How Are Fasteners Made? An Overview of Production Processes for Screws, Bolts, and Nuts

Fasteners have become an essential part of our daily lives. Common fasteners include screws, bolts, nuts, and washers, which are available in various shapes and sizes. They are typically produced from coil wire or round bar stock made from copper alloys (brass and bronze), aluminum alloys, titanium alloys, and nickel-based alloys.

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Manufacturing Methods

Generally, there are three primary methods for producing fasteners: cold forging, hot forging, or machining. The preferred technique depends on the type of fastener being produced and the metal used. Standard fasteners are typically manufactured through cold forging (without heating the material).

Cold Forging

This method allows for the rapid production of near-net shape or net shape parts, characterized by excellent mechanical properties and high-quality surface finishes. Another advantage of cold forming is work hardening. During cold plastic deformation, the material's crystalline structure undergoes significant changes, resulting in stronger and more resistant components. However, larger diameter parts require more compressive force to form; hence, another method may be more favorable.
The performance of cold forming machinery varies according to the size of the fastener being manufactured. Some machines can reach up to 300 parts per minute. The smallest screws and bolts used in electronics and clockwork can have diameters of less than 0.5 mm, while the largest cold-formed screws can have diameters of about 30 mm.

Hot Forging

Hot forging is typically used for large diameter bolts and screws, usually sizes M36 and above and lengths of about 300 mm or more. The bar stock is heated to high temperatures, making the material more malleable, and then supplied to a forging press. The temperature is determined based on the material composition, shape, and tolerances. This process enables the production of complex shapes and high formability. Hot forging is particularly suitable for manufacturing fasteners made from nickel-based and titanium alloys. The unique feature of hot-formed fasteners is their original surface structure, although hot forging is time-consuming.
Unlike machining (which will be covered later), both cold and hot forging are chipless processes.

Machining

Machining is a manufacturing technique that cuts the metal into the desired final shape and size through a controlled material removal process. This method is suitable only for non-standard fasteners and low-volume production, generating a significant amount of metal waste and consuming considerable time.

Cold Forging of Screws and Bolt Blanks

Typically, cold forging machines operate as horizontal presses at room temperature. The starting material in cold forming is coil wire, which is unwound by a feeding device, straightened, and fed into the press. There, the wire is cut to a fixed length, resulting in what is known as a blank, which is then picked up by a transfer device and carried to various workstations. Each station consists of a punching tool and a forming die. Each step progressively molds the material closer to its final shape, ensuring that each component aligns with the form of the last die. This process, also referred to as heading, produces screw blanks with heads and shafts.

Screw Formation

Specific devices are responsible for cleaning and polishing the blanks.
The next step is screw processing. Screws are generally formed in screw processing machines, where the components are positioned between two flat dies—one held in place while the other undergoes reciprocating motion—or between two to three rotating cylindrical die. The dies have surfaces with grooves matching the screw to be formed. Thanks to this combined movement, the dies form the screw from the blank with a head. The first method allows for the processing of hundreds of screws per minute, while the latter can create long screws.
Screw processing is a cold forming process that generates smooth, precise, and uniform external threads without altering the integrity of the microstructure. Thus, the mechanical properties of the fastener are improved.
Screws can also be created using taps or screw bars in a process known as screw cutting, involving the cutting of the material's crystalline structure.
Cut screws can be manufactured to virtually any specification, but many manufacturers prefer screw processing since the screws produced this way are often much smoother and more resistant to damage during handling.
Generally, screws are produced before heat treatment (e.g., hardening and tempering).

Cold Forming of Nuts

Nuts are square, round, or hexagonal metal blocks with a threaded hole that matches the external screw. The blanks can be cut from wire and formed in a cold forming press (as noted above). Nuts can also be manufactured by cutting blanks from hexagonal bars and drilling screw holes. Internal threads are cut in a tapping machine.

Heat and Surface Treatment

In many cases, fasteners undergo heat treatment to alter their microstructure and physical properties, such as strength and ductility. The process steps depend on metallurgical characteristics. For instance, steel fasteners are heated to specific temperatures based on carbon content and held there for a certain time. Afterwards, components are quenched in oil or water to enhance their strength and hardness. Following this, parts are re-heated at lower temperatures, achieving greater ductility with minimal distortion of the microstructure. Heat treatment lines for steel fasteners typically feature stations for cleaning, degreasing, hardening, quenching, washing, aging, and coloring. Generally, these lines operate with mesh belt furnaces, with fasteners moving according to the speed of each stage.
Occasionally, surface treatment may also be necessary. For example, special coatings can be applied to enhance the properties of fasteners. Case hardening may be used in situations such as with tap screws and self-drilling screws. Screws are heated and maintained in a carbon-rich atmosphere for some time. Carbon seeps into the surface, increasing the carbon content locally. The screws are then quenched to harden. Therefore, the surfaces of these screws become quite hard while the interiors remain ductile.
Also, special coatings can be applied to improve corrosion resistance. Such coatings are achieved through processes like galvanization.
Once treatment is complete, fasteners undergo a final cleaning stage before being prepared for packaging and shipment.

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