A cold-forming process in which a blank close to the thread's pitch diameter is rolled between flat reciprocating dies or round dies that push metal down into the roots and up into the crests. Rolled threads are fast, waste no material, keep unbroken grain flow and carry compressive stress in the root, so they have much better fatigue strength than cut threads. Rolling after heat treatment raises fatigue strength further and is specified on critical high-strength bolts.
These are rolled, look how the thread OD is bigger than the shank, they started from a reduced-body blank.
A shank noticeably thinner than the thread OD is a quick sign of a rolled thread on a reduced-body blank.
Also heard as
- rolled thread
- roll threading
Do not mix it up with
- Thread cutting: Thread cutting removes metal as chips and severs the grain; rolling displaces metal and keeps it.
Related terms
Thread cutting
Making a thread by removing metal with a tap, die, single-point tool or thread mill.
UNR
Unified National Round root: an external inch thread with a required rounded root, for better fatigue life.
Fatigue
Cracking that grows a little with every load cycle until a bolt breaks far below its static strength.
Pitch diameter
The diameter at which thread thickness equals the gap between threads, the dimension that controls fit.