Inside the Thread Rolling Process

Cold Forming Means Displacing, not Wasting Material

Cold forming is perhaps the best description of the thread rolling process.
Here's how it works:

1. A cylindrical blank having an outside diameter between the major and minor diameters of the finished thread is rotated between hardened steel dies bearing the reverse thread form.
2. The die threads penetrate the blank surface to form the thread roots and displace material radially outward to form the crests.
3. Unlike other threading processes, material is neither removed nor wasted, but displaced

Improved Tensile Strength, Shear & Fatigue Resistance

Static tensile tests on parts formed through thread rolling show overall increases in strength of about 30 percent.  Fatigue strength is reported to be improved by 50-75 percent.  Tests on bolts first heat-treated to a hardness of 35-40 Rockwell C and subsequently rolled show even greater fatigue strength.

In thread rolling, thread grain structure is not severed; instead, it's reformed in continuous, unbroken lines following the thread contours.  Rolled threads have increased resistance to stripping because such failures are compelled to take place across, rather than with, the grain flow.

Threads are produced with burnished roots and flanks, free from surface imperfections that might prove to be starting points for fatigue failure.  Surface layers of the thread, particularly those in the roots, are stressed in compression.  These compressive stresses must be overcome before the tensile stresses that cause fatigue failure can be built up.