Piercing produces holes, slots, or openings in a part by shearing a small piece — the slug — out of the sheet and pushing it down through a die opening. Unlike blanking, where the punched-out piece is the part you keep, in piercing the workpiece is the strip that remains, and the slug that falls through the die is scrap. The two operations share identical cutting mechanics; only the definition of "part" versus "scrap" differs.
The force the punch must deliver follows the standard cutting-force equation: F = L × T × S, where L is the cut perimeter (for a round hole, π × d), T is the material thickness, and S is the shear strength of the stock. For example, piercing a 10 mm diameter hole in 2 mm mild steel with a shear strength of roughly 350 N/mm² requires F = (π × 10) × 2 × 350 ≈ 22,000 N, or about 2.2 metric tons. That number, summed across every pierced feature in the die, drives the press tonnage you must specify.
Edge quality on a pierced hole is read from its cut-edge zones — rollover, burnish, fracture, and burr. The balance of burnish to fracture is a direct readout of die clearance: too little clearance forces secondary shear and raises tonnage; too much produces oversized holes with heavy burrs and excessive rollover. For piercing, clearance is typically held at 5–10% of material thickness per side, biased to the punch since the punch establishes the hole size.
Because the punch enters and exits the same slug-sized hole on every stroke, piercing is especially prone to slug pulling — the slug clinging to the punch on the upstroke and being carried back onto the strip, where it can be re-struck and wreck the tool. Sharp tooling, correct clearance, slug-retention features, and air or vacuum ejection all help keep pierced holes clean and the die running unattended.
Several practical limits constrain piercing. As a rule of thumb, the punch diameter should not be much smaller than the material thickness, because a slender punch is poorly supported against the side loads of cutting and tends to deflect or break; below that ratio, special guided or self-piercing techniques are used. Hole-to-edge and hole-to-hole spacing are likewise kept above roughly 1.5 to 2 times material thickness so the thin web between features does not distort or roll into the cut. When a hole must be pierced very close to a bend or a formed feature, the forming is usually done first or the hole is deliberately oversized and pierced after forming, since material flow during bending would otherwise pull a previously pierced hole out of round.