Bolt manufacturing process: Encyclopedia - Bolt manufacturing process

Bolt manufacturing process

The bolt manufacturing process consists of three stages:

• Heading
• Thread rolling
• Blackening

Bolt manufacturing process - Heading

The initial process is a cold forging process to produce a near net shape, called "heading", to produce the overall bolt shape. Bolts have a large head to shank ratio. Generally, they are longer than 1½ times the diameter, have multiple diameters, and require a geometrical shape (i.e. the rounded head).

Furthermore, this production method is probably used because cold heading has a very high production rate, and produces virtually no waste material. The process works as follows: the bar stock is fed into a machine, called a "former". The bar stock is stopped, and then gripped by a gripping die. The gripping die is shaped in such a way that it forms a carriage along the neck of the stock when its full gripping force is applied. An indented punch, the shape of a rounded head, will then proceed to move forward, colliding with the stock and causing the metal to flow at right angles to the ram force provided by the punch, increasing the diameter of the work billet, and reducing its overall length, thus forming a "rounded" head. The work billet is then sheared to the desired length.

Bolt manufacturing process - Thread rolling

The next process applied to the bottom of the work billet (the side without the head) is a bulk deformation process. known as "thread rolling". This type of cold-forming process was probably used because cylindrical parts (such as the remaining round part of the bolt) can easily have thread applied by rolling the work billet through two dies. The thread rolling process used was probably chosen over machining because thread rolling provides higher production rates, more effective material usage, stronger thread due to work hardening, and finally better fatigue resistance because the work billet undergoes compressive stresses during the rolling process.

Bolt manufacturing process - Blackening

The final process induced on the now almost complete bolt, is the application of an anti-corrosive layer, to stop oxidation, called "blackening". This process chemically coats the surface of ferrous metals, creating a strong barrier against humidity and corrosion. There are two types of blackening processes:

• hot blackening
• cold blackening.

Assuming that bolts are hot blackened; the implementation of this process is as follows: Hot blackening involves dipping the bolt into six different tanks. The bolt is usually "dipped" by automated part carriers for transportation between tanks. These tanks contain in order, alkaline cleaner, water, caustic soda at 455.4°C (the blackening compound), and finally the sealant, which is usually oil. The caustic soda bonds chemically to the surface of the metal, creating a porous base layer on the bolt. Oil is then applied to the heated bolt, which seals it by "sinking" into the applied porous layer. It is the the corrosion of the bolt. There are many advantages of blackening, mainly:

• blackening can be done in large batches (ideal for small bolts)
• zero dimension impact (the blacking process creates a layer about a micrometre thick)
• it is far cheaper than similar corrosion protection systems, such as paint and electroplating

Bolt manufacturing process - Materials

Bolt manufacturing process - Work-holding fixtures

During the "heading" process, the steel bar stock is gripped by gripping dies. In thread rolling, the work billet is held by thread dies. Finally, for the blackening processes the bolt, it is held by automated part carriers.

Adapted from the Wikipedia article "Bolt manufacturing process", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki