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Bending Basics: the Basics of Heavy Bending
Sheet metal gauges can range from 0.005 inch to 0.249 in thick, but aluminum and plate thicknesses start around 0.250 in. These thicknesses can go up to 13 inches. or even more. Also, plate steel comes in many strengths. These range from mild versions to extremely high-strength material like
Hardox . 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.
It is possible for the workpiece to be extremely thick or very strong. You must understand these variables and know how to work with them. The material's chemical composition and surface conditions as well as the thickness of the workpiece will determine if the bend runs with or against the grain direction.
Every form, no matter the scale, requires some degree of plastic deformation. Material expansion occurs at the outside edge of the bend. Compression happens inside. This is why it is essential to learn how to deal both. The minimum bend radius will be determined by the limits of material flexibility.
Strain-hardening can be caused by the strains that result from plastic deformation after cold forming. This can cause material to change in its mechanical properties at the location of plastic deformation. It is at this point that ductility, resistance to crack and ductility will need to be taken into consideration.
It doesn't matter what material you are using, or how thick it is, soft aluminums and mild steels are much more flexible than high-strength ones and can be bent to sharper angles. It is essential to keep the radius inside of bends when you are bending high-tensile or thick metals. This will reduce strain hardening, cracking and bending at the bend.
Material suppliers' product data sheets typically outline the range of plate forms that can be made without failures. They also recommend minimal bend radiuses based on the material type and its properties. A low-carbon-content steel, or soft aluminum, is essential for good formability, and a tight within radius. But, as the steel's hardness or carbon level increases, its ductility decreases and it becomes less formable.
Grain Direction is Important
When working with plate, you should pay attention to whether your grain direction is longitudinal (or transverse). The mill's roll process is what determines the grain orientation of a plate. It stretches the metalurgical structure and adds to the material. The grain directions are parallel to each other.
Because of the material's ductility, forming with grain requires less bending force. The grains will spread due to this stretching, which causes cracking around the outside radius. It is possible to reduce cracking by using a larger radius bend radius when you bend longitudinally in the grain directions. If you bend transversely to the grain directions, the decreased ductility will increase the required amount of forming tonnage. However, it can accept a smaller bend radius inside the bend without destroying its outside surface.
Localized Stress
Localized stress can impact forming results. This can limit how tight the inside bend radius may be. Flamming and laser cutting are two thermal processes that can harden the edges and cause stress concentrations. You may need remove surface gouging or sharp corners from sheared edges. Treating surfaces and sheared edges can help to reduce or eliminate microfractures in critical locations.
You may need to preheat the material between 200 to 300 degrees F before bending heavy plate. This is especially important if your goal is to bend thicknesses of 0.75 inches. You should not exceed 0.75 inches. The best results are achieved by heating the material evenly.
Springback
All steels, aluminums and plastics can exhibit springback once they are freed from the bending stresses. Springback is the result of elastic strain being released and is directly linked to material yield strength. This is why most aluminums and high-yielding steels require a wider bend angle to attain the desired angle.
Some sheet metal workpieces may have up to 2 degrees of springback. Therefore, you need a punch with at least 2° less angle than the included die angle in order to achieve the required angular clearance. Springback increases with increasing radius. It can also be more significant if the radius is larger than the sheet thickness.
A proper angle and width can help to compensate for excessive springback. This includes relief dies, that include angles of 78 to 73 degrees. Channel dies may have die angles perpendicular to the surface, straight up and right down. Both allow for the required penetration of the tool with minimal interference between die faces, punches, and materials.
Hot Forming of Steel
Hot forming happens when the plate is heated between 1,600 to 1,700 degrees F. This causes strain hardening, cracking in the radius, and distortions of the grain structure. The plate's molecular structure is altered by recrystallization due to the high temperature.
To restore the original plate condition, it might be necessary to process it again. Although hot forming can be more difficult than cold forming due to its greater formability and lower tonnage requirements it makes hot forming an appealing alternative when there is limited press tonnage. Although the press brake cannot form a plate coldly, it can form it warm.
Hot forming, like all other processes, has its limits. Hot forming is dangerous because of the high temperatures involved. It can also lead to surface decarburization. This is when the steel's carbon percentage changes or decreases. Most people view decarburization to be a defect. It makes the steel less durable, which can lead, in turn, to problems with products made from it. To determine the degree of carbon loss and to determine if the altered material can be accepted, material testing is possible.
Aluminum Hot Forming
To bend anything more than 5054 aluminum, it is necessary to anneal it using heating along the bend lines. It is important to anneal aluminum as hard as 5054 aluminum. Otherwise, it will crack and fracture during forming.
Aluminum melts between 865 degrees F and 1,240 degrees F. This means that it can't be heated as much as steel. Aluminum responds very differently to heat, bend and recrystallizes. Aluminum tends have a bit more springback after it is heated. Although you may achieve the desired radius or bend angle, once it cools, it begins to spring back slightly.
When steel heats, it first becomes malleable then melts. After aluminum heats, it first becomes malleable and then it becomes brittle before melting. It is possible for aluminum workpieces to crack or to break if heated to too high a temperature.
Another problem with hot forming aluminum is the fact that it does not change its color like steel when heated. You can use an oxyacetylene torch to anneal the aluminum. Keep moving back and forth until you get a golden color. You may also notice a black film (or soot) on the surface.
However, this can be removed easily later. It can take just a few flame strokes depending on the thickness of your plate. Be careful not to get too hot. This could cause it to become brittle and even melt.
Minimum Inside Bend Radius
For stainless steel, aluminium, and stainless there will be a range of minimum bend radiuses-to-thickness rates. This information will be provided by your material supplier. Be aware that the minimum radius will vary depending on whether the bend is transverse (across or with the grain). Transverse bending has a radius that is much larger than the one required for longitudinal bend.
As thickness increases so does the radius minimum. 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. Aluminum 0.375 in. thick has a minimum radius of 1.5 times its thickness. Aluminum 0.5 in. thick has a radius of 2 times its thickness.
With harder materials, the radius of the minimum radius increases. A material supplier may specify that the minimum radius is 3 times the thickness for 0.25-in. thick 6061 in T4 condition. Plates 0.375 inches thick may have a radius of 3.5x, while plates 0.5 inches thick can have a radius of 4x.
The trend is clear: The more thicker and harder a plate is, then the radius at which it must be bent is the larger. For 7050 aluminum of thickness 0.5, the minimum bend radius could be as high at 9.5 times the material thickness.
If you bend with the grain, the minimum inner bend radius will be even greater. Steel between 0.5-0.8 in. Thickness may vary between grade 350 and 400. The transverse bend radius may be 2.5 times the material thickness, while the minimum radius for longitudinal bending may be 3.75x the material thickness. There are also a range of 0.8 to 2 inches. Hot-form is required for thicker materials.
A Simple Rule of Thumb
A rule of thumb is used to determine the minimum bend radius for steel. It also applies to aluminum. Divide 50 times the material's specified tensile loss percentage. This value will differ by grade.
Divide 50 by the steel's 10 percent tensile loss value to get 5. Next, subtract 1 based on that answer. Multiply that answer by your plate thickness. If the material is 0.25 in. Thickness = 4 x 0,5 = 2. In this case, the minimum inside bend radius will be 2 times the thickness of the material.
This is only a general rule. This is a guideline. This information should be provided by your material supplier. You just have to look for them.
