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The press brake K factor, also known as the bend allowance factor, is a critical dimensionless parameter in the sheet metal bending industry. It plays an important role in determining the amount of material stretch during the bend, helping us calculate the bend allowance, which is the additional length of material required to make the bend.
The K factor takes into account the material thickness, the bend radius, and the properties of the material being bent. Essentially, it represents the ratio of the distance from the inside radius of the bend to the neutral axis of the material to the material thickness. A lower K factor means less material stretch during the bend, while a higher K factor indicates more material stretch. In sheet metal manufacturing, accurately determining the K factor is critical to accurately predicting the final bend angle and achieving the exact bend size.
It is indeed difficult to understand the concept of the K factor without understanding its unique associations. There are four key terms that are important to understanding the K factor and how it is calculated: apex, setback point, neutral line/axis, and bend radius. While you don’t necessarily need to remember these formulas or values to successfully design a bent sheet metal part, knowing this information will help you better understand how sheet metal behaves in a press brake and how to adjust your design to compensate for its deformation.
Let’s start with the basic terminology of bends and flanges. For example, a single 90-degree bend has two flanges, one above and one below, with the bend in the middle. The apex in sheet metal bending terminology, often called the die point, is located in the exact center of the bend. The apex is the theoretical point outside the bend’s tangent lines. If it is a perfect right angle with no radius, the intersection of the corners is the apex or die point.
The setback point (labeled “SB” in the example) is the distance from the apex to the bend line, which is where the bend ends and enters the flange. In this example, the bend has two setback points, and the distance is exactly the same on both sides. There are two main factors that affect setback: the material’s bend angle and the bend radius. Changing the radius moves the bend line down, and changing the angle moves the apex.
The neutral line is a line that runs through the center of the part, halfway through the thickness of the part. It is called the “neutral” line because during bending, the material on the neutral line is neither compressed nor stretched. During the forming of the part, the neutral line moves to the inside of the bend.
The bend radius is measured on the inside of the part, not the outside. This is because the part is compressed and stretched when it is bent, with the inside in compression, compressed and formed into the inside of the bend, and the outside in tension. When bending, the bend area deforms, and the stretched outside area moves inward toward the neutral line. In an exaggerated way, stretching the stretched outside area thins it, causing the neutral line to move inward as well. This inward movement and thinning is the source of the concept of the “K factor”. The K factor is equal to the ratio of the new reduced thickness to the original total thickness.
The K factor is determined by factors such as the physical properties of the material, the bending method, and the bend angle.
In precision sheet metal manufacturing, the K factor is a key factor. It is used to calculate the bend expansion diagram, which is directly related to the length of the sheet metal stretched during the bending process. It is a basic value for determining the bend allowance and bend deduction. Since the ratio of the distance to the neutral axis to the sheet thickness determines the location of the neutral axis in the metal sheet, understanding the K factor helps determine the location of the neutral axis after bending.
The K factor plays a pivotal role in sheet metal bending for the following reasons:
Since the K factor is based on the properties of the metal and its thickness, there is no simple way to calculate it before the first bend. Typically, the K factor is between 0 and 0.5. To find the K factor, bend a sample and deduce the bend allowance. Then substitute the bend allowance into the following formula to calculate the K factor.
First, prepare sample blanks of equal and known dimensions. The stock should be at least a foot long to ensure an even bend and a few inches wide to allow it to rest against the stop. For example, take a piece of 14 gauge material that is 0.075 inches thick, 4 inches wide, and 12 inches long. The length will not be used in the calculations. It will help to prepare at least 3 samples and take the average measurement of each.
Set up your press brake with the required dies for that metal thickness and make a 90° bend in the center of the material. In this case, that would be at the 2-inch mark.
After bending the samples, carefully measure the flange length of each sample. Record each length and average it. The length should be more than half of the original length. For example, the average flange length is 2.073 inches.
Next, measure the inside radius that was formed during the bend. A set of radius gauges can get you fairly close to the correct measurement, but for a precise measurement, an optical comparator gives the most accurate reading. In this case, the inside radius measured 0.105 inches.
Now that you have the measurement, you can determine the bend allowance. First determine the leg length by subtracting the material thickness and inner radius from the flange length (note that this formula only applies to 90° bends because the leg length is measured from the tangent point). For this example, the leg length is 2.073 – 0.105 – 0.075 = 1.893.
Subtract twice the leg length from the initial length to determine the bend allowance. 4 – 1.893 * 2 = 0.214.
Substitute the bend allowance (BA), bend angle (B<), inner radius (IR), and material thickness (MT) into the following formula to determine the K factor (K). For this example, the calculation result is:
Choosing the right K factor and calculating it accurately is critical to the quality and efficiency of sheet metal processing. If you need professional bending machines and related technical support during sheet metal processing, please feel free to contact us and we will provide you with the best solutions.
