How a bolt is made
Two very different routes make the bolts on this site. A hex bolt is cold-headed from wire at a mill, hundreds a minute, and MF imports those. An anchor bolt is cut, threaded, bent and welded from round bar, one at a time, and MF makes those at Umm Al Quwain. Press play on either line. Then look at the thread — because how it was made is the difference you cannot see from the outside.
What you can ask us
- Which route your bolts came by — headed at a mill, or made from bar here.
- Whether the thread is rolled or cut, and whether rolling was before or after heat treatment.
- The heat number, and the mill certificate behind it.
- To see the line. The plant is in Umm Al Quwain and visitors are welcome by appointment.
Rolled thread or cut thread
A cut thread is made by removing metal — a die or a lathe tool carves the profile out of a bar that is already the major diameter. A rolled thread is made by displacing metal — hardened dies press the profile into a blank that is between the major and minor diameter, and the material flows up into the crests. Nothing is removed. That one difference decides how the thread behaves for the rest of its life.
Cut. The grain of the bar runs along its axis, and the thread is carved across it. Every flank and every root ends on a severed grain — each one a place for a fatigue crack to begin. The surface is torn by the tool rather than smoothed.
Rolled. The grain follows the thread contour, unbroken, like a forging. The flanks are burnished by the dies and the root is left in compression — both of which resist fatigue. Industry sources commonly put the fatigue improvement at 50 to 75 percent over a cut thread of the same size.
Where it matters: any bolt that sees cyclic load — vibrating machinery, wind, traffic, thermal cycling. Where it matters less: a static anchorage that is tightened once and never moves. A rolled thread also uses less steel, because the blank is smaller than the finished thread, which is why a rolled bolt is not more expensive than a cut one.
Questions we are asked
Why does it matter whether the head is forged or machined?
A cold-headed bolt has its head upset from the bar, so the grain flows round into the head and the head-to-shank corner is the strongest part of the bolt. A head machined from oversize bar has the grain cut across at exactly that corner, which is where a bolt under bending or fatigue load fails first. The wedge tensile test in ISO 898-1 exists to catch a weak head.
Should the thread be rolled before or after heat treatment?
After, for fatigue-critical joints. Rolling before heat treatment forms the thread while the steel is soft, then the heat treatment relaxes the compressive stress the rolling put into the root. Rolling after heat treatment keeps that compressive stress in the finished bolt, which is what gives the fatigue benefit — and is why some specifications call for it explicitly and why it costs more, since the dies are working hardened steel.
Can an anchor bolt be cold-headed like a hex bolt?
Headed anchor bolts exist, but the L, J and U shapes and the long lengths that most foundations call for are not a heading job — they are cut from bar and formed. That is why an anchor bolt line looks nothing like a hex bolt line, and why MF, which makes anchor bolts, imports hex bolts rather than heading them.
How do I know which route my bolts came by?
Ask, and look at the certificate. A cold-headed bolt from a mill has the mill\u2019s identification mark on the head and a certificate in the mill\u2019s name. A bar-made anchor bolt has MF\u2019s certificate carrying the bar mill\u2019s heat number. Either is a proper product; what matters is that the route is stated and the paper matches the goods.
Related
How to read a test certificate · How anchor bolts fail · Galvanizing & thread fit · Build Your Bolt · Back to the Bolt Lab