Press tonnage calculator
The press force needed to blank or pierce a part, from its outline, sheet thickness and material.
How it is calculated
To cut a shape out of sheet, the press has to shear the metal all along the outline. The force is the length of the cut, times the sheet thickness, times the shear strength of the material.
Round shape: cut length = π × diameter
Rectangle: cut length = 2 × (length + width)
Tonnes = N ÷ 9,806.65
Worked example
Piercing a 50 mm hole in 2 mm mild steel: the cut length is 3.1416 × 50 = 157.1 mm. The force is 157.1 × 2 × 350 = 1,09,956 N, about 11.2 tonnes. With a 20% margin, choose a press of at least 13.5 tonnes.
The margin covers blunt punches, stripping force and harder batches of material. Add the force of every punch that cuts at the same time in a progressive tool.
Questions
How do I calculate press tonnage for blanking?
Tonnage = cut length (mm) × thickness (mm) × shear strength (N/mm²) ÷ 9,806.65. Add a margin of 20% or more when choosing the press.
What is the shear strength of mild steel?
Commonly about 300 to 380 N/mm², roughly 80% of its tensile strength. The calculator uses 350 N/mm² as a typical value; your mill certificate gives the real figure.
Do I add the force of all punches?
Yes, if they cut at the same time. In a progressive or compound tool, add the cut length of every punch that cuts in the same stroke.
Why add a safety margin?
Punches get blunt, material varies batch to batch, and the stripper and springs also need force. A press running near its limit wears faster.
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