Variables in metal bending
While sheet metal gauges run from 0.005 to 0.249 inch thick, aluminum and steel plate thicknesses start at 0.250 in. and go all the way up to 13 in. or even more. Likewise, plate steel varies in strengths from mild varieties to some very high-strength materials.
When it comes to very thick or high-tensile-strength material, traditional rules for determining minimum bend radii, minimum punch nose radii, die openings, bending force calculations, and tooling requirements may no longer apply—at least not in the same way that they do when working with thinner gauges.
- The most practical, accepted, and economical way to make large-radii bends generally is to cold-roll the material. Many variables have a bearing on the type of equipment you should choose, including wall, flange, and leg thickness.
- For large-radii bends, cold rolling achieves the desired radius with a minimum number of passes. Because of their size, wide-flange beams can be cold-rolled as well.
Because the workpiece can be extremely thick and strong, you need to understand the variables and learn how to work with them.
Plastic deformation in cold bending process
First, consider the material’s chemical composition, its surface and edge condition, as well as its thickness, and determine whether the bend is with or across the material’s grain direction.
All forming, regardless of scale, involves some kind of plastic deformation. Material expansion occurs on the outside surface of the bend, compression on the inside. The limits of material ductility will be the controlling factor for the minimum bend radius.
The strains associated with the plastic deformation when cold forming can cause the material to strain-harden. This can change the material’s mechanical properties in the area of the bend, where plastic deformation is occurring. At this point, ductility and resistance to fracture will need to be considered.
Why need to abide by a minimum inside bend radius?
No matter the material, its gauge or thickness, mild steels and soft aluminums are much more ductile than high-strength materials and, therefore, can be bent to a sharper radius. That’s why when bending thick or high-tensile metals, you need to abide by a minimum inside bend radius. This will minimize the effects of strain hardening and cracking at the bend.
The material supplier’s product data sheets normally outline the extent to which the plate can be formed without failures, recommending minimum bend radii by material type and properties. Generally, low-carbon-content steel or soft aluminum is necessary for good formability and a tight inside radius; but as the level of carbon in the steel or the hardness of the aluminum increases, its ductility and formability are limited, increasing the minimum radius that can be produced.
Importance of Grain Direction
When working with plate, pay close attention to whether you are forming with (longitudinal) or across (transverse) the grain direction . A plate’s grain direction comes from the mill’s rolling process, which stretches the metallurgical structure and inclusions of the material. The grains run parallel to the rolling direction.
Forming with the grain requires less bending force because the material’s ductility is readily stretched.
But this stretching causes the grains to spread, which manifests as cracking on the outside bend radius. To prevent or at least reduce this cracking when bending longitudinal to the grain direction, you may need to use a larger bend radii. When bending transverse to the grain direction, the reduced ductility will increase the required forming tonnage, but it will be capable of accepting a much tighter inside bend radius without destroying the outside surface of the bend.
A metal workpiece’s lattice structure has what’s known as slip planes that interact when forming.
When a tube, beam, or open section bends, compression builds on the inside radius and tension builds on the outside. Left uncontrolled, especially on thin-walled workpieces, these forces create localized distortion like wrinkling or buckling on the inside radius, wall thinning and shrinkage on the outside radius, and distortion and ovality of the overall profile shape.
Localized distortion in bending square tubes
A square tube can morph into a trapezoid, with excessive growth on the inside-radius dimension accompanied by shrinkage on the outside radius and to the cross-sectional profile in the plane of bending. Rectangular tubes, left unsupported during bending, might become concave, especially on the inside radius wall. The web and flanges of structural beams can ripple.
In a sense, compression and tension forces metal when under constant yield to “flow” in certain ways and to certain unconstrained areas. Consider the bending of a rectangular tube. If the tension forces pulling against the outside wall generate excessive stretching, that wall can shrink, which in turn affects how metal “flows” or moves elsewhere. The metal growth and shrinkage takes the path of least resistance. And in an unsupported situation, due to the counteracting forces of compression and tension, this path might be toward the member’s neutral axis and often offset to the inside of the bend; hence, the outside wall also can become concave. These tension forces, combined with compression on the inside radius, cause the inside wall dimension to grow. Left with no place to go, the metal on the inside wall buckles and, again, becomes concave.
Operators aim to control how tension and compression forces act upon a workpiece, through machine and tooling selection and movements throughout the bending operation, to control where the growth and shrinkage occur. It’s all done in a way that doesn’t affect the finished product’s design intent and strength requirements.
All steels, aluminums, and even plastics exhibit springback upon release from the bending forces. Springback is the release of elastic strain and is related directly to the material yield strength. It’s the reason you need a greater bend angle to achieve the required angle, especially for high-yield-strength steels and most aluminums.
The radius increases, so will springback, and the amount of springback can be significant when the radius is large in relationship to the sheet or plate thickness.
Minimum Inside Bend Radius
For steel, aluminum, and stainless you will find a variety of minimum bend radii-to-thickness ratios, and you will need to research these values in data provided by your material supplier. When researching these values, though, be aware that bending transverse (across the grain) or longitudinal (with the grain) will have an effect on the minimum bend radius required. Longitudinal bending requires a larger radius than those stated for transverse bending.
As the thickness increases, so does the minimum radius. For 0.25-in.-thick 6061 in an “O” condition, the material supplier may specify a 1-to-1 inside radius-to-plate-thickness ratio. In 0.375-in.-thick aluminum, the minimum radius is 1.5 times the thickness; for 0.5-in.-thick, it’s 2 times the thickness.
Minimum radius increases
The minimum radius also increases with harder material. For 0.25-in.-thick 6061 in a “T4” condition, the material supplier may specify the minimum radius to be 3 times the thickness; 0.375-in.-thick plate may have a minimum radius of 3.5 times the thickness; for 0.5-in.-thick plate it can be 4 times the thickness.
The trend is obvious: The harder and thicker the plate is, the greater the minimum bend radius. For 0.5-in.-thick 7050 aluminum, the minimum bend radius may be specified as much as 9.5 times material thickness.
Again, the minimum inside bend radius is even larger when bending with the grain. In steel between 0.5 and 0.8 in. thick, grade 350 and 400 may have a minimum bend radius of 2.5 times the material thickness when transverse bending, while longitudinal bending may require a minimum bend radius that’s 3.75 times the material thickness.
Notes: And between 0.8 and 2 in. thick, you likely will need to hot-form.
A Simple Rule of Thumb
There’s a rule of thumb to determine a steel’s minimum bend radius, and this generally works for aluminum too: Divide 50 by the material’s tensile reduction percentage as specified by your supplier. This value will vary by grade.
If the steel has a tensile reduction value of 10 percent, divide 50 by that value: 50/10 = 5. Next, subtract 1 from that answer: 5 – 1 = 4. Now, multiply that answer by the plate thickness. If the material is 0.5 in. thick: 4 × 0.5 = 2. So in this case, the minimum inside bend radius is 2 times the material thickness.
Note that this is just a rule of thumb. Finding the true minimum bend radius for steel or aluminum plate requires a little research. This should include data from your material supplier, whether you are bending with or against the grain, as well as information specific to the application.
Cold bending, as the name suggests, bends the workpiece in a cold state. On occasion, cold bending of large profiles occurs in a rotary draw machine.
The three-roll section bending machine and plate bending machine are the industry workhorse. The machine has three hydraulically driven rolls in a triangular configuration.
In a typical horizontal configuration, viewed from overhead, material is fed between the two top rolls and single bottom roll until the end of the material touches the far roll. The distance between the middle of the far roll and the middle of the bottom roll is called the grip length, which provides leverage to induce the force needed to create the bending moment.