Bending Force Calculation :

There are various ways to calculate the bending force of a sheet metal parts, most basic is to use the formula. Since it's a basic and everyone should know about theory, however there may be some variations in results due to error in input values such as Tensile strength, coeff. of friction (not considered in formulas), press type (Mechanical or hydraulic) and other practical aspects which are not possible to measure. Therefore these force values to be taken as reference and sufficient factor of safety to be considered, minimum 20% to be added to select the final press. 100% correlation of formula and actual press force is impossible, even FEA softwares have certain limitations.

Bending in a V-die with rectangular cross-section:

where kV = die opening factor, 0.75 to 2.5 (larger values are for smaller R/t ratios and vice
versa). A 1.33 value is used for a die opening of 8 times metal thickness.
W = width of the bent-up portion
L = distance between material supports
S = ultimate tensile strength

For instance, Let's assume a sheet part with thickness 3mm, Kv =2.5, more than 8x of thk., L=60, W=75, S=41

Force = (2.5 x 41 x 75 x 3 x 3) / 60 ⇒ 1153 Kgf

To check the accuracy of the formula, the co-relation has been made with the FEA force calculation :

FEA force is 10231N = 1044 kfg (almost nearby the formula)

Bending in a U-die, equipped with a spring-loaded pressure pad:

where kU = die opening factor, 0.4 to 10
RE = radius, die edge (see Fig. 8-53)
RD = radius, bottom of U channel
Ppad = pressure of spring-loaded support

Bending with bottoming (coining):