Making a custom metal form using sheet metal is a surprisingly delicate process. To get the best results, a design engineer has to account for many factors, such as the end use of the metal form, the material the sheet metal is made of (plain steel, stainless steel, etc.), and the dimensions/shape of the final product.
Getting one single thing wrong can make the final product practically useless. This is why design engineers spend so much time carefully reviewing designs and using virtual physics simulation software such as Autodesk to vet their designs before production.
One design element that many laymen tend to forget about is the radius of each individual bend in a sheet metal form. Where many people might assume that all metals can be bent at perfect right angles without an issue, this is not the case.
Each and every bend in a piece of sheet metal has a certain minimum internal radius that needs to be taken into account when planning the design of a sheet metal form.
The Problem with Bending Too Tightly
If a piece of material is bent too tightly, problems can occur.
The most common problem with bending a piece of sheet metal too tightly is that you can compromise the structural integrity of your sheet metal form. Basically, you might create a weak point in the metal that can break easily, lowering the maximum load the metal can take before shattering.
Another common issue with bending a piece of metal too tightly is that there can be excessive plastic deformity resulting from the stress of the bend, slightly altering the dimensions of the final work piece along the area where the bend was made.
With this in mind, what is the minimum internal bend radius for a piece of sheet metal?
Determining the Minimum Internal Radius for a Bend in Sheet Metal
There are a two major factors that affect how tight you can make the internal radius of a bend in a piece of sheet metal:
- The hardness of the metal.
- The thickness of the metal.
Generally speaking, the thicker a piece of sheet metal is, the wider the internal bend radius will have to be. Similarly, the harder a piece of metal is, the more room you’ll have to allocate for the bend as well, or risk fracturing the sheet metal.
In an article on thefabricator.com, Steve Benson, the President of ASMA LLC, shares a handy rule of thumb for determining the minimum bend radius for steel forms: “divide 50 by the material’s tensile reduction percentage as specified by your supplier. This value will vary by grade.” For this information, you’ll need to know the tensile reduction percentage of your steel, which you can typically learn from the steel supplier.
After dividing 50 by your steel’s tensile reduction percentage, “subtract 1 from that answer.” For example, if your steel reduction percentage was 10%, that would be 50/10 = 5, and then 5 – 1 = 4. Benson then advises you to “multiply that answer by the plate thickness” in inches. So, if the metal plate is .75” thick, you would multiply 4 x .75 = 3, so your minimum inside bend radius would be roughly 3 times the thickness of the material in this case, or 2.25”.
However, it is important to know that this is just a broadly applicable rule of thumb you can follow for making bends in steel. Getting the actual minimum bend radius for your sheet metal typically requires more detailed knowledge of the alloy being shaped, which comes with training and education in metal forming techniques as well as a familiarity with the material(s) used for a custom sheet metal form.
See how Marlin Steel’s engineers leveraged their expertise in designing custom metal forms to optimize the production process for other companies in the case study below: