How to Calculate Punching Force

If you punch round holes or square holes,or some other forms of holes through a given thickness of metal, you must know the force required to punch a hole in steel. You can calculate the punching tonnage been required with the help of the following punching force calculation formula (blanking force formula):

K : Safety factor, usually we will choose 1.3

L : Perimeter after punching , mm

t : Material thickness, mm

τ : Shear strength, MPa

If we want to converted the result into metric tons, we can divide the result of KN by 9.8

We can calculate the perimeter refering to the following chart

Thickness: the thickness is for the material which will be pierced through by the punching mold.

Shear strength: the physical properties of the plate, determined by the material of the sheet and can be found in the material manual.

The following chart is showing the shear strength for usual material:

                                                                                          Unit:kN/mm²

For example: 

If we will punch one square hole in the 3mm thickness, the material is  low-carbon steel plate, side length is 20mm, we can calcuate in this way:

Perimeter=10*4=40mm

Thickness=3mm

Shear strength=0.3447kN/mm²

Punch force(kn)=1.3*40*3*0.3447=53.77kN 

W can convert it into tonnage :107.55kN/9.8=5.49T

Punch and die clearance

The clearance between punch and die is represented by the total difference, which is one of the critical factors in the punching process.

For example, when use ∅12.25 lower die, the optimal clearance is 0.25mm.

Improper clearance will reduce the die service life, or burrs and lead to secondary cutting,the irregular opening will increase the demounting force,etc.

Besides, the die clearance is subject to the material and thickness, generally,for carbon steel plate,12%-18% of the thickness is best.

If no special requirements in CNC punch, you can refer to the following table for selecting the die clearance: