Sheet Metal K-Factor Bend Allowance and Flat Length

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K-Factor in sheet metal bending is a constant. It is used to calculate sheet metal flat length/ Flat-pattern. Mathematically k factor value is equal to the ratio of position of neutral axis and sheet thickness. In this article we will discuss sheetmetal bend deduction, bend allowance, K-factor, Y-factor and sheet metal flat pattern calculations.

Sheet Metal Bending

Sheet metal bending is a forming operation. Where sheets are deformed plastically to change its shape. During bending, material is stressed beyond its yield strength but below the ultimate tensile strength. 

After bending total length of sheet metal is more than flat length. This change in length can be represented as bend deduction or bend allowance.

Bend Deduction

During bending, the inside surface of the bend is compressed and the outer surface is stretched. As a result, total surface area of sheet metal part increases. In another words, Total length (A + B) is greater than sheet metal Flat Length.

Difference in total length after bending and flat length is known as Bend Deduction.

Use Bend Deduction Calculator to calculate Bend Deduction.

Flat Length = Length A + Length B – Bend Deduction

Sheet Metal Bend Allowance

Bend allowance is similar to bend deduction.  It is the material required to add to the Flange length (A1 and B1) to develop a flat length.

Flat Length = Length A1 + Length B1 + Bend Allowance

Bend allowance and bend deduction are directly related. Sum of bend allowance and bend deduction is equal to two times of outside set back.

BA + BD = 2 X OSSB

Outside Setback =  (Tan (A / 2) ) X (T + R)

Where ;

BA ; Bend Allowance

BD ; Bend Deduction

OSSB : Outside Setback

A : Bend Angle

T : Sheet Thickness

R : Inside Bend Radius

K-factor Calculation

K-Factor in sheet metal bending is calculated by following two Methods :

  1. Using Neutral Axis
  2. Using Bend Allowance or Bend Deduction
Neutral Axis and K factor

To understand K factor, firstly we need to understand what is neutral axis?

this image indicates the position of neutral axis when sheet metal is bend.

During sheet metal bending. Inside bend surface is compressed and the outer bend surface is stretched. Somewhere in between the thickness of material lies Neutral Axis. Neutral Axis is a line where material is neither compressed nor stretched. 

Position of neutral axis depends on the material, direction of material grain, tooling and environmental conditions.

Mathematically K factor is the ratio of position of neutral axis and sheet thickness. 

K-FACTOR = Position Of Neutral Axis / Material Thickness

Bend allowance and K factor

Bend allowance can be calculated using manual experiment.

BA = 3.14 (R+KT) X A/180


R : Inside Bend Radius, A : Bend Angle

T : Sheet Thickness, K : K-Factor Value

BA : Bend Allowance

Relationship between K-Factor and Y-Factor

Similar to K factor, Y factor is a constant used to calculate sheet metal part developed length. Y-factor and k-factor are directly related with each other..

mathematically y factor is equal to 0.5 times of pie of K factor

Facts about K-Factor
  •  Inside compression in sheet metal cannot exceed the outside tension. Therefore value of k-factor can not exceed “0.5” .
  • Neutral axis cannot go beyond the middle of the sheet thickness.
  • K factor Value is inversely proportional to sheet thickness. Increase in thickness, decreases k-factor value.
  • K factor value is inversely proportional to material hardness. Therefore with Increase in hardness, K factor value decreases 

Sheet Metal Flat Length Calculation

If material K factor value and bending flange length is known. Sheet Metal flat length can be calculated in three simple steps.  You can also use sheetmetal flat length calculator. 

Step-1: Calculate Bend Allowance

BA = 3.14 (R+KT) X A/180

Step-2 : Calculate Bend Deduction

BA + BD = 2 X OSSB

Outside Setback =  (Tan (A / 2) ) X (T + R)

Step-3 : Calculate Flat Length

Flat Length = Length A + Length B – Bend Deduction


To sum up, Sheet metal flat pattern is calculated using k factor. And k factor can be calculated by calculating the position of neutral axis or bend allowance. Calculation of neutral axis position is very difficult. But bend allowance can be calculated by reverse engineering method. 

Got Questions?  We will be happy to help.

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