Using K-Factor for Sheet Metal Flat Blanks

The K-Factor method is a versatile way of calculating sheet metal flat blank dimensions.  MEboost employs the K-factor method to calculate flat blanks in two directions.  Consider the flat blank below.  We have already designed the formed part and need to know the overall dimensions as well as the location of the bend lines.

Calculating Dimensions Using K-Factor

When a material is bent, some material on the inside of the bend is compressed, and some material on the outside of the bend is stretched.  There is a point in the material thickness where the material doesn’t compress or stretch.  This is the neutral axis.  The location of the neutral axis is specified with the K-factor.  It is the fraction of material thickness from the inside of the bend to the neutral axis.  Flat blank dimensions are calculated by determining the path length along the neutral axis.

For example, the following part that has a 90o bend.

To calculate the bend line location and flat blank overall dimension, the software calculates the path length along the neutral axis.  L1 and L2 are the straight legs of the part.  The location of the neutral axis affects the path length through the bend, L3.  The K-factor is defined as:

K = N/T

The length of L3 is the arc length through 90 degrees.  For non-90 degree bends, the π/2 term is replaced with the bend angle in radians.

L3 = (π/2)(KT + R)

The distance to the bend line is:

bend line dimension = L1 + 0.5L3

The overall flat blank dimension is:

flat blank dimension = L1 + L2 + L3

K-factor is a coefficient that is affected by the brake tooling as well as the material.  Publications such as Machinery’s Handbook give general recommendations for K-factors, but the best practice is to determine them from actual tool set-ups and materials.

Using MEboost to Calculate Flat Blanks

Clicking the Sheet Metal button on the MEboost ribbon will show the Sheet Metal form.

Flat Blank Report

Enter a name for the report.  A new sheet in the current workbook or a new workbook will be created and named using the report name.

General Conditions

Enter the material thickness and K-factor.

Select whether to use outside or inside dimensions.  Depending on the selection, all dimensions must be entered as inside or outside dimensions.  If a dimension is “mixed”, which means it’s partially inside and partially outside, it will need to be converted to either inside or outside by adding or subtracting material thickness.  Examples of dimensions types are shown below.

Dimension Types

Add Bends

Each bend is added using add bends.  Enter the bend radius, bend angle, bend type, and dimension.  Then select which direction to add the bend.  Click the Add button and the bend will appear either in Direction 1 bends or Direction 2 bends.

For angles that aren’t 90 degrees, the dimensions entered should be to the theoretical edge as if it were a 90 degree bend.

Non 90 Degree Bend Dimensions

Direction 1 and Direction 2 Bends

Bends for each direction will appear in each box.  Bends can be edited by selecting a bend a clicking the Edit buttons.  Bends can also be removed using the Remove buttons.

Last leg dimension is the dimension from the last bend to the edge of the material.

The Sheet Metal form with bends added is shown below.

Flat Blank Report

Clicking the Create button will generate a flat blank report.  The dimensions of the flat blank in each direction will be calculated as well as the location to each bend line from the starting edge of the material.

The report will show the total flat blank length for each direction.  The first column lists the straight leg length or the neutral axis arc length if there is a bend.

Bends That Are Too Close Together

If bends are too close together, the straight leg length between the bends will be negative.  That is, the second bend starts before the first bend ends.  When this happens, the straight leg length is highlighted in red.  Note: This only calculates if bends are too close but does not mean the bend is still possible.  Tooling geometry may not allow for bends that are theoretically possible.