K- Factor : Sheet-Metal Flat Pattern Calculations – SMLease Design

There is a direct relation between k factor and calculation for sheetmetal flat pattern.

K Factor is a very useful term in sheet metal part design and manufacturing. Manufacturer and designer use k factor for sheetmetal flat pattern calculations.

Designers use k-factor to make sure their designed part is manufacturabe.

Sheet-metal Bending and Bend Allowance

Sheetmetal 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.

Bending Process in sheetmetal changes total surface area, which results in increase in the total length. for example. (Length + Height) is not equal to Flat Length.

Flat Length = Length + Height – Bend Deduction

Why Bend Deduction/Allowance is Required?

When sheet metal is bent, the inside surface of the bend is compressed and the outer surface is stretched. Somewhere in between the thickness of material lies Neutral Axis: Line with-in thickness that is neither compressed nor stretched.

Location of neutral line varies depending on the material, Temperature, direction of material grain, tooling conditions.

Relationship of K-Factor & Neutral Axis?

 The K-Factor is the ratio of the position of neutral axis to the material thickness. Term k-factor is used in cad softwares to calculate sheet-metal blank size.
 
K FACTOR = Position Of Neutral Axis / Material Thickness

Relationship between K Factor & Y Factor ?

Y-Factor : Some Design Software use Y-Factor instead of K-factor to calculate flat pattern in sheet-metal design.

K and Y factor has a direct relationship..

Calculation for Bend Deduction / Allowance

Best way to calculate bend deduction / allowance is by reverse engineering. Following steps are used to calculate bend deduction manually and more accurately.

Step: 1 Take a sheetmetal blank of known thickness, size and material (For example 150X150X1mm)

Step: 2 Measure the Length A and B as shown in figure.

Step: 3 Calculate Bend Deduction

Bend Deduction = Flat Length – A – B

Step: 4 Repeat this iteration 2 to 3 times to get more accurate value of Bend Deduction

With change in material or thickness, k factor will also change. 

Relationship in between Bend allowance and Bend Deduction 

Bend Deduction = 2 X OSSB – Bend Allowance

Relationship between K- Factor and Bend Deduction

Facts about K-Factor:

  • When a sheetmetal part is bent with sharp inside radius, as compared to the thickness, It will stretch more on the outside, which 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.
  • With the increase in thickness value of k factor reduces.
  • With the increase in hardness value of k-factor reduces.

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