How to use this calculator
- Get the cut perimeter. π·D for rounds, 2(a+b) for rectangles, or sum every sheared edge for a custom profile.
- Enter stock and material. Thickness and shear strength — data sheet first, 0.7-0.8 × UTS as the fallback estimate.
- Compare against the press. The punch force plus a machine margin must sit inside the press rating at that point in the stroke.
- Size the stripper. The stripping allowance (10-20% band) sets spring or plate force for the return stroke.
How it works
Punching is shear on a wall of material: the punch drives the slug's entire perimeter through the thickness at once, so the force is the sheared area times the material's shear strength —
F = p × t × τ · p = π·D (round) · p = 2(a+b) (rectangle)
The same stock and machine arithmetic in bending form lives in the press brake tonnage calculator, the hole's downstream use in the bend allowance calculator's flat patterns, and hole-size practice near edges and bends in the K-factor calculator's territory.
Worked example
Verified against the live calculator
A 20 mm round hole in 3 mm mild steel at
300 MPa shear strength, 15% stripping allowance:
F = π × 20 × 3 × 300 = 56.5 kN ≈ 5.8 t · stripping = 8.5 kN
A 35-tonne ironworker shrugs at this hit. Swap to a
50 × 30 mm rectangular blank in the same stock and the
perimeter jumps to 160 mm — 144 kN ≈ 14.7 t — same
thickness, same material, 2.5× the force, purely from perimeter.
That is the design lever: punching force scales with the cut
outline, not the hole area.
Frequently asked questions
How do you calculate punching force?
Sheared area times shear strength: F = perimeter × thickness × τ. A 20 mm round hole in 3 mm mild steel (300 MPa shear) needs π×20×3×300 = 56.5 kN — about 5.8 tonnes — and a 50×30 mm rectangular blank in the same stock needs 144 kN (14.7 t).
What shear strength should I use for punching?
The data sheet value when you have it; the handbook estimate is 0.7–0.8 × tensile strength. Mild steel sheet is commonly taken around 280–350 MPa; stainless, HSLA and work-hardened tempers run substantially higher — which is exactly why presses sized on mild steel stall on them.
What is stripping force?
The force to pull the stock back off the punch on the return stroke — the sheet grips the punch after the slug breaks. Die-design references use 10–20% of the punch force as the starting band; it sizes the stripper springs or plate, not the press.
How can a small press punch a big blank?
Grind shear (an angle) on the punch or die face: the perimeter then cuts progressively over the stroke instead of all at once, trading peak force for working stroke. Staggering punch lengths in multi-hole tools does the same — the formula here is the flat-faced, all-at-once worst case.
Method & assumptions
- Flat-faced punch, full perimeter engaged at once — the worst-case peak. Shear ground on punch or die reduces the peak in exchange for stroke.
- Shear strength is user-entered (no material table embedded); 0.7-0.8 × UTS is the handbook starting estimate and the data sheet governs.
- Stripping at 10-20% of punch force is the die-design starting band — cluster layouts, galling alloys and tight clearances run higher.
- Die clearance selection, punch tip compressive stress, minimum hole-to-thickness ratios and slug control are die-design checks beyond this screen.