K Factor

Definition:

The k factor in sheet metal working is the ratio of the neutral axis to the material thickness. When metal is bent the inside of the bend undergoes compression while the outside is subjected to tension.  The hypothetical line where the transition from compression to tension occurs along the thickness of the metal is called the neutral axis. The location of the neutral axis varies and is based on the material’s physical properties and its thickness. The k factor is the ratio of the Neutral Axis’ Offset (t) and the Material Thickness (MT).

The k factor is useful for calculating flat patterns because it is proportional to the to the bend allowance and bend deductions.  Modern design software will incorporate the k factor of your material along with bend angles and radii to make reliable predictions about the part’s flat pattern.

Formulas:

Very simply the K Factor (K) is the relation of the Neutral Axis’ Offset (t) to the Material Thickness (MT).

The K Factor (K) is directly related to the Bend Allowance (BA), Bend Angle (B<), Inside Radius (IR) and Material Thickness (MT).

Table:

Developing A K-Factor Through A Test Bend

Calculating the K-Factor

Since the K-Factor is based on the property of the metal and its thickness there is no simple way to calculate it ahead of the first bend.  Typically the K-Factor is going to be between 0 and .5.  In order to find the K-Factor you will need to bend a sample piece and deduce the Bend Allowance.  The Bend Allowance is then plugged into the above equation to find the K-Factor.

1. Begin by preparing sample blanks which are of equal and known sizes.  The blanks should be at least a foot long to ensure an even bend, and a few inches deep to make sure you can sit them against the back stops.  For our example let’s take a piece that is 14 Gauge, .075”, 4” Wide and 12” Long.  The length of the piece won’t be used in our calculations.  Preparing at least 3 samples and taking the average measurements from each will help
2. Set up your press brake with the desired tooling you’ll be using to fabricate this metal thickness and place a 90° bend in the center of the piece.  For our example this means a bend at the 2” mark.
3. Once you’ve bent your sample pieces carefully measure the flange lengths of each piece.  Record each length and take the average of lengths.  The length should be something over half the original length.  For our example the average flange length is 2.073”
4. Second measure the inside radius formed during the bending.  A set of radius gauges will get you you fairly close to finding the correct measurement, however to get an exact measurement an optical comparator will give you the most accurate reading.  For our example the inside radius is measured at .105”
5. Now that you have your measurements, we’ll determine the Bend Allowance.  To do this first determine your leg length by subtracting the material thickness and inside radius from the flange length.  (Note this equation only works for 90° bends because the leg length is from the tangent point.) For our example the leg length will be 2.073 – .105 – .075 = 1.893.
6. Subtract twice the leg length from the initial length to determine the Bend Allowance.  4 – 1.893 * 2 = .214.
7. Plug the Bend Allowance (BA), the Bend Angle (B<), Inside Radius (IR) and Material Thickness (MT) into the below equation to determine the K-Factor (K).  For our example it comes to