Punching and blanking both use a punch and die to shear sheet metal, but they're opposite operations: punching keeps the sheet and discards the slug; blanking keeps the slug and discards the sheet. The geometry and force math are identical — only the intent differs.

The critical design parameter is die clearance: the gap between the punch OD and the die ID, per side. Get it wrong and you get burrs, excessive force, dished edges, or jammed slugs. Get it right and you get a clean shear with minimal deformation.

Clearance rule of thumb: per-side clearance = c × t, where t is sheet thickness and c depends on material:

• Mild steel: 5–8% of thickness
• Stainless steel: 8–12%
• Aluminum: 4–6%
• Copper/brass: 4–5%

So a 2 mm mild steel sheet wants roughly 0.10–0.16 mm clearance per side. A 10 mm punch runs in a 10.2–10.32 mm die.

What the sheared edge looks like: four distinct zones from top to bottom — rollover (plastic dishing as the punch enters), burnish (clean polished band where material flowed against the die), fracture (rough angled zone where the crack actually propagated), and burr (sharp lip on the exit side). With correct clearance, burnish is ~⅓ of thickness. Too little clearance: secondary shear, double burnish, and huge force. Too much: large rollover, big burr, torn edges.

Force calculation:

F = L × t × τ

where L is the cut perimeter, t is thickness, and τ is the material's shear strength (~80% of tensile strength for steel). Punching a 25 mm hole in 3 mm mild steel (τ ≈ 350 MPa):

F = π × 25 × 3 × 350 = 82,400 N ≈ 8.4 metric tons.

Real-world example: An electrical panel shop punching knockouts in 16-gauge (1.5 mm) galvanized steel sees frequent burr complaints from electricians whose hands get cut on conduit holes. The fix is rarely "sharper tooling" — it's checking clearance. Galvanized steel is gummier than bare cold-rolled; running 6% clearance instead of 8% causes the slug to weld to the punch, drag through the die, and tear the exit edge. Bumping clearance and adding a shear angle on the punch face (a small bevel, typically 2–5°) drops peak force by 30–50% and produces cleaner holes by shearing progressively rather than all at once.

Shear angle is why a punch press for thick plate sounds like a smooth thunk rather than a sharp bang — the work is spread over the stroke.