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Guide to Calculating Bend Allowance and Bend Deduction

SendCutSend's bending calculator displayed on a laptop with a bent part sitting in front of it.
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CNC bending opens up laser cut sheet metal to a new world of possibilities. We want to make these possibilities attainable, so we’ve created a Bending Calculator that calculates the complicated bend allowance and bend deduction value for you. This video and article go over how we make those calculations so you know exactly what’s happening when you order your bent sheet metal parts. 

Calculating Bend Allowance and Bend Deduction Video Guide

Note: it will be easier to understand the contents of this video and article if you’ve watched our previous video on the K-Factor in bending

Introduction to the Bending Calculator

Follow along with the video and the transcript below by opening up the SendCutSend bending calculator and inputting the values yourself. 

Screenshot of the SendCutSend bending calculator showing 5052 Aluminum

In our bending calculator, you can input your desired material, as well as the units of measurement your design uses (inches or millimeters). You can also input flanges. In this example, we’ve input two flanges and made our base length 2”. We’ve made Flange R1 6” with a 90° angle up, and Flange R2 2” with a 90° angle up. 

As you scroll down the page, you’ll see that there is a top and a side view for your part. In the side view, you can see that our example part is a C-channel Bend. 

Screenshot of the SendCutSend bending calculator side view.

If you scroll down a bit further, you’ll see the Desired Lengths and the Modified Lengths. These are two different dimensions because of the calculated bend allowance and bend deduction. 

Finally, the “Advanced Details” that you can see are the material type, the K-Factor, the bend radius, and the thickness. The Bending Calculator utilizes these values in addition to the material thickness to calculate the bend allowance and bend deduction.

Screenshot of the SendCutSend bending calculator Advanced Details.

The Background Calculations

Let’s take a look at what this calculator is actually doing in the background. This is easiest to understand with an example. 

In our example, we have two flanges that are 2” tall, with a base that’s 6” wide, so it has two bends in total. This part will be laser cut out of 0.080” thick 5052 aluminum. It will have a bend radius of .125”, a K-Factor of .4, and the two bend angles will be 90°. What we have to consider is if we flatten this bend out, we would end up having a flat part that is 10” long. 

However, because of stretching and compression on the apex of the bend, if we bent a 10” flat part to have two 2” flanges, the flanges would end up being slightly taller than 2”. The material science behind this is covered in our most recent bending video, “What is the K-Factor in Bending?” Ultimately, though, we have tension and compression forces elongating the material, stretching it like an elastic band. This causes the dimensions of your part to change post-bending. 

Screenshot of the Video Guide with Jake in front of a white board, drawing the tension and compression in the apex of a bend.

We fix this using a calculation called the “bend deduction.” This will be the amount that we need to subtract from the part dimensions in order to get the correct bend points and have a flange that measures correctly. But before we can calculate the bend deduction, we have to calculate the bend allowance. The bend allowance is the actual measurement of the stretched neutral axis, which is the invisible line that runs through the center of the material.

In order to calculate bend allowance, we need to know the K-Factor, which you can again find in our bending calculator for the material that you’re using. We’ll be using this equation to calculate bend allowance:

Bend Allowance = Angle(Π/180)(Bend Radius+KFactor(Thickness))

Screenshot of the Video Guide with Jake in front of a white board showing where the neutral axis is in a bend and a completed bend allowance equation.

Take this equation and plug in your own values for Angle, Bend Radius, and K-Factor. 

When we do this for our example, we get a value of 0.2466”. 0.2466” is the measurement of the neutral axis throughout the bend. 

Now that we know how long that bend is going to be after it has been “stretched,” we can calculate the bend deduction. The bend deduction tells us how much of the length we need to remove (deduct) from the part in order to put the bend lines in the right place. To calculate the bend deduction, we need the bend radius, thickness, bend angle, and the bend allowance that we just calculated:

Bend Deduction = 2(Bend Radius+Thickness)·tan(Angle/2)-Bend Allowance

Screenshot of the Video Guide with Jake in front of a white board showing where the neutral axis is in a bend and a completed bend allowance and bend deduction equation.

If we plug in those values from our example, we’ll get 0.1634”.

Using Bend Deduction to Adjust Your Design

How do we apply our bend deduction to the part in order to get the bend lines in the right spot?

With our example of a 6” base and two 2” flanges, we will need to subtract half the bend deduction (0.0817”) from both 2” sides and the full bend deduction (0.1634”) from the 6” base. This will shorten the total length of the flat part. When we actually bend the part, the “stretching” of the material will add the bend deduction dimension back into the total length of the part, enabling us to get the correct flange and base measurements.

Screenshot of the Video Guide with Jake in front of a white board showing where dimensions are removed from the laser cut part after calculating the bend deduction.

We’ve made it easier on you so you don’t have to calculate this out by hand before you upload your part. As we’ve stated, if you go into our bend calculator on the website and input the correct values there, you’ll see the bend deducted values calculated for you and you can change your design file measurements accordingly. If you’re using a CAD software, you can sometimes put the K-Factor and the bend radius values directly into the software and it will give you the same reduced values. It just depends on what software you’re using. 

Conclusion

Our precision laser cutting and CNC bending processes make it simple to get highly accurate cuts and bends within +/-.005″ and 1°, respectively. We still want you to know what we’re doing in our shop to machine your parts to spec, so we’re continually adding to our list of resource articles like this one. If you have any other questions about bending, processing, or machining, these articles and our guidelines are the best places to start.

Or if you’re ready now to have bent sheet metal parts cut, processed, and shipped to you within a few days, upload your file to our app and get an instant quote today!

Pre-flight Checklist

Before you upload your design files, be sure to go through our pre-flight checklist:

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