan head | Yes | Formed and trimmed on the header |
| Hex socket, Torx or Phillips recess | Yes | Punched during heading. Security drives need a dedicated punch |
| Reduced shank or extruded section | Yes, within the extrusion ratio | Formed in an extra station |
| Shoulder held to about ±0.02 mm | Not directly | Headed oversize, then turned or ground |
| Cross hole or slot | No | Drilled or milled as a second operation |
| Undercut or groove for a retaining ring | Partly | Rolled or turned after heading |
| Full thread up to the head | Not by rolling | A short runout of about 2 to 3 pitches remains unless the thread is cut |
| Head larger than about 2.5 x d | Hard | Extra stations, hot forging or machining from bar |
The four routes side by side, with typical size and volume ranges, are on the home page.
Finishing comes last
Coating goes on after heat treatment, and it changes the thread. Electroplated zinc to ISO 4042, zinc flake to ISO 10683 and hot dip galvanizing to ISO 10684 each add a different thickness, so the thread allowance has to be planned before rolling, not after.
For high strength parts the coating choice also decides the hydrogen risk. Hydrogen embrittlement in bolts covers when baking is required. The grade side of the decision is on the materials page.
How we use this when matching
We read your print for the route first: headed, machined and rolled, machined only or hot forged. Then we check size, grade and the secondary steps, and pass the RFQ to up to 3 makers that run that route at your volume. You hear from us within 24 business hours.
Process questions
Sector specific paperwork and parts are on the industries page.
Is a rolled thread stronger than a cut thread?
In fatigue, yes. Rolling compresses the thread root and keeps the grain flow intact, so rolled threads carry cyclic loads better. Static tensile strength is set mainly by the material and heat treatment. Rolling after heat treatment gives the largest fatigue gain but is harder on the dies.
Why does a headed part need a minimum order?
Each part needs its own dies and punches, and a header is set up for a full coil run. Below a few thousand pieces the setup and tooling dominate the price, which is why small lots are usually machined from bar and rolled instead.
When is heat treatment required?
For steel bolts of ISO 898-1 class 8.8 and above, quench and temper is part of the class. Classes 4.6 to 5.8 reach their properties from the steel and cold work. Self tapping screws are case hardened so the thread can cut its own mating thread.
Can plating weaken a high strength bolt?
Electroplating can introduce hydrogen, and parts above about 320 HV, such as class 10.9 and 12.9, are at risk of delayed fracture. ISO 4042 sets out the measures, including baking soon after plating. Zinc flake coatings to ISO 10683 avoid the electrolytic step.