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Correct calculation of an unfolded part for a press brake

Correct calculation of an unfolded part for a press brake

Understanding the relation between V opening, radius, bending allowance, and K factor is very important to better calculate an unfolded part before cutting the flat blank from the sheet metal.

Maybe you’ve received laser cut, punched, or sheared parts that seem “too long” or “too short” once bent on the press brake. Or maybe the parts that are designed on the computer do not meet reality once bent on the floor. Well, Rolleri is here to help! Read our tutorial below to learn more about the factors involved when trying to get the best results.


Understanding the bending process: simple facts

1) The radius obtained on a bent part affects the length to which we must cut that part (before bending).

2) The radius obtained on bending depends 99% on the V opening we choose to work with.


First, a simple conclusion

Before designing the part and surely before starting to cut the blanks, we MUST know EXACTLY what V opening we will use to bend the part on the press brake.


How the radius affects blanks

a larger radius will “push” the legs of our part toward the outside, giving the impression that the blank was cut “too long”. a smaller radius will require a blank that must be cut “a little longer” than if the radius were larger.


Bending Allowance

The unfolded blanks of the above figure would be calculated as follows:

B = 150 + 100 + 60 + BA1 + BA2

Now lets explain how to calculate BA1 and BA2.


Calculating the bending Allowance(ВА)

The portion we need to reduce from both legs once overlapping by becoming flat, is what we commonly know as “bend allowance” (or BA in the equation).


BA formula for bends up to 90°


BA formula for bends from 91° to 165°


iR= Internal radius


Β = Angle

Π = 3,14159265….

K = K-factor

You can find more information about sheet metal bending on our website! Please read an article about "Correct calculation of an unfolded part for a press brake"


When bending on a press brake the inner part of the sheet metal is compressed while the outer part is extended. This means that there is a portion of the sheet where the fibers are neither compressed nor extended. We call this portion the “neutral axis.”


The distance from the inside of the bend to the neutral axis is what we call K factor. This value comes with the material we buy and it can not be changed. This value is expressed in fractions. The smaller the K factor, the closer the neutral axis will be to the inner radius of the sheet.


K-factor - fine tuning

The K factor affects our unfolded blank. Not as much as the radius of the part, but we can think of it as a fine tuning calculations for blanks. the smaller the K factor, the more material is extended and therefore “pushed out”…. which means our leg will become “larger”.


K-factor estimation

Most of the time we can estimate and adjust the K factor when fine tuning our blank calculations. all we need to do are some tests (on the chosen V opening) and measure the radius of the part. In case you need to determine a more precise K factor, below is the calculation for determining the exact K factor for your bend.


K-factor formula


Solving an example:

B = 150 + 100 + 60 +BA1 + BA2

K-factor estimation

B1: R/S=2 => K=0,8

B2: R/S=1,5 => K=0,8

Both bends are less or equal to 90°:


Which means:

B1 = 3.14 x 0.66 x (6 + ((4×0.8)/2) – 2 x 10

B1 = -4.25

B2 = 3.14 x 0.5 x (8 + ((4×0.8)/2) – 2 x 12

B2 = -8.93


B = 150 + 100 + 60 + (-4.25) + (-8.93)

B= 296.8mm

Author: Julio Alcacer, manager of international sales, Rolleri Press Brake Tools

Commentary from Dreambird

Modern manufacturers use sheet metal fabrication to produce parts with tight tolerances. Moreover, the market of today dictates to shorten the lead-times as much as possible - the length of manufacturing is often the critical factor for the customer to choose between the subcontractors. Manufacturers are trying hard not to spend much time to perform manual repeated calculation, performing many tests and fixing errors during the production. The method that the article describes is precise and its formulas are useful, but using this approach on a regular basis with each order leads to spending additional time for manufacturing.

The most up-to-date press brakes are often equipped with CNC units and the bending sequence for a certain part may be designed in a software right after designing the part itself if a CAD package. If a user has a flat blank geometry file, he can calculate a bending sequence after importing the file in CAD/CAM software solution for bending.

Radbend, a standalone software solution for bending, a part of CAD/CAM Radan suite for sheet metal fabrication, is considered a worldwide market leader among similar applications. All the calculations mentioned in the article are implemented as Radbend's algorithms and thus no manual calculations are required. The bending is performed in Radbend exactly like it would be bent on the press brake - the "too long" legs are fitted to achieve maximum precision. Then, the bent part is sent into Radan3D module where a flat blank is created. When calculating the size of this flat blank, the bending is taken into account. Therefore, in production of the part, all the required parameters will be followed and the bending will come out correctly right from the first try.

Radbend is the answer to typical issues that manufacturers have in their bending applications like full collision checking which ensures costly errors are eliminated prior to production.

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