Metal Stamping

How metal stamping works

A flat strip or sheet of steel, aluminum, or copper is fed between two halves of a die. The upper half (the punch) descends with the press ram; the lower half (the die) sits on the press bed. As the punch contacts the sheet, it shears, bends, draws, or coins the metal against the die. The press ram returns; the strip advances; the punch falls again. Modern presses run 200 to 1,000 strokes per minute on small parts, 15 to 60 on body panels.

The defining trick of stamping — versus machining or casting — is that one stroke does many things at once. A single hit on a coin blanks the disk, coins the obverse and reverse images, and embosses the reeded edge in the same impact. A single hit on an automotive door inner-panel die forms a 1.5 m sheet into a complex stamping with stiffening ribs, beads, and trim cutouts. The press is doing in 50 milliseconds what a CNC machine would take 30 minutes to mill.

BLANKING       ──────►  PIERCING       ──────►  FORMING       ──────►  DRAWING

  ┌──┐              ┌──┐              ┌──┐              ┌──┐
  │↓ │ punch        │↓ │              │↓ │              │↓ │
  ├──┤              ├──┤              │  │              │  │
  │░░│ sheet        │░░│ ◯  ◯         │░ ╲              │ ╱╲
  ├──┤              │░░│              │   ╲             │╱  ╲
  │  │ die clearance│  │              │    ╲────►       ▼ cup ▼
  └──┘              └──┘              └──────┘          drawn into
  outline cut       hole punched      sheet bent        cavity

Stamping force math

The fundamental equation for shearing operations is

F = L · t · σ_shear

  F        = press force needed (N)
  L        = cut perimeter (mm)
  t        = sheet thickness (mm)
  σ_shear  = shear strength of metal (MPa)
           ≈ 0.7 × tensile strength

Worked example. A 200 mm perimeter outline blanked from 1 mm thick mild steel with shear strength 300 MPa requires:

F = 200 × 1 × 300 = 60,000 N = 60 kN ≈ 6 tonnes

So even a small 30-tonne press handles this part comfortably. Scale up to a body-panel cut perimeter of 8 m in 0.7 mm dual-phase steel (σ_shear ≈ 600 MPa) and the force becomes 8,000 × 0.7 × 600 = 3.36 MN, or 340 tonnes — and that is just for the trim. The blank-and-draw operation that creates the panel itself runs 1,000 to 2,500 tonnes.

For drawing operations, the force depends on the cup depth, blank diameter, sheet thickness, and material. The Siebel approximation for cylindrical drawing is:

F_draw ≈ π · d · t · σ_uts · (D/d − 0.7)

  d   = punch diameter
  D   = blank diameter
  t   = sheet thickness
  σ_uts = ultimate tensile strength

The (D/d − 0.7) term is the draw ratio penalty: blanks much larger than the punch require huge force to pull through. The maximum draw ratio for a single stage is roughly D/d = 2 — beyond that the cup wall tears.

Stamping variants compared

Variant	Operation	Volume	Tolerance	Best for
Single-stage stamping	One operation per stroke	1,000 to 100,000	±0.2 mm	Prototypes, low volume, very large parts
Progressive die	Multiple stations on a strip	50,000 to 100 M	±0.1 mm	Connectors, brackets, springs, washers
Transfer die	Mechanical fingers move parts between stations	10,000 to 10 M	±0.1 mm	Fuel tanks, oil pans, deep cups, large 3D parts
Fine blanking	V-ring + counter-punch, full shear	10,000 to 1 M	±0.02 mm	Clutch plates, locking mechanisms, gears
Deep drawing	Cup drawn from flat blank	10,000 to 1 B	±0.1 mm	Soda cans, battery shells, kitchen sinks
Compound die	Multiple operations in one station	10,000 to 10 M	±0.05 mm	Washers with simultaneous OD/ID, flat parts

Progressive dies dominate by sheer volume. A typical electrical contact terminal — the kind that crimps onto the end of a wire — runs through a 12-station progressive die at 800 strokes per minute, finishing 800 parts per minute on a single press. The strip carries half-finished parts through stations: pierce, partial form, full form, plate-tab cut, final detach.

