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Sheet Metal Bending Terminology: What Is K Factor in Bending?

SendCutSend's bending calculator displayed on a laptop with a bent part sitting in front of it.

Table of Contents

We’ve gone into a lot of in depth information on bending in some of our other articles, but let’s cover some of the basic sheet metal bending terminology that you should know. 

Follow along with the video and the transcript below to learn sheet metal bending terminology, from the apex to the K Factor.

What is K Factor in Bending Terminology? Video Guide and Transcript

What Is the Apex and the Setback Point in Bending?

Let’s start with talking about the basic terminology of bending and flanges. In this example, we have a single bend that’s 90 degrees with two flanges: a flange on the top, a flange on the bottom, and a bend in the middle. 

The first part of sheet metal bending terminology that we want to talk about is often called the apex or the mold point, and that’s going to be the very center of the bend. We’ll write apex here to indicate that. The apex is the theoretical point that’s off the tangents of the bend. So if we were to have a perfect corner without a radius, the point where the corners meet is where the apex or the mold point would be. 

Screenshot from the video describing where to find "the apex," a piece of sheet metal bending terminology to describe a theoretical point on a bent part.

The next term that we want to talk about is the setback point (labeled “SB” in the below example). The setback point is the distance from the apex back to where the bend line is going to be, where the end of our bend goes into the flange. The bend in our example has two setback points: we have one on each side of the bend that are the same exact distance. There are two things that really affect the setback: the angle to which you’re bending the material and the radius in which you’re bending it. If we change the radius, we move the bend line down, and if we change the angle, we move our apex. 

Screenshot from the video showing where to find both "setback points" off the apex."

The Neutral Line, Bend Radius, and K Factor

The next piece of sheet metal bending terminology we’re going to talk about is our neutral line (indicated by the dotted line in this example). Our neutral line is the line that runs through the whole center of the part, so it’s half of the thickness of the part.

Screenshot from the video describing the neutral line from sheet metal bending terminology.

We have one other term to discuss before talking about the K Factor, which is our bend radius. The bend radius is measured on the inside of the part, not on the outside of the part. The bend radius is measured on the inside of the part because the part goes under compression and tension. The inside of this part is in compression. This area is actually compressed and formed to create the inside of the bend. And then the outside of the bend is in tension. So when we bend, we actually end up deforming the area with the bend, and the area under tension ends up moving inward towards the neutral line. 

Screenshot from the video showing the inner corner of the bend, otherwise called the bend radius.

To exaggerate this, stretching the outside area that’s under tension, we end up thinning it, which causes our neutral line also to shift inwards. This shift inwards and the thinning is where we get the term K Factor from, which is a common term in sheet metal bending terminology. K Factor is equal to that new reduced thickness over the overall original thickness. 

Screenshot from the video showing the outer edge of the bend highlighted in blue to indicate tension, and the bend radius highlighted in red to show compression.

Utilizing the Bending Calculator

To help you calculate the K Factor and other important values, we have created a sheet metal bending calculator. Simply input your material, chosen thickness, and flange and base length, and the calculator will do all of the work for you. Be sure to utilize this tool before uploading your final design for machining.

If you have any other questions about sheet metal bending terminology or SendCutSend’s online CNC bending service, check out our bending guidelines

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