### Springback and inside radius calculator for sheet metal.

This Springback and Inside Radius Calculator estimates the amount of angular change and the relaxing of bend radius that occurs when metal is bent and released from pressure.  When the proportional limits of a specific material are reached or exceeded the material remains bent, figure one.

Stress-Strain Curve
Figure 1.

In the forming process, both sheet metal and plastic are bent to a specific bend angle. When the material is then released from the pressure of forming stresses within the bend (tension outside, compression inside) cause the bend angle to open and the radius to open. Both the angle and radius the material attempt to return to the flat position; relaxing as the pressure is released.  The inside radius also relaxes getting larger which in turn affects the numbers for Bend Allowance (BA), OutSide SetBack (OSSB) and Bend Deduction (BD). This phenomenon is known as Springback when applied to angle change and as the Springback factor (Sf) applied to the radius.

#### Springback and forming method

That was Air forming but when bottom Bending or Coining are the preferred methods of forming Springback becomes Springforward. Springforward uses increased pressure and depth of penetration to compensate for Springback. These processes force the material “open up” to the desired bend angle.  First being formed to an over bent position to compensate for the Springback. As with Air forming, Bottom Bending and Coining requires the material to be overbent bent to an amount equal to the bend’s Springback, figure 2.

Accurate numbers for Springback have been one of the more difficult values to predict due to the large number variables involved: material type, inside bend radius and method of forming to name a few. With all of those variables, Springback can be calculated. Using the correct data points; the thickness, inside radius, modulus of elasticity and the K-factors, etc. it is possible to predict a reasonably accurate number.

#### About the Calculator

Figure 2

All of that being said, please take note of the following instructions especially the unit of measure for each data point and make sure to input that data correctly:

1. Select appropriate golden input box to enter data.
Note the format of each required inputs, Inches, ksi, psi.
2. The degrees of bend angle are input as an included angles.
3. Locate the appropriate K-factor from Chart 1 below and enter it.
4. For other material specifications provided by Matweb.com provided by Matweb.com.
Enter your material type into the search bar (bottom right of the popup windowand transfer any appropriate data to the calculator, be sure the data is input in the correct format for the calculator.
5. Click Calculate the answers

For more details on the various inputs, please review the November 2017 edition of the Bending Basics column from the Fabricator magazine or under the Media Tab / Bending Basics from this sites Menu Bar.

#### Springback Calculator

Revision date 12/14/17 to version 2.2.1

K-factor chart

Please note that even on good day predicting Springback will never perfect a perfect science for the following reasons:

1. The material will change from bend to bend, grain direction for example.
2. The material will change from run to run, different batches of material.
3. Changes brought on by material coatings.
4. The material will not be constant throughout a given piece.
5. Material makeup will not constant throughout the sheet.

### Comments

#### Springback Calculator — 2 Comments

1. Knowing how many degrees I might have to over-bend a part to achieve the final angle helps me choose the right die angle.
Also I can see this tool being a big help in reducing the in-process trial and error to get the correct radius punch when forming profound radius bends.
The calculator told me I need a .875″ radius punch to get a 1″ radius in .059 CRS at 90 degrees. I believe that’s about right.
I like it! Thanks Steve!

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