Real-world specs

• Soda cans. Each aluminum can starts as a 0.27 mm thick blank, drawn into a cup, then ironed (squeezed between rollers) to 0.10 mm walls and 0.30 mm base. A modern can plant runs 1,800 cans per minute on each line — 100 million cans per year per line. Total stamping force per shot is around 10 tonnes.
• Automotive body side outer panel. A 1.4 m × 2.2 m steel sheet 0.7 mm thick, drawn into a deep complex panel on a 2,500-tonne tandem-line press. Cycle time 8 to 12 seconds per panel; tooling cost $5 million for the four-die set (blank, draw, restrike, trim).
• Battery cylindrical cell shells (18650, 21700). Deep-drawn nickel-plated steel cups 0.20 mm thick. The cups go through 5 to 8 progressive stations and finish at 200+ cells per minute per press. Tesla and BYD between them stamp billions of these per year.
• EI laminations for transformers. Silicon steel sheets 0.35 mm thick stamped into E and I shapes that interleave to form the transformer core. Stamped on 200-tonne high-speed presses at 600 strokes per minute.
• Coins. Modern US quarters are stamped from cupronickel at about 90 tonnes per coin, using a single coining press hit that simultaneously blanks the disk, coins both faces, and applies the reeded edge.
• Smartphone metal frames. Aluminum frames of iPhones and Galaxy phones are stamped/forged into near-net shape on 800 to 1,200 tonne presses, then CNC-finished. Each frame requires 3 to 5 stamping operations.

Press types

• Mechanical (eccentric/crank) press. A flywheel-driven crankshaft converts rotation to ram stroke. Fast (60 to 1,000 SPM) and energy-efficient. Force peaks at the bottom of the stroke, which suits blanking and forming. Most progressive-die work runs on mechanical presses.
• Hydraulic press. Constant force throughout the stroke at slower speeds (5 to 30 SPM). Better for deep drawing and forming where the press must hold pressure during a long stroke. Body panels run on hydraulic presses.
• Servo press. Programmable ram motion — the press can dwell at the bottom, slow down through critical strokes, or run a custom velocity profile. Doubles tool life for high-strength steel, reduces noise, and enables forming of materials that would crack on a fast mechanical press.
• Coining press. Slow, very high force (often 1,000+ tonnes for small parts), used for currency and precision flat parts where surface details matter.

Common failure modes

• Cracked draw beads. Drawn parts that exceed material formability tear at the punch radius — usually because clearance is too small, the draw ratio is too aggressive, or blank holder pressure is too high. Mitigated by deeper-drawing steel grades (DDQ, EDDQ) and lubricant.
• Wrinkles. The flange of a drawn cup buckles when the blank holder pressure is too low. Often the symptom of an undersized tonnage or worn holder springs.
• Springback. Bent parts spring back from the punched angle by 1 to 10 degrees once released. Compensated by overbending in the die geometry; high-strength steels and aluminum require more compensation than mild steel.
• Burr and rollover. The cut edge of a blanked part has a smooth zone (rollover), a sheared zone, a fracture zone, and a burr. Excess clearance expands the fracture zone and burr; insufficient clearance increases tool wear. Optimal clearance is 5 to 10 percent of sheet thickness.
• Galling. Aluminum, stainless steel, and high-strength steels weld onto the punch and tear off chunks of the part. Mitigated by coated punches (TiN, DLC, CrN), forming lubricant, and wider clearances.
• Tool fatigue. Punches and dies fail by surface fatigue (heat checking on hot stampings) or by chipping on hardened dies. Tool steels D2, M2, and powder-metal tool steels run from 50,000 to 5,000,000 strokes between regrinds.