Thread rolling vs thread cutting: rolled and cut threads compared

How each thread is made

Thread rolling squeezes a round blank between hardened dies (flat dies, cylindrical dies or planetary dies). The metal flows from the roots into the crests. Nothing is removed, so the blank is smaller than the finished major diameter.

Thread cutting removes metal from a blank of full major diameter, using a die head, chasers, a single-point lathe tool, a thread mill or, for internal threads, a tap. The finished thread form is the same; only the way it is made differs.

Grain flow, strength and fatigue

Cutting severs the bar's longitudinal grain flow at every thread flank. Rolling bends the grain flow around the thread profile, and the cold work hardens and burnishes the roots and flanks.

Rolling also leaves compressive residual stress at the root, where fatigue cracks start. Applied tension has to overcome this before the root sees tensile stress, so rolled threads have a markedly longer fatigue life under cyclic load. Trade sources report fatigue gains of the order of 50–75 % over cut threads.

The gain is largest when threads are rolled after the bolt is heat treated, because heat treatment after rolling relieves part of the compressive stress. The VDI 2230 bolted-joint guideline gives separate fatigue values for threads rolled before and after heat treatment for this reason.

Hot-dip galvanizing after rolling reduces the fatigue advantage of rolled threads, according to fatigue tests on tension rods.

Blank diameter and reduced-body parts

Because rolling displaces metal outward, the blank is turned or drawn to a diameter between the thread's major and minor diameters, close to the pitch diameter. The exact blank size depends on the thread, the grade and its ductility, and is set by trial on the rolling machine.

Where the unthreaded shank is left at blank diameter, the body is slightly slimmer than the thread major diameter. Product standards state whether this reduced body is permitted. Full-body bolts with rolled threads need a stepped blank or extrusion of the thread end.

Threaded rod and studs can be rolled from smaller bar than cut threads would need, saving material.

When each method is used

Rolling is the normal method for mass-produced external fastener threads and threaded rod, where fatigue resistance, consistent thread form and low material waste matter.

Cutting is used where rolling is impractical:

• Very large diameters or coarse pitches beyond the capacity of available rolling machines.
• Short runs and prototypes, where rolling dies are not justified.
• Steels too hard or not ductile enough to roll, and parts already hardened beyond the rolling range.
• Threads up to a shoulder or on parts that cannot be held in rolling dies.
• Special forms such as square threads, and internal threads, which are tapped or cut (thread forming taps are the internal equivalent of rolling).