Metal Spinning
Metal spinning presses a rotating metal disc against a mandrel with a roller or hand tool, forming it into an axially symmetric hollow shape.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Metal
- Typical tolerances
- Roughly ±0.010–0.030 in (±0.25–0.75 mm) on diameters, tighter on CNC machines with hard mandrels; wall thickness varies through the part in conventional spinning and follows the sine law in shear spinning
- Surface finish
- Spiral tool marks are inherent and are polished out where cosmetics matter; a burnished, work-hardened surface otherwise
- Typical volumes
- 1–5,000 parts; above about 5,000 deep drawing usually wins on cycle time
- Lead time
- 1–3 weeks including mandrel; days for repeat work on an existing mandrel
- Materials
- Metal
What it is
Metal spinning presses a rotating sheet metal disc against a mandrel with a roller or hand tool, working it progressively until it takes the mandrel's shape. It is the cheapest way to make an axially symmetric hollow metal part: the tooling is a single mandrel, often turned from hardwood, MDF, or aluminum, and it costs a small fraction of a draw die.
Typical parts are cones, hemispheres, dished heads, reflectors, tank ends, cookware, lighting housings, nose cones, and ducting transitions. Blank thickness usually runs 0.020–0.250 in (0.5–6 mm), and diameters range from a few inches to well over ten feet on large machines. Aluminum, copper, brass, low-carbon steel, and 304 stainless all spin well.
Cycle times of minutes rather than seconds cap the practical volume near a few thousand pieces a year, but with no die to pay for, spinning is economic from a single part and is the standard choice for prototypes of parts that will eventually be deep drawn.
How it works
- Blank and mandrel. A circular blank is cut and clamped between the mandrel (mounted on the headstock) and a tailstock follower block. The mandrel is the part's internal shape.
- Spin up. The lathe brings the assembly to speed, typically several hundred to a couple of thousand rpm depending on diameter — bigger blanks run slower to keep surface speed reasonable.
- Forming passes. A roller or hand tool is worked against the blank in a sequence of passes, walking the metal down against the mandrel a little at a time. The metal is pushed, not stretched: in conventional spinning the wall thickness stays approximately constant while the blank diameter reduces, which means every pass moves material inward rather than thinning it.
- Lubrication and heat. Spinning generates real heat, and lubricant is applied continuously. Thick blanks and hard materials are sometimes spun hot.
- Interstage annealing. The metal work hardens as it deforms. Stainless steel and heavily worked aluminum usually need annealing between stages, or they will crack.
- Trim and finish. The rim of the spun part is uneven and gets trimmed; the mandrel is withdrawn, and spiral tool marks are polished off if cosmetics matter.
Shear spinning and flow forming
Shear spinning deliberately thins the wall instead of feeding material inward. Wall thickness follows the sine law — the formed wall equals the blank thickness multiplied by the sine of the half-angle between the mandrel surface and the axis — so a 30° cone from a 0.100 in blank yields a 0.050 in wall. The blank diameter stays the same as the finished part diameter, which makes shear spinning far more material-efficient for cones.
Flow forming applies the same principle to tubes, extending a preform axially over a mandrel with rollers to produce long, thin-walled, very concentric cylinders — pressure vessels, rocket motor cases, and drive shafts.
Design guidelines
Axial symmetry is mandatory
Everything about the part must be a body of revolution. Flats, lugs, and off-axis holes are secondary operations. Re-entrant shapes — where the diameter reduces and then increases again — require a segmented or collapsible mandrel, which is a substantially more expensive tool.
Corner radii
Sharp corners at the transition between the base and the wall thin locally and are the usual failure point. Use a radius of at least 1–2× material thickness, and larger on hard materials. A radius equal to several times the thickness spins far more easily than the minimum.
Depth and wall angle
Shallow, open shapes spin easily. Deep cylindrical shapes with near-vertical walls are the hardest cases in conventional spinning, because material must travel a long way with little support. Cones and dished shapes are the natural geometry.
Wall thickness
In conventional spinning, wall thickness stays close to the blank thickness but is not constant — expect some thinning at the transitions and some thickening near the rim. If a specific finished wall is required, shear spinning gives a predictable result from the sine law; conventional spinning does not.
