Lesson 4.3DeformChapter 4 · Sheet and tube forming

Metal spinning

A roller presses a spinning disc over a form

Metal spinning shapes a spinning metal disc into a round, hollow part by pressing it onto a form, a bit like shaping clay on a potter's wheel. Lamp shades, stainless-steel bowls, the bells of brass instruments and rocket nose cones can all be made this way. The tooling is cheap, so it suits prototypes and small batches of round parts.

Photo: CC BY-SA 3.0, via Wikimedia Commons

metal spinning on a lathe · side viewThe main view shows the part halfway through; the box on the right shows it after the last pass. Circled numbers match the steps below.

How it works

Clamp. A flat disc (the blank) is clamped between the form, a solid shape called a mandrel, and a pad on the tailstock. The tailstock is the lathe's movable end support.
Spin. The headstock motor spins the form, the disc and the pad together, typically at a few hundred to a few thousand revolutions per minute.
Press in passes. A rounded roller, or a long lever tool in hand spinning, pushes on the spinning disc. It sweeps over the metal again and again, laying it a little closer to the form each pass.
Finish and trim. The last passes press the metal tight onto the form. The ragged rim is trimmed, often on the same machine, and the part comes off.

Key facts

Materials: Aluminum, copper, brass, low-carbon and stainless steel; also titanium and nickel alloys. Typically about 0.5 to 6 mm thick; heavy CNC machines spin much thicker plate
Typical parts: Cones, domes, bowls, cylinders, lamp reflectors, tank ends, nose cones, instrument bells
Part size: Diameters from a few centimeters to several meters
Precision: About ±0.1 to ±0.5 mm, depending on size and on hand versus CNC (computer-controlled) spinning
Surface finish: Smooth, with faint circular tool lines that are often polished away
Batch size it suits: One-offs to a few thousand parts; CNC spinning also handles larger runs
Tooling and machine cost: Forms from a few hundred to a few thousand dollars, often wood or plastic for prototypes; CNC spinning machines cost hundreds of thousands of dollars
Speed: Minutes per part by hand; under a minute to a few minutes on CNC

Strengths and limits

Strengths

Low tooling cost; a form can be made in days
Seamless parts with no welds
A design change only needs a new form
Works from small to very large diameters

Limits

Only round shapes (symmetric about the axis)
Slower per part than stamping at high volume
Hand spinning depends on a skilled spinner
Wall thickness varies a little; tight tolerances need CNC or extra machining

Where it's used

LightingLamp shades and reflectors
Kitchen and cateringBowls, pots, funnels, mixing bowls
MusicBells of trumpets and other brass instruments
Aerospace and defenseNose cones, engine inlet lips, pressure-vessel domes
Industry and HVACTank ends, hoppers, fan housings, duct parts

How to recognize it

A perfectly round part with a smooth, flowing profile: a cone, dome or bowl.
Fine concentric rings or a faint spiral on the surface, left by the roller.
A trimmed or rolled rim at the opening, and sometimes a small center mark where the tailstock pad pressed.
No seam or weld anywhere.