Metal Spinning
Also: spin forming, metal spin forming, spun metal parts, CNC metal spinning, hand spinning, shear forming, flow forming, spun cones and domes, metal spinners
Metal spinning forms a rotating disc over a mandrel with a roller into one-piece cones, domes and cylinders: low tooling cost, low to medium volumes.
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A flat disc, the blank, is clamped against a mandrel (also called a chuck or form) on a spinning lathe. As the blank turns, a roller on a lever or a CNC slide presses the metal against the mandrel in a series of sweeping passes until it takes the shape of the mandrel. In conventional spinning the diameter shrinks while the wall stays close to the starting thickness, thinning somewhat at tight radii and wherever the metal is stretched. The mandrel sets the inside surface; the outside shows the path of the roller. Trimming, beading, curling the rim and necking are done in the same setup.
Hand spinning with levers and CNC or playback spinning lathes make the same parts; CNC repeats better and handles heavier gauges. Shear forming reduces the wall deliberately in one pass and follows the sine law: the finished wall equals the blank thickness times the sine of the half-angle of the cone, so a cone with a 30° half-angle comes out at half the blank thickness. Flow forming extends the idea to cylinders, squeezing a thick preform along a mandrel with rollers into a long, thin, work-hardened tube with a closely controlled wall. Hot spinning, with a torch or induction heating, forms thick plate, titanium and other alloys with limited cold ductility.
Tooling is the reason to spin. A mandrel is a turned form, in wood or plastic for prototypes and in aluminum, steel or cast iron for production; it usually costs a small fraction of a deep-draw die set and can be made in days. Cycle times run from under a minute to tens of minutes per part depending on size, material and the number of passes, so spinning wins from one piece up to a few thousand a year, and stamping or deep drawing wins above that for parts small enough for a press. Very large diameters, such as tank heads and dish antennas, are spun at any volume because dies of that size are impractical.
The shape must be a body of revolution: cones, domes, hemispheres, cylinders, venturis, and flanged or stepped shapes. Re-entrant shapes, narrower at the opening than behind it, need sectional or collapsible mandrels. Work hardening limits how far a part can be reduced before it is annealed, especially in stainless. Wall thickness is not uniform, tolerances are looser than machining or drawing in hard dies, and non-round features such as holes, louvers and flats are added afterwards.
At a glance
| Typical tolerances | Diameter ±0.4-0.8 mm (±0.015-0.030") on parts up to about 600 mm (24") as a commercial standard, growing with size to ±1.5 mm (±1/16") or more on large parts; ±0.13-0.25 mm (±0.005-0.010") with care on CNC equipment and hard mandrels, or by machining after spinning. The inside diameter, which the mandrel sets, is controlled better than the outside. The wall thins by roughly 10-25% in conventional spinning and more at tight radii; shear forming and flow forming control the wall, to about ±0.05-0.1 mm (±0.002-0.004") in flow-formed tube. Height after trimming ±0.4-0.8 mm (±0.015-0.030"). Angles ±1-2°, allowing for springback. Tolerances vary with material, gauge and spinner, so confirm them on the quote. |
|---|---|
| Size limits | Diameters from about 25 mm (1") to 2-3 m (80-120") at general spinning shops, and to 6 m (20 ft) or more at specialists that spin tank heads and large domes. Thickness from about 0.5 mm (0.020") to 6 mm (0.25") in aluminum and mild steel at most shops, and about half that upper limit in stainless; plate of 25 mm (1") and thicker is spun hot on large machines. Depth is limited by how far the material forms between anneals and by the lathe; cylinders deeper than their diameter usually need more than one mandrel or an anneal. |
| Surface finish | The inside copies the mandrel and is smooth. The outside carries fine concentric roller marks, about 0.8-3.2 µm Ra (32-125 µin) depending on the feed and the roller, which can be left, smoothed with a finishing pass, sanded or polished on the lathe. Lubricant residue is washed off. Spun parts are commonly polished, brushed, anodized, powder coated, plated or painted; stainless parts are passivated, and annealed parts may need pickling or blasting to remove oxide. Work hardening raises the strength and hardness of the spun wall. |
| Lead time | Prototypes in 1-3 weeks including a simple mandrel; production orders in 3-6 weeks. Hardened steel mandrels, multi-stage tooling, annealing between stages and outside finishing add time. Repeat orders from existing tooling often ship in 2-3 weeks. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Ductile sheet in a soft temper. Aluminum 1100-O and 3003-O spin most easily, with 5052-O and 6061-O for stronger parts; 6061 is spun in the O temper and heat treated to T6 afterwards. Drawing-quality low-carbon cold-rolled steel spins well. Stainless 304 and 316 spin but work harden quickly and may need intermediate anneals. Copper C110 and cartridge brass C260 spin very well and take a bright finish. Grade 2 titanium and nickel alloys such as Inconel 625 and Hastelloy C-276 are spun hot or with anneals for aerospace and chemical parts. Hard tempers, high-carbon steel, and 7075 or 2024 in the T tempers crack instead of flowing.