Material selection
Annealed tempers spin best: 1100, 3003, and 5052-O aluminum, C11000 copper, cartridge brass, drawing-quality low-carbon steel, and 304 stainless. Hardened and precipitation-strengthened tempers crack. Specify the temper explicitly. Blank thickness selection from the sheet metal gauge chart; flat-pattern development for any secondary flanges from the bend radius and K-factor chart.
Trim allowance
Leave 0.25–0.5 in (6–13 mm) of extra length at the open end for trimming. The rim of a spun part is always uneven.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Geometry | Body of revolution | Re-entrant shapes need a collapsible mandrel | The mandrel must withdraw from the finished part |
| Base-to-wall radius | ≥ 3× thickness | 1–2× thickness | Sharp transitions thin locally and split |
| Wall angle | Cones and dished forms | Deep vertical walls are hardest | Unsupported metal must travel far |
| Wall thickness control | Accept variation | Use shear spinning for a defined wall | Conventional spinning does not control thickness |
| Material temper | Annealed (O temper) | Hard tempers crack | Work hardening accumulates every pass |
| Trim allowance | 0.25–0.5 in (6–13 mm) | — | The formed rim is always uneven |
| Interstage anneal | Required for stainless | — | Work hardening exhausts ductility |
Cost drivers
Spinning inverts the deep drawing cost structure: tooling is trivial and per-part labor is high. A mandrel in hardwood, MDF, or aluminum can be turned in a day or two, so entry cost is minimal. But every part takes minutes of skilled operator or CNC machine time, and multi-stage parts need annealing cycles between passes.
Volume breakpoints: 1 to a few hundred parts is where spinning is unbeatable — no other process gets a 40 in diameter dished head made this week. From a few hundred to a few thousand, CNC spinning with a hardened steel mandrel keeps it competitive. Above roughly 5,000 a year, deep drawing's seconds-per-part cycle overtakes spinning despite the die cost.
- Design a family of parts on one mandrel. Different depths and trim diameters from the same mandrel reuse the whole tool.
- Open the corner radii. Generous radii spin in fewer passes and crack less, which shows up directly in labor.
- Avoid re-entrant geometry. A collapsible or segmented mandrel can cost more than the rest of the job.
- Use shear spinning for cones. It starts from a blank the same diameter as the finished part, cutting material use substantially versus conventional spinning.
- Specify an annealed temper. Cracked parts from hard-temper stock are the most common avoidable scrap in spinning.
Questions
6 questionsWhat shapes can be metal spun?
Bodies of revolution — cones, hemispheres, dished heads, cylinders, and flared or stepped profiles. Re-entrant shapes, where the diameter reduces and then grows again, require a segmented or collapsible mandrel, which is much more expensive than a solid one.
Does metal spinning change the wall thickness?
In conventional spinning, thickness stays close to the blank thickness but varies through the part, thinning at transitions and thickening near the rim. Shear spinning deliberately thins the wall following the sine law: the wall equals blank thickness times the sine of the half-angle, so a 30° cone from a 0.100 in blank gives a 0.050 in wall.
What volume justifies metal spinning over deep drawing?
Spinning wins from one part up to a few thousand a year, because the mandrel is inexpensive and can be turned in days. Deep drawing wins above roughly 5,000, where its seconds-per-part cycle time overcomes the cost of a matched draw die.
What materials spin well?
Annealed tempers: 1100, 3003, and 5052-O aluminum, copper, cartridge brass, drawing-quality low-carbon steel, and 304 stainless. Hard and precipitation-strengthened tempers crack. Stainless and heavily worked aluminum usually need annealing between stages, since the metal work hardens on every pass.
What tolerance can metal spinning hold?
Roughly ±0.010–0.030 in (±0.25–0.75 mm) on diameters, and tighter on CNC machines with hardened steel mandrels. Diameters formed hard against the mandrel hold better than free-formed rim regions, which is also why a trim allowance of 0.25–0.5 in is standard.
What is flow forming?
Shear spinning applied to tube. A short thick preform is extended axially over a mandrel by rollers, producing a long, thin-walled, highly concentric cylinder. It is how pressure vessel bodies, rocket motor cases, and lightweight drive shafts are made.