Fastener Production Techniques: A Complete Guide

Overview of Production Techniques

How are screws produced? How are bolts manufactured? To answer these questions and discover how fasteners are created, we must start from the principle that production technologies vary depending on the shape of the product to be made and the materials used.
There are several production techniques that we will describe in the following paragraphs:

• Forging
• Thread rolling
• Machining
• Cutting
• Sintering.

Generally, the starting point is the raw material, in the form of bars or wire coils for the production of screws and nuts, and in the form of strip sheets for the making of washers.
In the first case, for the production of screws, it starts with the transformation of the wire rod, present in the form of wire wound on specific spools.
Through the heading process, the wire rod acquires the desired shape and can preliminarily be subjected to treatments such as pickling and lubrication, useful to prepare the material for subsequent steps.
In fact, subsequently, the wire rod undergoes a deformation treatment, called drawing, which must be performed before further processes such as forging or thread rolling.

It is also useful to know that among the main manufacturing methods of fasteners are hot and cold forming. The former is usually chosen for the production of large diameters while the latter is used for smaller sizes.
Thus, each technology has its steps to follow, its treatments to subject the material to, and its advantages related to the final use of the fastener element.

What is Forging and How it Applies to Fasteners

As mentioned in the previous paragraph, one of the basic technological treatments of fasteners is forging, which can be distinguished into:

• Hot forging
• Cold forging

Cold forging starts with the wire rod that, accurately drawn, allows optimal filling of the dies. This basic treatment is very fast because it is based on automatic and standardized steps: the metal wire is cut to precise length and then moved using mobile clamps in a series of successive molds.
It is in these molds that the material takes the desired shape. Cold forging exploits the malleability of materials and, consequently, respects their critical values. Indeed, during this production method, the yield strength remains below the critical threshold, and the final product is not compromised.
Although it is a technique used for small-size workpieces, technological evolution has allowed the use of increasingly sophisticated analysis and product simulation programs. Today it is indeed possible to use multi-station machines and reach significant diameters, up to 48 mm.

Forging can also occur hot. This technique is applied for the production of products with large diameters and, for this reason, the machine is fed by a bar, whose end is heated by induction heat before being cut.
Hot forging is used to deform very resistant materials. Indeed, through the press, the material is deformed to obtain the desired shape. The equipment, or dies, mounted on the hammer of the slide and on the anvil of the frame, are composed of many elements, some common to several screws, others instead specific.
For example, to stamp and produce a hex head M8 screw in various lengths, it is enough to adjust the different measures of the wire cut and lengthen or shorten the stroke of the tool.
Thus, the manufacturer will not have to replace the die from time to time but can keep the equipment intended to form the head mounted, thus saving significant production time. To avoid the breaking of the yield strength and, therefore, possible cracks, it is also useful to perform a preliminary thermal stabilization treatment.

Screws and Bolts Forging: Processes and Materials

The forging processes of screws and plastic deformation are generally the most widespread thanks to the good quality of the final product and the contained costs that this system requires, especially if the production reaches significant and constant numbers over time.

"Forging" is the term used to indicate this process because the creation of the product occurs through a mold. Depending on the product to be obtained, the procedures can vary, and it is important to consider some specific conditions, such as, for example, the temperature of the mold.

There are two main forging operations:

• Injection: the material, made liquid, is inserted into the mold and pressed; it will take the desired shape once cooled;
• Compression: the material is literally crushed in the mold and thus takes the desired shape.

In general, however, regardless of whether the technology is hot or cold, injection or compression, the materials most suited to this production method are those more malleable such as Steel, Aluminum, Copper, and Brass. Following forging, further processing may follow, including thread rolling or material removal, when, for example, it is necessary to create flanges, cavities, or obtain even more precise elements. It should be emphasized that, especially regarding hot forging, the malleability of materials influences the use of machinery, because not all screws can be made in a few steps.

This manufacturing technique is normally used to obtain numerous and different technical components. Among all, we remember the production of washers and seals.

Mechanical Thread Rolling

Mechanical thread rolling is a machining process that does not involve the removal of chips, but the deformation of the material occurs through its passage in continuously rotating rollers. In the thread rolling process, such rotating tools deform the surface of the material until the desired shapes and dimensions are obtained. The element is then passed between these two rollers or shaped plates, so that it assumes the desired silhouette under the effect of a consistent and continuous pressure applied during its passage.

This machining technique is used when it is necessary to obtain a strengthening of the element, which will be hard and resistant while retaining its mechanical properties. An additional significant advantage of this technique is the possibility to vary in succession the increasing and decreasing diameters, regardless of their position along the axis. At the same time, thread rolling is not to be preferred over other techniques, such as forging, if the goal is to achieve high deformations.

