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.
How Threads Are Formed
As the face of a thread die is forced against a rotating stock, the crest of the threads on the roll die begin to compress the work material. These initial points of contact will become the base or root of the rolled thread. As material is displaced by the penetration it is forced upward and outward from the side of the die thread flank, thus forming the crest of the roll thread on the blank. The nature of this
flow of material must be taken into consideration.
The 3 to 1 Ratio
When a thread roll has almost reached its maximum penetration, the work on the outside diameter of the part will grow three times for every increment of additional penetration. In other words, if it is determined that a part is being made .003 inches under size, it will only require .001 inches additional die penetration to bring the diameter of the threaded part up to size. This is a very critical adjustment, for if a die penetration is too great, the root area of the roll die threads will be rapidly filled up, causing rapid wear or breakage of the thread roll. The most effective way to control the diameter on the finished part is through careful preparation of the blank.