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Calculating Punching Force: A Step-by-Step Tutorial

Last updated:
April 2, 2024

Table Of Contents

Punching force includes blanking force, stripping force, pushing force, and lifting force. Calculating the punching force is fundamental to selecting the right press, as shown in Figure 1.

Blanking Force

F=L×t×r

where:

  • F – the blanking force (N);
  • L – the perimeter length of the blank (mm);
  • t – the material thickness (mm);
  • r – the material’s shear strength (MPa).
Figure 2-8  Discharge force, feed force, lift force.
Figure 1 Discharge force, feed force, lift force.

Stripping Force, Pushing Force, Lifting Force

a) Stripping force is the force required to remove the material clamped on the punch, given by

Funloading = kunloading * F

b) Pushing force is the force needed to push the blank along the direction of blanking out of the die cavity, given by

Fpushing = n*kpushing*F

c) Ejecting force is the force required to lift the blank against the direction of blanking out of the die cavity, given by

Fejecting = kejecting * F

where:

  • kunloading – the stripping force coefficient;
  • kpushing – the pushing force coefficient;
  • kejecting – the ejecting force coefficient;
  • n – the number of pieces in the die, n = h/t (h is the straight wall height of the die edge, t is the thickness of the workpiece);
  • F – the blanking force (N).

The coefficients for unloading force, pushing force, and ejecting force are provided in Table 1.

Table 1: Ratios of Unloading Force, Feeding Force, Stripping Force to Punching Force

Material Thickness/mmKunloadingKpunchingKejecting
Steel≤0.10.065 – 0.0750.10.14
>0. 1 ~0.50.045 – 0.0550.0630.08
>0.5 ~2.50.04 – 0.050.0550.06
>2.5 ~6.50.03 – 0.040.0450.05
>6.50.02 – 0.030.0250.03
Aluminum & aluminum alloys0.025 – 0.080. 03 ~ 0. 07
Pure copper & brass0.02 – 0.060.03 ~ 0.09

Selection of Stamping Equipment

When the stamping process involves simultaneous blanking force, feeding force, and ejecting force, the total stamping force F is calculated as follows: Total stamping force Ftotal = F + Funloading + Fpushing + Fejecting.

In this case, the tonnage of the press selected should be approximately 30% greater than Ftotal to provide the necessary margin.

When F, Funloading, Fpushing, and Fejecting do not occur simultaneously, Ftotal is calculated by only adding the forces present at the same instant.

In production, detailed calculations are typically performed for the punching force only, while the unloading force is estimated based on a fixed proportion of the punching force, plus the margin required for punching, totaling 50%.

Thus, the formula for calculating the tonnage of the press is:

Fpress = F × 150% = 1.5F

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