K factor in sheet metal is a constant, to calculate sheet metal part developed length. In another words k factor calculates sheet metal part flat pattern. Theoretically K factor is a ratio of neutral axis position and sheet thickness.
To understand k factor, firstly we will understand what is Sheet metal bending?
Sheet Metal Bending
Sheet metal bending is a forming operation, where metal is deformed plastically to change its shape. During bending, material is stressed beyond its yield strength but below ultimate tensile strength.
Now we will understand what happens when part is bend?
Sheet Metal Bend Deduction
During sheet metal bending, the inside surface of the bend is compressed and the outer surface is stretched.
As a result, total surface area or length of sheet metal part increases. In another words, Total length (Length + Height) is not equal to sheet metal Flat Length.
This difference in total length after bending and flat length is known as Bend Deduction
Flat Length = Length + Height – Bend Deduction
Relationship in between Bend allowance and Bend Deduction
Bend allowance and bend deduction are directly related. Sum of bend allowance and bend deduction is equal to two times of outside set back.
Bend Deduction = 2 X OSSB – Bend Allowance
What is Neutral Axis in sheet metal ?
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: A line that is neither compressed nor stretched.
Location of neutral line depends on the material, direction of material grain, tooling and environmental conditions.
K-FACTOR = Position Of Neutral Axis / Material Thickness
Relationship between K-Factor and Y-Factor ?
Similar to K factor in sheet metal, Y factor in sheet metal is a constant used to calculate sheet metal part developed length. Y factor and k factor in sheet metal are directly related.
Calculation for Bend Deduction / Allowance
Accurate value of bend deduction / allowance is calculated by reverse engineering. Following steps are followed to calculate bend deduction.
Step: 1 Consider a sheet metal blank of known thickness, size and material (For example 150X150X1mm)
Step: 2 Bend sheet metal part at 90 degree.
Step: 3 Measure the Length A and B as shown in figure.
Step: 4 Calculate Bend Deduction
Bend Deduction = Flat Length – A – B
Step: 5 Repeat this iteration 2 to 3 times for even more accurate values of Bend Deduction
Note :- Change in material or thickness, will change the value of k factor.
Calculation of K Factor in sheet metal using bend Deduction
Facts about K-Factor:
- When sheet metal part is bend with sharp inside radius, as compared to the thickness. It will stretch more on the outside, That means neutral axis will lie closer to the inside of the bend. Whereas when part is gradually bent will have less outside stretch that means neutral axis will lie closer to the centre of the part.
- Since the inside compression cannot exceed the outside tension, the k-factor can never exceed “.50” in practical use. That means, neutral axis cannot go beyond the middle of the sheet thickness.
- Value of K factor decreases with increase in thickness. In another words, k factor is inversely proportional to material thickness.
- K factor value decreases with increase in hardness. In another words, k factor is inversely proportional to material hardness.