Thread Rolling: A Non-Metalcutting Operation

Although I've used this column as a learning tool for typical metalcutting processes so far, I decided to delve into a different type of operation this month—thread rolling. Thread rolling is unlike a typical metalworking operation in that it does not cut metal. Instead, it forms a thread into a mirror image of a roller die.

Why use this operation in lieu of cutting threads? Rolled threads have improved physical properties. The cold working that takes place in the thread rolling process produces an increase in tensile strength and a better surface finish than what is achieved with cutting operations.

The thread rolling process occurs when the form of the roller, or rollers, in the thread rolling head imparts the shape of the thread into the workpiece blank. The compression of the parent material results in a work hardening condition from compressing the metal matrix. This process should be done after heat treating because any subsequent heat treating of a rolled thread will have an annealing effect.

Successful thread rolling depends on individual application parameters such as type of thread to be rolled, its major diameter, pitch and root depth. Also important is the ductility and cold-flow properties of the blank material. Thread rolling relies on the plasticity of the base material (its ability to deform) to permanently imprint the shape of the rolls on the workpiece blank.

Depending on the type of machine and thread that needs to be manufactured, shops can choose from axial, tangential or radial thread rolling configurations. A head carrying the rollers is placed axially, tangentially or radially to the workpiece. An axial thread roller moves from the tailstock end of the turning center along the workpiece blank centerline to create a thread. Tangential thread rolling makes its threads by approaching the workpiece blank from the side. Radial thread rolling uses two or three rolls to form a thread in a single rotation of the workpiece blank.