Q What are the differences between cut and rolled thread?
All America Threaded Products forms the majority of its threaded rod and bar products using the thread rolling process. Cold forming is the best description of this process. All America Threaded Products feeds a cylindrical steel blank, of diameter between the minor and major diameter of the finished thread size, into hardened steel dies bearing the reverse thread form to the finished parts. All America Threaded Products “matches” the dies to produce the correct thread form. The rotating die threads penetrate the blank surface, forming the thread roots and displacing material radially outward to form the thread crests. There is no material removal or waste.
This technology produces a significantly stronger thread than possible when the material is cut away in conventional cut-thread die methods. More specifically, the thread rolling (cold forming) process strengthens the thread in three ways: tension, shear, and fatigue resistance.
Unlike thread cutting which severs the grain structure in the steel, thread rolling reforms the grain structure in a continuous line along the thread form. Rolled threads then have increased resistance to stripping under heavy stresses as the material “closed-up” from work hardening along the root, flanks and crest. Rolled threads compel failures to take place across the grain flow, rather than with the grain flow in weaker cut threads.
Thread rolling improves fatigue resistance. The process results in “burnished” roots and flanks with consistent surface finish. Surface imperfections from thread cutting can become the starting points for fatigue failure.
Surface layers of the rolled thread, particularly those in the roots, are under compression stress. Tightening and other forces must overcome compression stresses before tensile stresses that cause failure can build up. Thread rolling improves a thread’s capacity to resist these fatigue stresses.
In summary, rolled threads have many advantages to offer versus cut threads:
- economical to produce at high rates
- increases tensile strength
- lower material cost and extended die life