Figure 1: The grain of a plate runs parallel to the rolling direction.
While sheet metal thicknesses range from 0.005 to 0.249 inches thick, aluminum and steel sheet thicknesses start at 0.250 inches and go all the way to 13 inches or even more. Similarly, sheet steel varies in strength from mild varieties to some very strong materials such as Hardox®. When dealing with very thick or high tensile strength material, the 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 as when working with thinner gauges.
Because the workpiece can be extremely thick and strong, you must understand the variables and learn to work with them. First consider the chemical composition of the material, the condition of the surface and edge, and the thickness, and determine whether the bending is with or across the grain of the material.
All shapes, regardless of scale, experience some form of plastic deformation. Material expansion occurs on the outside of the bend, compression on the inside, and you need to know how to handle both. The limits of the material's ductility will be the determining factor for the minimum bend radius.
The stresses associated with the plastic deformation during cold forming can cause the material to harden. This can cause the mechanical properties of the material to change in the area of the bend where plastic deformation occurs. At this point, ductility and resistance to fracture must be taken into account.
Regardless of the material, gauge or thickness, mild steel and mild aluminum are much tougher than high strength materials and can therefore be bent to a sharper radius. Therefore, when bending thick or very strong metals, you must maintain a minimum inside bend radius. This minimizes the effects of stress hardening and flexural cracking.
The material supplier's product data sheets typically outline the extent to which the sheet can be formed without defects and recommend minimum bend radii depending on the material type and properties. In general, low-carbon steel or soft aluminum is necessary for good formability and a narrow inner radius; but as the carbon content in the steel or the hardness of aluminum increases, its ductility and ductility are limited, increasing the minimum radius that can be produced.
Fiber direction when bending metal
When working with cardboard, pay close attention to whether you are multiplying by (longitudinal) or across (transverse) the direction of the fiber (seeFigure 1). The grain direction of a plate results from the mill's rolling process, which stretches the metallurgical structure and inclusions of the material. The grains run parallel to the rolling direction.
Molding with the grain requires less bending force because the ductility of the material is easily stretched. But this stretching causes the grains to spread, which is visible as cracks at the outer bend radius. To prevent or at least reduce this crack formation when bending lengthwise in the grain direction, it may be necessary to use a larger bending radius. When bending in the grain direction, the reduced ductility will increase the required forming capacity, but it will be able to accept a much narrower internal bend radius without destroying the outer surface of the bend.
Localized stress when bending metal
Localized stress can affect the deformation results and this limits how tight the inside bend radius can be. Thermal processes such as flame and laser cutting harden the edges and cause stress concentrations. It may be necessary to remove superficial cuts and sharp corners along cut edges. Finishing shaved edges and surfaces can help reduce or eliminate microfractures in critical areas.
When forming heavy plates for small bend radii, you may need to preheat the material between 200 and 300 degrees F before bending, especially if you are trying to bend thicknesses of 0.75 inches or more. For best results, heat the material evenly.
Figure 2: In the tool on the right, the mold space has been relieved. This allows the punch to penetrate deeply into the die space and, to compensate for springback, benefit from an included die angle of 78 or 73 degrees.
Jump back
All steel, aluminum and even plastic exhibit springback when released from bending forces. Springback is the release of elastic stress and is directly related to the yield strength of the material. Therefore, you need a larger bend angle to achieve the required angle, especially for high yield steels and most aluminum grades.
A given blank plate can e.g. have 2 degrees of setback, so you need a punch with a minimum included angle that is at least 2 degrees less than the die included angle to achieve the required angle clearance. However, as the radius increases, so does the springback, and the amount of springback can be significant when the radius is large compared to the sheet or slab thickness.
Proper nozzle width and angle can help compensate for this excessive recoil. This also applies to unloaded matrices (seeFigure 2), including angles of 78 or 73 degrees. Channel matrices contain matrix angles that are perpendicular, straight up and down. Both allow the necessary tool penetration without interference between die faces, punch and material.
Hot mold steel
Heat buildup occurs when the plate is between 1600 and 1700 degrees F. This reduces or even eliminates strain hardening, jet cracking and deformation of the grain structure. The high temperature recrystallizes the plate, which actually changes the molecular structure.
