Advantages of Rolled Threads
Rolled threads have improved physical characteristics, greater accuracy and a high degree of surface finish. They are uniformly produced at high rates of production with no wastage of material. These six major advantages account for the increased use of thread rolling:
The cold forging that threads receive during the rolling process strengthens them in tension, shear and fatigue.
The cold working of the surface increases tensile strength of the metal worked, and static tensile test have frequently recorded increases in the order of 10% in the breaking strength of the parts.
When a thread is rolled the fibers of the material are not severed as they are in other methods of screw thread production, as shown in the image below, but are re-formed in continuous unbroken lines following of the contours of the threads, as in any good forging.
Resistance to Fatigue
Thread rolling increases a parts resistance to fatigue failure in several different ways. Rolling between smooth dyes leaves the thread with smooth burnished roots and flanks, free from tears, chatter or cutter marks that can serve as focal points of stress and, therefore, starting points for fatigue failures.
Rolling also leaves the surface layers of the thread, particularly those in the roots, stressed, in the compression. These compressive stresses must be overcome before the tensile stresses can be built up, which alone, can cause fatigue failures. This increase in root hardness, of up to 30%, adds considerably to the parts resistance and fatigue.