How this instrument works
Punching a hole through sheet metal doesn't pull the material apart the way stretching or bending does — it shears it, forcing a punch through the material along the entire outline of the hole at once, the way a cookie cutter shears through dough rather than tearing it. The force required to do that depends on how much material has to be sheared through, which is set by two things working together: how long the cutting edge is (the perimeter of the shape) and how thick the material is, since a longer cut or a thicker sheet both mean more shear area to overcome.
The material itself sets the last variable: shear strength, a property that describes how much force per unit area it takes to shear that specific material apart. Shear strength differs from tensile strength — the figure more commonly quoted on material spec sheets — and while the two are related, punching calculations specifically need the shear figure, since punching is a shearing operation, not a pulling-apart one. Mild steel, aluminum and stainless steel each carry meaningfully different shear strengths, so the same hole punched through different materials at the same thickness needs a different force.
Multiplying perimeter, thickness and shear strength together gives the total force needed, in pounds-force, to shear cleanly through the material around the full outline of the punched shape in one stroke. This is the standard sheet-metal punching-force formula published in Machinery's Handbook, and it's what a press operator or die designer checks before running a job — punching a shape that needs more force than the press can deliver stalls the ram, damages the tooling, or simply fails to punch through.
- Enter the outline length of the hole being punched into Perimeter of the punched shape (in) — for a round hole this is π times the diameter; for a square it's four times a side.
- Enter the stock thickness into Material thickness (in).
- Enter the material's shear strength into Material shear strength (psi) — check a materials reference table for the specific alloy and temper being punched.
- Read Required punch force (lbf) for the total force the press must deliver to shear cleanly through the material.
- Compare the result against your press's rated tonnage (multiply tons by 2,000 to get lbf) before running the job — a press undersized for the job will stall or damage the tooling.
Worked example — a 4-inch perimeter through 1/8-inch mild steel
Enter 4 into Perimeter of the punched shape (in), 0.125 into Material thickness (in), and leave Material shear strength (psi) at its default of 40,000 psi — a typical value for mild steel. Required punch force (lbf) reads 20,000.0 lbf, or 10 tons.
By hand: 4 × 0.125 = 0.5, and 0.5 × 40,000 = 20,000 lbf exactly — the force a press needs to shear cleanly through 1/8-inch mild steel along a 4-inch cutting outline, such as a roughly 1.27-inch-diameter round hole (perimeter = π × diameter).
Questions
Why does punch force use shear strength instead of tensile strength?
Because punching shears the material along the punch's outline rather than stretching or pulling it apart, and shear strength specifically measures resistance to that sliding-apart failure mode. Tensile strength, the figure more commonly quoted on material spec sheets, describes a different kind of failure and would give a misleading force estimate if substituted directly — shear strength for common metals is typically a fraction of their tensile strength, roughly 60-80 percent depending on the material.
How do I find the perimeter for a shape that isn't a simple circle or square?
Add up the length of every edge of the shape being punched — for a circle, perimeter is π times the diameter; for a square or rectangle, it's the sum of all four sides; for an irregular outline, sum each straight or curved segment's length individually. The formula only needs the total outline length, regardless of the shape's overall form.
Where do I find a material's shear strength?
Machinery's Handbook and most metals-reference tables publish shear strength alongside tensile strength for common alloys and tempers — mild steel is commonly cited around 40,000-50,000 psi, aluminum alloys often 30,000-35,000 psi, depending on the specific grade. If a table only lists tensile strength, a rough approximation of roughly 60-80 percent of that figure is sometimes used as a stand-in, but a published shear value for the exact material is always more reliable.
How do I convert the punch force result to press tonnage?
Divide the force in pounds-force by 2,000 to get US tons — a press rated for 10 tons can deliver up to 20,000 lbf. Always compare the calculated force against the press's rated capacity with a margin, not right at the limit, since real punching also involves friction, tool wear and material variability that can push the actual required force somewhat above the theoretical calculation.
Does thicker material always need proportionally more force?
Yes, for a fixed perimeter and shear strength — the formula is a straight multiplication, so doubling the material thickness doubles the required punch force. That's why punching thick stock often calls for either a larger press, a smaller cutting perimeter per stroke (punching one hole at a time rather than a whole pattern at once), or techniques like shear angles on the punch that reduce peak force even though total work stays similar.