What drives the cost
- Size and thickness: blank material and spinning time grow with diameter and gauge
- Material: stainless, titanium and nickel alloys spin slowly, wear tooling and need anneals
- Depth and shape: deep or re-entrant shapes need several mandrels, breakdown stages or segmented tooling
- Tooling: the mandrel material and the number of stages, modest on small parts and significant on large ones
- Quantity: setup is short, but the cycle time per part stays high compared with a press
- Tolerances and finish: tight diameters, a controlled wall and cosmetic surfaces add passes, machining and polishing
- Secondary operations: trimming, piercing, flanging, welding, heat treatment and finishing
When to use it
- Round, axially symmetric parts: cones, domes, hemispheres, cylinders, bowls, venturis, funnels and reflectors
- One piece to a few thousand a year, where a draw die would never pay back
- Large diameters beyond press capacity: tank heads, hoppers, duct transitions, dish antennas
- Prototypes of parts that will later be deep drawn
- One-piece shapes in stainless, copper, titanium or nickel alloys for lighting, cookware, pressure and aerospace parts
When not to
- Shapes that are not bodies of revolution: boxes, rectangular pans, asymmetric forms
- High volumes of small parts: deep drawing or stamping makes them in seconds
- A uniform wall, or tolerances under ±0.13 mm (±0.005"), unless the part is machined afterwards
- Hard tempers and brittle alloys that crack instead of flowing
- Sharp inside corners: spinning needs a radius where the wall turns
Design tips
- Keep the part a true body of revolution, and add holes, slots and flats as secondary operations.
- Give inside corners a radius of at least two to three times the material thickness; sharp corners thin and crack.
- Dimension the inside surface, which the mandrel controls, and say where wall thickness matters; expect thinning elsewhere.
- Prefer shapes that open outward with a slight taper so the part comes off a one-piece mandrel; re-entrant shapes need segmented tooling.
- Add a curled, beaded or flanged rim to stiffen a thin-walled part.
- Specify a spinning alloy and temper: 1100-O, 3003-O, 5052-O or 6061-O aluminum, drawing-quality steel, annealed 304.
- If 6061-T6 properties are needed, call out heat treatment after spinning and allow for some distortion.
- Say which surface is cosmetic and what finish is wanted: as spun, brushed or polished.
Frequently asked questions
- What shapes can be made by metal spinning?
- Any hollow shape that is round about one axis: cones, domes, hemispheres, cylinders, bowls, funnels, venturis, bells and flanged or stepped combinations of them. Shapes that are narrower at the opening than behind it are possible with segmented or collapsible mandrels. Boxes and other non-round shapes cannot be spun.
- Metal spinning or deep drawing?
- Spinning needs one inexpensive mandrel and forms a part in minutes, so it suits prototypes, low and medium volumes and large diameters. Deep drawing needs a hardened die set for each stage but makes a part every few seconds, with tighter tolerances and more consistent walls, so it takes over from a few thousand pieces a year for parts that fit a press.
- How much does metal spinning tooling cost?
- Usually far less than stamping or draw dies. A simple mandrel for a small part can cost a few hundred dollars and a steel production mandrel for a mid-size part a few thousand; large or multi-stage tooling costs more. Wood or plastic mandrels serve for prototypes and short runs in soft aluminum. Prices vary by shop and part size.
- What is the difference between spinning, shear forming and flow forming?
- Conventional spinning shapes a disc over a mandrel in several passes and keeps the wall close to the blank thickness. Shear forming makes a cone in one pass and thins the wall by a predictable amount set by the cone angle. Flow forming thins and lengthens a thick cylindrical preform into a tube with a precise wall. All three use rollers and a rotating mandrel.
- Can stainless steel be spun?
- Yes. 304 and 316 spin well but work harden quickly, so deep parts are formed in stages with anneals in between, and they take more force and time than aluminum or mild steel. Expect a higher part price and a lower maximum thickness than the same shop offers in aluminum.
