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bolts Manufacturing Process

Bolts Manufacturing Process

Cold Forging:

This initial process is a cold forging process used to produce a near net shape work billet. An important consideration in cold forging is the tendency for the bar tobolts Manufacturing Process buckle if its unsupported length to diameter ratio is too high. This ratio usually is limited to less than 4:1 but with suitable dies, therefore, cold forging is the most likely production method because bolts have a large head-to-shank ratio, they are longer than 1½ times their diameter, bolts have multiple diameters and finally require a geometrical shape

Exaggerated process diagram of the bolt "cold forging process"

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.

Thread rolling

The next process applied to the bottom of the work billet is a bulk deformation process known as "thread rolling". This type of cold-forging process is used bolts Manufacturing Processbecause cylindrical can easily have thread applied by rolling the work billet through two dies. The thread rolling process is usually 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.

Blackening and galvanization

The final process induced on the now almost complete bolt is the application of an anti-corrosive layer, to stop oxidation. The usual process is called "blackening"; Either process chemically or physically coats the surface of ferrous metals, creating a strong barrier against humidity and corrosion. Hot Blackening is most popular & common blackening processes:

Hot blackening

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 450°C to 500°C and finally the sealant, which is usually crude oil. The caustic soda bonds chemically to the surface of the metal, creating a porous base layer on the bolt. Crude Oil is then applied to the heated bolt, which seals it by "sinking" into the applied porous layer. It is the crude oil that prevents the corrosion of the bolt. There are many advantages of blackening, following are major

  • Blackening can be done in large quantity at a time
  • zero dimensional 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

for attractiveness and superior corrosion resistance galvanization may be used. This incurs an additional cost but may extend the life of bolts deployed outdoors considerably.



Work-holding fixtures

During the "cold forging" 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 process the bolt is held by automated part carriers.



How a Headed Bolt is Manufactured

Raw materials-Fastener DimensionsStep 1
Manufacturing Process

Raw material is selected from bonded storage racks. Blanks are cut to size and lubricated before hot forging.
Step 2
Manufacturing Process
Fastener Dimensions'
Manufacturing Product Showcase

Manufacturing Process
Into the induction heater

Manufacturing Process
After heating, the blank is removed
Manufacturing Process
Bolt fastener part is placed in press
Manufacturing Process
Heading press is engaged and punches down
Manufacturing Process
...and forms head

Manufacturing Process
The headed component is now ready for the next process

heat treatment-Fastener DimensionsStep 3
Manufacturing Process
After the forging process, parts are routed to heat treatment for normalize and temper. Parts are then subjected to numerous secondary operations, depending on the customers requirements and the part configuration.
Hot Rolling Threads-Fastener DimensionsStep 4 (Hot rolling threads)
Manufacturing Process
Exotic material require hot rolled thread via induction heating in insure defect free threads and grain flow for safety critical applications.

Step 5

Manufacturing Process
Fatigue testing-Fastener Dimensions Stress Durability-Fastener Dimensions Penetrant Inspection-Fastener Dimensions Tensile Testing-Fastener Dimensions
Fatigue Testing Stress Durability Penetrant Inspection Tensile, shear
and double
shear testing
Step 6
Manufacturing Process
After the secondary operations are completed, destructive and non-destructive testing is performed per blueprint specification. Testing is extensive and includes the following processes; tensile strength, fatigue, grain flow and size, full metallurgical examination, stress rapture, micro hardness, penetrant and magnetic particle inspection along with hydrogen determination for titanium alloys.

Single Element Thread Gauging-Fastener DimensionsStep 7
Manufacturing Process
Single element thread gauging for final inspection which includes system 22 and 23 safety critical criteria per MIL-S-8879 and MIL-S-7742 specifications.

Inspection-Fastener DimensionsStep 8
Manufacturing Process
Helix, flank angles and root radius are inspected on optical comparators to insure parts exceed thread specification criteria.


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