Cold heading
Cold heading (also called cold forging) shapes the steel at room temperature. Wire from a coil is cut into blanks and pushed through a series of dies in a multi-station former. Each blow moves the metal a little further, until the head, the reduced shank and the hex are complete. Because the metal flows rather than being cut, there is almost no scrap, and the grain lines follow the shape of the head, which helps strength.
Cold heading is the standard method for small and medium sizes in large quantities: it is fast, accurate and economical. It also work-hardens the steel, which is how classes such as 4.8 and stainless A2-70 reach their strength.
Hot forging
For large diameters, very long bolts, special heads and small batches, the bar end is heated (usually by induction) until it is soft, then pressed into a die to form the head. This is called hot heading or hot forging. It needs much less force, so large heads can be made on smaller machines, and it suits short runs without expensive tooling. The trade-off is lower speed, a rougher surface and wider tolerances, so hot-forged parts are often machined or trimmed afterwards.
| Cold heading | Hot forging | |
|---|---|---|
| Typical sizes | Small and medium diameters | Large diameters and long bolts |
| Quantity | High volume | Low to medium volume, specials |
| Speed | Very fast | Slower |
| Accuracy and finish | Close tolerances, smooth | Wider tolerances, scale on surface |
| Material use | Almost no waste | Some trimming loss |
Thread rolling
A rolled thread is formed by pressing the blank between hardened dies (flat dies, or round dies on a cylindrical machine). The steel is displaced, not removed, so the blank starts close to the thread's pitch diameter rather than its major diameter.
- Grain flow. The grain lines follow the thread profile instead of being cut through.
- Fatigue strength. Rolling leaves compressive stress at the thread root, where fatigue cracks start. Thread-rolling suppliers report fatigue strength gains of around 50 to 75% over cut threads.
- Surface. The flanks and root are burnished smooth, removing tool marks that can start cracks.
- Speed and material. One pass instead of many, and no chips.
Thread cutting
Cut threads are made by a single-point tool, a die head or a chaser removing metal. They are used for very large diameters, special thread forms, short runs, and threads on parts that are too hard to roll. A cut thread is perfectly serviceable for static loads; the difference shows mainly under vibration and fatigue.
Rolling before or after heat treatment?
Most industrial bolts are rolled before heat treatment, while the steel is soft and easy on the dies. For the most demanding fatigue applications, such as aerospace bolts and some high-strength studs, threads are rolled after heat treatment, so the compressive stress at the root is not relaxed by the furnace. It costs more in die wear, but gives the best fatigue life.
Forming and threading in India
- Cold forming on multi-station bolt and nut formers is standard at the larger Indian makers for M5 to about M20 in volume.
- Hot forging is widely used for large and long bolts, foundation bolts, black (product grade C) bolts and short runs.
- Thread rolling is the norm for standard bolts. Thread cutting on lathes and threading machines is still common for large studs, anchor bolts and special threads.
- Turned parts from bar (on CNC or automatic lathes) are used for small batches and special designs.
For fatigue-loaded joints such as engine, crane, wind and machinery parts, ask for rolled threads. For static structural use, a well-made cut thread is acceptable.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.