Understanding Shearing Force
The **Punch Force** is the total force a press must exert to complete a punching or blanking operation. It consists of two main components: the **Shearing Force ($F_{shear}$) ** (the actual cutting force) and the **Stripping Force ($F_{strip}$) ** (the force needed to pull the punch back out of the material).
1. Maximum Shearing Force Formula
The maximum shearing force required occurs when the punch first engages the entire perimeter of the material. It is calculated as the area being sheared multiplied by the material's shear strength.
$$F_{\text{Shear (max)}} = L \cdot T \cdot S$$
- **$L$**: **Perimeter** of the cut (inches).
- **$T$**: **Material Thickness** (inches).
- **$S$**: **Material Shear Strength** (psi).
- **Resulting Force** is in pounds (lbs).
2. Force Reduction (Shear Angle)
To reduce the peak force required, a **shear angle** can be ground onto the punch face or the die face. This causes the cutting action to occur progressively, reducing the amount of material being cut at any one time.
**Note on Calculation:** If a shear angle is applied, the peak force is typically reduced by 30% to 50%. This calculator applies a conservative **$60\%$ effective force factor** (i.e., $40\%$ force reduction) when a shear angle is entered, as a common engineering practice for estimating press size.
3. Stripping Force
The **Stripping Force ($F_{strip}$) ** is the force needed to overcome the pressure exerted by the sheet metal clinging to the sides of the punch after cutting. It is usually estimated as a percentage of the shearing force.
$$F_{\text{Strip}} \approx F_{\text{Shear (reduced)}} \cdot (0.05 \text{ to } 0.15)$$
Total Tonnage Required
The total tonnage for which the press must be rated is the sum of the reduced shearing force and the stripping force, converted to tons (where 1 Ton = 2,000 lbs).
$$F_{\text{Total (Tons)}} = \frac{F_{\text{Shear (reduced)}} + F_{\text{Strip}}}{2000}$$