The process using rollers is a process that is usually used for the creation of screw threading. Indeed, thread rolling of threading guarantees a better outcome of the thread and, consequently, significantly lowers the risk of accidental dents during handling. The closer a thread comes to perfection, the smoother will be the application of the fastener element and the application of the preload during assembly, because friction will be minimal.

For all these advantages and the low production costs, this technology is particularly appreciated in various sectors, including aerospace, automotive, railway, oil & gas, and agricultural machinery.

Machining Processes for Special Screws and Nuts

Among the main machining techniques of fasteners is a particular one, namely machining, used to produce small series of fastening elements or for the realization of very complex and highly precise parts. This technology, which therefore allows greater accuracy of machining compared to others, is suitable for the production of so-called small metal parts, i.e., particular and small-sized elements with complicated geometric shapes.

Machining is a method that occurs by removal: the material is removed until the desired shape is obtained. The functionality of the machine varies according to the type of machining to which the material is subjected. Indeed, each creation involves different machining phases, all highly precise, since a simple vibration or speed variation could affect the final result.

Precisely because it is dedicated to the creation of small elements, this technique involves the use of materials able to flake off easily, thus creating small chips. Conversely, too hard or pasty materials would hinder production.

There are different types of machining that can be chosen based on the final shape to be obtained. Specifically, it is possible to distinguish between:

• Turning: using a lathe, the tool is brought close to the rotating element, and chips are thus progressively removed;
• Drilling: a type of machining that involves the use of a drill equipped with a chuck that supports a rotating spiral-shaped bit;
• Milling: similar to drilling for the use of the drill, this technique is distinguished because it does not work on the tip, but by abrasion on the sides of the piece; it is a machining suitable for creating complex shapes;
• Boring: it is a corrective technique that acts on the diameter and slightly but precisely modifies it; through the rotary motion of the reamer, the diameter of holes made with the drill or obtained from a casting will gradually approach the correct value;
• Broaching or reaming: this technology involves the use of a machine called a broaching machine or reamer, capable of working on holes or cavities and removing the material from the internal surface until the desired shape is obtained; the hexagonal cavity of a screw with a cylindrical head is a typical example of this machining.
  
  

The Cutting Process and its Applications

Like machining, metal cutting is a process particularly indicated for the production of small metal parts. It is indeed a technique used for the machining of flat sections and, therefore, of elements with limited thickness such as washers, low nuts, plates, retaining rings, etc.

The starting point is a steel, copper, brass, or aluminum sheet. The laminate, inserted into a press (usually vertical) with a punch or a series of punches and dies, is subjected to a force that fractures the piece.

There are different types of cutting, depending on the shape to give the material. For example:

• Drawing, suitable for the production of cylindrical products;
• Bending, for creating folds on the metal;
• Coining, for the production of knurling, chamfers, etc...

Cutting involves the use of advanced stamps and presses that do not damage the material. For this reason, it can be performed on any metal.
It is possible to talk about steel cutting, but also aluminum and copper.
In the first case, the final products are generally used in the food sector; regarding aluminum components, these find application in sectors such as electronics and mechanics, while copper elements are used in the field of electromechanics.

Regardless of the final application of the product, cutting is a machining that has the advantage of being fast and economical.
However, it is not a technique that guarantees precision and high quality of execution because, during the machining processes, the formation of burrs on the profile of the cut is frequent.
To remedy these inaccuracies, it is necessary for the element to be subjected to sandblasting to be chamfered. All these steps produce a considerable amount of waste, and this is another disadvantage of this machining.

Sintering: Innovation in Component Production

Sintering is a technology that allows the creation of small and medium-sized parts, especially if the final product must acquire complex shapes. Sintered metals are created from the compression of metal powders that are then sintered. Subsequently, such components are subjected to heat treatment to obtain porous and elastic metals.

In general, 3 types of sintering can be distinguished:

• Thermal sintering, it is a machining process at high temperatures. For the creation of the fastener part, the powder is inserted into a mold together with some binders; subsequently, it is brought to a very high temperature, but below the melting point of the material;
• Thermomechanical sintering, combines heating with the application of appropriate pressure (between 20 and 50 MPa) through a gas or liquid;
• Electrothermal and electromechanical sintering, is a technology performed with the aid of electric currents and/or specific electromagnetic fields that bring energy right on the part interested in the machining process. It is a solution that allows rapid and high-quality creation of the product, thanks to the use of 3D printers.

Specifically, regarding 3D printers, the processes normally used are 3:

• Stereolithography, mainly used for demonstration pieces;
• Selective laser sintering, used for products that require robustness and durability;
• Direct metal laser sintering, allows the creation of complex geometries and resistant and durable articles.

Finally, it is useful to emphasize that the sintering process allows combining materials that would not be combinable with metallurgical fusion. Among these, for example, iron-aluminum, copper-graphite, copper-tin-lead combos are included.

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