The threads of the bolts and nuts can be processed by machining or thread rolling. Some standards do not allow to use machining. What is the problems in machining?
Is simple. If you make a tensile test pulling only from the thread, you obtain a ultimate load at least 10% plus with forming thread that in machined thread.
Thread rolling is a material displacement process while thread machining is a material removal process. Therefore, machined threads will have discontinuity of 'grain-flow', and increased notch effect which will lead to inferior mechanical properties compared to rolled ones.
The grains of rolled threads will flow along or follow the thread contours and thus reduce the notch effect and increase the fatigue life up to 50% compared to the machined ones.
During thread rolling the material is plastically deformed which results in strain hardening, and increased strength. This gives better adhesive wear resistance.
You can achieve 20% savings on material and machining time with thread rolling compared to thread machining.
1. Joining done by threaded joints are vulnerable to failure to stress concentration near the hole.
2. Cost in manual tightening a screw can be six times the cost of screw.
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