The Thread Rolling Paradox

Theoretically, thread rolling is impossible because as a roll enters a blank, the ratio of diameters (roll to blank) is constantly changing. As a result, it would seem that the synchronization of the thread starts cannot be maintained. How then is thread rolling accomplished? The secret lies in the design of the thread roll holder, the selection of the roll diameter, and the design of the feed control cam on the machine tool.

Straddle type thread roll holders, for example, have a certain amount of gear wind-up or hack-lash built in, that will accommodate corrective synchronization motion of the rolls during in-feed. There is also a small but critical amount of axial float provided for the rolls to further accommodate these self-adjusting motions. Next, the roll diameters are chosen so that the thread starts are in perfect synchronization when the rolls have achieved 50% penetration of the blank, thus minimizing the mismatch in diameters. Obviously, once full penetration of the blank has been achieved, it is important to remove the rolls from the blank as quickly as possible. Otherwise, the small amount of backlash provided by the holder will be consumed and the roll will start to strip the threads from the blank. That is where proper cam design comes in.