Minimum Bend Radius Calculator

Figure 1
Cracking on the outside surface is caused by
using too small of an inside bend radius

This calculator predicts what the minimum bend radius should be to avoid chacking or orange peeling on the outside radius of the bend. Here again, a “Minimum Bend Radius” is not necessarily the same thing as a “Sharp bend.

A Sharp bend will always be less than a Minimum bend radius but, a Minimum Bend radius does not need to be sharp.

Minimum Bend Radius

What is the minimum radius? It is the smallest inside bend radius that you can place in a piece of material before the tensile forces on the outside of the bend begin to tear the grains in the material apart. It is normally expressed in terms of multiples of the material thickness, e.g. 1t, 2t, etc. If the punch nose radius is sharp enough it can even pull the grains apart when bending perpendicular to the grain.

The effect is most commonly seen in bends that run parallel to the materials grain, or the direction off roll. Once that separation begins to occur the bend to bend variations in angle and dimension will increase, sometimes dramatically.

You will also need to visit, another similar site, or material reference guide for the pieces of data that are required for the calculator to work correctly. In this case, you will need to find:

Material Tensile Strength in psi.
Material Thickness in inches
Area of Reduction as a decimal percentage
Optional Die Opening in inches

Minimum Bend Radius Calculator

Try our Springback Calculator and our Sharp Bend Indicator

Please leave a comment on the calculator; what you thought about it, along with suggestions on how we might improve it.


Minimum Bend Radius Calculator — 6 Comments

  1. It is the measure of the ductility of a metal, attained during a tensile test. It is the change in thickness between the original dimension and the dimension at the smallest measurement at the point of fracture caused by testing and is expressed as a percentage of decrease from the original measurement.
    It is listed along with tensile strengths, Yield strengths, Modulus of Elasticity, etc. for a given material at places like or other material datasheets.

    • OK, so I have a 1,5mm 1.4301.
      A Die of 12mm.
      Does that makethe entries right?
      Mat. Tensile 73200
      Mat. Thickness 0.059
      Area of Reduction 0.7 (70%)
      Closest the width used 0.472
      The Min iR is then 0.98mm (0.039in)

      When I input the data for S235JR.
      Mat. Tensile 63100 (52200-74000)
      Mat. Thickness 0.059
      Area of Reduction 0.17 (17%)
      Closest the width used 0.472

      I get a minimum iR of 0.170in
      This seems strange to me that iR of s235JR is so big.

      Where is my mathematical error?

      Greetings Hobie

      • The answers are correct, the first material is much stronger and more ductile, while the second material is not quite as strong and more brittle. Consequentially the material with the .17% Area of reduction will start to crack on the outside radius long before the material with a 70% reduction of area will.

        The calculator is telling you the approximant minimum inside bend radius in the brittle material to avoid cracking, that of.170-inches.
        The die opening that the calculator recommends will achieve that. Your input for die opening compares the calculators’ answers to the results of your choice of die opening.

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