
Sheet Metal Layout  Press Brake


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 Kfactor. Oh, and yes, you do need to know the Kfactor 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 Kfactor comes into play. The Kfactor 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 Kfactor of 0.42. The formula (See Bending Formulas) is: Bend Allowance = Angle * (PI / 180) * (Radius + Kfactor * 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. 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 Bend Calculator and get the answer. It is 0.1019, a negative number, which means you will subtract this amount from the total of the flange lengths, 5”, to get 4.8981.


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