With macros and canned cycles resident in the CNC on most contemporary turning centres, single point turning of OD threads can seem like almost a default process decision. However, for numerous applications, OD thread rolling has inherent advantages as an alternative to cutting threads.
Thread rolling is not a metal cutting process. It does not remove metal to create the desired profile, but instead it forms the thread into a mirror image of a roller die.
Metalworking manufacturers are under competitive pressures like never before. It's no longer sufficient to make parts “the way we always have.” Increasingly, matching the best processing technique to a specific application can mean the difference between making money on a job or losing it. Margins are that tight.
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. Good plasticity in a material being cut usually results in a gummy chip formation and built-up edge on the tooling. Cast iron, for example does not have a proper flow characteristic and therefore would not be a candidate for thread rolling.
Successful thread rolling is a function of several application parameters. Among them is the type of thread to be rolled, its major diameter, pitch and root depth. Additional thread rolling considerations involve the ductility and cold-flow properties of the blank material.
Depending on the type of machine and thread that needs to be manufactured, shops have several thread rolling configurations from which to choose. An axial thread roller moves from the tailstock end of the turning centre, along the workpiece blank centreline, to create a thread
As the name implies, the tangential roller head makes its threads by approaching the workpiece blank from the side. Sometimes called side rolling or cross slide heads, tangential thread roller are designed to roll threads by pushing, at a controlled feed rate, two fixed parallel rolls onto the rotating component. Mechanical or servo feed is required for tangential thread rolling. It is not possible to operate these rollers manually.
A third type of thread roller is the radial head. It uses two or three rolls to form a thread in a single rotation of the workpiece blank. The rolls on this type of thread roller are ground eccentrically. Starting with a flat on each roll, the thread form is progressive. A shallow thread form starts at one side of the flat and full form at the other side.
Accurate and precise machining of threads is a key skill set for any shop. Considering thread rolling as a potential tool for the manufacture of threaded parts should be among a shop's processing strategies.
Thread rolling requires a tooling investment to be made in the heads and rollers, which is higher than a single-point threading insert. However, for applications that involve hardened material, high surface finish and surface integrity as wells as production volumes, thread rolling technology may be more cost effective over the long haul. Moreover, since the heads can be run on a CNC turning centre alongside single point threading, thread rolling can be flexibly applied as needed by the application—the right tool for the job.