Metal spinning is a chipless forming process where a flat disc of sheet metal is clamped against a rotating mandrel and progressively pressed into shape with a roller or rounded tool. The disc spins at hundreds to thousands of RPM while the operator (or CNC tool path) sweeps the roller from center to rim in passes, bending the metal around the mandrel's profile. Think of it as the metal equivalent of throwing a clay pot — except the "clay" is 3 mm of stainless steel and the forces involved can reach tens of kN at the contact point.

Why spinning beats deep drawing for certain parts:

• Tooling cost: A single mandrel (often hardwood, aluminum, or steel) replaces an expensive matched die set. Prototype tooling can cost $500 instead of $50,000.
• Geometry freedom: Re-entrant shapes, necked-in bottles, and parabolic dishes are easy — deep drawing struggles with these.
• Material efficiency: Minimal scrap. The blank diameter equals roughly the developed length of the finished profile.
• Work hardening: The metal is cold-worked, increasing yield strength by 20–50% in the finished part.

Two main variants:

• Conventional spinning: Wall thickness stays roughly constant. The blank diameter shrinks as material flows axially along the mandrel.
• Shear spinning (flow forming): The roller deliberately thins the wall following the sine law: t_final = t_blank × sin(α), where α is the half-angle between the mandrel surface and the spin axis. A 30° cone made from 4 mm stock yields a 2 mm wall.

Real-world example: Stainless steel cookware. Look at the bottom of a high-end saucepan — the smooth, continuous transition from flat base to vertical wall, with no weld seam and a slightly polished concentric finish, is the signature of spinning. Same process makes satellite dish reflectors, brass musical instrument bells, rocket nozzle liners, and the parabolic ends of pressure vessels (called "dished heads").

Rule of thumb — blank diameter: For a simple cylindrical cup of diameter D and height H, the blank diameter ≈ √(D² + 4DH). For a 100 mm diameter, 50 mm tall cup: √(10,000 + 20,000) ≈ 173 mm blank. Add 5–10% for trim allowance.

Failure modes to watch: Wrinkling at the rim (roller too aggressive or insufficient backup), circumferential cracking (material exceeded its forming limit — anneal between passes), and "orange peel" surface (grain too coarse — use finer-grained stock). Spinnability correlates with tensile elongation: aluminum 1100-O (35% elongation) spins beautifully; 6061-T6 (12%) cracks unless you anneal first.