How do you calculate press tonnage for blanking?

Multiply the cut perimeter by the stock thickness and the shear strength, then add a reserve. 500 mm x 1.5 mm x 300 MPa = 225 kN; with 15% reserve that is 258.75 kN, or 29.1 US tons (divide newtons by 8,896.443).

What is the formula in inches and tons?

Tons = perimeter (in) x thickness (in) x shear strength (psi) / 2,000 for US tons. For 20 in of perimeter in 0.060 in steel at 45,000 psi shear: 20 x 0.060 x 45,000 / 2,000 = 27 US tons before any reserve.

What shear strength should I use for steel?

Use the supplier's shear figure if you have it; otherwise about 0.7 to 0.8 x tensile strength for carbon and stainless steels. A mild steel with about 400 MPa tensile strength shears at roughly 300 MPa, the figure in the worked example.

How much tonnage reserve should a press have?

Plan 10 to 20% above the calculated force for dull edges, material variation and stripping, and keep regular production below about 80 to 90% of the press rating. The example uses 86.25% of a 300 kN press, which works but leaves little margin.

Does the press tonnage formula apply to forming and drawing?

No. This formula covers cutting: blanking, piercing and trimming. Bending, forming and deep drawing depend on die opening, bend length, draw ratio and material flow, and need their own formulas.

Engineering and Process

Press Tonnage Formula

The press tonnage formula estimates the force needed to blank or pierce sheet metal from the length of the cut, the stock thickness and the material's shear strength. Use it to check that a die fits the press before tryout and to size a new press with a sensible reserve.

Formula

Cutting Force = Cut Perimeter x Thickness x Shear Strength

Variables

Cut Perimeter: Total length of edge cut at the same moment by all punches in the stroke, in mm
Thickness: Stock thickness, in mm
Shear Strength: Ultimate shear strength of the material, in MPa (N/mm²); for steels roughly 0.7 to 0.8 x tensile strength

Understanding the Press Tonnage Formula

Blanking and piercing shear the sheet along the punch edge, so the force is the area being sheared, perimeter times thickness, multiplied by the material's shear strength. In millimeters and megapascals the result comes out in newtons: 500 mm x 1.5 mm x 300 MPa is 225,000 N, or 225 kN.

Shear strength is the number that matters. Datasheets usually give tensile strength; for carbon and stainless steels shear strength is roughly 70 to 80% of it, and for aluminum alloys around 60 to 65%. Add stripping force and a reserve for dull tooling and material variation; 10 to 20% is common, and the example uses 15%, which takes it to 258.75 kN or 29.1 US tons.

Press ratings are quoted near the bottom of the stroke, so a press can deliver its rated tonnage only close to bottom dead center. A cut that happens higher in the stroke, or a die that also forms or draws, needs the press's tonnage curve, not just its nameplate. Staggering punch lengths or adding shear to the punch face spreads the cut over the stroke and lowers the peak force.

Worked Example

A blanking die cuts 500 mm of perimeter in 1.5 mm steel with a 300 MPa shear strength. The plant adds a 15% reserve and the press is rated 300 kN.

Cutting force = 500 mm x 1.5 mm x 300 MPa = 225,000 N = 225 kN
With a 15% reserve = 225 kN x 1.15 = 258.75 kN
In US tons = 258,750 N / 8,896.443 N per ton = 29.1 US tons
Press capacity used = 258.75 / 300 x 100 = 86.25%

Result: 29.1 US tons (258.75 kN) with a 15% reserve, 86.25% of a 300 kN press

Common Mistake

Using tensile strength instead of shear strength, or summing every punch's perimeter when the punches are staggered. Tensile strength overstates the force by roughly 25 to 40%; staggered or stepped punches cut in sequence, so only the perimeter cutting at the same instant counts.

Frequently Asked Questions

How do you calculate press tonnage for blanking?: Multiply the cut perimeter by the stock thickness and the shear strength, then add a reserve. 500 mm x 1.5 mm x 300 MPa = 225 kN; with 15% reserve that is 258.75 kN, or 29.1 US tons (divide newtons by 8,896.443).
What is the formula in inches and tons?: Tons = perimeter (in) x thickness (in) x shear strength (psi) / 2,000 for US tons. For 20 in of perimeter in 0.060 in steel at 45,000 psi shear: 20 x 0.060 x 45,000 / 2,000 = 27 US tons before any reserve.
What shear strength should I use for steel?: Use the supplier's shear figure if you have it; otherwise about 0.7 to 0.8 x tensile strength for carbon and stainless steels. A mild steel with about 400 MPa tensile strength shears at roughly 300 MPa, the figure in the worked example.
How much tonnage reserve should a press have?: Plan 10 to 20% above the calculated force for dull edges, material variation and stripping, and keep regular production below about 80 to 90% of the press rating. The example uses 86.25% of a 300 kN press, which works but leaves little margin.
Does the press tonnage formula apply to forming and drawing?: No. This formula covers cutting: blanking, piercing and trimming. Bending, forming and deep drawing depend on die opening, bend length, draw ratio and material flow, and need their own formulas.