How do you calculate the press tonnage a stamping job needs?

The short answer

The formula, term by term

Every cutting operation in a press, whether it is blanking, piercing, trimming or notching, needs a force proportional to the area being sheared. The working formula is tonnage equals cut perimeter in millimetres, times thickness in millimetres, times shear strength in kilograms per square millimetre, divided by 1,000. The division converts kilograms to metric tons, so the units stay consistent and the answer comes out in the number stamped on the press nameplate.

Each term has a trap in it. The perimeter is the total length of every edge cut in one stroke, not the outline of the part: a die that pierces four holes and blanks the outside in the same stroke adds all five perimeters together, and a progressive die adds every station that cuts simultaneously. Thickness should be the maximum in the tolerance band, not the nominal, because sheet arrives on the heavy side more often than the light side. Shear strength is not tensile strength; it is typically 0.6 to 0.8 of tensile strength for ductile metals, and using the tensile figure instead is a common way to arrive at a press that is 30 percent oversized.

Shear strength values to start from

For planning, mild steel runs 32 to 40 kilograms per square millimetre, medium carbon steel 45 to 55, stainless 50 to 60, soft aluminium 8 to 12, copper 18 to 25 and brass 25 to 35. Always take the top of the range for press selection, because material arrives harder than the mill record says more often than the reverse, and a dull punch raises the cutting force by 20 percent or more on its own. If the actual shear strength is known from a mill test record, use it; if not, the upper bound is the safe assumption.

Three worked examples

A 50 mm diameter hole in 3 mm mild steel: the perimeter is 3.14 times 50, or 157 mm; 157 times 3 times 35 gives 16,485 kilograms, or 16.5 metric tons. Add 30 percent and the requirement is 21.5 tons, so a 25 ton press is the smallest standard size that fits. A 100 by 60 mm rectangular blank in 2 mm stainless: the perimeter is 320 mm; 320 times 2 times 55 gives 35.2 tons, and with 30 percent that is 45.8 tons, so a 50 ton press. Four 20 mm holes pierced together in 4 mm mild steel: each perimeter is 62.8 mm, the total is 251 mm, and 251 times 4 times 35 gives 35.2 tons, or 45.7 tons with margin.

Notice that the third example produces the same tonnage as the second from a different combination of thickness and material. That is the useful property of the formula: it lets you compare a thick mild steel job against a thin stainless one on the same scale, and it explains why a shop can run a 50 ton press on both while a 25 ton press would stall on either.

What the formula leaves out

The cutting force is not the whole load. Stripping force, the force needed to pull the strip off the punch on the upstroke, adds roughly 2 to 10 percent of the blanking force, and more if the punch is ground with a straight land rather than a relief. Ejection force for the blank, blank-holder force in a drawing operation, and the force of any coining or embossing station all add to the total. The standard practice is to calculate the cutting force, add the auxiliary loads, then apply a safety factor of 1.2 to 1.3 for general stamping and 1.6 to 2.0 for deep drawing, where the load is sustained over a long stroke. Finally, multiply by the number of parts cut per stroke in a multi-outline die; a two-outline die doubles everything.

The tonnage curve is not flat

A mechanical press delivers its rated tonnage only near the bottom of the stroke, where the crank is close to dead centre and the mechanical advantage is highest. Higher up the stroke the available force falls away, and at 100 to 200 mm above bottom dead centre it can be less than half the nameplate figure. Cutting happens early in the stroke, where the punch first contacts the sheet, so the force available at that point is what matters, not the number on the frame. For thin sheet and a short stroke this is rarely a problem. For thick plate, deep drawing, or a die that has to cut high above the bed, the tonnage-stroke curve has to be checked explicitly, or the press has to be hydraulic, which delivers full force through the whole stroke. This is the single most common reason a press that "should" be big enough stalls mid-cut.

How to write tonnage into an RFQ

A stamping quote is more accurate when the RFQ carries the inputs rather than the conclusion. State the material grade and its shear strength if known, the thickness with its tolerance, the number of parts per stroke, the cut perimeter if the drawing is simple, and the annual volume with the batch size. Do not state the press tonnage you think is needed unless it is a constraint you are imposing, because a supplier who has a 160 ton press with a spare die space may quote a different and cheaper route than one who only has 60 tons. What matters to the buyer is that the chosen press has at least 20 to 30 percent headroom over the calculated cutting force plus auxiliary loads, and that the die is designed for the material's shear strength rather than a generic mild steel assumption.

See metal stamping for the press range and die work this calculation is used for, custom metal stamping for blanked and pierced parts quoted from a drawing, and laser cutting for prototypes where no tooling exists yet.