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Minimum bending radius when bending bars

Pre-bending and bending process of profile bending machine

Cold bending bending process

There is no mathematical formula to determine the minimum bending radius of steel sections.

To explain this better, let’s look at steel bending. The steel is bent using a cold bending rolling process.

The steel profile is placed in a profile bending machine (also known as an “angle bending machine”) with a three- or four-roll configuration.
The roller presses force on the feed path to force the metal blank into a predetermined curved path.

The force (stress) applied by the rolling mill transforms the structural steel into a curved shape (strain).

If you look at the chart below, steel is very ductile, and the material will undergo plastic deformation in a very brief elastic state. The goal is to achieve the required radius without reaching necking or eventually reaching the breaking point.

Minimum bending radius when bending bars
Minimum bending radius when bending bars

Looking back at the above chart, the goal is to maximize the elongation limit during the bending of the steel bar.

This is done by using a tougher steel.

A case study was recently conducted to bend a 2.25 inch diameter round rod to an inner radius of 3 inches.

Two different material grades are used.

The first is AISI 1144, which does not have a lot of malleability. The reason is that 1144 is a free-process material with sulfur added to make it more brittle. It is made in this way so that when the cutting tool encounters sulfide inclusions, the material is easier to shred. When the 1144 was bent to an inner radius of 3 inches, the material could not withstand the stress and cracked.

The second material is 4140 HR alloy with a typical elongation of 25%, while 1144 has a typical elongation of 10%. The 4140 successfully achieved a 3-inch inner diameter without fracture. This case study better illustrates that the minimum radius of different material grades is not the same.

Further reading:Bar Bending