  ### Tonnage Calculation

The tonnage required for punching any shape hole in a material is given by the following formula:

Required Tonnage = L x G x S where:-
L” is the total length of the cutting edge
G” is the material thickness
S” is the shear strength of the material
Hence required tonnage.

Example 1: For a 50.mm square in 2.mm mild steel
L = 50 x 4 = 200 G = 2 & S = 0.345
Hence required tonnage.
T = 200 x 2 x 0.345 = 137.95 KN

Punches with Shear
The tonnage required for punching with shear is given by the formula:-T = L x G x S x FWhere “F” is the shear factor (see ‘Calculating the effect of shear”)

Example 2: For a 3/8” x 3” Slitting Tool with 3/16” Shear Cutting ¼” Hard Aluminium
F = 0.63 (from graph below)
L = 2 (3/8 x 3) = 2.25 G = 0.25 S = 12 F = 0.63
Hence required tonnage:
T = 2.25 x 0.25 x 12 x 0.63 = 4.25 ton ### Recommended Clearances Shear is used on punches to reduce tonnage and noise whilst punching.
A full shear can reduce the tonnages by up to 50%.

#### Rooftop Shear:

• Ideal when punching at high loads

Taking large cuts (over 75%)  #### Inverted Rooftop:

• Very effective
• Easy to produce and regrind
• Avoid sharp corner in the centre, as this

provides a stress point, making the tool more susceptible

#### Concave:

• Best for nibbling
• More difficult to regrind

Than inverted rooftop  #### Four Way Shear:

• Ideal for large squares
• always avoid sharp inside corners, as these encourage breakage ﻿