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Lesson 6.1CutChapter 6 · Cutting and non-traditional machining

Punching and shearing

A punch shears shapes out of sheet against a die

Punching cuts holes and shapes in sheet metal by forcing a hard punch through the sheet into a matching die. The slots in a steel shelving upright and the holes in a perforated grille were made this way, one stroke per hole. Shearing is the same cut made along a straight line by two long blades, and it is how most sheet is cut to size.

Punching and shearing: a real photo
Photo: W. R. Schettler, Public domain, via Wikimedia Commons
A hard punch pushes the sheet into a die and shears out a hole in one strokeCross-sections, not to scale; the gap between punch and die is exaggeratedBefore the hitPunchStripperSheetDie1Punch shears the sheetShear zone23Slug drops, punch liftsHoleSlug54Shear zone, magnifiedClearancePunchDieSheetCracks meetThe cut edge, magnifiedpunch sidedie sideRolloverrounded edgeBurnishsmooth, shinyFracturerough, dullBurrsharp lipShearing: straight bladesHold-downUpper bladeLower bladecut strip fallsSheet being cutTooling: punch, stripper, die, blades
One punch stroke in three stages, with close-ups and a straight-blade shearTop row: one punch stroke from left to right; circled numbers match the steps below. Bottom row: close-ups of the cut, and the straight-blade version.

How it works

  1. Clamp. The sheet lies on the die (the lower tool: a hardened block with an opening shaped like the hole). The stripper, a plate around the punch, presses the sheet flat.

  2. Press. The punch (the upper tool: a hardened steel rod with the shape of the hole) comes down and pushes the metal into the die opening. The edge first bends and rounds over.

  3. Shear and crack. The sharp edges cut a smooth band into the metal. Then cracks start at the punch edge and the die edge and run toward each other across the clearance (the small gap between punch and die).

  4. Break through. When the cracks meet, the cut-out disc, called the slug, breaks free and is pushed down through the die as scrap.

  5. Strip and move. The punch lifts while the stripper holds the sheet down. On a CNC turret punch press (a computer-controlled machine with a rotating magazine of punches), the table then slides the sheet to the next hole.

Shearing

Shearing (bottom right) applies the same action along a straight line. A hold-down clamps the sheet, and a long upper blade comes down just past a fixed lower blade. The upper blade is usually tilted slightly, so it cuts progressively along the line, like scissors. An adjustable stop behind the blades, the back gauge, sets the width of the strip.

Key facts

Materials
Sheet steel, stainless steel, aluminum, copper and brass; also plastics, leather and gasket sheet with simpler tools
Typical parts
Electrical cabinets, brackets, chassis panels, perforated screens, washers, shelving uprights
Part size
Sheet usually up to about 6 mm thick; CNC turret presses take sheets up to roughly 1.5 × 3 m
Precision
About ±0.1 mm on a CNC turret press
Surface finish
Faces untouched; the cut edge has a shiny band, a rough band and a small burr (a sharp raised lip)
Batch size it suits
Prototypes to large series with standard tools; dedicated dies pay off in mass production
Tooling and machine cost
Standard punch-and-die sets cost tens to hundreds of dollars each; a CNC turret press costs hundreds of thousands
Speed
One stroke per hole, often several hundred strokes per minute

Strengths and limits

Strengths

  • Very fast: one stroke per hole
  • Low cost per hole; standard tools cover common shapes
  • No heat, so the metal around the hole is unchanged
  • The same machine can also form louvers (raised vent slots) and stiffening ribs

Limits

  • Each hole shape needs its own punch and die
  • Sheet only, usually up to about 6 mm thick
  • The edge is partly rough and carries a burr
  • Holes narrower than about the sheet thickness can break the punch

Where it's used

  • ElectricalControl cabinets, junction boxes, knock-outs for cable entries
  • Building and furnitureShelving uprights, perforated facade panels, ceiling grid parts
  • Appliances and air handlingOven and washer housings, vent grilles, duct parts
  • Electronics and ITServer racks, computer chassis, shielding plates
  • AutomotiveBrackets and reinforcements; sheared blanks for stamped body panels

How to recognize it

  • The hole wall has a shiny band near one face and a duller, rougher band near the other.
  • One face shows a slightly rounded hole edge; the other face has a small, sharp burr.
  • Large outlines with a faint scalloped edge come from nibbling: many small, overlapping hits.
  • Rows of identical holes, slots or louvers, and knock-outs (part-cut discs you break out for cables).

Compare with

Laser cutting6.2no tooling and any outline, but it traces every hole, so dense hole patterns take longer.Stamping and deep drawing4.2dedicated dies in large presses for mass production; punching is the flexible, small-batch cousin.Waterjet cutting6.4also no heat, but much slower; it cuts thick plate, stone and glass that a punch cannot.

Watch it

Roy Manufacturing · A beginner's tour of a CNC turret punch and its tools

Headland Technology · The machine's parts, taken apart in an animation

TRUMPF Inc. · Close-ups of the punching head and its tools at work

NCTOOLS CADCAM · A sheet perforated with long rows of holes

See real photos

Remember

  • A punch and die cut by shearing: the metal bends, is cut partway, then cracks through.
  • The clearance between punch and die sets the edge quality and the size of the burr.
  • Punching is the fastest way to make many holes in sheet; shearing does the same along a straight line.

More in Cutting and non-traditional machining

All 100 processes
6.2Laser cuttingFocused light melts or vaporizes a narrow cutOpen the lesson6.3Plasma cuttingA superheated gas jet melts through conductive plateOpen the lesson6.4Waterjet cuttingVery high-pressure water, usually with abrasive, erodes a cutOpen the lesson6.5Wire EDMSparks from a thin moving wire cut through-profilesOpen the lesson6.6Sinker EDMSparks copy a shaped electrode into a blind cavityOpen the lesson6.7Electrochemical machining (ECM)Electrolysis dissolves metal in the shape of the toolOpen the lesson6.8Chemical machining and photochemical etchingA light-printed mask lets an etchant dissolve only bare metalOpen the lesson6.9Ultrasonic machiningA vibrating tool hammers abrasive grains into the surfaceOpen the lesson

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