Thread Forming
What is thread forming?
Thread forming is a threading method in which a thread forming tap shapes the thread profile in the workpiece through plastic deformation without producing chips. Unlike conventional (cutting) threading, material is not removed from the hole but instead is displaced and compressed into the shape of the thread. Thread forming relies on the process of cold forming and is a commonly used method in machine shops aiming for strong and precise internal threads efficiently and cleanly.
Thread forming is also known as thread rolling, and it is closely related to concepts such as thread tap, threading, and plastic deformation, which are widely used in the machining industry.
Principle of thread forming operation
The thread forming tap used in this process is non-cutting, meaning it has no chip flutes for chip removal. The profile of the tap is designed to carefully press the material sideways to form the thread crests and roots without any material loss. The hole’s starting diameter must be precise; a hole that is too small will result in excessive force, while a hole that is too large will weaken the thread’s shape and strength.
Because no material is removed, chip-induced microcracks are not formed at the thread roots. Instead, the material structure is compressed and the grain follows the thread profile, which improves the thread’s fatigue strength. This feature is especially important in machining applications where parts are exposed to dynamic loads.
Advantages of thread forming in machining
Thread forming offers several benefits in machine shop production. Chipless operation means there are no chips left in the hole, which is particularly important for blind holes and in automated CNC machining. The need for a chip conveyor or compressed air cleaning is reduced, and the process remains controlled even for deeper holes—typically up to about 3–3.5 × D in depth.
Thread surface finish is usually exceptionally good, with surface roughness lower than that of traditional cutting threads. At the same time, the compressive and fatigue strength of the thread improves due to work hardening. Tool life is often long, since the cutting edges are not overloaded by chip formation and the load is distributed more evenly across the forming surface.
From a production speed perspective, thread forming enables higher spindle speeds and a smoother process, enhancing productivity—especially in mass production.
Requirements for thread forming
Thread forming is not suitable for all materials. The material must be sufficiently ductile and formable. Typical appropriate materials include structural steels, stainless steel, aluminum, brass, and copper. It is generally recommended that the material’s tensile strength does not exceed about 1200 N/mm², although the exact limit depends on the material’s toughness and lubrication conditions.
Lubrication is crucial because forming creates significant surface pressure and friction. The right choice of cutting fluid or forming oil reduces friction, improves surface quality, and extends the tap’s service life. The machine’s available torque is also important, since thread forming requires higher torque than cutting threading methods.
Pre-hole sizing is a critical part of the method. The hole diameter is typically made larger than with cutting threading, so there is enough material to form the correct thread profile according to tolerances.
Thread forming vs. cutting threading
In cutting threading, the tap cuts away material to form the thread, producing chips. This is suitable for hard or brittle materials, but creates the need for chip control and may weaken the structure at the thread root.
Thread forming, by contrast, shapes the material without removing any of it. The result is a dense and strong thread with a smooth surface that withstands load well. However, this method demands precise process control, adequate lubrication, and suitable material selection.
In machine shops, the choice between thread forming and cutting threading often depends on the material being machined, batch size, machine capacity, and quality requirements.
Summary
Thread forming is an efficient and reliable method for producing internal threads in machining. It is based on the plastic deformation of the material without chip removal, resulting in a strong and smooth internal thread. When done correctly, the method improves the fatigue strength of the thread, reduces chip-related issues, and increases shop productivity. A successful outcome, however, requires suitable material, precise pre-hole dimensions, sufficient force, and effective lubrication.
