Step-1: Raw material inspection & storage
Raw material for the bolt to be decided upon the designer based upon application. Material to be used to be inspected and then proper storage location to be identified in order to avoid rust formation and proper coverage needs to be provided.
Step-2: Unthreaded / Unfinished bolt manufacturing
Mostly casting and forging are used in this stage.Other methods may also be used:
1. Sintering
2. Prototyping (Rapid)
Step-3: CNC machining
After the part has been manufactured via forging/casting route, it is then machined to the required dimensions usually by CNC.
The operations followed here are: pointing, facing, grooving.
Step-4: Heat Treatment
Heat treatment is done after machining to make the fasteners stronger. Hardening & tempering operations are followed.
First, hardening is done such that bolt is heated to a temperature of 850-900°C and then quenched in the cooling media.
Secondly, the bolt is again re-heated to bring back the extreme hardened bolt less soft, so that the bolt remains stronger. Re-heating is done to bring down the brittleness of the bolt which happened during hardening.
Step-5: Surface Finishing
Next is the surface finish process. Usually, grinding process is carried out to make the surface smoother according to the surface finish specification.
Step-6:Thread Rolling
After finishing process, thread rolling is done with two dies. One is stationary and another is a moving die which actually exerts pressure on the bolts and forms threads.
Step-7: Coating
After thread rolling, bolts and screw fasteners are coated in order to prevent rust and corrosion. A fine example of bolt coating is geomet coating in bolts which will be tested to SST (Salt Spray Test) depending upon the number of hours as specified.
Machinery called FISCHERSCOPE is used to determine coating thickness along with other thickness measuring instruments such as coating thickness meter.
Step-8: Inspection for Form, Fit & Function:
After all the processes being done, finally the part goes to inspection. It should meet
1.Torque test, SST
2.Fitment with nut
3.Impact strength (Impact charpy test)
4.Tensile strength test (%elongation of bolt)
5.Core hardness of bolt
6.Coating thickness
Dimensional inspection etc.
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