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Sheet Metal Layout | Press Brake
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How to Calculate Bend Allowance for Your Press Brake
Calculating the correct flat pattern layout is crucial to getting a good quality finished part from your press brake. Yet, many CAD and CNC programmers have no idea how to calculate the required values. Years ago, the real experts created cheat sheets and tacked them to the wall. They only taught the new apprentice how to apply the results shown on the cheat sheet, not how to calculate the numbers. Well, now those experts have retired and it's time for a new generation to learn the right way to do the calculate the correct flat pattern layout. Calculating the flat pattern length from the 3D part really isn’t that difficult. Although you may find several different formulas that claim to calculate the Bend Allowance (See Bending Definitions), they usually are the same formula, only simplified by filling in the angle or a K-factor. Oh, and yes, you do need to know the K-factor to calculate the Bend Allowance. 
Let’s start with a simple L bracket. The picture shows that the legs of the bracket are 2” and 3”. The material thickness is 0.036”, the inside radius is 0.125”, and the angle of bend is 90 degrees. The flat length is the total of the flat portion of both flanges plus the length through the arc of the bend area. But, do you calculate that on the inside of the material or the outside? Neither! This is where the K-factor comes into play. The K-factor is the percentage of the material thickness where there is no stretching or compressing of the material, for example, the neutral axis. For this simple L bracket, I will use a K-factor of 0.42. The formula (See Bending Formulas) is: Bend Allowance = Angle * (PI / 180) * (Radius + K-factor * Thickness). Plugging in our numbers, we have: Bend Allowance = 90 * (PI / 180) * (0.125 + 0.42 * 0.036) = 0.2200999813105009. If you don’t have a calculator handy, try the Bend Calculator.
So the flat pattern length is 2” + 3” + 0.2201 which
is equal to 5.2201. So if you add up the flat length of all the flanges and add
one Bend Allowance for each bend area you have the correct flat length of the part.
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But look at the drawing. That is not how we normally dimension a sheet metal part. The dimensions are
usually to the intersection of the flanges or the Mold Line. This means that we
have to subtract two times the material thickness plus the bend radius (also
known as the Setback) for each bend area. For this set of dimensions, it would
be easier to calculate the Bend Compensation value. The Bend Compensation value
lets you add up the length of each flange using the Mold Line dimensions and
then add one Bend Compensation per bend area to the total. Don’t bother with
your calculator. Just go to the