The slab may need to be retreated to return it to its original condition. Nevertheless, compared to cold forming, hot forming allows for a much greater degree of formability and lower tonnage requirements, making it an attractive alternative when the tonnage capacity of the press is an issue. The press brake may not be able to form a sheet cold, but it can form it hot.
Like anything, thermoforming has its limitations. The high temperature required for thermoforming can cause oxidation. It can also cause surface decarburization - a change or loss in the carbon content of the steel. Most people view decarburization as a defect because the loss of carbon makes the steel less stable, which in turn can cause a number of problems with the products made from that steel. You can perform material testing to confirm the rate of carbon loss and whether the modified material is acceptable or not.
Thermovormen van aluminium
If you are bending something harder than 5054 aluminum, anneal it by heating it along the bend line. If you don't, hard aluminum will crack and break during forming.
Aluminum melts between 865 and 1240 degrees F, so you obviously can't heat it as strongly as steel. In some ways, aluminum heats, bends, and recrystallizes the same as steel, and in other ways it reacts very differently. When heated, aluminum tends to spring back a little more. You may achieve the desired bend angle and radius, but once it cools it will spring back a bit more.
When steel is heated, it first becomes malleable and then it melts. When aluminum is heated, it is first malleable, then it becomes brittle, and then it melts. If you heat aluminum too close to its melting point and then try to bend it, the part can crack or break.
Another difficult part of hot forming aluminum is that the metal does not change color when heated in the same way as steel. You can anneal the aluminum with an oxy-fuel burner with a neutral flame. Swipe back and forth until you see a golden color. You may also see a black film or soot, but you can easily wipe this away later. Depending on the thickness of the plate, only a few flame movements are necessary, so make sure that the plate does not get too hot. Doing so may cause it to become brittle or even melt.
Figure 3: Bending in the longitudinal direction, or bending with the grain of the material, increases the required minimum inside radius of the bend.
Minimum internal bend radius
For steel, aluminum and stainless steel you will find a range of minimum bend radii to thickness ratios, and you should check these values with your material supplier's data. However, when examining these values, keep in mind that bending in the transverse direction (across the fibers) or longitudinal direction (with the fibers) will have an effect on the minimum required bend radius. For longitudinal bending a larger radius is required than specified for transverse bending (seefigure 3).
As the thickness increases, the minimum radius also increases. For 6061 with a thickness of 0.25 inches in an "O" condition, the material supplier can specify a 1-to-1 ratio of inside radius to sheet thickness. In 0.375 inch thick aluminum, the minimum radius is 1.5 times the thickness; for a thickness of 0.5 inches this is 2 times the thickness.
The minimum radius also increases with harder material. For 6061 with a thickness of 0.25 inches in a "T4" condition, the material supplier may specify the minimum radius at 3 times the thickness; A 0.375 inch thick plate can have a minimum radius of 3.5 times the thickness; for a 0.5 inch thick plate this could be 4 times the thickness.
The trend is clear: the harder and thicker the plate, the greater the minimum bending radius. For 0.5 inch thick 7050 aluminum, the minimum bend radius can be specified as much as 9.5 times the material thickness.
Here too, the minimum internal bending radius is even greater when bending along the grain. In steel between 0.5 and 0.8 inches thick, grades 350 and 400 may require a minimum bend radius of 2.5 times the material thickness for transverse bending, while longitudinal bending may require a minimum bend radius of 3.75 times the material thickness are. And between 0.8 and 2 inches. is thick, you probably need a hot pan.
A rule of thumb when bending metal
There is a rule of thumb for determining the minimum bend radius of steel, and it generally applies to aluminum as well: divide 50 by the material's tensile reduction percentage, as specified by your supplier. This value varies by character.
If the steel has a tensile reduction value of 10 percent, divide 50 by this value: 50/10 = 5. Then subtract 1 from the answer: 5 – 1 = 4. Now multiply the answer by the sheet thickness. If the material is 0.5 inches thick: 4 × 0.5 = 2. So in this case the minimum inside bend radius is 2 times the thickness of the material.
Please note that this is just a rule of thumb. Finding the actual minimum bend radius for steel or aluminum plates requires some research. This should include details from your material supplier, whether you are bending with or against the wire, as well as information specific to the application. Nevertheless, the answers are there, waiting for you to find them.
