Cold Rolling Threads vs Cutting Threads - Thread rolling is the preferred method for producing strong, smooth, precise and uniform external thread forms. RHINO has developed a patented process for cold rolling threads on rebar that gives the RHINO’s Ultra System superior performance in tensile and fatigue applications. Cutting threads actually remove rebar material while rolling the threads will displace the rebar material with hardened dies. The result of moving material grains (molecules) into shape of the thread rather than weakening it by removing material, is that the grains become denser at the critical parts of the thread, especially in the root and on the flank below the pitch diameter. This effect improves the quality of the thread form. Additionally, the burnishing action of the steel dies produces an excellent micro finish. The superior finish improves assembly between external and internal threads.
RHINO’s Cold Roll Threader (CRT) is used in applications where superior splicing requirements are needed. RHINO’s CRT equipment is fast , consistent and easy to use allowing for optimal set-up and operations at a fabricator, end user or job site. RHINO’s CRT technology along our standard taper splicing system will meet or exceed the most stringent codes in the world.
Benefits of Cold Rolling Threads
Tensile Strength - The cold working of the surface increases the tensile strength of the metal worked, and static tensile test have frequently recorded increases on the order of 20% in the breaking strength of the parts.
Shear Strength - When a thread is rolled the fibers of the material are not severed as they are in other methods of screw thread production, but are re-formed in continuous unbroken lines following the contours of the threads. Rolled threads resist the stripping because shear failures must take place across rather than with the grain.
Resistence to Fatigue - Thread rolling increases the part’s resistance to fatigue failure in several different ways. Rolling between smooth dies 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, up to 30%, adds considerably to the parts resistance to fatigue.
Accuracy - The thread form of a set of thread rolling dies is faithfully reproduced on the parts, and does not change appreciably during the entire life of the dies. Thread rolling dies do not wear out in the same manner as do other threading tools. Wear, instead of being concentrated on a sharp cutting edge, is distributed over a broad surface, and the rolling action is relatively free from friction. Therefore, the thread form of a rolling die is not changed by erosion, nor does it fail to reproduce itself because of dullness or adhesion. It cannot be altered by improper sharpening as sharpening is never required.
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