What Is Piercing?

Piercing is hole-making. The punch shears through sheet metal and the cut-out piece drops into the die. Blanking does the same thing but you keep the piece that falls out. Piercing you keep the sheet.

Clearance between punch and die sits around 6% per side for cold rolled steel at 16 gauge. Thicker material needs more. Aluminum needs less or you get a lot of rollover at the entry. Stainless is its own problem.

The sheared edge has four zones. Rollover at the top where material got pushed down before cutting started. Burnish zone is the shiny part, maybe 30% of thickness on a good setup. Fracture zone below that where cracks from punch side and die side met in the middle. Burr at the bottom.

Punch Wear

D2 at 60-62 Rc holds up. Carbide for anything over a million cycles but you need rigid presses. Worn punches make oversize holes. The cutting edge rounds off and starts pushing material instead of shearing it. Hole diameter creeps up 0.02, 0.03 mm and quality rejects start coming back from the customer.

Resharpening takes 0.1 to 0.2 mm off the face each time. After five or six resharpenings the punch gets short enough that stripper timing becomes an issue.

Slugs

Slug pulling wrecks parts. The slug rides up on the punch face and gets stamped into the next hit. Leaves a witness mark or worse. Vacuum between punch and slug gets blamed but surface tension from cutting oil does it too. Cross-hatch grinding on the punch face helps. Ejector pins in the die help more.

Small slugs are worse than big ones. A 3mm slug weighs nothing and sticks to anything. Big slugs have enough mass to fall clear.

Progressive dies running 200 strokes per minute generate a lot of slugs. They pile up in the scrap chute. Chute angle matters. Under 45 degrees and slugs stack up and jam.

Hole Location

Distance from hole center to bend line needs to be 1.5 times material thickness minimum or the hole pulls into an oval during forming. More for high-strength steel. Distance from hole to sheet edge, same idea. Too close and the punch blows out the edge instead of making a clean cut.

Hole to hole spacing depends on what you can hold in the web between them. Two thicknesses is the number people use. Tighter than that and the strip starts to buckle between stations.

Pilots locate the strip at each station. Pilot holes get pierced first, pilots enter before piercing punches. Pilot diameter runs 0.02-0.03 mm under nominal so they actually fit the holes after a few thousand hits of wear.