What Is the K Factor, Bend Allowance, and Bend Deduction?

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K factor, bend allowance, and bend deduction are important values used to calculate the correct sheet metal flat length or flat pattern.

The length of the sheet in the bending state is different from the length of the flat pattern.

Because the sheet metal will stretch and compress during bending.

In this article, we will talk about the K factor, bend allowance, and bend deduction.

What Is K Factor?

First, we need to understand the neutral axis. When the sheet metal is bent, the bottom surface is compressed and the top surface is stretched.

The neutral axis is located inside the metal where it will neither be compressed nor expanded, so the neutral axis can maintain a constant length.

In flat sheet metal, the neutral axis is evenly located at half of the thickness of the sheet metal, but it will move when bending.

The position change of the neutral axis is determined by the material properties, thickness, bending angle, internal radius, and bending method of the plate.

Mathematically, the K factor is the ratio between the position of the neutral axis (t) and the plate thickness (T).

Where t is the distance from the inside surface to the neutral axis and T is the metal thickness(Figure 1).

The K factor is determined by the physical properties of the material, bending method, bending angle, etc.

It is usually greater than 0.30, but not more than 0.50.

In precision sheet metal manufacturing, the K factor is a very important factor.

The K-factor is used to calculate the bending flat pattern, which is directly related to the length of the sheet metal stretched by the bending.

It is the basic value for calculating the bending allowance and bending deduction.

Because the ratio of the distance to the neutral axis to the plate thickness is the position of the neutral axis in the metal plate.

Therefore, knowing the K factor can determine the position of the neutral axis after bending.

You can also determine the K factor through the table below:

What is Bend Allowance?

The Bend Allowance (BA) is the bending arc length measured along the neutral axis of the metal plate. Because the length of the neutral axis will not change after bending.

Once the bend allowance is calculated, it needs to be added to the flat length to calculate the sheet metal length required to form the desired workpiece.

As is shown in the below figure:

The following formula is used to calculate the flat length:

Sheet Metal Flate Length=Leg Length 1 +BA + Leg Length 2

Bend Allowance Calculation Formula


A - Bend angle

R - Inside radius

K - K factor

T - Material thickness

What Is Bend Deduction?

After bending, the flange length, that is, the total length is greater than the length of the sheet metal flat pattern length.

Bending deduction is the length of material that we remove from the total length of the plate in order to obtain the correct flat pattern.

Bend Deduction Calculation Formula:

According to the figure above, the bend deduction is the difference between the bend allowance and twice the outside setback.

Bend Deduction = 2 × OSSB - BA

Bend Deduction Calculation Formula:

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