How press tonnage is calculated
Blanking and piercing force is cut perimeter x material thickness x shear strength, divided by 2,000 to get US tons of 2,000 lb each, the rating used on American presses. Punching a 4 in diameter blank from 0.080 in mild steel at 50,000 psi shear gives 12.57 in x 0.080 in x 50,000 psi = 50,265 lb, about 25 US tons. Add the perimeter of every hole pierced in the same hit, not just the outline.
Stripping the slug off the punch typically takes another 5-20% of the cutting force. Air bending force is 1.42 x tensile strength x bend length x thickness squared divided by the V-die opening. The constant varies from 1.33 to 1.6 between sources; the familiar US shop rule of 575 x T squared / W tons per foot for 60 ksi mild steel sits near the top of that band, so cross-check against your press brake maker tonnage chart before committing. V-die width is normally 6-10 times thickness, and this tool uses 8 times when the field is left blank.
Specify the press with 20-30% headroom over the calculated value. Coil-to-coil strength variation, poor die clearance, punch wear and multiple stations hitting at once all push the peak load up. Have the die designer and the press supplier confirm the final tonnage and die set before you order.
Frequently Asked Questions
Shear strength runs about 0.7-0.8 of tensile strength. Use 35,000-50,000 psi for mild steel, 55,000-75,000 psi for stainless, 18,000-27,000 psi for aluminum and 30,000-40,000 psi for brass, then refine with the mill certificate.
Yes, anything shearing at the same instant adds directly. Stepping punch lengths so the stations break through at different times, sometimes called shear or staggered punches, can cut the peak load by 20-30%.