Crest Cracks in Rolled Threads: Blank Diameter and Wire Ductility in 304Cu

Thread rolling does not cut material, it displaces it. The thread is formed by the wire flowing between two dies, and every crest on the finished screw is metal that had to move there. When that flow is short, the crest is underfilled; when there is too much material to place, the excess folds over itself at the crest. Both defects read as a crack on an inspection bench, and both are decided before rolling by the blank diameter and the wire condition.

Rolling Demands Something Different From Heading

Cold heading is bulk compression: the whole section deforms and the loads are compressive in the centre. Thread rolling is surface flow. The strain is concentrated in the outer layer, it is tensile at the crest, and it is repeated by every pass of the die across the blank.

That difference is why a wire that heads cleanly can still crack at the thread. The heading operation is finished when the head is formed; the rolling operation asks the same material to flow again, into a much thinner and more sharply curved feature.

Underfilled and Overfilled Crests

• Underfilled crest. The blank is too small, or the material will not flow far enough. The crest is rounded, the minor or pitch diameter measures out of tolerance, and a No-Go gauge can pass while the thread is already too shallow to carry its rated load.
• Overfilled crest with a fold. The blank is too large for the die cavity. Excess material is pushed out at the crest and folds back, leaving a lap that looks like a crack and behaves like one in fatigue.
• Longitudinal crack at the crest root. Not a filling problem but a ductility limit: the wire work hardened during heading and has no reserve for the rolling strain.

Blank Diameter Starts at the Pitch Diameter

The rule of thumb is that the blank diameter is close to the pitch diameter of the finished thread, because the volume displaced from the flanks and root has to fill the crests. In practice the starting value is calculated from the thread profile area, then corrected by trial: rolled onto sample blanks, measured with Go and No-Go gauges, sectioned, and adjusted in steps.

A copper-bearing grade tolerates a blank slightly further from the ideal, because it fills the crest more readily and recovers from local overstress. That tolerance is useful in production, but it is not a licence to skip the trial: the correct blank diameter for a specific thread, die set and wire is established by measurement, not by a table.

What the Wire Has to Deliver

• Ductility after heading, not only before it. The relevant condition is the wire as it arrives at the rolling station, work hardened by the head. A soft annealed 550 - 650 MPa wire with 40 percent elongation is the starting point, not the guarantee.
• Uniform grain size, ASTM 6 - 8. A coarse grain flows unevenly and produces a wavy crest line along the thread.
• Surface free of transverse defects. A drawing scratch on the blank becomes a longitudinal crack at the crest root.
• A coating that survives heading. Oxalate and soap carriers hold lubricant through both operations; bare bright wire will gall on the die face at the rolling station.
• Consistent diameter. Since blank diameter decides fill, a diameter drift along the coil is a fill drift along the production run.

Die and Process Settings

Lubricant and the Chlorine Question

Rolling oils for stainless usually carry extreme-pressure additives, and some of those are chlorine based. They work, but chlorine residue left on a thread is the condition that starts pitting in 304 and can contribute to stress corrosion cracking in service. Where the fastener is used outdoors, near the coast or in contact with chlorinated water, a chlorine-free rolling oil with a clean post-rolling rinse is the safer route, and it costs nothing in thread quality if the additive package is chosen properly.

Verification

• Go and No-Go gauges on pitch and minor diameter, at the start, middle and end of a production run.
• Crest inspection at magnification. A 10 to 20 times loupe shows a fold as a line with an oxide shadow; a ductile tear has a rough, bright appearance.
• Thread section. Mount, polish and etch a longitudinal section to see the grain flow following the thread profile. Continuous flow is the sign of a correctly rolled thread; a cut or broken flow line indicates a forming problem.
• Wedge and tensile test per the applicable fastener standard, since a thread that passes gauges can still fail under load.

Wire Specification

A rolled thread is only as good as the blank that entered the dies. Set the blank diameter by trial, hold the wire ductility through heading, and the crest will fill without a fold.